A Last Look at Laetoli - The Eye American... · 2017-08-31 · Scientific American September 1998 7 P ercy Bysshe Shelley’s poem Ozymandias describes the cracked and toppled statue

SEPTEMBER 1998 $4.95

They are precious clues to the past,rare fossil tracks

left millions of

years ago by

humanity’s

ancestors.

So why has

science buried them?

A Last Look at Laetoli

WEIGHTLESSNESS AND HEALTH • NEW ELEMENTS • ATTENTION-DEFICIT DISORDER

Laetoli, 3,600,000 B.C.

Copyright 1998 Scientific American, Inc.

Preserving the Laetoli FootprintsNeville Agnew and Martha Demas

S e p t e m b e r 1 9 9 8 V o l u m e 2 7 9 N u m b e r 3

FROM THE EDITORS

7

LETTERS TO THE EDITORS

8

50, 100 AND 150 YEARS AGO

12

NEWS AND

ANALYSIS

SCIENCE AND THE CITIZEN

Resolving the universe.. . .

Medicinal marijuana?.. .

A river of acid.. . . Damnable

weather.. . . The burning season.

18

PROFILE

Rolf Landauer seeks the physical

limits of computation.

32

TECHNOLOGY AND BUSINESS

Electronic paper.. . . The first cancer

vaccine arrives. . . . Holographic

memories that hold up.

36

CYBER VIEW

Headaches from inscrutable

computers.

42

IN FOCUS

Violence in the classroom

proves hard to prevent.

15

44

58

The 3,600,000-year-old footprints found 20 years ago in the Laetoli area of north-

ern Tanzania vividly evoked how early human ancestors may have lived. To pro-

tect those tracks, scientists have now painstakingly reburied them. The authors,

who led the conservation project, explain why and how it was done. In addition,

with anthropologist Ian Tattersall and artist Jay H. Matternes, they describe how

views of the footprint makers have changed.

4

A rare dugong(page 20)

A rare dugong(page 20)

Astronauts suffer from motion sick-

ness, bone and muscle loss, puffy fac-

es and shrunken thighs. Nevertheless,

no ailment in four decades of space

travel suggests that humans cannot

survive long space voyages. Better

still, space medicine is providing new

clues about how to treat down-

to-earth conditions such as

osteoporosis and anemia.

Weightlessness and the Human BodyRonald J. White


Scientific American (ISSN 0036-8733), published monthly by Scientific American, Inc., 415 Madison Avenue, New York,N.Y. 10017-1111. Copyright © 1998 by Scientific American, Inc. All rights reserved. No part of this issue may be repro-duced by any mechanical, photographic or electronic process, or in the form of a phonographic recording, nor mayit be stored in a retrieval system, transmitted or otherwise copied for public or private use without written permissionof the publisher. Periodicals postage paid at New York, N.Y., and at additional mailing offices. Canada Post Internation-al Publications Mail (Canadian Distribution) Sales Agreement No. 242764. Canadian BN No. 127387652RT; QST No.Q1015332537. Subscription rates: one year $34.97 (outside U.S. $49). Institutional price: one year $39.95 (outside U.S.$50.95). Postmaster : Send address changes to Scientific American, Box 3187, Harlan, Iowa 51537. Reprints available:write Reprint Department, Scientific American, Inc., 415 Madison Avenue, New York, N.Y. 10017-1111; fax: (212) 355-0408or send e-mail to [email protected] Subscription inquiries: U.S. and Canada (800) 333-1199; other (515) 247-7631.

Attention-Deficit Hyperactivity DisorderRussell A. Barkley

Once viewed as simple inattentiveness or overac-

tivity, ADHD now appears to result from neuro-

logical abnormalities that may have a genetic ba-

sis. Behavioral modification training, along with

stimulant drugs, could help children and adults

with ADHD learn to exercise more self-control.

Creating superheavy atomic nuclei takes not

only tremendous energy but also a delicate

touch, because they last for only microsec-

onds. If they exist, elements 114 and be-

yond may prove surprisingly stable.

A math book for everybody.

Wonders, by the MorrisonsAlien constellations.

Connections, by James BurkeInsurance, stamps and stolen marbles.

102

WORKING KNOWLEDGEHow CD players turn light into sound.

109

About the Cover

Footprint made by an ancestor of Homosapiens proves that early hominids were

fully bipedal—long before the invention

of stone tools or the expansion in brain

size. Image by Slim Films.

ELEMENTARY MATTERS

Making New ElementsPaul Armbruster and Fritz Peter Hessberger

66

72

78

84

90

THE AMATEUR SCIENTISTExtracting DNA in your own kitchen.

96

MATHEMATICALRECREATIONS

Building the pyramids on schedule.

98

5

Mendeleev’s periodic table of the elements

is a brilliant document: an organizational

scheme for nature’s building blocks that

has withstood dramatic upheavals in

20th-century physics and that points the

way to new discoveries.

The Evolution of the Periodic SystemEric R. Scerri

Thermophotovoltaic devices convert heat from

fossil fuels, sunlight or radioactive isotopes direct-

ly into electricity. They may be ideal as generators

for deep-space probes, small boats, remote villages

and troops in the field that need compact, light-

weight, reliable power sources.

ThermophotovoltaicsTimothy J. Coutts and Mark C. Fitzgerald

Far beyond Pluto, almost halfway to Alpha Cen-

tauri, trillions of icy globes encase the solar system

in a diffuse spherical shell. Refugees from the for-

mation of the planets, these comets-in-waiting or-

bit in darkness until passing stars or clouds of in-

terstellar gas knock a few sunward once again.

The Oort CloudPaul R. Weissman

REVIEWS AND COMMENTARIES

THE SCIENTIFIC AMERICANWEB SITE

THE SCIENTIFIC AMERICANWEB SITE

Voyage to an undersea volcano on

board the submersible Alvin:http://www.sciam.com/explorations/

1998/070698atlantis/index.html

And check out enhanced versions

of this month’s feature

articles and departments,

linked to other science

resources on the

World Wide Web.

Voyage to an undersea volcano on

board the submersible Alvin:http://www.sciam.com/explorations/

1998/070698atlantis/index.html

And check out enhanced versions

of this month’s feature

articles and departments,

linked to other science

resources on the

World Wide Web.

www.sciam.comwww.sciam.com


Scientific American September 1998 7

Percy Bysshe Shelley’s poem Ozymandias describes the cracked and

toppled statue of an ancient potentate: “My name is Ozymandias,

king of kings: Look on my works, ye Mighty, and despair!” The

destruction of his once great empire might be mistaken as punishment for

hubris. The grimmer reality is that Ozymandias could have been the soul of

modesty and nature would have ground his works to powder just the same.

Three and a half million years ago a trio of furry bipeds walked across

an African savanna caked with damp volcanic ash. Maybe it was a happy

family stroll on a Saturday afternoon; maybe they crept fearfully through

a predator’s hunting ground. We will never know (but we can present the

best, most recent guess; see page 44 in “Pre-

serving the Laetoli Footprints,” by Neville

Agnew and Martha Demas). Odds are that

for those creatures, it was just another or-

dinary walk on another ordinary day. The

hidden struggles of their lives, the glim-

merings of hope and pride they nurtured

are all gone forever. Meanwhile their mud-

dy footprints have lasted 700 times longer

than recorded history. Where is the poetic

justice?

We moderns can expect no better treat-

ment. Wood rots, paper burns, stone splits,

plastic corrodes, glass shatters, metal rusts.

If humans disappeared tomorrow and no

one was left to mow the lawns, paint the

walls and fix the pipes, even the sturdiest of our concrete and steel struc-

tures would be mossy rubble in roughly 10,000 years. The irony is that al-

ready ancient stone monoliths like the Egyptian pyramids might be among

the last artifacts to vanish from view.

To put it another way, imagine watching the events of the next million

years on that uninhabited Earth, all compressed into a 100-minute

feature film. Don’t be late finding a seat in the theater: within the first 60

seconds nearly every large trace of civilization will have melted into the

terrain. Nothing to do then but watch the forests grow (talk about a slow

second act). Our world would survive as a stratum of buried junk.

So future anthropologists may not be assessing the heights of our ac-

complishments from the Mona Lisa, or Shakespeare, or the Golden Gate

Bridge, or a space shuttle. They may be measuring the tooth marks on our

chewed pencils; checking the metallurgy of old screwdrivers; deducing the

economy from phone books in landfills. Perhaps the act for which you

will be longest remembered was something you wrote in a wet cement

sidewalk when you were six years old: I WUZ HERE.

Go Ahead, Walk in the Mud

®

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AN EARLY STEPin human evolution

J. P

AU

L G

ETTY

TRU

ST


BRAIN TEASER

Iwas intrigued by the interesting arti-

cle “The Genetics of Cognitive Abili-

ties and Disabilities,” by Robert Plomin

and John C. DeFries [May]. I was puz-

zled, however, by a sample test that ap-

peared in the article (right). According to

the answer, the figure specified appears

in only a, b and f. But it also appears in

d, e and g. Although the test’s instruc-

tions state explicitly that the figure

must always be in the position shown,

not upside down or on its side, noth-

ing is said about the figure needing to

be the same size as illustrated. This

example demonstrates a problem

facing anyone who has taken a cogni-

tive ability or related test—having to

gauge the intelligence and perceptions

of the people who wrote the test.

Are they even aware of all possible

answers? In this case, the answer is

no, and the child pays the penalty.

JIM BAUGHMANWest Hollywood, Calif.

Plomin and DeFries’s article left me

skeptical as to whether the authors’

approach is likely to be scientifically

fruitful. For instance, their reliance on

tests of so-called cognitive ability (what

used to be called IQ) as measures of

general categories of intellectual func-

tioning is questionable. Psychologists’

claims about what such tests measure

rely entirely on assumed correlations

between test performance and cognitive

ability. In the physical and medical sci-

ences, inferring causes from correlations

alone is typically considered an error.

Imagine Galileo and Newton proceed-

ing similarly: on noticing a high (but im-

perfect) correlation between the speed

at which falling objects hit the ground

and the height from which they fall, the

scientists simply regard falling objects

as a new type of measuring instrument

for estimating height, forgoing a scien-

tific understanding of the reason for the

correlation (gravity), as well as its im-

perfect character (air friction).

J. M. CRONKHITEDepartment of Physics

Georgia Institute of Technology

Until tests are devised to measure the

full array of human cognitive abil-

ities, including teamwork and leader-

ship skills, rhythm, curiosity, attention,

self-confidence, imagination and so on,

we will have little luck in teasing apart

the various genetic and environmental

mechanisms of “intelligence.” It would

be more profitable to explore human

cognitive abilities from a different an-

gle. How can we help all children take

maximum advantage of their unique ge-

netic endowments? How close has any-

one come to reaching his or her genetic

potential, and how was this achieved?

BOB KOHLENBERGERBurlingame, Calif.

Plomin and DeFries reply:When tests of specific cognitive abili-

ties are actually administered, practice

items and examples clearly illustrate the

types of responses that are considered

to be correct, so the problem encoun-

tered by Baughman should not be an is-

sue. In response to the comment by

Cronkhite, the old shibboleth that cor-

relations do not prove causation is not

relevant. Tests of statistical significance,

including analysis of variance and co-

variance, can be incorporated as special

cases of multiple regression and corre-

lation analysis.

A more relevant issue is the experi-

mental power of the design. Although

twin and adoption studies are quasiex-

perimental in that people are not ran-

domly assigned to be members of a set

of twins or to be adopted, the studies

do provide considerable power to ad-

dress the questions of nature and nur-

ture in relation to cognitive abilities and

disabilities.

We agree that there is much more

to life than cognitive abilities, includ-

ing teamwork, leadership skills and

so on. But these traits are not highly

correlated with cognitive abilities and

thus were not discussed in our arti-

cle. And we are interested in trying

to help children maximize their ge-

netic endowments: our review con-

cerned the extent to which such en-

dowments for cognitive abilities and

disabilities are important.

X-RAY VISION

As an otolaryngologist, I was inter-

ested in W. Wayt Gibbs’s article

on radiation therapy in the May issue

[“Taking Aim at Tumors,” News and

Analysis]. I noticed, however, that the

caption for the picture on page 20 is in-

correct. Most standard x-rays and CT

scans show high-density areas, such as

bone, to be light, and they show low-

density areas, such as air, to be dark.

The black area in the picture, listed as

the esophagus, is actually air inside the

larynx. The walls of the esophagus are

generally collapsed on each other and

don’t contain any air.

JACK ALAND, JR.Birmingham, Ala.

Letters to the Editors8 Scientific American September 1998

L E T T E R S T O T H E E D I T O R S

Letters about the article by Robert Plomin and John C. DeFries on the ge-netics of intelligence began pouring in as soon as the May issue hit

subscribers’ mailboxes. Some readers looked forward to a day when suchresearch could, for instance, help teachers make “a preemptive strike againstreading problems,” as suggested by Jonathan Bontke of St. Louis. But manymore people expressed concern about potential misuse of these findings.Henry D. Schlinger, Jr., a professor of psychology at Western New EnglandCollege, even questioned the rationale for seeking an intelligence gene: cit-ing the authors’ assertion that biology is not destiny, Schlinger wondered,“Then why should we care about what the heritability of a particular trait is?”

4. HIDDEN PATTERNS: Circle each pattern below in which the figureappears. The figure must always be in this position, not upside down or on its side.

a b c d e f g

JEN

NIF

ER C

. CH

RIST

IAN

SEN

OFFICIAL ANSWERS(red) to this test of cognitive ability do not include the alternative answers (dashed

red lines) suggested by several readers.


REMOTE CONTROL

As a retired communications engineer,

I was naturally very intrigued with

your articles on the upcoming improve-

ment in the technology of television

[“The New Shape of Television,” May].

Now, if we could only see correspond-

ing improvement in the quality of the

programming, we would have some-

thing worth watching.

EUGENE V. KOSSOGualala, Calif.

Letters to the editors should be sentby e-mail to [email protected] or bypost to Scientific American, 415 Madi-son Ave., New York, NY 10017. Let-ters may be edited for length and clari-ty. Because of the considerable volumeof mail received, we cannot answer allcorrespondence.

Letters to the Editors Scientific American September 1998 9

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Advertising and Marketing Contacts

Solution to the Martin Gardner Puzzle

In “A Quarter-Century of Recre-

ational Mathematics,” by Martin

Gardner [August], the author pre-

sented his Vanishing Area Paradox,

illustrated by the two figures below.

Each pattern is made with the same

16 pieces, but the lower pattern has

a square hole in its center. Where did

this extra bit of area come from?

The key to the paradox is that the

large and small

right triangles are

not similar—their

acute angles are

slightly different.

Because of this

difference, the

upper pattern is

concave: the an-

gles at the cor-

ners are slightly

less than 90 de-

grees, so the sides

of the figure buckle inward. In the

lower pattern, the corner angles are

slightly more than 90 degrees, so

the sides bulge outward. The differ-

ence in area between the two figures

is equal to the area of the square

hole in the lower pattern.

IAN

WO

RPO

LE


SEPTEMBER 1948THE TRANSISTOR APPEARS—“Within the past few

months a group of physicists at the Bell Telephone Laborato-

ries has made a profound and simple finding. In essence, it is

a method of controlling electrons in a solid crystal instead of

in a vacuum. This discovery has yielded a device called the

transistor (so named because it transfers an electrical signal

across a resistor). Not only is the transistor tiny, but it needs

so little power, and uses it so efficiently (as a radio amplifier

its efficiency is 25 per cent, against a vacuum tube’s 10 per

cent) that the size of batteries needed to operate portable de-

vices can be reduced. In combination with printed circuits it

may open up entirely new applications for electronics.”

PRIMARY CARE—“Primitive medicine men learned long,

long ago what modern medicine is just rediscovering—that

distinctions between the mind

and the body are artificial. The

primitive doctor understands well

the nature of psychogenic illness.

Among pre-literate peoples, as

among those in more civilized

societies, these emotional discom-

forts are easily translated into

neurotic symptoms. This illus-

tration shows a sand painting

made by a Navaho medicine

man, designed to treat mind and

body in a curing ceremony. The

painting is made on the floor of

a hut, the patient is laid upon it

and paint is rubbed over him.”

SEPTEMBER 1898ON EVOLUTION—“At the

Cambridge Congress of Zoology

Prof. Ernst Haeckel read a fasci-

nating paper on the descent of

man. He does not hesitate to say

that science has now definitely

established the certainty that

man has descended through var-

ious stages of evolution from the

lowest form of animal life, during a period of a thousand

million years. ‘The most important fact is that man is a pri-

mate, and that all primates—lemurs, monkeys, anthropoid

apes, and man—descended from one common stem. Looking

forward to the twentieth century, I am convinced it will uni-

versally accept our theory of descent.’”

NO TRANSISTOR NEEDED—“Mr. José Bach describes in

L’Illustration an instrument by means of which the Brazilian

Indians communicate with each other at a distance. In each

malocca, or dwelling, there is a cambarisa, a sort of wooden

drum buried for half of its height in sand. When this drum is

struck with a wooden mallet, the sound is distinctly heard in

the other drums situated in the neighboring maloccas. The

blows struck are scarcely audible outside of the houses in

which the instrument is placed, so it is certain that the trans-

mission of the sound takes place through the earth, the drums

doubtless resting upon the same stratum of rock.”

SEPTEMBER 1848GOLD!—“News has reached us from California of the dis-

covery of an immense bed of gold of one hundred miles in

extent, near Monterey. It is got by washing out river sand in

a vessel, from a tea saucer to a warming pan. A single person

can gather an ounce or two in a day, and some even a hun-

dred dollars’ worth. Two thousand whites and as many Indi-

ans are on the ground. All the American settlements are de-

serted, and farming nearly sus-

pended. The women only remain

in the settlements. Sailors and

captains desert the ships to go to

the gold region.” [Editors’ note:The Sutter’s Mill find led to the1848 California gold rush.]

FOSSIL THEORY—“The fossil-

iferous rocks in the sedimentary

strata present us with the differ-

ent objects of bygone periods,

and it is astonishing what minute

and delicate objects have been

transmitted to us: the traces of

footsteps on wet sand; undigest-

ed food; even the ink bag of the

sepia [cuttlefish] has been found

so perfect that the same material

which the animal employed cen-

turies, nay, thousands of years

ago, to preserve itself from its en-

emies, has served for color to

paint its likeness with!—Alexan-

der Humbolt.”

FLOATING TUNNEL—“One

of the most extraordinary plans submitted to the French

Academy of Sciences is that of M. Ferdinand, engineer, who

proposes a floating tunnel from Calais to Dover, for the wires

of the electric telegraph, and large enough to be traversed by

small locomotives, for the conveyance of passengers. A tun-

nel for the wires of the electric telegraph we believe to be per-

fectly practicable and requires no great genius to conceive or

construct, but a floating tunnel for locomotives is as prepos-

terous as it is useless.” [Editors’ note: See News and Analysis,“Tunnel Visions,” July 1997, for an update on useful floatingtunnels now being planned.]

50, 100 and 150 Years Ago

5 0 , 1 0 0 A N D 1 5 0 Y E A R S A G O

12 Scientific American September 1998

Navaho sand painting for a curing ceremony


News and Analysis Scientific American September 1998 15

Heading back to school

brings to mind shiny new

notebooks, multicolored

pens, the latest clothes and some free

time for parents. This fall, however,

parents, teachers and students have an

additional concern: school shootings.

Although only 1 percent of all homi-

cides—and suicides—of school-age chil-

dren in the U.S. occur on school grounds, this statistic repre-

sents a dramatic increase. According to a survey by the Na-

tional School Safety Center (NSSC), the number of violent

deaths in schools rose 60 percent last year to a total of 41,

nearly half of which were multiple shootings. Experts worry

that an epidemic of school violence is under way. As Ronald

D. Stephens, executive director of the NSSC, describes it,

there have been attempted cases of “copycat killings,” partic-

ularly after the shootings in March at a Jonesboro, Ark.,

middle school that killed four students and a teacher.

Anxious to stop this trend, teachers and administrators

around the country have embraced a variety of preventive

techniques—everything from metal detectors to daily classes

in controlling anger. But in many instances, these programs

have not been graded for efficacy. Even more troubling is the

fact that, according to recent studies, certain popular meth-

ods simply do not work.

Preventing violence depends in large part on understanding

what causes it. The school shootings are not isolated but are

clearly part of a larger problem. During the past decade,

homicides and suicides among young people have more than

doubled; the rate of death as a result of firearms among

American children 15 years and younger is 12 times higher

than it is in 25 other developed countries combined. Al-

though the causes for these developments are myriad, studies

have documented that the standard complaints—ready access

NEWS AND ANALYSIS

32PROFILERolf Landauer

IN FOCUS

FORESTALLING

VIOLENCE

American youths are suffering anepidemic of violence, both in and out of the classroom. Designing effective prevention programs

is proving difficult

42CYBER VIEW

CONFLICT RESOLUTIONtraining courses are required in 61 percent of U.S. school districts. Despite the popularity of such programs, many have not been evaluated for effectiveness.

22 IN BRIEF

24 ANTI GRAVITY

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to guns as well as exposure to brutality, both at home andon-screen—do have an effect on kids. Initial results from theNational Longitudinal Study on Adolescent Health (an on-going survey of 12,000 adolescents) showed that childrenwho are able to get ahold of guns at home were more likelyto behave violently. The study also indicated that good pa-rental and family relationships correlated somewhat with re-ductions in violent behavior.

For its part, the correlation with television mayhem is long-standing. As far back as the 1960s, psychologist Leonard Eronand his colleagues at the University of Illinois demonstratedthat the more violence children watched on television, themore aggressive their behavior at school. The final report ofthe National Television Violence Study, conducted by the Na-tional Cable Television Association (NCTA) and released thispast spring, “confirmsthat TV portrays violencein a way that increases therisk of learning aggressiveattitudes,” says John C.Nelson of the AmericanMedical Association, oneof the organizations thatwas part of the NCTA ad-visory council.

Although experts havebeen able to make head-way in understandingsome of the roots of vio-lence, their efforts to fore-stall it have been less suc-cessful. Most violence-pre-vention programs are runlocally, often through theschool system. Becausepolicy at each of the some100,000 U.S. schools istypically set by localschool boards, there isconsiderable diversity inapproach. Yet “many ofthe programs being implemented [in schools] have not beenrigorously evaluated” by researchers, according to Linda L.Dahlberg of the National Center for Injury Prevention andControl at the Centers for Disease Control and Prevention(CDC). In 1992, to remedy this problem, the CDC began alarge-scale effort to review violence-prevention initiativesaround the country.

Preliminary results from the CDC and other studies are be-ginning to come in, Dahlberg says, and they are “a mixedbag.” For instance, intervention programs that start very ear-ly—some in kindergarten—can actually introduce children toideas about violence that might not have occurred to themotherwise. Young children in such programs have describedmore violent, aggressive thoughts and fantasies than research-ers anticipated. “We want to intervene early,” Dahlberg notes.“But when? And what should we do?” She suggests that ear-ly-intervention programs should focus not just on the childbut on the family and community.

At a conference earlier this year in Charleston, S.C., Del-bert S. Elliott of the University of Colorado’s Center for theStudy and Prevention of Violence reported that “the evidencefor programs that focus on family relationships and function-

ing, particularly on family management and parenting prac-tices, is quite strong and consistent.” His findings were basedon a study of more than 450 prevention programs. Elliottalso described conflict resolution training, peer counselingand peer mediation as ineffective when implemented alone:only when used as part of a more comprehensive preventionapproach did they did show positive results.

More extensive programs, however, require more resourc-es—money, people and time. Dahlberg points out that someof the less effective techniques were used in what she calls“schools in crisis,” where teachers and administrators werepreoccupied with other problems, such as overcrowding or de-teriorating buildings, or were not supportive of the program.

Quick fixes such as metal detectors do not seem to do muchgood either. Researchers point out that such sensors are often

expensive and will keeponly some of the weaponsout. At the same time, be-cause most violence oc-curs outside of school, theydo little to address thegeneral problem of youthviolence.

In the aftermath of re-cent school shootings, ex-perts emphasized the im-portance of watching forwarning signs of violence,but again, such monitor-ing is not foolproof. InJune the NSSC released alist of 20 potential indica-tors for violent behavior,including having a historyof bringing weapons toclass or having been bul-lied in school. Even so,NSSC executive directorStephens says, “for all thehigh-tech strategies wehave, there is not a scan-

ner around that can predict how and when a child might ex-plode” in anger and violence.

Some researchers are even concerned that this analyticalapproach could wind up harming kids. Edward Taylor of theUniversity of Illinois, who is developing a study for identify-ing predictors of violence in children, offers words of cau-tion: “We certainly don’t want a school system that everytime a child throws a temper tantrum, every time a child sayssomething aggressively, that they are immediately suspect ofbecoming mentally ill and violent.”

Notwithstanding the debates about prevention and thevarious attempts to reduce youth violence, many expertsworry that the broader context is being forgotten: until pro-grams consider youth violence against a societal backdrop ofviolence, they may have only limited success at best. MikeMales of the University of California at Irvine, whose bookFraming Youth: Ten Myths about the New Generation willbe published in October, argues that “the youth culture of vi-olence is the adult culture of violence.” Nearly 10 times asmany children die at the hands of their parents as die atschool. The tradition of learning by example has rarely hadsuch tragic consequences. —Sasha Nemecek

News and Analysis16 Scientific American September 1998

STUDENTS MOURNvictims of a shooting in Springfield, Ore., in May of this year.

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The exact location is a secret.But somewhere between Lon-don and Brighton a com-

pound ringed by high fences and razorwire will house the world’s only potfarm primarily devoted to commercialdrug development. In June the BritishHome Office gave a startup pharmaceu-tical company a license to grow 20,000marijuana plants of varied strains.

Geoffrey W. Guy, chairman of GWPharmaceuticals, intends to proceed toclinical trials with a smokeless, whole-plant extract, while also supplying mar-ijuana to other investigators interestedin medical research and pharmaceuticaldevelopment. The 43-year-old entrepre-neur-physician wants to capitalize onwhat he sees as the unexploited oppor-tunity to legitimize marijuana as medi-cine. “Cannabis has been much ma-ligned,” Guy says. “There are over10,000 research articles written on theplant, and there’s something well worthinvestigating here.”

The idea of giving this alternativemedicine a place alongside antibioticsand aspirin in the physician’s standardpharmacopoeia is by no means a newone. Marijuana and its chemical con-stituents have aroused interest as atreatment for conditions ranging fromthe nausea induced by cancer drugs tothe fragility of brain cells harmed bystroke. In the U.S., oral doses of delta-9-tetrahydrocannabinol (THC)—a syn-thetic version of the chemical in mari-juana that both relieves nausea and getsa person high—have been available onthe market since 1986.

But the makers of Marinol (the tradename for the THC synthetic) have hadtrouble competing with dealers on thestreet. A swallowed pill takes too longto relieve nausea. “The maximum levelsof THC and the active metabolites yousee after you swallow a capsule occur atanywhere from two to four hours,” saysRobert E. Dudley, senior vice presidentof Unimed Pharmaceuticals in BuffaloGrove, Ill., Marinol’s manufacturer.“That’s contrasted with a marijuana

cigarette, where the peak levels mightoccur from five to 10 minutes.”

Unimed and other companies are invarious stages of developing nasal sprays,sublingual lozenges, vaporizers, rectalsuppositories or skin patches that willdeliver THC into the bloodstream quick-ly. But new interest in marijuana aspharmaceutical goes beyond just substi-tutes for smoking. Guy’s motivation forestablishing GW borrows a page fromthe herbal medicine literature. He hy-pothesizes that the plant’s 400 chemicals,including dozens of cannabinoids suchas THC, may interact with one anotherto produce therapeutic effects. A fewstudies have shown that one cannabi-noid, called cannabidiol, may dampensome of THC’s mind-altering effects.And synthetic THC users sometimes re-port feeling more anxious than smokersof the drug, perhaps because of the ab-sence of cannabinoids other than THC.

GW Pharmaceuticals wants to testwhole-plant extracts for a series of med-ical conditions. A Dutch company, Hor-taPharm, will provide seeds to GW forplants that contain mainly one cannabi-noid. Different single cannabinoid plantextracts can be blended to provide thedesired chemical composition.

Interest in whole-plant medicinalmarijuana has even stirred in the U.S.,where research on the drug has beenstymied for 20 years. That bias may beshifting, as witnessed by a 1997 NationalInstitutes of Health advisory panel thatrecommended more research on thesubject. Robert W. Gorter, a professorat the University of California at SanFrancisco, has received approval fromthe Food and Drug Administration toperform a clinical trial on an orally ad-ministered whole-plant extract—and heis also organizing a separate investiga-tion with patients in Germany and theNetherlands. “Various cannabinoids inthe plant appear to work in a little sym-phony,” Gorter observes.

Pushing whole marijuana as medicineis not a task for the fainthearted. Financ-ing pharmaceutical development for acontrolled substance may not come easy.“I need the right type of people as back-ers,” Guy says. “I don’t want peoplefrom Colombia turning up with suitcasesfull of dollar bills.”

In addition, some scientists observethat evidence for cannabinoid synergiesis relatively slim. “There has never beenan effect of marijuana that has not beenreproduced with pure delta-9-THC,”says John P. Morgan, a professor ofpharmacology at the City University of New York. “Herbal medicine advo-cates think that plants are better be-cause there’s a mix of natural substanc-es. There’s not much basis for most ofthese claims.”

Ultimately, advocates of marijuana asnatural medicine may find their worksuperseded by developments stemmingfrom discoveries of cannabinoid recep-tors in the human body—and of mole-cules that bind to them. Some researchgroups are seeking analogues to thebinding molecules naturally present inthe body that might provide therapeu-tic benefits superior to those of plant-based cannabinoids.

Receptor research is also sheddinglight on the role played by the canna-binoids found in marijuana. NIH inves-tigators reported in the Proceedings ofthe National Academy of Sciences inearly July that THC and cannabidiolserve as powerful antioxidants. In labo-ratory rat nerve cells, the compoundscan prevent the toxic effects of excessglutamate, which can kill brain cells af-ter stroke. (After reading this report, le-


SCIENCE AND THE CITIZEN

HERB REMEDY

Exploring ways to administer marijuana as a medicine

PHARMACOLOGY

GOOD BREEDINGallows the Dutch firm HortaPharm togrow medical marijuana that contains

predominantly one cannabinoid.

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From the porch where I amslumped, exhausted by the heat,I stare in astonishment at a man

walking up the forest trail from thebeach, snorkel dangling from one hand.I have just arrived at Dugong Creek, aremote corner of Little Andaman Islandin the Bay of Bengal, to meet the Onges,a group of hunter-gatherers believed tobe descended from Asia’s first humans.I hadn’t expected to find other visitors.

“You know there are crocodiles,” Isay, indicating his snorkel.

“A hazard of the trade,” he grins.Himansu S. Das of the Salim Ali Cen-

ter for Ornithology and Natural Histo-ry in Coimbatore, India, is a sea-grassecologist. Because dugongs, Old Worldrelatives of the manatee, feed on under-water greenery, he had guessed thatDugong Creek would have beds of sea

grass nearby. The animals themselves,though, were likely to be long gone.Once seen in the hundreds or eventhousands along the tropical coasts ofAfrica and Asia, these sea elephants areall but extinct in most of their rangeand occur in reasonable numbers onlyin Australia. In five years of explora-tion, Das has gathered evidence of atmost 40 dugongs throughout the An-daman and Nicobar archipelago. To hissurprise, he has just learned from theOnges that a family of four still lives inDugong Creek, down one since theirhunt of two weeks ago.

The grass beds nourish not only theserare mammals but also marine turtlesand a variety of fish and shellfish. Withthe help of a grant from UNESCO, Dasis estimating the impact of humans onthe ecology. In fact, it is the local peo-ples who point him to the beds, morepredictably than do the satellite imageson which he initially relied.

The next afternoon, under a blister-ing sun, we set out for an Onge camp akilometer or so along the shore. At onepoint we have to ford a creek. Halfwayacross, in chest-deep water and withmy sandals held aloft in one hand, itstrikes me.

STALKING THE

WILD DUGONG

An undersea elephantremains elusive

FIELD NOTESgalization advocates reveled at the no-tion that marijuana may actually pro-tect brain cells.)

To proponents of legalization of thesmokable herb, arguments about alter-natives remain academic. “Because pa-tients are receiving full relief right nowfrom smoking the whole plant, weshouldn’t let them suffer while scienceplods along trying to come up with syn-thetic analogues that may not have thesame beneficial effect,” says Allen F. St.Pierre, executive director of the Nation-al Organization for the Reform of Mar-ijuana Laws Foundation.

Some medical users would ratherfight than switch from joints or brown-ies. Elvy Musikka, a glaucoma patientin Hollywood, Fla., is one of eight peo-ple enrolled in a federal program thatsupplies the drug for medical reasons.She maintains that if her legal supply iscut off she will move to a country whereshe can grow her own. “I think for thepharmaceutical companies to think theyproduce a better product than God istotally presumptuous,” she says. Phar-maceutical makers may find that Mu-sikka’s attitude—shared by thousands—

becomes the biggest impediment to suc-cessful drug development. —Gary Stix


“Crocodiles?” I ask.“Just keep walking,” he replies.I do. Saltwater crocodiles are the most

ferocious of them all, and I’ve seen theirtracks on the beach.

The Onges, we discover, have not seena dugong but have harpooned two tur-tles. One is being cooked, and the other

is secured at the end of a long ropestretched into the sea. When Koira, anOnge man, pulls on the leash, a headsticks anxiously out of the water as theanimal looks to see where it is beingdrawn. It is an endangered green seaturtle, small, about 15 kilograms.

Neither of us begrudges the Onges

their meal. They have lived on LittleAndaman for millennia with no harmto its biodiversity and now, because ofpressure from recent settlers, will prob-ably vanish long before the turtles. Themain threat to the sea-grass beds and tothe creatures that depend on them isthe silt that muddies the water as thedense tropical forest is cut down: themarine plants die of darkness. Overex-ploitation of fish, shellfish and othermarine species by immigrants frommainland India and by fishers from asfar away as Thailand is another press-ing problem.

As the grass patches shrink, the du-gongs become confined to ever smallerregions that are also the local fishinggrounds. Some fishers set their netsaround the beds to catch predators, suchas sharks, that come to feed on smallerfish, but the nets entangle turtles as wellas an occasional dugong. Das will berecommending to the Indian authoritiesthat some sea-grass beds be protectedas sanctuaries. But as we return—thetide has gone out, mercifully leaving thecreek just knee-deep—I realize with sad-ness that it’s already too late for the An-daman dugong. —Madhusree Mukerjee

in the Andaman Islands

DUGONG, A VEGETARIAN MAMMAL, needs fields of marine greens where it can graze in peace.

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Late into the night astronomers An-gelica de Oliveira-Costa and Max Tegmark worked to ana-

lyze their observations of the cosmicmicrowave background radiation. Thenext morning the young wife-and-hus-band team were due to present whattheir data revealed about the single mostimportant unknown fact in cosmology:the shape of the universe. Their previ-ous results, from a telescope in Saska-toon, Canada, between 1993 and 1995,had suggested that the universe is flat—the first observations to substantiate along-held belief among cosmologists.But intrinsic uncertainties in the mea-surements made it impossible to be sure.

So in 1996 the QMAP team (de Oli-veira-Costa, Tegmark and five colleaguesfrom the Institute for Advanced Studyin Princeton, N.J., and the University ofPennsylvania) flew instruments on a bal-loon 100,000 feet (30 kilometers) aboveTexas and New Mexico. When theyfinally processed the data—the night be-fore their announcement at the Fermi

National Accelerator Laboratory thispast May—the situation looked grim.The Saskatoon and the balloon resultswere completely different.

Suddenly, however, de Oliveira-Costarealized that Tegmark had accidentallyplotted the map upside down. Whenrighted, it matched the Saskatoon dataexactly. “That was my most excitingmoment as a scientist, when I realizedwe’d flipped that map,” Tegmark says.“It was then I realized, yes, Saskatoonwas right. The universe is flat.”

The QMAP balloon discerned muchfiner details in the radiation than theCosmic Microwave Background Explor-er (COBE) satellite did eight years ago.In some areas this radiation is slightlydimmer (blue, in illustration below); inothers, brighter (red). The red stripedown the middle represents the MilkyWay galaxy, whose own microwaveemission overpowers the cosmic signal;to avoid it, QMAP focused on a clearpatch of sky around the North Star.

When the brightness fluctuations areexaggerated 100,000 times, blobs be-come clear. They correspond to clumpsof matter that existed 300,000 years orso after the big bang. Their apparent sizedepends on the geometry of the universeand, in turn, on the cosmic density ofmatter and energy.

Combined with other observations,including those of distant supernova,the QMAP results corroborate the pre-

vailing theory of infla-tion—with the twist thatthe universe is only onethird matter (both ordi-nary and dark) and twothirds “quintessence,” abizarre form of energy,possibly inherent in emptyspace. Despite Tegmark’senthusiasm, however, thisconclusion is not defini-tive. Astronomers are stillwaiting for results fromtwo upcoming satellites,the Microwave Aniso-tropy Probe and Planck;meanwhile other groupsare flying balloons or tak-ing ground-based mea-surements. They all hopeto hold up or shoot downinflationary theory. “It’slike an Indiana Jones mov-ie,” says Paul Steinhardtof Penn. “Everyone seesthat holy grail.”

—George Musser


Alexander’s FateAn ancient conspiracy theory held thatrivals poisoned Alexander the Great,who died unexpectedly at the age of 32in 323 B.C. But a new analysis, publishedin the New England Journal of Medicineon June 11, finds otherwise: Alexander

probably fell victim to ty-phoid fever. The au-thors—including infec-tious-disease expert DavidW. Oldach of the Universi-ty of Maryland and histori-an Eugene N. Borza ofPennsylvania State Univer-sity—were puzzled by his-torical accounts statingthat Alexander’s body didnot begin to decay for

days after his death. They believe hemost likely succumbed to ascendingparalysis, a complication of typhoidfever that can slow down a person’sbreathing and make them look dead.

Science KnowledgeInterest in science is at an all-time high,according to a survey of 2,000 U.S.adults that was presented to Congressin July. But basic knowledge remainspoor. Jon D. Miller, director of the Inter-national Center for the Advancement ofScientific Literacy, conducted the sur-vey for the National Science Foundationlast year. Although 70 percent of thesubjects said they were curious aboutscience and technology, only 11 per-cent could define “molecule,” half be-lieved that humans and dinosaurs hadat one time coexisted, and only 48 per-cent knew that the earth orbits the sunonce every year.

Hello, SOHO?Engineers have tried to reach the Solarand Heliospheric Observatory (SOHO)since losing contact with the craft onJune 24 during routine maintenanceoperations. For the past two and a halfyears, SOHO has provided researchersfrom NASA and the European SpaceAgency with a wealth of informationabout the sun. Now the observatory isapparently spinning in such a way thatits solar panels do not receive enoughlight. In case SOHO comes out of thedark, the team is issuing frequent sig-nals to activate its transmitters.

IN BRIEF

More “In Brief” on page 24

THE FLIP SIDE

OF THE UNIVERSE

New cosmological observationsconfirm inflation

COSMOLOGY

COSMIC BACKGROUND RADIATION was remeasured with high resolution,

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In Brief, continued from page 22

Perfecting Microwaved Foods For the sake of convenience, most peo-ple put up with microwaves—eventhough the french fries get soggy, the

potatoes stay frozenand the eggs (don’t tryit yourself ) explode.But that could soonchange, thanks to newmathematical andcomputer models de-veloped by Ashim K.Datta and graduatestudent Hua Zhang ofCornell University. The

models predict how edibles of variousshapes and consistencies will heat, andDatta hopes they will help manufactur-ers devise more successful products.Ninety percent of new microwavablefoods introduced to the marketplaceevery year fail.

Treating TuberculosisDeadly multidrug-resistant strains of TBhave emerged in large part because thestandard therapy is so intensive. Pa-tients must follow two months of dailydoses, followed by four months oftwice-weekly treatments. Many simplydo not finish. But now the Food andDrug Administration has approved anew medication, rifapentine, thatshould make adherence easier. TB suf-ferers need only take rifapentine once aweek during the last four months of re-covery. The drug, which will be market-ed under the name Priftin, is the firstanti-TB agent approved in 25 years.

Triggering Tourette’s SyndromeThe ailment—which produces involun-tary movements or vocalizations calledtics—tends to run in families, but it ap-pears in only a small percentage ofthose children who inherit one copy ofthe responsible gene. Now scientiststhink they have discovered what canpush these kids over the edge into ill-ness: a streptococcus infection. HarveySinger and his colleagues at Johns Hop-kins University looked for antineuronalantibodies—which the body can makein response to bacterial infections andwhich attack brain tissue—in 41 Tou-rette’s patients and 39 controls. Theyfound that those in the former grouphad higher levels of the antibodies in aregion of the brain that helps to controlmovement. The finding could somedaylead to new means of prevention andtreatment.

More “In Brief” on page 26

A N T I G R AV I T Y

Tomorrow, Partly Froggy

Television evangelist and sometime presidential candidate Pat Robertson re-cently shocked the world by revealing that a science existed that he knew

even less about than paleontology. Delving into the latter discipline, Robertsononce contended that “there is no case where we have remains or fossils of an an-imal that died during the evolutionary process.” In fact, every fossil ever found isof an organism that died during the “evolutionary process.” But I digress. In June,Robertson added meteorology to the list of sciences about which he has theo-ries—and few facts.

The self-appointed forecaster took to the airwaves to warn residents of Orlan-do, Fla., about wicked weather possibly headed in their direction. Orlando stoodin the way of some righteous wrath, he maintained, as a result of the city’s deci-sion to allow gay organizations to fly rainbow flags in a local celebration calledGay Days. “This is not a message of hate; this is a message of redemption,” he in-sisted. “But if a condition like this will bring about the destruction of your nation,if it will bring about terrorist bombs, if it will bring about earthquakes, tornadoesand possibly a meteor, it isn’t necessarily something we ought to open our armsto. And I would warn Orlando that you’re right in the way of some serious hurri-canes, and I don’t think I’d be waving those flags in God’s face if I were you.”

Robertson the scientist might be expected to be an expert on hot air masses.But he’s not going out on a limb long enough to snap off in a 100-mile-per-hourwind by prophesying hurricanes in Florida. This just in: Buffalo gets snow in January! (And Floridaweather is even easierthan most. I lived in Mi-ami one summer, and Ican give you a fairly de-cent forecast for any dayin July or August with-out looking at a singlesatellite map: tempera-ture in the 90s with highhumidity, good chanceof an afternoon thun-derstorm. Rinse. Repeat.)

The issue of whetherGod has a face in whichto wave a flag is an openone for some people, but assuming He or She does, ears are probably attached,and assuming Robertson has God’s ear, maybe he could ask Him or Her for somemore detailed meteorological information. Which he could then include as a reg-ular feature on his 700 Club broadcasts, launching into something like this:

“Odious lectures by Stephen Jay Gould on evolution at the American Museumof Natural History will lead to cherub-size hail in the early evening in New YorkCity. Continued homosexual activity in the large cities of the East Coast and, ofcourse, San Francisco, will initiate a severe low-pressure front with associated tor-rentially heavy rains late in the day. Look for heavy flooding, especially in theaterdistricts. Because of some isolated pockets of freethinking, the Midwest will see a90 percent chance of frogs this afternoon with vermin, especially in low-lying regions, so please drive with the low beams on. Frogs diminishing towarddusk, followed by scattered murrain. Putting the map in motion now, we see thatgeneral sinfulness across the country will bring darkness after sundown.”

In the interest of an even-handed attack on wacky ideas, I’d like to point out theobvious fact that no religious group has a monopoly on them. Some of the morevocal members of some groups, however, do have their own broadcast outlets,so it’s easier to notice when they say something outrageous. The good news isthat such spokespeople serve the larger purpose of reminding everyone of thesimple and profound words written by Gould: “The enemy of knowledge and sci-ence is irrationalism, not religion.” Amen. —Steve Mirsky

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Against the dark stand of pine trees, the waters of the Rio Tinto

appear even more vividly redthan usual. Here, near its headwaters insouthwest Spain, the strong smell of sul-fur overwhelms even the fragrance ofthe dense forest. The crimson river—in-famous for its pH of two, about that ofsulfuric acid, and for its high concentra-tion of heavy metals—seems dead, a pol-luted wasteland and a reminder of theecological devastation mining can entail.

Yet the remarkable Rio Tinto is hardlylifeless, as scientists have discovered inthe past several years. Even in parts ofthe river where the pH falls below two—

and the water is painful to touch—green

patches of algae and masses of filamen-tous fungi abound. “Each time we gothere we find something new,” says Ri-cardo Amils, director of the laboratoryof applied microbiology at the Center forMolecular Biology at the AutonomousUniversity in Madrid, who discoveredthe river’s wild ecosystem in 1990. “Wehave now collected about 1,300 formsof life living here, including bacteria,yeast, fungi, algae and protists. But thereal number is surely much higher.”

Before Amils and his colleagues stud-ied the 93-kilometer (58-mile) river, itwas assumed that the acidic waters werepurely the result of the Rio Tinto coppermine, one of the world’s largest and old-est. The microbiologist now believes thatindustry—in particular, the sulfuric acidassociated with copper mining and thediscarded metal tailings—is not entirelyresponsible for the condition of the wa-ter. He has found that historical recordsrefer to the river’s long-standing acidity.Amils postulates that the river’s strangechemistry led its first miners—the Tar-


Asteroids on the InsideScientists have long looked far afield forasteroids. Now they have found one cir-cling the sun inside the earth’s orbit.

David J. Tholenand graduatestudent RobertWhiteley of theUniversity ofHawaii spottedthe object,named 1998DK36, using aspecialized cam-

era on the 2.24-meter telescope atopMauna Kea in February. Preliminary cal-culations show that nearly 1.3 millionkilometers always separate the earthfrom the asteroid as it passes throughthe daytime sky—good news, giventhat DK36 appears to be 40 meters indiameter. The asteroid that devastatedthe Tunguska region of Siberia in 1908was about the same size.

Phone HomeStill no sign of extraterrestrial life, ac-cording to SERENDIP III, the most sensitive sky survey to date. The search, led by Stuart C. Bowyer of the University of California at Berkeley, used a detector mounted on the world’slargest radio telescope in Arecibo, Puer-to Rico. Starting in 1992, the instrumentanalyzed 500 trillion signals, looking ina radio band centered on a wavelengthof 70 centimeters—a region typicallyreserved for communications. No luck.But the team hasn’t given up hope.SERENDIP IV, now in the works, shouldbe 40 times more sensitive than its pre-decessor; it will simultaneously examine168 million frequency channels every1.7 seconds.

Just Add WaterThe wonders of modern science nevercease. Ryuzo Yanagimachi and col-leagues at the University of Hawaii haveproduced live mice from dead sperm.The workers added water back to freeze-dried sperm and injected it into mouseeggs using a procedure called intracy-toplasmic sperm injection (ICSI). Theyfound that freeze-drying had preservedthe genetic information in the spermwell enough to regenerate healthymice. The tactic is expected to be an im-provement on previous methods ofstoring genetic information from miceused in research. —Kristin Leutwyler

In Brief, continued from page 24

SA

A World Aflame

Every year fire scorches some 71 million hectares (175 million acres) of forest andgrassland. In 1997 drought brought on by El Niño exacerbated fires, many of

which were deliberately set, the world over. In Indonesia, for instance, the devastationwas particularly extreme because of the worst drought the country had seen in 50years. According to the World Wildlife Fund’s 1997 report The Year the World Caught Fire,Indonesia lost two million hectares to flame. A satellite image from last year shows theextent of the damage (red in image at right). A composite of data from 1992 to 1995 (atleft) shows fires in the region in red and purple. Fires this year in the Amazon, Mexico,Florida and elsewhere promise to make 1998 another record year. The National Aero-

nautics and Space Administration has teamed upwith the National Oceanic and Atmospher-

ic Administration to provide weekly up-dates on fires around the world. In-

formation can be found at http://modarch.gsfc.nasa.gov/fire_

atlas/fires.html on the WorldWide Web. —Sasha Nemecek

ECOLOGY

RIVER OF VITRIOL

The Rio Tinto in Spain abounds inacid—and unexpected organisms

BIOLOGY

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INDONESIA


tessians in 3000 B.C.—to in-vestigate the banks for de-posits. Soon after, the Ro-mans, who extracted greatquantities of gold and silver,called the river Urbero, Phoe-nician for “river of fire.” Andthe Arab name for it was“river of sulfuric acid.”

Amils’s argument is bol-stered by other observations.The low pH and high con-centration of metals—includ-ing iron, arsenic, copper, cad-mium and nickel—is consis-tent throughout the entireriver, becoming less acidicwhere the Rio Tinto meetsthe Atlantic Ocean. Typical-ly, waterways that receivemining waste have acidicconcentrations only near thesource of pollution. Strongrains also cannot seem to re-duce the acidity of the river.

To explain how this amaz-ing condition came about—and is perpetuated—the re-searchers point to what theyhave learned about the spe-cies found there. They areconvinced that the extremeconditions are produced bybacteria. Thiobacillus ferro-oxidans, for example, is abundant inthe river. This microorganism is capableof oxidizing sulfur and iron—therebygiving the Rio Tinto its red hue andname. Amils and his colleagues recentlydocumented the presence of anotherbacterium, Leptospirillum ferrooxidans,which feeds exclusively on iron and iseven more abundant than T. ferrooxi-

dans. Evidence of its corrosive capabili-ty sits on the banks, where abandonedrailroad cars were flooded with riverwater. In service just 15 years ago, thesecars now appear as skeletons, devouredby the Rio Tinto microbes. “These bac-teria are a kind of metallic piranha,”Amils says.

Other bacteria—not as well under-

stood for the time being—ap-pear to feed on the immensedeposits of metal sulfides, cre-ating the sulfuric acid that,together with oxidized iron,produces the very conditionsthat lead to heavy metals insolution.

The most abundant organ-isms in the river appear to bealgae, which produce oxy-gen in champagnelike bub-bles that the researcherswatch float to the surface.How algae work in this acidinferno, however, has the sci-entists mystified. “We needmore research to explain howalgae can collect light andproduce organic matter andoxygen in such conditions,”Amils notes.

One possibility is that someof the Rio Tinto algae andfungi have established cer-tain symbiotic associations.“Through evolution we cansee that symbionts can thrivesuccessfully in a habitat thatotherwise would be inhos-pitable,” explains Lynn Mar-gulis, a biologist at the Uni-versity of Massachusetts atAmherst. “Long-term associ-

ation can create new species throughsymbiogenesis.”

Understanding this symbiosis—if it ispresent in the Rio Tinto—could helpMargulis, Amils and others understandthe development of early life on theearth. “In my view, the river is a bettermodel for the life that flourished in theProterozoic, with an abundance of oxy-gen and algae, than it is for the anoxicArchean eon,” Margulis says. Duringthe Proterozoic—between 2.5 billionand 600 million years ago—anaerobicand aerobic organisms survived in ex-treme conditions, perhaps assisting oneanother.

The Rio Tinto could also offer furtherinsights. Perhaps Amils and his creware seeing the kind of life that thrivedon Mars millions of years ago. The ver-satility of these bacteria—particularlythose that work in anoxic conditions onmineral substrates such as iron sulfides—make them good candidates for a modelof extraterrestrial life. “I cannot say thatMartians were like this, but Mars wouldbe a perfect bite for many bacteria liv-ing here, that is for sure,” Amils says.

—Luis Miguel Ariza in Madrid


DIATOMSfrom the Rio Tinto are among the 1,300 species found to live there.

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RED WATERSand high acidity of the Rio Tinto do not deter a wealth

of microscopic life, including blooms of algae.

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Many of the world’s problems stem from the fact that ithas 5,000 ethnic groups but only 190 countries. This

situation is illustrated on the map, which shows that fewstates are ethnically homogeneous and that many, particular-ly in Africa, have no majority ethnic group. Since 1945 some15 million people have been killed in conflicts involving eth-nic violence, although ethnic tensions have not necessarilybeen the catalyst. Among the worst incidents were the 1994civil war in Rwanda, which resulted in more than a milliondead and three million refugees, and the 1947 communal ri-ots in India, which left several hundred thousand dead and 12million refugees.

Why are some multiethnic countries plagued by violent,persistent ethnic conflict and others not? There are no com-pletely satisfactory answers, but it is evident that several fac-tors affect the outcome. One of these is the presence or ab-sence of political institutions that give minorities protectionagainst the tyranny of majority rule. Federal systems, such asthe one instituted after 1947 in India, can help dampen ethnictensions by giving minorities regional autonomy. Intermar-riage—between Thais and Chinese in Thailand, say, or Tai-wanese and Mainlanders in Taiwan—erodes ethnic differenc-es. And free-market forces tend to mitigate ethnic tensions.For instance, Russia has not adopted an irredentist policy—there are nearly 25 million Russians in neighboring re-publics—arguably because it would interfere with the goal ofachieving a Western-style market economy.

The region with perhaps the most intransigent ethnic rival-ries is sub-Saharan Africa, which has about 1,300 languagegroups in 42 countries, the boundaries of which were im-posed by the colonial powers with little regard for ethnicity. Inaddition to language differences, religious divisions exist—most prominently between Muslims and Christians. These

widespread ethnic and religious divides have contributedheavily to instability in countries such as Nigeria, where Hau-sa, Fulani, Yoruba and Ibo tribes contend for political power.Nigeria has suffered six military coups and two civil wars sincegaining independence from Britain in 1960. Following decol-onization, about three fourths of the sub-Saharan Africancountries have undergone coups or civil wars.

India has had a generally successful record in dealing withethnic tensions since independence despite its 300 lan-guages, thousands of castes and major religious fault lines.One explanation may be its extreme diversity: a country withso many divisions may be at less risk of violence than one inwhich just a few groups contend, because no single groupcan dominate. At its inception, India was blessed with a large,well-educated, democratically inclined elite that used its pres-tige to build a multiethnic political machine—the CongressParty—that was a potent force in mitigating tensions. Butnow there is uneasiness about India’s future because of therise to power of the nationalist Bharatiya Janata Party, whosemore extreme supporters shout slogans such as “For Muslimsthere are only two places, Pakistan or the grave.”

There is a widely held belief that ethnic violence in the for-mer Yugoslavia arose from ancient ethnic hatreds. But thisview ignores a history of peaceful coexistence and extensiveintermarriage among ethnic groups going back generations.It is unlikely that the recent conflict in the region would haveprogressed to genocide had it not been for political leaderssuch as Slobodan Milosevic, who distorted history to create amyth of a Serbia wronged by ancient enemies and now againthreatened by these same enemies. The notion of “ancientethnic hatreds,” at least in the Balkans, India and Africa, seemsto have limited basis in fact.

—Rodger Doyle ([email protected])

LESS THAN 50

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ACCOUNTED FOR BY LARGEST

ETHNIC GROUP

50 TO 69

70 TO 89

90 TO 97

98 OR MORE

NO DATA

SOURCES: The World Factbook 1997. Central Intelligence Agency, 1998. A Geolinguistic Handbook, 1985 edition, by Erik Gunnemark and Donald Kenrick. Country data are the most recent available as of early 1998. Ethnicity is defined on the basis of language used at home, on race or on religion, as seems appropriate for the country.

B Y T H E N U M B E R S

Ethnic Groups in the World

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In 1952 a young physicist visited anaging industrial building in Pough-keepsie, N.Y., that its owners

called the “pickle works.” The trim 25-year-old was looking for a new positionafter becoming disenchanted with histwo-year-old job at the National Advi-sory Committee for Aeronautics (pre-

decessor of the National Aeronauticsand Space Administration), where hehad some involvement with a project tobuild a nuclear-powered jet aircraft.

IBM, the company he was visiting, hadjust made the onerous transition fromcomputing machines that used elec-tromechanical relays to those that in-

corporated vacuum tubes. Even then, itwas looking warily ahead. The compa-ny needed physicists as part of a smallsemiconductor research team, estab-lished to guard against the unlikely pos-sibility that transistor technology wouldever amount to anything. “The futureof IBM is in semiconductors, and they

don’t even know it,” confided the man-ager who conducted the interview.

Rolf Landauer, the erstwhile job ap-plicant, recalls those words more than45 years later in his tidy office in thesweeping Eero Saarinen–designed glassedifice that houses IBM’s primary re-search facility in northern Westchester

County outside of New York City. “Iwas very lucky to have my career coin-cide with the period of high adventurethat followed,” he observes.

Moving from nuclear aircraft to mi-crocircuitry closely follows the techno-logical trajectory of the latter half of the20th century, a period in which fascina-tion with moon shots and nuclear pow-er has given way to a preoccupationwith the movement of electrons in smallspaces. Landauer has helped define atthe most fundamental level how muchuseful work a computing machine canperform in these lilliputian confines. “Hereally started the field of the physics ofcomputation,” says Seth Lloyd, a lead-ing theorist on using quantum-mechan-ical principles for computing and a pro-fessor of mechanical engineering at theMassachusetts Institute of Technology.

The basic tenet of Landauer’s world-view is that information is not somemathematical abstraction. Instead it is aphysical entity—whether it be stored onan abacus, on a punched card or in aneuron. Early in his research Landauerbegan to wonder how much energy isrequired for each computational step.Identifying a minimum would establisha basic limit akin to the laws of thermo-dynamics, which calculated the efficien-cy of 19th-century steam engines, or toClaude E. Shannon’s information theo-ry, which figures how many bits can beshipped over a wire.

The seminal insight of Landauer’s ca-reer came in 1961, when he challengedthe prevailing idea—put forth by math-ematician John von Neumann and oth-ers—that each step in a computer’s bi-nary computation required a minimumexpenditure of energy, roughly that ofthe thermal motion of an air molecule.Landauer’s paper in the IBM Journal ofResearch and Development argued thatit was not computation itself but theerasing of information that releases asmall amount of heat. Known as Lan-dauer’s principle, the idea that throwingaway bits, not processing them, requiresan expenditure of energy was criticizedor ignored for years. More recently, thiscornerstone of the physics of informa-tion has become the underpinning foradvanced experimental computers.

One person who did take notice of thisearly work was an unorthodox post-doctoral student at Argonne NationalLaboratory. At a conference in Chicagoin 1971, Charles H. Bennett explainedto Landauer how a computer might bedesigned that would circumvent Lan-


PROFILERiding the Back of Electrons

Theoretician Rolf Landauer remains a defining figure in the physics of information

LIMITS OF COMPUTATIONand the kinetics of small structures are ideas that physicist

Rolf Landauer prefers to communicate with pictures, not numbers.

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dauer’s principle by not discarding in-formation and therefore dissipating vir-tually no energy. Bennett expanded onLandauer’s work by showing that eachstep in the computation can be carriedout in a way that allows the input to bededuced from the output—in essence,the machine can be run backward. Sucha machine can first save the answer andthen put itself into reverse until each stepis undone. It avoids the energy lossesstipulated by Landauer’s principle; noinformation is erased, and accordinglyno energy is lost. Other researchers haveborrowed these ideas for reversible log-ic circuits that may help avoid the poten-tially fatal amounts of heat generated byvery small circuits in future computers.

Landauer’s relationship with Bennettbears a resemblance to the circularity ofreversible computation. At first,Landauer served as a mentor, con-vincing the younger man to take ajob at IBM. As Bennett refined andextended Landauer’s original work,Bennett became a mentor to Lan-dauer. The evidence of this inversioncan be seen in a 1996 paper pub-lished by Landauer in the journalScience. The reversibility of an in-formation-processing operation—

which Bennett suggested in responseto Landauer’s work—became a cen-tral idea in the paper, which dealtwith the limits of communication.The paper showed that no minimalenergy expenditure is required toship a bit of information. In con-trast, ordinary communications links,where the signal energy is thrown awayat the receiver, undergo a distinctly irre-versible operation. Landauer evokes theimage of a ski lift in which each chairhas two seats, each of which representsa 0 or a 1. Skiers—the bits of a mes-sage—sit in one of the two chairs for thetrip up the mountain. There the bits areall switched to the 0 chair so that theycan be hauled down the mountain forreuse in another message.

Landauer has always understood thegap between thought and practice. As atheorist, he has explored the bounds ofcomputation. Yet he has also served asa leading critic of the practicality oftechnologies—such as quantum compu-tation—that have pushed limits. He haseven voiced doubts about the fate of re-versible computers, a field related to hisown work. “All technology proposalscome with a penalty,” he notes. “A re-versible machine would require morecomplex and slower circuitry.”

M.I.T.’s Lloyd recounts that it is notunusual for him to receive a letter fromLandauer after publication of one of hispapers on quantum computation. (Aquantum computer is a type of reversi-ble machine that performs many calcu-lations simultaneously based on quan-tum-mechanical principles that allow asingle bit to coexist in many states atonce.) Landauer invariably suggests thata disclaimer should be affixed to the pub-lication: “Warning: quantum computersare unlikely to work in the real world.”

The contrast between his adventur-ousness as a theorist and his conserva-tive pragmatism stems from his tenureduring the 1960s as director of IBM’sphysical sciences department, where hehad to make judgments about fundingone technology or another. He managed

the beginnings of the company’s pro-grams for developing the semiconductorlaser and integrated circuits, even coin-ing the term “large-scale integration.”

His realist’s bent may also be rootedin his experience as a childhood refugeefrom Hitler’s Germany. Born in Stuttgartin 1927 to a well-to-do Jewish family,Landauer recalls the patriotism of hisarchitect father, who died in 1935, hislife shortened by a wound sustainedwhile fighting during World War I in theGerman army: “Like a lot of Jews whowere good Germans, he always thoughtthat this craziness has got to stop. I’malive today because he died in 1935. Wewould never have left Germany if he hadcontinued to be head of the family.”

The family settled in New York City,where Landauer made his way throughthe public school system, eventuallygraduating from the renowned Stuy-vesant High School. After his acceptanceto Harvard University, his uncle urgedhim to major in electrical engineering,

not physics. “He felt that physics was ahard way to make a living, particularlyfor someone who was Jewish at a timewhen the universities had official orunofficial quotas,” Landauer says in hisstill noticeable German accent. Lan-dauer eventually pursued courses thatmixed both physics and electronics. Butwhen given a choice later in life, he nev-er concealed his preference for theoryover management. Fortunately, his ca-reer coincided with the golden era of in-dustrial research, when musings on gal-axy formation or limits to computationdid not have to be tied to a specific tech-nology development, as they do today.

Besides his other work, Landauer be-came known for basic theories relatedto the physics of small structures. Hecan describe each of his theories in a

crisp, methodical manner, intelligi-ble to those who are not initiates inthe subtleties of quantum mechan-ics. Folders rest on one corner of hisotherwise clean desk, each contain-ing papers or charts he has set asidefor a visitor. One folder correspondsto electron transport—that is, themovement of electrons throughsmall spaces—and another to statis-tical mechanics, a contemplation ofthe tiniest device that can hold a 0or 1 bit without being knocked outof its state by noise in the environ-ment. Landauer eschews heavy useof mathematics in favor of a “low-brow, intuitive style” that relies onword pictures: “It’s like I’m sitting

on the back of an electron and watch-ing the world go by.” One major con-tribution, known as Landauer’s formu-la, calculates electrical conductance(how much current can be achieved fora given voltage) from the probabilitythat an electron entering a small struc-ture will make it to the other side ratherthan bouncing out of its entry point.

Although the physics of informationis now well established, Landauer re-mains preoccupied with a number ofquestions that he knows he may neverbe able to answer. How large, for in-stance, can one make a computer mem-ory in a finite universe? How preciselycan the world be described? “The vi-sion of a totally precise computer doesn’texist in the real world any more thanone can hypothesize detecting seven an-gels on the head of a pin,” Landauerdeclares. One of his legacies may be tounderline questions that will define theboundary between real computationand mere angel watching. —Gary Stix


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REVERSIBLE COMPUTATION allows operations to be run backward:

the input can be deduced from the output (horizontal chain). Therefore, no information

is lost, no energy is dissipated, and Landauer’sprinciple is preserved. Irreversibility occurs when one cannot track back to the input

from the output (two paths merging).


Twenty years ago Nicholas K.Sheridon got his big idea, thekind that scientists—if they are

talented and fortunate—get just once ortwice in a career. Sheridon hit on a wayto draw images electronically that wouldbe far more portable than heavy cath-ode-ray tubes, far cheaper than liquid-crystal panels. In theory, his inventioncould bring to digital displays many ofthe advantages of paper. They would bethin and flexible yet durable. Theywould consume only tiny amounts ofpower yet would hold images indefi-nitely. They could be used for writingas well as reading, and they could be re-used millions of times. Yet they wouldbe as cheap as fine stationery. Sheridonnamed his idea Gyricon, and he appliedfor and received a patent on it.

But there his good fortune failed him.Twenty years ago Sheridon’s managersat the Xerox Palo Alto Research Centerwere sitting on many of the inventionsthat would eventually propel the person-al computing revolution: the windowsand mouse interface, the laser printer,Ethernet. Like those innovations, Gyri-con drew only yawns from Xerox’sblinkered managers. “The boss said,‘Xerox really isn’t interested in displays.Why don’t you work on printing tech-nologies?’ So I did,” Sheridon recalls.

Fifteen years later the soft-spoken sci-entist returned to his inspiration, andtoday the incarnation of his idea sitsflashing the PARC logo in black-and-white on his desk. Although it is an ear-ly prototype, the 15-by-15-centimeterdevice is quite legible and thin. Moreimpressive is the fact that the device ispowered entirely by a pinky-size solarcell. When Sheridon removes the poweraltogether, the logo stops changingshade, but it does not fade.

Gently peeling a sheet off its plasticbacking, Sheridon shows me what Gy-ricon is made of. The material is no

thicker than a latex glove, and it feelsabout as rubbery. That is no coinci-dence: the substance is made by mixingtiny plastic balls, each just 0.03 to 0.1millimeter in diameter, into molten,transparent silicone rubber. Every ballis white on one side, black on the other.Cooled on slabs and cut into sheets, therubber is next soaked in oil, which itsucks up like a mop. As it does, thesheets expand and oil-filled pocketsform around each ball, which can thenfloat and rotate freely.

Through a chemical process that Xe-rox is holding as a trade secret, “eachball is given an electric charge, withmore on one side than on the other,”Sheridon explains. So when an electricfield is applied to the surface of the sheet,the balls are lifted in their oil-filled cells,rotated like the needles of tiny compass-

es to point either their black or theirwhite hemispheres eyeward, and thenslammed against the far wall of the cell.There they stick, holding the image, un-til they are dislodged by another field.At high voltages, the balls stick beforecompleting their rotation, thus produc-ing various shades of gray. Sheridon’sgroup has also produced red-and-whitedisplays and is working on combiningballs of various hues to produce full-color ones.

In his early work on Gyricon, Sheri-don had figured out everything except acheap way to make billions of plastic

balls, all colored on one side only andall the size of a pinpoint. “This is the se-cret,” Sheridon says, holding up a steeldisk slightly smaller than a CD. Thedisk is spun on a spindle at 2,700 rpm.White plastic is pumped onto the top ofthe spinning disk, black onto the bottom.The plastic streams skitter off into jetsthat join at the edge and break up intoprecisely bicolored, spherical droplets.

Sheridon has demonstrated that theGyricon material remains stable aftermore than two years and three millionerasures. His group has built displays atresolutions of up to 220 dots per inch(200 percent finer than most LCDs) andsizes up to a foot square. “We wouldlike to get higher resolution and betterwhiteness,” Sheridon admits. “But weknow how to do that: make the ballssmaller and pack them more closely.”

For certain applications, such as largecommercial signs, the technology ap-pears to be only a few years from mar-ket. Gyricon displays might find theirway into laptop and handheld comput-ers soon after that. “It would probablyallow you to run a laptop for six monthson a few AA batteries,” Sheridon says,because the device requires neither abacklight nor constant refreshing, asLCDs do.

But the real goal, Sheridon says, is alsothe most distant: an electronic surrogatefor paper. Engineer Matt Howard handsme a wooden pencil that is plugged into


TECHNOLOGY AND BUSINESS

PROTOTYPE OF XEROX’S GYRICON DISPLAY, as thin as seven sheets of paper, will hold its image for months without power.

THE REINVENTION

OF PAPER

Cheap, lightweight, low-power electronic displays have been

made in the lab

ELECTRONICS

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If all goes as expected, the first vac-cine against cancer will be ap-proved for sale before the end of

September. The vaccine will neither pre-vent cancer nor cure it, and it wouldfirst be sold in Canada, not the U.S. Itwill be a significant event nonetheless,because it will demonstrate that a long-held dream—of attacking cancer byguile from within, rather than assault-ing the body by brute force from with-out—is beginning to come true.

More than half a dozen large-scaletests of cancer vaccines are under wayin clinics around the world. Most aim atthe same malignancy as this first drug:melanoma, a fast-spreading skin cancerthat strikes about one person in 100.Ribi ImmunoChem Research, a biotechfirm in Hamilton, Mont., was simplythe first to file for market approval. Eu-ropean regulators are also evaluatingthe company’s clinical results, and Ribiplans to put its new medicine, Mela-cine, before U.S. Food and Drug Ad-

ministration reviewers later this year.What those experts will see is evi-

dence that nearly all the 70 terminal pa-tients injected with this cocktail ofripped-up tumor cells and bits of thebacteria that cause tuberculosis felt sig-nificantly fewer ill effects than the 70given standard chemotherapy. The vac-cine made patients’ lives easier but notlonger. At least not on average; the luckyfew who responded well to the vaccinedid survive longer than those who re-sponded well to conventional drugs.

There is good reason to hope thatother, more sophisticated vaccines stillin clinical trials will improve on thosemodest gains, in two ways. They maycontain more potent adjuvants, addi-tives such as the bacterial fragments inMelacine that awaken the body’s im-mune system to the fact that the cancerdoesn’t belong there. And they may usemore effective antigens, fragments oftumor cells that train antibodies andkiller T cells to recognize cancer whenthey see it.

In March, for example, Steven A. Ro-senberg and his colleagues at the Na-tional Cancer Institute reported goodnews about a vaccine they have madefrom a particular protein fragment andinterleukin-2, a chemical secreted by Tcells when they stumble on foreign bod-

ies. Of the 31 patients with widespreadmelanoma who were immunized withthe new medicine, 13 saw their tumorsshrink by more than half.

Rosenberg’s group went to great ef-fort to identify just the right section ofprotein to use in its vaccine, but a shot-gun technique may also work againstsome cancers. Michael G. Hanna, chair-man of Intracel in Rockville, Md., an-nounced in July that a decade-long testof its OncoVaxCL vaccine for colon can-cer had succeeded. Intracel borrowedparts of tumors removed from patients’colons, digested them with enzymesand then injected each patient with hisor her own tumor cells, along with abacterial adjuvant. In people sufferingfrom stage II colon cancer, the vaccineappeared to cut the rate at which thedisease resurged by 61 percent overabout five years, when compared withpatients treated by surgery alone. Intra-cel is planning to file for FDA approvalof its drug later this year.

Other large-scale vaccine trials arejust getting started. Progenics Pharma-ceuticals in Tarrytown, N.Y., had byJuly enrolled more than half the 800American skin cancer patients it wantsfor its study. They will test a concoctionof a carbohydrate antigen and an adju-vant derived from the bark of the SouthAmerican soap tree, says Robert J. Is-rael, the company’s chief scientist.

ImClone Systems in New York Cityis gearing up for a trial of similar size tosee whether its vaccine will prevent therecurrence of small-cell lung cancer.“Virtually all patients with this diseaserelapse after their initial treatment,”says Harlan W. Waksal, ImClone’s chiefoperating officer. “The disease usuallycomes back within a year—and with avengeance. We hope to stop that.” Im-Clone’s antigen might seem like an un-likely champion, constructed as it wasby making an antibody to an antibodyof a sugar-fat compound on cancer cells.But in small-scale trials, Waksal reports,about 40 percent of people given thevaccine survived five years, despite oddspredicting that fewer than 5 percent ofthem would hang in that long.

The John Wayne Cancer Institute inSanta Monica, Calif., is coordinatingan even larger and longer study, to spanfive years and eight nations and to in-clude 1,100 people whose melanomahas spread into their lymphatic system.The subjects will be given either irradi-ated melanoma tumor cells or interfer-on alfa-2b, a drug that forces tumors to


a weak power supply. As I write on thesheet, the tiny electric field conductedthrough the pencil’s graphite core dark-ens the screen wherever the tip touches.Howard is working on a handheldwand that will receive text and imagesfrom a computer and scan them onto aGyricon page, which would then be an-notated, photocopied, erased—but notdiscarded.

The effect of such an invention onbusiness—especially Xerox’s business—

is hard to overestimate. Had PARC

been more farsighted, or Sheridon moreambitious, would electronic paper havebecome commonplace a decade ago?Quite possibly. As it is, Gyricon nowmust compete with liquid crystals andelectrophoretic displays (which usecharged particles of one color suspend-ed in liquid dye of another) being devel-oped by the Massachusetts Institute ofTechnology and E Ink in Cambridge,Mass. Sheridon grimaces briefly as heconcedes, “It’s a horse race.”

—W. Wayt Gibbs in Palo Alto, Calif.

HEALING CANCER

Vaccines that prod the body to cure itself are finally being

readied for market

MEDICINE

TINY, BICOLORED PLASTIC BALLS, seen here under a microscope, flip to create black, white or gray-scale images.

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The burgeoning demand forstoring colossal amounts ofdigital data has spurred aca-

demic and industrial researchers to seeknew memory technologies. One prom-ising idea, dating from the 1970s, is touse holograms: “frozen” interferencepatterns created by lasers. A thumb-tip-size block of the right material can po-tentially store holograms representingthousands of billions of data bits, there-by offering a much greater density ofinformation than do today’s data stor-age devices. Moreover, holograms canbe read very quickly. So far, though,holographic memory has not becomecommercially viable. But recent effortsby a group at the California Institute ofTechnology may soon change all that.

Until now, the technology has beendogged by a key disadvantage: holo-grams tend to be “volatile.” In otherwords, reading them quickly degradestheir content. A hologram is createdwhen two laser beams—one of whichencodes data—interfere with each other.The interference pattern created by thebeams is captured as electric fields in asusceptible material—for example, lithi-um niobate that has been doped with atiny amount of some other metal. Toread the hologram, a single laser beam isshone at it; the hologram then diffractsthe light in a pattern that holds the storeddata. But the laser light also “washesout” the hologram as it illuminates it.

Although engineers have experiment-ed with various ways to make holo-grams more durable, all have had seri-ous limitations. One technique requires

that the storage medium be heated, andanother method needs very high powerlasers.

The Caltech group has discoveredwhat it claims is a far more practicalway to make nonvolatile holograms.Karsten Buse, Ali Adibi and DemetriPsaltis reported in Nature in June thatthey used some special, thin crystals oflithium niobate that incorporated traceamounts of iron and manganese atoms.When excited by different kinds of light,these doping atoms liberate electronsthat can be taken up by either nearbyiron or nearby manganese atoms to cap-ture the electric fields of a hologram.Iron gives up its electrons in response toeither ultraviolet light or red light,whereas manganese must have ultravi-olet light.

The researchers found that they couldrecord data durably in their crystals byilluminating them with ultraviolet light(not from a laser) at the same time thatthey made a hologram with two redlaser beams. The UV light stimulatedboth the manganese and the iron atomsto liberate electrons. Doing this ensuredthat the hologram created by the redlaser beams was stored by both ironand manganese atoms—despite the in-

sensitivity of manganese to red light.After recording, the UV light was

turned off. The resulting hologram couldbe read by illuminating it with red laserlight alone. The red light did not excitethe UV-triggered manganese atoms, sothey retained the imprinted data with-out loss. (The light signal from the ironatoms did diminish during reading, asexpected, but there were more thanenough manganese atoms to preserve astrong hologram for very long periods.)Turning the ultraviolet light back on al-lowed a new durable hologram to bewritten.

The work “is a step toward a practicalholographic storage device,” accordingto Hans Coufal of the IBM Almaden Re-search Center, who works in the field.But Coufal notes that materials sensi-tive to dimmer light will be needed formass-market devices. And LambertusHesselink of Stanford University sayshe doubts whether the Psaltis group’stechnique will still look promisingwhen the experiments are repeatedwith crystals thick enough to hold use-ful amounts of data.

Psaltis, however, says the technique—

which is patent pending—is in importantways adequate for a commercial read-write memory, although he hopes to im-prove it. The double-doped crystals theCaltech researchers used were made inEurope 20 years ago. Theory suggeststhat up to a 100-fold improvement inperformance should be possible withbetter crystals, states Psaltis, who is fab-ricating new versions. Double-dopedcrystals incorporating cerium instead ofiron look particularly intriguing, he adds.If Psaltis is right, the new age might bedominated by crystals after all.

—Tim Beardsley in Washington, D.C.

News and Analysis Scientific American September 1998 41

display the antigens that make themsusceptible to attack. (The two drugsneed not be exclusive; Ribi is running aclinical trial to see whether they workwell together.)

The largest trial of a cancer vaccineso far, however, is unfortunately an un-scientific one. In the past year, reported-ly upward of 50,000 people in Chinahave been injected with kang lai te, anextract from seeds of the herb Job’s tears

(Coix lacryma-jobi) that the govern-ment has endorsed as a treatment forcancers of the lung, liver and stomach.

It is too early to say whether sciencecan coax and coach the human body todefend itself successfully against itself.But at the very least, medicine nowseems poised to offer a more palatableexit strategy than poison, radiation orthe blade.

—W. Wayt Gibbs in San Francisco

THE DOPE ON

HOLOGRAPHY

A new technique could fulfill holography’s promise

for capturing information

DATA STORAGE

DOPED WITH DIFFERENT METALS, THE LITHIUM NIOBATE CRYSTAL(translucent block at bottom) can improve holographic memory.

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If the frustrations of all the world’scomputer users were brought to-gether, the resulting explosion

would make the big bang look like aRoman candle. This is true even thoughcomputers have come a long way in thedecade since the industry pronounced“usability” a necessity. Nevertheless, westill have a plethora of frustrating func-tions: inconsistent commands (drag afile in Windows, for instance, and youcould end up moving it, copying it orperhaps even creating a link back to itfrom another directory—who knows?),programs that rename files ac-cording to their tastes instead ofyours, “help” screens that explainoptions but not what they meanor what their consequences are,and inscrutable error messages.Home systems—such as Windows95 or 98—hide their inner work-ings from users, whereas profes-sional systems—such as WindowsNT—keep those inner workingsaccessible but design them onlyfor experts.

Nowhere, it seems, is there asystem designed for people whoknow how to use a computer butaren’t techies. I fall in that catego-ry, and the upshot is that I spendsome part of every day in an abso-lute rage at the bozos who de-signed the computer I live with,which is refusing to let me do one oranother simple thing.

The underlying presumption of soft-ware experts is that there are only twokinds of people: those who already knowall about computers and those whodon’t want to know anything aboutthem but just want to use them to com-plete tasks. The computer industry, withits usual fine grasp of language, callsthis last notion “transparency”—as in,“the computer should be transparent tothe user.” A good example of this wayof thinking is the Windows help system,which lets you click on a button to godirectly from the topic you looked up towhatever nested bit of the program youneed. Fine, but then the system nevertells you where that bit was or how togo there directly so that next time youdon’t have to go through the help sys-

tem. That’s like welding the trainingwheels to your bicycle: you can neveractually learn anything.

The notion that users should not haveto worry or care about how the innardsworked was first implemented in amainstream computer in 1984, whenthe Apple Macintosh changed people’snotions of what a computer could be.At the time, liberating users from thepetty bureaucracy of command linesmeant people were free to do all kindsof work they either could not have donebefore or could not have afforded to dobefore—desktop publishing is just oneexample.

From the mid-1980s to the early1990s, as graphic interfaces took hold

and computer companies set up usabil-ity labs, the idea that users should nothave to think about the computer lyingbetween them and their task was anenormous step forward. “The point can-not be overstressed: make the computersystem invisible,” wrote Donald Nor-man in his 1988 design classic The Psy-chology of Everyday Things. Normanwent on to imagine the perfect appoint-ment calendar, which looked like a pa-per calendar but which could send mes-sages and reminders to remote systems.

The usability efforts, however, ad-dressed only one of the two problemsevery computer poses: how to accom-plish tasks and how to manage the com-puter itself. The emphasis for the past15 years has been on tasks, and rightlyso. Most people do not buy computersbecause they think it will be fun to re-

arrange files and directories. Comput-ers, though, do still have to be man-aged—just as cars have to be serviced—

and the accessibility trend has madethis job far more difficult.

Some recent changes—such as Win-dows 95/98/NT’s registry, apparentlydesigned to be read and edited in theoriginal Martian by people who likewalking over a 1,000-foot canyon on atightrope with no safety net—seem tome purely lethal. Here is a single data-base that most people don’t know howto back up and whose corruption or losswipes out all your customization andconfiguration settings and makes yourcomputer forget it has any software in-stalled. It would have been perfectly pos-

sible to design the registry to beforgiving and to track all changes,so that you could go back andundo the last change you made orthe changes the program you justinstalled made that knocked outyour network. Instead changesare made on the fly, and there’s noway back.

The next leap in computing isto embed computers in objects allaround us. The new CrossPad is acrude example: you write on apad of ordinary paper with a spe-cial pen, and the device stores theimage and indexes the pages bythe keywords you circle. If onlyyou could use a fountain pen andthe pad came in jacket-pocketsize. . . . Even so, I want one.

But the whole mess still has tobe uploaded to a PC before you run outof storage space, and that introducesthe same old conundrum: how to get“inside” your computer so as to man-age it. As we increasingly talk about“smart” toasters and doorknobs andrefrigerators (imagine: they could tellyour computer to order milk before yourun out), we are not just talking aboutenhanced capabilities for these objects,we are also talking about putting inplace underlying systems to manage allthese things. I just spent four days try-ing to network three PCs that are sup-posed to network “right out of thebox.” Will installing your new toasterin a smart world be any easier than try-ing to network those PCs? As the comicactor Edmund Kean might have said,complexity is easy; simplicity is hard.

—Wendy M. Grossman in London


CYBER VIEWOpaque Transparency

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Preserving the Laetoli FootprintsThe discovery of hominid footprints in East Africa reshaped the study of human origins. Now conservators have protected the fragile tracks from destruction

by Neville Agnew and Martha Demas

THREE EARLY HOMINIDS cross a landscape coveredwith volcanic ash 3.6 million years ago in an artist’srendering of the Laetoli footprint makers. A largemale leads the way, while a smaller female walksalongside and a medium-size male steps in the larg-er male’s footprints. Other Pliocene animals—includ-ing giraffes, elephants and an extinct horse called a hipparion—also leave their tracks in the ash.

44 Scientific American September 1998 Copyright 1998 Scientific American, Inc.


One of the most remarkable events in the annals of anthropol-ogy occurred 20 years ago in an area of northern Tanzaniacalled Laetoli. A team led by famed archaeologist Mary D.

Leakey was searching for fossils of the early hominids that rangedthrough East Africa millions of years ago. In the summer of 1976, af-ter a long day in the field, three visitors to Leakey’s camp engaged insome horseplay, tossing chunks of dried elephant dung at one another.When paleontologist Andrew Hill dropped to the ground to avoid get-ting hit, he noticed what seemed to be animal tracks in a layer of ex-posed tuff—a sedimentary rock created by deposits of volcanic ash.On closer inspection of the area, the scientists found thousands of fos-silized tracks, including the footprints of elephants, giraffes, rhinocer-oses and several extinct mammal species. But the most extraordinaryfind came two years later, when Paul I. Abell, a geochemist who hadjoined Leakey’s team, found what appeared to be a human footprintat the edge of a gully eroded by the Ngarusi River.


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Excavations of the Footprint Tuff—as it came to be known—in 1978 and1979 revealed two parallel trails ofhominid footprints extending some27 meters (89 feet). The volcanic sed-iments were dated radiometrically tobe between 3.4 million and 3.8 mil-lion years old. The discovery settled along-standing scientific debate: theLaetoli footprints proved that early

hominids were fully bipedal—they hadan erect posture and walked on twofeet—long before the advent of stonetoolmaking or the expansion in size ofthe human brain. What is more, thetrackway provided information aboutthe soft tissue of the hominids’ feetand the length of their strides—infor-mation that cannot be ascertainedfrom fossil bones. For these reasons,the Laetoli footprints attracted a hugeamount of attention from scientistsand the general public. Leakey, whodied in 1996, regarded the discoveryas the crowning achievement of hersix decades of work in East Africa.

That the footprints have scientificvalue is obvious: they have answeredfundamental questions about human-ity’s past. But they also have a pro-found cultural symbolism. In a pow-erfully evocative way, the tracks ofthose early hominids represent thelong evolutionary history of human-kind. The footprints bear witness to adefining moment in the developmentof our species and speak to us directlyacross thousands of millennia.

For the past six years, the GettyConservation Institute—a Los Ange-les–based organization concernedwith the preservation of cultural her-itage—has worked with Tanzanianauthorities to ensure that the Laetolifootprints stay intact for years tocome. A team of conservators and

scientists recently completed a projectto protect the footprints from ero-sion, plant growth and other causesof deterioration that have threatenedthe trackway since its discovery.

A Pliocene Eruption

Skeletal remains stand a betterchance of survival in the fossil rec-

ord than impressions in mud or vol-canic ashfall. Yet traces of many ani-mals dating back to the Paleozoic era,some as old as 500 million years, areknown throughout the world. Becausean animal leaves many tracks duringits lifetime but only one set of boneswhen it dies, statistically it is not sosurprising that some of the tracks sur-vive as fossil imprints. The numberand variety of tracks preserved in theLaetoli exposures is nonetheless un-usual. At the largest of the 16 sites atLaetoli where tracks have been found,there are an estimated 18,000 prints,

CONTOUR MAP of hominid footprint G1-36 (right) was created by taking twooverlapping photographs of the print with a high-resolution camera. The deep im-pression at the bottom of the print indicates that the hominid walked like a modernhuman, placing its full weight on its heel. The length of the footprint is about 20centimeters (eight inches). On the next page, two views of footprint G1-25 showthat it suffered little damage between its discovery in 1979 and its reexcavation in1995. The reexcavated print (far right) is shown next to a photograph of the printtaken in 1979 by a member of Mary Leakey’s team.

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LAETOLI AREA in northern Tanzanialies in the eastern branch of the GreatRift Valley, where many hominid fossilshave been found. Other well-knownhominid sites include Hadar and Omoin Ethiopia, Lake Turkana in Kenyaand Olduvai Gorge in Tanzania.

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representing 17 families of animals, inan area of about 800 square meters.

Laetoli lies in the eastern branch ofthe Great Rift Valley, a tectonicallyactive area. About 3.6 million yearsago, during the Pliocene epoch, theSadiman volcano—located 20 kilome-ters (12 miles) east of Laetoli—beganbelching clouds of ash, which settledin layers on the surrounding savanna.At one point in the volcano’s activephase, a series of eruptions coincidedwith the end of an African dry sea-son. After a light rainfall, the animalsthat lived in the area left their tracksin the moist ash. The material ejectedfrom Sadiman was rich in the mineralcarbonatite, which acts like cementwhen wet. The ash layers hardened,preserving the thousands of animalfootprints that covered the area. Short-ly afterward Sadiman erupted again,depositing additional layers of ash thatburied the footprints and fossilizedthem. Finally, erosion over millions of

years reexposed the Footprint Tuff.The two parallel trails contained a

total of 54 footprints that could beclearly identified as hominid tracks.The soil covering varied from a fewcentimeters at the northern end of thetrackway—the area where the foot-prints had first been discovered—to 27centimeters (11 inches) at the south-ern end. To the north, the footprintsended at the wide, deep gully cut bythe Ngarusi River; to the south, fault-ing and erosion precluded any chanceof picking up the trail. The trackwayitself shows faulting, too, with a gra-ben—a section that had dropped 20to 40 centimeters because of tectonicactivity—near the midpoint. Part of thetrackway is also heavily weathered: inthis section the tuff had changed todried mud and the footprints werepoorly preserved. But in the less weath-ered part of the trackway the preser-vation was good, allowing clear rec-ognition of soft-tissue anatomical fea-

tures such as heel, arch and big toe.As so often happens in the field of

paleoanthropology, disagreement soonbroke out regarding the interpretationof the evidence. One point in disputewas the species of the hominids thatmade the footprints. Leakey’s teamhad found fossilized hominid bones inthe Laetoli area that were the same ageas the trackway. Most scientists believethese hominids belonged to the speciesAustralopithecus afarensis, whichlived in East Africa between 3.0 mil-lion and 3.9 million years ago. In fact,one of the Laetoli hominid remains—

a mandible with nine teeth in place—

became the type specimen, or definingfossil, for A. afarensis. (The famoushominid skeleton known as “Lucy,”discovered in 1974 in Ethiopia, is an-other representative of this species.)But Leakey did not accept that theLaetoli hominids were specimens ofA. afarensis; she resisted assigningthem to any species. (Leakey was cau-tious about interpreting her discover-ies.) She did believe, however, that themakers of the Laetoli footprints stoodin the direct line of human ancestry.

Another dispute concerned the num-ber of hominids that made the twoparallel trails. In one trail, the foot-prints were small and well defined,

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but in the other the prints were largerand less clear. Some scientists specu-lated that the trails were made by twohominids—possibly a female and amale—walking abreast or close to eachother. [For artistic representations ofthis interpretation, see “The Foot-print Makers: An Early View,” by JayH. Matternes, on page 52, and “TheLaetoli Diorama,” by Ian Tattersall,on page 53.] Other scientists believedthe trails were made by three homi-nids. In this view—which most paleo-anthropologists now share—the trailof larger footprints was made by twoindividuals, with the second hominidpurposely stepping in the tracks ofthe first [see “A New Look at Lae-toli,” at right].

The footprints prompted other in-triguing questions: Where were thehominids going? What caused themto break stride—which is indicated bythe position of four footprints in thenorthern section of the trackway—asthough to look back on where theyhad come from? Were they a familygroup? Were they carrying anything?And how did they communicate?These tantalizing questions will neverbe answered, but scientists can use theevidence gleaned from the Laetoli siteto attempt to re-create the moment

when the hominid tracks were made.Much of the controversy over the

footprints arose because few scientistshad the opportunity to study the printsfirsthand. At the end of each field sea-son, Leakey’s team reburied the track-way for its protection. But the teammembers made casts of the best-pre-served sections of the trails and docu-mented the site fully. Researchers cre-ated three-dimensional contour mapsof some of the footprints by photo-

graphing them from two perspec-tives—a process called photogramme-try. Leakey later published her workwith several co-authors in a monu-mental monograph that dealt not onlywith the hominid prints but also withthe many animal tracks and the geol-ogy of the Laetoli area. The evidencecollected by Leakey’s group—whichalso included fossilized pollen andimpressions of vegetation—providesan unparalleled record of the African


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A New Look at Laetoli

The artist’s rendering of the Laetoli footprint makers on pages 44 through46 reflects the widely accepted interpretation that the trackway was

made by three hominids.Many of the larger tracks at the site have features in-dicating that they may be double footprints.The evidence suggests that a rel-atively large hominid—about five feet tall,based on the size of its footprints—walked first,and a hominid four and a half feet tall deliberately stepped in theleader’s footsteps, perhaps to make it easier to cross the slick, ash-coveredground.A smaller hominid—about four feet tall—apparently made the paral-lel trail of well-defined footprints.The trackway indicates that this hominid ad-justed its stride to keep up with one or both of the other hominids.

The illustration shows the two larger hominids as males and the smaller in-dividual as a female, but this was not necessarily the case: the smallest mem-ber of the trio could have been a child.The female is shown walking slightlybehind the lead male because the two could not have walked abreast with-out jostling each other. —The Editors

made by two hominids walking in tandem. The two northern-most tracks (far left) were destroyed by erosion between theirdiscovery in 1978 and reexcavation in 1996. Four other tracks

in the northern section—G1-6, G1-7, G1-8 and G2/3-5—markthe point where the hominids apparently broke stride. Also pres-ent are the tracks of a hipparion.


savanna during Pliocene times and acontext in which to understand betterthe hominid trackway.

The Root Problem

Fieldwork on the Laetoli footprintsended with the 1979 season, and

Leakey’s team used local river sand torebury the site. Because the tuff is softand easily damaged, the mound of sandwas covered with volcanic boulders toarmor it against erosion and the animalsthat sometimes roam across the site—

particularly elephants and the cattle ofthe Masai people living in the area. Wenow know that seeds of Acacia seyal, alarge, vigorously growing tree species,were inadvertently introduced with thereburial fill. The loose fill and the phys-ical protection and moisture retentionprovided by the boulders created a mi-croenvironment conducive to germina-tion and rapid plant growth. Over thefollowing decade, the acacias and othertrees grew to heights of over two me-ters. Scientists who occasionally visitedthe Laetoli site began to voice concernthat the roots from these trees wouldpenetrate and eventually destroy thehominid footprints.

In 1992 the Antiquities Department ofthe Tanzanian government approachedthe Getty Conservation Institute, whichhas extensive experience in preservingarchaeological sites, to consider how thetrackway might be saved. The followingyear a joint team from the institute andthe Antiquities Department excavated asample trench in the reburial mound toassess the condition of the hominidfootprints. The assessment revealed thattree roots had indeed penetrated someof the tracks. But in the areas where noroot damage had occurred, the preser-vation of the prints was excellent. Lea-key’s intuitive decision to rebury the sitehad been the right one. With hindsightwe can now say that perhaps greatercare should have been taken in how thesite was buried. Also, periodic monitor-ing and maintenance—including the re-moval of tree seedlings before they be-came established—would have avoidedthe need for a long and costly conserva-tion effort.

The Getty Conservation Institute andthe Tanzanian government agreed tocollaborate on the project, but beforefieldwork could begin, various optionshad to be considered. Fossil bones areroutinely brought into the laboratoryfor study and permanent safekeeping.

Preserving the Laetoli Footprints50 Scientific American September 1998

REEXCAVATION began in 1995with the southern section of thetrackway (top right). Conservatorsextracted the acacia tree roots thathad penetrated the Footprint Tuff(middle right), then removed thefill from the footprints (top left).The reexcavated trackway (bot-tom left) was photographed with aPolaroid camera (bottom right) torecord conditions.N

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Indeed, to leave them in the field wouldbe irresponsible: they would certainly belost or damaged. But could the entirehominid trackway be lifted and movedto a museum in Tanzania? Was it tech-nically possible to do this without dam-aging the footprints? Some scientistswere vehement in their belief that thiswas the only way to save the tracks.

Removal would have been very risky,however, because the techniques for cut-ting out, lifting and transporting such alarge trackway had not been proved.The Footprint Tuff is far from being ahomogeneous stratum. It con-sists of many thin layers of vol-canic ash, each with differentweathering, hardness and cohe-sion. Without strengthening thetuff with resin—an interventionwith unknown long-term con-sequences—fracturing wouldprobably occur during removal.What is more, removing thetrackway or the individual foot-prints would separate them fromthe many animal tracks that hadbeen made at the same time.Part of the significance of thehominid trails—their setting inthe savanna landscape of EastAfrica together with the tracksof other Pliocene species—wouldbe lost.

An alternative proposal wasto shelter the trackway, erectinga protective building over it.The site could then be openedto the public, and the footprintscould be studied by visitingscholars. The Laetoli area, how-ever, is remote. There is no roadto the site and no water or pow-er lines nearby. Experience inTanzania has shown that with-out proper financing, trainedpersonnel and an adequate in-frastructure, sheltering the sitecould prove disastrous: it couldresult in the deterioration of thetrackway rather than its preservation.Even in countries where resources areplentiful, archaeological sites have beendamaged when planning has been inad-equate or when climate-controlled en-closures have not performed as expect-ed. Moreover, no shelter could fullyprotect the trackway from weathering:moisture from the ground below wouldrise to the surface seasonally throughcapillary action. Soluble salts in the wa-ter would crystallize on the surface,causing stress that would eventually

rupture the trackway. During the dryseason, dust accumulation in the printswould require frequent cleaning, whichwould inevitably lead to damage.

The third option was to reexcavatethe trackway, remove the vegetationthat had damaged it and then reburythe site more carefully, taking steps toprevent root growth that might harmthe footprints. Reburial is a proved pres-ervation method. The trackway sur-vived underground for thousands of mil-lennia; if reburied, it would be protect-ed from erosion, physical damage and

rapid fluctuations of moisture. Reburialis also readily reversible: the tuff can beuncovered in the future if the other op-tions become more feasible. For thesereasons, the Getty Conservation Insti-tute recommended reburial. In 1993Tanzania’s Antiquities Department de-cided to proceed with this recommen-dation, and a committee was set up toassist the implementation of the plan.Participating in the discussions wereLeakey and other eminent paleoanthro-pologists, Tanzanian officials and a re-

gional representative from the UnitedNations Educational, Scientific andCultural Organization.

Saving the Footprints

The conservation project began in1994. During that year’s field sea-

son, the trees and shrubs growing onand near the reburial mound were cutdown. To prevent regrowth, the conser-vation team applied the biodegradableherbicide Roundup to the tree stumps.In all, 150 trees and shrubs were killed,

69 of them directly on the re-burial mound.

Reexcavation of the trackwaytook place during the 1995 and1996 field seasons, beginningwith the southern section. Thissection was where the densestrevegetation had occurred and,coincidentally, where the bestpreserved footprints had beenfound in 1979. Archaeologistsand conservators used Leakey’sphotographs of the trackway tofind the exact positions of thehominid footprints. Also usefulwas the original cast of thetrackway, which was replicated,cut into conveniently short sec-tions and used as a guide for thefinal stages of reexcavation. Atemporary shelter erected overthe excavated area protected itfrom direct sunlight and shadedthose who were working on thetrackway.

In the southern section of thetrackway the trees had fortu-nately developed shallow, ad-ventitious roots rather than deeptaproots because of the hardnessof the tuff. As a consequence,there was far less damage thanhad been feared, and most ofthe footprints were generally ingood condition. In areas wherethe tuff was weathered, howev-

er, roots had penetrated the prints. Herethe conservation team surgically re-moved stumps and roots after strength-ening adjacent areas of disrupted tuffwith a water-based acrylic dispersion.Team members used miniature rotarysaws to trim the roots and routers toextract the parts that had penetratedthe surface of the trackway. The holescreated by root removal were filledwith a paste of acrylic and fumed silicato stabilize them against crumbling.

Recording the condition of a site is

Preserving the Laetoli Footprints Scientific American September 1998 51

LEAKEY’S CAST OF THE TRACKWAY was used toguide the final stages of the reexcavation of the footprints(top). Once the tracks were exposed and photographed,conservators recorded the condition of each print, notingany damage caused by root growth or erosion (bottom).

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one of the most important and chal-lenging conservation activities. The teamconducted a full survey of the exposedtrackway to provide the baseline datathat will allow future investigators to as-sess changes. Using a Polaroid camera,team members made eight-by-10-inchcolor photographs of the footprints.They then laid acetate sheets over thephotographs and noted the places wherethere were fractures, loss of tuff and in-trusive root growth, as well as any oth-er salient information.

During the reexcavation, the conser-vators noted dark stains in and aroundeach hominid footprint. This darkeningwas the result of the application of Bed-acryl, an acrylic consolidant that Lea-key’s team had used to strengthen thefootprints before making molds of them.(Silicone rubber was applied to the track-way to create molds, which were thenpeeled off and used to make fiberglasscasts.) The staining was an unforeseenside effect: although the Bedacryl didnot damage the footprints, it impairedtheir legibility and thus their scientificvalue. The Bedacryl could be removedby gently poulticing the footprints withacetone and tissue paper, but becausethere was a risk of damage to the printswhere the underlying tuff was fragile,only two prints were cleaned.

In consideration of the fact that fewresearchers had ever seen the exposedfootprints—most of the scientific litera-ture is based on casts and photographs—Tanzania’s Antiquities Department in-vited a group of scientists to reexaminethe trackway while the conservation andrecording work was going on. BruceLatimer, curator of physical anthropol-ogy at the Cleveland Museum of Natu-ral History, Craig S. Feibel, a geologist atRutgers University, and Peter Schmid,curator of the anthropology museum atthe University of Zurich, were nomi-nated by specialists in the field of paleo-anthropology to come to Laetoli. Theirstudies included a formal description ofthe footprints, stature and gait of thehominids and an examination of thethin layers of the Footprint Tuff.

Once the footprints were uncoveredand the root damage repaired, a team ofphotogrammetrists recorded the track-way to make new contour maps of theprints. The new maps are accurate towithin half a millimeter, which is farbetter than the maps made by Leakey’steam in 1979. The Laetoli trackway maynow be one of the most thoroughly doc-umented paleontological sites. The new

Preserving the Laetoli Footprints

Iworked on my painting of the Laetoli footprint makers during the early fall of 1978,shortly after the discovery of the hominid trackway. As part of my research, I flew to

Africa to confer with Mary Leakey and her associates at their base camp in Tanzania’sOlduvai Gorge. When I boarded the plane, the only information I had on the projectconsisted of a few photographs of the footprints and the surrounding area, along witha report on the geology of the Laetoli site and a list of the animal tracks found there.

While at the base camp, I consulted with Leakey and made a number of drawings ofproposed layouts. She drove me to Laetoli so I could familiarize myself with the mainfeatures of the terrain. The analysis of the Laetoli sediments indicated that there hadbeen several types of volcanic ashfalls in the area—some settling undisturbed on theground, some redeposited by wind—but all the ash had come from the Sadiman vol-cano. Geologists believe the color of this ash was light gray, not very different from thecolor of the hardened tuff in which the footprints were discovered.

I based my reconstruction of the two walking figures on the descriptions of Australo-pithecus afarensis. Fossil specimens of this species had been found at Laetoli and theAfar Triangle of Ethiopia; the bone fragments and dental evidence indicated that thetwo hominid populations looked roughly the same and lived at the time the footprintswere made. I inferred the limb proportions of the adults from the skeleton of “Lucy,” thefemale Australopithecus whose fossil remains had been found in Ethiopia in 1974. I as-sumed these hominids would have been lean, energetic bipeds, capable of exploiting avariety of habitats. For this reason, they would have probably had relatively little bodyhair, to ensure rapid heat loss. They would have also developed a dark skin to counter-act the injurious effects of ultraviolet radiation.

At the time I worked on the painting, only a few fragments of A. afarensis skulls had been found. I had to base the facial features of the female figure on those of A.

africanus, a species I had earlier reconstructed. Leakey wanted me to emphasize thesmall stature of these hominids, so I painted several guinea fowl near the figures. Themale figure carries a digging stick, presumably the only tool of this species (the earlieststone tools did not appear until much later). The female carries her toddler on her hip,probably the most convenient position for a habitual biped. The theory that the trailshad been made by three hominids was not put forth until after I finished the painting.

The final depiction (below) accorded with the few facts of the Laetoli site that werethen known. The painting first appeared in the April 1979 issue of National Geographicmagazine to illustrate an article by Leakey about the trackway.

JAY H. MATTERNES is an artist who specializes in the depiction of hominids and extinctmammals. His work has appeared in museums worldwide.


The Footprint Makers: An Early View

by Jay H. Matternes

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HOMINID FAMILY members leave their tracks in the ash from the Sadiman volcano.


photography, mapping and detailed con-dition survey have added an enormousarchive of data to the base record com-piled during the Leakey field seasons.This material is being integrated into an electronic database developed in col-laboration with the department of geo-

matics at the University of Cape Town.When conservation and documenta-

tion were complete, the trackway wasreburied under multiple layers of sandand soil from the surrounding area andfrom the nearby Ngarusi and Kakesiorivers. The fill was sieved to remove

coarse material and acacia seeds. Theconservation team poured fine-grainedsand on the footprint surface, thenplaced sheets of geotextile—a water-per-meable polypropylene material—aboutfive centimeters above the surface toserve as a marker. Then the team mem-


Only very rarely does the fossil record provide evidence ofan actual event in human prehistory. So in the late 1980s,

when we were considering subjects for presentation in dioramaform in the American Museum of Natural History’s Hall of Hu-man Biology and Evolution, the making of the Laetoli footprintsseemed an obvious choice. Constructing lifelike sculptures ofextinct humans involves many tricky decisions [see “EvolutionComes to Life,” by Ian Tattersall; SCIENTIFIC AMERICAN, August1992]. The decisions involving the Laetoli hominids were partic-ularly difficult because the 3.6-million-year-old creatures are soremote from modern-day humans. Our Laetoli diorama posedan additional problem: it was designed to represent a specificevent—the journey of the hominids across a plain of volcanicash—but the evidence from that event is a little ambiguous.

Willard Whitson, the museum hall’s designer, and I visited theLaetoli site in Tanzania and discussed our plans for the dioramawith Peter Jones, an archaeologist who was part of Mary Lea-key’s team when the trackway was discovered in 1978. We alsoconsulted paleoanthropologist Ron Clarke, who excavated manyof the footprints. Nobody disputes that the two parallel trailswere made by beings who were walking bipedally (althoughthey may have been tree climbers as well). The footprints in thewesternmost trail were much smaller and more clearly definedthan the prints in the eastern trail, but Jones pointed out that thestride lengths were the same. Clearly, the hominids were walkingin step and accommodating each other’s stride—which meantthat the two trails were made at the same time. What is more,the trails are so close together that the hominids must havebeen in some kind of physical contact when they made them.

Some anthropologists concluded that the trails had beenmade by a group of three hominids. The western trail, they

claimed, was made by a relatively small individual, whereas theeastern trail was made by two larger hominids walking in tan-dem, with one individual deliberately stepping into the tracks ofthe other. But Clarke disagreed with this view. He claimed thatbecause the footprints in the eastern trail had so many consis-tent features, they must have been made by a single large ho-minid. The larger footprints were more poorly defined than thesmaller prints, Clarke argued, because the feet of the large ho-minid had slid more in the rain-slickened ash.

These facts and theories were our starting point. The rest hadto be conjecture. Individuals of different body sizes could havemeant a number of things: male and female, parent and child,older and younger siblings. And although we suspected thatthe two hominids were in physical contact, we had no idea howthey were supporting each other. Were they holding hands?Walking arm in arm? Carrying something between them?

The scene as we finally rendered it (above) shows two Aus-

tralopithecus afarensis, a large male and a smaller female, walk-ing side by side through a sparsely vegetated landscape. Weopted for a male and a female partly to maximize visual interestbut also to show the large sex difference in body size that is be-lieved to have existed in A. afarensis, the presumed maker of thetrails. The male’s arm is draped over the female’s shoulder. In theexplanatory text we emphasize that this scenario is consistentwith the few facts we have but is not the only one possible.

Feminists have excoriated us for the “paternalistic” nature ofthe scene, but in fact we decided to show the figures joined thisway because it seemed to carry the fewest unwanted implica-tions. Indeed, a look at the faces of these creatures, brilliantlysculpted by English artist John Holmes, shows that both areworried, the male as much asthe female. Here are two small,slow and rather defenseless individuals moving through

open country that almost certainly teemed with predators.These early hominids were clearly bipeds when they were onthe African savanna, but this dangerous and difficult environ-ment was probably not their preferred milieu. Plausibly, theywere crossing this hostile territory to get from one more conge-nial region to another. Their tracks were headed almost directlytoward the well-watered Olduvai basin, where the lakeside for-est and its fringes would have felt much more like home.

IAN TATTERSALL is a curator in the department of anthropologyat the American Museum of Natural History in New York City.

Preserving the Laetoli Footprints

The Laetoli Diorama

by Ian Tattersall

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LIFELIKE SCULPTURES ofthe Laetoli hominids bear wor-ried expressions (right). Thediorama’s background showsthe stark landscape (below).


Preserving the Laetoli Footprints54 Scientific American September 1998

bers poured a layer of coarse-grainedsand and covered it with a special kindof geotextile called Biobarrier, which isdesigned to block root intrusion intothe burial fill.

Biobarrier is studded with nodulesthat slowly release the root inhibitor tri-fluralin, a low-toxicity, biodegradableherbicide. Trifluralin is not soluble inwater, so it is nonleaching and nonmi-grating: it inhibits root growth but doesnot kill the plants whose roots contactthe nodules. The effective life of Biobar-rier depends on the temperature of thesoil and the depth of burial. Based onthe manufacturer’s data, the materialwill have an effective life of about 20years at the Laetoli site. Above the Bio-barrier, the conservators added anotherlayer of coarse-grained sand, then laiddown a second covering of Biobarrierand a synthetic erosion-control matting.

The conservation team topped themound with a layer of local soil and abed of lava boulders to provide a physi-cal armor for the reburial fill. Themound, which is one meter high at itsapex, will be allowed to revegetate withgrasses; because they are shallow-root-ed, they will stabilize the reburial soilwithout posing any danger to the track-way surface. But the staff of the Antiq-uities Department will regularly moni-tor the site and remove any tree seed-lings that take root. The geotextiles are asecond line of defense should the main-tenance lapse. The shape of the mound,which has a slope of about 14 degreeson each side, will facilitate the runoff ofsurface water.

The entire process was repeated forthe northern section of the hominidtrackway during the 1996 field season.This section had suffered the most ero-sion because surface water from the sur-rounding area drains into the NgarusiRiver across the northern end of thetrackway. It was this drainage that ex-posed the first hominid footprint foundby Abell in 1978; unfortunately, the

same drainage resulted in the loss of thisprint and an adjacent one in the 18years between the burial of the track-way and its reexcavation. To preventfurther erosion, simple berms were con-structed from lava boulders around thetrackway to divert runoff from nearbyareas. Two gullies that were threateningthe northern end of the trackway werealso stabilized by placing lava bouldersand erosion-control matting on theirslopes.

Near the trackway, the team membersdug a monitoring trench, 2.5 meterssquare, which was reburied accordingto the same method used on the track-way. Parts of this trench will be period-ically reexcavated to assess the subter-ranean conditions and the continued ef-fectiveness of the Biobarrier. Acaciatrees have been permitted to survivearound the monitoring trench to seehow well the Biobarrier can block thetree roots. Although polypropylene ma-terials may be expected to last for manyyears underground, their use in tropicalenvironments such as Laetoli wherelarge numbers of termites live has notbeen properly evaluated. The monitor-ing trench will allow the AntiquitiesDepartment staff to check the perfor-mance of the geotextiles without dis-turbing the trackway itself.

A Sacred Ceremony

Experience has shown that successfulpreservation of remote sites requires

the cooperation of local people. If theyfeel excluded, there are frequently ad-verse results, from neglect to deliberateharm. Most of the people in the Laetoliarea are Masai. They have maintainedto a large degree their traditional way oflife, which centers on their herds of cat-tle. Cattle grazing on and around thetrackway site would cause erosion ofthe reburial mound and the destructionof the system of berms and drains fordiverting the surface runoff. While tend-

REBURIAL MOUND over thehominid trackway includes five lay-ers of sand and soil (diagram). Theconservation team poured fine-grained sand directly on the Foot-print Tuff (top photograph). Thereburial layers are separated bypolypropylene geotextiles and ero-sion-control matting (middle). Themound is capped with lava boul-ders to protect the trackway fromcattle and other animals (bottom).

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ing the cattle, herders with time ontheir hands might also be tempt-ed to interfere with the reburialmound. Everyone in the regionknows of the intensive activity atthe site in recent years, and somelocal people have been curiousabout the Biobarrier and othermaterials used in the reburial.

Laetoli lies within the Ngoro-ngoro Conservation Area, a vasttract set aside by the governmentto preserve both the natural envi-ronment and the Masai commu-nity’s way of life. This extraordi-nary undertaking, perhaps uniquein Africa, has a good chance ofsucceeding under capable man-agement. We frequently consult-ed the conservation area’s region-al coordinator—who became amember of the advisory commit-tee for the Laetoli project—andthe chairmen of the two closestvillages, Endulen and Esere. Ontheir recommendation, a meetingat the site was called by the Lo-boini of the region, the tradition-al religious leader and healer.

In a daylong meeting attendedby about 100 people, includingmen and women of all ages, theLoboini emphasized the signifi-cance of the trackway and explainedthe need for its protection. A sheep wassacrificed and a sacred ceremony heldto include the site among the placesrevered by the Masai people. In 1996,after the northern section of the track-way had been reexcavated, the ceremo-ny was repeated. Leakey herself attend-ed this meeting and was greeted bysome of the older people who recalledher work in the Laetoli area in the 1970s.

Ultimately, the survival of the site willdepend on the vigilance of the Tanzani-an authorities and the internationalcommunity. The Antiquities Department

has appointed two Masai men from thearea as full-time guards and instituted adetailed monitoring and maintenanceplan. The plan calls for regular photog-raphy from specified perspectives aroundthe site, periodic removal of all seed-lings—especially acacias—and repair tothe berms and drainage system.

Because the Laetoli site is not open tovisitors, we have installed a permanentdisplay at the Olduvai Museum, whichoverlooks the gorge where Leakey andher husband, Louis S. B. Leakey, madeso many of their famous discoveries.The museum is a short distance off the

dirt road that runs from the Ngo-rongoro caldera to the SerengetiPlain; it is accessible to both localpeople and international visitors.The room devoted to Laetoli con-tains the cast of the southern sec-tion of the trackway, along withtext and photographs that ex-plain why the site was reburiedand how it is being protected. Inthe past, the Olduvai Museumprimarily served internationaltourists en route to the SerengetiPlain. But the text of the Laetoliexhibit is in Swahili as well asEnglish, so it is hoped the localpeople—particularly Tanzanianschoolchildren—will come to themuseum to learn more about theLaetoli footprints and will be in-spired to care for the site.

Footprints are evocative. Whenastronaut Neil Armstrong trodon the surface of the moon, im-ages of his footprints were in-stantly recognized as symbols ofhumankind’s first steps into thecosmos. Between the Laetoli foot-prints and those on the moon liesa 3.6-million-year-long evolu-tionary journey. Looking at themyriad animal tracks at Laetoli,one has the sense that hominids

were not frequently encountered onthat landscape—their tracks are too fewin number compared with those of theother fauna. These creatures must havebelonged to an insignificant species thatsomehow escaped the inevitable extinc-tions in the harsh environment. Thewistful trail of three small figures care-fully making their way across the re-cently fallen ash from Sadiman is bothhumbling and stirring. These fragiletraces of humankind’s beginnings onthe plains of Africa deserve to be givenevery care and protection for their fu-ture survival.

Preserving the Laetoli Footprints Scientific American September 1998 55

The Authors

NEVILLE AGNEW and MARTHA DEMAS led the Getty Conserva-tion Institute’s project at Laetoli in Tanzania. Agnew received his Ph.D.in chemistry from the University of Natal in Durban, South Africa. Heheaded the conservation section of the Queensland Museum in Brisbane,Australia, before joining the Getty institute in 1988. He has undertakenconservation projects in China, Ecuador and the U.S. and is now the in-stitute’s group director for information and communications. Demasearned a doctorate in Aegean archaeology from the University of Cincin-nati and a master’s in historic preservation from Cornell University. Shejoined the Getty in 1990 and is currently involved in developing andmanaging conservation projects in the Mediterranean region and China.

Further Reading

The Fossil Footprints of Laetoli. Richard L. Hay andMary D. Leakey in Scientific American, Vol. 246, No. 2,pages 50–57; February 1982.

Disclosing the Past. Mary D. Leakey. Doubleday, 1984.Hominid Footprints at Laetoli: Facts and Interpreta-tions. Tim D. White and Gen Suwa in American Journal ofPhysical Anthropology, Vol. 72, No. 4, pages 485–514; 1987.

Laetoli: A Pliocene Site in Northern Tanzania. Editedby M. D. Leakey and J. M. Harris. Clarendon Press, 1987.

Missing Links: The Hunt for Earliest Man. John Reader.Penguin Books, 1988.

CEREMONIAL BLESSING OF THE TRACKWAYtook place in August 1996, when men and womenfrom the Masai community gathered at the Laetoli site(top). Leakey attended this event and reacquaintedherself with the local people (bottom). The great ar-chaeologist died just four months later.

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Weightlessness and the Human Body

by Ronald J. White


When a healthy Valeri Polyakov climbed out ofhis Soyuz capsule on March 22, 1995, after aworld-record 438 days on the Mir space sta-

tion, he had demonstrated that humans can live and work inspace for months at a time. It was not always clear that thiswould be the case.

In 1951, more than 10 years before Yuri Gagarin’s firstshort flight (108 minutes), Scientific American publishedan article by Heinz Haber of the U.S. Air Force School ofAviation Medicine that anticipated many of the medical ef-fects of space travel and, in particular, of weightlessness [see“The Human Body in Space,” January 1951]. Some of hispredictions, such as the occurrence of space motion sicknessat the beginning of a flight, have been borne out. Others, suchas the notion that space travelers would feel as if they werebeing jerked back and forth or that they would suddenly startto spin around during normal motion in space, have not.

As most doctors can attest, it is difficult to predict whatwill happen when a brand-new challenge is presented to thehuman body. Time and again, space travel has revealed itsmarvelous and sometimes subtle adaptive ability. But only inthe past few years have scientists begun to understand thebody’s responses to weightlessness, as the data—the cumula-tive experience of nearly 700 people spending a total of 58person-years in space—have grown in quantity and quality.Pursuit of this knowledge is improving health care not onlyfor those who journey into space but also for those of usstuck on the ground. The unexpected outcome of space med-icine has been an enhanced understanding of how the hu-man body works right here on Earth.

Feeling Gravity’s Pull

Although many factors affect human health during space-flight, weightlessness is the dominant and single most

important one. The direct and indirect effects of weightless-ness precipitate a cascade of interrelated responses that beginin three different types of tissue: gravity receptors, fluids andweight-bearing structures. Ultimately, the whole body, frombones to brain, reacts.

When space travelers grasp the wall of their spacecraft andpull and push their bodies back and forth, they say it feels asthough they are stationary and the spacecraft is moving. Thereason is embedded in our dependence on gravity for percep-tual information.

The continuous and pervasive nature of gravity removes itfrom our daily consciousness. But even though we are only

reminded of gravity’s invisible hand from time to time by,say, varicose veins or an occasional lightheadedness onstanding up, our bodies never forget. Whether we realize itor not, we have evolved a large number of silent, automaticreactions to cope with the constant stress of living in adownward-pulling world. Only when we decrease or in-crease the effective force of gravity on our bodies do we con-sciously perceive it. Otherwise our perception is indirect.

Our senses provide accurate information about the loca-tion of our center of mass and the relative positions of ourbody parts. This capability integrates signals from our eyesand ears with other information from the vestibular organsin our inner ear, from our muscles and joints, and from oursenses of touch and pressure. Many of these signals are de-pendent on the size and direction of the constant terrestrialgravitational force.

The vestibular apparatus in the inner ear has two distinctcomponents: the semicircular canals (three mutually perpen-dicular, fluid-filled tubes that contain hair cells connected tonerve fibers), which are sensitive to angular acceleration ofthe head; and the otolith organs (two sacs filled with calciumcarbonate crystals embedded in a gel), which respond to lin-ear acceleration. Because movement of the crystals in theotoliths generates the signal of acceleration to the brain andbecause the laws of physics relate that acceleration to a netforce, gravity is always implicit in the signal. Thus, the oto-liths have been referred to as gravity receptors. They are notthe only ones. Mechanical receptors in the muscles, tendonsand joints—as well as pressure receptors in the skin, particu-larly on the bottom of the feet—respond to the weight oflimb segments and other body parts.

Removing gravity transforms these signals. The otoliths nolonger perceive a downward bias to head movements. Thelimbs no longer have weight, so muscles are no longer re-quired to contract and relax in the usual way to maintainposture and bring about movement. Touch and pressure re-ceptors in the feet and ankles no longer signal the directionof down. These and other changes contribute to visual-orien-tation illusions and feelings of self-inversion, such as the feel-ing that the body or the spacecraft spontaneously reorients.In 1961 cosmonaut Gherman Titov reported vivid sensa-tions of being upside down early in a spaceflight of only oneday. Last year shuttle payload specialist Byron K. Lichten-berg, commenting on his earlier flight experiences, said,“When the main engines cut off, I immediately felt as thoughwe had flipped 180 degrees.” Such illusions can recur evenafter some time in space.

The lack of other critical sensory cues also confuses thebrain. Although orbital flight is a perpetual free fall—theonly difference from skydiving is that the spacecraft’s for-ward velocity carries it around the curve of the planet—spacetravelers say they do not feel as if they are falling. The per-ception of falling probably depends on visual and airflowcues along with information from the direct gravity recep-tors. This contradicts a prediction made in 1950 by Haber

Weightlessness and the Human Body Scientific American September 1998 59

The effects of space travel on the body resemble some of the conditions of aging. Studying astronauts’ health may improve medical care both in orbit and on the ground

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FREEDOM FROM WEIGHT, alluring though it is,has unfortunate side effects, such as motion sickness,head congestion and bone loss. On a space walk in1984, Bruce McCandless II tried out NASA’s newrocket pack—becoming the first astronaut to ventureoutside a spacecraft without a tether.


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and his colleague Otto H. Gauer: “Inthe absence of gravity there would nec-essarily be a sensation of falling in freespace. It is expected that one wouldgradually get accustomed to this state.”

The aggregate of signal changes pro-duces, in half or more of space travelers,a motion sickness that features many ofthe symptoms of terrestrial motion sick-ness: headache, impaired concentration,loss of appetite, stomach awareness,vomiting. Space motion sickness usual-ly does not last beyond the first threedays or so of weightlessness, but some-thing similar has been reported by cos-monauts at the end of long flights.

At one time, scientists attributed spacemotion sickness to the unusual patternof vestibular activity, which conflictswith the brain’s expectations. Now it isclear that this explanation was simplis-tic. The sickness results from the con-vergence of a variety of factors, includ-ing the alteration of the patterns andlevels of motor activity necessary to con-trol the head itself. A similar motionsickness can also be elicited by comput-er systems designed to create virtual en-vironments, through which one cannavigate without the forces and sensorypatterns present during real motion[see News and Analysis, “Virtual Real-ity Check,” by W. Wayt Gibbs; Scien-tific American, December 1994].

Over time, the brain adapts to thenew signals, and for some space travel-ers, “down” becomes simply where thefeet are. The adaptation probably in-volves physiological changes in both re-ceptors and nerve-cell patterns. Similarchanges occur on the ground duringour growth and maturation and duringperiods of major body-weight changes.

The way we control ourbalance and avoid falls isan important and poorlyunderstood part of physiol-ogy. Because otherwisehealthy people returningfrom space initially havedifficulty maintaining theirbalance but recover thissense rapidly, postflightstudies may allow doctorsto help those non–spacetravelers who suffer a lossof balance on Earth.

Bernard Cohen of theMount Sinai School ofMedicine and Gilles Clé-ment of the National Cen-ter for Scientific Researchin Paris undertook just

such a study after the Neurolab shuttlemission, which ended on May 3. Toconnect this work with patients suffer-ing from balance disorders, Barry W.Peterson of Northwestern Universityand a team of researchers, supportedby the National Aeronautics and SpaceAdministration and the National Insti-tutes of Health, are creating the firstwhole-body computer model of humanposture and balance control.

Space Sniffles

The second set of weightlessness ef-fects involves body fluids. Within

minutes of arriving in a weightless envi-ronment, a traveler’s neck veins beginto bulge, and the face begins to fill outand become puffy. As fluid migrates tothe chest and head, sinus and nasal con-gestion results. This stuffiness, which ismuch like a cold on Earth, continues forthe entire flight, except during heavyexercise, when the changing fluid pres-sures in the body relieve the congestiontemporarily. Even the senses of taste andsmell are altered; spicy food retains itsappeal best. In the early days of space-flight, doctors feared that the chest con-gestion might be dangerous, much aspulmonary edema is to cardiac patients;fortunately, this has not been the case.

All these events occur because thefluids in the body no longer have weight.On average, about 60 percent of a per-son’s weight is water, contained in thecells of the body (intracellular fluid), inthe arteries and veins (blood plasma)and in the spaces between the bloodvessels and cells (interstitial fluid). OnEarth, when a person stands up, theweight of this water exerts forces

throughout the body. In the vascularsystem, where the fluid columns are di-rectly connected, blood pressure increas-es hydrostatically, just as pressure in-creases with depth in water. For a quiet-ly standing individual, this hydrostaticeffect can be quite large. In the feet, botharterial and venous pressures can in-crease by approximately 100 millime-ters of mercury—double the normal ar-terial pressure and many times the nor-mal venous pressure. At locations abovethe feet but below the heart, the pres-sure increases by zero to 100 millimetersof mercury. Above the heart, arterialand venous pressure fall below atmos-pheric pressure [see illustration at left].

The hydrostatic effect has only a smallinfluence on blood flow through tissuebecause both arterial and venous pres-sures increase by the same amount. Butit does affect the distribution of fluidwithin the body by increasing theamount of blood that leaks from capil-laries into the interstitial space. Goingfrom a prone to a standing positionmoves fluid into the lower part of thebody and reduces the flow of bloodback to the heart. If unchecked, quietstanding can lead to fainting; soldierssometimes swoon when standing at at-tention. Two other hydrostatic effectsare varicose veins, which have becomepermanently distorted by the extra fluid,and swollen feet after long periods ofquiet sitting (such as an airplane flight).

In space, the hydrostatic pressure dis-appears, causing fluids to redistributenaturally from the lower to the upperbody. Direct measurements of leg vol-umes have shown that each leg losesabout one liter of fluid—about a tenthof its volume—within the first day. Thelegs then stay smaller for the whole timein space. (Actually, fluids begin to shifttoward the head while space travelersare still on the launch pad, because theysit for several hours in couches that ele-vate their feet above their heads.) Asfluid moves around, the body adapts byfurther redistributing water among itscompartments. Plasma volume decreas-es rapidly (by nearly 20 percent) andstays low.

These fluid shifts in turn initiate a cas-cade of interacting renal, hormonal andmechanical mechanisms that regulatefluid and electrolyte levels. For exam-ple, the kidney filtration rate, normallystable, increases by nearly 20 percentand remains at that level for the firstweek in space. In addition, returningspace travelers have a special form of

Weightlessness and the Human Body60 Scientific American September 1998

HYDROSTATIC PRESSURE in blood vessels changesdramatically when a person stands up. Pressure increas-es with depth below the heart, up to 100 millimeters ofmercury (mmHg) in a person of average height; abovethe heart, pressure decreases. As a result, fluid settlesinto the lower body and blood flow diminishes. Con-versely, in a prone position (or in weightlessness), pres-sure equalizes and fluid sloshes into the upper body.

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anemia, even after flights as short as afew days. Over the past few years, Clar-ence Alfrey of the Baylor College ofMedicine has shown that the loss ofplasma and the concomitant decrease invascular space lead to an overabun-dance of red blood cells. The body re-sponds by stopping production of anddestroying new red blood cells—using amechanism that was not fully appreci-ated by hematologists before Alfrey’sresearch on space travelers.

A third set of effects caused byweightlessness relates to muscle andbone. People who travel in space forany length of time come home with lessof both. Is this a cause for concern?

During weightlessness, the forceswithin the body’s structural elementschange dramatically. Because the spine

is no longer compressed, people growtaller (two inches or so). The lungs, heartand other organs within the chest haveno weight, and as a result, the rib cageand chest relax and expand. Similarly,the weights of the liver, spleen, kidneys,stomach and intestines disappear. As F. Andrew Gaffney said after his 1991flight: “You feel your guts floating up. Ifound myself tightening my abdomen,sort of pushing things back.”

Meanwhile muscles and bones cometo be used in different ways. Skeletal

muscle, the largest tissue in the body,evolved to support our upright postureand to move body parts. But in space,muscles used for antigravity support onthe ground are no longer needed for thatpurpose; moreover, the muscles usedfor movement around a capsule differfrom those used for walking down ahall. Consequently, some muscles atro-phy rapidly. At the same time, the na-ture of the muscle itself alters, changingfrom certain slow-twitch fibers that areuseful for support against gravity to

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EYES BECOME MAIN WAY TO SENSE MOTION

TOUCH AND PRESSURESENSORS REGISTERNO DOWNWARD FORCE

HIGHER RADIATION DOSES MAY INCREASE CANCER RISK

STRESS MAY COMPROMISE IMMUNE SYSTEM

The Effects of Space Travel on the Body

FLUID REDISTRIBUTIONSHRINKS LEGS

OTOLITHS IN INNER EARRESPOND DIFFERENTLYTO MOTION

FLUID REDISTRIBUTION CAUSES HEAD CONGESTIONAND PUFFY FACE

CHANGED SENSORY INPUT CONFUSES BRAIN,CAUSING OCCASIONALDISORIENTATION

LOSS OF BLOOD PLASMA CREATES TEMPORARY ANEMIA ON RETURN TO EARTH

KIDNEY FILTRATION RATE INCREASES; BONE LOSS MAYCAUSE KIDNEY STONES

WEIGHT-BEARING BONESAND MUSCLES DETERIORATE



faster contractile fibers, useful for rapidresponse. None of these changes pre-sents a problem to space travelers aslong as they perform only light work.But preventing the atrophy of musclesrequired for heavy work during spacewalks and preserving muscle for safereturn to Earth are the subject of muchcurrent experimentation.

Bone metabolism, too, changes sub-stantially. One of the strongest knownbiological materials, bone is a dynamictissue. Certain cells, the osteoblasts, havethe job of producing it, whereas others,the osteoclasts, destroy it. Both usuallywork harmoniously to rebuild bonesthroughout life. These cellular systemsare sensitive to various hormones andvitamins in the blood and to mechani-cal stress on the bone.

Bone contains both organic materials,which contribute strength and stability,and inorganic materials, which con-tribute stiffness and serve as a mineralreservoir within the body. For example,99 percent of the calcium in the body isin the skeleton. Stabilized calcium lev-els in the body’s fluids are necessary forall types of cells to function normally.

Joint Russian-American studies haveshown that cosmonauts have lost bonemass from the lower vertebrae, hips andupper femur at a rate of about 1 percentper month for the entire duration oftheir missions. Some sites, such as theheel, lose calcium faster than others.Studies of animals subjected to space-flight suggest that bone formation alsodeclines.

Needless to say, these data are indeed

cause for concern. Duringspaceflight, the loss of bone el-evates calcium levels in thebody, potentially causing kid-ney stones and calcification insoft tissues. Back on the ground,the bone calcium loss stopswithin one month, but scien-tists do not yet know whetherthe bone recovers completely:too few people have flown inspace for long periods. Somebone loss may be irreversible, inwhich case ex-astronauts willalways be more prone to bro-ken bones. A 1996 Spacelabmission was partly devoted tothese questions; a team of scien-tists from Italy, Sweden, Swit-zerland and the U.S. carriedout eight investigations relatedto muscle and bone changes.

These uncertainties mirrorthose in our understanding of how thebody works here on Earth. For exam-ple, after menopause women are proneto a loss of bone mass—osteoporosis.Scientists understand that many differ-ent factors (activity, nutrition, vitamins,hormones) can be involved in this loss,but they do not yet know how the fac-tors act and interact. This complexitymakes it difficult to develop an appro-priate response. So it is with bone loss

in space, where the right prescriptionstill awaits discovery. Up to now, vari-ous types of exercise have been tried[see “Six Months on Mir,” by ShannonW. Lucid; Scientific American, May]with little verified success.

Heavy Breathing

Disorientation, fluid redistribution,and muscle and bone loss are not

the only consequence of weightlessness.Other body systems are affected directlyand indirectly. One example is the lung.

John B. West and his group at theUniversity of California at San Diego,along with Manuel Paiva of the FreeUniversity of Brussels, have studied thelung in space and learned much theycould not have learned in earthboundlaboratories. On the ground the top andbottom parts of the lung have differentpatterns of airflow and blood flow. Butare these patterns the result only of grav-ity or also of the nature of the lung it-self? Only recently have studies in spaceprovided unequivocal evidence for thelatter. Even in the absence of gravity,different parts of the lung have differ-ent levels of airflow and blood flow.

Not everything that affects the bodyduring spaceflight is related solely toweightlessness. Also affected, for exam-ple, are the immune system (the variousphysical and psychological stresses ofspaceflight probably play roles in theimmunodeficiency experienced by as-tronauts) and the multiple systems re-sponsible for the amount and quality ofsleep (lighting levels and work schedulesdisrupt the body’s normal rhythms).Looking out the spacecraft window justbefore retiring (an action difficult to re-sist, considering the view) can let enoughbright light into the eye to trigger justthe wrong physiological response, lead-ing to poor sleep. As time goes on, thesleep debt accumulates.

For long space voyages, travelers mustalso face confinement in a tight volume,unable to escape, isolated from the nor-mal life of Earth, living with a small,fixed group of companions who oftencome from different cultures. Thesechallenges can lead to anxiety, insom-nia, depression, crew tension and otherinterpersonal issues, which affect astro-nauts just as much as weightlessness—

perhaps even more. Because these fac-tors operate at the same time the bodyis adapting to other environmentalchanges, it may not be clear which phy-siological changes result from which

Weightlessness and the Human Body62 Scientific American September 1998

LYING IN BED mimics weightlessness in its effecton the human body. At the NASA Ames ResearchCenter, volunteers lie head-down at a six-degreeangle—which, over a period of weeks, is not ascomfortable as it might look. Fluids drain fromthe legs to the upper body, muscles atrophy andbones weaken. These subjects try out variousrestorative exercises, diets and drugs. Seated on theright is astronaut Charles Brady, who carried outmedical tests during a 1996 Spacelab mission.

VIGOROUS EXERCISE, typically last-ing several hours, is a daily routine for as-tronauts. Terence T. Henricks works outon the shuttle Atlantis in 1991, whileMario Runco, Jr., wired with medical sen-sors, waits on deck. Although such exer-tion may slow atrophy of muscles, its ef-fectiveness is still not clear.

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factors. Much work remains to be done.Finally, spaceflight involves high lev-

els of radiation. An astronaut spendingone year in a moderately inclined, low-Earth orbit would receive a radiationdose 10 times greater than the averagedose received on the ground. A year’sstay on the moon would result in a doseseven times higher still, whereas a flightto Mars would be even worse. Suddenoutflows of particles from the sun, ofthe type that occurred in August 1972,can deliver a dose more than 1,000 timesthe annual ground dose in less than aday. Fortunately, such events are rare,and spacecraft designers can guardagainst them by providing special shield-ed rooms to which astronauts can tem-porarily retreat.

Obviously, the radiation hazard tolong-duration space travelers—and theconsequent cancer risk—is disconcert-ing. The problems of space radiationare difficult to study because it is nearlyimpossible to duplicate on Earth the ra-diation environment of space, with itslow but steady flux of high-energy cos-mic rays. Even so, researchers generallybelieve that with proper shielding andprotective drugs, the risks can bebrought within acceptable limits.

Down to Earth

When space travelers return to theworld of weight, complementary

changes occur. If the effects of weight-lessness are completely reversible, ev-erything should return to its normalcondition at some time after the flight.We now know that most systems in thebody do work reversibly, at least overthe intervals for which we have data.We do not yet know whether this is ageneral rule.

Space travelers certainly feel gravita-tionally challenged during and just af-ter their descent. As one person said af-

ter nine days in space: “It’s quite ashock. The first time I pushed myselfup, I felt like I was lifting three timesmy weight.” Returning space travelersreport experiencing a variety of illu-sions—for example, during head mo-tion it is their surroundings that seemto be moving—and they wobble whiletrying to stand straight, whether theireyes are open or closed.

Most of the body’s systems return tonormal within a few days or weeks oflanding, with the possible exception ofthe musculoskeletal system. So far noth-ing indicates that humans cannot liveand work in space for long periods andreturn to Earth to lead normal lives.This is clearly good news for denizensof the upcoming International SpaceStation and for any future interplane-tary missions. In fact, the station, as-sembly of which should begin late thisyear or early next year, will provide re-searchers with a new opportunity to in-vestigate the effects of space travel onhumans. On its completion in fiveyears, the station will have 46,000 cu-bic feet of work space (nearly five timesmore than the Mir or Skylab stations)and will include sophisticated laborato-ry equipment for the next generation ofmedical studies. Recognizing the needfor a comprehensive attack on all thepotential human risks of long-durationspace travel, NASA has selected andfunded a special research body, the Na-tional Space Biomedical Research Insti-tute, to assist in defining and respond-ing to those risks.

Many of the “normal” changes thattake place in healthy people during orjust after spaceflight are outwardly sim-ilar to “abnormal” events occurring inill people on Earth. For example, mostspace travelers cannot stand quietly for10 minutes just after landing withoutfeeling faint. This so-called orthostaticintolerance is also experienced by pa-

tients who have stayed in bed for along time and by some elderly people.Prolonged bed rest also results in mus-cle and bone wasting. The parallel is soclose that bed rest is used to simulatethe effects of spaceflight [see top illus-tration on opposite page].

Other age-related changes in func-tion also seem to match changes causedby spaceflight. The wobbliness follow-

ing flight resembles the tendency of theelderly to fall; the loss of bone duringflight is analogous to age-related osteo-porosis; immunodeficiency, poor sleepquality and loss of motor coordinationafflict both astronauts and the elderly.Although parallels in symptoms do notimply parallels in cause, the data are sostriking that NASA and the National In-stitute on Aging began an investigativecollaboration in 1989. The flight in Oc-tober of Senator John Glenn of Ohio,the oldest person ever scheduled to flyin space, should draw more attentionto the ongoing research in this area.


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JOHN GLENN, astronaut turned senatorturned astronaut again, emerges from ashuttle-training mock-up in 1989. Glenn isdue to fly on board the shuttle Discoveryin late October. Some of the medical effectsof space travel resemble the symptoms ofaging, such as poor sleep quality; Glenn,77, will participate in sleep experiments.

The Author

RONALD J. WHITE is associate director of the NationalSpace Biomedical Research Institute, a consortium of univer-sities led by the Baylor College of Medicine in Houston. Hehas been a professor at Baylor since 1996, before which heserved as the chief scientist of the Life Sciences Division at theheadquarters of the National Aeronautics and Space Admin-istration. A specialist in space life sciences and biomedical re-search, White served as NASA program scientist for Spacelabmissions in June 1991 and January 1992. He is the author ofvarious scientific papers and currently serves on the board oftrustees of the International Academy of Astronautics.

Further Reading

Proceedings of a Conference on Correlations of Aging andSpace Effects on Biosystems. Edited by R. L. Sprott and C. A.Combs in Experimental Gerontology, Vol. 26, Nos. 2–3, pages121–309; 1991.

Orientation and Movement in Unusual Force Environments.James R. Lackner in Psychological Science, Vol. 4, No. 3, pages134–142; May 1993.

Space Physiology and Medicine. Third edition. A. E. Nicogossian, C.L. Huntoon and S. L. Pool. Lea & Febiger, 1993.

Applied Physiology in Space. Special issue of the Journal of AppliedPhysiology, Vol. 81, No. 1, pages 3–207; July 1996.

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Attention-Deficit Hyperactivity Disorder66 Scientific American September 1998

Attention-Deficit Hyperactivity Disorder

A new theory suggests the disorder results from a failure in self-control.ADHD may arise when key brain circuits do not develop

properly, perhaps because of an altered gene or genes

by Russell A. Barkley

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As I watched five-year-old Keith in the waiting room of my of-fice, I could see why his par-

ents said he was having such a toughtime in kindergarten. He hopped fromchair to chair, swinging his arms and legsrestlessly, and then began to fiddle withthe light switches, turning the lights onand off again to everyone’s annoyance—

all the while talking nonstop. When hismother encouraged him to join a groupof other children busy in the playroom,Keith butted into a game that was al-ready in progress and took over, caus-ing the other children to complain of hisbossiness and drift away to other activ-ities. Even when Keith had the toys tohimself, he fidgeted aimlessly with themand seemed unable to entertain himselfquietly. Once I examined him more ful-ly, my initial suspicions were confirmed:Keith had attention-deficit hyperactivi-ty disorder (ADHD).

Since the 1940s, psychiatrists have ap-plied various labels to children who arehyperactive and inordinately inattentiveand impulsive. Such youngsters havebeen considered to have “minimal braindysfunction,” “brain-injured child syn-drome,” “hyperkinetic reaction of child-hood,” “hyperactive child syndrome”and, most recently, “attention-deficit dis-order.” The frequent name changes re-flect how uncertain researchers havebeen about the underlying causes of,and even the precise diagnostic criteriafor, the disorder.

Within the past several years, howev-er, those of us who study ADHD havebegun to clarify its symptoms and caus-es and have found that it may have agenetic underpinning. Today’s view ofthe basis of the condition is strikinglydifferent from that of just a few yearsago. We are finding that ADHD is not adisorder of attention per se, as had longbeen assumed. Rather it arises as a de-velopmental failure in the brain circuit-ry that underlies inhibition and self-control. This loss of self-control in turnimpairs other important brain func-tions crucial for maintaining attention,including the ability to defer immediaterewards for later, greater gain.

ADHD involves two sets of symptoms:inattention and a combination of hyper-

active and impulsive behaviors [see tableon next page]. Most children are moreactive, distractible and impulsive thanadults. And they are more inconsistent,affected by momentary events and dom-inated by objects in their immediate en-vironment. The younger the children,the less able they are to be aware oftime or to give priority to future eventsover more immediate wants. Such be-haviors are signs of a problem, howev-er, when children display them signifi-cantly more than their peers do.

Boys are at least three times as likelyas girls to develop the disorder; indeed,some studies have found that boys withADHD outnumber girls with the condi-tion by nine to one, possibly becauseboys are genetically more prone to dis-orders of the nervous system. The be-havior patterns that typify ADHD usu-ally arise between the ages of three andfive. Even so, the age of onset can varywidely: some children do not developsymptoms until late childhood or evenearly adolescence. Why their symptomsare delayed remains unclear.

Huge numbers of people are affected.Many studies estimate that between 2and 9.5 percent of all school-age chil-dren worldwide have ADHD; research-ers have identified it in every nation andculture they have studied. What is more,the condition, which was once thoughtto ease with age, can persist into adult-hood. For example, roughly two thirdsof 158 children with ADHD my col-leagues and I evaluated in the 1970sstill had the disorder in their twenties.And many of those who no longer fitthe clinical description of ADHD werestill having significant adjustment prob-lems at work, in school or in other so-cial settings.

To help children (and adults) withADHD, psychiatrists and psychologistsmust better understand the causes of thedisorder. Because researchers have tra-ditionally viewed ADHD as a problemin the realm of attention, some have sug-gested that it stems from an inability ofthe brain to filter competing sensory in-puts, such as sights and sounds. But re-cently scientists led by Joseph A. Ser-geant of the University of Amsterdamhave shown that children with ADHDdo not have difficulty in that area; in-stead they cannot inhibit their impulsivemotor responses to such input. Otherresearchers have found that childrenwith ADHD are less capable of prepar-ing motor responses in anticipation of

events and are insensitive to feedbackabout errors made in those responses.For example, in a commonly used testof reaction time, children with ADHDare less able than other children to readythemselves to press one of several keyswhen they see a warning light. They alsodo not slow down after making mis-takes in such tests in order to improvetheir accuracy.

The Search for a Cause

No one knows the direct and imme-diate causes of the difficulties ex-

perienced by children with ADHD, al-though advances in neurological imag-ing techniques and genetics promise toclarify this issue over the next five years.Already they have yielded clues, albeitones that do not yet fit together into acoherent picture.

Imaging studies over the past decadehave indicated which brain regions mightmalfunction in patients with ADHD andthus account for the symptoms of thecondition. That work suggests the in-volvement of the prefrontal cortex, partof the cerebellum, and at least two ofthe clusters of nerve cells deep in thebrain that are collectively known as thebasal ganglia [see illustration on page69]. In a 1996 study F. Xavier Castel-lanos, Judith L. Rapoport and their col-leagues at the National Institute of Men-tal Health found that the right prefron-tal cortex and two basal ganglia calledthe caudate nucleus and the globus pal-lidus are significantly smaller than nor-mal in children with ADHD. Earlierthis year Castellanos’s group found thatthe vermis region of the cerebellum isalso smaller in ADHD children.

The imaging findings make sense be-cause the brain areas that are reducedin size in children with ADHD are thevery ones that regulate attention. Theright prefrontal cortex, for example, isinvolved in “editing” one’s behavior, re-sisting distractions and developing anawareness of self and time. The caudatenucleus and the globus pallidus help toswitch off automatic responses to allowmore careful deliberation by the cortexand to coordinate neurological inputamong various regions of the cortex.The exact role of the cerebellar vermis isunclear, but early studies suggest it mayplay a role in regulating motivation.

What causes these structures to shrinkin the brains of those with ADHD? Noone knows, but many studies have sug-

Attention-Deficit Hyperactivity Disorder Scientific American September 1998 67

CHILDREN WITH ADHD cannot con-trol their responses to their environment.This lack of control makes them hyperac-tive, inattentive and impulsive.


gested that mutations in several genesthat are normally very active in the pre-frontal cortex and basal ganglia mightplay a role. Most researchers now be-lieve that ADHD is a polygenic disor-der—that is, that more than one genecontributes to it.

Early tips that faulty genetics underlieADHD came from studies of the rela-tives of children with the disorder. Forinstance, the siblings of children withADHD are between five and seven timesmore likely to develop the syndromethan children from unaffected families.And the children of a parent who hasADHD have up to a 50 percent chanceof experiencing the same difficulties.

The most conclusive evidence that ge-netics can contribute to ADHD, how-ever, comes from studies of twins. Jac-quelyn J. Gillis, then at the University ofColorado, and her colleagues reportedin 1992 that the ADHD risk of a childwhose identical twin has the disorder isbetween 11 and 18 times greater thanthat of a nontwin sibling of a child withADHD; between 55 and 92 percent ofthe identical twins of children withADHD eventually develop the condition.

One of the largest twin studies ofADHD was conducted by Helene Gjoneand Jon M. Sundet of the University ofOslo with Jim Stevenson of the Univer-sity of Southampton in England. It in-volved 526 identical twins, who inherit

exactly the same genes, and 389 frater-nal twins, who are no more alike genet-ically than siblings born years apart.The team found that ADHD has a heri-tability approaching 80 percent, mean-ing that up to 80 percent of the differ-ences in attention, hyperactivity andimpulsivity between people with ADHDand those without the disorder can beexplained by genetic factors.

Nongenetic factors that have beenlinked to ADHD include prematurebirth, maternal alcohol and tobaccouse, exposure to high levels of lead inearly childhood and brain injuries—es-pecially those that involve theprefrontal cortex. But even to-gether, these factors can ac-count for only between 20 and30 percent of ADHD casesamong boys; among girls, theyaccount for an even smallerpercentage. (Contrary to popu-lar belief, neither dietary fac-tors, such as the amount of sug-ar a child consumes, nor poorchild-rearing methods have

been consistently shown to contributeto ADHD.)

Which genes are defective? Perhapsthose that dictate the way in which thebrain uses dopamine, one of the chemi-cals known as neurotransmitters thatconvey messages from one nerve cell, orneuron, to another. Dopamine is secret-ed by neurons in specific parts of thebrain to inhibit or modulate the activityof other neurons, particularly those in-volved in emotion and movement. Themovement disorders of Parkinson’s dis-ease, for example, are caused by thedeath of dopamine-secreting neurons in


NORMAL BRAIN image outlinesthe right caudate nucleus (yellow)and the globus pallidus (red),brain structures that regulate at-tention and that are reduced insize in children with ADHD.

Psychiatrists diagnose attention-deficit hyperactivity disor-der (ADHD) if the individual displays six or more of the fol-

lowing symptoms of inattention or six or more symptoms of hy-peractivity and impulsivity. The signs must occur often and bepresent for at least six months to a degree that is maladaptiveand inconsistent with the person’s developmental level. In addi-tion, some of the symptoms must have caused impairment be-

fore the age of seven and must now be causing impairment intwo or more settings. Some must also be leading to significantimpairment in social, academic or occupational functioning;none should occur exclusively as part of another disorder.(Adapted with permission from the fourth edition of the Diag-nostic and Statistical Manual of Mental Disorders. ©1994 AmericanPsychiatric Association.)

INATTENTION• Fails to give close attention to details or makes careless

mistakes in schoolwork, work or other activities• Has difficulty sustaining attention in tasks or play activities• Does not seem to listen when spoken to directly• Does not follow through on instructions and fails to finish

schoolwork, chores or duties in the workplace• Has difficulty organizing tasks and activities• Avoids, dislikes or is reluctant to engage in tasks that

require sustained mental effort (such as schoolwork)• Loses things necessary for tasks or activities (such as toys,

school assignments, pencils, books or tools)• Is easily distracted by extraneous stimuli• Is forgetful in daily activities

HYPERACTIVITY AND IMPULSIVITY• Fidgets with hands or feet or squirms in seat• Leaves seat in classroom or in other situations in which

remaining seated is expected• Runs about or climbs excessively in situations in which

it is inappropriate (in adolescents or adults, subjective feelings of restlessness)

• Has difficulty playing or engaging in leisure activities quietly

• Is “on the go” or acts as if “driven by a motor”• Talks excessively• Blurts out answers before questions have been completed• Has difficulty awaiting turns• Interrupts or intrudes on others

Diagnosing ADHD

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a region of the brain underneaththe basal ganglia called the sub-stantia nigra.

Some impressive studies spe-cifically implicate genes that en-code, or serve as the blueprint for,dopamine receptors and trans-porters; these genes are very ac-tive in the prefrontal cortex andbasal ganglia. Dopamine recep-tors sit on the surface of certainneurons. Dopamine delivers itsmessage to those neurons bybinding to the receptors. Dopa-mine transporters protrude fromneurons that secrete the neuro-transmitter; they take up unuseddopamine so that it can be used again.Mutations in the dopamine receptorgene can render receptors less sensitiveto dopamine. Conversely, mutations inthe dopamine transporter gene canyield overly effective transporters thatscavenge secreted dopamine before ithas a chance to bind to dopamine re-ceptors on a neighboring neuron.

In 1995 Edwin H. Cook and his col-leagues at the University of Chicago re-ported that children with ADHD weremore likely than others to have a partic-ular variation in the dopamine trans-porter gene DAT1. Similarly, in 1996Gerald J. LaHoste of the University ofCalifornia at Irvine and his co-workersfound that a variant of the dopaminereceptor gene D4 is more commonamong children with ADHD. But eachof these studies involved 40 or 50 chil-dren—a relatively small number—sotheir findings are now being confirmedin larger studies.

From Genes to Behavior

How do the brain-structure and ge-netic defects observed in children

with ADHD lead to the characteristicbehaviors of the disorder? Ultimately,

they might be found to underlie im-paired behavioral inhibition and self-control, which I have concluded are thecentral deficits in ADHD.

Self-control—or the capacity to inhib-it or delay one’s initial motor (and per-haps emotional) responses to an event—is a critical foundation for the perfor-mance of any task. As most childrengrow up, they gain the ability to engagein mental activities, known as executivefunctions, that help them deflect dis-tractions, recall goals and take the stepsneeded to reach them. To achieve a goalin work or play, for instance, people needto be able to remember their aim (usehindsight), prompt themselves aboutwhat they need to do to reach that goal(use forethought), keep their emotionsreined in and motivate themselves. Un-less a person can inhibit interferingthoughts and impulses, none of thesefunctions can be carried out successfully.

In the early years, the executive func-tions are performed externally: childrenmight talk out loud to themselves whileremembering a task or puzzling out aproblem. As children mature, they inter-nalize, or make private, such executivefunctions, which prevents others fromknowing their thoughts. Children with

ADHD, in contrast, seem to lack the re-straint needed to inhibit the public per-formance of these executive functions.

The executive functions can begrouped into four mental activities. Oneis the operation of working memory—

holding information in the mind whileworking on a task, even if the originalstimulus that provided the informationis gone. Such remembering is crucial totimeliness and goal-directed behavior: itprovides the means for hindsight, fore-thought, preparation and the ability toimitate the complex, novel behavior ofothers—all of which are impaired inpeople with ADHD.

The internalization of self-directedspeech is another executive function. Be-fore the age of six, most children speakout loud to themselves frequently, re-minding themselves how to perform aparticular task or trying to cope with aproblem, for example. (“Where did Iput that book? Oh, I left it under thedesk.”) In elementary school, such pri-vate speech evolves into inaudible mut-tering; it usually disappears by age 10[see “Why Children Talk to Them-selves,” by Laura E. Berk; ScientificAmerican, November 1994]. Internal-ized, self-directed speech allows one to


BRAIN STRUCTURES affectedin ADHD use dopamine to com-municate with one another (greenarrows). Genetic studies suggestthat people with ADHD mighthave alterations in genes encodingeither the D4 dopamine receptor,which receives incoming signals, orthe dopamine transporter, whichscavenges released dopamine forreuse. The substantia nigra, wherethe death of dopamine-producingneurons causes Parkinson’s dis-ease, is not affected in ADHD.

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reflect to oneself, to follow rules and in-structions, to use self-questioning as aform of problem solving and to con-struct “meta-rules,” the basis for under-standing the rules for using rules—allquickly and without tipping one’s handto others. Laura E. Berk and her col-leagues at Illinois State University re-ported in 1991 that the internalizationof self-directed speech is delayed in boyswith ADHD.

A third executive mental function con-sists of controlling emotions, motivationand state of arousal. Such control helpsindividuals achieve goals by enablingthem to delay or alter potentially dis-tracting emotional reactions to a partic-ular event and to generate private emo-tions and motivation. Those who reinin their immediate passions can also be-have in more socially acceptable ways.

The final executive function, reconsti-tution, actually encompasses two sepa-rate processes: breaking down observedbehaviors and combining the parts intonew actions not previously learned fromexperience. The capacity for reconstitu-tion gives humans a great degree of flu-ency, flexibility and creativity; it allowsindividuals to propel themselves towarda goal without having to learn all theneeded steps by rote. It permits childrenas they mature to direct their behavior

across increasingly longer intervals bycombining behaviors into ever longerchains to attain a goal. Initial studiesimply that children with ADHD are lesscapable of reconstitution than are otherchildren.

I suggest that like self-directed speech,the other three executive functions be-come internalized during typical neuraldevelopment in early childhood. Suchprivatization is essential for creating vi-sual imagery and verbal thought. Aschildren grow up, they develop the ca-pacity to behave covertly, to mask someof their behaviors or feelings from oth-ers. Perhaps because of faulty geneticsor embryonic development, childrenwith ADHD have not attained this abil-ity and therefore display too much pub-lic behavior and speech. It is my asser-tion that the inattention, hyperactivityand impulsivity of children with ADHDare caused by their failure to be guidedby internal instructions and by their in-ability to curb their own inappropriatebehaviors.

Prescribing Self-Control

If, as I have outlined, ADHD is a fail-ure of behavioral inhibition that de-

lays the ability to privatize and executethe four executive mental functions I

have described, the finding supports thetheory that children with ADHD mightbe helped by a more structured envi-ronment. Greater structure can be animportant complement to any drug ther-apy the children might receive. Current-ly children (and adults) with ADHD of-ten receive drugs such as Ritalin thatboost their capacity to inhibit and regu-late impulsive behaviors. These drugsact by inhibiting the dopamine trans-porter, increasing the time that dopa-mine has to bind to its receptors on oth-er neurons.

Such compounds (which, despite theirinhibitory effects, are known as psycho-stimulants) have been found to improvethe behavior of between 70 and 90 per-cent of children with ADHD older thanfive years. Children with ADHD whotake such medication not only are lessimpulsive, restless and distractible butare also better able to hold importantinformation in mind, to be more pro-ductive academically, and to have moreinternalized speech and better self-con-trol. As a result, they tend to be likedbetter by other children and to experi-ence less punishment for their actions,which improves their self-image.

My model suggests that in additionto psychostimulants—and perhaps an-tidepressants, for some children—treat-


A Psychological Model of ADHD

A loss of behavioral inhibition and self-control leads to the following disruptions in brain functioning:

IMPAIRED FUNCTION

Nonverbal working memory

Internalization of self-directed speech

Self-regulation of mood, motivation and level of arousal

Reconstitution (ability to break down observed behaviors into component parts that can be recombined into new behaviors in pursuit of a goal)

CONSEQUENCE

Diminished sense of timeInability to hold events in mindDefective hindsightDefective forethought

Deficient rule-governed behaviorPoor self-guidance and

self-questioning

Displays all emotions publicly; cannot censor them

Diminished self-regulation of drive and motivation

Limited ability to analyze behaviors and synthesize new behaviors

Inability to solve problems

EXAMPLE

Nine-year-old Jeff routinely forgets important responsibilities, such as deadlines for book reports or an after-school appointment with the principal

Five-year-old Audrey talks too much and cannot give herself useful directions silently on how to perform a task

Eight-year-old Adam cannot maintain thepersistent effort required to read a story appropriate for his age level and is quick to display his anger when frustrated by assigned schoolwork

Fourteen-year-old Ben stops doing a homework assignment when he realizes that he has only two of the five assigned questions; he does not think of a way to solve the problem, such as calling a friend to get the other three questions

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ment for ADHD should include train-ing parents and teachers in specific andmore effective methods for managingthe behavioral problems of children withthe disorder. Such methods involve mak-ing the consequences of a child’s actionsmore frequent and immediate and in-creasing the external use of promptsand cues about rules and time intervals.Parents and teachers must aid childrenwith ADHD by anticipating events forthem, breaking future tasks down intosmaller and more immediate steps, andusing artificial immediate rewards. All

these steps serve to externalize time,rules and consequences as a replace-ment for the weak internal forms of in-formation, rules and motivation of chil-dren with ADHD.

In some instances, the problems ofADHD children may be severe enoughto warrant their placement in specialeducation programs. Although suchprograms are not intended as a cure forthe child’s difficulties, they typically doprovide a smaller, less competitive andmore supportive environment in whichthe child can receive individual instruc-

tion. The hope is that once childrenlearn techniques to overcome their defi-cits in self-control, they will be able tofunction outside such programs.

There is no cure for ADHD, but muchmore is now known about effectivelycoping with and managing this persis-tent and troubling developmental disor-der. The day is not far off when genetictesting for ADHD may become avail-able and more specialized medicationsmay be designed to counter the specificgenetic deficits of the children who suf-fer from it.


The Author

RUSSELL A. BARKLEY is director of psychology and pro-fessor of psychiatry and neurology at the University of Massa-chusetts Medical Center in Worcester. He received his B.A.from the University of North Carolina at Chapel Hill and hisM.A. and Ph.D. from Bowling Green State University. He hasstudied ADHD for nearly 25 years and has written many scien-tific papers, book chapters and books on the subject, includingADHD and the Nature of Self-Control (Guilford Press, 1997)and Attention-Deficit Hyperactivity Disorder: A Handbookfor Diagnosis and Treatment (Guilford Press, 1998).

Further Reading

The Epidemiology of Attention-Deficit Hyperactivity Disor-der. Peter Szatmari in Child and Adolescent Psychiatric Clinics ofNorth America, Vol. 1. Edited by G. Weiss. W. B. Saunders, 1992.

Hyperactive Children Grown Up. Gabrielle Weiss and Lily Troken-berg Hechtman. Guilford Press, 1993.

Taking Charge of ADHD: The Complete, Authoritative Guidefor Parents. R. A. Barkley. Guilford Press, 1995.

Dopamine D4 Receptor Gene Polymorphism Is Associated withAttention Deficit Hyperactivity Disorder. G. J. LaHoste et al. inMolecular Psychiatry, Vol. 1, No. 2, pages 121–124; May 1996.

PSYCHOLOGICAL TESTS used in ADHD re-search include the four depicted here. The tow-er-building test (upper left), in which the subjectis asked to assemble balls into a tower to mimican illustration, measures forethought, planningand persistence. The math test (upper right) as-sesses working memory and problem-solvingability. In the auditory attention test (below),the subject must select the appropriate coloredtile according to taped instructions, despite dis-tracting words. The time estimation test (lowerright) measures visual attention and subjectivesense of time intervals. The subject is asked tohold down a key to illuminate a lightbulb on acomputer screen for the same length of time thatanother bulb was illuminated previously.

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You can call it a gamble: there’sjust a narrow path to reachour goal—or to miss it. In this

spirit, we start an experiment to makenew, superheavy elements. These ele-ments have to be produced in a lengthy,complicated procedure in which wesmash atomic nuclei into one anotherat very high speeds and hope they un-dergo fusion. The resulting productswill be extremely fragile, and most willbreak apart immediately.

Only under very extraordinary con-ditions will a new element have a chanceto survive the production process andland in a stable configuration—what wecall the ground state. But even whenthese conditions are met, the productionrate of a new element is tiny. To makeelement 112, the heaviest artificial ele-ment produced to date, we conductedan around-the-clock experiment for 24days and created only two atoms of 112,which lasted for only microseconds.

When we began hunting for element113 this past spring, we expected theproduction rate to be a factor of two orthree times lower. So we attempted tomake 113 in an experimental run thatlasted 42 days. We found nothing. We’re

still asking ourselves, Why didn’t wesee anything? Did we choose the wrongenergy setting? Is the production ratesmaller than we expected? Is theresome unusual property of element 113that makes it difficult to detect with ourcurrent equipment?

Despite the difficulties of making newelements that last for such a short time,these lingering questions make it impos-sible for us to give up the search. Overthe past six decades, researchers havemade 20 artificial elements. The ques-tion is, How many more can we create?

Artificial Elements

In 1936 physicist Emilio G. Segrè wasworking at the cyclotron in Berkeley,

Calif., in the laboratory of his friendErnest O. Lawrence. Segrè irradiated asample of molybdenum with particlescalled deuterons; afterward, he took thesample back to the University of Paler-mo. There Segrè discovered the firstman-made element, technetium, ele-ment number 43 (this so-called atomicnumber indicates an element’s positionin the periodic table as well as the num-ber of positively charged particles, or

protons, found in an atom’s nucleus).The inspiration for making techne-

tium came from experiments carriedout by another Italian physicist, EnricoFermi. In 1934 Fermi, then at the Uni-versity of Rome, proposed that new ele-ments could be made by bombardingan atom’s central core, or nucleus, withuncharged particles known as neutrons.Typically neutrons reside within atomicnuclei, but a lone neutron can penetratethe nucleus, where it may be captured.The resulting nucleus may be stable, orit may be radioactive. In the latter case,the neutron will, through a processknown as beta-decay, change into aproton, an electron and an antineutrino(a chargeless, virtually massless, sub-atomic particle often released duringnuclear decay). As a result of this pro-cess of neutron capture and beta-decay,the number of protons in the nucleusincreases, the atomic number goes upwith each proton and higher elementsare formed.

This idea of irradiating the nuclei ofvarious elements with neutrons waspicked up by other research groups. In1940 Edwin M. McMillan and PhilipH. Abelson, both at the University of


Making New ElementsThree new elements—110, 111 and 112—have been produced

over the past several years. Scientists are now struggling to create 113 and 114. How many elements can they add to the periodic table?

by Peter Armbruster and Fritz Peter Hessberger

a b

FERMIUM

ZINC

LEAD


California at Berke-ley, jumped aheadto the next un-known elementand, by neutron ir-radiation at the cy-

clotron there, syn-thesized element 93—

the first element beyonduranium, the heaviest nat-

urally occurring element knownat the time. (Appropriately, element 93was eventually named neptunium, be-cause Neptune is the first planet beyondUranus.) During the 1940s and 1950s,the Americans—working primarily withGlenn T. Seaborg of Lawrence BerkeleyNational Laboratory and continuing thescientific work that began during war-time research into nuclear weapons—

discovered a string of new elements: plu-tonium (element 94), americium (95),curium (96), berkelium (97), californi-

um (98), ein-steinium (99) and

fermium (100).All eight of these

elements can be pro-duced in weighable quantities ac-

cording to Fermi’s suggested combina-tion of neutron capture and beta-decay.But the quantity of each that can be syn-thesized varies widely. Today the world’sreserve of plutonium is more than 1,000tons (1030 atoms); the reserve of fermi-um, however, was never more than sometrillionths of a gram (1010 atoms). Un-fortunately, Fermi’s recipe ends at fermi-um—beyond this point, beta-decay doesnot take place, so new elements cannotbe produced by the technique. A newapproach was needed.

International Rivalries

The option at hand was to bring to-gether at very high speeds light ele-

ments such as carbon (element 6), ni-trogen (7) or oxygen (8) and transura-nic elements—such as plutonium (94)through einsteinium (99)—with the ex-pectation that the energy of the colli-sion would prompt the fusion of thetwo nuclei, creating even heavier ele-ments. But the enormous practical dis-advantages that arise when trying toslam two nuclei into each other posedmajor challenges in the laboratory.

To carry out these experiments, scien-tists improved on existing technology(namely, the cyclotron) and, for the firsttime, built linear accelerators. These de-vices allow researchers to accelerate high-intensity beams of ions at well-definedenergies. The use of transuranic elements


ELEMENT 112 forms when a zinc nucleussmashes into a lead nucleus with enoughforce to overcome the natural repulsion ofthe two positively charged nuclei (a). Thetwo nuclei fuse together (b) to form a com-pound nucleus that is very unstable. The nu-cleus immediately releases a neutron to pro-duce isotope 277 of element 112 (c). Thenew element lasts several hundred microsec-onds before undergoing radioactive decay toproduce a chain of six “daughter” elements(d–i): element 110, hassium, seaborgium,rutherfordium, nobelium and fermium.

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ELEMENT 110

ALPHA PARTICLE (2 NEUTRONS AND

2 PROTONS)

ELEMENT 112

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meant that this technique could be car-ried out only where there was access tothe large nuclear reactors found in coun-tries with nuclear weapons. As a result,during the cold war, the two researchsites involved—Lawrence Berkeley Na-tional Laboratory in the U.S. and theJoint Institute for Nuclear Research inDubna, Russia—were competing notonly scientifically but also politically.

By 1955 the Berkeley group had pro-duced element 101, mendelevium, by

the fusion of helium (element 2) and ein-steinium (99). Between 1958 and 1974the two groups created the elements no-belium (102), lawrencium (103), ruth-erfordium (104), dubnium (105) andseaborgium (106). Those days were sofull of tension that the U.S. and Russiastill argue over who was first to discov-er the elements as well as what theirnames should be. The names mentionedhere reflect the ones recognized by theInternational Union of Pure and Ap-plied Chemistry [see table on page 76].

After making element 106, scientistshit another roadblock: the standard fu-sion technique did not succeed in pro-ducing additional new elements. It wasat this time that Germany entered therace. With the founding in December1969 of the Gesellschaft für Schwerio-nenforschung (GSI, or the Institute forHeavy-Ion Research) in Darmstadt, Ger-man chemists and physicists had begunto participate in this field that had been

exclusively the domain of Americanand Soviet researchers. Several yearslater, in 1975, the heavy-ion acceleratorUNILAC (Universal Linear Accelera-tor), an idea conceived by ChristophSchmelzer of the University of Heidel-berg (the first director of the institute),was put into operation at GSI.

This novel machine was the first thatcould accelerate all types of ions, includ-ing uranium, at continuous, adjustableenergies, offering great flexibility in at-

tempts to fuse two nuclei.One of our main objectivesfor UNILAC was to makeelements 107 through at least114—what we call the su-perheavy elements. Why aimfor 114? According to theo-retical calculations, element114 should be particularlystable because its nucleusshould exist in what physi-cists call a closed shell.

In 1948 Otto Haxel, J.Hans D. Jensen and Hans E.Suess of the University ofHeidelberg, as well as MariaGoeppert-Mayer of ArgonneNational Laboratory, ob-served interesting regularitiesin the number of neutronsand protons found in atomicnuclei: certain combinationsof these two subatomic par-ticles produced nuclei thatwere quite stable comparedwith their neighbors. Similarpatterns had been recognized

in the number of electrons found inatoms as well—specific configurationsof electrons produced chemically stableelements. Scientists determined that suchpatterns reflected the way electrons fillsuccessive energy shells surroundingatomic nuclei. Some shells hold onlytwo electrons, whereas others can carryup to 14. Researchers found that in themost stable elements—the chemicallyinactive noble gases—the outermost oc-cupied shell was completely full, orwhat they termed “closed.”

Drawing on the idea of the filling ofelectron shells, Goeppert-Mayer and,independently, Jensen developed theshell model of the atomic nucleus andexplained how many protons and neu-trons together produced closed shells.The two shared the 1963 Nobel Prizein Physics for the discovery.

For every atom of a given element,the number of protons in the nucleusremains the same. But various forms, or

isotopes, of a particular element can ex-ist, each with a different number of neu-trons in the nucleus. Isotopes are distin-guished by what is known as atomicweight, a figure equal to the sum of thenumber of protons and neutrons. Stableisotopes with extremely stable nucleiinclude calcium 40 (20 protons and 20neutrons), calcium 48 (20 protons and28 neutrons) and lead 208 (82 protonsand 126 neutrons). In addition, one iso-tope of element 114, with 114 protonsand 184 neutrons, should also have aclosed shell. Physicists described ele-ments neighboring these closed shells asexisting on an “island of stability” in asea of otherwise unstable nuclei.

So once UNILAC was up and running,we were anxious to see if we could reachthat island of stability. In the beginning,everything seemed promising. Theoreti-cians initially told us that calculationsindicated elements close to 114 wouldhave long half-lives, on the order of bil-lions of years, comparable to the half-lives of certain lighter elements such asuranium and thorium. (An isotope’shalf-life is the time required for half of agiven number of atoms to undergo ra-dioactive decay.) We expected to be ableto produce considerable quantities of thesuperheavy elements—possibly leadingto novel materials for chemists to inves-tigate and new atoms for nuclear andatomic physicists to study.

But in the early 1980s it became clearthat the production of superheavy ele-ments around element 114 was not go-ing to be easy. All attempts to synthesizethem using various combinations of pro-jectiles and targets in nuclear reactionsfailed. Even further efforts to find theseelements in nature yielded nothing.

Cold Fusion

After learning about an important 1974 discovery by Yuri Oganessian

and his research partner Alexander De-min of the Dubna facility, we at GSItook up their new strategy for produc-ing the superheavy elements. Oganessianand Demin bombarded a target madeof lead (element 82) with ions of argon(18) and produced element 100, fermi-um. Oganessian recognized that duringthis process, the heating of the newlyformed nuclei was much lower than thatin the higher-impact collisions neededwhen irradiating the heaviest transura-nic isotopes with very light ions, as hadbeen common practice until then. As aresult of the lower excitation energy,

Making New Elements74 Scientific American September 1998

TEAM EFFORT is needed when creating elements.The authors are pictured with the team that pro-duced element 112, the heaviest artificial elementsynthesized to date. Peter Armbruster is standing inthe front row, third from the left; Fritz Peter Hess-berger is standing in the back row at the far left.

GSI


Oganessian concluded, far more nucleicould survive the fusion process andwould not subsequently undergo fissionand break apart.

Our GSI team became interested inthis technique, which we termed “cold”fusion because of the lower excitationenergy—and hence, reduced heat—ofthe nucleus during the procedure. (Ofcourse, this method has nothing to dowith the discussion some years ago ofpurported room-temperature fusion ofdeuterium nuclei in a test tube.) Re-searchers at Berkeley regarded cold fu-sion as a curiosity and did not take itseriously, but for us it was our onlychance to enter the field. For one, thestarting materials, lead and bismuth,were readily available in nature and didnot need to be produced by nuclear re-actors. In addition, the configuration ofUNILAC allowed us to use all kinds ofions as projectiles and to vary their en-ergies in small steps that could be easilyreproduced at any value.

We also had the right device for iso-lating and detecting products from a fu-sion reaction. As UNILAC was beingbuilt at GSI, Gottfried Münzenberg leda team of scientists from both GSI andthe Second Physics Institute of the Uni-

versity of Giessen in Germany that builta special filter, called the separator forheavy-ion reaction products, or SHIP,for this purpose. The filter removes pro-jectiles and unwanted side products ofthe fusion reaction and efficiently focus-es desired products onto a detector thatenables us to identify them by their ra-dioactive decay products.

Elements 107 and Beyond

In the early 1980s our group was ableto prove convincingly that cold fu-

sion works: we identified bohrium (ele-ment 107), hassium (108) and meitner-ium (109). Soon after, we described ourtechnique in these pages [see “CreatingSuperheavy Elements,” by Peter Arm-bruster and Gottfried Münzenberg;Scientific American, May 1989].

Following this success, however, werecognized that taking the next step toelement 110 would require improve-ments in our experimental techniques.To produce a single atom of meitneri-um, element 109, for instance, we had tooperate the accelerator for two weeks.Furthermore, our ability to producethese superheavy elements falls off by afactor of three with each addition to the

periodic table—making them increasing-ly difficult to detect.

Despite these potential problems, wecontinued to pursue the method thathad worked for us in the past. Teams atBerkeley and Dubna had by then re-turned to the previous method of “hot”fusion, so at least we had no competi-tion at that time. Through a variety ofstructural changes to UNILAC, we wereable to triple the intensity of its ion beam.We also enhanced by a factor of threethe sensitivity of SHIP, the filtering de-vice. Finally, our colleague Sigurd Hof-mann built a new detector system withincreased sensitivity.

Our team had changed somewhat aswell. In addition to the old group (thetwo of us, Hofmann, Münzenberg, Hel-mut Folger, Matti E. Leino, Victor Ni-nov and Hans-Joachim Schött), a newcrew joined us: Andrey G. Popeko, Alex-andr V. Yeremin and Andrey N. Andre-yev of the Flerov Laboratory of Nucle-ar Reactions in Dubna, as well as Ste-fan Saro and Rudolf Janik of ComeniusUniversity in Bratislava, Slovakia. Wehad stayed in close contact with theDubna team since 1973, but changingpolitics now allowed these scientists tocollaborate with us directly.

Making New Elements Scientific American September 1998 75

FOCUSINGMAGNETS

FOCUSINGMAGNETS

TARGET WHEEL

ION BEAM

BEAM STOP

MAGNETICDEFLECTORS

MAGNET

VELOCITY DETECTORS

SILICON DETECTOR

GAMMA DETECTOR

ELECTROSTATICDEFLECTORS

ELECTROSTATICDEFLECTORS

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FILTERING APPARATUS allows researchers at GSI toisolate the nuclei of superheavy atoms. The equipmenttakes up an entire room, as seen in the photograph above.

These devices divert ions thatdo not match expected charac-teristics out of the beam path, leav-ing only the desired fusion products.

Once the ion beam leaves the target wheel, wherefusion takes place to form new elements, it pass-

es through a series of focusing magnets andelectrostatic and magnetic deflectors.

This beam then passes through additional deflectors andmagnets that focus the nuclei onto a series of three de-

tectors, which identify various characteristics ofthe nuclei produced during fusion. Knowing

such factors as velocity and the productsof radioactive decay enables scien-

tists to identify what elementhas been made.


When we resumed operation of UNI-LAC in 1993, we conducted a series oftest runs with beams of the isotopes ar-gon 40 (element 18) and titanium 50(element 22), which had been used toproduce various isotopes of both men-delevium and rutherfordium. We alsoconducted other runs to determine theright energy setting for making element110. In all these trials, the success of ourstructural improvements was apparent:both the beam intensity and detectorsensitivity were definitely better.

On November 9, 1994—after a breakof more than 10 years—we were finallyable to identify the decay products of

yet another previously unknown ele-ment: during the fusion of lead 208 (ele-ment 82) with nickel 62 (element 28), a new compound nucleus was formedwith 110 protons and an atomic weightof 270. This nucleus promptly dis-charged one neutron, resulting in isotope269 of element 110. This new isotopehad a half-life of 170 microseconds; weidentified it by monitoring how the nu-cleus decayed into so-called daughterproducts. In this case, the nucleus dis-charged a series of four alpha particles,each a helium nucleus with two neu-trons and two protons, to generate thedaughter product isotope 257 of ruther-

fordium (element 104). In subsequentexperiments, we also created isotope271 of element 110. This isotope of theelement was somewhat easier to make:its rate of production was about a fac-tor of four higher than that of the previ-ous isotope.

Just over one month later, on Decem-ber 17, 1994, the 25th anniversary ofGSI, we discovered element 111 afterirradiating bismuth 209 with nickel 64.This experiment provided some impor-tant confirmation of the theory of nu-clear shells. Two of the decay productsof element 111 (isotope 268 of meit-nerium and isotope 264 of bohrium)

Making New Elements76 Scientific American September 1998

D isputes over who created elements 102 through 109 re-sulted in an international controversy over their names.

In 1994 the International Union of Pure and Applied Chemistry (IUPAC) appointed the Commission on Nomenclature of Inor-ganic Chemistry (CNIC) to determine official names for these ele-ments (many had been informally named by their discoverers).

The list proposed by the CNIC, however, did not settle the con-troversy—it simply inflamed the debate. Last year the IUPAC so-licited comments from chemists around the world concerningthe naming of these elements and in August 1997 settled on fi-nal names, as shown below. The names for elements 110 through112 have yet to be officially determined. —P.A. and F.P.H.

ELEMENT

102

103

104

105

106

107

108

109

DISCOVERER(S)

Initial claim: Nobel Institute in Stockholm, Sweden

Additional claims: University of California, Berkeley; Joint Institute for Nuclear Research, Dubna, Russia

Disputed; both Dubna and Berkeley teams claim priority



Berkeley (undisputed)

GSI, Darmstadt, Germany (undisputed)

GSI (undisputed)

GSI (undisputed)

SUGGESTED NAMES (proposed by)

Joliotium (Dubna)Nobelium (Nobel Institute, CNIC)

Lawrencium (Berkeley, CNIC)

Dubnium (CNIC)Kurchatorium (Dubna)Rutherfordium (Berkeley)

Hahnium (Berkeley)Joliotium (CNIC)Nielsbohrium (Dubna)

Rutherfordium (CNIC)Seaborgium (Berkeley)

Bohrium (CNIC)Nielsbohrium (GSI)

Hahnium (CNIC)Hassium (GSI)

Meitnerium (GSI, CNIC)

OFFICIAL NAME (chemical symbol)

Nobelium (No); in honor of Alfred Nobel, Swedish inventor and founder of the Nobel Prizes

Lawrencium (Lr); in honor of Ernest O. Lawrence, inventor of the cyclotron

Rutherfordium (Rf ); in honor of New Zealand–born physicist Ernest Ruther-ford, whose work was crucial to the early understanding of the nucleus

Dubnium (Db); in honor of the Dubna laboratory

Seaborgium (Sg); in honor of Glenn T. Seaborg, American chemist who was co-discoverer of 11 artificial elements

Bohrium (Bh); in honor of Danish physicist Niels Bohr, whose research contributed significantly to the modern understanding of the atom

Hassium (Hs); named for the German state Hesse, where Darmstadt is located

Meitnerium (Mt); in honor of Lise Meitner, the Austrian physicist who first conceived the idea of nuclear fission

The Politics of Naming


proved to be more stable than previous-ly observed lighter isotopes of these ele-ments, just as the theory predicted.

In our next set of experiments, weplanned to use the neutron-rich zinc 70as a projectile, hoping to generate ele-ments 112 and 113. And in February1996 we succeeded in producing ele-ment 112, the heaviest element evermade in the laboratory.

Our yield was much lower than ex-pected; as mentioned earlier, we madetwo atoms in 24 days. The half-life ofthis isotope of element 112 was 240microseconds. We were able to identifyisotope 277 of element 112 by observ-ing its radioactive decay and followingits successive emission of six alpha par-ticles to yield the daughter product fer-mium 253 [see illustration on pages 72and 73]. In the process, we identified anew isotope of element 110 and a newisotope of hassium. The isotope of 112was the first nucleus we made with over162 neutrons, a quantity that constitutesa deformed closed nuclear shell. Thisfactor contributed to the increased sta-bility of certain products of the element’sdecay. Specifically, we found that hassi-um 269, produced during the decay ofthis isotope of 112, had a half-life of 9.3seconds. In contrast, hassium 265, whichhad been observed in earlier experiments,had a half-life of only 1.7 milliseconds.Such findings further confirmed theo-reticians’ predictions that there wouldbe a closed shell at 162 neutrons.

The creation of superheavy elementsis the result of careful long-term plan-ning. Success has come intermittently,and recent developments are no excep-tion. The next step for us to take—mak-ing elements 113 and 114—is provingquite difficult. As mentioned earlier, labshere in Germany, as well as in the U.S.

and Russia, have not, as of this writ-ing, been successful in creating 113or 114, despite lengthy and repeatedefforts. Nevertheless, we have comea long way since the 1940s, whenNiels Bohr predicted that fermium,element 100, would be the last ele-ment of the periodic table. We cannow identify new elements with half-lives of less than 10 microseconds. If,out of 10 billion trials, two nucleifuse together once to form one super-heavy atom, we will find it.

Yet all evidence suggests that eachnew element in the periodic tablewill be harder to make than the pre-vious one—a trend that shows no signof changing. Continued improve-ments in our experimental techniquesmight bring us to the elusive element114 using the successful cold fusionmethod. But the main obstacle toreaching higher elements reflects afundamental law of physics: positive-ly charged nuclei naturally repel oneanother, and these repulsive forcesincrease as the nuclei become larger.Although we have managed to cir-cumvent these forces so far, we willnot be able to continue indefinitely.

Today all laboratories searching forsuperheavy elements collaborate closely,and political competition between ourcountries is no longer a major drivingforce. But we hope that even without thepolitical pressures, the countries involvedwill continue to support research on su-perheavy elements. We know that these

efforts probably cannot be convertedinto practical applications, but the intel-lectual and technological achievementsgained from this work have certainlyjustified our efforts. We know that thenumber of elements in the period tableis finite. The question to be answered is,How far can we go?

Making New Elements Scientific American September 1998 77

The Authors

PETER ARMBRUSTER and FRITZ PETER HESS-BERGER work together at Gesellschaft für Schwerio-nenforschung (GSI, or the Institute for Heavy-Ion Re-search) in Darmstadt, Germany. Armbruster has been atGSI since 1971. As senior scientist of the nuclear chem-istry department, Armbruster was head of the researchproject to synthesize elements 107 through 112. His cur-rent interests also include developing methods for incin-erating nuclear waste. Hessberger arrived at GSI in 1979after studying physics at Technische Hochschule inDarmstadt. He worked with Armbruster on the synthe-sis of elements 107 through 112 and received his doctor-ate in 1985. Hessberger is also studying nuclear reac-tions and nuclear spectroscopy.

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HIGH-TECH LABS at the Joint Insti-tute for Nuclear Research in Dubna,Russia (top), and at Lawrence BerkeleyNational Laboratory in the U.S. (bottom)have been home to many new elements.

Further Reading

On the Production of Heavy Elements by Cold Fusion: The Elements106 to 109. Peter Armbruster in Annual Review of Nuclear and Particle Sci-ence, Vol. 35, pages 135–194; 1985.

Recent Advances in the Discovery of Transuranium Elements. Gott-fried Münzenberg in Report on Progress in Physics, Vol. 51, No. 1, pages57–104; January 1988.

An Element of Stability. Richard Stone in Science, Vol. 278, pages 571– 572; October 24, 1997.

Meine 40 Jahre Mit Rückstoßspektrometern. Peter Armbruster in Phys-ics Blätter, Vol. 53, pages 661–668; 1997.

New Elements: Approaching Z = 114. Sigurd Hofmann in Reports on Prog-ress in Physics, Vol. 61, No. 6, pages 639–689; June 1998.

GSI site is available at http://www.gsi.de/gsi.html (also available in German athttp://www.gsi.de/gsi.welcome.deutsch.html) on the World Wide Web.

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The periodic table of the ele-ments is one of the most pow-erful icons in science: a single

document that consolidates much ofour knowledge of chemistry. A versionhangs on the wall of nearly every chem-ical laboratory and lecture hall in theworld. Indeed, nothing quite like it ex-ists in the other disciplines of science.

The story of the periodic system forclassifying the elements can be tracedback over 200 years. Throughout itslong history, the periodic table has beendisputed, altered and improved as sci-ence has progressed and as new elementshave been discovered [see “Making NewElements,” by Peter Armbruster andFritz Peter Hessberger, on page 72].

But despite the dramatic changes thathave taken place in science over the pastcentury—namely, the development ofthe theories of relativity and quantummechanics—there has been no revolutionin the basic nature of the periodic system.In some instances, new findings initiallyappeared to call into question the theo-retical foundations of the periodic ta-ble, but each time scientists eventuallymanaged to incorporate the resultswhile preserving the table’s fundamen-tal structure. Remarkably, the periodictable is thus notable both for its histori-cal roots and for its modern relevance.

The term “periodic” reflects the factthat the elements show patterns in theirchemical properties in certain regularintervals. Were it not for the simplifica-tion provided by this chart, students ofchemistry would need to learn the prop-erties of all 112 known elements. Fortu-nately, the periodic table allows chem-ists to function by mastering the prop-erties of a handful of typical elements;


The Evolution of thePeriodic SystemFrom its origins some 200 years ago, the periodic table has become a vital tool for modern chemists

by Eric R. Scerri


all the others fall into so-called groupsor families with similar chemical prop-erties. (In the modern periodic table, agroup or family corresponds to one ver-tical column.)

The discovery of the periodic systemfor classifying the elements representsthe culmination of a number of scien-tific developments, rather than a suddenbrainstorm on the part of one individu-al. Yet historians typically consider oneevent as marking the formal birth of themodern periodic table: on February 17,1869, a Russian professor of chemistry,Dimitri Ivanovich Mendeleev, complet-ed the first of his numerous periodiccharts. It included 63 known elementsarranged according to increasing atom-ic weight; Mendeleev also left spaces

for as yet undiscovered elements forwhich he predicted atomic weights.

Prior to Mendeleev’s discovery, how-ever, other scientists had been activelydeveloping some kind of organizing sys-tem to describe the elements. In 1787,for example, French chemist AntoineLavoisier, working with Antoine Four-croy, Louis-Bernard Guyton de Mor-veau and Claude-Louis Berthollet, de-vised a list of the 33 elements known atthe time. Yet such lists are simply one-dimensional representations. The pow-er of the modern table lies in its two- oreven three-dimensional display of allthe known elements (and even the onesyet to be discovered) in a logical systemof precisely ordered rows and columns.

In an early attempt to organize the elements into a meaningful array, Ger-man chemist Johann Döbereiner point-ed out in 1817 that many of the knownelements could be arranged by theirsimilarities into groups of three, whichhe called triads. Döbereiner singled outtriads of the elements lithium, sodiumand potassium as well as chlorine, bro-mine and iodine. He noticed that if thethree members of a triad were orderedaccording to their atomic weights, theproperties of the middle element fell inbetween those of the first and third ele-ments. For example, lithium, sodiumand potassium all react vigorously withwater. But lithium, the lightest of thetriad, reacts more mildly than the othertwo, whereas the heaviest of the three,potassium, explodes violently. In addi-tion, Döbereiner showed that the atom-ic weight of the middle element is closeto the average of the weights for thefirst and third members of the triad.

Döbereiner’s work encouraged othersto search for correlations between thechemical properties of the elements andtheir atomic weights. One of those whopursued the triad approach further dur-ing the 19th century was Peter Kremersof Cologne, who suggested that certainelements could belong to two triadsplaced perpendicularly. Kremers thusbroke new ground by comparing ele-

ments in two directions, a feature thatlater proved to be an essential aspect ofMendeleev’s system.

In 1857 French chemist Jean-Bap-tiste-André Dumas turned away fromthe idea of triads and focused insteadon devising a set of mathematical equa-tions that could account for the increasein atomic weight among several groupsof chemically similar elements. But aschemists now recognize, any attempt toestablish an organizing pattern basedon an element’s atomic weight will notsucceed, because atomic weight is notthe fundamental property that charac-terizes each of the elements.

Periodic Properties

The crucial characteristic of Mende-leev’s system was that it illustrated

a periodicity, or repetition, in the prop-erties of the elements at certain regularintervals. This feature had been ob-served previously in an arrangement ofelements by atomic weight devised in1862 by French geologist Alexandre-Emile Béguyer de Chancourtois. Thesystem relied on a fairly intricate geo-metric configuration: de Chancourtoispositioned the elements according to in-creasing atomic weight along a spiralinscribed on the surface of a cylinderand inclined at 45 degrees from the base[see illustration on next page].

The first full turn of the spiral coin-cided with the element oxygen, and thesecond full turn occurred at sulfur. Ele-ments that lined up vertically on thesurface of the cylinder tended to havesimilar properties, so this arrangementsucceeded in capturing some of the pat-terns that would later become centralto Mendeleev’s system. Yet for a numberof reasons, de Chancourtois’s system didnot have much effect on scientists of thetime: his original article failed to includea diagram of the table, the system wasrather complicated, and the chemicalsimilarities among elements were notdisplayed very convincingly.

Several other researchers put forward


DA

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THREE-DIMENSIONAL TABLE transforms the traditional periodic chart into a mul-tilayered structure. The traditional vertical columns, which correspond to a group orfamily of elements, can be seen running down the central core of this structure (for ex-ample, H, Li, Na and so on) as well as through the layers. Elements that are positionedon top of one another in layers, such as He, Ne, Ar and so on, belong in the samegroup and thus have similar chemical properties. The horizontal rows, or periods, ofthe traditional table correspond to the multiple layers of the three-dimensional table.This version highlights the symmetrical and regularly increasing size of periods, afundamental chemical feature not yet fully explained by quantum mechanics.


their own versions of a peri-odic table during the 1860s.Using newly standardizedvalues for atomic weights,English chemist John New-lands suggested in 1864 thatwhen the elements were ar-ranged in order of atomicweight, any one of the ele-ments showed properties sim-ilar to those of the elementseight places ahead and eight

places behind in the list—a feature thatNewlands called “the law of octaves.”

In his original table, Newlands leftempty spaces for missing elements, buthis more publicized version of 1866 did

not include these open slots. Otherchemists immediately raised objectionsto the table because it would not beable to accommodate any new elementsthat might be discovered. In fact, someinvestigators openly ridiculed New-lands’s ideas. At a meeting of the Chem-ical Society in London in 1866, GeorgeCarey Foster of University College Lon-don asked Newlands whether he hadconsidered ordering the elements alpha-betically, because any kind of arrange-ment would present occasional coinci-dences. As a result of the meeting, theChemical Society refused to publishNewlands’s paper.

Despite its poor reception, however,

The Evolution of the Periodic System80 Scientific American September 1998

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EARLY VERSION of an organizing sys-tem for the known elements was designedin 1862 by French geologist Alexandre-Emile Béguyer de Chancourtois. Knownas the telluric screw, it was the earliestdiscovery of chemical periodicity.

FIRST PERIODIC TABLE was developed by Russian chemist DimitriIvanovich Mendeleev in February 1869. This draft shows groups of ele-ments arranged horizontally rather than in the more familiar vertical col-umns. Mendeleev produced many tables of both kinds.


Newlands’s work does represent thefirst time anyone used a sequence of or-dinal numbers (in this case, one basedon the sequence of atomic weights) toorganize the elements. In this respect,Newlands anticipated the modern or-ganization of the periodic table, whichis based on the sequence of so-calledatomic numbers. (The concept of atom-ic number, which indicates the numberof protons present within an atom’s nu-cleus, was not established until the ear-ly 20th century.)

The Modern Periodic Table

Chemist Julius Lothar Meyer of Bres-lau University in Germany, while

in the process of revising his chemistrytextbook in 1868, produced a periodictable that turned out to be remarkablysimilar to Mendeleev’s famous 1869version—although Lothar Meyer failedto classify all the elements correctly. Butthe table did not appear in print until1870 because of a publisher’s delay—afactor that contributed to an acrimo-nious dispute for priority that ensuedbetween Lothar Meyer and Mendeleev.

Around the same time, Mendeleevassembled his own periodic table whilehe, too, was writing a textbook of chem-istry. Unlike his predecessors, Mende-leev had sufficient confidence in his pe-riodic table to use it to predict severalnew elements and the properties of theircompounds. He also corrected the atom-ic weights of some already known ele-ments. Interestingly, Mendeleev admit-ted to having seen certain earlier tables,such as those of Newlands, but claimedto have been unaware of Lothar Mey-er’s work when developing his chart.

Although the predictive aspect ofMendeleev’s table was a major advance,it seems to have been overemphasized byhistorians, who have generally suggest-ed that Mendeleev’s table was acceptedespecially because of this feature. Thesescholars have failed to notice that thecitation from the Royal Society of Lon-don that accompanied the Davy Medal(which Mendeleev received in 1882)makes no mention whatsoever of hispredictions. Instead Mendeleev’s abilityto accommodate the already known ele-ments may have contributed as muchto the acceptance of the periodic systemas did his striking predictions. Althoughnumerous scientists helped to developthe periodic system, Mendeleev receivesmost of the credit for discovering chem-ical periodicity because he elevated the

discovery to a law of nature and spentthe rest of his life boldly examining itsconsequences and defending its validity.

Defending the periodic table was nosimple task—its accuracy was frequentlychallenged by subsequent discoveries.One notable occasion arose in 1894,when William Ramsay of University Col-lege London and Lord Rayleigh (JohnWilliam Strutt) of the Royal Institutionin London discovered the element ar-gon; over the next few years, Ramsayannounced the identification of fourother elements—helium, neon, kryptonand xenon—known as the noble gases.(The last of the known noble gases, ra-don, was discovered in 1900 by Ger-man physicist Friedrich Ernst Dorn.)

The name “noble” derives from thefact that all these gases seem to standapart from the other elements, rarelyinteracting with them to form com-pounds. As a result, some chemists sug-gested that the noble gases did not evenbelong in the periodic table. These ele-ments had not been predicted by Men-deleev or anyone else, and only after sixyears of intense effort could chemistsand physicists successfully incorporatethe noble gases into the table. In the newarrangement, an additional column wasintroduced between the halogens (thegaseous elements fluorine, chlorine, bro-mine, iodine and astatine) and the al-kali metals (lithium, sodium, potassi-um, rubidium, cesium and francium).

A second point of contention sur-

rounded the precise ordering of the ele-ments. Mendeleev’s original table posi-tioned the elements according to atom-ic weight, but in 1913 Dutch amateurtheoretical physicist Anton van denBroek suggested that the ordering prin-ciple for the periodic table lay instead inthe nuclear charge of each atom. Physi-cist Henry Moseley, working at theUniversity of Manchester, tested thishypothesis, also in 1913, shortly beforehis tragic death in World War I.

Moseley began by photographing thex-ray spectrum of 12 elements, 10 ofwhich occupied consecutive places in theperiodic table. He discovered that thefrequencies of features called K-lines inthe spectrum of each element were di-rectly proportional to the squares of theintegers representing the position ofeach successive element in the table. AsMoseley put it, here was proof that“there is in the atom a fundamentalquantity, which increases by regularsteps as we pass from one element tothe next.” This fundamental quantity,first referred to as atomic number in1920 by Ernest Rutherford, who wasthen at the University of Cambridge, isnow identified as the number of pro-tons in the nucleus.

Moseley’s work provided a methodthat could be used to determine exactlyhow many empty spaces remained inthe periodic table. After this discovery,chemists turned to using atomic numberas the fundamental ordering principle

The Evolution of the Periodic System Scientific American September 1998 81

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CHEMISTS Mendeleev (left) and Julius Lothar Meyer (right) developed the modernperiodic chart almost simultaneously in the late 1860s. Mendeleev’s table was publishedfirst, and he receives most of the credit for discovering the periodic system because heused it to make many successful predictions and vigorously defended its validity.


for the periodic table, instead of atomicweight. This change resolved many ofthe lingering problems in the arrange-ment of the elements. For example,when iodine and tellurium were or-dered according to atomic weight (withiodine first), the two elements appearedto be incorrectly positioned in terms oftheir chemical behavior. When orderedaccording to atomic number (with tel-lurium first), however, the two elementswere in their correct positions.

Understanding the Atom

The periodic table inspired the worknot only of chemists but also of

atomic physicists struggling to under-stand the structure of the atom. In1904, working at Cambridge, physicistJ. J. Thomson (who also discovered theelectron) developed a model of the atom,paying close attention to the periodicityof the elements. He proposed that theatoms of a particular element containeda specific number of electrons arrangedin concentric rings. Furthermore, ac-cording to Thomson, elements with sim-ilar configurations of electrons wouldhave similar properties; Thomson’swork thus provided the first physicalexplanation for the periodicity of the elements. Although Thomson imaginedthe rings of electrons as lying inside themain body of the atom, rather than cir-culating around the nucleus as is be-

lieved today, his model does representthe first time anyone addressed the ar-rangement of electrons in the atom, aconcept that pervades the whole ofmodern chemistry.

Danish physicist Niels Bohr, the firstto bring quantum theory to bear on thestructure of the atom, was also motivat-ed by the arrangement of the elementsin the periodic system. In Bohr’s modelof the atom, developed in 1913, elec-trons inhabit a series of concentric shellsthat encircle the nucleus. Bohr reasonedthat elements in the same group of theperiodic table might have identical con-figurations of electrons in their outer-most shell and that the chemical prop-erties of an element would depend inlarge part on the arrangement of elec-trons in the outer shell of its atoms.

Bohr’s model of the atom also servedto explain why the noble gases lack re-activity: noble gases possess full outershells of electrons, making them unusu-ally stable and unlikely to form com-pounds. Indeed, most other elementsform compounds as a way to obtainfull outer electron shells. More recentanalysis of how Bohr arrived at theseelectronic configurations suggests thathe functioned more like a chemist thanhas generally been credited. Bohr didnot derive electron configurations fromquantum theory but obtained themfrom the known chemical and spectro-scopic properties of the elements.

In 1924 another physicist, Austrian-born Wolfgang Pauli, set out to explainthe length of each row, or period, in thetable. As a result, he developed the PauliExclusion Principle, which states thatno two electrons can exist in exactly thesame quantum state, which is definedby what scientists call quantum num-bers. The lengths of the various periodsemerge from experimental evidenceabout the order of electron-shell fillingand from the quantum-mechanical re-strictions on the four quantum num-bers that electrons can adopt.

The modifications to quantum theorymade by Werner Heisenberg and ErwinSchrödinger in the mid-1920s yieldedquantum mechanics in essentially theform used to this day. But the influenceof these changes on the periodic tablehas been rather minimal. Despite the ef-forts of many physicists and chemists,quantum mechanics cannot explain theperiodic table any further. For example,it cannot explain from first principlesthe order in which electrons fill the var-ious electron shells. The electronic con-figurations of atoms, on which our mod-ern understanding of the periodic tableis based, cannot be derived using quan-tum mechanics (this is because the fun-damental equation of quantum mechan-ics, the Schrödinger equation, cannot besolved exactly for atoms other than hy-drogen). As a result, quantum mechanicscan only reproduce Mendeleev’s origi-

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The Evolution of the Periodic System82 Scientific American September 1998

POPULAR PERIODIC TABLE—known as the medium-long form—can be found in nearly every chemistry class-room and laboratory around the world. This version has theadvantage of clearly displaying groups of elements that havesimilar chemical properties in vertical columns, but it is notparticularly symmetrical. (The different colors of the table in-dicate elements with the same type of outer shell of electrons.)

*These elements have been discovered but not yet officially named.

†These elements have yet to be discovered.


nal discovery by the use of mathemati-cal approximations—it cannot predictthe periodic system.

Variations on a Theme

In more recent times, researchers haveproposed different approaches for

displaying the periodic system. For in-stance, Fernando Dufour, a retired chem-istry professor from Collège Ahuntsicin Montreal, has developed a three-di-mensional periodic table, which displaysthe fundamental symmetry of the peri-odic law, unlike the common two-di-mensional form of the table in commonuse. The same virtue is also seen in aversion of the periodic table shaped as apyramid, a form suggested on many oc-casions but most recently refined by Wil-liam B. Jensen of the University of Cin-cinnati [see illustration above].

Another departure has been the in-vention of periodic systems aimed at

summarizing the properties of com-pounds rather than elements. In 1980Ray Hefferlin of Southern AdventistUniversity in Collegedale, Tenn., deviseda periodic system for all the conceivablediatomic molecules that could be formedbetween the first 118 elements (only 112have been discovered to date).

Hefferlin’s chart reveals that certainproperties of molecules—the distancebetween atoms and the energy requiredto ionize the molecule, for instance—oc-cur in regular patterns. This table hasenabled scientists to predict the proper-ties of diatomic molecules successfully.

In a similar effort, Jerry R. Dias of theUniversity of Missouri at Kansas Citydevised a periodic classification of a typeof organic molecule called benzenoidaromatic hydrocarbons. The com-pound naphthalene (C10H8), found inmothballs, is the simplest example.Dias’s classification system is analogousto Döbereiner’s triads of elements: any

central molecule of a triad has a totalnumber of carbon and hydrogen atomsthat is the mean of the flanking entries,both downward and across the table.This scheme has been applied to a sys-tematic study of the properties of ben-zenoid aromatic hydrocarbons and,with the use of graph theory, has led topredictions of the stability and reactivi-ty of some of these compounds.

Still, it is the periodic table of the ele-ments that has had the widest and mostenduring influence. After evolving forover 200 years through the work ofmany people, the periodic table remainsat the heart of the study of chemistry. Itranks as one of the most fruitful ideasin modern science, comparable perhapsto Charles Darwin’s theory of evolution.Unlike theories such as Newtonian me-chanics, it has not been falsified or rev-olutionized by modern physics but hasadapted and matured while remainingessentially unscathed.

The Evolution of the Periodic System Scientific American September 1998 83

The Author

ERIC R. SCERRI studied chemistry at the universitiesof London, Cambridge and Southampton. He later earneda Ph.D. in history and philosophy of science from King’sCollege, London. Since arriving in the U.S., he has been aresearch fellow at the California Institute of Technologyand, most recently, has taught chemistry at Bradley Uni-versity in Peoria, Ill. In January 1999 he will take up a po-sition in the chemistry department at Purdue University.Scerri’s research interests lie in the history and philosophyof chemistry, and he is the editor of the interdisciplinaryjournal Foundations of Chemistry (http://www.cco.caltech. edu/~scerri/ on the World Wide Web). His e-mail addressis [email protected]

Further Reading

The Periodic System of Chemical Elements: A History of the FirstHundred Years. J. W. van Spronsen. Elsevier, 1969.

The Surprising Periodic Table: Ten Remarkable Facts. Dennis H.Rouvray in Chemical Intelligencer, Vol. 2, No. 3, pages 39–47; July 1996.

Classification, Symmetry and the Periodic Table. William B. Jensen inComputing and Mathematics with Applications, Vol. 12B, Nos. 1–2, pages487–510; 1989.

Plus ça Change. E. R. Scerri in Chemistry in Britain, Vol. 30, No. 5, pages379–381; May 1994. (Also see related articles in this issue.)

The Electron and the Periodic Table. Eric R. Scerri in American Scien-tist, Vol. 85, pages 546–553; November–December 1997.

For information on Fernando Dufour’s three-dimensional periodic table,send e-mail to [email protected]

H

Na Mg SiAl P ClS Ar

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Mn CoFe Ni ZnCu GaCaK Sc VTi Cr

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HePYRAMIDAL TABLE is another form of theperiodic chart that displays the symmetry ofthe periodic law, particularly as it relates to thelengths of successive periods. The one here is amodified form of the table recently developedby William B. Jensen of the University of Cin-cinnati. Heavy solid lines indicate elementswith the same type of outer shell of elec-trons; light solid lines show ele-ments belonging to thesame groups; dashed linesshow secondary relationsin chemical properties.

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*These elements have been discovered but not yet officially named.

†These elements have yet to be discovered.


It is common to think of the solarsystem as ending at the orbit of themost distant known planet, Pluto.

But the sun’s gravitational influence ex-tends more than 3,000 times farther,halfway to the nearest stars. And thatspace is not empty—it is filled with a gi-ant reservoir of comets, leftover materialfrom the formation of the solar system.That reservoir is called the Oort cloud.

The Oort cloud is the Siberia of thesolar system, a vast, cold frontier filled

with exiles of the sun’s inner empire andonly barely under the sway of the cen-tral authority. Typical noontime temper-atures are a frigid four degrees Celsiusabove absolute zero, and neighboringcomets are typically tens of millions ofkilometers apart. The sun, while stillthe brightest star in the sky, is onlyabout as bright as Venus in the eveningsky on Earth.

We have never actually “seen” theOort cloud. But no one has ever seen an

electron, either. We infer the existenceand properties of the Oort cloud andthe electron from the physical effects wecan observe. In the case of the former,those effects are the steady trickle oflong-period comets into the planetarysystem. The existence of the Oort cloudanswers questions that people haveasked since antiquity: What are comets,and where do they come from?

Aristotle speculated in the fourth cen-tury B.C. that comets were clouds of lu-

The Oort Cloud84 Scientific American September 1998

The Oort Cloud

On the outskirts of the solar system swarms a vast cloud of comets, influenced almost as much by other stars as by our sun. The dynamics of this cloud may help explain such matters as mass extinctions on Earth

by Paul R. Weissman


minous gas high in Earth’s atmosphere.But the Roman philosopher Seneca sug-gested in the first century A.D. that theywere heavenly bodies, traveling alongtheir own paths through the firmament.Fifteen centuries passed before his hy-pothesis was confirmed by Danish as-tronomer Tycho Brahe, who comparedobservations of the comet of 1577 madefrom several different locations in Eu-rope. If the comet had been close by,then from each location it would have

had a slightly different position againstthe stars. Brahe could not detect any dif-ferences and concluded that the cometwas farther away than the moon.

Just how much farther started to be-come clear only when astronomers be-gan determining the comets’ orbits. In1705 the English astronomer EdmondHalley compiled the first catalogue of24 comets. The observations were fairlycrude, and Halley could fit only roughparabolas to each comet’s path. Never-

theless, he argued that the orbits mightbe very long ellipses around the sun:

For so their Number will be determi-

nate and, perhaps, not so very great.

Besides, the Space between the Sun and

the fix’d Stars is so immense that there

is Room enough for a Comet to re-

volve, tho’ the Period of its Revolution

be vastly long.

In a sense, Halley’s description of com-

The Oort Cloud Scientific American September 1998 85

CELESTIAL PIED PIPER, the red dwarfstar Gliese 710, will crash through theOort cloud in 1.4 million years—reani-mating dormant comets, luring many outof their orbits and hurling some towardthe planets. Such incursions, the result ofhaphazard stellar motions in our galaxy,occur every one million years on average.In this artist’s conception, the distantcomets are not to scale.

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ets circulating in orbits stretching be-tween the stars anticipated the discov-ery of the Oort cloud two and a halfcenturies later. Halley also noticed thatthe comets of 1531, 1607 and 1682 hadvery similar orbits and were spaced atroughly 76-year intervals. These seem-ingly different comets, he suggested,were actually the same comet returningat regular intervals. That body, nowknown as Halley’s comet, last visitedthe region of the inner planets in 1986.

Since Halley’s time, astronomers havedivided comets into two groups accord-ing to the time it takes them to orbit thesun (which is directly related to the com-ets’ average distance from the sun).Long-period comets, such as the recentbright comets Hyakutake and Hale-Bopp, have orbital periods greater than200 years; short-period comets, less than200 years. In the past decade astrono-mers have further divided the short-pe-riod comets into two groups: Jupiter-family comets, such as comets Enckeand Tempel 2, which have periods lessthan 20 years; and intermediate-period,or Halley-type, comets, with periodsbetween 20 and 200 years.

These definitions are somewhat arbi-trary but reflect real differences. The in-termediate- and long-period comets en-ter the planetary region randomly fromall directions, whereas the Jupiter-fami-ly comets have orbits whose planes aretypically inclined no more than 40 de-grees from the ecliptic plane, the planeof Earth’s orbit. (The orbits of the otherplanets are also very close to the eclipticplane.) The intermediate- and long-pe-riod comets appear to come from the

Oort cloud, whereas the Jupiter-familycomets are now thought to originate inthe Kuiper belt, a region in the eclipticbeyond the orbit of Neptune [see “TheKuiper Belt,” by Jane X. Luu and Da-vid C. Jewitt; Scientific American,May 1996].

The Netherworld beyond Pluto

By the early 20th century, enoughlong-period cometary orbits were

available to study their statistical distri-bution [see illustration on page 89]. Aproblem emerged. About one third ofall the “osculating” orbits—that is, theorbits the comets were following at thepoint of their closest approach to thesun—were hyperbolic. Hyperbolic or-bits would originate in and return to in-terstellar space, as opposed to ellipticalorbits, which are bound by gravity tothe sun. The hyperbolic orbits led someastronomers to suggest that cometswere captured from interstellar spaceby encounters with the planets.

To examine this hypothesis, celestial-mechanics researchers extrapolated, or“integrated,” the orbits of the long-pe-riod comets backward in time. Theyfound that because of distant gravita-tional tugs from the planets, the oscu-lating orbits did not represent the com-ets’ original orbits [see bottom illustra-tion on opposite page]. When the effectsof the planets were accounted for—byintegrating far enough back in time andorienting the orbits not in relation tothe sun but in relation to the center ofmass of the solar system (the sum of thesun and all the planets)—almost all the

orbits became elliptical. Thus, the com-ets were members of the solar system,rather than interstellar vagabonds.

In addition, although two thirds ofthese orbits still appeared to be uni-formly distributed, fully one third hadorbital energies that fell within a nar-row spike. That spike represented or-bits that extend to very large distanc-es—20,000 astronomical units (20,000times the distance of Earth from thesun) or more. Such orbits have periodsexceeding one million years.

Why were so many comets comingfrom so far away? In the late 1940sDutch astronomer Adrianus F. van Wo-erkom showed that the uniform distri-bution could be explained by planetaryperturbations, which scatter cometsrandomly to both larger and smaller or-bits. But what about the spike of com-ets with million-year periods?

In 1950 Dutch astronomer Jan H.Oort, already famous for having deter-mined the rotation of the Milky Waygalaxy in the 1920s, became interestedin the problem. He recognized that themillion-year spike must represent thesource of the long-period comets: a vastspherical cloud surrounding the plane-tary system and extending halfway tothe nearest stars.

Oort showed that the comets in thiscloud are so weakly bound to the sunthat random passing stars can readilychange their orbits. About a dozen starspass within one parsec (206,000 astro-nomical units) of the sun every one mil-lion years. These close encounters areenough to stir the cometary orbits, ran-domizing their inclinations and sending


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COMET HALE-BOPP, which made its closestapproach to Earth in March 1997, is an ex-ample of a long-period comet. It last visit-ed the inner solar system 4,200 years agoand because of the gravitational influenceof Jupiter will make its next appearance in2,600 years. In the meantime, it will travel370 times farther from the sun than Earthis. Like most long-period comets, Hale-Bopp has a highly inclined orbit: the planeof its orbit is nearly perpendicular to theplane of Earth’s orbit (inset).

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a steady trickle of comets into the innersolar system on very long elliptical or-bits [see illustrations on next page]. Asthey enter the planetary system for thefirst time, the comets are scattered bythe planets, gaining or losing orbital en-ergy. Some escape the solar system alto-gether. The remainder return and areobserved again as members of the uni-form distribution. Oort described thecloud as “a garden, gently raked bystellar perturbations.”

A few comets still appeared to comefrom interstellar space. But this wasprobably an incorrect impression givenby small errors in the determination oftheir orbits. Moreover, comets can shifttheir orbits because jets of gas and dustfrom their icy surfaces act like smallrocket engines as the comets approachthe sun. Such nongravitational forcescan make the orbits appear hyperbolicwhen they are actually elliptical.

Shaken, Not Stirred

Oort’s accomplishment in correctlyinterpreting the orbital distribu-

tion of the long-period comets is evenmore impressive when one considersthat he had only 19 well-measured or-bits to work with. Today astronomers

have more than 15 times asmany. They now know thatlong-period comets enteringthe planetary region for thefirst time come from an av-erage distance of 44,000 as-tronomical units. Such or-bits have periods of 3.3 mil-lion years.

Astronomers have also re-alized that stellar perturba-tions are not always gentle.Occasionally a star comes soclose to the sun that it passesright through the Oort cloud,violently disrupting the com-etary orbits along its path.Statistically a star is expect-ed to pass within 10,000 as-tronomical units of the sunevery 36 million years andwithin 3,000 astronomicalunits every 400 million years.Comets close to the star’spath are thrown out to interstellar space,while the orbits of comets throughoutthe cloud undergo substantial changes.

Although close stellar encountershave no direct effect on the planets—theclosest expected approach of any starover the history of the solar system is900 astronomical units from the sun—

they might have devastating indirectconsequences. In 1981 Jack G. Hills,now at Los Alamos National Laborato-ry, suggested that a close stellar passagecould send a “shower” of comets to-ward the planets, raising the rate ofcometary impacts on the planets andpossibly even causing a biological massextinction on Earth. According to com-puter simulations I performed in 1985with Piet Hut, then at the Institute forAdvanced Study in Princeton, N.J., thefrequency of comet passages during ashower could reach 300 times the nor-mal rate. The shower would last two tothree million years.

Recently Kenneth A. Farley and hiscolleagues at the California Institute ofTechnology found evidence for just sucha comet shower. Using the rare helium3 isotope as a marker for extraterrestri-al material, they plotted the accumula-tion of interplanetary dust particles inocean sediments over time. The rate ofdust accumulation is thought to reflectthe number of comets passing throughthe planetary region; each comet shedsdust along its path. Farley discoveredthat this rate increased sharply at theend of the Eocene epoch, about 36 mil-lion years ago, and decreased slowly

over two to three million years, just astheoretical models of comet showerswould predict. The late Eocene is iden-tified with a moderate biological extinc-tion event, and several impact cratershave been dated to this time. Geologistshave also found other traces of impactsin terrestrial sediments, such as iridiumlayers and microtektites.

Is Earth in danger of a comet showernow? Fortunately not. Joan Garcia-San-chez of the University of Barcelona, Rob-ert A. Preston and Dayton L. Jones ofthe Jet Propulsion Laboratory in Pasa-dena, Calif., and I have been using thepositions and velocities of stars, mea-sured by the Hipparcos satellite, to re-construct the trajectories of stars nearthe solar system. We have found evi-dence that a star has passed close to thesun in the past one million years. Thenext close passage of a star will occur in1.4 million years, and that is a small reddwarf called Gliese 710, which will passthrough the outer Oort cloud about70,000 astronomical units from the sun.At that distance, Gliese 710 might in-crease the frequency of comet passagesthrough the inner solar system by 50percent—a sprinkle perhaps, but cer-tainly no shower.

In addition to random passing stars,the Oort cloud is now known to be dis-turbed by two other effects. First, thecloud is sufficiently large that it feelstidal forces generated by the disk of theMilky Way and, to a lesser extent, thegalactic core. These tides arise becausethe sun and a comet in the cloud are at


CENTRALGALACTIC BULGE

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TIDAL FORCES arise because gravity becomes weak-er with distance. Therefore, the central bulge of ourgalaxy—a concentration of stars at the hub of the spi-ral pattern—pulls more on the near side of the Oortcloud (not to scale) than on the far side. The galacticplane exerts a similar force in a different direction.The galactic tides are analogous to lunar tides, whicharise because the side of Earth closest to the moon feelsa stronger gravitational pull than the antipode does.

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LONG-PERIOD COMET is so weaklybound to the sun that the planets have adecisive influence on it. Astronomers canusually see the comet only while it swingsby the sun. When they apply Kepler’s lawsof celestial motion to plot its course—its“osculating,” or apparent, orbit—the com-et often seems to be on a hyperbolic tra-jectory, implying that it came from inter-stellar space and will return there (a). Amore sophisticated calculation, which ac-counts for the planets (especially the mostmassive planet, Jupiter), finds that the or-bit is actually elliptical (b). The orbitchanges shape on each pass through theinner solar system.

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slightly different distances from themidplane of the disk or from the galac-tic center and thus feel a slightly differ-ent gravitational tug [see top illustra-tion on preceding page]. The tides helpto feed new long-period comets into theplanetary region.

Second, giant molecular clouds in the

galaxy can perturb the Oort cloud, asLudwig Biermann of the Max PlanckInstitute for Physics and Astrophysicsin Munich suggested in 1978. Thesemassive clouds of cold hydrogen, thebirthplaces of stars and planetary sys-tems, are 100,000 to one million timesas massive as the sun. When the solar

system comes close to one, thegravitational perturbations ripcomets from their orbits and flingthem into interstellar space. Theseencounters, though violent, areinfrequent—only once every 300million to 500 million years. In1985 Hut and Scott D. Tremaine,now at Princeton University,showed that over the history ofthe solar system, molecular cloudshave had about the same cumula-tive effect as all passing stars.

Inner Core

Currently three main ques-tions concern Oort-cloud re-

searchers. First, what is the cloud’sstructure? In 1987 Tremaine, Mar-tin J. Duncan, now at Queen’sUniversity in Ontario, and Thom-as R. Quinn, now at the Universi-ty of Washington, studied howstellar and molecular-cloud per-turbations redistribute cometswithin the Oort cloud. Comets atits outer edge are rapidly lost, ei-ther to interstellar space or to theinner solar system, because of theperturbations. But deeper inside,there probably exists a relativelydense core that slowly replenishesthe outer reaches.

Tremaine, Duncan and Quinnalso showed that as comets fall infrom the Oort cloud, their orbitalinclinations tend not to change.This is a major reason why as-tronomers now think the Kuiperbelt, rather than the Oort cloud,accounts for the low-inclination,Jupiter-family comets. Still, theOort cloud is the most likelysource of the higher-inclination,intermediate-period comets, suchas Halley and Swift-Tuttle. Theywere probably once long-periodcomets that the planets pulled intoshorter-period orbits.

The second main question is,How many comets inhabit theOort cloud? The number dependson how fast comets leak from thecloud into interplanetary space.

To account for the observed number oflong-period comets, astronomers nowestimate the cloud has six trillion com-ets, making Oort-cloud comets the mostabundant substantial bodies in the solarsystem. Only a sixth of them are in theouter, dynamically active cloud first de-scribed by Oort; the remainder are inthe relatively dense core. If the best esti-mate for the average mass of a comet—about 40 billion metric tons—is ap-plied, the total mass of comets in theOort cloud at present is about 40 timesthat of Earth.

Finally, from where did the Oort-cloud comets originally come? Theycould not have formed at their currentposition, because material at those dis-tances is too sparse to coalesce. Norcould they have originated in interstel-lar space; capture of comets by the sunis very inefficient. The only place left isthe planetary system. Oort speculatedthat the comets were created in the as-teroid belt and ejected by the giant plan-ets during the formation of the solarsystem. But comets are icy bodies, essen-tially big, dirty snowballs, and the as-teroid belt was too warm for ices tocondense.

A year after Oort’s 1950 paper, as-tronomer Gerard P. Kuiper of the Uni-versity of Chicago proposed that cometscoalesced farther from the sun, amongthe giant planets. (The Kuiper belt isnamed for him because he suggestedthat some comets also formed beyondthe farthest planetary orbits.) Cometsprobably originated throughout the gi-ant planets’ region, but researchers usedto argue that those near Jupiter and Sat-urn, the two most massive planets,would have been ejected to interstellarspace rather than to the Oort cloud.Uranus and Neptune, with their lowermasses, could not easily throw so manycomets onto escape trajectories. Butmore recent dynamical studies have castsome doubt on this scenario. Jupiterand particularly Saturn do place a sig-nificant fraction of their comets into theOort cloud. Although this fraction maybe smaller than that of Uranus andNeptune, it may have been offset by thegreater amount of material initially inthe larger planets’ zones.

Therefore, the Oort-cloud cometsmay have come from a wide range ofsolar distances and hence a wide rangeof formation temperatures. This factmay help explain some of the composi-tional diversity observed in comets. In-deed, recent work I have done with


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Beyond a distance of about 20,000 astronomicalunits (20,000 times the Earth-sun distance), the vari-ous outside influences are capable of throwing thecomet back toward the planets.

Once the comet reenters the inner solar system, theplanets may pull it to a new orbit, so that it reap-pears on a regular basis.

HISTORY OF A LONG-PERIOD COMET beginswhen it forms near the planets and is catapulted bythem into a wide orbit.

There the comet is susceptible to the gravitationalforces of random passing stars and giant molecularclouds, as well as the tidal forces of the galactic diskand core. These forces randomly tilt the orbital planeof the comet and gradually pull it farther out.


Harold F. Levison of the Southwest Re-search Institute in Boulder, Colo., hasshown that the cloud may even containasteroids from the inner planets’ region.These objects, made of rock rather thanice, may constitute 2 to 3 percent of thetotal Oort-cloud population.

The key to these ideas is the presenceof the giant planets, which hurl thecomets outward and modify their or-bits if they ever reenter the planetary re-gion. If other stars have giant planets,as observations over the past few yearssuggest, they may have Oort clouds, too.If each star has its own cloud, then asstars pass by the sun, their Oort cloudswill pass through our cloud. Even so,collisions between comets will be rarebecause the typical space between com-ets is an astronomical unit or more.

The Oort clouds around each starmay slowly be leaking comets into in-terstellar space. These interstellar com-ets should be easily recognizable if theywere to pass close to the sun, becausethey would approach the solar systemat much higher velocities than the com-ets from our own Oort cloud. To date,no such interstellar comets have everbeen detected. This fact is not surpris-ing: because the solar system is a verysmall target in the vastness of interstel-lar space, there is at best a 50–50 chancethat people should have seen one inter-stellar comet by now.

The Oort cloud continues to fasci-nate astronomers. Through the goodfortunes of celestial mechanics, naturehas preserved a sample of materialfrom the formation of the solar system

in this distant reservoir. By studying itand the cosmo-chemical record frozenin its icy members, researchers arelearning valuable clues about the originof the solar system.

Several space missions are now beingreadied to unlock those secrets. TheStardust spacecraft, due for launch nextyear, will fly through the coma of cometWild 2, collect samples of cometary dustand return them to Earth for laboratoryanalysis. A few years later the CON-TOUR probe will fly by three cometsand compare their compositions. TheDeep Space 4/Champollion mission willsend an orbiter and a lander to cometTempel 1, and the Rosetta mission willdo the same for comet Wirtanen. Thenew millennium is going to be a won-derful time for studying comets.


The Author

PAUL R. WEISSMAN is a senior researchscientist at the Jet Propulsion Laboratory inPasadena, Calif., where he specializes in studiesof the physics and dynamics of comets. He isalso the project scientist for NASA’s Deep Space4/Champollion mission, which is scheduled toland on short-period comet Tempel 1 in 2005.Weissman has written more than 80 refereedscientific papers and is one of three editors ofthe Encyclopedia of the Solar System, to bepublished by Academic Press this fall.

Further Reading

Comets: A Chronological History of Observation, Science, Myth and Folk-lore. Donald K. Yeomans. John Wiley & Sons, 1991.

Comets in the Post-Halley Era. Edited by Ray L. Newburn, Jr., Marcia Neuge-bauer and Jurgen Rahe. Kluwer Academic Publishers, 1991.

Dynamics of Comets: Recent Developments and New Challenges. Julio A.Fernandez in Asteroids, Comets, Meteors 1993: Proceedings of the 160th Symposiumof the International Astronomical Union. Edited by A. Milani, M. Di Martino and A.Cellino. Kluwer Academic Publishers, 1994.

Completing the Inventory of the Solar System. Edited by Terrence W. Rettigand Joseph M. Hahn. Astronomical Society of the Pacific Conference Series, Vol.107; 1996.

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ORBITAL ENERGY of known long-pe-riod comets, as shown in these histo-grams, reveals the Oort cloud. Astron-omers first calculate the osculating orbitsof the comets—the orbits they would takeif their motion were entirely caused by thesun’s gravity. One third of these orbitshave a positive energy, making them ap-pear interstellar (a). But when correctedfor the influence of the planets and ex-trapolated backward in time, the energyis slightly negative—indicating that thecomets came from the edge of the solarsystem (b). A few comets still seem to beinterstellar, but this is probably the resultof small observational errors. As the plan-ets continue to exert their influence, somecomets will return to the Oort cloud,some will escape from the solar systemand the rest will revisit the inner solar sys-tem (c). Technically, the orbital energy isproportional to the reciprocal of the semi-major axis, expressed in units of inverseastronomical units (AU–1).PA

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Photovoltaics is a technology thattypically transforms sunlight intoelectricity. Radiation from the

visible part of the spectrum is, after all,abundant, nonpolluting and free. Butphotovoltaics can also provide usefulamounts of electricity from infrared ra-diation—that is, radiant heat generatedby a source of energy such as fuel oil.

This lesser-known approach, calledthermophotovoltaics, offers a majoradvantage in certain settings: a genera-tor can operate at night or when thesky is overcast, thereby eliminating anyneed for batteries to store electricity. Thetechnology is also preferable in someways to conventional electricity-gener-ating technology based on burning fos-sil fuels. Its efficiency—the percent of fuelenergy converted to electricity—can besubstantially higher than that of electricgenerators powered by natural gas oranother fossil fuel. Moreover, a semi-conductor-based thermophotovoltaicsystem can be designed to minimizepollutants. And because it contains nomoving parts, it will run silently and re-liably, requiring little maintenance.

In spite of these advantages, thermo-photovoltaic technology has not enjoyedthe success of solar photovoltaics, whichtoday constitutes a thriving, albeit spe-cialized, segment of the energy market.But the divide may soon shrink. Thetechnology, which has its roots in thesame research that produced solar cells,appears to be coming into its own.

The ideas that underlie thermophoto-voltaics stretch back 40 years. Pierre R.Aigrain of the École Normale Supéri-eure in Paris first described some of thebasic concepts during a series of lec-tures in 1956. In the early 1960s U.S.Army researchers at Fort Monmouth,N.J., created the first documented pro-totype of a thermophotovoltaic genera-tor. Its efficiency was less than 1 percent,

however. Efficiencies of 10 to 15 per-cent were needed for a usable generatorthat could be deployed by troops oper-ating in the field.

Throughout the late 1970s and intothe early 1980s, research supported andcarried out by the Electric Power Re-search Institute in Palo Alto, Calif., theGas Research Institute in Chicago, Stan-ford University and other organizationsachieved some improvement in perfor-mance. But the components of early sys-tems could never channel enough heatto the units that convert infrared energyinto electricity. Recently a new set ofmaterials has taken the technology pastthe development stage.

Commercial Debut

Thermophotovoltaics is about toreach the commercial marketplace.

A company in the Pacific Northwestplans to market a thermophotovoltaicgenerator to run electrical equipmenton sailboats. Other applications underdevelopment include small power unitsthat would supply electricity in remoteareas or for roving military troops. Thetechnology could also assist in runninghybrid electric vehicles, in which anelectric battery complements the powerfrom an internal-combustion engine.Ultimately, thermophotovoltaics couldproduce megawatts of power, furnish-ing some of the requirements of utilitiesor helping industry meet its electricityneeds by recovering the unused heatfrom industrial processes.

Production of electricity from radiantheat requires several functional ele-ments. A source of heat must be cou-pled with a radiator, a material thatemits desired wavelengths of infraredradiation. A semiconductor device con-sisting of an array of interconnected cellsmust be engineered to convert these se-


ThermophotovoltaicsSemiconductors that convert radiant

heat to electricity may prove suitable for lighting remote villages or powering automobiles

by Timothy J. Coutts and Mark C. Fitzgerald

THERMOPHOTOVOLTAIC GENERATORturns radiant heat into electricity. The burner(center) generates heat, whose infrared pho-tons hit a radiator (green enclosure). The radi-ator channels certain infrared wavelengths toarrays of photovoltaic cells (grid) that convertthe energy to electricity.


lect wavelengths to electricity, which isthen sent through a circuit to performuseful work—running a refrigerator ona boat, for instance. Finally, to performefficiently, a thermophotovoltaic systemmust rechannel unused energy back tothe radiator. In some applications thewaste energy can also be used for otherpurposes, such as heating a room.

Sources of heat for a thermophoto-voltaic system might range from fossil-

fuel burners to sunlight to a nuclear fis-sion reaction. As a practical matter,most systems under development de-ploy fossil fuels. Solar energy boostedto high intensities by “concentrator”devices can be used to operate a ther-mophotovoltaic generator, but the room-size concentrators and devices neededto store heat for nighttime use are stillin the early development stages. Manyexperts consider the other alternative,nuclear fuel, less acceptable because ofpublic fears of radioactivity. Thus, thenuclear option may be relegated tohighly specialized applications, such asunmanned space probes to the outerreaches of the solar system, where solarcells would cease to function for lack ofsunlight.

In constructing a burner to supplyheat, researchers have begun to con-

sider taking advantage of the metal-mesh or ceramic containers used

in industrial drying of paper,inks, paints and agriculturalcrops. With a large surface

area, these burners achievethe necessary temperaturesof above 1,000 degreesCelsius (1,832 degreesFahrenheit).

Radiators are need-ed because the semi-conductor converterthat transforms radi-ant heat to electricitycannot efficiently usethe infrared energyproduced by com-busted fuel. The con-verters can operate ef-ficiently only within aspecific range of wave-

lengths, whereas the heatfrom a flame transmits

infrared energy at wave-lengths and intensities that

can shift unpredictably (becausethe flame is subject to air currents

and variations in temperature). The ra-diators transform the available heat en-ergy into a well-defined range of wave-lengths of uniform intensity.

A radiator may be built as a flat orrounded surface or as an array of tinyfilaments. The material properties ofoxides of rare-earth elements such asytterbium, erbium and holmium allowheat to be radiated from the burner inrelatively narrow bands of wavelengths;silicon carbide, in contrast, emits abroader spectrum.

In constructing a converter, thermo-

photovoltaic researchers choose semi-conductors that match the infrared spec-trum of the wavelengths emitted by aradiator. These wavelengths correspondroughly to the energy needed to freeelectrons that would otherwise remainbound within the crystalline solid of thesemiconductor converter [see box onpage 93]. The most energetic immobileelectrons reside in the so-called valenceband of the semiconductor crystal, whichdescribes the range of allowed energylevels of the outermost bound electrons.Electrons in the valence band are notfree to move through the crystal. Whenan atom absorbs an infrared photon ofjust the right amount of energy, an elec-tron is boosted to the conduction band,where it can flow in a current in the crys-tal. (A photon is a unit, or quantum, ofelectromagnetic energy. According tothe laws of quantum mechanics, all elec-tromagnetic energy has the propertiesof both a wave and a particle, and theenergy of a photon determines the wave-length of its corresponding wave.) Thephoton energy required to move an elec-tron from the valence to the conductionband is known as the band-gap energyand is expressed in electron volts.

Once in the conduction band, elec-trons cross a junction between two dis-similar areas of the semiconductor crys-tal, moving more freely in one directionthan the other. The resulting congrega-tion of negatively charged electrons onone side of the junction causes a buildupof negative electrical potential, whichacts as a force pushing the electrons in acurrent that flows through the photo-voltaic cell. Each converter consists of aseries of cells wired together to increasethe power output. The current generat-ed from the converter can then movethrough a wire connected to a lamp ora household electrical system.

Advances in converter materials haveenabled engineers to improve electricaloutput. They can select converters thatbecause of their particular band gapsrespond best to the range of wave-lengths emitted by a given radiator. Inpast years, the absence of appropriateradiator-converter combinations hadposed a key obstacle to furthering thetechnology.

The first generation of thermophoto-voltaic devices used radiators that pro-duced a narrow band of wavelengths.Radiators made of the rare-earth mate-rial ytterbium oxide often accompaniedsilicon semiconductor converters with aband gap of 1.14 eV. In theory, a selec-


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tive radiator—which emits a narrowband of wavelengths—should be moreefficient than broader-spectrum (broad-band) designs. The photons in a selec-tive radiator should provide the mini-mum energy needed to lift an electronin the semiconductor converter into theconduction band; any excess energywould otherwise be lost as waste heat.Selective radiators should thus supplymore power at a lower cost per watt ofelectricity. In practice, the systems havenever performed as expected. The radi-ators fail to emit enough of the energyfrom the burning fuel at the precisewavelength needed by a material likesilicon to make the conversion processefficient or to produce enough power.

Moreover, temperatures of 2,000 de-grees C are required to provide sufficientintensity to achieve worthwhile poweroutput. The heat can stress the materialcomposing the radiator as well as othercomponents, thereby shortening its lifespan. In addition, polluting nitrous ox-ide emissions can also result from fuelcombustion at these torrid temperatures.

Thermophotovoltaics has advancedbecause researchers have learned howto pair radiators that transmit a rela-tively broad range of wavelengths withsemiconductors able to cope with thatwide spectrum. Broadband radiators,such as silicon carbide, are capable ofworking efficiently at cooler tempera-tures of up to 1,000 degrees C. Semicon-ductor materials developed for the solarenergy industry from the third and fifthcolumns of the periodic table—so-calledIII-V materials, such as gallium anti-monide and indium gallium arsenide—

perform the photovoltaic conversion atthe wavelengths emitted by these radia-

tors. The band-gap energy required togenerate power from III-V materials—

0.5 to 0.7 eV—is much less than the1.14 eV necessary for silicon.

No thermophotovoltaic system canconvert all the infrared energy to elec-tricity. Any photon with energy lowerthan the band gap of the converter can-not boost an electron from the valenceto the conduction band and so does notgenerate electricity. These unused pho-tons become waste heat, unless somemeans can be found to use them. A pho-ton-recuperation system is a componentof a thermophotovoltaic system thatsends lower-energy photons back to theradiator, which reabsorbs them, helpingto keep the radiator heated and to con-serve fuel. Then, more of the emittedphotons reach or exceed the band gap.

Retrieving Unused Photons

Investigators have explored severalapproaches to photon recovery, in-

cluding the use of a grid of microscopicmetal antennae. These antennae, whichmight be a thin metal film atop a con-verter cell, transmit the desired infraredwavelengths to the converter while re-flecting other photons back to the radi-ator. Many photon-recovery schemeshave fallen short: some systems detecttoo narrow a range of wavelengths; oth-ers are too expensive. The most promis-ing option is the back-surface reflec-tor—so named because the unabsorbedphotons move entirely through the lay-ers of the semiconductor converter andare then returned to the radiator by ahighly reflective gold surface on theback of the converter.

Around the world, researchers are

Thermophotovoltaics92 Scientific American September 1998

CONVERTER CELL transforms heatinto electricity when infrared photonswith the right energy penetrate the cellnear the intersection of two unlike areasin a semiconductor crystal. When a pho-ton encounters an atom there, it dislodgesan electron and leaves a hole. The electronmigrates into the n-type layer (one thathas more electrons than holes), and theholes move into the p-type layer (one thathas more holes than electrons). The elec-tron then moves into an electrical contacton the cell and travels through an exter-nal circuit until it reappears in the p-typelayer, where it recombines with a hole. Ifthe photon has less than the desired ener-gy, or band gap, it is reflected off thequartz shield and back to the radiator.

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GENERATOR (photograph and dia-gram) consists of a burner that combinesfuel and air to produce heat and of an in-frared radiator surrounded by thermo-photovoltaic converter cells with attachedcooling fins. When the fuel ignites, the ra-diator heats up to 1,250 degrees Celsiusor more. Then 48 gallium antimonidecells, connected in series, convert the in-frared energy emitted by the radiator toelectric power. At the same time, some ofthe current starts the fan, which blows airupward to cool the photovoltaic cells. Ex-cess electric power produced by the gener-ator is delivered to a battery for later use.At its optimal operating temperature, thiscircuit can supply 30 watts of power. Ex-tra heat can be used to warm a room.


exploring various technical avenues fordeveloping and commercializing ther-mophotovoltaics. Since 1994 they havegathered for three international confer-ences sponsored by the U.S. Departmentof Energy’s National Renewable Ener-gy Laboratory (NREL). The DefenseAdvanced Research Projects Agency(DARPA), the DOE and the U.S. ArmyResearch Office all have funded pro-grams to develop the technology.

Scientists are excited by laboratorymodeling that suggests the promise ofthis technology. With a radiator that canoperate at 1,500 degrees C, it appearspossible for semiconductor cells with asingle junction (the site where electricpotential builds up) to obtain power-

density outputs of three to four wattsper square centimeter of converter area.

Cells with multiple junctions are alsobeing considered, an approach bor-rowed from the solar photovoltaics in-dustry. Multijunction cells would allowthe converter to capture a wider spec-trum of wavelengths, thereby makingbetter use of broadband radiators. Eachof the separate junctions would gener-ate a current after absorbing photons indifferent energy ranges. Theoretically,multijunction devices might achieve fiveto six watts of energy per square cen-timeter. Compare these projections withthose for the typical flat-panel solar-cellarray, which typically achieves 15 milli-watts per square centimeter. Whereas

these estimates derive from computermodels, and actual power outputs willundoubtedly be smaller, prototypeshave exhibited power densities greaterthan one watt per square centimeter.

The slow and painstaking work of de-signing and integrating the componentparts of a thermophotovoltaic systemhas begun at several private and gov-ernment laboratories. To obtain usableamounts of power, each converter cellmust be wired to other cells. Tradition-al semiconductor manufacturing meth-ods can pattern, etch and wire togethermultiple cells on a single surface, calleda wafer. Researchers at the NREL andthose working separately at Spire Cor-poration in Bedford, Mass., and the Na-

Thermophotovoltaics Scientific American September 1998 93

Thermophotovoltaics takes advantage of the semiconduc-tor properties that allow a current to be generated when a

photon is absorbed within the crystalline material. The electronsin a semiconductor crystal inhabit a range of defined energy lev-els, each one known as a band (left panel in diagram). Each bandspecifies the amount of energy needed to free an electron fromthe crystal. The outermost band in which electrons are still re-stricted from flowing freelyin a current is known as thevalence band. To moveabout, valence electronsmust ascend into a higher-energy band (arrow), theconduction band. Betweenthese two bands lies arange of energies that can-not be occupied by elec-trons, known as the forbid-den gap, or band gap.

In a semiconductor theconduction band is partiallyfilled with electrons, whichconstitute negative chargethat originates from impu-rity materials added to thesemiconductor. Meanwhile empty energy levels in the valenceband—areas where electrons are missing, known as holes—areequivalent to positive electrical charge.

If the semiconductor has more electrons in the conductionband than holes in the valence band, it is called n-type, with nsignifying negative. If it has more holes than electrons, it is calledp-type, for positive.

When a contact is made between n- and p-type semiconduc-tor material, a diode forms. At first, negatively charged electronstend to move to the positively charged holes on the p side of thejunction and the holes flow toward the n side. Negative chargebuilds up on the side of the junction where the electrons con-gregate, and positive charge is concentrated on the oppositeside. An electrical potential difference, a steep energy barrier, op-poses continued current flow (“equilibrium state,” in right panel ).

The equilibrium condition is upset when photons—the funda-mental particles of electromagnetic radiation—fall on the semi-conductor junction. When photons with energies equal to orgreater than the band gap are absorbed, they excite electronsfrom the valence band across the forbidden gap into the con-duction band. The altered concentrations of electrons and holesthen cause some electrons to move into the n side of the junc-

tion and some holes tocross to the p side. This re-duces the potential differ-ence across the junction(far right). This phenome-non is called the photo-voltaic effect.The electronsmay then move to an exter-nal circuit in the form ofphotogenerated electricity.

The band-gap energiesneeded to lift electrons intothe conduction band areamong the factors that dis-tinguish thermophotovol-taic cells from solar cells. Ina solar cell the band gap isabout one to 1.5 eV. For the

thermophotovoltaic cell the energies are typically in the range of0.5 to 0.7 eV—the infrared rather than the visible spectrum.

The difference in band gap does not mean, however, that solarcells can necessarily produce more power. In the case of a solarcell, photons radiate from the sun at a temperature of about6,000 degrees Celsius, whereas infrared photons destined for thesemiconductor diode in a thermophotovoltaic cell are muchcooler, in the range of 1,000 to 1,700 degrees C. The optical ener-gy incident on the thermophotovoltaic cell is much larger thanthat falling on a solar cell, though, because the distance betweenthe sun and the solar cell is 150 million kilometers. The separa-tion between the thermophotovoltaic cell and the surface of theradiator is likely to be about two centimeters. Hence, the electricpower output of the thermophotovoltaic converter is potential-ly far greater than that of a solar cell. —T.J.C. and M.C.F.

The Photovoltaic Effect

N-TYPELAYER N

DIODEJUNCTION

P N PP-TYPELAYER

N-P JUNCTION EQUILIBRIUMSTATE

WHEN APHOTON HITS

CONDUCTIONBAND

BAND GAP

VALENCEBAND

ELECTRON

PHOTON ELECTRICAL POTENTIAL DIFFERENCE

HOLE BOUND ELECTRON

PHOTON absorption in a semiconductor triggers the photovoltaic effect.

SLIM

FIL

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tional Aeronautics and Space Adminis-tration Lewis Research Center havedemonstrated these techniques for arraysof thermophotovoltaic cells. One nota-ble example comes from NREL research-ers Scott Ward and Mark Wanlass, whohave interconnected small thermopho-tovoltaic cells, each wired in series. Thewiring connections move along the topof one cell and then run down alongthe back of an adjacent one in a config-uration that reduces current, increasesvoltage and minimizes power loss.

This design could eventually reducethe converter to a wafer populated withcells that have only two external con-tacts—the ones needed to create a cir-cuit—that could power a water pumpor a cabin in the woods. Many such wa-fers could be connected to achieve de-sired power requirements. The integra-tion of photovoltaic cells may reducethe high cost of the technology, becausethe cells can be manufactured usingstandard semiconductor manufacturingmethods. In their prototype, Ward andWanlass also devised a novel design forrecirculating unused photons. The elec-trically active areas of the wafer rest atopan indium phosphide substrate that is

semi-insulating. Because the material isnonconductive, and most electrons re-main relatively tightly bound within thesemiconductor crystal, low-energy pho-tons can move through the substratewithout getting absorbed by free elec-trons moving through the conductionband. The photons then reflect off a goldsurface and return to the radiator. Inother prototype designs, many unusedphotons are absorbed by the converters.

While developmental work continueselsewhere, the first thermophotovoltaiccommercial product is about to reachthe market. JX Crystals in Issaquah,Wash., has created a product—MidnightSun—primarily for use on sailboats.The 14-centimeter-wide-by-43-centime-ter-tall cylindrical heater, powered bypropane gas, can produce 30 watts ofelectricity and is targeted as a means ofrecharging batteries that run navigationand other equipment. The unit not onlyprovides electricity but acts as a co-gen-erator, supplying space heating for theboat cabin. It uses a partially selectiveradiator made of magnesium aluminateand has gallium antimonide photovol-taic cells connected in series.

Although its current $3,000 price tag

makes it more expensive than a con-ventional diesel generator, MidnightSun runs silently and is expected to bemore reliable, because it lacks any mov-ing parts.

The product may also prove attrac-tive to owners of recreational vehiclesor wilderness homes, who could takeadvantage of a substantially less costly

Thermophotovoltaics94 Scientific American September 1998

10

9

8

7

6

5

4

3

2

1

01.5

BROADBANDRADIATOR

BAND GAP

SELECTIVERADIATOR

WAVELENGTH (MICRONS)

2 2.5 3

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(WAT

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CEN

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ETER

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BAND GAP SPACING BETWEEN ATOMS (ANGSTROMS)

2.0

1.5

1.0

0.5

0

0.7

5.4 5.6 5.8 6.0 6.2 6.4 6.6

BA

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GALLIUMARSENIDE

INDIUMPHOSPHIDE

ALUMINUMANTIMONIDE

DESIREDBAND GAP

GALLIUMANTIMONIDE

INDIUMARSENIDE

INDIUM ANTIMONIDE

PHOTON RADIATORS emit either a narrow band of wave-lengths (selective) or a wider range (broadband) that can betransformed into electricity by a semiconductor converter. Selec-tive radiators, made of rare-earth materials, produce a smallernumber of photons at or above the band gaps at which electrici-ty can be generated (red area). Broadband radiators, in contrast,can take advantage of more of the usable power reaching thecell (blue area). Lower-energy photons—those to the right of theband gap—become waste heat.

ILLU

STRA

TIO

NS

BY

SLIM

FIL

MS

BUILDING CONVERTERS by alloying different materials fromthe third and fifth columns of the periodic table can achieve adesired band gap—the amount of energy needed to free an elec-tron so it can flow in a current. Band gaps of 0.5 to 0.7 eV (shad-ed area) are ideal for thermophotovoltaic devices and correspondto increasingly shorter infrared wavelengths from the radiator.These band gaps can be obtained by combining compoundssuch as gallium antimonide with indium antimonide in incre-ments represented by the dots along the red line. Calculating thespacing between atoms (bottom axis) allows designers to jointogether different compounds with distinct crystalline structures.


unit than the stainless-steel and brassthermophotovoltaic generator neces-sary for the marine environment.

Despite the present drawbacks of se-lective radiator systems, researchers stillstudy them. DARPA has funded ThermoPower’s Tecogen division in Waltham,Mass., to create a gas-fired generator asa power supply for troop communica-tions or for powering laptop computersin the field. In the civilian realm thesame unit could keep a home furnacerunning in a power outage. The 150-and 300-watt-power modules use arraysof ytterbium oxide fibers at a wave-length of 980 nanometers. These selec-tive radiators are matched with siliconphotoconverters. A multiple-layer insu-lating filter recovers unused energy.

Beyond the Niche

Though still in its infancy—actuallyjust barely out of the research labo-

ratory—thermophotovoltaics holds greatpromise for many niche markets. Long-er term, the technology could play a

broader role in the global energy mar-ket. The terminology “niche market”can itself be deceiving, evoking the im-age of a few buyers served by a smallcottage industry. Yet the world is re-plete with billion-dollar niche markets—many of which had humble beginnings.Thomas J. Watson, the founder of IBM,originally saw a market for only a smallhandful of computers worldwide but de-cided to pursue the opportunity anyway.

The recovery of industrial waste heatcould establish a huge market for ther-mophotovoltaics. Many industrial sec-tors—manufacturers of glass, aluminum,steel and other products—generate enor-mous quantities of heat through theirproduction processes. The glass indus-try estimates that two thirds of the ener-gy it consumes emanates as waste heat,which could amount to a gigawatt ofpower. Thermophotovoltaic converterscould possibly be used to generate elec-tricity from waste heat, affording enor-mous savings in electricity costs. Anoth-er promising application is suggested bya hybrid electric vehicle introduced by

Western Washington University. The ve-hicle supplements the power from elec-tric batteries by supplying 10 kilowattsfrom a thermophotovoltaic generator.

Today the funding devoted to ther-mophotovoltaics is not more than $20million to $40 million a year. That isthe money researchers receive for theirstudies on converter devices, radiatorsand the integration of the various com-ponents into working generators. A re-cent market study financed by the NREL,however, indicates that thermophoto-voltaics could produce $500 million incommercial sales by 2005. Revenues forthis nascent market would come fromsubstituting this technology for dieselgenerators of less than two kilowattsused in the military and in recreationalvehicle and boating markets. Thermo-photovoltaics may lead to the possibili-ty of cleaner, more efficient and inex-pensive solutions for a number of alter-native energy markets. A technologythat had its roots in the 1950s mayfinally get a chance to prove itself at theturn of the new millennium.

Thermophotovoltaics Scientific American September 1998 95

The Authors

TIMOTHY J. COUTTS and MARK C. FITZGERALD have workedon renewable energy and related technologies for many years. Coutts,who has studied solar-cell technology since 1971, is a research fellow inthe National Center for Photovoltaics of the U.S. Department of Energy’sNational Renewable Energy Laboratory (NREL) in Golden, Colo. Hewas educated at Newcastle Polytechnic in Britain, where in 1968 he re-ceived his doctorate in thin-film physics. At the NREL, he has broad-ranging responsibilities in both solar and thermophotovoltaics technolo-gies. Fitzgerald is president of Science Communications, a photovoltaicconsulting firm in Highlands Ranch, Colo. He has 19 years of experienceproviding education services about renewable energy.

Further Reading

The First NREL Conference on Thermophotovoltaic Gen-eration of Electricity. Timothy J. Coutts and John P. Ben-ner in AIP Conference Proceedings, Vol. 321. AIP Press, 1995.

The Second NREL Conference on ThermophotovoltaicGeneration of Electricity. Edited by John P. Benner, Timo-thy J. Coutts and David S. Ginley in AIP Conference Proceed-ings, Vol. 358. AIP Press, 1996.

The Third NREL Conference on ThermophotovoltaicGeneration of Electricity. Edited by Timothy J. Coutts,Carole S. Allman and John P. Benner in AIP Conference Pro-ceedings, Vol. 401. AIP Press, 1997.

EXPERIMENTAL CAR at Western WashingtonUniversity gains some of its power from a ther-mophotovoltaic generator (inset).

PHO

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If you’re a man who’s looking to

get married, here’s some friendly

advice. Only consider women who

are smarter than you are. I followed this

prescription four years ago when I mar-

ried Michelle Tetreault, a charming and

brilliant biophysicist. Now my wife al-

ways intrigues me with her insights and

never lets me get away with anything

dubious at home.

At work Michelle employs the latest

techniques in biochemistry to unravel

the secrets of photosynthesis. By manip-

ulating the smallest units of inheritance,

the individual base pairs on a single

strand of DNA, she can change one by

one the amino acids that make up a key

protein and then study how well this al-

tered molecule can do its job.

Hearing about Michelle’s research so

often at the dinner table recently prompt-

ed me to try my hand at molecular biol-

ogy. Although the many cutting-edge

techniques she uses are probably beyond

the range of amateur dabbling, recent

advances have opened up intriguing av-

enues for informal explorations into

biotechnology. To help clear the way,

this column explains how anyone can

do what biotechnologists do routinely:

extract and purify DNA.

DNA is the largest molecule known.

A single, unbroken strand of it can con-

tain many millions of atoms. When re-

leased from a cell, DNA typically breaks

up into countless fragments. In solution,

these strands have a slight negative elec-

tric charge, a fact that makes for some

fascinating chemistry. For example, salt

ions are attracted to the negative charg-

es on DNA, effectively neutralizing them,

and this phenomenon prevents the many

separate fragments of DNA from ad-

hering to one another. So by controlling

the salt concentration, biologists can

make DNA fragments either disperse or

glom together. And therein lies the se-

cret of separating DNA from cells.

The procedure is first to break open

the cells and let their molecular guts spill

into a buffer, a solution in which DNA

will dissolve. At this point, the buffer

contains DNA plus an assortment of

cellular debris: RNA, proteins, carbo-

hydrates, and a few other bits and piec-

es. By binding up the proteins with de-

tergent and reducing the salt concentra-

tion, one can separate the DNA, thus

obtaining a nearly pristine sample of

the molecules of inheritance.

My profound thanks go to Jack Chi-

rikjian and Karen Graf of Edvotek


Spooling the Stuff of Life

by Shawn Carlson

T H E A M A T E U R S C I E N T I S T

KITCHEN LABORATORYincludes most of the items needed to isolateDNA. A drinking straw, for example, can beused to add alcohol to the solution (a), anda coffee stirrer serves to spool the DNA (b).

a


(www.edvotek.com), an educational

biotech company in West Bethesda,

Md., for showing me how anyone can

purify DNA from plant cells right in

the kitchen. You’ll first need to prepare

a buffer. Pour 120 milliliters (about four

ounces) of water into a clean glass con-

tainer along with 1.5 grams (1/4 tea-

spoon) of table salt, five grams (one tea-

spoon) of baking soda and five millili-

ters (one teaspoon) of shampoo or

liquid laundry detergent. These cleaners

work well because they have fewer ad-

ditives than hand soaps—although do

feel free to try other products as well.

The detergent actually does double

duty. It breaks down cell walls and helps

to fracture large proteins so they don’t

come out with the DNA. The people at

Edvotek recommend using pure table

salt and distilled water, but I have used

iodized salt and bottled water success-

fully, and once I even forgot to add the

baking soda and still got good results.

In any case, try to avoid using tap wa-

ter. To slow the rate at which the DNA

degrades, it’s best to chill the buffer in a

bath of crushed ice and water before

proceeding.

For a source of DNA, try the pantry. I

got great results with an onion, and the

folks at Edvotek also recommend gar-

lic, bananas and tomatoes. But it’s your

experiment: choose your favorite fruit

or vegetable. Dice it and put the materi-

al into a blender, then add a little water

and mix things well by pulsing the blades

in 10-second bursts. Or, even simpler,

just pass the pieces through a garlic press.

These treatments will break apart some

of the cells right away and expose many

cell walls to attack by the detergent.

Place five milliliters of the minced veg-

etable mush into a clean container and

mix in 10 milliliters of your chilled buf-

fer. Stir vigorously for at least two min-

utes. Next you’ll want to separate the

visible plant matter from the molecule-

laden soup. Use a centrifuge if possible.

(To learn how to build a centrifuge, see

the January Amateur Scientist column.)

Spin the contents at low speed for five

minutes and then delicately pour off at

least five milliliters of the excess liquid

into a narrow vessel, such as a clean

shot glass, clear plastic vial or test tube.

If you do not have a centrifuge, strain

the material through an ordinary coffee

filter to remove most of the plant refuse.

With luck, any stuff that leaks through

should either sink or float on top, so it

will be a simple matter to pour off any

solids into the sink and then pour the

clear liquid into a clean vessel.

The solution now contains DNA frag-

ments as well as a host of other molecu-

lar gunk. To extract the DNA, you will

need to chill some isopropyl alcohol in

your freezer until it is ice-cold. Most

drugstores sell concentrations between

70 and 99 percent. Get the highest con-

centration (without colorings or fra-

grances) you can find. Using a drinking

straw, carefully deposit 10 milliliters of

the chilled alcohol on top of the DNA

solution. To avoid getting alcohol in

your mouth, just dip the straw into the

bottle of alcohol and pinch off the top.

Allow the alcohol to stream slowly

down along the inside of the vessel by

tilting it slightly. The alcohol, being less

dense than the buffer, will float on top.

Gently insert a narrow rod through the

layer of alcohol. (Edvotek recommends

using a wooden coffee stirrer or a glass

swizzle stick.)

Gingerly twirl back and forth with

the tip of the stick suspended just below

the boundary between the alcohol and

the buffer solution. Longer pieces of

DNA will then spool onto the stick,

leaving smaller molecules behind. After

a minute of twirling, pull the stirrer up

through the alcohol, which will make

the DNA adhere to the end of the stick

and appear as a transparent viscous

sludge clinging to the tip.

Although these results are impressive,

this simple and inexpensive procedure

does not yield a pure product. Profes-

sionals add enzymes that tear apart the

RNA molecules to make sure they do

not get mixed up with the coveted DNA.

Even after the most thorough extrac-

tion, some residual DNA typically lin-

gers in the vessel, forming an invisible

cobweb within the liquid. But with a lit-

tle more effort, you can see that materi-

al, too. Some dyes, like methylene blue,

will bind to charged DNA fragments. A

tiny amount added to the remaining so-

lution will thus stain tendrils of uncol-

lected DNA. I don’t know whether any

household dyes, like food coloring or

clothing or hair dyes, will also work, so

I invite you to find out. Add only a

drop: you want all the dye molecules to

bind to the DNA, with none left over to

stain the water.

Exciting as it may be, extracting an

organism’s DNA is only the first step in

most biological experiments. You’ll

probably want to learn what further in-

vestigations you can do—for example,

sorting the various DNA fragments ac-

cording to their lengths. This depart-

ment hasn’t described suitable methods

for separating large organic molecules in

decades. (See the Amateur Scientist col-

umns for August 1955, July 1961 and

June 1962.) But I’ll be revisiting some of

these techniques in coming months to

help you bring your kitchen lab into the

modern age of biotechnology.

The Society for Amateur Scientistsand Edvotek have joined forces to cre-ate a kit containing cell samples, labware, enzymes, buffers and detergentsthat can help you create higher-qualitypreparations. To order, send $35 to theSociety for Amateur Scientists at 4735Clairemont Square, Suite 179, San Di-ego, CA 92117. For more informationabout this and other amateur scienceprojects, visit the forum hosted by thesociety at http://web2.thesphere.com/SAS/WebX.cgi on the World Wide Web.You may also write the society at theabove address or call 619-239-8807.


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171

1,180

2,060

2,950

3,860

4,690

5,630

6,490

140

120

100

80

60

40

20

0

450

2,820

4,440

5,640

6,440

6,810

6,770

6,310

HEIGHT

OF

PYRAMID

(MET

ERS)

WORKERS NEEDED FORHAULING AND LIFTING

WORKFORCE NEEDED TO BUILD PYRAMID

TOTAL W

ORKFO

RCE

WORKERS NEEDED FOR QUARRYING AND OTHER TASKS

621

4,000

6,500

8,590

10,300

11,500

12,400

12,800

The pyramids of ancient Egypt

rank among the most enig-

matic of archaeological mys-

teries. The largest, the Great Pyramid at

Giza, was built by Egyptian king Khufu

in about 2500 B.C. and is still mostly in-

tact. Its original height was almost 147

meters (482 feet), and it weighed more

than seven billion kilograms (7.7 mil-

lion tons). The pyramid was made from

huge blocks of stone, which were quar-

ried, trimmed to a fairly regular shape,

transported to the construction site and

then piled on top of one another with

astonishing precision.

How did the Egyptians put together

such giant edifices? And why were they

built? Historians and archaeologists do

not know for certain. Many pyramids

were used as tombs for kings, and there

are plenty of theories about their con-

struction. And now historians can make

a good guess at the size of the work-

force, thanks to some mathematical de-

tective work by Stuart Kirkland Wier,

an amateur Egyptologist who belongs

to a study group sponsored by the Den-

ver Museum of Natural History. His

calculations appeared in the CambridgeArchaeological Journal (April 1996).

Some two millennia after the Great

Pyramid was completed, Greek histori-

an Herodotus reported that it took

100,000 men to build the structure.

Herodotus, however, was not a reliable

source on ancient Egypt. It now seems

that he overestimated the workforce by

an order of magnitude. According to

Wier, the true figure was surprisingly

small: only about 10,000. How did Wier

come up with this number without

knowing how the pyramid was built?

He estimated the total amount of work

needed to construct the pyramid, then

made a few assumptions about how the

Egyptians scheduled the project.

Wier performed his calculations for

Khufu’s pyramid, but the same meth-

od can be applied to others. The first

step is to work out how much

“energy” a pyramid contains.

Energy in this context means

potential energy—the ef-

fort required to lift a

given mass to a given

height. Wier divid-

ed the potential

energy of the pyramid by the number of

days spent building it. This calculation

yielded the amount of work required

each day to lift the blocks of stone. The

sole source of power for the Egyptian

workforce was human muscle. Divid-

ing the required work by the energy

output of a typical Egyptian laborer,

Wier determined the number of work-

ers needed to perform the lifting tasks.

The biggest uncertainty is time. How

long did it take to build Khufu’s pyra-

mid? Khufu reigned for 23 years. Con-

struction of his pyramid probably did

not begin before his reign, but it could

have ended near the time of his death

or many years before. Wier assumed

that Khufu’s pyramid took 23 years to

build—the same length as the king’s

reign. That amounts to some 8,400 days

if the work continued year-round. The

actual time might have been half as long;

because of this fundamental uncertain-

ty, we can make only a rough estimate

of the size of the workforce.

Khufu’s pyramid, when new, was

146.7 meters high, and the sides of its

square base were 230.4 meters long.

The volume of a pyramid of height hand base side s is s2 × h/3, which works

out to be 2.6 million cubic meters for

the Great Pyramid. The material was

limestone with a density d of 2,700 kilo-

grams per cubic meter, so the mass was

about seven billion kilograms. The po-

tential energy of a pyramid is h2 × d ×

by Ian Stewart

M AT H E M AT I C A L R E C R E AT I O N S

Mathematical Recreations

Counting the Pyramid Builders


ROG

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bis

THE GREAT PYRAMIDat Giza (above, in center) could have been built by as few as 10,000 workers (below).

DM

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s2 × g/12, where g is the acceleration

caused by gravity (9.81 meters per sec-

ond squared). For the Great Pyramid,

this amounts to 2.52 trillion joules, a

joule being the standard unit of energy.

The average amount of useful work

provided by a man in a day is about

240,000 joules. Assuming perfect effi-

ciency, 1,250 men would be needed to

raise the blocks into position over 8,400

days. This estimate is clearly too low;

Wier multiplied it by a fudge factor of

1.5 to account for the inefficient use of

muscle power. He also factored in the

amount of work needed to raise the

stones from the bottom of the Khufu

quarry to the level of the pyramid site—

a distance known to be 19 meters.

Of course, pyramids are not just fea-

tureless heaps of stone: they have pas-

sages and chambers, some of which are

remarkable feats of engineering in their

own right. But the major part of the

construction work was simply piling up

the blocks, so we can ignore the struc-

tural details. The pyramids

were built in layers from

bottom to top. Certainly no

block could be added to the

structure until the blocks be-

low had been put in place.

Nobody knows how the

blocks were raised. Some

experts believe the Egyp-

tians hauled the blocks up

vast ramps of sand. Others

say the laborers used a cun-

ning arrangement of levers.

Workers transported the

stone blocks from the quar-

ry to the construction site by

dragging them on wooden sleds. (Egyp-

tian carvings show this being done.) The

Khufu quarry was several hundred me-

ters from the base of the pyramid. To

calculate the number of men required

for hauling, Wier estimated the friction

between a loaded wooden sled and the

ground, then determined the amount of

work needed to overcome the friction.

Manpower was also needed for other

tasks: quarrying the stone blocks, trim-

ming them to shape, installing them in

the pyramid. Wier assumed that all such

tasks together required a maximum of

14 men per cubic meter of stone per day.

These calculations give the average

number of workers at Giza. But did the

pyramid builders employ a workforce

of fixed size? Or did they enlist extra

workers when the project demanded it

and dismiss them when they were no

longer needed? The records don’t say,

but several possibilities can be inferred.

Dividing the volume of the Great

Pyramid by the time available to build

it shows that an average of 310 cubic

meters of stone must be put in place

each day. As the pyramid grows, how-

ever, the stone blocks must be raised

higher, so more work is required to lift

them. And the amount of workspace

available to the lifters—the area at the

top of the unfinished pyramid—decreas-

es as the structure gets higher. These fac-

tors make it clear that a steady rate of

310 cubic meters a day is not sensible.

Instead the construction rate should be

greater when the pyramid is low and

should drop off as it gains in height.

Wier considered three possible con-

struction schedules [see illustration atleft]. Under schedule A, the workers in-

stall the blocks at a constant rate of 315

cubic meters each day until day 8,110.

At that point, the construction rate

plummets because of the limited space

at the top of the pyramid. Under sched-

ule B, the construction rate starts at

462 cubic meters a day and declines

steadily until day 8,000, after which it

falls off more rapidly. Under schedule

C, the construction rate starts at 500

cubic meters a day but drops sharply af-

ter day 2,000 and tails off to zero after

day 7,000. All three schedules result in

the completion of the pyramid in 8,400

days, but they require different num-

bers of workers as the job progresses.

The bottom illustration on page 98

shows the number of workers required

at each stage of the pyramid’s construc-

tion under schedule B. At the start,

about half the workers are hauling or

lifting the blocks, and half are quarry-

ing the stone, installing the blocks and

undertaking other tasks. By the end,

though, nearly three quarters of the

workers are moving the blocks. At no

stage does the number of workers ex-

ceed 12,800 men—about 1 percent of

the estimated Egyptian population at

that time. Schedules A and C lead to

similar results.

Perhaps the simplest schedule is to

employ a workforce of constant size,

except near the end when there is not

enough room at the top for more than

a few workers. Under this schedule,

10,600 men would suffice to build Khu-

fu’s Great Pyramid. Even fewer workers

would be needed to build the other large

pyramids at Giza and Dahshûr. Despite

the tremendous size of these monu-

ments, there were more than enough

workers in Egypt to build them.

Mathematical Recreations100 Scientific American September 1998

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DAYS AFTER BEGINNING OF CONSTRUCTION

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SCHEDULE C

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THREE POSSIBLE SCHEDULES lead to the completion of the Great Pyramid in 8,400 days.

FEEDBACK

In “Tight Tins for Round Sardines” [February], the column onpacking circles into regions of various shapes, I wrote that

“nearly all the information we have about such questions datesfrom 1960 or later.” One reader misunderstood this remark, com-plaining that I had cavalierly disregarded the great classical work

of Gauss, Lagrange and others. By “such questions” I meant thepacking of objects into finite regions, like 11 circles in a square(above) or 10 circles in a larger circle (left). (The circles are repre-sented by points marking their centers.) The classical work is onpackings of the infinite plane, and it also assumes that the objectsform a regular array. In the problems discussed in the column, the

region is of limited extent, and the arrangement of circles need not be regular.Martin Trump of Newbury, England, has explored the analogous problem of

distributing charged particles inside a sphere. His World Wide Web site—http:// www. geocities.com/Paris/3142—has pictures of these distributions. —I.S.

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Reviews and Commentaries102 Scientific American September 1998

Sixty-one years ago W. W. Norton

and Company published Lance-

lot Hogben’s Mathematics forthe Million, one of the truly great popu-

larizations of this century. It has sold

more than 600,000 copies and is still

selling thousands of copies a year. With

the publication of Jan Gullberg’s Math-ematics: From the Birth of Numbers,Norton seems to have done it again.

The book is an enthusiastic and ut-

terly amazing popularization that

promises to be in print for de-

cades. After 18 months, it is

in its sixth printing.

The two books have

much in common. Both

were written by non-

mathematicians: Hog-

ben was a zoologist and

Gullberg a surgeon.

Neither was an Ameri-

can, but each ended up

being published by an

American company. Both

saw mathematics as a so-

cial equalizer and essential

for helping the masses to es-

cape control by society’s “clev-

er people”—mathematicians, sci-

entists and government officials.

Both authors richly appreciated the

historical development of mathematics.

And both books are big: Hogben at

649 pages and Gullberg at 1,093 pages.

Hogben cuts a broad mathematical

swath, from the development of count-

ing to calculus and statistics. Gullberg

covers nearly all that and much more.

When Hogben’s classic was published in

1937, it was praised for its illustrations.

The number and variety of pictures in

Gullberg’s opus dwarf those of Hogben.

Both books are serious, although Gull-

berg also makes generous use of humor.

But here is a sharp contrast: Hogben

claimed to have written his book in six

weeks; Gullberg took 10 years for his.

An itinerant surgeon (or “Dr. Saw-

bones” as he sometimes referred to him-

self), Gullberg seemed to be drawn to

small towns, each a bit off the beaten

track. In the years that he worked on the

book, he lived in Angola, Ind.; Forks,

Morton, Moses Lake and Sequim in

Washington State; Moab, Utah; Møsjen

and Rjukan in Norway; and Karlskoga,

Sweden. He claimed that his peripatetic

way of life was crucial to the gathering

of data and the development of ideas.

Certainly his book project provided

something of an anchor during that pe-

riod. Gullberg died at age 62 in May

1998 of the consequences of a stroke

suffered a few days earlier while at work

at a hospital near his home in Nord-

fjordeid, Norway. Fortunately, he lived

long enough to see his book become a

publishing success.

The scope of the book is huge—num-

bers and language, combinatorics, logic,

set theory, theory of equations, geome-

try, trigonometry, fractals, sequences and

series, calculus, harmonic analysis, prob-

ability and differential equations. Few

mathematicians would undertake to

write a survey so ambitious. A full quar-

ter of the tome is devoted to calculus top-

ics. Given the tremendous influence of

calculus since its invention more than

300 years ago, perhaps it deserves

that much space. Bits of humor

and other ancillary material

are present in the calculus

chapters, though not with

the same frequency as in

other chapters.

The book’s large pag-

es are loaded with eye-

catching graphics and

illustrations, frontis-

pieces from original

works, cartoons and

jokes. The illustrations

reveal much about Gull-

berg’s remarkably broad

and deep curiosity about

mathematics, his appreciation

of mathematics in history, his

fondness for languages (he was flu-

ent in English, Swedish, Norwegian,

French and German, and he could read

Greek and Latin) and his humorous

side. The opening illustration is a pho-

tograph of a sculpture by Swedish sculp-

tor Axel Ebbe. Depicting a young man

emerging from an imposing mass of

stone, it is entitled Breaking Away fromthe Darkness of Ignorance, to which

Gullberg forcefully adds, “and then

there was Mathematics.”

Within the first five pages, as the au-

thor explores “The Origins of Reckon-

ing,” we encounter lovely whimsical

drawings by his wife, Ann, and twin

sons, Kalin and Kamen, then nine years

R E V I E W S A N D C O M M E N T A R I E S

MATHEMATICS FOR THE MANYReview by Donald J. Albers

Mathematics: From the Birth of Numbers

BY JAN GULLBERG

W. W. Norton and Company, New York, 1997 ($50)

MATHEMATICS’S DEEP HISTORYpervades Gullberg’s book. This illustra-tion is from the frontispiece of Leibniz’sDissertatio de arte combinatoria, a workthat strove to reduce all truths of reason

to a simple system of arithmetic.


old, and apt quotations from Thomas

Hobbes, Umberto Eco and Carl von

Linné (Linnaeus). And he never lets up

throughout the almost 1,100 pages—

history, brief profiles, more than 1,000

technical illustrations done by his son

Pär, and excerpts from an astonishingly

wide range of sources, from daily news-

papers to Newton to Winnie the Pooh.He is even bold enough to include some

of his own verse and jokes, most of

which are quite good.

The illustrations alone make the book

worth the price—and at less than five

cents per page, it is a bargain. Gullberg

searched through an amazingly large

number of classics and reproduced from

them a wonderful collection. Among

them are the frontispiece of Gottfried

Leibniz’s Dissertatio de arte combinato-ria (1666); Pascal’s original triangle

from his Traité du triangle arithmetique(1665); Johannes Kepler’s drawings of

13 semiregular polyhedra in his Har-monices mundi (1619); the frontispiece

and table of contents of the first printed

textbook on calculus, Analyse des infini-ment petits by L’Hospital (1696); brief

selections from John Napier’s A Descrip-tion of the Admirable Table of Loga-rithms with a Declaration of the MostPlentiful, Easy, and Speedy Use Thereofin Both Kinds of Trigonometry, as Alsoin All Mathematical Calculations; and

a map of Königsberg circa 1740 with its

seven bridges that inspired the famous

problem of the same name (solved by

Leonhard Euler in 1736), which helped

to launch graph theory.

A third of the way through, Gullberg

presents a chapter entitled “Overture to

the Geometries.” He begins the chapter

with a chilling quotation from the book

of Roman civil law (circa A.D. 650) that

serves as a reminder that mathematicians

have not always been held in

high regard: “The study and

teaching of the science of ge-

ometry are in the public in-

terest, but whosoever prac-

tices the damnable art of

mathematical divination shall

be put to the stake.” In the

same chapter, and on a decid-

edly lighter note, we find in-

structions on “How to Catch

Elephants—An Elephanta-

sy,” which appeared in the

Copenhagen newspaper Ber-lingske Tidende.

By the time we have fin-

ished the chapter, we have

been treated to wonderfully

readable sections on history,

geometric abstraction, per-

spective and projection,

form and shape, survey of

geometries, topology and Euclidean

and non-Euclidean geometries. The

next chapter continues with more ge-

ometry, which is followed by chapters

on trigonometry (plane and spherical),

hyperbolic fractions, analytic geometry,

vector algebra and fractals.

Gullberg gives the reader plenty of

motivation and a multitude of worked-

out examples, which are especially

helpful. He derives many familiar for-

mulas and provides many proofs, but

he is careful not to overwhelm the read-

er with detail. He was particularly keen

on reading the masters, as is reflected in

the opening remarks to his extensive

“Works Cited” section: “The strength

of those who have gone before should

not be gleaned solely from histories or

random quotations. True inspiration

and power lie in the original works.”

Not only did Gullberg write Mathe-matics: From the Birth of Numbers, he

built the book using his Macintosh Plus.

He set the type and lovingly designed

the pages, deciding where to place each

illustration, drawing and cartoon. His

son Pär, now a computer graphics spe-

cialist, provided vital technical assis-

tance, including the computer-generat-

ed illustrations. After the manuscript

was reviewed by several specialists in

different areas of mathematics, correct-

ed, copy edited and the pages put into

final form by Gullberg, all the publisher

had to do was start the presses. The vol-

ume is remarkably clean for a book of

this size and scope. Typos and mathe-

matical glitches can be found, but they

are rare. Few of us, whether amateurs

like Gullberg or professional mathema-

ticians, could write (or even read) a

manuscript of this size without occa-

sional lapses.

About a year ago, when I asked Gull-

berg why he wrote Mathematics: Fromthe Birth of Numbers, he responded: “So

that people might develop confidence in

doing mathematics and an appreciation

for how long the development of math-

ematics has taken.” In the same inter-

view, when asked about his target group,

he said, “I can’t write for a target

group—only for myself—as a means of

exploring mathematics and its history.”

Readers of Scientific American, stu-

dents, teachers of mathematics and all

others who care about my favorite sub-

ject will want to own this book. It is an

important reference and a book that is

plain fun to dip into. If a family is to

have only one mathematics book on

the reference shelf, then this is the one.

It is a great present for anyone who likes

mathematics. But if you buy this extra-

ordinary tome for someone, there is con-

siderable danger that you will end up

keeping it for yourself—and then you

will need to buy another copy.

DONALD J. ALBERS is director ofpublications of the Mathematical Asso-ciation of America. His profiles and in-terviews of mathematicians have ap-peared in two books, Mathematical Peo-

ple and More Mathematical People.

Reviews and Commentaries Scientific American September 1998 103

WHIMSY from the discussion of perspective and projection

PLAYFUL TAKE on circular functions in the chapter

on trigonometry was drawn by Gullberg’s wife, Ann.

How to Catch Elephants – an Elephantasy

If you want to catch elephants, this is how you do it accordingto the Copenhagen newspaper Berlingske Tidende.

All you need is a blackboard, a piece of chalk, a mariner’s tele-scope, a pair of tweezers and an empty jampot.

You begin by writing 1 + 1 = 3 on the blackboard and set it upin a place rich in elephants, and hide yourself in a nearby tree.

Very soon an inquisitive little baby elephant will draw near tomuse upon this remarkable statement and by and by several ofher elders will join her.

When there are enough of them, you just turn your telescopethe wrong way around, so that the assembled elephants be-come quite small. Then you pick the small elephants up withyour tweezers, one by one, and put them in the jampot.


Scientists have discovered some

eight or 10 planetary systems be-

yond our own. So far the homi-

est example attends a dim, single star in

the constellation of Ursa Major, the

Great Bear. That sunlike star is desig-

nated as 47 UMa, entered long ago as

just one more star by the first ever as-

tronomer royal, John Flamsteed. (You

can see it on a clear, dark night, as one

of a sprinkle of faint stars. Extend the

line of the Pointers backward from the

North Star by about three times the sky

separation between the two Pointers to

find the little grouping.)

The distance of 47 UMa from Earth,

about 45 light-years, is known by trigo-

nometric measurement; from its spec-

trum we find that it is nearly the size

and mass of our sun, though somewhat

older, brighter and hotter. An unseen gi-

ant planet circles 47 UMa every 1,100

days: a super-Jupiter, yet too faint for

the best telescopes. Our indirect evidence

is compelling; tiny, elliptical motions of

that sun-star in response to the wide

swing of its planet have been mapped

in detail since 1990. Alas, we lack the

sensitivity to detect any sun’s minute re-

sponse to an Earth-like orbiter, which

would be hundreds of times lighter than

a giant gas planet.

But imagination stands ready. Suppose

(here we leave fact to voyage on a single

fantastic journey) our space probe has

toted a good video camera into an Earth-

like annual orbit near 47 UMa. That

would shift our point of view about 50

light-years out, a minor hop on a galac-

tic scale, although a tedious journey,

lasting some hundred thousands of

years, with any of NASA’s 20th-century

solar system vehicles. Out in the Big Bear

system from our small probe we peer at

an airless, star-studded sky, night-black

even close to the brilliant disk of 47

UMa itself. That sunlit orb moves an-

nually—don’t ever point your camera at

it!—around a belt in the sky, and a single

shimmering planet rounds there, too.

Expect comets and asteroids, but they

will probably be too faint to notice. The

Milky Way will look familiar. But what

constellations will enliven that dark sky?

This is no alien sky, for we have not

yet left our galactic neighborhood.

The conspicuous figure of the brilliant

constellation Orion will be quite recog-

nizable from the Bear system. Orion’s

stars are so far from us here at the sun

that our shift to 47 UMa will simply

edit the shape of the hunter a little, his

belt and sword hardly changed. Other

constellations—such as the Big Dipper

(still far away from 47 UMa even though

we have journeyed almost straight to-

ward it), Scorpius and Cygnus—will be

recognizable as well, although one of

the Swan’s half a dozen defining stars is

much diminished.

Yet the very brightest star in our home

port, Sirius, or Dog Star, will no longer

be a skymark in the Bear

world. It will dwindle to

resemble 50 or more

stars of the second mag-

nitude, known out there

only to adept stargazers.

Our own sun will pre-

sent an obscure, souther-

ly object for binoculars.

Canopus is the second

brightest star in our

night sky, easily viewed

from Florida any winter

evening. Out in the Great

Bear it is Canopus—not

Sirius—that is the bright-

est of all.

The explanation of

this spotty change is neat. Stars appear

faint or bright for two wholly differing

reasons. Even a dim star can appear

bright if it is close by. (After all, although

our sun is a dwarf among stars, for us

here it is not merely bright but sends

life-giving heat, at the tiny distance of

only 0.0000158 light-year, or 500 light-

seconds.) A really distant star can be vi-

sually bright if it is sufficiently lumi-

nous; ample optical power overcomes

the distance. Among the 100 brightest

stars as seen from Earth, our sun com-

pares in light output only with its two

nearest neighbors, both in Centaurus far

south: one star a fair match for the sun

and its reddish companion rather faint-

er; all the rest are at least 10 times

more luminous than the sun.

A shift of 40 or 50 light-years

will reduce to the commonplace

any neighbor under 10 light-

years away, like Sirius, but would not

even halve the powerful gleam of Can-

opus, a massive, bright giant around

300 light-years out that is intrinsically

thousands of times more luminous than

the modest but enduring sun. Among

the well-seen stars from Earth a dozen


DU

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COMMENTARY

WONDERSby Philip and Phylis Morrison

A Minor Shift in Point of View

Our own sun will be an obscure,southerly object for binoculars.

Continued on page 108


Iwas in one of London’s oldest cof-

feehouses the other day, sipping a

cup (of Folger’s, as it happens) and

thinking about the first such watering

holes. Like the one in St. Paul’s church-

yard, where those who supported the

American colonists’ cause used to meet

and greet during the 1760s. One such

dangerous coffee-drinking liberal was

mathematician Richard Price, who put

actuarial studies on a modern basis with

his analysis of the Northampton Bills of

Mortality. From which he was able to

deduce life-and-death matters well

enough for the new insurance firms to

be able to charge realistic premiums and

not go broke before their clients did.

Price’s statistical work attracted the

French finance minister, Anne-Robert-

Jacques Turgot, who had the unenvi-

able task of balancing the books at a

time when the French economy was

heading rapidly toiletward. A pal and

adviser to Turgot was the Marquis de

Condorcet, whose claim to fame was

the way he took the new statistical view

of things to the next stage and invented

what he called “social mathematics.”

Thus armed, he intended to predict all

aspects of behavior and thereby set the

study of society onto a scientific footing.

Turgot and Condorcet both came out

of the physiocrat school of economic

thought, which looked to the English

example of a lightly regulated agricul-

tural market as the only way to save

France from ruin. For them, the price

of a loaf of bread held the key to politi-

cal stability. Well, they never made it out

of the bakery in time to save their heads.

But en route to his eventual death in a

French Revolutionary prison, Condor-

cet tried hard to popularize the agro-

nomic ideas of an English lawyer, Jethro

Tull. In 1711 Tull had gone to the south

of France for his health and had seen

peasants in the vineyards around Fron-

tignan hoeing between furrows and (al-

though he didn’t know it) in this way

aerating the dirt and making it easier for

water to permeate. As a result of which

they were getting great crop yields with-

out having to resort to the use of expen-

sive cow dung. When Tull, on native

soil, found that with this trick he could

grow corn in the same, unmanured field

for 13 years, he was in like Flynn with

his farmer friends. His “how-to” book,

The New Horse-Hoeing Husbandry,became an instant hit.

When Tull’s groundbreaking work

went into a second version, the editor

was a journalist so famed for his prick-

ly style he was known as Peter Porcu-

pine. William Cobbett by name, he had

begun his writing career while teaching

English to French immigrants in Phila-

delphia. With remarkable disregard for

reality, he wrote diatribes against up-

start American ideas about democracy.

In 1794 the eminent English scientist

Joseph Priestley arrived, on the run from

anti-American lab-burning mobs in

England, and the loyalist Cobbett greet-

ed him with an article lambasting the

liberalism of both Priestley and his pro-

American science cohorts back home.

Priestley himself had started life as a

church minister and at one time headed

a Sunday school. A member of his staff

was a guy named Rowland Hill, who

would go on to renown when in later

life he reformed the English mail service

and introduced the first lick-it-and-stick-

it, prepaid postage stamp: the Penny

Black. Hill’s teaching career took off

when he became master at the radical

new Hazelwood School in Birmingham.

The place was gaslit and had central

heating as well as a swimming pool; the

curriculum included such off-the-wall

stuff as applied mathematics and mod-

ern languages. In 1822 Hill and his

brother published a modest work enti-

tled Public Education, which brought

Hill to the attention of the left-leaning

great and good, including Robert Owen,

enlightened mill owner and founder of

what would end up being the British

Socialist Party.

Like many of his stripe before him,

Owen went to America to establish

one of the many (temporary) utopian

communities fashionable at the time.

His was in New Harmony, Ind. When

the commune failed, Owen left, but his

sons stayed behind and became U.S. cit-

izens. One son, Robert Dale Owen, be-

came a leading figure in Indiana politics,

spending two terms in the House of

Representatives (where he introduced

the bill to set up the Smithsonian). In

1888 his daughter, Rosamond, married

(for one week, whereupon the new hus-

band inconsiderately died) fervent Zion-

ist supporter and spiritualist Laurence

Oliphant.

Earlier in life Oliphant had set up as

a travel writer for the Times of London,

covering the Crimean War. He also

worked as a British spy. At one point

he took a job as secretary to Lord Elgin,

who was governor general of Canada

but who went down in history as the son

of the man who, in 1803, snitched the

Elgin Marbles. Or, as Elgin pater would

have put it: “Removed them for their

own good.” The marbles (today in the

British Museum) were mostly large bits

of the Acropolis frieze, dating from the

fifth century B.C. In Elgin’s time they be-

longed to the Turkish occupying power,

who could not have cared less about

Reviews and Commentaries Scientific American September 1998 107

CONNECTIONSby James Burke

Rebellious Affairs

COMMENTARY

Continued on page 108

Banks did have an impeccablescientific background—that is, he was born rich.

DU

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or more are as far away as a couple of

thousand light-years. Like Canopus,

they will remain as interesting out there

as they are to observers here. (The new

Hipparcos survey allows for a more

precise mapping of the faint stars out

there; anyone who would draw us a

map of bright stars from 47 UMa will

be gratefully noticed here.)

The apparent path against the stars

that our sun and planets trace during

the year is the zodiac. It is the relic of the

flat, whirling disk of gas and dust out

of which observation and theory to-

gether suggest that planets are formed.

Their orbits still recall that common

motion, even though most material was

lost to space or to the sun. It is natural

to set our imaginary orbit into the zodi-

acal plane of 47 UMa. That far-off sky

has its differences, but it is only a revised

version of the naked-eye sky on Earth.

The super-Jupiter in the Bear (we don’t

know whether it is the only wanderer

visible in that night) circles in three years,

not in 12 as Jupiter does here. From time

to time, it will shine about as bright as

Venus shines in these parts.

But shift our viewpoint out 10 times

as far as the 47 UMa system, and our

starry neighborhood is left behind. Stars

are plentiful, but only the most improb-

able stellar events—near collisions or

supernova explosions of unprecedented

proximity—much affect planets. Only

that central sun holds real power over

its small siblings, the planets. Given a

sun similar to our own, the orbits and

chemistry of the planets and the chancy

interactions among them are what de-

termine events. Big, nearby planets are

influential. They can eject a smaller

planet from orbit by rhythmic gravita-

tional disturbances. During the youth of

a star—the sun’s, too, we believe—plan-

ets, once formed, were often expelled as

orphans into space or into a fiery merger.

The most likely proposition is that

this Great Bear system, with its super-

Jupiter close in, has never been home to

any keen skywatchers. But we estimate

that there are tens of millions of other

planetary systems in this galaxy, a me-

tropolis of a few thousand star neigh-

borhoods like our own, a quarter-mil-

lion candidates in each. We do not know

the range of variety among those as yet

undetected planetary systems in our

vast spiral. We have plenty to learn.

Reviews and Commentaries108 Scientific American September 1998

Wonders, continued from page 106the building and certainly not one being

gradually pulverized for lime.

It took 13 years to transport the mar-

bles to England and sell them to the

British government for a bargain price

that would ruin the Elgin family for

two generations. But because Elgin was

in dire straits, he was in no position to

haggle. A promoter of the acquisition

deal was Sir Thomas Lawrence, painter

to the king and general artistic biggie.

He had started life as a child prodigy

and by this time, 1816, was charging

such eminences as the Duke of Welling-

ton and the Prince Regent an arm and a

leg to do their likeness. Already famous

back in 1792, when he was an associate

of the Royal Academy, he had also been

elected Painter to the Dilettanti Society,

entry to which required that you were

(a) an aristo and (b) had crossed the

Alps in search of culture. Lawrence

wasn’t and hadn’t, but he was a friend

of Sir Joseph Banks.

Banks was president of the Royal So-

ciety at the time and pals with the king,

so the Dilettanti rules were waived for

Lawrence (you get the idea). But Banks

did have what was considered an im-

peccable scientific background—that is,

born rich with good family connections

and an obsession for botany. So early

on, in 1768, it wasn’t hard for him to

win the coveted post of naturalist on

Captain James Cook’s first voyage of

exploration to the Pacific. Cook’s vari-

ous landfalls inspired Banks later to

promote the notion of Australia as the

absolutely perfect spot to dump convict-

ed criminals as well as to organize an-

other kind of transplantation: that of Ta-

hiti breadfruit trees to the West Indies.

The first ship sent on this food-aid

mission was the HMS Bounty, of muti-

ny fame. After the mutineers had set

Captain Bligh adrift in a rowboat, they

sailed the Bounty off into the blue, and

nothing further was heard of them until

their descendants were discovered 30

years later on Pitcairn Island, where lo-

cal scuttlebutt maintained that their

leader—the Bounty’s first mate, Fletcher

Christian—had secretly made it back to

England and a life of obscurity, thanks to

a passing American ship and its oblig-

ing master. He was Captain Folger, the

great-uncle of the man who founded

the company that made the coffee I was

enjoying at the start of this column.

Connections, continued from page 107

SA SA

ON SALE SEPTEMBER 24

Also in October. . .Unveiling the Galaxiesbehind the Milky Way

“Designer” Estrogens

Violations of Charge-Parity

Symmetry

The Hagfish

ESH

EL B

EN-J

AC

OB

COMING IN THE OCTOBER ISSUE. . .

SPECIAL REPORT

ON COMPUTER

SECURITY

SPECIAL REPORT

ON COMPUTER

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The Art ofMicroorganisms

The Art ofMicroorganisms


Working Knowledge Scientific American September 1998 109

W O R K I N G K N O W L E D G ECOMPACT-DISC PLAYER

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TIME

AM

PLIT

UD

E

a

b

c d

by Ken C. Pohlmann

Professor of Music Engineering Technology, University of Miami

The release of the compact disc

and its player in 1982 revolu-

tionized the audio world by

introducing optical digital technology.

Unlike the analog format of the old-

fashioned LP record, in which a stylus

traces a groove pressed into the record’s

surface, a CD player retrieves data by

shining a focused laser beam on the un-

derside of the disc. Because nothing ex-

cept light touches the disc, playing a

CD causes no more wear to the record-

ing than reading does to the words

printed on this page. Furthermore, the

audio quality of a CD is extremely

high, and any part of the disc can be ac-

cessed quickly for playback.

When music is recorded digitally,

sound is sampled and represented as a

series of numbers that measure the am-

plitude of the source signal. Thousands

of numbers are needed to describe even

a brief sound. These numbers are en-

coded in binary form (as strings of 0s

and 1s) and stored in the form of mi-

croscopic pits and smooth areas called

lands on the disc’s data surface. During

playback, the CD player’s laser light

reflects back from the rotating disc at

varying intensities as it strikes the pits

and lands. A photodiode array detects

these fluctuations, which are then trans-

lated into 0s and 1s. This binary stream

is decoded through demodulation and

error correction and converted back

into a variable electric signal, which is

amplified and played through head-

phones or loudspeakers.

Other types of CDs have also been

introduced, requiring specialized play-

ers. CD-ROM (compact disc read-only

memory) stores data primarily for read-

out through a computer, and other for-

mats allow one to record and erase in-

formation on a disc. The DVD format

is the successor to the CD and is al-

ready widely used to store feature films

with multichannel sound tracks. Both

DVD and CD have the same basic opti-

cal storage technology, but DVD offers

dramatically increased storage capacity.

SLIM

FIL

MS

PHOTODIODEARRAY

SEMI-SILVEREDMIRROR

FOCUSINGLENS

LABEL

LASERPOLYCARBONATEPLASTIC

PROTECTIVE ACRYLIC

ALUMINUM

LAND

PIT

DECODING THE DISC begins with thedata track’s pits and lands, which aretranslated into 0s and 1s (a). This binarystream consists of 16-bit strings, eachcoding for a numerical value between–32,768 and +32,767. These numericalvalues represent amplitude at pointsalong a wave (b). A digital-to-analogconverter takes the numbers and pro-duces a correspondingly variable elec-tric signal (c). This signal is amplifiedand fed into a loudspeaker, where it ischanged into sound waves (d).

OPTICAL PICKUPfocuses a semicon-ductor laser on the discand receives reflectedlight. If the laser lightfalls on a pit, the lightscatters. If the light hitsa land, or smooth area,it reflects back at ahigher intensity to thephotodiode array.

SEMICONDUCTORLASER

ELECTRIC SIGNAL

CD’S LAYERS include a polycarbonate data layer,which features billions of microscopic pits ar-ranged in a spiral. The aluminum reflects the play-er’s laser light. Depending on the playing time, a data spiral might contain three billion pits andstretch about five kilometers (three miles) long.


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