Astronomers claim to be terribly em-barrassed that they can’t find 90

percent of the matter in the uni-verse. But they usually say this with a bigsmile. Something swirls in the heavensand streams through our bodies withouta whisper; it cannot be seen; in short, it isas compelling a mystery as any scientistcould hope for. And the fun has notstopped despite the recent announcementby Italian physicists that they had found

the first hint of the elusive dark matter.The concept of dark matter resides in

the gap among three ways of weighingthe universe: a direct census of stars, dustand gas; measurement of the relativeamounts of light elements; and analysisof the dynamics of galaxies and galaxyclusters. The discrepancies indicate thatthe universe is filled with some kind ofextraordinary material.

As it happens, such matter would solveone of the biggest mysteries in fundamen-tal physics: Why do the fundamentalforces of nature—gravity, electromag-netism, and the weak and strong nuclearforces—vary so widely in strength? Forthe forces to differ, the quantum effectsthat tend to equalize them—namely, abattle between particles of force and par-ticles of matter—must be neutralized.One way to do so is supersymmetry,

which arranges a dynastic marriage be-tween the two particle families. The pho-ton (a particle of force) pairs off with a“photino” (a particle of matter), thequark with the “squark” and so on. Foreach known type of particle, physicistsbelieve there is a more massive “sparti-cle” that remains to be discovered.

The search for sparticles is a centralgoal of particle physics today. The easi-est one to find should be the lightest one,the “neutralino.” To have eluded detec-tion so far, it must be weak both in theusual sense of the word (that is, unlikelyto affect anything else) and in the techni-cal sense (able to interact only via theweak nuclear force). The particle shouldweigh 50 to a few hundred times asmuch as a proton—hence the monikerWIMP, for “weakly interacting massiveparticle.” (Imperfections in the big bangcould even have created “WIMPZIL-LAS,” a billion times heavier still.)

WIMPs are physicists’ best candidatefor the astronomers’ dark matter. By anintriguing coincidence, the number ofsuch particles that would have been cre-ated by the big bang approximatelyequals the amount of extraordinary darkmatter deduced by astronomers. The elu-sive neutrinos may also contribute to thisdark matter, now that they are known tohave a small mass, but alone they couldnot suffice; they are too footloose to haveseeded galaxy formation.

WIMPs are comparatively static,which is how the Italian group claims tohave found them. The team, known asDAMA and led by Rita Bernabei of theUniversity of Rome, relies on scintilla-tors, a type of particle detector that looksfor the light given off as particles strikeatoms. To pick WIMPs out from otherparticles, such as those given off by natu-rally occurring radioactivity, DAMAwatches for the seasonal fluctuation pre-dicted by American astrophysicists An-drzej K. Drukier, Katherine Freese andDavid N. Spergel in 1986.

Whereas the solar system orbitsaround the center of the Milky Way,WIMPs should have no organized mo-tion. To exert the gravitational influenceinferred by astronomers, they must bedistributed spherically, and revolutionwould distort that shape. Consequently,the solar system should encounter aheadwind of WIMPs—which should beslightly faster in June, when Earth’s or-bital motion around the sun adds to itsmotion around the galactic center, thanin December. The flux of other particlesshould remain constant. DAMA ob-

News and Analysis30 Scientific American March 1999

lab must rely on the time-honored obser-vational science of morphology. “I thinkmorphology will always have a place be-side DNA technology in our lab,” saysdeputy director Edgard O. Espinoza.“The techniques complement each oth-er.” In one memorable case, investigatorshad worked for days without luck tryingto extract DNA from a contraband boneto identify the animal. By chance, mam-mologist Bonnie Yates wandered by.“Look at that cool giraffe vertebra!” sherecalls exclaiming. Case solved.

Yates and her co-workers search forsubtle details of structure, color and pat-tern that are indicative of a given species,guided first by visual memory and laterby painstaking measurements. The dis-tinctive look of a crocodile hide immedi-ately gives away one $6,400 handbagseized from an upscale department store,for example, and the striking pattern of

python skin that lurks be-neath the black dye of anoth-er bag is easily recognized un-der infrared light.

The morphologist’s task be-comes more difficult when thesample is just a bit of an ani-mal processed into a commer-cial product and there are noclues to its geographic origin.A stool crafted from an ele-phant foot may be evidenteven to the untrained eye, buta pair of average-looking hik-ing boots fashioned from ele-phant hide, say, are less obvi-ous. “I had one person calland ask if I could do driednoses,” Yates chuckles. (She

could—the nose turned out to be from aserow, a protected Asian bovid that lookssomething like a goat.) Goddard esti-mates that in the end, the lab can deter-mine whether evidence comes from aprotected animal for about two thirds tothree fourths of the samples it receives.

As for the mysterious deaths of thewalruses, the scientists cracked that case,too. Their experiments indicated that theexposed, bleached neck bones of the de-capitated animals had been submergedin saltwater for weeks—meaning thatthe animals were first beheaded and thendumped in the water. The carcasses laterfloated up on the beach as they decom-posed. Another crime solved.—Mia Schmiedeskamp in Ashland, Ore.

MIA SCHMIEDESKAMP is a free-lance science writer based in Seattle,Wash.

REVENGE OF

THE WIMPS

Italian physicists have found themissing dark matter—or maybe not

PHYSICS

IGNOBLE END FOR BUFO MARINUS,

as a coin purse, did not violate animal-protection laws,but the fates of others identified by the Ashland lab did.

MA

RK G

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Copyright 1999 Scientific American, Inc.

served just such a variation during its firsttwo periods of operation. This suggests aparticle 60 times as massive as the protonand a trillion times less likely to interactwith other particles.

The result, though consistent withprevious theoretical and experimentallimits, has met with widespread skepti-cism. As Bernard Sadoulet of the Uni-versity of California at Berkeley,Jonathan R. Ellis of CERN and othershave argued, DAMA showed only thatthe energy of particles was higher inJune than in December. They have notyet looked for a decrease from Decem-

ber to June. Moreover, because the en-ergy resolution of the scintillators iscoarse, instrumental noise could mas-querade as WIMPs. Other physicists,such as Gilles Gerbier of the SaclayCenter of the French Atomic EnergyCommission, have complained thatDAMA has not provided them the rawdata needed for corroboration. “I’mcertainly not saying they are wrong,”Sadoulet says. “I’m asking them to pro-vide the proper evidence.”

Several competing teams are nowchecking the result using a different typeof detector, one that looks for the elec-

tric current that incoming particles letloose in a crystal of germanium. To dif-ferentiate particles internally, ratherthan rely on the controversial seasonaleffect, Sadoulet’s group also monitorsthe crystal for telltale vibrations set upby WIMPs as they hit atomic nuclei.Another system, used by Laura Baudisand her colleagues at the Max PlanckInstitute for Nuclear Physics in Heidel-berg, sorts particles by using multipledetector layers. If confirmed, the detec-tion of WIMPs would open up a wholeunseen universe. No one knows quitewhat to expect of it. —George Musser

News and Analysis34 Scientific American March 1999

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

Divorce, American-Style

The late social scientist Jessie Bernard of Pennsylva-nia State University once observed that “there aretwo marriages … in every marital union, his and

hers. And his … is better than hers.” The growing awarenessof this particular perspective among women most likelycontributed to the dramatic rise in divorce rates in the 1960sand 1970s, along with urbanization, the growing role ofwomen in the workforce and more liberaldivorce laws. But why is the U.S. the worldleader in divorce?

A possible explanation lies in the rest-lessness of Americans, who are far moreapt to migrate than, say, Europeans. Thosewho move, particularly a long distance,may be more likely to divorce because theinhibitions of traditional family and com-munity ties have been left behind. Divorcehas colonial roots, too: Puritan courtsgranted divorces, and disgruntled hus-bands and wives often simply abandonedtheir spouses.

The map shows the estimated propor-tion of Americans 18 and older who weredivorced as of March 1997. The reasons forthe marked regional disparities are notdefinitively known, but they probably re-flect several factors, including churchmembership, which may reinforce mar-riage ties. Not surprisingly, therefore, Flori-da and most of the western states, wherechurch membership is low, have a higherproportion of divorced people. Migrationmay contribute to the high proportion of divorced people inthe West and Florida, which have a larger proportion of peri-patetic individuals than other areas have. The broad swath ofcounties stretching from North Dakota and Wisconsin downto the Rio Grande is an area with few divorced people, whichmight be expected in view of high church membership andthe relatively few migrants to this area. The low prevalence ofdivorce in Virginia, North Carolina and South Carolina maystem in part from fairly high church attendance. The huge tri-

angular area with its apex in Michigan and its base from east-ern Texas to southern Georgia shows a mixed pattern in theproportion of divorced people. This area has wide variationsin migration.

There is little doubt that divorce rates rose sharply in the1960s and 1970s, but there have been some difficulties in in-terpreting divorce statistics since the early 1980s. Larry L.

Bumpass of the University of Wisconsin, who has done themost extensive work on this point, concludes that the divorcerate has stabilized in the past two decades. As of March 1997,the U.S. had more than 19 million divorced people, or 9.9 per-cent of those 18 and over. The median age of divorced peopleis about 50, and 58 percent are women. Among whites, 9.8percent are divorced, compared with 11.3 percent of blacksand 7.6 percent of Hispanics. Divorce rates in urban areas arehigher than in rural areas. —Rodger Doyle (rdoyle2@aol.com)

LESS THAN 8 PERCENT

ESTIMATED PROPORTION OF AMERICANS 18 AND OVER WHO WERE DIVORCED AS OF MARCH 1997

SOURCE: Estimates based on 1990 U. S. Census data by county and 1997 Bureau of Census data for the U. S. Because the method of estimation is subject to substantial error, data for individual counties may not be accurately coded;however, the broad regional patterns are believed to convey an accurate pattern.

8 TO 9.9 PERCENT 10 PERCENT OR MORE

ROD

GER

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YLE

Copyright 1999 Scientific American, Inc.