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A maverick's quest for quantum gravityMichael Duff

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Physics WorldDecember 2000

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CHRISTMAS BOOKS

Michael Duff

A maverick's quest for quantum gravityThree Roads to Quantum GravityLee

2000 Weidenfeld and Nicolson 196pp£16.99hb

There is one central quandary to 21 st-cen-tury physics. It is that the two main pillarsof 20th-century physics - quantum me-chanics and Einstein's general theory of rel-ativity-are mutually incompatible. On thesub-atomic scale, Einstein's view of gravityfails to comply with the quantum rules thatgovern the elementary particles, while onthe cosmic scale, black holes are threaten-ing the very foundations of quantum me-chanics. "Quantum gravity" is the name wegive to the solution to this problem, butthe way to achieve it and the question ofwhether such a solution even exists arethe subjects of hot debate. Three Roads toQuantum Gravity by Lee Smolin is a layper-son's guide to the different routes on whichtoday's theorists are currently embarked.

Smolin has himself worked in the field ofquantum gravity for the last 25 years, but itis fair to say that his ideas have always re-mained outside the mainstream. This bookis no exception. Indeed, the "true heroes ofthis story" are not the names that most prac-titioners would have chosen, such as Ed Wit-ten or Stephen Hawking, but rather othermavericks such as Alain Connes, DavidFinkelstein, Chris Isham, Roger Penroseand Raphael Sorkin.

So the reader should be aware thatSmolin's views are highly idiosyncratic andare by no means representative of currentthinking. This said, the book is written inthe same lively, anecdotal and readable stylethat those familiar with his previous book,The Life of the Cosmos, will have come to ex-pect (see "Holes in a final theory?" PhysicsWorld December 1997 pp39-40).

The first of the three roads that Smolinidentifies is superstring theory and its suc-cessor, M-theory. According to superstringtheory, the fundamental building blocks ofnature are not point-like elementary parti-cles, but tiny one-dimensional strings that livein a universe with one time dimension andnine space dimensions. Just like violin strings,these relativistic strings can vibrate, with eachmode of vibration representing a differentelementary particle. Most importantly, thesestringy particles include gravitons, the hypo-thetical carriers of the gravitational force.

Indeed, superstrings satisfy one of themain requirements of a consistent quantumgravity: we can calculate the probability oftransitions from one quantum state to an-other involving gravitons without encoun-tering the infinities and anomalies that haveplagued all previous attempts based on ap-

Road to nowhere? - simulation of a model thatrealizes some of the aims of loop quantum gravity

plying ordinary quantum field theory toEinstein's general relativity. Superstringsnevertheless subsume Einstein's theory, inthat it is recovered in the limit where theenergy of the gravitons is sufficiently small.Moreover, the superstring equations admitsolutions for which six of the nine dimen-sions are curled up to an unobservably smallsize so that the theory is compatible with oureveryday experience of a world with onlythree space dimensions.

Unfortunately, there is not one but fivemathematically consistent superstring the-ories, each competing for the tide of the"theory of everything"; clearly an embar-rassment of riches. This problem is cured byM-theory, a unique all-embracing theorythat subsumes the five superstring theoriesby requiring eleven space-time dimensionsand incorporating higher-dimensional ex-tended objects called membranes.

Among the achievements of M-theory isthe first microscopic explanation for theentropy of a black hole, first predicted in the1970s by Stephen Hawking using macro-scopic arguments. There is a cautionary talehere. In the years between the superstringrevolution of 1984 and the M-theory revo-lution of 1995, the mavericks who advo-cated eleven dimensions and membraneswere scoffed at by the orthodox superstringtheorists. So perhaps we should be more tol-erant of Smolin's version of unorthodox

speculations - although some of them, suchas the formation of new universes insideblack holes, will take some swallowing.

The second road Smolin describes is"loop" quantum gravity, the road that hehimself walked for many years (see "Thenew universe around the next corner" byLee Smolin Physics World December 1999pp79-84). According to this picture, if wewere to probe space-time at very smalllength scales, we would discover that it is notthe smooth Riemannian geometry of gen-eral relativity but quantized in tiny discretevolumes. The theory grew out of an ideain quantum chromodynamics (QCD), theaccepted theory of the strong nuclear force.

In a version of QCD due to KennethWilson, the fundamental entities are notfields but loops. Smolin and others attemp-ted a Wilson-loop description of gravity.Their motivation was to get away from theidea of fundamental quantum processestaking place in a fixed-background space-time and allow the space-time to be part ofthe quantum dynamics.

Critics of this approach will argue that al-though there is nothing wrong with Wilsonloops as a calculational technique, it mustbe applied to the right equations if it is toget anywhere. And although QCD no doubtprovides the right equations of the nuclearforce, the right theory of quantum gravityhas to go beyond the 1916 Einstein equa-tions upon which loop quantum gravity isbased. Perhaps not surprisingly, Smolin'sattitude is more forgiving, and he advocates asynthesis of these ideas and string theory.

The third road, trod only by the heroes,involves "wrestling with the fundamentalprinciples" and questioning the foundationsof thought and even of mathematical logic.

There is always a danger when followingthis road that it can easily become so neb-ulous as to invite the famous put-down ofWolfgang Pauli: it is not even wrong. To saythat die final dieory will be completely differ-ent from what we have now, with conceptswe have not yet even dreamed of, is an emptytruism. Much more can be learned from aconcrete proposal, however flawed, thanfrom a sterile contemplation of the quan-tum-gravity navel. Smolin is not one to heedsuch warnings, however. The more he canfeel mat his musings are "deep", "profound"and "philosophical", die happier he is.

Read this book. You will be informed,provoked and sometimes irritated - butalways amused.

Michael Duff is Oskar Klein professor of physics at

the University of Michigan, Ann Arbor, US, e-mail

mduff@umich.edu

PHYSICS WORLD DECEMBER 2000 51

CHRISTMAS BOOKSStringy stuff ties up best-seller lists

Top ten titles for 2000 at Amazon.co.uk1 The Elegant UniverseBrian Greene (Vintage)2 A Brief History of TimeStephen Hawking (Bantam)3 Just Six NumbersMartin Rees (Phoenix)4 In Search of Schrodinger's CatJohn Gribbin (Corgi)SHyperspaceMichio Kaku (Oxford Paperbacks)6 ChaosJames Gleick (Minerva)7 The Pleasure of Finding Things OutRichard Feynman (Allen Lane)8 Strange BeautyGeorge Johnson (Jonathan Cape)9 The End of TimeJulian Barbour (Phoenix)10 Lucifer's LegacyFrank Close (Oxford University Press)

Top ten titles for 2000 at Amazon.com1 The Elegant UniverseBrian Greene (Vintage)2 A Brief History of TimeStephen Hawking (Bantam Doubleday Dell)3 Just Six NumbersMartin Rees (Basic Books)4 Hypers paceMichio Kaku (Anchor)5 The End of TimeJulian Barbour (Oxford University Press)6 ChaosJames Gleick (Penguin USA)

Strange BeautyGeorge Johnson (Knopf)8 The Holographic UniverseMichael Talbot (Harperperennial Library)9 At Home in the UniverseStuart Kauffman (Oxford University Press)10 RelativityAlbert Einstein (Crown)

Columbia University stringtheorist BrianGreene can put his feet up this Christmas, safein the knowledge that his popular-science titleThe Elegant Universe is this year's best-sellingphysics book on both sides of the Atlantic,according Amazon, the on-line bookstore. Thebook already scooped the Aventis Prize forScience Books earlier this year.

"I think it is wonderfully gratifying thatideas about the universe, which thousandsof physicists worldwide have devotedthemselves to developing over many years,are now being widely shared with the generalpublic," Greene told Physics World. "I amsurprised at how well my book has done and Ifeel its success directly reflects the depth ofcuriosity that people have about the workingsof the universe."Matin Durrani• The Elegant Universe was reviewed inPhysics World by John Charap (July 1999 p43)

KurtHoselitz

Mysticism, physics and psychoanalysisThe Triad: The Physicists, the Analysts, the Kabbalists

Tom Keve2000 Rosenberger and Krausz 362pp£18.50/$29.50hb

This historical novel treats the developmentof both atomic physics and quantum theory .and of psychoanalysis from their early daysat the start of the 20th century to the out-break of the Second World War. It is difficultto review because the action moves about,both in terms of subject and location. Muchof the story takes place in Budapest, butwe are also taken to Manchester, Vienna,Berlin, Copenhagen, Princeton and else-where, before ending up in London. Thenovel is written in the form of an autobi-ography of Sandor Ferenczi, a Hungarianpsychoanalyst who was an admirer andpupil of Sigmund Freud. He was also per-sonally acquainted with many of the famouspeople mentioned in the book.

Although definitely a novel, the authorhas gone to a great deal of trouble to makethe events in the book authentic and true tohistory. Many original letters, publicationsand conference reports are quoted, and thefact that the author is a physicist who knowsand understands the scientific content of thebook, makes it read almost like a factual bio-graphical history. The fictional dialoguesand discussions are therefore almost as trueas if they were verbatim reports.

The book describes the intellectual atmo-sphere and the general ethos in the early partof the 20th century of the creators of psy-choanalysis and of atomic and quantum

Cafe society - the coffee houses of Budapest arethe scene for much of this latest physics novel

physics. The feelings and culture prevailingin central Europe - especially Hungary,where the author comes from — are broughtout. Famous chemists and physicists of theAustro-Hungarian empire, such as FritzPaneth, George von Hevesi and StephanMeyer of the Radium Institute in Vienna,come to life, as does their friendships withFreud and his family. We are also given aflavour of this cultured and intellectual com-munity with its ethos of the coffee-housesociety, including the Cafe Royal in Buda-pest, where many of the characters lived.

These people knew each other well, asmany of them were Jews who shared a com-mon family background as descendantsof great Rabbis, who themselves had beenstudents of the Kabbala. The Kabbala wasan ancient Jewish mysticism based on stud-ies of the Torah. It consisted of an amalga-mation of the principles of mathematicsand of natural science in the soul of the stu-dent. The author manages to show the closeconnection between this mysticism and thethoughts of some of the great scientists ofthe modern age.

The book begins with a "who's who" thatboth lists the personalities in the text andprovides a foretaste of what is to come. Thelist clarifies the family connections andfriendships, making it indispensable for thereader to gain a full understanding of thestory. Beginning with meetings of psycho-analysts, we soon enter into episodes withHevesi, who worked closely with Rutherfordand became one of the "band of physicists"around him and Niels Bohr.

The description of early meetings of Rut-herford and Bohr in Manchester in 1912 andthe beginnings of Bohr's model of the atomare recounted in an intimate and interestingmanner, as is Hevesi's interaction with Bohr.(Indeed, a letter from Hevesi to Bohr con-gratulating him on the detailed theory ofthe behaviour of the hydrogen and heliumatoms in August 1913 is quoted verbatim.)

The narrative continues with some of thehistory and ideas of the most notablefamous Rabbis, especially Chatam Sopherof Bratislava. There follow vivid accounts of

52 PHYSICS WORLD DECEMBER 2000