The end of time: the next revolution in our understanding of the universe: Julian Barbour, Weidenfeld and Nicholson, London, 384 pp., $16.95, ISBN 0195145925

phenomena persuaded physicists that the Ising model was not needed, in partbecause an erroneous conjecture by Ising himself convinced people that the failure ofa one-dimensional Ising model to capture ferromagnetic properties was also true oftwo- and higher-dimensional models. An interesting analogy might usefully bedeveloped between this case and the proof by Minsky and Papert that single-layerperceptrons cannot calculate simple mathematical functions such as parity. Their‘intuitive judgement’ that the extension to multiple-layer networks was sterileeffectively halted the development of neural net theory for fifteen years.

This collection is a valuable addition to the growing literature on models. I haveraised certain criticisms here, but if willing to think differently about how scientificknowledge is applied to the world, the reader will learn much from reading theseessays.

Thanks to David Freedman for comments on an earlier draft.

References

Axelrod, R. (1997). The complexity of cooperation: Agent-based models of competition and collaboration.

Princeton: Princeton University Press.

Epstein, J. M., & Axtell, R. (1996). Growing artificial societies: Social science from the bottom up.

Cambridge, MA: The MIT Press.

Hand, L. N., & Finch, J. D. (1998). Analytical mechanics. Cambridge: Cambridge University Press.

Humphreys, P. (2003). Extending ourselves: Computational science, empiricism, and scientific method, in

press.

Maynard Smith, J. (1982). Evolution and the theory of games. New York: Cambridge University Press.

Skyrms, B. (1996). Evolution of the social contract. New York: Cambridge University Press.

Paul HumphreysCorcoran Department of Philosophy, 512 Cabell Hall, University of Virginia

Charlottesville, VA 22904-4780, USA

E-mail address: [email protected]

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The end of time: the next revolution in our understanding of the universe

Julian Barbour, Weidenfeld and Nicholson, London, 384 pp., $16.95, ISBN0195145925

The life of the cosmos

Lee Smolin, Oxford University Press, New York, 358 pp., $16.95, ISBN 0195126645

Just six numbers: the deep forces that shape the universe

Martin Rees, Weidenfeld and Nicholson, London, 208 pp., $14.00, ISBN0465036732

In the early part of this century, physicists, led notably by Albert Einstein and thepioneers of quantum theoryFin particular Neils Bohr, Werner Heisenberg, and

Book reviews / Studies in History and Philosophy of Modern Physics 33 (2002) 357–385 377

Paul DiracFdiscovered that the underlying nature of physical reality is strangerthan anyone had ever imagined. A series of brilliant insights led to the realisation, onthe one hand, of the relative nature of space and time measurements, and hence ofour basic concepts of space and time (ultimately leading to the discovery of nuclearenergy), and on the other hand, of the quantum nature of matter, with its associatedquantum statistics and uncertainty of prediction (leading to transistors and lasers).Combining these views ultimately led to a realisation of the necessity of the existenceof anti-matter, and of the dynamic nature of the vacuum. Further developments ledto an understanding of the existence of symmetries characterising the variousfamilies of elementary particles, and of the unified nature of the fundamentalinteractions when described as gauge theories with forces mediated by exchange ofgauge bosons. These properties have all been confirmed by carefully controlledexperiments.

This series of successes has opened the floodgates of speculation, with theoreticalphysicists engaging in ever more daring hypotheses about the nature of reality in thehope of unlocking ever-deeper aspects of its nature. However, there is a fundamentaldifference from what has gone before: almost all of these later speculations have notbeen tested, and most may not even be testable. Thus grand unified theories,supersymmetry, superstrings, M-theory, loop quantum gravity, chaotic inflation,quantum cosmology, pre-big bang cosmology, and brane universes, to name but afew, are daring speculations that may reflect the true nature of the universeFbut,then again, they may not. Indeed, they cannot all be correct, for they are oftencompeting accounts of the nature of reality. (For example, there are now at least 30competing inflationary accounts of the early history of the universe.)

The three books under review represent a spectrum of such speculations relating tocosmology and the nature of the universe. All three are similar in that each, on theone hand, gives an interesting and informative account of our current understandingof the way the universe is organised that is presented in a well-written, richlytextured, and informative narrative. On the other hand, each book stronglyadvocates an unverifiable theory of ultimate reality as a fundamental way of solvingthe major theoretical problems facing us. The book by Rees is the most conservative;the books by Barbour and Smolin are linked in sharing strong relational views aboutthe nature of the universe (indeed, the authors have had a profound influence oneach other). All the three books are non-technical (i.e., they are without equations),but all contain serious, well thought out presentations of major parts of modernphysics. Barbour and Smolin also interweave the text with interesting accounts oftheir personal journeys of scientific discovery, and people they have encounteredalong the way.

Barbour’s proposal is at once the most revolutionary and the most difficult tocomprehend. It is focused on the nature of timeFone of the least understood aspectsof physics, and certainly worthy of deep consideration. In a daring move, he deniesthe obvious by claiming that time is an illusion: ‘‘We shall come to see that time doesnot exist y quantum gravity will yield a static picture of the quantum universe y

we must go to a deeper reality in which nothing at all, neither heavens nor the Earthmoves. Stillness reigns’’ (p. 14). Later on, Barbour observes that ‘‘a small but

Book reviews / Studies in History and Philosophy of Modern Physics 33 (2002) 357–385378

growing number of physicists have begun to entertain the idea that time truly doesnot exist. This also applies to motion: the suggestion is that it too is pure illusion. Ifwe could see the universe as it is, we should see that it is static. Nothing moves,nothing changes’’ (p. 39).

Barbour’s move is based upon both philosophical and physical considerations.Philosophically, he states, ‘‘I believe in a timeless universe for the childlike reason thattime cannot be seenFthe emperor has no clothes’’ (p. 251). Curiously, he does notapply the same argument to many other concepts where it seems equallyvalidFenergy, the electro-magnetic field, or the gravitational force, for example. Inphysical terms, his argument is based on an interesting analysis of the relationalnature of time measurements, on the ‘‘block’’ view of the universe suggested byrelativity theory (where we view space–time as a single unit rather than as a successionof times), and on the Wheeler-de Witt equation of quantum cosmologyFwhich is anequation governing the wavefunction of the universe. Because the Hamiltonian ofGeneral Relativity vanishes, this equation predicts that the wavefunction thatdetermines the probabilities of what happens should be time-independent. Startingfrom this point, Barbour essentially elevates the time-independent Schr .odingerequation to a more fundamental status than that of the time-dependent equation, andbases his views on the resulting time-independent theory.

Now this is obviously a rather precarious thing to do, in view of the way that theapparent passage of time dominates macroscopic physics, chemistry, biology, andeveryday life. Barbour has to explain all of this as an illusion conjured up by themind, which reads the records of apparent events in a particular orderFa move thatleaves at least as many issues obscured as it answers. Barbour’s move is one of greatcourage, but puzzling in that it is based on a controversial equation which (as hehimself admits) is very problematic; indeed, the equations at issue are not even well-defined (see pp. 38–39). Moreover, his move is based upon a particular version ofquantum theory that is untested in the domain to which it is applied. In physicalterms, the move addresses the problem of measurement (which is nicely described inthe book) by adopting the very controversial many-worlds interpretation ofquantum mechanics, with its infinitely branching structure of reality. Thisinterpretation is necessary because only the Wheeler-de Witt equation (with its fullpredictability and time reversibility) occurs in the theory. A more conventionalinterpretation would add to this deterministic equation a process of quantummeasurement or wavefunction collapse implying changes in the wavefunction withtime that would break both the time reversibility and the predictability of the theory(cf. Penrose, 1989). On a macroscopic scale, taking into account the phenomenon offree will would have the same effects. The relation of the theory to the concept of‘‘records’’ or ‘‘time capsules’’ that are supposed to underwrite it, as well as thetheory’s relation to current quantum cosmological ideas, is analysed in detail inButterfield’s (2001) recent lengthy review. This review ends by concluding thatquantum gravity, as developed so far, does not provide any evidence for Barbour’sdenial of time.

The appeal of Barbour’s theory is that, in the end, it is philosophically based in arelational view of the nature of time, developing to its limits a theme proposed by


Ernst Mach, Albert Einstein, John Wheeler, and Barbour himself. To some, these arecompelling philosophical arguments; but in the end, the issue should be whetherthese ideas, notwithstanding their intrinsic attractiveness, relate to the physics of theactual universe around us. So, I suppose, ultimately one must simply ask for someexperimental consequences of Barbour’s theory that we can check. If some crucialtests could be made and were verified, one might have no choice but to go along withthis view, radical as it is. But as far as I can see, there are no such experimentalimplications proposed; indeed, Barbour gives us no experimentally testableconsequences whatever of his proposed worldviewFso the pickings are slim indeed,in view of the price of abandoning common sense about one of the mostfundamental physical issues of all: the apparent irreversible passage of time at themacroscopic level. I cannot imagine many physicists or philosophers followingBarbour down this stony path.

What is more, Barbour’s approach ultimately involves sneaking in a standardconcept of time through the back door. For the reading of records by the mind insome order presupposes a concept of time operational in the mindFwhich surelycannot occur if the brain is part of a static, unchanging world. Barbour is exemptingthe mind from the physics that he proclaims governs the way things work. He makesgreat play of what he calls ‘‘time capsules’’, that is, static records of events in thepast; but he does not explain why the brain reads them in a continuous and uni-directional manner, when according to his view, the brain too must be static if indeedits operation is based upon the laws of physics (as most neuroscientists suppose) andtherefore, in his view, unable to ‘‘read’’ anything at all. He states ‘‘it is well knownthat much processing goes on in the brain, and employing normal temporallanguage, we can confidently assert that what we seem to experience in one instant isthe product of the processing of data coming from a finite span of time’’ (p. 266) and‘‘my brain contains at any one instant several ‘snapshots’ at once. The brain,through the way in which it presents data to consciousness, somehow plays themovie for me’’ (p. 267). But, of course, neither ‘‘processing’’ nor ‘‘presenting thedata’’ can happen in a fully static world, for these imply some change in the state ofthe brain that takes place in time. You can’t solve the problem that way if the brain istruly part of a physical world with the features that Barbour attributes to it.Returning to the quote given above, ‘‘If we could see the universe as it is, we shouldsee that it is static. Nothing moves, nothing changes’’ (p. 39), the fundamentalinconsistency in Barbour’s approach is again apparent: the act of ‘‘seeing’’ takesplace in time; yet Barbour is again allowing the human mind to transcend the basicworldview that is supposed to govern the way nature is. In addition, Barbour’s viewapparently fails to explain the inviolability of the direction of time. If time is indeedrelational, why can’t we read the records of events in a different order and, indeed,choose whether to go forwards or backwards in time? What constrains the mind tobehave in the particular uni-directional way we experience? Until these questions areanswered, Barbour’s theory (like that of Price, 1996) simply displaces the majorproblems of time from fundamental physics to the human mind, and leaves themthere, unanswered.


The End of Time is flamboyantly presented as ‘‘The next revolution in ourunderstanding of the universe’’ (and I must admit to reacting against the subtext ‘‘Iunderstand more than Einstein’’; see pp. 136, 164). A skeptic will rather view it as adaring but ultimately flawed exercise in speculation. It is arrived at by taking to itslogical conclusion the implications of certain hypothesized but untested equations offundamental physics. Given the bizarre nature of the consequences drawn, combinedwith its evident failure to explain our experience of time, the more plausibleinterpretation of these deductions is that they show the proposed underlyingequations must either be false, or at least incomplete. Perhaps the point is that theseequations, as usually presented, fail to take self-interaction into account, assuggested by Rohrlich (2000); or, maybe it is because these equations are not thewhole story, either because they ignore ‘‘collapse of the wavefunction’’, or because,being differential rather than integral equations, they represent bottom-up effectsbut do not make explicit top–down effects (which certainly are of major importancein the real universe, and should resonate profoundly with the relational viewpointthat Barbour propounds); or, maybe, as suggested by Prigogine (1996), somemore fundamental revisions of our understanding of fundamental physics arerequired in order to make it more compatible with the time-irreversibility ofmacroscopic life. I would suggest that all these avenues be thoroughly exploredbefore one follows the unlikely path suggested by Barbour, even though that pathdoes resonate in a way with certain mystical religious views that also propose thattime is an illusion.

Barbour summarizes his ideas as follows: ‘‘The central insight is this. A classicaltheory that treats time in a Machian manner can allow the universe only one value ofits energy. But then its quantum theory is singularFit can only have one energyeigenvalue. Since quantum dynamics of necessity has more than one energyeigenvalue, quantum dynamics of the universe is impossible. There can only bequantum statics. It’s as simple as that!’’ (p. 253). Thus, Barbour assumes that all thepremises of this argument are correct, and adopts the resulting strange outlook. Iprefer to run the argument the other way about. Since the outcome is so improbable,and indeed contrary to our experience, at least one of the premises of the argumenthas to be wrong (for example, quantum theory may not apply in this specific form tothe universe as a whole). In fact, even the author does not seem to truly believe hisown theory. Earlier he emphasizes ‘‘In a timeless world, verbs of becoming like‘happen’ have no place’’ (p. 45). Yet, later he suggests that creation takes place allthe time: the apparent persistence of objects is an illusion, for they never remainexactly the same, and so, what appears to be the same object at different times is, infact, a different object at each instant, created anew, almost identical to what wasthere before (pp. 229, 251–252).1 Thus, the apparent continuity of things is illusory:things do not exist continuously. Instead, creation is continuously taking place,

1This reminds me of the eccentric old man who called the police in great excitement to report a burglary:

everything he owned had been stolen. On arrival, everything seemed in perfect order. His reply to the

puzzled police was, ‘‘Well, you see, they replaced everything with identical replicas.’’ This is essentially

Barbour’s argument, and is similarly both irrefutable, and beyond credibility.


rather than having taken place once-for-all at the origin of the universe.Unfortunately, the idea of creation simply makes no sense in a fully static universe.In that situation, things either exist or don’t exist, but they can’t ‘‘be created’’ in anymeaningful sense, for that is a verb related to the progression of time. In advancingthis idea, the author is abandoning his central premise of nothing ever changing.Coming into and out of existence is as radical a kind of change as one can have!

To sum up: my differences with the author’s understanding of the nature of timeare wide-ranging and fundamental. Still, the issues he raises are important, and areraised in a thought-provoking manner. In particular, it is most useful to have theargument for Barbour’s extreme view, based on a particular approach to quantumcosmology, laid out in this way.

Smolin’s densely written book is radical in a rather different sense. He gives acomprehensive summary of the present ideas of particle physics and cosmology whilerelating his discussion to major philosophical issues. He moves from asking ‘‘What isthe universe?’’ and ‘‘What is life?’’ to a rather thin chapter on ‘‘Philosophy, Religion,and Cosmology’’ (one cannot really do justice to these themes in such a short space)and, from there, to a useful discussion of the anthropic principle in its various guises.Like Barbour, he strongly supports a relational view of the physical world (and,indeed, he was strongly influenced by Barbour in adopting this view).

Smolin’s theme is that a theory of quantum cosmology cannot be consistent unlessit describes a complex, self-organising universe; thus, he links two rather differentmajor themes of modern physics in an interesting way. His central proposal is amodification of a common suggestion: if matter locally collapses to form a blackhole, as yet unspecified, quantum field effects could lead, on the other side of theevent horizon (and hence hidden from the parent universe region), to a change fromcollapse to a re-expansion into a new space–time region, but with marginallydifferent values of physical parameters. The crucial observation is that a form ofDarwinian selection then comes into play: namely, those space–time regions wherethe constants of physics are such as to maximise production of black holes will cometo dominate, because they will succeed in producing more progeny (‘‘babyuniverses’’) than others. Thus, a process of natural selection, in the sense ofevolutionary biology, will result in an effective ‘‘tuning’’ of the fundamentalconstants of nature so that they take values that maximise black hole productionthrough astrophysical processes. The universe region in which we live should thencorrespond to such values of those constants. Thus, Smolin proposes using aprinciple of self-organisationFnatural selection in the Darwinian senseFas acentral theme in cosmology.

This is a very interesting blend of ideas, giving a nice twist to the conceptsembodied in the broad family of chaotic inflationary universe models. Smolincombines two of the major ideas of modern science (the evolving universe idea andDarwinian evolution) in a new form, enabled by two other major recent discoveries(the formation of black holes, and the possibility of negative energy fields arisingthrough quantum field theory). This integrative theme yields a fundamentallydifferent perspective on the universe, but depends on a series of unproven physicalassumptionsFmost particularly, (a) that the present dispensation does indeed


maximise black hole production, and (b) that the dispensation with this result willalso allow complex life forms to existFa far from obvious conclusion. Additionally,(c) the re-expansion mechanism is hypothesized rather than modelledFit ispresumably possible in terms of our present understanding of quantum fields, butone would like to see the details spelled out. Overall, the idea is intriguing andinnovative; indeed, it is one of the most creative accounts I have seen of the possiblebasis of cosmology. Yet many of the details needed to turn Smolin’s ideas into a well-defined physical theory are missing. And it is very unclear how the full nature of hisproposal can be tested. He does link it, to some degree, to observations. However,the key problem is that there is no obvious way to show that the central theme of theprojectFphysics leading to re-expansion after collapse to a black holeFis actuallycorrect. Until well-defined observational tests of this aspect are articulated, it shallremain a metaphysical rather than solid astrophysical proposal. But it certainlydefines a very worthwhile research programme that should be pursued with vigour.This book provides a strong motivation for such a theory, and many ideas on how itmight be developed.

Rees’ book is the most conservative, but in the end it too is propounding anunprovable idea. He gives a superb, concise overview of present-day cosmologicalunderstanding, and the way it depends on the values of specific cosmologicalparameters: two related to the basic forces, two fixing the size and texture of theuniverse, and two fixing the properties of space itself. Specifically, the parameters areN; the ratio of electrical to gravitational forces; E; nuclear fusion efficiency; O; whichdetermines the amount of matter in the universe relative to the critical density forrecollapse; l; the cosmological constant (or ‘‘dark repulsive energy’’); Q; the bindingenergy of clusters and superclusters of galaxies; and D; the dimension of space. Reescarefully relates each of these numbers to properties of the observed universe (theexistence of atoms, planets, and stars, for instance), discussing such issues as darkmatter and the evidence for an accelerating universe (indicating a non-zerocosmological constant). He also shows how particular values of these constantsare crucial to the way the universe is, and, in particular, to the formation of structurein the universe.

Rees then considers the fundamental metaphysical issue of what underlies theexistence of a favourable ‘‘cosmic habitat’’ for life and, thus, why these six numbershave the favourable values we measure today (the ‘‘fine-tuning’’ problem): is it due tocoincidence, providence, or a ‘‘multiverse’’? He opts for the latter, hypothesising thephysical existence of an infinite ensemble of universes, in order to provide a basis forexplaining the present day values of these parameters: ‘‘Our Big Bang may not havebeen the only one. Separate universes may have cooled down differently, ending upgoverned by different laws and defined by different numbers y [this] is a naturaldeduction from some (albeit speculative) theories, and opens up a new vision of ouruniverse as just ‘one atom’ selected from an infinite multiverse’’ (p. 150).

Rees presents the argument in an interesting and, indeed, persuasive way.Nevertheless, in terms of the basic criteria for a theory, Ockham’s razor has gone outthe windowFan infinite ensemble of universes is hypothesized to explain the singleuniverse we can actually observe. Additionally, it is not a well-defined move until one


has precisely defined the nature of this ensemble, and produced a solidly basedmetric of probability on itFwhich has never been done. However, this metaphysicalmove is the only ‘‘scientific’’ approach to ultimate causality that gives some kind ofproper foundation for applying probability arguments to the concept of the universe(essentially, by denying the uniqueness of the universe and, hence, all the problemsassociated with that uniquenessFcf. Ellis, 1999, 2000). The price is that one has alsoabandoned any solid relation to testability, and hence the scientific (as opposed tometaphysical) status of what is proposed becomes dubious.

The problem is simple. I can presuppose any properties I like for my infiniteensemble of universes, and you can propose any others for yours. Yet we cannotdecide between these proposals observationally, and there is no scientific way toprove what those properties are, as the ensemble is not available for inspection orinvestigation. In particular, you may assume that something like the usual laws ofphysics hold throughout your ensemble, together with the usual laws of logic, andI may not. (I might, for example, envisage an ensemble in which many universes arecompatible with the kind of magic and wizardry associated with Harry Potter.) Noone can determine which proposal is correct, if indeed either is. We may succeed indefining an ensemble that will make testable predictions in a probabilistic way,and hence claim we had verified that such-and-such a particular kind ofensemble actually existed; but this is a very indirect kind of ‘‘proof’’. We will never

be able to experimentally test any proposed physical laws underlying such aproposal.

To make a reasonable case, someone would have to show that their pet ensemblehypothesis was the only ensemble, or family of ensembles, that was possible. Butsince we cannot, for example, assume the usual laws of logic would hold for allmodels in the ensemble, this would seem a rather hard case to make, for we may wellseverely disagree on the meaning of ‘‘what is possible’’ in this extreme context, andwe cannot assume that our usual laws of argumentation would hold in thisdiscussion. Moreover, given the ‘‘impossibility’’ proofs of Godel, Turing, andChaitin (see Chaitin, 1999), even if we could empirically determine the nature of suchan ensemble, and disregarded the insoluble issue of verifying that it existed, we couldnever prove thatFas a logical conceptFthe ensemble had the unique propertiesdesired. Further, even if one could do all this, in the end it leaves open yet again theunderlying ultimate question: why has this particular ensemble been instantiated,rather than another one that would issue in different properties on average(supposing one could give a well-defined meaning to that idea)? In other words, theissues forcing a choice between coincidence, providence, or a ‘‘multiverse’’ would stillnot have been laid to rest.

Thus, while the idea of an ensemble of parallel worlds is well motivated and worthdeveloping in depth, it seems that major questions will inevitably remain about itsrelation to testability and observation. Nevertheless, the reader is sure to find Rees’spirited defence of the scientific credentials of this idea to be both educative andintriguing.

All these books are well written and thought-provoking. They each indicate waysthat physics might proceed in the future, and are each very useful, both in terms of


their descriptions of present day fundamental physics and its relation to cosmology,and in terms of opening up our minds to interesting possibilities. Yet the reader will,in each case, also need to continually question the degree to which these intriguingideas are susceptible to experimental test. Insofar as that is not possible, we are beingpresented with metaphysical speculations rather than testable scientific theories. Thisis a necessary step when one addresses ultimate issues, as these books attempt to do;but it must be identified for what it is. The need in the future is to make thisdistinction very clear, and to develop sophisticated philosophical approaches thatwill help us successfully tread this slippery terrain, and sensibly choose between thetheories being presented to us in a context where they are indeed not testable in anyordinary sense.

References

Butterfield, J. (2001). The end of time? Forthcoming in British Journal for Philosophy of Science. Also at:

Los Alamos archive gr-qc/0103055; and at PITT-PHIL-SCI00000104.

Chaitin, G. (1999). The unknowable. Berlin: Springer.

Ellis, G. F. R. (1999). The different nature of cosmology. Astrophysics and Space Science, 269–270,

693–720.

Ellis, G. F. R. (2000). Before the beginning: Emerging questions and uncertainties. In D. Block, I. Puerari,

A. Stockton, & D. Ferreira (Eds.), Toward a new millenium in galaxy morphology. Dordrecht: Kluwer

Academic Publishers.

Penrose, R. (1989). The emperor’s new mind. Oxford: Oxford University Press.

Price, H. (1996). Time’s arrow and Archimedes’ point. New York: Oxford University Press.

Prigogine, I. (1996). The end of certainty: Time, chaos, and the new laws of nature. New York: The Free

Press.

Rohrlich, F. (2000). Causality and the arrow of classical time. Studies in History and Philosophy of Modern

Physics, 31B, 1.

G.F.R. EllisDepartment of Mathematics and Applied Mathematics

University of Cape Town, Private Bag

Rondebosch 7701, South Africa

E-mail address: [email protected]

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Documents

The end of time: the next revolution in our understanding of the universe: Julian Barbour, Weidenfeld and Nicholson, London, 384 pp., $16.95, ISBN 0195145925