Space-Time Turn Around Lee Smolin Marvels at Roger Penrose’S Masterly and Imaginative Argument That Our Universe Is One of a Succession

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cosmology Space-time turn around Lee Smolin marvels at Roger Penrose’s masterly and imaginative argument that our Universe is one of a succession.

o living physicist has yet made a reassemble, for example — he lays out his His work in the discovery as great as those of Isaac thinking on the origin and fate of the Uni- 1960s on quantum Newton or Albert Einstein, but verse. Penrose addressed this problem in his gravity has borne NRoger Penrose is in a better position to do first popular-science book, The Emperor’s dramatic fruit within so than most. Combining a mastery of math- New Mind (Oxford University Press, 1989). the past five years. ematics with trust in his own research com- His latest volume describes a new way of Penrose introduced pass, Penrose — a mathematical physicist resolving that problem. It is an astounding two influential con- at the University of Oxford, UK — is driven idea, which, if true, would revolutionize cepts: spin networks, by a heroic obsession to understand fun- physics and cosmology. Cycles of Time: which in 1988 seeded damental puzzles about nature. The depth We should pay attention because Penrose An Extraordinary an approach called of his thinking and fertility of his creativity has repeatedly been far ahead of his time. The New View of the loop quantum gravity; concerning the mathematical foundations of most influential person to develop the general Universe and twistor theory, a modern physics place him above his peers. theory of relativity since Einstein, Penrose RogeR PenRose recasting of space- In Cycles of Time, Penrose introduces established the generalized behaviour of space- Bodley Head: 2010. time geometry that 320 pp. £25 his most outrageous and subtle idea yet. time geometry, pushing that theory beyond has generated a recent Answering the question of why the future special cases. Our current understanding of breakthrough in our understanding of gauge is so different from the past — why eggs black holes, singularities and gravitational theories, the basic ingredients of the stand- crack into pieces that never spontaneously radiation is built with his tools. ard model of particle physics.

Readers will not be disappointed with the matter — and hence energy from ordinary Neil Turok and their colleagues. But these audacious ideas in his latest book. It starts stuff — wanes. But the ‘dark energy’ asso- exotic proposals involve theories of quantum with a masterful explanation of the direc- ciated with the vacuum of space remains gravity, which Penrose has no need for in his tionality of time. A gifted popularizer of constant (at least in simple models of it) and hypothesis. science, Penrose skilfully breaks the normal eventually dominates. Dark energy accel- rules by including equations and describing erates the expansion, further diluting the inflation popped subtleties and uncertainties. He is honest matter. All black holes will evaporate and Penrose’s proposal has another advantage, too, clearly distinguishing established sci- any other space-time features will be ironed in common with other hypotheses that ence from his own speculations, and relat- flat by the exponential expansion. Stars and eliminate the singularity. It suggests that ing opposing views and alternative ideas galaxies will dissemble if, as Penrose postu- before the Big Bang, there would have been with balance. lates, elementary particles eventually decay plenty of time to set up the correlations seen Penrose then sets out his proposal. It rests to photons and other massless particles. in observations of the cosmic microwave on the puzzle that the apparent initial state If these hypotheses are true, then at very background and distributions of galax- of the Universe is highly improbable — a late times the Universe will look a lot like it ies. Consequently, there is no need for the quandary he has emphasized for years. By did at very early times — its spatial geometry hypothesis of rapid inflation of the Universe running the laws of physics backwards from is homogeneous and flat, and it is filled with very early in its history. This is potentially the Universe’s present state, we can work out a gas of photons. There is one difference: a good thing, because inflation is hard to what it looked like just after its birth. But the temperature and density of the early stop once it is started, and can easily lead to given all of the possibilities a multiverse with an infi- conjured up by physics, it nite number of universes is extremely unlikely that a like our own. randomly picked universe it is possible that our early universe is The multiverse scenario will resemble our own. raises challenges because The initial state of our the late universe of a previous era. the explanation for why Universe is special, Pen- this is penrose’s big idea: our Universe is like it is rose argues, because it is must then rely on untest- simultaneously very hot able assumptions about an and very cold. The mat- deliciously absurd, infinite ensemble of unob- ter and electromagnetic but just possibly true. servable universes. This in radiation are exceedingly turn raises puzzles about hot, at a temperature that applications of probability, approaches infinity as we go back in time to Universe differ by an enormous factor from and requires use of the anthropic principle — the singularity of the Big Bang. But because its end point. This can be understood as a further decreasing the empirical content of the there is no energy in gravitational waves, he change of scale, such that an act of compres- theory. The anthropic principle posits that our says, the geometry of space-time has a tem- sion — by a vast factor — could turn the late Universe is one among a vast ensemble, most perature of essentially zero. Both extremes Universe into the early one. of which cannot contain life. Because one is mean that we can simplify our description of Penrose pulls one more trick out of his hat: free to make arbitrary hypotheses about the the state of the Universe. the insight that physics in both the early and other universes, which are neither observable late regimes is insensitive to scale. Briefly, nor need be like our own, almost any property cool geometry this is because massless particles move at the of our Universe can be explained away. All of At extremely high temperatures, the ele- speed of light, at which point time stands still these problems are avoided by hypotheses mentary particles that comprise matter and for them. Because there is no clock ticking, such as Penrose’s that invoke a succession of radiation are indistinguishable and their there is no reference against which they can universes rather than an unobservable infinite interactions negligible because their energies measure a scale of length or time. simultaneous plurality. are tiny compared with the Universe’s heat. So if the only difference between the very Despite this, inflation has so far proved The newborn Universe is essentially a hot early and late Universe is scale, and physics successful in accounting for the observed pat- gas of photons, and everything that happens in both of these extremes is insensitive to terns in the cosmic microwave background. to that gas is determined by one number: its changes of scale, then it is possible that our The challenge of scenarios of succession such temperature. The coldness of the space-time early Universe is the late Universe of a previ- as Penrose’s is to account for those observa- geometry also means that we can simplify its ous era. This is Penrose’s big idea: deliciously tions and make a prediction that differentiates structure — at zero temperature there are no absurd, but just possibly true. Moreover, it it from inflation. Then experiment can decide. black holes and space is uniform. doesn’t matter if such a transition took an Penrose’s proposal therefore needs develop- Penrose argues that the direction of time eternity — photons are insensitive to the ment and reflection as a scientific idea. is explained by the evolution of the Universe passage of time. Cycles of Time starts off as a masterpiece from this special, simple and improbable Penrose’s concept joins several other of pedagogy and becomes more challeng- state to more probable ones. The unfolding of proposals, such as loop quantum cosmol- ing as the book progresses. But it is worth increasing numbers of random events drives ogy, that replace the Big Bang singularity reading to see Penrose’s extraordinary mind the arrow of time. This is an expression of and allow time to run before the Big Bang working to confront one of the fundamental the familiar second law of thermodynam- occurred, suggesting our Universe is the puzzles of our present understanding of the ics that randomness — or entropy — tends progeny of a previous one. Other ingenious Universe. ■ to increase. The problem of explaining the mechanisms for making the history of the arrow of time is then reduced to the question Universe cyclic — so Lee Smolin is a faculty member at the of why the early Universe was so special. that it repeatedly swells nature.com Perimeter Institute for Theoretical Physics, Penrose tries to answer this by turn- and contracts — have for hawking on the Waterloo, Ontario N2L 2Y5, Canada, and ing from the very early Universe to its been proposed by physi- multiverse, see: author of The Trouble with Physics. extreme future. As it expands, the density of cists Paul Steinhardt and go.nature.com/ZhEGpZ e-mail: [email protected]