The Fabric of the Cosmos: Space, Time and the Texture of Reality

  • Brian Greene
Alfred Knopf: 2004. 512 pp. $28.95 Allen Lane: 2004. £25 0375412883 0713996773 | ISBN: 0-375-41288-3

The most apocalyptic pronouncement in the history of science was made by the physicist Hermann von Helmholtz in 1854. The Universe, von Helmholtz declared, is dying. His pessimistic prediction was based on the second law of thermodynamics, according to which the entire cosmos is on a one-way slide towards a state of complete disorder, or maximum entropy, from which it will be unable to extricate itself.

The basis of the dying Universe is easy to explain. We are surrounded by processes that have a definite directionality to them: people grow old, cars rust, cliffs are eroded, sandcastles get washed away. Take a movie of any familiar scene and run it backwards — people will laugh, because the world in reverse looks so preposterous.

The same directionality pervades the cosmos. The Sun, for example, is not the immutable orb it may seem. Over billions of years it has consumed much of its fuel, and eventually it will burn out and die. The same goes for all stars. Irreversible decay and degeneration spell universal heat death, just as von Helmholtz proclaimed.

But this incontestable fact conceals a deep mystery. The world about us may display a conspicuous ‘arrow of time’, but the underlying laws of physics are, with a minor exception, completely symmetric. At the level of individual atoms, there is nothing to distinguish future from past. So somehow, between atom and cosmos, the temporal lopsidedness of the Universe emerges. How this happens is one of the longest-running debates in science.

I became fascinated by this mystery after hearing a lecture delivered at the Royal Society in London by Fred Hoyle in 1968, and I have returned to it again and again throughout my career. Early on it became obvious to me that the subject was marred by widespread misconceptions and muddled thinking, even among some of the giants of theoretical physics. It is therefore extremely heartening that Brian Greene has written such a sensible and consistent account of this tantalizing topic, where so many other authors have merely added to the confusion.

Following the success of his book on the unification of physics, The Elegant Universe, Greene has turned his attention to the nature of space and time. Before zeroing in on the vexed issue of time's elusive arrow, he covers some familiar territory: the theory of relativity, basic cosmology, Mach's principle, quantum non-locality and the Bell inequalities, Wheeler's delayed-choice experiment and the many-universes interpretation of quantum mechanics. Greene even tackles, but does not resolve, the vexed issue of the flow of time — that most familiar of human experiences, which is bafflingly absent from physicists' descriptions of the world.

One way to think of the cosmic arrow of time is to regard the Universe as a gigantic clock slowly running down. The problem is to explain how it was wound up in the first place. Cosmologists naturally look to the Big Bang as the start of the whole show. But this immediately presents a puzzle. We have available a snapshot of the Universe shortly after its birth, in the form of a thermal map of the sky taken from satellites such as the Wilkinson Microwave Anisotropy Probe and various ground-based instruments. The data show the fading afterglow of the Big Bang, and enable cosmologists to reconstruct the details of its primordial thermodynamic state. The result is striking: just 380,000 years after its birth, the Universe looked virtually dead already, consisting of uniform hot gas spread through space in a state extremely close to the heat death envisaged by von Helmholtz. So, if the young Universe was close to thermodynamic equilibrium then, how has it come alive again and achieved such a low-entropy state today?

Greene correctly identifies the solution of this puzzle. The key lies with gravity. This universal force seizes any tiny irregularity in the distribution of matter and amplifies it. Over billions of years, the smooth hot gases of the early Universe have become clumpy assemblages of galaxies, stars and black holes. This trend from smooth to clumpy constitutes gravity's very own arrow of time and trumps the rise in the entropy of matter. The upshot is that a Universe that starts out smooth, with matter in thermodynamic equilibrium, can end up clumpy, with matter dragged far from equilibrium. Since the time of the Big Bang, the entropy of matter has risen, but the maximum possible entropy has risen even faster, mostly due to the expansion of the Universe. Our very existence depends on the entropy gap that this has created.

The quest for the source of time's arrow can therefore be traced back, somewhat tortuously, to the fact that the Universe began in a relatively smooth state. When I worked on this problem in the 1970s, the initial smoothness had to be simply accepted as an unexplained initial condition. Today, however, there is a ready explanation in the form of the ‘inflationary Universe’ scenario, involving a burst of frenetic expansion shortly after the cosmic origin. Any pre-existing irregularities would have been smoothed away by this abrupt and huge swelling of space.

This pleasing conclusion immediately begs the question of what preceded inflation and why it happened at all — which provides a convenient jumping-off point for Greene to embark on a discussion of the physics of unification. This is familiar territory for him, as he is first and foremost a string theorist. He treats the reader to a review of the current state of string/M theory, with digressions into a bewildering array of topics including higher dimensions, brane worlds, the holographic paradigm, teleportation, wormholes and time travel. If I have a criticism of this book, it is that it packs in so many challenging and abstract topics that it can leave the reader's mind reeling. Sensibly, Greene repeatedly cautions that most of the ideas he gallops across are extremely speculative and are unlikely to be tested experimentally any time soon.

The nature of space and time has exercised the minds of the world's greatest scientists and philosophers for centuries. Mostly they have regarded space and time as simply there — a given. Modern physics has shown that they are, in fact, things, just as particles of matter are things. The heady developments described in this book hold out the promise that we may one day explain how space and time have come to exist, and why they possess the properties that they do.