Brian Greene

THE FABRIC OF THE COSMOS

Space, time, and the texture of reality

In his earlier book, The Elegant Universe, Greene revealed himself to be a science popularizer of the best kind, combining acknowledged expertise in his field with the ability to write vividly and clearly. He continues the good work in his new book. He has a remarkable ability to render complex ideas in non-mathematical language, making use of stories, illustrations, and analogies to explain the often complex ideas he wants to convey. However, he thinks the book will also be suitable for students, teachers, and professionals, and the notes provide mathematical underpinning for those who are able to make use of it. If I have a complaint (and it is only a minor one) it is that some of his folksy analogies depend on allusions to (presumably) cartoon characters who will doubtless be familiar to an American audience but were unknown at least to this British reader. But the meaning is clear from the context, anyway.

This is a wide-ranging book but at its core is a set of questions about space and time. Are they real physical entities or simply useful conventional ideas? If they are real, are they fundamental or do they emerge from more basic constituents? Does time have a beginning, and does time really have a direction, flowing from past to future? Such questions used to be the province of philosophers but these days they are increasingly being addressed by physicists.

Plenty of questions, then, and at least some answers, or partial answers. In Part 1 of his book, Greene considers the startling implications of quantum particle entanglement across space. These show us that space is not what we thought it was. And the same is true of time, as he demonstrates in Part 2: in a quantum universe we have to accept that there is a kind of entanglement across time as well as space. Language and common sense find it hard to accommodate relationships of this kind. Here Greene does an excellent job of explaining the meaning of John Bell's insight, a notoriously difficult task; typically, he does this by constructing a dialogue between Mulder and Scully of X-Files fame.

It takes Greene nearly half the book—over 200 pages—to explain these difficult and paradoxical ideas. In the remaining chapters he considers what the modern view of spacetime implies for cosmology. And, since he is one of the leading exponents of string theory, he naturally gives it a lot of space. The discussion of strings inevitably recapitulates some of the material in his earlier book and readers of that book are invited to skim this section if they wish. But most, I suspect, will be glad of the chance to refresh their memories.

The book deals with deep questions that, as Greene acknowledges, verge on the metaphysical. So-called post-modernists would no doubt say that the answers physicists give to such questions are no more valid than those of the past, but there is an important difference. Traditional cosmologies were static, expressed in mythological terms that changed only slowly if at all. There was no reason they should change since there were no tests that could be applied. Even within the ranks of physicists there are those who say that the theories espoused by Greene and other string theorists are too much like metaphysics, not capable of being tested empirically. But Greene is emphatic that it is possible to test his ideas.

One fascinating, if speculative, way this might be done emerges from a well-established idea which is remarkable enough in its own right. According to the theory of inflation, the universe expanded unimaginably fast from a tiny size at a very early stage. The early universe was so small that quantum fluctuations dominated, and this phase has left its traces for us to see in the night sky today. "The sudden burst of inflationary expansion stretched space by such an enormous factor that what initially inhabited the microscopic was drawn out to the macroscopic." So the universe we see today is a hugely expanded version of quantum events in the distant past. The analogy Greene uses is of tiny scribbles on the surface of a balloon, which expand hugely when the balloon is inflated.

According to inflation, the more than 100 billion galaxies, sparkling throughout space like heavenly diamonds, are nothing but quantum mechanics writ large across the sky. To me, this revelation is one of the greatest wonders of the modern scientific age.

Greene's sense of astonishment here is surely justified, but its potential implications are even wider. At the end of his book he returns to this literally seminal idea and speculates that it could be taken even further, to reveal traces of his beloved strings.

Perhaps cosmic expansion can stretch the imprints of even shorter-scale processes or features—the physics of strings, or quantum gravity more generally, or the atomized structure of ultramicroscopic spacetime itself—and spread their influence, in some subtle but observable manner, across the heavens. Maybe, that is, the universe has already drawn out the microscopic fibers of the fabric of the cosmos and unfurled them clear across the sky, and all we need do is learn how to recognize the pattern.

So perhaps we shall not need to build ever bigger and more powerful particle accelerators to verify string theory. Perhaps we shall simply need to look at the stars. Mindblowing stuff.

3 September 2004