Brian Greene

THE ELEGANT UNIVERSE

Superstrings, hidden dimensions, and the quest for the ultimate theory

Greene is one of the leading researchers in superstring theory and is professor of physics and mathematics at Columbia University and Cornell University. He is thus formidably well qualified to write about his subject, but, luckily for non-physicists, he is also a brilliant popularizer, in the best sense of that word. His approach is non-mathematical but he goes into considerable detail about his subject, and he expresses the hope that what he has written will be of interest to readers with some scientific background, such as science teachers and students, because he is writing about matters that are right at the forefront of current research and speculation. Readers wishing for more technical details can find these in the extensive notes, which amplify some points in the main text and also include mathematical treatments of some topics. There have been other books about string theory, but few to match this one in respect either of depth or of clarity.

The book begins by identifying a basic problem in modern physics: the incompatibility between Einstein's general relativity theory and quantum mechanics. It is well known that Einstein spent three decades trying to find a unified theory of physics but failed. String theory, according to its advocates, promises to provide this, although it is still incomplete. The first part of the book sets out the basics of general relativity and quantum mechanics in a fair amount of detail and explains exactly why the problem of reconciling them is so difficult.

After this scene-setting, we come to string theory itself. This is based on a geometry of ten dimensions, which obviously cannot be directly visualized, but Greene resorts to simple diagrams to explain the ideas, using pictures of a garden hose as the basis for his descriptions. The underlying theory is, of course, highly mathematical, and this leaves it open to the objection that it is not testable and is thus not genuinely scientific. The difficulty here is the minuteness of the strings: probing these hypothetical structures would require energies some 17 orders of magnitude greater than those currently available, and this implies building an accelerator the size of the galaxy. Greene, however, suggests various ways in which it may be possible to obtain indirect evidence for the theory.

Having established what string theory means, Greene goes on to show how it can provide explanations for the particles of physics and may also help to reconcile gravitation and quantum mechanics. The theory depends on mathematical transformations of Calabi-Yau shapes, which are spaces in which the extra spatial dimensions required by string theory can be curled up; these shapes are illustrated with rather beautiful diagrams. Profound though string theory is, however, it is not the ultimate, and in his next section Greene takes us further, to confront questions that seem to verge on the philosophical or even metaphysical, yet which are now beginning to come within the purview of physics.

Once more we are back with Einstein, who raised the question of whether there is any freedom in the way the universe is constituted. Recent work, since 1995, implies that there may be a way of resolving this very basic issue, through what is called M-theory or superstring theory. This requires eleven dimensions in place of the ten needed in string theory and also the inclusion of other elements in addition to strings. If M-theory can be shown to be correct, it will have major implications for cosmology, which Greene develops in his final section. Exploration of M-theory, he believes, will be a major part of physics in the twenty-first century. We may be able to answer questions such as how the universe began and why it has evolved to reach its present form: "… the development of a quantum theory of gravity through superstring theory lends credence to the hope that we now possess theoretical tools for pushing into the vast regions of the unknown, and, no doubt after many a struggle, possibly emerging with answers to some of the deepest questions ever posed".

And yet an element of uncertainty remains in all this. Exciting although all these new ideas are, Greene is doubtful whether we shall ever be able to explain all the features of the universe, such as particle masses and force strength. "Is it possible that we [shall] still have to resort to experimental measurements, rather than theoretical calculations, for their values? And, moreover, might it be that this failing does not mean that we need to look for an ever deeper theory, but simply reflects that there is no explanation for these observed properties of reality?" We cannot yet know whether there will ultimately be a limit to our understanding.

This is one of those books that generate a real feeling of intellectual adventure and demand to be reread, probably more than once.