The Fabric of the Cosmos: space, time and the texture of reality by Brian Greene

A universe with strings attached

Brian Greene's first book, The Elegant Universe, was a worldwide bestseller. Greene's unique combination of clarity and humour shed a brilliant light on the darkest recesses of modern theoretical physics. At the core of The Elegant Universe was a description of the quest for a Theory of Everything (ToE), a single mathematical formula that would describe not just the properties and interactions of matter, but the structure of space and time. For Stephen Hawking, its derivation would be tantamount to "knowing the Mind of God".

There is no such theory, not yet anyway. But many physicists, including Greene, feel that it may be within their grasp. These scientists think that the most promising route to a ToE is through the idea that at a fundamental level matter is not composed of point-like particles but extended one-dimensional objects known as strings.

String theory began as an esoteric attempt to describe the behaviour of subatomic particles known as quarks. It went out of fashion until it was realised that strings could help unify quantum physics with gravity, thus removing the biggest stumbling-block for a ToE. It was then realised that there were several different, but equally viable string theories.

This is not good news, because there should be only one ToE. The idea went out of fashion again, only to return when it was realised that all the versions of string theory were descended from and related to an overarching mathematical structure called M-theory: a kind of Big ToE.

The Elegant Universe focused on the properties of matter and the forces of nature, and how scientists have described them, using mathematical laws of increasing elegance and sophistication on the way to string theory. The development of string theory into M-theory and the insights this gives is also the background to Greene's new book, The Fabric of the Cosmos. It surveys similar territory, but with quite a different emphasis: on the role of space and time in physics.

In the first great work of theoretical physics, the Principia, Newton formulated his theory of mechanics using concepts of space and time as absolute quantities that he saw as manifestations of an eternal and omnipresent deity. In 1905 Einstein demolished that view. In his special theory of relativity, temporal duration and spatial distance are not absolute but depend on the observer, and are not separately meaningful. To make sense, one has to weld the three dimensions of space and one dimension of time into a four-dimensional space-time.

Einstein's general theory, a decade later, constructed a description of gravity in terms of warped space-time: space tells matter how to move, matter tells space how to curve. In Newton's earlier conception of the stately dance of the heavenly bodies in their orbits, space and time provide the stage and the tempo but are not part of the show. In general relativity, bodies in motion flex the fabric of space-time which, in turn, guides them in their path.

The second great development at the beginning of the 20th century, quantum mechanics, further muddied these waters. We're used to the idea of empty space; in quantum physics there is no such thing. The "vacuum" is filled with an indeterminate soup of virtual particles, ceaselessly springing into and out of existence. Even when it's not there, matter seems to be inextricably linked with space.

The two views of nature represented by general relativity and quantum theory have so far resisted all attempts at reconciliation. It is with an eventual unification in mind that physicists have turned with optimism to M-theory. But there are problems. In order to be mathematically self-consistent M-theory and its descendants require more dimensions of space-time than the four we are used to.

Something has to be done with the spares. These could be wrapped up so small we can't see them. Alternatively, we could actually live in, say, a 10-dimensional Universe but be confined to a four-dimensional slice of it called a brane. Either way, it seems a bit contrived. Every time I think about these superfluous dimensions I have a vision of Occam angrily sharpening his razor.

Another problem with M-theory is that there isn't a shred of experimental evidence for it. This is why I completely disagree with Greene's assertion that M-theory reveals the "true texture of reality" (space-time woven from strings). How can something be "true" and "real" if it can't be tested?

Despite my scepticism towards the theory itself (and the fact that many of the diagrams are too small to be observed), I thoroughly enjoyed this book. Greene has an obvious flair for communicating difficult ideas as well his own enthusiasm for the subject. The Fabric of the Cosmos manages to be both challenging and entertaining: it is highly recommended.

Peter Coles is professor of astrophysics at Nottingham University