麻豆传媒

Three Roads to Quantum Gravity by Lee Smolin, Weidenfeld & Nicolson, 拢16.99, ISBN 0297643010

IN SCIENCE as in life, people tend to band together when the going gets rough. And thus as research funding and careers become ever more precarious, the groves of academe now reverberate to the sound of bandwagons carrying hordes of physicists on what they hope will be a voyage of discovery 鈥� but which could take them round and round in circles.

Nowhere has the bandwagon effect been more apparent than in the search for the holy grail of physics, a unified account of the two most successful theories of all science: quantum theory and general relativity. The former deals with events on the smallest scale, while the latter 鈥� Einstein鈥檚 theory of gravity 鈥� comes into its own on the largest, cosmological scales.

Unifying the two would give us a single theory spanning the whole range of physical phenomena. For nearly fifty years this enticing prospect has motivated a growing number of physicists to search for such a unified theory. In spite of their efforts, their successes so far have been modest. The problem lies in the utterly different vision of reality of the two would-be constituents of the longed-for unified theory. The quantum world is one of discrete packets and jumps; Einstein鈥檚 vision of gravity is of smooth plains of space and time, warped by matter to create what we call gravity.

In the mid-1980s, theorists found that unification seemed more likely if subatomic particles and forces such as gravity were regarded not as zero-dimensional points but as tiny vibrating one-dimensional objects called strings. With both gravity and subatomic forces thus cast into the same quantum format, this 鈥渟uperstring鈥� approach offered the prospect of real progress. Superstring theory soon became a double-decker bandwagon, with thousands of theorists piling aboard.

And it remains the only bandwagon going anywhere, at least, according to popular accounts of the search for a unified theory. But as the title of Lee Smolin鈥檚 personal account of the search, Three Roads to Quantum Gravity, makes clear, there are at least two other approaches.

The first focuses on making the very fabric of space and time discrete, as quantum theory demands. This task was made easier by a mathematical breakthrough in the 1980s that has led to so-called loop quantum gravity, which views space-time as a 鈥渟pin network鈥�. And no, don鈥檛 even think about asking what that is. Like so much in this field, it defies ready description.

It鈥檚 the same story with the other alternative to superstring theory that Smolin seeks to bring to a wider audience. This centres on fundamental issues such as how to describe a universe without stepping 鈥渙utside鈥� it. His account of how attempts to solve similar questions have advanced knowledge is very tough going, even if you鈥檙e familiar with the basic ideas. It is, however, fascinating to learn how philosophy is emerging as a crucial signpost on the road to quantum gravity.

It is also refreshing to have a mainstream scientist highlighting the failings of the modern research milieu, with its insularity, fragmentation and derision of those not aboard the biggest bandwagons. Had it not been for the refusal of a few theorists to join the superstring circus, the solutions of some of its most recalcitrant problems would have been no closer now than they were in the mid-1980s. There is now a growing suspicion that all three roads to quantum gravity hold the key to solving each others鈥� problems. Smolin is optimistic enough to predict that unification will be achieved by 2010.

But it鈥檚 hard to avoid the conclusion that we don鈥檛 hear too much about the two other roads to quantum gravity simply because they are incomprehensible to anyone but mathematical physicists. As someone who has reported in 麻豆传媒 some of the developments Smolin describes, I sympathise with him about how difficult it is even to convey a glimmer of the logic behind the various approaches.

Even so, Smolin鈥檚 book could certainly have benefited from some decent editing. Anyone who got to the end of Stephen Hawking鈥檚 bestseller should certainly have a crack at Three Roads to Quantum Gravity. But be prepared to be fascinated and frustrated in equal measure.