Three Roads To Quantum Gravity

THE SOUND OF SPACE IS A STRING
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can be no triumph without an equal amount of foolishness.
When the problem is as hard as the invention of quantum
gravity, we must respect the efforts of others even when we
disagree with them. Whether we travel in small groups of
friends or in large convoys of hundreds of experts, we are all
equally prone to error.
Another moral has to do with why Einstein made so many
mistakes in his struggle to invent general relativity. The
lesson he had such trouble learning was that space and time
have no absolute meaning and are nothing but systems of
relations. How Einstein himself learned this lesson, and by
doing so invented a theory which more than any other
realizes the idea that space and time are relational, is a
beautiful story. But it is not my place here to tell it ± that
must be left to historians who will tell it right.
The subject of this chapter is string theory, and I begin it
with these re¯ections for two reasons. First, because the main
thing that is wrong with string theory, as presently formulated,
is that it does not respect the fundamental lesson of
general relativity that spacetime is nothing but an evolving
system of relationships. Using the terminology I introduced
in earlier chapters, string theory is background dependent,
while general relativity is background independent. At the
same time, string theory is unlikely to be in its ®nal form.
Even if, as is quite possible, string theory is ultimately
reformulated in a background independent form, history
may record that Einstein's view of Newton applies also to
the string theorists: when it was necessary to ignore fundamental
principle in order to make progress, they had the
courage and the judgement to do so.
The story of string theory is not easy to tell, because even
now we do not really know what string theory is. We know a
great deal about it, enough to know that it is something really
marvellous. We know much about how to carry out certain
kinds of calculations in string theory. Those calculations
suggest that, at the very least, string theory may be part of
the ultimate quantum theory of gravity. But we do not have a
good de®nition of it, nor do we know what its fundamental
principles are. (It used to be said that string theory was part of