Three Roads To Quantum Gravity

THE SOUND OF SPACE IS A STRING
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of duality discussed in Chapter 9. Also, it cannot be overemphasized
that in the language in which it is understood ±
that of diagrams corresponding to quantum particles moving
against a background spacetime ± string theory is the only
known way of consistently unifying gravity with quantum
theory and the other forces of nature.
What is very frustrating is that in spite of this, string theory
does not seem to fully incorporate the basic lesson of general
relativity, which is that space and time are dynamical rather
than ®xed, and relational rather than absolute. In string
theory, as it has so far been formulated, the strings move
against a background spacetime which is absolute and ®xed.
The geometry of space and time is usually presumed to be
®xed for ever; all that happens is that some strings move
against this ®xed background and interact with one another.
But this is wrong, because it replicates the basic mistake of
Newtonian physics in treating space and time as a ®xed and
unchanging background against which things move and
interact. As I have already emphasized, the right thing to do
is to treat the whole system of relationships that make up
space and time as a single dynamical entity, without ®xing
any of it. This is how general relativity and loop quantum
gravity work.
Still, science is not made from absolutes. The progress of
science is based on what is possible, which means that it often
makes sense to do what is practical, even if it seems to go
against established principles. For this reason, even if it is
ultimately wrong, it may still be useful to follow the background
dependent approach as far as it will go, to see whether
there is a consistent picture in which we can answer
questions such as what happens when two gravitons moving
in empty spacetime scatter from each other. As long as we
remember that such a picture can give at best an approximate
description this can be an important and necessary step in the
discovery of the quantum theory of gravity.
Another main shortcoming of string theory is that is not one
theory, but a whole class of theories, so it does not lead to
many predictions about the elementary particles. This shortcoming
is closely related to the problem of background