Three Roads To Quantum Gravity

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THREE ROADS TO QUANTUM GRAVITY
mentioning that many of the leading people who made key
discoveries in quantum gravity did not work on string theory.
I take the fact that this kind of criticism came from both
sides as evidence that I did not completely fail to present
an evenhanded view of the successes and failures of loop
quantum gravity, string theory, and the other approaches to
quantum gravity.
At the same time, I cannot help but notice that as time goes
on, it appears that the close-mindedness that characterizes
the thinking of some (of course, not all) string theorists does
appear to have inhibited progress. Many string theorists
seem disinterested in thinking about questions that cannot
be sensibly posed within the existing framework for string
theory. This is perhaps because they are convinced that supersymmetry
is more fundamental than the lesson from general
relativity that spacetime is a dynamical, relational entity.
Nevertheless, I suspect this is the main reason for the
slow progress on key questions such as making string theory
background independent, or understanding the role of the
dynamics of causal structure, problems that cannot be addressed
without going beyond current string theory. Of
course, other people can and do work on this problem, and
we are making progress on it, even if we are not considered
by the orthodox to be “real string theorists.”
My own view remains optimistic. I believe that we have
on the table all the ingredients we need to make the quantum
theory of gravity and that it is mostly a matter of putting the
pieces together. So far, nothing has changed my understanding
that loop quantum gravity is a consistent framework for
a complete quantum theory of spacetime, and string theory
does not yet provide more than a background-dependent approximation
to such a theory. I believe that some aspects of
string theory might nevertheless play a role, as an approximation
to the real theory, but given a choice between the
two, loop quantum gravity is certainly the deeper and more
comprehensive theory. Furthermore, if the atomic structure
of spacetime predicted by loop quantum gravity requires
modifications of special relativity such as a variation in the