Three Roads To Quantum Gravity

EPILOGUE:
209
And quantum gravity is nothing if not high risk. The
unfortunate lack of experimental tests means that relatively
large groups of people may work for decades only to ®nd that
they have completely wasted their time, or at least done little
but eliminate what at ®rst seemed to be attractive possibilities
for the theory. Measured sociologically, string theory seems
very healthy at the moment, with perhaps a thousand
practitioners; loop quantum gravity is robust but much less
populous, with about a hundred investigators; other directions,
such as Penrose's twistor theory, are still pursued by
only a handful. But thirty years from now all that will matter
is which parts of which theory were right. And a good idea
from one person is still worth hundreds of people working
incrementally to advance a theory without solving its fundamental
problems. So we cannot allow the politics of the
academy too much in¯uence here, or we shall all end up
doing one thing. If that happens, then a century from now
people may still be writing books about how quantum gravity
is almost solved. If this is to be avoided, all the good ideas
must be kept alive. Even more important is to maintain a
climate in which young people feel there is a place for their
ideas, no matter how initially unlikely or how far from the
mainstream they may seem. As long as there is still room for
the young scientist with the uncomfortable question and the
bright idea, I see nothing to prevent the present rapid rate of
progress from continuing until we have a complete theory of
quantum gravity.
I should like to close this book by sticking out any part of
my neck which is not yet exposed, and making a few
predictions about how the problem of quantum gravity will
in the end be solved. I believe that the huge progress we have
made in the last twenty years is best illustrated by the fact that
it is now possible to make an educated guess about how the
last stages of the search for quantum gravity will go. Until
recently we could have done no more than point to a few good
ideas that were not obviously wrong. Now we have several
proposals on the table that seem right enough and robust
enough, and it is hard to imagine that they are completely
wrong. The picture I have presented in this book was