Three Roads To Quantum Gravity

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other. They wrote to each other about the telescope and what
it revealed, but Galileo seems never to have mentioned
ellipses, and to have gone to his grave believing the planetary
orbits were circles. Nor is there any evidence that Kepler ever
thought about falling bodies or believed them to be relevant to
explaining the motions of the planets. It took a young scientist
of a later generation, Isaac Newton, born the year of Galileo's
death, to wonder whether the same force that made apples fall
drew the Moon to the Earth and the planets to the Sun. So,
while my story is fanciful, it really did happen that scientists
with the stature of Galileo and Kepler each contributed an
essential ingredient to a scienti®c revolution while remaining
almost ignorant of and apparently uninterested in each
other's discoveries.
We can hope that it will take less time to bring the different
pieces of the quantum theory of gravity together than it did for
someone to see the relationship between the work of Kepler
and Galileo. The simple reason is that there are many more
scientists working now than there were then. Whereas Kepler
and Galileo might each have complained, if asked, that they
were too busy to look at what the other was doing, there are
now plenty of people to share the work. However, there is
now the problem of making sure that young people have the
freedom to wander across boundaries established by their
elders without fear of jeopardizing their careers. It would be
naive to say this is not a signi®cant issue. In many areas of
science we are paying for the consequences of an academic
system that rewards narrowness of focus over exploration of
new areas. This underlines the fact that good science is, and
will always be, as much a question of judgement and
character as it is a question of cleverness.
Indeed, over the last ®ve years the climate of mutual
ignorance and complacency that separated the string theorists
from the loop quantum gravity people has begun to dissipate.
The reason is that it has been becoming increasingly clear that
each group has a problem it cannot solve. For string theory it
is the problem of making the theory background independent
and ®nding out what M theory really is. This is necessary both
to unify the different string theories into a single theory and to