Three Roads To Quantum Gravity

HOW TO WEAVE A STRING
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thing!" In fact the speaker had given a very measured
presentation full of careful quali®cations and caveats and
had not made a single claim that went beyond what they had
done. The problem is that such quali®cations have to be
presented in the terminology speci®c to the theory, and the
person next to me, from the opposing theory's camp, was
unable to follow it. This has happened to me in both
directions. Even now, one can go to a conference and ®nd
that string theory and loop quantum gravity are the subjects of
separate parallel sessions. The fact that the same problems are
being addressed in the two sessions is noticed only by the
small handful of us who do our best to be in both rooms.
There are many remarkable aspects of this situation,
including the fact that almost every one of these people is
quite sincere. Just as the existence of Moslems does not deter
some Christians from the sincere conviction that theirs is the
one true religion, and vice versa, there are many string
theorists and many loop quantum gravity people who do not
seem to be troubled by the existence of a whole community of
equally sincere and smart people who pursue a different
approach to the problem they are spending their lives
attacking.
But this is a problem not of science but of the sociology of
the academy. Sometimes, rushing from the loop room to the
string room and back again, I have wondered what would
have happened had physics in the seventeenth century been
carried out in the same sociological context as present-day
science. So let us wind back time and consider an alternative
history of science. By 1630 there would have been two large
groups of natural philosophers working on the successor to
Aristotelian science. At conferences they would have divided
into two parallel sessions with, as today, little overlap. In one
room would be those who thought that falling bodies
provided the key to the new physics. They would spend
their time in profound re¯ections on the motion of bodies on
the Earth. They would launch projectiles, experiment with
pendulums and roll balls down inclined planes. Each of them
would have their own personal version of the theory of falling
bodies, but they would be united by the conviction that no