Three Roads To Quantum Gravity

HOW TO WEAVE A STRING
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may be the birth of a new universe inside the horizon. This
idea has been studied using approximation techniques in
which the matter forming the black hole is treated quantum
theoretically, but the geometry of spacetime is treated as in
the classical theory. The results do suggest that the singularities
are eliminated, and one may hope that this will be
con®rmed by an exact treatment. But, at least so far, neither
string theory nor loop quantum gravity, nor any other
approach, has been strong enough to study this problem.
Until 1995 no approach to quantum gravity could describe
black holes in any detail. None could explain the meaning of
the entropy of a black hole or tell us anything about what
black holes look like when probed on the Planck scale. Now
we have two approaches that are able to do all these things, at
least in some cases. Every time we are able to calculate
something about a black hole, in either theory, it comes out
right. There are many questions we still cannot answer, but it
is dif®cult to avoid the impression that we are ®nally understanding
something real about the nature of space and time.
Furthermore, the fact that both string theory and loop
quantum gravity both succeed in giving the right answers
about quantum black holes is strong evidence that the two
approaches may be revealing different sides of a single theory.
Like Galileo's projectiles and Kepler's planets, there is more
and more evidence that we are glimpsing the same world
through different windows. To ®nd the relation of his work to
Kepler's, Galileo would only have had to imagine throwing a
ball far enough and fast enough that it became a moon. Kepler,
from his point of view, could have imagined what a planet
orbiting very close to the Sun might have looked like to
people living on the Sun. In the present case, we only have to
ask whether a string can be woven from a network of loops, or
whether, if we look closely enough at a string, we can see the
discrete structures of the loops. I personally have little doubt
that in the end loop quantum gravity and string theory will be
seen as two parts of a single theory. Whether it will take a
Newton to ®nd that theory, or whether it is something we
mortals can do, is something that only time will tell.