Three Roads To Quantum Gravity

WHAT CHOOSES THE LAWS OF NATURE?
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is indeed something that requires explanation. If the explanation
is not to be that some outside agency chose the state of the
universe, there must have been some mechanism of selforganization
that, acting in our past, drove the world into this
incredibly improbable state. Cosmologists have been worrying
about this problem for some time. One solution which has
been proposed is called in¯ation. This is a mechanism by
which the universe can blow itself up exponentially fast until
it becomes the ¯at, almost Euclidean universe we observe
today. In¯ation solves part of the problem, but it itself
requires certain improbable conditions. When in¯ation begins
to act, the universe must already be smooth on a scale of
at least 10 5 times the Planck scale. And ± at least as far as we
know ± in¯ation requires the ®ne tuning of two parameters.
One of these is the cosmological constant, which must be
smaller than its natural value in a quantum theory of gravity
by a factor of at least 10 60 . The other is the strength of a certain
force, which in many versions of in¯ation must be no greater
than 10 76 . The net result is that for in¯ation to act we require
a situation with a probability of at most 10 781 . Even if we
leave the cosmological constant out of it, we still require a
situation whose probability is at most 10 721 . So in¯ation may
be part of the answer, but it cannot be the complete answer.
Is it possible that some method of self-organization accounts
for the fact that space looks perfectly smooth and
regular, on scales hugely bigger than the Planck scale? This
question has prompted some recent research, but as yet no
clear answer has emerged. But if we are to avoid an appeal to
religion, then this is a question that must have an answer.
So, in the end, the most improbable and hence the most
puzzling aspect of space is its very existence. The simple fact
that we live in an apparently smooth and regular threedimensional
world represents one of the greatest challenges
to the developing quantum theory of gravity. If you look
around at the world seeking mystery, you may re¯ect that one
of the biggest mysteries is that we live in a world in which it is
possible to look around, and see as far as we like. The great
triumph of the quantum theory of gravity may be that it will
explain to us why this is so. If it does not, then the mystic who