Three Roads To Quantum Gravity

WHAT CHOOSES THE LAWS OF NATURE?
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have different sets of elementary particles which interact
according to different sets of laws. If there are adjustable
parameters, it is possible that they are set at random each time
a new universe is created.
So there is a simple answer to the anthropic question.
Among all the possible universes, a minority will have the
property that their laws are hospitable to life. Since we are
alive, we naturally ®nd ourselves in one of them. And since
there are a great many universes, we need not worry that the
chance of any one of them being hospitable to life is small,
because the chance of at least one of them being hospitable to
life may not be small. There will then be nothing to explain.
Martin Rees likes to put this in the following way: if one ®nds
a bag by the side of the road containing a suit that ®ts one
perfectly, that is something to wonder about. But if one goes
into a clothing store and is able to ®nd a suit that ®ts, there is
no mystery because the store carries lots of suits in many
different sizes. We may call this the God of The Gap. It is also
sometimes called the weak anthropic principle.
The only problem with this kind of explanation is that it is
dif®cult to see how it could be refuted. As long as your theory
yields a very large number of universes, you only need there
to be at least one like ours. The theory makes no other
predictions apart from the existence of at least one universe
like ours. But we already know that, so there is no way to
refute this theory. This might seem good, but actually it is not
because a theory that cannot be refuted cannot really be part
of science. It can't carry very much explanatory weight,
because whatever features our universe has, as long as it can
be described by one of the large number of string theories, our
theory will not be refuted. Therefore it can make no new
predictions about our universe.
Is it possible to have a theory which gives a scienti®c
answer to the anthropic question? Such a theory may be
framed around the possibility that the universe can make a
physical transition from one phase to another. If we could
look back into the history of the universe to before the big
bang, it may be that we would see one or a whole succession
of different phases in which the universe had different