Three Roads To Quantum Gravity

198 THREE ROADS TO QUANTUM GRAVITY
Notice that this argument is valid only if there is no way to
explain how the laws of nature might have been chosen
except by invoking the action of some entity outside our
universe. You may recall the principle with which I started
this book: that there is nothing outside the universe. As long
as there is a way of answering all our questions without
violating this principle, we are doing science and we have no
need of any other mode of explanation. So the argument for
the strong anthropic principle has logical force only if there is
no other possibility.
But there is another possibility, possibility 3. This is like
possibility 2, but with an important difference. If the different
string theories describe different phases of a single theory,
then it is possible that under the right circumstances there
could be a transition from one phase to another. Just as ice
melts to water, the universe could `melt' from one phase, in
which it is described by one string theory, to another phase, in
which it is described by another. We are then still left with the
question of why one phase rather than another describes our
universe, but this is not so hard to resolve because in this
picture the universe is allowed to have changed phase as it
evolved in time. There is also the possibility that different
regions of the universe exist in different phases.
Given these possibilities, there are at least two alternatives
to the God of the Gaps argument. The ®rst is that there is some
process that creates many universes. (Do not worry for the
moment about what that process is, for cosmologists have
found several attractive ways to make a universe which
continually spawns new universes.) The big bang is then not
the origin of all that exists, but only a kind of phase transition
by which a new region of space and time was created, in a
phase different than the one from which it came, and then
cooled and expanded. In such a scenario there could be many
big bangs, leading to many universes. The astrophysicist
Martin Rees has a nice name for this ± he calls the whole
collection the `multiverse'. It is possible that the process
creates universes in random phases. Each would then be
governed by a different string theory. These universes will
have different dimensions and geometries, and they will also