Three Roads To Quantum Gravity

200 THREE ROADS TO QUANTUM GRAVITY
dimensions and appeared to satisfy different laws. The big
bang would then be just be the most recent of a series of
transitions the universe has passed through. And even though
each phase may be governed by a different string theory, the
whole history of the universe would be governed by a single
law ± M theory. We then need an explanation in physical
terms for why the universe `chose' to exist in a phase such as
the one in which we ®nd ourselves, which exists for billions
of years and is hospitable for life. There are several different
possible explanations of this kind, which are described in
detail in another book of mine, The Life of the Cosmos, soI
shall be brief here.
One idea is that new universes could form inside black
holes. In this case our universe would have a large number of
progeny, as it contains at least 10 18 black holes. One may also
conjecture that the changes in the laws from old universes to
new are small, so that the laws in each new universe formed
from our own are close to those that hold in our universe. This
also means that the laws in the universe from which ours was
formed were not very different from those of our own. Given
these two assumptions, a mechanism which is formally
analogous to natural selection operates, because after many
generations those universes that give rise to many black holes
will dominate the population of universes. The theory then
predicts that a randomly chosen universe will have the
property that it will make more black holes than will
universes with slightly different values of the parameters.
We can then ask whether this prediction is satis®ed by our
universe. To cut a long story short, up to the present time it
seems that it is. The reason is that carbon chemistry is not
only good for life, it plays an important role in the processes
that make the massive stars that end up as black holes.
However, there are several possible observations which
could disprove the theory. Thus, unlike the God of the Gaps
and the God of The Gap theories, this theory is very vulnerable
to being disproved. Of course, this means that it is likely
to be disproved.
The important thing about this theory is that it shows that
there are alternatives to both the strong and the weak