Three Roads To Quantum Gravity

MANY OBSERVERS, NOT MANY WORLDS
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language. But this one world contains many different, equally
consistent histories, each of which can be brought into being
by the right set of questions. Each history is incompatible
with the others, in the sense that they cannot be experienced
together by observers like ourselves. But each is, according to
the formalism, equally real.
As you might imagine, there was a huge, if mostly friendly,
disagreement over what to make of this. Some of us follow Fay
Dowker and Adrian Kent in their conviction that this in®nite
expansion of the notion of reality is unacceptable. Either
quantum mechanics is wrong when applied to the whole
universe, or it is incomplete in that it must be supplemented
by a theory of which set of questions corresponds to reality.
Others follow James Hartle and Murray Gell-Mann in embracing
the extreme context dependence that comes with their
formulation. As Chris Isham says, the problem lies with the
meaning of the word `is'.
If this were not trouble enough, there are other dif®culties
with this conventional formulation of quantum cosmology.
It turns out that one is not free to ask any set of questions:
instead, these are constrained by having to be solutions to
certain equations. And, although we had solved the equations
that determine the quantum states of the universe, it
proved much more dif®cult to determine the questions that
can be asked of the theory. It seems unlikely that this can
ever be done ± at least in any real theory, as opposed to the
toy models that describe little yo-yo-like versions of the
universe. Perhaps I should not comment on the likelihood of
®nding the right set of questions, given that our solving the
equations for the states was itself a total accident. Still, we
have tried, many smarter people have tried, and we have all
come to the conclusion this is not a stone that can be
moved. So conventional quantum cosmology seems to be a
theory in which we can formulate the answers, but not the
questions.
Of course, from the perspective of the last chapter, this is
not surprising. We saw there that to formulate a theory of
cosmology we must acknowledge that different observers see
partly different, partial views of the universe. From this