Three Roads To Quantum Gravity

34 THREE ROADS TO QUANTUM GRAVITY
universally accepted physical interpretation of it. There are
many different points of view about what quantum theory
really asserts about reality and its relationship to the
observer. The founders of quantum theory, such as Einstein,
Bohr, Heisenberg and SchroÈdinger, could not agree on these
questions. Nor is the present-day situation any better, for now
we have extra points of view that those guys, smart as they
were, were not imaginative enough to foresee. There is now
no more agreement about what quantum theory means than
when Einstein and Bohr ®rst debated the question in the
1920s.
It is true that there is only one mathematical formalism for
the quantum theory. So physicists have no problem with
going ahead and using the theory, even though they do not
agree about what it means. This may seem strange, but it does
happen. I have worked on projects in quantum gravity where
everything went smoothly until the collaborators discovered
one day over dinner that we had radically different understandings
of the meaning of quantum theory. Everything went
smoothly again after we had calmed down and realized that
how we thought about the theory had no effect on the
calculations we were doing.
But this is no consolation to the layperson, who does not
have the mathematics to fall back on. With only the concepts
and principles to go on, it must be very disconcerting to
discover that different physicists, in their different books,
offer very different versions of the basics of quantum theory.
Quantum cosmology helps rather than hinders because, as
we are about to see, it limits the scope for possible interpretations
of the quantum theory. If we stick to the principles
introduced in the ®rst two chapters, several of the approaches
to the interpretation of quantum mechanics must be abandoned.
Either that, or we must give up any idea that quantum
theory can be applied to space and time. The principle that
there is nothing outside the universe and the principle that in
the future we shall know more do point to a new way of
looking at quantum theory that is both simpler and more
rational than many of the older ideas. As a result of applying
quantum theory to cosmology, there has emerged over the last