Three Roads To Quantum Gravity

MANY OBSERVERS, NOT MANY WORLDS
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different from what seems to be coming out of the equations of
the conventional approach to quantum cosmology. There,
each solution seems to have within it descriptions of an
in®nite number of universes. These universes differ, not only
in the answers that the theory gives to questions, but by the
questions that are asked.
Everett's relative-state form of the theory must therefore be
supplemented by a theory of why what we observe corresponds
to the answers to certain questions, and not to an
in®nite number of other questions. Several people have
attempted to deal with this, and some progress has been
made using an idea called decoherence. A set of questions is
called decoherent if there is no chance that a de®nite answer
to one is a superposition of de®nite answers to others. This
idea has been developed by several people into an approach
to quantum cosmology called the consistent histories formulation.
This approach lets you specify a series of questions
about the history of the universe. Assuming only that the
questions are consistent with one another, in the sense that
the answer to one will not preclude our asking another, this
approach tells us how to compute the probabilities of the
different possible answers. This is progress, but it does not go
far enough. The world we experience is decoherent but, as has
been convincingly shown by two young English physicists,
Fay Dowker and Adrian Kent, so are an in®nite number of
other possible worlds.
One of the most dramatic moments I've experienced during
my career in science was the presentation of this work at a
conference on quantum gravity in Durham, England, in the
summer of 1995. When Fay Dowker began her presentation on
the consistent histories formulation, that approach was
generally regarded as the best hope for resolving the problems
of quantum cosmology. She was a postdoc under James
Hartle, who had pioneered the development of the consistent
histories approach to quantum cosmology, and there was
little indication in her introduction of what was coming. In a
masterful presentation she built up the theory, elucidating
along the way some of its most puzzling aspects. The theory
seemed in better shape than ever. Then she proceeded to