  In the canonical approach to general relativity it is customary to
parametrize the phase space by initial data on spacelike hypersurfaces.
However, if one seeks a theory dealing with observations that can be made by a
single localized observer, it is natural to use a different description of the
phase space. This results in a different set of Dirac observables from that
appearing in the conventional formulation. It also suggests a possible solution
to the problem of time, which has been one of the obstacles to the development
of a satisfactory quantum theory of gravity.
