Events and observables in general relativity

Hans Westman (Perimeter Institute)

Classical general relativity is a gauge theory with respect to
reparametrizations. Observables are defined to be gauge independent, i.e
they should remain invariant under arbitrary one-to-one smooth
reparametrizations. This means that in the construction of observables
the parameters must be eliminated one way or another. We discuss, given
a model (M,g F,phi,...), ways to eliminate the parameters in order to
extract observables and discuss under which conditions it is possible.
With these conditions met one may also construct a manifold from the
model denoted the event manifold M_{e}. This manifold is conceptually
distinct from the manifold M and if the model does not meet the
conditions it can have a different topology than M. We suggest that it
is the event manifold M_{e} that has ontological significance and not
the parameter manifold M.