The Problems of Time and Observables: Some Recent Mathematical Results
We present 2 recent results on the problems of time and observables in canonical gravity. (1) We cannot use parametrized field theory to solve the problem of time because, strictly speaking, general relativity is not a parametrized field theory. (2) We show that there are essentially no local observables for vacuum spacetimes.
C.G. Torre, The Problems of Time and Observables: Some Recent Mathematical Results, gr-qc/9404029, Apr. 1994.