Physical Review D
American Physical Society
Using a recent classification of local symmetries of the vacuum Einstein equations, it is shown that there can be no observables for the vacuum gravitational field (in a closed universe) built as spatial integrals of local functions of Cauchy data and their derivatives.
C.G. Torre, Gravitational observables and local symmetries, Physical Review D, vol. 48(6), 1993, p. R2373.