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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.
Originally published by the American Physical Society. Publisher's PDF can be accessed through Physical Review D - Particles, Fields, Gravitation, and Cosmology.