Physical Review D
American Physical Society
We classify all generalized symmetries of the vacuum Einstein equations in four spacetime dimensions. They consist of constant scalings of the metric, and of the infinitesimal action of generalized spacetime diffeomorphisms. Our results rule out a large class of possible ‘‘observables’’ for the gravitational field, and suggest that the vacuum Einstein equations are not integrable.
C.G. Torre and I.M. Anderson, Symmetries of the Einstein equations, Physical Review Letters, vol. 70, Jun. 1993, p. 3525.