Symmetries of Trautman retarded radial coordinates

Abstract

We consider spacetime described by observer that uses Trautman's retarded radial coordinates system. Given a metric tensor, we find all the symmetry maps. They set a 10 dimensional family of local diffeomorphisms of spacetime and can be parametrized by the Poincaré algebra. This result is similar to the symmetries of a Gauss observer using the Gauss simultaneous radial coordinates and experiencing the algebra deformation induced by the spacetime Riemann tensor. A new, surprising property of the retarded coordinates is that while the symmetries are differentiable, in general they are not differentiable twice. In other words, a family of smooth symmetries is smaller than in the Gaussian case. We demonstrate examples of that non-smoothness and find necessary conditions for the differentiability to the second order. We also discuss the consequences and relevance of that result for geometric relational observables program. One can interpret our result as that the retarded radial coordinates provide gauge conditions stronger that the Gaussian 4d radial gauge.