Signs of Lorentz violation in electrodynamics: variable speed of light and the photon mass

Abstract

Recent studies of spacetime anisotropy in the context of local Lorentz invariance (LLI) based on classical Michelson-Morley experiments, as well Kennedy-Thorndyke tests, pointed out the existence of terms first order in v/c and of angular signatures independent of v. This contribution replaces the Lorentz symmetry by a velocity gauge transformation following an argument centred on observability. Results show even and odd order terms and indicate that motion is always underestimated in the spatiotemporal platform. Though LLI is not recovered in exact special relativistic terms, the alternative looks compatible with the relational aspects of general relativity (GR) with variable speed of light models as well a nonzero photon mass. This raises the hypothesis that Einstein equivalence principle, and consequently LLI, is a cornerstone of GR, but not necessarily a fundamental one of SR