Detecting Lorentz Violations with Gravitational Waves from Black Hole Binaries

TL;DR

This paper proposes that detecting scalar waves traveling at different speeds from gravitational waves could serve as a clear indicator of Lorentz symmetry violation, complementing existing constraints from astrophysical observations.

Contribution

It introduces the idea that scalar excitations are a necessary consequence of Lorentz violations and suggests their detection as a smoking-gun signature, which has been overlooked.

Findings

Scalar waves are predicted to accompany Lorentz violations.

Current astrophysical constraints limit differences between tensor wave speeds and light.

Scalar wave detection could provide direct evidence of Lorentz symmetry breaking.

Abstract

Gravitational wave observations have been used to test Lorentz symmetry by looking for dispersive effects that are caused by higher order corrections to the dispersion relation. In this Letter I argue on general grounds that, when such corrections are present, there will also be a scalar excitation. Hence, a smoking-gun observation of Lorentz symmetry breaking would be the direct detection of scalar waves that travel at a speed other than the speed of the standard gravitational wave polarisations or the speed of light. Interestingly, in known Lorentz-breaking gravity theories the difference between the speeds of scalar and tensor waves is virtually unconstrained, whereas the difference between the latter and the speed of light is already severely constrained by the coincident detection of gravitational waves and gamma rays from a binary neutron star merger.