  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.
