We demonstrate that Michelson-Morley tests, which detect direction-dependent anisotropies in the speed of light, can also be used to place limits upon isotropic deviations of the vacuum speed of light from $c$, as described by the photon-sector standard model extension parameter ${\tilde{\kappa }}_{\mathrm{tr}}$. A shift in the speed of light that is isotropic in one inertial frame implies anisotropic shifts in others. Using observer Lorentz covariance, we derive the time-dependent variations in the relative resonance frequencies of a pair of electromagnetic resonators that would be generated by such a shift in the rest frame of the Sun. A new analysis of a recent experimental test of relativity using this result constrains ${\tilde{\kappa }}_{\mathrm{tr}}$ with a precision of $7.4\times {10}^{-9}$. This represents the first constraint on ${\tilde{\kappa }}_{\mathrm{tr}}$ by a Michelson-Morley experiment and the first analysis of a single experiment to simultaneously set limits on all nine nonbirefringent terms in the photon sector of the minimal standard model extension.

• Received 9 June 2010

DOI:https://doi.org/10.1103/PhysRevD.82.076001