Improved constraints on isotropic shift and anisotropies of the speed of light using rotating cryogenic sapphire oscillators

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 ~ 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 ~ tr with a precision of 7:4 10 . This represents the first constraint on ~ 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.