  Currently operating gravitational-wave interferometers are Michelson
interferometers with effective arm length L ~ 4x10e5 m. While the
interferometer remains in lock, data at the fsr sideband frequency encode
information on slow phase changes in the f ~ 10e-5 Hz range, with a fringe
sensitivity ~ 10e-10. Preliminary LIGO data presented in 2009 show no Lorentz
violating signal at the second harmonic of the Earth's sidereal frequency. This
sets a limit on a possible change in refractine index, dn/n < 2x10e-22, an
improvement of more than three orders of magnitude over existing limits.
