Electromagnetic cavity tests of Lorentz invariance on Earth and in space
  We present a Michelson-Morley type experiment for testing the isotropy of the speed of light in vacuum and matter. The experiment compares the resonance frequency of an actively rotated monolithic optical cryogenic sapphire resonator against the resonance frequency of a stationary evacuated optical cavity made of ultra-low-expansion glass. The results yield an upper limit for the anisotropy of the speed of light in matter (sapphire) of Delta(c)/c < 1 x 10^-16, limited by the frequency stability of the sapphire resonator. 
M. Nagel, K. M\"ohle, K. D\"oringshoff, E. V. Kovalchuk, and A. Peters
physics.class-ph physics.ins-det