A Criticism of "Gas Mode" Reinterpretations of the Michelson-Morley and   Similar Experiments
  It has been argued by R. T. Cahill and others that a Michelson interferometer in "gas mode" - in which the light paths are through an included gaseous medium - are able to detect and have detected an absolute frame of reference. It is shown here that this argument supposes incorrectly that the reduced velocity of light in gas is an observer-independent invariant. This velocity is not invariant, but given in a frame with respect to which the interferometer moves with velocity v by the usual relativistic formula for the addition of velocities, these being in this case the velocity v and the reduced velocity of light in the inertial frame of the interferometer. It is suggested nonetheless that though the absolute frame urged by Cahill may be undetectable, there are persuasive grounds for considering the alternative Lorentzian Relativity that did suppose the existence of such a frame. 
Daniel Shanahan
physics.class-ph