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When the phase velocity of light in a moving medium is used to predict the phase velocity of that light relative to a stationary observer then the Einstein low speed approximation to his velocity addition equation and the Fresnel drag equation both predict the exact same result. Therefore existing interference fringe shift experiments do not differentiate between the Fresnel and Einstein predictions. However, when the group velocity of light is used, the Fresnel prediction and the Einstein prediction are different. When Fresnel predictions based on group velocity are made for the proposed experiment, the difference in round trip arrival times for the two halves of a split laser pulse is substantial. When Einstein predictions based on group velocity are made the difference in round trip times is, of course, zero. Thus a positive result for the proposed experiment would violate Special Relativity Theory (SRT) with regards to the relativity of simultaneity and suggest that the Lorentz-Poincare’ physical viewpoint on the SRT equations is a better viewpoint. If the Einstein prediction for group velocity is not correct, then when positive data for the experiment configuration shown in Figure 1.0 is collected at various orientations at different times of the day it is possible to identify a local preferred reference frame in which the speed of light is actually (not just apparently) isotropic. This reference frame could be used in place of star reference frames for spacecraft navigation. Other implications of a local preferred reference frame are explained in the paper.

Fresnel vs. Einstein - a New Direct Test