Experimental Investigation of the Second Postulate of Special Relativity*

The addition-of-velocities hypothesis, which Kantor recently claimed to have verified, has been investigated using a different experimental arrangement. A laser beam was passed through a moving mica window, which might be considered as a Huygens-type moving source of light. An interferometer was built with which to detect and measure any change in speed of the emitted light. The apparatus allowed investigation of: the effects of window speed, which could be varied continuously up to 63m/sec; direction of window motion with respect to direction of propagation; and the effects of air in the beam path. Emission theory as used by Kantor predicted a $\frac{1}{3}$ fringe shift; however, no shift was observed under any conditions. The estimated sensitivity of the method is 1/20 fringe. Our results are therefore consistent with the second postulate of special relativity, but not with Kantor’s hypothesis.