Velocity of Gamma Rays from a Moving Source

Serious difficulties from extinction are shown to exist in the interpretation of past experiments on $\gamma$ rays from moving sources. We have measured the relative speed of the two $\gamma$ rays emitted forward and backward by a ${\pi }^0$ meson decaying in flight. The velocity of the neutral pions, which were produced in the reaction ${\pi }^{-}+p\to {\pi }^0+n$, was $v=0.2c$. We have compared our results with what would have been expected, taking account of extinction, on the assumption that the initial photon velocities were $c+v$ and $c-v$. The results were in complete disagreement with this assumption.