Propagation of Light in Non-Inertial Reference Frames

Summary

This chapter revisited the question of the constancy of the speed of light by pointing out that it has two answers — the speed of light is constant in all inertial reference frames, but when determined in a non-inertial frame, it depends on the frame’s proper acceleration. (The local velocity of light, however, is always c.) It has been shown that the complete description of the propagation of light in non-inertial frames of reference requires an average coordinate and an average proper velocity of light. The need for an average coordinate velocity was demonstrated in the case of Einstein’s elevator thought experiment — to explain the fact that two light signals emitted from points A and C in Fig. 8.1 meet at B′, not at B. It was also shown that an average proper velocity of light is implicitly used in the Shapiro time delay effect; when such a velocity is explicitly defined, it follows that, in the case of a parallel gravitational field, the Shapiro effect is not always a delay effect.

The Sagnac effect was also revisited by defining the coordinate velocity of light in the non-inertial frame of the rotating disk. This velocity naturally explains the fact that two light signals emitted from a point on the rim of the rotating disk and propagating along its rim in opposite directions do not arrive simultaneously at the same point.