Rotational Doppler Effect and Spin-Rotation Coupling

The Rotational Doppler Effect (RDE) involves both the orbital angular momentum of electromagnetic radiation as well as its helicity. The RDE phenomena associated with photon helicity go beyond the standard theory of relativity. The purpose of this paper is to elucidate the theoretical basis of the helicity-dependent RDE in terms of the general phenomenon of spin-rotation coupling. The physical implications of this coupling are briefly pointed out and the nonlocal theory of rotating observers is described. For an observer rotating uniformly about the direction of incidence of a plane wave of definite helicity, the nonlocally measured amplitude of the wave is larger (smaller) if the observer rotates in the same (opposite) sense as the electromagnetic field. The implications of the nonlocal helicity dependence of the measured amplitude of the radiation are briefly discussed.