Relativity and the Doppler effect

The Doppler effect is a familiar velocity dependent pitch shift known from acoustics, which occurs for electromagnetic waves, too. Since light doesn't need any medium to propagate, in 1905 Einstein deemed the Doppler shift for light a purely relativistic effect, arising under Lorentz transformation of the propagating waves' phase, and tied it to the inertial reference systems. Nowadays, a non-relativistic technique of the same name, useful at speeds as low as a few cm/s, finds applications to Doppler radars and in laser Doppler velocimetry. Here, we review this issue, differentiating between relativistic and low-speed Doppler effect.