Photon Doppler Shift??? I am a practicing EE who has used the standard Doppler shift equation for thirty years in radar design; therefore, I am not disputing the correctness of the equation but the "explanation" that is so often given in text books. The general explanation always discusses relative velocity between source and receiver. I have always wondered how the "photon" that was emitted or absorbed "knew" the relative velocity in question. The pictorials always describe the wavefronts being bunched closer or further apart as the source/receiver moves. But at the time of emission or absorption how does the photon know its velocity?? And relative to what?? The effect is such that on the relative velocity is important but the explanation of how this info is imparted to the photon leaves a lot to be desired. If anyone on this forum has a good reference to this I would greatly appreciate hearing about it. My own thoughts are that there is a background FOR to which all source/receiver velocities are referenced. It is the difference in the effects that the velocity relative to this FOR has on the photon that is eventually made known at the receiver. The FOR maybe unnecessary from the point of view of SR but the explanantion of how this relative velocity info is imparted to the photon is surly lacking. Also could this be a case for the quantum mechanical argument that it is only observables that are important in a theory since one can really only measure energy of the photon directly??