I don't know how to understand doppler shifts and yet conserve energy. Consider a red photon is emitted by a torch. This photon is let free to travel through space. Any observer at rest relative to the torch (which by now may be millions of miles away) will see it as a red photon. Another observer aproaching the torch from a distance at a high speed will see this photon as blue shifted; let's say as a UV ray. This observer will conclude that this photon has enough energy to cause a photo-electric emission of a Sodium metal electron. Still another observer receeding form the torch at high speed will conclude that this photon, severely shifted into the red, say an IR photon, cannot bring about the photo-emission of an electron from the surface of a piece of Sodium. Questions: 1) It seems that whether or not a photo-electron is emitted depends on which frame of reference you are in....i.e. whether the freed electron exists outside from the metal or not depends on how the incident photon looks...UV..ok!. IR no effect. Well, the electron can and can't exist freely simply by virtue of what frame of reference we look at a photon from. Worse still; what about using that damned photon for electron pair creation? If I view it as a gamma ray, I'll get the pair creation...not so if I view it as IR!! We must all agree, no matter what our inertial frames of references are to each other, that the electron-positron pair either does or doesn't exist!!! 2) The photon energy will differ from one observer to another because of the relative motion between observers. This is due to the Doppler effect for light. Where does the photon get it's extra energy from if it's seen as UV; or where does the photon loose it's energy to if it absorbed as IR?