1. The problem statement, all variables and given/known data When considering conservation of energy and momentum in the collision between a photon and an electron (in Compton scattering for example), is it reasonable to worry about 'stationary' electrons? 3. The attempt at a solution From what I can recall the derivation of the Compton scattering formula was based on a stationary electron. But how is this justified? Are electrons in stationary atoms always stationary? Thanks for any help!