1. The problem statement, all variables and given/known data Using energy and momentum conservation requirements, show that a free electron cannot absorb all the energy of a photon. 2. Relevant equations Einstein energy-momentum formula: (E)² = (mc²)² - (cp)², where m = mass, E = energy, c = speed of light, and p = momentum vector. 3. The attempt at a solution I expanded this by setting the initial energies of the photon and electron equal to the final. However, I am not entirely sure what significant expression I'm supposed to be looking for here in order to answer this question. Thanks.