1. The problem statement, all variables and given/known data "Use the laws of conservation of relativistic momentum and energy to show that an isolated electron cannot emit a photon." 2. Relevant equations 3. The attempt at a solution So far I have tried to answer using the p = p' + q where q is the momentum of the photon, along with E^2 = p^2*c^2 + m^2*c^4, but am lost on how to prove that it is not possible to emit a photon.