1. The problem statement, all variables and given/known data A 2.5 MeV photon is moving in positive x-direction and an electron in the opposite direction at a velocity of 0.99c. Calculate their common total energy, momentum and total rest mass. 2. Relevant equations Relativistic Equations 3. The attempt at a solution I have some concerns with the statement of the question. First of all how is it relevant that the particles are going in opposite direction? Not sure how to go about calculating the momentum of a photon since it has no mass, and would it ever get a relativistic rest mass? When asked about the rest mass of the electron, how is that not just the table value? It's the relativistic mass that would need calculating. So I would like some help with this early exercise to get me going in the right way to think about such a problem. All examples in the text book has been about observing a moving inertial frame of reference and finding the relativistic variables. This is quite different. Especially since a photon always moves at the speed of light.