I read somewhere else on this forum that photons have zero invariant mass, only relativistic mass. Here is my problem, Einstein's equation for relativistic mass says that is invariant mass is zero then so will relativistic mass at all velocities. If a photon does have relativistic mass, then invariant mass would be greater than zero and before a photon could reach light speed it's mass would diverge to infinity. What am I missing here? Thanks in advance.