>>1
Particle/wave duality might make it easier to understand. A particle nature photon has infinitesimal mass, but it is a mass nevertheless so it would bend to the will of gravitation. But when it behaves as wave things become quite complicated. I can't really get into that, it is already too hard to imagine a wave like motion. A sine function graph may come to your mind but that is hardly anything like it. You follow a trajectory while fluctuating your electric and magnetic fields while having no mass and volume but a position...