> Is momentum the only type of energy a photon has?
Well, momentum is not a type of energy.
But then, for photons and any other massless thing, the momentum is a function of the particle's energy. It's mass that make it possible[1] for the momentum to vary.
[1] IANAP and don't know the exact causation relation here. Talking about causation probably does not even make sense, but I'm not sure of that.
Not quite. It's the ability to travel at different velocities that allows the momentum to vary for particles with mass. But massless particles like photons always travel at the speed of light, so that option is not open to them. Therefore, their energy and momentum are always proportional.
Well, momentum is not a type of energy.
But then, for photons and any other massless thing, the momentum is a function of the particle's energy. It's mass that make it possible[1] for the momentum to vary.
[1] IANAP and don't know the exact causation relation here. Talking about causation probably does not even make sense, but I'm not sure of that.