Actually, photons do have mass, although some "purists" insist they only have a momentum, or as we used to say in the Navy "all vector, no force!" :). But we know they have mass because photons are affected by gravitational pull (see "light bending," as well as black holes).
Yes. I should have said c=speed of electromagnetic waves *and* massless particles.
As for photon mass, I don’t know how to square this with:
“In modern quantum physics, the electromagnetic field is described by the theory of quantum electrodynamics (QED). In this theory, light is described by the fundamental excitations (or quanta) of the electromagnetic field, called photons. In QED, photons are massless particles and thus, according to special relativity, they travel at the speed of light in vacuum.”
Maybe it’s a wave/particle thing again.
But then when it gets down to quantum, I get lost pretty easily.