I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is frozen in time and is frozen in its state. It can never change states." If that is the case, then how come we can see photons coming into and out of existence on Feynman diagrams? If they travel at the speed of light how is it possible for them to interact and change states?