About the Zero Mass of Photon
The photon, that is the messenger of the electromagnetic force, is considered with a zero restmass. Yet, just as there is no energy with a zero value, so we talk about a "Zero Point Energy", for the equivalence between mass and energy (datum point of the Theory of Relativity and foundation of modern Physics), there should exist also a "Zero Point Mass". That is, no particle, with energy, though extremely small, as the energy of the quantum of light, the Planck's grain, can have a zero mass. In other words, just for the equivalence Mass-Energy (E = mc2), to any particle with energy should correspond a mass equal to the energy carried, divided the square of the speed of light. Of course when we consider particles like the photon this value will be extremely small, however it should be ≠ 0. Thus, a lot of the behaviours of the photon, in which it shows a clear sort of mechanic action (see photoelectric effect, Compton effect, or the Raman effect), so far ascribed to a mere energetic effect, may probably be considered as real "mass effects".