✔ 最佳答案
One point that you need to note is that a photon has no "rest mass". If it had, it would not be able to travel at the speed of light according to the Special Theory of Relativity.
In the equation m = E/c^2, you could treat such calculated result of "m" as the "mass" of a photon. But this is only a mathematical calculation result, it has no physical significance at all.
Strictly speaking, the classical definition of momentum, mass x velocity, may not be applicable in the quantum world. The derivation of the equation p = E/c using the classical approach that I gave is just an "illustration", it cannot be regarded as a formal proof. This is similar to the case that Bohr's model of the hydrogen atom can be proved using a classical approach with principles in electrostatics and circular motion. But the formal proof requires the use of Schrodinger's equation in quantum mechanics.
All in all, it is commonly accepted that a photon has energy and momentum but no mass. More specifically, you can only write down the momentum of a photon
as E/c, but not mc.
