Is photon simply a massless particle？——The concept of photon revisited

Photon is often referred to as a massless particle, but this is in no sense an adequate designation. The physics for photon can never be recovered from that for massive particles by simply setting m=0. The energy-momentum relation, the (in-)dependence of velocity on the reference frame, spin and the projection of spin, to name just a few, are some aspects that show radical differences between photon and massive particles. Even in terms of Bose-Einstein statistics which both may observe, the difference does not lie in the fact whether the objects of concern are with or without mass. Photon is de facto referred to the energy quantum of the radiation field, which was proposed in the effort to fit experimental results. Quantization for the light field has followed a different scheme as for massive particles, it is simply realized by replacing the amplitude of the radiation field with annihilation and creation operators. Since it is impossible to define a position operator, thus the countability, being relevant to the locality problem, of the photon turns out to be rather questionable. The current article tries to provide a comprehensive review over the concept of photon in the framework of quantum mechanics, relativity and statistical physics, in the hope of provoking a re-examination of the precise formulations and underlying pictures concerning this fundamental notion.