I believe, nobody knows exactly what photon is. Instead we have several MODELS of photon. Let us summarize these models. 1. Classical Physics. There is NO photon in classical physics. There reason is that there is NO QUANTIZATION of field in classical physics. However there is an electromagnetic radiation in classical physics that is closely related to photon. 2. Atomic Physics (or Bohr model of atom) There is some preliminary idea of photon. It is known that atoms can radiate during transition from level of energy E2 to E1. Total radiated energy should be (E2 - E1), however the radiated wave has not particle-like behavior. 3. Schrodinger model of atom. Schrodinger equation does not describe photon, but photon does exist in the Schrodinger model. Energy of Photon is hw, photons are created when atom goes down from E2 to E1, photons are annihilated when atom absorbs them and goes up from E1 to E2 FREE photons exist as electromagnetic radiation 4. Heisenberg model I think that is similar to the Schrodinger model, except there is NO FREE photons. Because one of ideas of Heisenberg was to make a theory that describes ONLY what can be measured. We cannot measure photonic field without destroying it. 5 QFT Inherits basic features of the Heisenberg model and generalize it for relativistic case. I suggest add more details to these models. I think in result we will see clearly that what we are talking about is not REAL PHOTON, but our simplified understanding of it, because a specific theory put some limit on what we can think about photon and what we cannot.