[Moderator's note: Spun off from https://www.physicsforums.com/threa...city-in-a-medium.843312/page-2#post-5294258.] The web page http://plato.stanford.edu/entries/quantum-field-theory/ says: "Quantum Field Theory (QFT) is the mathematical and conceptual framework for contemporary elementary particle physics. In a rather informal sense QFT is the extension of quantum mechanics (QM), dealing with particles, over to fields, i.e. systems with an infinite number of degrees of freedom." In my understanding from above, quantum mechanics should be included in modern QFT. Quantum mechanics is developed based on Einstein light-quantum hypothesis; for example, Rayleigh scattering, Stokes Raman scattering, and anti-Stokes Raman scattering are all explained based on Einstein light-quantum hypothesis: E = hf. Now I have a question for you: In modern QFT, photon energy is derived or is taken from Einstein light-quantum hypothesis?