The question I am asking comes from a side issue in another thread. I am seeking an explanation that will explain my wrong understanding of the physics. https://www.physicsforums.com/threads/q-re-wiki-equation-for-t-x-t-2.903984In post #19 of the above thread I describe my uncertainty about my understanding. I have been assuming that the concept of equilibrium at a given temperature, with respect to a mixture of particles, including photons and matter particles, means that for each kind of matter particle, the average kinetic energy of those particles is the same as the average photon energy. Is this wrong? If this is wrong, can you suggest where I can find out what the correct relationship is?In post #20, mfb responded. It is wrong. Classically (and the protons behave like classical particles here), you have the same average energy for every degree of freedom. The degrees of freedom of the electromagnetic field are a bit more complex, they do not correspond to photons moving around. In addition, you get a Bose-Einstein statistics due to quantum mechanics, which differs from the classical statistics. So I now know I was wrong, but I still don't understand what the correct relationship is. I would much appreciate someone (1) posting an explanation of the correct relationship of the average kinetic energy among the particles of a particular matter species, e.g., protons, and the average kinetic energy of the photons in the mixture, OR (2) citing a reference that has such an explanation.