This post is not related to my E=mc^2 post but is a follow-up on the thread: photon collision by misnoma. The following questions were not addressed in that post but I wanted to ask anyway since that thread is now locked. This is better a high energy question anyway. A) Is it possible for two photons to merge into a single photon of higher frequency all by themselves without help from at atom, nuclei, vacuum, particle of mass m? B) Can a high energy photon ever be split into two lower energy photons? Specifically: Is this reaction possible with only the interaction of the photons: hf1 + hf2 = hfT where the resulting photon has the combined energy of the two previous photons that combined to form it? Would the reverse reaction occur I looked up the following articles mentioned on this website from the previous thread: http://www.hep.ucl.ac.uk/opal/gammag...-tutorial.html [Broken] http://2physics.blogspot.com/2006/03...cattering.html [Broken] The former link is confusing and seems to involve particles of mass. The second link requires a vacuum. I also looked up Delbrück scattering, but you need an atomic nuclei to be involved. Compton also needs an atom. Quantum transitions involves energy levels of electrons. If the probability for such an event is non-zero in any higher order correction, does anyone know where I can find that information? If the answer to my questions are "No", that photons can't merge into photons or seperate into lower energy photons without help, would I be right in saying that I would therefore absolutely need an atom, nuclei, particle, or vacuum in order for photons to merge or seperate into other photons?