1. The problem statement, all variables and given/known data Calculate the photon energy threshold for the reaction γ + p → p + π. Assume that the initial state proton is stationary. <thats really it> 2. Relevant equations < unknown, but suspects include: standard special relativity equations > -- other useful relations: proton rest mass, mp≈938MeV/c2 neutral pion rest mass, mπ0≈135MeV/c2 3. The attempt at a solution < havent yet, see below > Alright so I don't need help solving the problem yet, rather I need help understanding the problem. I admit, I may have missed class the day this was introduced, but I may not have... I'm still learning how to follow this professor. (wonderful person, bit of an accent, clears overheads before I can finish writing down notes, also often uses less-oftenly-used, granted commonly accepted, language ex: center of mass frame vs center of momentum frame. ). I am not clear on what is meant by the "photon energy threshold". Is this the minimum energy of the photon needed to produce the reaction? or might it have something to do with the reaction carrier photon (think Feynman diagrams). Instead, is it somehow related to QM scattering... I really don't think this is so, I certainly can't see how, but that about covers what we have started so far this quarter. I realize not many, if any, of you are in my class-- and therefore probably have minimal ideas of what my instructor my want from us. I might ask a fellow student but there are few I really know, and none whom I know how to contact. The index of my textbook makes no reference to "photon energy threshold" or variations there-of. Any, and all, suggestions you have on how to interpret this question will be full-heartedly appreciated and explored. Thank you.