I am trying to use FeynCalc package for Mathematica to check some of my cross sections computations. Is there somewhere a tutorial with examples how to use the package?
Since E = mc^2, how can photons be massless? If a photon has no mass, then, according to Einstein's formula, its energy is given by E = 0 x c^2, which is 0. Yet, photons do have energy. This seems to be a complete contradiction. Please explain! Thank you.