1. The problem statement, all variables and given/known data High energy photon of energy E collides with a low energy photon of energy 1eV. It forms an electron-positron pair. I have found the threshold energy as E=261GeV. I am asked to find the speed of the centre of mass at this threshold energy. 2. Relevant equations E=γmc2 3. The attempt at a solution I can use conservation of energy to get the energy of the electron (or positron) say then use the above formula to find v from γ. I believe this then should be the CM speed. However it returns c. I don't believe that this is an acceptable solution (I don't have the answers though). Any thoughts would be helpful, thanks.