1. The problem statement, all variables and given/known data Find the velocity of the centre of mass of two photons with different energies (they happen to be approaching). 2. Relevant equations E = hf p(photon) = E/C E=gamma(mc^2) 3. The attempt at a solution I have calculated the energy of the two photons in the centre of mass frame (using conservation of 4- momentum) and also the momentum of the centre of mass. However I have no idea how to get a velocity out of this. Thank you.