[SOLVED] Annihilation, perhaps a defination problem Problem: An electron and an anti-electron undergo mutual annihilation. Which of the following does not have to be conserved? A. Electric charge B. Kinetic energy C. Lepton number D. Momentum Attempt to question: The answer is B but I why not also d. I would consider that the mass of photon is zero. So in both B and D, kinetic energy and momentum are perhaps zero. Problem is: it is impossible to divide by zero. Therefore, this problem seems to a definatition problem. It must be that momentum conserves but why? What is the assumption in the theory? Thank you for your answers!