1. The problem statement, all variables and given/known data Show that although gamma rays can have energies greater than the energy of a e+e- pair at rest, the process gamma --> e+e- is forbidden. 2. Relevant equations Noether's theorem 3. The attempt at a solution I know that to show that the process is forbidden, I should be able to show that it violates a conservation principle. But I'm not at all sure what formalism to use for the problem. Can someone give me a nudge in the right direction?