For some reason (most likely lack of coffee and sleep), I can't figure out these questions. I assume they all have to do with conservation of energy and/or momentum, but I'm lame and need help. Explain clearly why the following processes are impossible. 1) A single photon strikes a stationary electron and gives up all its energy to the electron. 2)A single photon in free space is transformed into an electron and a positron. 3) A fast positron and a stationary electron annihilate, producing only one photon. I also have a million other questions, but I'll keep my first post short.