Show, using spacetime diagrams and also using frame-independent calculations, that the law of conservation of 4-momentum forbids a photon to be absorbed by an electron, e + γ → e, and also forbids an electron and a positron to annihilate and produce a single photon, e⁺ + e⁻ → γ (in the absence of any other particles to take up some of the 4-momentum); but the annihilation to form two photons, e⁺ + e⁻ → 2γ, is permitted.