Furry's theorem
In quantum electrodynamics, Furry's theorem states that if a Feynman diagram consists of a closed loop of fermion lines with an odd number of vertices, its contribution to the amplitude vanishes. As a corollary, a single photon cannot arise from the vacuum or be absorbed by it. The theorem was first derived by Wendell H. Furry in 1937, as a direct consequence of the conservation of energy and charge conjugation symmetry.