The story
The theorem was derived by Wendell H. Furry in 1937.
Etymology from Wiktionary, CC BY-SA 4.0.
Furry's theorem
Today it means: In quantum electrodynamics, a theorem stating 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 (and therefore a single photon cannot arise from the vacuum or be absorbed by it). Full definition →