“…In [3] Gell-Mann showed that in the four-Fermi limit of the standard model the amplitude is exactly zero to order G F ; this is because, according to Yang's theorem [4], which is based on rotational invariance, two photons cannot couple to a J = 1 state. Therefore the amplitude is suppressed by the additional factors of ω/m W , where ω is the photon energy and m W is the W mass [3,5,6,7]. As a result, the typical vacuum cross sections are exceedingly small (e.g., see [8] for the process γν → γν).…”