Hi everyone, I have a doubt which was actually raised by a recent discussion on this forum. We know that Furry's theorem says that diagrams with an odd number of photon vanish. Roughly the reason behind that is the charge conjugation symmetry, since under such operation the photon field behaves like [itex]A_\mu\to -A_\mu[/itex]. However, I have seen many times the "soft bremsstrahlung" diagram, i.e. e+e- scattering with the emission of an additional photon. This diagram is usually used to deal with infrared divergences in QED. Why doesn't it vanish? Thank you very much.
As was said in that thread, Furry's Theorem applies only when an odd number of photons is connected to the same internal fermion loop.