Recent content by noether21

The analytic continuation argument used to derive crossing symmetry (Itzykson & Zuber) seems to require that there are no massless particles (vacuum is an isolated point). Further it appears that the external legs must be on mass-shell in the crossing symmetry derivation. In the 3-point...

Thanks for the pointer to Weinberg's book. Weinberg's discussion also seems to merely make the claim without offering any proof; he refers to Feynman's 1949 paper (section 8). Feynman has an argument to show that the divergence of the amplitude of a process that emits a photon...

Since the 4-vector A couples to the conserved current j^\mu = \psi-bar gamma^\mu \psi in QED, k_\mu Fourier-transform(j^\mu) = 0. The Fourier-transform of j is a mess. Is there an easy way to see why terms containing photon momentum k_\mu drop out of the photon propagator in practical QED...

If the momenta on the three external legs p(incoming fermion), p'(outgoing fermion) and p-p' (photon) of a vertex diagram are replaced by -p, -p' and p'-p respectively (i.e., all the external momenta are multiplied by -1) does the amplitude remain unchanged?