QFT Question: What is meant by "dipole form"? Hello physics people! Probably a very basic question, but here goes. I'm taking a course on QFT based on Ryder. I've heard my professor refer to propagators as having a "polar" or "dipole" form. Things like (k^2 - m^2 + ie)^(-1) For anyone who has a copy of Ryder hand, the specific case I'm puzzling over at at the moment is equation 1.20 on page 16, where the text above says that the form factor G_M has a dipole form. ( G_M = (1+q^2/M_q^2)^(-1)). I'm not sure what is meant by something having a polar/dipole form. I can't seem to find anything on google or wikipedia, but I might not be digging deep enough. Can someone shed some light on this?