Let us assume that we want to describe the full process of photon emission by electron A and absorption by electron B. Therefore electron B must be on the forward lightcone of electron A. In the normal forwards in time description a virtual photon propagates from A to B depositing a certain amount of energy and momentum onto electron B. But does this process alone also describe the recoil of electron A? Should one also include the backwards-in-time virtual photon which propagates from B to A depositing an equal amount of negative energy and momentum onto electron A? One could say that the product of the forward-in-time and backward-in-time propagators give the full probability of photon emission by A and absorption by B. Is this where the Born rule comes from?