A sensible introduction. A snippet:
The assumption of effects running back in time is not only consistent with the experimental results, but also simplifies the physical description of electron-photon interactions. … Feynman posed however the question if effects running back in time could also be a possible mode of interaction. He thought specifically about incoming, so-called “advanced” waves, that converge simultaneously from all sides ending up as if by magic in the center. They represent the time-reversed version of the outgoing, so-called “retarded” waves, that we know from our everyday experience, for example in the form of radio or water waves. He found out that the effects of advanced and retarded waves compensate each other so that the overall effect corresponds exactly to the observable phenomenons [sic]. More precisely, Feynman’s analysis revealed that they compensate each other almost completely. The difference results in an increased resistance opposing the electron acceleration that is actually measurable. There is no convincing alternative explanation for this increased resistance to the present day.
Some compressed background discussion and references here.