As this is referring to photons I'm reminded of a concept that comes from the use of Feynman diagrams to model interactions. I think this answers your question.

As this is referring to photons I'm reminded of a concept that comes from the use of Feynman diagrams to model interactions. I think this answers your question. If you're calculating the probability that a photon goes from A to B you can calculate the probability (or more technically, the amplitude) that it goes straight there (this is the crude approximation), but you also have to account for the case where it went a bit off the way, hit another photon and then arrived at B (a slightly less crude approximation), and you can account for the path where the photon interacted with two other photons for a more accurate approximation, and with three other photons for an even more accurate approximation. People stop the calculation from being infinitely complex because it appears that the probability of the cases with many interactions is incredibly unlikely. I guess the conjecture here was that if you have some error bound for the N interactions case, then the N+1 interactions case would have lower error. And that's been proven to not always be true. I may be way off the mark, my knowledge of QED comes purely from watching Feynman and Susskind on YouTube.