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I was reading though a Phys.SE thread on Feynman’s QED and FTL photon travel over short distances where the most popular answer explained how Feynman was not implying photons can travel faster than the conventional speed of light, and said they would be interested in a direct quote of Feynman referring to photons. I am about to start undergrad phys so I am curious as to a heuristic explanation of this as Feynman says on page 89 of QED:

‘There is also an amplitude for light to go faster (or slower) than the conventional speed of light’ ... ‘It may surprise you that there is an amplitude for a photon to go at speeds faster than conventional speed,$c$. The amplitudes for these possibilities are very small compared to the contribution from speed $c$; in fact they cancel out when light travels over long distances.’

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I believe Feynman tended to view the virtual photons as real physical things. While the modern view tends to be that the virtual photons are merely a symbolic way of working out integrals in a perturbative expansion.

It's probably a philosophical matter whether you view the virtual photons as real or not.

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  • $\begingroup$ I assumed it was in reference to a virtual photon to begin with as well but there’s no reference of the photon being virtual, rather, he’s just describing a photon going from A to B $\endgroup$ – DanHJEV Oct 6 at 19:24
  • $\begingroup$ If fields are real then all particle interactions are “virtual,” since they’re all simple models of field interactions. $\endgroup$ – JPattarini Oct 6 at 19:31
  • $\begingroup$ Well QED was not really that old at the time so people didn't really know what the right interpretation is of it. What he's talking about is the exchange of a photon from an electron to another electron. (Both coming in from infinity and then going off back to infinity). $\endgroup$ – zooby Oct 6 at 20:06
  • $\begingroup$ @DanHJEV The fact that Feynman does not actually use the word "virtual" does not mean that he is not talking about virtual photons. The last sentence : "in fact they cancel out when light travels over long distances." essentially means that those photons are indeed what is usually understood as "virtual photons". $\endgroup$ – Alfred Oct 7 at 9:05

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