[Physics] How does charge work if photons are neutral

electromagnetismquantum-electrodynamics

How can an electron distinguish between another electron and a positron? They use photons as exchange particles and photons are neutral, so how does it know to repel or attract?

Best Answer

Well, one quick answer, if you want to answer this at the level of QED, is that there are two Feynman diagrams (one where an electron scatters off of a photon, which then scatters off of a positron; another where the electron and positron annihilate, and then the photon decays into an electron and positron) describing $e^{+} + e^{-} \rightarrow e^{+} + e^{-}$, while there is only one that describes $e^{-} + e^{-} \rightarrow e^{-} + e^{-}$ (electron scatters off of a photon, which scatters off of another electron). The short of it is that you can't consider just QED vertices--you have to look at entire Feynman diagrams of processes, and at least know how many of them there are.

Of course, another answer is that, simply, the classical theory distinguishes these two processes--Maxwell's equations plus the Lorentz force law tell me that one process is attractive, and the other is repulsive. Naïvely, one would expect that the quantum process, by land large, would mirror the quantum process.

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