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Steve D'Aprano wrote: > I've seen (or at least, I remember seeing) > diagrams of matter/antimatter annihilation with the two particles coming > together and a single photon coming out: a simplified and strictly wrong view > of the physics. It may or may not be wrong, depending on what the diagram was supposed to represent. Each vertex in a QED Feynman diagram involves one electron, one positron and one photon. That much is perfectly true. But if you work out the probability of that process happening on its own, it turns out to be zero. You need a diagram with at least two vertices to get a non-zero probability. Another way to say it is that a single-vertex diagram correctly represents a virtual process (involving virtual particles, which don't have to conserve energy or momentum), but it doesn't correspond to any real process (since real particles have to conserve both). -- Greg

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