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I've been reading about Feynman diagrams lately and I'm trying to understand the pair annihilation diagram. The picture's here: http://en.wikipedia.org/wiki/File:Feynman_EP_Annihilation.svg

I don't understand the following things about the diagram:

1. Why anti-patricles have to travel backward in time

2. What the horizontal solid line represents

3. Why gamma rays come out of both vertices.

Any help would be greatly appreciated.

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# Feynman diagram for pair annihilation

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