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Feynman diagram for pair annihilation 
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#1
Mar2414, 03:58 AM

P: 339

Hi,
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:Fe...nihilation.svg I don't understand the following things about the diagram: 1. Why antipatricles 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. 


#2
Mar2414, 09:05 AM

Sci Advisor
Thanks
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#3
Mar2414, 02:03 PM

Sci Advisor
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#4
Mar2414, 10:21 PM

P: 339

Feynman diagram for pair annihilation



#5
Mar2414, 10:27 PM

P: 339

I'm trying to figure out why the internal line has to point in the direction of the positron's vertex. Is it because the virtual particle is emitted by an electron and absorbed by a positron? 


#6
Mar2514, 04:46 AM

P: 857

forward/backward in time is just an interpretation of the negative energy solutions of Relativistic quantum mechanics. In a very rough sense, instead of having negative energy, you have traveling backward in time. An electron traveling backwards in time is seen as a positron traveling forward in time.
There is no actual meaning in where the virtual particle points at... in physics, we integrate the propagator over all points in spacetime. Thus you can grab the 2 vertices, move them around and get the same image the other way( emission from positron and absorption by electron). 


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