Feynman Diagram

**Moose352** · Jun1-04, 11:36 PM

I'm pretty new to the particle physics world, and modern physics in general, so bear with me. I also don't have a book yet, so it's kind of messed up I'm a bit confused by a few apsects of feynman diagrams, although my confusion actually lies much deeper. Looking at this diagram http://www.mov.vic.gov.au/scidiscove...01708_w150.jpg which shows two electrons repelling, I don't understand why the vertex of the electron on the right is higher than that of the left. Actually, I don't think I even understand the concept of photons mediating the em force. I guess I don't really understand when a particle emits photons, and what implications that has. Can anyone point me to a good online source (until I get a book) or explain?

thanks

**Janitor** · Jun2-04, 01:35 AM

As far as the part of your question about one vertex being higher than the other, I believe it is conventional for particle physicists to let time be the vertical axis and a dimension of space be the horizontal axis when they draw Feynman diagrams. So you could interpret the diagram as meaning the particle on the left emits a virtual photon, and some time later the particle on the right absorbs it. When they calculate the associated amplitude, I don't think it really matters what the temporal ordering "really" was, and in fact if you tried to do something experimentally to detect which particle emitted the photon and which absorbed it, you would affect the process badly enough to create all sorts of mischief which would in turn have to be accounted for in your calculations.

**Norman** · Jun2-04, 12:41 PM

Originally Posted by Janitor

As far as the part of your question about one vertex being higher than the other, I believe it is conventional for particle physicists to let time be the vertical axis and a dimension of space be the horizontal axis when they draw Feynman diagrams. So you could interpret the diagram as meaning the particle on the left emits a virtual photon, and some time later the particle on the right absorbs it. When they calculate the associated amplitude, I don't think it really matters what the temporal ordering "really" was, and in fact if you tried to do something experimentally to detect which particle emitted the photon and which absorbed it, you would affect the process badly enough to create all sorts of mischief which would in turn have to be accounted for in your calculations.

Janitor is exactly correct... the verticle axis is time, so the lower left hand particle emits a (virtual) photon and at some time later (up on the verticle axis) it is absorbed by another particle. All that matters when you calculate the amplitude, which in turn allows us to calculate the cross section, is what kinda of reaction channel it uses (in this case it is the t channel). When you know the channel and the correct momentum and masses you can calculate the amplitude using the rules for Feynman diagrams