>> ^offsetSammy:

Here's something I have never understood about Feynman diagrams, and I hope someone can explain it to me.
A Feynman diagram represents one possible way that two particles can interact, and from a single diagram you can work out the probability of that event occuring. But wouldn't there be an infinite number of ways an interaction could play out, and therefore an infinite number of diagrams? How do you know which one to draw?


It depends exactly what you mean. For an electron-positron annihilation/scatter, there are a couple of basic diagrams as he showed in the film. These are called "second order diagrams", indicating that there are two vertices. You can add in extra loops and vertices in the middle of the diagram to create third, fourth etc. orders, but each one contributes a very quickly decreasing amount towards the whole picture. I'm no physicist, but I think after the fourth or fifth order they're pretty much just ignored.

If you mean there are an infinite number of "things that can happen" for each input, then no; it's very limited by the rules of the diagram (mostly based around conservation rules - charge, momentum etc.). Drawing out the diagram, twisting it around and swapping the joins and vertices is a very good way of determining what the possible outcomes are.