*promote

Self promoting this video and sending it back into the queue for one more try; last queued Wednesday, April 11th, 2012 5:11pm PDT - promote requested by original submitter GenjiKilpatrick.

very approachable, thank you

in particular i hadnt explored this term
http://en.wikipedia.org/wiki/Phonon

Being a nerd but not a scientist, i've read about how awesome Feynman Diagrams are but this is the first time i've understood them!

I love the Sift. Whenever there's something in physics that I'm mulling learning more about, someone posts a video about it within a few weeks.

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?

>> ^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.

One of the best things about Feynman was his ability to explain stuff to ordinary people. So, here he is explaining stuff to other physicists in a way THEY can understand. That's a Boss.

@offsetSammy (I'm a physicist.):

Basically what @GlasWolf said. There are in principle an infinite number of things that can happen between the input and the output (you may for instance always add an extra self-energy term as shown in the video at 5:30 onwards), but usually only a few of all these infinite possible processes make up most of the total process.

To understand what I mean by that, it's important to remember that we are talking about quantum physics here. When we say that the different subprocesses have different probabilities, we don't mean that the particles choose (with a certain probaility) one of the possible Feynman diagrams to follow. No, in fact all the possible diagrams are followed at the same time. But the fact that some of the diagrams are more likely than others means that they are weigthed more heavily in the calculations. All the diagrams that contribute a tiny part to the total process can therefore be ignored, making the calculation much easier. And if your calculations turn out incorrect, you can simply add a few extra diagrams and try again.

This is also related to why the Feynman diagrams at first were met with scepticism by the physics community. Particles behave according to quantum mechanics, and don't go along straight lines as in the diagrams. But it turns out that they are very useful for translating the inherently counter-intuitive quantum physics into a language that is easier for the human brain to understand. They were therefore accepted as a very valuable tool, even though they are technically unphysical.

*learn

Adding video to channels (Learn) - requested by messenger.