@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.

>> ^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.

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?

Your video, Quantum Field Theory Made Easy! - Feynman Diagrams, has made it into the Top 15 New Videos listing. Congratulations on your achievement. For your contribution you have been awarded 1 Power Point.

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!

>> ^Sagemind:

OK, Just a minute, I'll be back once I've finished my Masters in Physics and Mathematics and learn Feynman Diagrams - Then I will make a comment.
Now..., Where do I register for the courses?


I was thinking the same thing, I didnt understand any of this. With that said, in the amazing age of the internet, there are quite a few paths to courses atleast leading up to this, for example, check out the Kahn Academy, it's brilliant: http://www.khanacademy.org/

OK, Just a minute, I'll be back once I've finished my Masters in Physics and Mathematics and learn Feynman Diagrams - Then I will make a comment.

Now..., Where do I register for the courses?

He doesn't really setup why people want to use feynman diagrams. The reason is quite interesting. It turns out the simplest (i.e. the fewest lines) interactions are the most probable and dominant. There are also specific conservation rules you have to follow when drawing them. The genius of these diagrams is that we draw all the solutions to a given QED problem and solve for the wave functions in an intuitive way. It is a clever trick.

It's a bit simplistic to say that the conservative physicists like Bohr and Dirac rejected Feynman's diagram because they loved their complex mathematics so much. Fact is that Feynman diagrams are very dumbed down versions of reality, as quantum particles do not move in such simple straight lines as implied by the diagrams. Their power lies in how they are an exquisite intuitive tool for keeping track of how the complex equations should be put together. When you first hear of them there is a danger of mistaking them for being meant as accurate pictures of reality, and I think this was a big part of Bohr and Dirac's misgivings.

Anyhow, when you keep in mind that the diagrams are always to be used in conjuntion with the complex mathematics, they are perhaps the most useful tool ever discovered in physics.

This has nothing to do with Feynman diagrams. This is
a trick way to do long multiplication.