Feynman Videos

Hey all. I recently found a set of videos on YouTubefrom a workshop on Quantum Mechanics by Richard Feynman in 1983.

Here's the link to the first one: https://www.youtube.com/watch?v=72us6pnbEvE

I think the most interesting point raised during this first video is the talk about Feynman diagrams (aside: think how weird it must be for Feynman to be talking about Feynman diagrams to people. They're actually using that term to the man himself, and I just think that must feel disorientating). The question someone asks is: if the successive Feynman diagrams for an interaction (say, light scattering off an electron. There's a series of diagrams involving more and more virtual photons and electron-positron loops etc) represent some sort of infinite sum, then why do we observe each term in Nature? If the whole set of diagrams is a mathematical trick, then how come we actually see each term occur in reality? The answer is that it's a confused question: we don't see each term seperately.

If you're not too familiar with Feynman diagrams I'll use a very simple concrete model. (This also helps me, because I'm not overly familiar, and it's easier on us all if we just accept that the details following this are probably wrong, although the point is correct.) Let's have an electron and a photon scatters off of it. We want to calculate the probability of that happening, so we sum successive Feynman diagrams representing each possible way that the event could occur.

Diagram 1: The photon collides with the electron. The electron absorbs it, moves through time a little, then re-emits it.

Diagram 2: The photon collides with the electron. The electron absorbs it, the re-emits it AND THEN re-absorbs the photon and finally re-emits it again.

Diagram 3: Same as 2, but when it re-emits the photon, the photon turns into a positron-electron pair, which annihiliate each other, regenerating the photon which gets re-absorbed by the electron and the re-emitted.

The point is that each diagram has a physical meaning, and that they get progressively more complicated. Each diagram represents one way something could occur, and the associated probability; the probability of any of them happening is simply their sum.

So the question is: if the probability of the electron scattering the photon is some infinite sum - how come each 'term' in the sum actually occurs? Answer: it doesn't. The diagrams are a mathematical trick to break up a complicated (or maybe impossible) calculation into smaller parts. But what happens in reality doesn't have to be those things happening seperately, and of course you can't actually watch a photon go and get emitted and absorbed by an electron; what really happens is some kind of 'combination' of every possible way that happens to add up to what really happens. This is very similar to Feynman's path integrals where the particles 'sniff out' every possible path - what it actually does is a matter for philosophers, but maybe it goes along all of them, maybe it goes along none of them. But it doesn't really make sense to say that you can observe it going along just one of them.