Freeman Dyson - The Feynman diagrams (72/157)

To listen to more of Freeman Dyson’s stories, go to the playlist: • Freeman Dyson (Scientist)
Freeman Dyson (1923-2020), who was born in England, moved to Cornell University after graduating from Cambridge University with a BA in Mathematics. He subsequently became a professor and worked on nuclear reactors, solid state physics, ferromagnetism, astrophysics and biology. He published several books and, among other honours, was awarded the Heineman Prize and the Royal Society's Hughes Medal. [Listener: Sam Schweber; date recorded: 1998]
TRANSCRIPT: So Feynman has this path integral picture of the world, as if the world was a kind of a tapestry in which all kinds of things could go on and all you had to do in order to predict the future was start with a known state in the past, allow everything to happen in the intermediate time in all possible ways, every particle or every field could jiggle around as much as it wanted in all directions, and then at the end, in the future state, you want to calculate the probability amplitude for a particular configuration in the future, you simply add up the contributions from all the histories in between. Each history contributes a certain probability amplitude and the amplitude is just the integral of the Lagrangian over the space time volume between the past and future. So that was Feynman's picture and it made sense, it was understandable as a physical picture. But then he had a practical version of this which was a sort of a crude approximation which was the Feynman diagrams, which were very different actually, although they were supposed to be an approximation. The Feynman diagram just consisted of a set of straight line tracks which were supposed to be individual particle tracks, and joined at, vertices where two or three lines would intersect, and each vertex corresponded to an interaction and each straight line corresponded to a particle track. And then you had propagators which were telling you the probability amplitude for the particle to move from A to B, and then instead of a path integral you had just a sum over the propagators. And that was supposed to give you the answer, and the amazing thing was that it did, the amazing thing was that this very simple diagram method gave you the right answers although the connection between that and the path integral wasn't at all obvious. So what I was always trying to persuade Feynman was that it's not enough to get the right answers, you have to understand what you're doing. And so we had big arguments about that, and I told him that he ought to learn some quantum field theory if he wanted really to understand this, and he said it just was a language he never would learn and he didn't think it was worth it. As far as he was concerned he thought in pictures and he didn't think in terms of equations. I thought in terms of equations and not in pictures. So we never agreed, but we just had fun talking.

[@mrnarason]

"it's not enough to get the right answers, you have to understand what you're doing" savage

[@andrewbudiman1310]

Victor P. shut up

[@grahamblack1961]

Shut up and calculate.

[@DumbledoreMcCracken]

That is the problem, and the difference between Mathematics and everything not Mathematics.

[@pokeman123451]

There’s nothing to understand if someone doesn’t theorize about things no one else understands.

[@foreyfriend145]

From fan of F.D: Hmm sounds like F.D saw Faynman diagramas as abstract paintings, but I think that also we cannot fall in sort of mathematic absolutism ( what is not translatable into equations is not worth or right).

[@clayz1]

RIP Freeman Dyson. It is my wish that God has by now shown you and every other deserving scientist to step to the other side just how it all works.

[@NothingMaster]

Dyson being essentially more of a natural mathematician than a physicist per se, in the strictest sense of the word, always preferred the mathematically rigorous approach (hence the path integral formulation preference) and the QFT recipes. Whereas, Feynman was first and foremost an intuitive physicist with an incredibly curious and creative mind, a creator of original ideas who instinctively preferred to deal with the diagrams. That is NOT to say that Feynman wasn’t an incredibly gifted and competent mathematician/theorist; he was after all the one that fully established the path integral formulation. It’s just that Feynman’s creativity more readily lent itself to the pictorial representations of mathematical expressions and the journey of the fundamental underlying picture.

[@Dubickimus]

oh, Its just the integral of the Lagrangian over the space-time volume in the past and the future. lol simple stuff, really.

[@u.v.s.5583]

Does that integral exist, does it converge in some strong, absolute sense?

[@Cotonetefilmmaker]

@@u.v.s.5583 it does exist. It almost certainly it does not converge, but it does give out the right answers every time.

[@u.v.s.5583]

@@Cotonetefilmmaker Oh, let's notify the press, let's call it the God's Integral!

[@The268170]

What the fuck is a integral >:\

[@ShazzPotz]

@@The268170 So you understand "the Lagrangian over the space-time volume between the past and the future". You just don't know what the Integral is?

[@master_rajeev]

Amazing

[@foreyfriend145]

Popular Quantum Mechanics, IMHO you need two guys playing in the same league to put it (whatever the matter/disagreement is/was) that clear.

[@coolcat23]

If there is one thing that bugs me about Feynman (apart from stories about his ego), it is that AFAIC he was too comfortable with not knowing why something works. He was very accepting of phenomena and apparently did not feel the urge to peek under the hood. I also think in pictures but I wouldn't do physics to get results (or approaches that deliver), I would do it to understand what is under the hood. Quantum field theory looks like it has a much higher chance of being ontologically accurate than probability-based symbol games like Feynman diagrams.

[@maxwellsdaemon7]

0:41, Dyson must have been simplifying (forgetting?) since the amplitude is NOT simply the integral of the lagrangian (i.e the Action), but rather the Exponential of the integral of the lagrangian...