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. [Listener: Sam Schweber; date recorded: 1998]
TRANSCRIPT: So I got on the bus in Berkeley and I always loved these long Greyhound bus rides, especially because I never stopped on the way. In those days of course it was very different from Greyhounds today. They had these long rest stops where you could go to the bathroom and get meals. They didn't have bathrooms on the bus. And so you could actually see a lot of the country as you went across. You'd have usually half an hour or an hour at the rest stops, so I saw a good deal and, and I remember Wyoming and I remember Salt Lake City and I remember Kansas. But anyway, somewhere around Kansas, after about 48 hours of being sort of half awake, half asleep, suddenly everything fell into place and I understood Feynman and Schwinger altogether; how they fitted together, and... Feynman was essentially talking the same language as Schwinger, only putting the time in a different order, and... so the Feynman propagators were simply time ordered Green's functions and they were also multiple commutators, so all three languages actually were the same.
[SS] May I interrupt you? Then this was done without paper? This was really done on the bus?
Yes, it came absolutely just in my head only, and I knew very well that I couldn't write on the bus; I knew very well that once I got to Chicago I could write all this down and it would make sense. And the equations were sort of already well-formed in my head. So we droned on through Iowa and finally ended up in Chicago. And there I stayed also in the International House in Chicago, spent a week there, and walking the sand on the shore of Lake Michigan, I explained all this to Christopher Longuet-Higgins, drawing diagrams in the sand like in the style of Archimedes! We had a great time, and I was able to make it all clear, first of all on the sand and then afterwards on paper.
[SS] And clearer to yourself?
Yes. So the first, then the first written account of it was done in the International House in Chicago. I also learned a lot from Christopher about what's going on in chemistry. He was doing the hydrides of boron which were really interesting because it has, you know, it has anomalous valences and the normal rules don't apply. Nevertheless it has a lot of stable hydrides. So he was able to understand that. And then, after a week in Chicago, I took another Greyhound bus to Princeton and settled down here and then wrote up the official version of this work, which was the paper called: The Radiation Theories of Tomonaga, Schwinger and Feynman. And meanwhile, of course, we'd heard about Tomonaga's work which was, I think, also in the spring of '48, when Tomonaga sent his first two papers from Japan, and these came as an absolute total surprise, that somebody in the rubble of Tokyo was actually able to do physics. I hadn't heard of Tomonaga previously and he wrote to Oppenheimer from Tokyo and Oppenheimer sent a copy of the papers to Hans Bethe in Cornell, and so we saw them there. And these two papers of Tomonaga, it was called, I don't remember... on the many...
[SS] So he sent you - I mean so you had available a copy of The Progress of Theoretical Physics?
Yes. This was the new Japanese journal which was published in English. It was published on brown paper which was all they had in Tokyo at the time, and it was just like a voice from the deep. I mean we thought of Japan as being a total ruin and there was this man who had somehow or other kept physics alive all through the war, and there it was. And he'd in fact done all this long before Schwinger and essentially arrived at the same results as Schwinger three or four years earlier, with more or less the same techniques. I mean, his techniques were very similar to Schwinger, but actually crystal clear, much, much clearer that Schwinger. So that helped again. Anyway, so I wrote up the paper and just explained why all these things were the same and the time ordering was essentially the key to it and wasn't all that difficult really. And so once you had this time ordering method you could translate Feynman into equations, and then... so anybody could actually do it. I mean, all you needed was to write down the equations in the way normal physics is done, and then the Feynman rules would follow. So that was published in the Physical Review around November '48. [...]
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