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Interview of George Uhlenbeck by Thomas S. Kuhn on 1962 March 31, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4922-2
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This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Henri Abram, Niels Henrik David Bohr, Max Born, Louis de Broglie, Max Delbruck, Paul Adrien Maurice Dirac, Tatiana Ehrenfest, Paul Ehrenfest, Walter M. Elsasser, Enrico Fermi, Ralph Fowler, Samuel Abraham Goudsmit, Werner Heisenberg, Oskar Benjamin Klein, Hendrik Anthony Kramers, J. P. Kuenen, Otto Laporte, Hendrik Antoon Lorentz, J. Robert Oppenheimer, Wolfgang Pauli, Isidor Isaac Rabi, Harrison McAllister Randall, Julian Schwinger, Arnold Sommerfeld, Llewellyn Hilleth Thomas; American Physical Society meeting (Boston), Huygens Club, Kapitsa Club, Rijksuniversiteit te Leiden, Technische Hogeschool Delft, University of California at Berkeley, and University of Michigan.
George, I would be very grateful for anything you can do to illuminate this question of knowledge and conviction about the anomalous gyromagnetic ratio of the spinning charge.
Ja, well let me tell you. Let’s not talk completely about the spin, perhaps yet. But I can tell you that when we thought about it, we had no idea of publishing. It was too strange, and we were so young. This I have already told at several occasions. What we did, of course, was tell it to Ehrenfest. Ehrenfest was immediately interested. He said we should write something up, and that was the Naturwissenschaften letter, which was really completely written by us. Now about this gyromagnetic ratio, we simply postulated it. Because, taking it over from the core, we had of course to make the same assumption for the electron. I still remember that Goudsmit was not with me all the time that fall. I was assistant of Ehrenfest, Goudsmit was assistant of Zeeman. He was three days of the week in Amsterdam, and then he always came to Leiden for the colloquium and, stayed also, I think, a few days in Leiden. There is a certain vagueness in my memory at which time we started really to write it up. That means at which time Ehrenfest told us to do it, because he surely did. I remember that Goudsmit wrote me a letter, in which he made very primitive models. This whole Abraham business was really quite straight-forward. If you have a (point) electron moving in an orbit then you get gyromagnetic ratio of e over 2mc. That was of course standard. Sam wrote me a letter in which he did something that was very primitive, and he says “I don’t know why, but for a continuous distribution,” — he certainly didn’t use these words! — “might it not be different?”
Continuous charge distribution?
Continuous charge distribution, and mass distribution too. I thought about it, and I talked with Ehrenfest… He immediately said that had been done. He said to look in the early papers of Abraham. I was, of course, at the same time in charge of the library. So I made a search of all the early papers. There was this paper, and there was the factor two, for the case that the charge distribution was a surface distribution. You see, the charge had to be only on the surface of the sphere. The mass, of course, had to be uniform. Abraham had also made calculations for a uniform distribution; you then get another factor. In fact, by changing the distribution, you can get anything between one and two. Anything. Well, that you got at least two for one case, that was very encouraging at that time. That was why we mentioned it in the Naturwissenschaften note. Now Lorentz knew that right away. We gave this note to Ehrenfest, and he says, ‘Now you must also talk with Lorentz about it,’ and we did. Lorentz was retired already. He was in charge of the Teyler Institute in Haarlem which was a kind of retirement job… He lived in Haarlem, but he came every Monday to Leiden and he gave a lecture from 11 to 12 Monday morning. It was the Monday morning lecture of Lorentz. There you had to go. Ehrenfest simply drove you to it. People came from all over.
Lorentz there talked always about his recent work or recent literature. It was always very beautiful, beautiful lectures. Anyway, that was why he came to Leiden. One of these Monday mornings then we saw Lorentz and told him about this idea. Lorentz was not discouraging. He was a little bit reticent. He said that it was interesting and that he would think about it. He immediately, of course, knew about Abraham, and he also then thought he should do some calculations about it. And he did. It was so typical of Lorentz, that he immediately made very extensive calculations on the classical theory of rotating electrons. I think already the next week, but maybe two weeks later, he gave me such a stack of papers with long calculations. Large white paper, I still remember. He tried to explain it to us — to me, but it was so learned for us that I — By the way, he published these things. It was one of the last things Lorentz published. It was this contribution to the Como Congress. The only thing which was clear out of his explanation, the only thing I really remember, is that he painted out this famous difficulty, that the magnetic energy would be too large. If you believe in equivalence of mass and energy, you have to take the radius of the electron extremely large, you see, in order to get in. Otherwise you have, beside the electric energy, magnetic energy, and it would be much too heavy. I took the old radius, and then the electron was as heavy as a proton. So it looked nonsense.
This was later pointed out also by Fermi, in a short note to Nature, I think. But this was the difficulty. I remember that we were not even very depressed by it, because we thought it was probably nonsense anyway. Otherwise people would have done it already. And so we told it to Ehrenfest, and that I still very well remember. We told it to Ehrenfest. Lorentz was, of course, the god nearly, also for Ehrenfest, and certainly for everyone in Holland — the absolute authority. “Lorentz has shown us that this is nonsense.” And so we said to Ehrenfest, “We’d better not publish that note.” And then Ehrenfest said, “I have sent it away already weeks ago and it will appear next week” And then he said to us – I don’t know whether Sam remembers, but I remember — “Well, Sie Beide sind jung; Sie konnen eine Dummheit leisten.” And he went on, and that was all. Then the next week really it came out. And the funny thing was that three days after that Sam got a letter from Heisenberg. Sam got a letter from Heisenberg! In which he says, “very interesting,” and he said, “what do you do about the factor two in the fine structure formula?” That was the Thomas factor, but at that time we had not derived the fine structure formula… In the note we only said that qualitatively everything was then in order, especially that the fine structure was proportional to the fourth power of z, and that, as also mentioned by Lande, the doublets were inverted. Which was right, you see. I have made the note in there too.
I know that this whole passage that Lande mentioned — I remember it quite well. It happened, of course, after this note in Nature, because St. Nicholas Day is December 5 in Holland, and then it had already appeared. So he must have not remembered that it had already appeared. So he must have not remembered that it had already appeared. Anyway, we got this letter of Heisenberg, and he says, “What do you do with the factor two?” Now this was much more precise, of course. But we had only these qualitative considerations… And then we had to derive it ourselves. That was quite a struggle. I think I did that finally. I really learned to do a little bit of perturbation theory, which I didn’t know at that time very well. I did it only for circular orbits. And there came this remark, which Van der Waerden makes so much of, but I don’t think it was so important really. That was, namely, that if you sit on the electron you don’t have a magnetic field because of the relativistic transformation. Or much more simply, because of the Biot and Savart Law of the rotating positive charge. I remember that Einstein mentioned that. Then it is relatively straight forward, of course, although still not obvious, how to do the perturbation theory. You have to compute the average energy then of that thing. And I did that, and then I got, of course, also the fine structure formula with the factor two wrong. And so, I don’t know whether Sam answered Heisenberg. I certainly did not do that…
What was the relation of the piece in Nature and the piece in Naturwissenschaften?
I will tell you. That came about as follows. Bohr came to Leiden. It must be either November or December of that year. You see, this all happened in the fall.
This was after the Naturwissenschaften piece was out?
Ja, Ja, after that. It was the golden jubilee of Lorentz’s doctorate in Leiden, a big occasion. Bohr came then and he always stayed at Ehrenfest’s house. We talked then with Bohr really very long — whole mornings — and Bohr especially urged us to look back at hydrogen spectrums. Now we had done that before of course, but only on a formal basis. That was mainly Sam’s contribution. That was the first paper we wrote together. It appeared in Dutch only… We postulated that the spectrum of hydrogen had to be, as it is now. That appeared already, I am sure, in August or so of that year. That was the first paper we wrote together, and Sam mainly wrote it. The only contribution I made to it was really because I was so dissatisfied. Sam gave me lectures all the time, because I was, of course, in these things a complete beginner. But I was so scathing about treating hydrogen one way and the alkali spectra another way. I said that must be nonsense, you must do it at least in the same way. And then Sam, I think in a very short time —
This was really the fine structure, the difference in the treatment of the fine structure.
The fine structure, yes. Then Sam, because he had known so much about this formal spectroscopy, guessed how it was. And then, I remember that I still looked through the (dissertation cards). I had all these things. The only further contribution I made to the paper, was that I saw there was a line in the hydrogen fine structure which Kramers did not explain. He did the Stark effect for hydrogen, but of course without spin. He explained all kinds of things of the Paschen measurements, but he did not explain this outstanding line. He didn’t say anything about it. It was just this line which was possible in a new interpretation, because the selection rules changed. You see, it was a forbidden line previously, but now became allowed. And it was clearly there. We mentioned that in the paper as an argument for this new interpretation.
… What is the gist of that paper? Is it a treatment of hydrogen in terms of a Rumpf rather than in terms of the relativistic —?
No, no. It was purely formal. It was simply how the energy levels should be, and how it then could be conducted with alkalis. Without interpretation of what quantum numbers meant. Just a j, then there was an l, of course, but what they meant, we didn’t say. Of course the connection with the relativistic Sommerfeld formula was not in this first paper. That appeared in the Nature. Of course that was on the insistence of Bohr. I remember that in these discussions with Bohr, we finally came then to this famous picture about how it was that it split and that you just got the old one back, but now with the double degeneracy.
That all came out during these conversations with Bohr?
Yes, and that was in Leiden. Then Bohr left Leiden and he went to Germany. He wrote then also to Ehrenfest that he felt as a pop-gun on the spin. He gave lectures about it, talked about it to everybody, and really because of that everybody accepted it. Although there was still the factor two. Except Pauli of course. He didn’t believe it.
What did Bohr say about the factor two? What was the situation?
Well, he says, that will come in order. You see, he was also completely uninterested in the classical difficulties. Completely uninterested! I remember that I talked with him about what Lorentz said. He said, “this is of course not classical, and so that one has not to think about it in those terms.” He was, of course, worried He always was worried about such difficulties, but he had such a feeling that that was the answer to many of these difficulties. I think with regard to the Nature letter, in memory, he essentially wrote it. After these computations. Probably Kramers helped. We did really very little with it except that we had this picture, and we had also told him about Heisenberg’s calculation and the factor two and so on. That was all in this letter. But the style of that letter is essentially Bohr. You must ask Sam whether he wrote anything of it. My memory is that I certainly didn’t write one word of that letter. Although it was signed by Sam and me with an appendix of Bohr.
Yes, I know.
Which was of course very nice. Although the contents, we knew, completely in Leiden. That was in January, I am sure. Then, because of this, Sam went to Copenhagen. This was January or February I think of ‘26 — must be ‘26. It was the Lorentz fellowship. At the 50th doctors’ jubilee thing, there was a Lorentz fellowship created, and he was one of the first Lorentz fellows. And there he worked, unfortunately not successfully, with Bohr, on the helium spectrum. There was this great difficulty which now we know is exchange energy. Sam tried in all possible ways to make models by which it could be understood. So he came back a little bit discouraged, I think, in the spring of that year, without further success. But at that time, when he was there, Thomas was also in Copenhagen. Thomas had this factor two. That paper of his was so learned at that time. He was also such a remarkable — he gave, then, a speech about it in Leiden. I still remember it very well, because he couldn’t write on the blackboard. It was just physically impossible for him to write on the blackboard. Everything came out so large. It was very remarkable. Kramers then also tried to simplify it very much. Anyway, we were able to simplify it sufficiently — Sam and me. Then we wrote our third paper on the spin, which is not very well known. That was a review paper, which Sam was supposed to write but which we wrote together. That appeared again in Dutch, and in it is the simplified derivation. There, so to say, also the whole connection with x-rays and with alkali spectrum, and how everything then tied together was presented. That was the most complete presentation of the spin, as we knew it. That must have appeared around June of ‘26. Then Sam went home, because then he went to Tubingen. That’s where he did, in the fall of ‘26 and ‘27 already, hyperfine structure with Back. I think that was the time. While I, together with Ehrenfest, in the fall of ‘26 started to do essentially the quantum statistics. That became my dissertation. Then Sam and I were together again in Copenhagen. We both, of course, had to write our dissertations.
Returning to the question of the anomalous gyromagnetic ratio, if one doesn’t worry about self-energy, electromagnetic mass, the derivation is almost trivial.
Absolutely! It was trivial.
And in that sense one doesn’t need the Abraham paper. In fact the Abraham paper is vastly more difficult and is almost about something else.
I don’t think that had much influence at all. It was only that, you see, in our naivete, we thought that it was impossible to get it, because one always thought of point electrons moving. Ehrenfest remembered this paper. He knew the literature very well, of course, and he remembered this paper. We looked it up, and there was the factor two. And it was a sort of encouragement that it was therefore not so classically impossible to get it.
But even though you then supposed a body with continuously distributed mass with all the charge concentrated on the surface?
This didn’t seem so odd as the —
Well, this was always the classical picture. We always thought that it was a little sphere. It was only a kind of small encouragement, and that was why we mentioned it. But then Lorentz of course came along, and says, “But if you think of it that classically, then you have all these problems.” … So I don’t think it had further any effect at all afterwards. I think that both in the Nature paper and in the third paper we didn’t even mention Abraham any more. We simply postulated that these were probably intrinsic properties of the electron.
This also points to another problem that’s bothered us. Why didn’t anybody get hold of this idea for the. Rumpf? Is it that the Rumpf was never thought of, conceptualized as a single body?
Right. It was always separate particles moving.
And the Rumpf angular momentum, the Rumpf quantum number, represented a sum of the angular momentum for a group of discrete particles.
You never thought of it as a core or a sort of equivalent single particle rotating?
Never. Never. I don’t think that was ever an idea. And therefore factor two was always funny.
What sorts of problems were most on your and Sam’s minds, that seemed most unsatisfactory about the Rumpf model, problems in which you saw that the spinning electron might do something for you?
About that it is so difficult to say. Since we are at it we might as well tell it. I can tell you how it came about, you see. I was of course in Italy for three years, teaching the son of the Butch ambassador. That was a job I had gotten from ‘22 to ‘25. I was a very dutiful student always. I took my exams in time. When I had this job, I also did my exams. I worked on it there, and then I came back on a vacation and did the intermediate exams and final exams and so on.
These were the exams for the doctorate?
These were the doctoral examination, which was before the thesis. That is, in Holland, separated. You see, there is a doctoral examination and then there is a dissertation.
How many subjects did you have to cover? How broad were those doctoral exams?
Oh, quite a lot of mathematics, and theoretical mechanics, which was taught by a mathematician. Then there was theoretical physics. It depended on the major and the minors. But the traditional one was if you were theoretical physics. Then you took surely mechanics and mathematics as minors. You had to do quite a lot of mathematics — function theory and what not.
No electromagnetic theory?
That we did with Ehrenfest. The mathematics was function theory, advanced analysis, so to say; differential geometry; and then this mechanics…
Then the theoretical physics exam included the electromagnetic theory?
It was Ehrenfest. Ehrenfest was always extremely cavalier about it. I mean if he trusted someone, then you had hardly to do any exams. And he said, well you know it.
What did he want to make sure you knew?
He essentially gave only two courses. The one year it was Maxwell’s theory, which always ended up with a little bit of special relativity; the other year he gave statistical mechanics, which ended up with a little quantum theory and, atomic structure. And that was essentially all that he taught. It was four hours a week. I had done the exam, and so I was what they call a “doctorandus”, which means I was ready for my dissertation. I did this relatively early, but then I liked it so much in Italy that I stayed an extra year. It was against the advice of Ehrenfest, but I did it anyway. The third year I didn’t do any physics whatsoever. The second year I knew Fermi, and I had a seminar with Fermi in Rome. But the third year I didn’t do anything. No physics whatsoever. So then I came back. That was in June. We always came back in the summer because the student had to do his examination to the next grade. This was, in fact, the last one, because this time he had to do the final exam for the gymnasium and he went through. So I was really finished. It was clear that I wanted to get my dissertation. But I was really at that time in a kind of a shaken position. I was so fascinated by history I thought I should study it. This was even practically impossible, because for that you had to have classical languages, which I didn’t have. So I began to study Latin. While I was assistant to Ehrenfest I did that. But then that was all soon over of course — because of the spin, essentially. I came down with Ehrenfest, and he says, “all right.” I think he had his doubts at that time about me. I was too elegant, because I had lots of money in Italy. This was because of the devaluation of the lire. You see, I was paid in Dutch guilders. I had, thank God, saved money. But then he spoke of the possibility of assistantship. He says, “Anyway, now, you have to learn a little bit of what is going on.” I knew Sam, who was younger than I was. He was two years after me really, but he was at that time already an expert.
You had known him before you left?
Oh yes, sure. Only as a younger student, so to say — not very intimately. Now Sam was not a dutiful student. He never could do exams. He had, not even done his doctorandus exam, which I had done already a few years ago. This was because he was so afraid of the professor in mechanics. That’s a very interesting sidelight. Finally we had to push him through his doctorandus exam. Ehrenfest was only finally able to do that by letting him drop mathematics or, rather, mechanics as a minor. But he had to have two minors, so he took experimental physics and astro-physics as his two minors. This was of course at that time very strange, because as a result he was not allowed to teach mechanics or mathematics in the Dutch high schools. Of course as soon as he was in Ann Arbor, he was the one who always gave the course in theoretical mechanics… He did it always with great pleasure. I always kid him that he has no right to teach it. According to the Dutch law he is not allowed to teach mechanics. Anyway, he had already published several papers. He knew Heisenberg and Hund, and he knew all the spectroscopy of course very well.
Well he was already clearly driving toward spectroscopy?
Oh yes, that was his specialty. Especially this kind of formal business, this looking with the help of numbers through the experimental material, and getting some regularities out. In that he was really a past master.
And in this you had never been interested?
Not interested? I didn’t know it. I just didn’t know it. Then Ehrenfest gave Sam the task of teaching me. That was one of Ehrenfest’s pedagogic principles. He always wanted to have people work together in pairs. That was his method. You always have to do it together — that was his principle. So that summer from June on, Sam came. I think we came together twice a week. The other days I went to Leiden to start working with Ehrenfest on something quite different — partial differential equations, the wave equation, properties of the wave equation, which I was also very much interested in. Sam just lectured to me. We were together sitting in a room, and he told about all these successive points, and he did that very beautifully.
This was all on spectroscopy and atom models?
Spectroscopy and atom models, and so on. I learned it quickly because of that, you see. I remember that then very soon came about all these questions of the old duality, Nichtmechanische Zwang. Somehow there was something mysterious going or because of the two orientations of the core. There were papers by several people, I think also by Bohr — which really escaped us altogether. But we studied them — especially Pauli’s papers. Especially Pauli’s papers on the exclusion principle. That was already known to us. There, especially, the four quantum numbers appeared, which for the first time, in a formal way, are attributed to the electron. There was this hocus-pocus of combining them so as to explain the periodic system, the Zeeman effect, and the transition to the Paschen-Back effect. This was all in the Pauli papers already. Sam had studied those, and we always came back to them. Now the date is — I am in conflict with Sam about the date. He has a very good way of dating it, because he says it; was at the same day that there was a tornado in Holland. I don’t remember that quite. But anyway, I thought it was earlier. He may have been right that it was August. It was one of these times. Then came this idea that if there were four quantum numbers then it couldn’t be solved with the three degrees of freedom. Now that would only be there if it was really rotating, if it was not a mass point, but something which rotated. We always looked upon it as really a rotating thing. It was quite clear that if you then have it rotating, and if you give it as nowadays half quantum number — in those days it was still one — either way it had two orientations only. This was then the explanation of the alkaline doublets. That occurred immediately to us. The anomaly of the gyromagnetic ratio was still difficult, but then came this period — which must have been in September — when we talked with Ehrenfest about it. The academic term had already started. I was already then in function, because Ehrenfest had then begun to get confidence in me, and I became his assistant right away. And then Sam went to Zeeman, so it must have been in that period that Abraham and also this connection with Lorentz developed. That must have been, I would think, the end of September, beginning of October.
You told me before, in talking about the hydrogen paper, that in your discussions with Sam you kept insisting it was nonsense to treat hydrogen one way, the alkalis another. As you learned the whole of spectroscopy and models with Sam, or from Sam after the return from Italy, were there other things which you or he or both of you felt to be wrong?
Well, that’s so mysterious, you see. The thing with hydrogen was, of course, that you had the Sommerfeld theory. This was a difficult theory, but it was solved, so to say. One didn’t think about it anymore. Then there was this other one, where you had the vector model. This seemed to be completely different. Every other word was the vector model, of course, because that was the only thing at hand. And then you had to do such hocus-pocus, you see, changing the quantum numbers once in a while from j2 to j2-1/4. Nobody was satisfied with it, but that was, so to say, the spirit of the times. You had to guess it somehow. I only remember that my great dissatisfaction was that hydrogen was, so to say, left aside. I thought it was terrible that in the simplest case there, somehow, the model did not apply. There you had another theory. That was this paper which I have, surely, here. It must have appeared in August, really. Sam may have been right, that all this appeared a little bit earlier than I think. But that, surely, is the first paper we wrote together. It appeared in Physica.
One of the problems with the Rumpf that Bohr and Lande raised was that if you specify the quantum numbers and then ionize the atom, you ought to be able then to predict what the total angular momentum of the next element over in the periodic table is.
That was the Aufbau principle.
Right. And that doesn’t work. Was this a bad problem for you people, also?
Oh yes, that was one of the great riddles. They had several names for it, we mentioned it also in our letter… It certainly was a difficulty, but not so sharp, because one did not know what to do with it. It was certainly not with us the starting point for our theory. I think the starting point was really Pauli’s paper, and the four quantum numbers to one electron. Of course then, already, the Aufbau principle became that of two (electrons). Of course Pauli had also a very important paper — which we knew too — showing that the rumpf couldn’t have a regular momentum because of the relativistic effects. Of course these are isolated things, so to say. There was of course discontent with them, but one didn’t know what to do with it for sure. We certainly didn’t know what to do with it.
I take it that very often what precedes a basically new idea is a generally diffused sense that something’s wrong. I’ve been trying to give more structure to this sense of something wrong, even though it isn’t a particular thing being wrong that leads to a particular idea, but the general sense that things are not going as they should be.
There was, of course, this general feeling. First of all, it was a central problem just what the proper quantum theory should be. That was general; that everybody had.
Do you mean quantum theory for the atom, or does this sweep in the whole question of black-body radiation, specific heats?
For the atom, the structure of the atom. How, really, the correspondence principle should be sharpened, how one could get all these remarkable regularities in the spectra, that was the central problem of ‘24, ‘24 for sure. How it had to be done, that of course maybe some of the great people, as Heisenberg, had some vague, ideas, but we certainly didn’t know. And in such a case, you see, there were also, in a sense; no sharp contradictions. The situation was always really such that you had to take for granted certain things which somehow seemed strange. It was not such that you could say, as with the Michelson-Morley experiments, there was a sharp contradiction.
There wasn’t with that either.
Well, with a certain disrespect to Lorentz, I think there was. At least for some of the people there was. The situation is there similar as it is today really, I think. Now one knows also that this is not the final theory, and therefore no one knows what to do about it. At that time one had not the feeling — at least we didn’t have the feeling that one was very close. But then I was of course a complete beginner, and in a sense Sam was too. Sam knew the spectroscopy, but he didn’t, know all this theory of the multiple periodic systems, and what not and what not. Celestial mechanics, the helium spectrum — all these things were for Sam, and at that time for me too, completely new. We just didn’t know those things.
Were you concerned about some of the other outstanding problems? The dispersion problem was particularly messy.
No, we didn’t know that very good.
You were aware that it was troubled.
We knew about Kramers’ paper, but did not know it very well, because that was very difficult stuff. For us that was very difficult stuff, you see. And the Raman effect and so. We knew that it was there, and we knew roughly how to describe it, but the mathematical formulas were not known to us at that time — to Sam and me. Of course it was known to other people. This was also interesting what Lande says about De Broglie. That surely was unknown to everybody.
The thesis of De Broglie. Nobody spoke about it, not even Ehrenfest. Nobody! I didn’t even know of its existence before Schrodinger. And I am sure that it was not discussed in a colloquium. Ehrenfest’s colloquium was, in that respect, perfect. Every new thing which came up. So I think he must be wrong.
I think very probably; though, clearly, there is a difference. Einstein got excited fairly early about it. You knew Einstein wrote to him about it?
There must be people who knew about it, all right. Maybe Ehrenfest knew it too, but I don’t remember that he ever drew our attention to it in any way.
What about the problem of the corpuscularity of light?
I discussed this with Ehrenfest quite often, because he had this paper with Epstein. We had several of these Duane-like explanations. I still have notes about that. We tried then to get intensity formulas, which was not so easy as one thinks. But nothing came out of that… But it was still in that sense separate from atomic structure. Atomic structure was one way. All those questions of photons was another way, another part which was perhaps also dark, but had nothing to do, or not in an immediate sense to do, with the spectral problems. There you have, of course, not only the Land interval rule, but you have also the intensity rules of Ornstein and Burger, which Sam had worked on. He and then Kronig. Fermi wrote about it. All from the correspondence principle guessing from the correspondence argument, … which one was remarkably successful with. You see Sam was there really very good. He had these formulas, guessed by means of certain measurement that you have to have. Still, it was clear that this was all, so to say, clever guessing. That there should be a theory, everybody thought, surely necessary all right. But it was far away; we thought it was far away.
A little bit like the renormalization problem when people learned how to get rid of (infinities.)
Ja, ja, I mentioned that in my talk at Leiden too. Because in the atmosphere at the time there was such a remarkable analogy, too. The years of ‘26, ‘27 and the years ‘48, ‘49. There was the same type of example. Now it will really go. It had also the following effect, that both the quantum theory time and that time simply produced the generation. There was the quantum theory generation, and there was the renormalization generation. A whole generation of theorists really came to the fore around ‘48. Kroll, Karplus, Case — the whole present theoretical staffs in all American universities come from the ‘48 period.
But that one hasn’t been nearly as big and fruitful…
No, because it petered out. It petered out. It turned out not to be such a break-through. That’s a terrible word to use about this.
That’s very expressive.
Ja, but only now you think of missiles. But it was not, of course, like the quantum mechanics was.