Oral History Transcript — Dr. George Uhlenbeck
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George Uhlenbeck; March 30, 1962
ABSTRACT: 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.
Ehrenfest was a man who always had to get it out of his toes. He had, somehow, no technique. Nothing was in his fingers. He always had to think it out completely from the beginning. Although he knew mathematics it was not simple for him. He was not a computer. He could not compute. That’s the one thing I never learned from him. I had to learn it all by myself later on. There were people like Sommerfeld and Born, who immediately took up an idea and made calculations. He was a little bit frightened and also disgusted by them. Always “diese komplexen Integrale.” Sommerfeld, as soon as an idea comes out, in the next paper does some complex integrals, computes all kinds of things. That was also true of Born. In several of my visits with Ehrenfest to Gottingen the contrast between these two was very striking. Born, on the other hand, was slightly afraid of Ehrenfest. Born was afraid that he’d make the wrong calculations, and then Ehrenfest would, by simple models, by simple logical algebra, show him that it was wrong. Of course I don’t know whether that happened, but he was somehow a little bit cagey about it. Ehrenfest on the other hand was of course impressed by all the mathematics.
We had the colloquia and seminars — Wigner was also there — he can still remember that too, I am sure. Here Ehrenfest always wanted to have the simple point, “what is the point”. Can one say that, so to say, in a few words or with a very simplified model so one could see what the point was. Often he was unable to do it, usually unable. And then Ehrenfest … jumped around. It was always so. It was disgust, really, all the time, all the time. And he never stopped. In a certain sense it was also his tragedy to be interested, because you see when he got older it got more and more difficult for him to get it out of his toes, to learn it all. People like Sommerfeld and also Lorentz, and surely Born, could always fall back on the technique. They could make long calculations. Maybe in the fundamentals it was not very new, but it was always interesting. The electron theory of metals of Sommerfeld was not at all original with him. The idea came from Pauli and Lorentz originally. But then he worked it out so far that it was a whole school after a while. The Sommerfeld electron theory of metals was due to this remarkable facility of Sommerfeld of working things out completely and in great detail.
Then was the problem of quantum mechanics and Ehrenfest. I think he always hated it. Oh, and then there was this whole generation which came with it. All these youngsters, who had, with great facility, made these calculations because it was, so to say, a technique which was given you, and you didn’t have to understand much. You just computed, and you did this and you did that and everything came out. Ehrenfest said. “Diese Klugscheisser.” “Always so clever they were! And nobody understood anything.” Which was partially true and partially wrong of course. There was, therefore, a mathematical apparatus built with Hilbert’s basis, with operators, which had a sort of abstractness. It was so against his creed that I am sure that he suffered from it. I remember that we talked. so much about one of his last papers. He was rather proud of it. That was the “Erkundigungsfragen”. That was a typical paper of Ehrenfest. That was, I think, the last one he wrote. It was in ‘30, I think. I was in Leiden. I went with him to Gottingen. Then he wrote this paper in his inimitable way. He wrote a series of questions. Very humbly he says, “Einige Erkundigungsfragen”. He wanted ‘only a few points of information.’ Then he wrote, typically, number 1, number 2, etc. The paper is still worth reading. Pauli took it up, and answered it, point by point, very respectfully and, nicely, and also very clearly. It was so nice of Pauli. It was very sad that it was not yet known. The answers to these questions were not quite known hen. That gave Ehrenfest quite a kick, “that it nicht tun was. Gar nicht tun, nicht tun.” This is typical now too — this remarkable facility that certain people have with the mathematical formulas. There is, of course, always this tendency. It’s like a machinery which runs wild. It doesn’t churn up anything of real value. That was not true of quantum mechanics, of course, because it was really something which any child could use if he knew anything.
Kuhn:Did he see this about it?
Uhlenbeck:Well he understood it all right. He said he was just too old. It was against his creed to really take part in it. Although he did take part in it. We, in those later years, did all these calculations. Ehrenfest had a good education all right. So he could finally work with spherical harmonics and what not if it was absolutely necessary. He did it too! But mainly I was the one who did all these exercises following the papers of Schrodinger when they came out — the perturbation theory and what not.
Kuhn:Was there an intervening period in which you started doing the matrix mechanics and then switched?
Those came quickly after each other. We studied matrix mechanics too. The Drei Manner Arbeit — Born, Heisenberg, Pauli — written by Born, Heisenberg, and Jordan. But with it you still couldn’t make little problems for yourself, because it was all so unwieldy. As a result the young people did not have the feeling then that they had the key. That came really only after Schrodinger. Then people had the feeling. The equivalence of the two was I think first shown by Pauli… Well it was Eckart who showed it too, but he was, of course, still in the wilds. America was in the wilds. He did it remarkably on his own. But Pauli in a letter to Schrodinger showed the identity. That’s also mentioned in one of the Schrodinger papers — that Pauli saw it right away. I still remember that I then computed the intensity of the Balmer lines. You had all these formulae already. It only was those damn difficult integrals! I liked, already at that time, to compute such integrals, and I did it. But I made a mistake. I made a mistake. And out of the mistake it followed that even for the principle quantum number there would be a selection rule.
Now that was of course nonsense, it couldn’t be. Ehrenfest says, “The complete theory, pfft.” He couldn’t see what the error was. He said, “well you write it all down, nicely, as a letter to Born.” I was a young student and I did it. I wrote these integrals nicely, and wrote it all out. And Ehrenfest sent it off to Pauli with a little footnote, in which he said, “Bitte, behandle die Tiere sanft.” This is only said in wildlife, … Pauli then answered right away. I still have that letter of Pauli. He says, “Ja, ich habe mich auch mit diese Integrale umgeschlagen.” Then he pointed out very carefully what my error was. It was just a substitution. I had forgotten to take something into account. Then he gave the answers, which were also the first answers. He had found those in Copenhagen — typical Paul — when he was in one of these cafes. He was always computing in these cafes. He said … “How to do it.” And he gave me the answers. Then there was also a little postscript, so far as I remember, to Ehrenfest, saying that this letter was very sanft. It was really very nice of Pauli to write me without any sarcasm whatsoever.
Kuhn:I didn’t know he could do that. We hear so much of his sarcasm.
Oh yes, he did. He was really at the bottom a very friendly fellow. He was a man who in his early days always — and it was always clear, at least to me — tried to see whether a person had an area of sensitivity. If he found it, then he pushed, certainly. If he didn’t find it, well, then he looked at you. He always said something, and then he looked at you to see whether that hit something or not. I always had to laugh because I saw that he wanted to do that. I remember in Ann Arbor then I told him, “ja”, I was working on Brownian motion. Paul remarked: “Desesperance physique! Typische desesperation physique!” Then he looked at me. I said, “Es ist schon wahr. Es ist schon wahr.” Then he saw that I was completely untouched about it because I realized that it was so. So afterwards he always said to Elsa, “Ja ja, that Uhlenbeck, such a starken (grad)!”, because he couldn’t get my goat. He was a charming man. I was very close to him. Especially during the war. I have such a beautiful letter. He was at Princeton and I was in the Radiation Laboratory. He had a rather bad time. He was here and was quite unhappy. He was alone at Princeton, really, and there was really nobody to talk to. He was a refugee in a sense, and he didn’t think that they treated him very well.
Well we were both of course in the midst of the war. Ivy whole family was in Holland and in the Dutch East Indies. No I liked him very much really. He was much sharper to the older people than to the younger people. Born was really afraid of him. Ehrenfest too, although he got along with Ehrenfest very well. They were always kidding each other –- in a sharp way all right. Of course, with regard to that kind of wit Ehrenfest was his equal. He could do it back just as readily. There was this famous story about what Ehrenfest said of the Pauli effect. I don’t know whether you have heard of it. This was a famous story. The Pauli effect was that when Pauli came into the laboratory, everything went wrong. He was extremely proud of that. That, he thought, was really remarkable, and he believed it really true. I still remember that in Ann Arbor we went to the lake. On the way back the car broke down. Pauli got out of the car muttering, “Pauli, it is the Pauli effect.” Although it was a joke, of course, yet he felt, “well, maybe something — .” Not at the same time, but an analogous happening Ehrenfest said, “Ah, Pauli. This Pauli effect is very trivial. You know, this is only a special case of a very well-known effect, that ein Ungluck kommt niemals allei.” And Pauli objected to say “It comes naturally.” Ja, that’s a famous “Ehrenfestis.” Ein ungluck kommt niemals allein. When Pauli comes in, and everything else comes also bad. He believed in Jung, and he was a very good friend of Jung. He had a little bit of mystical connection. I had long talks about that with him. One of the remarkable things in which he believed was extra-sensory perception. He believed in that. Whether it was proved, he was not sure. But that it existed, he had no doubt. And that it could exist. He says that there was something like that which is so completely separate from physics. He had also a remark he once told me when we talked about biology, and about the problem of life.
This was in his youth, that I talked with him about that. He told me he did not believe that in all these questions of evolution everything went by random mutations. That was just at the period that Bohr and Heisenberg also and of course Delbruck began to think about complementarity and these things. And Pauli said, “Ja, diese wage are just as strange, just as separate from physics as extra-sensory perception.” “Delbruck could believe! Delbruck believes it!”, he said, but he couldn’t believe that by just random mutation, by just pure chance, one could get this whole development. He could not believe it. He had read a lot in these things, too. He was a very cultured man, a very cultured man. He had all kinds of technical examples which I didn’t know anything about, but apparently about which he had talked with Heisenberg and with Bohr. These three were rather close. They talked about many other things. Bohr and Pauli were very close.
Kuhn:It bothered him not at all that there should be realms quite outside of physics?
Kuhn:He felt no conflict?
Uhlenbeck:I don’t think he had really systematically thought about these things as if trying to make a unified philosophy. That I don’t think he did. Maybe he did. But anyway, he was not convinced that with the quantum theory, so to say, the last word was spoken, or that it was even possible, in that sense. Pauli’s paper on Kepler is most remarkable. I recommend it to you if only for the foreward. I read it with great interest. There is first, in this book, a paper by Jung. This is a typical Jung paper. It is so mystical and also a bit confused, that one doesn’t know one way or the other what this really asserts. After he gives this account, Pauli’s paper is a kind of an appendix which is written with the typical Pauli clarity. At least you know what he asserts. It has these mystical points, but at least one knows what he asserts. And, of course, it is a very nice paper. He has also papers — which I have never really seen — on the development of physics in the 19th century. Always with Pauli you at least know what he says. Which is very often not the case with some.
Kuhn:Particularly with people who take so much of that attitude. Did you see signs of this same thing in his physics?
Uhlenbeck:No. I don’t think so.
Kuhn:You would say that these were totally different aspects of the personality and kept very separate.
Uhlenbeck:Well, he had this feeling of depth. One of the nicest things of his is in the Bohr Festschrift in which he writes a paper on the CPT theorem. Then he gives a quotation which perhaps would please Bohr, because it shows what Bohr always says a quotation from Schiller — …, “even if it is not quite clear, if it has only depth.” Only by such remarks can you see that he had these things. Here, this is this interesting quotation: //This is about the neutrino. This is a letter which he wrote to Lise Meitner: “The gravity of the situation … is illuminated by the pronouncement of my respected predecessor … who recently said to me …, “Oh, it is best not to think about it at all.”// No, the influence of Pauli should not be underestimated. It’s really so deep, you know, on whole generations. More than anybody else, almost more than anybody else. Certainly more than Heisenberg and Dirac. He knew everything, and had a critical opinion about it. In addition he had of course enough original thought … — maybe no finally fundamental contribution after the exclusion principle, but —…
Kuhn:When you say influence, then you mean mostly direct influence on people?
Uhlenbeck:On people, yes. He was the conscience of physics. That’s what everybody said. He was the conscience of physics. Ehrenfest was the Socrates of modern physics. He always questioned and said that he did not understand. But Pauli was the conscience. Pauli really, finally, accepted it. He was not easy to convince, of course, not easy to convince. But finally he accepted it. Then it was surely right, then it was really very strong.
Kuhn:How did he feel about the relation between the new and the old? How did he feel about complementarity.
Kuhn:Was he quickly convinced?
I think so. I don’t know, precisely, that went, because I wasn’t there at that time. The matrix mechanics he took up immediately, and he gave the solution to the one problem that was really hard, namely the hydrogen atom. That’s what he did right away. The second Born book of quantum mechanics, which was completely matrix mechanics was, therefore, already a little bit out of date when it appeared. There is a rather scathing review by Pauli of that book. It came out in Naturwissenschaften. Born was very unhappy about it. In the book wave mechanics was not mentioned, although it was already known at that time. Pauli said that all these problems like the hydrogen spectrum were clone in this complicated manner by means of matrixes, which was really not to be defended. And this criticism, he said, is not due to the impotence of the man who writes the review. A typical Pauli paper! [coughing] He then immediately took up this wave mechanics.
Rather interesting and ironic in a sense is the Pauli spin paper, which was really a rather profound paper, because of the two valuedness of the wave functions. Pauli told me that he never thought that that was such an important paper, afterwards. But he says, “Ja, Es war viel wichtiger als ich dachte.” He has told me that. And the point was that he was the last who accepted the spin, the last! He only did it after the Thomas precession was elucidated. Then he said “all right,” but he still did not quite like it. At this time I was in Gottingen. Oppenheimer was there, and also Lande, and they all said ‘well now we should make a quantum theory of it.’ It was, of course, completely dark to me, how all that should be done in wave mechanics. Then Pauli did it. It was very profound really, because that was a big step to go from the scalar psi to the two component psi. I remember that I studied that paper very hard, and it was very clearly written. Except that it was so full of these transformations of these wave functions — the Caley parameters as I remember, which, of course Pauli knew. It sounded profound to me, and difficult too. Although again, it was that it was so clear that afterwards you could use it. Many people did of course.
Kuhn:How did he feel about Dirac?
Uhlenbeck:He had a great admiration for Dirac. This kind, of dream-like way of using mathematics and then getting something out. ‘The Dirac papers’, he said, ‘you must read all of them.’ He did, and he studied them backwards and forwards. One of the remarkable things about Pauli, really — in contrast to anybody else that I know of among these great physicists — is that he read everything. Everything! I’ve still got memories. He was at Princeton and there was the Physical Review. He began to read it, certain papers of coarse only. He didn’t only read it, but he had paper there with a notebook and he made calculations. The whole paper I think he did that until the end, you know. All the renormalization, all these things. He followed them precisely. He knew them in detail. Which really very few people do. Who nowadays reads a paper? Very few people. And he did it until the end really. I was so surprised when I was in Holland, and I wrote a few papers on the quantum theory of the non-ideal gas. They were all right, I was rather proud of them too. But there were still difficulties. And then Pauli came out, and he said “Ja, dies war nicht tun.” He had given colloquium lectures on them. He had read them precisely. Now, as a consequence of his following the literature so carefully everybody wanted to know his opinion. He fitted it right in where it belonged.
Kuhn:How did other people feel about Dirac? Delbruck relates that to Bohr it was pure formalism.
Uhlenbeck:That was of course quite different. Bohr has also not mathematical. Pauli was — just this dream-like mathematics. Mathematics guided him. Ehrenfest even read. Dirac’s book. He studied it very hard — “ein greuliches Buch” — he thought it was simply terrible. “You can’t tear it apart,” he says, “You can’t tear, it apart!” He had to follow it completely from beginning to end to understand it. He says “Ja, Dirac’s book, genau wie Gips.” So smooth. You couldn’t tear it apart. He wanted of course to chop it up and say “What’s all this assertion? What has he not done? Can’t one —?” And it had this classical character that you could only say it this way and no other way.
Kuhn:Then he said “Genau wie Gips”, did he mean the character of the writing was the same?
Uhlenbeck:I think he was speaking of the presentation. This kind, of didactic way, and smooth, as if it was absolutely necessary that this was the only way to develop the subject and there was no other way. That was something which Ehrenfest couldn’t stand. He always says you must be able to say everything in three different ways. “Komme nicht auf einen (???)”, he’d say. In Dirac’s book, everything was so linear in the logic. And that he disliked, but he admired it too, of course, tremendously. Ein gremliches Buch, but he studied it very hard. I still hope that I can get it out of Ehrenfest’s wife a copy of Dirac and a copy of Gibbs, which are simply full, full with Ehrenfest’s notations. Everywhere. The whole thing — it’s full. I think it is an historical document. But to get them out of Mrs. Ehrenfest is not so easy. She is not easy to handle. It was so difficult to get the letters out of her.
Kuhn:I take it from what Martin says that there are still quite a few letters.
Uhlenbeck:Sure, quite a number left. I only hope that they won’t be destroyed. Still, Dirac was so different from the other people because he was always a little bit off to the sides — and because he was so silent. He didn’t discuss very much ever as I remember, although I saw him only once in a while. He was in Leiden several times. Oppenheimer of course knew him rather well. Although I think he also had trouble with him in his Cambridge time.
Kuhn:You know, I showed Maria Mayer that chapter of the Born biography about Oppenheimer. She says she doesn’t remember it at all. Maybe it was she, but she doesn’t remember.
I was in Gottingen, after I wrote my dissertation. That was a rather hectic year — the second year I was assistant to Ehrenfest. I was assistant to Ehrenfest for two years. Really, the second year I worked on the statistical mechanics of quantum statistics, the subject of my dissertation. Then it was very clear that if I stayed in Leiden as his assistant — you know you worked every day — I would never write it. I had to write it, because I had a job already at Ann Arbor. That was open to me before I got my Ph.D. I wanted very much to do it. Sam, after some hesitation, did too. So we both had to write our dissertations. Ehrenfest said, ‘well, you go to Copenhagen. Get out of here and write it there.’ Then under high pressure I wrote it, in two months I think. After that was over, I went to Gottingen — from Copenhagen to Gottingen, and after to Leiden. There was Oppenheimer. He was, so to say, clearly a center of all the younger students. I knew him there already, and in the early Oppenheimer period he was really a kind, of oracle. He knew very much. He was very difficult to understand, but very quick, and with a whole group of admirers. So that must have been the year that Born talks about. Maria was there too, and Joe Mayer, and Nordheim was there, and several other people. Robert was really one of the leaders there among the younger students. He had done, I think, his degree with Born, maybe half a year before. Again the one that appreciated it immediately was Pauli. He was the only one who understood it.
I doubt that Born understood it properly because it was really very complicated. It was on the continuous spectra, the normalization of the wave function of the continuous spectra, and so on. That of course was very good. Later one could do these things so much simpler. I stayed in Leiden for about a month together with Oppenheimer. He was, for a while, assistant of Ehrenfest… I don’t know exactly the time. I went to America in ‘27, in August, together with Sam — on the same boat. In New York Robert received us. He was extremely nice, and put us up in a hotel and took us out to a remarkable place in Brooklyn. It was all so strange. Alice was rather ill. She was sea-sick all the time. But anyway, afterwards we went to Ann Arbor and he went to Berkeley. But then he went back again to Europe, and he was Ehrenfest’s assistant, I think, it was for about three or four months. Then he got this terrible pneumonia. He had terrible trouble with his health, and he had to go to the mountains. He went to Chile. I think he still has a very good memory of Leiden and of Ehrenfest. In contrast, before him Ehrenfest had had as his assistant Elsasser. Elsasser was a typical German, in a sense… Ehrenfest didn’t think so in the beginning, but found out while Elsasser was there that he was a ein “Klugscheisser.”
Uhlenbeck:A “Klugscheisser.” One with these cracker jacks which Ehrenfest couldn’t stand very much. Ehrenfest was so sensitive. I wasn’t there, but I heard from good authority that Ehrenfest was so afraid of Elsasser that he didn’t dare come to the institute. Elsasser had to leave because it was an impossible situation. Ehrenfest stayed home. He didn’t come to the institute. Now Elsasser had clearly to resign, and he did — after half a year, I think. That was the end of Elsasser, and then Oppenheimer came. I don’t know precisely the time though. Oppenheimer liked Ehrenfest, and he was very patient, very patient. Ehrenfest didn’t understand Oppenheimer at all well, but he at least was willing to try. He was very patient. Only the people who were so clever and didn’t want to talk further about it or try to make it clear were of the type that Ehrenfest couldn’t stand. Robert wasn’t like that, although he was certainly —
Kuhn:Well he’s not always patient with people.
Then he was. Yes. That’s why he had so many students. The impatient Oppenheimer was the post-war Oppenheimer… He was quite different before the war. I had much contact with him before the war, before I went to Utrecht in ‘35. A typical example of the situation was this famous Ehrenfest-Oppenheimer paper. Martin Klein had so much trouble with it. Ehrenfest was in Ann Arbor and there was this question: how to prove that if you have composite particles of which each component has the Fermi statistics, then the compound particle could have Bose statistics if composed of an even number — how do you prove that? There was a theorem which, also, was usually attributed to Elsasser. And there was a little proof of Wigner, which was only for very special cases. Ehrenfest had great trouble with it, how to prove it. He did not like to say, “Well, if you interchange the two, then it is an even number of interchanges, and an even number minus one to the even power is plus one, so it must be plus.” That he didn’t like. It had a certain phony character, you see. He says, “But now suppose I take this one and do it another time,” and he says, “It is clearly approximate only, because it must be so that the states of these two compounds are the same. If they are not in the same state it is not true. Then they are different particles.” He says, “How does that come about?” We talked about it at the summer school, and then Ehrenfest went to the west coast. There he again was very much together with Oppenheimer. And then they wrote this paper which is an always quoted paper about the theory, because it is ‘proof’ of the theory.
This is a paper which nobody has read, nobody. And now Martin Klein tries to read it, and it is extremely difficult. I, unfortunately, have no reprint of it. I think one could probably figure it out, but it was completely written by Oppenheimer. The proof is in none of the books, but there is a very interesting passage about it, which I mentioned to Martin, in Kramers’ book. Kramer knew it was a bit of difficulty, but all the other Klugscheisser just think not. Very interesting. But this shows that they must have talked very long with each other about it. Then, perhaps, Ehrenfest understood what Oppenheimer wanted to do, and left it with him to write it. It is in absolutely unEhrenfestian style. An Ehrenfest article you cannot miss because it has a grossen, grossen style… Oppenheimer and Ehrenfest got along very well. They liked each other very much. Robert has also great respect for him.
Kuhn:How well did you know Born?
Uhlenbeck:Not terribly much. I was never very close to him. In the old days I met him very often. In the late ‘30’s we were both working on condensation theory — the Mayer theory. I had worked on it with Karl, you see. Born did it at the same time (with Fuchs). Several papers. They were not very good in my opinion. Furthermore, one of the nicest proofs was one I had given him. He gives me reference, but I had of course intended to write it in the paper with Karl. I told him that, and he just printed it in his paper. This point was mathematical proof, really, which we succeeded in doing extremely elegantly by means of (Lagrange’s) theorem. I told it to Born at the (Von Laus) Congress and he was very interested. He says, “Please write it to me.” And I wrote it to him. And then, well, he must have considered that — I mean it was not that he didn’t mention me. He, of course, mentioned me all right. Still, it was a little bit tedious, I thought, because our paper appeared afterwards, and in it, the same proof appeared… It was something which I think still now that I would never have done myself without asking beforehand. But he was then also in a difficult period. He was then at Edinburgh… He wanted to do too much. He wanted to do his theory of liquids and of superconductivity and of field theory — he wanted to do all the things. He was ambitious all right… None of them panned out, really. And that bothered him. He was of course also (half-professor) all right. When I was in Gottingen I went often to his lectures. His lectures were always like colloquium talks. It was always recent papers which he simply read and somehow digested a bit, and did all the calculations on the blackboard. And this was very instructive. It was always very mathematical. One had the feeling that (you took part more). But I didn’t really know him. Neither did I know Sommerfeld really very well, although I met him several times. We were anti-German, everybody around was anti-German. That was just the (???). We were too close to them. Sommerfeld was in the Ann Arbor summer school, and there he was extremely nice. He was really a remarkable gentleman. So Prussian! He made a Prussian impression. He was very short. But he was always so straight — this military bearing he had.
Kuhn:But he was very different with students?
He was very nice with students. La Porte was one of his students too, of course… La Porte always tells that they worked in the evenings together. Around 10 o’clock Herr Professor stopped, and they drank a glass of wine together. They did that so all the time, or they met in a cafe with the students. No, he must have been a very nice person. Pauli and Sommerfeld were once together in Ann Arbor. Firstly, Sommerfeld was the only one to whom Pauli didn’t say “du”. Secondly, as Sommerfeld came in, Pauli always stood up. It was very striking. He always stood up, because it was Herr Professor. That was still so strong in him that he could not. He had all kinds of reservations about many of the things, but he had this inherent respect from his early days in Munchen.