Oral History Transcript — Dr. H. G. Kuhn
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H. G. Kuhn; May 3, 1963
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: Birkenhead, David Hubert, Frederick Lindemann, Lothar Nordheim, Robert Wichard Pohl, F. Simon, C. P. Snow, Tammann, John Sealy Edward Townsend, Emil Wiechert; Universitat Gottingen, and University of Oxford.
Thomas Kuhn: Was Bohr’s influence on Franck as strong as these remarks of Born’s would have indicated?
H. G. Kuhn: My direct recollections refer to a later period where Franck was mostly interested in the Franck-Condon principle, briefly speaking, the coupling of vibrations and electron transitions in molecules. I’m pretty sure that at that time Bohr’s influence was not a direct one, so I think this might be true for the earlier period. I know that he most certainly never spoke of Niels Bohr in any other tern than with great veneration, and what happened beyond that I don’t know.
Thomas Kuhn: Well, that would certainly apply to the later periods which I have also talked with him about.
H. G. Kuhn: Bohr went to Gottingen from time to time and they were on very friendly personal terms certainly.
Thomas Kuhn: How much did people like yourself at Gottingen know about what was going on among the theoreticians at the time of the development, or just before and during the time of the development of matrix mechanics?
H. G. Kuhn: We knew that something important was going on, but most of us were rather frightened off by the mathematical apparatus. I certainly do remember trying to study Heisenberg’s first paper, so there was certainly some connection, but I don’t think people like me were very familiar -- younger people in the experimental group were very familiar with the actual mathematical development. There were, of course, joint seminars and colloquia where often talks were given in which one often didn’t understand very much. Quite often I think people like Born and Franck didn’t understand one another. The language was probably a little different.
Thomas Kuhn: Did they systematically come, regardless of whether it would be an experimental or a theoretical paper? Or did you tend to get a different group at the colloquium if it was Franck’s?
H. G. Kuhn: No, as far as I remember this colloquium was held by the three professors: Franck, Pohl, and Born, and generally they were all there, and almost everyone was there. Occasionally even Professor Tammann turned up, usually with an expression of protest on his face. I still remember once a talk was given, I think, by Wichert, who reported on some paper on excitation, and at the end of it one could feel it was boiling in Tammann. He got up -- he was a great, mighty hulk -- and he said, “Humpf, have you read the paper which was written about this in (???)?” -- I forget, some very early date –- “Do you know what it said there? Wenn es leuchtet dann leitet el auch.” (When it is luminescent, then it conducts.) “That’s all there is to it; haven’t read it, have you! Humpf.” This describes the attitude of Tammann to these developments.
Thomas Kuhn: How was Pohl’s attitude?
H. G. Kuhn: Pohl’s attitude was one of -- well, I can’t describe it -- grumbling protest against what he called “Kreidephysik”, “chalk physics”. He really did not believe very much in this, but he just did not follow the developments. One could feel this attitude rather well -- these abstract things going on there are all very nice on paper, but they don’t mean very much. This may be unfair, but I think this is the recollection I have.
Thomas Kuhn: Did you experimentalists particularly go to Born’s “Atommechanik” lectures which were in ‘24, ‘25? I’m not sure when they started, ‘23, ‘24. They were the ones which were then published as the book Atommechanik.
H. G. Kuhn: I’m not sure. As an undergraduate, when I was studying, I went to Born’s lectures on classical branches of physics, but I cannot remember having gone to these lectures. I’ve certainly heard him talk about it.
Thomas Kuhn: He says in the introduction to Atommechanik, “I’m calling this “Volume One” because this whole thing ought to change very soon now, and I think there will be a ‘Volume Two’ which will look very different.” Among other things, he develops in the book in particular the case of the helium atom and shows that the theory just isn’t coming out; and the more accurate the approximations the worse it gets. So that with Born, as with some other people in this period, there is a very clear sense of ‘We’re right up at the edge of having to have something absolutely new’. Did that feeling spread to the experimentalist also?
H. G. Kuhn: Oh, very much so, yes. One realized that everyone was searching for a really radical solution and that all these theories were provisional. They were nice as far as they went, but obviously something different is wanted. Oh, the feeling was very strong in the atmosphere throughout the laboratory.
Thomas Kuhn: To what sorts of problems would you have pointed, do you suppose, if asked, “Well, now why are you so sure that this is all wrong?”
H. G. Kuhn: This is very difficult to say. It’s difficult not to mix up what I know about it now, and the way I looked at it then. But I still think one was strongly aware of the internal inconsistencies, the occurrence of frequencies of orbits which were not directly observable. I think this must have been very much talked about, by people like Born presumably. I think one tine Heisenberg was there, too. But this is all I remember. I’m afraid there’s very little I can contribute.
Thomas Kuhn: You said that you were somewhat put off by the mathematics of the early matrix mechanics. Was it different with the Schrodinger equation?
H. G. Kuhn: It was rather different with the Schrodinger equation, but even then the Schrodinger equation was built up on the basis of Hamiltonian theory, and I’m afraid that the experimental physicist at that time was not very conversant with it. Well, one was to some extent familiar with it from books like Born’s Atommechanik. Schrodinger’s first paper I think was generally looked on as more digestible. I certainly remember studying this together with colleagues and trying to make sense of it.
Thomas Kuhn: How long was it before, in any very direct way, you found that new formulations, matrix or wave mechanics, were affecting your own work in any significant way -- I mean, began to play a role for you?
H. G. Kuhn: It was probably the Franck principle when it became the Franck-Condon principle which affected me directly, because this affected thesis work with the distribution of intensities in band spectra. Then also the interpretation of the continuous spectrum of hydrogen was one of the striking interpretations. Later of course it came in all sorts of ways, in work on atomic spectra.
Thomas Kuhn: Was it hard to become familiar with it as a working tool?
H. G. Kuhn: Yes, it was certainly very much harder than it is now for anyone who has a textbook of quantum mechanics. There were just no textbooks building it up in a simple way, and the statistical interpretation was still rather bard to swallow. It certainly was hard.
Thomas Kuhn: What happened to the people who did not follow the new leadership? I was interested in what you said about Townsend.
H. G. Kuhn: I think the general impression is that Townsend really was very much under the influence of his feeling of ill treatment in that his work was not recognized compared with Ramsauer’s work. He always felt so bitter that he refused to go with the time. He thought he had discovered the Ramsauer effect. I only once tried to look at the papers, and I believe they are just written in a very strange and, well, almost muddly way. Maybe he did discover it; anyway this was his feeling. And also he did not like to have quantum theory, and at one time in his laboratory quantum theory was, so to speak, forbidden. Isn’t that strange?
Thomas Kuhn: And this really was the old quantum theory?
H. G. Kuhn: Oh, it was the old quantum theory, which he referred to as “these new theories,” in the 30’s he referred to it as “these new theories,” meaning Planck, Bohr and Franck and such people. It was very sad. Therefore in his laboratory, developments were rather stifled then. Work on discharges went on, but it really had a very deadening effect; whereas in the other laboratory, under Lindemann, there was quite a different outlook. Lindemann, as you know, was very interested in quantum theory, and he has himself written a book on it, and up to the very last days he was always very interested in the fundamental aspects of quantum theory.
Thomas Kuhn: I’ve never read. Lindemann’s book, the quantum theory book. In his early career he was rather against the quantum; be was repeatedly producing semi-classical explanations of quantum phenomena.
H. G. Kuhn: Yes, this sounds quite likely, and I think he, almost all the time, never quite liked the way quantum theory developed. He had his own ideas, which I think were a little vague perhaps. He wanted to build up quantum theory in a different ways and he could be extremely stubborn. For instance, he was very reluctant to accept the Lamb shift. He often collared me and asked me, “Now, could you explain what people mean by this?” And when I explained to him about the zero point energy of radiation, he really was very unhappy and didn’t take it seriously. When I showed him any of my work which was ready for publication, intending that he should submit it to the Royal Society for publication, he was always unhappy until I tried to explain it to him in terms of Bohr’s orbits, which of course was not always possible. In a way he was advanced in his views, but never quite advanced in the way of the advances in the rest of the world at that time. The same was true, for instance, when cosmic radiation was discovered. Up to the very last he tried to explain it as an effect in the atmospheres so he had this stubbornness.
Thomas Kuhn: But be was entirely content by then with Bohr orbits?
H. G. Kuhn: Oh, yes. Oh, yes.
Thomas Kuhn: Because I think initially he had not been at all in favor of this. Lindemann said of the Franck-Hertz experiments, “That’s a wonderful piece of work; only I don’t see why you drag the quantum in.”
H. G. Kuhn: Yes, I see. Yes, that may well be. But, as I said, the outlook in the laboratory was very good; though he might disagree with the way other people looked at it, he would never really impose his opinion. He would sometimes ask me, “Don’t you think one could do this and that experiment?” And I often then said, “No, I think it’s not worthwhile, for this and that reason.” And he was really very good about it; he was never dictatorial, and this is why on the whole the atmosphere in the laboratory was very good. It was only a terribly snail, badly equipped and understaffed laboratory before the war. But he was certainly an interesting figure, and a very good head of the department.
Thomas Kuhn: Was he still doing much research of his own then?
H. G. Kuhn: No. he was mostly interested in atmospheric problems; he took a lively interest in those, but he didn’t do any research of his own. He was not actively engaged in the laboratory, even in the direction of the laboratory work. But he was never frustrating.
Thomas Kuhn: You said something before about the resistance he met in trying to expand his facilities at Oxford. Could you tell me more about that?
H. G. Kuhn: Well, in a way of course, this is now pretty well-known. This is all in his biography, the official biography by Birkenhead, which represents the facts apparently very well. I read the book and I certainly couldn’t say that I found anything contradicted by own experiences; whereas looking as the work of C. P. Snow, I just couldn’t see any relation to the man he was talking about. It just had nothing to do with Lindemann. So I don’t know if I can add much to this.
Oxford was just a university which was regarded as the university for mainly the scholarly subjects: classics, history, philosophy. Chemistry was admitted to some extent and had a slightly older standing than physics, though even this standing was not very old; it went back perhaps to the beginning of the century, when some chemists were very reluctantly admitted as College Fellows. People were very worried because they might have dirty hands or bad manners; that is how they thought of people who worked with their hands. Physics came in very much later, but Lindemann certainly did a great deal in preparing this. Especially when Professor Simon came to Oxford, the combination of these two did a great deal to introduce physics, really to put it on the map.
Thomas Kuhn: Well, the feeling here was that this was really Cambridge’s business and that perhaps they had made a mistake?
H. G. Kuhn: Yes. This only broke down slowly, except after the war. Atomic energy then changed it very distinctly. And now there is very little left of this feeling against science in Oxford; there are pockets of resistance left, but only pockets.
Thomas Kuhn: What was the feeling about mathematics? Clearly that is an old subject. But it’s also one in which Cambridge’s leadership has been pretty marked.
H. G. Kuhn: Yes. Well, there was certainly not this resistance, because mathematicians don’t work with their hands. I think there is a lot of this old attitude of classical Greece, that the man who works with his hands is despised. And I imagine this is why mathematics has no standing in the antique times.
Thomas Kuhn: Am I wrong in the impression that even though this aspect of the attitude would not apply to mathematics, nevertheless the Cambridge mathematics tradition is a much stronger one than the Oxford tradition?
H. G. Kuhn: I suppose this is true, but I wouldn’t like to guess why it is. It may be connected with the fact that Cambridge had physics as well; there was this close connection with physics. I don’t know what the origin of this is.
Thomas Kuhn: Going back briefly to Gottingen, to what extent was there a meaningful division between the experimentalists on the one hand, the Franck or Pohl students on the one hand, and the Born students on the other? Did one observe these people in all sorts of combinations, from the people who did both to the people who did just one or did just the other?
H. G. Kuhn: Yes, I think the division consisted of this fact, that some people had a training in mathematics and did theoretical physics, and the other people had no proper training in mathematics and did experimental work. I couldn’t really remember anyone who was half-and-half. On the other hand the connection between the departments could not have been better and friendlier, largely due to Professors Franck and Born themselves, who were very great friends. This affected the relation between the departments; one saw quite a lot of one another. But I think in the work there was a fairly marked division. People like Nordheim and Heitler and Heisenberg and Hund worked with mathematics; they wouldn’t have dreamt of carrying out an experiment, and vice versa.
Thomas Kuhn: Did, they come around and ask you what results you were getting? Were they concerned with what was going on in the experimental part of the field?
H. G. Kuhn: I think to a very limited extent, because they were so concerned with the mathematical formulation of the fundamentals that they probably were not sufficiently interested in the details. This is exaggerating it; of course they took an interest in general terms, but there wasn’t really very much daily contact. Franck and Born discussed matters a lot with one another, and in colloquia there was very often this discussion. But there again it was quite often that something would first be explained from the blackboard by Born; and when nest people wouldn’t quite understand it, then Franck would come and explain it in his way, which was probably the same thing but more direct, a pictorial way. And then there may have been an expression of dissatisfaction from Pohl expressing his doubts about the whole matter. This is n recollection, but you know sometimes one remembers individual colloquia; it may be a little colored.
Thomas Kuhn: Is there a particular colloquium or a particular group that you do remember? You said before that you particularly remembered errors that weren’t there when Franck talked about the Compton effect.
H. G. Kuhn: Yes, that is one thing I remember. But I couldn’t offhand mention anything else of a similar kind, although we often had distinguished people giving interesting talks. I was then a very young man, and was really only learning physics. And an institution which was very important for us young people was the seminar, as distinct from the colloquium. The seminar was done by Born and Franck together, and this was the means of recruiting young researchers. If you wanted to work with either Born or Franek, there wasn’t really an examination you could pass. I never passed an examination except after my doctoral, well, a physics examination anyhow; there wasn’t any examination to pass. So the test was this; one bad to give a talk at the seminar. According to this -- well, you were probably also subjected to a discussion, to an interview -- you were either admitted or you were rejected. This was very useful; it could be quite an ordeal for some people.
Thomas Kuhn: What did you talk on?
H. G. Kuhn: I gave several talks. These talks were not only for the purpose of being admitted. I vaguely remember that one talk I gave was somehow connected with potential theory. This sounds very mathematical, but it was of course elementary. Another talk I remember was this: I had a chemical past and probably it was owing to this that Franck had set me the task of reporting on the then new and very exciting development of -- what are they called -- these Werner compounds. The book by Werner had then appeared on the complexes: cobalt, iron, and all these complexes. This seemed very interesting. They’re connected with the arrangement of atoms in space, so I remember reporting on this; just anything they would choose. It was chosen and then one talked about it. But this was a very valuable experience. Probably Hund might have been there too sometimes.
Thomas Kuhn: Hund was likely, I take it, to have been one of the people among the theoreticians who was perhaps closer on occasion to the experimentalists than say Heisenberg or Nordheim were?
H. G. Kuhn: Yes. Oh, Nordheim was in the beginning very much on the mathematical side; he was at first linked more with David Hubert. I still remember his sitting in the lecture with Hilbert. I went to Hilbert’s lectures partly in order to learn mathematics, but partly in order to hear the great man. Nordheim had the task always of sitting in the lecture and of helping Hubert out when he got stuck, which happened quite often. He got confused with his own mathematics, and then Nordheim very politely had to say, “Excuse me, Herr Geheimrat, I think this should be so and so.” But later Nordheim joined Born’s group in order to assist --.