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Interview of Oskar Klein by Thomas S. Kuhn and John L. Heilbron on 1962 September 25, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4709-1
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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: Svante August Arrhenius, Pierre Victor Auger, Carl Benedicks, Christian (Niels’s father) Bohr, Harald Bohr, Niels Henrik David Bohr, Max Born, Louis de Broglie, Walter Colby, Arthur Compton, Charles Galton Darwin, Peter Josef William Debye, Paul Adrien Maurice Dirac, Paul Ehrenfest, Albert Einstein, Hilding Faxen, Richard Feynman, James Franck, Erik Ivar Fredholm, Walther Gerlach, Werner Heisenberg, Harald Hoffding, H. H. Hupfeld, Frederic Joliot-Curie, Ernst Pascual Jordan, Kaluza, Hendrik Anthony Kramers, Ralph de Laer Kronig, Rudolf Walther Ladenburg, Hendrik Antoon Lorentz, Mrs. Lorentz-Haas, Lise Meitner, Yoshio Nishina, L. S. Ornstein, Wolfgang Pauli, Harrison McAllister Randall, Leon Rosenfeld, Svein Rosseland, Erwin Schrodinger, Manne Siegbahn, John Clarke Slater, Arnold Sommerfeld, Otto Stern, Llewellyn Hilleth Thomas, Pierre Weiss, Eugene Paul Wigner; Kobenhavns Universitet, Stockholm Tekniske Hogskola, and University of Michigan.
Should I begin perhaps with this first question: “How did your interest in science arise?” … because I shouldn’t take long about this. My home was rather intellectual. My father was a rabbi and had a very deep interest in the history of religion, especially the relation between Judaism and Christianity. He spent as much time as he could spare working on these things. He had very strong general interests of different kinds. He studied as a young man at Heidelberg, and then he went to science lectures also. He told me, I think, that he had heard lectures of Bunsen and Helmholtz and Kirchhoff. He always tried to find books for me that interested me. But my science interest was very early. It came mostly from being glad to hear and see things about animals. For a time I thought I would be a biologist. That state lasted rather long. So when I was something like fourteen or fifteen, I read about Darwin. My father thought, of course, that Darwin was a little radical, but he found it quite good; so he found books of Darwin in the library; and so I read such things. And then I read stories, but mostly it was to read to very young children who could not read for themselves.
The interests which were more like those in which I worked later started, I think —. I used to go around with my mother’s opera glasses and looked at stars. That was when I was about fourteen. I wasn’t permitted to stay out very long, so it took quite a time before I saw Sirius. And I remember that was a very great event. We came home from some party and then I saw Sirius. Then the next year I think I started with chemistry by trying to do fireworks. Some young friends of mine who lived near us had a book by Ostwald translated into Swedish—in German it was called Schule der Chemie; it was a dialogue between a teacher and pupil on chemistry. And then I think my parents gave it to me, so I read it very eagerly. And that was really an interesting book, because it gave some idea on the scientific things also behind the chemistry in a very nice way. I tried to make as many experiments as I could in connection with that. I think I knew the book almost by heart at that time. But then I began to read a little more advanced books by Ostwald also. I had very great trouble. Then I knew very little mathematics. I had great trouble, I remember, one week, to understand how one reduced the different temperatures to another. But after that I found mathematics rather easy. But that took me quite a time.
How much science was there in your school at this time? And how much arithmetic and mathematics?
We actually hadn’t started I think with algebra. I think we started that a little later. Because I found that very easy when I started it. But I remember that this was difficult at that time as I hadn’t had any. The only interest I had in mathematics was the fact that I felt very badly about not understanding things. One learned many rules about how to calculate, and I felt badly about them. But I had no positive interest but just that I felt that I ought to understand better. But then that came together so that I spent very much time then in connection with chemistry on the gas laws, PV=RT, and how the different things were connected together; that was very difficult also. I found it a great problem how to put Boyle’s law and Gay-Lussac’s law and the general law together. My father knew Svante Arrhenius a bit. Then I also read Ostwald’s books very eagerly. And then in the summer of 1910, just before I was sixteen, there was a (peace) conference.
My father was rather interested in such things, but not practically, having been very much impressed by the prophets of Israel and their ideas. But then he decided to take part in that conference. I think the main reason was that he heard Ostwald was coming. He became curious because I was interested. He was invited to have lunch at Arrhenius’, together with those people. And then he asked if he could ask his two boys to come after lunch, and we came, my younger brother and I met Arrhenius and Ostwald. That was of course a very great event. And then Arrhenius asked me if I wouldn’t come in the fall and work a little bit in his laboratory here. By that time he had started to work on radio chemistry a little bit. He had a very primitive electroscope and then he had some small platinum plates which he made radioactive with the decay products of a very weak actinium source. And then he put them into some salt solution and some acid solution, and he studied the change in the radioactivity. Rather primitive way of chemistry. And then I made such measurements on the electroscope together with these things. That was quite interesting, but didn’t bring anything of importance. But then during the next term, it must have been in the spring of l911—I had met the chemist Riesenfeld, and he was then Deputy Head of the Institute—Arrhenius was in the United States at that time—so I just went for a little visit to him once and he asked if I didn’t want to do some little work.
I did that in my free time from school, and that came to the little work that you have seen in my first published paper. But during that time, as I mentioned before, I did some reading on both mathematics—this book by H. A. Lorentz—and I read some theoretical physics. I hardly knew the word at that time. I got held of the first volume of Helmholtz’s lectures on theoretical physics—mechanics—and that was not so difficult. So I read this with interest. And I read some of Clausius’ work on thermodynamics. Of course how much I understood, I don’t know. I was rather fascinated.
Were you conscious of or troubled by the differences between Clausius’ approach and. Ostwald’s?
Oh, yes, I understood that, and gradually I had to laugh a little bit at Ostwald. You know he had this idea about the energy. During the summer of  I had got hold of a book by Ostwald on general subjects. I think it was called Die Forderung des Tages. Then there was a cousin from Germany with us, and we used to do mathematics together, and we also read a little bit of that together. I became astonished, that he took it a little seriously, but I had to laugh very much. I think Ostwald had written there some thing like “Theorie des Glucks”. He tried to devise a formula for the “Gluck”, which was something like the energy expended minus the energy which one did not expend against one’s will and then multiplied by the sum of the two, and so on. That made me to be less fond of Ostwald after that; but still I like his chemical books very much. I was quite aware, I think, that Ostwald was not very deep in such questions as Clausius and Helmholtz. The next spring, 1912, I finished high school and then I started my university—the regular studies. And then I took chemistry, which took most of my time the first year, and mathematics lectures.
Most of your scientific education until you got to the university you got by yourself?
Mainly, as I said. Of course Arrhenius gave me some hints. I remember that he tried to teach me van der Walls’ equation, and he gave me some exercises. He gave me that equation and he wanted me to—I didn’t know it—he wanted me to work it out so that I should be able to see that curve. But to begin with, I chose some bad values and I didn’t get that minimum and maximum. But that came up gradually. He was very nice and very helpful, and then he lent me some books also. One book he lent me I read with very great pleasure. That was Rutherford’s book on radioactive transformations. I used to take that to school. Once I sat and read it just before class began. When the mathematics teacher came in, he took it and then he said: “I am reading the same book”. I had been wondering what he read. But his was the German translation of it; I had the English one.
How much, meanwhile, was the school doing about science and mathematics?
I went to one of the first co-educational schools and it was run in a very leisurely manner, which I rather liked. So I had much free time. Of course, during the earlier years, I just did the regular courses which one should do, and I was not particularly good in mathematics. But we had some very nice books which we sometimes read where there were some simple experiments on sodium and potassium in water, and things of that kind. But that was at a very early stage. But then, later on, I didn’t do so much in school. I started school at an age which was not at all early, just when I was seven. But then when I began to have strong interests, I was eager to finish school. And once my elder sister—who had also been to that school earlier—came up and had some talk with the lady who was at the head of the school—we always called her “Tante Anna”. She was quite an interesting kind of a person. My sister said that I thought of perhaps trying to finish school during the next last year; but then she was always very enthusiastic and eager and said that I should not do that but should rather jump to the next class. So then suddenly I got the order to do that. I was not very glad for it because I felt very much at home in my own class and didn’t know very well the boys and girls in the other. But still it was very tempting of course. I did it, and then at once I thought I would do it again, and during the summer I took some in English and read some history and Swedish literature and thus got into the last class. In that way I spent only two years in the gymnasium and therefore I hadn’t had so very much of the science teaching. I had biology earlier on the elementary stage that influenced me very much. We had a very nice lady—first one and then another—who taught us biology and that impressed me very much. And she read us stories about animals and made dissections, and once when I missed a class I went to her in the evening with another boy and she showed the dissection of a rabbit to us. We had to buy a rabbit, and then the next problem was how to kill it. So we went to a woman in the market place who slaughtered such things, and had her kill it while we were absent. But physics and chemistry, these things I learned mostly by myself. And in this way through (???), and later at the University. I learned a good bit of mathematics studying by myself in the same way… It was very convenient, but it might not have been educational because I got used to doing things in my own way.
When you speak of biology as having influenced you particularly deeply, what have you in mind?
First, it was just the behavior of animals, and I am still very much interested in that. Recently I read Lorentz and Tinbergen. Then also it was anatomy and the structure of flowers, but we went never very far in that. But I have always kept my interest, and in the thirties helped one of our good cytologists with the mathematical side of reactions and we even published a paper together. But to begin with, it was just children play, like chemistry. But then in the spring of l9l1, when I had finished my first examination, which we call “fil. kand.”—my father had died that spring—I went with my mother to visit some relatives in Germany. My father had already had the idea, which my mother then took up, that after I had finished my examination I should spend some time abroad. I had a cousin who was professor in Groningen, Laqeur. He set me to work on some experiments, measuring which I did very badly, and so he thought I was no good at experiments. But I very much liked him and I think that was mutual.
There was also at that time a conference there which Arrhenius attended. He wrote a letter to Perrin the Elder asking whether I could not stay for a year in Paris with him. Then through my relatives I stayed with some people in Brittany to learn a little better French. But when I had been there two weeks, the war broke out. I knew very little about politics then, and therefore that was quite unexpected. I tried to stay on in France, but had to go home. I went back to Arrhenius. I tried to play around measuring dielectric constants by means of long electromagnetic waves. But nothing came out of such simple devices. Then I did my military service. I went back to Arrhenius again and began measuring dielectric constants of solutions with the help of a school fellow of mine who was an engineer. I thought I had found some rules and even wrote a paper on this which is the second on your list.
How long was the military service?
It was comparatively long for us because it was during the war. It began in June, 1915, and finished at the end of September or beginning of October, 1916. I used this paper on dielectric constants of solutions for what we call the “Licentiat” in the end of 1917. During the spring I did some laboratory work in physics, for that exam. During that time I began to read more theoretical physics, and I began to read Bohr’s papers of which I didn’t understand very much. I thought I would understand more after I had read the papers of Sommerfeld. I began to have an idea of quantization, but it was rather vague. At the same time I was very much interested in electrolytes and the forces between ions, but in my free time I always read about the other things. I remember that in 1917 Arrhenius had a visit from Wilhelm Bjerknes, the meteorologist.
He had been professor at Leipzig during those years and when he came back there was a big dinner. Suddenly Arrhenius introduced me as a mathematical physicist and I didn’t know I was that. I took my examination in physics—it may have been after that. Then in 1918 Arrhenius put me to work on some ideas he had on absorption. I had a great deal of trouble with the making of glass apparatus and breaking it. I got a few results, but they didn’t fit into his formula and so he somewhat lost interest in it. At the same time I think he thought that it would be a good idea if I went abroad. Arrhenius’ son helped me apply for a fellowship from the University—there was one such. I thought I would go first to Bohr and then on to Gottingen. I did not know at that time that one should want to go to Bohr most of all, but only that it was the closest. I wanted to go on to Gottingen to Debye. I got interested in his work on dielectric constants. I thought also about going to Einstein. In the fall of 1917, Kramers came up to Stockholm.
He impressed us very much at that time. First of all, he looked much older than he was—he was my own age—and he gave a lecture in Danish so that we thought he was a Dane. He had only been one year in Denmark! He talked about quantization, the adiabatic principle and things of that kind. He came out to Arrhenius’ Institute and we had some private talks together. I already knew about Bohr, of course, but he told me more about him and that played a role in my deciding to go to Copenhagen. I wrote to Bohr, and he wrote back very kindly, so I got there in May, 1918. I don’t remember when I first heard of Bohr’s paper. I must think a little bit. I think it was in my second University year in the fall of 1913. I remember that two other university students and I went up to Uppsala to hear Svedberg give his inaugural lecture for his professorship. I think he must have mentioned Bohr, but I am not quite sure. I know I heard of Rutherford’s discovery of the nucleus for the first time in that lecture.
Even before you heard about Bohr, how aware were you of the quantum?
I tried to read Thomson’s book about the atom, and like everybody else I was aware that in some way one should be able to understand the atom and the formation of molecules. I thought that would be an ideal against which to work, but I did not know that the solution was so near at that time. When one first heard about the quantum it was very mysterious. One heard of energy quanta, but thought of a moving body and did not understand how energy could be other than continuous. Then after I began to read a little, I understood a little on the formal side, but my insight into the physics was very vague. I read a bit of those quantization papers of Sommerfeld and Schwarzschild. Then there were papers by Debye, and I think also Einstein.
I remember that I had not heard anything of the Hamilton-Jacobi equation in my University years, but learned that from Kramers when I came to Copenhagen. I learned a great deal from Kramers—which he had gotten in turn from Bohr. I heard from Bohr largely generalities as I had not much occasion to see him at this tine. Kramers was my teacher in these new things; Hamilton-Jacobi, the correspondence principle and beginnings of the Copenhagen philosophy. I remember that I went to the library in Copenhagen and got hold of Jacobi’s big book and read about it there. Later I got Whittaker’s book. Quantization depended upon separation of the Hamilton-Jacobi equation, and Kramers had the mathematical idea that it should be possible to prove that separation was only possible in elliptical coordinates. At the same time I had some ideas about electrolytes. After my stay in Copenhagen, Kramers came to visit me in Sweden. We went on a skiing trip to (Dalarna). It was warm weather and there was not much skiing, but all the time we discussed these two problems.
We did manage to get something done on both problems, but on this Hamilton-Jacobi problem nothing was published. I worked quite a bit on this Hamilton-Jacobi problem during the following spring, 1919—winter, 1919. I had also been working a little bit on the electrolyte problem in the summer with Bohr. He put me first on some problems about molecules, and the first one was, I think, an ionized hydrogen molecule. Bohr thought that one should have just two nuclei and one electron moving in a ring, and I made some numerical calculations on that. I think it was on the ionization energy. The calculations showed that this could not —-. The results were all negative. And then there were some more general calculations on such ring molecules. I also did some numerical calculations during that summer, but the result was also negative. But, of course, it was quite a good experience. I had occasion to talk to Bohr during those times. And I got a very, very much stronger impression of course of Bohr himself, and his way of reasoning, which made an enormous impression on me.
If you would tell us more about what you saw of Bohr’s working at that time—
First, it was, of course, largely through Kramers, who gave me a review. He had been working very intimately with Bohr on his papers then. But then I remember also that Bohr took me on a walk and then he told me a bit on his general ideas, both on physics and on general philosophy also. On physics, I think, it was the correspondence point of view, which be called at that time analogy principle. That came out very strongly, and I remember one special connection: he criticized a paper by Debye, who had tried to treat the dispersion of light by the hydrogen molecule by means of such a model. And then Bohr said that that could not be done because we know that the dispersion formula must contain the spectrum frequencies which at that time came out as differences of course between the energy of stationary states which had no simple connection with the mechanical frequencies. At that time one had the mechanical model.
That was one side of the correspondence which of course came to play a very great role later. That Bohr saw very clearly: that one could not use the mechanics in these little (cells). The mechanical frequencies could up to now turn up in the dispersion formula. But it must be the spectral frequencies, it must be the energy differences. He was also, I think, at the same time mentioning that the whole use of mechanics was rather provisional, and that it was very strange that one should be able to quantize by means of mechanical orbits; but that it might be that deviations from mechanics would vanish through the averages when one had to do with periodic motion. And he made it very clear at that time already that the quantization of the stationary states had to do with periodicity. I don’t think that at that time yet—but not very far from that time—he had this first instance of the indeterminacy principle, namely that the stationary state could not be better defined than h divided by the life time. I think that came a little later, but it came rather early also.
When you speak of the correspondence principle for this period, is it still largely, do you think, of correspondence at high quantum numbers?
That was the basis, but there was always the attempt to stretch it further. And Kramers at that time made a very nice paper where he calculated the Fourier coefficients on an elliptical orbit with Stark effect, you know. And then he tried to draw conclusions—and of course Bohr did that, too, having given the basis for it—also for low quantum numbers. He tried to use some kinds of averages between these two states which were implied in the transition, averages of those things which one should calculate from the classical treatment. At that time also Einstein’s paper of 1916, on the derivation of Planck’s formula by Bohr’s ideas, played a very great role; so that Bohr and Kramers following him tried then to estimate the transition probabilities—these coefficients of Einstein—by connecting them through Fourier coefficients. And then in the same way came such selection rules; so that if in any orbit a certain Fourier coefficient was missing, then Bohr concluded that such transitions would really be forbidden, for instance, that would give the right spectrum for the oscillator and, for instance, in rotation also. At that time Rubinowicz had used some other way to come to the same conclusion about angular momentum… And then Bohr helped me very kindly on some little work I was doing on the electrolytes in the same summer. This interested me very much. That was quite a different problem—it was about thermodynamics of the dielectric constant. He made a very nice reasoning which I quoted then and mentioned his name in the little paper which came out in 1919. But this very, very clear way in which he made that I remember impressed me. I remember that when I met Kramers then in Stockholm, I said something to Kramers that it had impressed me so. I think I compared it little bit with Arrhenius, whom, of course, I admired also. Kramers also said, “Yes, but Bohr has a stronger (light),” or something of that kind. We both admired him.
The work that was going on in 1918, or thereabouts, at Copenhagen with the development of the correspondence principle—did it still seem at that time as though the final answer would lie along the line that was then being pursued? Did it seem that a still more radical break would be necessary?
Yes, there were some a little bit diverging views. Bohr and Kramers believed rather strongly that one should get on and try how far one could get with mechanics, but always be prepared that in the real theory one should have some kind of translation of mechanics one didn’t see how to get. But the aim was so that I think when Heisenberg’s paper came, Bohr was immediately convinced that this ought to be, the way which he, so to say, had aimed at. But then there came of course other ways also…I think we all, everybody, speculated on possible ways to interpret it, because we understood that first of all quantum theory was not completed, couldn’t answer all the questions that could reasonably be asked. At the same time, we in Copenhagen knew that certain questions were meaningless.
What sort of questions do you think of when you say you were aware that it could not answer all the questions?
For instance, to calculate the transition probability between two states. In only very special cases one could do it quantitatively. One could do it in the limit of large quantum numbers, but one should have a rule for calculating it generally. One had no such rule. One tried averages, but they were not satisfactory, and in very special cases one could do it. But then, of course, there were other people, but we who had learned the thing through Bohr, we were very strongly against attempts to ask, “In which way does the electron jump from one state to another.” Without being able to say exactly why we didn’t believe in such a question, we were quite certain that such questions were meaningless. And that was also shown that one should have a deeper basis for the whole theory than these mechanical pictures. I think that was clear. Then one was searching in different ways. From my way I began to play around with (waves) rather at an early state.
May I ask if there were other questions that were particularly troublesome in Copenhagen, besides the general question about the transition probabilities? What about specific heats, or making a good helium atom model, or something of that nature?
One had great troubles then. Everything about molecules was very, very uncertain and vague—you know Bohr had tried to bring in models, but he had given that up already by 1919 I think, or something like that. But by that time about, he began to start that work on the periodic table, and that gave, of course, a new insight into the quantum theory also. I remember he came one morning—was it 1919 or 1920?—and Rosseland was there too at that time, and told us about this way of changing the quantum numbers in the alkali atoms, so that the outer electron was forbidden to have that lowest quantum number. And he didn’t get it the first time, he didn’t change it sufficiently, but it was in the way which then proved correct. And that gave the first indication, I think, of what became then the Pauli principle. And then he began—oh, that must have been earlier, I remember in the summer of 1919 he was already very eager to speculate about how one could think of these closed groups being arranged. He thought of geometrical ways of polygons and things of that kind; but I think he was quite clear also that the electrons for instance in the—I may confuse it a little bit with the summer of 1922. In 1922 he was very deep in the periodic table. In 1919 he was very deep in the spectrum theory. He gave a lecture in the spring in Berlin which be began to work out. I. was with him then, and he began to work that in the way he does-he dictates, re-writes, and. so on. This Berlin lecture must have appeared in 1920, and there, yes, I think already there—he had seen this for the sodium spectrum that the excited electron could never go down to the lowest quantum number. And that was the beginning of his work on the periodic table.
How literally did he take those models he had of the shell structure?
I think he played sometimes with more literal models, but he always took them very cautiously. (He would never take the little ones and say, “little this”—but he played around with different things.)
As a group those in Copenhagen, those close to Bohr, dismissed problems like: Where is the electron between the time it’s in this orbit and the time it is in this orbit? But I take it that while you discussed this you also had the feeling that one does need a new formulation in which it will be impossible to ask the question.
Now, the more technical problem, like the problem of finding the proper intensity formulas—did you suppose there too that it was the lack of the more fundamental theory that was preventing that?
I believe so, yes. I believe so.
Take your failure to calculate the ionization energy of the hydrogen; was a failure of that sort again taken to point to some fundamental difficulty? Or was that like not having found quite the right model yet?
I think that, as far as I understand, that was what Bohr had in the back of his mind. The rest of us also very soon got it, i.e., that the whole is not satisfying as it is. One should try how far one could go with mechanics, but there were obvious limits and one obvious limit I mentioned before, this work on dispersion. When Debye tried to calculate mechanically how light acts on hydrogen molecules, he tried even to compare it to some experimental results; and I think he got a little bit of a check even. But Bohr said this is quite impossible because in this way you could never get any other dispersion formula than one where the mechanical orbital frequencies appear; but we know that they are not those which really come in the dispersion problem. So that was a very clear indication against a too extensive work in mechanics. And there was very much discussion about the adiabatic principle, you know, which Ehrenfest had stressed so strongly.
I think Bohr came to it independently of Ehrenfest first. Bohr went down to Leiden when Kramers had his doctor’s disputation in the spring of 1919, and then he and Ehrenfest became very close friends. But Bohr already knew about (the main thing) at that time. That was when an extension of the use of mechanics in the stationary states, i.e., that you could use them also in very slow (transformations). But the whole was doubtful so that it was strange that one could get so far in (checking) it. Then you know that later on there came one example after the other where one got into conflict in fixing the stationary states by means of the adiabatic principles. One case was just this cross field. So there came more and more examples of the limitation of that. I remember that Pauli was rather early very dissatisfied. Bohr’s view was, you remember, more or less, not to speak too much about the difficulties.
Of course one should always know that nothing is certain, and that you must keep yourself very much on the guard, but still one should try to go on as far as you can. However, Pauli had more the temperament to be very angry when he had not a clear solution of a thing. And then he always spoke about Bohr’s “Beschlichtigungsphilosophie”, that is to say that he tried to declare that the difficulties were not. But that was not “Beschlichtigung” in any other sense than that Bohr hoped that gradually one should be able to see clearer and clearer into the problem and that one should use what one had in the meantime. That was just a different temperament, because in the long run they agreed very well. Pauli was, of course, very much impressed by Bohr. But that was a little bit later. Pauli came to Copenhagen in ‘22, after his Gottingen (summer). And then he got into the very important work on the anomalous Zeeman effect which led into the spin. And there were very strong reasons for dissatisfaction also, and they got stronger and stronger. But I think there was always in the back of Bohr’s head and through him also with the rest of us, that we thought there must be in some way a more fundamental formulation of the theory.
How did the people of Copenhagen regard what was going on in Munich?
Yes, there was a little criticism, of course. One thought that—with all admiration of Sommerfeld, and I think the admiration is stronger perhaps now than then—that they took things a little too literally. I remember in 1919, in the fall of 1919, Sommerfeld was invited to Lund in Sweden, and they then asked Bohr and also those who were with Bohr to come to Lund. And Sommerfeld was invited to give a lecture and Bohr was invited to give a lecture. Then of course we thought that Bohr saw so much deeper in the things than Sommerfeld; but I think, like young people often do, we underestimated Sommerfeld. Because Sommerfeld was a very great man; and Pauli, who knew him better, had always a very great admiration for him. But we were a little bit critical. I remember at that time Sommerfeld gave a lecture about the anomalous Zeeman effect, and Sommerfeld liked very much to play with numbers. Pauli later compared him with Kepler. He had two or three cases and then he tried to extrapolate that about how these levels would go. I said (an aside) to Bohr, and Bohr laughed a little and said that he didn’t believe in (the extrapolation) at all, and Bohr was right in that case. (But often, of course, Sommerfeld was right in very fine things.)
Was there any attempt with the old quantum theory by Professor Bohr to think about measurement problems, that later did so much take a place in his work, from 1927 on?
I think the first example was that which I mentioned, about the necessary lack of definition of the stationary state because the stationary state has a finite lifetime… He wrote very clearly in one of his papers,…I think it must have been the one on the theory of spectral lines; I think it was the lecture given in, England. I think it must have been that, but also he wrote a big paper in two parts which appeared first in German—”Grundpostulate der Quantentheorie”, and I think there that he pointed this out… But I remember also his talking about these things in these years. It was very clear to him that you could not define a stationary state better than the periods. I believe that that was the nearest to that kind of measurement problem. That was also a bit the background when he took this up so strongly in ‘27.
I have been very curious to know how early that began to appear as an important technique.
The theory was in some way too undeveloped, I think, to describe the measurements, so that I don’t think it came so explicitly. Very much discussion about the relation between waves and quanta of light, which Einstein started so strongly and about which Lorentz had made very strong remarks, I think they were rather important for Bohr. That was one side of the quantum theory in which I think just Lorentz’s remarks made Bohr rather skeptical against taking light quanta too literally.
I wanted to ask particularly about the attitude in Copenhagen in those years toward the photon hypothesis.
I think there was a bit of skeptical attitude. Bohr pointed always very strongly to this—that the (wave) optics shows that the wave description is correct. You remember that Einstein sometimes tried to make something intermediate between quanta waves by having some deviations from the superposition principle. But then Bohr never believed in such attempts. The whole was very paradoxical, so that I don’t think that he even attempted a solution. I think he only hoped that it might come in future when one proceeds from the correspondence point of view.
Millikan’s measurements on the photo effect—the ones he did in 1916, which were supposed to verify the Einstein formula—were they known in Copenhagen?
They were known. And of course one didn’t doubt such results. But that was very close to Bohr’s description of the hydrogen atom. Bohr was very clear that above these discrete levels there was a continuous range of states and therefore when you illuminated with the quantum up in that range, then the electron would be liberated. So far Einstein’s law was quite compatible with Bohr’s general ideas. But it was just to say that light (???) were existent in the vacuum in some little (way)—he would rather say there is this wave propagation and then there is this probability of changing the state of an atom. Einstein tried at the time to express that by using the word “Gespenster field” which played a role in the discussion. And that came to play a role for Bohr also, but that was at a later stage, along the Bohr, Kramers and Slater.
When you thought of the problems that seemed to indicate that one had to go deeper, were the set of problems about light and the photon also among them?
They were among them also…But I remember that that was very much among them. I remember that in the winter of ‘23—that was a little late in these times— I studied some work of Poincare where he said that the Planck formula would not agree with the ordinary theory of the scattering of light. I did not know the Compton effect at that time. So I only knew that there was a problem there. But then the Compton effect theory came about that time. And then Pauli in the summer of ‘23 made some very nice work where he connected the Planck formula with (???) by the Compton effect. I mention this only to say that we were often very puzzled about the whole behavior of light and electrons and so. Now I remember what I wanted to say before—that Bohr’s doubts about the energy principle came up now and then, also in earlier years, also in connection with this question of light. Then already, I think very early, he was thinking that in some way the light gave a probability for a transition. But then he thought that that might go against the detailed energy balance. You remember that came up later then. But as far as I remember, both this and dispersion played a role already in the first. He mentions such things as a possibility. That, I think, he will remember himself.
Was anybody at Copenhagen then working of dispersion—Kramers, of course, was a bit later.
That was later. No, I think nobody was working at it. Bohr had these general ideas which I think were important for Kramers later, but I think that one had not the mathematical mechanics technique for it, which Kramers found—which he really invented, which was very ingenious I think.