Oral History Transcript — Dr. Victor Frederick Weisskopf
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Interview with Dr. Victor Frederick Weisskopf
Victor Frederick Weisskopf; July 10, 1965
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: Hans Albrecht Bethe, Niels Henrik David Bohr, Max Born, Paul Ehrenfest, James Franck, Werner Heisenberg, Fritz Houtermans, Lev Davidovich Landau, J. Robert Oppenheimer, Wolfgang Pauli, Rudolf Ernst Peierls, Léon Rosenfeld, Erwin Schrödinger, Otto Stern, Hans Thirring, F. Uhrbach, Gregor Wentzel, Eugene Paul Wigner, E. J. Williams; Kbenh?avns Universitet, Universität Göttingen, and Univeristät Wien.
Kuhn:How did you get interested in science?
Weisskopf:That was very early, mostly as a reaction against an extremely humanistic family.
No, not professionally. My father was a lawyer. Traditionally humanistic: arts, literature, music, and so on. As a second son, wanted to stand on my own feet. I joined, pretty much on my own initiative, with one or two good friends of mine, one of these little circles called Urania in Wien, a kind of Volksbildungsverein. There I took a course in differential and integral calculus from Friedrich Waismann, a man whom you might know, who went eventually to Oxford. At that time he was one of those very poor intellectuals with no job, no money. He earned his money by giving evening courses of this kind. He picked me out during the course and actually told me a lot of physics and mathematics, and good science.
Kuhn:This vas before you were at the University?
This was high school. I was also influenced by reading an enormous amount of popular literature on Bohr; in fact the 10th anniversary issue of Naturwissen schaften I read word by word, not understanding very much. At that time I was fifteen years old. I also read Kramers-Holst's book, and the like. There was also a magazine on a somewhat lower level than Scientific American which is called Kosmos and which still exists in Germany. Quarterly they issued a popular book in the field of science and I devoured those. There were good and bad books in physics — not too many popular books. [Arthur] Haas, a theoretical physicist in Vienna, wrote a few smaller books of medium difficulty. Anyhow, at that time I caught very much the spirit of the Bohr atom essentially on my own, with a few friends, and was also an amateur astronomer, and it was clear to me from the beginning that this was my career; there was absolutely no question about my choice of profession at all. In the last year of high school I spent a great deal of time in the Radium Institute where I worked with Franz Uhrbach who is a physicist now with Kodak in America. I discussed many deep philosophical problems; this was the sort of atmosphere in which I was living at this time.
Kuhn:Was there much family resistance?
Rather incredulity more than resistance. My father tried weakly to persuade me to go into engineering and study at the Technische Hochschule rather than the University. But I was at that time — and I hope I still am — a man who wanted to go for the full thing, so I didn't follow his advice. The resistance wasn't very strong, however. My father died before I went to the University, but I don't think he would have opposed it.
At the University I knew a few people from the Radium Institute: Professor Przibram and Stefan Meyer, mainly through Franz Uhrbach who was at that time a young research associate there. He brought me into touch with the higher ups, so to speak. So right from the beginning I was not merely one of a great many students, as things are today; I had the right so to speak. I did not do much class work; in fact I hated class work. I read and really studied a lot of books. One in particular, Theoretische Physik by Clemens Schaefer which still exists today, was my Bible and I went through it chapter by chapter, page by page; even now my notation is influenced by that.
Kuhn:Was there anybody who gave you advice on that?
Yes. Franz Uhrbach mainly, and Hans Thirring as soon as I got to the University. The first volume of Clemens Schaefer, on mechanics, had been done in my last year of high school with the support of Uhrbach. As soon as I got to the University I got in touch with Thirring who had been told about me. I enjoyed his lectures enormously; in fact his were the only lectures I followed from beginning to end and I still have some of the notes. Later on, as I said, I was always too impatient with lectures and learned from books instead. Thirring was very helpful; I was there for two years in Vienna, from 1925 to 1928, and he helped me personally with explanations and really led me onward. I wasn't doing much experimental physics at that time; Ehrenhaft I disliked intensely, as many of the other people did, and 1 did not go to his course although I should have. I took the introductory chemistry course, which I hated very much and which was at a very low level, I think. Mathematics, however, I did enjoy at that time although I did not go systematically, unfortunately, to the courses. There was the famous mathematician Hahn, and also (Menger)Furtwängler, and a few others.
Kuhn:Did you again cover mathematics systematically in your reading?
Not enough, no. I was too impatient. I studied theoretical physics; if it was necessary I went to mathematics. Clemens Schaefer in fact contained a great deal of mathematics. I read at that time an introductory book, Mathematics for Theoretical Physicists, by the famous Nernst and Schoenflies, which contained fundamentals. But I am still weak in mathematics which probably comes from my never taking mathematics very seriously in the beginning. I should say, because I am so proud of it, that by this time I had two publications to my credit. One I made with a friend of mine, George Winter, when I was 14, which was an experimental observation of the color of (Perseides); that was during my period of interest in astronomy; and the article was published in the astronomical Nachrichten. Then I published another paper during the first semester, when I was a freshman, on the expression for the energy of sound. I think the paper is all wrong now, nonsense in fact. I looked at it now and think it's just nonsense. But I remember submitting it to Stefan Neyer who sent it to the Phsicalische Zeitschrift where it appeared. It's very strange to notice that complete completely wrong papers can be accepted; this one was naively wrong, I think.
Kuhn:Did you read Atombau during this period?
Yes, that's a very good point. The old Atombau — it did not include the Schrödinger equations. Absolutely, no question. I was quite familiar with these things, although not with the relativistic parts. I had a certain difficulty with relativity theory. Again I worked a little in the spirit of taking things one by one, and then I got sort of stuck and never took in relativity at that time. I have the impression that, in contrast to today's youth, I learned very little but this very thoroughly. I was amazed sometimes in the early days at how little I knew, but what we knew we knew quite well and discussed thoroughly. I think we made at that time the opposite of the mistake made in today's teaching — diametrically opposite, and that has its advantages too.
At the end of the second term I asked Thirring what I should do — Thirring was sort of my teacher — and he told me to get out of Vienna as soon as possible; he said it for two reasons, scientific training and future possibilities for employment. I asked him where I should go and he said to Göttingen, which, in hind sight, was a mistake. He should have sent me to München to Sommerfeld; and if he had sent me to Sommerfeld I probably would have been a better physicist. I would, have been better trained.
In Göttingen I came into the following group, listing them from the top down: Born, Franck; younger assistants Heitler, Nordheim and Herzberg; my fellow students — the former were teachers — Delbrück, Maria Mayer, Edward Teller and more which I can't remember off-hand. I was overwhelmed right from the beginning by the complete difference in approach, level, and heat of discussion from Vienna. There we had a few people like Unrbach and maybe Thirring, but for the most part the atmosphere was completely stagnant.
Kuhn:You had had no contemporaries either?
Weisskopf:In Vienna, no. Then suddenly I was in the midst of a bubbling atmosphere extremely active seminars; there was at that time a very close collaboration with the mathematicians in Göttingen — such well-known men as Hermann Herglotz, Hilbert — who was a little too old for me to really have much contact with — but Courant, of course, very much so. Of course this was paradise for me, or at least a new world. Paradise is not the right word because I was completely bewildered in the first seminars; in Vienna I was accustomed to being the brightest boy, the one who understood everything while the others required explanations. The moment I got to Göttingen I didn't understand a word. There was also a completely different tradition of under the influence of Born, which was complicated and a little formal for taste. The first thing to do was of course not to begin any research work but to learn. This was when I first started to learn quantum mechanics. When I spoke before about the Schrödinger equation — I certainly knew it — that would be silly — but I don't remember having really thought about the Schrödinger equation because I wouldn't have known with whom to talk about it in Vienna.
Kuhn:Had you already been reading original papers while you were in Vienna?
Weisskopf:No, I had not. Somehow this wasn't done. I read books; as I said, Atombau and Spektralinien, but I never read many original papers. This habit still sticks with me. However, I learned to absorb knowledge through discussion at that tine. The people who taught me the spirit of quantum mechanics, complementarity and also techniques, were really the thing. Again, no courses. I don't remember any course to which I went systematically in Göttingen except perhaps the famous course of Herzberg of which the present book, the first volume, is an almost exact copy. An incredibly good course. I learned quantum mechanics, non-relativistic quantum mechanics, from Heitler to whom I owe very much, to some extent from Nordheim, to a great extent Delbrück who is a little older than I and was at that time one of the older students. We were very close and we discussed things together a great deal. Then of course there was Herzberg with whom I had a great deal of personal contact apart from the course.
Kuhn:Wigner was not there?
Weisskopf:No he was not; he might have been there the first semester, but I didn't really have contact yet. When I started actually being a member of the society Wigner was no longer there. My association with him begins later on. Now at that time I could have gone into mathematics, but somehow I neglected this again. I don't mean mathematics as a mathematician, but I could have gotten thorough mathematical training as most other theoretical physicists got and I never got. The opportunity was there — Herglotz, Courant with his famous mathematical practicum, which I must have taken because I couldn't have gotten a degree without doing so; I must have taken it very sloppily because I don't remember much of it. You know that the German educational system is completely different; you're not supervised and you can do what you want.
Kuhn:Did you study Courant-Hilbert?
Weisskopf:Not very much. I knew what was in it and heard a lot of talks about it, but I didn't study it systematically. Again I should have but I didn't. I would say that at that time I learned, so to speak, the lingo; from the discussions with Heitler, Herzberg and Delbrück I really learned a lot of formal things at that time. I think there was quantum theory of crystals, which occupied Born at that time, and the molecular problem of Heitler-London. I heard a lot of seminar talks but I don't think I really understood what was going on; I learned the lingo a little too fast so that I could talk about it without really understanding.
Kuhn:What sort of an issue at this point was the matrix versus the wave approach?
Weisskopf:I think it was already settled. It was not exciting, though I'm not sure I can really judge. I was a little young to know what really was going on in the minds of the "big" at that time. I think, or rather I know, that it was Dirac's radiation theory that came up at this time and was very very widely discussed and exciting; the Dirac equation was there but it was so ununderstandable that I didn't even touch it. That is, in the spirit of systematics I felt that I had a hard enough time with the Schrödinger equation and I would leave Dirac's for later. I know it excited people, but no one really understood it. I'd like to tell an anecdote, which impresses me more and more because of the time. You know better than I; when did Otto Stern discover the anomalous magnetic moment of the proton?
Kuhn:In the early thirties.
Well then, it was in the early thirties that this anecdote took place. It must have been late 1930 because I wasn't in Göttingen longer than spring, 1931. Stern came to Göttingen where we were all under the influence of the Dirac equation. He attended the theoretical seminar where he asked each person, including myself, as I was then an advanced graduate student and a recognized member of the group, what he thought was the magnetic moment of the proton. Everyone said, "There's no question; it's one magneton. It's a particle with spin one-half, what else should it be?" He took our names, he wrote down what each of us said, and each had to sign it. Then he told us to wait until next week for the Gautagung in Hangover, where he announced the 2.5, or whatever the value was at that time; I think he wanted to cash in on a few bets. Well, you have this anecdote now; it illustrates the tremendous impact of the Dirac equation — why should it not work for the proton?
But even at the time when this anecdote took place, which was at the end of my stay in Göttingen, I would not have been able to give a coherent account of the Dirac equation. In a little in the spirit of systematics, I was enough to understand and to wonder about, to speculate and to discuss far into the night the ordinary non-relativistic quantum mechanics; I had the feeling I had to understand that before I could worry about the rest.
Now let me mention one extremely important event for me. As I said before, I had learned the foundations of quantum mechanics thoroughly and quantum statistics from Heitler. I didn't take the lecture in there systematically, but I remember very nice discussions and I read a good deal about it. However, as far as modern problems are concerned, the more complex problems such as dispersion theory, f-values, which were then so important, radiation theory, or even many particle problems, crystals, and so on, I learned the lingo but really didn't understand it. And then a really decisive thing happened to me in the year 1929 when Ehrenfest came as a guest professor, replacing Born who had a slight stroke.
Perhaps I should say a word about Born. Born was supposed to be the man with whom I worked; but partly because of his health and perhaps more because of a certain incompatibility in our attitudes, I never heard much of Born. Born's lectures were famous, but they were not suited to my taste; they were much too formal. On the other hand, I was a good deal in touch with Born; for example, it was at that time that he wrote Optics and I wrote a chapter in it, a classical physics chapter on the theory of refraction. It says in the forward which chapter it is. I liked it very much; it was a sort of task, which he gave me. It had nothing to do with modern things, but I was very much interested in physics 'as such'; still under the influence of my Vienna upbringing, I thought of classical physics as still something very much worthwhile. This is why I dug into the molecular continuum theory of refraction — a very beautiful theory which I worked out for his book. I remember giving it to him, and I think I did quite a good job, but Born didn't like it very much because my own approach was so much different from his own formal one. For example, Born reproached me quite seriously, and justifiably perhaps, for my mathematical sloppiness. I had at that time the feeling that he misjudged me and now I'm convinced; he didn't see the physical part of it, which was good. I think I still have the manuscript.
Then came Ehrenfest, and for me this was an enormous experience because in spite of all this very good training I got with Heitler and so on, I felt somehow uncomfortable in Göttingen. If this was physics, it wasn't quite what I had been counting on — too formalistic, too much mathematics.
Then came this breath of fresh air, Ehrenfest. He came to the seminars and he asked those questions I never would have dared asking; I had thought of some of them, but I just didn't dare. But when I saw a great man like Ehrenfest asking 'stupid' questions, it impressed me no end. He selected me right away because he is also from Vienna and he took me on walks and treated me almost as his child. I think this is probably the greatest influence anyone ever had on me and I notice it now in everything I do — when I ask stupid questions in seminars — this is all from then. In a way it was influence, but also it was as if I had found a kindred soul. I had the feeling that this was the way I would like to do physics, but I had never seen it done before — going to the primitive things, the essential things, trying to simplify, to make models. This was very much not [sic] against the spirit of Göttingen at that tire.
Now I'll come back to the question you asked me: why should I have gone to Ehrenfest. When I went to Göttingen I did not get a good formal training; on the contrary, I got a certain aversion to formal mathematics in physics and went then, after some detours, to Bohr who has somehow the Ehrenfest approach, though in a different sense. Therefore I never got a good grinding in mathematics, whereas had I gone to Sommerfeld I would have. That's what people do there; they sit in rows and work from 9:00 to 5:00. That's what I needed at has time, and I don't think the other spirit would have been killed in me. I would have been more rounded. This is what I meant before about going to Munich. With Ehrenfest I began to feel more at home in physics. Many things which I learned from Heitler and Herzberg grew clearer to me; suddenly I saw the real problems much more clearly, the problems of radiation coupling, and of course the deeper philosophical problems which always concerned Ehrenfest. But I must say that the main influence of Ehrenfest was in directing my attention away from formalism and toward the essentials. Then Ehrenfest left again, unfortunately, after one term.
After this, partly because of the sickness of Born, I was rather in touch with Franck, but not as an experimental physicist. Somehow, I don't know why, pushed me a little out of experimental physics. I think it was mainly the present wife of Franck, Fraulein Sponer, who was at that time running practicum in experimental physics and treated those who she thought were theoretical physicists rather contemptuously. "This is nothing for you. You don't need to do this." That was bad; I think with a little more love they could have made an experimental physicist out of me easily at that time.
Kuhn:Would that have been good? Would you have liked that?
Weisskopf:That I cannot tell, but it's the same as with mathematics. I think I probably could have done more in physics. I'm not sure about the experimental question, but as far as mathematics is concerned I'm 100 percent sure that I would have done more if I had had the mathematical training. This is jumping ahead, but I can prove it to you; I would have calculated the Lamb shift before it was discovered. It would have saved theoretical physics from that "blamage."
Kuhn:You talked about the great interest in the Dirac paper, the emission-absorption paper, and. I told you of my sense that that, at the time it was written, was not at all a field-quantization paper.
Weisskopf:This was why I was able to understand it, because — well, you know the dates better — the Jordan-Wigner papers, when did they come out? That is the Fermi statistics paper. But when did the field quantization of the Bose gas core out. It was before the Fermi gas, wasn't it? It was Jordan and Klein. I can't tell you much of what you really want to hear because these are the papers I did not understand at all; they were just impossible. I had a lot of seminars about them at that time but I just did not understand them. I would say that Ehrenfest helped me so much morally because he admitted that he didn't understand them either, and that of course was a great help for a young man to find out. But we could understand the Dirac paper; that was clear — oscillators quantization of oscillators, you have the Hamiltonian; I could write this down and understand what spontaneous emission is, whereas this other stuff I could not understand. I could see all these people writing equations and that one follows from the other, but what really was behind I did not understand. I cannot tell whether the others did, but I had no possibility of finding it out.
Kuhn:Was there discussion of this field quantization approach? Were there other people opposed? You say you didn't understand it; was there direct opposition to it?
I don't know. I can't tell. I just don't know. I was a very young boy — just 20 — at that time, and I'm not a Pauli. I had the feeling that it was all beyond me but that I would get to it eventually; meanwhile there was no point in wasting my time with it when I couldn't understand it anyhow. I wanted first to understand the ordinary quantum mechanics; the other would come later and maybe by that time people would be able to formulate it better. Therefore I probably didn't even listen much to those discussions. I really can't tell you. One was a little one-sided in those days, compared to now — at least I was. "I'll learn this and then I'll wait for the next. Why waste my time." The graduate students now do the same, in fact, and I've reproached them for this.
I can tell you what kind of problems excited me at this time, and they were more or less Franck's problems. I remember endless discussions, but deep discussions, about the difference between absorption and dispersion. For example, imagine that you have light scattering at an atom. The frequency is not the resonance frequency. Then you have, so to speak, the classical dispersion theory, the forced oscillations. The moment you get into resonance, however, you divide the problem into two. You have absorption and then emission. Is this qualitatively different? Is there a transition? What is the transition? What is a lifetime — in the case of a resonance absorption you have a lapse. It stays there 10-8 seconds and then emits. Is there a lifetime in the non-resonance? Is it also excited and emits it afterwards? These questions excited me enormously, and in fact I think most of my deep thinking after I absorbed quantum mechanics was related to problems like this under the direct influence of James Franck.
Kuhn:As you phrased these questions in this form, you're phrasing them still largely in terms of the old Bohr atom. Now was this the terminology which you used?
Kuhn:Very definitely a correspondence principle terminology and not yet a wave mechanical?
Weisskopf:Yet still we tried to answer these questions wave mechanically. There is a paper of mine, by the way, which sort of summarizes these questions, in the Physikalische Zeitschrift of the Soviet Union... Well, we formulated the problems with the old terminology but when we really wanted answers we had calculated them quantum mechanically. For example this very interesting problem about the turning of the polarization in a magnetic field where you can sort of measure the lifetime. Of course you get turning corresponding to 10-8 seconds when you are in resonance, but the turning of polarization in a magnetic field when you are out of resonance is very small; it corresponds to 10-10 seconds, the reciprocal of the frequency. Now does it really mean that is short lifetime? These are the questions we discussed endlessly but deeply, not in the old fashioned Bohr way, but as one would discuss them now. I think that now I cannot give a better description of this than I gave at the time.
Kuhn:How deeply had Franck taken in the new way of doing this?
Completely, but in a very interesting way. I would say completely unformal. Franck could not do one calculation with the Schrödinger equation, at least at that time, but he had a Fingerspitzengefühl; he knew exactly what it was. In fact I remember at that time that I would have guessed something completely different, but he told me it would come out the way he said and it did. He felt these things. Take for example the question of absorption and subsequent emission; it can only be done if the incoming frequency band completely includes the line width. The most interesting case is what happens if it doesn't, if you irradiate with a sharp line, sharper than the line width. Then it is much more like the case out of resonance. Many of the things I've forgotten, but these are the kinds of problems which I discussed with Franck. In fact I think Franck used me as his unofficial assistant in getting these things clear, which was a tremendous boon for me because there is hardly a man who is more Pleasant from a personal point of view than Franck. This collaboration with him and the time with Ehrenfest are probably my most enjoyable periods from this time. Then came the time for me to write a thesis. Again I had trouble with Born —the combination of his illness and the fact that we didn't look at things the same way. I didn't even ask him for a thesis topic; there was pride in it, too — I wanted to choose my Doctor's thesis myself. Under the influence of Franck I was always around those line width problems. We often speculated together with the help of Heitler and some of the younger people there. Maria Mayer and I knew each other very closely and we discussed all these problems connected with dispersion, f-values, line widths. I myself found at that time the quite simple method of getting the line widths out of Dirac's theory of radiation, but only for a single transition, the transition from the first excited state to the ground state. That is how one can get around the approximation, the expansion in e2/hc, by means of these two simultaneous equations.
I was extremely intrigued by this and so were others; I went to Born and told him that this was what I wanted to work on. Born did not show special interest, but he didn't discourage me either. He accepted it as a subject for my doctoral thesis. "I accept this. Go ahead. It's fine."
Mostly I worked with Franck and Franck at this time suggested that I find out how it is when the transition goes between two excited states. He admitted that he didn't know; he said he understood that one gets the classical line width, of course, but between two there were many possibilities and he encouraged me to find out what they were. I tried very hard, but I was simply unable; was just not clever enough. Then one day Wigner came to visit and to give a seminar. I'm not sure that I knew him before, at least not very well, but I certainly used him during the time he was there. Perhaps it was a week. I told him that I had this method for determining the line width and that I knew the system of equations — one has to take the three simultaneous equations: the ground state, the excited state, and the second excited state — but I couldn't solve it. Again, if I had been trained with Sommerfeld, I might have been able to.
Wisner said, "Look, that's really not very hard." I remember sitting with him in the Gasthaus (zur Krone) where he figured this out for me on a piece of paper and together we found out this funny thing — that the line widths add up. I was tremendously proud; of course it was really Wigner, but then he was solving my equations after all. Wigner also was very happy and excited about it. At that time he was on his way to Princeton. He was alternating. But he told me to think some more about this discovery and perhaps we could publish a paper about it when he got back. Then I took it to Franck and said, "Look, we've found the solution to your question." He found this very interesting and we started speculating on the uncertainty principle in physics and found it rather plausible that this is the sum of the two. I think I made some more calculations, but I didn't add much to it. There it was, the whole thing.
When Wigner came back, we published it and this is the so-called Weisskopf-Wisner affair. By the way, this is not important, but I'm always proud to tell this story. With the name of Weisskopf one is not in very good shape in the case of double publications, but I was lucky because Wigner comes after Weisskopf; then, to thank the gods for it, I always kept painstakingly to the alphabetical rule, as you will find in literature — in order to pay back the gods for the considerable favor of letting my name be first at that time. There is one exception, but it wasn't my fault — a paper with Ewing at Rochester, which he changed without my knowledge.
I would like to emphasize one point since it is probably very interesting; I like to tell the story in the following way. When I came into Physics I came one or two years too late. All of my older friends, such as Bethe, Heitler, Bloch, and Wentzel — who were only two or three years older than I — took their Ph.D.'s in the golden period; that means every Ph.D. was the opening of a new field of physics; London-Heitler opening chemistry, Bloch opening the solid state, Wentzel the photo effect. When I came it was like Alexander, the son of Phillip: the world is conquered; what shall I do? In fact, the problems which Wisner and I attacked were the beginning of the end. Our paper, as Wick pointed out this morning, was the first paper where there was a divergence, the first paper in the literature where there obviously was an infinite term, which we didn't know what to do with so we threw it away. In other words, it was the beginning of a problem. It was the first problematic paper and the problem is not solved up to now. I would like to emphasize that I was not aware of this although Wigner was. I have talked to him about it afterwards. I remember vaguely that Wigner was always talking about some difficulty, which was beyond me to understand.
Kuhn:You say you weren't aware of this; you mean you weren't aware that there was an infinity you were throwing away?
No, at first it wasn't as clear as we have perhaps intimated today. There an integral, which was the shift of the line and which, with a little arrangement, can be made zero. I thought this was perfectly right; also, being a non-relativistic man, I didn't understand too well what happens at very high energy. I thought this probably didn't contribute and somehow the matrix elements would go to zero and this integral is either zero or extremely it was, at that time, a kind of reasoning which would have been believed by many people. I was convinced that there was no problem — "of course, why should there be an infinity there?" We were calculating line widths so there should be. Wigner, in his own strange way — he never argues — kept saying, "You know, there's something wrong here," but he didn't really explain to and maybe he couldn't. Looking back now it was really quite clear; you could show that because of the sum rules this matrix element cannot go to zero fast enough, and this integral is obviously infinity. I don't know and I would like to ask Wigner whether he knew it was infinity or not; I think it's an interesting point in the history of field theory. I would not doubt it if he said he knew it exactly and knew the right reason.
There was a second point in this whole business, namely, what happens if the two transitions have the same frequency, as it is in the case of the oscillator. Then the rule of the sums of the widths no longer applies. Wigner saw this point right away and asked me in a letter from America to put it in the paper; I never understood it and put it very badly in the paper so that in the end we had to write a second paper and Wigner was very angry with me. That was a point I never understood until a few years later when I understood from the correspondence principle that the oscillator cannot have that additional sum rule.
This paper of course was not my thesis because one is not supposed to offer joint work for a thesis. I applied the same ideas, under the influence of Preach, to resonance fluorescence, to the problems I mentioned before, because with this method it was much easier to answer some of those questions about the non-difference between scattering and absorption and the resonance. Then I wrote a thesis which is in the Annalen der Physik, "Theory of Resonance Fluorescense," which applies the same methods to the problems of resonance fluorescense in a very straightforward manner; this was simple work. This was written mainly as a sort of "house theory" for Franck's experiments, with the exception of the last chapter which treats the reaction with not one but many atoms, with the crystal lattice.
This chapter was all wrong but wrong in a very interesting way. It stems from my interest, due to Born, in refraction problems and is the beginning of many particle problems which later on was a very important thing for me. I didn't really have a good "Doctor Father" for my thesis. As I said, Born just accepted it and didn't really study it, Wigner was in America, and, essentially, I had sole responsibility for it with the exception perhaps of Franck. That of course was very good, being on my own as early as that.
Kuhn:When you speak about this paper with Wigner as having the first of the infinities what about the infinities that were coming through, say, in the Heisenberg-Pauli which was the year before this?
Weisskopf:These are different infinities. These are the zero point oscillation infinities?
Kuhn:And also the self-energy infinities.
Weisskopf:Well, there might be the electro-static self-energy, but in a rather trivial way. Our infinity was the coupling between the transverse radiation field and the — I might be wrong, you might prove me wrong, my present answer is that Heisenberg and Pauli's trouble was the following: first their trouble was the zero point energy of the empty space, second their trouble was the treatment of the Coulomb field, but that is minor. They have the ordinary Coulomb infinity, simply the self-energy of the static charge, but I don't think they had the self-energy of the coupling with light. I might be wrong. They had two papers of course, one in '29 and the other in '31, later than ours.
Kuhn:Yes, I was thinking of the'29 paper. I wasn't saying that it was the same infinity, but I meant to say that in problems of interaction of matter and field infinities are now beginning to emerge.
Weisskopf:My point, in order to support my statement, is this: the electro-static infinity is old. Lord Rayleigh had that already, so if this appears again it is not really the quantum mechanical difficulties of the field coupling. Now the zero point energy is something else; in this respect I am not correct.
Kuhn:I'm not trying to challenge the statement; I'm only trying to find out what it means.
It's correct to challenge it. I should perhaps restrict the statement in this way: it's the first time that an actual physical effect is calculated and in this calculation an infinity had to be left out. This is probably the test way of formulating it. In order to come to a finite result you have to go away to a infinite integral.
So I got my Ph.D. after taking one of these incredibly mild exams because one knows everybody; my examination committee was composed of Courant, Franck, Born and Eucken, all of whom I knew personally. The exam was a farce, in fact; instance, Franck knew I had just returned from a meeting in Hannover where a few new things were discussed and he asked me to tell them about that. I think was bad, because after I had received my Ph.D. I was really ignorant in physics. I knew my own field-very well, but about the rest I knew nothing. I did not know a Compton effect was — I am serious — and was a Ph.D. in Göttingen in 31. It's a shame. It was due to this systematic approach. Well, then I went to Heisenberg, I guess.
Heilbron:You had a paper with Born at some point.
Weisskopf:Yes. What year was that? It was during my University years so it must be '30 again. It was a very nice thing; what was it called again?
Heilbron:"The Quantum Mechanics of Absorption."
Weisskopf:Yes, that was during my student days when I was very much in love with Dirac's radiation theory. Born was working on these solid state problems end he wanted to make an application; it is simply how an atom absorbed at [the surface of] a crystal loses its energy into the crystal lattice, and, then this complete analogy between space radiation field and sound field in the crystal. Through this work with Born I must say I learned some solid state crystal technique; 'Koch and Peierls' and so on I did learn in Göttingen. Then I went to Leipzig just to learn some more physics. Perhaps it is important to know that at that time one did not get any money for such things, so I went to Leipzig on my own; I had also been on my own in Göttingen. Heisenberg and his group were there. The main people were perhaps Placzek, Sauter, and Wick. I spent only one term there, and was mainly busy writing this paper, this Zusarmenfassender Bericht, about the line widths. We had originally done the natural line widths and I had decided to go on and get all possible line widths, also under the influence of Franck. I had started this in Göttingen already, getting all the different line widths under one way of looking at them. So at Leipzig I mostly wrote on this article and at the same time assisted in discussions with Heisenberg, but I did not do any work. These discussions were partially on Bloch metals, whether the electrons are really free, and also on nuclear physics. I remember one gnawing question on beta radioactivity:, are the electrons in the nucleus or not? And I remember sitting outside having lunch with Heisenberg on a very hot day. We were next to the entrance to the swimming pool. He said, "These people go in and out all nicely dressed. Do you conclude from this that they swim dressed? So he was very doubtful about what the electron does when it goes into the nucleus and comes out of the nucleus.
Kuhn:This was now when, the spring of 31?
Weisskopf:Yes. The neutrino was already there. But the neutron was not yet there, so there was the problem of how the nucleus gets the negative charge. At that time I would have answered to it that somehow the electrons are in there; that has 16 protons and 8 electrons. And this was part of our discussions, that, in fact, you cannot really understand how the electrons can be there.
Kuhn:I gather that Heisenberg concluded from this that very possibly quantum me simply did not hold in the nucleus?
Weisskopf:Yes, that was often said. I must say I do not have a very good recollection of the discussions. This is what I meant when I said that I will probably disappoint you somewhat. I cannot reproduce these discussions, partially because I was a one-track mind. I worked on my line widths, and this was just coffee klatsch.
Kuhn:Do you remember how you yourself reacted to the idea that quantum mechanics might break down just like that?
I really cannot remember. It was a wonderful year from a personal point of view. It was a wonderful crowd. Heisenberg was very pleasant, and I made really great friends, Placzek, for example, and Bloch. It was the first time I had met them. Then I got an offer from Schrödinger to replace London, who had gone to America for a term; I accepted with pleasure, it being my first paid job. 200 Mark/month - not very much. So I went to Berlin. I did not do much physics there; I finished my line widths article. It turned out that it was very difficult to have any contact with Schrödinger; in fact I had no contact. I had to give courses for him sometimes and correct his Ubungen. I was perhaps a little tired of physics, very interested in other things. It was '32, Berlin, 1/2 year before the Nazis; it was a very exciting time in r- y respects. At that time I also applied for a Rockefeller Fellowship; I wanted to go to Copenhagen. I got it, but for the next fall, that is fall, 1932, but I couldn't stay in Berlin longer than the end of that term, about January 1932. If I had stayed a little longer I would have had to leave because of political reasons. It was a very, very disagreeable, but in a way, interesting time. I was more involved in politics than in physics, actually; at least in the sense that I talked to many people in Berlin and a lot of non-physics friends and interests.
Schrödinger was not at all inspiring; he was a completely 'lone-wolf' — absolutely no personal contacts in physics. He worked on some 5-dimensional relativity theory that I wouldn't have understood anyway. So I did the little physics I wanted to finish up and nothing else. I did have some contact then with the Haber Institute, mainly with Kopfermann who later on became one of my best friends. He was working at that time with Ladenburg on the f-values. Coming of course, from Franck and having a theoretical training from Franck, I was asked a lot of questions. I liked the time at Haber's Institute, where the Franck problems were taken up and also a lot of solid state and crystal theory; but I was not very active.
Kuhn:Did you go to the colloquium there?
Kuhn:Was there any real excitement?
Weisskopf:No. There were only two exciting people there at that time, and they were both Hungarians. One was Neumann and the other was Szilard. Wiener was in America. Both I met at that time, and was deeply impressed. With Neumann I discussed a lot of fundamental philosophy of quantum mechanics; he wrote that book at that time. And Szilard always has great ideas. Berlin is too big a city. It is like Columbia in a way; like Columbia versus Cambridge.
Kuhn:Did they train themselves?
Weisskopf:In small circles, not the colloquium. Around Neumann you found Szilard, and others, but the colloquium itself was too formal an affair. With Planck and Einstein in the first row who dared say anything? Also, perhaps I was not interested enough, I do not know, After that I went to Russia for one-half year, from February until my Rockefeller started, which was October, November. I was there altogether about nine months.
Kuhn:Did you go on your own?
Weisskopf:No; they invited me. I had a number of friends in Kharkov; Viennese who went there, communists who moved over. It was a very bad tine. A lot of people who were either communists, half-communists or non-communists who just went there because it was the only place where you could stay alive. There was a new institute in Kharkov; Landau was there as was Alexander Weissberg, who wrote a book on Russia recently. He is a converted man now. He is a very interesting man: mostly politician but started off in physics. He, at that time, was a big shot there; he invited me. Placzek also came and Houtermans was there.
Kuhn:Was this a job?
Weisskopf:Yes. Houtermanns was an assistant of Richard Becker during the Berlin ties, and I was very much in contact with him there. At Houtermans' house in Berlin I kept up in physics, but mostly I was busy with politics and met many people of the leftist-liberal type. That was an exciting time in Berlin. At any rate, Houtermanns was then later in Kharkov, which was sort of a receptacle of refugees, either depression refugees or Nazi refugees. The months there were not too interesting from a physics point-of-view. I wrote the paper I mentioned earlier and I did have contact with Landau, but my recollection is overshadowed by the political experiences. I would not say that this time had much significance for me as far as physics is concerned.
Kuhn:Is Landau older than you are?
Weisskopf:No. Exactly my age. However I am not a prodigy; he was. He started much earlier. Landau was, at that time, and still is, very obviously my superior in physics. Tisza was also there. I cannot even remember what Landau was thinking about then.
Heilbron:You just missed Landau at Leipzig I suppose?
Was Landau in Leipzig? Not in my time. He was in Zurich, then in Copenhagen. I met Landau later on abroad and had more physics contact with him then than I did in Kharkov. But I-must admit that I did not go to Russia because of physics; I went because I wanted to see Russia and I did so.
Then after the Russian stay, which was in many other respects a very important time for me, but not physics, I went to Copenhagen to start with my Rockefeller. In fact I decided to divide my Rockefeller year into one-half year at Copenhagen and one-half year with Dirac at Cambridge. The emphasis was very strong on Copenhagen, however. I went to Copenhagen in the fall of '32, November. I was just engulfed by the Copenhagener Geist and by many other things. I met my wife. Delbruck and Williams were there, Teller, Bloch, Chandrasekhar and Bhabha. Plesset was later. I don't know who was from America; I don't think there was much from America at that time.
I was sort of blank as far as my work was concerned. I had done the line width and had had enough of it and I wanted to go on to basic questions. First is, of course, the impact of Bohr: the whole philosophy of quantum mechanics. I was "in" right away due to my old friends who were already there. Delbruck had been there a year or so and Bloch also, and all my good friends from before. It is very difficult to get into Copenhagen; I have seen cruel things happen if you come and cannot get through the 'Guard'. Bohr was surrounded by five or six, maybe even more, of his disciples, who were a very arrogant crowd. If you were not accepted by them you would have a very difficult time with him. That was always so, and I can give you a few examples. Rabi is one; a number of Americans had a very bad time here.
Kuhn:Did they simply wind up on their own, doing what they could?
No. They might even not notice it, fortunately; but some did, the more intelligent ones.
Just "not in" you know? They would see Bohr very little because we watched it. I know because I was one of those disciples; we were not nice. Well we did it out of tremendous enthusiasm, to keep the level high. We stayed together in the evenings and discussed things. We went to the movies together. A new fellow who was not well known had a very hard time. Some of them could get "in"; Williams for example, was not known by anybody but he had such a wonderful, buoyant personality that he was accepted right away. Others did not have this luck, and it was not always dependent on ability in physics, although it was, of course, to some extent. I was extremely lucky that I had known Delbrück, Bloch and Teller before and I was accepted in the midst right Gamow was not there at that time. I was sucked into the atmosphere; in the midst of discussion with Bohr and with the others and the Kopenhagener Geist. How one talks about these things, for you see one talks there quite differently from Göttingen. In Göttingen I got the 'hand-craft' of quantum mechanics; complementarity and all this I learned here.
Kuhn:Was complementarity something that was talked about in Göttingen?
Weisskopf:A little, not much, in its own slightly formal way. Not with the kind of life-and-death attitude one had in Copenhagen.
Kuhn:What about Leipzig?
Weisskopf:Not at that time. They were too occupied with special problems. The Leipzig crowd felt they knew this already, and therefore it wasn't interesting to them. What time was this? When did Rosenfeld and Bohr write their paper?
Kuhn:They wrote it in 32.
You see it was this spirit that pervaded the rooms; the endless discussion about observability. You came right into the midst of it because of Rosenfeld-Bohr. I remember that Rosenfeld was not always there; he had to go from time to time, then Bohr took me as a "dictatee". You know how Bohr works, he always wants one man who writes, so to speak, with him. So when Rosenfeld wasn't there I was it. It was toward the end of my stay.
It is a strange feeling when you work directly under Bohr as his collaborator for a short time. You lose your personality; you have no responsibility for anything; you know you have to be at the institute at ten o'clock, your mind is blank until you come and then it is his mind that fills your head and you discuss with him and help him express things. Actually you are only there as an echo, still a deep echo that goes through your mind. You had to go to Carlsberg and stay late in the night; when you leave the house you go to movies; you can't even discuss with other people because it is too much. Therefore I always thought it was a terrible thing to be caught by Bohr. Of course if Bohr says, "you work with me?" you say yes with enthusiasm. I consider myself lucky that this didn't happen often. I do not think I would be what I am if it had happened a little more. I have seen many people sucked out completely and I am sure —. How confidential is this?
TSK It is as confidential as you want to make it. In any case, if you say nothing about how confidential it is, it goes into libraries for the use of scholars. If you want it retracted, let us know.
Weisskopf:I know of people who, to my mind, would have been much better physicists had they not spent so much time as Bohr's slaves.
Kuhn:Were there other people who felt this way about it? I have been waiting for somebody to say this to me, nobody has yet.
Weisskopf:Who would say that? I am not sure whether Bloch wouldn't say that, did you talk to Bloch?
Kuhn:Not yet. I will when we get back to Berkeley.
Weisskopf:Teller might if he still remembers the tines and is not too occupied with his present problems.
Kuhn:Is this something that some of you said among yourselves?
Kuhn:Would Pauli have said it?
Weisskopf:Yes. Oh, Pauli, of course:
Kuhn:Did he say it?
Weisskopf:Yes, yes. He probably said it even to me: "You are lucky that I got you away from there," because that is what he did. I am not sure whether he didn't say anything to me. Pauli would certainly have said it; almost one of the 'pet' subjects of Pauli's intimate conversations, although he loved Bohr. Kalckar died; he knew this very well.
Kuhn:What effect do you think this may have had on Kramers?
Weisskopf:I cannot swear to it, but I am almost sure that Kramers said this to me at a dinner at which Kramers and I exchanged our impressions about this. Pais will tell it to you; Klein I do not know, he could if he wanted to but he probably won't. Of course it is clear that this does not at all detract from my deepest admiration for the man; on the contrary, it shows what tremendous personality he had. Yes, Kramers would have said it. At that time I was entering what one now calls field theory in a very deep way —
Kuhn:Before you get to this there is something I very much want to hear about. I take it, from what you began to say before, that there was still great excitement generated in Copenhagen by complementarity, by the measuring process —
Weisskopf:Due to the Rosenfeld-Bohr paper, yes.
Kuhn:And you were all involved? Nobody was saying, "That isn't physics."
Absolutely no. On the contrary, this was the great subject all the time. There is a famous joke about a zoology professor who examines his student, and one knew that he always likes to ask about the worms. One only had to get to that subject somehow and then everything is fine. Now this is a pun, a Viennese pun, it's not even German. There was one student who said, "Herr Professor, da wär'mer. Die Würmer teilt man ein in das und das." "Da wär' mer " means "here we are." "We are," you see, means "worms". It was generally among our group there and particularly under the influence of Placzek, who is a man who creates atmosphere, that this subject was called the worms, "die Würmer". Er spricht schon wieder über die Würmer. Because with Bohr you can start with anything and you will end up with complementarities, especially at that time. And so this is called "die Wilmer." But this was by no means — well, it was making fun, but in the relevant sense.
You know that whenever I talk about fun in Copenhagen I must tell the famous story that I myself experienced. When I came, even in the first weeks, here were Placzek, Delbrück, and Gamow — I think Gamow was there in the first few weeks — always making practical jokes about everything and I was not yet accustomed to this atmosphere. I went for a walk with Bohr and Bohr said, "How do you like it here," And I said, "Also wundervoll, but it is something which worries me, why do people always make fun about these things?" And Bohr said then this statement that became famous, "Es gibt Dinge die eben sind so ernst, dass man darüber nur scherzen kann." I learned this very quickly myself, of course, and with this sentence all these jokes are to be interpreted because every single one of those men would have given his life to defend the seriousness of the question of complementarity.
Heilbron:Was there anyone who took the position of Peierls and Landau in this?
Weisskopf:Yes, in fact Landau came at that time to Copenhagen. Of course. In fact, wasn't that the beginning of Rosenfeld and Bohr? Peierls was not in Copenhagen, but he appeared there quite often, and I think I remember, though not clearly, Bohr's being extremely unhappy about Peierls and Landau.
Heilbron:Yes, very unhappy. I just wondered if they obtained any sympathy from others there.
Weisskopf:Oh, yes. Absolutely. I think that Landau particularly was a very convincing man. I would say that Bloch and Placzek, as I recall, were very strongly partisans of the Peierls-Landau attitude, and Bohr fought like a devil. By the way, I should say that these people, the group, "us" — we were not at all "yes men", absolutely not. On the contrary. Out of sport rather, we liked to contradict the great man; also in order to start a good discussion apart from the fact that we really wanted to understand the things. We were not "yes men" at all, and Bohr had a very hard time at that time with us. I was still very young, but the great "fighters" against Bohr were people like Landau and like Bloch. I'm not sure whether Bloch was involved in this, but he could have been, and Teller. There was not at all a "yes" spirit, and Bohr had to fight extremely hard, especially about this paper.
Heilbron:Did he win by the time the paper was done?
Oh, yes. He won. He always fights — he comes back time and again until you say completely "yes" — "I know he only said 'yes'" — until you accept the formulations. But he won, after a long and drawn-out battle.
Now at that time I studied field theory papers — Pauli, Heisenberg, Dirac — these papers by Dirac about what one now calls the Feynman function. And so the time went by; I wasn't there very long. Looking back on my life, I find it a strange thing to imagine that I was only eight months in Copenhagen and what that eight months meant: It's tremendous. I would have needed five years now to assimilate and go through all this. Of course I never can distinguish between the eight months I was here and the constant visits I made here because of personal and scientific reasons; I was practically always in Copenhagen for a long time to come. I went, then, in the summer of '33 to Cambridge, because on paper I war half in Copenhagen and half there and I had to show up in Cambridge to please the authorities in America.
There I heard zero from Dirac, as everybody has, but very much from Peierls, whom I knew before — but we hadn't been close. I spent most of my time with Peierls and he taught me the "handcraft" of field theory, how one calculates things — Casimir operators, alpha-gymnastics, Dirac equations — the real meat. I must say Peierls was wonderful in this respect; I spent much time with him. He knew this very well of course.
Kuhn:How did you happen to decide? I mean, you were looking for fundamental problem was it just sort of obvious that field theory was it?
Weisskopf:Yes, because field theory was "the thing everybody talked about it, so let's get into it."
Kuhn:Would there have been any alternatives in this period?
Weisskopf:Yes, there would have been. For example, I did not know anything about nuclear physics at that time, but really nothing: It's perhaps worth mentioning, because at that time, after all, the neutron was discovered, the beta decay, the positron,— and all those kinds of things were extremely exciting, but I was completely disinterested. This one-track mind business again.
Kuhn:Was everybody in Copenhagen disinterested?
Weisskopf:Not everybody. Nuclear physics always was simmering. Maybe Teller was more interested in nuclear physics. Bohr was; we had seminars about it, but it didn't sink in, you know. I was just introduced into hole theory, into positron theory, field theory, radiation theory and that kind of thing.
Kuhn:You must have been somewhat in on this opposition of Bohr's to the positron.
Weisskopf:No, I think that was earlier. You mean against the hole.
Kuhn:Against the hole was earlier. In fact I have now been assured by several people that in March or April of '33 Bohr was looking at the picture and saying it might be something like they got the direction wrong.
Now when you say it, I remember that. But it didn't impress me very much. In '33 I just learned trade; it comes back to me, but I don't have a very clear recollection about discussions at that time. But again my memory is so bad, I just don't know. There are things I just can't remember — what the discussions were in the corridors at that time. I didn't write anything at this time because I learned field theory; I mean the Dirac equation. I must say that because I am a non-relativistic man, this was the first time I began to get the idea of the Dirac equation.
Then when I was in Cambridge I got an offer from Pauli to be his assistant. Casimir was my predecessor and Peierls was the year before; Casimir had to go back to Leiden and I was to become Pauli's assistant, which was, of course, fantastic. To be asked to be Pauli's assistant is the great thing; I had the feeling "I'm in, nothing can happen to me". In a way that's right. So I care in the fall of '33 to Pauli. Now come all these well-known stories about my reception by Pauli —
Kuhn:Tell it anyway; I'd like to have it from you.
Weisskopf:They are on another tape. By the way, you should know about this. There was a tape taken of me by a Viennese man whose name I have forgotten, but you can find it out. There is at the University of Vienna a sort of project for the intellectual history of Vienna and because I am from Vienna this man interviewed me about my life.
Kuhn:How extensive was this interview? How long did it go on?
Weisskopf:Well, about two or three hours. It had a slightly different aspect, dealing more about Vienna. It's in German, of course, but you really ought to get it. When he asked me, I wondered, "What shall I do with this fellow?" And I thought that this was the time to get all the Pauli jokes I know on tape; they are on that tape. The main point about my reception by Pauli was that he told me he wanted to get Bethe as his assistant, that would have been much better; Bethe was working in the solid state at present and since Pauli hated the solid state he took me.
Kuhn:Did he tell you this right smack at the beginning?
Kuhn:Had you known him at all before?
From a distance. From Copenhagen conferences. The first time I came to see him, I knocked at the door — no answer. He was in a very bad mood at that time; the whole period was a difficult one for him for personal reasons. When he didn't answer, after a few minutes I opened the door and he was sitting at the other end of the room. He figures and figures and figures and says "no" all the time with his head — something didn't work probably — and I said, "Herr Professor"; but his only response was, "Warten, warten, warten," "Wait, wait, wait." So I stood there in this disagreeable situation for ten minutes, and then he said, "Ah, are Weisskopf; yes, you will be my assistant. I will tell you that I wanted to take Bethe...;" And then comes the story. The sequel to it is that he gave me some problem — I really don't know what it was — and after a few weeks I showed him what I had done; he was very dissatisfied with it and he said, "ich hätte doch Bethe nehmen sollen."
I was prepared by Peierls, though; it didn't come unexpectedly to me. I knew what I would 'get', and I enjoyed it absolutely. The point is that you learn very much from Pauli. I learned the primitive handcraft from Peierls, but the real things about field theory, light, and this whole problem of electro-dynamics I really learned from Pauli. Relativity. By the way, I told him that I would come to him only under the condition that he never ask from me anything concerning general relativity. He was just working on that with his five dimensional representation. I said, "I don't understand it, I'm not interested in it," and, "I would like him to know that if he wants to work on this he has the wrong man." And he accepted that; he never asked me to. I learned from him what was really relativity theory: invariances, the ideas of groups, the whole of what now is important.
Kuhn:Had groups become an issue for you before?
Yes, but in a different way. I was very much aware of groups of classification because of Heitler. When I was a student in '28 and '29 and Göttingen, groups were the great thing. When did Wigner's book come out? It was Weyl, and also van der Waerden made an impression. I was introduced into group theory by Nordheim and Heitler, so I was pretty well grounded in physics rotational groups and permutational groups. One did the consequences and group characters very complicatedly at that time, much more so than now. It was about the different symmetry operations, (Young) operators, all this. This was very well grounded in Göttingen; it was typical Göttingen fare and the kind of thing-I didn't like very much, but I learned it there. What I'm referring to now in the Pauli time, in '33, is rather the invariance of fields, the Lorentz invariance; essentially at that time it was only Lorentz invariance. Problems of this kind.
Then came a very fertile time under Pauli. I published three papers in '41, and it is very hard to time them. They are the self-energy, the Pauli-Weisskopf, and the Danish one, the polarization the vacuum. The self-energy was the first? It was a typical paper that I did under the guidance of Pauli. Pauli said, "Now you should sit down and calculate the self energy." Now this, by the way, is a great blamage; I always get slightly red faced when I'm quoted for this discovery. Pauli told me that we must re-calculate the self-energy of the electron because so far it had only been calculated by Ivar Waller for a single particle, that is, not with the Dirac theory of the vacuum. "The vacuum will probably change it; try to calculate it, - Pauli said. So I sat down to calculate it and made a calculation mistake, a very primitive, bad mistake, which made me terrifically unhappy at the time, and got the result that it diverges as badly as Waller's diverges. And that was published. Then I got a letter from Furry saying that he had recalculated and had found a mistake; it only diverges logarithmically. Great discovery. And I published, of course, a Berichtigung. At that time I was about ready to give up physics.
Kuhn:Had you known Furry before?
Weisskopf:No; that was the first time I had heard of the name. Pauli of course is rather strange about these things and his attitude was, "Well, I didn't expect much more from you; I'm not surprised. I never make mistakes, but of course you people do. That happens when one has to work with people like this. And, "I said that it must be different." One must be careful, after all, when one tells Pauli stories, because he is the kindest man in the world. Not only was he perhaps despisingly friendly, but he was really friendly. He said, "This sort of thing happens with people, but that's no reason a man isn't a nice fellow and a very useful man. And physics is interesting. After all, I'm mainly interested in the result and what came out was sort of what I had felt before, "because Pauli had said it would be different. Afterwards, to make up for it, I published this paper much later in '39 in which I analyzed very carefully why it really is that it diverges logarithmically. This was on my mind for a very long time...
Heilbron:How much importance was attributed to this Wentzel business about the removal of the self energy for the classical case by some limiting procedure?
Weisskopf:That was considered to be a swindle. It cannot be done for the logarithmic divergence I am sure; it can only be done for the single particle. Not much importance was attributed to it.
Kuhn:At any rate, in your Berichtigung you're not so clear about it as you were in the first paper.
No, I don't think we considered this an important point; it was a kind of freak, and I think it was considered so by Wentzel, too. Wentzel, by the way, was a very, very interesting man at that time and very important; I learned a lot from him. Anyway, this paper was work done under Pauli's orders. Now I come to the work about the vacuum polarization, which I published in the Danish Academy. I very rarely complain about not getting enough recognition because I think I've received in my life more recognition than I really deserved, but for this paper I don't get enough recognition. In my opinion, this paper is really the beginning of re-normalization; and when you read it you'll find, it. The first purpose of the work, and the one for which it is perhaps best known, was the recalculation in a very much simpler way of the Euler-Heisenberg vacuum polarization for slowly varying fields, which Wick mentioned today. It was really only a recalculation, although with very nice methods suggested by Pauli. This is why I didn't publish it in Zeitschrift fur Physik; I thought I should have something in the Danish Academy, and so I published this there because it was really only a simplification.
However, this same paper contains a study in which I was not very sure of myself, which is also why I published it in the Danish Academy, but which excited me very, very much. The study was to show that all the infinities that come about in calculation are in fact infinities that you cannot measure, namely, infinities of charge, infinities of mass, and infinities of what I called there the "dielectric constant of the vacuum." As it says there explicitly, one could assume that the total result is given by nature and one can forget about these infinities. What is given there is the recipe for re-normalization. Again I say that if I had had my Sommerfeld training I could have done much more with this. In fact I used rather primitive methods there to prove my point and perhaps that is another reason that the work was not too well known. But you could directly quote from there a recipe for re-normalization. There is a paper by Dirac in which he says the same thing for the mass, I think, but I knew that already. I directly say there are three magnitudes which are essentially nowadays the three "z's", the three infinities; and I say in there that these are the three infinities; but is characteristic that were they finite, you wouldn't be able to notice them.
This is why you can forget them, and I say this explicitly in this paper. I did this work rather independently; Pauli was of course interested in it and he advised me in many things. By the way, I was always a sloppy man, and this paper was one that has the greatest number of calculating mistakes of any paper ever written; it's terrific what's wrong in there, but in principle it was right. The third paper of this period is one which Pauli and I get too much credit for, the famous paper we wrote together on the pion dynamics. This has a rather interesting story. I wrote at that time a sort of review article for the Naturwissenschaften as I have done quite often in my life, explaining the situation for non-experts — an activity, by the way, which was strongly despised by Pauli. In fact he told me once, after I had mentioned I was writing this article, "That you can only do in off hours." I think it's quite a good article; you'll find it in the 1934 Naturwissenschaften. In writing this, and also through conversation with Pauli, I was struck by the fact that the Klein-Gordon equation does admit that charge could disappear.
No, I'm sorry. It is an absorption of light quantum. I could show that light quantum can produce two charges; it is simply a matter of taking the Gordon equation and putting in an electromagnetic field as a perturbation. Through this perturbation a meta-field is created and this has positive and negative charge. I saw this, and I'm not sure but what it was known to some people before. It was certainly known to Pauli, but I think it worried me more than it did him. Pauli's attitude was, "I's not interesting, it's not very important."
But I said, "There is something funny, isn't there? Because not only can I create' these pairs, but I can also annihilate them, though only as an induced effect. In the presence of a light quantum this double charge, positive-negative charge, can also disappear or can be created. Light has an influence on it. Isn't this analogous to pair creation and isn't this a way —?" But I couldn't get it right. Why? Because I was not able to quantize both the radiation field and the meta-field. I did it sort of in the old Schrödinger way with perturbation theory, but not with quantized fields, in particular not with quantized meta-field. I simply had no idea that one could quantize this field, or that one even should.
By the way, I'm wrong; if one doesn't quantize the meta-field, one only gets pair annihilation and not pair creation. If you already have positive and negative charge, this charge disappears by the emission of light. But out of nothing the creation doesn't occur because one hasn't quantized the meta-field, one doesn't have the zero point oscillation. So I did not quantize the meta-field; it never came into my mind, but I saw that we had here a pair creation-annihilation Process without hole theory) without Dirac's whole idea, and that intrigued me.
Pauli didn't want to hear anything about it; he was busy with other things... Well, I tried to tell this to Pauli for about a half hour and he wouldn't listen. He said, "It's silly, go away; I don't want to hear it; again it's one of those silly ideas; leave me in peace." Then I got very annoyed and, said, "Let me tell you a quotation," and then I quoted to him a German quotation which goes, "Ach, warum so viel Eifer, warum so wenig Ruh? Mich dankte Euer Urteil waren reifer, hartet ihr besser zu." It means, "Master, why so much excitement and so little quietness. I think your judgment would be more mature if you would listen better." And he said to me, "What is this?" I said, "This is Meistersinger." "Meistersinger?" he said, "Wagner nag ich iiberhaupt nicht." And then the afternoon was gone; I had to leave, you see. Wagner had to leave. The next day I came to him again and said, "Now listen, Pauli," and he said, "Oh, is that what you mean; why didn't you tell me right away?"
So it was difficult, but when he caught this idea he said, "Clearly we can do this; we just have to quantize the meta-wave also and then we'll get everything." He was very excited then, because he called this our "anti-Dirac paper," you see, he didn't like the hole theory. At that time one didn't have the symmetric formulation; one always had to fill the vacuum, and he hated this. He said, "This is wonderful: Now we can show Dirac that the pair creation and pair annihilation is all there and there's no need to fill the vacuum." And with great enthusiasm he worked it out quickly, because it really comes out very simply once one quantizes the field. He asked me then to do the details: "Here is the formalism. Now you go and calculate exactly the pair creation and pair annihilation and you'll see how it differs from the electron." Of course it doesn't differ from the electron very much except by typical spin terms.
I remember one other story. I was just in the middle of these calculations, which were not terribly easy; and I came to Copenhagen and met Hans Bethe for the first time. Or perhaps not for the first time, but I didn't know him too well. I knew that he was busy with similar problems because he calculated the pair creation with Heftier. So I said to him, "Look, I have very similar integrals. How did you do this? I'm sort of stuck." And he said, "Well, you have to do this and that, and that, and that." "Fine," I said, "How long do you think it will take to do this kind of work?" He said, "If I do it, it takes two days; if you do it, it will probably take three weeks." This was true, and I got a wrong result in addition. But in principle it was the same calculation as Bethe-Heitler, though at that time it seemed to come out of a completely different formalism. That was the nice thing about it.
Pauli liked it very much so it was written; and if you read it now, you will see an introduction written by Pauli telling how he was arguing against Dirac. What it really is at present is a first quantization of boson systems and therefore, so to speak, the basis of the present physics.
These were the three papers of '34. What did I do then? I think it was about this time that I began to be interested in nuclear physics. What's the next paper?
Kuhn:Before you get to nuclear physics, let me ask you this. I'm perplexed about the change of attitude, which must have been quite different from one person to another, toward the state of field theory and quantum electrodynamics over the infinity problems. Some people seemed to have felt from a very early day that it just wasn't going to work. Other people seemed to have felt that it was working all along, but was just mathematically difficult.
Weisskopf:Yes, I think the situation was not so different from today. If you asked what I thought, I wouldn't really know. I must say I would like to re-read my Naturwissenschaften article because I'm sure I mentioned it there. I mentioned the difficulties of the infinities; one cannot so easily assume that the electron is finite because that gets into trouble with the relativity so there is a definite problem there. I'm not too sure which of the two views Pauli and I took at that time. I think it was too new; one didn't take sides. One said than there were these possibilities: either it's really all wrong, or there's only a mathematical problem, or there is some lambda, some cut-off. The future will show; let's keep our minds open. I don't think anyone was very Partisan in this respect. We were all very much aware of this infinity, as all these papers show.
By the way, I calculated the self-energy of those pions right away. I didn't publish it then but waited to put it into a paper I wrote later in America on self-energy to show that the pions diverge even more than the electrons.
Heilbron:There's just one paper, isn't there, in the Danish Academy?
Kuhn:Let's get that chronology right on the tape now.
Weisskopf:The papers are, in order, as follows: '34, the self energy paper ordered by Pauli; then the boson paper, the Klein-Gordon equation with Pauli; and in '36, the polarization of the vacuum and the re-normalization proposal, which was done partially with Pauli but was really written in Copenhagen. That's why it was published here.
Heilbron:There is also some paper about the polarization of the electrons that scatter off a crystal?
Weisskopf:Oh, yes. That's the "wrong" paper. This one of the famous papers of mine that is completely wrong. Pauli said, "I told you right away you should not write it." This paper I did on my own. There was the Bohr idea that one cannot measure the spin of a free electron, which is of course right. I thought that one could easily show that the spin of an electron can be measured when scattered by crystal lattices, which might even be true, but the way it's done in the paper is completely wrong.
Kuhn:That was written when you were still with Pauli?
Weisskopf:That was written in Zurich, yes.
Kuhn:How did you find that it was wrong? Did you recognize it yourself or did someone point it out to you?
I think someone pointed it out to me, and I'm not sure but what it was Pauli himself. After it appeared. I did another paper with Kemmer on the scattering of light by light. I think this was in a letter to Nature. We connected the scattering of light by light with the Delbrück scattering. Today it's a triviality; one light quantum is replaced by the Coulomb field, but at that time it was not so trivial. That thing was in fact the beginning of this later paper on the vacuum polarization where very similar problems are treated. That just shows that these were the things one was worrying about. No, I know what it was about; it had deeper significance, that letter to the editor. Euler and Kockel at that time, under Heisenberg, calculated the scattering of light by light, but had to do a lot of subtracting because there were a great many terms that were infinite. They did this in the usual clever way and got the result. And Kemmer and I showed that you can do the calculation without raking any subtractions, because you can show that it is equivalent to the Delbrück scattering, replacing one light quantum by the Coulomb field, and the Delbrück scattering doesn't diverge. This is a special case of what one does now every day if one calculates these things.
But it shows how strange it is, in a way, that the Schwinger-Feynman electrodynamics hadn't been discovered at that time. There was no reason whatsoever; I think it only attests to the weakness of the minds of theorists that they had wait for Lamb and Rutherford, for the Lamb shift. In fact I didn't wait, but I wasn't good enough; we tried to calculate it before, but we didn't succeed.
Kuhn:Did you actually' try to calculate it?
Later. I haven't reached that point in my life yet. That was after the war, in '46. But I was interested from the beginning in this, probably from the Danish paper and this paper with Kemmer — how to get rid of those infinities. I had heard through Kopfermann, who is a spectroscopist, about these Pasternak shifts before Lamb. Pasternak had some vague indication that something was wrong there. He had the wrong sign in fact, but anyhow Kopfermann always drew my attention to the fact that there was something there a theorist should look into. Then, after the war in '46, French, a student of mine at M. I. T., and I tried to do it. We said, "Well, let's really try to calculate the problem and subtract everything which one can subtract"; that is, according to the extremely pedestrian rules that one should always compare it with the free electron. Nowadays I would say that any diagram which appears in both has to be subtracted so that the finite difference is left. We calculated and calculated; it was too hard for us. And I wasn't too impressed by the problem either, because I thought, "If we get a result, so what? Nobody has observed it."
Since both Schwinger and Feynman got the same different result, I thought "who am I" and went back to the drawing board and spent half a year with French looking terrific mad scramble. Everyone wanted to calculate it, and of course we doubled and tripled our efforts and got it, completely right. I showed it to Schwinger and Feynman, who both said, "This is wrong because we get a different result." If people get two different results, then there must be something wrong with the method. Feynman and Schwinger probably felt the same way. None of us at that time had any room for mistakes and didn't find any until Schwinger came and said, "You were right after all; I made the mistake." I called Feynman long-distance and he admitted it too. But then it was already too late for our paper because Lamb and Knoll had sent in the same results a few months earlier.
So I learned two things from that experience: first, I should have learned more mathematics from Sommerfeld and I would have made this calculation, maybe even before the war, and I would have had this result before observation; second, when you get a result, stick to it, show self-respect, and don't admire Schwinger and Feynman too much. Then at least you would have published it first. So these are the lessons one learns only afterwards. But really the main point I wanted to bring out in this conversation is that these are actually all old problems and it is really surprising that it needed the push given by this experiment to establish a reasonable "subtraction physics". Everybody knew it was necessary. Why not do it?
Kuhn:I don't know these papers — or really this subject — at all. Schwinger and Feynman didn't publish because the answer didn't agree, or wasn't interesting and experimental point of view?
Weisskopf:They weren't yet satisfied. I think the experiment was so inaccurate at that time that agreement wasn't the point.
Kuhn:You couldn't tell your result apart from theirs on the basis of its agreement with the experiments at that point.
Weisskopf:No. Also I had the feeling that, since nobody was really sure of his method, if time applied any reasonable and elegant methods; there were very bad methods, mostly hunches and, as I said, it was a matter of selecting the right Feynman graphs. It was only later on that this was worked out so that it is really good and clear and unique. You haven't spoken with Bethe yet?
Weisskopf:Because the history of the new quantum electrodynamics after the war is, of course, a very interesting story.
Kuhn:I'm delighted to have you go on and talk about it, but so far as the project is concerned we're not going to treat it.
You're not going that far. You see the Shelter Island Conference.... If people think this sort of thing is useful enough and somebody else wants on, that's fine; but I don't think you can really do this right without the papers. We just aren't getting ourselves past the very early 1930's. There's no official stopping point, except I've said definitely from the start I wouldn't go past '39. We're not getting to '39. If anybody will go on past the things we know about and tell us about them, so much the better; but in terms of what we're prepared to dig for it varies from one place to another in different parts of the field. And the longer the project goes on, the more papers we know something about. I'm delighted to have people tell me things; but so far as actually trying in any systematic sense to get dope on it, we're certainly not going to go into the new quantum electrodynamics.
Then I'll go back to '36. I wrote this paper in Copenhagen about the polarization of the vacuum and the re-normalization, and at the sane tine I got into nuclear physics. I personally didn't do anything in nuclear physics. Nuclear physics in Zurich wasn't anything which was discussed much; Pauli didn't like it. There was lots of experimental nuclear physics going on — Scherrer and Staub — but I didn't have much contact with them. In Copenhagen I took it up again because of Bohr, of course; he was in the midst of it and wrote his compound nucleus paper.
Kuhn:In what status did you come back?
Weisskopf:Pauli wanted another assistant and I had been in Zurich three years — longer than anybody else — which was very unusual. I had no job, and bohr offered me an Orsted fellowship for the time being. Placzek was here in Copenhagen and a number of other people. some of the names I mentioned before might well have been '36 instead of '32. Rosenfeld was here. At that time nuclear physics started; and I was very much interested in the alckar-Bohr work so I took up nuclear physics very strongly, partially because one felt a little stuck in electrodynamics, and partially for a very practical reason. First of all, it was a new kind of physics; you had to learn it. also I had the impression that if I really wanted to get a job, I must go to America; and in America nuclear physicists are very much sought after. So I thought I'd do something in this field. I got into a lot of discussions with Kalckar and bohr about the compound mucleus and, in particular, about the nuclear temperature of the statistical model. And at that time I wroe this paper about the statistical model,the evaporation. Now the evaporation idea is of course not mine; it was Landau's and Frenkel's. I don't know what was definitely original in this first paper, but it was published in the Physical Review, and I wrote the paper from Copenhagen definitely because I wanted to advertise in America that I existed. I think the treatment was perhaps original and the direct application of thermodynamics. At that time I remember Otto Stern told me, "the nuclear physics in this paper I don't believe and I don't think very highly of. But what I really like about the paper is that it is an exercise in statistical mechanics of small systems." Yes, I remember; I know what my contribuition there was! I found that one must be really very careful with the temperatures, that due to the fact that the evaporation ofone particle takes so much energy away from the system, one must distinguish between the temperature before evaporation and after the evaporation. I found out that the temperature that is characteristic for the Maxwell distribution is not the temperature before, but rather the temperature after. This was not known before and this impressed Otto Stern simply as an example of statistical mechanics of small systems.
Kuhn:At what point had you really learned statistical mechanics?
Weisskopf:Well, as I told you, I had a good grounding in Göttingen from Heitler; it was one of the things I learned at the University and I always liked the subject the problems connected with it.
Kuhn:It never comes into your work in the interim?
Weisskopf:Since then, no. That is the first time it care into my work. I liked it very much. This paper was published, and right afterward — probably completely independent of this — I got this job at Rochester, mostly due to Bohr, who recommended me there. At Rochester I went back to electrodynamics, I guess because I felt this guilt; I felt this guilt that we do not understand the self-energy and this logarithmic energy. I have to really investigate thoroughly where this self-energy comes from. Then I wrote this paper 'in the physical Review of '39 which, I must say, really interested me very much. Even now I think it is a worthwhile paper. It investigated all the details of why the self-energy is logarithmic, how it comes about that the influence of the vacuum on the electron makes the electron broader and therefore increases the Coulomb energy, and all these detailed analyses of what's going on — perhaps a little in the spirit Wick has advertised today. I felt rather lonely when I wrote this paper; I had the impression that nobody around me was interested in it. Everybody did a lot of nuclear physics, including myself. Bethe and all the exciting people in America did nuclear physics, and I did this. I remember that I was touched by Wigner, who once visited Rochester; when I told him about the paper he said how wonderful this was and how interesting. And I was really touched because this was the first time that anybody was even interested in this. But that, if you're only going to '39, the last thing I did in electrodynamics.
Kuhn:By the time you came to the United States, had there been enough people who had come ahead of you that physics was now clearly established, or did you have a sense of loneliness and America as a great desert in respect to theoretical physics?
Kuhn:If it had been ten years earlier it would have been totally different?
Weisskopf:It would have been totally different. Still, one came over with the spirit of a missionary — one had this feeling. But I was not the first. You see, all good friends were there: Bethe, Teller, Bloch, and so on. I would say that everybody there had a very high opinion of Oppenheimer; I mean, "Oppenheimer, there is really good physics." The rest not so much. So we felt there in the East a little like missionaries, people who had to start something. But the feeling was really nob very strong because there were so many of our friends around and our families. One had a little of the feeling of an American desert, but not very much. Oppenheimer was considered as completely on a par; Oppenheimer and his group were completely on a par with European physics. It is true that when I wanted to discuss anything seriously at Rochester, I had to go to Ithaca and talk to Bethe r to Princeton where I usually could find some people, European friends. This changed pretty fast. I remember that then Marshak care from Cornell to Rochester; in fact I got him there, I appointed him. I asked Dubridge about him. With him one could really do good high-level physics. We did this nuclear force theory, though nothing came of it. So after a few years things developed; Rabi and all the nuclear physics development was so fast; and so much experimental material was going on, particularly in Ithaca, which is near Rochester. Bacher and the little cyclotron, for example.
I came to America, I would say, at a time when one was not afraid; one could already see that this was the place where physics was going to live. It was quite obvious. But that probably is not so for the people who came in '34, such as Bloch. Totally different. I have letters if I could find them still — it would be nice to find them still. Before I decided to go, of course, I wrote to Bethe and I wrote to Bloch, "Well, how is it really?" The answers were mostly very encouraging. This is essentially the end of it. There was a second nuclear paper in Rochester, a paper with Ewing, which was an extension of the first one; two charged particles, this potential barrier concept sort of worked into the evaporation model. Then came the war. That's really it.