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In footnotes or endnotes please cite AIP interviews like this:
Interview of Eugene Wigner by Charles Weiner and Jagdish Mehra on 1966 November 30,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Arrival in the U.S. in 1930; comparison of social, scientific, general intellectual climates in U.S and Europe; early interest in nuclear physics, relationship with graduate students; beta decay, compound nucleus model, Breit-Wigner formula, early shell model; review articles by Bethe; relation of early meson theory to nuclear physics; nuclear forces; charge independence; journal literature of physics ca. 1937; effectiveness of group-theoretic models in nuclear physics; effectiveness of quantum mechanics for nuclear physics; significant early experimental discoveries in nuclear physics: neutron, deutron, artificial radioactivity; fission, shell model of Mayer and Jensen; rotational levels in nuclei; the specialization of physics; effect of World War II on nuclear physics research; work at Chicago; conferences after the war; branching off of high-energy physics from nuclear physics; work personally regarded as interesting.
I’d like to start by reminding you that in your interview with Professor Kuhn, you ended up by telling him of the circumstances of your coming to Princeton in 1930, and that it did come as a surprise to you, and the offer was very proper and very much a surprise in terms of the salary offered.
I never have seen as much money in one heap as was offered at that time.
When you translated it into German marks, I think it must have sounded like a tremendous amount.
Well, it was.
You mentioned coming here in 1930. I would like to ask some general questions about that: your general impression of Princeton once you arrived, and your contacts with people here, how they developed, whom you got to know. Of course you knew people from Europe who were here, but how had these relationships developed? Through this I hope to establish the circle that you moved in and the environment that you experienced.
My relations developed very slowly. I am not a very good man to start social contacts, and particularly I wasn’t at that time. I really knew very few people here. There was Dr. H. P. Robertson, called “Bob,” whom both Johnny von Neumann and I knew quite well. I was introduced to Veblen, who was of course much my senior, both in experience, knowledge and reputation, and also in age, and I knew him reasonably well. But I led a very solitary life here. I saw the Neumanns reasonably often, but they were newly married, and naturally they wanted to be alone, and I did not feel that I should intrude too much. I was used to a solitary life from Germany.
Did you immediately get into teaching when you arrived?
No. My English was very moderate. I spoke a little bit of English on the boat which took me 11 days. I read some novels — Thackeray, I believe, and Walter Scott.
In “British.” However, my English was miserable. So for two months at least I did exactly nothing. I remember, when I gave the first colloquium in English, it was in chemistry, and I learned the first two sentences so that I could speak them clearly and in a definite voice. Then I got stuck, and for about 20, 30 seconds, I could not utter a single sound. Then Hugh Taylor, who was a very kind and helpful person said, “Why don’t you continue in German?” This encouraged me so much that I regained my composure and could continue in English, but it was probably a miserable English. So I did not give classes at all during the first four months. I had contact with some of the instructors, but altogether my teaching was very moderate. And I think they understood that. Johnny spoke much better English, a hundred times better English.
Why was it that you received the offer? In other words, it came as a surprise to you in Europe, but when you arrived here, did you discover what it was they expected of you, or why they were motivated to ask you to come?
I think they wanted to look both Johnny [von Neumann] and me over. I think that the incentive for it came, as I think I told Dr. Kuhn, from Ehrenfest. Ehrenfest was consulted by many American institutions as to how to modernize their scientific spirit. And they were not primarily interested in teaching or teachers, but in the scientific spirit. And Ehrenfest recommended to them not to invite a single person, but at least two of them who already knew each other, who wouldn’t feel suddenly put on an island where they have no intimate contact with anybody. Johnny’s name was of course well known by that time the world over, so they decided to invite Johnny von Neumann. They looked: who wrote articles with John von Neumann? They found: Mr. Wigner. So they sent a telegram to me also. I think that was the story.
Your work on group theory had been published by that time, were they aware of it?
Yes, but you know that was at that time very unpopular, extremely unpopular. You probably know the phrase about abolishing the group pest.
Unpopular or unknown?
Unpopular; people resented it. And it’s not surprising. If somebody comes with an idea that you should learn something more, and you should learn some more mathematics, you won’t like that. People don’t like to learn. There is an enormous pleasure in inventing things. In learning there is a certain pleasure—but not what somebody else tells you to learn.
May I ask a question about your solitary life? You mention that you were used to it from Germany. Was this some kind of an enforced solitariness, or self-imposed, or is it part of your temperament?
It’s probably part of my temperament, but it was also part of the lack of ability to engage in easy conversation. Also, you know that foreigners were not very much liked in Germany. It’s not a country where you have immigration on a large scale. If every other person spoke with a terrible accent it would have been much resented.
But you had your Hungarian circle in Berlin. You were close to von Neumann and to Szilard.
Yes, and I saw them quite often. Johnny led a life very different from ours. He was sort of a bon vivant, and went to cabarets and all that. Szilard was also different from me. Very different.
You mentioned about coming here, the idea of coming in pairs and that one of the reasons was that there would be someone to communicate with. Did this also have something to do with the state of physics, even in this case the state of mathematics as well, in this country, where it was a question of intellectual communication as well as social communication?
In that case meant intellectual communication as far as modern physics was concerned. Robertson was here, but he was the only one who was (as far as I can remember) interested in quantum mechanics that first year. And his interest was largely a formal one. He was a very skillful manipulator with symbols, but I don’t think he was as deeply interested in quantum mechanics as he was, for instance, in relativity theory. So that Johnny and came, so to say, in a sense, as the pioneers who break new ground. I could quote you a Hungarian poem on this, May I? I will try to translate it into English: “Shall I dare to break in with the knowledge of the West”—in this case, of course, it’s East, but the poem says “with the knowledge of the West, to the country which is used to the life of the ancestors.” That’s a famous poem.
Meaning traditional life? In that sense?
Yes. Yes, it was traditional. But the most important theoretical physicist here in Princeton at that time was E. P. Adams, and his interest was in classical mechanics, somewhat in electrodynamics, but it was entirely in the macroscopic field. Now, since that time have learned that the classical field still has many interests. But it is different, of course, from the microscopic one with which quantum mechanics deals. Now, there was Eisenhart, who had a very sound knowledge of Riemannian geometry, and its applications to relativity theory were not foreign to him at all, but still that wasn’t his love. There was Veblen, who had sort of a dilettantish interest in physics, and there was Tracy Thomas, a student of Veblen, but his interest in modern microscopic physics was also quite dilettantish. You know, a mathematician is familiar with the theory of theoretical physics, but he’s not familiar with the fact that there is a level in boron-l0 at 1717 Key which is the partner of the ground level of carbon-l0, etc. You know, they don’t have facts. They are not strongly aware of what the basis of the theories is, and how it connects, and how the theory is verified. They know the theory as an abstraction, rather than as a link to experience, or rather to what we call experience.
Was there anyone here at the time, including John von Neumann, who could converse with you then and go from the mathematical to the physical and back again?
Very little, and perhaps I did miss this. At Berlin we had a colloquium, as I remember, every Wednesday afternoon. Schrodinger organized it. And after the colloquium, we always went to what they call a coffee house. I don’t know whether this institution is familiar to you? You sit at a table, order a cup of coffee…
That’s very European.
Yes. And then we talked, about physics, about everything in the world almost. And that missed. You see, there’s no coffee house, to begin with, at Princeton.
Now there are coffee houses but they play folk music.
There are very few coffee houses in the United States, in that sense.
Yes, very few. I don’t know any.
Did this atmosphere in the state of physics exist only in Princeton, or did it sort of characterize the state of physics in the United States?
Well, you see, Princeton was prominent in mathematics, and as for the mathematical sophistication, there were few places in the United States which could vie with it. But the physicists—the theoretical physicists for whom I had a great, a very, very high regard, were Gregory Breit, whom I am sure you know, and you may not even have to put down his name, and Van Vleck. Now, Van Vleck only knew from a distance, but had great admiration for him. Well, I knew about Condon in a vague fashion—Millikan was a myth-and Oppenheimer, of course. How can one forget Oppenheimer? Oppenheimer was so close to a European theoretical physicist that I didn’t even think of him as an American theoretical physicist. But he was in California, and it took four days to travel to California, and don’t know how many dollars. [Telephone interruption]
You were discussing contacts with other physicists in this country. You mentioned that California was rather far away. Did you travel to see Gregory Breit?
Yes, or he came down. I think we saw each other reasonably often.
At meetings or by individual arrangement?
Oh, he came down to visit or I went up to visit. We managed somehow. Not at meetings.
I see. On an informal basis, then.
Just to pursue this a little bit, what about other arrivals from Europe? Did they sort of check in with you when they came? Or did you make contact with them soon after?
ft’s difficult to remember. Of course, you are quite right, there was the famous quadruplet at the University of Michigan—LaPorte, Uhlenbeck, Goudsmit and Dennison. Dennison of course was as American as they come, and was a splendid person. I don’t think I even saw them. You know, a distance of a thousand miles at that time, and for a person coming from Europe, was unbelievably much. And at the very end of my sojourn here, I went out on a little trip, and I think I went out as far as Chicago, but that was a crazy idea and took me a week. I thought, “I may never get back to the United States, I’d better see.” I forgot another thing. I had some contacts with C. T. Zahn. Do you know his name?
No, I’m sorry. Where was he?
Charles T. Zahn. He was at Princeton, an experimental physicist, but unmarried, and we saw each other occasionally, and I think I even took a little trip with him in his automobile, and we were friends.
I see. What about the Michigan summer schools? You didn’t go?
No, frankly, I wanted to get back to Europe when the term was over. I had parents, a brother, a sister, cousins, and I felt at home there.
You did go back, didn’t you, when the semester was over, but you maintained a part time instructorship here at the same time?
Yes. Just before we left, we received an offer, both Dr. von Neumann and I, for a half-time visiting professorship. They expected us to spend half a year in Berlin and half a year in Princeton. And there was a good deal of wisdom in that. They realized that even though there were two of us, we were somewhat isolated here. Perhaps they also found out from Johnny, and probably I would have said the same thing, we were not ready to abandon Europe or our friends, our families, for good. Hence, they said, “Well, why don’t you spend half a year in Princeton for the coming five years, and half a year wherever you want, Europe, or Berlin? It will benefit both of us.”
It was a remarkable arrangement to have been proposed by Princeton.
Yes, it was, and I admired them for their wisdom no end, and still admire them.
It was the reputation that had developed of the abilities of yourself and Professor von Neumann. The recognition was already established.
I don’t know. Perhaps so. I don’t want to argue that, because I sure don’t want to argue that I was not a recognized physicist.
Well, how did this work out then? Did it work out as well as you thought it was going to?
Yes, except after about two and a half years, it became pretty clear that our days in Germany were numbered. As soon as this became clear, Johnny at once decided to stay in Princeton for good. In a sense he fell in love with America on the first day. He thought: these are sane people who don’t talk in those traditional terms which are meaningless. To a certain extent the materialism of the United States, which was greater than that of Europe, appealed to him. I am much more traditional-minded, much more dependent on emotions, love of a way of life, of—what I can’t express—than Johnny was.
Does it have to do with relationships with people and things of that sort?
To a large extent, yes.
in this period, as you returned in successive years to Germany, were you aware of the changes taking place, progressively developing, in terms of the political situation? Could you see definitely a difference between one year and the next?
Oh, very well. You see, I followed the events. I was even subscribing to a German newspaper which was sent to me to Princeton. There was no question in the mind of any person that the days of foreigners, particularly with Jewish ancestry, were numbered. Neither was there any question in the minds of people who could admit this at that time, and had another choice, that the regime would either precipitate a blood bath in Europe, or would subjugate the other nations without a war.
Was Szilard one who was particularly perceptive and aware of this?
It was so obvious that you didn’t have to be perceptive. I don’t think he saw it any more clearly or any earlier than the rest of us did. It’s like, “Well, it will be colder in December.” Yes, it will be. We know it will.
You know it. At this time was there any active arrangement? I know Szilard for one was involved in England in helping people who had no apparent way out, to arrange a way out.
Well, that was considerably later. I think that started—I could look up my notes, but without those, I would estimate that it started in ‘32.
I see. ‘33 was your last year in Europe, then?
I have the most miserable memory.
I have a note that you held the position in Berlin until ‘33, from ‘30 to ‘33.
That must be correct, then.
Then when you did come back to Princeton, you were still on a part time basis, is that right?
When you severed the European tie, what did you do with your other half year? You just stayed in Princeton the full year, is that it?
I think I stayed the full year, but I just don’t remember it. I did spend half a year, at Polanyi’s invitation in Manchester.
I know there was one year in Wisconsin. That was a little later?
That was later.
I think more important is the question that will get us, since we’re going to have to break soon, as we indicated, to the major topic that we’ll pick up later—that is, you were in a position, hopping back and forth across the Atlantic, to observe differences in the development of the new field of nuclear physics. Can you characterize these differences?
I think I became interested in nuclear physics soon after it started. That was, however, a good deal later. Breit spent a year in Princeton, and that is when we started on nuclear physics.
Nuclear physics? You started already in 1932? Your first paper is in 1932.
You are much better than I on this. [Looking through papers]
You wrote on the neutron immediately, on the theory of the neutron, “Beitrage zur Theorie des Neutrons.”
Yes, but that was—that was not serious.
And then in ‘33 you were writing, “Uber die Streung von Neutronen und Protonen?”
That was serious. But this I wrote, I thought, in Wisconsin.
Is that so? You know, I thought that was written when I spent half a year in Wisconsin.
What paper number is that on the bibliography?
This is 20 and 21, which I wrote in Princeton, very soon after I came.
But that was with Weisskopf?
Yes. He was not here. We discussed that with each other before I left Berlin and when I came here, I wrote it down.
I see. And yet it was published in Europe. You felt it was more appropriate to publish it there?
I couldn’t have written it in English. That is one reason. There is something missing here, I published a paper with Gregory Breit.
And yet in ‘32 you were writing papers in English. Your paper in Physical Review, is here, in ‘32, on the “Quantum Corrections for Thermodynamic Equilibrium.”
The paper with Weisskopf was written in ‘30 and in the following two years I learned a good deal of English. You are absolutely right, the Quantum Correction paper was my first paper in English. I wrote it in the United States, on one of the occasions I was here, and I remember it very well. It was sort of a break away from group theory.
What number paper was that?
This is a very important paper too, “On the Quantum Corrections for Thermodynamic Equilibrium.”
Looking at this reminds me of things. This in ‘33 wrote in Wisconsin. Then I went back to Hungary, and I knew that Germany doesn’t exist as far as I was concerned. But this paper which I wrote in Hungary on a vacation was published in Germany nevertheless.
Were you in contact with Kirkwood at this time?
No. Kirkwood visited me, not much later, and he wrote a paper—
Yes ... soon after …
You surprise me with the details of your knowledge of papers.
We have copies of your bibliography. That helps.
It also happens to be my field of work, you see, the Wigner Kirkwood Expansion.
Oh. You must give me a reprint of your paper.
I shall indeed.
But Dr. Breit and I published a paper about lithium-8, And that was here in Princeton.
Maybe, since we’re going to have to break in a few minutes, we can do some homework in between. But let me ask another question, for a moment, that is, how did you first get interested in nuclear physics? How did you first become aware of nuclear physics? You mentioned before that you got into the field very soon after it started. In your opinion, when did it start, and how did you learn of it?
For me it started with Heisenberg’s paper. Heisenberg wrote a paper (I don’t know the date of it, but again we could look it up) in which he pointed out that now that the neutron has been discovered, and of course that was an experimental discovery, that now that the neutron has been discovered, one can think of starting a theory of the nucleus. This impressed me very much. I was working at that time on solid state. I thought, well, why don’t you look at it—Heisenberg’s paper was very, very general. He wanted to have a bird’s eye view of the totality of the elements, I asked myself: Why don’t you look at what you can do with the deuteron and helium? Hydrogen 3 was not known at that time. What kind of forces would explain them? I think I saw it qualitatively, that the calculation will unquestionably yield a much higher binding energy for helium than for the deuteron. And so I thought, well, that may be a nice thing.
Were you aware, before the discovery of the neutron, of experimental work that had been going on in the field? Were you aware of the attempt to discover a neutron?
Not of that, but the whole area of experimental nuclear physics was not foreign to me—the papers of Rutherford and so on. Another thing which contributed much was the paper of Gamow, and of Condon and Gurney.
‘28, I think.
That also attracted a great deal of attention. Perhaps I will tell you something which will be useful to you, and which I don’t think I have articulated before. This is, that the great experience for me in the United States, and the condition which made work here very attractive and very pleasant was the possibility to work with graduate students. I don’t know if you realize that Seitz was my first graduate student, Frederick Seitz. And collaborating with him was a really great experience. At that time I was not in very good standing with the university here [Princeton].
In what sense?
Well, they were not at all sure that they wanted to continue my appointment.
Why? What was the basis of their doubts? Did they not appreciate or understand your work?
Well, you may put it either way. They did not appreciate it. At that time they were somewhat better equipped with theoretical physicists, because Condon came back. And Condon and Robertson formed a reasonably solid nucleus for theoretical physics in Princeton. And they were not sure at all that they wanted to have me, in addition to that. It was a very difficult time for me because I had nowhere else to go.
I see. Does that account for the year at Wisconsin?
Then it was just a question of considering a new position. You were considering it, as well as other institutions considering you.
Breit recommended me, recommended to Wisconsin to invite me for the year, for half of the year that he spent at Princeton. He said I would be a good substitute, and also told them, I am absolutely sure, though I never discussed it with him in these words, he told them, “Why don’t you look him over? He may be a good addition to the department.” They did offer a job to me, and, as you know, I accepted it.
You mentioned, about graduate students, that one of the most important things to you was the opportunity to work with graduate students. Was this because the relationship gave you sort of an intellectual circle, even though a small one?
Not only that. It was very pleasant to work with somebody who is deeply interested in the subject, who is interested in exactly the same thing in which you are interested at that time: namely, the structure of sodium and the binding energy of sodium. It was a close collaboration which I never had before. I’d published papers with Johnny, but the collaboration was not so close. For instance, the first paper which we published jointly was written when I was in Gottingen and he was in Berlin.
Not quite a close collaboration.
Not the daily intellectual struggle there?
No. And that meant a great deal to me. And I was enormously lucky in the people whom I had, namely, Fred Seitz was the first one, John Bardeen the second one, Conyers Herring the third one—it was simply fantastic.
This is the solid state galaxy.
Yes, it is. Really fantastic.
This is the time we agreed to break, I think. [Interruption in tape) We’re resuming now after an interval of about an hour and a half. Let us just pick up then where we left off, if you like.
I only remember what I said last, namely, that the collaboration with many of the graduate students caused me intense satisfaction, because one could work with them closely and on one subject, and not jump into several different questions at the same time. This collaboration I had not known before. I was also enormously lucky in the graduate students who happened to collaborate with me, as I mentioned.
Did Bardeen work with you?
Yes, Bardeen was the second graduate student working with me. Seitz, as mentioned, was the first. Herring was the third. You just can’t have it better.
Was there any overlap?
Very little. That was also a great advantage, that you could work with one, and that you knew him and could really collaborate. Now, I often have three or four graduate students. Nobody’s bright enough to keep abreast of three or four students, if they are any good. So it is a detached collaboration, and it is not the same thing. One shouldn’t have nostalgia for irretrievable times, but I thought I’d mention that, because that was one of the things which was very, very happy during the first few years in America. It has subsided, unfortunately. Already when I returned from Wisconsin, oh, there were fifteen people and things to attend to. The war came pretty soon, and as I told you, I knew what was coming. I prepared for it. I suggested that we work on uranium fission. And all this takes time.
Did you also have a human contact with these people?
Not only intellectual but human contact?
With Seitz, very much. Seitz was—well, this is again something I probably shouldn’t say—Seitz was at that time, as a graduate student, more mature than I was, in many ways, and I admired him very much.
Was there a large difference in age?
Not much. It was close. But in some ways, he was more settled, more content with what he could achieve and what he could not achieve. Of course his future was much more certain than my future, because, as I mentioned, my future was quite uncertain at that time. And that of course has an unsettling effect on everyone.
In these relations were you able to overcome that feeling of loneliness that you had known before?
To some degree, yes. But if we look honestly at this, there are relations which a man cannot supply. You want to have some contact with women also, and I was missing that very deeply in the United States, because I did not know any women.
When did you get married? What year?
I was married in Wisconsin in ‘36. My first wife died. Here is her picture. She died after less than a year of marriage.
Your first wife was American?
This was at Wisconsin.
In Wisconsin the situation was entirely different. From the first day I felt at home.
Why is that? What was the difference?
It’s very difficult to tell. A small cause—a small cause was that it is a more agricultural community, and I knew agriculture. Wheat fields mean something to me, and meant something to Wisconsin people also. But this was a small cause. Another cause was that in Princeton everybody felt terribly important, and terribly important people are not as natural as not-terribly-important people. Many people think now I am important. I hate to think of it that way, because I don’t think it does something good for your soul, to be important. I think that if we are just like the next fellow, and that we don’t have to maintain some semblance of knowing more than we do, it’s better for the soul.
Have you always felt this way?
Yes, I always felt that I wanted to be a soldier, not a general.
In Wisconsin, Breit was there at the time? No, you were his replacement.
For half a year; but then for a year we were together there, in fact, for two years.
I see. So you went there in—
—I think in ’35.
Well, the record I have: ‘37-’38 you’re at Wisconsin, but from what you say it sounds as if there was another semester in addition.
Yes, and before that I was there on a leave of absence for a semester. I can give you this date because I remember what happened. In 1936 I went to Wisconsin.
And then you stayed another year.
Then I came back for a year, because I still had that arrangement. I resigned and left for Wisconsin.
I see. Did you have the understanding that you would stay at Wisconsin?
Yes, I had a permanent position there.
Now, Breit was there. Van Vleck was no longer there? He’d been gone a long time.
How did the physics community itself and your colleagues there differ in their approach to physics from Princeton? It wasn’t only the wheat fields that accounted for the difference?
No, it wasn’t only that. The physicists had a community spirit. Do you know Ray Herb?
He is a wonderful person. He kept the group together. He was enormously unselfish, enormously enthusiastic, could work day and night, and infused a spirit of community into his people. And he took me into this group, somehow, which was really wonderful.
That accounts for a lot of things.
You made a remark about Princeton, as consisting of some very important or self-important people. Has the situation changed over the years?
I don’t know. I can’t tell. Ask a younger man, because I can’t tell any more. I am always afraid that I am now just a silly cipher, but I don’t know, I have no way of telling. You don’t understand that, because you are young.
No—that is a fair question for you to put to us, to ask someone else, differently.
It is very difficult. I try not to be an important person with a beard; but whether I don’t give that impression just the same, I can’t tell.
We’ll ask a young man.
Ask a younger man.
On this situation, just before we broke we started to ask about your interest in nuclear physics and how it began. You talked of the Heisenberg paper, commenting on the discovery of the neutron, and how this affected you. I think it’s appropriate that we pick up that thread.
The first serious paper that I wrote on this was a paper with Breit which you have probably heard of, since you know so much. People now call it the Breit-Wigner formula.
Yes. I would like to come to that in a moment, but before we do, could I ask one or two questions? I would like to ask about the fact that you were doing very important work in the field of atomic physics and molecular physics and quantum mechanics, before you came to work in nuclear physics. You’ve already partially answered the question, but was it the bad state of affairs in nuclear physics that excited you to work in this field? Because nuclear physics was hardly in a good state of affairs.
It hardly was in a state.
Right. I don’t think people ever—at least, I don’t—work that way. I read the papers on slow-neutron capture. And a picture emerges as to what it probably is. Then put that picture—I say “I” because I relate my experience, but this was of course done in collaboration with Breit—on paper. Acquisition of the picture comes automatically; you don’t work on that. You work on writing it down, having it typed, putting in the formulae, mechanically. But the essential part is not what happens when you are sitting at the desk. The essential part is that you acquire the picture. You read—I don’t know—Bjorge and Westcott’s paper—and you realize what probably has happened. Then you read—well, I don’t want to enumerate the experimental work which went into it, but reading the experimental papers and thinking about them quite automatically—those are the relevant methods by which you acquire a picture. And then you put that on paper.
But you were part of the discussions of these things that were going on around 1932, in nuclear physics. In 1932 you were partly in Princeton and partly in Berlin. Were you a part of this group that was discussing these questions?
I don’t think we discussed nuclear physics in Berlin, I remember a statement of Schrodinger which—now, perhaps, again, I should not remember it. He said, “After all, all this talk about the neutron is very boring.”
This was already when ‘32, ‘33?
More than that, ‘34 probably, I don’t know. I think that the paper with Dr. Breit was published in ‘35, but I don’t really recall.
Just before 1935, in fact in 1934, Fermi’s theory of beta decay had appeared. Were you involved in these discussions at all?
No. I, of course, read Fermi’s paper. In those days you could read every important paper, which you can’t do now with the proliferation of the literature.
Yes. All papers were important at that time, or many of them.
A much larger fraction. I thought about beta decay, and I tried to make a theory of beta decay, but it certainly didn’t work. Some statements about it are even contained in one of these papers. When was Fermi’s paper?
Then something is wrong, because I remember—my recollection is that in 1937, I thought of a way to explain beta decay, which did not work.
Had you had some contact with Fermi before then?
No, I never set eyes on Fermi—
—until he came to this country?
Until he came to this country.
Soon after the work on beta interaction by Fermi, the ideas in nuclear physics began to develop very fast. There was Bohr’s liquid drop model, and the idea of strong interactions, and the name “compound nucleus” was very soon proposed. Were you involved in these discussions?
The paper by Breit and me appeared I think on the same day as Bohr’s paper on the compound nucleus, and it also used the picture of the compound nucleus. So we did not have to learn that from Bohr. But I had never seen Bohr up to that day and did not know about it.
The important thing is that these things were going simultaneously in different parts of the world, and you were at that time in Princeton?
The Breit paper we thought out together in Princeton, and I think I wrote it out in Wisconsin; then sent it back to Breit, and he returned it. It doesn’t matter who writes the first draft of a paper.
Will you tell us of the circumstances attending that work? The recognition of the problem, and how you went about doing it? This was a very successful effort. It was an original calculation, the kind of which—well, not quite the same, but the compound nucleus and the calculations based upon it were done in Copenhagen, and this was done here, so I’m interested in finding out about the recognition of this problem and the attack on it.
You see, I read the papers, though I can’t enumerate them, but Westcott and Bjerge, Fermi, several others come to mind—it became clear (and I am sure this happened to Breit simultaneously—I can’t speak for him to the same extent that can speak about myself)—that this is very similar to optical absorption, where you also have reasonably sharp, in fact, much sharper lines; and that it is a process very similar to the one which first described in an article jointly with Polanyi, in Antiquity. We thought at that time—and that was my thought—that the chemical reactions go by this mechanism. This was in ‘25, I think. But I realized, when I read the experimental papers on slow neutron capture that the picture which was developed for chemical reactions, and which did not work very well for those, was probably appropriate for the neutron capture process. We stopped for awhile working on neutron capture because Bethe published a paper in which he explained it in a very different way. Well, we thought we’d give this theory a chance to prove itself or not to prove itself.
Is this the paper on neutron resonance?
No, Bethe’s paper was before our paper. If I find our paper could give the reference to it. He said that the cross section is always proportional to 1/v.
Excuse me. It’s a shorthand way of talking. He said it’s always l/v, and he also said that scattering should be at least as large as capture. Well, with that interesting idea, and interesting picture, we gave it a chance to see how it worked. As the experimental results came out, and they came out thick and fast, we realized increasingly that the picture which we had, that Dr. Breit and I had, is much more appropriate. So we decided to publish it.
During this period from, say, 1932 to 1937, what were your other major interests? You had become now quite excited about nuclear physics, and the compound nucleus model, shall I say. It seems to work very well?
Well, for a long time it was not clear how well it worked because the neutron spectroscopy was very rudimentary. The experimental data were more qualitative than quantitative. The picture looked right, but whether it was right was not clear. There was the question that in cadmium, the slow neutron part was not 1/v and we did not understand why, but it turned out subsequently that the resonance is too close to the thermal region, so that it overlapped with the 1/v region. But this wasn’t very clear at first.
Since we want to be talking about the different stages of development in nuclear structure as represented by the models, may I ask about the shell model. Are you familiar with the first attempts to explain the nucleus in terms of the shell model? A proposal of Harkins, I think, came in 1930.
Yes. At that time, though, we were not familiar with it. Today somebody called me up from the Institute. He had written a very thick paper on something in which I also was once interested and, well, somehow he learned about it. He told me about his work and its connection with my earlier paper. He said, “I thought that you also worked on it, and perhaps I should have studied the literature first.” And I told him, “No, if one studies all the literature first, one never does any work.” And this was also true for us. I did not study Harkins’ paper. Whether it was a mistake or not is irrelevant. I did not. And neither did Feenberg, with whom I wrote a paper on the structure of nuclei between helium and oxygen, which used the shell model.
Yes. That was in 1937.
But I did study a paper by Bartlett, which said—now I forget exactly what it said—but I think it made the point that the p-shell ends at oxygen-16.
So you were discussing about, in some way, the concept of the shell model.
Yes. But that shell model was very different from the present…
We also felt that the shell model—and Bohr felt that too, articulating it beautifully—that the shell model cannot be accurate because we have short range forces, and with short range forces no average potential can be set up. We still do not understand why the shell model works so well. It is apparent that the shell model is not correct verbatim. In other words, the wave function does not look like the shell model wave function. But so many of the consequences of the shell model wave function are valid in spite of that. You asked that?
Excuse me. I say things that you may already be familiar with.
I think it’s necessary to explore it, and to explain it as it comes to you logically, without thinking of us as your audience, just thinking of it as it has developed.
Because the potential of this is greater and beyond the immediate moment.
In 1936 and 1937, Bethe’s articles in Reviews of Modern Physics had appeared. Would you recall something of the excitement or interest they created?
Bethe’s articles in Reviews of Modern Physics were just marvelous. Just unbelievably good. At the same time, Feenberg, who worked with me at that time and was not much younger than I, was mad at it, because there were crazy mistakes in it. And he had a copy marked, unhappily. But I was just ravished by it.
Had you felt, prior to that time, a need for such a synthesis of the state of the art and the state of knowledge in the field?
Probably I knew 95 percent of what was in that paper. But to have it all together, to have it printed, was very, very useful, nevertheless. It is difficult to explain, but if one knows that 3, 5 and 7 are prime numbers, it means more than to know that 3 is a prime number, 5 is a prime number, and 7 is a prime number.
For the first time it gave a composite knowledge of what was known about nuclear physics.
Yes. And even if you knew every single thing—which I did not, probably—but even if you did know, the fact that it was put together was useful. Excuse me.
No, excuse me. I was just going to say about the period leading up to this, that you had been in contact I assume with Bethe when he came to this country, and were you in contact with him during the period of his writing this three-part paper?
You know, my correspondence at that time was very meager. First of all, I wrote out every letter in longhand, and hence wrote very few letters.
Did you correspond with Bethe at that time?
Perhaps one or two letters.
Had you known him before in Europe?
I think I must have. Oh, yes, of course—I met him in Manchester.
I see. Before he came to this country.
Was this on one of your European return trips?
Yes. Now I can answer one of the questions you asked before. One of the half years spent in Manchester, when I did not know what to do.
I see. This is probably 1933, as he was there then.
I wrote an article in Manchester.
The bibliography comes in very handy.
The bibliography and biography are inextricably combined.
It does seem so. But I was in Manchester. As I said, my future was uncertain, and Polanyi wanted to help me out and invited me for half a year to Manchester. So of the unaccounted half years, perhaps two or three, this accounts for one, Wisconsin accounts for a second. So I think I have accounted for my life.
So you knew Bethe then, and then when he came here—was it ‘35?
Yes. He was here. You must have visited, instead of corresponding. Was he then part of your widening circle at the time? Or you part of his?
Well, he was at Cornell. I saw him at Physical Society meetings.
You had no intimate contact with him at that time? I would like to ask about 1935 still because meson theory was formulated around 1935. Would you recall in what practical manner did it affect nuclear theory?
This was one of my great errors. I always thought that that was a high-powered theory which wouldn’t do much good for nuclear physics. And thought that if somebody wants to write about meson theory, it’s perfectly fine, but I don’t see that it adds much to the picture. But you understand, that was my thought.
Were you aware of Yukawa’s work shortly after it came out?
In a vague fashion, yes; he said it is like this: “If there is a pulse and I can attribute it to mesons, why not?” I mean, this was my impression. It’s a good idea.
Was this question discussed here in Princeton about nuclear forces and the origin of nuclear forces?
Was this because in general there weren’t many people here discussing nuclear physics?
That was part of it. Breit and I were the ones who were interested in it. I don’t think Condon was, much. Breit shared this view about meson theory, but we never discussed it. I mean, I never told Breit my views about “high power” theory—and I don’t know that I ever formulated it before in this way. But that was my general impression: “Oh, that’s very good, very good.”
The question of nuclear forces had already come to exist in 1932, when Heisenberg proposed that the nucleus consisted of neutrons and protons. Did you think about what this new force might be?
You see, now we believe that there are four or five types of forces, but at that time, things were quite different. The existence of four or five types of forces is a recent discovery; this discovery had not been made at that time. People thought that there were 55 different kinds of forces which someday would be explored and so on. However, we did not feel that this question was ripe for discussion,
You were familiar with exchange forces in your work in chemistry.
Yes. But, and this is very odd: I discussed that once with Teller, and I told Teller that I think at short distances the nucleons repel each other just as atoms do. But of course, again, that was a very strange thing to say, and to think that that was the origin of saturation. You can say such things, and you have no support for it, no support against it, it is a rather useless thing to say and I never did do anything about it. It turns out, and this is the strangest thing, that it is true. But I assure you I never claimed any credit for it. I don’t know, Teller probably remembers that I told him this.
Was this back in Berlin?
As a matter of fact, this conversation took place in Budapest.
Oh, that’s further back in time.
It was on a visit of us both to Budapest.
I see. Do you remember when, in the early 30’s?
I think it was ‘35. I felt that this idea of exchange forces may be very artificial, but we explored it, and I came to believe in it more and more.
This wasn’t just a casual remark, but something that stayed in his mind?
Yes, it did. But you see, I also wrote a number of papers pointing out that it is very difficult to explain the total binding energy, if the ratio of Majorana to ordinary forces is 3 to 1, which is the necessary ratio for saturation. I wrote several articles on this, saying I can’t understand it. I did not say that one way out of the difficulty would be to explain saturation by repulsion at short distances because there were fifteen other ways and there’s no point in pointing to one explanation if there are many others.
You referred to your guess about short-range, what would happen at short-range between nucleons. May I ask, when was the short-range character of nuclear forces recognized, and when did charge symmetry and charge independence come to be recognized?
Let me answer the questions one by one. The short-range character of the nuclear forces I think was obvious to everybody. It is often said that I recognized it. I think that’s nonsense. Everybody knew that. But the very short-range repulsion was recognized much later. You know, now the nuclear forces are considered to be essentially attractive outside of a radius of 0.4 fermis, but repulsive inside. This is what I meant. This very short-range repulsion is what I thought, and mentioned to Teller, that this is probably the explanation. It was put forward and given good arguments for by Jastrow. The other question was…
Charge symmetry and charge independence.
The charge independence was the only one which I ever felt to be important, and that was due to the analysis by Breit and Present, I believe, of the experiments of Milton G. White and some other people in California. It was an entirely experimental discovery, and it so happens that a short time ago I glanced at one of my papers, and then I said, “It is extremely unlikely that the forces between protons and protons or neutrons and neutrons, are comparable with the forces between protons and neutrons, because the preferred nature of the Z equals N speaks against it.” This was entirely incorrect. But the way we learned about it was through the experimental work of Dr. M. G. White and some other people, and the analysis of the experiments was due, I think, to Breit and Present. I hope I am not misquoting Breit. Then a paper was also written by Breit and Condon. I remember that very well. And then Feenberg and I realized what this really means, that there is charge independence. I think that was in collaboration with Feenberg.
That was in 1937. That is when you introduced the symmetric Hamiltonian and its application to the theory of nuclei.
I would like to ask about your collaboration with Feenberg, and your use if isotopic spin. The concept of isotopic spin was also, I believe, introduced by Cassen and Condon, was it?
Cassen and Condon. You’re right, said Breit and Condon. Cassen and Condon did not introduce the concept of isotopic spin. They introduced—but should look that up—I think what they introduced was the isotopic spin variable, but not the quantum number. As a matter of fact Heisenberg introduced the variable also, so that I don’t really know now what Cassen and Condon did.
We’ll look that up too.
Heisenberg was among the first people to talk about the charge independence of the—
No, excuse me, it was Breit and Present. I may be wrong.
But who used the phrase, “isotopic spin,” the first time?
I don’t know, but my impression is that it was first used in the paper of Feenberg and myself. But this is isotopic spin as a quantum number similar to total spin. Not introducing formally a variable to distinguish proton and neutron. That was done in the very first paper of Heisenberg. He wrote the one which I mentioned.
1932, is it?
Well, no, at least it was after the neutron.
Did people realize already at this time (I’m talking about 1937 or so) that different models of the atomic nucleus were necessary to explain the properties for different mass ranges of nuclei, for light, medium light and heavy nuclei?
Certainly not at this time. In fact, an erroneous belief of mine was that in the heavier nuclei) when the ratio of proton to neutron number changes, there are electrons and neutrinos in the nucleus which are emitted in the beta decay. (This is the false theory of beta decay which I mentioned.) And I thought that the first two are added at chlorine, the first time it happens that the neutron number exceeds the proton number, chlorine-37. But this was an erroneous idea. And the picture—at least never heard of it.
This raises a question. Now you say it is an erroneous idea. At the time when the paper was published was there any comment on it, any criticism of it?
That was a hidden remark in one of these papers, perhaps in the paper on the mass defect of helium or some paper of that sort—a rather early paper. I can find it, perhaps.
“On the Mass Defect of Helium.” l933.
I thought it would be later, because you mentioned 1937.
You are right, here is the paper. In that paper I said that, and the conversation with Teller was very soon after that. So it may have been in ’34. It doesn’t matter.
Well, it’s just placing it in context. But what I was getting at here was more of a point about criticism and discussion through the literature. It’s apparent from what you say that you had conversations and a minimum of correspondence with colleagues, but did the literature itself, the publication of results, serve to further the conversation very much, in terms of feedback from it, criticism? I don’t know. Was this the case? Did you get reaction to your papers when they were published? Or once it was in the literature, did people just assume that that was…
Everybody read everybody else’s papers.
Did they let you know whether they agreed with it or not? That’s the point that I’m trying to get at.
People on the whole agreed. I remember that I discussed Bethe’s paper with him—you know the one which eventually turned out to be erroneous. Now you understand that when I say “erroneous” it doesn’t mean that he made a mistake, but that the picture which he projected, or which he thought of, did not prove to be the useful picture. It is not a derogatory statement, I was questioning it, as you realize, because my ideas were different. I discussed it with him. I did not say, “I don’t believe your paper.” I asked him, “Now, this is a consequence, and this is a consequence; well, with your theory you never will have a cross section higher than about 50,000 or so barns,” and he said, “No, I won’t. Well, we will see how experiments will turn out.” And I did not know that his picture was inadequate.
Right. What about outsiders? Because you normally would be in touch with Bethe, whether it was through a published paper or through private correspondence or personal conversation. But what about others? Presumably there were other people reading the journals in that period, and there’s a larger physics community. I’m probing here to find out if the literature was effective in reaching a larger audience.
I think it was. I remember Peierls and Bethe wrote articles, and they still wrote in Manchester, and they knew the paper which I had written on scattering of neutrons by protons—in 1933. They must have known it because they criticized it, in my opinion incorrectly.
There was also at that time a question of time delay. Preprints as such didn’t exist. People used to write letters. One notices from various correspondences that letters served the purpose of reprints. And what was happening in Europe, if my impression is correct, is that at the more important centers—say Copenhagen, Leyden and Zurich—one morning Pauli would be in Copenhagen, the next morning he would be in Leyden, the following day in Zurich, and the whole thing would be passed around like gossip.
I heard a certain amount of gossip, but I never was at the center of things, in this sense.
Was it happening in the United States to some extent?
Not to the same extent. One of the first impressions which I got in the United States is that people here worship solid work much more than they worship it in Europe. Therefore ideas counted for less.
Solid work meaning a rigorous mathematical demonstration and experimental…
A solid calculation, a long calculation, with three decimals put in, and, you know, solid work.
Not the speculative spirit.
Did you have any feeling of this as being a continuing tradition?
To some degree, yes. I may have been a little bit mistaken, because my principal contact was Breit. And Breit is a little addicted to this.
What about the experimental centers here? You mentioned that in the mid-30s, by this time, experimental results began to be important, but earlier they were more of a qualitative nature. When was it that they first began to be a serious factor? Secondly, where did you look for these results? At what centers of research?
I remember that the paper on the mass defect of helium was much influenced by the measurements at Berkeley, which incidentally were—not wrong, inaccurate. Inaccurate. And also, the mirror nuclei that you mentioned, the isotopic-spin multiplets were discovered to a large extent at Caltech. Is that correct?
Yes, think so.
And that, of course, had a great influence upon us.
May I ask a question still about the l930s? You had done extensive work in atomic physics, and you were one of the first people who introduced group-theoretical methods. You mentioned earlier that this was not very popular because people did not want to learn group theory. When you went into nuclear physics, did you consider employing group-theoretical methods right away?
You see, I thought that it is a useful tool; and that if there is a need for it, fine. If there is no need for it, not. Now when it turned out that protons and neutrons are similar, and I thought for a long time that the spin-dependent forces were weak, then of course it was a wonderful opportunity to introduce a type of group-theoretical results which I considered much before I really understood atomic spectroscopy correctly. And I used that. But it’s a tool, not a purpose.
But it was a very powerful tool. The important problems of symmetry in atoms and in solid state had been explored using group theory. Did one worry about the questions of symmetry in nuclear physics?
I did. But not much.
When was the parity quantum number first introduced?
That is very old. That I can tell you. The quantum number experimentally, as you know, was introduced by LaPorte and Russell.
I didn’t know that.
Well, I’d better—excuse me—tell you that, because that was an order of magnitude more difficult—no, not one, five orders of magnitude more difficult than to recognize what it is theoretically, read in particular LePorte’s paper on the iron spectrum with intense pleasure. Then I started working on the symmetry of atomic wave functions. It was pretty obvious that, in addition to rotation, there is also reflection and I thought it was pretty obvious at that time that it’s the LaPorte quantum number. That was done in 1927.
When was it recognized that the parity was conserved?
That was obvious.
Oh, yes wrote a paper about it—as a matter of fact, also in 1927.
Is it about the conservation of parity?
What number is that on your bibliography? That way we don’t have to give it the exact title here, but just give it a quick reference.
Number 10 [in bibliography]: “The conservation laws in quantum mechanics.”
So far as the acceptance of this idea was concerned, was it a question of education or time?
There was no question that it was correct, because it’s a mathematical deduction of the Schrodinger equation. That the mathematical deduction was correct, was pretty obvious. Now, people didn’t like it. Of course, you tell people that, depending on what your political views are, we should protect the South Vietnamese people, or that we should not oppose the North Vietnamese people—people don’t want you to say the one which they don’t want to hear. But the fact that it is conserved was obvious. It was a mathematical deduction so there’s no question on this. Nobody could contradict it. They may not like to hear it.
It was one of the first questions of symmetry that was recognized.
Well, really not; because I thought that the rotational symmetry was so powerful, and ever since I have found that it is really beautiful how the spatial symmetry, which plays no role in pre-quantum mechanics, now suddenly explains so nicely the transitional properties. I thought that that was a really remarkable accomplishment. Let me quote Jordan.
Pascual Jordan. Jordan said to me once that it is very good that you worked out the symmetry of group-theoretical considerations because, had they not agreed with experiment, this would have been the last occasion to introduce changes into quantum mechanics. Because, you see, suppose the helium ground state did not come out right, what would you have done? It would not have given a hint what to do. Right? Because suppose—I don’t know about the energies, think 24.2 volts—if there had been a discrepancy of a volt, let us say, what would you have done? It just would have stood there. Something would have been wrong, but with no indication how to change it. But if something with the structure of the spectra—with the group theory had been wrong, you would have had an indication how to change it.
Yes. This leads me to ask a question about the fact that at what state of the development of wave mechanics were you able to discern that it was not just a theory of atomic spectra, but it could completely describe all atomic phenomena?
You know, one had the impression from the paper of Born, Heisenberg and Jordan…
From their very first paper?
Yes, I had that. Now, it did not give the impression that it does more than give a theory or a method to calculate the stationary states—that it is more than that became evident to me with the publication of Heisenberg’s paper: “Uber den anschaulichen Inhalt der quantumtheoretischen Kinematik und Mechanik.” You know, the uncertainty principle paper. And I remember having read that, and even though I realized that in detail it did not hold water, I felt: “This is it. This is it.”
You refer to your excitement about the paper by Heisenberg, Born and Jordan, in the Kuhn interview, that it meant very much to you. You discussed it with your friends in Berlin. So I assume that it was already at this point that you recognized that wave mechanics was the theory that would describe these phenomena.
This was not wave mechanics. It was quantum mechanics. But you see, the paper of Born, Heisenberg and Jordan was a paper which only gave stationary states and transition probabilities. The fact that more can be obtained was valid when Schrodinger published his paper. Because Schrodinger’s paper went further than the Born, Heisenberg, Jordan paper went, by what I used to call the Second Schrodinger Equation, namely, the time dependent Schrodinger equation. And that was a greater step. So was the observation of Ehrenfest, which nowadays seems trivial that the wave packet moved according to classical mechanics. He compared the motion of wave packets according to classical mechanics and quantum mechanics and found that their centers of mass move in the same way.
That’s right. But it really solves things. That was conveyed to me. I should have understood that much before, but it was conveyed to me by Heisenberg’s paper, the uncertainty principle paper.
Just ten years later when the questions of nuclear forces were being discussed, was there any question in anybody’s mind whether quantum mechanics would be the correct theory to describe these interactions?
Yes. Bohr published a paper saying that he thinks not; that it is not the right way, that quantum mechanics did not give a proper description of nuclei. This was not said in so many words, but if you read the article, you gain this impression. I did not know what the situation was, and thought that I would try out what one obtains from quantum mechanics. Not to play with it because somebody had suspicions which might be well-founded that quantum mechanics was not adequate, didn’t appeal to me. I was not convinced that Bohr was wrong. Not at all. But thought, well, he may be right, he may be wrong, I’ll try it out. Does that answer your question?
Yes. The important thing is that a certain amount of doubt had already been introduced, almost 30 years ago, that quantum mechanics may well not be the theory to discuss the forces between nucleons.
Well, don’t know. I thought people believed it to be the theory to discuss the forces between nucleons.
About the nucleons I should like to ask a question: Was it always apparent that the nucleons inside the nucleus behaved like the free nucleons?
No, Bohr said no, and he said—well, the way people read Bohr… Bohr is not so easy to interpret. He speaks so that the statements are not clear-cut. He does not say “yes” or “no” or “it is 50-50.” He says, “The question of the validity of quantum mechanics, for the close contact between nuclei went,”—well, he used a long sentence, and at the end he says, “all has to be subject to careful scrutiny,” or something like that. I don’t want to criticize him.
This was his style…
... But this was his style, and it was a beautiful style.
But from this, how does one conclude that Bohr is saying that the nucleons inside the nucleus are going to behave in the same way as when they are free?
He said the opposite, rather.
That they do not?
Yes. He expressed doubt. He expressed doubt.
But until now there is no question of being able to say that they don’t or that they do?
People are reasonably successful in explaining nuclear structure by quantum mechanics. Isn’t that right?
Yes. What I’m saying is that in macroscopic physics you find that objects, when they are put together, have the same properties as when they are taken singly. Now, if I think of the nucleons packed in the nucleus, and when they are free outside the nucleus, whether they have the same electro-magnetic and nuclear properties and so on?
This he questioned completely. You see, Bohr spoke only in great generalities, and not in clear cut definite sentences, but he expressed doubts in his very intuitive and very charming way. He did not say, to my recollection, anything concrete. Some concrete statements on this were made, in particular by Sachs, who investigated the nuclear magnetic moments, and he thought, and many other people thought, that the magnetic moment of nuclei in the nucleus may be different from what it is outside. This does not mean that crudely speaking they don’t behave the same way.
I’d like to ask now a different type of question, sort of a change of pace. I’d like to ask you to think back and try to put into periods the development of nuclear physics, assign a starting date and try to reflect on the major significant developments, and then what happened at any particular point to bring about a transition to a new stage, where you are either considering different problems or using different techniques, and then to help us to focus on what these things were in the transition period, and what accounted for the change from one stage to another. It’s a tall order.
Where do you want me to begin, with Becquerel?
No, I meant in the 30’s. Let’s say the period when you became aware of nuclear physics, theories of nuclear structure, as representing a legitimate and interesting field of physics, not only for yourself but for other good physicists.
I think experimental work was decisive. The discovery of the neutron. The discovery of the deuteron. I remember a conversation with the physicist whom I esteem as highly as anyone, Dirac, and I told him, how nice that a deuteron has been discovered. And he said he was skeptical of it. He said, “Then you can fill Zeppelins with it [deuterons] and they won’t rise so fast.” I said, “Yes, but you also can have a more direct indication of the size of the magnitude and strength of the nuclear forces.” Well, he wasn’t very interested in that. Of course, his work was so elegant and refined that such crude things didn’t much interest him. But that was what started me, and the knowledge of the great difference between the deuteron binding energy and helium binding energy. However, also very important was the paper of Heisenberg, which I mentioned. The next great step was again an experimental discovery, I would say, namely, the proton-proton scattering. I think you probably can get better historical evidence or historical records of this than my recollections.
I think it’s important to find out how you perceived it or at least in retrospect perceived it.
Well, I learned about it from Milt White who came to Princeton after having made these experiments. I had heard, of course, about the experiments already. Then, the important theoretical work, in my opinion, was that of (I think it was) Breit and Present. But surely I have quoted that several times. This opened up the way to look at nuclei, nuclear structure, from a different point of view. Now, the next thing was—I did not mention it because you put it further back—the very important work of Gamow and Condon and Guerney.
Yes. You said I should start in ‘30. But that, of course, was very important, very important. Next came the slow-neutron experiments, with artificial radioactivity, which convinced me that all that about the electrons in the nucleus was nonsense; that chlorine entering that was a personal reaction. I don’t think many people thought that the electron neutrino enters the nucleus at chlorine. But anyway, the neutron experiments were then very important. I think the explanation of that was given by Breit and me. I don’t know whether you agree with it, but that’s my theory. Now, the next—you see, from there on the discoveries did not change everything. They changed part of the picture, but not everything.
So it was still the same type of problem you were confronted with, and it was just a question of changing a little emphasis here and there.
Not little, perhaps. The next thing was, of course, the fission discovery, which again was an experimental discovery, and in some ways furthered physics enormously.
May I ask here a question? How did you learn about the fission discovery?
Bohr came to Princeton and broke the news.
Yes. I would like to recapture the excitement of those days when it was talked here.
Well, it’s a very queer thing. Can you guess where I found out about it? In that building there—it is the infirmary. Because I contracted jaundice and was in the infirmary for six weeks. It was a wonderful period, because jaundice doesn’t hurt really. They feed you miserably—entirely miserably—and at that time I didn’t know that shrimp has no fat in it. They fed you on potatoes, beans, and everything boiled in water, and the food was not good. But the rest and the detachment were wonderful. In addition to that, Szilard was in Princeton, and he came to visit me every day, and we discussed fission problems and this and that. Well, the theory of Bohr and Wheeler of course occupied us very much, and I think it contributed to the development of our ideas about nuclear structure a great deal. I am not an enthusiast for the liquid drop, but even I can’t deny that it contributed very much.
Let me ask, then—we’re talking now of 1939—the Bethe articles came out in ‘36 or ‘37. Would you say that they then were—you’ve told us already your reaction to them—but then these appear to be a summing up of the state of development at that time, and do you think that really prepared the way for any of these newer developments?
In some ways, unquestionably. People wouldn’t have—you see, it fixed things in your mind. It’s just like the knowledge of the small multiplication table prepares you to do multiplication with—it’s difficult to say. Not in an obvious way. Not in an obvious way.
Now, in ‘37 there were results from cosmic ray studies, which were related to the Yukawa ideas. Were these regarded really as part of nuclear physics—the fact that it was believed that a meson had been discovered? Or was it thought that this was somewhat of a separate phenomenon?
It’s highly problematical. As I said, it was a highly problematical idea. At least I thought so.
Also hypothetical, because the meson had not been discovered.
Oppenheimer believed in the meson right from the first.
Yes. Well, he was at Caltech where the work was being done, and evidently had discussed this in 1936. The Harvard Tercentenary had discussed a lot of things in relation to penetrating radiation. That’s an interest of his.
Yes. You’re absolutely right.
I don’t mean to interrupt the narrative. I’d like you to complete it.
Well, the next thing, which was very important, was the shell model.
About the shell model, if I may interrupt you—something very important happened—
What I’m trying to do, though, if we can agree, is to get an overview from you. Then we can go back and fill in. To get you in the frame of mind where you are thinking of the sequence. Then we can fill in. Obviously there are going to be things left out, and then we can go back to these things in a few minutes.
I think the shell model was really a tremendous progress, and it is a most successful model. I remember having heard about it first when I walked by, in Eckart Hall, an office where I had worked a good deal before, during the war, and Teller and Maria Mayer were sitting there, and they told me about it, and I said, “You know, there were indications for that already in an old paper of Hund, when he plotted the spins of nuclei as against the mass number, and pointed out that certain cycles of spins appeared at one place, definitely, and then the jump to the next spin comes later.” That was already an indication of a shell structure. But nobody understood it. It was an indication which stuck in my mind, and there were several other indications which stuck in my mind, but they were parts for the formation of a picture, but not yet a picture. And the picture was formed by the discovery of the magic numbers. You know that Elsasser discovered them much before; nobody believed it. But this was under the influence of Bohr, because Bohr said, “No shells in nuclei.” That was very important. The next one was of course the rotational levels. The order. And I think I would stop here, at present. This was the last of real importance. So often I think that Talmi’s work on the elaboration of the shell model is very important, but think not as important as those points.
The collective model.
I mentioned the rotational states. Whether you call that “collective model” or “unified model” doesn’t matter here. The phenomena attendant to the rotational levels.
So then you would feel that the history, to recapitulate what you said, that in the period of about 1932, with the series of events, with the deuteron and Heisenberg’s paper on the neutron, in a sense opened up a new stage in this field. Let’s not say it was the birth of the field, for which we always have to go back further, but given the Condon-Guerney and Gamow work and other work earlier, then these papers and this work in 1932 opened up a new stage. And you feel that although many things happened in the years before fission, that they were variations and they didn’t change the tone of it. They didn’t change the main stream of work. The fission work did. You didn’t tell us exactly how.
That was really more practical.
It added a dimension to the field, would you say, rather than a new direction?
To the interest. You know, people got excited about it. Johnny von Neumann said that the trouble with nuclear physics is that it started at the wrong end of the periodic table. We interrupted you?
No, no, not at all. I meant to recognize there a very important peak. One of the first things to come out after the war was your work with Heisenberg, the Wigner-Heisenberg theory of the R-matrix. Was it not the most complete attempt to define these pi-nuclear reactions?
Yes, Yes. It’s very amusing. You know, it’s very amusing, and I still like it a great deal. I think also the mathematics which is attendant on it is fascinating. But I don’t think you can compare it with the discovery of the rotational states. It’s a theoretical description, and it’s very useful. Similarly the peripheral reaction theory of—was it Salpeter? Who discovered the peripheral reaction?
The peripheral model?
No, the peripheral, the V-P reactions.
V-P, the direct reactions?
Yes. I can look it up in Heisenberg and Wigner.
Butler, of course. Yes.
I’ll give you a point for that.
I didn’t know his first name was Stewart, because I don’t know him. But, Stewart Butler ... that was very important also.
Well, I would like to inquire about this question, what was the—and still is—a complete, consistent description of nuclear reactions, in the low-energy range, of course?
But that is true. I think that the R-matrix theory is very nice and very interesting. It’s essentially a “black box” theory, as I call it. In other words, it reduces the whole continuous dependence of the cross-sections to a number of parameters. Now, this is very useful and also conceptually interesting, and if you want to take a deprecatory view of the R-matrix theory, don’t come to me. There is Mr. Rosenfeld in Manchester. He is opposed to it.
The Rosenfeld of Copenhagen?
Yes, the same Rosenfeld. But that’s childish, in a way, of course, because you don’t get mad with theory.
This is the nature of discourse—a dispute.
But, you see, I don’t feel that it is on the same level as the discovery of the rotational levels, which is just an entirely new element added. You see, nobody suspected. The article of Feenberg and myself had rotational levels in beryllium 8, and Hund pointed it out again. And we felt, yes, that’s very strange, very impressive—but it was not recognized as an important and fundamental phenomenon. And that was a real discovery that shook my belief in many of the older theories.
May I ask about these two peaks you mention, the discovery of fission and the discovery of the rotational levels in nuclei. In the case of fission, the liquid drop concept proved to be extremely successful.
Well, it proved to be, reasonably.
It showed that the characteristics could be defined in terms of the liquid drop, the deformation of the nucleus, and so on.
You disagree. Why don’t you express your disagreement?
Not very well. It gives a beautiful picture, but if you look at the details of this picture, they are very inadequately fulfilled. You see that the spontaneous fission is strong in those nuclei in which one would not expect it to be strong. There are many resonance levels in uranium, there are grades of—I’m only telling you what surprises came with the drop model. The many resonance levels in uranium-235 were an enormous surprise. In fact, so much so that even I was amazed at it, although I was not one of the main believers in the liquid-drop model. But it was a nice picture and a beautiful picture.
With apologies to my friend Charles Weiner, may introduce a metaphor which would like to know whether you think it is correct: that Bohr was often an impressionist in his physical theories, in broad strokes.
And this was the description of the fission process as a liquid drop. In details it would not bear intent scrutiny, and yet the broad strokes were somehow correct.
Yes, but I don’t know what in it was correct. It is correct that liquid drops also separate, if they are electrically charged. I don’t think we have a really good theory of fission.
In the case of rotational levels, one finds that the nucleus behaves more like a rigid body than like a liquid drop.
Yes, and there the experimental support is massive. Look at the Hafnium spectrum. One level after the other, within I think 2 percent, const. J (J + 1) for the energy.
Excuse me, the tape is about to run out, so instead of catching you in the middle of a sentence, I’d like to stop and flip it over. Have you completed the filling in?
Here is a question on that. Meson theory was proposed in 1935, and until the shell model came along, I would think there were many people who were common to high-energy nuclear physics, mesons and nuclear forces, low-energy physics. There was not much compartmentalization. There was a certain fluidity, and people thought that they were somehow working on common frontiers of physics. Now, this question is as much a question of opinion as of physics. I would like to inquire from you how this fluidity helped matters, in looking at physics.
This is a very difficult question, as I am sure you know better than do, I think it gave physicists an elation, that they know physics. Now this elation-do I pronounce the word correctly?
Thank you. That elation has largely disappeared. Nobody knows physics. And that’s sad. Does that answer your question?
It would be nice if you could expand on it a bit.
You know, when I was, I don’t know, 28 or 30, I felt ashamed that I did not know simple Planetary theory, that I did not know the theory of—now, what did I not know? You know, I knew physics on the whole reasonably well. However, I did not know electromagnetic theory, for instance, as well as I should have: I felt embarrassed by this and wanted to learn it. And often hoped, “just finish this article and then I’ll have a little time and I will learn about it.” Now this feeling has disappeared. I know that I will never learn, let us say, the present thoughts on high energy physics, that a large part of physics will always remain a closed book to me. I know the Feynman diagrams, but I am not as at home with them as I would like to be. And perhaps I can learn that. But as to the whole of physics, I certainly cannot. Now, this illusion, to be able to know physics, or that one would like to know physics, has disappeared with the tremendous development and fragmentation of physics. And that is a sad thing. It’s a different science, emotionally.
Certainly this difference is felt by physicists who have been in physics for a full generation. Do you think that the new entrants to the profession are aware of the difference and if they are, are they bothered by it at all?
I think that is similar to the question of whether the children who grow up in Russia or in China or in Hungary are missing freedom. Probably not, consciously, but their outlook on life—I think you know of it. Os homini sublime dedit, coelumque tuere inssit et erecta at sidera tollere vultus. “He gave man an erect stature, begged him to look at the sky, and to raise his face to the contemplation of the stars.” My translation is probably not very good, but, you see, there is something there. Something has been said. Now, this type of elation is smaller in present day physics. It’s more down to earth. It’s a more special interest. Similarly, I did not miss not knowing all the physiology that could have been known, all the biology and so on. Newton did know all the science of his time. I did not. In a sense I missed it, but just as the Russian child…
May I ask, you said that earlier on you were not interested in meson physics, but later on you became interested in meson physics, did you not?
Yes. Critchfield, Teller and I did write one paper on it, which was unsuccessful. But yes. I never worked on it seriously.
Why don’t you follow up on the fission announcement. You learned of it here. And then I wanted to know about some events following that, and then the effect in general of the war on theories of nuclear structure, whether in fact—well, let’s get to that one when I come to it. Is that agreeable?
You learned of fission through Bohr, as did everyone else, on his arrival in this country. Then what type of discussion ensued at Princeton?
Most of it I heard indirectly because I was in the infirmary. Szilard and I discussed it very much. And we also knew about the ideas of Bohr and Wheeler. I think we drew their attention to the fact that the calculation can be done by the so-called transition state method. You probably knew about that, the so-called transition method? It’s a method to calculate the rates of chemical reactions. Two of the people who developed it were Pelzer and myself, for chemical reactions, and of course I knew the method, because everybody reads his own papers very carefully. We drew their attention to that. I think it was a very interesting time and quite exciting. Szilard came to me one morning and said, “Wigner, now I think there will be a chain reaction.” And I said, “No, it’s just the other way around,” and then I realized, “No, it’s right.” Evidently, the fission fragments are either too neutron rich, or too neutron poor. They are neutron rich but in the first moment making an error of sign, I thought they were neutron-poor. But he came to me with that idea one morning.
Was he here for a semester or just as a temporary visitor?
I don’t know. He was, at that time, in the United States. There was an enterprise similar to the enterprise which he founded in England and this supported him. Do you know the Academic Assistance Council?
This was to provide jobs for refugee scholars from Europe.
There was one in America also, but it was not formally organized. The way I remember it, K. F. Herzfeld collected the money, and he was here along that line. But that’s recollection.
I see. Well, this is important, because this particular chapter of Szilard’s role in this thing is really undocumented.
I wasn’t referring to that, though that may very well be true also, but certainly his role in enabling and assisting an entire generation of physicists and scholars in getting across.
Wasn’t it wonderful, what he did? He could do things which I surely could not do. He could approach strange people and persuade them to give money for some purpose, a purpose which those people didn’t have in mind at all.
Were you at the subsequent Washington meeting? Of course, you had already had discussions on fission at Princeton, so it wasn’t a question of a tight little secret, because in fact it was talked about on the campus. At the Washington meeting, there again it was talked about. Was the only difference that it was a more formal presentation of it in Washington? At the Washington meeting?
No. Just that it was a much larger group, more people.
So it’s not a question of one being public, one being private, but one was more public than the other.
Yes. You see, here, there was a seminar in that little room, and Bohr spoke in that seminar. My recollections are not terribly clear on the details. I think I was in Washington, but I could not swear to it.
I didn’t bring it with me, but we have lists of attendance at that meeting.
I went to both meetings—if I didn’t go it was because of my jaundice, but jaundice is a good reason.
That brings another question before I proceed with fission. Did you feel that any of these international conferences or larger public meetings, I mean larger than the Princeton ones, were very important during the ‘30s?
In the same sense that Bethe’s review article was important. It fixed something. It made it public opinion. It crystallized it. Perhaps that’s the best word, even though it’s a metaphoric word. It crystallized it. I don’t think I ever came away with new ideas, or things that I did not know. But this doesn’t matter. This crystallization, this settling down in one’s mind, are very important processes.
Do any particular conferences stand out as being especially effective as crystallizers in that period of the ‘30s?
When Gamow and Teller spoke about the Gamow-Teller selection rules, altogether about the theory of beta decay.
Do you remember which conference it was? One of the Washington conferences?
You don’t remember which one?
My recollection for years… I happen to know when was born, but not because I remember it.
May I ask a question on beta decay? As you have done before, Professor Wigner, could you point out the major peaks in the discussion of the universals of interaction? In the theory of our understanding of beta decay, and—there was a time when Yukawa introduced mesons, that he thought it might also be the data for explaining beta decay.
Maybe it is. We don’t know that yet. But I think the great discovery was Fermi’s paper, and that was of overwhelming significance. That had overwhelming significance. The Gamow-Teller selection rules were perhaps interesting and focused attention on a number of things, and it was a particularly good meeting, but I think it was not of the same order, and I’m sure Teller would be the first to agree with me. I don’t hesitate at all to say it. Critchfield and I thought that there is a universal beta decay and that it is an anti-symmetric interaction. I don’t know whether you are familiar?
Wait a minute. It would be that, scalar and tensor? Good. Now, our work was—no, think it was vector and axial vector, wasn’t it?
It turned out to be that. Did you propose that it would be vector-axial vector?
We proposed that it is anti-symmetric, and now—what does that mean really? We proposed that it is—no that was—it would have been vector. Now, our great work was that the experimental results were wrong, and indicated that it is tensor, so we abandoned it, which was lucky, because it is not right, even though the interaction is vector.
When was that? That would be in the middle ‘50s some time? Even before that?
Oh, much before that. Much before that.
This is “The Anti-symmetrical Interaction in Beta Decay Theory.” Now, after that, which was the most significant?
I don’t think it has any significance, because it turned out to be wrong.
And Gamow-Teller comes much later?
Much earlier. Much earlier.
At one of the Washington meetings, which was in either ‘38, ‘39, or ‘40.
And after this?
This had no significance. Now, what was the next thing? Well, of course, the great discovery, I don’t need to tell you, of Lee, Yang and Wu, of the parity violation. And the subsequent discovery of the V minus A interaction, which is now very well established.
Did you take part in the discussion of V minus A theory?
Yes, in the discussion, but not contributing to it.
Were you at the Padua Conference in 1957-58?
No. No. It did not surprise me, the V minus A, but you see, so many other things don’t surprise one.
Now, before we jump into the ‘50s, I’d like to pursue these events in the late ‘30s. It was about that time, through your discussions with Szilard, that you realized the potential implications of fission.
And the story’s been told, and perhaps even over-told and over-dramatized in many ways, but is there anything you feel hasn’t been expressed? Have you ever really expressed yourself on this and given an historical account of it? If not, this might be a good time.
I have a set of notes on the subject, on the uranium project and my participation in it, also Szilard’s role, which may some day be published. But it has many personalities involved, and many times it says, that “Bohr was entirely wrong” or things of that sort, which of course—he was not entirely wrong, but it says in the notes, so…You understand that these were notes to myself, sort of a diary.
I hope that you will preserve this, under whatever circumstances you think are best. This, in time, whenever you’re prepared to allow it to be used, would be a very important historical document.
Well, it may be interesting. I don’t know how you feel about this particular word, but many things I recognize completely; what happens now has an enormous similarity to what happened in those days.
Well, here I remember, if I may—
—then it would be worth, it would have a public impact, wouldn’t it?
No, it would not. People don’t react. Somebody who does not want to believe that Hitler wants to attack France will not believe it, no matter what you say or do.
That’s true, because it’s not a question of a scientific thing.
Well, look here, I can show you documents in which—you know, I’m interested in civil defense; I am not sure whether you know that.
I can show you papers where somebody says, “You can’t build shelters because the radiation will get into the shelters and will destroy the electric motors which drive the fans.” Now, you can’t be more idiotic than this. But if somebody does not want to believe that civil defense can be established, he will find some reason, however idiotic. It is incomprehensible.
It gets out of the realm—it gets into an emotional state. Let me ask a question, beyond that period. What do you think was the effect of the war on the development of the field of nuclear structure? There are several factors of course to consider. You can consider that it accelerated the work in the field, or sort of left everything pending, or provided a thinking period, or brought people together to communicate. There are many things that one with no knowledge of it might suggest as possibilities. I’d like to get your feeling on it.
Well, on the whole the last way is what is my impression, that people’s attention was diverted from nuclear physics, and they worked on something that was immediately realizable. They became, so to say, ingenious developers. And nuclear physics was delayed. Now, occasionally it is good if you go away from a subject and let it settle in your mind. You forget your prejudices and this and that. But the favorable effect was only that way. You mentioned before the R-matrix theory; the beginnings of the R-matrix theory were conceived at the end of the war, and I wrote it as we finished. Our work had become entirely of an engineering nature and calculated how thick an 3-beam has to be, and such things which are not usual for theoretical physics. But in the idle hours—because that doesn’t take up all your attention and interest—in the idle hours I thought about nuclear reactions and the R-matrix theory’s beginnings were formulated in that way.
One of the first derivations after the war was your discussions of the R-matrix theory, and Feshbach, Peaslee and Weisskopf’s paper on the derivation of the Breit-Wigner formula.
Yes. Yes. And that was a very important paper, as it turned out.
Can you think of other work that was brewing at that time, during this war period?
The theory of the multiplication constant of criticality, which, of course, is not physics—not physics of the frontier, but it’s interesting physics—was developed then. And that’s an interesting part of statistical mechanics, of transport theory. Also, the considerations on the so-called radiation damage—that turned out to be very important for solid-state physics. And finally, of course, the theory of material under very high pressure which was developed at Los Alamos. Those were very important things. Also, people learned about shock waves. Did you know that there were shock waves before? You were a baby, perhaps, at that time. But the whole knowledge of shock waves.
What about the effect of bringing large numbers of people together—including a number of people from Europe—together with the younger generation? One factor was bringing people who had been in Europe before together with people who have been in America for a long time, who were native Americans. Secondly, bringing together people from various parts of the United States, and thirdly, bringing together senior men and the younger men. Was it effective? Did it mean anything?
I made a great many friendships at that time which have lasted very well, Alvin Weinberg, whose name is probably familiar to you—is one of my closest friends—I met him through the uranium project. Similarly Gale Young, whom you probably don’t know equally well. He’s a very thoughtful man. He’s closer to engineering and practical applications. (Young is an easy name to spell, Weinberg is probably familiar to you.) I’m sure there are many others. Los Alamos’ history is not as fortunate as Chicago’s history. There was much friction there, as you probably also know.
At Los Alamos.
Well, the environment itself was quite different. It was an artificial ,contrived environment, whereas Chicago was a university atmosphere.
Yes, although we did not see the university.
That’s true. You knew Cyril Smith then, in that period, too.
Cyril Smith I think I met first after the war. You see, I never was in Los Alamos until, oh, I don’t know, probably ‘50.
I thought of him as being at Chicago, too, for a while.
You see, we were terribly busy. We worked very intensely.
You were at Princeton or Chicago?
At the Metallurgical Laboratory. You were head of the Theoretical Physics.
Yes, I was head of the “Fourth Floor Communists,” as we were called.
Why was that name given?
Because we did not go along with the standard ideas. That’s my interpretation. You can probably have another interpretation from those who called us “Fourth Floor Communists.”
Is the standard interpretation in terms of the scientific work or in terms of the administrative procedures?
No, in terms of the technical possibilities. We thought that the heat transfer agent should be water. I don’t need to explain to you. We felt that people had been using water for cooling for many years. On the other hand, the engineers wanted to use helium cooling, which we thought would delay things unreasonably, and would introduce problems which are difficult to foresee and difficult to master, and we also felt that we knew how to calculate a heat transfer coefficient, which of course was considered by some of the engineers as interference in their prerogatives. It’s a complex situation.
Right after the war, you returned to Princeton.
And in l946 there was a conference here, the Bicentennial Conference, and the theme as expressed was “The Future of Nuclear Science.”
That was a very poor title.
I want to know something about the conference and whether it was significant. It did assemble a large number of people, young Feynman at the time and Dirac was here, and people from all over the country.
Oh, yes, and all over the world. We made every effort to get Russia to cooperate. It was a very interesting experience, actually. I don’t know whether you are interested in it, but anyway, it was really more a semi-political conference. We felt we should get into equilibrium with this fact that science contributed a very important new weapon, and that this new weapon should be somehow controlled, that war should be discontinued, if that is a good word.
But there were technical papers at the meeting dealing with theoretical developments with quantum electrodynamics perhaps.
Yes. But they were on the surface. That was not the principal interest.
How about the conferences that followed? Were you involved in the Shelter Island Conferences?
As a matter of fact, Shelter Island I think I missed, but there were several conferences, principally organized by Oppenheimer, which were very important and very successful, and at those I learned a great deal. I presume others did also, but of course I can’t speak so well for them.
You differentiate between these conferences and the ones that you classified as “crystallizing” conferences. You said just now that you learned a great deal. Was this different from other conferences that you attended?
You know, I think you are right. I am not consistent. The fact is that I happened to learn about quantum electrodynamics at these conferences, and in that way for me it was different.
Now, these conferences were what Oppenheimer described as small, intimate conferences.
And the first at Shelter Island, the second I guess at Pocono Manor in ‘48, and then a third on the Hudson somewhere, Old Stone-on-Hudson.
I just don’t remember these locations.
We had to dig them out to find out for sure.
I don’t remember, but I am sure you are right. I really don’t remember. I would have said that it was in Princeton, but that’s wrong.
Well, there may have been others, and that’s what we’d like to find out.
These were the conferences arranged by Oppenheimer: Shelter Island, Pocono, and Old Stone or Hastings-on-the-Hudson.
These are the antecedents of the Rochester Conference.
Yes. That’s an important point that I’d like to get to. But these conferences were small and were a select invited group. Now, did they represent the beginning of a new stage, or the end of an old one?
The beginning—well, you see, he felt that physics should be kept together. I think he will give you a more eloquent motivation for that. But he was a natural leader, and he somehow organized them. You know, I just don’t remember the locations.
He has indicated that he has some files on these that he’s going to look at with me, and we’re going to piece together the history of it in this way, using his letters organizing it and so forth. But the point that I was getting at is the developments after the war. If we take your statements about the effect of the war, then the next question is, what happened after the war? You mentioned some theories, some ideas developing during the war. What were the major events then that characterized the development of this field of nuclear structure?
Yes, and I know why you ask this, because of your concern with high energy.
Electrodynamics is not nuclear structure.
Right. But these are developments in ‘46, ‘47, ‘48, ‘49. By 1950 you are having a Rochester High Energy Physics Conference. Evidently these were important years. What characterized them that led up to the changes that we’re living with now?
The number of physicists has increased very much. They have much more money. Physics has expanded greatly.
Was this evident in ‘46, until ‘50?
Well, yes, more or less. You know, I remember a conversation with Franck, James Franck. That was during the war, and I told him, “Oh, I am getting into the older age bracket. I don’t think I will have much to do with modern physics after the war.” And he said, “Oh, Wigner, you are entirely mistaken. After this rapid development which has taken place before the war, there will be a time to settle all this, and the progress of physics after the war will be much less rapid.” Well, we were both equally mistaken. Because I kept up with physics much better than I expected. I did not keep up well, but much better than expected.
And you didn’t keep up because physics slowed down, so in that sense you were wrong and he was wrong. In 1950, though, the High Energy Conference at Rochester was held, and I’d like to ask you to explore a bit this transition point, the branching off of high-energy physics. Although we’re considering nuclear structure, we’d like to know at what point and for what reasons high energy physics began to develop as a separate field, clearly defined as such?
That’s a difficult question. I don’t think it was a sudden event. But people in high energy physics lost interest in nuclear structure. Nuclear structure became something that you had to learn. The shell model was something you had to know, that it is a seven-halves shell that closes—as a matter of fact, a twenty-eight—but I don’t want to go into details. There are concrete things, and it wasn’t that you had to figure it out every time. You can’t work that way. And the high energy physicists and a number of physicists were not interested in that, and they said that fundamentally new discoveries will come at high energies. Oppenheimer was one of them.
And who else in that group?
The most interesting person with whhom you should speak is John Wheeler, because during the war he said nuclear physics is for the engineers, and after the war it’s cosmic ray physics and high energy physics and it’s entirely different. I asked him, “How about beta decay?” and he said, “Beta decay is all right.” And then he did of course a great deal of work in nuclear physics himself. So he did not remain—you know what Bismarck said? “Only us oxen are consistent.”
Was this the general pattern of the older group? By older I don’t mean necessarily age, because Wheeler was relatively young, but I mean the people who had been active in nuclear structure before and during the war. Would you say that this was a mass movement over into high energy?
No, it’s that many of the young people went into high energy, because—well, it was a glamorous field, wonderfully supported by the government.
Then in turn, this was part of the fragmentation you were talking of.
Yes, I don’t have reprints here today, but the number of reprints on high-energy physics that come in on an average day is about four.
If we’re going to stick to your deadline we’re just about at it. Can I suggest a final few questions, to bring it to a logical close? First let me ask my colleague if anything glaring has been left out.
I would like, if I may, so far as physics is concerned, to complete the picture to mention, on the nuclear structure, one or two minor points. This was Feenberg’s wine-bottle model, and that was a type of a shell model to get L-dependent force. Then of course came Mayer and Jensen’s work, and then Rainwater proposed that the nucleus was deformed, which led to the…
You are absolutely right. I should have mentioned Rainwater…
I wanted to identify its importance as a suggestion after the war.
The wine bottle model—I have a high admiration for Feenberg—but this was essentially an error, similar to many errors which I mentioned, by Bohr and by myself. The magic numbers were known and he felt that that was the easiest way to explain them.
Another question which occurred to me was that nobody has successfully given a connection between the nucleon-nucleon force and the binding energy of nuclei.
You know, that is so, and I mentioned that this was a problem which was always on my mind, and I wrote several papers. I pointed out that we still do not understand the magnitude of the nuclear binding energy. I understand this situation changed a short time ago, and you should be familiar with it, because Bethe says he developed…
Bethe’s work with Read in which he is employing the nucleon-nucleon potential to calculate this. [Wigner note: I do not know this name.]
So, you see, that is very nice, but is it what we ask nuclear physics to explain? I don’t know… Yes, it’s nuclear physics.
Just on that tone, and if you’ll give us a minute or two…
It has no color to it. You see, it has no color. Yesterday I listened to somebody explaining the analog states. Well, the analog states was a beautiful discovery also. Isotopic spin, what I call isotopic spin multiplets in very heavy elements. The clouded crystal ball model is a very useful model, which I also participated in. The many-body theories—Brueckner, Bethe, Goldstone—were very important and significant discoveries.
For the record I should make a minor correction in my statement about the d, p reaction, direct reaction. I’ve been thinking about it. I think Oppenheimer and Phillips had predicted first of all in the case of light nuclei, and later on it was Guth who predicted it for heavy nuclei, and it was Stewart Butler who verified that that was so.
Guth—Oak Ridge, yes—and I see him very often. No, I did not know that, thank you. That Oppenheimer did it, I did know.
I would like if I may ask one question, and it is the following: This is a remark attributed to Professor Wigner, and I always have believed it to be correct. Here is the opportunity to verify it. It quotes you as saying that you have always wanted to work in significant problems, and that you have always followed a path which led to significant problems.
I don’t remember. I have said so many things.
What’s the source of that remark?
Well, in some essay, I don’t recall exactly the title, but I would have liked to ask here, at least in the field of nuclear physics, if Professor Wigner would identify the more significant contributions which you yourself and your students have made.
I don’t know. It’s awfully difficult to tell that a contribution is significant. Also, a contribution is not really a contribution, because it has antecedents; it is exploited then by others and they add to it, and that gives it more life and content, I was happy, interested in it; enjoyed it. Whether it was significant or not is for others to judge.
That’s an interesting statement, and I understand it in the way you mean it. May I ask from which of the major pieces of work that you have done have you received the most personal enjoyment?
I think the R-matrix theory gave me very much pleasure. It was great fun. Isotopic spin introduction gave me much fun. I just made a long calculation on isotopic spin but I don’t know when I will be able to publish it, because I have too much to do. That gave me a great deal of fun. (Just about 12) And it’s interesting. One should not work in a field if the work, the down to earth work and thinking about it, even unsuccessfully, does not give you pleasure. The great weakness of Szilard was that he had no pleasure in work, and he did not draw the consequence of this, not to be a physicist.
Does this explain why after the war he went into biology?
He went into biology because he did not want to learn mathematics, the mathematics which was necessary for continuing physics. He liked to contribute ideas, but to work on them did not give him pleasure. And, as pointed out to somebody else, you can’t expect other people to work on your foolish ideas. They have their own foolish ideas.
A final question. Do you feel that the field of nuclear, structure is essentially complete, or that it’s entering a new stage, or that in fact it’s on a plateau?
Well, it is on a plateau. I think that’s the best way to put it. I hope that it will come off the plateau, but it is difficult to expect it. There are many interesting points and many interesting ideas floating around, but I don’t feel a prospect for what people call a breakthrough. I don’t feel, by anything that I’ve heard recently, elated—”Well, now, isn’t that wonderful!” The last thing I heard on that is Talmi’s work, that gave me that impression. I don’t know if you are familiar with that, Talmi and Thieberger.
Yes. Bethe recommended that we talk with him for a real view of some recent developments.
He’s here, you know.
Yes. We stopped by his office just to introduce ourselves, but he was out at the moment.
He’s a very wonderful person also, personally. I think you would enjoy talking to him.
I think we’ve kept you longer than we said we would, and I thank you.
Thank you very much.
E. Wigner, Phys. Rev. 43, 252 (1933).
Charles L. Critchfield and E. P. Wigner, Phys. Rev. 60, 412 (1941).