Oral History Transcript — Dr. Victor Weisskopf
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Victor Weisskopf; December 5, 1966
ABSTRACT: Origin of interest in nuclear physics, discussions of compound nucleus, Copenhagen, 1936; work on interaction of evaporation and nuclear temperature, 1936; Breit-Wigner formula; application of evaporation model to nuclear reactions; postwar work in electrodynamics and nuclear reactions; relative merits of compound nucleus and shell models, 1950-51; explanation of independent particle motion by Pauli principle, 1951; estimates of shell model radiative transition probabilities; optical model and relation to compound nucleus models, 1953-55; emigration to U.S., 1937; initial impressions of American physics community; teaching and research at University of Rochester, 1937-43; visits to Cornell University; elaboration of evaporation model; effect of Bethe-Bacher-Livingston REV. MOD. PHYS. review articles, 1936-37; contact with physicists at American institutions; summer schools in U.S. and Europe; role of conferences; centers of nuclear physics research, 1930s; the atmosphere and social aspects of life at Los Alamos and subsequent effect on physics, 1943-47; effect of war work at MIT on electrodynamics and nuclear models, 1947-55; visits to Brookhaven; collective model; role of nuclear spectroscopy; origins of high-energy physics; relations among nuclear structure, meson physics, and field theory; current (1966) work in nuclear physics in Europe; the future of nuclear physics.
Session I | Session II
Weiner:When we last talked, we left off at the point where you had arrived in the United States. You were describing your reactions to the contacts with other physicists in this country, both with some of the Europeans from previous acquaintanceship, and some of the newer American group. You commented about how much at home you felt in this new community of physicists. We didnít get into too many specifics about your contact with other people. You mentioned Bethe for one. Now, he was very close geographically as well as in field of interest. Perhaps you can tell a little bit more about how you arranged to see him and what you talked about at the time.
Weisskopf:Well, my memory is very bad. Iím not Telegdi. I donít know even how often I visited, but I would suppose it must have been probably once a month that my wife and I came over [to Cornell] or he came to Rochester, mostly to discuss problems. For example, when I had a new idea or got stuck with an idea, I just went over to Bethe to have simply somebody with whom I could talk. Talking always helps. Apart from this, of course, there were regular opportunities to meet, either by being invited to give a colloquium talk or common seminars of interest. I think Iím not wrong in saying I was there every month once, letís say, for a day. You asked me also what kind of collaboration it was. I do not remember that we ever wrote a paper together. In fact, I donít think there exists a paper ďWeisskopf and BetheĒ as far as I know. However, as you said yourself, we were working in the same field, and thereís hardly a paper in that period, in the pre-war period, that I wrote which wasnít thoroughly discussed with Bethe; and most of the Bethe papers he discussed with me — in particular, in the field of nuclear physics, but I think also in other fields.
Weiner:Did this apply to the three-part [series of articles in] Reviews of Modern Physics
Weisskopf:No, that was much earlier, You see, the three-part [series] was finished before I came to this country.
Weiner:I see, It was finished in Ď37.
Weisskopf:In early Ď37. It was published in Ď37. It was finished in Ď36, and I came to this country in September Ď37.
Lubkin:By that time you had evolved the evaporation model.
Weisskopf:Yes. In fact, the evaporation model is treated in Betheís articles due to the fact that I had corresponded with him about it and sent him the manuscript before publication.
Lubkin:He mentioned that he, too, was doing some work on the theory. Were you corresponding on it?
Weisskopf:Yes, definitely. We were corresponding on this. As he implies, he calculated the level density of nuclei. Now, the level density of nuclei plays an important role in the whole thermodynamic or statistical theory of nuclear reactions, and I was using in my papers a slightly different formula — a more simple formula than he has derived. And he, in fact, in his review article, applies some of my formulas by using his expression for the level density. As far as I know, but I might be wrong, the idea of evaporation does not come from Bethe. It came from me, but not exclusively from me. Independently, it was also proposed by Landau and Frenkel in ways rather similar to mine and in some rudimentary form proposed by Bohr and Kalckar but not in an explicit form. But Frankel and Landau did about the same as I did. They didnít apply it perhaps in so much detail as I did in my paper, but it is essentially the same idea.
Lubkin:Did they then go on with the evaporation ideas to the extent that you did?
Weisskopf:No, the evaporation models — not the Russian authors — to my knowledge; I donít know. I mean as far as the published work is concerned, I would think that they did not apply it to this extent, as we did, and also as Bethe did.
Lubkin:Bohr and Kalckar, tooÖ
Weisskopf:No, Bohr and Kalckar only put [forward] the foundations of this idea. In some ways my paper on evaporation, after all, came from Copenhagen and is an outgrowth of the Bohr and Kalckar initiative and was also considered by them as such, and therefore they in a way left to me this part of their initial idea.
Lubkin:What was the response of physicists to the paper originally?
Weisskopf:I think it was extremely positive. I wouldnít be surprised if it was one of my most popular papers. I mean the application of the compound-nucleus theory in this detail to nuclear reactions comes from this — this together with the paper I wrote with Ewing. So the response was extremely big. And I wouldnít be surprised if it was the most quoted paper. Iíve never made statistics of that, but I suppose there are such statistics somewhere.
Weiner:Thereís a citation index, I donít think it goes back that far.
Weisskopf:I wouldnít be surprised if this and my paper with Ewing are the most quoted papers I wrote.
Weiner:During this period the first two parts of the Bethe article had come out while you were in Europe.
Weisskopf:I think all three for that matter.
Weiner:Yes, according to what you said before. What was the reaction to these articles in Europe? They appeared in Reviews of Modern Physics.
Weiner:How much of a time lag was there in reading it? Was it received and read regularly?
Weisskopf:Sure, sure. Itís the same time as now — two weeks transportation.
Weiner:By that time things had changed. They didnít wait for a year to get the bound copies.
Weisskopf:Oh, no. And I should say that the mail service at that time was much better than it is now — the surface mail.
Weiner:Was it felt that there was a need for this type of article?
Weisskopf:Sure. In fact, there was at that time after all, a European tradition of writing such articles and it was started with the famous Handbuch der Physik where Bethe himself in fact created that tradition by his articles on metals and on the single-electron problems, and he just continued that tradition in America. So this was extremely important. It was a milestone.
Weiner:How did it become effective? Did people regard this as a sort of clearing of the air in the field, or as a summing up?
Weisskopf:A summing up. I would like to refer to what I say about these articles in my description of Betheís life in the memorial volume. I consider this unique. Betheís way of writing review articles is something unique. I wish other people would do it that way. He doesnít only review — he also reworks the papers, so there is not a single conclusion or statement that he hasnít calculated from beginning to end, including the evaluation of the experiments, so that he reshapes the work done by others in his own style, and that makes these Bethe reviews tremendously valuable.
Weiner:We, by the way, talked to him in great detail about the processes involved in compiling these articles and writing them, and we did it by making a Xerox copy of the table of contents of each of them and went down item by item, asking him to indicate which was reworked, which was new, which was his work, and which was someone elseís work. It was tedious. We spent more than an hour on it — just to go down and have him rethink the whole thing because it was such an important article.
Weisskopf:By all means. It was, I think, perhaps more important than a book. More important because it came at a time when perhaps one would not have been able to write a book, although these three articles would make a perfect book — in fact, a better book than many books that have been written. But the fact that it came so quickly at a time when this field was completely new and in the center of interest made these articles the most important piece of literature in this field. I really wish that there would now be a Bethe in the field of high-energy physics, particle physics, who would write three articles of this kind about the present state of particle physics. It would be enormously useful, but there is nobody.
Lubkin:Is it possible? Isnít there so much more data that would have to be absorbed?
Weisskopf:No, there are just no Bethes.
Weiner:Did this article have any effect on attracting people to the field?
Weisskopf:Definitely. Iím absolutely sure. I wouldnít be surprised if I am one of them. I canít remember clearly enough, but Iím absolutely sure that many people were attracted to the field by this.
Lubkin:Could you sort of assess the theoretical ideas and the recent experimental results that Bethe had to put in there?
Weisskopf:Itís hard to say because itís the whole field. This really contains everything that was known and that was worth knowing in nuclear physics at that time. So if you want me to do this, I would have to give you a complete description of nuclear physics in 1936, which is a little hard for me. I would have to refer to the table of contents.
Lubkin:Sure. But, for example, we know that the compound nucleus had just been found.
Weisskopf:Yes, and this was in.
Lubkin:Right. But can one introduce major experimental evidence or categories of evidence that existed, and were there any other major theoretical ideas?
Weisskopf:Such as the compound nucleus, you mean?
Lubkin:Yes, something of this magnitude.
Weisskopf:Yes, I would say the treatment of the two-body problems, the deuteron, the radiation of the deuteron, the proton-neutron scattering, the ideas of Wigner on proton-neutron scattering and deuteron structure, which he also reworked and reformulated. This is one other field — the field of the two-particle problems. But you catch me unprepared. There are probably many other things that I just donít remember. I would have to leaf through these reports to give you any more information.
Lubkin:As far as the experimental data, aside from the Fermi resonancesÖ
Weisskopf:Oh, enormously. As I said, the neutron-proton scattering data, the nuclear spectroscopy data of level spectra.
Lubkin:How well known were levels?
Weisskopf:Not much, but enough to make a few statements. Again, I am not Telegdi. He would be much better able to give you an exact answer to this question. In fact, this is a typical question for him.
Lubkin:Did one talk about isobaric multiplets?
Weisskopf:Yes, the Weizsacker theory of isobars is an example. Since you mention this, the mass formula comes to mind. You know, the Weizsacker empirical mass formula is treated in great detail and with great skill by Bethe. And then the three Bethe articles have the interesting feature that they are not only theoretical; they are also experimental. They contain many theories that are important for experiments such as the theory of the stopping power of a charged particle in matter in great detail — electron stopping, proton stopping, where Bethe himself has also contributed a lot. So I would say it is much wider than just the compound nucleus.
Weiner:Returning for a minute to your contacts with people, you mentioned Bethe; who were some of the others? Did you talk to anyone else at Cornell when you went there?
Weisskopf:Yes but not comparably as much — with Bacher, to whom I was also personally quite close.
Weiner:Livingston was there.
Weisskopf:Livingston was there and the experimenters who worked at the cyclotron. In fact, I was in close touch with the experimenters and tried to induce them to do experiments in connection with my paper with Ewing.
Lubkin:Yes, it sounded as if you had done some of that experimental work yourself.
Weisskopf:No, not with my own hands, but I was always very close — and particularly at Rochester — with the experimenters because I wanted them (and they wanted also) to work on the problems connected with our theory. So I was with them evaluating the curves and I was spending the evenings looking at them — how they take data and so on. But I didnít work with my own hands. In fact, I remember one thing. I was very close, in particular, to the experimenter who was in charge of the cyclotron, Sydney Barnes. He is still a professor at Rochester. I was also personally very close to him. I spent many days and evenings with him at the instrument. One day, I remember, when the beam was out, I accidentally got my hand in the beam and got on my finger a radiation which was probably above the safety rules. Nothing happened to me, but I remember I was very proud about this. As I mentioned in the previous conversation, I had a lot of contact with Edward Teller in Washington, not as close as with Bethe, but I also liked to discuss problems with him. These are probably the most important contacts outside, although you know how it is: one goes to meetings and meets a lot of people. I should say also that I was close to the circle around Oppenheimer — Oppenheimer himself and Furry, who now is at Harvard; then Serber and Phil Morrison.
Weiner:Now, you saw these people you mention sometimes at Oppenheimerís ranch, but sometimes in California.
Weisskopf:Sometimes at meetings and in California. We spent the two first summers, Ď38 and Ď39, in Stanford, where, by the way, I also had very close contact with Felix Bloch and the other people in Stanford like Yearian and Rabi who was there. I should have mentioned Rabi and his collaborators, Norman Ramsey and Zacharias, with whom I also had a lot of discussions.
Lubkin:Was this at Columbia or at Stanford?
Weisskopf:Both at the Columbia visits and meetings and in the summer at Stanford, Rabi was with me in Stanford for a summer.
Weiner:Had you been invited there as part of the special program?
Weisskopf:Yes, summer school. I gave a course in the summer school there. I think they have it now still. They invite the summer people out from the East; and, as I say, it happened that Rabi and I were there at the same time and gave courses.
Weiner:Was this then in the same tradition as the Michigan summer schools?
Weisskopf:Yes, itís an imitation of the Michigan pattern. I was also at the Michigan summer school in Ď41 I think.
Weiner:That was about the last one because of the war.
Weisskopf:Thatís right. It was, in fact, the last before the war.
Weiner:Was this a particular American innovation?
Weisskopf:Schwinger was there, too, by the way at that summer school. That is a typical American innovation Ė- yes.
Weiner:The Europeans copied that.
Weisskopf:I should make a distinction because the Europeans introduced another innovation after, the war. The American innovation is the summer school at the university, and Michigan, as you know, was the one that started it with Pauli and Fermi in Ď31 I think.
Weiner:It started as early as Ď27.
Weisskopf:Thatís right, by Goudsmit and Uhlenbeck, and that then was copied by many other schools, particularly in the Middle West and Far West, not so much in the East. They organized symposia or short courses and invited prominent people. Now, what was invented in Europe in the Ď5Oís — and I know who it was; it was Cecile Morette DeWitt, who is now a professor in Chapel Hill. I think she invented the present type of summer school — that is, to go to a very nice place, which is not a university but which is distinguished by its external charms. Cecile Morette, to be specific, organized the first summer school of this kind in 1950 in Les Houches, which is an extremely romantic resort (summer and winter resort) on the foot of Mount Blanc in France, a most beautiful landscape. She organized this with an enormous amount of initiative. It was the last year she was in France before she married and then came to the United States. It was, as she said herself, kind of paying a debt to Europe. She did it against enormous difficulties. It was extremely hard to get money and to transform a little mountain inn with an abandoned barn into a summer school. The first summer I helped her a lot. I was at that time Visiting Professor at the Sorbonne. The first summer was really most romantic because at that time there were some 30-odd students and a few professors, including myself and Rosenfeld and a few others. The students had to peel potatoes between the courses and do the cooking and washing and cleaning and everything. It was a really wonderful spirit of pioneers in the whole thing. Nowadays there are summer schools that are just plush in service and so on. Now, this summer school, as I said, was the first. It still goes on every summer; however, under much more luxurious conditions.
Weiner:No more kibbutz.
Weisskopf:No more kibbutz spirit. Right, ďkibbutzĒ is the word, yes. That one was imitated in Europe. In fact, I think there are now at least something like six or ten summer schools. One is in Corsica, the French one. Every year there is one in Sicily and in Greece and so on. So this really has sort of spread. In this country they also have something of this kind from time to time. This is now the new version of summer school.
Weiner:Do you think in the recent decades this represents an attempt to supply some integrative mechanism back into physics again?
Weisskopf:It is certainly a reaction against the kind of documentation that we have today — I mean these short, incomprehensible letters to the editor that only the expert understands. Therefore, the student, both the young and the more experienced scientist, likes to spend four to six weeks in the summer to be introduced systematically to this field by experts. In Varenna by the way, there is a famous summer school every year — the Enrico Fermi School organized by the Italian Physical Society — under most pleasant circumstances in an old chateau, in an old mansion on the lake. So this has really spread a lot, Cecile Moretteís [Dewitt] idea really corresponded to a need.
Weiner:Iíd like to go back to an earlier period. You mentioned Teller in Washington. What about the Washington conferences that Gamow and Teller had organized?
Weisskopf:They were based on a different idea. The summer schools are things that last four weeks or six weeks where one systematically goes through a field. The Teller-Gamow conferences are rather what one calls now ďtopical conferencesĒ. They chose a subject which was of great interest — stellar development or particles, something like that — and invited something like 50 people for, as far as I remember, only a few days. I think it is a three- or four-day affair, Iím not sure. But certainly not more than a week. These things still do happen, as you know.
Weiner:The topical conferences do happen, yes, but they no longer can be kept quite that small.
Weisskopf:No, they are no longer on this nice, intimate, personal level. Actually, I would say that the Shelter Island tradition probably can be traced to these Teller-Gamow ventures. It was roughly the same idea.
Weiner:I see. Now, those were supported by the Carnegie Institution of Washington. How did individuals travel there? Did you pay your own way if you were invited?
Weisskopf:Sometimes, yes. Sometimes the department paid. These were not the lush days of today where no physicist moves from one city to another without being paid.
Weiner:What Iím getting at here is trying to determine the primary source of support for physics research in all of its aspects — the experimentation, the meetingsÖ
Weisskopf:Again, Iím not really the man to ask this, because particularly at that time I was completely out of touch with the way, for example, the experiments were supported. I couldnít tell you a single thing about who paid for the cyclotron at Rochester. I donít know.
Weiner:Who would be good to ask — not necessarily about Rochester but in general?
Weisskopf:Bacher or, in the East, Rabi or Kusch. In New York you have a lot of these. You must ask experimental physicists who still were active before the war. In fact, I would be very interested. I donít know — there was no AEC at that time and no Science Foundation, so I wonder where they got the money.
Weiner:There is a report that Iíve ordered. Itís a listing of research funds over the years. The NRC put it out.
Weisskopf:Itís very interesting. I wouldnít be surprised if itís the university itself — sort of private funds at Rochester. I donít know really. Now, the traveling was always a great difficulty. I remember in the lean days at Rochester we were allowed, I think, one or two meetings a year, and the rest we had to pay for out of our own pockets. We drove to Washington or took the Greyhound.
Weiner:What was the main result of these meetings? Iíll give you several alternatives. One, of course, would be contacts with your colleagues and where discussions might ensue informally on a number of topics, other than the one being discussed.
Weisskopf:Right. You get stimulated into new problems.
Weiner:But did you feel that these meetings actually advanced the state of knowledge? WEISSNOPE: Oh, definitely. Thereís no question about it.
Weiner:Can you point to any specific instances?
Weisskopf:Particularly my own experiences. You see, there are different people. Certain people work very much by themselves. An example is Julian Schwinger. The other people, like myselfÖI couldnít work by myself. I can only work by talking. I get an idea and I have to talk it over with somebody. And by talking it over I get further ideas of how to go on, not necessarily from the man but simply by telling him I myself get the idea, by having been forced to make it clear to somebody — and very often, of course, by the critical remarks I get back. Now, I think that the majority of people are rather like me and not like Schwinger. This is why I think these meetings are of such great importance where all work in relation to others and not only to your awn work; you go there and find out: ďAha! This might be a problem I could solve. I could contribute.Ē
Weiner:How would you characterize these years? I guess your parents arrived soon after you arrived.
Weisskopf:My mother arrived two years later.
Weiner:And you were then settled. You were very soon a part of the American physics community. What was the outstanding characteristic of this period? How would you describe it in terms of your own development from 1937 to the beginning of the war?
Weisskopf:I donít quite understand your question.
Weiner:Well, you had different stages in your life. Sometimes youíre assimilating knowledge.
Weisskopf:Well, it was a very productive period, an extremely productive period for me. I worked as you know in nuclear physics and in field theory. I learned a lot. I was very close to experimental physics. I had for the first time systematic teaching duties, which broadened my knowledge a lot. It was the first time I really had many graduate students to work with. So I would say the six years from Ď37 to Ď43 certainly were probably some of my most productive years.
Weiner:Did you return to Europe at all during that period?
Weisskopf:No, we couldnít. We had no money.
Lubkin:What about the development of nuclear physics in this period from the time you arrived?
Weisskopf:That was the great development of nuclear physics. It came of age during these years.
Lubkin:Would you say that there was any outstanding experimental evidence developed in this period up to the time of the discovery of fission?
Weisskopf:Which I do not consider such a tremendous, outstanding discovery. I would say one cannot put oneís finger on it, but there was a constant development of accelerators and of detection devices — counters, ionization chambers. In these three years our knowledge of reaction cross sections, angular distributions and nuclear spectroscopy — essentially these two items — simply became enormously larger.
Lubkin:Was neutron spectroscopy at all possible then?
Weisskopf:I think yes. The time-of-flight measurements had started at that time.
Lubkin:Yes, I think Alvarez worked on that. But nuclear emulsions had not yet come intoÖ
Weisskopf:I think so. Iím not good on this. But I believe yes.
Weiner:Then by this time — certainly in Ď36, Ď37 and on — nuclear physics was both a very well-defined field and very much of a thriving field.
Weiner:This is before fission or even in spite of it — before and after.
Weiner:And this took place mostly in this country or at least thatís the impression we get. I donít know how you feel about that. Many Europeans had come here but, of course, there was work going on anyway in this country.
Weisskopf:And in England.
Weiner:Thatís what I wanted to get at. Where else? England and where else in particular?
Weisskopf:Well, which period are you considering?
Weiner:Iím concerned now with Ď37 on.
Weisskopf:Well, you know, donít forget from Ď37 on there were not many years available because from Ď39 the war started. So clearly there was not much going on, even in England, when the war started. Up to then there was a lot of development in England and Italy, in Fermiís school, and Copenhagen. Not in Germany. Also in France — there was the Joliot school. So I would say the European contribution up to the war was quite substantial with the exception of the downfall of Germany and Austria.
Lubkin:At what date would you say Germany no longer did much?
Weisskopf:1933. You see, the expulsion of the Jews started right away. I wouldnít say that all nuclear physicists are Jews, although it is to some extent justified. However, the expulsion of the Jews disrupted the whole scientific life in Germany, especially in nuclear physics. The whole thing just came to a standstill.
Weiner:Just at the point where it could have taken off.
Weiner:Just to backtrack for a while, you were in England in 1933.
Weisskopf:Yes, for half a year.
Weiner:Did you sense any real difference — other than the immediate experience you had had in Germany, knowing the problems there — any difference in the style of doing physics and the types of problems people were concerned with?
Weisskopf:No, I would not say that it was so different from the other centers, like Copenhagen or Zurich, except that of course you noticed a tremendous influence of the Rutherford tradition, which had nothing like it anywhere. At Cambridge, of course, scientific life was fantastic, comparable to Copenhagen because of this overpowering influence of personality — Bohr there and Rutherford here.
Weiner:How about comparisons with pre-Nazi Berlin?
Weisskopf:I am not too able to judge pre-Nazi Berlin because I was just a small fellow at that time but I would say that pre-Nazi Berlin was absolutely first class. I personally worked only with Wigner and then with Schrodinger. So I really did not have the feeling of the whole, but with all the people there — Szilard and Nernst and Planck and Einstein — it must have been a terrific time. I know it only second hand; I wasnít immersed in it. Itís true I lived there for half a year. I think it was a wonderful place, but I didnít take so much advantage of it as I did in Copenhagen and Cambridge.
Weiner:You were aware of the so-called Hungarian circle in Berlin?
Weisskopf:Oh, sure — Wigner, Szilard and so on and Von Neumann. I was rather more aware of the similar situation in Gottingen because there I spent many years as a student with Teller, Born, Franck and Heitler, London, Herzberg. This I really was able to enjoy as a student.
Weiner:And yet you characterized the scientific community that you found yourself in upon coming to this country as a very good one, a very healthy type of environment.
Weisskopf:Yes. It was in many ways different — in some ways better, in some ways worse. It was better because it was a more informal society as is the American society in general. You had easier access to everybody, to anybody. It was perhaps not so dominated by extremely impressive personalities, as in England, Denmark, Gottingen and Berlin.
Weiner:And you were also older and more established by that time.
Weisskopf:Yes, older and established. I was one of the leading people here and over there one of the students.
Weiner:I wanted to get on to something about the Washington meeting, the announcement of fission...
Lubkin:Before you get on to fission, I wanted to ask when did people first start thinking about nonspherical nuclei?
Weisskopf:Very late. Thatís Bohr-Mottelson.
Lubkin:Until then did one feel it had to be a sphere?
Weisskopf:I believe so. There again Telegedi is probably going to quote you a paper of somebody who said it already before. To my knowledge, I donít know. I believe it was Bohr-Mottelson who did it first, but Iím not sure. Wait a minute. That is of course not quite true because the discovery of the quadrupole moment of the deuteron by Rabi in Ď39 was obviously the first proof that the deuteron is not a sphere. I take it back. There were quadrupole moments discovered experimentally in nuclei before Bohr-Mottelson. So that the nuclei were not spherical was experimentally known before — some of them were not.
Lubkin:But there were no theoretical ideas to account for this.
Weisskopf:No special theoretical ideas.
Lubkin:But it was just one of the loopholes.
Weisskopf:Yes, perhaps. One wasnít quite clear where it came from.
Lubkin:Was there any serious talk about the shell model in the pre-war period?
Weisskopf:No, except at a very early date — in the early Ď30ís. There was Elsasser who in fact invented a kind of shell model. And then in 1935 there was a paper by Bethe on the neutron absorption and scattering on the basis of the shell model which I described in great detail in the article with Friedman. Bethe calculated the neutron absorption and scattering on the shell model and got completely wrong results because the compound-nucleus feature wasnít considered. And then on the pressure of the compound nucleus, the shell model then was completely discarded — wrongly. As it later turned out, the two pictures are not mutually exclusive. This again is in my paper with Friedman where it was shown that the two can live together.
Lubkin:I think we are now ready to go on with fission.
Weisskopf:This again is my bad memory. It plays me tricks. There is so much written about fission. I just read the other day the book by Ruth Moore, which by the way is a very interesting and good book. And there is so much written — such as the Smyth report — that I canít give you a personal report. I was involved, as you know, in some of it. I was involved in the exciting days at Princeton when fission was discovered and when the possibility of an atomic bomb was on the horizon. And then we tried to induce the French physicists not to publish, and there was a famous telegram, which, in fact, over my name went to Halban, the collaborator of Joliot, and it didnít work because they already had published. Anyhow this is all in the literature. I personally at that time did not work too much on fission. I was just interested in it because of the exciting possibilities. We all hoped, of course, it would not give rise to an atomic explosion. And my work in fission started actually only when I went to Los Alamos.
Lubkin:How did you choose to go to Los Alamos rather than anywhere else?
Weisskopf:Thatís very simple. I was a relative latecomer. Therefore, I was an enemy alien because I wasnít a citizen yet when the war started. So it was not possible for me to go where most of the physicists went — namely, to the radiation lab, to the radar efforts — although I did work to some extent by certain arrangements on semiconductor problems during the early war years and in conjunction with Purdue University. Lark-Horowitz, who was a good friend of mine, brought me into some solid state physics, very interesting semiconductor problems. I still have an interest in solid state, dating from this time. So I was excluded from the radar business. I was always in touch with my good friends like Fermi. By the way, Fermi came later, but Fermi should be listed among those I visited very often in New York. Through Fermi and Bethe and so on, I sort of knew what was going on about fission and knew probably more than an ďenemy alienĒ should know. Then when Oppenheimer gathered all those nuclear physicists — in the summer of Ď42 — in Berkeley to start discussing it, I couldnít go there. Bethe went there. Bethe was already a citizen. However, at that time, they had sort of discussed the whole fission question, and then Los Alamos started and Oppenheimer tried to collect people. At that time, the government was much more reasonable towards enemy aliens than later on. He was allowed to ask certain enemy aliens to work with him before they became citizens. That was done on a mutual guarantee system. If two or three people guaranteed for him, then it was done. Let me say it this way: in the year Ď42 - Ď43 most of my friends went into war work, either in the radiation lab here in Cambridge, Massachusetts, or to Berkeley on these fission things. I was one of the few who were left at the universities, and I had a big teaching load. I taught graduate courses both at Rochester and at Ithaca. I commuted between Rochester and Ithaca. And so I and a few others were the last ones left, so to speak. Then early in the winter of Ď43 I got a telegram: ďCome to Santa Fe.Ē It was very strange. I first got a visit from one of the people — I cannot tell you who — working with Oppenheimer asking me to join and telling me that everything will be arranged, that ďyou will be going to Santa Fe, but this is highly secret and you must not tell anybody about it, where it is and what it is.Ē But I knew what it was. Then the F.B.I. got interested in me evidently and wanted to clear me, and I remember an F.B.I. agentÖIn fact it was even different; I was asked to come to Santa Fe to a meeting. It was January or February of l943. And this was sort of a gathering of people, a number of people, where they were told what the plan is and what the whole present state of knowledge is, that an explosion is possible and what the critical mass is and so on. We were sort of given a primer course of a week in the stuff. And we were told this is all extremely secret and we must tell nobody even where it is. And I remember at that time an F.B.I. man, who was investigating me naturally, came to my wife, who was at Rochester when I was there, and asking her, ďWhere is your husband?Ē And she said, ďI cannot tell you.Ē And he showed his F.B.I. papers. He said, ďYou have to tell me. This is treason if you donít tell me.Ē And she said, ďNo, I cannot tell you.Ē And he got very rude with her, but she didnít tell him. It was so funny because we were told how dangerous it is even to mention the word Santa Fe to anybody. So, I came home then after all this meeting and I went on with my courses, and every week I got a long distance call from Santa Fe and everybody would be shouting, ďProfessor Weisskopf, a call from Santa Fe!Ē So there were a few very funny episodes. Anyhow, it was decided that I was supposed to go to Los Alamos, and I went there something like April or May, again to very pioneer-like conditions. Houses were not yet built. The whole thing was a big mess. But it was very interesting and romantic. My wife and my children, who were very small at that time — one year and three years — came then in July when the first houses were constructed. We were really among the first settlers. We felt like Mayflower immigrants. The first settlers of Los Alamos looked down on those who came later when things were easier. Another funny thing. At that time I was, again, an enemy alien with a special dispensation, and therefore I was under the War Department or something like that and had this special clearance. Then I got citizenship at the earliest possible date, which was in the fall of Ď43. In fact, I had to go to Rochester to get my citizenship. Then when I came back to Los Alamos I was from then on under the Justice Department. Therefore, my clearance was revoked. So as soon as I became a citizen my temporary clearance as a foreigner was no longer valid. So I had to stay three weeks off from the laboratory until I was recleared and then could enter the area. This was one of these many, many things that are so silly. I donít want to go into detail on Los Alamos, which of course is a chapter one could write a book about.
Weiner:There have been many books written about it, but the point is: does any of them capture the whole atmosphere?
Weisskopf:No, and Iíve read them all.
Weiner:I get saturated with this stuff.
Weisskopf:Itís absolutely terrible, because none of them really captures this incredible atmosphere in Los Alamos. First of all, the international angle; and then this whole magic mountain atmosphere, of there being an elite society secluded from the rest. We always had the feeling that the fence didnít keep us from the others but kept the others from us. The whole social life, the cultural life, the excitement of the work, the incredible tension with the work; and, in particular, the story of the test in Alamagordo is really worthy of a good writer. I would love to sit down with a good writer and sort of tell him and he should write it. Itís a tremendous thing.
Weiner:What about the William Laurence account? Did that capture some of the flavor?
Weiner:It was a journalistic type of account.
Weisskopf:No. This is off the record, but he is such a dope, and he always thinks of himself when he writes things. Itís terrible. I donít like this account at all. It doesnít give the atmosphere at all.
Weiner:This is interesting because when you tell a person whoís not really aware of what weíre doing about this project, they immediately think of it as of the least interest because it has been done.
Weisskopf:But, in a different way, I think youíre right. In the long run in the history of science the Los Alamos effort is not interesting at all. Itís a sideline. Technologically it might be interesting. But I would say there is here more of a human-interest story — for a sociologist, not for a historian of science.
Weiner:Again, a historian of science can and should be a bit of both.
Weisskopf:Should be a bit of both. And I would say that Los Alamos is not documented. Let me say it this way.
Weiner:Well, come to think of it, I remember some interviews Iíve done with Feynman and with others, and a great deal has emerged from that, of relationships and the atmosphere and walks into the mountains — this sort of thing. It reminds one of a Utopian community. There are a lot in American history, where people seclude themselves as an elite group into the mountains.
Weisskopf:As I mentioned to you last time, it was for me especially a tremendous experience because I was the mayor of that city for two years and I had an enormous amount of personal, human, sociological experience. I would say the Alamagordo test would make a fantastic subject for a good writer.
Weiner:Letís agree that weíll talk about that some other time. Iíd like to talk about it.
Lubkin:But how did nuclear physics alter during the war?
Weisskopf:In many ways. First of all, probably the most important way in the long run was the development of instrumentation. It was necessary to develop nuclear detection equipment of much better quality in order to get these neutron measurements for the bomb. And the development of that detection equipment has completely changed the experimentation. Iíll give you an example. Before the war, the theory of neutron evaporation was a very nice theory but it was very hard to check it because there were practically no methods to measure the intermediate-energy neutron spectra. Now, because of the development of the bomb, the spectroscopy was necessary to measure them; it was developed. And afterwards — in fact, during the war even — those neutron spectra were measured and were shown to agree pretty well with the evaporation theory. This is only one example of many: the tremendous development of technical detection equipment. As far as ideas are concerned, clearly the main development was just in that field of the theory of nuclear reaction of heavy elements. See, the main nuclear physics problems in atomic bomb building — I donít mean technical problems — were the theory of fission itself; and then, almost more important, the reaction of materials to the bombardment of neutrons because in order to construct a bomb, you have to know what the neutrons do, to keep the neutrons in and so on. It means inelastic scattering. Now, this is all based, since they are mostly heavy elements one has to deal with, on the compound-nucleus theory. This is why it was a heyday for a man like me. It was the field where all these things can be applied. So this kind of physics was developed very much then. I would say that, to speak for myself, that many of the ideas that we developed later or shortly after the war — with Feshbach, Peaslee, Weisskopf, all the theory of nuclear reactions — were in fact started in Los Alamos. Did I tell the story of the cross section of nuclei?
Weisskopf:So this was a typical problem.
Lubkin:It sounds in a way like you as a theorist then benefited more than anybody else from Los Alamos.
Weisskopf:No, but I would say that I have certainly profited very much. Many others profited, too.
Lubkin:Many of them seem to have gotten involved in engineering problems then.
Weisskopf:Right. You see, I was less involved — youíre quite right. Most of the theoretical physicists were involved in fields in which they did not work before — the theory of explosions, which I was also involved with but not so much; the theory of matter at very high temperatures and this kind of thing; whereas I was in a way fortunate or not (I donít know) I was mainly the expert of nuclear reactions and cross sections. I donít know whether you know that my office was called ďthe cave of the hot winds.Ē It was actually written there. People came to me and asked me for cross sections. You know, hot air, I just gave them my estimates, and they usually turned out to be right.
Lubkin:You were called the oracle also.
Weisskopf:In a sense.
Weiner:Who else in your circle was concerned with the same questions?
Weisskopf:Bethe certainly. In fact, Bethe was the director of the theoretical division, and I was deputy director of the theoretical division. There were Serber, Ulam (heís now a mathematician; he worked very closely with me), Wayne Bowers (heís now at Duke University). Of course Teller was there — but you probably know that there was a certain conflict between Teller and Bethe — Oppenheimer. There was Bacher. And then Niels Bohr and his son. Bohr was there continuously and particularly his son Aage Bohr, was there continuously. I think I mentioned the most important people.
Weiner:Letís just for a minute get into this mayoralty. For example, what motivated you to run for mayor?
Weisskopf:I think they asked me. It was not my initiative. It was the initiative of a group of people.
Weiner:Did you have opposition?
Weisskopf:Not much — I mean not openly, you know.
Lubkin:You mean you were elected by acclamation?
Weisskopf:No, there was a vote. Most of the people voted for me, but there was a vote.
Weiner:Was this a political office or just an unofficial type of assignment?
Weisskopf:It was unofficial in some respects. The community was of course outside the legal order. Legally it was under a military administration. In fact, the military themselves — General Groves and his people who were there — actually urged the civilian population to have a representation. Iím not sure whether the initiative came from them or from us — probably from us first. There were constant frictions between the civilians and the military as to administration, as to services. We agreed that the best thing would be to have the civilians represented by a mayor and an assembly with deputies. I wasnít the first mayor. The first was Bob Wilson of Cornell. I was the second one after him.
Weiner:What year was that?
Weisskopf:It must have been Ď44ĖĎ45.
Weiner:What sort of questions were you concerned with?
Weisskopf:Everything: prices at the PX, rents, service (oh, I could tell you stories for hours), supplies of Indian house help, everything had to be rationed, fights with the military about paving the roads, building sidewalks, schools, hospitals, prostitution, everything.
Weiner:Typical urban problems.
Weisskopf:Typical urban problems, yes. I remember one story. (Well, thatís off the record.) At one of the town council sessions two topics were discussed. One was that the ceiling prices in the PX were not kept and the other was prostitution. And then one fellow who was especially upset about the ceiling prices said he had nothing against prostitution as long as the ceiling prices were kept.
Weiner:During this period there must have been some thought when success — if youíll pardon the use of that word — was in sight about returning to research and to new pursuits perhaps.
Weisskopf:Oh, very much so. In fact, it was even organized. Before the final experiment, nobody thought of anything else. In fact, people didnít think enough about other consequences of the bomb. But after the experiment, after the summer of 1945 (after all, Los Alamos still went on) there were systematic courses given for the transition to ordinary life. People had forgotten physics. There was sort of a University of Los Alamos, where I gave a course in nuclear physics, and other people gave other courses. It was a self-retraining effort of quite interesting dimensions.
Weisskopf:Not only. It was official. It was part of our duty.
Lubkin:What kind of transition period was there for most people before they returned?
Weisskopf:Itís hard to say. I think most people stayed until the beginning of Ď46 simply because there was a lot to do there, mostly writing up. Before that nobody ever wrote anything about what they did. In fact, it is even now not enough documented. The whole documentation of Los Alamos leaves a lot to be desired because people ran away afterwards. But most of the activities after the bomb experiment were concerned with documentation. And people went away from Christmas Ď45 on. Some left right away. I left relatively early. Others stayed longer.
Weiner:During this period, people from different universities were in contact and new relationships were made. Was there a lot of jockeying going on?
Weisskopf:Tremendous. From the fall of 1945 on, Los Alamos was sort of a playground of all heads of departments to get the new, bright, promising younger people. People got promoted from assistant professor to full professor in one week by having better jobs, one after the other, offered to them.
Weiner:What institutions emerged as the main contenders and why?
Weisskopf:M.I.T. and Cornell got a lionís share. They worked very systematically on it, because they had practically no nuclear physics before the war. M.I.T. mainly through the initiative of Zacharias, built up a big nuclear physics group purely from Los Alamos.
Weiner:Was Zacharias there?
Weisskopf:Yes. Many other places got people too. Chicago got a big bunch and Berkeley. The slower universities like Harvard did not get enough people from there.
Weiner:And then there were occasionally people picked up at other places. Cornell picked up Feynman. But youíre talking numbers now.
Weisskopf:Evidently, if a big shot from a university was at Los Alamos, that university gained a lot. I mean Bethe clearly took most of his friends along — younger collaborators along, like Feynman, Bob Wilson. The whole bunch that now works with Bob Wilson. These are now of course all full professors and older people, but they were all taken from Los Alamos. So Cornell, M.I.T., Chicago because of Fermi — the big shots who were there got the lionís share.
Weiner:How about Berkeley? Who was their chief representative?
Weisskopf:Oppenheimer. And Alvarez.
Lubkin:What brought you to M.I.T.?
Weisskopf:Look, I was a little disappointed with Rochester, I think primarily because of the financial treatment I got there, and to some extent also because of the small-town aspect. I always wanted to get into this area.
Weiner:Did you feel isolated there, even with these contacts?
Weisskopf:Not so much in physics as in other matters — the general cultural atmosphere.
Weiner:And being with the kind of people you enjoyed.
Weisskopf:Yes. I must say right from the beginning I said, ďI want to get into the Cambridge area,Ē and I took the first opportunity.
Weiner:What was on the agenda of people there? They knew they wanted to go back and do some nuclear physics, but what was it they were doing? Were they just picking up the old threads?
Weisskopf:Yes. Well, it was obvious that the old threads were extremely profitable because, as I said before, there were hundreds, of new possibilities because of the tremendous advances of the techniques that were just waiting to be used. That is, in experimental physics. In theoretical physics I would say most of us, including myself, went into other fields — not that we gave up nuclear physics. In fact I already made a plan in Los Alamos to write a book on nuclear physics because I had the feeling that since we had gathered so much more knowledge, now is the time to write a book about it. But still when I came back I first postponed the book project and jumped into electrodynamics.
Weiner:This was true of other peopleÖ
Weisskopf:Many people. Yes. The Shelter Island conferences are perhaps an effect of this.
Weiner:You also mean the Shelter Island, PoconoÖ? A whole seriesÖ
Weisskopf:Yes. They were an effect of this. Here we wanted to face the outstanding problems. Most of us went into this.
Weiner:Was it apparent at Los Alamos that there would be greater government support of physics research?
Weiner:And did you have in mind then the large accelerators?
Weiner:Was this race for high energies already apparent then?
Weiner:It was apparent, of course, in the Ď30ís. But it seemed that after the war there was pent-up demand.
Weisskopf:It was evident that physics was going to enter a new phase with tremendous support and popular demand. That was quite clear.
Weiner:That gets us to M.I.T. Then thereís a whole series of questions.
Lubkin:In 1947 you apparently returned to nuclear physicsÖ
Weisskopf:In 1946 actually.
Lubkin:Yes, the Feshbach, Peaslee and Weisskopf appears in Ď47. In the paper you donít consider non-zero nuclear spins. Is that right?
Weisskopf:That was simply a simplification. We wanted first to get the principles. However, we discussed in the paper, that the spin cannot make any fundamental differences. So that was a kind of simplified version of the theory.
Lubkin:So, in other words, you were aware of your limitations.
Lubkin:In the errata to that paper you correct yourself for the case of indium.
Weisskopf:You know, I donít even remember anymoreÖ
Lubkin:Itís probably a minor point.
Lubkin:Then in Ď49 itís the paper of Weisskopf and Feshbach. Is this an outgrowth, would you say, of the boundary-condition models two years earlier?
Weisskopf:No. You see, the boundary condition model was really the compound nucleus. That was the Feshbach-Peaslee. And it was also incorporated in the book. Which one are you asking me now?
Lubkin:Feshbach and Weisskopf, Ď49.
Weisskopf:The schematic theory of nuclear cross sections. That is a new thing. That is the cloudy crystal ball, which was new, which was in factÖNow wait a minute, [consulting papers]. Iím wrong. Thatís the boundary-condition model. That is not the cloudy crystal ball. This is really the boundary-condition model.
Lubkin:Is this the first time that the transport cross sections are introduced?
Weisskopf:No, the transport cross section is a very well-known concept which goes back to Maxwell, not in work with nuclei of course.
Weisskopf:No, it is an old concept. It is used mostly in scattering problems. Actually that is probably one of the concepts which became popular in Los Alamos. It is a concept like diffusion length. Itís a very basic concept which has been used in physics since Maxwell. This paper is still based on the extreme compound nucleus assumption.
Lubkin:Now, in 1950, the magic numbers started coming in.
Weisskopf:Right. In fact, a little later Ė- Ď51.
Lubkin:Well, the evidence is presented otherwise.
Weisskopf:Yes, 1950. You asked a question about the book. [Questions prepared by interviewer and previously shown to Weisskopf.] The book was, as I said, published at the wrong time. This is true. The book is written essentially on the basis of the compound-nucleus idea, although there is a chapter on the magic numbers on the shell model. However, in the book the shell model is not explained. In the book the shell model is still introduced as a model that works for reasons unknown to us. No reasons given. Now, the first time (I donít want to say that Iím the first one to give reasons, although I think Iím one of the first) but the first paper in which Iíve given these reasons wasÖI think, have you got ďThe Shell Model in Nuclear StructureĒ in Physica? I wonder if thisÖ
Lubkin:No. Is this before the Science paper in Ď51?
Weisskopf:No, thatís Physica. [consulting papers]
Weiner:What paper are you referring to now in the bibliography? Is there a number assigned to it?
Lubkin:Physica 18, 1083 (1952).
Weisskopf:This is í52. It has no number. I must say Iím not quite sure myself what the order of events was there. First must be the book in which Feshbach and I considered the cloudy crystal ball model, the optical model. The first optical-model calculations here were published February, l953, as an explanation of the Barschall curves (Iím speaking of a letter now). And the bigger paper came a little later.
Lubkin:So it would be Physical Review, 96.
Weisskopf:However, I find here an article which I mentioned before in Physica. That is probably a talk I gave at some meeting, which also has it, but essentially itís the same thing. In other word, it was already published in December of Ď52. But then I have here a thing which I think is important in Ď51 — Science: ďNuclear Models.Ē This, to my mind, is a very important paper. This is January of Ď51. Iím not sure whether Iím actually the first, but at least I try here to give an explanation of the shell model. This is not in the book.
Lubkin:Right. Why is it that you didnít put it in the book?
Weisskopf:Because the book was already out.
Lubkin:Oh, yes? The publication date is Ď52. You must have already turned the manuscript in.
Weisskopf:You know, one doesnít like to put too many undigested results in a book. How do you know it will be right a year afterward? The publication date of the book is 1952, but it is a little bit of a swindle. The publication date was really Ď51. You know, they always do that.
Lubkin:The introduction probably has the date you did it.
Weisskopf:It says here June, Ď52. [Looking through book] I say here, ďIt is assumed that the neutrons move under the influence of a common potential and that the interaction between the neutrons can be treated as small perturbation. This model was discussed in Chapter 7 and it was pointed out that its validity is very questionable. It is therefore very surprising and so far unexplained that this model can be used successfully.Ē So I have no explanation. But this is earlier and this is simply because I donít like to write undigested ideas.
Lubkin:But you certainly had become convinced that the spin-orbit coupling was important.
Weisskopf:Oh, absolutely. The spin-orbit coupling is in here, but it isnít explained. The ďScienceĒ paper is the beginning of an explanation. And out of this, then, came the optical model in Ď52 and Ď53 and then the unification, to show that the optical model and the compound nucleus is not a contradiction. This is the Friedman paper which came out in Ď55.
Lubkin:What about the papers with Feshbach and PorterÖ?
Weisskopf:This is the paper about the ďcrystal ballĒ model.
Weiner:You say one came out of another. By what process?
Weisskopf:Itís a natural process. First, you want to explain the shell model because itís here. So I came to these ideas of the optical model and saw that the Pauli principle was the reason for it. And then you tell yourself, ďItís absolutely impossible that all the work I did for 20 years can be wrong. After all, the experimental evidence for the compound nucleus is pretty much there. These two things seem to contradict each other. Which is right?Ē And therefore we discussed for days and weeks and months on end with everybody and his brotherÖ
Weiner:At M.I.T. you mean.
Weisskopf:At M.I.T. and also with Bethe, And then finally we could show of course in a very vague and more or less mathematical way that these two things are not really contradictory. And that is this Feshbach-Porter paper, which contains one essential recognition. It is not so much the compatibility of the two models. Thatís in it but that wasnít yet clear. The main point in the Feshbach-Porter paper is the distinction between coarse structure and fine structure, and the discovery of coarse structure resonances. I would say that this is probably one of the most important discoveries in which I was involved, That is, if you take the coarse results — the average cross sections at zero energy — and plot them against the radius of the nucleus, against the size of the nucleus, you get these resonances. And we explained these resonances: they occur just when the size of the nucleus becomes such that a standing wave develops inside, then there is a resonance. I must say that was one of the nicest discoveries, the one that gave the greatest pleasure to me, because here was a phenomenon that could be predicted. We could predict there would be resonance at this and that, and then Barschall and others made the observations and found this resonance. So that was of course very pleasing. Hereís a resonance predicted by us and found in the experiments.
Weiner:When did you feel that you had it?
Weisskopf:I donít know. Well, at that time.
Weiner:What Iím trying to get at is the time it took.
Weisskopf:To find such a thing? Oh, probably half a year.
Weiner:You worked on it continuously during that time.
Lubkin:What made the idea of the complex potential?
Weisskopf:Thatís a pleasant idea. Yes. That has something to do with our previous theory. Let me perhaps try to explain this. In our old way, Feshbach-Peaslee, that was in the book, we tried to describe, to formalize sort of geometrically, the Bohr theory of the compound nucleus by assuming that the nucleus is black, the nucleus fully absorbing. That means that any wave that comes into the nucleus doesnít come out anymore. Itís absorbed. That was the basis of our mathematical development of boundary conditions. The boundary conditions just express this physical fact. That was the basis of the whole development of Blatt and Weisskopf. Then it was soon recognized, also by comparing it with the previous papers by Bethe and by discussion with Bethe, that this means nothing else but an assumption that the nucleus has a very strong absorption coefficient, that it is black. ÖThen of course came the shell model. Now, the shell model is an obvious contradiction to blackness because there the particles move back and forth and are seemingly not absorbed. So the next jump is very easy — itís always easy afterwards. But it was sort of clear that one would then think, ďWell, letís take an intermediate situation where we say the nucleus is not black but gray. Maybe that works.Ē First we said, ďPerhaps the nucleus is gray, letís calculate it.Ē Feshbach is very good in mathematical calculations of problems like this. So ďLetís just go to the end and say the nucleus is grayĒ and then all these things came out, you know — these resonances that I was talking about. And we were very glad. It was obviously a very good theory. I said that the compatibility of the two theories came up later. In the paper itís only sort of vaguely contained. Later on we were thinking, ďMaybe that grayness is already enough to save the compound nucleus.Ē And that was Weisskopf and Friedman. And if you then go on and develop it with the gray nucleus, you can get both features — namely, the resonance features and in the long run (in terms of the time scale of the nucleus), if one waits long enough, you get real absorption and all the features of the compound nucleus. And so we were able to put these two pictures together.
Weiner:This sequence of events occupied how many years?
Weisskopf:I would say that was one of my most systematically productive periods. That is from Ď49 to Ď55. In Ď49 we first formulated that black nucleus theory. That is this paper with Feshbach, ďA Model for the Nucleus.Ē And then all through to the optical model, the gray nucleus and then the unification of the two.
Lubkin:Meanwhile, you were collaborating with experimenters, having them check outÖ?
Weisskopf:Yes, mainly with Barschall and with everybody who did these neutron experiments.
Weiner:How was the work received? You got the pleasure out of it. You knew what you were doing.
Weisskopf:Oh, it was very well received.
Weiner:It caused quite a stir.
Weisskopf:It causes a stir also because it was so simple. It is logically and mathematically very simple.
Lubkin:You mentioned a paper by Bethe.
Weisskopf:Yes, thatís very old.
Lubkin:Is this a Ď35 paper?
Weisskopf:No. Actually I meant a paper of Ď40. Let me get the bibliography of the Bethe book and I can tell you exactly.
Lubkin:In his interview he mentioned that in Ď35 he tried to explain the Fermi resonances and postulated the existence ofÖ
Weisskopf:Thatís a very important paper. Thatís the paper where I said that he was using the shell model essentially, and he got the wrong results.
Lubkin:But they later turned out to be right?
Weisskopf:Well, that was completely cleared up by these papers of ours. The resonances which he got there are in fact not the actual resonances but the coarse resonances, the resonances of the average cross sections. This is it, ďContinuum Theory of Compound Nucleus,Ē Bethe, Physical Review 57, 1125, (1940). Here he has something which is actually rather similar with Feshbach and Weisskopf, as we found out afterwards.
Lubkin:Then would you say that the crucial experiment in the postwar period through 1955 was the finding of the resonances?
Lubkin:The Barschall resonances.
Weisskopf:I wouldnít say only the resonances.
Lubkin:What other important experiments were being done that youíd have to explain?
Weisskopf:Almost anything. You see, the whole systematics of nuclear reactions, particularly at that time in what one called the direct reactions, the limitation of the compound nucleus model came up. They found out that many reactions do not comply with the compound-nucleus predictions and you have to introduce what one calls direct reactions. Have you read that survey article I wrote on nuclear physics? I think it appeared in Physics Today.
Weisskopf:There I speak about this. The Barschall measurements were very important and also these resonances at low energies I was talking about. Who did these measurements? Seidel, Hughes, Palevsky. It was the school at Brookhaven: Harvey, Sailor, Hughes, who died.
Lubkin:The barn-book man. Was that done at Brookhaven?
Weisskopf:Brookhaven, Hughes and his collaborators. Yes, I was very much in collaboration with Hughes. I saw a lot of Hughes. I made lots of visits to Brookhaven at that time. I spent half my life in Brookhaven. They loved this because it was the theory of their measurements. I was an extremely well received guest at Brookhaven at that time.
Lubkin:In what years would you say this was?
Weisskopf:Oh, Ď53, Ď54, Ď55.
Weiner:You were their pet theoretician?
Weisskopf:Yes. I spent one summer there. I had many, many visits.
Lubkin:At the same time that you were developing the cloudy crystal ball, the Bohr-Mottelson work was being done. What was the impetus for their theory and why did that develop at the same time?
Weisskopf:This is sort of a different thing. We were mostly worrying about cross sections and nuclear reactions. They were worrying about spectroscopy, which was developed at the same time from the experimental point of view. In spectroscopy, up to Bohr-Mottelson, one could more or less explain at least the levels of very simple nuclei, up to oxygen and fluorine, but not the heavy nuclei. That was just a mess. In fact, one said usually, ďWell, this is just so many levels and they donít mean anything.Ē And they simply tackled this problem and said, ďThere should be something one can say.Ē In particular, they were most interested in that problem of the rotation: can one speak of rotation in the nucleus? What does it mean? And they finally, I must say to everybodyís and probably their own surprise, found you do find rotational spectra in nuclei. It was known that there are some regularities but nobody really paid much attention to them, but they really took them seriously.
Lubkin:And as a result of the Bohr-Mottelson theory, then people started looking for more rotations?
Weisskopf:Yes, of course. This has a tremendous influence on the experimental technique afterward.
Lubkin:And how did you feel about the Bohr-Mottelson theory?
Weisskopf:This was great. In fact, I had the feeling that itís too bad one hasnít got enough time. I could also have discovered it if Iíd worried about it. You have the feeling this is wonderful. In the first place, Aage Bohr is a very close friend of mine and we are extremely close in method — in other words, not very mathematical but with more insight and a simple way of putting things. Both Mottelson and Bohr were of this character. It was one of these theories that I loved from the beginning, as you feel when you see a child of a friend of yours that it could be your child. It has all the characteristics. I find it a really wonderful discovery. It is far from a thing like Einsteinís but itís just beautiful.
Lubkin:They did much of that work in Denmark?
Lubkin:But you collaborated earlier with Aage BohrÖ
Weisskopf:In a completely different problem. It has nothing to do with that.
Lubkin:You became friends from the very beginning.
Weisskopf:First of all, through my friendship with his father, and then through Los Alamos.
Lubkin:Was he by then a Ph.D.?
Weisskopf:Well, maybeÖFirst, I knew him very well. He came for a visit, I think to California and spent some days here in Cambridge. You know how it is. He said, ďWhat are you worrying about these days?Ē He told me a little about these questions of nuclear magnetic moments, and we got to discussing it. And then suddenly we said, ďLook, the explanation of this effect might be this.Ē And we sat down and calculated a little and found it works and then we wrote a paper together.
Lubkin:Do you think you would have continued collaborating if he had remainedÖ?
Weisskopf:Yes, probably if he had been here. Here it is: Ď49 — ďThe Influence of Nuclear Structure on Hyper-Fine Structure.Ē Itís a very nice idea, which I think I would give him more than 50% credit for. Iím sure if he had been here we would have collaborated.
Lubkin:Did you take up the collective-model ideas at all in your own work?
Weisskopf:Well, yes, to some extent the work with Friedman is on the collective model. But do you mean especially the rotational?
Weisskopf:No, I have never worked in this field. I canít tell. I donít think I really wrote a paper on it, but I studied it a lot and I gave talks on it and so on. I donít think I wrote a paper.
Weiner:Do you have something, Gloria, before we jump ahead on nuclear spectroscopy?
Lubkin:This is a rather general question. How would you trace the influence of nuclear spectroscopy on your own ideas over the years? You seem to have concentrated most on nuclear reactions.
Weisskopf:Yes, but you see the two canít be separated. Take an example. As you know, in nuclear reactions, the concept of level density plays an important role. Now, how can you talk about level density without using spectroscopy? Itís different sides of the same coin. You canít separate these things. But it is true that Iíve never been actively busy with finer analysis of the spectra, maybe because of my general laziness. I donít like detailed work, detailed specialized work, and for spectroscopy you need this. Every single level has its significance. Iím more for the great sweep. If you wanted an explanation like this, it may be why I never worked in detailed spectroscopy.
Weiner:Do you find thereís been a group of people consistently working in spectroscopy regardless of whether they worked in nuclear physics or not?
Weisskopf:Yes. This detailed work in spectroscopy is sort of a character. Itís a disease, though a very useful disease. But the people who do it have typical character properties. You find them all in molecular, atomic, nuclear and now even high-energy — you know, in particle spectroscopy — the same kind of pedantic detail people.
Lubkin:Are you speaking now of theorists or experimentalists?
Weiner:Is this a question of a group dominated by an older generation or has there been new blood coming in?
Weisskopf:Absolutely. Itís just a certain attitude, which is very important because science consists of details. If no one were interested in details, the whole thing would be wishy-washy. Itís a very important attitude, even if I donít share it. This is not a value judgment — on the contrary. If there is something I canít do, I have usually even a higher opinion of it because I am unable to do it. Then it must be very difficult.
Lubkin:But your own work has been influenced by the nuclear reactions more.
Weisskopf:But even there I work, for example, less on direct interactions than on statistics of interactions — for the same reasons. Direct interactions are still too detailed. But that is a shortcoming of my work. For example, you asked me about spin-zero. I didnít consider those spins because it was too much detailed work but other people did it like Hauser and Feshbach. They did the extension of the theory to detailed cases of different spins. It was sort of clear how to do it but I never did it myself because I was only interested in the general sweep of the field.
Lubkin:On nuclear physics development in general, can one trace how nuclear reaction evidence led to new concepts of the nucleus?
Weisskopf:You begin with Fermi and the slow neutrons and that the idea of the compound nucleus essentially came from there. Then you have the overwhelming influence of the compound-nucleus idea during the war because the war problems dealt with heavy nuclei where it was mostly valid. Then came the discovery of the shell model which showed that the compound- nucleus model isnít always right. In fact, there are many reactions that are direct reactions. And then came the synthesis. So thatís really the history.
Lubkin:But meanwhile what was nuclear spectroscopy showing?
Weisskopf:Nuclear spectroscopy had not too much bearing on the question of nuclear reactions except as a tool for analysis of scatterings.
Lubkin:Right. But how did the information gained from nuclear spectroscopy improve the understanding of what the nucleus was in general?
Weisskopf:Well, Bohr-Mottelson is an example.
Lubkin:But is there anything in the earlier period?
Weisskopf:Sure. The whole development of the shell model. I mean the shell model couldnít have been discovered without nuclear spectroscopy because all the evidence of the shell model comes from nuclear spectroscopy. What does the shell model tell you? The shell model tells you what the levels are in the different nuclei. Thatís spectroscopy.
Lubkin:In the Ď30ís there was nothing?
Weisskopf:In the Ď30ís there was also not much spectroscopy yet.
Weiner:I have sort of an earlier question which takes us back to the period after the war where you were beginning to develop the basis for branching off of high-energy physics. What contributed to this? What were the symptoms that a new branch of physics was developing?
Weisskopf:First of all, the experiments — the discovery of the mesons — that happened just shortly after the war. First it was the problem of the two mesons, and then the meson resonances by Fermi and then the discovery of the strange mesons. Hereís a new field. You have to study it. You have to understand it.
Weiner:Did this appear to be a very exciting thing?
Weiner:More so than the other developments that were going on.
Weisskopf:More so than the nuclear physics. Thatís a matter of character. For some people yes, for some people no. Some people feel that you always have to go to the extreme spearhead of the development of physics. They go to particle physics. Other people feel that this is too much of a quicksand, you know, and letís wait a little until more knowledge is there and let us till the ground where there are more results to be expected. These people stay in nuclear structure. This is a matter of character. I did both because I donít like to specialize. You didnít hear my talk at Yale?
Weiner:No, do you have a copy of that?
Weisskopf:Iíll send you a copy.
Weiner:You told me about the mesons and the feeling about it, but, for example, at the Shelter Island conferences and Pocono — and the other one on the HudsonÖ— did all this peter out beforeÖ?
Weisskopf:When you look at theoretical physics, letís make three divisions. One is nuclear structure, the kind of thing we talked of here; the other is meson physics, and the third is field theory — essentially electrodynamics, but not exclusively electrodynamics. Now, these are really almost three different subjects. Shelter Island was to a great extent, but not exclusively, devoted to the developments of electrodynamics and Lamb shift and new calculations dealing with infinities. And the other side is the digestion of the new experimental results, and in particular this problem of the two mesons, which in fact first was a problem of the strange behavior of pions — that they are absorbed but not emitted, all these contradictions which then could be explained by the discovery that there are pions and heavy electrons, and the pions decay into heavy electrons. This is sort of almost phenomenological theory. Thatís the kind of thing that, for example, Gell-Mann has done. Well, that was before Gell-Mann, but a statement like: we can explain the phenomenon by two kinds of mesons is like the introduction of the strange quantum number. This is, so to speak, an intelligent reaction to the newest experiments which should be distinguished from work like Schwingerís and Feynmanís and many others, including myself at the time, to the problems of electrodynamics and field theory, which is not, so to speak, the intelligent reaction to the newest discoveries but it is rather an attempt to improve and reformulate the foundations of our understandings of interactions with certain applications like the Lamb shift and refinements.
Weiner:I see. This is a good way to put it. What does happen, though — and itís an interesting coincidence in time — is that there are three conferences, Ď47, Ď48 and Ď49.
Weisskopf:At Shelter Island, and other places.
Weiner:Yes. What Oppenheimer has called ďthe small intimate conferences after the war.Ē And then in 1950 is the first of the Rochester conferences. And you donít see these small intimate groups anymore. You see physics practically in a new stage, at least on the surface.
Weisskopf:Then this went completely into that field of intelligent interpretation of new discoveries.
Weiner:And the fashion in a sense shifted to this, too?
Weisskopf:Well, the fashions shifted into thisÖNot exclusively. Exclusively is exaggerated. Even now those conferences at Rochester have a few sessions of fundamental field theory, too. It is not so much fashion, but rather that not much more could be done. The field is stuck since the new electrodynamics.
Weiner:This raises a question of your time. There are a lot of important things I want to ask. One is some questions to fill in on the electrodynamics. You talked about the earlier stages of it in the Kuhn interview, but I want to talk about the post-war stuff. And then the other is to ask something about the subsequent development of high-energy physics, although we agreed not to get into it — its interactions with nuclear structure and whether, in fact, ideas from one have crossed over and how people from one field come in or leave one field to go to the other. And then the final thing on CERN.
Weisskopf:Well, maybe we better postpone the two second points. We could also interrupt here, and have those three things later because we wonít cover it anyhow.
Weiner:Is there any one of the three that we could start on? You know whether they are long or short answers. The quantum electrodynamics Iíd rather see you develop at length. I donít want to have to compromise on that. The CERN thing I think also is a unique story.
Weisskopf:Yes, that is a unique story. The last was this business ofÖ?
Weiner:Of once high-energy physics develops as a defined area of inquiry, whether people who work in nuclear structure cross over? Maybe we should pursue this.
Weisskopf:I would say there that itís a big problem. First, there are a lot of common problems both ways. I was always emphasizing this: that high-energy physics can help nuclear physics and nuclear physics can help high-energy physics. And in fact, since I am at CERN every year thereís a conference on that subject, where the two come together. The conferences are in Geneva or Rehovath. The next one is Rehovath in February, but I wonít be able to go unfortunately.
Lubkin:There is no real nuclear physics going on at CERN per se, is there?
Weisskopf:Well, yes. Surely there is. I have introduced this. In fact, there is now nuclear physics with high-energy tools. This is a special division. And of course Rehovath was alwaysÖThere are a few places like the Weizmann Institute where people are interested in both. And Iím trying to introduce it at CERN, in Europe so to speak, to be interested in both, because Europe is rather strong on nuclear structure.
Lubkin:Will the meson factories help at all?
Weisskopf:Yes, definitely. The meson factory is one of these tools. There should be a lot of overlap because one should use high-energy beams for the investigation of nuclear structure, and nuclear structure itself is very important for high-energy experiments because there are many experiments you canít do with hydrogen, so you have to use the nuclei. You have to know the nuclear structure. But there is a certain difficulty in it because of the nature of these two fields. One is extensive and the other is intensive. The elementary-particle physicists just donít want to bother with what they call the uninteresting problems of nuclear structure — the complicated problems — and want to have a pure case. But of course nature isnít always pure. The nuclear structure man is sort of afraid of high-energy physics. He says, ďIt has lots of problems which we canít solve anyway. I would like to be more in the well-trodden field of nuclear structure where you have to deal only with Schrodinger equations and not too much relativistic effects and no unknown particles.Ē On the whole the trouble is, to my mind, that the very intelligent people are usually attracted too much to the high-energy field leaving the nuclear structure field to some extent to second-rate physicists. I say this by no means as a general statement. I can give you a whole list of first-rate physicists who work in nuclear structure, particularly here in Cambridge.
Lubkin:Who? Could you give us some names?
Weisskopf:Herman Feshbach, Arthur Kerman, Lee Grodzins, Aron Bernstein, to name only a few here.
Lubkin:What about elsewhere?
Weisskopf:Bruce French, Harry Gove at Rochester, Allan Bromley at Yale, and Europe is full of them.
Lubkin:Would you say more so than here now?
Weisskopf:More so, yes.
Lubkin:And this is because high-energy physics is so expensive?
Weisskopf:First that, and second that Europe is slightly less fashion-minded. People stick more to their fields. But there is the same problem now, particularly since high-energy physics is now starting in Europe at CERN. The same problem comes up there also.
Weiner:Is this a problem of the young people, the best of the young people?
Weisskopf:Yes. But wherever there are good people in nuclear structure the effect is of course mitigated. In Europe itís de Shalit who is one shining example of a first class and extremely inventive fellow who is in nuclear structure. Talmi, an Israeli, is another. Then there is a French school. And then of course Bohr and Mottelson are shining examples.
Lubkin:Is the Bohr Institute still a center of nuclear physics?
Lubkin:More than anything else would you say?
Lubkin:Field theory is secondary, for example.
Lubkin:Then in Europe you could characterize the Weizmann InstituteÖ
Weisskopf:The Weizmann Institute, Saclay in France, Copenhagen, and Harwell in England, although thatís a little on the decline.
Lubkin:Now, CERN you would not sayÖ
Weisskopf:CERN is not yet. CERN is mainly the application of high-energy techniques to nuclear physics, which is slightly different.
Lubkin:And where are the United States centers now?
Weisskopf:Cambridge, (M.I.T.), Rochester, Columbia, Yale, Los Alamos and Berkeley — Berkeley. Strangely enough in the chemistry department, but this is Seaborg —
Weisskopf:And Friedlander and a number of people.
Weiner:It started with G.N. Lewis in Berkeley in the chemistry department in the Ď30ís.
Weisskopf:Then donít forget Brookhaven. Mrs. Goldhaber among other people, and Goldhaber himself and Palevsky — quite a number of people. They have a new tandem there.
Lubkin:Is Chalk River in on this expansion?
Weisskopf:Itís a little in the background. It was better in the past.
Weiner:Let me ask a question which might serve as a concluding one, and that is how would you characterize the present state of the field of nuclear physics? Would you say that itís the end of a period, the beginning of a new period or a plateau?
Weisskopf:I think itís the beginning of a new period in respect to the application of high-energy tools to the nucleus. Since Bohr-Mottelson in this period we were talking about, there has been a certain solidification. Therefore the field needs a new push, and this new push is probably coming from the high-energy tools, meson factories and so forth.
Lubkin:In between Bohr-Mottelson and your own optical model there has been no real outstanding event.
Weisskopf:Oh yes, there has been. First, lots of work was done in working out those things, an enormous amount of work — Then, there are new concepts: the nuclear matter concept of Brueckner and Bethe. I do not think it is as important, as Bethe thinks it is, but this is only a small difference of opinion. Another new concept is the Analog state. In spite of this, I think there is, since Ď55, a certain lack of new ideas, I must say. This is certainly true.
Weiner:Would this then account partly for the fashionableness of high-energy physics?
Weisskopf:I donít know whatís the cause and whatís the effect. I didnít mention Wilkinson, who is part in Oxford and part Brookhaven, and one of the main ingenious people in nuclear structure. But especially important is the application of high-energy physics to nuclear structure. So I have a feeling that there will be a new period when these new machines will come into use.
Weiner:I just thought of another concluding question, a sociological one that will give us an order of dimension here. Sociologists and historians talk of relevant communities, of the people whom you need to talk with at a particular period. How would you characterize this change in terms of numbers — a relevant community from the early Ď30ís down to today, first starting with the early Ď30ís and then give the number today?
Weisskopf:Itís all so different because the whole structure of the scientific society has changed. The number of first-class people on the level of Bohr-Mottelson I do not think has changed much. Maybe it is 50% higher, maybe even twice as high.
Weiner:Are you talking of a group here that is under 50 anyway — in terms of numbers?
Weisskopf:Of numbers in a given age group.
Lubkin:How many are there in a given age range?
Weisskopf:In a given field, I donít know — maybe 20, 30. I donít think that number has changed much. But previously there was not so much below but now there is this terrific pyramid below of competent but of not extremely ingenious, scientists. There are now thousands of competent nuclear physicists. I find it bad, I think itís wrong. Itís cluttering up the field. Theyíre not doing very useful work.
Weiner:What else should they be doing?
Weisskopf:They shouldnít get into physics. Itís too well paid. They get too much money. I think the best thing to do would be to reduce the salaries drastically so it would drive out all those of the field except those that are really interested — in other words, reestablish the situation of before the war. This is of course impossible, but this is the only measure that will help.
Weiner:A sort of Goldwater philosophy, go back to the simple, pure days.
Weisskopf:Itís pure Goldwater philosophy, but I think that thereís an enormous amount of unnecessary work done that clutters up the field.
Lubkin:But how would the experiments get done if there weren't...?
Weisskopf:Iím speaking now of theory. In the experimental field it is also that way, but much more ingenuity instead of mere brawn should be put into it. Every experiment is done that can be done now just by sheer force. Thatís also bad.
Weiner:But thatís been the historical trend, hasnít it, ever since the days of Lawrence?
Weiner:So this is the one thing you could say has characterized the development of the field, this increase in the brute force approach?
Weisskopf:Yes. Now, it has certain results. Thereís no question that high-energy physics is based on all this. By the way now, we will see whether a drastic drop in funds destroys science or not. Iím afraid we will make this experiment whether we want it or not.
Weiner:I think that really covers this stage of it. Next will be the CERN thing and then the quantum electrodynamics.
In fact, I have a certain self-interest in talking to you about CERN because Iím supposed to write a history of CERN for a Rabi memorial volume and that is sort of weighing on my conscience. I havenít even thought about it.
H. A. Bethe and R. F. Bacher, Rev. Mod. Phys. 8, 82 (1936); 9, 69 (1937); M.S. Livingston and H.A. Bethe, ibid. 9, 245 (1937).
H. Geiger and Karl Scheel, editors (Jules Springer — Verlag, Berlin, 1933).
F.L. Friedman and V.F. Weisskopf, "The Compound Nucleus," Niels Bohr and the Development of Physics, edited by Pauli, Rosenfeld and Weisskopf (Pergamon, 1955).
Session I | Session II