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Interview of John Wheeler by Charles Weiner and Gloria Lubkin on 1967 April 5,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/4958
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Early interest in science; training in physics and graduate work at Johns Hopkins, courses, books ,seminars,informal atmosphere,1927-33; summer work in spectroscopy labortory at National Bureau of Standards,1929; interest in elementary particles, attraction of nuclear physics,1933;post-doctoral work with G.Breit on pair theory; New York University 1933-34; potential well model and its application to experimental results,1933-35; studies at Bohr Institute,Copenhagen 1934-35; discussions in Copenhagen on electrodynamics; role of experimental results in formulation of Bohr’s concept of compound nucleus, 1935; reasons for initial sceptical reaction to compound nucleus model; significance of Bethe-Bacher-Livingston review articles in REV. MOD. PHYS. 1936-37; development of compound nucleus idea after 1935; use-of compound nucleus and Breit-Wigner formula as guide for experiment; development of collective model; reaction to Yukawa’s 1935 meson theory and subsequrnt new cosmic ray particles; establishment of cosmic ray laboratory at Princeton, 1945; work considered personally satisfying; scattering matrix and other work at Univ. of North Carolina, 1935-38; close relationship of physics and mathematics departments at Princeton, 1938- ; collaboration with Bohr on theory of fission,1939; relation of fission physics to other aspects of nuclear physics; plans of physicists for postwar work.
This is a taperecorded interview of Professor John A. Wheeler, the Joseph Henry professor of physics at Princeton, in his office at the Palmer Physical Laboratory on April 5, 1967. The questions are being asked by Mrs. Lubkin and Charles Weiner. I’d like to start by asking you when you first became interested in physics and how this came about.
I think it was at the age of three when I got concerned about where one went on the world, and I was told that you came around the world the other way—the world was round. Then: “What happened if you went out in space?” Well, you went on and on. “And then what?” Well, you went on and on.
Do you remember asking these questions?
I remember that my mother told me a few years later about my having asked these questions. I can remember the scenario under a table.
Who provided the answers?
What was the next step?
Well, actually, nobody in a family that’s devoted to books, as my family was, could avoid becoming interested in the things that were coming out. J. Arthur Thompson’s Outline of Science I can recall at the age of ten was one of the exciting books, I can remember another book, the author of which I don’t recall now, on the nature of electricity, which was exciting; but more fascinating than any of those things to me at that time at the age of ten-was reading about explosives and what could be done with them. And the fact that on the farm where we lived we were trying to install electricity, and at the time the dynamite caps and dynamite were installed in the pig barn where I fed the pigs every day, gave me a chance to get access to a dynamite cap.
And you used it.
I used it.
Your father was a librarian. Did this account for the many books around the house?
Right. And my mother had been a librarian and my brother a librarian and my sister a librarian.
How about the science books you mentioned? Where they included in the family library?
In the family library right. And my father was very much interested in invention and in Yankee ingenuity, as it was called in those days. The fact that he had been a student at Brown University and had to work his way through by working at the library there gave him an idea of the needs and demands of a community like that—an industrial community. There was lots of silver working, brass working, machine shop work; and people all the time coming in to get answers to questions, and he was deeply interested in the library as the university of the people; so that anybody, no matter how poor he was, could work his way up in the world.
People like our friend Micheal Pupin were a kind of family ideal. It was really, I suppose, a carryover of this ideal that we had in this country coming from England of science as with the common welfare, the university of the common people, a Cooper Union idea sort of thing. He also prepared a booklist, a list of books in science to be distributed through the American Association of Science all over the country.
The American Association for the Advancement....
The American Association for the Advancement of Science—right—a science booklist. Dad got that project rolling. It continues today. I don't know how many hundred thousand have been distributed.
They publish in paperback now I think. Did they approach him to do this?
He approached them.
Was he a school librarian, a university librarian, a public librarian?
A public library, right.
So each of these moves that you made in your earlier years— these were various positions?
In public libraries in different cities.
Yes, right: Jacksonville, the very first library that he had a position at after his marriage, and Los Angeles as assistant librarian, and then Youngstown, Ohio as librarian-in-chief. Then there was an intermission there because he had a bad heart condition, and we went to live on a farm in Vermont, coming back after a year and a half—great for the health but poor in the pocket book—to Youngstown, Ohio, and then going on in Baltimore as director of the public library there—the first American public library to be built like a department store so that everybody could look in the window and see what was going on inside and all these exhibits in the windows. So it was attractive to a reader to go in—again, this idea of a library as a university of the people.
By the time you moved to Baltimore, how old were you?
I moved to Baltimore so that I did my last year of high school in Baltimore. So I was at that time 14.
By that time had you distinguished physics from the rest of science as a separate field of special interest for you?
Well, I had had a physics teacher in Youngstown, Ohio, who was not especially exciting but quite adequate. I had had two wonderful teachers in mathematics at this school in Youngstown, Ohio—the Rayen High School—Miss Doershuk and Miss Aleida Baldwin, And when Goldberger a few years ago was stolen away by Princeton from Chicago to come here as a colleague of ours, I was delighted to discover in talking with him that he had been educated in the same town in the same high school with the same mathematics teachers.
Had you been interested in radio during that period at all?
In Youngstown, Ohio, the distance to Pittsburgh was about 60 miles, and there was one of the pioneer radio stations, KDKA, and no small boy could read the Sunday supplements with their instructions of how to put together your own radio without trying to do something like this himself. Of course, it was a little appalling to go down to the department store downtown and ask for a "roehesid" and find that the man didn’t know what you were talking about and then he told that it was a “rheostat.” [pause in recording]
You mentioned in response to my question about an interest in radio that in fact it was hard to keep away from radio in that period and in that locale.
And then when you bought parts, you constructed radios, Did you work with anyone on this?
No, this was all my own. But there were two friends who were interested in similar things—one of them, the son of an inventor, Steckel, who developed a process for rolling steel at high speed.
I know of a Steckel mill for chrome plate.
Right. Well, he was doing this on his own, and his poor wife had to do a lot of drudgery that other wives wouldn’t have to do because they lived in such a poor way. Every morning he’d go down the street with his dog, and we’d say, “There’s Steckel and Pep going to work,” in the little garage where he did his experiments. But his son had an interest in similar things, and so we had a telegraph that went back and forth between our houses so we could do Morse code back and forth with each other.
Then there was another friend, Burdett Moke, who’s now teaching geology at Wooster College, Wooster, Ohio, with whom we used to do experiments on making guns, making safes with combination locks—a three-combination lock with the parts whittled of wood—and we formed what we called the Wheeler-Moke Gun and Safe Company, but we never did any business except make things for ourselves. We also were interested in other things. I built at that time a calculating machine of about the normal size of a calculating machine in which you pushed down the key and you had the addition and the carrying mechanism, but all the parts were whittled out of wood by Yours Truly.
That was inspired by reading a book called The Comptometer by a man named Feld, who had developed this machine, so that created great interest in it. Another inspiring book at that time was a book called.... The trouble is it's been republished since that time, and I'm afraid what I'm going to be giving you is the more recent title: Ingenious Mechanisms and Mechanical Devices—all sorts of clever machines to do repetitive mechanial processes or close doors. Another inspiration was a visit with my father to see the Waltham Watch Company. He had been born and grew up near that suburb of Boston, and on a visit back there one time with him he asked the company if we could come and look what they were doing, and it was marvelous to see these little machines turning out parts and picking up parts and moving from one place to another and assembling them.
Was this visit while you were living in Vermont?
I can't remember, I suspect it was when was living in Vermont.
But the Gun and Safe Company and the other work was done prior to your 14th birthday in Youngstown?
Right, between the Vermont sojourn and the move to Baltimore.
All of this was touched off by my question on distinguishing physics as a special field of inquiry from the rest of science.
Right. So I think engineering was really my interest more at this time—engineering and mathematics. When I got to Baltimore and went to Johns Hopkins, to the University, I signed up as an engineering student. I can remember meeting Ames on the campus at that time. He of course was the president and had been the president of the American Physical Society and a great physicist, and he inquired what I was going to be in, and I said engineering. “Well," he said, "maybe you'll get interested in physics."
What prompted him to do this? Had he had any knowledge of you before this meeting?
Not that I know of.
This was an encounter.
When did you enter Hopkins?
I entered Hopkins in September of 1927, as a freshman in engineering.
Wasn't that young?
Yes, I was 16 then.
Evidently you had accelerated your high school program.
Yes, I think that partly was because I moved from place to place, and in this little one-room country school in Vermont I went to, you could go as fast as you wanted to. There were, after all, only about 25 students all the way from first grade to eighth grade, so each student pretty much worked on his own. The teacher would listen to him recite and so on, and then he’d go on to the next thing. So you weren’t boxed step in with everything else,
I see, So then entering Hopkins at the age of 16 in 1927, majoring by choice in engineering—what happened?
So here was engineering and here was physics, and there were some very nice people in the physics department who gradually drew my interest into that subject more and more. Another inspiring feature was that the engineering library in those first days contained also the physics library, and here were all these things coming out with the vector model of the atom and such things, so that one could look at these journals and see these mysterious diagrams and try to puzzle out what they meant. And then that first summer between freshman and sophomore year, my father was going over to Washington—he took me with him—for a visit to the Bureau of Standards. I had read some of the writings of Paul Heyl, not only a good physicist, of course, but a popularizer of science.
So I had a chance to meet him and see the tide-predicting machine or harmonic analyzer, which was absolutely fascinating with all these wheels and gears and pulleys. But then I found out about there being a program of student assistantships, so that one could work at the Bureau of Standards during the summer and get paid $35 a month, So applied for one of those and had the good luck to end up with Meggers, the spectroscopist, working with him in the summer. So that was the summer of 1928, and a book of Hund was either available that summer or the following summer—I don’t remember which—on line spectra, and so it was quite inspiring to see this because Meggers was deeply interested in the application of these new ideas to explaining the spectra that he was studying so carefully. So my first scientific paper was written and published with Meggers on the band spectra of scandium, yttrium, and lanthanum monoxides.
This was in 1931 that you published.
Was this the first research that you had done inside a professional laboratory?
Right. I hadn’t really had an opportunity to see what a laboratory was like, to the best of my recollection, before that time. In my freshman year at Hopkins there were people like Hubbard, who was giving a course in physics—John Hubbard; and one of the instructors was Bowling Barnes, who’s now with the Barnes Company up in Connecticut. Then there was George Collins, who went to Brookhaven afterwards. So this made a lively discussion. I can remember being in the office of the opthalmologist being fitted for glasses, and there ahead of me was Bowling Barnes. We had a chance to talk physics, and he was talking about being able to calculate the motions of electrons and atoms, and this was what he was doing to explain some of his observations.
This was absolutely charming to me, that one could calculate things in all this detail and analyze these small things. The next year, of course, gave more chance to go on in physics. I found it especially wonderful that Herzfeld was there. If I recall correctly, I took nothing the first year with Herzfeld—-only knew of his existence—-but the second year took a course though I can’t tell you what it was, but the beginning of every course that he gave—not only that first one but every other one—had this in common: that they all gave one a kind of a survey of the whole of physics and then how this particular branch of physics that he was going to be talking about tied in with the rest of physics and what the great ideas were and then leading into the beginning of the subject this way.
Even on an undergraduate level? This was on an undergraduate level.
Right. The best teacher I ever had at any other time as far as exposition is concerned and clarity was Mernihan. Mernihan gave the course in calculus that I took in my freshman year, and I had a number of other courses with him subsequently. He always had an interest not only in mathematics as pure mathematics, but also mathematics in its application to engineering, and in these last years since his retirement he’s done something towards getting an engineering school going in Brazil, which I understand has a very high-caliber mathematics program with it. So that was that. Then there came seminars. I remember the seminar of Herzfeld that met every week and how at one point Ehrenfest came on a visit, my one and only occasion to see Ehrenfest.
They were discussing the question of the wave functions and pattern of wave functions: Could one hope to separate variables, given a general problem in quantum mechanics, or was this restricted to certain very special problems. Herzfeld talked about the pattern of nodes on a plate when it’s set into vibration by drawing a bow string over it and the sand separating out. And at this point, if I remember correctly, he said something about how this made one think of it as being conceivable that one could separate variables in a general case. At this point Ehrenfest rose and said, “Herzfeld, my dear Herzfeld, you are absolutely crazy.” But the liveliness of a discussion like that was of course inspiring, not that other colloquia weren’t also interesting.
Were you already in graduate school at that time?
Well, one of the interesting features of Hopkins was that one could switch over from undergraduate to graduate with an unnoticeable transition, They had what’s known as the new plan,” so you could make a nonstop flight, if you wanted to, to a doctor’s degree with nothing in between. This is what I did. But I should say that probably for formal purposes the undergraduate training stopped at the end of the first two years in my case, although I went on taking other undergraduate courses like history and English literature that were interesting. But I was really taking graduate courses, so that made it possible to go through in six years from entry to getting a Ph.D.
When in the course of those six years did you make the decision to switch from engineering to physics?
Actually, I’m very glad you ask that question because it means that I can put myself straight on record. I already said my memory is very bad. That first summer between the freshman and sophomore years was not really working at the Bureau of Standards. It was the second summer, between the sophomore and junior year. The first summer was working in an engineering occupation in the Pittsburgh Vita Grande Mining Company in Zacatecas, Mexico, about halfway between the border and Mexico City. The mine there was run by my mother’s brother, John Archibald, after whom I am named. He had a problem of electric motors in a mine that pumped the water out to keep the mine dry. Of course, if you’ve no motors, you can’t run the mine. And I was then occupied in rewinding some of the motors that had worn out or had short circuits in them. So I found this a very interesting occupation, but it also gave me a chance to reflect a little bit more on the difference between engineering and physics. I remember saying this to myself when I came back.
It gave me a chance to reflect on the fact that an engineer builds a bridge or whatever it is that lasts 20 or 50 years, but if somebody discovers something in science, well, that’s a permanent acquisition of the human race. And somehow that made an appeal. I’ve neglected to say that there was another occupation that I had. I’ve talked only of summer occupations. During the year while I was in high school I delivered papers in Ohio, so as a consequence one shoulder is permanently a little bit higher than the other. But the move to Baltimore made it possible for me to work in the technical division or industrial division of the public library Saturday nights, because the normal people had Saturday off and some substitute was needed. So here I was doing reference work on Saturday nights in the public library, and did that I think pretty much straight through college—the six years in college. And here is the greatest variety of questions that people bring in to you: “Where can find out how to build such-and-such?” or “Where can I get the best information on reinforced concrete?” or “How can I tell about anticorrosion metals?” So this business of feeling that anything could be tackled, and, by George, if you just gritted your teeth hard enough, you could find one way or another some information that would help somebody, was very inspiring. I kept on with that and kept learning from it, of course.
One person I found very helpful. Although I had taken German in school and always kept an interest in German, I didn’t feel that my German pronunciation was good; and this German-speaking woman kindly gave me instruction in pronunciation in return for my talking to her some about physics. Another occupation was tutoring students who had problems. I have always agreed since those days that the only way to learn something is to teach somebody else. Then still another occupation: Mernihan had an acquaintance who with the changeover from prohibition to permissiveness on alchoholic beverages wanted to start a brewery. He, however, wanted to do it as economically as possible. He didn’t want to go to a consulting engineer to get the story about the design of the cooling pipes and the flow. And so Mernihan sent him to me. So I acted as his consulting engineer in the design of this brewery of his.
This decision to specialize in physics came about then after the second year in college. What did you have in mind when you thought of physics? Did you think of it as physics with a capital “P,” or was there a particular subject matter area, a particular type of inquiry, that interested you and you found that that area belonged in a certain field of physics?
I would say that I hadn’t really very much idea of what it would mean to be a physicist. couldn’t think ahead in career terms very clearly. I think it was just the absolute fascination with this collection of topics and with all these mysteries. Of course everybody was concerned at that time with trying to understand the quantum principle better, and one of the colleagues from whom I derived the most was a fellow student, Robert Murray. He and I used to discuss these questions about why you have half quantum numbers, why sometimes you have integer quantum numbers sometimes; we saw a paper that came out on quarter quantum numbers. Then another thing that think really more than anything else for a period—perhaps about 1928—was a kind of guide was the book of H. A. Lorentz called Problems in Modern Physics.
It was based on his lectures that he’d given at the California Institute of Technology and was kind of a survey for an audience of people who knew some significant amount of physics, about comparable to what knew but not too technical, on what were the real problem areas. And I think that somehow gave me some guidance in what to think about and what to worry about in the years ahead. The informal atmosphere of Hopkins I think was a very fine thing. We had a little library—you’ve probably visited there—where the students could work with desks in the library and, so to speak, feeling at home in the library rather than orphans. And the classes were often given on a seminar basis in the sense that students gave the reports instead of the professor talking all the time. So that made a person feel a sense of commitment to what he was talking about.
Were there journal clubs or something similar?
Right, there was a weekly journal club or rather a seminar, a weekly seminar where somebody would generally talk. There was conducted also a seminar focussed on a particular subject. Each year the subject changed. That seminar was conducted primarily by Herzfeld and Maria Mayer and Dieke. Joe Mayer also took some part in it, and of course other members of the department would come in. But it was more a theoretical seminar, and one year it was focussed on the book of Born on quantum mechanics. Chapter by chapter we went through, different people taking turns reporting it. There was great democracy between undergraduate students, graduate students, and staff in this thing, taking part in it.
Which you subsequently called “colleagueship.” Maybe thats where it started. So they did keep up with the new developments in quantum mechanics certainly.
Very much so.
And then because of the informal atmosphere you were able to absorb it without waiting for a formal course in the subject.
Right. One was learning as one went along. So, unlike many of the same generation, I never had a course in quantum mechanics because we learned it as it came out.
Was there one offered as you recall?
I don’t remember whether there was one offered. There was, of course, a course in spectroscopy, which in many ways had much of the same material—Dieke's course.
Your choice of dissertation topic ended up as “The Theory of the Dispersion and Absorption of Helium.” How did that come about? When did you decide this would be the thing to do?
Well, actually, Herzfeld suggested it to me. The reason he suggested it to me was that he had written this article in the Handbuch der Physik with Woolf on the theory of dispersion and absorption, and it was clear that the two simplest atoms were hydrogen and helium, and there was a fairly satisfactory treatment of hydrogen, and his Handbuch article showed there wasn’t available a satisfactory treatment of helium; so that it seemed natural to get into that. Actually, looking back on it, I can’t think of any happier topic to have got into in the sense that it, first of all, involved electromagnetism, which was such a beautifully general subject, and then the quantum mechanics of a relatively simple atom and yet not so simple as to be trivial.
And then it involved the idea of dispersion, the connection between absorption and refractive index; so that I‘ve always continued to feel that that offers a kind of way into the understanding of atomic structure that I can believe would do a lot of good if it were used more in the teaching of physics at the undergraduate level. In fact, I‘ve always hoped that one could some time do a book called Elementary Processes of Atomic Physics that would, among other things, show the points of view that you get by looking at things from the dispersion approach. Well, I can remember sending copies of this, The proof had just come at the time I was making application for a National Research Council postdoctoral fellowship to go and work with Breit, and one had to have supporting material to send in with that, and so I sent the proof in with that. I can remember also having met Van Vleck shortly after this time, and Van Vleck was explaining to me how I could expound the subject more clearly than I had done in my write-up there and give a little more background. So I always appreciated his comments.
How did you meet him?
I think it was through Breit that I met Van Vleck.
This then was by 1933 when you were completing your work.
Right. I’ve forgotten when the due date was for applications, whether it was January or February, whatever it was. I had actually to make a decision between Breit and Oppenheimer as the two people that I might think most naturally of working with, and I did take occasion to meet Oppenheimer at one of the meetings of the Physical Society, but various considerations influenced me to feel that Breit would be the person most appropriate in my case to work with. Of course I feel that Breit is one of the unappreciated people in physics—all that he has contributed.
I just talked with Merle Tuve last week, and he urged me definitely to see Breit. Certainly the period in the late ‘20s is a tremendously important period, and then other things build onto that.
If you’re thinking of Breit, I have written up a little statement about him, which I’d be very happy to send. It’s only a small fraction, but one man’s appreciation. Breit doesn’t know about it. It was for another purpose.
I’d like that. Then you made this decision on the basis of the interests of the man or on the type of relationship you thought you could develop. Were both of these mixed up?
Yes, both factors there, and it’s a little hard at this time to take them apart. But I would say that ... I would have to go back to look at the write-up of my application for the fellowship to find out my statement at that time as to exactly why I wanted to work with Breit. I don’t have a record of that now. I’m sure they do. I believe that the factor was the idea that nuclear physics was the exciting forefront in physics, the exciting area for making progress in physics, and that out of the interactions between particles as shown by scattering and nuclear binding one could hope to learn the forces between elementary particles, and this would put one on the road. And Breit had been so active in that area that it seemed to me a very appropriate sort of thing to be in. Of course Oppenheimer was getting into pair physics. Looking back at it later, I can see that it would have been also enormously valuable to get into that. However, with Breit I did several things in pair physics, too, so I feel as if I had got that side of my education to some good extent looked after.
How did this interest in nuclear physics start? You were at Hopkins in 1932 when a number of important events were taking place in the field. It might be good to try to recall what the reaction was to a number of events in 1932—the early accelerator work of Cockcroft and Walton and the work that was being done at Berkeley by Lawrence, the discovery of the neutron and the discovery of the positron. Do these stand out as discrete events in your mind?
It would be very hard to date them, and have no consciousness of the time ordering of the events in my own experience except reading back in the books. I think that’s perhaps an illustration of how one at the time didn’t appreciate the importance of the different features. I can remember in a seminar giving a report on the paper of Bothe and Gentner, which really foreshadowed the work on the neutron. It was mysterious, the kind of results that showed up in that experiment, and of course one could really have almost read the neutron out of those experimental results if one had had enough insight to do it.
Do you remember feeling puzzled about the theoretical problem, the lack of having the neutron in the nucleus? That is, did the electron-proton model trouble you much at that time?
No, it hadn’t. I’m very glad you bring that up, because I can’t remember a great deal of consciousness at that time about what the problems were of nuclear structure. The only person we had on the staff who was actively concerned with nuclear physics to the best of my memory was Norman Feather. He had come for a year from England. He had just gotten his doctor’s degree at Cambridge, if I’m not mistaken. He was a young man of great ambition and drive; and I think he felt that his future really lay in England but this was an interesting interlude, to visit in America. And happily it was a period when the staff at Hopkins felt that something new ought to be done to give the graduate students a better education. Every student ought to have the chance to work with somebody on a research project. And so I found myself working not only earlier with Bearden on x-rays, but a little bit later with Feather on counting alpha particles in scattering.
So that gave some acquaintance, but I cannot recall Feather having been worked up over the Heisenberg picture of the nucleus as made out of neutrons and protons as compared to electrons in the nucleus. And I can’t remember until I got to Breit discussion about these issues of electrons not being in the nucleus. As a matter of fact, the work with Breit on pair physics convinced me that the great white hope of theoretical physics was the electron-positron theory and that people had been too early and too glib and too facile in ruling out the idea of the electron in the nucleus, that pair theory offers mechanisms for binding electrons in very small regions of space that never got a thorough discussion in these offhand comments of why there couldn’t be electrons in the nucleus. So my period of work with Breit was really one, on the one hand, of trying to work out nuclear forces from scattering and binding and learning that that was no great avenue to fame and fortune, and we know today people are still beating their heads against that wall and feeling that there has to be some other approach to this thing, and the feeling in my bones that I was going to dedicate myself for some time to see what could be done about electrons being the building block of everything, despite of course the obvious issues of principle about the spin of nitrogen that comes with that.
So this led in later years, but I don’t want to jump ahead of the story, but it was a hope that was built into me so strongly that I couldn’t stay away from it. And I didn’t leave it from that period of 1933 until 1947. I didn’t leave the idea that electrons were the basic building material until 1947. And the fanaticism with which I pursued that view is shown I guess not least by the fact that I felt that if electrons were the building blocks of atomic nuclei, the forces that were involved would have to be not the static electric forces but the radiative component of the forces. Therefore, it was of very great importance to understand the influences set up by a rapidly accelerated electron, and this led to interest in, first of all, just the most elementary problem of a positive and negative electron going around each other, but allowing for the retardation of forces, allowing for the acceleration component; and then led to the work on half advanced, half retarded potentials.
And then that finally got on the road when Feynman and I were working here at Princeton. And what finally turned off working on that was the conclusion out of pair theory and renormalization theory that the electron is not a simple thing. The idea of action at a distance I gave up, not because the action and the distance was complicated but because the particle was complicated. It was just the wrong basic starting point for the description of physics, to think of a particle. Pair theory made clear, and renormalization theory, that what one thought was an electron was really an infinite number of pairs of positive and negative electrons indeterminate in number and that the whole of space is filled with pairs.
Was this a disappointment in a sense?
Yes. And of course nobody gets religion like a reformed drunkard. As I’ve often said about this subject, the fanaticism, if you would like to call it that, with which I pursued the opposite approach— that it’s a pure field theory explanation of nature that one ought to work at—comes from having worked so hard at a pure particle explanation of what one sees.
But the basic drive, the basic motivation, is still the same except you’re looking for a different answer. The type of question that you’re interested in in physics hasn’t changed; it’s just that you’ve explored one avenue that didn’t take you to the ultimate goal.
Were other theorists working with a similar theme in mind in that period through the ‘30s, would you say?
Nobody was as crazy as I was, to think that you could explain everything in terms of electrons. And this I think illustrates a weakness of my approach at that time, to have this secret hope nursed internally and talk about it occasionally with close friends but not feeling particularly at ease about bringing it out on a public platform except insofar as one could talk of some specific thing that was a clean cut result that one could write up and publish, but publish it without making clear what the longer term goal is, for example, these objects called polyelectrons made out of positive and negative electrons, which were later observed experimentally. That was interesting and it was fun and so on, but the real motivation doesn’t show there. These things actually dont have the slightest relevance to the elementary particle problem, although that was why I got into them in the first place.
Did you discuss this work with Breit at NYU in 1933 when you went there as a National Research fellow?
Well, in fact, it was through being with him that I learned enough about pair theory to see how marvelously beautiful it was and to get inspired to follow this approach. I can’t provide an exact date in that period from September 1934 to September 1935 when I first got feeling very strongly about this way of looking at things, and it’s of course true that one finds it a helpful way of working at physics to have a white hope area of problems that are very difficult and then another are of problems which are—what shall we say?— more down-to- earth, where it’s quite clear what needs to be done and where one can go ahead and make progress. I think anybody would go crazy if he tried all the time to work on far-out problems.
Let’s Jump as long as we’re on this to today’s scene. Do you think that this same white hope factor is operative in today’s field of elementary particle physics? Do you think that the same thing motivating you for a 13 or 14 year period is the same factor that’s operative now with many people in elementary particle theory? Or is it a different orientation?
No, I think that there’s an increasing number of people today who are dedicated to the idea that there’s some simple principle that can be discovered and clear up everything. In fact, I’m troubled that many of the important fields of physics that are of very great practical importance aren’t receiving as much attention by good people as they might. Still I’m not so troubled about that. You and I know that there are these up and downs economicially and that one’s outlook in a period of prosperity can be of one kind and a period of depression another. I was a child of the Depression, and in those days there was a lean and hungry look on the faces of the physicists that one met at meetings, and also work on the fundamental issues was really much more secondary, I would think, to work on things that were more down-to-earth.
For example, what would be more down-to-earth in that period of the ‘30s?
Well, spectroscopy at Hopkins was a very strong field, and people were concerned with it. And my own experience at the Bureau of Standards let me see a little bit about how people were using it, and then Beardon’s work in x rays was clear. And then the work of Pfund and Wood in optics was most interesting. It was perfectly marvelous to have their lecture demonstrations showing this and that beautiful effect in optics, and one got the feeling that these were things that could be used. And then there were these colleagues—for example, Bowling Barnes working on the infrared and building their own gratings by winding wires onto bars, the most elementary way you could think of making a grating for the study of the infrared. And here were the hopes of these people that these things could be put to some practical use. The study of reflecting power of different kinds of material and how the reflecting power was influenced by impurities on the surfaces; that was one of the lively subjects at that time.
Did anyone go into the new field of nuclear physics—let’s say the experimental end of nuclear physics—with the hope that this, too, would be a practical field? I’m talking now of the early ‘30s.
I certainly don’t remember anybody going into it, but I do remember the “Breit feeling” that this was an exciting field, and I remember of course Tuve. Breit had had a lot to do with getting the work going of Tuve in the inspiration and guidance as to what in nuclear physics could be done. And people were trying to do this on a shoestring with not at all the feeling that this was something of an industrial application.
Maybe I can clarify my question. You’ve answered that question, but let me ask a more refined one. I can understand why a theorist would go into nuclear physics at the time. Certainly in your case you’ve explained it. Would it be a similar motivation that would be involved for an experimentalist at that time?
How did I make the first acquaintance of or at least see the Norwegian who worked with Tuve? His name escapes me right now. [Odd Dahl]
Not Larry Hafstad, although Hafstad had been one of my fellow students at Johns Hopkins and was interested in nuclear physics. This was a real Norwegian who Joined the Carnegie Institutft for Terrestrial Magnetism through Breit. It was natural to meet the people there of course. But he had been on an expedition in the Sahara Desert. He’d been on expeditions in Asia, nearly lost his life in some of his exploits. But he had said that nuclear physics was the thing for him from now on. That was the really pioneering thing. I think that reflected a feeling of some of the young enthusiasts. This was an experimental man.
Well, the closest nuclear physics laboratory, equivalent of a nuclear physics laboratory, to Hopkins was the Department of Terrestrial Magnetism I think at that time for experimental work.
Did you visit Tuve’s laboratory while at Hopkins?
I did not visit it while at Hopkins to the best of my recollection. I suspect that I visited it while I was with Breit. It was a very nice thing with him to make visits. I made more than one visit with him to Princeton while I was at New York University, and through him of course met Einstein and Wigner here. But the meeting with Tuve, I honestly can’t remember whether that came then or later.
Getting back to an earlier remark you made on the lean and hungry looks on the faces of physicists because of the Depression years, you were able to secure in this period, though, a National Research Council fellowship. Did this involve teaching?
No, that was a pure research fellowship to work with Breit. I was a very lucky person.
What would have been the other alternatives? If you wanted to go directly into a teaching position, would you have found difficulty in obtaining a suitable position?
I suspect that I probably would have got a teaching position at Hopkins, though I don’t really know that, as an instructor. I would imagine that Herzfeld must have talked with people there of alternatives in case I didn’t get it. Perhaps I had something more definite in mind at that time, but I don’t remember now. I do remember Herzfeld speaking about how hard it was to get positions, and I can remember talking about that later because it became of course quite relevant to me at the time when I had to get a position after my fellowship ran out. It was especially bad to run out when you were abroad.
In this early period before the compound nucleus model was developed, what sort of picture did a theorist have of the nucleus, and what sort of picture would an experimentalist have in mind that he would be trying to use in his experiments?
Let me add, if any.
If any, right. Did he have a model in mind when he did a measurement?
The picture that was very much in mind working with Breit in this period from 1933 to ‘34 as a background—and I think through Breit and others was communicated to the experimentalist—was the picture of a potential well, because, after all, the Gamow model of alpha particles escaping from nuclei was very clear, and also very clear was the idea of a similar barrier picture for protons getting into the nucleus. So there was this picture of a barrier with a hole and position for states, and these were particle states, and an individual particle came in and got into one of these states. But there was of course the very deep mystery which puzzled us of these experiments of the Cambridge group when they shot particles into the nucleus at one or another energy and got resonances going in and they got different groups coming out.
So it was kind of a double-energy spectrum, both the spectrum for the incoming particle and the spectrum for the outgoing particle; and how you fitted all this together was really quite baffling. But all one had was this picture of a barrier and particles going around inside of a potential, but where the potential came from was not very clear. Of course by this time the paper of Heisenberg had come out, but I can’t remember that influencing the considerations of Breit and myself and others working with Breit at New York University anymore than to this extent: that it said that neutrons and protons were the particles. But as regards the statistical approach to the nucleus that Heisenberg had, this was not very much in the spirit of the work that was being done at New York University at that time, and certainly one was very far from either a compound nucleus model or a collective model or a shell model. One can say, “Why wasn’t one talking about a shell model?” and I can’t really answer that. I think that would be a very interesting question to ask.
Apparently some people were. Elsasser had offered something along those lines. The question is: why wasn’t it caught up?
Were you aware of the early shell model papers at the time?
I confess I don’t remember them at the time. I do remember sitting at lunch at New York University, and I think it was Cox telling me—and I’ve forgotten whether the other person or persons at lunch were Breit or Lowan (Lowan was one of the capable young men—he has since died) about his experiments in beta decay, and he had this anomaly in the angular distribution. One of us—certainly not me; somebody who was more aware of it than I was—pointed out that if this were correct, then parity wouldn’t be correct; and this was so preposterous that we all felt there must be something wrong with the experiment. And he didn’t have the faith or whatever it took to say, “But you theorists, you’re only theorists.” He never said that.
And he went back and doubted the experiment.
As far as the experimentalists were concerned, would you say that they were looking for predictions of the potential well model in their experiments or did they not have this in mind?
It was a good deal the study of the penetration factor of the Incoming particles getting into the nucleus. That was quite a center of attention: how this would be affected by the shape of the potential that one used so that the calculations in many respects were testing only a very limited aspect of the model—the aspect of the one particle coming in and its getting through this potential and how it moved after it got inside. And as regards the story of what happened after it did get in and what it did to other particles, this was pushed to the side. So the experimentalists were, I think, perhaps a little meeker than they needed to have been in the sense of looking at the experiments that they were doing—at least many of the people that one was acquainted with—as ways to test these theoretical calculations of barrier shapes and not a whole lot more.
But you say they were using experiments to test—
Test for the shape of the barrier.
Without challenging a whole model
And without being really interested in nuclear forces as another example per se.
Well, there were of course the people at the Carnegie Department of Terrestrial Magnetism who were studying the scattering of protons by protons and so on.
Were they the only nuclear force group? That’s the feeling I get—that It was Tuve’s motivation to study nuclear forces; that’s why he went into that experimental work. Was this an exception, though, to the general experimental scene?
I certainly remember very much the spirit in the air; that once you could find out the force between particles, then all the world would open before you. I can’t remember any more people doing it experimentally at that time than the people in Washington. But It was certainly a hope of many more people, and I think that the reason that the experimentalists that I was acquainted with were doing such things as these penetration experiments was not so much that they thought they were the most exciting things. I think, if they had had their choice, they would have been preferring to work on the interaction between neutrons and protons and protons and protons, but they didn’t have the tools to do it. This is an impression looking back on it.
It’s a basic issue in the period I would like to learn more about, and this is a first interpretation of your impression. Where were the significant experimental results coming from in this period of the early ‘30s?
Well, the people in England were doing experiments with the Cockcroft-Walton equipment, and that provided a good tool. One of the projects on which I spent quite a bit of time was the scattering of alpha particles in helium. I was also interested in other scattering processes, but the raw material in all those cases came from the Cavendish laboratory group in England.
What was the group that found the puzzling resonances before the compound nucleus?
I should be able to tell you that. It was either the Cambridge group or the Bethe group.
The Fermi work
The Fermi work didn’t happen till ‘35, right, on these resonances. That was in Copenhagen. And of course there was a completely different spirit between Bohr’s approach to nuclear physics and Breit’s—Bohr looking over the whole thing without getting down to detailed calculation on any one aspect and always looking for a paradox that would throw light on a whole new approach, and Breit, on the other hand, focussing on a very careful comparison of a detailed model with experiment and the soul of integrity and giving one the feeling that any part of physics should in principle, if one understood it properly, be subject to calculations so you could really hope to check the theory against your experiment and not just talk.
Did you make a conscious choice of one approach as opposed to the other after being exposed to both of these men?
Well, I feel very lucky to have been exposed to both, and I don’t think one can get along without both. Bohr certainly would never have proposed to get along without it. He was most conscious of these checks but content to let other people make them.
How did that transition come about from NYU to Copenhagen?
Well, I had a second fellowship year, and clearly I wanted to learn as much as I could, and I talked with Breit about the possibilities. I already had a high respect for Bohr and he did. An alternative would be to stay with him, or an alternative would be to go and work with Oppenheimer. And I can remember the application in general terms. As I wrote to the National Research Council Fellowship Board, the motive for going to work with Bohr was that he had more insight, and he could see ahead deeper into the explanation of the puzzling problems that lay ahead than anybody else that one could name in the world, so that he was the logical person to try to work with. I don’t have the exact words. I don’t have any written records of this. The National Research Council people I'm sure do. Anyhow, Breit did write to Bohr, and Bohr said that would be okay; and consequently I did go. Of course it was an unhappy time to arrive because Bohr had just lost his son that summer in a sailing accident, and he was really pretty knocked out for a couple of months, But then he came back into it.
This the beginning of a whole new period.
Yes, I wonder if we don’t want to get some lunch now. [Pause in recording]
Resuming now after a break for lunch.
I was commenting as we were returning from lunch about how the character of what one produces in an interview depends so much on who does the interviewing and what kinds of questions he asks. And that reminded me of Edward Teller visiting in North Carolina at the time was there for a three-year stay. I remember an evening party where the game was played in which a person is sent out of the room and those behind agree on a word, and then the person comes back and has 20 questions to find out—yes or no. But I noticed that when I was sent out and came back and started the usual question: “Is it something in the animal kingdom?” As each successive question was asked, the answers came slower and slower and caused more and more trouble to those who were answering. Finally came to the final word and it was "cloud," after I‘d asked a number of questions. Then at last they broke down and told me why the game looked so strange: because they had agreed in advance that this would be one where no word was agreed upon to start with. Every answer, however, would have to be consistent with all the answers that had gone before. So it was really harder for the people playing the game than it was for me. The point was the word “cloud” that had been produced really came more out of the questions that were asked than out of anything that they had agreed upon before the thing started. And so it may be with this interview.
With that warning and disclaimer, we’ll proceed with restraint. When we left off, I believe we were talking about your going to Copenhagen and arriving during the summer at a sad time for Niels Bohr because of the loss of his son. Then you indicated that very soon after that though, within a few months, he was back in the swing of things. It’s just at that point I‘d like to explore what you mean by “back into things.” What sorts of things were going on?
The liveliest interactions of that time were with Rosenfeld and with E. J. Williams, who subsequently died. Bohr and Rosenfeld were concerned about the problem of measurement in quantum electrodynamics, and Bohr and Williams were concerned about the theory of the loss of energy by high-energy particles. This was that marvelous time when the background of the cosmic rays was being sorted out, and one was trying to understand whether these positive particles that were observed in the cosmic rays were electrons or something else.
The predictions of the Bethe-Heitler calculations based on quantum electrodynamics were that they could not be electrons that penetrated 10 centimeters of lead because such a particle would radiate away almost all of its energy. But could these predictions be believed? And here was where the discussions of Williams and Bohr and Williams and Weizsaecker led to the conclusion that the arguments about radiation, although they had flowed out of what looked like quite complicated calculations, could actually be derived by the most elementary arguments, and there was no escape from them; and so one had to conclude that these particles could not be electrons—they must be something else. As far as the positive ones were concerned, they could of course be protons, but the negative ones clearly then had to be some new kind of particle. So the stage was set for the discovery of the meson by this work and the foundations of electrodynamics.
Did Yukawa’s work come into discussion during this period? it came out in 1935.
I cannot remember Yukawa’s work receiving discussion at this time, 1934 to ‘35 when was there.
May I ask a question just to clarify something? When did you leave the States?
I left in Jun 1935. I arrived in September ‘34.
I don’t remember the exact date of Yukawa’s paper. It may have been just after that. But these discussions were going on then at the time. What form did they take? Was this in a group or in private discussions?
Discussions of about four or five of us. There were Williams, Rosenfeld, Bohr, myself. Plesset was there at the time, and occasionally other visitors would come in. It was a fairly quiet year, should say, judging from the Institute of Theoretical Physics as it is in these days. Franck was at that time staying at the Institute, but he wasn’t taking part so much in these discussions except occasionally. So three or four or five of us would meet and argue on the blackboard and discuss. One of the real issues at this time, still on this question whether electrodynamics was correct, was connected with experiments of Gray and Tarrant. Most young physicists of the present time couldn’t tell you what Gray’s and Tarrant’s experiments were all about if you asked them today, I suspect, but they had to do with the scattering of gamma rays of 2.6 million volt energy by lead, and the amount of scattering was far larger than could be accounted for in terms of any known process.
The scattering of gamma rays by the atomic electrons would not suffice to explain what was observed because this was a large-angle scattering, and what was observed at large angles had quite substantial energy, whereas Compton scattering would produce protons of quite low energy. There was work in progress at Leipzig. A student of Heisenberg named Delbrueck was hard at work trying to calculate the coherent scattering of gamma rays by lead nuclei or heavy nuclei with a view to explaining this process. And Moller at Copenhagen was interested in this topic, too. Delbruck also came to Copenhagen later in the year, but that was already at the time when Bohr was having his effect on Delbruck, and perhaps also the difficulty with his calculations was having its effect on Delbruck, and he was gradually switching from physics to biology, in which Bohr was deeply interested, too. But, at any rate, the story was that I got into the act of trying to explain what was going on.
It was a fascinating analysis of the detailed mechanisms. I never published it because I guess in the end it looked like too much of a bookkeeping task. But it turned out that one could account for this anomalous scattering in terms of a whole variety of processes going on in the lead: an incoming photon coming in, knocks an electron on; this electron gets scattered into a new direction; this electron reradiated a photon in a new direction—of if the electron had a positive charge, the electron would be annihilated by a negative electron, to give you two photons going in a new direction. In these ways one could account in a quite reasonable way for the observation, so that there was no basis for saying that the Gray and Tarrant experiments, puzzling as they had seemed at the time, showed anything paradoxical or new about quantum electrodynamics.
Have you preserved the notes, the papers, on this?
Especially since they weren’t published, it would be very interesting to have them in the long run for historical purposes.
Did Bohr start to talk about the compound nucleus ideas while you were there?
Right, yes. It was along in the spring. Miller had been to Rome, if I remember, for an Easter visit; and there he had seen these experiments going on and all this mystery of the resonances and came back very excited, and there was this seminar—maybe there were eight, maybe 15 people there—in which the results were reported, and then this very lively discussion. Then this led up to the idea of the compound nucleus. I don’t remember actually seeing the dramatic moment when Bohr came up with the concept of a simple potential with a particle moving in it was completely the wrong way to approach this, but instead this billiard ball picture. But then that developed very actively.
Do you know the story behind it? Did he have a sudden insight or was this something that he developed slowly?
I wish could tell you that, but can’t. That would be a wonderful story to have.
Who might know? Of course it might be in his papers, his correspondence, but who today might be able to give us some insight into that?
I would suspect Rosenfeld would be the best person because he was around at the time, and Moller, because Moller had brought back the report. I think they’re both so conscientious that between the two of them, they would help.
But you feel that Bohr came up with the idea in response to the Fermi resonances.
Right. It was not cooking in his mind before that time. It was one of these cases where he was just looking for a paradoxical result and here it was and it led to a new view.
And apparently all this occurred within the space of a few months.
What was the reaction of the group there to the idea once it was full-blown and formulated?
Well, of course, a deep interest. Also, I forget to mention Kalckar was there. Unhappily he died since then. But he worked with Bohr on this, and they subsequently published a detailed article on it following Bohr’s Nature shorter article. But Bohr really relied very much on having around people who were able to work along with him on some enthusiasm, who weren’t so committed to their own work that they couldn’t pull off and collaborate with him. Kalckar was very good this way, and Rosenfeld was always a great source of support to Bohr in this way, to pitch in on his interest of the moment.
What about your own interests? Prior to this, during the earlier part of your year there, you were working in quantum electro dynamics. Did you continue or were you caught up in the compound nucleus discussions and considerations?
Well, I was working on quantum electrodynamics and also I had a good deal of interaction with Plesset, who was interested in quantum electrodynamics at that time. In connection with analyzing this Gray and Tarrant work, Plesset and I took a look at some of the scattering processes that one could have in atomic nuclei, and we found ourselves using the dispersion formula to predict how much scattering there would be at that time. And this we tried to sell to Bohr, but he has a very deep and wonderful sense of caution about things, too—daring even-numbered days of the month and caution odd-numbered days of the month—and this sense of caution said to him: “What guarantee has one that the dispersion formula makes any sense at energies like this, at relativistic energies, 2.6 million volts?” So although if you accepted the dispersion formula, it would allow you to predict that you could not explain the observed effect that way, still that did not seem to him an adequate argument.
So we never published that. However, later on I‘m very happy that came back to dispersion theory and the relativistic domain with John Toll when we were the first to apply it in the relativistic domain and to show that the arguments of causality that Kramers and Kronig had developed in a nonrelativistic domain held also in a relativistic domain so that one could predict some of these quantum electrodynamical processes in this way. So if we’d only had that causality argument in the relativistic domain at this time, why we could have firmed up this point. But thanks to the caution of Bohr—and a justified caution—we didn’t publish this.
When did you finally publish your dispersion relation paper then?
We published that about 1953. It was just a brief abstract, and then Toll’s thesis is a complete development of dispersion theory.
Bohr then of course was involved in the discussions of work that you were doing. Do you think there was anything in that discussion that might have been a contributing factor in his own considerations on the compound nucleus?
I don’t think so.
There was nothing that would give him any....
We were talking about states of excitation in the nucleus and scattering of the nucleus, but the idea of exchange of energy between the particles— I would say that wasn’t there.
And the most important experimental results that he used in his work were apparently the Fermi resonances?
In addition to the general disturbing results that had been building up in previous years.
Yes, but it was this paradoxical result that a nucleus can show such a huge cross section for capture of a neutron, while, according to the picture of an elementary field of force that the neutron went through, it would just go through and out with a very small probability that it would lose any significant amount of energy by radiation; so it just shouldn’t be captured. So there was a direct contradiction.
How many months would you say elapsed between the experiment and the theory?
Two weeks, I’d say. Lubkind: Nobody had time to think about it probably before he had the answer.
Right. He saw so much more clearly than anybody else how absolutely inconsistent it was with existing views, so that the paradoxical nature of it stood out to him, I think, with a vividness that drove him on in a way that nobody else appreciated or felt.
What kind of response did the compound nucleus have among theorists and experimentalists?
Well, of course, there at Copenhagen, Kalckar got into calculating on it, and later on got into calculating on it after Bohr visited the States in 1937. However, the group of physicists around the country, this country, of whom I saw the most after my return, I think were rather skeptical about it, They were still much more captivated by the concept of a potential in which a particle was moving; and all the more so because somebody, for example, like Wigner, deeply concerned about the magnetic moments of nuclei, was concentrating on an aspect of nuclear structure where shell effects would stand out more strongly and where any such liquid drop or collective aspects would be very subsidiary. Maria Mayer, of course, was skeptical of these considerations, too. But think that there was a deeper element in the skepticism or failure of large numbers of people in this country to take up this point of view, and that was the fact that the model didn’t lend itself to simple, analytical treatment—all these particles banging into each other is bad enough in classical physics, but to do it in quantum physics is even worse.
Was this an aesthetic consideration as well as a practical one—the fact that it didn’t lend itself to simple analysis?
It seemed so to me. At least if I can permit myself to be for a moment a psychologist, I would say that it was this lack of aesthetic appeal that kept many colleagues from getting enthusiastic about participating in it. Somebody like Weisskopf, who had much more of the point of view that one gets from Bohr of order of magnitude analysis and rough and ready calculations, was much more capable of both reacting affirmatively and doing something about it than somebody who likes to say, “Well, first let me see the Hamiltonian; then let me see the zeroeth order approximation; then let me see the first order perturbations and so on.”
Well, can you characterize a difference in response to the compound nucleus on one side of the Atlantic as opposed to the other? The question is directed to the response in this country that you had direct knowledge of. Did you have any feeling that it was received differently in Europe? Weisskopf, whom you cited as an example, I think was still in Europe or had just come, but he’s a European in training. Is this characteristic of the European response?
I think it’s not so much Europe versus America as it is people who have been exposed to order of magnitude reasoning and horseback analysis of physical mechanisms, Peierls, for example, who also had been in the Copenhagen group, took up work on the compound nucleus model. Teller was interested in it, although I’m not sure that he did anything in detail on it. And in the Soviet Union ... I am really in no position to say how it was in the Soviet Union, was about to say that Landau and Fock were attracted by it, but can’t cite chapter and verse on that.
When would you say that the average theorist was won over to the compound nucleus then—in what year?
Well, one very great step forward that helped to bring a much larger group of people into looking at nuclear physics this way was the Breit-Wigner treatment of resonances in slow neutron capture, because here, at any rate, was a simple calculation based on simple principles that everybody could follow and that didn’t appeal to these order of magnitude estimates at all, So it provided a way to treat the capture cross section of a resonance in a way that steered clear of the details of this liquid-drop model and the compound-nucleus model.
Actually, in time the publication date of the Breit—Wigner formula is ‘37. It was about the same time as the Bohr-Kalckar paper. Of course Bohr’s ideas were being bruited about for a while before then—for perhaps a year or so. Then the theorist, at any rate, would be willing to throw over the potential well model. Is that correct?
Right. And the great series of papers by Bethe and Bacher about this time did a lot, I think, to make people feel at home with this new point of view.
You mean the Reviews of Modern Physics papers, beginning in ‘36 and then ‘37?
Yes. Bohr felt always ... if I’m not speaking out of place. I shouldn’t really be quoting him.
Why don’t you characterize?
Well, that this thunder had been stolen a little bit in those papers. A good many things that he was going to say in his paper with Kalckar showed up in this series of papers. But that’s only in certain limited respects of course. Bohr always thought that series of papers was very great and wonderful, and also was certainly one that was most appreciative of all there was in the papers.
What effect do you think the existence of these three review papers had on the field?
It marked really the coming of age, I think, of nuclear physics more than any other single publication that one could easily cite, a whole series of developments expounded there.
Was it a sort of a summing up of things, of showing what was known, or did it have the effect of suggesting new problems?
It summed up various models, what one knew about the interaction of neutrons and protons; it summarized what one knew about the lightest nuclei; it treated all this complex of questions about the capture of neutrons; it produced a kind of order in the field that made it a natural starting point for anybody going on. I can’t recall any decisive new points of principle that came out, but many new points of calculation and many nice new ideas which I think were very beautiful.
Could you trace for us in rough outline how the compound nucleus developed, the subsequent developments that grew out of the original idea?
Yes. Well, of course, the idea of the liquid drop and the idea of the compound nucleus were in many ways closely connected with each other and yet in other ways distinct, The liquid-drop model furnished a kind of idealized version, you might say, of the compound-nucleus model; so that from it you could calculate the states of excitation and evaluate the differences between acoustic modes in which the density of the nuclear material changed and surface tension modes in which the shape changed—whereas the compound—nucleus picture was more an abstract picture of a system which maintained no memory of how it had been formed and which therefore would break up in one or another way purely dependent on built-in probability factors and not at all dependent on how the system had been created, by what act of bombardment.
The liquid-drop picture, of course, gave one a certain faith that this was an actual way to do business, and the Breit-Wigner formula, coming along so quickly after this general model of the liquid drop, provided a central point of reference in the discussion—namely, the idea of a level of the compound nucleus endowed with transition probabilities: a probability for radioactive decay emitting a neutron or emitting an alpha particle or emitting a gamma ray, so that all the considerations of transition probabilities could be applied in a nice straightforward way. This of course left still very much the question: How were you going to calculate these transition probabilities? And that called on a much more detailed picture, and the compound-nucleus model didn’t purport to supply an answer to that question, but only to give you this picture that not only were there levels of the nucleus below the level of dissociation, but also resonance levels up in what one previously had always thought of as a continuous spectrum with these levels, each level with its characteristic energy, each level with its characteristic probabilities of decay by this, that or the other mechanism; so that it for the first time gave one a kind of ideal of what he could hope to have in nuclear physics: namely, a bookkeeping of what these energies are and what these transition probabilities are. And if you once knew that and had them in a handbook, why then you’re set to be a nuclear engineer and build what you wanted to build.
Are you saying then that the Breit-Wigner formula predicted more discrete states than anybody had found experimentally?
Well, no. So to speak, it provided a treatment of how the probability of scattering or the probability of reaction would depend upon the energy of the incoming particle near the resonance without your having to know any more about that resonance or the nucleus that it belonged to them simply (a) the position of the level and (b) the various decay probabilities of the level. Once you knew them, then you could evaluate, with the help of the Breit—Wigner formula, these scattering and reaction cross sections—without knowing anything more. So there was a beautiful division then of nuclear physics, which had been two very different problems—one, the reaction aspect of nuclear physics and the other, the structure aspect of nuclear physics—that reduced them to the one single aspect: the structure aspect. If you can only know the levels and their resonances, then you could predict these reaction cross sections without entering into all sorts of complicated issues of what orbit was followed by what particle moving as it came in and what orbit was followed by another particle going out. So the Breit-Wigner formula was of course seized on by Bohr as just the marvelous way to give concrete form to this general concept of the compound nucleus.
Here you saw in these transition probabilities exactly the things he was talking about—how a nucleus did this or that independent of how it had been formed. Now, this isn’t to say that there weren’t people who were deeply worried about the question of whether this was an adequate account because it sounded preposterous that you could get away so cheaply, and this worry had its reaction on not only Bohr but Peierls and Placzek, who were responsive to the feelings of the times; and they with Bohr, working at Copenhagen but not when I was there—this was after had left—considered the issue of what happens if the resonances are so close together that one resonance level overlaps on another. Then they appreciated that really you no longer had the compound nucleus in its simple form. You would have some memory of how the system had been formed, and you couldn’t get by so cheaply in predicting what the consequences of bombardment would be. But that was only spelled out in general terms in their paper. There was an enormous amount of work to be done, and of course even today still to be done, to work out these details.
The implication, though, is that the compound nucleus, as illuminated by Breit-Wigner and Peierls and Placzek, made it possible to organize experimental work in a meaningful way. Certainly the theorists felt this way. Did the people doing the experiments feel this way, and, in fact, did they act on it? Did they use this model in designing experiments and in testing the model?
There was some work at Wisconsin going on, some work at Caltech going on, on these low-energy resonances; also at the Carnegie Institute for Terrestrial Magnetism in Washington, using this general picture to account for nuclear reactions as they are normally conceived. In addition to that, there was the work going on of people who were following Fermi’s track on slow neutrons. So perhaps one could very well say there might have been altogether, if you count up the places I’ve left out, eight places where this picture of the compound nucleus plus the Breit-Wigner formula was used as a guide to doing work on these resonances and providing an ideal type of program to guide what one should be doing in nuclear physics.
In broader terms, would you cite, for example, your own work with Bohr on fission as a direct outgrowth of the compound nucleus?
Very much so.
What other highlights besides that subsequently grew?
Well, there was the question of loss of energy by fast particles in colliding with atomic nuclei, the slowing down of fast neutrons. Of course we’re approaching the time when a lot of nuclear physics had to be discontinued because of the war, but the behavior of fast neutrons in some of the wartime devices and their slowing down—this came into use there.
What about the collective model? Is this an outgrowth or an independent concept?
Well, the collective model really was forced by the fact that one was making progress with the shell model, and there were clearly shell-model effects that work. And the single most important step in breaking ground into the collective model was the idea of Rainwater to explain why atomic nuclei had such big quadrupole moments. No single particle picture could explain it, because the charge was too small for one particle to give you that much effect, even though it was highly unsymmetrical in its location, Surely the liquid-drop picture wouldn’t work because that was spherically symmetrical, but when you put the two together and had this neutron motor-cycling around the inside of the nucleus and pushing at pressure on the wall and deforming it so that it wasn’t a spherical drop, then you got this huge quarupole moment. And this made sense, and his order of magnitude calculation showed that this made sense.
I had the pleasure of, or as Eugene Wigner would say, the idea wasn’t so surprising to me as one might have thought, because I had already had the same thing. I remember working it out on the train going between Paris and Copenhagen in the late fall of 1949 after one of the discussions with Bohr on nuclear physics. But it just shows that these ideas are lying around. We, of course, were concerned also to try to see a little bit more about how similar ideas could come into a larger spectrum of problems, not only the ground states of atomic nuclei but also reaction processes and excited states. And this led to this picture of the kinetic energy of the nucleus in a vibration, which one had previously estimated on the liquid-drop picture as coming not from the bulk motion of the liquid, as one would say in a purely hydrodynamic picture, but in terms of the effect of the wall motion on the orbits of the individual nucleons inside. I think that it was an inspiration in that work—l949 and very early 1950—that there did exist this paradox: How could one reconcile the compound nucleus, which was so needed to account for a variety of effects, with the single particle picture, which you also needed to account for magnetic moments and quadrupole moments? This of course was the point of going back in ‘49 and talking with Bohr about these things.
You mean you made the trip especially because you were becoming concerned over the qudrupole moments?
No, no. As a matter of fact, if I get historically correct at how I got into being back in Copenhagen again in ‘49; I would have actually liked to spend the year in Copenhagen, but I thought that for my children it would be better if they spent the year in France from the point of view of language. But then there was another factor at work: What I really had in mind to do at the beginning of that year of leave of absence in 1949 was not nuclear physics at all and not to work with Bohr primarily on nuclear physics, but to make occasional visits to him to talk over these issues about the lay of the land, about physics in general. I was still deeply interested in this idea of action at a distance, which had gone back to the electron theory of matter.
But the question was: Since one had already discovered by now how to formulate all of electrodynamics in terms of interaction between two electrons, forgetting the field and saying you’d swept out the electromagnetic field, could one do the same thing—this was my issue—to sweep out the gravitational field between particles and since gravitation really means space-time, could you eliminate space and time from physics and have a physics that would only deal with particles with no space-time around? Well, I made quite a bit of progress in my period in Paris toward the formalism of such a thing, but I’ve never published it because, as I have remarked, I don’t feel that that’s the right approach to physics— the particle approach. Nobody gets religion like a reformed drunkard. But I did make several visits to Copenhagen to talk with Bohr. I think it is probably, although I have no diary or anything to show how things went, I suspect that it was discussions with him and his concern about how to reconcile these two pictures of nuclear physics that brought my attention back into that field because I had never felt that nuclear physics, after my first decade in it, was the answer to making progress into these deeper issues. But nuclear physics is, of course, a beautiful subject as regards seeing mechanisms. And if you say that so much of physics is a story of mechanisms and processes and not simply laws and equations, it’s fascinating to track down these mechanisms and see how you can find a mechanism which will include both the collective model and the individual particle picture. I talked to Bohr and Mottelson at Copenhagen on a more recent visit. If remember correctly, that was about 1962. Now I‘m talking about Aage Bohr. I particularly talked to Mottleson.
The question was: Could one have calculated by elementary considerations how the atomic nucleus would behave and what a queer mixture it would be between collective and individual particle pictures, starting from first principles? And he laughed and remarked—because, you know, I had more experience than he—that the kinds of effects you get are so extremely sensitive to the exact choice or the exact parameters of nuclear forces as they show up in nuclear physics, that you could have gotten one type of effect or another or another or another, and really it was only by a fantastically detailed analysis of an enormous amount of experimental material that one could really ever in the end in these later years find out exactly where he lies in this middle ground. The mean-free path, after all, of the nucleons in the nuclear material is neither extremely short compared to nuclear dimensions nor extremely long, and the fact that it’s just comparable to nuclear dimensions is what makes all the interest and gain.
When was this conversation?
This conversation was around, I would say, '62.
But I gather that your paper with Hill had some impact on the Bohr-Mottelson development afterwards, How did this develop?
Well, this was the first reasonably systematic account of the collective model of nuclear physics. You could say that following that 1949 work, Rainwater found this deformed nucleus. And then this work was doing with Bohr: If I had had my time free, the natural thing would have been to just stay on and finish that up with Bohr that coming spring; and this is what I would have done. But I had all this series of cablegrams and long-distance telephone calls from Washington, and had to give up the physics thing for a while; but in nooks and crannies at times I could fit in a little. So since I was at Los Alamos and Hill was able to work with me, we were able to fit this in. And this paper on the collective model that appeared in Physical Review would have of course been much more reasonably done, and much more extensive, if we weren’t trying to do it along with some emergency things. However, that in a way I think paved then the ground for the subsequent work of Aage Bohr and Mottelson. But of course this is a territory open for everyone, and I certainly wouldn’t pretend that if that paper had never been written, they would never have done their work, because physics was ripe for this at that time.
Did your paper with Hill predict rotational levels or just vibrational?
Just vibrational levels, and Aage Bohr had been interested in the rotational side, I don’t know the exact time sequence on this, as to when he came out with the story on rotational energy levels, but that was of course a really decisive point in getting this point of view accepted of collective motions based on single-particle interactions with the nuclear surface.
We've covered a lot of ground, because it was logical to do it in this way. Now perhaps it’s logical to backtrack and to take you back to some circumstances leading up to this work. You mentioned earlier that your fellowship ran out while you were in Europe in 1935, and the next thing we know about you is that you ended up as an assistant professor at the University of North Carolina. How does one go about getting a job in the midst of a Depression when he’s in Europe and the job is in this country?
I think it’s primarily that at a Physical Society meeting in Washington in the spring of 1934 I had given a paper on scattering as a mechanism to determine the law of force between particles, and I had beaten the drum for that idea, which was my enthusiasm of the moment and on which I had some new results to give, so much that it sounded so attractive that Arthur Ruark, who happened to hear it, thought that he would ask me to come to Chapel Hill, where he was the chairman of the department at that time. So that was really how I ended up in North Carolina, although the salary that I got there was less than the salary that my wife was offered as a schoolteacher at the same time.
Did you actively seek that offer from Ruark, or did he know that you were going to be leaving Copenhagen ...?
For all I know, Herzfeld may have written him.
When did you get married?
I was married about four days after I got back from Copenhagen. I had been engaged in New York. I’d been out five times with this girl and we were engaged and then I went to Copenhagen. Then we had these arrangements to be married as soon as I got back. It was of course the craziest thing in my life that I didn’t marry her and take her with me to Copenhagen, but those were the days when nobody had any money.
Going back to the question of nuclear forces, you arrived back just before the Yukawa paper on the meson theory. Would you say there was much significant work on nuclear forces prior to this? In other words, how far had Breit gotten?
It was a phenomenological analysis at that time that he was engaged in, and he had no theory of course of nuclear forces. I must say, to be absolutely honest about my own reaction to the Yukawa paper—of course, it’s had in many ways a great influence on point of view: however, perhaps because of this built-in idea of the electron theory of elementary particles, which I certainly don’t subscribe to in the slightest at this time. I at least was aware that you could have all sorts of mechanisms inside of particles that could have a very dramatic effect on the forces between them of such a kind that you would never be able to tell just from the forces what was the machinery that was giving rise to them. And around Johns Hopkins I’d seen enough of the people like Herzfeld and the Mayers who were considering the Van der Waals forces between atoms and molecules to realize that a force could look quite complicated, and yet the inner machinery that gave rise to it could be quite simple. So the idea of dragging in a new particle to account for nuclear forces rather than going down to the bottom and looking for the underlying machinery behind all of the particles never really made an appeal to me. Perhaps 20 years from now it will be a good time to go back and look back at the meson theory of nuclear forces and see what it really amounted to.
Did you have any change of heart at all or any slight vacillation when Neddermeyer and Anderson announced the discovery of the particle in cosmic rays that apparently was the Yukawa meson?
Well, this was of course wonderful, to have a new particle; and of course it followed so closely along with these developments in quantum electrodynamics—I think these developments in quantum electrodynamics that said that the particles couldn’t be electrons contributed as much as anything to saying they had to be mesons. But when it turned out that the mesons observed in the cosmic rays weren’t captured by nuclei with all the probability that one had been led to expect on the Yukawa theory, everybody was bothered. I was, I must say, one of the least bothered people.
When did people begin to be bothered, those who were vulnerable?
Oh, well, with the experiments of our colleagues in italy on the capture of slow mesons—Pancini, Conversi and Piccioni. Then I ininediately started working in that field. This was of course right after the war, and it was perfectly marvelous the way the Italians got physics going again so quickly, and these bright young men with simple equipment did very important experiments. I had given a paper in the fall—I believe it was the fall of ‘45, but it could have been the fall of ‘46— on the elementary particle problem as one was looking at it for the American Philosophical Society: “Problems and Prospects in Elementary Particle Physics.” In that I tried to forecast how things would go. I felt that although accelerators were important, still....
You mentioned this paper in 1945 on elementary particle physics, and I think you mentioned that still the best source of the highest- energy particles would be the cosmic rays.
Would be the cosmic rays, and so I resolved that the way to get going on this was to set up a cosmic ray laboratory here at Princeton, and so I did get a laboratory going, thanks to the support of other members of the department, and we were able to take over a building down the hill here that had been used for wartime work. So we had work going on spark counters and on meson capture and on absorption of cosmic rays, and here was a theoretical physicist guiding experimental work about five different projects going on. As time went on, I got more back into the theoretical side but with Tiomno and others on the mechanism of meson capture and so on, which looked very interesting. But then I came to the feeling that really, interesting as all this was, it wasn’t going to be the way into the really fundamental issues. So this leave of absence in ‘49, when I went to work on action at a distance from the gravitation point of view, furnished an occasion when I could gradually ease out of that work, and happily there was a colleague, George Reynolds, who was sufficiently interested in it so that he was willing to take over the directorship of the cosmic ray laboratory from then on, and he has remained in charge of it since.
Let me challenge you on the decision to build a cosmic ray laboratory as a source of high-energy particles when other people had come to some different conclusions after the war. What were the considerations leading you to this as opposed to people who were developing newer accelerators on new principles?
The motto of the Duke of Burgundy is: “Je suit presse” which means, “I’m in a hurry.” And in elementary particle physics, to build your own mechine is a long row to hoe. Cosmic rays are right there all day, every day. And it’s the same principle—I’m in a hurry—that made me give up cosmic ray physics and go back to the theoretical side. One is really of course enormously dependent on the efforts of his colleagues all over the country, and nobody could possibly get along without them. But just the fact that there are so many wonderfully gifted colleagues in experimental physics means that one can often make faster progress doing theoretical physics.
Can I characterize your quest over these years as pursuing a specific type of questioning and going in and out of various fields where it was possible to find answers to these questions you raised? In other words, go into cosmic rays because you think at the time it will give you the best answer and go out of it because you feel that some other field will be more productive—not as a field of physics, but in terms of the questions that you have a deep personal interest in.
Right. So that’s how, since 1953 onward, I have just stayed in this field of relativity because I feel now that’s really the promising way. It may well be just as mistaken as any other approach, but at least I’ve never seen any field in my life that just keeps on opening up and opening up wider and wider vistas as one goes on.
Would you say that a substantial number of your colleagues also have reacted the same way and pursued questions in and out of fields?
Right. I think Wigner is certainly a person to whom no field of physics is foreign. He’s of course covered a far wider range than I have, and yet I think that we share similar motivations.
What work have you done that gave you the most personal satisfaction?
I think that the thing that I have found the most excitement in was the point of view that electric charge is not a place in space where Maxwell’s equations break down and not a place in space where there’s some mysterious thing that you call electricity, but instead can be explained from this third point of view as electric lines of force trapped in the topology of a multiply—connected space. I think that is the development that’s given me the most excitement because it, more than any other single thing I can point to, makes me feel that the idea of space that’s topologically complicated in the microstructure is really a far—reaching and powerful and useful idea in describing physics. It makes me feel that one is on the right track in pursuing the implications of Einstein’s theory.
When did you develop that?
And this is still your present approach and your present line of work has led out of that?
Let me ask you another variation of this question. In terms of its impact on physics, what was the most significant work you’ve done?
There would be three things I would think about in that connection. I wouldn’t know quite how to size them up. One was the scattering matrix, which I had introduced simply for describing nuclear processes, but of course has since found a much wider field of application.
This is in ‘37?
I think so, I guess that was about it—-right. I think I’d put the scattering matrix or the work on the compound nucleus, I think those two. I don’t mean the compound nucleus, I mean the collective model of the nucleus as a starting point for work in this field, What I’m saying really is that in terms of the impact on physics, work that I‘ve done in relativity is still restricted to a small group of people that are working in that field. I don’t think that most physicists see anymore than I do how to apply such ideas out of the realm of relativity to the everyday work that they’re doing in nuclear physics or what have you.
I wanted to draw the distinction between the personally satisfying work, the fulfillment of a personal quest, and the larger scene,
Of course one can give different answers to different questions. If you’d asked me what had the most influence, I have the feeling that the period of work on the hydrogen bomb was the most timely, because I think I responded to the call to work on that and the urgencies to take part more readily than anybody in this country would do because I was in Europe at just that time in 1949 when Europe looked like a house of cards that could be just blown down. I was really very much worried about it.
And you responded on the basis of that immediate experience.
That was really a very deep worry.
Let me go back. The reasons we’ve jumped around is because of the limitations on time, and there were certain questions we wanted to cover because they just fell into place. But in this period after you returned to this country and your marriage and going to North Carolina in ‘34, you then pursued a line of work in nuclear forces in ‘36. And in ‘38, I guess it was, you came to Princeton.
Was that period special in terms of a new interest or was this the thing that you really had been doing in ‘34 before you left for Copenhagen? Was this just a continuity of the same type of work except in a new environment?
Well, the period in Chapel Hill was a continuation of the interests I’d had from New York and from Copenhagen, and I remember the first graduate student I had, Katherine Way, was working on the problem of rotating nuclei and what would be their properties. And it turned out that if the nucleus rotated too fast, there was no solution to the equations. Well, it would have been nice to follow that up, because it would have led immediately to the concept of fission. But we didn’t follow it at that point. But that was also the period of the development of the scattering matrix—again, as an outgrowth of this problem of trying to treat collisions between complex systems; alpha particles- alpha particles, which had really developed from the work with Breit in analyzing the experiments made in England on scattering, trying to understand scattering.
Did anybody else use the scattering matrix at that time, or did it need high-energy physics to become a greatly used tool?
No, it was used. Wigner was very interested in it, and then he began to publish papers in the field of the scattering matrix after that.
You were collaborating with Breit in the period when charge independence was recognized. Is that right?
How did this come about?
Well, I must say that I would find it very hard to recall the details of who did what on that charge independence. This was the period when these empirical expressions were being introduced for nuclear forces—the Heisenberg force, the Bartlett force and the Wigner force and so on. One still hoped to account for a saturation of nuclear forces and the fact that heavy nuclei didn’t collapse purely on such symmetry grounds. It’s interesting to me how very long it took before those hopes were given up, and people frankly leaned on a hard core to prevent it as a cheap approach to the problem. But to introduce the hard core in those days would have been regarded as cheating.
But were people led to charge independence on a symmetry argument or was there any real experimental evidence in—when was it—36?
Well, there was the semiempirical mass formula, showing how masses depended upon the number of neutrons and the number of protons, and there were measurements made, particularly here at Princeton, on the betadecay energies between one nucleus and its near complement to check that the energy difference could be accounted for purely as a Coulomb effect. So one could say that there was no real difference between the nucleonic forces.
In other words, this is something a lot of people suspected at this same time.
Right. It was really built into the Heisenberg paper of 1932, should say, as a kind of a tacit assumption. The subsequent work was more a refinement of that idea and a checking out, would say, than producing the idea in the first place. I would say the Heisenberg paper had it, Well, am absolutely honest there or am absolutely correct? No, I think lm confusing two thoughts. Im confusing the idea of equality of protonproton force and neutron-neutron force with a more extensive idea that youre bringing up: How do those two forces compare with the third force, the neutronS-proton force? And that third topic really came more to the fore when Wigner got going on the idea of supermultiplets. And that was already, I think, in the period 1937 that he was working on that idea,
And this was on symmetry grounds rather than any real evidence, right?
Who else in this country at the time was working on problems of this type?
Yes. Or capable of reacting....
Well, I would say Bethe. Van Vleck had been doing some work on interaction of neutrons with nuclei, and he had a general interest in the subject, and certainly Breit had a general interest in the subject. But I think that symmetry in the larger sense that you're speaking about-Wigner was the real driving force.
By this time was most of the work in theoretical nuclear physics beginning to be concentrated in this country? Was that trend apparent?
Very much so.
Was it because of the influx of refugees partially, do you think?
Right: Placzek at Cornell and Weisskopf coming, Bethe being at Cornell.
There are a number of people, of course, during this period. What brought you in 1938 to Princeton?
Well, had already visited Princeton in 1937 on leave of absence from Chapel Hill for three months I’m not stating it in quite the correct order. I had got a leave of absence to come here half-way through my three-year appointment at Chapel Hill for a semester, so I could work here; and then was invited back to give some lectures the following year. I’ve forgotten whether it was for a two-week period or something like that. This was at the time when Princeton was making negotiations to get Wigner here from Wisconsin, and their plans for the future depended really very much on whether he was going to come or not. And until that decision had been reached in his case, the University hadn’t come through with a decision as to what they wanted to do about inviting me here, I knew that they had thoughts in that direction because several of the people here had told me that.
It was particularly difficult to know exactly what one was going to be doing for the coming year in my case, because there was also at that time an invitation to go to Johns Hopkins. But I waited on that until the Princeton decisions had come through. Then I came here. The combination of people in physics and mathematics here is unique, and that’s what then appealed to me and what continues to appeal to me as being quite marvelous. In our new building that’s going up across the street here, the physics and mathematics are again right next door to each other and again linked by a common library.
Why did they wait to find out what Wigner would do before inviting you? Did they expect to develop a certain type of department?
It’s a question of working out the proper balance in the department. And since normally departments work these things out in their own inner councils,I don't know all the factors that went into it.
I have another question about the atmosphere here when you came here in 1938, You mentioned this close tie of mathematics and physics, was this through any formalized seminar? I know there was a journal club with that title, I imagine, here.Was this a journal club for physicists, for example? Or did the mathematicians also participate?
The journal club that met Monday nights was a journal club quite for physicists. Of course anybody was welcome to come, but the mathematicians wouldn’t have been attracted. The ties really come from the kind of individuals that there have been here, people like Veblen, a pure mathematician who was interested in physical problems. Of course later he left the University for the Institute for Advanced Study; Von Neumann, with his interests across the board; Wigner; Marston Morse, who had through his work in the calculus of variations an interest in physics problems.
How far back does it go? I can’t tell you the whole story, but I would say that Veblen had a great deal of influence on the favorable way things worked out in that he had a good many of the negotiations on the building of this mathematics building right next to the physics building and how the building would be constructed and the fact that the library would be in there; it would be a joint library in this union between the two buildings. The tea was a common tea at four o’ clock every day until the size of the two departments grew so big that now there are two separate teas. That’s lost us some contact, but some of us occasionally drop in at the other tea. And you remember how, when the Institute for Advanced Study was being built, the architect would come and sit in the tea room here so that he could get a little flavor of the sociology of the place so he would know what to do. And then you remember how Oppenheimer later stated that “tea is the place where we explain to each other what we don’t understand.”
These were happy years then at Princeton. They were interrupted because of the war, The story has been told many times—mostly recently, I guess, in this latest book on the Manhattan Project—of Bohr coming over with Rosenfeld in 1938, in the latter part of ‘38, I think, was it? And you met him at the ship here, and it was then that you learned of fission, Then you apparently worked with him very intensely on it. What were the circumstances of this work? How much did you do independently? How much did you do collectively? And when you did work together, what sort of interchange was it? Although you had interacted before, this was the first time that you...
We collaborated, Right, that’s correct, yes. I would say that even the discussions that I had with Bohr in '37 on his visit to the States were foreshadowing collaboration on some kind of nuclear physics. If he came here, I would have suspected we would have found ourselves involved in it; but fission of course made it especially urgent. But the whole thing was really to discuss all the issues, we generally met for our discussions in my office, which at that time, before Fine Hall had become so crowded, was over there—and also his office was there, both on the same floor, and he would generally come into my office for these discussions. And in the meantime, when he wasn’t in there, because discussion would perhaps be an hour or two every second or third day, I would be calculating this or that thing. We would often bring somebody else into the discussions. Of course when he was here, there were many calls on his time to visit one or another person and talk about one or another issue, and there was always written down on his blackboard on the lefthand column a long list of the things or people that he had to see or do something about. He always had this terrible conscience about them. By coming into my room, he didn’t have to look at that first.
Was he working under some sort of compulsion during the period? The implication was that there was a double compulsion—one, the urgency of the question of fission in general, and then the other one, the urgency of the problem of proper credit for Frisch and Meitner. Was this apparent to you?
Yes, this question about being concerned about the credit for Frisch and Meitner. That part of the paper he dictated to me, the beginning part about how to put the proper credit for the people and so on, the wording that he thought would be good for it. The other parts we discussed a lot, but for the most part it was easy enough for me to write them up. The biggest single development in there was the theory of the transition state complex, the rate of going over the nuclear barrier. And there I had dug around, talked with chemical friends and also with Eugene Wigner, who had been in older days concerned with nuclear reactions. He was in this next building, the infirmary, with hepatitis at this time. It rather knocked him out. But just to be referred to his long-ago work on the rate processes in chemical reaction gave some insight which was helpful in arriving at this final formula for the cross section in terms of the number of channels that are accessible over the top of the fission barrier, a beautifully simple formula— only at that time the word “channels” had not even come into use, so you won’t find it called the “channel formula for fission cross sections.”
So there was some discussion with him on this?
Yes, yes. You were asking more about this method of collaboration. Of course I don’t want to repeat these stories which were all so much fun at the time: rushing up to the library to look up Rayleigh’s Collected Papers; also on another occasion rushing up to the library and consulting various dictionaries and thesauruses to see if there was a better word than fission so that you could have a verb that went with it better than”this nucleus fissions.”
“To fish” would be the verb.
Well, it was the verb for a while.
Was there a serious concern about this?
He was concerned about this question of terminology.
It’s interesting. In the course of interviews and in looking through correspondence we’ve come across similar debates on the word “positron” and the word “mesotron”—very serious discussions. People apparently have strong feelings about what’s proper and what makes good physical sense.
The deuteron is another case and the dipon.
Yes, and it’s a question of how you could change the name and get something that was better suited to the real meaning of the language and yet not do violence to the feelings of the people concerned with the discovery.
One decision was fortunate, and that was avoidance of the term “yukon.” I guess people wanted to name the original meson the “yukon” after Yukawa. I’d like to ask another general question. I think this subject of the human elements of the reaction here and the response is something that should be explored at length, so I’d rather not pack it into a few minutes and perhaps return to it sometime. I’d like to ask about the effect of fission itself on the development of nuclear physics.
Right. It’s very interesting in that connection that survival of the species is dependent in some ways on nuclear fission, in some ways on sex, and people are equally unwilling to talk about both. But in the post-war period nothing struck me as more remarkable than the scant mention of fission in the literature of nuclear physics. Of course there was a very good reason for it for a long time. In the whole period from 1945 to 1955, the key work on the subject was classified, so nobody in his senses working in a free laboratory would work on it for fear that what he might do would already have been duplicated in a government laboratory.
So this meant that really fission physics made far less a contribution than it might otherwise have made to the clearing up of ideas in nuclear physics in particular. And it’s all the more remarkable because of all the processes that you know in the whole of nuclear physics, it’s the one, par excellence, where a collective mode of deformation is the whole works—separating. So it’s a field of physics where in my view there’s still an enormous amount to be found out. It’s still a mystery why the charge divides unequally.I think that stands to tell us a great deal about the relative importance of average forces versus shell-effect forces in nuclear division.
You did do some subsequent theoretical papers on fission in the ‘50s—right?
Right. And then the most comprehensive account of the subject is in Fast Neutron Physics by Fowler and Marion. There’s a thing called “A Channel Analysis of Fission,” For a few friends took that chapter and had it bound up as a book for people working in that area of nuclear physics. The real paradox today is this channel number that’s supposed to give a cross section for fission, in fact, has not been checked with anything like adequate precision. If you look at some of the numbers, you have the feeling that there are discrepancies—maybe by a factor of five. And people produce new measurements, and the factor of discrepancy changes. Is there something wrong about the way the measurements are being made or is there something wrong with the formula? We aren’t really in a position to say yet.
Let’s return to your earlier statement. I think it leads us into a concluding area of discussion, at least for this interview.
Could I just take one minute? I don’t want to forget one thing. At the time the fission paper came out with Bohr and myself in ‘39, there were many people who felt that there were lots more specificities, differences between one level and another, than were envisaged in that paper. That marked a difference in point of view that it would be interesting at some time to explore, but let’s not do it now.
Let’s make a note to do that. You made a statement about the fact that fission physics did not play as effective a role in the development of nuclear physics as it could have in the post-war period, and you gave some reasons for it. At the same time, though, it seems that people who had been in nuclear physics prior to the war and in its applications during the war left nuclear physics per se and went into what we now call elementary particle physics— for different reasons, I gather, than the ones you mentioned about not exploring fission. How do you account for this branching off in this post-war period? Let me give you a prelude to it and ask: Do you remember any discussions towards the end of the war when people were planning on what they would do in peacetime when they got back to normal operations? Were there such discussions with plans on the agenda for what kinds of problems one wanted to tackle? In your case I think it was clear. You mentioned you immediately went into the cosmic ray laboratory. But do you remember discussions in general of this type?
Well, at first let me say that although one has talked a great deal about the influence of science on wartime developments, the converse effect of the Radiation Laboratory at MIT and Los Alamos on the outlook of scientists was a very powerful one, too, in the sense that people could see that somebody with a vision, a dream, could accomplish something quite marvelous if the vision and the dream made sense. So people were encouraged to think big in a way that had never been possible before, and consequently there was a discussion going on about accelerators after the war and already before the war ended—what one could build in the way of new accelerators. About the kind of physics that one would do in this period, I think that I was really in a poor position to be a reporter on that because I was in the state of Washington from July of 1944 to September of 1945, most of the time on the Hanford plant, so I couldn’t consult with colleagues in the same way that other people would be doing.
Yet in your own view you were interested in higher energies because this was a way to an understanding of fundamental particles, and you were still pursuing that. Do you think the interest in accelerators on the part of this other group was motivated by similar desires, to achieve again higher energies for that purpose? In retrospect it makes sense, but it’s not always clear that that was the feeling at the time.
I think it would be a very good thing to see this explored, but, as I say, to get a really representative story. I am a very poor person to give this to you.
Your own reaction was a good one. I mean that part of it is clear.
I remember Fermi at this time was interested in mechanisms by which the cosmic rays got accelerated into interstellar space. We had talked some about such issues when he was at Hanford on visits, but we didn’t talk, to the best of my recollection, at that time about what he was going to do himself in the field of physics in the post—war period.
If you take the Shelter Island conferences, the three conferences, in ‘47, ‘48 and ‘49 (we characterize them as Shelter Island type conferences), the topics of discussion vary, but quantum electrodynamics certainly was one of them. Do you think that it might be useful to examine those conferences at close range to determine the answer to this type of question?
I think it would be exceedingly important because just as the big article of Bethe and Bacher marked a certain turning point in nuclear physics, I would say those Shelter Island conferences marked a turning point in the physics of the post-war era. I’ve always felt that the contributions of Kramers were not sufficiently recognized in producing a point of view—an outlook which then others could develop the mathematics for—on the post-war developments in quantum electrodynamics. His paper is in Dutch unfortunately, but the point of view was certainly circulated around and particularly at the first Shelter Island conference.
In this last minute do you feel that these conferences, although they certainly are interesting from many points of view, are really significant in terms of the transition from nuclear physics being a self-contained single field for a good many people to the state where it is no longer a field of interest for a large group of people who had previously been deeply involved in it?
Yes, I think it marked electrodynamics becoming the prestige subject.
I’d like to explore what you mean by “prestige.” I think that perhaps since there are too many other questions of this type that maybe we better leave them for open discussion at the conference itself, perhaps with some clues to them, and quit right here. Thank you very much.
[ *Natl. Bur. Stand. J. of Research 6, 239-275 (1931) ]