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In footnotes or endnotes please cite AIP interviews like this:
Interview of Frank Hoyt by Thomas S. Kuhn on 1964 April 28, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4683
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This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Henrik David Bohr, Paul Adrien Maurice Dirac, Albert Einstein, Werner Heisenberg, Ernst Pascual Jordan, Hendrik Anthony Kramers, Walther Nernst, John William Nicholson, Wolfgang Pauli, Erwin Schrodinger, John Clarke Slater, Arnold Sommerfeld, Harold Clayton Urey, John Von Neumann, Eugene Paul Wigner; Kobenhavns Universitet, and Universitat Berlin.
… I might ask you first about the origin of your trip to Copenhagen.
Well I was at that time an instructor in physics at the University of Wisconsin. Theoreticians were a rarity and I was attempting experimental work under rather difficult conditions; very heavy teaching schedules, as you know, were quite common in those days. For a long time I’d been interested in the Bohr theory, although I was actually working experimentally in X-rays and crystals. I made an application for a National Research Council Fellowship to study with Bohr. It seemed a reasonable idea. I think that was about the first year that the National Research Council had decided to give fellowships for study abroad, with the feeling that American physics needed contact with Europe, particularly with theoretical physics, in Europe. Strangely enough I was successful in getting this fellowship. I had previously made an application for the Scandinavian Foundation Fellowship, and I’d gotten turned down on that one. It went to [R. B.] Lindsay, who was already in Copenhagen at the time that I arrived, which must have been about September, 1922, I think. I went over with my family and spent a couple of months — well this is extraneous, but —.
When you say your family, you mean your parents?
Yes, yes, I wasn’t married at the time. And after a couple of months I guess of traveling in Europe, I went up to Copenhagen by myself. I remember Bohr telling me when I got there that they had been somewhat puzzled about this application that I had made to study there. It wasn‘t entirely usual. But they looked around a little bit and found I had done some experimental work on the excitation potentials of X-rays, and I seemed to be a reasonable person to take in.
When they said that it wasn’t entirely usual, did they mean that the form of the application wasn’t usual, or was it that the institute was so new at this point that they weren’t very used to having visitors at all?
It was very small, as you know.
You said you’d had a long-standing interest in the Bohr theory. How much acquaintance with it in detail do you suppose you had?
Well probably quite detailed I would say in terms of what had been published and was available at that time. I was really led to it through thinking about the intensities of the X-ray lines, you see. I think that I must have written this to Bohr as the thing I wanted to work on when I was there. That through the correspondence principle there should be an approach to the intensities of the X-ray lines. This is actually what I spent the first — well I guess really the whole — time I was in Copenhagen on. It was an attempt to understand the intensities of spectral lines in terms of the correspondence principle. This was a fruitless quest, as you know, in that —
Well they didn’t come out terribly well, but they led to some interesting work.
Well, it had to depend on the harmonic components of initial and final state, and what combination of these two or what integral between the two states would give a correct account of the intensities. It sounds very childish now, but Bohr took it extremely seriously and was very much interested. I remember his making the remark, “Well someday this just has to be worked out, if the theory is at all correct.” Then I think towards the end we both came to some sort of realization that the intensities were themselves a property rather than something to be derived by any integration over the orbits. At least this was just a glimmering of what came just shortly afterwards in picking out the frequencies and the intensities as the important things.
You’re putting your finger exactly, at the moment, on what I had in mind when I said the answers may not have come this way but it led to some extraordinarily interesting developments, and I hope we can give that more structure, exactly that transition. If you were there from the fall of ‘22 through the spring of ‘24, you were there from the time of the work you yourself were doing on the correspondence principle right through the origins of the Kramers dispersion formula. That is very much this particular transition, and one about which any conversations or intermediate stages in that transition we can get would be tremendously helpful.
I wish my memory about it were clearer, but it’s a little fuzzy.
Well let me ask you some questions and see what comes… You do two papers in the Phil. Mag. [46 (1923)], one in which in particular you are dealing with intensity of X-ray lines… and a second paper [Phil. Mag. 47 (1924)] where you really turn from X-ray spectra to hydrogen and really to absorption spectra. You had to do absorption spectra because there you’ve got the advantage that you’re proceeding always from the same initial states so that you don’t have to worry about weights in various states.
Hmmm, you remember more of it than I do!
’ Well I have looked at it, however superficially, considerably more recently than you have. These papers raise for me a number of problems, or at least a number of interesting areas of exploration. …For example as you spell out the correspondence principle for intensities, which in one sense is the principle that had existed at least since 1918, since Bohr’s “Quantum Theory of Line Spectra,” where they’d been using Fourier components to get some indication of intensities, you make a good deal of tying the correspondence principle into the Einstein coefficients.
With some hint, and I think possibly an appropriate one, that this is a rather different way of developing it from what has been standard in the past. What I’m fishing for here is this: Jordan in particular, in talking about certain of the background for his own work, has said what I think is hinted elsewhere too, that until fairly late in the game, the notion that the Einstein fluctuation formula, and the Einstein coefficients really tied deeply and closely in with the correspondence principle, was something that Bohr rather held at arm’s length. I had wondered whether this also fitted that sense of novelty which I get in your own pointing to the manner in which you’re developing the correspondence principle, or whether you felt at the time that you were simply spelling it out again for purposes of your own paper.
I think it’s more the second. I don’t think it was anything at all new in the way in which the correspondence principle itself was applied. It was only that there were data in the intensities of the lines that should be made use of, and that what it was doing was really looking for the exact connection between the intensities, or you may say between the Einstein coefficients because the two were the same thing, and the dynamic characteristics of the orbits. Did it depend, for example, only on the harmonic component in the initial orbit, in emission, or did it depend on an average of the initial and the final, or was there possibly an integral of some sort which averaged continuously between the two orbits? It’s nothing deeper than that.
Linear, quadratic, logarithmic —
Yes, yes, [laughing], I guess so. Then the last paper that I did publish on that subject — well I don’t know whether it was the last—it was not quite completed at the time that I left. It was on the harmonic analysis of the rotating orbits you see. And I had worked this out—it wasn’t entirely evident, how one would treat the penetrating and rosette type orbit—that one could do this in rather a nice way in terms of the Bessel function analysis of elliptic motion. That appeared in the Physical Review [25 (1925) pp. 174-86. Submitted from Chicago, 3 Oct. 1924] after I returned, to this country but that work was done in Copenhagen and I did have some correspondence about that after I came back to America. Unfortunately I don’t think I have kept it, I might take one look at home, but I’m quite sure that I didn’t. I do remember talking this over with Pauli.
He was there, what, the second of the two years you were, or just in and out?
Yes. Well he was in and out; not when I first arrived, but somewhat later, for rather extended periods of time as I remember it. I remember one time that he was making very serious efforts to learn the Danish language. I have the recollection of an evening in which we were sitting around; Kramers was there. I don’t remember whether this was at Bohr’s house or where it was. Pauli refused to speak anything but Danish all evening, and whenever he came to an unusual word he would turn to Kramers and ask him what it was. He would give the word in German, and Kramers would give it back to him in Danish, and it nearly always turned out to be almost the same thing because these were technical terms of one kind or another.
How was your own Danish?
Pretty good at that time. As I found the last time I was in Copenhagen, it’s deteriorated greatly since that time. But then I lived with a Danish family and spoke Danish continuously for quite a while. But Bohr and I always spoke in English. I made some attempt to suggest that maybe we should speak Danish when we were traveling, but of course he didn’t like this very much. He preferred to practice his English. Heisenberg was there too, I think it must have been towards the end of his stay. One little picture that remains in my mind is Heisenberg sitting reading Gibbs Statistical Mechanics in the library while he was waiting to see Bohr. Bohr had told him that he should read Gibbs, and there he sat, Gibbs in front of him for hours on end at the table in the library, reading Gibbs from cover to cover.
Did he see why it was that Bohr liked this so much better than Boltzmann?
No. At least we had no particular discussion on that. It puzzled me a little bit. But Bohr was giving I think a course in statistical mechanics at that time, and was very much enthralled with Gibbs. Let’s see, the other people who were there at that time, I presume you have or will contact — Lindsay, Harold Urey for a relatively short time, Slater — I think these are the only Americans.
Who else do you think of among the Europeans? Heisenberg and Pauli, Kramers, of course, Klein I take it was again in and out.
Yes, Klein was there; Rosseland and Hevesy too.
Was Coster there while you were there?
I don’t think so. May have been for a day or so.
You must have been in Copenhagen or near abouts, when word first arrived about the Compton effect.
Yes. Well, I just don’t have any recollection of this, I’m sorry to say. You asked it in the letter and I’ve been thinking about it since, but —.
Yes, well I raised it there thinking that perhaps if I did this would remind you of things in the interim.
Yes, well I should have, but I just don’t.
Nothing about what sort of a surprise that was, or whether everybody believed it or not? It‘s curious of course that now everybody tells you, “Well yes, it was hard to believe but of course it had to be right.” Then you look back — where one can find original literature at the time one notices that really not everybody believed it. In particular you know in Cambridge there was the Kapitza Club. I’ve seen the minute book of the Kapitza Club, and there were a couple of meetings where the members informally voted as to whether Compton was right or not, and on the whole — I don’t know that everybody voted, but those whose votes were recorded show a small majority against the Compton effect.
I would say at that time there seemed to be very little in the way of, say, group discussion. I hardly recall any seminars. It was more individual contact with Bohr, and of course between two individuals.
In the second of those two papers in the Phil. Mag., you point particularly at one point, to the Ladenburg and Reiche formula, the formula with which they have — and I think it’s really mostly Ladenburg’s work in the first instance — related dispersion to absorption and to the f coefficients for the electrons in the atom. I had wondered a good deal about the extent to which that work, the Ladenburg-Reiche work, was known and discussed. To put perhaps too much of the point too soon, it’s terribly important by the end of your time in Copenhagen, the extent to which people are thinking of the intensities as the intensities of harmonic oscillators that operate between states. This seems to come out of the Bohr-Kramers-Slater paper. On the other hand, it’s been perfectly possible to take some piece of it out of the Ladenburg-Reiche work earlier, and the extent to which that was a working tool for people before the extended correspondence principle and the Bohr-Kramers-Slater paper, is a question of considerable interest to me.
Yes, but I’m afraid I can’t help very much. My mind seems to be something of a blank on that. I was perhaps too much concerned with the details of trying to find the harmonic components for the penetrating orbits. I think it was the second paper in the Physical Review, too, after I got back, but again this was a fairly routine sort of business. I don’t think it began to touch anywhere on the basic questions. I remember in my own mind thinking about something like the Heisenberg idea of concentrating on the intensities and the frequencies as the important entities, but nothing really came out of this. I think one interesting thing is how seriously Bohr took these rather crude ideas, and his attention to all the details, lie was very much interested in this work. I also remember doing something, I think it was in a mercury spectrum, and he asked me to make up a complete diagram of this spectrum by putting in the lines in color.
Yes. [laughing] It sounds a little naive, but he said, “Well, you never know”; maybe one would really get something valuable out of this, just looking at all the details of this kind, you see.
Did you get any sense that this was something he’d done before with other spectra, and that this was a fairly standard way of working?
Yes, yes. He really liked to see the spectrum in color.
And it was the spectrum he wanted in the first instance? You weren’t making up a term-level diagram and then —.
Yes, it was like a term-level diagram with thick broad lines for the intense lines to indicate the intensities.
That does interest me. Were you in at all on any of the discussions that went on around Slater’s idea that then came out in the Bohr-Kramers-Slater paper?
No, I seemed to have missed out on that. I had some discussions with Slater about it, but was not involved in the preparation of that paper.
Do you have any recollections of your discussions with him — with Slater?
No. Not anything that I can recall. We did have some discussion and it seems to me they were more about details of some of Bohr’s papers that were obscure to both of us, and we were trying to disentangle them. Slater wasn’t there very long.
He went off I think to the country fairly early, and then on from there. He rather indicates now the sense that he’d come in with a good idea that was being shoved in directions he wasn’t altogether enthusiastic about, and I think this may have had something to do with the fact that he was not there longer than he was.
You have talked to Slater?
Have you talked with Urey?
I haven’t, but my colleague Heilbron … fairly recently has talked with Urey. I have not yet seen the outcome of that conversation, but be had quite a good and interesting time with him I know.
He was working on somewhat the same problem Lindsay was at that time, kind of foreshadowing the Hartree procedure, the consistent orbits.
I take it that if you don’t remember or didn’t take part in discussions of the Bohr-Kramers-Slater paper, the same thing would be more or less true about the Kramers dispersion formula.
Did you talk with Kramers about that?
Well, I did, I talked quite frequently with Kramers but my memory is failing me, I’m sorry.
You said you traveled with Bohr in this country during the course of his trip. I’d be very much interested in hearing anything you do remember about that trip.
Well, Bohr came over here for two things, primarily to give a series of lectures at Amherst and the Silliman lecture at Yale, but of course he visited a number of laboratories at the same time. At that time the Bohr theory itself was not as veil established as one might have believed. You remember these earlier approaches by Nicholson and Nagaoka and also by Langmuir, the so-called “static atom.” Bohr was quite frequently sort of pushed to the wall on these things. I remember a visit to General Electric. Or was it Langmuir … Wasn’t it Lewis?
Well Lewis, if you’re speaking of the “static atom” in particular, this would be more nearly Lewis’s atom than Langmuir’s, though I think there’s some overlapping of the two.
Yes, yes, I think it was Lewis though the two do overlap somewhat. I remember their having a model of this “static atom” at G.E. with lights and so forth to illustrate various things.
When you say Bohr was pushed very hard on this visit to G.E., would this have been Langmuir in particular who was doing that?
That I can’t quite remember either, I’m sorry. I do remember that he was very much impressed by Robert Frost who was at Amherst at that time, and there were several discussions with him. Unfortunately I can’t remember very well the content. But this apparently was something that Bohr always remembered, because whenever I would see him later on he would always bring up the name of Robert Frost, until the very last. This was something that was always on his mind.
That interests me very much.
I remember more of these personal anecdotes than I do about —.
Well they also are quite helpful. I hope you will, whether they’re physics or more personal things of this sort. Ultimately, particularly of course so often with Bohr, the physics is so much part of the man that I think it would be a terrible mistake to try to separate out the strictly technical from the mere personal. Where else did you go?
Well I think the only two places — well those three are the only ones that I remember having been with him at — Amherst and Yale and this trip to General Electric. I think this was at General Electric in Schenectady, although I can’t quite be sure. At any rate it was with the General Electric people.
That sounds like the right place for them to have had a model.
Then Chicago I remember, but I can’t remember whether he gave a lecture at Chicago or not. I know he had Thanksgiving dinner with us, with my family in Chicago, and I have one picture of him from that time on a very cold and raw day in a park in Chicago. It’s the only photograph that I think that I have of Bohr of my own.
Were there many people in this country at the time who shared your own concern really with the Bohr atom, with these developments?
Oh, yes! There was a great deal of interest. This was a little uneducated and uninformed perhaps at Amherst.
Who had been responsible do you suppose for bringing him to Amherst?
That I’m not sure. It was some lecture foundation or some fund that Amherst had. And then there were the Silliman lectures at Yale — which were never published incidentally. I used to spend a good deal of time helping Bohr write papers. The agonies he went through in trying to formulate his thoughts I will always remember.
Do you remember which papers you particularly worked on with him?
I think actually we spent some time trying to write the Silliman lectures although they never got published, and also the third part of the ‘Atombau & Spektrallinien’, that series of papers, which again I think never got published, did they?
There’s a very very small piece of it that was published about ‘23, but that was really a substitute for writing the third and fourth part. There were originally to have been four parts. The first two of course come out in ‘18, and then the third part just about, I would say 15 or 20 pages, as a gesture towards the announced subject matter, and then there was nothing. I think I’m right in saying there was originally to have been a fourth part.
I know I translated a number of papers from Danish into English, but gee I just can’t seem to remember what these were. The one that I do remember was on the subject matter, at least, of the third paper, I think, on line spectra. It was after he returned from America. I remember spending an evening with him in his house at Tisvilde where we had a big bowl of shrimp and a bottle of port, and worked through some little detail of an interpretation of more complex spectra. Bohr was very happy about it, it came out so nicely. He said, “Now, well now we can let the mathematicians puzzle over this!” As you know he was not very sympathetic to some of the mathematical work in physics. I think he was sympathetic to pure mathematics, but he never liked to see mathematics at all emphasized. You could see this in his first paper on the alpha particle, stopping power for alpha particles where he missed a more or less conventional mathematical representation. I’ve forgotten now just what it is, but one of the integrals that he has there I think he did numerically — well, no, how is it?
There is an integral there that I think he does do numerically.
If there is, then that’s perhaps the one. You might like to look into that. I think that is a tabular integral, or at least expressible in terms of known functions.
That’s interesting. I hadn’t realized that. It’s a very plausible story.
I discovered this many years ago in going over that paper. Not so many years ago — much later, in the forties some time. Along the same lines — again you probably know, he was never an admirer of Sommerfeld. He once remarked to me — I guess this is all right even for the recorder —
At this stage of the game, nobody is going to be offended, and this whole subject of the difference between the Bohr and Sommerfeld approaches to these problems is really of great historical significance.
Yes. Well the one remark he made to me was that he thought in every fine point that came up, Sommerfeld was wrong.
Now what sort of point did you take him to mean by fine point? He didn’t mean the fine points of mathematics.
No. Oh dear. I have a vague thought as to what it was, and yet I can‘t dredge it up. I think it was in connection with the interpretation of the relativistic fine structure of hydrogen, just of hydrogen. But just precisely what it was I’m afraid I just can’t —. And ‘wrong’ is perhaps not quite the correct word to intimate, because he never quarreled with his mathematics.
You speak of Bohr’s interest in these ‘crude’ methods of the day. In certain ways it was obviously perfectly clear to Bohr that these needed to be replaced with other methods.
What is less clear to me is how deep Bohr supposed the reformulation was likely to go.
I think he thought it was likely to go very far. Towards the end of my stay there he was quite convinced that fundamentally the orbital theories were insufficient. I do remember him saying to me, “Now everything is in Heisenberg’s hands — to find a way out of the difficulties.”
Oh, really. “Now everything is in Heisenberg’s hands.”
It was not a question of any competition between Bohr and Heisenberg or of any reluctance on the part of Bohr to a radically new approach. He saw it coming, hoped that it would come, and evidently had faith or the belief that Heisenberg was the one who would find it. That was about the last remark that I can remember before I left Copenhagen.
This would then have been sometime in the spring or early summer of ’24.
I guess so. When was Heisenberg’s first paper written?
Heisenberg’s first paper was not written until 1925, sometime in the summer of ‘25, so this would be a whole year before.
And I left in the summer of ‘24 or the early fall of ‘24 I think.
So that this would really be a full year before the first of the Heisenberg papers was done.
But presumably there was some exchange well before the paper actually appeared. Is this the time that it was written or the time that it was published?
I would say it was actually written early in the summer. I’d have to check. It certainly was not in Heisenberg’s mind, yet in anything like the form in which it appeared. On the other hand, during the time that you were still there, this would be the period in which Kramers and Heisenberg were working very closely together on the extension and derivation of the big paper on the Kramers dispersion formula. And a lot of the ideas that ultimately worked through and that grasp of the extended correspondence principle was presumably one that Bohr sees in action during the work on that paper. But he really had sort, of ‘picked’ Heisenberg already while you were there —
— as the person who was to do that? Did it surprise you at all that he should pick Heisenberg rather than either Kramers or Pauli?
No, I don’t think so. I remember also one remark he made to me, as a way of comparison between Pauli and Heisenberg. He said, “Of course Heisenberg is the one who has made the great discoveries.” I think this was before the exclusion principle, it was quite early. He said, “On the other hand, I think Pauli is the one who really feels the poetry of physics.” He separated them as sort of different individuals.
They were of course terribly different. That’s also very interesting to me.
And he [Bohr] didn’t feel very sympathetic to Sommerfeld. I don’t think Sommerfeld ever visited Copenhagen.
Not to my knowledge, no. He of course sent students there, he interacted with Bohr, particularly at Gottingen at the meetings just before you arrived, I think, the summer of ‘22, and they corresponded a reasonable amount. I’m much interested in this remark about the extent to which Bohr was convinced by the end at least that the thing had to be radically recast and the likelihood that Heisenberg would be the person to do it. To what extent do you suppose this feeling of his about the necessity for a really deep recasting had developed actually during the period you were there?
I think it did develop during that period, particularly towards the end of that period.
What do you suppose was particularly responsible for this changing attitude? The paradoxes of quantum mechanics had been there from the start and he was quite aware from the very beginning that at least you couldn‘t reconcile these quantum methods with classical mechanics and electrodynamics. So that aspect of it he points to from the start. Now what is it in the years ‘22, ‘23, ‘24, that is leading him further?
Well I think it was the difficulties in the Zeeman effect and its relation to the fine structure of spectral lines. Again details of such escape me, I haven’t thought back to these things for a long time. I’m sure there are other people who can probably help you a great deal more on this, but I’m sure it was in that area that it became rather clear that there were inconsistencies. And I think the difficulties in getting an exact theory of the intensities of spectral lines had something to do with it too.
Were there many discussions in Copenhagen of the Zeeman effect problems?
Who would be particularly involved in them?
Well I think Kramers. Bohr always worked very closely with Kramers and relied on him for all details. Also in England — I remember on the way to America we stopped in England for a meeting of the British Association, and I remember him having discussions with the English physicists at that time, sort of shaking his head over the puzzles of the Zeeman effect and the fine structure.
By the time you reached there, had he really pretty well finished work on the periodic table?
So that was pretty much behind.
That was pretty much behind I think, although he was always concerned with it.
Do you remember anything particular that he said about Stoner’s work in the same area?
No I don’t.
Did he react well or badly on the whole to the reception on this trip to America? Compared to the other areas he’d been in, it was a relatively backward one in physics, wasn’t it?
Yes, it was. Well, I think it’s possible that he was a little disappointed in not finding, shall we say, more intelligent criticism of his ideas.
To what extent was he able to communicate ideas on that (level)?
Well of course (this suffered). He had a terrible time lecturing in English. This may have had something to do with it. But I wouldn‘t want to give the impression that he was really unhappy or that he felt that he wasn’t well received.
Do you have a copy of this very early book by Kramers?
Ja. I don’t know my way around in it at all well. Excuse me, which one do you mean, the Kramers thesis on the hydrogen atom, or the [H.] Holst and Kramers book on —?
No, this sort of semi-popular book [the Holst & Kramers book] … This touches I think on these rival theories which were still not completely out of the picture at that time. I don’t have a copy of that, I think it was in English not Danish.
I think that book is in Dutch actually. I’m not sure it wasn’t translated. I think it may have been translated, but it’s Holst and Kramers. I think, not just Kramers, and originally, I think, in Dutch. Let me check one thing you said. When you mentioned these alternate theories, the theories of Lewis and Langmuir, you also said Nicholson. Was Nicholson taken at all seriously in this country still at the time you were here?
He probably was. I seem to remember him way back at M.I.T. before I went to Stanford. One thought of these as alternatives, the Bohr orbital theory and some of these others, as possible.
And Nicholson would have been one of the viable alternates?
I think so, I think so; all these rings and things, I’ve forgotten much of it now but —.
That interests me a good deal because I’ve never been clear how well the Nicholson model was known and how long after Bohr anybody took it at all seriously. It’s a perfectly plausible sort of scheme and it shares a good deal — plausible until one gets down to details I should say — and it shares certain of the essential characteristics with the Bohr model of the day.
I think in this little book that was put out — you must have a copy of it — at that [1963 memorial] meeting, that in there there’ll be a good deal of background information and that sort of thing.
Oh yes, there certainly is, including one very important message on a Christmas card from Bohr — Niels to Harold — with direct reference to —.
I haven’t read that as carefully as I should have, I just glanced through it.
Yes, there is some quite interesting material. Tell me now what happens when you get back? You get back and you go to Chicago to the Ryerson Laboratory. Do you at this stage of the game go there now as theoretician or as experimentalist again?
What about that contrast in American physics between the theoretician and the experimentalist? Are you an experimentalist in your own mind up to the time you go to Copenhagen?
Well I’d say this was a transitional phase, that year at Wisconsin.
Had you wanted to be doing more theoretical work?
I think I was during that year. I’d been feeling that I wanted to devote myself to theory and the only way I could do this was by means of a fellowship.
How much of the sort of “basic mathematics” and relatively advanced mathematical physics had you been able to get yourself during your education? Did you do physics at M.I.T., or did you go there as an engineer?
I started out in chemical engineering, and then went to chemistry. I actually took my Master’s degree in chemistry, although it wasn’t a very chemical subject, it was X-rays and crystals. The work of Bragg was attracting a good deal of attention at that time and I did some early work on that.
That was in 1920 then?
Let’s see. I was the class of ‘19, but the class of ‘19 was graduated in ‘18 on account of the speed-up due to the war. I think I took my Master’s degree in ‘19, and then was there for one additional year, working also experimentally with D. L. Webster on the excitation potentials of X-ray lines. Then when D. L. Webster went to Stanford I went with him. I was his only graduate student.
Ah, that’s why you came here. Stanford was not a usual place to take a. Ph.D.
No, no. Well I finished up my experimental work for the thesis here at Stanford with D. L. Webster.
So really the transition to physics was really a transition to Webster while at M.I.T., and then you took the degree with Webster in the physics out here.
Webster wasn’t primarily a theoretician of course, although he was pretty good in theory. But my inclination, leanings towards theory I think came primarily from E. B. Wilson.
Was he at M.I.T. at that point? I hadn’t realized that.
Yes, my interest in theoretical physics as such is really due to E. B. Wilson.
How much theory had you had? How much math and how much theory had you been able to get by the time you took your degree?
Well quite a bit as a matter of fact. Although, it was more formal mathematical physics than what we would now call atomic physics.
Ja. What sort of subjects had you been through?
Oh, electrodynamics, classical radiation theory, Planck’s Warmestrahung, and that sort of thing.
Did you work through Jeans do you suppose?
Yes, Jeans was one of the standard books one should read at that time. I also remember that with E. B. Wilson we had a kind of reading course on Maxwell; we read through a good deal of Maxwell, the electromagnetic theory. So I really had a pretty good foundation in what was thought of at that time as theoretical physics.
At what point do you suppose you caught up with Sommerfeld’s Atombau?
Well it came out, I remember now, while I was in Wisconsin.
It came out in English while you were at Wisconsin.
No, I’m sorry. There was a second edition that came out.
The third actually came out while you were in Wisconsin.
Yes. Well that must have been while I was at Stanford, yes, that the first edition of the Atombau und Spektrallinien came out.
Did you read it then?
Oh yes, yes. From that point on, everything was on me, I had to work it out for myself; also the mathematics for quantum mechanics, matrix mechanics and so forth I had to dig out for myself as everybody else did at that time.
Nobody I take it around you — did Webster have any interest in the Bohr theory when you were with him?
Oh yes, yes, but he wasn’t primarily interested in the theoretical questions.
Well now you’re back to Chicago and you’re there as a theoretician.
Actually still as a National Research Council Fellow I think the first year.
You had that for three years?
Yes, that’s my recollection.
Was it fairly clear from the start when you went back there that this was going to be turned into a faculty appointment?
Yes, that’s right.
Who else there was doing theory?
At that point I was the theoretician. There was no one else. Carl Eckart of course came about 1927. The leading spirit was of course A. H. Compton, although Gale was head of the department. Compton was the important person in physics. I remember one summer lecturing on the new quantum mechanics — this was pre-Schrodinger — a class in which I had both Comptons. They were both, of course, very interested in going into all the mathematical detail for the matrix theory. I also remember — I can’t quite recall how it came about — giving a little talk at Princeton to Birkhoff and another mathematician there. So I sort of helped to diffuse the ideas.
Well I take it from what you said that you must have immediately followed the matrix mechanics just as soon as it began to come. Do you remember at all your feelings about the first of the Heisenberg papers in the interval before these were recognized as matrices? Of course those came close together.
Yes, they came practically simultaneously as I remember it.
But from your point of view you were quite interested in matrices and lecturing about them really before Schrodinger came along and before you were conscious of that at all.
Because you know, many people paid no attention to any of it until the Schrodinger came along, and then learned the Schrodinger, and maybe learned the matrices after they —
No, I went very much through the matrix phase.
What sort of problem did you have selling it? What sort of reactions did you get to this?
Oh I think they were — quite sympathetically received I would say. I think that people always had the feeling that this was really it.
What sort of a difference then did the appearance of the Schrodinger equation make?
Well, I think it engendered a hope that things were really a little simpler and a little less abstract, and there was a tendency to hope that one had found a truly classical interpretation. But this I think everyone would agree on both here and in Europe.
When the equation first came out, at least in Schrodinger’s mind, it looked a good deal more classical than it was possible to keep it.
I don’t know what Bohr’s reaction was at that time, this you can perhaps find out from someone.
I think not so good to the interpretation.
That would be feeling. Certainly one knows that he didn’t accept this. But whether he went through any waverings in his own mind, what his personal feelings were about Schrodinger, I don’t know.
Were there significant debates on those issues in this country also?
I’m sure there were.
But you have no particular recollections of them.
I don’t remember anyone holding out really very long for a literal classical interpretation of the Schrodinger waves, at least among the people I came in contact with — which were not so very many. You see physics was a very small group, and I don’t seem to have had a very great deal of contact with other theoreticians at that time, not to the extent that one would now if a new idea of this sort came up. I remember talking to Van Vleck a good deal, who was at Wisconsin that year. But physicists didn‘t travel around then the way they do now. I was a little bit isolated at Chicago, more or less fascinated by following through the mathematical details of these new things.
Let me take you back to your early involvement with X-rays and simply use it to set up a question. You’re interested in X-rays… Were they just waves for you, just plain electromagnetic radiation?
Well, I think I will have to admit that I was not at that time so much concerned with questions of this sort I was building apparatus to make the measurements. And the reason I got into X-rays was that D. L. Webster was there and it was a matter of trying to confirm experimentally the existence of the energy levels.
What sort of a ‘dent’ in Chicago, or in this country did the Heisenberg uncertainty paper make? Was that the sort of issue, that whole interpretation issue that people cared that much about?
No, I would say that it probably was not, among the people that I was in contact with at that time. But this could have been quite different elsewhere, as you probably will find out.
What about your own impressions of it? Was it a paper you read with great care?
It seemed to you a reasonably satisfactory way? This was again it so far as you were concerned?
Yes, yes, I would say so. Let’s see, a good deal of that period must have been just before or during the year that I was in Berlin.
…That paper was out late in the spring, June or July, so you may well have seen it before you left for Berlin.
Yes, I think so.
Did you go to Como by any chance on your way to Berlin?
How did you happen to pick Berlin?
Schrodinger. Oh yes, I begin to remember now. Schrodinger came to Chicago and gave some lectures, or Wisconsin and Chicago also I guess, both places. I guess I thought that I needed another contact than Bohr, which I think was a mistake. I should have gone to Copenhagen that year, and could have probably just as well if I’d wanted to. It was a Guggenheim Fellowship. I think Bohr was perhaps a little piqued at this. I remember that I did make a visit up to Copenhagen, and Schrodinger had been working out some of these interpretations of the uncertainty principle in terms of waves. I was talking to Bohr about this and he said, “Well, we know all that.” I thought I was bringing something a little new!
What sort of a group was there in Berlin in ‘27 - ‘28?
Well, it was pretty inhomogeneous I would say. There was very little room for contact between Schrodinger and the other people in Berlin at that time or in Germany at that time. Schrodinger had been appointed professor and there were these colloquia in theoretical physics, but I had the impression that Schrodinger himself was a little lost in a good deal that was going on there at that time.
Lost in what sense? Personally lost?
Yes. He didn’t take very much part in the discussions, and he was rather critical. The one issue or subject at that time was the application of group theory in quantum mechanics. It was a little unfortunate perhaps, since this has been something of a side issue as regards the main development of theory and its applications. Von Neumann was lecturing, and Wigner. I can remember colloquium after colloquium devoted entirely to working out the mathematics of the group theory applications.
Really! Well now was that the colloquium?
No. That was not the big colloquium. This was the theoretical one.
This was more nearly then the graduate students than the faculty theoretical colloquium?
Yes, yes, yes. They were all people from various places who were primarily interested in theory.
I ‘m interested that there was that much going on quite that early.
Well this was ‘28-‘29 I guess.
Was it? … Let’s check that date. I’ve got you down as ‘27-‘28, on the Guggenheim in Berlin.
Well now that might be right. I do have records on that, it could be checked.
Let’s assume for the moment that it’s ‘27 – ‘28 … The important point at the moment is what was going on there. Who would you remember besides Wigner and Von Neumann and Schrodinger, to the extent that he was involved, who were really much involved with the new quantum theory in Berlin at that point? Berlin had so much dropped out of the picture after the war as a place for the problems of Quantum theory, except for Einstein himself, whose relations to it were rather strange.
Oh, I remember. London. London was just beginning to work out the elementary things about chemical interactions and homopolar bonds and things of that sort. There were some very good talks by him at that time. There must have been other people. Heisenberg was quite frequently there. I think that was the time that he first presented the theory of ferromagnetism. And the Dirac electron papers came out during that time.
What sort of a dent did those make in Berlin? All of these things add together, so it was ‘27 – ‘28 that you were there.
Again, this was something that was immediately accepted by the theoreticians. Von Neumann was very much interested also in the group theory aspects of a relativistic electron. In the big colloquium quite other things were going on. This was a period in which there were a lot of people trying to disprove Einstein.
Disprove what part of Einstein?
Oh, experiments that would detect the motion of the earth through ether.
I don’t know how well this is realized or known. It might be interesting to fix this date a little more accurately for that reason. This was the beginning of anti-Semitism in a big way in Germany, because it was certainly there.
And you got a lot of this in the big colloquium.
A lot of this.
Do you remember who gave papers of that sort?
Nernst. Well, I’m not sure that Nernst gave papers, but Nernst was quite active. Einstein was there. I can remember just one thing out of these discussions, in which some evidence of this sort had been presented, and Nernst getting up and saying, “Nun, Herr Einstein, machen Sie eine Theorie!”
What proportion of the time — I mean already — must say I’m shocked at the notion, not that it happened from time to time, but that your residual sense —
It wasn‘t an isolated occurrence, though I couldn‘t give you exact numbers. My impression, which of course isn’t infallible, was that maybe something like a third or so of the colloquia were devoted to subjects of this sort. I probably didn’t attend them all. This was on the Reichtagsufer — the big colloquium building there.
In this group that was more concerned with the quantum mechanical topics, was Szilard also a part of that?
Yes, Szilard was also there, that’s right.
Now I get the impression from what you say, that there was this group of bright young men deeply involved with new problems, growing to a great extent out of the Schrodinger equation, and that Schrodinger himself may have been somewhat isolated from them.
Yes, this was true.
Do you have any sense as to what his own concerns were that set him apart from the group?
I think he was still looking for a more completely classical interpretation.
And that this had already separated him from the group that were simply accepting the n-dimensionality of the Schrodinger wave equation space.
Ah, that’s a sad story.
Carl Eckart was there that same year, but he was in Leipzig.
Did you get to Leipzig at all that year?
No, I never did. Eckart was up in Berlin more than once. I saw him occasionally, but I never got to Leipzig.
Of course there were all sorts of things going on there.
Yes, yes. I remember one lecture that Wigner gave on group theory. I guess it was just at the time that he had hit on this interpretation. He started out to give this lecture, but being — at that time at any rate, and I guess to some extent still — a little bit shy, he was somewhat hesitating in beginning this lecture. Von Neumann was there, and immediately jumped in and went up to the blackboard and started in, and Eugene sat there tossing the chalk up and down in his hand waiting for a chance to say something, for al1 the rest of that lecture!
What was your own response to group theory?
Well, I didn’t, I wasn’t too interested at the time I think. Later I dug it all out and gave some lectures on it in Chicago after I got back, but —.
What did you dig it out of?
So it was really only when Wigner’s book came out that you put it together.
Yea, I think so. I didn’t do more with the original papers, though I did get the general idea.
I’ve also wondered to what extent people ever really dug it out of Weyl rather, which was available before Wigner‘s book.
Was it? Yes, it was, of course.
I gather there was even some feeling about this.
Well, yes, but when I actually gave a course on it I based it considerably on van der Waerden’s little book, for which I still have very great admiration. It’s a little less diffuse and more direct in its approach.
Can you guess when that course was given? I’m curious about how long it took people — an awful lot of people held group theory at arm’s length for some time.
Well, I don’t, but it was not immediate. It was after the ideas had been pretty well established and accepted. It wasn’t a ‘first’ of this sort.
What did you work on yourself when you were in Berlin?
Oh, something that turned out to be rather trivial. Polarization of resonance radiation.
It hadn’t been trivial a little while before, had it?
Well, perhaps it wasn’t completely trivial.
Do you remember how you happened to take that? Had you been concerned with that problem at all already in Copenhagen!
No, no. I don’t remember whether I suggested it or Schrodinger did. I worked with him on it, and he thought it was perhaps more significant than it was. I remember his telling me that he thought this was more important than everything that was going on in the colloquium at the time. But I think this is not a good objective judgment. It was a little indicative perhaps of his frame of mind at the time.
I went up to Copenhagen, and presented it at a colloquium in Copenhagen, I remember.
Is it your sense in telling me this now that he felt this way about it because it was likely to provide insight into the physical significance of the wave function — if one could see what happened to it in an electromagnetic field?
Yes, I think so, something of that sort. I’ve forgotten the details even of what I was trying to explain in that work.
I think now what I’d like to do — so far as the essential areas are concerned we have been in most of them —
I might add. There was the Bohr meeting in 1952 in Copenhagen, which I attended. It was a little like the last July meeting. Bohr was there of course. There was a good deal of fancy field theory thrown around by Dirac and Heisenberg, end I remember Bohr mumbling to me in his usual more or less incomprehensible fashion, shaking his head, sort of implying that all of this business was nonsense. “There’s no real fundamental progress here,” I think was the thought in his mind. Then I saw him again, I think it ‘58 — I could pin this down if it was important — then he seemed to be — well, I had lunch with him out at his villa. He told me a lot of things, but he was more interested in politics at that time; at any rate he talked to me only about, well his personal experiences in America. That particular day he was somewhat concerned with the racial question in America. Strangely enough, he felt this as a very difficult question and one that had at least two sides to it.
That does interest me.
Yes, this is rather interesting. He seemed to feel that this presented a very difficult question to the people in the South. There’s certainly no question of what his basic feelings were in the matter of civil rights — it wasn’t called that in those days, but he was aware that this was a very complex question. He also tried to tell me some things about his reasons for his trip to America. He felt some disappointment in the way in which he was received in his attempts to discuss political questions and so forth. Unfortunately I missed a good deal of what he said. He used to smoke cigarettes, but in his later life, I presume under doctor’s orders, he took a pipe, and this didn’t in any way improve the audibility of what he said. I remember sitting beside him on a little bench there, and his voice trailing off. But he was enormously interested in the Atomic Energy Commission in Denmark, and insisted on taking me with him to a meeting. Not that I attended the meeting, but he wanted me to see the quarters that they had in the Parliament Buildings.
This was in Copenhagen; you didn‘t go out to the installation at Raskilde.
Yes, I did, but that was later. Not with Bohr. But be wanted to provide a car for me and send me out there. Actually I went out with someone else in his car. Bohr had set up the visit, and was very anxious that we should see it all. That was the last time that I saw Bohr. So I guess we have more or less covered everything.
I’d like to take you back a little bit to the very beginnings of your interest in science.
Now, one thing before I forget. I don’t quite know whether one should bring this up, whether it has any particular significance or not. There’s a rather obscure reference that you may never have looked into, in which Bohr makes a very curious statement. The statement is that everything we learn about the nucleus makes it less likely that nuclear energy will ever be released.
I haven’t ever come across that.
If you’re interested in statements of this sort. “Misstatements by famous people.” I’ll try to chase this one down, because it’s even possible that n memory is wrong. Now this is not in the Physical Journal but in The Journal of the American Chemical Society. There’s a paper by Bohr, I’ve forgotten the subject, in which he makes this statement. It’s sometimes interesting to know that the great can —. It’s a little like the statement Dirac makes at the end of one of his papers, that one can never understand the widths of spectral lines without going into the structure of the electron. Also, the famous statement that now all problems are solved except those of life. I was particularly interested in the statement about the widths of spectral lines because it was just at the time that I was myself working on a theory of the width of lines which came out almost simultaneously with the Weisskopf-Wigner. But don’t quote me on the statement of Bohr unless you can find it in print.
No. Well I’m very glad for that lead. Actually as you’ll gather, the development of nuclear physics we’re doing substantially nothing with. This is for somebody else to follow. But one can’t cut Bohr up into pieces from this point of view, so I’m certainly very glad to have this. There are two areas, one of them largely biographical, but which also relates to the second. I’d like to ask you first a little bit more about how your own interest in science grew up, how you got into it. I’d also like to ask you — and these questions tie to some extent together I think — something about your general observations about this transformation which you’ve watched in American physics, or in physics in this country as a field.
Well, my interest in science dates back to my last year in high school. Up to that time I was more interested in literature.
Had there been science in your family? Was either of your parents interested?
No, my father was an organist. In those days Latin and Greek were still really taught.
It was a private school in Chicago, the Harvard School for Boys. Famous in one way which you may or may not recall: Leopold-Loch. I don’t know whether you’re interested. But my last three years of high school were spent there. I was quite enthusiastic about learning Greek. It was only last year at high school that I took courses in both physics and chemistry. I had to make the choice at that time whether to go in for science or to start in on Homer. I remember I got as far as buying a copy of Homer, but decided to give it up in favor of these two courses in physics and chemistry, so my interest up to that point — my only inclination in that direction had been geometry, plane and solid geometry. Algebra always bored me, but the logical beauty of plane geometry as it was taught in those days out of (Wentworth) had quite an impact on me. So I decided to go to M.I.T. I think my father in particular was rather anxious that I should do something practical, and I had been more interested in chemistry I think than in physics the year that I took the two courses simultaneously. I decided to go into chemical engineering. Then I failed a course in mechanical drawing — that was the only course I failed at M.I.T.! I got sort of fed up with that and I went into the pure chemistry, ‘Course 5’. Then I gradually shifted over towards physics I think initially largely due to the influence of E. B. Wilson. His lectures on vector analysis I remember particularly. I had a course in advanced calculus under him that was one of the routine courses there. Then for the very few of us, probably not more than four or five, who were interested, he gave his lectures in vector analysis, stemming directly back to this work with Gibbs. There were practically no graduate students. I can only remember two of us who were graduate students at M.I.T. at that time.
No. Yes, there must — there were more graduate students in chemistry, I’m sorry; but only about two who were interested in physics and following through more fully into physics. Then D. L. Webster got an appointment there and then there wasn’t anything to do except follow along with what his interests were, which was building what would now be called a Cockroft-Walton device for getting the high voltages necessary to measure the potentials of the series lines. From then on I guess we sort of followed it.
Right… It’s clear in your own career, and a number of other people have been fairly explicit about it, that despite the extent to which one could take courses and read Maxwell, and read Jeans, and so on, a career in physics meant a career as an experimentalist in these days. For example Kemble, the year before Van Vleck, had to get special permission from the Harvard department, which he ultimately didn’t use, to write a purely theoretical thesis. Van Vleck’s was the first theoretical thesis that bad ever been accepted by the Harvard department. Dennison was having a somewhat similar experience at the same time at Michigan. And your career is going to identify you with theoretical physics, but your graduate work, or at least your thesis, is very definitely experimental. You feel you have to go to Europe to convert yourself to being a theoretician. I’d be interested in any remarks you can make, either about the resistance to the notion of theoretical physics in this country, or about how that transition came about, about the growth of theoretical physics in this country —
Well, I don’t think there was any opposition to it. But, as you know, there was evidently a feeling that theoretical physics was not developing properly in this country and that the National Research Council had definitely wanted to take steps in the direction of building it up. But I wouldn’t have felt that there was any antagonism to it. It was something of a novel idea that a physicist didn’t go in the laboratory and build things. Oh, I do remember one person who was always rather — two people perhaps — who were somewhat anti-theoretical, H. G. Gale in spite of the fact that he was the one who induced me to come to Chicago, and Arthur Dempster. They really felt that you weren’t doing physics unless you could do something with your hands, putting things together in the laboratory. This is what stamped you as a true physicist. But I wouldn’t say that this was a strong influence that retarded things in this country. Very soon after that America began to develop very fine theoretical physicists.
What do you suppose accounts for that transition? It was a very rapid one.
Well, don’t you think that it was just the close relation between theory and experiment in the development of atomic theory? This was perhaps more evident in the very early days, but still conspicuous in the development of quantum mechanics. There was perhaps a time when theoretical physics ceased to be essentially a way of calculating things out of established theory, and began to have a really intimate interplay in suggesting new discoveries and explaining them. Now this isn’t the first time that this has happened in the history of physics; if you go back to Faraday and Maxwell you have the same sort of situation. Maxwell was both an experimentalist and a theorist in a sense. You see pictures of galvanometers in Electricity and Magnetism. So it may have been a transient period in this country, due partly to the state of physics at that time. Well, not entirely. There was the development of the special theory of relativity for example.
If you look at America’s role in that, you notice that it’s exclusively experimental, and at a time when Europe was producing some people in each. On the whole the American physicists of any prominence between about 1880 and the 1920’s really were entirely figures in experimental physics. Some of them extraordinarily good experimentalists.
Tolman was perhaps one.
Yes, Tolman would be one of the exceptions.
But there weren‘t radical theoretical developments originating here. It was more what we would now call mathematical physics rather than theoretical physics.
That’s true enough. Well, sir, I’m very grateful to you.