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In footnotes or endnotes please cite AIP interviews like this:
Interview of George Uhlenbeck by Thomas S. Kuhn on 1963 December 9, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4922-5
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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: Henri Abram, Niels Henrik David Bohr, Max Born, Louis de Broglie, Max Delbruck, Paul Adrien Maurice Dirac, Tatiana Ehrenfest, Paul Ehrenfest, Walter M. Elsasser, Enrico Fermi, Ralph Fowler, Samuel Abraham Goudsmit, Werner Heisenberg, Oskar Benjamin Klein, Hendrik Anthony Kramers, J. P. Kuenen, Otto Laporte, Hendrik Antoon Lorentz, J. Robert Oppenheimer, Wolfgang Pauli, Isidor Isaac Rabi, Harrison McAllister Randall, Julian Schwinger, Arnold Sommerfeld, Llewellyn Hilleth Thomas; American Physical Society meeting (Boston), Huygens Club, Kapitsa Club, Rijksuniversiteit te Leiden, Technische Hogeschool Delft, University of California at Berkeley, and University of Michigan.
George, I think there’s one thing that is sort of a complete blank in the way we worked before and which I’m trying to pick up systematically with people because it’s useful comparative material. This is this question of early life, as though you were going to give us a biography, but obviously with an idea to the sorts of things that determined your interest in science -– I know that it wasn’t very clear for a long time that you were going to be a scientist – how this related to family background, and things of this sort. So you could start pre-natally, or in the crib, or in kindergarten. Tell us something about the sort of family you came from, who was interested in the sciences.
You see, I of course was born in the Indies, in Batavia, and the whole family is military, both on my mother’s side and on my father’s side. My grandfathers on both sides were all military men and, as a consequence, had their careers in the Indies where I was born. A consequence of that was also that they always got out early because one year in the Indies counted as two years’ service so after twenty years my father retired although he was only forty-two, and he did it mainly for the education of the children.
You mean he retired?
He retired and went to Holland because as a military man the last place we were at was a small village in Sumatra where there were hardly any schools, so I came to Holland when I was six years old and we lived in the Hague. My father had all kinds of accessory jobs because the pension was not very high but still it was enough. So my whole early education was in the Hague where I went all through elementary and high schools; and, ja, I was a very dutiful student, very dutiful. I always worked very regularly and I was always very good in the class and so on. I was certainly not clear until the last years in high school what I was going to do; then mainly through my sister who was five years old, —
Was that your only sibling, or did you have others?
No, I have two younger brothers, but that was my older sister who had great influence on me at that time. Well, where were we? We were about at my high school. Now let me tell a little bit about one teacher I had: that was Mr. (Borgesius) whom I mentioned in the introduction of my thesis. He was a very shy man and also he had no order in his class, but he had the reputation in the school — mainly through my sister — that he was the most learned man in the whole staff; and for me at that time, and for quite a while, anything which was learned was good for me. So I tried to get in touch with him, but since he was very shy and I was, of course, much shyer even, it was a very strange connection I had with him. Once in awhile we came together, but neither of us said very much, but it had the result that I learned calculus when I was in high school because he gave me books, you see.
Was his field mathematics?
No, he was a physicist, really an experimental physicist, and had done a very excellent dissertation; he took extreme care with demonstrations which were simply beautiful but were lost to the students because he had no discipline. I still remember that he gave these lectures and worked these demonstrations and only a few people would listen, so to speak. But because of him I learned calculus and he also gave me lectures of Lorentz; you see, for many years, Lorentz gave elementary physics lectures in Leiden, just as Feynman does now at Caltech. He thought that was what he did. These lectures came out in a two-volume book. They were really remarkable — I don’t know whether I have it anymore, I don’t think so. They used only a little bit of calculus which was taught by Lorentz himself in the beginning few pages and it was completely original — original lectures really. I got the Feynman lectures; they are quite different but they have that same quality of suddenly someone who is really a very great physicist suddenly thinking that he should tell it to the youth, so to speak. This book, by the way, afterwards, after many editions, came into the hands of a committee and now it is a committee book and just as bad as everyone else, but the first editions I still think were classics. He called it Beginselen der Natuurkunde. I went through that while I was still in high school, mainly through this teacher, which had as its consequence my knowing calculus and this thing; I had practically copied this book with all its notes. No problems, there were no problems, which was also very typical.
Typical of Lorentz?
Yes. There were problems in the calculus book which was also by Lorentz. [Lehrbuch der Differential und Integralrechnung (1907)]
Yes. He wrote a book on calculus in which I did all the problems and those went very far really and were very interesting. I finished high school and then I think, I told you already, I didn’t start in physics but rather in chemical engineering.
I wanted to ask you a question. You did tell me that; how did it happen, George, that you went to high school instead of gymnasium?
First of all, because it was one year shorter and no one of our family, so to speak, had gone to gymnasium, and —
Had they gone to high school and then to military school? You had an uncle in the University —?
Yes, I had an uncle in the University, of course, who had also great influence on me, but a bit later really; he was the great linguist. But it was not discussed; it was just the obvious thing to do that one went to the high school. I was not clearly a man who was interested in languages and so on and I also didn’t want to go into medical school or law or something like that; that was also very clear. Well, when I was 13 I just went to the same high school my sister went to and that was it.
But as of that time it meant that you were closing off the possibility of going to the University?
Yes, I could go to the technical high school, of course, to which I went. Then, as I told you, this law was changed and I was very unhappy in Delft mainly because it was such a mechanical sort of business, all these many lectures which one had to go to and chemical laboratories which one had to take and at which I was not very good at all. I didn’t like that, so after a semester, then again because my sister knew that I was unhappy, she persuaded my parents that I should go to the University then.
We talked a reasonably large amount about the University already; I’d like to see if we could explore more of this earlier period. Your family was generally a good deal oriented toward learning? Would you say they were interested in scholarly pursuits?
No, they weren’t. I think one of the disappointments of my parents was that none of their sons went to the military schools; only one of them wanted to, but he couldn’t because of his eyes, and I didn’t want to so he [my father] thought I should just become an engineer, which seemed much better to him than to be a high school teacher. But it was clear that since there was very little money, I had very soon to get some kind of a job. It was already quite a sacrifice for the family to let me study, you see. My sister never studied; she got an education to become a biology teacher, which she did. But I was then supposed to go to the University.
Did she go on after high school?
She went on after high school and then studied biology with a teacher in the Hague to get this degree which gave her the right to teach biology in high schools. When I finally went to Leiden she went then too for a year.
To the University?
Yes. But the University was seven years while Delft was five years. These were the arguments in which it seemed to my parents very important that I get through with that as soon as I could; therefore there was a little bit of reaction from my going to the University, but not really. I didn’t have to fight very much about it because it was clear that I was unhappy and so I was allowed to go to Leiden at that time. I stayed, of course, always at home; we commuted by train to Leiden or to Delft in those cases, and that was really the way it went. Then when I came to Leiden that was, so to speak, paradise for me because there were hardly any lectures there. I didn’t get in touch with Ehrenfest at all in the early years, but I worked by myself very much and the lectures didn’t bother me very much because there I was helped by the fact that I knew calculus already so most of it was very simple, and so I had lots of free time, an enormous amount of free time -– which I didn’t have in Delft at all. You had to do a bit of experimental physics, but you were not allowed to do it more than one afternoon a week; it was forbidden to do it more than one, which was all right with me. I did it once a week, one afternoon a week. Those two years before my first examination -– there were no examinations -– one must not underestimate that business. There were no course examinations, no constraints whatsoever; nobody even knew whether you went to the lectures and I very often didn’t at that time.
You could just do what you wanted and what I did at that time mainly was to study Boltzmann’s gas theory which I picked up second-hand -– I still have the copy — in the Hague, I thought, “Jesus,” that looked to me the most learned theory; if I could do that, then I would really know something. Now with regard to the kinetic theory, my interest was already aroused in high school, at least when I studied these lectures of Lorentz’s; I was very much impressed by kinetic theory, mainly because I thought it was an example of theory, it explained something. Also, in that same book of Lorentz’s there were in the electricity part, models, so to speak, which were part of the mechanical models of the ether which he used mainly to illustrate the various relations. Those I thought were just swindle — those had nothing to do with reality — but the kinetic theory I thought was a theory, so I learned that already, and when I got this gas theory I really studied it; I copied the book practically and, as a result, I had to learn analytical mechanics because that was needed for it, so I studied that a bit for myself.
What did you study that in?
I studied it out of a book by [P.] Appell not a big book but rather a precis which I only went through enough so that I could follow Boltzmann again with it. Also I studied Maxwell theory a little bit out of this little book by Schaefer; it was also a very thin book.
It was Clemens Schaefer?
Clemens Schaefer, yes, but he had this very thin book at that time which was in a red cover. All that I did essentially for myself, but you see, there were no lectures in physics because Ehrenfest lectured after the first exam. The only lectures in physics were the general elementary course which still used Lorentz’s book which at that stage was diluted, so to speak, and already terrible. There was one lecture by [J. P.] Kuenen in thermodynamics. That was the only one I followed in physics, really, and although it was all right he was such a hesitant lecturer and went so slow for me that I learned something about thermodynamics, but it //had little effect on me//. The one thing I worked very hard in for the University was the experiment, because there you had this way as I told you, where the first year you could go once a week and the second year twice a week. You had to do, I think, about forty experiments in which for each of them there was written out a little syllabus in which all the formulas and so on which one had to use were mentioned. Then you had to write a little report for each thing, and those I took enormously seriously because, as I said, I was very dutiful. I derived all these formulas; I wanted to derive them all. That’s one of the reasons I did this Maxwell theory. And I wrote it all at length. That made for me personally a good impression because Kuenen got so impressed by that; he saw those things, and as a result, the third year, through the efforts of Kuenen, I got what you call a fellowship which was quite exceptional in those days. It was a state fellowship, which meant, on the one hand, that you didn’t need to pay the tuition, which amounted, I think, to about a thousand guilders, and that was a godsend for my parents. Towards the end of the second year I had to take this exam, and that was terrible for me. But I finally did it together with a good friend of mine -– we took it on the same day -– a man named Nijhoff. I especially had connections with Nijhoff who is the son of the big publisher and who was the brother of one of the great poets of Holland. I admired that poetry very much. I was always with him; he was older than I was but started to study philosophy, then went into experimental physics. I learned lots of things from him; I remember that he talked about philosophy, Spinoza, with me and we were always together and took exams on the same day. I didn’t know Sam [Goudsmit] very well yet.
When you say the exams were dreadful for you, why?
Well, because there you have to do these orals; they were all oral, and that means you have to know a lot of things ready and especially in mathematics it was very hard for me that I had to know all the convergence criteria, series, and whatnot, because, of course, the second year the mathematics went further than calculus. It was just that this kind of cramming I thought was very disgusting but I did it and it was precisely on time. I took the exam on time, but afterwards I was a little bit overworked; I was very tired. That’s the time when I first got in contact with Ehrenfest; that was after. I had very little contact with Ehrenfest all the time and he told me once that he was so surprised at himself that he always had some confidence in me although he didn’t really know me at all, but I think it was mainly because of Kuenen and because I was so dutiful.
You mean you had little contact with him then really until you came back from Rome?
I talked with him all right, once in a while, and he had a little seminar that semester in my third year which I took part in and which I even gave a little talk and all that, but I was at that time rather tired and I didn’t, so to speak, do anything more than follow the lectures of Ehrenfest, and the mathematics tool. It was at that time that Ehrenfest suddenly mentioned this job in Rome. He had had a request from the Dutch ambassador and I jumped at it.
I forget; did you finish your third year before you went to Rome or did you just do half of it?
No, I finished my fourth year, I’m sorry, because during my fourth year I was a teacher in the gymnasia in Leiden for 12 hours, and I didn’t like that either because I didn’t have much discipline in my classes and it was quite a strain on me. That was the fourth year, and it was at the end of the fourth year -– so it was not the third year — that Ehrenfest mentioned this job, which implied, of course, that I had already done a year and a half after my first exam. At that time you were supposed to do your second exam after five years so he said I had to do the second exam at the end of this year and I worked very hard in Rome. I came back at the end of this year and I worked very hard in Rome. I came back during the vacations to do the exams and I just managed to do that somehow.
You and other people keep referring to Huygens’ [Club] as though I knew all about it and I don’t.
Well, that was a dispute as they call it; it existed quite a long time and I became a member of it in my third year as also Sam did some few years later. There were students in it all about the same level, some a little older, after the first exam. In my memory we got together once every week or once every other week at each other’s rooms and one of us gave a little talk; then there was usually a second little talk, the main talk, a little one, and then what you call improvisations and so on. It was very nice, and we —
Was it all in the sciences?
Yes. There were always a few chemists, astronomers and mathematicians in it. I gave several talks in it and worked very hard at it. We had a little blackboard which we carried about and once a year there was a big yearly festival.
How large a group was it?
I would estimate about 20, not more certainly.
How long did one stay in it before one made room for some others?
Until the doctoral exam; then you became “outlaid” and you came once in a while. Even went I was working for my dissertation I still came there and then I was one of the “outlaid”. Ehrenfest encouraged it very strongly and, in fact, he said it had to be extended and that was one of the things I did when I was his assistant. I started, together with others, to adopt for the young student a similar one which was called the Leiden Jar. There what he wanted — what he always wanted — was the students to teach each other, so the Leiden Jar meetings were essentially meetings of the young students in which either part of the lectures or questions about the lectures which the students followed were, so to speak, repeated, or where they did some other things; and the people who took charge of it were members of Huygens. It was a hierarchy, you see, and the best of the Leiden Jar became members of Huygens; that was the scheme. But as a result, the people were always talking together. Ehrenfest came once in a while, suddenly popping in at a Huygens meeting or coming to such a Leiden Jar meeting. He was very much in favor of that; this was, so to speak, his method of teaching because although he would always (with) one man he still had this responsibility and he thought that he could just let it go on from there.
At what point did you make major contact with your uncle who, you say, influenced you so much?
During my third year and also the fourth year.
He was at Leiden?
Oh, yes. He was in very delicate heath and he was very often overworked; he somehow managed to go on pension when he was only 61 and after he became a pensioner he became so fine! Then he did lots of work and died only when he was 85 and had a wonderful time -– lived in Switzerland most of the time. But this duty as a professor he just couldn’t stand although he was a wonderful lecturer according to everybody. Anyway, for quite a while, during the days I was in Leiden, I had lunch at his house and he was very family-conscious. He was the one who made the whole family tree and took care of it as it came in this little book on families; he went and looked through the old documents in Germany and everywhere else.
Was he your mother’s brother or your father’s?
He was a cousin of my father; he was on the same level as my father.
Not really an uncle then?
No, although we always called him Uncle Cornelius, C. C. Uhlenbeck. He had no children. He was an expert in linguistics, a great Dutch linguist at that time, and he was a man who was very fond of literature and so on, all the literature. I still remember that when I came there I went to his study which was on the first floor and was full of books and there he sat; then I talked with him and very often he started reading. I still remember his reading all these Sanskrit things to me; I didn’t understand a word but it sounded simply magnificent. Then we had lunch and took a little walk in the garden where we talked on everything; he was a very great influence on me, of course, as a young man. After lunch I went back to the school.
One thing I realize I don’t understand here: in your last few years at least in high school you were sure you wanted to do science?
Somehow science, yes. Physics already; you see, because of this (Borgesius) it was clear that I had to do physics and really also theoretical physics was also clear because that looked much more learned than experimental. As another example of my -–
But later you were a lot less sure of it?
At Rome. The last year in Rome shook me up very hard; then I didn’t know anything about what I was going to do.
What was it about the experience in Rome that shook you so?
I got completely dissociated from physics; I didn’t do any physics at all because I didn’t know anything. The second year at Rome I still had contact with Fermi but not later and the people I saw were all of another direction. So somehow the study of physics disappeared and I didn’t do anything for a whole year, no reading. I, or course, had my job to do which was relatively simple but I read all kinds of Burckhardt and Mommsen and history of art -– I had a very good friend who was an art historian with whom I went around — and all kinds of things for a whole year, you see. So at the end I didn’t know if I should do it at all? Thank God, the one that that kept me back was that I didn’t know Latin and Greek and it was clear that if I wanted to do history — which I wanted at that moment mainly through the influence of the books of Huizinga which I then thought were so magnificent that that was really the thing to do — I had to learn that. I still remember that I talked with my uncle about it; he thought very highly of Huizinga and was a colleague of his at Leiden. He said, “Ja, that is very fine; of course, that is very profound stuff, but you have to learn Latin and Greek.” So I started to learn Latin; I took lessons in it as soon as I came back but my uncle said, “Try anyway also to see whether you can still get a Ph.D. in physics because that sounds, so to speak, more practical.” And that’s why I went to Ehrenfest and Ehrenfest said, “All right.” He needed an assistant at that time and so he asked me after a month of so if I wanted to become his assistant. Then I started working with Sam and with Ehrenfest all the time and in the fall I still started to learn Latin, but very soon it was far too difficult for me and I had so many things to do that I never even went over the hump with Latin and then whole thing disappeared. Then I was simply back in the groove again. But that was the year which made me uncertain about it.
Through all of this time it was most likely that you would wind up as a high school teacher?
Yes, but that was very, very terrible to me because I thought it was a terrible job. I had done it a year and I thought it was a terrible job. That was also one of the things which bothered me very much at that time, the future of it, you see. Thanks God, there were no financial difficulties anymore; that I never had any more.
Because of the assistantship and that sort of thing?
First, the high school teaching, then, of course, Rome -– I got a princely salary for that and so I saved a lot — and then I became an assistant and even had money saved and so on, so I was completely independent from the end of my third year on.
Did you still live at home?
Not any more, no. I lodged in Leiden and came home on weekends. During this Leiden period I was always together with Nijhoff and with Wiersma. Wiersma was a very good experimental physicist; he became a professor in Delft later on. Both are dead now, Nijhoff died very early, Wiersma just after the war. But then at that time, of course, after the spin which happened the first three months I was back essentially, then, of course, there was no doubt anymore about what I was going to do. Then, of course, it was a godsend for me that we got this offer to come to America; I didn’t have to be a high school teacher!
I gather from Sam that you were clear from the start that you wanted to take that?
Yes! Right away, I said right away; Sam was a little bit more doubtful but he did it too finally.
Did you think of that at the time as being a permanent move to America, or were you —?
Yes, more or less. Ehrenfest always said —. We got the job partially through Ehrenfest. Sam may have told you about how we finally got this offer because we were relatively well-known then among the younger generation. Randall at Ann Arbor wanted to build up theory -– maybe you know it already?
Yes, you’ve talked a bit about it and Sam has also talked about how Colby came, and —
So after the decision was made, Ehrenfest said, “Of course, you should go there; all the younger people who are really good should go away, and then you should afterwards come back if they want to make you professors somewhere.” That was, so to speak, another scheme of Ehrenfest’s; he thought you should send them all away and then you could call them back, but the last one didn’t happen, of course. That was the trouble with his scheme, although I did it, partially because I had that in mind and partially through Kramers who also, so to speak, pulled the “duty-racket” on me. He said it was my duty to come back to the fatherland in ’35 and I did, although I didn’t want to at all.
You didn’t want to even then?
But I thought it was my duty. Something which was perhaps another small point and which was, I think, characteristic of my views was that everything which was learned was good. So in my third or fourth year -– I don’t recall which -– I studied [Newton’s] Principia Mathematica extremely hard; it has three volumes and I didn’t go through the three volumes, but I went almost through the first volume only. It was really because it absolutely looked like abracadabra — all these things which looked very learned to me. So together with Nijhoff I studied it. In a mathematics colloquium I also gave a couple of lectures about it and I still very much remember the reaction of the old mathematician, [J. C.] Kluyver. He was really the best mathematician in Leiden; he was an old man but he was really an old-fashioned analyst, so he listened and he was a very sarcastic man. He said, “Well, that’s very interesting that you can do this all by these symbols on the board, but now, if all the mathematics really is just this combination of symbols, then one could do it on a machine! We could do it on a machine and we would never have to think again!” And that shocked me so much, so terribly, that I thought, “Jesus, if it is that mechanical —!” That put a damper on the whole enthusiasm for Principia Mathematica. “If a machine can do it, then it is clearly not very learned.”
Where did you get, do you suppose, this deep interest and value for anything learned?
Very young, I think, and I think mainly through my sister; she was always impressed by it and it was because of her, I think. I don’t know why or how it came, but it was just somehow that anything which —. Well, also perhaps through my uncle who was considered by the family as an enormously learned man and everybody in the family looked up to him.
But he was admired for his learning in the family?
In the family, yes. My father sometimes made fun of him a little bit, too, but he was still Uncle Cornelius. Ja, he was clearly so learned and knew everything, so to speak, and could learn —. There were all sorts of stories in the family about him -– how many languages he spoke; they were all apocryphal. He didn’t know so many languages, although he, of course, spoke most of the languages very fluently and also Russian. But he knew many, of course, like a linguist knows them. So I think mainly because of these two influences on the side and also because of Nijhoff for whom I had enormous respect in my youth because he had all this knowledge about philosophy which I tried to read but had not —.
Had you know him before the University?
No, not before, but the first years at the University; I knew him right away when I came. The point was that in a certain sense it went contrary to Ehrenfest and I think it was extremely healthy for me that I then got in contact with Ehrenfest who did not want to do anything learned; if you couldn’t say it simply, if you couldn’t be to the point, then he didn’t want to hear it, and anything which was, so to speak, long-winded and learned, he made immediate fun of. And as a result, and since he was finally the man who certainly had the most influence on me, this counter-acted this thing of mine considerably. You see, in mathematics for instance, in the old days, I was the one who wanted to have it absolutely rigorous; now, after Ehrenfest, I think it is bad when it is very rigorous! So it was very good for me when Ehrenfest really took me in hand in that respect. Every time I worked with him —. He also used his assistants sometimes to read papers which he wanted to know. Then we had to tell him about them from notes we made and so on, and then, of course, if I didn’t say the point — Jesus! -– or if I hadn’t understood the point!
Do you remember any of the papers you read for him this way?
Oh, yes, many. About the Compton effect and about these experiments of Bothe and — what was it?
Geiger. Well, if you read the Compton effect this would be not right at the time but later papers on the Compton effect?
Later papers, yes.
Did you read the Dirac paper?
No, not Dirac. Of course, what we went through right in the beginning was all the Schrodinger papers, one by one, all of them.
But this would already be a little later?
That was in ‘26; my assistantship really began in ‘26. In ‘25, the first three months there was the excitement of the spin thing and everything, but in ‘26 when Sam really went to Copenhagen, then I was really the one with whom Ehrenfest worked.
Did you, do you think, spot the Schrodinger papers with the very first one, or did it wait for the second?
No, right from the first, of course.
Do you think you went over the first paper carefully even before the second one came out?
No, that is not so, because the first one had, of course, the hydrogen atom in it and that was already a bit learned. Even Ehrenfest did not know that quite. But then the next one, which had Hermite polynomials, we went through and then came back to the first one.
Do you think you were interested in the first one from the start?
The first one was very strange and it’s the reactions to this that I’m trying to get at. The derivation of the equation seems to be nothing; I mean, it looks like nothing at all. At least I haven’t talked to anybody who felt as though he understood it.
I don’t recall that in detail. I think we studied it. You see, at that time the man who knew a lot of mathematics and whom Ehrenfest always asked was [J.] Woltjer, a theoretical astronomer. His son is now a professor in astronomy at Columbia. Ehrenfest always used him instead of the professional mathematician with whom he didn’t get along so well. I still remember that Woltjer talked to us about the first paper and about the mathematics, this complex integral, but Ehrenfest of course -– “Ach, die komplexen Integrals” -– that was always hard on him.
What I’m really trying hardest to spot is to see whether one could still remember whether one was interested in the first paper before the second one came out.
Well, I think one was extremely impressed that the Balmer series came out and that, of course, came from the first paper so it was clear; while from the matrix mechanics it was, as Pauli would say, an “analytisches Kanstatuck” to get the Balmer formula out. Pauli did it, of course, too. But here it was, so to speak, with more or less traditional mathematics. And then the next one — you know, they came issue by issue — we went through very, very carefully. Perturbation theory. Then, of course, I learned for the first time what the spherical harmonics was; we didn’t know that.
Tell me something about the nature of the interest in these problems with the wave equation that you worked on with Ehrenfest. What was that about? What motivated that work? What was the point of it?
Ehrenfest always read or wanted to know the recent developments. It was also clear that this was a very important recent development; furthermore, it used mathematics with which everybody was more familiar, and I think that anybody at that stage couldn’t help saying when you saw the papers of Schrodinger, “Golly, this is suddenly a breakthrough.” Then also very soon came the connection with matrix mechanics.
Actually I didn’t mean those papers. You’re involved presumably even before the Schrodinger equation with this problem of a theorem of Lorentz’s on wave equations.
Oh, the wave equations. That was a completely different sort of —. Ehrenfest’s great trouble was always to see whether we could get a dissertation together, and he was a man who thought that at least one should try to get a subject which was not, so to speak, in the center of interest, but in which he was interested enough. Already there had existed for awhile his paper with [H.] Bateman [“The Derivation of Electromagnetic Fields from a Basic Wave-Function,” Proc. Eat. Acad. Sci., 10 (1924), 369-374]. He was interested in just the mathematical properties of the wave equation, and his friend Herglots had also done some method. He thought it would be very good if we together would try to study all these methods of solutions of the Cauchy problem, essentially, and see how they were related. Purely mathematical.
That’s not like him, is it?
Yes, that was like him -– the systematic touch he had, especially when there were two methods which gave the same answer; then he wanted to know why. What is the point of the similarity, so to speak. Of course, then he wanted to know what the point was in each method and how it hung together, and that was the way he started it already that summer; then immediately he had to do it with n-dimensions because it was typical of Ehrenfest that he was always interested in the fact of dimensions. His famous paper why does space have three-dimensions; it’s a famous paper which made an enormous impression on me. [“In What Way Does It Become Manifest in the Fundamental Laws of Physics that Space Has Three Dimensions?” Proc. Amsterdam Acad., 20 (1917), 200-209].
When was that done?
That was around the same time, I’m sure.
So this whole questions about the difference between odd-dimensional and even-dimensional spaces –
That was very important for him, you see: why is Huygens’ principle not valid in even dimensions? So that’s the way we started it, and then out of it really came these two papers. Finally we put it together that it clearly was not going to be my dissertation, after three months. So we wrote it up and out of it we also had to make this Lorentz Featschrift for Lorentz and then this “Stelling” of Lorentz [“Over een Stelling van Lorentz en hear uitbreidung voor meer-dimentionale Ruimten,” Physics, 5 (1925), 423-8.] was really an outgrowth of that same study, which I wrote up alone for this. So that was a purely mathematical intermezzo in my work which had nothing to do with Schrodinger. Then in ’26 Schrodinger came and we worked very hard at it, very hard. I told you this story about Pauli.
I don’t know that you did.
Well, that was a very typical story. Since I then learned at that time —
Oh, the computing story, the integral —. Yes, yes, that story you did tell me. You did a paper with Ehrenfest on de Broglie’s phase waves in five-dimensional space. [“Graphische Veranschaulichung der De Broglieschen Phasenwellen in den funfdimensionalen Welt von O. Klein,” Z. fur Phys. 39 (1926), 495-8.]
Yes, that was because of Oskar Klein. I have, by the way, a manuscript here of a paper by Ehrenfest, Oskar Klein, and myself; it never got published, but I have it here and it might be of interest.
I would think it would be.
Oskar Klein came to Leiden on a Lorentz fellowship, I would think, in June or so of that same year.
’26, yes, and stayed about a month together with me. We stayed together in the same apartment. You know surely from him that he came from Ann Arbor and that he had the wave equation, of course, the relativistic form. He had done quite a lot and we talked about it with Ehrenfest every day. Out of that these things came out, you see, and I was very much impressed by Oskar Klein at that time.
Were you impressed with this approach, the five-dimensional approach?
Yes, with the generality of it, because it seemed then that one was very close to a world formula — one equation containing everything, you see. I remember that I had the feeling that “Golly, we now perhaps know everything.”
Did Ehrenfest share that?
No, of course not; it was I in my youthful —. Of course, Ehrenfest was very much interested in these five dimensions because of this view that quantum conditions could somehow be understood as a periodicity condition in the fifth dimension, and that is when we wrote these papers. Then all three of us would write up what we did in these discussions and that’s what I have, but it was mainly Klein; that is why Ehrenfest didn’t want it and so it was dropped, but it may still have some interest.
Yes, it would be worth having. If I remember correctly, and I’m not sure on this five-dimensional relativity version that I do, one of the reasons that Klein kept on with it after Schrodinger was the hope that it wouldn’t demand the transition to configuration space. Now I may have this wrong.
No, I don’t think so; I think it was mainly the connection with general relativity which he always had in mind. I have forgotten a little bit how it was, but I think it was always this connection with general relativity which Klein is still thinking of. In his last thing he says we have to go back to the absolute space of Newton.
Oh, does he? That I didn’t know.
Because the vacuum really has everything in it nowadays and that would play the role of the absolute space of Newton; it’s one of his last statements.
I’ll be damned; that I didn’t know. In Leiden, as you were doing the Schrodinger equation, what about the question of the interpretation of the wave function?
That also came relatively quickly. That it was probabilistic we knew very soon.
Well, of course, the Born paper itself comes fairly soon.
Right. And although Born and Ehrenfest were so antipodal, still that was one of the papers which we discussed very much and which I had to study -– the Born collision paper. I don’t think we had very much our own opinion about that; at least I didn’t have. I accepted the probabilistic interpretation very soon. It was probably in connection with that — that was in the summertime — that Ehrenfest said what we should now do was to see the consequences of all these things to statistics. And that was what we did. That was then the whole task which came in the fall of ‘26. It was until ‘27 that we worked on that.
When the Bose-Einstein statistics came out, you were still in Rome, weren’t you? That was ‘24, ‘25.
Yes, I hardly knew it.
When you got back, however, clearly Ehrenfest was already deeply involved with that?
Oh, he would have been since he knew about it; he always had his doubts about it.
Yes, well that’s what I wanted to ask you. I mean, initially that comes out without any benefit of the wave equation, no question of symmetry properties or anything else of the sort, but a new game to be played for possibly non-existent particles. How did he feel about that?
Oh, he hated all these arguments that this was the right way of counting. There were several of these arguments that because the particular were indistinguishable you had to count it this way. I still remember that he said, “If Boltzmann heard that, he would turn over in his grave.” Just because they were identical you had to count like Boltzmann! But then he was, I think, impressed by the pre-quantum mechanics Schrodinger paper nad that is what we studied very hard because he knew, of course, that somehow you have to have the Planck radiation formula and that therefore if you quantize the waves, then the Bose statistics is the natural way; so we thought for awhile.
“Natural way” in what sense? That it’s the one that gives the right results?
It gives the right results for Planck, right? And therefore, since that was right, and since matter was also waves, it seemed natural also to quantize.
Well, to say that matter is also waves is not yet quite clear?
Well, with Schrodinger and de Broglie and so it was clear. You see, the particle-wave duality was, of course, in everybody’s mind, but still everybody was more inclined to say that it was either one or the other. It was clear that if there was a wave motion and a wave equation for an ideal gas, so to speak, then one should use the same way as one does with electromagnetic waves and one then got the Bose statistics formula. That was in Schrodinger’s paper. Schrodinger’s was a very fine paper, — really, one of the very clear and very good ones. This connection then with the symmetry properties in configuration space was troublesome to us for awhile and I remember that for quite awhile we had some doubts whether there was only one anti-symmetric and one symmetric —. It was all in that summer that we learned that from Dirac and Heisenberg and that we got back to the Fermi paper.
You hadn’t known the Fermi paper?
I knew it, but it looked strange to us, so to speak; it was clearly —. Furthermore, he did it so strangely at that time because he put — as a vessel he took a harmonic oscillator which was already a bit strange for us to do. But then it was clear how it hung together, that there was an interpretation in μ-space, an interpretation in γ-space, that are all in my dissertation and which I then clearly, a la Ehrenfest, distinguished. That was a very fine time for me, this fall of ‘26; we worked all the time on these statistical questions. We had the disappointment that the results, at least, which we had, were also published by R. H. Fowler and typically, as Ehrenfest said, he just made it learned and unclear, but he had the same results all right. As a result we didn’t publish anything except for the flurry of the interpretation of the Pauli principle, the impenetrability of matter. I think I have the postcard from Ehrenfest about that which was wrong, which was just plain wrong.
It’s the thing that works in one-dimension but not from there. In one of the papers you do do with Ehrenfest, there is a remark that the Pauli principle was “anschaulich” before it came to depend on symmetry properties, but now it’s no longer clear why there should be such a thing, which is presumably before this postcard. Now, one of the things I’m curious about — and it’s the sort of mental transition that it’s hard to pin down or get information about — is the concern of a number of people — I know Heisenberg was concerned for some time -– to understand ‘why the Pauli principle’?
And clearly the transition to symmetric versus anti-symmetric wave functions was no answer to it; it changed the problem, but it left you with the old questions. By the time I get to learn physics, this is no longer a problem; there is no answer to it but now one speaks of Fermions and Bosons and that’s all. How did that transition take place? Where does that vanish as a problem without ever being solved?
Well, I think it became clear, and surely slowly, that the choice of the statistics, so to speak, could not be decided on the basis of quantum mechanics alone, that it lay outside of it. People felt that they couldn’t find an explanation and therefore they gave it up. People slowly gave up trying to find some kind of an explanation although Pauli, of course, kept going at it and his paper on the connection of spin and statistics was one of his answers, which, at least, he thought very highly of. I think it was a very important paper also in the ‘40’s; that was, of course, Solvay Congress of 1940.
But there’s a perfectly real sense in which the spin and statistics connection still doesn’t answer the question and makes it more fundamental if you like; it relates things, but really if you asked more physicists today what the answer to this question was, they wouldn’t tell you it was unanswered; they’d tell you it wasn’t a question.
Oh, I think it depends whom you ask; I mean, I think that it is certainly a question. I would still call it a problem but it is such a deep one that perhaps only later one can see; it is clearly not that one can do it simply. People very soon gave up and Ehrenfest, too, after the debacle of the impenetrability; this is one of the things, you know, with Ehrenfest. Nowadays and also in the old days people didn’t do that; if they made an error they didn’t take it back in print; he did that; he wrote a little note and he mulled this through and why and so on. I thought it was very fine that he did that. He put it all out, so to speak; nobody worried about it. Although it went on afterwards for quite awhile. Jaffe worked on it and you still can [see] that it’s not wrong, what Mr. Jaffe did; by making the forces very strange, entirely strange, you could still enforce this anti-symmetric business.
You were really already gone, or about to leave, by the time of the Como Conference and the ‘27 Solvay Congress, weren’t you?
Yes. By the way, these papers that Lorentz gave me [i.e. Lorentz’ calculations on a rotating charge] were finally published; they are in his collected papers. I think he gave them in the Como Conference, or at least a reworked version of it, but he did that in less than a week or two weeks at the time I talked with him — all these calculations, you know.
I asked you particularly about Como and Solvay because the whole question of the working out of the interpretation and then the relative end of the argument sharply after Solvay is a very interesting sort of point. But really you escaped that entirely. So as far as you were concerned, this was never a great big issue. Did you ever get concerned with measurement problems, for instance?
I tried to read Bohr and somehow it was clear that that was very profound, but I was not, so to speak, conversant with it at all end I never became very conversant with it at all.
When did you see the Heisenberg uncertainty principle paper? After you got to this country?
No, I think I saw that while I was still in Leiden, but I can’t be quite sure about it.
You might well have seen a manuscript or a proof.
I don’t think that it made an enormous impression on me somehow; it looked very fine but I was at that time quite pragmatic about things, you see, and also for my personal work. After coming to Ann Arbor, it appeared very clearly that I knew very little; I knew this quantum mechanics a bit and I had dutifully followed lectures, but then suddenly I had not only to give lectures but suddenly I also had Ph.D. students and that worried me sick! I didn’t know what to do with them. So it was clear that at that time I went through a minimum and I had to learn all kinds of things; I was therefore mainly interested in getting certain problems which I could do with the student preferably, and there, of course, I finally got out of it all right with the help of Fermi, who came for the summer schools there and with whom I worked very hard. Then I got positron calculations; I learned Dirac theory only at Ann Arbor, I learned the Dirac paper. I knew it —.
Which of the Dirac papers?
The electron paper. It came out in ’28 and I saw it, of course, but I couldn’t understand it. I didn’t know what to do with it, but then finally through Fermi and the positrons and so on, I got familiar with it and I made long calculations with it, trying to check every Oppenheimer calculation about it because he always published it with only the result which was very nice, if you don’t like problems. Once I worked a whole summer with Fermi on these things, and then I had students —.
Now, when you say ‘the positron’, you mean the hole theory before the positron really?
Well, they came almost very soon together; there was about six months, maybe, in between.
Oh, no. More than that. That is, the equation itself is ‘28 and the hole theory is ‘30 and the positron isn’t until late ‘32.
Ja, maybe so; I don’t have the dates in my head.
But there is a lot of intervening work, the Klein paradox and so on.
I learned that then immediately afterwards because there was even a Physical Society meeting in Boston in which there was a series of invited papers on the positron theory. I was one of the speakers; I gave the general introduction and then came Anderson and then came Oppenheimer. But then I was more or less conversant with it.
That would have been ‘32 or ‘33.
It was at that time that I worked very hard on these things; and then I was completely, so to say, on my own feet; then I knew what to do.
Were you involved with that, though, enough earlier to have been bothered by the hole theory?
Well, I knew that there were difficulties but I didn’t actively take part in it at all. I’m sure that Robert [Oppenheimer] talked with me about it because he was then, of course, in the midst of it, but I had no real opinions about it. That was this little note with Fermi; we wrote a note to it. This summer with Fermi was a wonderful experience because he was so concrete, you see; he has one of the most concrete minds I know. We did all these calculations on the blackboard and then made notes and then it was very clear that to explain something all these approximations were, so to speak, uncertain, and that we should do the solution of the Dirac equations for a potential which was not Coulombian but was, so to speak, the statistical potential. Well you can’t do it, of course; I said, “Fermi, you can’t do it.” He said, “What do you mean you can’t do it? You do it numerically.” I said, “numerically!” He said, “yeah,” he thought a little bit, and said, “It will take about a week for one wave function.” And we worked every afternoon — he punched and I wrote down the numbers and really in a week we had the curve and we knew how far therefore, for this wave function, it compared with all these approximations which one turned out; these approximations were quite bad, you see.
We left it at that; it was at the end of the summer that we did that. But that one could push it that way was again a revelation for me — that you could simply sit down and compute. And he was so fast, you see — boy, was he fast! -– he was really astonishing. I’m sure that because of that I really read immediately and in great detail his beta radioactivity papers which I worked through backwards and forwards and then I went on with it with [E. J.] Konopinski; we did the derivatives which were all wrong, but it was still a very great excitement for the moment. Yes, those were really the main influences for me, I think. Ehrenfest in the beginning, then I had to learn how to compute, which took me a couple of years. Thank God for the Brownian motion; it was for me very fine that there was such a field that people didn’t work at but in which there were still many things to do, so I wrote then these papers on Brownian motion. I still remember that Pauli said, “Brownian motion – Desperazions Physik,” he said, “Desperazions Physik.” But that didn’t bother me.
Why did you do that? Because it offered fascinating puzzles still?
There were definite questions, there were various questions, you see, which were not answered, and it came also partially through the work in Michigan on the noise questions, Johnson noise. They made very fine experiments there and I was then clearly the man, since it was statistics and I was supposed to be the theoretical adviser about it. So I studied all that very carefully and that brought me to the Brownian motion. I got a dissertation out of it for someone and then when I went back to Holland — the second time, I think, or the first time — I worked with Ornstein on it where I did most of the work, but it was the methods which he had invented, all right. At that time, of course, nobody looked at that paper really; it only became known in the war when all these noise questions came up again.
Tell me about your impression of physics in this country when you got here. What was the transition from Europe to Ann Arbor like for you?
Well, we were on the outskirts, we were in the provinces, that was very clear. In a respect it was very nice because you were not in a center -– nowhere in America, not only in Ann Arbor. The only feeling of being in the center again was during the summer schools because of all the people who came.
Did you two start the summer school?
No, no. Randall started it and there also was a summer school before we came, but as soon as we came, then it became really quite a center of activity of the department. Of course, Ehrenfest came, and Fermi, Kramers, Pauli, Sommerfeld, Dirac, and everybody came; then, of course, the summers were extremely busy and you worked very hard because you went to the lectures, too, but at the same time, you had to give lectures because that was, so to speak, the way it was set up. Then, of course, during the fall you started to digest what happened during the summer with all these things: Quantum Electrodynamics of Fermi which I edited completely because he sent me the notes and I wrote part of it, correcting his English and so on because his English then was not very good. That was this Reviews of Modern Physics paper which then I learned of course very well. So the center was still Europe. That’s also why Randall said we had to go to Europe every two years and he convinced the administration of that: that’s what I also did. Then really the shift came when I went back to Europe because all the people like Bethe and so on came slowly to this country and, of course, nuclear physics started. I still remember that at one of the last Physical Society meetings before I went back to Utrecht, they asked me to give them an after-dinner speech. Then the main point I tried to make there was just this change, because I said when I was in Leiden up to ‘27 the Physical Review was one of the funny journals just like the Japanese, where you looked at it once in awhile but you never really considered it very much; then in ‘35 when I left it was quite different and it was then already one of the central journals. That was an example of how things had changed; just in these eight years really the whole shift came.
Rabi made an interesting and, I think, controversial point yester in discussing this rapid development of American physics. He said he felt that in spite of the great contributions made by the refugee and the emigrant physicists that actually they scarcely played an important role until the war and after the war because they were in many ways still isolated enough and unaccustomed enough to the situation here: And it’s his impression really that the first great impact was made by the Americans who, like himself, had spent a year or so abroad, had come back, and been able to teach this very large number of students who were here; so he would say that there was a really major affect from his own experience, Condon’s, Oppenheimer’s, and so on.
I think Oppenheimer’s school was very, very decisive because Oppenheimer in those days, as soon as he came back, I think, in ’28 or so, had an enormous number of students and also [post-graduate] people, research fellows and so on, and he had an enormous activity. There then, at Berkeley, was one of the great centers — [W. H.] Furry was there, and [A. T.] Norsieck and [Willis E.] Lamb — all these guys came from Oppenheimer so Oppenheimer was able to bring what he had learned, to bring this excitement, over there, and I think what Rabi says is true. It was only much later that, so to speak, the real refugees – we were not refugees —, but people like Bethe and so on, really took part. Rabi himself then, I think, started a school at Columbia which also had a large influence. He really discovered Schwinger. Did I tell you? I always had to tell the people at Columbia that Schwinger was a great man. I was always the one Rabi asked, you see, and I still remember in ’35 — I think it was before I went back to Holland or maybe earlier — oh, no, about ’36 when I went to the summer school in Ann Arbor – he wanted to say, “Well, you have to talk to Schwinger; here’s a young man and he says that the Bethe-Heitler formula is not correct and you’d better talk to him about it.” And I remember that I said, “Fine”, and Schwinger came and he was such a young man; this may be wrong, but it looked to me as if he were still in knickers! He was a very young fellow and he began to talk about this Bethe-Heitler calculation which I had also been through very carefully because that was one of the things I had to do when I worked with Enrico [Fermi]. I think I convinced him that it was not wrong and that there was on some point in his argument which I didn’t quite understand but which I thought could not be right.
The only remarkable thing was that after ten minutes, we talked as complete equals; I mean, he knew clearly just as much as I did about it and it was very, very nice. And at the end I said to Rabi, “Well, you’d better take him; he is really one of the very good men.” Then I was, of course, again in Columbia in ’38 and Schwinger was again in trouble; he could get his Ph.D. because he didn’t go to lectures of the mathematicians and he didn’t have enough credits. So Rabi had told Schwinger that he had to go to my lectures at Columbia; of course, he didn’t because it was early in the morning, and I asked Rabi, “What shall I do?” because I was, of course, perfectly willing to give him an “A” on that because he needed the credits. Rabi said, “No, that you shouldn’t do; you give him an exam and you make it a tough one.” So I did, we made an appointment, and, of course, he knew everything; he had somehow gotten the notes! I also cleaned up a couple of derivations which I had done a little bit sloppily and which he had done much better and again it was so that he knew everything, so I said, “Fine, I will tell Mr. Rabi.” I could now with good conscience give him an “A” and that helped him in getting his degree.
What were the lectures on?
Statistical mechanics, which was not his field. And then he did the deuteron work and on that he got a dissertation; I think they by-passed the mathematicians because he had gotten the credits partially from me. So I was always involved in saving Schwinger somehow; even during the war I was involved in it because then he was at the radiation laboratory. He was at Chicago with the atomic energy business but he didn’t like that, so, typically Julian, he just hopped in a car and went to Boston. He didn’t tell anybody -– he just went! He came to Rabi, who was then at the radiation lab. And Rabi said, “Well, all right, of course, you can work here.” And that he liked; he was in my group and he did all these mathematical problems on wave guides which was very good, of course.
Did he ever do the computations on those?
Oh, yes. He was a real computer, really remarkable. He was mathematically and technically really remarkably good.
I meant did he do the actual numerical work?
As far as it was necessary, yes; the experimentalist finally only came to him in the evening at about 4 o’clock. I finally got him to give a seminar on it at 4:30. Julian was always out of breath when he came in, but then I had a certain influence on him so he did it and very conscientiously. But then the people in Chicago got mad that he had left and through channels it was told that Schwinger should be sent back to Chicago, that he was needed there, and that especially I should be reprimanded for having taken him away. So I was brought to Lee DuBridge and he said, “Well, they tell me you have seduced Schwinger into working at the radiation lab.” I said, “Nothing of the kind! He just came. I had nothing to do with it: he just wants to do it.” “Well,” he said, “people are very mad and you’ve got to go to Chicago and put the thing right.” So I had to go and spend a day or so in Chicago talking to Eugene Wigner and all those guys; I just told them how it happened and, thank God, they weren’t mad anymore and dropped the thing. I was complimented by Lee DuBridge for my great diplomatic gifts in dealing with Schwinger who didn’t even know about it; he was just sitting there! So I had to save him from going to atomic energy work which he just somehow didn’t like.