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Interview of C. J. Gorter by John L. Heilbron on 1962 November 15, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4639
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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: Felix Bloch, Ludwig Edvard Boltzmann, Hendrik Brugt Gerhard Casimir, Dirk Coster, Paul Ehrenfest, Adriaan Daniel Fokker, Samuel Abraham Goudsmit, George Hartwig de Hass, Werner Heisenberg, Petr Kapitsa, Willem Hendrik Keesom, Hendrik Antoon Lorentz, L. S. Ornstein, Wolfgang Pauli, Erwin Schrodinger, J. Tinbergen, Anton Edvard van Arkel, Pieter Zeeman, F. Zernike; Flask Club, Rijksuniversiteit te Groningen, Rijksuniversiteit te Leiden, and Teyler Foundation.
Well, perhaps we can begin with some short account of your own beginning interest in science; what your family was interested in, how you got interested in science, when you decided on a scientific career, and so on.
Well, my family is third generation intellectuals, one might say; I mean, not belonging to the old governing classes of this country, but my grand father were a doctor and a notary, and my father was a government official. I’m a third generation so to speak in intellectual circles. Among scientists there arc many who are not; their fathers or their grandfathers were working people; but I’m slightly older than they. I was at quite the best school of the country; it was in the Hague. And the rector of this secondary school was Casimir, the father of the physicist Casimir. And he was a remarkable man; he was professor of education at this same time in the University and rector of our school. And he was creating quite a modern atmosphere there. It was the most progressive school, in a way, and at the same time the most expensive school. All the well-to-do people with progressive sympathies sent their children there. So I was educated in this group also.
What were his particular methods?
Well, we had much more freedom. Originally we had the gymnasium in which Greek and Latin are the central subjects, like in Germany at the gymnasium. Then you had the (Burgerscholen,) and this is a sort of gymnasium without Greek and Latin, a sort of Realschule, as in Germany. And these were always separate; but he just made a combination of them. First there were two years general education and then you could split up. This was an invention of his, and many schools are now of that type in this country; they are called (lyceum) like 1ycee in France. Now he created that school, and then be had all sorts of modern things for societies of children; they had a council of the school composed of representatives of all the classes, and all sorts of sport clubs and other things. There were many clubs and self-activities of the children which you now find in every school in this country. But he was the first one to start this. He was a very modern man and he could convince the well-to-do people to give money for that. And also behind the school were mainly a combination of well-to-do people and clever people. Because first came the well-to-do people; they had shares and so on. Then they took a number of other people who were clever, and so the level was very high. It was considered to be the best school in the country.
There were then scholarships for —?
No, there were no scholarships, no, no. This was a secondary school, so this was a school where the average age was between 12 and 18. And Casimir was also in this school, but he is two years younger than I, and so at school I didn’t know him very well. But at that school I then chose the direction without Greek and Latin because I was against it as a boy of 12 or 13. 1 was very modern, and I thought, I’ll learn Esperanto.” So I learned Esperanto; that’s much easier. I have forgotten it since, but I learned it. And then I thought, “I am there, and I don’t need this Greek and Latin.” At first I was most interested in history, but then in the higher classes we had a very good school teacher, (Korfer) was his name—he was a pupil of Lorentz. And he was a fascinating teacher, and this teacher got mc into science, in a way. One of the things I remember of him was that when he didn’t know something he said, “I don’t know.” Other teachers don’t do that; they always have some reasons: it’s not easy, or it’s too difficult for you, or other things. But he said, “I don’t know.” And this is one of the things which impressed me. I was interested also in many other things. I was interested in politics and all sorts of social problems and all sorts of things.
Did you get much mathematics?
Yes. In school we got much more than in the States, for instance, here. It was just like in a German gymnasium; in a way just like what Kronig had at the gymnasium, I think. This is much more than my son had during the year he spent at a high school in America a few years ago.
It didn’t ruin him completely did it?
He forgot whatever he knew. He learned also some things: American history and such things and English literature. He is much more interested in literature and such things, so I think it was very good for him. But there was no science compared to what we get here, and no mathematics. So we get much more. I was 16 when I got my final degree here from the school, end then I was just 17 when I started here at the University, so I was quite young also, in a way. But Casimir was still younger. Casimir is the best, cleverest fellow I know. He did the same school, and he jumped over classes and he got his final examination when he was l5 while the average is 19. But then he did another year; he also did the gymnasium mostly in the Greek and Latin direction; and there he also had the final examination. And then he came to the University two years later than I— our difference in ages also was 2 years, so we started at the same age of l7 years at the University.
Had you decided on physics when you entered?
Oh, yes. Yes, yes. Well, this teacher just convinced me. He had always the best boys in his classes go into physics. So there is quite a generation of people from this school who studied at the University; for instance, (Koeller) who built the liquid air machine at Philips, and Belinfante who is now at Purdue. Every year the people started studying physics because he was such a marvelous teacher. He was, as I said, a pupil of Lorentz, and he told us much about it. Well, Lorentz and Kamerlingh-Onnes and Zeeman and van der Waals were very central personalities when we were young, and this attracted the young people in a way.
Well, did he have you talked into theory at that point?
Well, he didn’t talk me into it; he just gave us lessons, but he just impressed me that way. I think there was one other thing; when I was in school, I read then a book on special relativity, a popular book. This fascinated me also very much, so that when I was at the University, I thought even more about theoretical than about experimental physics. I mean, it was about equal, but it was more theoretical, at that time.
What kind of courses did you pursue then at the University?
Well, here it’s just a standard scheme. You see, you have to choose a certain direction, and my direction was mathematics and physics, at equal levels, not one more than the other. You have two main subjects and one small subject, and the small subject was chemistry. This does not mean that chemistry is small, but I didn’t learn much chemistry. So this was the combination. You could choose between physics and mathematics and astronomy and physics and mathematics and chemistry, and I chose chemistry. So that was my direction, and then I got my bachelor’s degree here, and such things.
Do you remember how much modern physics you got, and how much of the new physics: matrix mechanics, and so on?
There was not quantum mechanics because it was l924, so there was not quantum mechanics yet. I was born in 1907, and I entered the University in l924.
Oh, then I see there are some errors in our outline.
Yes, there are quite a few.
So you were in school during that entire period when the new quantum mechanics came?
Yes, you may say so, yes. But in the first years, and this is still true in the universities, you are quite isolated from the research side. We had a thing which turned out to be very good; the main lectures were given to us and to all groups of the sciences and the medical people. They all sat at the same lessons; and this meant that they had to be so easy that the physicists didn’t go there, so we just dropped them. And then we worked for ourselves, and we had a very active student society here, which was called The Leiden Flask, and I was very active in that. And so we developed ourselves, more or less, in physics. These courses were of no importance; we passed the examinations, but we didn’t follow the courses because we learned it from books. And this was a very active student society. During my whole student life these societies have been very important. Ehrenfest was the man who mainly protected them, and he was very enthusiastic about then; he was the guest of honor at their meetings, and so on.
What sort of program did they have?
Oh, they were just clubs. At first in The Leiden Flask, I think, we met about three, or four, or five hours a week and. a seminar or something was given— a speech or something. We went over the courses, or other things. We discussed things, and there was a council which arranged them. Every year we had about 20 students in physics and mathematics here, so we were 20 of the same year. And then perhaps 8 or 10 were active, but we met 3 or 4 or 5 times every week to discuss different things and prepare ourselves for examinations and so on, just as a group together. And there was also the group over us, who also came. The first two years are considered to be just the starting, the pre-bachelor degree people. And so we had 10 or 15 people. I remember I lectured there on things I was interested in. Non-Euclidian geometry I remember was one of the subjects; it was not physics at all, it was mathematics. ... Another subject I was very keen on was chemistry; it was the homo-polar bond of Lewis in the American literature. You know Lewis from California. My favorite subjects were these two. So none of them were physics.
You never were exposed formally to modern physics in your course work?
No, no. We11, we had general courses which we didn’t attend. But than we had special subjects, and then we went. And the main special subject was thermodynamics; we got lots of thermodynamics and radioactivity. And radioactivity I preferred. And this was a course given, I think, by de Haas which greatly impressed me also. And then there was some course also on atomic physics and X-rays, and such things, but that wasn’t very modern. But I still remember that the radioactivity attracted me very much. And then, of course, we had chemistry courses and mathematics— quite a bit of mathematics again.
So you can’t really say that the new developments in physics were reflected in any curriculum change?
No. But de Haas impressed us a bit more than Keesom. Keesom was a systematic man who gave thermodynamics. It was very good, a very good course, but very slow (and ???). But de Haas always had something with genius. I mean he always had mysterious things, on the construction of the universe, and so on. He always impressed me with all sorts of mysterious things which often were very witty also. But he was very ill; his health was not good. He was often ill, and so often he didn’t come to the course. We would be sitting there and if he didn’t turn up, we would go out. A European university is much less organized than an American university, and this is one of the examples. If I don’t turn up in one of my courses the same thing happens—the students go home. Nobody cares, “He isn’t coming, so we’ll go home.
At what point did one start getting a little more detailed in all of this?
My own starting points in the way of getting a bit more interested were two things: one that I was elected as a fellow of another student society, Christian Huygens. This was a student society to which the most active people were elected. So it was an honor to be there, and I was very happy to be elected there in my third year as a member, which was rather early. The central personalities there were, for instance, Uhlenbeck, and Goudsmit, and so on, and through this we got contact with it. The second thing was that in the second year already— it was earlier perhaps—we had one course, one hour a week, which was also (called) among this Leiden Flask.
But this was given under the special protection of Ehrenfest by the assistant of theoretical physics. So he explained the modern things of theoretical physics and gave courses, and sometimes younger people spoke also there, as it should be done in the Leiden Flask. But about half of the time he talked; it was on Friday at 1:00 o’clock, I remember. And there we got interested in the modern problems. And I think one of the persons was Uhlenbeck and there were also Tinbergen and Goudsmit and others who taught just a bit more than physics. But it was not an official course, it was just a group again. The third thing was this. I was very sure of myself, and there was a lecture given by van Arkel who is now a professor of physical chemistry here.
He gave a lecture on the chemical bonds, and he did it in a purely electrostatic way with hetero-polar bonds. He tried to explain everything with electric fields. But I had been reading Lewis, and I was against it, and I attacked him. I attacked him rather fiercely, and I said that he couldn’t explain the hydrogen molecule, which was true, and the chlorine molecule, and so on. Well, he tried to put these things through, and he would probably have succeeded if Kramers had not gotten up—he had just then come to Utrecht as a professor from Copenhagen— and he took my side. And there was quite a discussion. I was out. It was between Kramers and van Arkel, but I had started the whole thing. And the next day Ehrenfest came to me, and he scolded me terribly. He said that I had behaved very badly because I was just starting as a student, and I had been so impolite to this important speaker.
But at the same time he invited me to be a permanent guest at his colloquium, you see. So he thought, “I want this man.” And so I was very happy about that. Ehrenfest was always looking for young people whom he liked and whom he could get on further. He was most interested in boys of 17 and 18 who would be future coming men. And so he got me out of this group. And these things meant that in my third year already I was getting in touch with the modern developments at that time which did not include quantum mechanics, but quantum theory, of course.
Do you recall the sort of things that were discussed about quantum theory? What were felt to be the main problems, the main difficulties7
Well, we were very interested mostly in X-rays and the Bohr model, and those things were discussed. Spectra was discussed a little bit. And then, of course, we came immediately to Atombau und Spektrallinien of Sommerfeld. And this was the Bible of everything. After my examination when I got ray bachelor’s degree—I was 19— I started reading in Sommerfeld; this was a great thing. It was 1927. I was 19, almost 20, when I started that. Then with three of my friends I went on bicycle to Norway to see a sun eclipse. This was my first contact—you mentioned this— with astronomy, so to speak. Two of my very best friends were astronomers. One was Kuiper, you probably know his name; he was director of Yerkes for some time, and now he is in Tucson, Arizona; he is director there. And the other one is now the director of the largest observatory in Australia, Bok.
He was at Harvard for quite a time. And they were two of my very best friends. And the third one was a geologist. And we went on bicycles to Norway to see the sun eclipse there. And so we bicycled for (50) kilometers or so, and we got there. It was raining, of course. And this was then immediately after the examination, so I did the examination at a rather early time, and then we went there together, us four friends. And it was, quite a thing; we were in the newspapers everywhere because at those times boys of 20 years of age going so far for a scientific expedition was rather a thing to put in the newspapers. So we had a very nice time there. I was the cook. We were camping, of course, with a tent.
How did you happen to go to Sumatra?
I had these connections with the astronomers, and then later when I was here in this laboratory, they wanted someone to go on a real expedition. The expedition to Norway was not a real expedition because we wanted only to investigate the color of the corona, so we had lamps and resistances to vary the color on screens to look at the corona. But we didn’t look because it was raining. We had our things sent over by mail from the observatory while we went on bicycle. When they went to Sumatra they just invited me to be a member of this expedition, and there were five people from Holland going there. Two were doctors; this was Minnaert. You probably don’t know his name, he is a professor of astronomy at Utrecht now. He was a Flemish fellow who was in a German university during the First World War and was condemned to death by the Belgians for that. Then he fled to Holland, and he became a physicist; he was first a biologist; he completely recovered. He is now a 100 percent Dutchman, rather a left-wing man, and he behaved very well in the. Second World War—no sympathy with the Germans whatever. He was a senior man of the expedition; the other senior man was an astronomer from the (Blitz). There were three assistants: one of them was Kuiper whom I mentioned to you, my old friend who is now in Tucson, Arizona; the second one was van Wijk who is now professor in Wageninen, of agricultural physics, and the third one was myself. And those five people went by boat then and set up an observatory in Sumatra in 1929.
Were you the cook again, or were your activities more scientific?
Well, I was the electrician. The soldering of the wires, and so on, this was my task. But essentially what we were doing was observing sun eclipse and optical work. So then I learned a bit of optics. Here I hadn’t been doing much optics— I was a low-temperature man here— but there we worked with spectrographs, and so on. And there again the weather was not very good. There was no rain, so we got some photographs of the flash outside of the sun, but not very much. It was an interesting voyage, so I saw that part of the world, Indonesia, which is certainly most interesting.
You saw it while it was still Dutch.
It was still Dutch, yes. I was there once later when it was Indonesian after the last war to compare it; it was quite interesting. So we discussed for a year because I first had to prepare myself in Utrecht. I saw Burger and Ornstein and all those people in Utrecht—I was sent there for learning the optics. I worked under Minnaert for three or four months, and then we went by boat there to Indonesia. Then we were for two months setting up this observatory. We had a whole government ship at our disposal; it was quite a thing. We had about 25 white people helping us and lots of coolies, and so we had quite an observatory there. We had five instruments through which the sun was observed. Then later I made a voyage through India to Indonesia; I saw Java and different places. Clay I met there; there was a Pacific Science Congress there at the same time, so I enjoyed being there. Then we came home again, and started studying again. It was between my bachelor’s examination and the second university examination which is a so-called doctoral examination. It is not a doctoral degree; we have a system which is different from other countries; so you might say it’s a bit of heavier master’s degree or such. But this was the doctoral examination, and I didn’t have that yet, so this was during my student’s time.
What did your teachers think about it? Did they think it was a good idea for you to take your time off at that point?
Well, as a matter of fact, Professor de Haas—he was my professor; I was working under him, he was my supervisor—he sat in this room in this chair and asked me whether I would be willing to go there. The professors had just arranged it, and they sorted out the people whom they wished to go there. There is a committee of the Royal Academy of Sciences which goes over the Ec1ipse Expeditions, and this commission had just decided to ask these five people. One was an optical man which was van Wijk; he was a student of Ornstein; one was Kuiper, an astronomer, and the third one was me who was the electrician of the group.
When did you become a student of de Haas? After the bachelor’s decree you immediately —?
Oh, yes; then I came into this laboratory. I had thought much about theoretical physics, but Ehrenfest was against that. It’s always in Holland that when the students of mathematics and physics start, about 30 or 40 percent wish to do theoretical physics because this attracts them, with Einstein and Bohr, and so on. And they are rather selected, also for the mathematics, so they have a sort of theoretical tendency. And then Ehrenfest discouraged them, most thoroughly; this was his system. And they still (get some) discouragement but not so radically as Ehrenfest did it. So I remember him just refusing Casimir for theoretical physics; Casimir was by far the best student who was there. And then Casimir insisted that he wished to do theoretical physics— he was 18 then. But Ehrenfest said, “No.” And then he said, “Wouldn’t it be much better if you should become an architect.” And Casimir said, “Why an architect?” And he said, “Well, when you were a boy you could so nicely play with wooden things, and so on, and make big buildings, and so that would be much better than theoretical physics.” Ehrenfest’s idea was that the real good theoretical physicists would do it anyhow, so he couldn’t keep them back. So he tried to keep them back with all the means he had.
Everybody. And then Casimir insisted, and Ehrenfest said, “All right, if you now really wish to do it”—it was now 1927—“then I have here a book, Weyl Group Theory and Quantum Mechanics, just tell me what’s in it.” And so he sent him home; and that was at the age of 18. And Casimir worked for three months— I remember, it was the only time in my life I’ve seen Casimir working— and then afterwards he told Ehrenfest what was in it. It was too difficult for Ehrenfest; he couldn’t read it, but Casimir just told him what was in it. Then he was accepted. He was also invited to the colloquium, as I had been, so he was officially accepted. Then he sent Casimir very soon to Pauli and to Bohr, and so on, to be taught at other places because Ehrenfest also had the idea that people should be sent to other places.
What were Goudsmit and Uhlenbeck preparing for at this point; had they been accepted as theoretical students?
Well, they were preparing for their doctors’ degrees; they both got their doctor’s degree in ‘27. Uhlenbeck had been for some time in Rome; he was the teacher of an ambassador’s son there in Rome. And he came back, I think, in l924. And Tinbergen was Ehrenfest’s main student. Tinbergen was a famous econometric man. I don’t know whether you know his name; he was quite an internationally famous man, - and he had worked with Ehrenfest on mathematical economy for quite a time. So Ehrenfest didn’t do much physics at that time— in l924. Tinbergen is now quite a famous man; he sits always in the United Nations, and so on. He is a big shot in mathematical economy. And then when later Uhlenbeck came, Ehrenfest came back a bit to physics after some time. There was at that time in theoretical physics a professor and one assistant, and Uhlenbeck was the assistant. And then he was the head of this Friday afternoon meeting, as I told you, The Leiden Flask for senior people, senior students. That’s what they called it.
And Goudsmit and Uhlenbeck had been accepted as theoreticians?
Goudsmit was mostly away; he was an experimental physicist, essentially. And he worked in Tubingen with Zeeman and Back, so he was the man who knew all the spectral lines and wave lengths. It is the story that at his doctoral examination, he didn’t know anything. All the questions which he was asked he failed. And they finally asked him just to write down Maxwell’s equations, and he couldn’t write them down. And then finally he said, “What do you wish then?” And then Ehrenfest said, “Could you tell us the wave lengths of some spectral lines?” And then he drew the whole blackboard full of spectral lines and explained the whole thing because he knew everything about spectral lines. At that time you were much more independent as a student; the professors said all their things and you learned other things. I also failed my first examination at the University. It was a (???), the small examination we call it, after the course. And I fell down because I hadn’t learned the things they had been teaching me; I had learned other things. But they wished to know what they had been teaching, so I fell down.
What happened under those circumstances, you just took them again?
No, no; I got really nervous then; then I had to do some work because I was always convinced that I had rather good brains so I could do things. Then it was a change for me. And I was doing much in politics then. I was a member of and later President of the Labor Party of students here in this University, and then Ehrenfest got me completely out of that. He said, “Well, you must do one thing. Later you can do what you wish. My best friends, like Einstein and so on, they did lots of other things, but first they had to be good physicists.” And he tried to get us to be good physicists first. I dropped a number of things: I played chess and all sorts of other things, and did a little in politics, and learned Esperanto and such things.
How long was Ehrenfest interested in mathematical economics?
Tinbergen, who was his pupil then, refused military service. He was quite a revolutionary. He was in the same club as I, of the Labor Students. He wrote his thesis on maximum-minimum problems in physics and economics. This was his thesis; it covered two things. But, of course, he got his doctor’s degree in physics. But essentially the main thing was economics in his thesis.
Did Ehrenfest publish anything on economies?
No, I haven’t seen anything. He worked with him, and there were complaints. I think Uhlenbeck complained, and so on, that he gave all his time to other things than physics.
And that was in ‘24, ‘25?
I think ‘23-‘25, at that time. And then, of course, he got the spin— that was then in late ‘25, I think, as far as I remember things. He still was interested in those things, and in late ‘26 he got interested in quantum mechanics; (he didn’t take it very seriously) at the beginning, I think.
Which, the matrix mechanics or the wave mechanics?
I don’t know; I think both; both of them (had come out already). And I remember at this old institute which was called Institute (??) which is now called Institute Lorentz, Institute of Theoretical Physics that they were laying on the tables. I remember Uhlenbeck and Goudsmit lying on the tables and discussing together very lazily. All their energy had to go to the brain work. And we came in sometimes and they were lying there and making all sorts of (difficult stories). And then later they told us about it also and it was very interesting.
What were these stories? Were these the spin discussions?
No, this was the quantum mechanics. For the spin discussions, I was too young. I think then it was ‘25, and I was just in my second year, and then I had no connection with it. I had been reading it— I mean the whole story of what happened; Kronig told me, of course. (We were at Groningen together, so we worked much together then.)
That’s quite a story.
Oh, yes. He got the Planck medal, I don’t know whether you noticed that; I think, it was in this connection. Because he had really made the invention first, and then they talked him out of it. I knew it; he had told me. He hadn’t published it; but he published it in Pauli’s Memorial Book, as you know. And I knew it, and Uhlenbeck and Goudsmit knew it too, that they had not invented it, and that Kronig was originally the man.
Well, you’ve given some quite interesting details of Ehrenfest as a teacher. What sort of methods had he that made him so renowned?
Well, he had the enormous idea of first searching for good people and then making those people enthusiastic. The remarkable thing was that Lorentz, who certainly was a greater scientist than Ehrenfest, had no people of that level. I mean, Ehrenfest had so many good people; he picked them out everywhere. He had Burgers, the hydrodynamics man; and then he had Coster, the X-ray man, who was an excellent man. Then he had Tinbergen, he had Kramers; that was the first generation. Then he had Uhlenbeck, and Goudsmit, and Casimir, and Rutgers; there are very few theoretical physicists who have so many first-class people. Sommerfeld perhaps had and Born; I don’t know, and Bohr, of course. But Ehrenfest could do it. He picked them up, and these were the important people. And he always gave a considerable part of his time to these young people. He was not interested in senior people, so he got in trouble with all the senior people.
I mean, I had trouble with him, Kramers had trouble with him, everybody. When we were getting older, then he was more interested in the boys of 17 than in the senior people. He thought, “Well, let them do it themselves.” He wasn’t interested. Sometimes he didn’t listen when you told him something. He was interested in the young people. And then he was, well, a very good teacher; he was quite informal. He was an Austrian Jew who came here to Holland; I mean, he didn’t know Dutch. In a way, he fit in very nicely here, but he was completely revolutionary—once a year he cut his hair; that was in April. The whole year he would let it grow, and then he cut it again. He was completely unconventional. But then he had good friends here in this University; one of the people who was most fond of him was an old Baronet hero who was in international law, van Eysinga; he just died last year. When I met him, I was a senior colleague then in this University, and then he always wished to talk: about Ehrenfest and what Ehrenfest did; he was one of his friends. So here in the University he was quite a central personality; he made people enthusiastic.
How did he do that?
Well, he was different from everything else. We were a sleepy country, you see, in those times, in physics also. He just stirred things up by his extravagancies. First he didn’t speak any language; I mean, ho spoke a mixture of Dutch, German and English and everything else. I remember people asking him, “What language are you speaking?” and he said, “I speak my own language.” That was him, you see. He was a character, you can believe me. And then he had all sorts of ways of comparing things; he was a very good speaker in all the things. I remember, for instance, one of the things he had always were statistical problems; he was a great man of statistics with Mrs. Ehrenfest in his early things on the H theorem. And then he said he had this system with two dogs—he always taught with two dogs—and there were fleas on them.
And these fleas were jumping from one dog to the other, and then he would make statistics about one dog and how many fleas there were on this one dog and the probability of their getting on the other dog and what happens when a third dog comes there; and this way he taught us statistics. I remember this was the normal way— he talked always of the dogs and the fleas. But then I remember once when Einstein was here— Einstein had a special chair in this University in those times, and he gave some lectures from time to time. Einstein gave a lecture and things were discussed and there were many guests and people, and Ehrenfest wished to be official. And then he started talking again about these statistics, but then he couldn’t use his dogs. And he said, “Well, suppose we have two boxes of matches, and we think that the matches jump from one box into the other.” And everybody was wondering because it wasn’t as amusing as the dogs, but anyway he had matches jumping from one box into the other. And so he was a character. I had to have all his courses. And he didn’t like to give examinations, and what he did was this: the course started in September and went until June, and then he had a spcia1 examination which he gave after the Christmas holidays, in January.
He asked only about the subjects which he had been teaching between October and January. If you passed through this examination, you didn’t have to have an examination over the rest because he said, “Well, the people will follow this course anyhow; (the people will get through it and will get through it enthusiastically) and it’s very important for them that they get a good basis and don’t wait until the end of the year. So they have to pass this examination; then they are completely free, and I am no longer interested; they have done what they should do for me.” And then he was very careless in mathematics also; he was not a mathematical man, like Kramers or Casimir, or most theorists. I mean, he wasn’t interested in numbers; like Landau, you know, who just got the Nobel Prize; he is the same kind. Landau is a man who is most like Ehrenfest in many respects. And he would sometimes put in his equations with 4(pi) in parentheses; he didn’t care about a factor of 4(pi) or of other constants. This just gives the sense of the story. Well, and this fascinated young Dutch people enormously, and so he was a central personality here in Leiden, Ehrenfest. And he had many more pupils than Lorentz; Lorentz came here sometimes. He was also an excellent speaker, but very different; he was complete1y precise.
He would make a whole calculation on the blackboard, and everything was marvelous; you just could copy it and print it. I remember once in Ehrenfest’s course when he was just speaking; he went away at a moment and he said, “I am not going to come back here to people who have such dull faces.” And he went away and left us sitting there. I know one boy (he didn’t like) was sitting in the front row looking very silly, and Ehrenfest couldn’t stand that. Well, he was very emotional, a very emotional man. I also had some friction with him, and Kramers and all the people who were there at that time had it too. But he was a marvelous man. You know he committed suicide?
Yes.—-It was said during the time of the Bohr-Einstein debates over complementarity, and so forth, that he felt that very strongly since his friends were involved.
Oh, yes. When something new was done in Gottingen or Copenhagen— which were the centers of theoretical physics at those times—then he often went there; he was asked to come. They would send him a letter or telegram asking him to just come, and then he went there. And he was at these discussions, also at other places—(???). And then he told us later about it, so we have still a colloquium which is called the Colloquium Ehrenfesti here and it’s Wednesday evening. So people come together to do theoretical physics essentially or sometimes a bit of experimental physics with theoretical interest, and this is on Wednesday evening from half past 7 until half past 9. And it is still there, and it’s still full of people; people come from Utrecht and Amsterdam and Delft, and they add to the Colloquium Ehrenfesti. And this was one of his most central activities, and in all these colloquia he told us about these discussions, so from those times we were very well informed about those things.
This colloquium, when Ehrenfest ran it, was mainly for the students, is that it?
Well, you can’t say that. You had to have an invitation. This is the colloquium to which I was invited, the colloquium of Ehrenfest. And there you had to be invited, but, well, half or three quarters were students All the senior people also came there: de Haas came often, and Keesom and (Frenkel) and a few chemists and other people; it was also such a colloquium. When there was a speaker from outside, sometimes he invited more people, but usually we were about 20 or 30 people. And there we also had all the people who were coming to visit Leiden and who were there. Oppenheimer was there for quite a time, and Fermi was there and Tamm, the Russian, and Elsasser. They were all working for some time in Leiden, and they also sat in on the colloquium. Also the visitors who came to Leiden were always invited.
I mean, if you would be there on Wednesday evening when they were holding the colloquium, you could just be there at the discussion. Then he just encouraged people to make silly remarks and to have a free discussion. He was very aggressive, and so the discussion was also. Still it is like that; we continue a bit the tradition of Ehrenfest. Like Landau; I mean, just interrupting the speakers and so on, asking them difficult questions. I remember Felix Bloch once in 1929 was telling about his electron theory of metals there. Well, Ehrenfest attacked him so much, he couldn’t move forward nor backward, and Bloch is not a man who is easy to stop. But he couldn’t move he was just completely pinned down on certain weak points of, well, this single electron model which was too simple according to Ehrenfest. He just proved this to Bloch during the colloquium; we enjoyed it. And I remember I was giving a lecture, and one of my subjects was then X-rays—intensities of X-rays, intensities of reflections—and I had a speech written down on the things I wished to tell, and I had four chapters. One chapter was the introduction, and there were three other chapters. And the whole evening I was speaking, and I got no farther than the introduction because Ehrenfest and other people attacked me all the way in such a way that I couldn’t get out. But still it was an interesting evening, also for me because, I mean, this was the style. A very lively and aggressive life there was.
Do you remember what Ehrenfest’s reasons for being uncomfortable about the new quantum physics were?
Well, he was just interested; I don’t think he was much for or against it. Originally he didn’t like the whole thing, I remember. But very soon, I think in 1926 already, he got convinced that there was something in it. And then he followed it up very much. And he was on the side of Bohr and Born. He was on those sides he was not on the side of Einstein though he just told us about it.
Was he very much torn by this antagonism?
Well, I don’t know. He was always very enthusiastic about these things; he was a very lively man, so I don’t know whether this really personally hurt him very much. I don’t know. I remember, for instance, Schrodinger was also visiting here, speaking at the colloquium, and so on. But then, before this, he had been telling us all the wrong things Schrodinger had been doing, because in his youth Schrodinger had been doing the wrong things. And he said, “Now you can see a great man and see that even if you do something wrong in your youth it doesn’t matter if you do something. Later you can still become a great man like Schrodinger.” He had all sorts of strong ideas; he was very left politically— rather left-wing. A great thing which also perhaps contributed to his suicide was the anti-Jewish movement in Germany which he was very sensitive about and very unhappy about at that time.
Could you see him getting progressively more disturbed during the later Twenties?
Yes, yes, yes. I had a discussion with him on suicide also and things. Boltzmann also committed suicide; Boltzmann was his teacher, and this influenced him greatly. He had the idea that one should commit suicide between the ages of about 50 and 60. I mean, you have had the best part of your life then, you have done what you should do, you have been useful to the world perhaps a bit, and you have enjoyed it and why go on and be an old man and get all sorts of pains in your back and be at places where you really shouldn’t be. And he said, “Now, I’m feeling really that Kramers or Uhlenbeck should be in this place.” But he said, “I can’t go away for financial reasons; that’s why I have to stay here while there are younger people who are better than I am.” So this was his reasoning. But I never thought he would do it because he was so afraid of getting a cold that you would never think that he would commit suicide.
Did he recur to that theme?
Twice I think I talked with him about it, but other people have also said the same thing. But, I mean, one of the things was this son of his. He just was a child who couldn’t move, and so on, and it took half his salary just to keep this boy alive. And he just thought also that this boy shouldn’t be in the charge of the family later. So this was another factor. And then this situation with Mrs. Ehrenfest was not so very good. She was in Russia when he was here, and when he went out to America or anywhere else she came to the house here in Leiden. And she went to the colloquium then.
So she didn’t live in Leiden most of the time?
She was in Russia, in Moscow. She was teaching and had a position there. But she had also great qualities; we liked her. Both of them were much against smoking. And when someone had just lighted a cigar or cigarette in this institute before leaving in the evening, he smelled it the next morning, and he got very angry about it. You didn’t dare to do it. There was one story which we had about Schrodinger. There was a man Wiersma, who was my boss in the laboratory, under de Haas, but over me, and he was quite a character—a friend of Ehrenfest’s also. He died during the war. And be had once told Schrodinger that Einstein had the permission to smoke in this building. And then, I mean, Ehrenfest was so terribly angry to think that they had, well, refused Schrodinger what they allowed Einstein to do. It spoiled the whole situation; I remember it was a problem we had. And then we had another thing; we had this colloquium, the 500th Colloquium we had. And then we people—the regular people— thought to make some feast out of it for Ehrenfest because the colloquium was his central thing.
He always lived for the colloquium. And I found it out—that it was the 500th time— I think. Then we made a sort of nice evening for him; we all gave him a book with lots of photographs in it of his family and everything and of the history, and we made poetry, a sort of children’s poetry in it. Then a number of people had been invited, and we had been writing letters to Bohr and to Einstein and to Born and all his old friends, and Pauli, and they all sent telegrams to congratulate him on this evening, and they all were read. Then the professors from Delft and here Professor Keesom and de Haas and a number of people came there for this colloquium. And Ehrenfest was so angry; he just detested it. He just scolded—. And the worst thing we had done was we had given him a sort of flowery wreath and he said, “Wel1 you have just given this to me for my funeral already!” He was very angry about it, and the only thing he liked was the album of the photographs. These photographs we had stolen from his own house. His son, who was a physicist, you know—young Paul Ehrenfest who died in the avalanche later— helped us and had given us the photographs which his father had in some cupboard and never did anything with. Then we had some other photographs and we all glued them together in a book and wrote all sorts of things under them, and this he liked, this book. But the rest he detested— this whole 500th colloquium with all the troub1e we had for it.
When was his son killed?
He was killed in 1938 or 1939 I guess it was.
Did the other son live at home?
No, he was in a hospital; it was a home or hospital, but he was not in the family house. Nobody has ever seen him; he was about 15 or 16, or something like that.
And you say Ehrenfest was much impressed by Boltzmann’s act?
Yes. Oh, yes, yes. Well, he just defended it; he said it was a principle. But he had his personal problems also. I only once made a trip together with him; he took me to Gottingen once. And then we visited Franck and Born. I was there some time. That was 1931, and then the Nazi tide came up, and he was very sad about that, I remember; it was a bad situation. It weighed very heavily on him, but he would perhaps have committed suicide anyhow because it was his principle. I done t know.
What about some others of these gentlernen with whom you came in contact?
Well, we had, of course, many physicists here always; I remember Dirac and Fermi and Tamm, as I mentioned, and those people; they were studying here for some time. Elsasser was a longer time there, but he was an assistant. And there was Oppenheimer; Ehrenfest couldn’t stand Oppenheimer because he coughed. He coughed in the colloquium and attracted the attention by coughing; he had a bad cough. And Ehrenfest didn’t want it; he didn’t wish Oppenheimer to be there. He said, “You’d better go home; you cough so much in the colloquium—.” So he went away after some time, but he was interesting. So all those people I met and talked with and so on. But, well, I was younger than they, of course, and I had not been developing so rapidly as people like Kronig and so on, and Casimir; I was a bit slower. Casimir got more into contact with the theorists than I did; you should perhaps see Casimir. I was here working in the laboratory experimenting with de Haas and working on the para-magnetism; I got my thesis on para-magnetism.
What kind of a gentleman was de Haas?
Yes. Well, he was a man with enormous intuition— no mathematics; he had enormous intuition and good ideas, and he was a good experimentalist. He had a wife who was very clever. She was much more clever at mathematics than he was. And he was a great man who knew how to treat people and to see people. He had excellent ideas, but he didn’t read literature, for instance, so ho was very badly informed about things. He learned from the students what had been published, and so on. He didn’t read papers. But he was quite a character, and a good experimentalist. And he had a view of people. I remember, for instance, that once there was a boy who came with a thing— a sort of concept for a publication. De Haas said, “Oh, well,” he just looked at it and said, “Yes, well, all right,” and he put it in the cupboard. And then half a year later, I came and he said, “You see, I put it in the cupboard. Do you know why?” he said. “Well, I do it because I’m not quite sure about it; if in half a year’s time he still will say the same thing, it will be all right.” I mean, the character of the man just was like that. I mean, it just gave to everyone his own treatment and way of behaving; and he was a character also.
How did he get along with Ehrenfest?
Good. I once criticized de Haas in the presence of Ehrenfest. I mean, I was just speaking as you do about professors, I said, “Oh, well, de Haas doesn’t know his literature,” which was true. And then he just fell on me very fiercely. He said, “You shouldn’t say such things about de Haas; he has excellent ideas—.” And he had really; he had something of a genius, de Haas, experimentally. He had a room opposite here in this laboratory and he was working all by himself there with one assistant—he was a technician. And he did all sorts of experiments, and so on, on the fundamental things of physics. I mean, for instance, he said, “Well, we’ll just check Maxwell’s equations in five decimals.” Or he wanted to see, for instance, if the charge of an electron is just equal to the negative of the charge of a proton, so he would have them together to see whether there is not a small difference of one in 10.
Such things he wished to do. And he wished to see interaction between light and gravitation, and all sorts of experiments. He wished to know whether the susceptibility of a solution was the same when it moved as when it was standing silent. And he did all sorts of experiments. And he always discovered effects, and he was very enthusiastic about the fact that he had discovered something. But he was so critical that he never published anything either before he had proved that it was wrong, or he had explained his mistakes. This was also a great example for us. I’ve been finding many effects in my life, too, but usually you have to go on in order to find out that they are wrong; and usually they are wrong; I mean, when you wish to do something new. And this we learned from him, also, among other things. I was very fond of de Haas, too. . Keesom was more the hard worker. I mean, he was the thorough man, the hard worker; he knew his mathematics well. He was a stiff Catholic; he had a family of nine children; he had a real family (???)—- a sort of dictator. He was not interested in people, I mean, in what they did. He just said there is a free place there and a free place there, and he wasn’t interested in their characters and their special problems; he just was interested in doing physics and getting things done. In some points he was a better physicist than de Haas because he knew his things much better; he studied the literature. But, well, he had nothing of the genius; he was just the hard working scientist, just like anyone.
Did Lorentz lecture very often, during those years?
He had a Monday morning lecture, from 11:00 to 12:00, and I attended it only a few times, because, wel1, he died in 1927. Well, I was a student then; I started in ‘24, so the first year I was too young for it. Then he was some time in America, I think, in ‘26, or so. I heard perhaps 8 of his lectures. He was a very good speaker. I remember once one of the things which I saw which was quite different from Ehrenfest. I remember, people were afraid of him a bit; he was so very kind. He was so kind it was difficult; I mean, he just overflowed with sugar and kindness, and his wife [daughter], Mrs. de Haas, was just the same. They were so kind that you get embarrassed by it. And I remember once that a boy came in too late to one of his lectures, and what did Lorentz do? He just went to the boy, shook hands with him, and said, “Oh, I’m so very sorry; I must have been starting too early.” And everybody was sitting there.
This was not meant with great irony?
Not in the least. Well, of course, but, I mean, (you didn’t know it); he was just so kind himself, and so on. Then he was like that. He was Dutch; much more Dutch than Ehrenfest was, of course, and than de Haas was, in a way de Haas had something (???), I think.
Lorentz didn’t lecture on any of the modern physics?
Oh, yes. He gave quantum theory, even quantum mechanics. I think they went on the side of how Schrodinger came to quantum mechanics originally. It was about some mechanical theory which played a big role then in the coming of the quantum theory; I’ve forgotten now since from some time ago I haven’t been doing any mechanics…This was a sort of principle that you can describe waves by particles, and particles by waves, the way they are connected to each other; this was also what Schrödinger did essentially.
Oh, Hamilton’s ana1ogy between –
Yes, yes; this was it. I’m not 100 percent sure, but I think this was one of the things Lorentz taught us. Lorentz was following everything— the newest developments anyway, and he wasn’t against it. It wasn’t his field. I mean, he hadn’t made it, but he was very much interested.
Do lecture notes exist on these lectures of Lorentz?
I don’t know; I simply don’t know.
Were there usually appointed Ausarbeiters there in the Dutch schools, as there were in the German lecture courses?
No. Kramers did that. In Kramers’ courses there were always students taking notes, and then you could read them, and they were nicely typed out. Then the students studying could just look at them, and so on. Kramers did do that.
But that was unusual?
Yes, yes. I don’t think Ehrenfest did that, and neither did the other professors,. but maybe they did in other places; I don’ t know.
How frequently did Einstein exercise his rights as…
Well, not very often. I think perhaps every second or third year he came for a few days, and so on. Then they [Ehrenfest and Einstein] walked together arm in arm around in the street here before the tram, and everything stopped for Ehrenfest and Einstein walking together there. And the tram went “Ring-ting-ting” but they didn’t care; they were important people, and so on. And Pauli, of course, was a great friend of Ehrenfest. Pau1i was his best friend, in a way, you can say in many respects. They were just calling each other and attacking; Pauli was a very fierce man, as you know, a very aggressive man, and Ehrenfest was also.
It must have been quite something when they were both present at the colloquium.
Pauli was (chairman), sitting there all the time and coming with acid remarks. I talked much with Pauli later about Ehrenfest. I went often later to Zurich, for instance, where, in those days, Casimir was the assistant of Pauli. Pauli also scolded me very strongly on many things, and so on.
How would Ehrenfest react when he was shown to be wrong after one of his aggressive attacks? Presumably he wasn’t always correct.
Oh, no. He could just completely admit it; he would say, “You are perfectly right; you are perfectly right.” He wasn’t so that he had an idea in his head, he was more setting up a discussion, and so on. And he thought the play of discussion was to stick to your point of view as long as it wasn’t proved that you were wrong. But he was certainly not a man who would—. After the discussion, .the discussion was over; you were good friends and you went— well, he didn’t drink any beer— but he would have been a man to drink a glass of beer somewhere. You know there have been very rnany stories about Ehrenfest around.
Weil, I have been anxious to hear as many as possible.
Well, one thing. You know Zernike who later got the Nobel Prize? Do you know German? Well, Zernike was a young professor in Groningen, and Ehrenfest was the big man. Ehrenfest came there one time and they were sitting together in (Baulink) — a nice restaurant; I have also been in my Groningen tire sitting together and drinking a bit and talking about all sorts of things. And then it came out that Zernike could read the newspaper upside down. And Ehrenfest said, “This is marvelous; can you really do this?” “But do you think you could take a German book, written with German letter could you also read that?” And Zernike said, “I don’t know.” Well, then they asked the chief waiter, “Wouldn’t you have a book written in German?” this is something you wouldn’t do in America. He said nothing, and the boss finally came and he said, “Oh, yes I have one.” He lived in the same house where the cafe was, and he went up to his room and after ten minutes he came back with Goethe’s Faust. And then Goethe’s Faust was taken and put upside down, and Zernike read the thing like that. And Ehrenfest said, “Ah, so das ist Ihre Begabung”, “That’s your specialty!” I remember Pauli once told me that he and Ehrenfest were sitting in a car together in Copenhagen, and they were talking a bit Ehrenfest was going away in the taxi. And then Pauli said, “Oh, yes, well, I 1eft my light burning in the room there, upstairs.” There were lots of rooms and he thought one was his room, and he said, “I must have left the light burning upstairs.” And then Ehrenfest went into the taxi—this is what Pauli said—and Ehrenfest opened the window, and said to Pauli. “Nicht alle Lichter sind von mir angesundet worden, Pauli.” “He shut the window, and I couldn’t say anything back,” Pauli said.
Do you recall other stories?
I don’t know. You know, of course, of this story about the Encyc1opedia article? It’s very famous; everybody knows that. Pauli had been writing on relativity, I think. And when Ehrenfest and Pauli met for the first time in 1926, Pauli wasn’t behaving very well in those times; he was a very disareeab1e young man, and so on. Ehrenfest wished to criticize him a bit and he said, “Well, I’m very happy to meet you…Es freut mich sehr Sie kennen zu lernen, aber ich muss sagen, Sie gefallen mir nicht ganz so gut wie Ihr Enzyklopaedie-Artikel.” And then Pauli immediately retorted, “Und Sie, Herr Ehrenfest, gefallen mir viel besser als Ihr Enzyklopaedie-Artikel.” … I went skiing in those days in Switzerland and Austria, and on my way to go skiing, I went often to Zurich, and then I visited Casimir and other people. (Holborn) another friend of mine was there then, and there were others. Or I went to Munich whore Gerlach was; then I was with the German people a bit there, Meissner and Gerlach and those people. And then I went skiing. Those were some of the points I passed by. That was a bit later, of course; that was in early 1930.
I was quite interested in finding out a bit about the Teyler Stichting.
Oh, the foundation, yes. It was (Peter Teyler von Rost); he left his money to this foundation in 1784, I think. Then they had a museum there and big collections of scientific things and art, and in the 19th century everything was art. But then there were some very wealthy descendents usually of (Peter Teyler von Rost) and they decided about 1910 that they wished to go back a bit to science and not have the main thing be pictures. They went away from art and concentrated more on science because Teyler was really more interested in science than in art. And then they asked Lorentz to come there. And Lorentz in l911 then left his position here and went to the Teyler foundation as curator, as it was called. Then Ehrenfest got the chair here. There was a single chair of theoretical physics. From that time on, Lorentz was the central personality there in Teyler’s foundation; he was sitting there spinning his web. He received people there and made it a center of activities. And at that time he also made those calculations about the Zuider See, and his big works and so on. He was a great man; he was sitting on all sorts of committees and things; he was president of the Academy of Sciences, and he was the central personality. He came one morning a week to Leiden and gave his Monday morning course then; he sometimes came to the colloquium also.
How many positions were salaried there?
There was one position; there was the curator and. there was a conservator as the second man in command. And then there was a technician, and he was sitting there. And the practice, was that the young conservator was a future professor, so Lorentz just selected someone and then later he made him a professor somewhere, and then he just selected, another one; this was the system…If he said, “This is a professor,” then it was all right. He was very critical, and people know that. Some of the people who had been there were for instance, Elias, he was a professor at Delft later, and van der Pol, you know probably that man, and Coster was there, the x-ray man, and Fokker was there and de Haas was there also for some time, and, well, there were others. Burgers, the hydrodynamics man, was there; and they all had been there for one or two years—E1ias, the longest, perhaps 6 or 7 years. They had been in the conservator position, then they were appointed there. And then when Lorentz died in 1927, Fokker became the curator. So Fokker gave up his position as professor in Delft and became the successor of Lorentz.
And then also he wished to have someone, and I am the only one who has been his conservator. And later (Ratenauer) was there, but he didn’t get the title of conservator, but, in fact, he also had the position. But the money went down a bit; the capital lost its value a bit because he had fined capital and money and life got more expensive, and they had agricultural land, and so on, and it went down in value. So financially the situation went down, so (Ratenauer) was not officially conservator, I think, as I was. But I remember in the slump time, Fokker and I both just gave back to the foundation 10 percent of our salaries so that new instruments could be bought, because they said, we can hardly have any money for research and instruments. That was when I was working there. And then Fokker just gave the example— he had a very well-to-do wife— and he said, “Well, why shan’t we give something up of our salary?” And we both dropped 10 percent of our salary, I remember, so that we could have some more money for instrument buying. I think it was only once that we did that, but we did it.
So the entire fund came from the estate originally?
Yes. It was a private foundation. And there were lots of people who wished to have this money. And these people were disputing it all through the 19th century; they were descendents…
But it no longer has this status that it did?
No, not quite. There is still one— the successor of Fokker, Kistemaker, who now has the special chair of the Teyler Foundation in this University. So he is a professor appointed by Teyler’s foundation at Leiden University.
So they now just maintain another chair here essentially?
Yes, the chair is here. But they don’t give much money for it because really Kistemaker’s main job is (director of mass separation) in Amsterdam. So he is one day a month in Haarlem. The thing is closed. There is a geographical laboratory now working in the buildings there, not in the museum, but in the other buildings where the lab was where I worked. Kistemaker goes there sometimes to give a lecture— once a month or once a fortnight. Then he gives also one lecture a week here in Leiden. But this is his job also, and so he is the successor of Lorentz and Fokker; these are the only two who had the chair. This is possible in Holland; you see, a private foundation gets the permission of the queen to create a special chair, and this was Teyler’s chair.
And before Lorentz was there, it didn’t support science?
That’s right. Oh, well, there was the museum; they had a science museum a very big electrostatic machine, and all sorts of things. They had lots of optical researches and things, and well, things they had in the 19th century in physics. (I was never very much interested in those things. )
We haven’t talked about van der Waals.
I didn’t know him. I mean young van der Waals I knew, but the old van der Waals was much older than Lorentz even, so he had died before I just arrived in the University. But he was the man behind Kamerlingh-Onnes in a way. I mean when Kamerlingh-Onnes was here, the two theoreticians who just influenced Kamerlingh-Onnes were van der Waals and Lorentz. And after quite a bit of hesitation he chose van der Waals and just concentrated on the interaction between molecules and so on. He took this thermodynamic line of van der Waals rather than the optical and magnetic line of Lorentz which was continued by Zeeman, who discovered the Zeeman effect in this place, as you know, in 1896.
And you stayed at Teyler’s then until the chair opened up at Groningen?
I was (entitled) from 1931. I was already appointed there before I got my doctor’s degree. That is, my thesis was very good, but de Haas was so slow—.
He put it in the cupboard for awhile?
Well, I had to read it to him in parts, just read it to him. And then he said, “Well—.” I didn’t understand it at the time, but there was a senior man, and (de Haas) wished him to have his doctor’s degree first, you see. This was Wiersma. He was a very gifted man, but he didn’t write things down; so he was four years older than I, and de Haas didn’t wish me to have my degree before him. And he was right, and so he just let me wait. And then Fokker asked me, “Could you come to Teyler’s foundation,” and I accepted. So when I got my degree, I already had my position there. And I was there about 4 and 1/2 years, to 1936; then I went to Groningen.
How did Fokker know of your work?
Well, he was at this University, and I followed his courses. I mean, he gave one course a week, like Lorentz did, and I attended this course. And he knew me from the colloquia, because there was always the colloquia. So Fokker and I were always discussing, and I knew him quite well.
So once you got this position there as conservator, you were then established and in line for a chair?
Yes. Well, I would have been if Fokker had been Lorentz, but Fokker was not Lorentz, and so I wasn’t quite sure. Although I wasn’t much interested. I had originally thought— this was also one of the questions you asked about what I intended to be when I studied—I thought I would be a school teacher in physics. This was the thing; I mean the only way of development for people studying physics was being school teachers, or professors. But there were already three professors of theoretical physics in the country, so we didn’t count on that. So I intended to be a school teacher, in a way, that’s what I intended, and only later I just thought, well, scientifically, but it was much later that I really thought about it. Well, he asked me to come to Teyler’ s foundation.
I thought a few times of going away from there because working alone in a laboratory is not easy. I mean, we had only one technician and he was not a good technician; later I had a better one, but at the beginning it was a bad one. Well, I did all sorts of silly things; I liked being there because of the independence. And I never again had so much time for thinking as I had there, but after all, it was good that I left. I had two other invitations to go—two other possibilities. Once I had an invitation to come back here [to Leiden] and have Wiersma’s position as the second man to de Haas. Wiersma was invited by the Russians to go to Kharkov. (B—-) was there at those times, and (Shubnikoff) and those people. Wiersma planned to go there, everything was arranged, and I was to be his successor— that was in 1934. And I had put my conditions to de Haas also; that I would have certain possibilities here, that I could do adiabatic demagnetization and those things; and he had agreed.
So I was planning to come back here and would have been very happy to have been back here. But then Wiersma just changed his mind when he didn’t get the visa in time because the Russians were already then getting difficult. And then a half a year later this whole business in Kharkov exploded; people were killed; (Shubnikoff) was killed even, and there came the great purge of the scientists in Russia particularly at Kharkov. This was one thing, and then I had another possibility; Fokker just advised me, or recommended me for going to Rutherford in Cambridge. And this was in l934, and I almost got that job there. Kapitza had left, and then this place of Kapitza which was a very good place was cut into two. And then I was going to have one of those places; that was the idea, but I didn’t get it. And the reason was given by Rutherford to Fokker; Fokker had been working under Rutherford, so he knew him quite well. And it was given in this way; they had appointed Peierls for the first place, who fled from Germany he came through Holland and then was in England. And then they said for the second place they had to take someone from the Empire, they couldn’t take another foreigner. So then he took Allen, and he worked there on liquid helium and did excellent work there. I would have liked in a way to have been there one or two years with Rutherford, but I just missed that.
Wasn’t quite a bit of the equipment from Kapitza’s laboratory sent to Russia?
No. Let me think. I think later, quite a bit later, they sent some things. Kapitza did two things; first he had this liquefier, and then he had his high magnetic fields with his generator. He produced pulsed fields by short circuiting a condenser bank. And this pulse machine was away when I was in Cambridge for the first time, as I remember; maybe he tool: that. The other one, the liquefier, was still there originally, but then later they made another one also. Yes, maybe, there wasn’t very much. But the building was there; they had the Royal Society Mond Laboratory, a very nice lab. Then later Schonberg was there with Allen, and they managed it. First Allen and then Schonberg, and now Schonberg is the head, and now Pippard is there again in Cambridge. But then, so, I would have been in Cambridge then, one or two years before that. Then I accepted this place in Gröningen, I think. I was the successor of Prins; I don’t know whether you know Prins.
How were things at Groningen? Was it a reasonably well-equipped place?
Oh, yes; oh, Groningen was quite a center. At those times, I think, Groningen was quite a good place; Coster was an excellent man; he was also one of the best pupils of Ehrenfest, but an experimenter. He was one of those who was a first generation intellectual, so his father was just a worker in the harbors of Amsterdam. And he had worked himself up; he had done all sorts of things. He had enormous energy and a very great intelligence, and he worked with Bohr and with Siegbahn in Lund and in Copenhagen, and he discovered Hafnium together with Hevesy. He had quite a name, and then he was appointed around l925, I think, as a professor of physics in Groningen, and he concentrated on X-ray work, and so on. Zernike was the theoretical man there. And then later we had two lecturers—readers is a better word; it was a bit higher position than lecturer; it’s a sort of associate professorship, this position.
There were two positions, and Dieke was there, for instance, from Johns Hopkins, whom you know. And then later Prins and Kronig were there. When Kronig was there, he was under Coster; then he was the theorist. Zernike, although he was a professor of theoretical physics, he didn’t give any modern theory; he was against it. So he didn’t feel anything for quantum theory; he didn’t teach it. He just thought, “Well, I’m not giving it.” Zernike was quite a character. So Coster, in order to get a reasonable education for the students, just asked a good theorist to be there to give the lectures. First he asked Dieke, who was a theorist in those times, and then Kronig. Kronig gave the theoretical lectures, really, modern theory. And Zernike gave his own things—the physical mechanics—which was quite good— and Maxwell theory.
That’ s what Zernike gave, and Kronig gave the modern lectures: atomic physics and quantum theory, and so on. Then there was a second man, an experimental man, and that was Prins. Prins was the man who with Zernike made the intensity of X-ray things; you know probably those things. And he was a mathematician and physicist, an excellent man also. Prins was then appointed as professor in the Agricultural University, Wageninen, and this almost killed him because this was an agricultural university. I was also on the list, and I was very happy I never got it. But I was then appointed as his successor in Groningen. There I had a very nice place because Kronig was there, and I worked together with Kronig a great deal. I was very fond of Coster also, with whom I had an excellent time. I even worked together with Zernike, so I had quite an interesting time there. We had then discovered the paramagnetic relaxation, and so on, and we worked together on that. I had some young people there, and had a marvelous tine.
Do you know what Zernike’s reasons for opposing the quantum theory were?
He was an individualistic character. He was much more interested in the techniques; he had already invented then the microscope—the phase— contrast method— for which he got the Nobel Prize. He was interested in technical things. He had the Zernike (galvanometer) and later the Zernike magnet, and so on. And he wished to do that, and these old things he had learned from Lorentz in theoretical physics. He was very clever; everybody knew his intelligence was very high, but he wasn’t interested in those modern things. He thought, “This quantum mechanics someone else can do, why should I do it?” And then he experimented in the lab during the night usually. He was a very good experimenter and made all those different inventions a.nci constructions and things. And then he very often took away your things. I remember once I had a lens system and I found the lens was out of the middle.
I immediately said, “Zernike must have done that.” I went to Zernike and there it was; he just stole everywhere in the lab and worked on those things. And then he apologized; he wasn’t a bad natured man; he was very good natured. And he said, “Oh, sorry I forgot that; thank you very much for lending this to me.” I remember even once when the secretary came to the lab and the whole typewriter was gone; the one typewriter we had in the lab was gone. And Coster came there and be was very fierce, and he said that we should investigate, and so on. But nothing was broken. Finally someone said, “Let’s telephone to Zernike.” And he said, “Oh, yes, I’m very sorry. My sister” — she was a writer — “just wanted a typewriter and I took it from the lab yesterday.” He was quite a character. But he had very bad relations with Coster. Coster didn’t like this. So this way I was placed in a position in between. But I was good friends with both, so I enjoyed it. I liked Zernike in a way, but he was also quite a character. European professors are much more characters than the average American professor. Not the modern professors; we are just like everybody. Would you like same tea? 1ihere are you from personally?
From Berkeley in California.
From Berkeley, oh, that’s a nice place. I’ve been there; I have good friends there. I’ve lectured there; I think it was three years ago. This summer I was only as far as Stanford; I was mainly in La Jolla, but I went up to Stanford. Usually I go to Berkeley, also, when I am in California, of course.
We have been having great difficulty finding out information about Dutch scientists. Information is little available in the general literature; the Dutch Who’s Who for instance, is not very good, or very informative. This misinformation on you comes from there.
Well, I’m very often in America and in other countries also. I like La Jolla; that’s a nice place. But I like Berkeley also; MacMillan I also know, I know a number of people there. Segre I knew quite well; he worked on the Zeeman in Amsterdam many people were working in this country. Bloch, for instance, worked for a year with Kramers, and we were very good friends in that time; he’s a very old friend of mine. Placzek, you know that name also probably, he was working with Ornstein in my time.
Did Ornstein used to attend these Ehrenfest colloquia?
No, because he was against Leiden. He had been chased away from the Practicum once as a student— he was also a Leiden student, of course, of Lorentz. But he was on bad terms with de Haas, in particular, so he was against Leiden, so they never talked together. And Utrecht and Leiden were things apart. But then with this ease of the astronomical thing there was a committee of the Royal Academy of which they both were members, and this was all right. But Ornstein never came to Leiden. He was a theoretical physicist, but later he became the director of a laboratory; he was a boss, a very big boss; I mean, he was a bit bossy. But, well, he had certainly many qualities. He was a good organizer and manager and a good theorist.
Do you recall— I guess the work done at Utrecht was not very much discussed then in the colloquia?
Well, these intensities of the spectral lines were; yes, sure, we knew about it.
Well, I was thinking more of the work going on simultaneously, about which I’m going to talk to Burger later in the week, on the photon. They did the most remarkable things with the photon in Utrecht about that time. I was wondering if that ever became a subject of conversation?
No. I don’t think so; what was it?
Well, they assigned a cross section to the photon and did all kinds of statistical mechanical calculations using the cross section they had attributed to it.
Well, there was an old line of research on photons; I mean, Ehrenfest himself worked on it and many other people, so, I think, Ornstein also continued part of this. But this was an old line of thought about these statistics of photons—
This work was inspired by the Compton effect, and so forth.
No; it wasn’t much discussed as far as I know. We had the Netherlands Physical Society which was a very active body which was founded by Fokker, and there we met. But the Utrecht people didn’t go there very often. I mean Bloch did, and Placzek and those people, but the Utrecht students and senior people didn’t go very often— until 1938. In 1938 Ornstein just took leave of the Physical Society and reformed it completely, and he made it much better. Then he was interested; before that he just boycotted, slightly, the Physical Society. It was more on the Leiden and Amsterdam interests. But the connections over all were not very good; and Kramers also couldn’t fight it with Ornstein. They had friction at this time.
Well, when Kramer’s went to Utrecht there must have been considerable competition for young theorists.
Well, Ornstein didn’t do any theory any more. Ornstein was the experimental physicist there; he just changed the chair. There were two chairs, one for theory and one for experiment and director of the lab. Julius was there before, I think, and then, Ornstein went over to experimental physics, and the chair of theoretical physics was free.
What were the relations between Utrecht and Amsterdam, then, with Zeeman’s laboratory?
Zeeman was a very fine old gentleman, huh? I don’t know whether they had any relations, maybe, I don’t know. I mean, Zeeman had his own atmosphere and this nice laboratory of his which is now called the Zeeman Laboratory, and which was called Laboratoria Physica in those times. I don’t know whether there was much relation. Ornstein had his own group, and I got into touch with that because of the sun eclipse; I worked there. But on the whole there was not much contact with these other people. Leiden or Amsterdam or Groningen; they were much more tied together. Ornstein was a difficult man; I mean he was the only man who had trouble with Lorentz. It was very difficult to have trouble with Lorentz, but he had friction with him, apparently; I don’t know. But Zernike was, in a way, a collaborator of Ornstein; they worked together once in Groningen on those statistical things. But from what you are telling me now I have some vague idea that I have heard about it; I can’t remember.
Well, they are very curious papers; I should think Ehrenfest would have disliked them very much.
Ehrenfest would certainly know it because he was interested in those things himself. I have once tried to do an experiment myself about combining photons. It was in Teyler’s foundation that I did it. I did lots of silly experiments there, and so this was one of them which didn’t give any result.—But now they’ve done its.
I meant to ask you about that rather remarkable title you gave that paper: “Negative Results of Doing This and That.” And you, later published another paper with the same sort of title.
I could have done many because I’ve done many experiments with negative results. There were the nuclear magnetic moments I tried to observe here at Leiden in 1936 yes, the nuclear resonance.
It’s not very often one publishes a negative result.
No. Not in those times. Here, nowadays, people even publish the idea of doing an experiment. They just have the idea and they publish it; they send news letters to the Physical Review. But at those times to publish something negative really was a thing. People just advised me to do it; they said, “Well, after all, it’s nice to do it.” I just thought that if people said it’s worthwhile to write it down, then “All right, let’s write it down.” And I put in one page and two pages and so on. But, yes, this has changed. I knew sometimes the things Burger did and Ornstein, but this I don’t remember. They were interested in galvanometers and. so on, and noise measurements and those things.
Well, when you left Groningen and went to Amsterdam, things were beginning to get a little difficult, I would imagine. How was Dutch science during the Second World War?
I was appointed in February before the German occupation, and then I went there afterwards. I was appointed when the Germans were not there, but I went when the Germans were there. And in the beginning things were not very noticeable; the Germans kept out of things, and so on. But later it got worse and worse, of course, and we all got our share. I mean, Leiden University was the best. They just immediate put a stiff leg against the Germans, and there was a reason for that. There wore old traditions hare at Leiden, Lorentz and so on. I mean, you feel it much stronger. Go when the first Jewish professor was dismissed, Meyers, who was president of the Academy of Sciences, then one of the professors gave a speech on the importance of this man. He was arrested immediately afterwards, and the students just made a strike, and the University was closed down already. This was at a very early moment— the 26th of November, l94O.
Did it reopen?
No. But they asked a number of professors to continue, and they just continued, and even some courses were given. But there were no examinations, and so on. Well, let me think. No, no, that’s not true; it wasn’t closed; I was talking nonsense. It was the 26th of November that we had this speech of Cleveringa, who then was arrested, and a number of her people were arrested. Then the University went on, and then later there was a second professor, Kranenburg, who was dismissed. Then the professors made an agreement among one another that when another one should be dismissed, then they would all resign - or a number of them would resign. Then about 40 Leiden professors, which, I think, was about half of them, all resigned together in one block. Then the University was closed; this was in l94l; this was a year later. And then most of the students went to Amsterdam from Leiden. I was a professor at Amsterdam, and we got students from Leiden in considerable numbers. So it wasn’t so bad for the students after all, I mean, they could just as well study another place. We treated them decently. Then the University was closed here. De Haas was not among those who resigned; he didn’t resign, and Keesom also did not, so this was a rather difficult position.
And de Haas didn’t because he said, “Well, if I had done this, the whole thing would be closed down. We have a large school of instrument makers here in this laboratory and they probably would be sent immediately to Germany because they are excellent technicians. It is much better to keep them here and keep the laboratory going.” Those were his reasons. So de Haas didn’t resign and didn’t close it down. But Kramers did resign. Those who resigned were paid, I think, half pay, and they could continue some things like being directors of their institutes without giving courses. We resigned in Amsterdam; that was later in ‘43. That was much more serious then, the Germans got much more difficult. We would have been shot if we had not taken it back; they told us, “Well, if you don’t take this back, then you will be court-martialed. And we took it all back officially, but we didn’t have any courses any more. That’s what you do in war-time. I mean, you don’t let people be shot, but you just take it back and do what you wish, so that’s what we did. It was sometimes difficult to get out of it because they wished me, also, and other people to give courses.
Why did they wish that just to keep the people occupied?
Oh, no. It was because they had asked the students to sign a declaration of loyalty that they would do nothing against the Germans. The students had refused that. But when l5 per cent of the students had signed, then the Germans said, “Well, only for those who have signed may the teaching go on, but not for the others.” And then we said, “No, Sir, we have been appointed by the queen to teach any of the students and not 15 percent who have signed your declaration. And so we didn’t do it. First we resigned and then took it back, and then sabotaged the things a little. The graduate students worked, but it’s just the elementary courses that were the problem, you see. Very few people in this country, physicists also, had any sympathy with the Germans.
Did the Germans interfere much with what was taught.
Sometimes. (Once the mayor of Amsterdam) criticized me because I had made, well, not jokes, but illusions to certain things. Other people also criticized, but they really didn’t care very much. They took some laboratories; they took some things away when the atom bomb came, and those things. They wished to make an atom bomb by themselves; a fellow by the name of (Bucher), I think, once did this. He came here and took things from the University, and took then to certain places and. set up another place, a laboratory, and: so on. They interfered with chemistry, for instance, but this laboratory they just left.
They made no effort to encourage scientists to come to work on their Uranium projects.
They sent Heisenberg and Becker. But later we found out they had been sent to feel out how we felt. But we just told them what the situation was and that we hoped that the Allies would win the war, so they had not much idea that we should do mach reasonable work for them. And apparently they reported in their country that there was nothing to be done with us.
They had intended at this point to—
Well, they came here just as visitors. I mean, they wrote to Kramers and so on. (Heisenberg [Kramers(?) ] wrote back and said they could come, he wouldn’t say no.) So they just were here, but then everybody told them what the situation was, particularly about the Jewish situation. I lived: in a Jewish quarter in Amsterdam; I happened to be there because the lab was there, and I was just in the middle of it. I showed Becker around and talked to Heisenberg, also. I just told them what we thought about it.
How did Heisenberg respond, do you remember?
Heisenberg was very witty, in a way I have— well, all physicists have—an enormous admiration for Heisenberg. Well, he didn’t think at all about it; neither did Becker, by the way. We complained about the newspapers, that there was nothing in them. Heisenberg said, “Well, you mustn’t expect to find any truth in the newspaper. When you read the newspaper, you must think, ‘Why have the people been writing that; what, do they want me to believe’ That’s the way you have to read the newspaper.” That’s what I remember that Heisenberg said. So he didn’t say there was anything true in the newspapers; he didn’t say there’s anything wrong in newspapers, but he was telling us that we were considering them with the wrong mind, lie should see ‘what they want us to believe,’ when we read them. Actually there was something in it that was certainly true. Yes, he was very witty, but he just wished to feel out how the situation was. He lived with Clay. We just received them; I mean, they were not Nazis, so we just could receive them, but we did it in such a way that they knew we were not interested. Well, I don’t know in which way they were sent. In a way people liked so much to travel a bit. For instance, de Hans, also and other, people, just wanted to go once outside of the border and see other people. Perhaps Heisenberg and Becker just wished to see how things were in Holland. It was just that they had to report, of course, to the authorities what happened, but it was not that they were spies or anything of the kind; I wouldn’t suggest that.
Had you any knowledge of how the American bomb—the Manhattan project was developing?
No, no, no. I was great friends with Halban, and he was with Joliot working in Paris, and I visited them just before the war. And they were just putting the water in, and the uranium and so on, and saying, “Well, be careful now, the bomb will go!” So I knew about it; I had been there with (Kowarski) and Joliot and Halban, who was my friend really. Then we talked about it, so it was not a secret then, I mean, among scientists. But I had completely forgotten about it. But when the bomb went off, I had a friend, this Kuiper, whom I mentioned, who was then in the American army. He was in this group of Goudsmit, Alsos, and he came to visit us in his jeep. Then this thing came up — it was in the newspapers about this whole explosion. “This must be uranium,” I said to him. And he said, “How do you know!” And I said, “Well, I know because it must be,” But I had not thought about it for five years. -The same thing happened to me with radar. I mean, I knew exactly about radar, the whole set up, because I was always working with high frequencies. So I knew about radar. And I remember when I was sitting at the radio, which was forbidden during the war because we were supposed to deliver all our radios to the Germans, there was the battle of (Cape Matapan), I think, in 1942. Then it was stated in the British radio that they had sunk a number of Italian ships at a distance of 35 miles although they had never seen each other. Then I knew this must be radar, and it was. I mean, when I hear of a thing I think there must be a reason for it. Those two things are similar in the way I reacted myself. I hadn’t thought about it, but when I heard what happened, I drew the right conclusion from it.
Mien Heisenberg was here trying to feel out the Dutch scientists, he made no reference to a German bomb project?
No. I knew nothing about it. The only thing I knew was—yes, this man (Bucher) who was here and wished to work. Then he said he was going to do something and it would not— “Es ist nicht von kriegwichtiger Bedeutung, es ist von kriegsentscheidender Bedeutung.” And this threat I remember. Then very vaguely I thought a moment about uranium arid possibly that this is what he might have meant with “kriegsentscheidender Bedeutung.” For the rest we didn’t know anything. And just the date this bomb fell on Hiroshima when Kuiper was in my house. Then I just immediately knew it; I could tell him what it was. Well, what else is there?
Well, anything else you can think of which would be—
The time was from 1898 until 1938.
Those are to be viewed as maximum.
Wel1, as I say I have just seen the people lying on tables in (???), and hearing the exp1osion from the Como Conference; this I remember very well. I mean, Ehrenfest and Kramers came often to us, and we had discussion. The first quantum mechanics I learned myself was from Elsasser, who is now in La Jolla. He was the man who first suggested the interference of electrons, Walter E1sasser. And he was an assistant of Ehrenfest, and they couldn’t stand each other. But he gave us courses in quantum theory; this was in 1928, or so. And so from him we learned it a bit.
Do you recall the process of acceptance and adjustment to the idea of conrplementarity; the results of the Como Conference?
Well, it was all explained to us by Ehrenfest, and we all were on the side of (???). I think, that Ehrenfest and the people all the time felt that Einstein did it very cleverly, but that he was not right. I mean, in this discussion, in a way. I mean, at the same time, at this Como Conference there was also the Kramers-Kronig Relation. This was a side-line in which I also later was much interested, and this also came up there, I think.
But for the rest, I was much interested later, I mean when I was at Teyler’s foundation, in the coming up of nuclear physics. I had the idea that there would be a neutron gas everywhere. One of the silly experiments I did was to take a top and rotate it very fast around an axis. Then I had a piece of paper over it, and above that then, on a long wire, I had a disc. I wanted to see if this disc would start to rotate in the same direction as the top. If there would be neutron gas everywhere, then, of course, the whole thing would start to rotate, and then also the disc would get a small motion; and it did. But this was one of the things I had learned from de Haas that you must give it up [Put it aside for a time]. Then finally it was found out that when you turned the top the other way, it remained almost silent. It would go slightly in the same way, but much less.
But they found out it was the pressure of sound. Noise went through this paper — it just lifted this a bit, and because the wire was slightly. (???), it just got a thing there, and the sound was different in a way, and that’s why you got the effect. So the one thing is to be careful with your (???).
And it’s nice to do experiments like that. But when you’re old later, you know, in a big laboratory, and so on, that you have to run, then it’ s very different. But we had a nice time then in Teyler’s Foundation.
One thing which I did earlier was to bend the crystal in such a way that you get a strong intensity, so a bent crystal gives a much stronger intensity of an X-ray spectrum. This is a method which has often been used by (Dumond) and by (???) to measure the intensities of weak gamma rays and weak X-rays. And this I also did here in the laboratory, I remember, just bending a crystal around and having X-rays interfere so that not just one point of the crystal contributed, but all the radiation came back to one point. But it didn’t work; the techniques weren’t quite good enough. You must do it technically quite well, and I was just a student and did it clumsily, you might say. So I had many very nice experiments, and I liked doing it. And in a way, I would still like to do those experiments, but I have no time; there are too many other things.