Leon Brillouin - Session I

Kuhn:

I hope to be silent almost the entire afternoon. I would really like very much to leave this conversation up to the two of you, Dr. Brillouin and Dr. Ewald…

Ewald:

May I ask a few questions?... First of all, when did you come to Munich? After the war?

Brillouin:

Before the war. It was December, 1912. And I spent the winter up to July of 1913. That was the time I was there.... My training in France had been at the Ecole Normale in Paris, which is a part of the University. So I took all my degrees at the University of Paris. And in 1913 I was through with the degrees, including what they call the “aggregation.” The “aggregation” is an examination for students who want to become professors in the Lycees. I took this examination, and I was getting ready to start working on the PH.D. when I went to Munich… I had done some research with Perrin by that time. It was one of the rules of the University that before finishing the degree you should write a small thesis, you should do a small piece of research. And I had done some research on Brownian motion in the laboratory of Perrin — something of no great importance — which gave me the privilege of working in the laboratory for six or eight months, and getting acquainted with Perrin. If I may remember the people I knew best, the professors which were really impressing me most, I should say that the first one was Poincare, and I was terribly impressed by his teaching… It was on a subject which is completely forgotten now. It was cosmogony, and what Poincare could say on cosmogony in 1910 is absolutely of no importance and no value now. Cosmogony before radioactivity and (cosmic radiation) — a big if. There were just question marks and question marks. I was very impressed by Poincare by one thing. He always carried a bundle of notes which he left on the table. Then always at the moment of the lecture he would pace up and down before the blackboards for a few minutes, keeping silent. He would pause a moment and say, “Well, I have a new idea. We will see together if it works or not.” I’ve never seen anybody who could do this, but he did it. Within half an hour we knew whether the idea was any good or simply nonsense. He would do it very promptly like that. And sometimes he would completely forget that there was a class and simply do it for the fun of it! All unrehearsed. And that was very typical of Poincaré.

Ewald:

It was a class for what type of student?

Brillouin:

Oh, these were all older students, people of my age who were two or three years in the University.... That was my last year of undergraduate, and the beginning of graduate work. And this was his last year, because just the year after he died. And I knew him very well because he was a great close friend of my father. As a matter of fact as a young boy I remember going to the Poincare’s home and playing with the children of about my age. I knew them very well by the time I had contact with Poincare.

Ewald:

Had you any contact with Mauguin?

Brillouin:

Very much. Yes, I (write) Mauguin very much. I never took any course with Mauguin. He was not teaching when I was a student; or just beginning. No, I knew Poincare, Perrin, Langevin, besides my father. I went to the first lectures of Langevin at the College de France when he was still not regularly appointed. He was just replacing my grandfather who was. Ill. You know my grandfather was Mascart?

Ewald:

You are the fourth generation?

Brillouin:

Yes, Briot, Mascart on my mother’s side, my father and myself the fourth generation. My grandfather Mascart, was also Professor of the College de France, and he was very sick in 1909, 1910. He also had cancer. He asked Langevin to take his place for a few months. Langevin started lecturing on electromagnetic theory, and for three or four years in succession he did all the problems of electromagnetic theory from the beginning to relativity. So I attended the first lecture of Langevin. He had a large audience. There were a number of young students coming then to his lecture. There was also something very (interesting) right at the end of 1911 or 1912. I remember one year he started lecturing on transformation of variables and change of coordinate systems in electro-magnetism. He had discussed the relativity principle for electromagnetic theory, and tried to extend it to other problems besides the electro-magnetic ones. He gave the generalization that energy equals mc² and a few more results. The paper by Einstein came just a few months later in Annalen der Physik… I can remember some years later when Einstein came to Paris and gave a series of lectures, I compared their notes. Langevin had not published anything. I don’t know if you have seen any papers signed by Langevin — there may be half a dozen altogether. He always lectured and never published. A number of the things he did, results that he gave in the lecture course at College de France, were sometimes written by some students and published in the (dee do sheck). And so you have no work of Langevin. Now by that time Langevin and Einstein compared notes. That must have been around ‘23 or ‘24. I don’t remember… They found that they had exactly the same result but Langevin had made the proof in one way and Einstein in another way. Einstein was amazed at the simplicity of the general proof of Langevin by that time. It had never been published, neither by the one or by the other, but they happened to be there. I’ll tell you one work about the visit of Einstein. Einstein was terrified by newspaper men. So he wanted to be hidden in a place where nobody could find him. Langevin had found an old lady who had a room available in one apartment in the far corner of Paris. And there was Einstein, and only Langevin knew where to find him, where to call him. In many cases it made things very difficult, but he wanted to keep alone and not be disturbed by the newspapers.

Uhlenbeck:

Did you know the Curies?

Brillouin:

Yes, I knew Madame Curie very well, and I also remember meeting Curie himself. That was when I was a young boy, about 13 or 15, at the Lycee. It happened that the Curies lived in (Voulerin) west from Paris; and my parents had a summer house not very far from there. So very often we met the Curies on the train and we traveled back and forth together. When Curie was (talking) physics with anybody else he wouldn’t pay any attention to anyone around him. He certainly never noticed me and never gave me a word.

Ewald:

That was one of my early impressions when I was a student in Cambridge. During the Xmas vacations I went to Paris to meet my mother who was working with (Herr Glader) there, painting. And I went to the Sorbonne to some lecture in physics, and I happened to run into a lecture by Pierre Curie. So I at least have an optical and acoustical impression of Curie I thought he was very dull, monotonous.

Brillouin:

I never heard a lecture by Curie, but I heard a number of lectures by Madame Curie and they were terribly dull. She would lecture in a very monotonous tone, very low tone, everything on the same level. Furthermore the lecture was at 1 o’clock, right after lunch, and after a few minutes everybody was asleep. And it was too bad, because the lectures were very good and very well-prepared. I did what I could to keep awake and to take notes. We had to compare our notes afterwards to be sure what was missing!

Ewald:

Tell me. I once paid a visit to Langevin. My recollection is, that this was in the Ecole de Chemie et Physique — is that correct?

Brillouin:

He was Director of the Ecole de Chemie et Physique for many many years… He was a Professor there before he came to the College de France, and then he still kept teaching at the Ecole de Chemie et Physique while being at the College de France. He was dismissed by the Petain government as being too liberal for the situation. The case of the Ecole de Chemie et Physique was a very strange one, because after all that he was also dismissed from the Ecole [College] de France at the same time. At the College de France this chair was taken over by Maurice De Broglie for the few years. Many people in the University resented very much the fact that Maurice De Broglie had accepted to take over the chair of Langevin in war time. They said it was not fair. And in the same way, at the Ecole de Physique et Chemie they wanted to appoint Thibaud. I think he was appointed and stayed there during the war, and was dismissed with a great deal of bad feeling right after the war…

Ewald:

Had you done any particular work on optics before you came to Munich?

Brillouin:

I had been working with Perrin on Brownian motion. By that time the director of the laboratory of Perrin was Edmond (Bauer) and (Bauer) had built a very ingenious and easily transportable optical device with which he could make measurement of the blue color of the sky and compare it with the intensity of the sun. The idea was to get Avogardro’s number from the Rayleigh scattering of the sky.... I remember that when I was working at Perrin’s laboratory, (Bauer) took his equipment on top of Mont Blanc, stayed there a couple of days to make his measurements, and came back. Everything was working right but he found that the intensity of the blue color of the sky was much too strong for the theory and that he had to assume a reflective power from the earth which was about 80 or 90 per cent. This looked to him very fantastic, so he wanted me to do it again. I did it the next summer — that was in the summer of 1911 just before going to Munich. I had been working for a month on the top of the Mont (Derovre), making all sorts of measurements there. That was very convenient, because there was a good organization on Mont (Derovre). Not very comfortable, but all the comforts you find in a mountain cabin, with a garden, with some cooking and a place to rest. I did all sorts of measurements, but instead of doing the measurements right away by optical means, I did it with photographic plates. I had about 100 of these photographic plates still in my drawer when I came to Munich. As a matter of fact I spent a number of afternoons and evenings in the basement of Munich laboratory looking at these plates, measuring the intensity of different lines, and seeing where they would agree to give me (the figure). This was never published because I also found what Bauer had seen at Mont Blanc. I found that there was a reflection from the earth which was at least 80 per cent and sometimes 90 per cent. Otherwise it gave (2⁴ very well). In 1914 before the war, just at the moment when I was getting ready to write it out for publication, there were two or three long papers written by Americans with similar measurements. They had obtained exactly similar results…But theirs were so much better and so much more complete than mine that I never published it. So I had experiments with optical instruments, but that’s about all.

Uhlenbeck:

We are much interested to hear if there was discussion of modern physics in Paris at that time? Was there a colloquium for instance?

Brillouin:

It was organized later. I don’t think there was anything regular. Langevin was beginning to do it, but didn’t do it regularly. There was no regular or systematic organization. We had meetings from time to time — Perrin, Langevin in their laboratories, or at the Curie laboratory — but only occasional meetings. There was no regular colloquium.

Ewald:

Was there much discussion about the quantum theory?

Brillouin:

Yes, a good deal. There is one part I can remember very well. You remember there were the two first Solvay Congresses in Brussels — the first in 1911…Langevin reported on it in his lecture at the College de France. He took two sessions to discuss all the readers at this Solvay Congress of 1911. And he gave Sommerfeld’s point of view, and he gave us his computation of the l/4 quantum magneton… I remember that I knew about this work of Sommerfeld much before I knew Sommerfeld himself, because my father was very much impressed with the work of Sommerfeld.

Ewald:

Your father was at the 1911 Solvay Congress?

Brillouin:

Yes. And I believe that Poincare was there also. They were all very much impressed with this Sommerfeld discussion, and with the addition of Langevin, and this was one of the reasons I decided to go to Munich. I was wondering after my studying in Paris, whether I should go to Munich or to Berlin. My father was afraid that Berlin was too big a place, that I might have more difficulty making contact with scientists in Berlin. And so I decided to go to Munich. This connection between my father and Sommerfeld was the reason why I decided to go to Munich.

Ewald:

Very interesting. This was one of the questions I wanted to ask you. It was very rare for French students to come to Germany to work.

Brillouin:

I had been in Germany many years before as a young student in 1909. I spent a year in the Black Forest with Arthur [Otto?] Lehmann.

Ewald:

Yes, of course. The man with the liquid crystals with whom Mauguin also worked.

Brillouin:

So I spent a month or two in the Black Forest at that time, because I wanted to study German better than I knew from school. And after that something very queer happened. I saw in one of the local newspapers a short article of a young fellow from Stuttgart who wanted to travel on bicycles through the Alps. So I got in touch with him. I had my bicycle. We took a long trip from the Black Forest to Munich to Tyrol all through Tyrol and back to Lyon, the rest of the time. He was a German student, and so during all this trip we used to speak one day French and one day German.

Kuhn:

…Had there been as much consciousness of the quantum problem in France before the 1911 Solvay Congress?

Brillouin:

There was a great deal of interest in the quantum theory, but very few people did believe in it, except for Langevin. Langevin was really the only one who advocated relativity first and the quantum second, and he was the only one who introduced it to the students. The students were extremely interested, but older people were not very much interested. My father was very much interested.

Uhlenbeck:

Poincare?

Brillouin:

Poincare of course. There was a good deal of discussion with Ehrenfest about who did it first — Ehrenfest on one side and Poincare on the other side had found a way to prove that you couldn’t find the black-body radiation law without some sort of quantum condition. And Ehrenfest said to me once, “Well Poincare did it wonderfully, but he did so many wonderful things, and this is one of the few things I have done nicely.” “So please, please say that I did it!”

Ewald:

Now, coming to your work with Sommerfeld…

Brillouin:

…Sommerfeld wanted me first to discuss some problem of propagation electromagnetic waves; not around the earth but around a cylinder, with expansion in (Bessel) function, and so on. By that time I had pretty good mathematical instructions from Frankfurt. I knew what a Bessel function was. Mathematicians in France were interested only in pure mathematics — applied mathematics were below their level. They wouldn’t care, they wouldn’t teach it, they wouldn’t even speak of it. So I came there and Sommerfeld asked me, “Well you must know very well the Bessel function?” I replied: “A little bit, not too much.” Sommerfeld was interested in the propagation of electromagnetic waves around the earth. Since somebody had made pretty reliable laboratory experiments on the propagation around a conducting cylinder, he wanted to see whether in that case theory and experiment agreed. I tried and tried and tried. I didn’t know enough to get started. So after two months Sommerfeld said, “Well, I see you don’t have the background for that. Let’s try something else,” and he came up with this problem on light propagation. There I found immediately what I wanted. I found immediately the application of the theory of the Saddle point method, which Sommerfeld had given us a week before in a lesson. Isn’t it wonderful — here I have an application of the Saddle point method. I gave the complete solution right away… The time it took me to write it! My German was so bad that Sommerfeld had to have one of his students put it in better German before it could be sent for publication. Finally the two papers appeared in Ann. d. Phys. I think in August, 1914. And so it’s not to be found in most of the libraries. That’s why I decided to have it translated in English and re-published as a book.... Nobody knew it, nobody ever read it.

Kuhn:

DeBroglie refers to some of this work of yours in his thesis.

Brillouin:

Yes. The case of De Broglie is the following: I first met De Broglie during the war. Maurice De Broglie I also met during the war. I was all during the war with the radio engineers in the French army, and we had our laboratories in the (Invalides) in the middle part. By that time Maurice De Broglie, who was much older and was a retired Navy officer, thought of a new method for receiving radio signals in submarines. His idea was that even when the submarine is under water, it could receive radio signals and radio messages with a large coil outside the submarine and a powerful amplifier inside. Maurice De Broglie built the large coil, and I at that time happened to have built the most powerful amplifier in Paris so we put the two things together. We went to Toulon and experimented in a submarine. We found that radio signals from all the long-wave stations in Europe, and even from America, could be received very well in the submarine under water, and very clearly. This was a surprise. We could receive the signals at a depth which was much greater than the theoretical penetration depth. Then, afterwards, I found the explanation, which was very simple. In order to be quiet and not to be disturbed by the machinery in the submarine, we used to go outside Toulon to a place where the sea was not too deep. We lay down with the submarine at the bottom of the sea in order to hear something. They were not coming from above down, they were coming from below up! But all together this was very successful, and a few months later all the submarines of the Allies had these reception sets. Now they are built with the coil, and amplifier inside to receive long-wave signals… I was at the laboratory for the Signal Corps and Louis De Broglie was at a transmitting station at the Eiffel Tower, which was just a few blocks away. I used to see him, and that’s where I met him first. We did a few minor experiments together on one thing and another. Then, when the war was over, he was writing his Doctor’s thesis. His brother, Maurice, encouraged him very much. It is very surprising that Maurice De Broglie, a experimentalist, was so much interested in the possibility that particles may be waves and waves might be particles. At the same time Louis De Broglie was coming regularly to the Ecole de Physique et Chemie to meet Langevin and myself. I think we were the only three people to whom he talked during the time he wrote his thesis. At the same time, to be complete, I should say that my father had the queer idea that some sort of waves could be used to give the quantum conditions, that some sort of standing waves around the nucleus could give something similar to the quantum condition. He did develop a scheme — be didn’t develop it completely — which was very close to the one which Louis De Broglie used afterward to get the quantum condition. Louis De Broglie quoted my father about this. Maybe he quoted me about the group velocity, I am not sure.... We had so many discussions together. He came and we had long discussions at the Ecole de Chemie. I was working at the Ecole de [Chemie et] Physique after the war, Langevin had decided to reorganize the French Journal de Physique, which had suspended publication during the war. They asked me to be the secretary of the Journal de Physique, and the offices of the Journal were at the Ecole de Physique et Chemie. I was working there two or three days a week, to be close to Langevin to get advice whenever I had a problem…

Ewald:

How did actually Louis De Brogue get this electron wave idea?

Brillouin:

Well, he got it from (???). He looked at the experiment. You see particles emitted from a radio-active source. You put on a magnetic field and some are bent to the right and some are bent to the left, and some travel straight ahead. So his idea was: “Well, all these must be very similar. Either they are all waves or they are all particles.” Since the idea of the photon being a particle had already been used by Einstein, he tried the opposite idea, to see if he couldn’t make everything waves. That’s how he got the idea of wave mechanics. But I remember seeing him sit down in front of me and point to a picture of particles — some going right, left, and some going straight ahead....

Kuhn:

When did that start with him? Do you know?

Brillouin:

I don’t know exactly. Of one thing I am sure, that the Journal de Physique was reorganized and I took care of it in 1920 — in the spring of 1920. It took a few months before we had an office organized at the Ecole de Physique et Chemie. My memories of Louis De Broglie coming to see us over there must be of the fail of 1920. By that time I had just finished my research — that was a sad experience. I had been writing my thesis in 1911, and it was practically, in my opinion, finished and terminated in July, 1914. I put all my papers away in a drawer when I had to leave to go to the Army, and I was sure that on my way back I would just have to take the trouble to write it. Well, I was demobilized in 1919. I rushed to the drawer, found all my papers, and tried to read them. They didn’t make sense. I couldn’t understand a word of the notes I had been making five years before. Not a word! I knew really what main ideas along which I had been working, but I could not rebuild the type of discussion — the notes were so short. So I had to rebuild it completely, and this took me a full year. So I had my thesis finally done in 1920…The thesis was on a quantum theory of a sort of ideal solid. I tried to build it the same way as you have in ideal gas, trying to get the general scheme which would apply in a general way to all kinds of solids without going into too much detail. I assumed the classical theory of solids and quantification of waves, and I computed the radiation pressure of elastic waves on the (border) of the solid. That gave me the (thermal dilation?)… What I am telling you about Louis De Broglie was the winter of 1920 to 1921. And I think he wrote his thesis in 1921…It may have taken him some time to get it written because this was the sort of thing difficult to write and to explain before the official examination. But he had it practically done in ‘22, I am sure of that.

Kuhn:

How did people feel about it? Was there a good deal of discussion about it?

Brillouin:

There was a good deal of interest from many people. Of course most people were terribly skeptical. Generally people were skeptical, and not only in France. I don’t think very many people took it seriously before Schrodinger… Einstein took it seriously. Langevin and I took it seriously. My father was interested, but it was not exactly what he had been thinking of so he didn’t like it too much. People were interested but nobody was convinced that it could be the link to a final solution. Everybody felt that there was too much uncertainty in his theory to believe it. [A description of De Broglie’s attempt to retain determinism in quantum mechanics by means of particles guided by a wave is here omitted.]

Ewald:

I would like very much to bring up the topic of Nadelstrahlung and the beginnings, which I thought went back to Einstein. You see this is the stage where the photon was not yet fully accepted, and one still thought of a Maxwellian interpretation. Sommerfeld’s work on the generation of gamma rays and X-rays confirmed the idea of Nadelstrahlung, to a certain extent, from Maxwellian considerations. But it seems to me that this Nadelstrahlung concept has not been stressed sufficiently as an intermediate state between the full acceptance of photons as a kind of particle and the previous field concept.

Brillouin:

…Well, in France I must say that either people were not interested in quanta, or accepted it immediately. Langevin and Perrin and Madame Curie — all this group — accepted the photon of Einstein literally right away without any discussion. They found it wonderful, they liked it and they used it. I don’t think there was any objection. [From luncheon conversation: Bouasse was a Professor from Toulouse whose texts were widely used throughout France. They were excellent, but Bouasse had forced his publishers to let him put in whatever he liked, and he often included long vituperative forewords directed against those who opposed classical physics. This was itself ironic because Bouasse had himself castigated Mascart for using the old elasticity theory rather than Maxwell’s theory in his Optics. As I told you at lunchtime Bouasse, who was a very prominent figure in French science, was very much against relativity and quanta, and in that respect he had a very bad influence on the younger generation. Too many young students did read the books of Bouasse which were the best in French by the time, and so took the ideas of the old professor who denies any kind of physical meaning to all this quanta material. But in the group of Langevin, Perrin, and Curie this was accepted right away. Especially, I remember Langevin lecturing on the problem of Brownian motion of a particle emitting and receiving light quanta, the momentum exchange needed to main Brownian motion, and things of that kind. That was one of the discussions which most impressed us. So we said, “all right it works, that’s what we need.”

Uhlenbeck:

This has to do with the fact that quanta are necessary for the derivation of the fluctuation of the radiation law, which you refer to earlier, which Poincare and Ehrenfest did at the same time.

Brillouin:

I remember personally because I was very much interested with these discussions. I wrote two papers of no importance whatsoever now, and, later, two papers in 1918 or 1919 just after the war — to show that you could vary the conditions of absorption, remission and so on, and that you always got the right Brownian motion from interaction between the particle and the radiation field. That was just repeating what Einstein had said in a much simpler way to show that it worked all the way through, even for large (photons) or large particles. I’m sure that this special discussion had been a great point for our group in accepting the photon idea. When this was settled that way, we said: “That’s certainly a good. idea. How far it will go nobody knows, for this is so new, it’s hard to say exactly how far you can use it, but it’s certainly right in principle.”

Uhlenbeck:

How does it come about that in France there was so little done in spectroscopy and in the Bohr theory?

Brillouin:

Well, I couldn’t say. Spectroscopy and experimental optics was done in France especially by the group of Fabry’s. Fabry was a typical experimenter, an excellent experimenter, but not interested at all in theoretical problems, not at all. He was interested in using spectroscopy to see what was going on in the stars and in the sky. But for the interpretation of his experiments, he didn’t care! So the only work that was done was done much later by Cabannes… I had predicted in 1920 in my Doctor thesis a sort of Raman effect, which nobody took the trouble to discuss at the time. The explanation was very simple. We have an elastic body, a piece of glass. This glass is vibrating with a certain frequency, and so if you shine light upon it, the frequency of the scattered light will be an initial frequency plus or minus the frequency of the elastic waves in the glass. I discussed that very carefully. The result was that the change in frequency depended upon the angle of scattering, and I had given all these formulas in 1920. It was much beyond experimental possibility at that time, and it was proven by Cabannes about 10 years later. In between the Raman effect had come in. The Raman effect was about 100 times greater than the one I had predicted. Nevertheless, everything was found in order. I also predicted at that time the fact that an ultra-sonic wave should scatter optical waves, and that they also would be shifted in frequency. This was observed by Debye and (?) much later. As a matter of fact Debye had published a paper by that time which was almost a word by word reproduction of a chapter of my own paper 10 years before…

Uhlenbeck:

But there was no work on the Zeeman effect and the Stark effect and the interpretation of the fine structure of spectra…Was Langevin interested in these things?

Brillouin:

No. Langevin gave all this in his lecture to the College de France, but later, not before the war. That I really don’t remember too clearly… I heard about quanta, photons and the like at Munich just before the war, and then during the war I lost contact completely with physical work. When I was demobilized in 1919 I took the book of Sommerfeld and read it like a novel. Thinking of Sommerfeld, I have the story which I explain in my book on wave propagation, how when I came into his office one morning be pointed to the and said that there’s a very extraordinary paper by Niels Bohr in this issue…

Kuhn:

Can you remember any of the immediate follow-up on that? Were there colloquia on it, were there other discussions of that Bohr paper in Munich while you were still there?

Brillouin:

I don’t remember. There must have been, but I don’t remember. Do you remember any discussion at that time?

Ewald:

I said this morning already that I think the first report on this paper was the one I gave on the British Association meeting in Birmingham. But I reported on quite a number of papers, and the paper which had been of main interest to me had been Bragg’s paper on diamond. Of course I pointed out this marvelous numerical result of explaining the Rydberg constant, but I wasn’t a theoretician enough really to see the implications. At that moment I don’t think that Sommerfeld really was impressed by this numerical concordance, but I think we all waited for the paper to appear.

Brillouin:

Then maybe he was waiting for this paper because he knew before.

Ewald:

That is quite possible… Another paper on which I reported and of which I did not see the value really, was Darwin’s paper on the reflection of X-rays by perfect and imperfect crystals. This was really the beginning of the dynamical theory. But I had forgotten all about it, and didn’t read up on my whole dynamical theory while I was in Russia — I was an X-ray mechanic and entirely isolated. Only at the end when it came to writing it up I remembered this paper and said “Now there is this paper by Darwin. He should have something of these results, and how do mine compare with his?” And, then I found out that he had actually predicted the reflection curve the same as I had, except he had not made a graph of it. He had only given a formula, which was not so easily interpreted.

Uhlenbeck:

How did you come to Russia?

Ewald:

It was during the war. I had an X-ray station. I was sent to the Hindenburg Amy and stayed three years in the environment of (Dringst). There I developed my whole dynamical theory. Without the war it would never have been done!

Kuhn:

There was much more science in Germany during the war than there was in France, wasn’t there?

Brillouin:

Oh yes. Certainly science still progressed very much in Germany during the war, while in France everything was at a standstill and everybody was working — either mobilized and unable to do anything or working for the war… The only one who had some appreciation of scientific work was General (Trevier) who had charge of research in radio propagation. He was the first man who managed to collect a number of people together among whom I was…

Ewald:

Well you see in Germany there was one center in Berlin. That was the A.P.C., the Artillerie-Prufungs-Kommission. Here there were Born and Madelung and Paul Hertz, who was impossible as a soldier — I don’t know whether you knew him. And Lande was there. So that was one group. Everybody of course did war work of kinds. Sommerfeld was, I think quite engaged in developing the gyro-compass, together with Anschutz. I remember one funny question which came from the editor of one of the chief German quarterlies. He wrote a letter to Röntgen saying: “With his very penetrating X-rays, wouldn’t it be a rather good means for exploding enemy airplanes in the air?” That sort of question was still being asked in World War II, Doctor Brillouin, you speak of immediately after the war, sitting down and reading the Atombau as a novel, did many other people in France do that? Was it widely read in France after the war?

Brillouin:

Well, Sommerfeld’s Atombau. Yes, that was a book that was widely read. And among the younger people, everybody wanted to know it. Older people were rather afraid by a certain amount of mathematics there. It was very popular…

Kuhn:

There was, you think, no inclination in France after the war to stay away from some of the subjects or some of the books that had been done in Germany?

Brillouin:

No, certainly not, certainly not.

Ewald:

Still it is a bit surprising that someone like Langevin was not connected with the development of the quantum theory in the twenties.

Brillouin:

Well, Langevin did a good deal in his teaching. As I was explaining a moment ago, Langevin after 1920 never wrote one paper. Everything he did he gave in his lectures at the College de France, to a group of students. He had a very good class of from 30 to 130 students regularly year after year at the College de France, and that was the place where he communicated his new work. Then after he had given it to the students, he was tired of the whole thing. He wouldn’t take a minute to write it up…

Ewald:

When was the Institut Henri Poincare founded after the war or before?

Brillouin:

After the first World War. I think I can tell you exactly, because I was in this country. I came to the U. S. for the first time in 1924. That was for the meeting of mathematicians at Toronto. I came back and taught at Madison, Wisconsin in 1928. By that time I had been offered a position in the United States, and since I had no regular position in France I was wondering whether to stay here or go back to France. So I wrote to my wife, who was in Paris “what would you think of this or this possibility?” She said, “Don’t do it. There is an institute being organized in Paris, Institut Poincare, and they are going to offer you a position.” And the week after I got a letter from the Institut Poincare. So it was founded in the spring of 1928. I came back to Paris and I started teaching at this new Institute. The building was not built. We were teaching in classrooms at the Sorbonne, and the building was started in the fall of 1928. It was built with the help of the Rockefeller Foundation. They gave half of the money, and the French government gave the other half. It was built at the same time as the Gottingen Institute. The Rockefeller Foundation wanted to divide the money between the two groups, Paris and Gottingen.... ||Mathematics Institute at Gottingen||.

Ewald:

You know, this is a subject which I think really needs a chapter by itself. The influence on the development of modern physics which was exerted by the Rockefeller Foundation. What was it called? The International Education Fund. There was a Dr. Tisdale, is that right?

Brillouin:

There was Tisdale.

Ewald:

He was the important man at that time. All the young people went through it and got fellowships, and I think that had an enormous influence.

Brillouin:

In the case of the Institut Poincare, the influence came from old Birkhoff of Harvard, the mathematician. Birkhoff was a great friend of (Bauer). Birkhoff came to lecture in Paris — it must have been in 1927 — and they discussed the matter with (Moyle). They made all the arrangements — (Bauer) keeping in touch with the French government and Birkhoff with the International Education Board. So they managed to bring each of them half of the money for the building, for the new chair, for everything.

Uhlenbeck:

This Institut Henri Poincare had a separate staff didn’t it?

Brillouin:

Well the professors had been reorganized as part of the University, so it is just one of the many institutes belonging to the University of Paris. It’s not separate. All the money which had been provided before was devaluated with the devaluation of the franc, so the French government had to put in new money to keep everything running. In between a number of new institutes and laboratories were organized. The University of Paris is scattered all over you know… At that time we were two in theoretical physics. There was Louis De Broglie and myself. There were a few assistants. There was Bard and (Darmois). So we were four professors tightly attached to the Institut Poincare. And furthermore, all the courses in mathematics were transferred to the Institut Poincare so that the whole department of mathematics of the Sorbonne had its offices and lecture rooms at the Institut Poincare… Poincare was actually part of the Sorbonne, for the internal organization. There was never any decent organization for courses of mathematics at the Sorbonne. There were just a few lecture rooms on the top of the Sorbonne, in the roof and the attic and places like that. Terribly hot in summer and freezing in winter. This was the opportunity for moving all the department of mathematics to a decent location in a new building.

Kuhn:

…The first years of the century is still a period in which from the point of view of the historian, there are a few famous holdouts on the reality of atoms. I wonder to what extent that is an issue in anybody’s mind, still in the first decade or two of the century?

Brillouin:

Well I can give you my answer. I was not interested in chemistry, as a matter of fact I was a very poor student in chemistry. How I managed to pass the examination I still wonder. We had a professor at the Ecole Normale giving us theoretical chemistry, and the whole course for the whole year was devoted to the very strange idea of atoms in chemistry. Shall we believe in atoms or shall we not? And how much do we have to go in that strange direction, and how can we keep back with (energetics) and forget about atoms whenever possible.... This was in 1910. This was one side. The other side was Perrin.

Kuhn:

To what extent was the man who gave this chemistry course you speak of widely regarded as somebody who was totally behind the times?

Brillouin:

No, he was regarded by chemists as someone who was very cautious, not to mix up experiments and theories. That theories are something not to be believed and experiments are the real thing.

Ewald:

Was he a pupil of Ostwald’s?

Brillouin:

He must have been a pupil of Ostwald. His name was (Simon) if I remember correctly. He was not a very famous chemist, but he was a very good lecturer and he impressed us very much. And so when after these courses I went to hear the lectures of Perrin, and I saw the atom jiggling around like an old friend, I was very much amazed and I thought, “Well after all they must exist.”

Kuhn:

What about physics? What about statistical mechanics? Kinetic theory? Were these pursued seriously by physicists or did the doubts about the atom affect physics too?

Brillouin:

Statistical mechanics — Langevin knew it. But besides Langevin and my father, nobody knew about statistical mechanics in France.

Ewald:

What about Poincare?

Brillouin:

Oh, Poincare, yes. But Poincare wrote most of his books on thermodynamics with old-fashioned thermodynamics, and no statistics. One of the first books in French on statistical mechanics was my father’s book on the viscosity of gas and liquids — one of the very first in the French language…

Kuhn:

Was non-statistical thermo-dynamics then a part of the standard French curriculum?

Brillouin:

That was the first year of physics at the University, and it was done on a very classical and elementary basis. We all had to read old papers of Carnot to start with, and then to proceed with some text books in thermodynamics. We had to use one principle, and make a number of problems which always used the principles, and see what happened…

Kuhn:

Dr. Ewald, just taking this question about atoms, was the situation quite different in Germany?

Ewald:

Let me say that the first time I heard of statistical mechanics was probably in 1908, in Gottingen. I came to know Zermelo and Caratheodory and (Nelson). They lived in the Privatdozenten-Haus, on the Heinhauser Weg or something. “Wiederkehr Einwand” was much discussed. I didn’t understand it at the time…

Brillouin:

…In France as far as the statistical thermodynamics is concerned, I am sure they learned it from my father — and in private discussions with him about theory of gases — from Langevin; and from a series of remarkable lectures given by Lorentz at the College de France in 1912 or something like that. That was the end of it for me, that was the last word — after Lorentz it was perfect.

Ewald:

Should we call it a day?