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Oral History Transcript — Dr. Leon Brillouin

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Interview with Dr. Leon Brillouin
By Thomas S. Kuhn
At Professor Brillouin’s Apartment,
New York City
April 5, 1962

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Leon Brillouin; April 5, 1962

ABSTRACT: Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Henri Abram, Edmond H. Bauer, George D. Birkhoff, Bouasse, Marcel Brillouin, Louis de Broglie, Maurice de Broglie, Jean Cabannes, Marie Curie, Pierre Curie, Paul Ehrenfest, Albert Einstein, Charles Fabry, Paul Langevin, Hendrik Antoon Lorentz, Jean Perrin, Henri Poincaré, Arnold Sommerfeld; College de France, Ecole Normale Supèrieure, Ecole Polytechnique, Institut Henri Poincaré, Universitat München, and Université de Paris.

Transcript

Session I | Session II

Kuhn:

Many of the things I want to talk about are things that came up briefly last time, particularly the whole question of the early work of De Broglie.

Brillouin:

I met him first occasionally during the war when he was in the Eiffel Tower and I was in the service of the Signal Corps. But only occasionally at that time, I really met him right after the war…when I was working at the Journal de Physique… I am sure that in the discussions I had with Louis De Broglie when he came to his first idea of waves and mechanics, were from 1920 to ‘22 and no later… I didn’t see him much after that, and by that time he had already given lectures to different groups and he was already writing or having written and trying to publish his doctor’s thesis. I am sure that the most important part of the work was finished in 1922.

Kuhn:

It’s not evident in the early papers, which are the ‘22 papers, that there is nearly as much development as there is going to be by the time of the thesis. There’s no hint of the quantum conditions, for example, in the ‘22 paper. There really is nothing in the ‘22 paper on group and phase velocity yet.

Brillouin:

Well yes. But the very important thing was the moment when he discovered the relation between momentum and wave-length. And this must have been in --

Kuhn:

That was in the ‘22 papers.

Brillouin:

Everything started from there. That was really the first mark. And the rest had to follow it. Each step took a long time of thinking and wondering before he made up his mind about the way to adjust a new term. But I am sure -- we agree on that -- the p relation, the lambda relation, was ‘22.... And then I remember there was some time before his thesis could be printed. I don’t remember what was his difficulty, but it was, by that time, very expensive to get something printed in Paris. De Broglie had made arrangement with Gauthier to get the printing done, so the printing date of the thesis is certainly one year after the writing was done…

Kuhn:

How often do you suppose you spoke with him about these ideas?

Brillouin:

I don’t remember, but somehow after a dozen times. A number of times. Sometimes he came to see some papers in our library, sometimes we talked together for five minutes, sometimes for a longer time. I don’t remember. He found it convenient, whenever he came to the left bank, to come to our library, because by that time the Institut Poincaré was not yet organized, and this was one of the few places where he could find a number of books which he needed.... He was always a very quiet person; very interesting, but rather self-conscious, which I must say he is still. I can just give you a personal remark, I don’t know whether you would like to know… He hated to travel. Hated it. He never moved away from Paris. He had a wonderful property and a big house in Normandy, but he never went there. He just stayed in his office and in his apartment in Paris, and never moved. When he had to go to some of the meetings of the Solvay Congress, it was a great emotional problem for him, but he came. Then he was told to go to Stockholm for the Nobel Prize -- he first wanted not to go. But his brother told him that he had to go, and he went there. I saw him at the Institute Poincare when he came back. He said in slight (???), “Oh, if I had known how much trouble it would be to go there, I would have refused that Nobel Prize.” Travel was too much strain on him; he looked awful when he came back. He had been always really modest and self-conscious, although he is no more. But he is still a very quiet person. I’ve never seen him excited.

Kuhn:

Do you have any recollection yourself of how you felt about these ideas?

Brillouin:

I can give you what I remember. I remember discussing with Louis Broglie and with Langevin. And I remember discussing the matter with Langevin after Louis De Brogue had left. I remember discussing also, the matter with my father, who had some original, ideas which were very close to Louis De Broglie’s structure. I discussed it also with (Bauer); you could ask (Bauer) about it. We in this group were immensely interested, but rather skeptical… The solution of De Broglie for the spectra was only approximated. So it could be just right or it could be another of these approximations which drop out of the picture later on. While the Schrodinger solution was so good that everybody was impressed with it. Really I think this was the general consensus of opinion at that time. For the people we knew. But there were people who never read the paper and then were afraid of what it involved right from the first page and, didn’t try to read any further. I could say one more thing. Altogether, Langevin was extremely interested, but he was never really convinced of the extent of the waves. He liked particles. He wanted to play with particles, and he was very much impressed by Schrodinger. The cloud tracks of the particles. This, here we have a particle. This must be a wave, this is a particle. I remember a discussion where I told, him, “Yes, all that, but compute the thickness of the (ray) from the quantum principles, and it’s just about the right thickness, so you cannot say whether it’s a wave or a particle.” He didn’t like that. He wanted a particle to play with. That was his inner feeling… Einstein didn’t like too much the waves, and liked better to play with the photon and particles. Perrin was involved in many discussions also. What Perrin’s opinion was I can’t remember clearly. I don’t know. I think Perrin was ready to accept anything, provided it could lead to some experiment. He wasn’t much of a theoretician. He was interested in practical results. If a theory could give him something to experiment upon, he would do it.

Kuhn:

Let me switch away from this whole area and go back again to the question of scientific education…

Brillouin:

I would say that in my time, and for many years later, courses at the University were not very good. In physics it was mediocre. At the Ecole Normale, which had previously been an independent institution but by that time was attached to the Sorbonne, we had a number of younger teachers who were really excellent teachers and excellent scientists. There was Henri Abraham. I don’t know if you have ever heard of him. One of the people who didn’t like him too much used to say “Abraham is supposed to be a wonderful physicist when you speak to an engineer, and a splendid engineer when you speak to a physicist.” He was one of the first examples of people interested in applied science, and he was very brilliant in the field of applied science in electricity especially, and electronics and that sort of thing. He was, at the same time, a brilliant teacher, and open to any kind of a discussion. With him there was another one I knew very well, Eugene Bloch and his brother Leon. They had been working together on all sorts of problems of optics, making very precise measurement of all sorts of optical spectra. Very good experimenter -- not so much interested in theory -- but brilliant experimenter. At the Sorbonne we had a few old teachers who were just giving the regular course according to the textbooks they had written themselves -- for years and years. All older. The best courses at the Sorbonne were given by Perrin in my point of view, and by (De Kreuse).

Kuhn:

What was the relation of the faculties -- the department of physics at the Sorbonne and the physicists at the Ecole Normale?

Brillouin:

Don’t forget that we never had anything like a department in any French university. Each professor had his own laboratory with his own register, his own mechanic, and his own organization, even his own library very often. The next professor would have another laboratory -- each one separate from the neighbor. The best libraries were at the Ecole Normale and in one of the physics laboratories at the Sorbonne -- we used to borrow books from either one when we needed. There was not too much contact except going to hear the lectures.

Kuhn:

Now a student coming into this situation, and wanting to study physics would take courses indiscriminately from Ecole Normale people and Sorbonne people?

Brillouin:

Those at the Ecole Normale could take any course they wanted either at the Ecole Normale or at the Sorbonne. As a matter of fact there were a few courses at the Ecole Normale which everybody had to take. We also had to go to the Sorbonne because some courses were required for examination. But there were a number of courses which were free for anybody who chose to go. The students at the Sorbonne usually couldn’t come to the lectures at the Ecole Normale. It was a separate group… There was an entrance examination for the Ecole Normale and there was none for the Sorbonne. For the Sorbonne you could just have a Baccalaureate after finishing your high school, your lycee, and then go straight in at the Sorbonne. That depended on personal matters. After Baccalaureate it required usually two years -- sometimes just one but usually two years -- to pass the examination to the Ecole Normale. Some people didn’t want to spend that time preparing for the examination, and went straight to the Sorbonne. One way or the other.

Kuhn:

Could they then transfer to the Ecole Normale?

Brillouin:

No… They could go on and take all the examinations at the Sorbonne. The Ecole Normale was especially useful for people coming from outside of Paris, because at the Ecole Normale all the students coming from outside had dormitories and living organizations at the Ecole. Those who were already living in Paris, like myself, got a small fellowship from the Ecole Normale to provide for expenses. It was an additional fellowship which you didn’t get at the Sorbonne.

Kuhn:

Where does the Ecole Polytechnique fit in? Was it by this time pretty strictly an engineering and military school?

Brillouin:

It’s completely an engineering and military school. It’s completely outside. It had no connection whatsoever with the Sorbonne or the Ecole Normale. With the Ecole Normale none at all. With the Sorbonne there was no connection to the point that students from the Ecole Polytechnique who wanted to go to teach in (???) had to take all their examinations over at the Sorbonne. The examinations at the Ecole Polytéchnique didn’t count. Some professors were teaching in both places, but the Ecole Polytechnique was administered by the War Ministry, and the Sorbonne was administered by the Department of Education. They were completely separated from each other.

Kuhn:

But people could get the doctorate either at the Sorbonne or at the Ecole Normale, could they?

Brillouin:

All doctorates were given by the Sorbonne. You could work in any laboratory, or in any place even outside to make your Ph.D., but the degree was given by the University…

Kuhn:

What would a typical curriculum be like, what would one start out by taking, what courses, what lectures?

Brillouin:

I try to remember. The first year at the Ecole Normale I had all the regular physics courses, and I took in addition a few courses with Perrin and Langevin. Langevin was teaching at the College de France, which was also an independent organization. Then the second year I had chemistry, and since I didn’t like chemistry very much I took a great deal of mathematics in addition.

Kuhn:

Did the physics go on?

Brillouin:

Yes, with Perrin, Madame Curie, and Langevin mostly. Then in the third year we were supposed to do a small piece of personal research. This I did at Perrin’s laboratory, also going on with the courses of Madame Curie, Perrin and Langevin. Then the fourth year was almost entirely preparation for the final examination, called aggregation, which was a pretty hard piece of work. Then after that I started for the PhD… The courses were usually two lectures a week, so we could take a half a dozen different courses. On some of them I would take an examination and on some of them I would just come to the lectures and not take an examination because I didn’t feel any reason for writing a (partial) examination which were of no special use… On some of the elementary courses there were regular problem sessions, but otherwise many of the courses had no problems.

Kuhn:

Did Langevin believe in problems as a mode of training?

Brillouin:

No. At the College de France he never had any problems of any sort. Furthermore the College de France was never directed toward any degree. It was completely free teaching, with no degree and nothing, to the point that we got into trouble at the College later on account of foreign students. Foreign students just coming to study at Paris had some fellowship. They had to send back home an account of what they had been doing. Since we had no examination at these courses, it was usual for the College de France to deliver a paper with the signature of the director and of the professors, that so and so had been attending the lectures regularly during the year and working at the laboratory…

Kuhn:

How far these courses would go? Was this very much up to the professor, or was there a pretty set curriculum for a student?

Brillouin:

It was very much according to what the professor wanted to do. French professors were really completely free. There were theoretically some programs to be followed for each type of course, but usually when the professor died and was replaced by another one, the program would be changed according to the wish of the new professor. The professor was really very independent in his subject matter and the level on which he wanted to do it. He could give it on the lower level, or just direct a more difficult course, and be satisfied with half a dozen students. They were good and independent.

Kuhn:

Was this also true of the doctorate? Did you work with a single individual finally on a problem, or was there again subject matter on which you had to be examined?

Brillouin:

There was nothing like the predoctorate examinations. You could take the doctorate only if you had passed a certain number of examinations, through what we call the license which is something like the master’s degree. With the master’s degree you could start working for the doctorate, and you would be examined only on the subject matter of your own research.

Kuhn:

What subject matters did one need for the license?

Brillouin:

You had to take three different examinations in three directions. These would be changing all the time. It was a rule that three had to be taken, but not three arbitrary subjects. There were a number of combinations, derived by the professors. In my time I had to take a general physics, general chemistry, and another one, for which I selected classical mechanics. Some of the people took a more elementary course for the third one. [A long exchange on the subject matter presupposed for the license is here omitted].

Kuhn:

How much math would most people take? The standard calculus I take it everybody had?

Brillouin:

Yes, everybody had that, and then it all depended on everybody’s curiosity and what he wanted to do later. Some were really going to be just experimenters and didn’t want to be troubled with too much mathematics. We were even always teasing one of the teachers who wasn’t able to do a good integration.

Kuhn:

How did laboratory and experimental work get worked in?

Brillouin:

Oh yes. With general physics and general chemistry courses there were very elaborate experiments to be performed. Now the course of general physics was two professors. That meant four lectures a week, and certainly three or four laboratory sessions a week three mornings a week.

Kuhn:

That’s very much.

Brillouin:

I remember we were always working a few together on the laboratory sessions. The friend with whom I was working agreed with me that he hated to take an experiment which had been all prepared in advance and just pushing the buttons to see what happened. So every time we came in we took everything apart and would rebuild it our own way. This made a good deal of fight with the assistants. I remember once one of the (assistants) came in. We had taken apart a whole electrical machine -- I don’t know what it was, an experiment with ballistic galvanometer or something like that. We had taken it apart on the table. We were going to rebuild it after the drawing given us. So we called for the assistant who said: “Please don’t touch it any more. The drawing is not right. Of course, I know it, but I will put it…” Then he took all the wires “Ah, here is where this one goes.” We worked on without knowing what he was doing. We rebuilt it and it did work!... Then at the Ecole Normale during the second year we had a number of experiments to perform in the laboratory of the Ecole Normale. These were at a higher level, and they would keep us working for two or three weeks on one experiment, setting up the experiment, putting everything together, checking the apparatus. I remember one of the experiments we had to do by that time was already about cosmic rays. Nobody knew what it was, and this was not called cosmic rays. It was called natural discharges in gases. We had not knowledge enough.

Kuhn:

A good deal of laboratory work went right on through training until the license? Did the license give a good deal of emphasis to it?

Brillouin:

There was no examination, but in each group there was an assistant who would grade the people, “This one is a good experimenter.” “This one is not too cautious,” and so on. We had to write a small report after making the experiment. The assistant would watch the people during the day, just to be sure they had, done it, or whether they had put numbers in at random just to see what would happen… There was not much on experimental physics on the examination for the license, but the grades from the laboratory were taken into account for the final grade. I must say that when I worked with Sommerfeld at Munich for one year after completing my studies at the Sorbonne, I was very much surprised to realize that there was a good deal more organization and supervision and control in the German university; and not the same sort of freedom and anarchy that we had in the French university… For instance, Sommerfeld used to give problems every two weeks or every week. He would see whether the problems were done or not done. Yes, there was closer supervision than there was in French universities.

Kuhn:

And was the curriculum also more systematic? You could be sure what people knew, had in common?

Brillouin:

Yes, Yes. Much better organization, much closer organization than we had. And I know that the sort of anarchy which was the rule at the university was very disturbing for many young people, because they came out of the Lycee or the high school where there is very strict control of everything, and then suddenly they were thrown in a place where they could go anywhere, attend any courses, not come, go to the cafe, complete freedom. For many of them it was a very disturbing experience, and some of them were not ripe for it.

Kuhn:

You surprised several of us the other day, and I understand better now, when you said there had been a 100 people at Langevin’s lectures at the Collage de France. How many of those people would go on and do very much more physics? How many people studying for the doctorate, doing physics at the Sorbonne, would have been there in any particular year?

Brillouin:

In my time, almost three out of four of the students working for their Ph.D. would come to the lectures of Langevin. Perhaps for one year, perhaps they would go for two years. Some of them would go to the lectures of Madame Curie instead. But all the people working for Ph.D. would take these graduate courses… However, there were not always 100 people. There were 100 people at that first lecture of the year, and then there would remain 20 or 25 or 30 by the end of the year. Many of them would be there at the beginning, and then drop out if it would be too difficult.

Kuhn:

All of the 25 or 30 would then be candidates for the doctorate?

Brillouin:

Sooner or later, I should say so.

Kuhn:

So that there may have been 5 or 6 people taking doctorates each year?

Brillouin:

Yes.

Kuhn:

Were you at places other than Munich? Did you travel in Europe or in England to other institutions?

Brillouin:

No, I stayed in Paris and Munich. I was to some other places occasionally for just short trips.

Kuhn:

We are trying to understand why there is not more French involvement than there is in the development of quantum mechanics…

Brillouin:

It is difficult to explain why it was so. There were really few people working in theoretical physics at that time, and there was a pretty good reason for it; there were only, I think, two or three regular chairs of theoretical physics in the whole of France. There was one chair of theoretical physics in Paris, there was one at the Collage de France -- that’s two. There used to be one in Bordeaux, but by my time I just wonder whether it was still there. Maybe there was another one in southern France. That was two, and that was all. So there was really no career open for a theoretical physicist in the French organization. People who had a curiosity for theory would go right away to pure mathematics. There were dozens of chairs of mathematics. Even the smallest university would have one or two mathematics chairs. So the competition was unfair between theoretical physics and mathematics. Many of my colleagues told me “Are you crazy? To go into theoretical physics, there is no future.”… Most of the pure mathematicians were interested only in pure mathematics. There was a glory in pure mathematics which was so much above applied mathematics or theoretical physics, that if one was able to do pure mathematics he would not condescend to do anything else. The situation was serious. It is in many respects even serious now in France. I give you an example. There was no regular course of applied mathematics when I entered the Sorbonne -- nothing at all. I didn’t know about the Bessel function, I didn’t know about any computations with expansions and different kinds of orthogonal functions. I had no place to learn it. This was taught to a certain extent at the Ecole Polytechnique, but not at the University. After the first World War it was felt that something like that would be needed and a course was organized. A new chair was created at the University. And at the beginning for the first few years it was held by someone who really gave a course in applied mathematics. But then very soon this one went to pure mathematics and a pure mathematician came in. This was just a new chair of pure mathematics in addition to the preceding one, because the professor wouldn’t be pleased with having to teach all these elementary things. Now, when the Institut was organized we managed to have another chair of applied mathematics created. But it has always been drifting in the same way. A new chair of applied mathematics was refounded every five or ten years, and after five or ten years it would drift to the pure mathematics… You see it works in two ways. First, the prestige of mathematics made it that every young man wanted to do the purest sort of mathematics; and second, he wanted to do it because all of the committees for the examinations were composed of pure mathematicians. He knew that his talents would be recognized if he were doing that, and that he couldn’t get a position anywhere if he wasn’t… My father was a theoretician and Langevin was a theoretician. They happened to have the two chairs of physics at the College de France. And there was great criticism among other physicists that they were blocking the way or experimenters. Theoretical physics was really at a low level, represented by very few people, when I was a student.

Kuhn:

Do you remember any of the earlier efforts to get something like the Henri Poincare? That, I take it, is a problem of persuading the government?

Brillouin:

Yes. It was complicated stuff, because you had to get the support of the government and to get the credits voted by the deputies and the Senate. All this was a very complicated business, and the real scientists didn’t like to spend too much time going to talk with politicians. That’s the trouble of any organization completely under the supervision of the government -- any change is so difficult to make. I have a story for you. The problem of adjustment of the salaries was a terrible problem. Every time the government wanted to adjust the salaries of the professors according to certain written or unwritten rules, it was written that a professor of such and such a category should be paid exactly the same thing as a captain in the army or the navy. So you had to raise the salaries of a few thousand people at once if you raised one… The whole body of civil servants and military have to be raised together. So it’s easier to leave it the way it is. Nothing was done. We had a great deal of inflation even before the war, so all the professors were very poorly paid.

Kuhn:

I realize one thing I also wanted to ask you. You told me last time about the books of Bouasse. I wonder what other books were widely used?

Brillouin:

|| I can show you some written later by one of my colleagues.|| He rewrote all the good books for physics, because he felt that those available were no good, and he was right. The ones he wrote are classical books, and not much new, but nevertheless pretty well written and very complete. He was not a good research man, but he was an excellent teacher… He started writing them around 1920 -- immediately. I think his first book must have been on thermodynamics, and then he wrote the one on optics. He was a very hard worker. As soon as he was a teacher instead of being a student he said that he needed the books…

Kuhn:

If you go back now before these books started to come out, before 1920, into the days of your own education.

Brillouin:

Bouasse wrote a number of books, maybe 20 of them. They were the best ones of the times. There were some older books written by the teacher we had at the Sorbonne. His name was Bouty, and they were very poor. All the books before Bouasse were terribly old fashioned. There were some very good books on the advanced level, which had been written by my grandfather Mascart. He had a whole textbook in two volumes on electricity and magnetism, and another one on optics, also in two volumes. These were very good books.

Kuhn:

These were still widely used when you were a student?

Brillouin:

Yes. They had been written around 1900, so they were still used. But they were too difficult for the first year student at the University.

Kuhn:

So the Bouasse books were generally for the first year, or the first two years?

Brillouin:

No, no! They were also difficult for the first year. He pretended to write them for the first year, but no first year student could really follow them.

Kuhn:

Were books in other languages widely used, or were almost all the books used French?

Brillouin:

I used to read a number of books in foreign languages. What did I read? Not in the first year, but in my third year and fourth year I read two of Planck’s books, the Warmestrahlung and Thermodynamics. Then I read also some English. There was a big book on optics which was very -- by Wood… And then I read a number of books by Kelvin because they happened to be in my father’s library. [Kuhn adds that as he was leaving Brillouin told him that there had been an immense amount of trouble over relativity; that this reflected immediately upon Newtonian mechanics which was the standard subject that was widely, widely taught in the schools. It had been deeply resisted for a very long time -- unlike quantum mechanics. But on the other hand, once the quantum mechanics had come along people got used to relativity -- simply said, “All right, but we’ll go on teaching things the way they were before.”]

Session I | Session II