Walter Elsasser

Elsasser:

I think you’re mixing up here my personal biography with this particular little incident in the history of physics. I wonder whether one shouldn’t just talk about this particular item of the electron waves first.

Heilbron:

Well certainly that is our main interest, but it’s not clear that these things are entirely separable.

Elsasser:

Well then afterwards we can talk about my personal life… I’ll give you this historically. In 1923 Davisson and Kunsman published a little paper in which they shot slow electrons against a surface of platinum or some such metal, and showed that they are scattered in such a way that maxima and minima appear. This was a polycrystalline substance. This aroused quite some interest in Gottingen of course. I came to Gottingen in l921. When this came out, Max Born put a student of his, Hund, onto this particular problem. This was Hund’s dissertation. The idea was this, that electrons are arranged in shells and hence there is a variation of charge density. First you have, a shell, then you have an inter(stitial) space, then you have a shell of charge again, and so forth. Well obviously when an electron hits a thing like this, depending on the distance from the axis where it hits classically, there will be more or less defection, so you get maxima and minima. Since you are quite free, you can stick enough parameters into the field, and you can get this out. And I think the remark I read some place recently about Born’s saying that he suggested to me that I do this other thing on diffraction — I think that’s a little bit of a lapse of memory from his later age. Now what happened to me was this.

There was somebody talking about Bose’s statistics, and I was at that time working with James Franck. I wanted to become an experimentalist. I was always really much interested in theory, and in about a year, or somewhat over a year later, I switched over to theory entirely. Anyway, I got myself the original paper of Einstein on the Bose statistics which appeared in the Berlin Academy. And then Einstein said the following. He said he calculates the fluctuations of a Bose gas. And these fluctuations are expressed by a formula containing a sum of two terms. One of these terms is just the conventional fluctuation of particles in a box, and the other one is exactly the fluctuation that you get when you have a cavity filled with standing waves. This is a well-known formula for the fluctuation of an assembly of waves. So then he remarks that this was very, very strange.

Because after all he was dealing here with material particles. Well, he didn’t quite know what he was dealing with, it was quite a general scheme. But in case of material particles, this was very strange. And so he referred to a footnote to the thesis of de Broglie. Well, I got curious, and I said, “Darn it, I must see whether I can get this out of the library.” And much to my surprise, the University library in Gottingen had it right there. It was very simple to read. And there were these well-known de Broglie formulas that f = h/p and so forth, and de Broglie’s suggestion that one can express orbits by standing waves which give the quantum conditions. Well, then I started thinking, and I finally came up with this idea that maybe these waves are more real than we think. After all, at that time the duality between particles and quanta was well established. So I took a slide rule and figured out what would you get from a polycrystalline metal if you had de Broglie waves falling upon it and there were a vacuum and it came out the correct order of magnitude. Then of course I also followed the Ramsauer effect immediately, and you asked here in your letter whether this had quite a considerable role in my thinking. It had, obviously. This is a very strange thing that electrons can go through atoms, without finding any resistance — with a cross-section very much smaller than you should expect. It certainly looked pretty darned much like a diffraction phenomenon. Of course you can’t get it out of any classical model.

Heilbron:

Was the Ramsauer effect pretty well-known as an anomaly at the time? Much more widely known than the Davisson-Kunsman results?

Elsasser:

It was very widely known, yes. It was well-established and widely discussed in seminars and so forth.

Heilbron:

And was Hund’s thesis also to elucidate the Ramsauer effect?

Elsasser:

No, no. His thesis was just this matter of variation of charge density in shells which was just an ad hoc construction to explain the maxima and minima. So when I ended up with writing this little note, I showed it first to James Franck, and he showed it to Max Born. And then they said, “Well it is crazy, but you ought to send this to Mr. Berliner,” who was the editor of Naturwissenschaften at that time. So I did, and I heard the story later in detail. Berliner sent it first for reviewing to Pringsheim, an experimental physicist. Pringsheim didn’t know quite what to do with it. And they sent it to somebody else, I forget who. Then they finally agreed we ought to show it to Einstein. So Berliner showed it to Einstein, who lived in Berlin at that time. And Einstein is supposed to have said, “Well, darn it, I didn’t take my theory of the Bose statistics quite that literally, but I think the man should be given a chance. It should be published.” So they published it. Davisson saw it, and apparently it made no impression on him — it seemed too wild.

I never found out the detailed story, and I always suspected these people of a little bit of plagiarism, but when one does this, one is almost invariably wrong. The biographer of Davisson in the Biographical Memoirs of the National Academy explains exactly what the story was. Namely that when he saw it first, he thought it was just nuts. Then later on Davisson traveled in Europe, just about the time he made his final experiments with Germer. There he talked with Schrodinger and Thomson, all kinds of people, and they all told him to look for wave interference. By that time he had pretty much forgotten about my paper. So by that time Schrodinger’s work was known, you see, and then he just gave me a nice footnote and that was that. As a matter of fact, you see I was just 21 years old when I wrote this, and I was just too young to exploit it.

Heilbron:

Franck and Born had no interest in exploiting it?

Elsasser:

No, Franck was very generous. I tried for two or three months to design an experiment to find those things. Franck said I could have access to his shop and so forth, but that he was not able to take any of his more experienced men out of their work to help me, which is what I had asked him to do. So I gave up after I think three months because this is a very difficult job for a 21-year-old student who has no experience. It’s an intricate experiment.

Heilbron:

Did no one make an attempt at that time to call your paper directly to Davisson’s attention?

Elsasser:

Oh he saw it. I had a good many talks later on with Karl Darrow — he is a very good friend of mine. I know that they knew this thing when it came out, they just didn’t know what to do with it at that time.

Heilbron:

The experiments to which you refer at the end of this paper in Naturwissenschaften were then your own?

Elsasser:

Oh, ja. I gave those up after three months. I just couldn’t handle it, that’s all.

Heilbron:

Had you done detailed calculations to show that the measurements on the Ramsauer effect that were then around were in conformity with your theory?

Elsasser:

No, this is very hard to do you see. Because all you can say, which one can see readily, is that if there is a suitable resonance there, this may happen. It’s just effectively impossible to tie this down except by having a very specific model with lots of problems.

Heilbron:

Well let me ask you about de Broglie’s work in that first year. You say that the library had his thesis. Was it widely known at the time?

Elsasser:

No, I don’t think anybody had ever heard of it.

Heilbron:

And these remarks that Born makes about it being thoroughly known are at best just slips of the memory?

Elsasser:

Well I would think so. I think that Born might have known about de Broglie’s work, but I think people were pretty much occupied with the standard type of atomic mechanics, which you find, in Born’s book. And Heisenberg’s paper came out in ‘25 too… I remember when Heisenberg used, to go around and talk about his paper. He called it “the great saw.” It was very amusing — a psychological sidelight. I asked him why he called it a great saw. He said, “Well, because it’s sawing off the limb on which classical physics sits!” Interesting sidelight on the subconscious psychology of an innovator, you see. So, I think people were just too busy to think about my idea. I think this pretty much concludes the story.

Heilbron:

Well tell me about what you can recall of the development of matrix mechanics at the time. You were a graduate student then.

Elsasser:

I was still a student. I was in my fourth or fifth year of study, and I remember regular seminars. There was a weekly seminar called the structure of matter, of which Born and Hilbert were the main-stays, in which all the stuff was discussed at great length. –- Everything that had to do with the basic structure of the atom.

Heilbron:

Do you recall what were considered to be the main problems in this seminar just before 1925?

Elsasser:

Well, I remember that people weren’t at all sure how long it would go until they found the solution. It was very clear at that time that the Sommerfeld-Epstein quantum conditions worked for systems with one degree of freedom and that they didn’t work for anything as simple as the helium atom — so people were just stuck, you see. Then Heisenberg came out with this idea that replaced the differential calculus by a difference calculus. And of course he didn’t realize nor did anybody else at this time that this was matrix calculus… Yes, it was very clear even when I began school that classical mechanics and the Bohr-Sommerfeld theory weren’t enough. I don’t think there was a very great difference in tone between the two places I attended. First in Munchen there was Sommerfeld, who had his regular seminar, and things were reported. I remember when I was in Munchen there was one period when Sommerfeld was very much concerned with the so-called sum rules for multiples, intensity rules. He used to play with these numbers. They used to delight to get integers out of this without doing any dynamics, and. they could not fit it into any dynamic scheme at that time. It was clear from this that any future quantum mechanics must be closer to something that can produce integers than the conventional theory.

Heilbron:

And was there great concern with the sum rules at Gottingen as well?

Elsasser:

Well, no this was not so pronounced. I don’t think this is in Born’s mind so large. Now I do remember however a seminar and a course in 1926. I don’t quite remember who was teaching this. I think it was alternately Hilbert and von Neumann. It was about linear operator calculus, in which they tried to apply the methods of linear operator calculus to these new theoretical findings. Now I also remember Hilbert once telling me, in one of these seminars or on some social occasion, that he and Ritz had tried 20 years earlier to make an eigen value theory of the atom. I’ll tell you the story. They tried to find an eigen value problem, you see, which would give the Balmer series. And then they found that any eigen value problem that they chose would always give them as point of convergence — that is the point where the distinct spectrum ends and the continuous spectrum begins — zero. And I think Hilbert probably could prove this. And so, since the Balmer series of course converges to a finite point in the spectrum — they could prove that this simply could not be represented by any known linear eigen value problem.

It was only a few years later that Ritz found the combination principle, which cleared this up, but by that time he had probably forgotten this article. I remember these lectures on operator calculus were very enlightening; I learned a great deal from them. And then this whole development I presume interested me so much, and I was not making much progress in experimental work, so I decided I had better become a theoretician. That was in the spring of ‘26. And then Max Born said, “Well, now you’ve already spent a good deal of time here trying to get a thesis done, I’ll give you a comparatively simple problem so you can get out of here.” And he gave me this problem of applying the Born scattering theory to scattering by hydrogen atoms, which was a very, extremely simple problem. It was just a matter of grinding it out, which I did and I got my degree on this.

Heilbron:

Do you recall anything in the way of differential reactions to the wave versus the matrix mechanics at Gottingen? How was the one received and was it different from the way in which the other was regarded?

Elsasser:

No, I don’t remember any great arguments or fights. After all, this all went very fast, you see… Heisenberg’s paper appeared in ‘25. And with Schrodinger’s paper showing the equivalence, the physics of the thing was fairly clear. It was just a matter of elaborating it. You see the whole darned thing took place in about one year… After the identity was shown, nobody could possibly have any qualm with the theory, and so then it was just a matter of catching the band-wagon. Born then immediately developed this Born approximation of scattering theory.

Heilbron:

What was the relationship between the scattering theory and the statistical interpretation which Born announced I think in 1927?

Elsasser:

This must also have been worked out in ‘26, because he gave me this thesis subject at a time when he had worked out the Born scattering theory, which must have been in summer of ‘26. Jordan was I think one of the principal fathers of this whole theory. He was very close to Born and Heisenberg at that time. Of course Heisenberg was not in Gottingen; he just dropped in periodically. I think this must have all been worked out by the end of ‘26, at least in outline. But there was one thing at that time at Gottingen. There were so many brilliant people there that a student who had not much experience got an inferior impression as to what the world of science was like. If you hadn’t known what science was before, you see, you thought all science was like this.

Heilbron:

Then may we go back, with this as an introduction, to the differences between the schools you attended, Munich and Heidelberg?

Elsasser:

Well, I was born in southwestern Germany. From 1915 on I think, my father, who was a lawyer, was a judge in the city of Heidelberg. I was always interested in science. I had a very good mathematics and physics teacher with whom I was very close. He was an unusually educated man. He had a Ph.D. in mathematics, quite unusual for a high school teacher. I wanted to become an engineer, because I had to make a living, you see. He told me there was now a new profession, namely that of industrial physicist. This would permit me to stay at the university rather than to go to engineering school — you know in Germany the engineering universities are separate from the rest of the universities. So this appealed to me very much, because I was interested in science anyway. I used to monkey around with experiments in mathematics and what not in high school a great deal. And my father always used to say, “Well, I have a peculiar kind of boy. I know that people go to football games on Sunday afternoons. He sits home and studies calculus.” This teacher induced me to take up calculus at the age of 16. By the time I was out of high school I knew calculus very well.

This teacher’s name was Reinig. He came from a small village somewhere in southern Germany. And once in a while I played hooky from school and used to go up and listen to classes in physics at the University. Then I entered the University. It was very convenient of course, with my parents living there in Heidelberg. But then there were political problems at that time. I entered the University in the fall of ‘22. Arid I was in a fairly abnormal position, because although my parents and I were good protestants, it was well-known in the town that we were of Jewish ancestry. There was another relative who was a well-known local doctor to whom every child in town went. This caused a great deal of trouble, and of course trouble was compounded by the fact that the head of the physics department, or the director of the Physics Institute, was Philip Lenard, who as you know later on became Hitler’s right hand man. Well, I went to Lenard’s first lecture in my first semester. By the way, his lectures were magnificent experimental demonstrations. He appeared there with a swastika on his lapel three or four inches big and got the most tremendous applause — I think it must have lasted for about five minutes. Tremendous applause from the students, which was obviously directed towards the swastika and not just Mr. Lenard. And. I remember going home then and breaking out in tears and saying, “I refuse to go to this University to study physics.” And my father, who was a good German patriot, said one must not let himself be cowed by this political extremism.

So I stuck it out for a year, but I was told by a number of people that to stay on would be just suicide, because he was known to make nasty personal scenes with people he didn’t like in the laboratory. The second year I had to take the laboratory, and I might be exposed to being thrown out bodily or at least to arguments and other unpleasant things. So, then I went to Munich. This seemed more quiet at first. I saw a good deal of the Sommerfeld group, but I wanted to become an experimentalist, and the head of the Physikalische Institute was Wilhelm Wien, the discoverer of Wien’s law. A very correct, pleasant Prussian type, precise, a stickler, but very nice. And I found out eventually that all members of the regular staff of this Institute, about 70 or 80 in number, except one, and except Wien himself, were card-carrying members of the Nazi party. This was, remember, in 1923, the year of the Beer Cellar Putsch. And this one man who was not in it discussed it with me and urged me to go away, because I was just ruining my future if I stayed there. So he said, “Why don’t you go to Gottingen? All my associates recommend that you go to Gottingen. I’m neutral, I just want to help you. It is well-known that this is a Jewish university anyway.” So I finally talked to Wien, and he said that he would do everything that was his duty, but he obviously could not keep me from being not well treated by this majority of his associates. He tried to be as correct and neutral as he could.

He was evidently not a crusader, though he was very nice. He tried his best. It was obvious that he didn’t like it, but he was powerless. So I went to Sommerfeld. Sommerfeld wrote a very nice letter of introduction and recommendation for me to James Franck. And I left Munchen in the spring of 1924 and appeared in Gottingen. Franck put me to work in his laboratory. I spent two years there with comparatively little success. At that time one had to have pretty good manual skill to be an experimentalist. I finally switched over to theoretical physics in the fall or summer of ‘26. And Born said, “You’ve been around for quite a while. Why don’t you take up something that is safe so you can get your degree and get out and do some work.” So this is what I did, and I did this thesis on scattering by the Born theory. Then … I spent a semester in Zurich arid a semester in Leiden in Holland, for post-doctoral work, and I had a little job in Berlin for a short while. I also spent several months in Russia in the spring of 1930. They paid me extremely well as a foreign specialist. It was finally a little too tough for me. This was the time of the great famine, the second five year plan. It was very interesting, but I shouldn’t go into it. It doesn’t belong in the history of quantum mechanics. So, then I got a job with Professor Madelung in Frankfurt. From 1930 I stayed there until l933 when Hitler came into power. I had my passport taken away you know. This was in March, and it was clear that I would be out anyway by July or August. Poor Madelung. He was very nice, but he already had a bad reputation as being what the Nazis called a left wing liberal.

He had been rector of the University for a while and had expressed rather strong views about personal freedom which the Nazis disapproved of, so he let me know that he could hardly keep me beyond that time. So I simply packed up and went to Zurich. I knew Pauli quite well and Pauli said, “This is fine. They need a theoretician in Paris to do nuclear physics. I will write Joliot.” He did, and Joliet wrote back that he would make the necessary preparations and so forth, which took a long time. I finally got tired and took a French visa and went to Paris. They were just about ready to give me the job, and I had a small, tiny little fellowship for a year, and then they put me on the payroll at a fairly low level, but it was enough to live on, better than most refugees. Well I never liked nuclear physics too well; it didn’t appeal to me. The theory is rather formalistic, and the experimental end is highly technical and expensive. But I was supposed to do nuclear theory. I was working directly under the supervision of Francis Perrin, the son of Jean Perrin, who is now head of the French Atomic Energy Commission. He was already a professor at the time, and we published a couple of papers together. That’s when I did this work on nuclear shell theory. However this did not appeal to me. I remember even when I was a student, Max Born used to tell me that I was better on conceptual problems than analytical ones. This has always been true all my life. And so I decided I ought to go into a field where I can practice this. In nuclear physics you have to be more of a mathematician than I proved myself to be. I got a visa to come to this country in 1935.

Being the depression, I traveled all the way to Chicago and then back, and I couldn’t get a job. So I came back in 1936, and that time I travelled all the way to the west coast, and Robert Millikan fixed me up with a promise at least of a job in geophysics. This suited me fine, and I slowly worked my way into geophysics, and I’ve gone quite a way since then in geophysics. Geophysics at that time consisted of Gutenberg in Gottingen, and a few retired colonels in the military, and that was about the size of it, except for a couple of people in the department of terrestrial magnetism. I hope I helped make it popular. It is now a little too popular, in terms of billions of dollars. Then biology has always interested me. I had a friend who was a biologist in my Paris days, and I had many bull sessions with him about the fundamentals. Bohr’s paper on these things, which was entitled “Light and Life” and published in Nature in 1933, fascinated me very much. And I decided sooner or later I am going to come back to this, but at that time I felt it was too speculative. This was not a way of making a living. I had had some astronomy, quite a bit of astronomy in fact in my student days, because I acquired two minors, math and astronomy. So I thought geophysics was a good way of making a living, and I did fairly well in this. Somehow I had forgotten all about biology, but it was evidently somewhere in the back of my mind. And so when I felt I was really settled at the University of Utah, where I came as a full professor, I began to browse in the library and to think about it. I published a couple of notes, and eventually in l956 I wrote my book, which was published in l958. I’m still writing papers on it, and I think it has a very, very great future. But this is a different problem from physics, because there are so many taboos. You must not say that an organism is a machine; and you must not say that an organism is not a machine. And this is very serious, and even some very distinguished physicists have absolutely wretched opinions about it, and you can’t argue with them. If you talk about the formal structure of our experience in relation to organisms, no matter how formal and rational you try to make it, you arouse deep-seated emotions. You are right in the midst of controversy.