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In footnotes or endnotes please cite AIP interviews like this:
Interview of Paul Epstein by John L. Heilbron on 1962 June 2,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with circa 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: Niels Henrik David Bohr, Arthur Compton, Peter Josef William Debye, Carl Henry Eckart, Paul Ehrenfest, Albert Einstein, Peter Paul Ewald, Folsom, Kasterin, Felix Klein, Hendrik Anthony Kramers, Max Theodor Felix von Laue, Petr Nikolayevich Lebedev, Hendrik Antoon Lorentz, Robert Andrews Millikan, Walther Nernst, John William Nicholson, Max Planck, Adalbert Wojciech Rubinowicz, Erwin Schrödinger, Karl Schwarzschild, Arnold Sommerfeld, Johannes Stark, Timiriazev, Umov, Theodore von Kármán, Wagner, Hermann Weyl; California Institute of Technology, Rijksuniversiteit te Leiden, Universität Leipzig, University of Moscow, and Universität Munich.
Professor Epstein, you mentioned at the conclusion of our last talks that you had been in California for the first time in 1913. What was the occasion for that visit?
The occasion was to visit a few American universities and to see how conditions were there. You see, that was at the time when I was still studying in Munich under Sommerfeld, and I knew that I didn’t want to go back to Russia. Also Germany was not too congenial to me either, because of the political conditions there. I liked the men all right. So I wanted to expand my knowledge of countries. And the reason why I went there is that a friend of mine, Koch, went here by invitation to the Mt. Wilson observatory. So I took the opportunity and made a tour of America, and a tour here also I had the connections. I spent about two weeks in Pasadena then. It was a very beautiful place, far more beautiful than it is now. Of course through Koch I had the run of the Mt. Wilson observatory and so on. I became acquainted with everybody, it was very interesting. And then when this position in Pasadena became open and Millikan told me about it, I took it with great pleasure, because I knew what a fine place it was.
Then the Bohr paper came out when you were in the United States, or had you seen it before?
No, it was not the whole year that I was in the United States — the whole trip took only four months. I was back in Munich by fall 1913. So that the first Bohr paper came out presumably when I was in Munich, but I don’t remember it or anything. You see, I started in June I think, and was back by October.
Do you recall this curious paper of Bohr on the Stark effect, which I think you’ve been looking at?
Ja. I don’t remember that paper at all. If I mention it in my work, then of course I have seen it, but it certainly did not influence me in any way.
It almost seems to be a Thomson atom that he has considered here — this oscillation through the positive core.
Ja. Well, you know that in my theory this oscillation — I call them for some reason Pendelbahnen I think — also plays a role. I worried a little about why they will not collide with the center, and I mentioned them as very narrow ellipses. But you see, that is connected to the other question, namely the attribution of the lines in the old theory and the new quantum mechanics. The remarks which I make in that paper on the hyperbolic orbits about comparison with Rutherford’s observations, are based on the two-dimensional model. There the third dimension is not considered, and that is an idea of degeneration. And therefore the quantum conditions also are formulated in two dimensions. Now that cannot be done in quantum mechanics, and that shows it was wrong. For instance, in Sommerfeld’s theory, take the quanta he called there “azimuthal.” Now in the general dynamics you can’t do that. There is the azimuthal, which is described by trigonometric functions, and then there is the one that is described by spherical harmonics. That corresponds to the old azimuthal, but in a different character. And the statistics must also be carried out in three-dimensional states. So that all that based on two-dimensional hyperbolae statistically is in principle not correct — these conclusions concerning the statistics, which I tried to compare with Rutherford’s work cannot be maintained.
But did this paper have much of an effect?
I really don’t know. You see of course it was published while I was at Leiden, but it was done a couple of years before really. I intended to apply the correspondence principle to that, and do what later was done by Kramer’s, the radiation of an electron going through matter. Only I had a lot of personal worries in that time. … I remember I even wrote a letter to Bohr. I don’t know whether I told him exactly what it was I intended, but I told him that he will not like the paper. But I was wrong, because he suggested the same thing to Kramer’s, I think without knowing that what I intended was essentially the same.
The question of oscillations through the nucleus and the collision between electrons and the nucleus was a problem that came up in other respects as well; the crossed-field problem, for instance.
Ja, now you see how it was resolved: the whole attribution of the orbits had to be reversed completely. The n equals one, which was considered the zero azimuthal quantum is just the other, the completely symmetric one now. And therefore that which goes through the kernel is not necessary any more. The azimuthal is at least by one unit smaller than the total, while according to Sommerfeld it could be the total.
Was the problem of eventual collision in some of these orbits considered to be a grave one?
Not to my knowledge, except by mathematicians. You see, I remember that I talked about this point to Weyl, that is whether it makes sense to use such an orbit, and he thought no, it should be excluded. But we just kept it in.
I have heard somewhere a story about your solving the crossed-field problem by finding a system of coordinates in which it was separable. I believe it had stumped people for a number of years.
I never published the calculation. I published the result in a letter to the Physical Review, I don’t remember exactly the date. But that was when I was in America, it must have been 1921 or so.
But that one had been quite a thorn, hadn’t it?
Ja. Well, somebody solved it then after I had already published the results. And I remember that Bohr pointed out to that man that it was already done. But I just applied my general theory of systematical approach by successive approximation; it was I think a straightforward application of it.
When you finally came to the United States, we discussed last time some of the differences you found between American and German universities. What about the publishing situation? Was that similar or not?
The publishing situation was bad and necessarily difficult here, through personal idiosyncrasies of the then editor, (Folsom), of the Physical Review. He insisted that the abstracts which preceded every paper had to be written exactly as he wanted it. And nobody knew what he wanted really, so that it was an endless struggle. He kept returning the abstract to be rewritten, and finally he wrote the abstract himself, and so it was from the point of view of the author much poorer than his own. That was one difficulty. Then another was that they wanted to save on type setting. You see, in Germany of course the wages of printers were not very high and the type-setting was all hand-done. Here they tried to use machines, even for the formulas.
Therefore the formulas must be written in a definite way, especially fractions which are difficult to set and so on, which would be quite all right if the author had known it and had written himself in that way. But they tried to doctor it up in the office there in New York. And that was simple butchery, because the formula then had no sense whatever, or it was wrong. And that led to great expense, because they set it out in that impossible way and then it all had to be reset again. And they did not judge the paper in the office, but sent it around the country to referees, which is a very doubtful procedure from the point of view of priority, because before they begin to set it, they have sent it all over the country, and a number of people have already read it. Now the Americans seem to be exceptionally honest, but in Germany it wouldn’t have done, it would have appeared somewhere else, without the author’s name. Anyway, the delay was very considerable, and also they required an abridgement. And that is really what is very necessary, and that problem isn’t solved yet. It is a means of publication where you can write a really detailed and extended paper, the kind in the Philosophical Transactions in England or in the Verhandlungen of the many academies which exist in Germany.
Let me ask you another question about the early days at Cal. Tech. Millikan was of course an experimentalist, but Cal. Tech, rapidly became a fine place for theoreticians. Was it originally intended, to make it strong in theory as well?
Well, it was not intended to give the first place to the theory. It was only intended to bring it up to an appropriate level. Now I don’t know, the fact that there was a theoretician of some reputation here, that was of course an attraction. People of abilities did come then, Eckart and Zwicky and so on. And of course (Tolman) was also here, though he was not really strong in mathematics, but he had a very clear physical insight, and was very stimulating. And now Bateman was considered a physicist. In my opinion he was not; he was a very brilliant mathematician. They also intended to have every year a visiting physicist from Europe, but of course that did not have much effect on the developing of a school of physics, because the time was too short. In the beginning they were a year here but later they were only usually part of a year, and they spent most of their time becoming accustomed to the country, traveling and so on. So that although Lorentz was here twice for a year, and Ehrenfest and Darwin each once, I don’t think that any of them had any pupils here. That program did not continue in the original way, because there are so many first-rate physicists in America that there is no point in importing them.
About when did this stop?
It went down automatically. You see, there is also the general question about the relation of the theoretical physicist to the experimental. And that is different from Europe’s in one respect, namely that the experimentalists know more theory than they did there, and they think that they know quite a lot. And that makes a collaboration between theoretical physicists and experimental very difficult. In Europe for instance when I worked with that Koch I mentioned, it was understood that he left the theory to me; here they meddle the whole time themselves. And they set themselves good ideas, but after all they bring it into a form which you cannot subscribe to; you can’t publish such a paper, even if it is in some respects valuable. For instance, there are the questions which Millikan and Bowen and later too other collaborators, treated: the cosmic rays and so on. Now always Bowen wanted to do the theory, so finally I let him and got out of it. If I had the field for myself, I don’t know whether it would have been very much better in the end, but it would have been more elegant in form. Then of course we got here very good younger theoretical physicists: Oppenheimer, for instance, though he didn’t stay long here, and Houston, who was good in experimental physics and in theory.
You mentioned last time that you had talked to Schrodinger later in life about the work he had done just prior to the publishing of his first paper on the wave mechanics, and that at the time he admitted to having incorrectly solved his equation for the hydrogen atom! Do you know how long he had that equation before he published it?
Not very long, it’s a matter of a few months. But he never thought at first that he had solved it correctly –- [he studied differential equations during those months.] You see, that method of constructing matrices by means of complete orthogonal sets was actually already known to mathematicians. It is in one of the lecture courses of Klein, for instance, but of course none of the physicists knew about it at that time.
Were you familiar with that notion?
No, I was not, though I had that lecture course of Klein’s in my library in Europe, which burned out. But I didn’t get around ever to studying it.
You must have been in Europe when complementarily and the uncertainty principle were first being discussed.
Ja, that must have been in ‘27. I don’t remember whether I read it before I went or there in Aachen. Anyway, when I finished in Aachen, it was a summer semester, and I went to the British Association before I returned to America, and there was Heisenberg also. And he spoke about this principle.
Do you recall the reaction there at the time?
I don’t think that they completely realized yet all the implications. I mean, they sensed that it was something important, but no battle developed. You see, the general probability idea was already stated by Born earlier. I for one accepted it with great pleasure, because you see I had already come to these questions of the hyperbolic orbits. Now the elliptic did not make much trouble from the point of view of the correspondence principle, but the parabolic did. How to interpret, the functions was a very difficult thing, and this probability interpretation was the only suitable one. You know perhaps the paper of that German physicist — what is his name — who even went so far that he found that the electron dissolves and its distribution is continuous in the atom. No, I can’t tell you about other reactions to the probability interpretation because I was at that time here in Pasadena. That is an interesting question, but you will have to ask some other people about it.
Why did you happen to go to Aachen?
Oh, that was just Institute policy. At that time we started the department of aeronautics. The money we had already from the start when I came here, but then we started to build an institute and we needed an adviser especially. On European matters Millikan always consulted me, and I told him the only man to ask was von Kármán. There was of course Prandtl, the great aerodynamicist, but Prandtl was too unworldly, and Kármán was a much more practical man. So he agreed, and I knew Kármán from old times and wrote him a letter, and he came. Then the question was how to make him a permanent adviser. He had an important position that he didn’t want at first to relinquish, so we made it so that he came every other year to Pasadena, and in the intermediate years I came to Aachen. And that went on for four years. I was two times there and he was two times here, and then he accepted the permanent appointment.
Was the political situation at Aachen better than it had been at other places?
It was the general situation. It was not that the government was bad, the government was reasonably liberal. It was the Germans, the population, and they were the same in Aachen as everywhere else. You see, I was in Germany during the whole war until 1919, and I saw the Hitler era coming even then. If it hadn’t been for the occupation there, I guess Hitler would have come right in 1919.
Things were different at various institutions though, weren’t they? For instance, I understand Heidelberg was very bad from the beginning.
Heidelberg was bad in physics because they had that crazy Lenard, who was a Nazi. And Johannes Stark was also a Nazi. But that was an exception, Heidelberg. In general the university people were just anti-Semitic, but not virulent.
When did your interest in subjects other than strict physics and mathematical physics come to the point that you began to publish on these matters? You have a paper I think in the early ‘30’s on certain interpretations of the principle of complementarily.
Well, I considered that a physical paper. I have interests, as you know, in psychoanalysis and art, and I have a great head for philosophy. I thought that subject was a really good thing, but philosophers just can’t handle it. So that publication was to set the philosophers right.
But the psychoanalysis is an interest of old standing?
Ja, well, I have here a paper on psychoanalysis which is quite revealing, but I can’t, give it to you except if you want to read it here, because it is in long-hand, and I have no other copy.
I would very much like to. I wonder if in connection with your interest in psychoanalysis you’d be willing to say a few words about Ehrenfest, whom you had known for so long.
Ja, well, I shouldn’t like to say it on the record, but…