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Interview of Rudolph Minkowski by Thomas S. Kuhn and John Heilbron on 1962 April 1, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4780
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This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with 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: Max Born, Albert Einstein, Werner Heisenberg, Rudolf Walther Ladenburg, Otto Lummer, Wolfgang Pauli, Erwin Schrödinger; and Universität Breslau.
I’m delighted to see that you have brought what I take to be the notes on Bohr’s lectures. Tell us a little bit about how they came into your possession.
This is a set of six lectures, which Bohr gave in June, ‘22, in Gottingen. I was, at that time, in Gottingen with Franck. Together with Huckel, I got the task of taking notes and working out these lectures. This is the result of it. There is, also, here in front a copy of part of a letter from Bohr, which has to do with the contents. Somebody had raised a question about the correctness of the assignment of the quantum numbers.
Do you remember who raised the question?
I am not quite sure. It may have been Ladenburg, but I am not quite sure.
You told me something about your own feelings at the time. I remember your saying that it was the most difficult job you’d ever done.
Well, you see, these lectures were in the afternoon. We both took notes, and then in the evening after dinner we sat down and tried to write it down. Usually what happened was that everything was smooth sailing except for those places which were difficult, and there both our notes were inadequate. Then we had to work very hard to assemble the line of thought.
This is something I want to come back to. Had you at this stage of the game already had a good deal of training and done some work in this general area of atomic structure?
Yes. You see my thesis was — or rather, turned out afterwards to be — a determination of the f value of the sodium D lines. I say “turned out later to be,” because when I started on this thesis it wasn’t at all clear what the result was to be. This work was with Ladenburg — and it was still formulated in classical terms — the number of dispersion electrons. Neither of us had any idea what would come out. Eventually, when the result was surprisingly simple, Ladenburg started to think about it. This is how he came to formulate the f value… You see, the classical formulation of the problem was in terms of the number of dispersion electrons, and the problem was to determine the number of dispersion electrons per atoms. That’s the classical formulation. And when it turned out that the sum of the numbers for the D lines was about one, this led to thinking on the problem. That is what led Ladenburg to formulate the f value.
Was this, so far as you know, the first step of Ladenburg’s in this direction? How early does his interest in the dispersion problem come about?
Oh, the interest in the dispersion problem was, I think, relatively old. Of course this was probably an interest which was stimulated by Reiche. There was the problem of line intensities and line widths. I don’t know exactly when his interest in dispersion started, but it must have been there before. I forgot when(Rochdestvensky’s) experiments were made. They were, I think, before the war — I am sure. And I know that Ladenburg was interested in that… There was of course — I think this was before the War — Reiche’s work on absorption. This also, I think, goes back to work which had been done in Breslau by (Senftleben) and (Mescault) on flames; and this probably went in itself back a little bit to Lummer interests. So there was a certain line of thought in the Physics Institute at Breslau at that time which went back to such problems.
I hadn’t known that Ladenburg’s interest in it went that far back.
It must have started at that time. He got active after the war. That was the time when he started to make his experiments on neon, and when I made my thesis. So that was when the active interest started.
Your thesis was theoretical?
No, it was observational.
One knows Ladenburg as an experimentalist, but there are also important theoretical papers…
Yes, yes. I think he was primarily, of course, an experimentalist. But he had a very great interest in theory, and was not interested in the type of experiment which is not accessible to promotion of theory.
I wondered, why did you go to Breslau? What took people to Breslau? What was its reputation and where did it fit into scientific education?
In my particular case the situation was very simple. I was living in Breslau.
And you went to college there as well as taking your Ph.D.?
Yes, yes. Well, I started in Breslau. Then came the war. During part of the war I was in Berlin in Born’s group, and at that time I attended some lectures in Berlin. Then I went back to Breslau and got my degree there.
You started then in Breslau when?
In 1913… I was in the army from about late ‘14, and came to Berlin in ‘16.
How did you get to Berlin?
This was the scheme whereby people in the research establishments could ask for certain people to be transferred to their department.
You already, on the basis of three years of college at Breslau before you went into the army, had reached a level at which you would be asked for?
Yes. There were not so many people then, you see. Physicists weren’t as numerous then as they are now. I think it was a state of affairs where anybody who had some connection and education in physics would be gathered in in these research establishments. This was I think the main scientific establishment of the German army, the so-called Artillerie Prufungs-Kommission. That means artillery testing commission, which, originally, had been mainly interested in ballistics but during the war got into some other obvious problems. Ladenburg, actually, was a reserve officer, and he got in this thing. And he was the one who gathered Born, Lands, and whoever else was there. Now this group did some say military science work, but the group also remained interested in physics. The point of attraction in the Einstein-Planck-Laue rooms was still alive. This was quite an active research group.
When you speak of going to lectures during the war in Berlin, were these regular university lectures?
These were regular university lectures.
And these men went on with regular university courses?
How about the colloquium?
Well, you would, of course, go regularly to the colloquium if you could go…
To some extent it was, therefore, a richer life, scientifically in Berlin than it had been before?
Yes, there was a very strong group. This is I think one of the factors which made for very little interruption in the development of physics.
To take you back to Breslau… Did Breslau have a major reputation as a scientific center?
Yes, in physics very definitely, because Lummer was there. Lummer — who in his later years got somewhat old — was after all a first-rate experimental physicist. Lummer was involved in starting quantum theory… This was one of the things in which Lummer felt, somewhat, that he was slighted, and that his contribution was not adequately recognized.
Really. Because certainly his name is closely associated with –-
Yes, yes. Well, I think really what happened is that when he made his black body studies, the research was at first somewhat disregarded. I mean this contradicted what seemed to come from sound theory. It’s not unusual, where the experiment seems to contradict the theory, to disregard it as suspicious. Lummer before the war was actively directing his institute, although not in the way which is very typical for many German places. I mean where the full professor is king and determines entirely what is being done. Even before the war Lummer ran the institute rather on an open basis, leaving the people whom he had to do their own problems. You had then a group in Breslau where the interests were very widespread. You see, you had acoustical research going on with (Waetzmann); you had Schaefer, who was interested in infra-red research; you had Ladenburg, with his interests in, say, physical theory of emission and dispersion and that type of thing; they were essentially free to do what they wanted. Lummer was very liberal in making funds available for everybody’s experiment, in entire disregard of the budget.
What about the theoretical side?
Schaefer, of course, was at the same time a theoretical physicist. Reiche — … who came just when I left … There was a very short stay of Schrodinger. As far as I remember, Schaefer’s successor was Schrodinger, who was in Breslau for about one year. He was there in ‘21, at the time when I had my examination… Schrodinger was there for too short a time, I think, to come in very close contact with anybody. He was very pleasant personally, and I think everybody liked him personally very much, but he was not there long enough to really have any effect on what was going on.
Did he have a great reputation already at this early time?
Yes. He was considered very highly then.
Let me ask you, did this great mathematical tradition at Breslau still endure in the ‘20’s, at the time you were there? Several people have spoken of it as having been a great center for mathematics.
Yes, but my impression is that it was somewhat dying out. There was of course a time, and this was before the war, when it was very strong. Carotheodory was in Breslau then — not at the University — but at the technical high school. There were also Gerhardt Schmidt, and Kneser, but by that time it had died out, I think. Carotheodory had left, Gerhardt Schmidt had left, and I forgot who followed them.
Was the curriculum pretty standard, was there a big course in experimental physics?
This was the standard arrangement of all German universities. There was a big course in physics, which you took in the first two semesters.
From whom did you take that?
Lummer. You see, this was always the main task of the full professor. The longer these went, the more Lummer made a circus out of this course. To do so was very tempting. After the war this got worse and worse. Occasionally he would feel a need to give a really good lecture. Word about this would usually be passed around, and everybody would go there. I have heard lectures then which really beat anything which I ever heard anywhere else. One I remember particularly in which he did the whole of crystaloptics in one lecture, perfectly and completely. At such times the outsiders or, say, the students of medicine who had to go to that lecture, didn’t realize that they were not really listening to a circus act, but that there was really lots of good physics behind it… This was the only big course. Then you had a course in theoretical physics, which probably extended over four semesters or more, I don’t remember it quite. In Breslau it was by Schaefer. I mean he was the theoretical physicist. Then there would be lectures on special fields.
By the Dozenten?
By the various people.
Were there books used in conjunction with these courses?
Not really. In the big lectures, some book was recommended, but they were not courses based on books.
And one could go through it without looking at a book?
I think you could.
Were Schaefer’s books yet available?
No. Schaefer’s books were the result of his lectures. You see, after having given these lectures for a good many years, he finally published them as books.
What about the Planck book?
… You would of course use Planck’s Thermodynamics, which was out then. But what was done in a lecture was not necessarily what was in Planck’s book. Now Planck’s own lecture course, which I heard, was very similar to his book.
What about books in mechanics, electro-magnetic theory, optics?
Electro-magnetic theory I think the book most widely used would have been Abraham’s. To some extent, although that was not translated into German that I know of, Lorentz’s Theory of Electrons. Optics? I think Drude’s was probably the commonly used book. Mechanics? I don’t quite remember. But these were not, in general, lectures tailored after some book. You might read Boltzmann or Kirchhoff…
How much mathematics would you normally take as a student of physics?
A student of physics would of course take calculus and differential equations. Beyond that I think it would depend a little bit on the personal interests. I think that most people took a course in analytical geometry, because that was necessary if you wanted to get teacher’s credentials.
Did the students, at least in their more advanced training, think of themselves as living in a particularly exciting and troublesome period in the development of physics?
I think that would vary greatly according to the field in which you were working. A student whose interest was in acoustics would not be as much aware of the developments which were coming as somebody working in a field more closely connected with them. So Sommerfeld knew that there were great things happening, and others probably didn’t realize this as much.
Did it seem, at the time you took your degree, that things were going all right? What I mean is that at least by ‘23, some people were saying, ‘It’s all wrong, something absolutely basic has to be done.’
I think, while this may be somewhat of an oversimplification, that the problem which really led to the recognition that there was something basically wrong was the attempt to make a model for the helium atom. This was attacked at first with the idea — after the hydrogen atom worked so beautifully — that difficulty in doing the helium atom was just one of handling the three-body problem. This led to such efforts as Pauli’s work on the H2+ … which is a three-body problem, but one which can be easily handled. That turned out to be manageable. Attempts to find a model for the helium atom — you will find it in these lectures, which were just about in the middle of that period. I remember when I came to Gottingen in ‘21, Born was working on a model for the helium atom, which of course did not work. I know Pauli was interested in that problem. I think that this was the main point that finally brought recognition that it was something more basic. The helium atom played a very great role, I would say, in the development of that attitude.
The dispersion problem was clearly another one that could have bothered people as much.
That did not play so great a role, I think, in the consciousness of people. There were some problems which turned out to be spurious, but which somehow gave a direction to the thinking anyway. This famous problem of the hydrogen atom in a crossed magnetic and electric field, which, as you probably know, was considered as unsolvable. You couldn’t carry out the integrations. Pauli for a while was convinced that this had physical meaning, and tried to persuade experimental physicists to try and observe it.
This was at Hamburg?
It started in Gottingen, and it was also in Hamburg. The thing found its very sudden end when Epstein came to Hamburg in ‘23 and wrote the solution for the problem on the blackboard in 10 minutes. It was just a problem of finding the correct coordinates to separate the equations.
How did Pauli feel when that happened?
People were a little bit ashamed that they hadn’t noticed this… You see things like that, even if they afterwards turn out to be rather trivial problems, still influence your thinking, because they set your mind in the direction in which there may be something.
The absorption frequencies of anomalous dispersion should be the spectral frequencies and they’ve got to be the mechanical frequencies. Some people were deeply depressed, or impressed, by this problem. You were involved, somewhat, with Ladenburg, in this area.
I think that this was regarded as something which should be handled with the aid of the correspondence principle and Fourier developments. While ne didn’t see how to handle this quantitatively, I don’t think that this originally was a point of great worry for most people. You got, of course, the intensities in the hydrogen spectrum correctly, and so I don’t think that … this was very strong in people’s minds generally. I know that it was very much on Heisenberg’s mind before he developed quantum mechanics… For Heisenberg it was a very strong problem, at least before he really came at this quantum mechanics. I know this because Heisenberg developed quantum mechanics while he was in Helgoland one summer. Helgoland is a little island in … the middle of the ocean. It was a place where sufferers from hay fever went for the summer because the air was free from pollen. This is why Heisenberg went to Helgoland, and on his way to Helgoland he stopped in Hamburg and the whole group was together. He was then very actively worried about this problem.
Was Pauli there at that point?
I don’t know whether Pauli was still in Hamburg at that time. I don’t know.
I just wondered whether you had any recollections of Pauli having actually been in Hamburg when Heisenberg came?
Yes; I think he must have been there at this visit… At that time this was the formulation of the problem in which Heisenberg was interested. But he did not see the solution then. The solution must have come during his stay of about six weeks or so in Helgoland, because when he came back he stayed again in Hamburg, and then he had the basic elements.
Had he been to Cambridge in between?
No. Not at all.
But you know he did go there in the summer of ‘25?
Yes. He may have gone to Cambridge afterwards.
He was in Cambridge for a bit, and then he came back via Copenhagen.
Yes. That must have been later — later he may have gone to Cambridge and then to Copenhagen.
Do you recall what he felt about his own solution to the problem when he came back to Hamburg from Helgoland?
Well, he did not have the complete formulation, but he had the basic elements to it.
Do you remember anything that he said about it, or that other people said about it?
I would not want to say anything. It’s too far back… I think the general reaction was that he had something.
His reputation I take it by this time was already tremendous, was it?
I want now to get you from Breslau to Hamburg, via Gottingen, via anywhere else you were in the interim.
From Breslau I went to Gottingen for a year… That was ‘21-’22…
Did you go as a student still?
Well, no. I had my degree. I just went. I knew Born from the war and before, and so I went there to learn a little bit more about these things. Franck was there then.
Did it take an independent income to be able to just take off for a year and not have a job?
So this was not something that most people could have done?
Was the reputation of Gottingen as a leading center already very clearly established?
Oh, that was very old. You see the reputation of Gottingen as a leading center goes back to the early years of the century…
But you didn’t go there for the more mathematical sorts of physics?
No, no, no.
You went there for Franck or for Born?
But you see I went there for Frank and Born.
Were you struck by differences between what seemed to be the main problem at Breslau and what seemed to be the same problems at Gottingen? Did the picture of what was going on in physics change for you at all?
I don’t think so, because I was familiar with these things from Breslau. I knew about these problems, so I didn’t find anything say unexpected in the line of work and line of thought that was going on at Gottingen.
The Bohr lectures were that year? Was that a special series?
That was a special series. You see this was the time when Bohr thought he understood everything about spectra, and he had been invited to talk about this.
In these lectures, as you recollect, does Bohr seem to feel that this is right? Because just about a year later some people are going to be feeling that it isn’t right.
No, I think this is essentially from the point of view that one really understands the thing basically, and what one deals with are really technical difficulties. That is, I think, in ‘22 essentially the attitude. Well, I don’t know. I don’t want to look now. But I think the thought that there is something very fundamentally wrong was not active yet. Undoubtedly some people may have considered this, but it was not outspoken and the basic attitude was still, I think, that it is a problem which could be solved if one would be smart enough.
This year was the year of the Stern-Gerlach experiment. You were still at Gottingen when those experiments were reported. Was this a great surprise?
I think the surprise was that one could make the experiment.
Lande says that Stern was the person most surprised of all.
I don’t know that that is true. When the result came, the research was so convincing, that this just showed it is that way. I have, somehow, the feeling that for many people the fact that one could make the experiment was more surprising than the result of the experiment.
There are paradoxes about it though.
Yes, Yes. But I think while one was aware of these paradoxes — and while one might not expect it — it was not an entirely paradoxical result. Having the result, it fitted into the general picture one had very easily.
What about the experimental difficulties themselves?
Well, I may be wrong, but I think many people may have been more surprised that it was possible to make the experiment than —
Just in terms of the mechanical problems?
Just in terms of the observational problems, yes. Just in terms of the difficulty of the experiment.
What I really want to do is go on now and talk a bit about Hamburg. Just for the record I put in couple of things you said here. It was really then through Pauli that you went? Pauli was at Gottingen in ‘21-‘22. The two of you went together. And it’s your feeling that he actually was instrumental in arranging this.
Yes, yes I know that.
Is that a function of the rapid build-up at Hamburg, that made this possible? Or Pauli’s already gigantic reputation?
No, this was the build-up in Hamburg. Somebody was needed there, and he arranged it.
When I had lunch with you you spoke of Hamburg as being a really new school. Was it the Institute that was new?
No, some of the institutes had existed before as State institutes with essentially the tasks of the bureau of standards. The university was started after the war.
There had been no university in Hamburg?
There had been no university in Hamburg; I don’t know when it actually was opened but it must have been around ‘20 or so. These institutes were of course taken into the university. There were of course all possible frictions.
Well then, who was in charge of building up the department?
You see, there were two full professors, one for experimental physics and one for theoretical physics. The experimental physicist was Koch, Peter Paul Koch, who was a pupil of Rontgen’s. The theoretical physicist at that time was Schottky, who I think about a year later went to Rostock as Stern’s successor, and Lenz came to Hamburg.
The moves were often very rapid were they not, in the German universities? This business of a man’s coming with a real appointment but being there only a year happens quite often I notice.
Yes. That was inherent in the whole system of things. The universities were not federal institutions but state institutions, and the several states would compete with each other by offering incentives.
And there was no sense of obligation? If you went to a place with a professorial appointment you somehow obligated yourself morally to stay for a few years?
Well, it depends on quite a few circumstances. It is, of course, a question of personal attitude. It is also a question of the conditions under which you took a certain position. If you took a certain position without any special treatments, you were of course not bound to stay if someone else offered you better conditions, which could be in the form of a higher salary, or in the form of new buildings, new equipment.
Did people bargain with the place that they were at if they received a call. ‘I will stay if you will do the following?’
Oh yes, oh yes.
Yes. But I think this was much more usual because you had a somewhat different system of getting positions. I think in this country you are much more likely to advance at your own university than to go to another one; while in Germany you would normally advance by being called to some other position, and the chance of advancement in your own university was small. But this depended. There were great artists who would succeed in getting offers of a position again and again, and trade out some advantage and stay where they were.
You were a privatdozent when you went to Hamburg, and you were later professor there.
No, I was only this thing, the titular professor… This was just a title. You started as Privatdozent and after a number of years you got this titular professor. Which did not mean anything but the title.
I had not heard of that, that is the situation. Now when you went there, you and Pauli went together?
I came about a year after Pauli. Pauli went to Hamburg first… We were together in ‘21-‘22 in Gottingen, and Pauli went to Hamburg probably early in ‘22, I would think. I followed him in the fall of ‘22. Stern must have come in ‘23. Then there was Lenz.
In setting up a new department, or the new department at Hamburg, was the thought that one would cover the entire area, or were there deliberate specialties for research?
No, not really. This was partly a question of people who were there…
Were you close to Pauli?
Yes, I was together with Pauli a good deal.
You told me when we had lunch about Pauli’s working habits.
Yes, Pauli’s working habits were nightly. He started to work after movies in the evening, and possibly after walk or visit to a night club. He would then work through the night… He would start around midnight and work from midnight to five o’clock. He then would sleep and get up for lunch.
Were there many people who did that? I know a few now who do.
No, I mean there are of course always people who work better in the even in than in the morning, but I think Pauli was an extreme example of it… I think Pauli is the only one I remember who had this habit. One of the things I remember were long walks at night with Pauli, worry in about the problems which led to the Pauli principle.
Was his interest at this time pretty exclusively spectroscopic?
At that moment I think it was.
Where had he gotten that sort of concern? That is, he’d been to Munich, but there’s much more numerology than one ever finds in Sommerfeld.
That is partly, I think, Bohr’s influence. What Bohr did at that time was a good deal numerology. I think that Bohr’s influence was very strong. I think there may have been some influence from Gottingen. I mean Born’s and Frank’s interests were somewhat on the observational side. I don’t know that (Romer) played a great role, but it might have played a little role. I would think it’s mostly Bohr.
What were these talks like?
I would say it was to a large extent a swinging back and forth of questions — why certain lines don’t occur and others occur, and that type of thing.
Would this be worrying about it in terms of Zeeman patterns?
Oh yes, this would be in terms of Zeeman patterns, just trying to sort out the situation and. trying to find some leading thought which would explain the whole thing.
This is Zeeman patterns still just before the g factor? …
No, I think that this very likely was after the g factor. I think that was there.
Then one could worry very hard about the patterns. When you say worrying about why such and such a line did not occur, was this to worry why a particular line in a particular spectrum didn’t occur, or to worry why lines of a certain type didn’t occur?
I think it was more of the general kind, not individual things. I think this extended over quite a while. I would say a good my weeks, in which this problem was turned around, looked at from various angles…
Do you remember any of the angles?
No, I would not know. Nothing of that really comes back.
And, so far as you can tell, the answer came gradually?
I don’t think very suddenly, but also not very gradually. I would say it was a matter of days rather than weeks until it finally crystallized. You see it is very difficult to tell anything about these conversations. This was late at night and it was to a large extent thinking out loud which I tried to stimulate by remarks and criticism. It’s very difficult to reproduce. This is the type of thing where if you knew beforehand that something important is going to come out, you would sit down and take notes at home on what went on.
The Stern-Gerlach experiments had already occurred, of course, before Stern came to Hamburg. And what’s famous about Stern at Hamburg is the work of the institute and the continuing molecular beam experiments. Does he himself get totally absorbed in that, or was he interested in a wider variety of problems than that?
I think that his main interest at that time were the molecular beam experiments. This doesn’t mean that he didn’t think about anything else. There were some problems of thermodynamics about which he was thinking, but his main interest then, very definitely, was molecular ray experiments.
One thing that’s puzzled me about those. I may be wrong. My own impression is that Stern’s earlier career was almost entirely in theoretical physics.
Yes, that is one of the remarkable things about Stern’s history, that he started out entirely as theorist, and suddenly became the experimentalist.
Now how much was Stern himself involved with the experimental detail?
Oh, very much…
Was there any major role, do you think, of specially good technicians, or something of the sort at Hamburg?
I would not say. I mean, he had, of course, assistants. Estermann was, of course, in some way the most outstanding one, but I think it was essentially Stern who set the thing and who thought out the experiments, and. this was not just the paper and pencil…
Did the whole group at Hamburg follow these beam experiments with great interest?
No, this was Stern’s group. You see Stern had his own group, actually, after a long while when he was housed in inadequate rooms. In the physics institute he got a building of his own, and this was his group…
Was there a good colloquium at Hamburg?
Yes, there were two. One was the general colloquium, and then there was a smaller one — the theoretical colloquium. This was a small group of people —
Well, some graduate students. Both were on an unusually high level, I would say. The debates in both were very lively.
How often did they meet?
They met once a week.
Each one of them?
Yes… The main colloquium was visited by everybody. The astronomers would come, very often the mathematicians would come. It was a large audience, so it was a little bit more formal, but the discussions were very lively; lively enough to make a graduate student, who gave his first talk, faint.
Were most of the talks given by graduate students?
Oh no, that was usually the faculty and assistants.
How often visitors from outside Hamburg?
Oh, not too often. I think there were — after ‘23, when the effects of the inflation were over — maybe two invited visitors a year. Although visitors — visiting theoretical physicists — were more likely to talk in the theoretical one. This did not mean that in the other one there were no theoretical problems reported. I remember one in which Pauli reported on Sommerfeld’s paper on the theory of metals, and tore it down completely.
On what basis?
Well, he thought the whole thing was wrong — which was, in general, a very good sign that something was very good. If Pauli tore it entirely to pieces he probably found out a bit later that he had overdone his criticism. The theoretical colloquium was an exceedingly lively discussion group. That was very violent. I remember one session when Von Neumann, who was visiting, wanted to give a talk. He never got beyond the first half sentence.
What was the talk on?
I forgot what it really was on. But after the first half sentence, somebody interrupted … with a question. Then the discussion started, and he never got to his formal talk again. That was the type of sessions these were. This was complete free-for-all. If anybody was dissatisfied with any remark he would speak up… This was the colloquium in which Epstein reported his solution of the crossfield problem.
You don’t remember how that hit Pauli? One wonders whether it may not have hit him fairly hard.
I don’t know that it hit him very terribly hard. Well, probably he felt like anybody feels who did not see the solution for a problem.
You talk about this colloquium, and of course I immediately think of the great many things that right in this period could have been discussed. What about the Compton effect, for example? This comes right in at the very beginning of your time at Hamburg.
Yes, the Compton effect actually was reported first at Hamburg by Sommerfeld. Sommerfeld came back from a trip to this country and told the story. That of course was a very exciting, new thing.
Was there skepticism about it? Sommerfeld was not skeptical, he thought it terribly important from the start.
No, I don’t think there was any skepticism, because the mere fact that Sommerfeld reported it as something important.
Did he report it already with the Compton-Debye theory? Was this photons?
I’m not quite sure.
What about the De Broglie papers, was anybody talking about these at Hamburg?
No, I think the Dr Broglie papers were largely neglected until Schrodinger. I think the attitude toward the De Broglie papers was largely one that this was a little bit abstruse to handle these things, and that nothing would come from it.
What about the photon generally? I have no sense yet of how seriously different people took the notion that light has particles. Do you run into this idea? Are people talking about it, at Berlin, Breslau, Gottingen, Hamburg?
No, I would not say that this played a great role in discussions. It may have come up, but not a very great role — until Schrodinger.
There again it was just in the background. What about, well, electron spin. Do you remember who reported that?
I have a slight feeling that this was one of the things to which Pauli’s first reaction was that it was wrong.
Yes, it was. He was in fact the longest hold-out. But do you remember why? Do you remember other people’s reactions, your own reaction? I mean, this was in your field.
The thing is, my own reaction was probably quite a bit influenced by Pauli’s, and I really don’t remember too well how that developed.
What about Bose’s paper, the Bose-Einstein statistics, and the photon gas? This is again ‘24-‘25.
‘25, yes. I don’t think that this caused say any particular discussions. So far as I remember, I’d say there were some discussions about it. This was of course a thing in which Stern was much interested, but I don’t think that there were particularly violent discussions about it.
What about Einstein? Was he already felt, perhaps, to be out of date, a member of the older generation who was not going to contribute anything more fundamental. Or did one still grasp his every word, every paper?
I think that very largely the feeling was that he somehow did not have the right contact with the quantum problems. I remember, not in detail, one session of this theoretical colloquium when Bohr was there. A discussion arose having largely to do with questions of causality and non-causality. Einstein’s and Planck’s feeling was that there should be some causal connection behind it. I think, on the whole, the feeling was that this was not recognizing the true nature of the problem.
Have you any notion when that colloquium would have been?... Was it before the new quantum mechanics?
I think that was after; I think that was after.
But probably before 1930? Or was it perhaps even after 1930?
No, I think it would be before 1930.
You speak of Einstein’s having the wrong contacts. What had happened to Berlin?
I think it is largely a question of personalities. Neither Planck nor Laue were very out-going personalities, and Einstein had become somewhat involved in politics.
Already? How early was Einstein involved in politics?
I think the, worst political talk I ever heard was one that he gave at some election meeting in Berlin in 1918… I mean it was a very good talk. It was not long, it was very good. But politically entirely ineffective.
This would have been in favor of some candidate?
This was really in favor of democracy and the democratic list of candidates. Einstein was a bit interested in politics then, and of course there were political attacks on him through that time. Somehow, what happened in Berlin, I think, is that there was no real group. It. had become too large and there was no personality really who drew the thing together.
Now we talked earlier about ‘22, and your pretty, strong feeling that people were not yet feeling the need of a renovation. By ‘23 some of them are. Does that come out in the colloquia — the sense that it’s now off the tracks?
Yes, well. That sense starts, of course, the moment you make up your mind that there is something very fundamental to be changed.
Exactly. Who does that, and when do they do it? How do other people say, “no, it will come out, if we just keep working.”? You see, I think we agree entirely that that’s the situation. Now I’d love to be able to illustrate it, to give it more structure and to illustrate it.
Now this is very difficult to say. The people working on it in that period were in very close contact, and I don’t know who first definitely came to the conclusion that there was something fundamental wrong. However, from my recollection I would say that it was a fairly sudden transition. I would say that some time in ‘25 or so, maybe early ‘24, the general conviction was that there was a fundamental problem involved — not just a technical problem. Up to that time it was a technical problem. I think that must have been sometime late in ‘23 or ‘24. It must have been a fairly sudden transition, because I don’t remember any great debates — one person saying that there is something fundamentally wrong, and somebody else violently fighting that one could not say this yet. So I have a feeling that this was pretty generally recognized in a very short time.
Did people behave differently when they were convinced that there was something fundamentally wrong?
Well, this is difficult to say.... The time between the recognition that one has to deal with a fundamental problem, and the answers to it — Schrodinger and Heisenberg — was a relatively short time. And there were some other problems in that period which were exciting.
What do you think of when you speak of these other problems?
Well, these problems of the Pauli principle, the spin, and that type of thing. This fitted, somehow, in this interval, and gave people something to think about. Of course probably everybody wondered what could be fundamentally wrong, but somehow the solution came fairly soon.
How did you feel when the solutions started to come? You said that when Heisenberg came back from Helgoland and talked, people felt that it wasn’t all there yet, but that this probably was the answer. Did people get terribly excited about the Born-Heisenberg-Jordan paper?
Yes, I think that was a very exciting one, because there one had the feeling that we now really had made the step which was necessary.
You think people did not get this from the Heisenberg paper?
Well, probably many did.
Then the Schrodinger paper is of course not terribly long after that. Did that draw a different reaction, was it more exciting?
It was probably individually different… I would really not want to say from recollection. It certainly must have played a role in influencing Stern, for instance, to try and make diffraction experiments with atomic beams.
There’s one question about education that I didn’t ask you when we talked about the curriculum at Breslau. By the time you did your mechanics, was Hamilton-Jacob theory and contact transformations already a standard topic?
I don’t know how much it was a standard topic, but we certainly got it.
And this would have been from Schaefer?
Yes, yes. I don’t know how unusual this was. Whether this was standard at that time or not I don’t know. … A rather interesting; incident historically is that in Schaefer’s lectures on electro-dynamics, it still played a considerable role that light was electro-magnetic radiation, and optics a part o electro dynamics This is, I think, historically rather interesting. At the time when you already have quantum theory from Planck for more than 10 years, in lectures on electro-dynamics this still is an important point.
But there wasn’t any other approach, was there?
No, no, but I mean this is rather interesting, how long this — as a problem — stayed alive in lectures on theoretical physics.
Did Schaefer present it as a problem? Did Schaefer say there obviously must be something the matter with this because of the quantum?
No, no. It was still just the attitude that this was something remarkable, a somehow relatively new problem…
And one didn’t quite know what to do with it. Did the quantum come into the regular theoretical physics course?
The quantum? No. Not at all.
Not at all into the regular physics course. So you might have had special lectures on this, but it would not have been part of the curriculum.
Not in the theoretical physics course.
Even with four semesters?
No, I don’t think that it came in any way to speak of.
How about relativity?
Well, that came in on a very elementary level.
That does interest me.
That of course in 1913 was still a very new thing.