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Oral History Transcript — Dr. Otto Laporte

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Interview with Dr. Otto Laporte
By Thomas S. Kuhn
At Ann Arbor, Michigan
January 29, 1964

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Otto Laporte; January 29, 1964

ABSTRACT: 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 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: Joseph Sweetman Ames, Ernst Back, Niels Henrik David Bohr, Max Born, Gregory Breit, Louis de Broglie, Constantin Carathéodory, Eugene Charles Catalan, Peter Josef William Debye, Paul Adrien Maurice Dirac, Paul Ehrenfest, Albert Einstein, Walter M. Elsasser, Walter Grotrian, Werner Heisenberg, A. S. King, Rudolf Walther Ladenburg, Alfred Landé, Max Theodor Felix von Laue, L. S. Ornstein, Friedrich Paschen, Wolfgang Pauli, Arthur Pringsheim, A. Schoenflies, Erwin Schrödinger, L. A. Sommer, Arnold Sommerfeld, Otto Stern, Merle Antony Tuve, Gregor Wentzel, Wilhelm Wien, Robert Williams Wood; Universität Frankfurt, Universität Göttingen, Rijksuniversiteit te Leiden, Mount Wilson Observatory, Universität Munchen, and Universität Munster.

Transcript

Session I | Session II

Laporte:

Now you want me to talk first about pre-University interests that started one to get into sciences at all.

Kuhn:

You asked me at lunch if I weren't interested in this transformation of American physics; I said yes to the extent that I talked to people who were involved but not as a central concern. Certainly one of the things I want with some regularity in any country is how the people came into the sciences, what their notion of a scientific career was, from what sort of families people were drawn into the sciences. This is interesting material; it illuminates, for example, certain differences, I think, in the French-German situation which one discovers this way. So if you're willing to do it, I'd really be very glad if you'd start by saying, "I was born in such and such a place, my father did such and such, and it was odd for me to be a scientist, or it wasn't—"

Laporte:

My family is a Huguenot family, French Huguenot, and they fled from France to Switzerland in the late 17th century, then stayed in Switzerland and were finally allowed into Prussia by Frederick the Great, practically for the purpose of becoming civil servants; so my antecedents have all been civil servants in the Prussian state. My father was an officer in the Prussian army in the particular field of heavy artillery; that means that I was born in Mainz which was a heavily fortified city, I had my early schooling in Cologne which is a heavily fortified city, and my father was shifted back to Mainz once more and from there to Metz, similarly heavily fortified. Then the First World War broke out and we were evacuated and, went back to Mainz where my mother's family lived.

The second time in Mainz I had, I would say, my most formative years. I went through the later stages of the gymasium and it was then that my interest in science was particularly stimulated. What first got me interested in science is the usual small boy's desire to build some gadget or other, then I read up on them to see what I was doing and why I was doing it, and I got pretty far with that; this was accompanied by comparative excellence in the gymnasium studies in physics and chemistry, although not particularly in mathematics because in mathematics they emphasized the numerical to such an extent that I became thoroughly sick of it. We got five-place logarithms at a very early age and for years we had to do numerical examples of plane and spherical trigonometry with logarithms, interpolating to the fifth place, and I found that extremely boring and galling.

Kuhn:

Was this a particular characteristic of this gymnasium?

Laporte:

No, it was general. If you had asked Heisenberg he would have said the same. I don't mean that he may not have enjoyed it more than I did.

Kuhn:

He did talk a bit about his own mathematical interests and education. Now it may be that a good deal of the time he was simply working in this field beyond the class, working by himself, but this heavy emphasis on computing and numerical work did not come out. This particular as pect of German mathematical education had not struck me before and I would not have realized it was a general thing.

Laporte:

I used to read occasionally in Felix Klein's Elementare Mathematik vom höheren Standpunkt and I would realize with indignation how little the teaching which I received was touched by that. What we had was not from a "höheren Standpunkt" but from the lowest Standpunkt possible, so that was no fun.

My interest in the sciences became so great, coupled, you see, with good grades in physics and chemistry, that I once asked my mother—my father was in the wars—if I couldn't get as a Christmas present a series of special lessons from our physics teacher in more advanced physics. I used to occasionally have to have private lessons so as not to get too lousy grades. My special bates noires, as I said, were mathematics and English; in those two fields I sometimes had to have private lessons. So I was accustomed to traipsing off an afternoon to the home of some retired teacher and getting some extra lessons. In those days, you know, not to have been advanced to the next class would have been a terrible disgrace to myself and to the family, so the family prestige was involved. So I got these lessons and I approached my physics teacher who, I knew, had a degree from the University and so on and he and I read for several months certain chapters in Kirchhoff's lectures on theoretical physics; they were much too difficult but it gave me a sort of a sniff of what theoretical physics could be like.

Then there came another using sort of impetus in the direction of science which you might be interested in. My father by that time was a somewhat senior officer in the field of heavy artillery and he was given a special appointment, namely, to be in charge of the German sound ranging, to locate the enemies' guns from the noise of the muzzle detonations; so after having gone to a special series of courses in Berlin, he came back on a few days furlough and told me that he had had this new application of science to artillery, something brand new, and that he now had as his special charges two scientists, one Max Born and the other one Rudolf Ladenburg. Those were his special people; he had to see that their ideas would be put into practical use.

So I knew Born and Ladenburg when I was perhaps only sixteen years old. Now every boy had heard the name of Einstein and Gauss and so on, perhaps even Riemann, though I am not so sure, but certainly Born and Ladenburg were sufficiently unknown; they were also fairly young, you see. My father knew them quite well. I might say—now I'm skipping ahead—that in 1940 I was in a long symposium with Ladenburg and he recalled all that very well. My father had then been dead for a long time end [Ladenburg] said that my physical similarity to my father was

Laporte:

(cont.) so great that he was really quite transported back into the old days. So I don't know whether the fact that my father and Born had worked together during the war had anything to do with my first semester at Frankfurt; that may just have been an accident. We lived in Mainz, you see, it was the post-war period, and it was economically very restricted. It was the beginning of the inflation and so it was the cheapest and most immediate thing to take university courses at Frankfurt and commute back and forth, which is what I did.

Kuhn:

That was '20-'21 then?

Laporte:

That was in 1921. I got my gymnasium degree, the Abitur, in the spring of 1920 and started right away taking courses.

Kuhn:

By the time you got the degree how wide a range of science subjects had you had and to what level?

Laporte:

The education in physics and chemistry in the asium was really quite thorough; it could well be compared with the general physics and general chemistry undergraduate courses here. The only thing that was perhaps a little different is that over here the numerical is somewhat more stressed; that is to say, the problems with numbers in them, and also because of course, the system of units have still more proliferated in the English speaking countries. Our students spend a tremendous amount of time just changing from one system to the other, with or without g in it, that is, hiding the essentials of the phenomena behind transformation of units—a well-known sport of the undergraduate teacher. So there wasn't quite that much in the German gymnasium but we learned a good amount. They drilled us on Ohm's and Kirchhoff's laws in the gymnasium and certainly geometric optics; I knew geometric optics better than they know it here when they've gone through the general physics course.

Kuhn:

How far had your mathematics gone by then? Had you had any calculus?

Laporte:

I hadn't had any calculus but my older sister was going to a girls' gymnasium in the neighboring city of Wiesbaden. There was no girls' gymnasium in Mainz; they only had schools there. And for some reason, their mathematics went a little further, so I studied calculus with her, strictly in application to analytic geometry. We took the typical "pragmatic" engineering approach to calculus which, indeed, is what we use in physics. We had enormously careful training in trigonometry and spherical trigonometry and algebra, of course. Mathematics was rich more static and uost ossified than the physics. That may be due to the individual teacher; I don't want to generalize on that. As I say then, my own knowledge had gone so far that I could understand, let me see, Gauss's and Stokes a theorem by the time I got there.

Kuhn:

What were you reading, since you clearly were doing a good deal of work by yourself?

Laporte:

I was reading textbooks in calculus; I was doing such things as find ing the envelope of a straight line whose two ends can move on the positive x, positive y axis and things of that nature. [interruption]

Kuhn:

We were talking really about curriculum and you were telling me about the mathematics. Just a couple of other things on this pre-University period. To go back to your interest in gadgets, what sort of things were you interested in? Did you actually build them?

Laporte:

Yes. Many of them, in typical boy fashion, I just started and didn't finish. I once started a very ambitious project of building an enormous spark coil, but I never finished it; I became too impatient about winding the damn thing.

What I did do was a considerable number of experiments with optics, with lenses, image formation, spectra. I read in the Ostwald's Klassiker, I read Newton's original papers in German translation and I repeated some experiments. Why I chose Newton rather than going to a better book of optics I don't know. I never had enough dispersion from my prisms, that was one of my great sources of grief. You see, it was during the war and you couldn't really buy anything and everything was under military restrictions and so on, but my father slipped me a 45-45-90 degree prism once from a trench periscope and I used that, but of course it didn't give me much dispersion and I was quite unhappy about that. I tried to make prisms, hollow prisms with carbon disulfide, for which I used plate glass and glazier's putty. The carbon disulfide always dissolved the glazier's putty and a great stink would sweep through the house.

Then I once built an electric arc which was a great success; I really produced a very intense light source. I used a resistance and winding of the resistance was a most time-consuming thing but then I found out I could use a tub with a saline water which would do just as well; and by not putting the one electrode too close to the surface of the water I was able to get that Wehnelt interruption effect, a discharge right there between the water and electrode and with great sputtering a humming would start in the arc and so on.

Kuhn:

These are all fairly elaborate; what had you done before you did these?

Laporte:

Before that I inherited from a cousin who was killed in the war in the trenches one of those electrostatic machines with oppositely rotating discs and I did many experiments with that. I placed my kid sister on four champagne bottles with a board on top and charged her all up— things like that. But that is really more playing than experimentation. I did do a little bit of work too, as a high school kid, with wireless telegraphy from one end of the room to the other but I never had power. You see, in this country the children can do that so much better; I had to build every doggone thing myself, including the spark coil—a little spark coil I built and that's what inspired me to build the bigger spark coil.

Kuhn:

Did your family encourage this?

Laporte:

I told you the history of the family briefly. I was the first scientist We've never had anyone like that in the family; they were all people who entered the civil service or the foreign service or something like that, so they let me do it, just out of sort of a friendly indulgence.

Kuhn:

Did they resist your decision for a scientific career at all?

Laporte:

No. No, my mother died just about that time, the end of the war, and my father then was the sole head of the family. Then, I suppose, the fact that he himself had worked with Born and Ladenburg caused him to see that there was something in this. But, of course, you know that there were no scholarships. When I went to the University it was clear that he would have to bear the cost of the whole thing and both my sister and I went through the University without a penny of help from any foundation or government.

Kuhn:

Was she older than you or younger?

Laporte:

She was older, but I also have a younger sister who did not go to the University.

Kuhn:

What did one think of at this time as the likely outcome of training in physics?

Laporte:

That's a good question. To me it was always clear that I wanted to be a university professor; that was the thing. The unmentioned fate, of course, might be that if you couldn't make the grade there you would have to be a gymnasium professor—the same thing; after all, Riemann was that and so was Grassmann; Grassmann, I think,never made it to a university. Sometimes nowadays one sees comparatively elaborate papers, " The Yearly Program of the So-and-So Gymnasium," around 1870 or '80 or '90. I don't know.

Kuhn:

What sort of confidence could you feel that there would be a university job?

Laporte:

I don't know; I never gave it very much thought.

Kuhn:

Do you expect that you were influenced at all by the fact that some new universities were opening up?

Laporte:

I'm not sure whether that was an influencing factor.

Kuhn:

Frankfurt itself was relatively new.

Laporte:

Frankfurt was new, yes, and Cologne was starting: those were just two universities. It's hard to say. I suppose I went there with sufficient enthusiasm and if I had been a failure very soon, I suppose I would have had to give this some additional thought; as it was, fortunately, through circumstance and Sommerfeld's friendly advice, I was not a failure there right away, so it was all right.

Kuhn:

You start out with this great interest in building things; you do a lot of experiments at home, some of them very elaborate. You go, though, to Sommerfeld and your career is pretty much in theory. Was there a conflict here? Were you always clear that you wanted to be a theoretical physicist? How did these two sides of things relate?

Laporte:

I think it is due to the fact that more formal teaching goes on in theoretical physics. Experimental physics is not really being taught except on a much lower level. When I went to Frankfurt, right away I was under the influence of some very great men. The oldest one there was a mathematician named [Arthur] Schoenflies whom you know, of course, as the man who first formalized the theory of the space groups of lattices, but who has many, many other great achievements. He was then an old man, but he gave beautiful courses. And then there were two younger mathematicians; one was the notorious Ludwig Bieberbach and the other, a small wizened man named [Ernst] Hellinger, to whom the theory of the eigenfunctions with continuous quantum numbers and their normalization and orthogonalization is due, the eigendifferential. Those are the mathematicians who taught.

Kuhn:

You went to lectures by all of them from the very beginning?

Laporte:

Yes.

Kuhn:

You were at Frankfurt for only one year, yes?

Laporte:

Yes. Then in physics I took the lectures of Born and Lana; Landé was then a young Privatdozent. And from Born I took heat, which was what he was doing in his cycle. Now it should perhaps be said at this moment that it was a great disadvantage of the German system that you don't get any advice from appointed advisors. I just took the courses whose subjects interested me and they were all much too difficult. Oh, I forgot one mathematician: [Otto] Szász who came to this country as a refugee, as did Hellinger and many others, of course. S-Z-A-S-Z, it was, pronounced Shash—a Polish name. He was a wonderful man and from him for the first time I got an introduction into mathematical rigor. He gave the course in differential calculus and he gave it as the mathematicians like to give it; that is, with a great deal of set theory and so on. I must say that I had my doubts about that right then and there.

Kuhn:

What did you take with the others? You went to more lectures than most people do in the first year at the university; this is a big heavy dose.

Laporte:

Yes. From Schoenflies I had differential geometry and that's the course I enjoyed most. 'By diligent working, reading and so on, I kept up that course completely and I got a great deal out of that. From Born, as I say, I had heat, which was first heat conduction and then became thermodynamics; that was already a little bit too difficult for me. He introduced the second law of thermodynamics with the Carathéodory method and I must say I found that very tough going. Then Bieberbach had a course in theory of functions of a complex variable and of that I only understood the surface. Now if I had been in the clutches of some sufficiently energetic advisor he would have just told me not to take these courses, but I wasn't told that; I was completely on my own.

Kuhn:

Did you have Born then for just one semester?

Laporte:

Yes.

Kuhn:

What did Landé talk of?

Laporte:

Certain chapters of Maxwell's theory, radiation theory and so on. Is this confidential?

Kuhn:

It can be.

Laporte:

This is confidential now: it was a lousy course. He was not very clear; I think he was learning it himself. There is one thing I can say in summarizing about Frankfurt: either the physics group in Frankfurt were not as much interested in quantum theory, or they kept that interest hidden from the beginning students; we never talked so much about it, we never got into it. Now in Munich that was all different. Even the youngest graduate student was right in the thing.

Kuhn:

Was Otto Stern in Frankfurt at this time?

Laporte:

Yes. I'm not quite sure whether he was at Frankfurt then or later.

Kuhn:

I think he left shortly after this, but you didn't take a praktikum from him this year, an experimental physics course?

Laporte:

No; that was because the chairman of the department of experimental physics was well-known to be a dud—his name was Wachsmuth and I was warned against him. I intended to take that in Munich and I did, under (Willy) Wien.

Kuhn:

So you intended really from the start to go to Munich after the first year, did you?

Laporte:

I guess so. I guess we had to work on my father; it meant that the whole family would have to leave, moreover. You have no idea of the economic difficulty to which people had to go.My father was then a pensioner from the army and we weren't particularly rich. My relations with Born were of the best; he occasionally gave me some little extra problems which I did all right, and when I told him we were moving to Munich, he said he would speak to Souuerfeld about me, which he did, even though I was a sheer beginner. Sommerfeld knew about me when I came and that was very pleasant, and I had no difficulty as far as getting into things was concerned.

Kuhn:

In the summer of 1921 the whole family packed up and moved to munich.

Laporte:

This was very difficult.

Kuhn:

Was your father by then retired from the army?

Laporte:

Yes. He had to retire in 1918 with the breakdown of the German Empire. He was then pensioned and things were pretty tight.

Laporte:

(cont.) Our apartment was taken over by the French occupation and we were literally without a home and had a great deal of trouble getting the permission to move into Munich. You had to have permission because of the great housing scarcity, so we got this permission and moved to Munich—the whole family.

Kuhn:

How did you yourself pick Munich?

Laporte:

There are two things: first of all, it depended on the permission to move in. We thought of Heidelberg and my father tried to get into Heidelberg as well as Munich, but I took a very dim view of Heidelberg. In Heidelberg there was a great experimentalist but his things did not appear to me particularly spectacular—I was a beginner, you see. That was Lenard. And of course, I was fortunate since, by golly, he is certainly one of the most disagreeable and pathological persons of the century.

Kuhn:

Yes. You would not have been a figure in quantum mechanics if you'd gone to Heidelberg.

Laporte:

That would have been true later on too; it would have been a curse to have been in any way associated with him ; and also the theoretician whose name I cannot recall was a complete creature of Lenard's and I knew that, so if we'd gone to Heidelberg, it wouldn't have been good for me. I might have studied a semester there and then gotten away independent of the family.

Kuhn:

Did you think of Göttingen at all?

Laporte:

Yes, I thought of Göttingen very much but it was not ever possible to move to Göttingen. So we moved to Munich and it was certainly wonderful. Right away I took part in a seminar; we had an organization meeting and Sommerfeld said, "Oh, yes, here's that young man about whom Born talked. Now there is a fine paper by Einstein and de Haas and I'd like you to talk about that." I just sat there trembling, having no ideas, never having seen the paper, never having heard of the whole subject; but I had to talk about it and the person who helped me understand this paper was Sommerfeld's personal assistant, Pauli. I don't know how much more you want me to reminisce.

Kuhn:

In this area and this period and these people I want you to reminisce just as fully as you can.

Laporte:

I see.

Kuhn:

Sommerfeld's seminar and the whole Munich curriculum and relations with these people had such a bearing, and the people individually and collectively are of such importance, that anything that will establish either atmosphere, detail, things that were said in the seminar, what the questions were—any of this is altogether fair game. Now I'd say you do have a remarkable memory and to the extent that you will do it, I'd like anything; anything you've got I'm interested in.

Laporte:

This first seminar in Munich was on magnetism and so I led off with that talk on the Einstein-de Haas effect.

Kuhn:

You had had no electromagnetic theory really.

Laporte:

I had had no electromagnetic theory and I knew that if I ever said this to Sommerfeld—. He used to frequently say, "We're going home now," so that meant that I had to walk with him. We had the same road for about one mile and he walked on a little further. When he said, "Now what courses are you taking?" and I would tell him, he frequently would say, "That's all very well; I prefer, however, that people take intensive training rather than extensive training." You see, he was not opposed to people's having a rather irregular course program and picking it up when they could. I forget whether I said to Pauli or to Wentzel or to Sommerfeld that I hadn't had this course but I'm sure it wouldn't have made any impression. Now you want to know who was there, of whom the Munich school consisted?

Kuhn:

Yes.

Laporte:

Physics was extremely lively while mathematics was comparatively sterile, although there were great names. I'll start with mathematics. In mathematics they had all the people who sort of publish in the Enzyklopädie der mathematischen Wissenschaften, circa 1910. There was [H.] Lindemann, you know, with e and pi, and Voss and Perron of the continued fractions; those are all great names among the mathematicians. In astronomy we had the great Seeliger and in physics, on the other hand, besides Sommerfeld there was, on the experimental side, Wien—a separate institute. In Sommerfeld's institute, of course, the senior in was Herzfeld with whom you've talked, no doubt, and Ewald. They had regular positions; they were "habilitiert". And then as assistants there were Wentzel and Pauli and another man named [Heinrich] Ott who later on went to Würzburg, a crystal man. Sommerfeld always had a little crystal tradition which he always tried to keep up since Laue, you see.

Kuhn:

How large a student group was it?

Laporte:

It was in the immediate post-war period and for many of the boys, to go to the university was the only thing to do. There were no jobs, so people went to the university. Sommerfeld's big lectures were quite crowded but there was a great deal of dropping out. There were no exams and you just came when you wanted.

He often had around 80 or 90 students in a big lecture and he would have one big lecture with exercises, too, an extra exercise meeting, which was usually conducted by an assistant and Sommerfeld would come in toward the end. They took that very seriously. Then he had an additional course on some special topic, usually of atomic spectra or something like that, and then there was also a seminar. Then more basic courses, perhaps of a slightly more special character, were given by the younger appointees, the extraordinary professors like Ewald and Herzfeld. Ewald gave a course on theory of radiation and things like that. I took many of those courses.

Laporte:

(cont.) I forgot to mention among the mathematicians—I mentioned that mathematics largely was sterile—one man there,although Sommerfeld disapproved of him very much, was a fascinating old codger, 80 years old. You see, they had no retirement in those days. That was [Alfred] Pringsheim, I think, the father of the physicist [Peter] who was later at Berlin; he was also the father-in-law of Thomas Mann. A very wealthy man: he had one of the finest collections of Italian majolicas in the whole world, Renaissance majolicas, the beautiful Renaissance pottery plates and vases you can see in the Metropolitan. He was a function theory man of the most rigorous Weierstrass type, but such a fascinating lecturer that I was completely captivated; and Sommerfeld disapproved completely. He said to me, "Don't do Weierstrass; only do Riemann!" and it was absolutely true, of course. Sommerfeld was completely borne out. While this sort of has cultivated my mind and provided general information, it hasn't done me any good at all for physics. Pringsheim occasionally spent a half hour just to poke fun at the notion of the Riemann surface and such things.

We never had an integral; we only did it with mean values and things like that. We had only infinite series and so it was very peculiar and Sommerfeld disapproved highly of that. While I was there we had a couple of retirements—Pringsheim and Lindemann both retired—and Sommerfeld made himself rather disliked, I heard, among the faculty in the Senate by wanting to see that a physicist's mathematician would be called to Munich. But he succeeded in that and Carathéodory came, but he came just about the moment I got my Ph.D. degree and all I got out of him ever was that he examined me in my final doctoral examination; from that there sprang a friendship which we kept up until he died. A wonderful man, Carathéodory. I know he did both mathematicians' mathematics as well as the other type. A great fellow. Let me see, shall I speak a little bit more about the—.

Kuhn:

Well, there are any number of lines you can take, but I do want you at some point to say more about the seminar, about this first semester on magnetism and how that went on from there. I'd like to know at what point, for example, you first read Atombau. Had you read that before you got to Munich?

Laporte:

No! This is very funny—disgraceful in a way. I can almost still remember beginning my presentation of the Einstein-de Haas paper, trying to reproduce, I shudder to think how crudely, the introduction of Einstein and de Haas on the ampere currents and the conclusion, that with sudden magnetization there would have to be imparted an angular momentum to the body as a whole and how that was inescapable. I talked about that and Sommerfeld said, "Of course, if this were a gas then we would replace the ampere current by the atom model of Bohr." It was obvious, as he put it. I just nodded feebly! I had no idea about the atom model of Bohr except that it existed, you see, whereupon I very soon hot-footed it to the nearest book seller and bought Atombau und Spektrallinien which was already the second edition.

Kuhn:

What did it feel like when you read it? Was it all new to you?

Laporte:

Yes, I think so. This is somewhat vague, however, whether it was the first or second semester, but at the same time Sommerfeld also gave a course on quantum theory of the atom and so on; of course, he would explain this and extremely clearly.

Kuhn:

You don't conceivably still have your notes on that course?

Laporte:

No. He had absolutely a wonderful clarity of presentation and he enjoyed it too, totally different from what we often see nowadays where a person doesn't mind being known as a poor speaker; although we had those too in Munich.

Kuhn:

In addition to problems for quantum theory about the magnetic properties of materials, there are other problems that clearly people were upset about. For example, the gases in particular didn't behave in the way one thought that they ought to behave. Did this get a lot of attention in the seminar? Was this a source of real trouble?

Laporte:

I remember that the greatest sources of trouble in discussions were always those which had to do with radiation. That is where I remember distinctly, hour-long discussions.

Kuhn:

Now tell me what sort of problem you mean here? There are various ways that this can be a problem.

Laporte:

First and foremost was always the question, "Why doesn't the electron radiate while it is in the Bohr orbit?" Secondly—

Kuhn:

One's impression from Sommerfeld's writing is that he himself did not worry terribly much about this, but that may be quite wrong; I mean, that may simply be the way it comes through in his writing and I'm curious to know who in fact really worried hard about this problem.

Laporte:

I think he worried about it. He had a deep interest in aperiodic problems, too. You know, everybody has certain domains which they think are their own and they want to work in them pretty undisturbed and an example of that was Sommerfeld's never-dying interest in the continuous x-ray spectrum, not the discrete x-ray spectrum. The discrete x-ray was paid much attention to and was being largely cleared up by the efforts of Wentzel and others.

The experimentalists in the North country, Siegbahn, the older Siegbahn, of course, and Wentzel and many others worked on it and gradually that funny recognition came that the x-ray spectra were all like alkalis, which we found very hard to understand, of course. But Sommerfeld was always interested in the continuous spectrum and I remember one of those two-hour-a-week advanced lectures in which he gave us his own theory, I think, going back to about 1911, of the electron which is moving and losing its speed very rapidly and then the advancement of the maximum, these lobes which shoot forward if you do it with the relativity correctly. Then he said the first part of the semester would be devoted to the classical theory of the radiation of a decelerated body and the second part of the semester would be devoted, with Dr. Pauli's help, to the quantum theory. They worked very hard and finally had a few lectures on that. The attempt was to quantize the hyperbolic motion.

Kuhn:

Here there is a question of date. This was still your first year, you think?

Laporte:

No, that was perhaps the second, 1922. The fact that the Fourier decomposition of a classical motion had something to do with the radiation was clear because Bohr's correspondence principle was always being used and, of course, the fomula from Hamiltonian mechanics— what is it—if you express the energy as a function of the phase integrals only, the integral pdq, for a periodic system, H as a function of all the J's, then the partial of H with respect to a J gives you omega, and that's the classical analogue of the Bohr frequency relations; we made a great deal of that and that was regarded as something very, very sacred. As I was saying, the respect which Bohr's publications aroused in Munich was just unanimous. Sommerfeld had the greatest respect for them. In later years I understand the relations between Sommerfeld and Bohr were not good but whenever I was a witness to it there was just great respect and recognition.

Kuhn:

Yes, it's fairly clear, just watching successive editions of Atombau, that, although there was no question that there was great respect, their approaches were often quite different.

Laporte:

Yes, oh yes, they were; they were decidedly different. Bohr was almost fanatically opposed to using mathematics, any kind of mathematics, and his son has continued that; I mean, that's a definite style they have. Whereas Sommerfeld tried to solve the problems, hoping to be guided by beautiful mathematics, Bohr brushed that aside as formalism. There isn't any doubt that Sommerfeld was much more of a mathematician than Bohr ever would be. Sommerfeld, you know, quantized all the motion by using the phase integral condition for every degree of freedom; this gives you the right results always but, as you know, there is the phenomenon of degeneracy already in the phase integral quantum theory. Somehow Bohr always held that against that method and he tried to get all of the results by coming up with only one quantum number where others had the sum of quantum numbers.

Kuhn:

Was that still a real issue at the time that you were doing that with Sommerfeld?

Laporte:

Oh, that was a real issue.

Kuhn:

On the other hand, it's also clear that Sommerfeld in this same period, or just at the beginning of it, is still trying to stay away from the correspondence principle. You do see this very clearly, I think, in the very slow pace at which it creeps into Atombau and still in the third edition, the 1922 edition I guess, where the main treatment of the correspondence principle is in one of the appendices and Sommerfeld tries to get selection rules for example out of the Rubinowicz treatment and will go to some lengths trying this thing. And he'll say, "Bohr gets this and can go beyond it by his very powerful methods," but he won't elaborate; he'll still try to do everything the other way. The fourth edition is rather different and I wondered whether there were tastes of that?

Laporte:

No, I never thought that he was so opposed to it. If you want me to give you some piece of memory about the interplay between Bohr and Sommerfeld, somewhat later Bohr had an article in Nature, I believe it was, and he had sent Sommerfeld a typescript of that or it had reached us somehow. He came out with new occupation numbers of the various shells and that, I think, appeared first in Nature. I remember sommerfeld treated that as though suddenly a curtain had been removed; that was regarded as something quite great. As soon as you saw it you knew it had to be right; where formally the shell of eight was subdivided into four plus four, Bohr subdivided it into two plus six, and this of course was then the forerunner of the Pauli exclusion principle. I remember that that was regarded as the thing. He always admonished us to read the Bohr papers which Bohr published in the Danish Academy.

Kuhn:

"The Quantum Theory of Line Spectra."

Laporte:

Yes, and I found those very painful to read, not only because Bohr is very painful to read anyway, but also, of course, my English wasn't good. I told you English was my bête noire in the gymnasium. No, he had no feeling in that respect—I think.

Kuhn:

That's very interesting. You speak of the concern with radiation problems and describe one of them. Was there any concern with the photon wave problem? Here of course, if you can, the difference between your first year and second year may be important because the Compton effect intervenes. If you can, tell me whether this fitted in at all before that End what the impact of the Compton effect in Munich was?

Laporte:

We were, of course, very much aware of the photon problem, and I can tell you one bit from memory. You know that So erfeld and his whole institute were always very much interested in hydrodynamics. The engineers even nowadays speak of the (Or Sommerfeld) equations, you know, for the stability of certain flows and so on. He induced Heisenberg to take his Ph.D. problem in hydrodynamics. So the relations with the Swedish school of hydrodynamicists was quite intimate; there was this very eminent applied mathematician named Oseen and Oseen once came to Munich and presented to us solutions of the Maxwell equations which he claimed had the properties of light quanta, that is, could give extremely concentrated radiation. I remember that we all went to this talk, listened to it with extreme respect, but it was not generally regarded as the way out of it because we always talked about Einstein's paper on the photoelectric effect. We had Einstein's paper of 1917 with the A and B coefficients, so I think we all realized that a purely classical solution could not be found.

Kuhn:

Was this a problem that was much discussed in the way that the problem of why the electron doesn't radiate was much discussed?

Laporte:

Yes, but let me get back to this previous thing. I said that Sommerfeld regarded the continuous spectrum as his thing, and he and Pauli tried to do something about the continuous spectrum quantum mechanically but the trouble was, as we know now, aperiodic phenomena couldn't be quantized. So they hoped desperately that the following scheme would work: You have an electron moving on an hyperbola, and then in a certain while a long-distance oscillator force takes it back again; then in some way, by cutting off and letting this oscillator force act later and later, they would arrive at a limit of quantizing a single hyperbola. That was the program. And Sommerfeld in a way was very naive, naive like Goethe, with the naiveté of the truly great man. He announced in class that they were going to present that and then it blew up! It didn't work. Pauli and Sommerfeld worked very hard and it didn't work. I was never led into where it blew up, but it finally was announced that that would have to be dropped.

Kuhn:

That's fascinating.

Laporte:

But it shows that Sommerfeld, the man who solved the most rigorous boundary condition problems, his diffraction on the straight edge and things like that, was not afraid to try the most radical extensions of this scheme, bringing in an oscillator force and then letting it come back less and less, so to speak.

Kuhn:

This is the sort of thing that nost nobody remembers. This is what I mean when I say that if you can do this thing this way I'd like just as much as you can remember about this period and what was going on in Munich.

Laporte:

Oh, that's just a single instance.

Kuhn:

I don't expect that anyone will say to me, "And the next day he came in and we did this," but this is really very good and just the sort of thing we're looking for and much too rarely get. Was there great excitement about the Compton effect?

Laporte:

Oh, yes; yes indeed. God help me if I tell you something wrong, but it seems to me that Wentzel and I were looking at this paper of Debye's in the Physikalische Zeitschrift and Heisenberg wandered in. I said to him, look here—a remarkable thing—the light quantum falls in and is scattered away and the wave length shift contains the cosine of the angle." "Oh," he said, "Look, here it's red and over here it's blue; it must be wrong!" He translated it, you see, into very concrete visible terms, clearly just to be funny, but—. Of course, like so many young people we were quite intolerant of so many things; an expression like, "Schöne,aber falsch," was quite common.

Kuhn:

In this case, did people pretty generally accept this very quickly the Compton effect?

Laporte:

Oh, yes, it had to be accepted. I forget exactly whether the Schrödinger paper on the Doppler effect came very shortly afterwards.

Kuhn:

It's not long, but I'm not quite sure how quickly.

Laporte:

Yes, it fitted in. Are you aware of the Schrödinger paper which I have always regarded as particularly miraculous? It's in the Zeitschrift rather than in the Annalen. He takes a circular orbit, I believe, a circular ball, and he says that the electron moves with this speed, therefore there is this de Broglie wave length associated with it. He fits the de Broglie wave length into the orbit and it closes onto itself; I remember that that impressed people. Everybody shrugged their shoulders and said, "Ah!"

Kuhn:

This paper I'm not aware of, but when is this?

Laporte:

1924, before the first Annalen paper.

Kuhn:

It's before the first Annalen?

Laporte:

I think so.

Kuhn:

But he does this in terms of de Broglie's wave length?

Laporte:

I think so, yes.

Kuhn:

Because, you see, de Broglie himself also does it in terms of de Broglie wave lengths. Let's put it this way, it's not quite so simple a treatment as one later uses, but de Broglie does derive the phase integral condition from constructive interference of the waves.

Laporte:

Well, I will have to look to see just to what extent that differs. Well I hope I'm not giving you anything that's wrong.

Kuhn:

If the paper doesn't exist, we'll find out what other paper you're remembering but I have not had time to search. Very likely there is such a paper; I don't mean to be challenging your memory on it, but it interests me and I really don't know it at all. In this general area, was there any attention paid to the little note by Elsasser in Naturwissenschaften, the one that picks up the de Broglie material through Einstein and relates it to electron defraction before it's recognized? Do you remember discussions of that in Munich?

Laporte:

Yes, but now my memory is not very good on that. I believe Elsasser was in Munich.

Kuhn:

He may have been at some point; the paper itself was done, was conceived and worked on, at Göttingen.

Laporte:

I think Sommerfeld once told me that Elsasser had come to him—I was still in Munich—and said that he was going to leave Munich because he thought the atmosphere there was not good; they were too anti- Semitic. This is very interesting. You see, that was just about the time when one occasionally could see a big placard of the National Socialist Party on the columns with all the things like the opera and so on. Underneath was the swastika and one saw, "Juden ist der Eintritt verboten." At the same time the very eminent, the most eminent organic chemist in Munich suddenly quit his job—a full professor and holder of a chair named Willstätter—because of anti-Semitism.

Kuhn:

When was this?

Laporte:

Between 1920 and 1924.

Kuhn:

As early as that! I didn't realize that. Come back again to the subject of the seminars; the first one you were in was mostly concerned with magnetism. Are there any other things which you particularly remember about that seminar, other papers that were reported on or subjects that were particularly discussed?

Laporte:

We always talked a great deal about paramagnetism of the incomplete shells, the inner shells. The interest of all spectroscopists was in that same direction, you see.

Kuhn:

If you were talking about the paramagnetism already—this was now the fall, of 1921—you barely had the Bohr theory.

Laporte:

Oh, no, no. We used to talk a great deal about the Bohr magneton versus the Weiss magneton.

Kuhn:

How live was that issue?

Laporte:

It was very much alive. We all knew that the experiments did not decide definitely in favor of the Bohr magneton, but one somehow had the feeling that it had to be right; that had to be the way, and the Weiss magneton after all was purely empirical. So we had discussions on that and on the Curie point and on all those questions: to what extent would the Debye-Langevin theory have to be modified because not all positions, in space quantization, were allowed? Then in radiation we were always occupied by such questions as even if the atom does not radiate, then when the jump takes place what exactly happens in the jump; how will the phase of the orbit at that moment make itself felt; does it go over from the one place to the other or on a tangent or what? And we all knew that; it was a very peculiar feeling. Everybody felt that to ask for such questions was in a way wrong, you see, and that in a way establishes the atmosphere for such thoughts from which Heisenberg's very first paper came, his paper which was as yet without matrices.

Kuhn:

Matrices without matrices, yes.

Laporte:

Yes, Fourier coefficients without sines and cosines.

Kuhn:

What I had in mind when I said that you didn't yet have the Bohr paper was not of course so much the Bohr magneton or Bohr's previous discussions of masi etism of ato . as the whole s•tion of closed shells

Kuhn:

(cont.) on the Bohr model in which the individual electrons were still in elliptical orbits as against something like Ellipsenverein or cubical distributions, the asymmetric models which nevertheless had shells. The letter to Nature comes out in August or something of the sort in 1921 and the full, paper is, I think, early in 1922. So you would have been at Munich when the big atomic structure papers of Bohr's came out.

Laporte:

Yes, the theory of atomic structure.

Kuhn:

And I'm terribly much interested in knowing how great the impact of these were. I have the feeling that Sommerfeld had largely been working previously in terms of symmetrical models in which there was a strong correlation between the positions of all the electrons in a given shell. He gives strong arguments why that should be the case and then gives it up and takes the Bohr—. Now the nature of that transition and the problems that arose in Munich is a terribly important subject also because it leads on to the next spectroscopic develo ents so quickly.

Laporte:

That was called Ellipsenverein, wasn't it?

Kuhn:

That's one of the things that was used, the Ellipsenverein, but it's not the only one. There were also the cubical atoms that, I think, Born and Landé, and Landé particularly, started out.

Laporte:

Even Heisenberg, I think, had something to do with the helium atom.

Kuhn:

That was later, at least in so far as I know. Now there may be some unpublished work of Heisenberg's on the helium atom. He and Born did a paper later on the helium atom in which they say at the end, "We've tried every possible approach and none of them work, so this will not be resolved by—."

Laporte:

Well, I don't know whether I can contribute to answer that question. I know we used to speak about pulsating cubes and then the papers by "diese amerikanischen" chemists used to be talked about, namely Lewis and Langmuir.

Kuhn:

In your time these were still discussed?

Laporte:

Oh, yes; yes indeed. We discussed them and the business of heteropolar bonding was very much discussed. Everybody had that vision that for chlorine you have a cube with one thing missing and then the hydrogen or the sodium would quickly supply that one thing and put it in the corner of the cube, but either way this was more chemistry than physics. We drew a picture of a cube more as a way of counting—.

Kuhn:

But did you ever stop doing that? You see, what I have in mind is that that model or models like that probably came initially more from chemistry than from physics, but my' ression is that, with a few exceptions, until 1920-1921, if people tried to do complex spectra from a model they tended to use this sort of model also, so it was a physics model too. That is, certainly the Ellipsenverein, the pulsating cubes, one tried to do spectra and energy levels with also.

Laporte:

Most of the time that sort of thing came to grief quite quickly because of questions of stability. They did speak of this and that being unstable. In retrospect it seems rather surprising that no one ever thought they could get those famous Larange solutions of the three-body problem.

Kuhn:

Landé in 1919 did a cross-orbit helium paper in which the mathematics is very crude—it was wrong and I gather Sommerfeld may have dismissed it on that basis—but it was the first attempt to take the two independent electron orbits, let one of them perturb the other, and quantize the problem; and it's got an essential role in the route to the vector model, too, because in trying to find quantum conditions Landé tries to add the angular momentum and so on. Do you remember any discussion of that approach? You're almost forced to take it after the Bohr papers, but there appears to me to have been an important transition right in the period of your early years as a student in Munich.

Laporte:

I honestly don't know whether there was any serious discussion of those papers. Shortly afterward, you know, Sommerfeld went into the discussion of the alkaline earths and that culminated in his discovery of the quantum number j, for which he has never received sufficient credit. He's the one who assigned the quantum number j, the total angular momentum.

Kuhn:

Let me then ask another question. He may not have received sufficient credit, but let it be said that it's very clear in the historical development that it's a terribly important development. But, you know, when he assigns it it isn't the total angular momentum; that is, his initial 1920 I think it is—.

Laporte:

Oh, inner quantum number.

Kuhn:

It's the inner quantum number and the total angular momentum is still the azimuthal quantum number. And he's got some remark that the inner quantum number may have something to do with the magnetic interaction with the Rumpf, but it isn't total angular momentum; and the man who really does that identification, and over some resistance, I think, is Landé.

Laporte:

Is that right? Landé.

Kuhn:

Now this is not to deprive Sommerfeld of anything—.

Laporte:

Oh, Landé certainly has not ever received the proper recognition.

Kuhn:

Right. Now here again is an area: this whole question of the changing significance of the inner quantum number which again ties in with the whole question of selection rules; that is, one has selection rules for total angular momentum,one has selection rules for m. Which was the inner quantum number and for, let's say k, and they don't quite add up. Now there must have been feelings of malaise about that.

Laporte:

Yes, but one almost used to change the selection rules to fit the particular spectrum, you know.

Kuhn:

Again, I think you were at Munich at the time the inner quantum number finally becomes identified as total angular momentum, which it had not been before.

Laporte:

I'm not so sure whether that ever became so positively identified, you see, because the fact that the azimuthal quantum number was not the total angular momentum could only be understood after you had ascribed an additional degree of freedom to the electron, and that didn't come until Goudsmit and Uhlenbeck.

Kuhn:

Except that prior to this you assign it to the Rumpf, so that it doesn't solve everything; but if you've got a quantum number R as you do with the vector model it will at least give you the additional degree of freedom. In the fourth edition of Atombau the spin and the Rumpf won't work quite right, but there's no question in drawing vector models in which you're adding R and a J, so you've got the degree of freedom, and in that sense the difference between K and J and that it's J that's the total angular momentum is by then clearly understood, but there's been a difficult transition in between whose structure is not clear to me.

Laporte:

Quite frankly I had forgotten about the Rumpf, that is, that one ascribed to it more than the role of just a rare gas shell or something that was responsible for—.

Kuhn:

Heisenberg in '21 or '22 does this first version of the Rumpf model in which he shares the angular momentum between a valence electron and the Rumpf and then talks about magnetic interaction between the two and does a funny sort of relative space quantization of the Rumpf angular momentum and the orbital angular momentum. Do you remember that model at all?

Laporte:

No. There Heisenberg was involved in a rather special problem, but you know there was Heisenberg's great attempt, before the paper which we mentioned a moment ago on the matrix elements without matrices, where he wanted to derive the spectral frequency not just from energy levels as their parents but the energy levels themselves were going to be derived from something higher up yet. I mean, it was quite ad hoc and unrealistic, but somehow it just showed that tremendous desire to burst the shells and to arrive at the right thing.

Kuhn:

How did people feel about that paper? Of course, there were a couple of them really, a whole series; I'm never clear how closely that Spectroscopic paper on the new formulation to take account of the anomalous Zeeman effect relates to what also plays in here which is surely the Verzweigungsprinzip, which is again a splitting up.

Laporte:

We used to speak about "unmechanischer Zwang."

Kuhn:

Now how much was that talked about at Munich? I'm very unclear about what the attitude was in Munich where I have the feeling that the atmosphere was rather different with respect to problems of this sort than at Copenhagen or at Göttingen.

Laporte:

I don't know whether I can be too helpful in that respect.

Kuhn:

When you say you used to talk about "unmechanischer Zwang," did one talk about it at Munich?

Laporte:

Yes.

Kuhn:

Did one sneer at these crazy papers that people were doing?

Laporte:

Yes, I had the feeling there was a certain amount of sneering. What year was that?

Kuhn:

Those were mostly '24. That was the great year, that is.

Laporte:

Now Sommerfeld came back from America; you know he had been Carl Schurz professor at Madison and had travelled all the way to California like Boltzmann in the earlier century and he came back loaded with spectroscopic information. I remember that in our Munich colloquium, instead of having a blackboard full of fancy equations and maybe curves, he had enormous plates of the calcium spectrum which he had gotten in Pasadena, I think, and they were painfully pushed through the projecting apparatus. Sommerfeld would say, "Here now we have that combination," and he had written the inner quantum number transitions in ink and we were all very much down to earth and realistic about that.

Kuhn:

Now this would have been what, '23?

Laporte:

'23, maybe winter '23-'24.

Kuhn:

Is this then where you yourself got started on the problems of unravelling these spectra?

Laporte:

Yes.

Kuhn:

It was really in some part using data that Sommerfeld had picked up on his trip to America?

Laporte:

Oh, yes, quite. We were then very highly committed to actual spectroscopy, you see, because Sommerfeld took this interest in calcium and from calcium we went on to other spectra and suddenly there appeared from England, under the influence of Fowler, not the statistical mechan ics Fowler but A. Fowler I believe, one single but very fundamental paper by the Spainard Catalán.

Kuhn:

That was a little earlier, I think.

Laporte:

Was it earlier? Well, it might have been '23 because, after all, it took me a little while to write my thesis. But this Catalán paper had a great influence and that was for—I forget—manganese, I think.

Kuhn:

'22 actually, I think, was the Catalán paper, and I think that Sommerfeld had seen a copy of it before it actually appeared.

Laporte:

Catalán must be given great credit; I don't think he received very much help on that from Fowler; and Fowler, like so many others, couldn't really see very much else besides the alkalis and the alkaline earths. That was it. Then I think it must be said that Sommerfeld then was firmly of the belief that that was the general structure of spectra with high multiplicity and he saw in that, of course, a brilliant justi- fication of the inner quantum number, whatever its meaning.

Kuhn:

I think it might be a good idea, before we get into that, simply to talk about a few of the other bigger things that were going on and then come back and concentrate simply on spectroscopy itself. One of these, which does of course relate terribly closely to spectroscopy, is the vector model. Again I simply throw this out as a name; it reaches its final form in '23 but it's been coming through a series of previous papers. Do you remember discussions of this or things that were said about it? The whole question as to Sommerfeld's somewhat ambiguous attitude toward models anyway may have played a role in the reception of this at Munich. But you were by this time certainly deeply involved in spectroscopy?

Laporte:

Yes. I don't think that we ever had any opposition to vector dels, I believe. I don't know; Wentzel might contradict me on that, I don't know. I don't think we had any opposition. I forgot to mention one name of a man who was there in Munich at that time, too, a very meritorious man in the history of early band spectra, and that was [Adolf] Kratzer. Then for some damn reason Kratzer received a chair in Munster, and as far as I can see, he hasn't done a damn thing since, so he has, so to speak, vanished from the scene; but he had a great influence on all, of us, constantly driving home this question of the integer versus the half-integer problem. This occupied us a great deal.

Kuhn:

Can you tell me more about that?

Laporte:

Heisenberg was concerned with this. We didn't know whether the half-integer quantum numbers had physical reality or whether it was the result of some averaging of something on top of the levels; this may very well have been where Heisenberg's idea came from.

Kuhn:

It was certainly one very important source of it. Heisenberg told me a very interesting story which ties in with another rather curious thing in Atombau. Around 1920 I think it is, Sommerfeld introduces the Zerlegungssatz as a device for analyzing the anomalous Zeeman effect. He gives this thing to Heisenberg and says, "Find some energy levels," and Heisenberg clearly works on a doublet and says, "It works out beautifully if I use half-integer quantum numbers." Sommerfeld says, "This is mad, it can't be right. Interestingly enough, in the 1922 edition of Atombau, Sommerfeld doesn't use half-integer quantum numbers; instead he uses the strange selection rules, delta n equals plus or minus 2 or zero. Then you can stick with integers just by multiplying all your quantum numbers by a factor of two. Does this remind you of anything? You must all have known about this and been talking about this?

Laporte:

No, here I'm afraid I have to leave you.

Kuhn:

Just one or two things and then back to the spectroscopy again. What about the Bohr-Kramers-Slater paper?

Laporte:

Yes. I know that we were all very much bothered about the realities of the whole question. I at least used to be highly bothered—this shadow presence of these oscillators always. It uost seemed as if there were two worlds, one which was the physical world and the other was the world of almost-physical character.

Kuhn:

Now were you bothered about this before the Bohr-Kramers-Slater paper or was it the model of the extended correspondence principle using virtual oscillators that came out after?

Laporte:

Well, I don't know; it may have been aggravated by the Bohr-Kramers-Slater paper.

Kuhn:

The element of virtual oscillators, which a lot of people, I think, take out of the Bohr-Kramers-Slater paper, exists for some people before this through—.

Laporte:

Through Planck, I think.

Kuhn:

Yes, certainly Planck's oscillator. What I was thinking of rather than Planck was Ladenburg and the Ladenburg-Reiche paper, the dispersion paper.

Laporte:

I see.

Kuhn:

Did the dispersion problem bother people?

Laporte:

Oh, yes. Yes, the dispersion problem bothered us.

Kuhn:

Do you remember discussions of it? Did it come up in colloquium or in seminar?

Laporte:

Yes. When the Kramers extension of the dispersion formula came up, of course, that was highly interesting and in a way very satisfying. We all liked that. Then when the Raman thing ultimately arrived, then it made less of an impression. But, of course, the dispersion was always a great problem because, as you said, we have the oscillator frequencies in it and actually they are spectroscopic frequencies.

Kuhn:

You're certainly right that this was the case; furthermore, that I think is the other place where Debye and Sommerfeld overlapped. It

Kuhn:

(cont.) was around 1916 or '18, in there; in the dispersion theory papers there is at least a big paper by Debye in which the electron frequencies, not the optical frequencies, appear. Debye seems relatively pleased with this, yet the problem drops out of sight then in published literature. I wonder what happens to it?

Laporte:

Well, of course that's sort of true; physics is a little bit like engineering in some respects because problems on which the experimentalists work have to be dealt with by the theoreticians; on the other hand, it's more like mathematics in that it follows the line of least resistance. If a problem proves to be really very, very hard, then you find it's just not being talked about in the literature.

Kuhn:

Yes, now I think this is very apt. This is a problem that's deeply involved with Heisenberg's route to matrix mechanics or something like matrix mechanics.

Laporte:

Kuhn's sum rules—a relation of yours?

Kuhn:

No, but in this country it's usually known as the Kuhn-Thomas sum rule and my name is Thomas Kuhn and every time we had this in lecture, everybody turned and looked at me.

Laporte:

I see! Isn't that amusing. That's L. H. Thomas?

Kuhn:

No, that's a Thomas who was a student of Reiche's.

Laporte:

Oh, a German Thomas.

Kuhn:

A German Thomas who was killed; he died very young. I forget, but I guess he died rather than was killed; anyway, it isn't L. H. Thomas. But it is curious, it's one of those things of which I am particularly conscious; that is, who paid attention, who was still really aware, that there was a fund- ental incongruity in the structure of the theory around this problem. How far underground that went when it turned out to be very difficult is something I have very little idea of.

Laporte:

One of the people who kept closest watch over all the difficulties and was highly critical was Pauli. There isn't any doubt that Pauli's merit, apart from his great creative stuff, also lay in his two wonderful contributions, one the relativity article in the Enzyklopädie and the other the early quantum Handbuch article. I remember one thing that impressed us at this time too and that was this paper by Ladenburg on the color of the chemical compounds of all those atoms where, according to Bohr, the inner shells were being completed. Ladenburg first pointed out that those are the ones that have color, colored ions, and those are the ones that have par eti that must be the same phenomenon.

Kuhn:

I didn't know that.

Laporte:

In a way, you see, Ladenburg was a very down-to-earth physicist, more so than Born.

Kuhn:

Much more. I'm not sure how much it gets tangled up with your own work but it's in the area in which you work: you spoke a little while ago about this problem of the recognition of the similarities of the x-ray spectra and the alkali spectra and of course what this then is theoretically also is the problem of this magnetic interaction, or is it a relativistic effect? Do you remember debates or discussions about that one?

Laporte:

Yes. We were completely on the side of the relativistic effect. I mean, there of course we were almost, you might say, partisan to the extent of being prejudiced and that was because of the Sommerfeld fine structure formula which, as you took more and more and more terms, kept on representing the L doublet very accurately. Now you ask Wentzel about that question.

Kuhn:

I will.

Laporte:

He was strongly involved in that.

Kuhn:

I know he was, and indeed I am going to ask him about it but I'm going to ask you, too; it's worth comparing notes because nobody remembers it all. What about the problems that this presented? For one thing, the need to treat the quantum numbers differently in optical spectra from the way one had treated them before, if one was to carry the relativistic treatment out and use it on the alkalis.

Laporte:

Yes. Wasn't it Landé who first came out very clearly without an explanation and said the x-ray spectra are of the same shape?

Kuhn:

I'm not sure who first did it; in fact, it wouldn't surprise me if a little paper of Goudsmit's that almost nobody knows was the first one in which that was said. There are a number of places in which this pops in, in which this suggestion is made. But, you know, it's a good clear idea, but the successful treatment of things like the interval rule, a lot of the treatment of the Zeeman effect, has been magnetic interaction treatment.- And just at the level of labelling with quantum numbers these work differently if you take it to be a relativistic effect. Nevertheless, you would say that the Munich group as a whole stuck definitely to the idea that it was to be relativistic, these optical—.

Laporte:

I seen to have that memory.

Kuhn:

What was the impact of the Pauli paper then, the first of those two papers in '24-'25, the one that leads into the exclusion principle and the electron quantum number but which starts out by showing that you don't have enough relativistic interaction with the Rumpf?

Laporte:

I don't know but what I might have been gone from Munich then already. 849

Kuhn:

Ah, yes, you left in the fall of '24.

Laporte:

I left in the summer of '24.

Kuhn:

You were already gone from Munich then?

Laporte:

I distinctly remember that time. When these first papers appeared I said to myself immediately, "Here is where the fact that we don't have a low triplet term in helium has to be explained, and all these other things."

I'll tell you one amusing thing since we are reminiscing; it's not very important but to me, in retrospect, it's highly amusing. The very first paper I ever published, also under Sommerfeld's interest who was always one for boundary value problems and partial differential equations, was on the diffraction of waves around a sphere. Debye had worked on it.

So I was doing my own little work there—18 year old—with spherical harmonic series of the diffracted wave around the sphere and round me the talk would always be about, "Is it integer quantum numbers or is it half-integer quantum numbers?" One day I said to Wentzel, I think it was to Wentzel, "Isn't it very strange that when you have a spherical harmonic [goes to board to demonstrate] Pl is symmetric with P-l-1.

If we were to use half-integer l's here, then it would become completely symmetric. Also these spherical harmonic series always have this 2l+1 in front. Wouldn't it be nice now if we systimatically halve them? This would become l+1/2." He said, "Spherical harmonics! What have they to do with quantum physics? You're dreaming!" So there.

Kuhn:

That is a lovely story. An anecdote. And, after all, defraction of a plane wave around a sphere is already very close to the Born theory of scattering.

Kuhn:

Through this period when you were there, through the summer of '24, on the one hand, and I suppose this was particularly apparent at Munich, it's a period of gigantic new successes in quantum theory, particularly with atomic spectra. On the other hand, it's a period when, at least in some places, people are becoming more and more deeply convinced that it isn't working. You know Born publishes the Atommechanik and calls it Volume I. He's quite explicit that the reason he calls it Volume I is that this had to be all wrong, things had to be done very differently and it wouldn't be very long before there would be Volume II. Did people feel that way at Munich at all also?

Laporte:

That's the little book, rather small size in format.

Kuhn:

It's bigger than the MIT lectures which are the Problemen der Atomdynamik, but it's a fairly small book.

Laporte:

Actually the second volume turned out to be that thing with Born and Jordan which was an almost pedantic tour de force in using only matrix mechanics.

Kuhn:

That's right, it is that book. Now Born at least, and some of the people around him, were altogether prepared for something as radical, I think, what as what came; I don't mean they expected it to be matrices or anything of the sort, but you'd say that here was a sense of very clearly defined crisis in Göttingen. I'm not clear that there was the same sort of feeling in Munich.

Laporte:

Well, I was only in one place at a time, so—. Well, I think that was the same in Munich because due to the fact that we were interested in the continuous x-ray spectrum and in questions of dispersion, quite apart from atomic physics, the recognition that one could really quantize arid work only in a very limited number of problems—that anything aperiodic didn't work—I think that gave one the feeling that things were in a pretty bad state.

Kuhn:

Was there any great concern with some of the atom problems which weren't working, that is, helium, H2+, Pauli's work, and the fact that these just didn't come out, just didn't yield the energy levels one thought they would?

Laporte:

Oh, yes, of course. What one could do was just nibble at the problem like explaining a p - d transition by assigning inner quantum numbers; nothing was really done. I remember that after the Lands paper with the formula appeared, Heisenberg said—"yes, this is good." I think Heisenberg was asked to present the Landé paper in a seminar. I take it back, it might have been that he was presenting one of his desperate attempts before the matrix stuff. He said, "That something must happen we can see from the following consideration: take the Landé formula"— you know it, of course; full of L times L plus 1, S times S plus 1, J times J plus 1—"how we know that all these quantum numbers actually are phase integrals. A phase integral is equal to the Planck constant times the quantum number in question, so now let me take the Landé formula and multiply it by h in the numerator and denominator. Then instead of J times J plus one divided by S times S plus one I get explicitly appearing h's all the time. The h does not drop out, in contrast to the normal Zeeman effect in other cases. The h will, have to appear explicitly; therefore, we can never in a million years hope to explain this in any simple way and some new idea has to come in."

And like so many things, you know, which are rather simple, we all nodded sagely and agreed; that is, sometimes very simple statements do not really get you until much later. You see, if you call the phase integral to which J belongs Pj, then it's Pj times Pj plus h.

Kuhn:

Yes, it's a good point; it's a good story too. That's very nice. Should we go on now or should we arrange to come back to this whale thing once more while I'm here? I want to talk about your own work, the whole problem of gestrichene and ungestrichene terms, the unravelling of this—I want also to hear about your transition to America. So if you're willing to do it, although we could push on now, I'd really like to come back and try this once more, spend an hour or so at some time before I go.

Laporte:

Sure. It's around 4 o'clock and we might even be ready for a cup of coffee. (Coffee-break)

Laporte:

—of [Willy] Wien, you see. Wien just at that time was doing his most fundamental experiments about the Abklingungszeit, the decay time of the states, by letting canal rays go into an almost complete vacuum; he measured the time it would take for the Balmer lines to decay and, therefore, he got the first determination of the time an atom would stay excited. We used to debate that for a long time how that would influence the width of the spectral lines, you see—the Fourier—.

Kuhn:

I hadn't realized that this was going on just at that period. Was there then a reasonable amount of interaction between Wien's group and Sommerfeld's group over this, because I have the impression that to a very great extent relations were not so good?

Laporte:

I have the feeling that perhaps personal relations might have been a little cold, but actually we were on good terms. Our colloquium would hop from the Theoretical Physics Institute to the Experimental Institute to the Technische Hochschule. There was re interaction between physics and, engineering than there is nowadays; we did quite a lot of talking to engineers.

Kuhn:

Which engineers in particular?

Laporte:

Well, there was [Jonathan] Zenneck, the wave man; he was quite in with everybody. Then there was [George] Joos the theoretician, who was a great admirer of Sommerfeld and influenced by him. These are the names I can recall right now. Strangely enough, interaction with astronomy was very, very little, perhaps because Seeliger was once and for all opposed to relativity, and that Sommerfeld couldn't very well take; he somehow knew that relativity was right, and, I think ,Sommerfeld had a streak of intolerance in him. He used to frighten many students before they knew him better, a very Prussian exterior but he wasn't really that way. A wonderful old character. [Break]

Session I | Session II