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Oral History Transcript — Paul Peter Ewald

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Interview with Paul Peter Ewald
By G. Uhlenbeck with T. S. Kuhn and Mrs. Ewald
At the Rockefeller Institute, New York, N.Y.
March 29, 1962

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Paul Peter Ewald; March 29, 1962

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: Niels Henrik David Bohr, Peter Josef William Debye, Paul Sophus Epstein, David Hilbert, Max Theodor Felix von Laue, Wolfgang Pauli, Arthur Pringsheim, Fritz Reiche, Arnold Sommerfeld, O. Wallach, J. Zenneck; Universität München, and Universitat Göttingen.

Transcript

Session I | Session II

Ewald:

Let me tell you of my own making. I was born in Berlin, in the year of grace 1888. With three 8's and three Emperors: Wilhelm I, Friedrich, and Wilhelm II. So this was all my political experience in my very first year already. I went to school in a very famous gymnasium in Berlin, the Wilhelm's Gymnasium, and had one very interesting schoolmate there. He was the son of the famous psychiatrist (Remarck). He was an outstanding mathematician who developed his own knowledge of differential calculus before I left Berlin, which was when I was 12 years old ... So this was certainly a very great stimulation to me ... I was telling you what kind of mathematics course was in the gymnasium. It began with geometry, and we were told to get a (Ziehdeckel). A (Ziehdeckel) consists of two paperboard covers joined by parchment ribbons. Now you can put in between these any amount of sheets -- it pulls apart. Then we were told to use our best (Unschrift) -- (Unschrift) is an artificial script which we had dabbled in at school, and, to put down the following sentence: "Der Raum ist a) unendlich, b) statisch, und c) unbegrenzt teilbar." Well that was the beginning, and after having understood these concepts over which the mathematicians have fought for centuries, we began to get down to more Euclidian type of stuff. So there was not very much useful geometry. I liked the construction part, where you are given certain sides of angles and have to construct triangles. But you see I didn't stay in Berlin beyond my twelfth year. Then we settled in Potsdam, which is not very far away. But it was an entirely different type of school. Both were Gymnasium -- that is to say classical schools with Greek. The Victoria Gymnasium in Potsdam had a good tradition, because it was the school where Helmholtz had been taught, and where C. T. Jacobi had gone to school, before Helmholtz. So there were two great scientists who had come from that gymnasium. And a very good thing about the German gymnasium at that time was that it gave you -- or me at least, who was not a good pupil -- any amount of free tine. I had not very much to do. I spent my days either rowing and sailing on the beautiful Havel Takes, or being with my friends who were interested in chemistry. And so, since I had a room for myself, I rigged up quite a little chemical lab, and thought I'd learn something of chemistry, which was not really the case ... One of my friends was the son of a manufacturer of perfumes, and, they had a chemist in their factory. He learned his chemistry from this man. And he gave it on to me. So it was second or third hand, but it was very stimulating ...

Kuhn:

But again there was not much science or math in the curriculum, or was there more?

Ewald:

There were perhaps two hours per week of mathematics and one hour of either physics or chemistry.

Mrs. Ewald:

There were more.

Ewald:

You think there were three hours of mathematics? -- certainly not more. In contrast to Latin, which was six hours a week, and Greek which was probably about about as much ... [Entrance of Uhlenbeck]. The science curriculum is very quickly dealt with, because it didn't go very far. In chemistry I think all we learned was that Fe plus S is FeS. That's about the limit of the curriculum. And in physics it was not very impressive either. Besides there was always the difficulty of discipline in the physics lectures. None of the physics teachers were able to interest the boys to that degree that they were ready to pay attention. They were always playing tricks, etc., in the physics. But I am always quite impressed what we learned in mathematics, in spite of this being a classic school. This included logarithms, of course, trigonometry, and, spherical trigonometry, which I think is not done here. It included the complex numbers, the exponential function and de Moivre's theory. About Diophantine equations I am not quite sure, because I was very fond of reading Euler's algebra. That of course is full of Diophantine equations, so I may remember them from there rather than from school. I think this is more than is done here, even in a non-classical school. But the main thing was that I had any amount of time on my hands to do my own reading. For instance, one of the books I read at the time was the biography of Helmholtz by Krönigsberger. This is quite interesting, because it contains a description of how Helmholtz tried to use the theory of dispersion to learn something about the ultimate particles of matter and their (reaction). This apparently impressed me very much, because I remember a walk with my mother in Potsdam when she asked me what I was going to do with my life. And I said, "Well, I think what I would like to do is to see whether one can't get farther in this direction." And the funny thing is that actually I've spent my whole life doing that, except that instead of light I take X-rays ... I had forgotten this whole thing entirely until long after I had written out my dissertation, which was just on this topic. But I don't think I dre t it up. I still see the street in Potsdam in front of me as we walked along. I think it actually took place. But certainly I had forgotten all, about it for a long time. Well, I was always terribly bad in history. I never managed to learn history, although my father was an historian. I never knew my father -- he died before I was born. I grew up in the remnants of his historfcal library, with all his big historical works on the shelves, but I never opened one. And when it came to reading an historical novel or something in school, I just couldn't do it. It was awful. So it's very queer that I sit now in history of science. I always find it a joke. We had good relations with Cambridge -- a number of friends at Cambridge, England. I had really meant to go and study chemistry in Göttingen. But just before I was due to leave,both my grandparents died, and so we were free. We were no longer bound to Potsdam. We decided on the spot that I would try to get in at Cambridge, and my mother, who was a painter, was going to study in Paris. So in the fall of 1905 I found myself in Cambridge -- Caius College -- and my mother was in Paris studying painting. I stayed in Cambridge for half a year two terms and then I went to Göttingen. I tried to take a course on calculus in Cambridge, but I failed entirely. This was a snail college course, given by a man in Caius College -- one of the senior fellows, (Geller). He was a very timid soul. He gave it in his own room to a group of perhaps five or six students. He was so afraid -- you know the Cambridge way. There was a blackboard not much bigger than a bridge table top -- and this was on an easel. He was so afraid of covering up anything that at first he stood at the side, and gradually he disappeared back behind the blackboard and all you could see was his hand writing a formula. He had a stammer too. But the main trouble was that these English boys all had had calculus at school, and. I had not. You see our school training stopped just short of calculus. There was a strong movement toward giving the elements of calculus at schools also, and I think that in the modern schools, gymnasiums etc. they include calculus -- Felix Klein always was a great advocate for getting calculus into the schools. So since Geller gave no explanations, but only made us do problems, I just failed entirely. And, when I came back to Göttingen my first attempt was to learn calculus. I still had in mind becoming a chemist, and I went to hear Wallach's lectures. Wallach was a very famous chemist. He lectured from 7 to 8 in the morning. This was because of the medical students -- who had no other time. Big lecture, big lecture room, and I always fell asleep. Wallach had a kind of nasty look when I made some disturbance, and then this happened for the third time and, his look became nastier. I stayed away. So that was the end of chemistry for me.

Kuhn:

Was calculus something regularly taken by chemists in this period? ...

Ewald:

I don't know what the regulations were, but I can tell you that in Stuttgart I don't think that even in the 1920's calculus was demanded from chemists.

Kuhn:

Although you received your secondary education in and near Berlin, seem to have avoided it when chosing among the universities for an education in science.

Ewald:

Actually I had picked. Göttingen even before I went to cambridge. We had friends in Göttingen, and Göttingen was known to be one of the best universities for science in general. I think that's all I can say. I didn't like Berlin as a town anyway, and I never considered going to Berlin to study. I didn't consider Munich at the time, so it was just Göttingen. We had several friends in Göttingen. One was a chemist, Porsche, who I think became the successor of Wallach , and the other was the philosopher (Nelson -- both personal friends. This gave a kind of attachment. And I was very lucky at Göttingen, because through Nelson who was on very good terms with Hilbert, I was made Ausarbeiter for Hilbert's course on calculus. You see in Göttingen under the organizational power of Felix Klein there was a first-rate mathematical library. Part of Klein's organization was, also, that the main courses were written out long-hand just the way given, and deposited in the mathematics library. Students who had missed a course -- you know there is no checking in Germany of attendance -- could read up, or students who hadn't understood a point could read up. I must say that it was also very essential to correct errors made by the professors during the course. Errors of s and other things which were very frequent. So this was so to say, my first official academic position. The assistant to the assistant to Hilbert for working out the lecture course on calculus. Well this did me no end of good, because first of all, Hilbert's assistant was a perfect teacher. It was [Pause] the man who died in Northwestern -- Hellinger! Hellinger had a real innate pleasure to teach. When I came to him with my worked out Ausarbeitung, he of course found many faults, partly with me and partly faults Hilbert had made. He set it right, and I really had a private tutor of the first quality. I must confess that nearly all mathematics I know goes back to this introductory course from Hilbert on calculus. You see Hilbert would give examples which were so interesting in themselves, and he would carry on the theory of calculus to such a degree that you really got a survey of nearly all analysis. The complex integration, multiple integration, space integration came there. I remember his examples were so well-chosen. For instance, for complex integration he gave the determination of the sign of Gaussian sums. I don't know what Gaussian sums are any more, but it was a paper by Gauss' where he used the contour integration in order to determine the signs of some sums which were otherwise very hard to determine ... This was in a two semester course. The first calculus ... Then I took other courses. There was Carathéodory, who gave the problems course paralleling Hilbert's course, and Herglotz and others. Herglotz gave the introduction to analytical geometry and there I couldn't follow at all. Herglotz is a Viennase and speaks very fast. He used to write down his transformations as a11x1 plus a12x2 etc. I always had to look between my book and the board to find out whether these were correct, and so I failed entirely. You see it's interesting that just this outer and very trivial difficulty really threw me overboard. After having had three semesters in Göttingen, I was keen to learn what epsilotic is. The Göttingen mathematics given by Klein was the visualizing kind, getting onto problems, not being too meticulous. The Weierstrassian way was usually called epsilotic, because it used the small quantity epsilon which goes to zero on every occasion. Now the only man who really did it the epsilotic way as far as I had heard, was Pringsheim in Munich -- Arthur Pringsheim. By the, way, I had taken a physics course, which not interested terribly, and I had dropped chemistry, so I was now bent on mathematics. And I went to Munich partly to hear Pringsheim and partly because I had never had any experience in south Germany. The third reason was that all the good friends we had in Göttingen warned me and my mother, "I should for heaven's sake not go to Munich, because so many students had gone to the dogs there." It was so dangerous to be there, they shouldn't let me go to Munich. So of course that set my head to Munich. And in Munich I actually heard nearly exclusively Pringsheim for at least two semesters, maybe three. Theory of functions. And at the end of the second semester Pringsheim very tentatively, and excusing himself, so to say, came to Cauchy's theory, in theory of functions. Which of course in the other aspect would be the very beginning of the whole theory. I enjoyed Pringsheim's course immensely. It was a great aesthetic pleasure and when after a few years it came to his examining me in mathematics, I couldn't answer a single question. But I'd enjoyed it, and I'd really tried to work. It was absolutely counter to my brain, this whole algebraic approach. I am very much stronger on the visualizing side, than on the formal side ... When I came to Munich I wanted to hear algebra and calculus of variation, and mechanics. I think algebra was not given that term when I was mature for it; and calculus of variations was given by Lindemann, who was sick; and mechanics was given by Voss, also a famous man, but he was sick. And so I never heard anything of these subjects in all my life. And I sometimes miss it. But you can do with very little mathematics. I once complained to Sommerfeld that I knew so little mathematics. He said, "Why the devil do you complain? It isn't that you know so ch thematics. The much more important thing is that you you can apply what you know, and find a way And I think that's quite true. You can't just store it.

Kuhn:

... In the middle twenties Pauli expressed the feeling that mathematics and physics at Göttingen are particularly ridden by formalism. If this is true at all, and it may not be, it obviously speaks to in some part almost the reverse of the contrast that you have found in your own education much earlier between Gottingen and Munich. Is this in part a difference between the later influence of Hilbert and the earlier influence of Klein? Or does this correspond to nothing that rings any bells with you? ...

Ewald:

Born has an excellent mathematical education which he got in Breslau. This was a great school of mathematics, traditionally, since Steiner's days, with many great thematicians there. Born would sit down, and, what he himself called, "ein Problem durchixen" -- you understand to 'x' it through That is to say, to write down equations and to make approximations and to get results and to work it out. And Pauli probably in his youthful enthusiasm, had some low opinion of "durchixen" and wanted to get things done with more physical background. Probably. It may be that, but I couldn't be sure.

Kuhn:

Then at least for your first semesters at Munich you were principally concerned with mathematics. What got you to physics?

Ewald:

An old friend whom we visited last fall in Athens, Hondros . He was a pupil of Sommerfeld. He worked on electromagnetic waves traveling along dielectric cylinders. That was his thesis work. We met together in Göttingen [munich?]. I had even met him before I went to Göttingen [Munich?]. He told me, "Sie sind ein Esel. Kommen sie in Sommerfeld's Vorlesung." You can stay away again if you don't like it, but come and hear it. I said I had no reason to go, I wanted to be a mathematician. Well, he told me I was an ass, so finally I gave way and I went to Sommerfeld's lecture on hydro-dynamics. This was the two hour lecture in the short summer semester, I think from the very first lecture this captured me entirely. You see Sommerfeld had this beautiful way of not assuming anything. I had not done mechanics, not even point mechanics. And I went straightaway to hydro-dynamics. And I had no idea of vectors, I had never heard of vectors. And Sommerfeld developed the whole vector algebra, scalar product, vector product, and so on, and that vector analysis, in conjunction with the hydro-dynamic concepts. Hydro-dynamics is really so important because it's the physical field theory which you can see in nature. And so this really fascinated me. From that moment on I was quite convinced that this was my future. This was probably my fifth semester. Then probably one or two semesters later Sommerfeld gave the first course ever given on the theory of relativity. I think it was either a small special course or it was within the framework of his general course on Maxwellian theory. But I know that he was quite proud because it was the first example of a course where relativity was given to a class ... This would have been in 1909 I should suppose. Einstein's paper was from 1905. I worked that out again in Göttingen fashion. So I learned something. But, on the other hand, it did not attract me very much, and I have never taken it up again. I did some experimental work under sommerfeld. You see when Sommerfeld came to Munich, which had been 2 or 3 years in advance -- in 1905, I suppose -- he made it a condition to have a small institute for theoretical physics. This was rather unusual, because theoretical physics needs a desk and a library, but not an institute. Of course Boltzmann had been the predecessor of So erfeld in Munich and I don't know whether he had any institute or not. I think that Boltmann probably was the curator of the collection of instruments which belonged to the Bavarian Academy. When I first came to Sommerfeld, he was still installed not in the University, but in a building next to the Bavarian Academy, a very old-fashioned building. He and his assistant Debye were sitting there. He had one pupil, Ludwig Hopf, who was doing hydrodynamics. Also, Guth has recently told me that Debye had made thermodynamic measurements. Anyway, I was put to do something useful with the instruments which were to be found in this collection. There was one enormous glass bowl, and Debye set me to evacuate that with a Töpler pump which gave me much headache. In order to study the electrode-less discharge in nitrogen or in air you wrapped a copper wire around the equator of this big sphere. If you passed the discharge of an induction coil through it, there was a faint glow in this bulb. He wanted me to rake a spectrum of this.

Kuhn:

Sommerfeld or Debye?

Ewald:

Well Debye was Sommerfeld's assistant, and he was the leading spirit in this respect, I think. You see the question was whether this could be regarded as a direct proof of Maxwell's displacement currents, which they were still a bit uneasy about, -- Debye thought that maybe this could be used to prove something. Well to me it only proved that I was not an experimentalist. And after a while I had had enough of it, because I found it always filled with water when I came back the next morning. It was awful. But now you see, a couple of years later, Zenek was appointed professor of physics at the polytechnic institute in Munich ... Previously he had worked in the electrochemical industry. And the year after he had received the chair in Munich he invited us to a colloquium just to show us some of the experiments he would be showing. And to my horror and amazement, and I must say delight too, I saw this same experiment, which I had been trying to do. It was performed with a resonance transformer and a copper strip around this thing carrying 10,000 amps, the whole bulb glowing so that the lecture hall seating perhaps 500 students was illuminated for a minute to the last bench. This is the difference between the technical aspect and the physicist's sealing wax and string aspect. [Discussion of early attitude towards the quantum in Munich here omitted].

Uhlenbeck:

Was there a colloquium in Munich?

Ewald:

Oh yes, of course there was a colloquium. Actually in a way I was the founder of the Munich colloquium. You see when we went down for lunch down to Ludwigstrasse, I didn't like the idea that Debye and Hongros, and Hopf and (Hirschelmann) -- all these elderly students -- talked about things which I didn't understand at all. And so I said to Hongros, I urged very strongly, "Couldn't we get together to talk over things, so that we young people could learn a bit quicker what the problems of real actual interest were." Hongros eed that would be a great plan. He talked to Debye, and Debye talked to Sormerfeld. Sommerfeld said "Oh, that's fine," and Debye said yes. Sommerfeld said "I will not be there, so you are quite among yourselves." Which I think was a very wise decision. But he bought a box of cigars to be put on the table and to be smoked during the colloquium, which was his gift to the colloquium. The first two or three colloquia were really more or less of the type I had in mind. Then there came an invitation to Laue, who was in Berlin, to come and give a colloquium talk. The talk was on the entropy of radiation.

Kuhn:

Can you think what year, or about when in your Munich career this would have been?

Ewald:

Well,this may have been 1909. At that time I had just heard hydro-dynamics and perhaps one more course of Somm erfeld. I didn't what radiation or entropy was, so I was looking forward to this talk very much. But Laue of course didn't explain anything of entropy or radiation, and besides it was entirely unintelligible to most. Laue has not a very good way of explaining, so at the end, after some questions had been asked in rather holding-back asked: "Couldn't you at least explain what entropy of radiation really means?" And Laue said, "That is the essential question: This is what we are really trying so hard to find out." After that I held my mouth closed. But in away this was the beginning of the Munich colloquium.

Kuhn:

Is it because there was not much concern with thermo-dynamics and entropy at Munich, or was it because you yourself had simply not gotten that far?

Ewald:

I would say that both is true. First of all I had not gotten that far, and secondly I think thermo-dynamics was Sommerfeld's weak side. It was not a particularly good lecture course he gave, and certainly weaker than his other ones. Laue of course was very firm on thereto-dynamics, coming from Planck's school. I got my first really good introduction to thermodynamics from a small lecture of Laue's. I think we were five students who had inscribed for this lecture course. Two very soon dropped out, and then we were three. We took turns at making out who had to be there, because Laue's way of lecturing was not the best. He hadn't exactly a stammer, but he spoke very (verhaspelte sich) -- I don't know how you say that in english. His writing on the blackboard was as bad as his talk, so it was really quite difficult to follow Laud's lectures. But it was a beautiful course, and I learned quite a lot from it.

Kuhn:

Laue came to Munich very shortly after this colloquium. Was a deep concern at Munich with black-body radiation and specific heat perhaps due to Laue's arrival?

Ewald:

No, definitely not. The coming of von Laue was certainly in keeping with the general interest, but Sommerfeld's interest in this was much earlier. You see, at Munich there were the big course lectures, and then there were particular lectures, small lectures one hour or two hours a week. One of the topics that came up fairly regularly at that time was theory of black-radiation, and that of course contained Planck's work. [In response to a question on the connection seen between the photo-effect and the black-body radiation problem, Ewald recalled that the photo-effect was discussed in colloquium talks on Halwach's work. Returning to the early days of the colloquium:] Sommerfeld came in after Laue's talk. I believe that Reiche gave one of the quite early talks. And this was very funny. You see, Reiche had worked out a theory of the light wave going through a focus. And this was, like most of Reiche's papers, a very learned paper, with very, very stiff mathematics. I don't know whether so body else reported on it or whether Reiche himself came to Munich to report on this at the colloquium -- still in the AlteAcademie. Only a few people could make anything of it. The problem was the distribution of intensity at the so-called focus, and especially the rather mystical loss of 1/2 wave-length as the light wave goes through the focus. So Reiche gave his paper for an hour or two, and at the end Debye stood up and said, "Well, yes, this is very interesting, if you write down this integral and instead of taking it over all angles, you just limit it to a zone, or to a cap of the sphere, you see how it all comes out." He did it on the spot: This was Debye's treatment of (Bessel) functions. As a matter of fact, you know that Debye's dissertation which I think has never been published, was on the diffraction of light waves on rain-drops. It was really the theory of the rainbow. He was always urged by (Condon) to write-to write a book, but he never found time to do it. So Debye had treated these diffraction problems of spherical waves and so on and had a marvelous grasp of Bessel functions. And so he saw right away what Reiche's derivation amounted to. [Discussion of the first Solvay Congress and the Wolfskehl lectures here omitted].

Uhlenbeck:

Was there at that time a feeling that there was an (obstacle), so to say that there was a basic frontier which was certainly clearly linked around? Was so to say the quantum theory clearly the frontier of physics?

Ewald:

I would say that the aspect and interpretation of the quantum theory at that time was dominated by radiation theory, black-body radiation. There were the Rayleigh and the Mien ends of the curve, but the intermediate part belonged entirely to Planck's quantum thesis. This I think was the central aspect.

Uhlenbeck:

It was conscious to the people?

Ewald:

Very conscious ...

Kuhn:

... Can one try to find the shift in the profession's feelings about this problem, from the time it's a nuisance to the time when this is something we've got to find, another way to get at?

Ewald:

This is not a development along a curve, but it's really a development along a band. There may be a day of the week when Soutuerfeld had an idea, "Oh, perhaps if I did that and that in the classical style, it would get me somewhere." Then he would try it out, and if it didn't, well then it confirmed his idea that quantum was a fundamental aspect. But on the whole I think that especially among Debye and Lenz -- the younger people -- also of course there was very much discussion. It was held at the time you had these two different approaches.

Uhlenbeck:

And the quantum riddle, as Landé calls it, was the basic problem,much more than relativity?

Ewald:

Relativity seemed to be more or less worked out by that time. Not general relativity of course, but special relativity.

Kuhn:

Very briefly, I know you were at Munich until 1912. I know you were back in Munich I guess from 1918 to 1921.

Ewald:

I got my degree in the spring of 1912, and then I became assistant to Hilbert in Göttingen. Summer semester 1912 up to the fall -- for two semesters, I believe -- and then I returned to Sommerfeld ... In Gottingen I met my wife, and we married May 1913, supposedly. Then I went back to munich expecting to obtain assistant's position with Sommerfeld because Sommerfeld's assistant, Lenz, was to be called up for the military. But Lenz had a weak stomach and on the day when he was to be examined by the military doctors, he ate so much porridge that the doctor declared him unfit. So he now claimed that he would have to remain in his assistant's post. And I, who had married on the hope of becoming assistant to Sommerfeld, sat on the (dry). I think we finally somehow divided the assistant's salary, which didn't get any of us very far. Then soon after I became assistant. However, we hadn't really had a wedding trip yet. I had always wanted to go to the south of France, which I didn't know. So I was looking forward to going to the south of France. But then I read the announcement of the British association eting in Birmingham. I saw that Professor W. H. was going to talk on the structure of diamonds. That really interested me very much. And so instead of going to the south of France, we took a trip to England. We went and took part in the very youthful British Association meeting in Birmingham. And I am very happy, because my wife didn't know England and it was her only chance before the outbreak of the war to be in England. In the Birmingham meeting there was still Lord Rayleigh, Oliver Lodge and other quite impressive people. I met Bragg and Bohr, and Darwin and Moseley, and Bohr gave his paper on the atom.I gave a report on the British Association at our colloquium in Munich in 1913, the beginning of the fall semester ... I think this is how it [Bohr's work] first came to the knowledge of the munich people. Of course the in point there was the explanation he ve of the Rydberg constant.

Kuhn:

Tell us about how this was received in Birmingham and Munich.

Ewald:

I think in both places it was recognized as something of the greatest interest. You see the fact that the numerical result- 109,000 and then so many decimals -- could be obtained by a combination of general universal constants -- this was quite upsetting. It was a proof that there was something in it. On the other hand of course, nobody felt very easy at the conflict between classical field theory, where there should be a radiation by the electron ...

Kuhn:

Was there a lot of talk in the corridors about this particular paper?

Ewald:

There was over most anything. We all were very keen d eager to discuss things.

Kuhn:

Bohr,after all, was very young.

Ewald:

... Anything that came from Rutherford of course was something which you would consider as important. This, although it didn't come directly from Rutherford, had his sanction and it came from his school. [Mr. Kuhn raised the question whether there was work on the Bohr model, particularly in the Munich school, ediately after its announcement].

Ewald:

Before the war started Sommerfeld was in a kind of depression, and didn't feel that he had achieved anything. Then when the war broke out it him a certain stimulus, and then I think he soon be to work on the book [Atombau] It appeared very early -- 15 or '16 as far as I remember, but you could look that up.

Ewald:

Now you see one date which I don't really know but which we should find out is the time when Kossel was in munich, because Kossel was quite essential in that he worked in very close connection with Sommerfeld ...

Mrs. Ewald:

We pushed our perambulators with our babies together, but I don't remember exactly when it was ...

Ewald:

And he was full of ideas ...

Kuhn:

... Thinking of Hamilton-Jacobi theory, I take it that in 1912 this was just not part of the education of a physicist.

Ewald:

No, certainly not. And you see the difficulty was, there was only one book from which to learn celestial mechanics, and, actually, the Hamilton-Jacobi method. This had been lost to the physicists, and it was still preserved in astronomy. You had to go to Charlier's book on celestial mechanics. Possibly Poincare, I don't know, but the in book was Charlier ... [A discussion of the utility and knowledge of the Hamilton-Jacobi method is here omitted].

Uhlenbeck:

When did Pauli come to Munich? ...

Mrs. Ewald:

During the summer in 1919. But I remember him first from 1919.

Uhlenbeck:

There is such a nice story about Pauli and. Epstein. Epstein was working on the helium atom, and Pauli was already then sure that it couldn't be done. He imitated Epstein so beautifully, he said "You've only to close your mouth - thro your nose, there you have it." [Uhlenbeck imitating Epstein] ... This was in one of the summer school sessions. He told me about it. Epstein had said, "Now I will go to the helium atom. For that I need 18 contact transformations." "I have already done 12. The last 6 are a bit more difficult." Pauli immediately said "The 12 are absolutely trivial." He said it could not be done, and of course it was not done.

Ewald:

Yes, I remember the period when Epstein said that. "I still need 6 contact transformations ..."

Kuhn:

Let me ask just one more question, since you are a hard man to get hold of, and I will save the others in the hope that I will be able to do this once more with you. We talked about mathetical education. What about metallurgy and crystallography? ... You yourself studied this, yes?

Ewald:

Yes, but I never intended to become a crystallographer.

Kuhn:

No. But to what extent was crystallography itself a part of the education of a physicist, or to what extent was this a local and accidental situation?

Uhlenbeck:

I took crystallography as a student. In Leiden it was required. I don't think any of my co-students took crystallography. And I wouldn't have done it except for the advice of some of my more world-wise colleagues. I wanted to take astronomy, as a minor subject. When I said that, I was told that Seeliger, a famous astronomer, was a very hard examiner. He really wanted the students to know something. And so that was my test. And they classed me correctly as being lazy and not knowing very much. So they told me, "Why don't you go to Groth. Groth won't do you any harm in your examination." And so I said, "Well it's much too late, I can't begin now, the year before my examination, to take up a new subject entirely unknown to me." "Oh yes, you have still time, you can go to his practical class for one semester and then read up a bit. They won't ask any more." And that's what I did. So I made some measurements on the goniometer, and tried to determine the system of crystals which were laid on the table. I don't think I ever did anything useful or correct, but Groth always came and looked over my shoulder and was very nice. And that's the end of the story!

Kuhn:

This was relatively unusual, or did a lot of people do this?

Ewald:

No, certainly not a lot. Not ng the physicists I know of ... Landé also took Groth's course ...

Kuhn:

I think we will stop at that point. In fact I think I've pushed disgracefully far.

 

Session I | Session II