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Oral History Transcript — Dr. Paul Peter Ewald

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Interview with Dr. Paul Peter Ewald
By R.A. Young
At M.I.T., Cambridge, MA.
April 1, 1959

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Dr. Paul Peter Ewald; April 1, 1959

Abstract: Professor at Universitat Stuttgart in 1921-1937; visits the U.S. in 1936 (University of Michigan Summer School). Political climate in Germany and Ewald’s dismissal from Stuttgart; works on foundations of crystal optics while living in mother’s house. Leaves Germany on a research fellowship from University of Cambridge in 1939-1939; professorship at Belfast University, 1939-1949. Relationship with Hans Bethe; comments on his children. Discussion of Ewald’s scientific contributions. In particular the dynamics theory; comments on Max von Laue. Worked as x-ray technician during World War I; stationed in Russia for three years (Otto Blumenthal). On the rewriting of the dynamics theory and problems still to be solved.

Transcript:

Ewald:

I had to give some biographical material to the people in Paris who gave me an honorary degree and so I sat down and wrote my necrologue.

Young:

So, anyone who wishes to know your "vita" should look at your obituary?

Ewald:

Yes. A cheerful thought.

Young:

Professor Ewald, let's follow your travels for a moment. You took your doctorate at Stuttgart. Is that correct?

Ewald:

No, I got that at Munich.

Young:

When did you go to Stuttgart?

Ewald:

When I was made professor there. I took my doctorate in Munich and I became Privatdozent in Munich and began academic teaching in Munich. This was in 1919, I should say. Then, in 1921, I was made Professor in Stuttgart, first an assistant professor. And then I got a call to the University of Muenster, I think half a year after I had arrived in Stuttgart.

Young:

Where is Muenster?

Ewald:

In Westphalia, It's a fine old town. I refused that call. There was a very fine system in Germany, which every part of Germany, every “land” has its own Ministry of culture and education and so you could play them out against one another. Since Muenster belongs to Prussia, I had to deal with the Prussian Ministry of Education, and they offered me very nice conditions, a good salary and so on. And so I was quite tempted to go there. But my Minister in Stuttgart had told me, “Don't tie yourself down before you can let me know what they offer you. So I came back and reported to him, and he said, “We should be able to do better than that.” And so they gave me a chair there.

Young:

At Stuttgart?

Ewald:

Yes, at Stuttgart, and there I remained from… This was in 1922, I should say, and there I remained until 1937.

Young:

Why did you leave then? Were there political reasons for your leaving?

Ewald:

Yes. This was the time of the Nazis, and I had no pleasure at the thought of being involved in a war which was very likely to break out soon, and having to work for the Nazis.

Young:

How early did you see the oncoming of this Hitlerism? How early could you see the handwriting on the wall, really?

Ewald:

Well, I think I saw the first handwriting on the wall when we were still living in Munich. This must have been 1919 or so…Oh, no, not quite as early as that, but 1921-22. I guess I was already professor in Stuttgart, but my family could not move to Stuttgart then, because we had no apartment there, owing to the great housing shortage. But there were the big posters of the Nazi Party, of their rallies in the Lowen Keller — crimson red posters with black writing on them, and then at the bottom JUDEN IST DEB. ZUTRITT VERBOTEN, Jews are not permitted to come. If I thought this was so awful, that since then I knew what to think of the Nazis. It shocked me very deeply.

Young:

I believe that in previous discussions you pointed out to me that the thing that you noticed was that brutality became tolerated, particularly brutal on the party of the police, was it? Or the government, or what?

Ewald:

Well, these party organizations. It was not the police so much, nor the Government. But there was the Nazi Party system, you see it was a regime by itself and they dictated to the police. And, as long as they were not in power they dictated to the Government. They would just knock you down, and knock a few teeth out of your mouth. Or it may cost you an eye, or I don't know what. And of course you didn't like to risk that.

Young:

Did they ever knock you down? How many teeth do you have?

Ewald:

Forty-two. They never knocked me down.

Young:

Did this happen to people you knew? Did you see it happen?

Ewald:

Yes, I saw it, it happened to people we knew.

Young:

And what would be the year of that?

Ewald:

I really don't remember exactly.

Young:

In the 20s?

Ewald:

No, I think this would probably be '33, which is the year when Hitler took over. But let's get away from politics.

Young:

Then in '37 you went where?

Ewald:

In 1937 I went to Cambridge. I had been to America for the first time in '36. The University of Michigan had invited me to their summer school, and I could just make it. I could conclude my German lecture course and leave. They wanted me to be there on the 29th of June. In fact I think I arrived there on the 3rd or 4th of July, not having been able to make it earlier. I went straight to New York and to Ann Arbor, and I must say this was a very nice experience. I have always had a kind of sentimental feeling for Ann Arbor, which I consider as my American hometown.

Young:

Because of that experience?

Ewald:

Yes. And I think it's quite fortunate for an outsider to meet America first not in New York or in any of the big towns in the East, but inland.

Young:

Having KB grown up in the Midwest and having spent four years in New York, I heartily agree with you.

Ewald:

Then I had another great surprise, because I came there with all the haughtiness of a German professor, thinking that I could teach these people what real physics is, et cetera, and I listened in to the other courses which were given. And I found that I had the greatest difficulty in following, that I could hardly understand what was going on. These were courses given by Breit, by-Wigner, by Bethe, and I think Fermi (who only came later, I am not quite sure), and Lawrence from California was there, E.O. Lawrence, the cyclotron man. They had just constructed their first cyclotron, and had asked Lawrence to come there and give them a hand. And Condon gave a course on lattice dynamics, and he did it at such a rate that I could hardly follow what he was doing. I could, certainly… not take any notes simultaneously. It just flabbergasted me. I thought at that time that it would be a nice thing to write up my course — which was already a course on X-ray optics, and contained, although not quite consciously, a lot of the Fourier transform stuff not as well formed as I have it today. I wanted to write it down. I came halfway, and then it grew too hot in Ann Arbor, it was awful. It was a very hot summer. I remember sleeping naked on the floor because I could not stand lying in my bed.

Young:

I heard some other stories about that summer at Ann Arbor.

Ewald:

Rabi was there.

Young:

A fellow at Georgia Tech, whose name was Ewalt — spelled with a T in the end — apparently was there that same summer, and his mail and yours were always getting mixed. Then the story about you taking on the graduate students around the tennis court and running them ragged.

Ewald:

We played tennis, yet. The astonishing thing is that although it was so hot, there was no other hour tree, and I played tennis from 2 to 4 in the afternoon.

Young:

Apparently you were the only one who lasted the whole two hours.

Ewald:

I don’t know.

Young:

Then you went from Ann Arbor back to Cambridge or did you then go to Belfast?

Ewald:

I was still in Stuttgart.

Young:

Oh, I am sorry; it was the next year that you went to Cambridge.

Ewald:

I'll tell you an interesting thing. Just before I started out from Stuttgart I got an invitation from Harvard University, who were celebrating their tercentenary, and would I come to the celebrations, and represent the Technische Hochschule, or come on my own. I didn't want to do it. I had to come back to Germany, where my family was, and all my belongings, and I still had my position in Germany. I was not thinking at that time of just evading my duties and fleeing Germany. I meant to return in orderly fashion, and I had to make up my peace with the Minister of Culture, who was meanwhile no longer the local Swabian Wurttemberg Minister of Culture, but it was all consolidated in Berlin as the Reich's Minister or Culture, Dr. Rust. So before boarding the train I wrote to the Ministry in Berlin, and told them I had received this invitation to the tercentenary, and was it all right if I went there. I waited for an answer for a long time, as I couldn't answer the Harvard people before knowing whether I was allowed to go. And, then in Ann Arbor, through the German Consulate here, the answer reached, me that he did not allow me to go there. So I went. I bought a car, to tour the Eastern Universities and ended up at Harvard just at the time of the tercentenary. But actually the car broke down pretty soon and I went by rail. I went to the tercentenary. A man yesterday — probably President Stratton here, of MIT — told me he had first met me at the Harvard tercentenary, and I told him I had come there rather clandestinely because I wasn't allowed officially to take part, and I only stayed for a day or two. But at that time also I saw Bridgman's laboratory and was very deeply impressed seeing Bridgman and speaking to him.

Young:

When did you then go to Belfast?

Ewald:

I returned to Germany, and at the time they had instituted… the Nazis were now in power in 1936, and they had a Nazi Dozentenschaft Fuhrer, a Fuhrer of the staff at every university. This was a young Nazi boy, of good standing in the Party, who ruled the Dozentenschaft. That is to say, he could announce a meeting of the staff and lecture to them, and of course he got his instructions from the Reich's Ministerium. Everybody had to go there. When I came back from the states, the first meeting, in October — I guess — of that year, 1936, was announced by this Dozentschaftsfuhrer who was an assistant in the Department of Electrical Engineering, and he lectured on the lack of value and the non-existence of objective truth in objective science, which was one of the great themes of the Reich’s (???) Minister, which he had propounded. I think, even at some congress in Paris in those years, there were delegations, and there were Nazi delegations, and I think they talked of that theme too. Anyway, it was the slogan of the Nazis, “Away with the so called objective science, we have volkische Wissenschaft, it's all to be looked upon from the point of view of national interests.” I was so annoyed by all this that I stood up, went through the hall, and went out through the door. And all the people were there, partly trembling, and partly… Well, nothing happened. I went from there straight to the American Consul General, in order to learn whether he could do something for our librarian, who was Jewish and had been dismissed, and wanted to emigrate I felt very happy at having escaped this business of being lectured at by this young boy, and doing something on my own. Nothing happened. But then it took about six weeks, and then I got a letter from the Rector of the Technische Hochschule that I had let this meeting, which was the convened meeting at which I was expected to assist, and I had not given him an explanation, so it was a conjecture of his whether this had been on private grounds or because I objected to the subject of the meeting. This was a rather tricky question. I meant to get away in peace, and not get my teeth knocked out, so I told him that I had left because I had an appointment, but besides I objected very strongly. This developed into a conversation which then led to my breaking away from Stuttgart, asking for my dismissal, and then I was dismissed.

Young:

On the spot?

Ewald:

Well, it took several weeks or so. Finally I said, "l don't want to retain my chair under these circumstances, so please, will you relieve me of my duties?" And this was done.

Young:

What were you teaching there?

Ewald:

Theoretical physics. And then I retired to my mother's house near Munich, on the Ammersee, and I felt very happy. And I began to work. There is a paper in Kristallographie which is the fourth of a series of papers on Krystal Optik which dates from that time.

Young:

What was that?

Ewald:

On the foundation of crystal optics. I had hardly arrived there when I got a letter from friends in Cambridge, who said they had heard from America that I had been dismissed in Stuttgart, and wouldn't I come for a visit to Cambridge. I answered; I can’t really come for a visit. If you can arrange something for me to stay there permanently I can come but I will not be given permission to leave Germany just like that. I can't go on a spree, because I shan't get that permission very often. Wooster — who was the one who had written to me — within three or four days. It just happened to be the day before a meeting of the senate or the committee which had to decide on fellowships and so on. And he just brought it in in time, and. I was offered a fellowship there — a research fellowship in Cambridge. This I began in October 1937.

Young:

How did you leave Germany then? Did you get permission?

Ewald:

I wrote to the Minister and told him I had been offered this fellowship to continue my work, and may I please have your permission to stay in Cambridge for the time of this fellowship. And this permission was given to me, which in a way was quite kind of them. Of course, it didn’t cost them anything, but they might have been nasty and said no, which I had expected, in which case I would have had to go without their permission and wouldn't have been able to come back. But as it was I left my family in Stuttgart. My younger daughter still had to finish school (high school), she was just on the verge of the final examinations and we wanted very much that she should finish that.

Young:

Which one is Mrs. Hans Bathe?

Ewald:

That’s the older daughter, and she was already in America.

Young:

Where did she meet Bethe? Did she come with you to Ann Arbor in 1936?

Ewald:

No. she didn’t come with me to Ann Arbor. Bethe had been my assistant in Stuttgart.

Young:

I didn't know that.

Ewald:

He had been my assistant only on loan, so to speak, while Sommerfeld was in America, only for half a year. But then there is an old family tie, because his father had been assistant to my uncle, who was professor of physiology, and his father was a very famous physiologist.

Young:

I see. How long did you spend at Cambridge then?

Ewald:

A year and a half.

Young:

Did you finally go to Belfast then?

Ewald:

Yes, then I went to Belfast. I would have nearly gone to Canada, to Laval University, where there was also a position open. I had the choice between Laval University, where I would have been teaching in French, and Belfast. I preferred Belfast.

Young:

What did you teach at Belfast?

Ewald:

Mathematical physics, it was called.

Young:

Particularly elasticity, or lattice dynamics?

Ewald:

No, just the ordinary course, mainly for engineers, mechanics; and then the other parts of physics — you know, aero-dynamics, optics, electricity and magnetism. I remained there till 1949.

Young:

Who enticed you to Poly?

Ewald:

I would have been retired in Belfast quite soon. I was 61 in '49, and at 65 I would have had to retire in Belfast. And besides, Belfast is a bit of a dead end in the British University system. It's a bit too eccentric, geographically, and, it's difficult and expensive from there to get to meetings in London — traveling takes more of your time than being at the meeting does and, it's very laborious. I was very glad to change over to America, where I thought the conditions for obtaining work even at a later date would be better. Which I still think is the case.

Young:

Where did you learn English? Did you learn it as a child from an English governess?

Ewald:

I learned it as a child, from my mother.

Young:

Who was English?

Ewald:

No, but she spoke English very well. She was a very good linguist.

Young:

I don't think I heard any German accent in your voice. It surprised me when I first met you. But I do hear a very British accent. Well. Mr. Bienstock here wishes to know about the rest of your family.

Ewald:

I have tour children. The oldest is living in Connecticut, and has a little factory or workshop for making spot-welders — miniature spot-welders. That is very nice. He was the last one to marry. He has a very nice Dutch wife and a daughter and since we are in New Milford we are quite close to them, and can go and see them, every week. The second child is Rose, who married Bethe and is living in Cornell. She is so well known that I needn't describe her. The third one is also a daughter, Linde. She is a medical doctor, and took her degree in Belfast which has a big medical school. She married. a Belfast man, who is an educator, and they are living in near London — that's the end of one of the electric railways from Liverpool street Station, about 20 minutes ride. They have two children. My youngest boy lives in Sidney. He is a chemist, and does high pressure physical chemistry.

Young:

Sidney. Australia?

Ewald:

Yes. And he is married and has two boys and I hope very much that we will go to Australia next year and pay him a visit.

Young:

You are fairly well scattered, aren't you?

Ewald:

Yes, we are.

Young:

According to Longsdale and Van Coogan and others, your Ph. D. thesis played a very prominent role in the origins of X-ray diffraction. I'd like to ask you a few questions about that. What was the title or the main subject of your thesis?

Ewald:

The title was "Dispersion und Doppelbrechung einens elektronen Gitters”

Young:

What is that in English?

Ewald:

"Dispersion and double refraction of a lattice of electron."

Young:

What was the original part of this thesis?

Ewald:

Everything was original. What do you mean by your question? Do you think I stole it? (laughing, in a bantering tone)

Young:

You considered the lattice as a collection of diapoles.

Ewald:

Not a collection, an arrangement.

Young:

And considered the coupling between these diapoles, the effect of this on the scattering of electromagnetic radiation, or just the calculation/index of refraction?

Ewald:

Yes, it’s a calculation of the index of refraction.

Young:

Now, my information is probably not very accurate, so I may ask the wrong questions, but I am sure you can straighten that out. You came to a point in the calculations at which you were having trouble getting conversion of a series. Was it this that you came to Von Laue with?

Ewald:

No.

Young:

How did Von Laue become cognizant of the content of your thesis?

Ewald:

Not on account of convergence. Oh, well, it has in a way to do with convergence too, so in a way it is quite correct. The sums are slowly convergent, and therefore if you have a finite crystal, you would expect the surface to have an influence even far in the interior of the body. That is to say, the way in which you cut off your crystal, would still be of influence quite deep in the crystal, and this necessitated some conceptual refinements of the then usual theory of dispersion. And with these I came to Laue. (At this point a third party joins the conversation. His name is not available to transcriber.) During the years that I have known this gentleman [???] gets told several stories about the early days of X-ray diffraction, and people around him have told me stories about his part in it, and I am trying to get the record straight on his part in it as much as his modesty will permit. I would think that what is here would be of general interest to crystallographers, and to people interested in X-ray diffraction. I can name many people who would be quite interested, and I think that you, Professor Warren, probably have good questions to ask him that I wouldn't know about.

Young:

I was now in the process of asking him about his thesis. I very naively asked him the title of it, which came back in German, of course.

Ewald:

I couldn’t very well give it in Latin.

Young:

My last question was how Von Laue became cognizant of the content of the thesis. I am leading up to asking him about the first time that he just missed the Nobel Prize.

Ewald:

I wanted to consult Von Laue on these questions and concepts. To my astonishment, Laue did not know of the subject of my thesis and of the work. He had been married a short time before, and he took me to his home in the Bismarckstrasse. I remember leaving the Institute in Munich, and walking through the Englischer Garten, and I began telling him of the assumptions of my thesis. And he was so astonished at hearing, for the first time, it seemed that the crystals are considered to be made up of a regular way of atoms, or rather, to be a periodic media. He kept asking, “What's the distance of the atoms?” I told him, "I can't tell you what the distance is, because the distance is very small, certainly, but we don't know whether there are atoms, or molecules in the parts of the lattice, or whether there are groups of molecules, and without knowing that we don't know what the distance is. " Laue pressed on this point, and I rather evaded it because actually I didn't know the answer myself. I hadn't calculated it. I knew it was sure to be a very small distance. Laue was rather inattentive, distracted, and asked several times whether I could further explain. After supper I showed him my manuscript, and showed him the thesis of the formulae et- cetera, and told him of my difficulties. He was rather distracted, and asked a few times, "What happens if you take very much shorter waves than the light waves and if they pass through the crystal?” And I said, “This can be answered very simply because the formulae I have here are strict formulae, they hold in that case, so it's easy to discuss it, but I want to finish my thesis, and at present I am not interested in this.” So I finished my thesis, I didn't get anything out of Laue — no help I wrote up my thesis, and was glad to get rid of it.

Young:

When was this?

Ewald:

Probably in January 1912, maybe in December 1911, I don't know. And I don't know when I handed in the thesis — probably not very early. Probably it was in 1912. And I had to work for my final exam, for my oral exam, so I was kept busy, and Laue never spoke of it again. I took my exam, and I retired out to my mother's house in the country and I was very happy there. She painted my portrait, which was burned up in Stuttgart during the war. Meanwhile Sommerfeld had sold me to Hilbert and I was to be his physics tutor in Gottingen — his private tutor, so to speak. So I went to Gottingen, and the next thing I heard was that Sommerfeld came to Gottingen — I think about Whitsuntide — and gave a colloquium in the Physics Society in Gottingen, telling of Laue's discovery and Laue’s work. I had never heard of it. The night after his talk I sat down, and discussed my formulae and invented the sphere and the reciprocal lattice, and saw that it comes out very easily and sweetly.

Young:

Was it with Sommerfeld that you sat down and discussed this?

Ewald:

No, in Gottingen.

Young:

Oh, you discussed them with yourself.

Ewald:

Yes.

Young:

And you produced the Ewald Sphere and the Reciprocal Lattice. ? The Reciprocal Lattice and the Ewald sphere were at that time? I always thought that came later.

Ewald:

No, it was at that time. It was a restricted reciprocal lattice, because my formula dealt only with an orthorhombic crystal, for convenience. Then later on Laue generalized that to the general reciprocal lattice. What astonishes me with regard to this early work in X-rays, is the difficulty we had at the time, of combining the two ways of looking at it: the continental way of speaking of diffraction, and the Bragg way of speaking of reflection, which seemed simply to fall apart, in two entirely different ways. And very few people recognized the coincidence of the two things. For instance, there was a paper by the Russian crystallographer Wolff, explaining that he had taken pictures with zinc blenders, along the tube diagonal, and he had obtained six spots in a triangular ray, and so on. Of course this comes out at once in reflection (?) of the reciprocal lattice, but it was every simple, stupid little detail that required so much ingenuity and work at the time. It's really remarkable how slow progress is on things which later on, or very soon, become quite obvious.

Young:

How long did this take in your course? One lecture or two? The same material when you teach a course in advanced theory.

Ewald:

I did that offhand.

Young:

Was the Ewald method of lattice sums used in your thesis?

Ewald:

First of all, Ewald method is not correct, insofar as the idea is old. It is contained in papers by Riemann, as different types of transforming the sums. First of all, there is what we now call the Fourier transform. This term was not invented at the time, and nobody considered this as a special type of transformation. I don’t know from whom the term "Fourier transformational" stems. I wonder whether that might be Wiener who has written on Fourier transformati0n. But I never found out who was the originator of the term Fourier transform. You see there is such a term, because it consolidates your thinking. Well, this was one type of transforming the lattice sums. The second and more difficult one was the one that leads to the lattice summation formulae, and which applied to the transformation of theta functions. This was an idea which Debye gave us, to Sommerfeld and me, that we could use the same method that Riemann had used in that paper, I don't know on what occasion namely to write down a certain sum in form of an integral, and this integral is shown to be a theta of theta function, and then you can use the transformation theory functions in order to transform the integrant, and then integrate one part in one form, and the other part in the other form. So this goes entirely back to Debye. It's a very useful method.

Young:

It sounds much better in the original, Die Stella Methode. Stele? Now, to me you are best and most widely known for the method, of lattice sums, and next for the dynamical theory of X-ray diffraction. Professor Warren, how do you know him best?

Young:

I would put it the other way around. Dynamical theory.

Ewald:

Yes, you and I being interested In X-rays; do recognize him for the dynamical theory most. Are there other things besides the dynamical theory that you know him for? Well, the Ewald theory, of course, of the reciprocal lattice, which we have covered. Anything else?

Young:

Don't you think that the place where your name is thought of first today is in connection with the dynamical theory?

Ewald:

Yes, I think that is really the valuable work live done.

Young:

But I think you still have one big job to do, and that is to rewrite that dynamical theory so that all people can understand it. Won't you do it?

Ewald:

Yes, I am going to. But you see, my form of the dynamical theory has been replaced very largely by the Laue form. It was very funny. This whole theory originated during the war, and I worked it out in Russia, sitting there at my X-ray table, when there were no patients lying on it. I handed this is as a "Habilitationsschrift” — a paper for becoming a lecturer in Munich. Sommerfeld read it, and sent it back with some comment which I don't remember; probably that something might be changed. I guess I did that, and I sent it back again.

Young:

What is “it” that's going back and forth?

Ewald:

This manuscript of the dynamic theory. I know that Sommerfeld told my mother that he had tried in vain to understand it, but that it probably was a very good one. So he accepted it.

Young:

Where were you when you developed it? Was it on the Russian front? I was interested in how you got the X-ray table.

Ewald:

Laue evidently never understood. it, because he rigged up his own form of the dynamic theory, attaching it to wave mechanics, and introducing boundary connections which don't occur at all in my form. So, actually Laue's theory doesn’t go one iota beyond mine in what it achieves. Both have a limited application.

Young:

The Japanese prefer yours, don't they, the electron diffraction people?

Ewald:

Not for electron diffraction you have to use something which is closer to the Laue form, which was developed by Bethe. It's Bethe's theory which was modeled on mine.

Young:

When did you go to the Russian front?

Ewald:

In 1915.

Young:

And how did you happen to come by the X-ray table?

Ewald:

Because I was an X-ray technician of the army. So I spent a happy three years in Russia, where the fighting had practically ceased, and there was nothing gruesome to be done. There were rather old peasants who were brought there from the Western front, where they had been half killed over to the Eastern front to recover and there they broke their bones, and liked it, because they couldn't be sent back to the Western front. This is important. For three years, experimental X-ray work. (laughter)

Ewald:

Don’t you know that I was experimental assistant to Sommerfeld? On graphite, for instance, there is an early paper of mine, and that was a very difficult subject, because the graphite crystal was very, very thin, and you had to use an exposure of 200 hours or something like that, and, with the old type of tube you had to regenerate every quarter of a minute.

Young:

So you were an X-ray technician through a typical bit of army maneuvering, then, I suppose. How did the word. X-ray get connected with your name? From your thesis?

Ewald:

Glocke, I think, brought up this idea that we people who were working on X-rays — he was also a student of Roentgen is, of Wagner's, also in the X-ray field — and he brought up this idea that it would be a good thing if we used our special knowledge of handling X-ray equipment for this purpose, because there was a lack of X-ray technicians for medical purposes.

Young:

As I heard the story before, there was a weather station nearby, too, and it was fully manned.

Ewald:

Yes. This was very nice. At first, I was on a big farm, a big estate, a rather isolated place. Then after a year or so I was transferred to a little town, just west of Duinsk — Noi Alexandros [???] where there was a weather station, and this weather station's chief was Professor Otto Blumenthal, who was the editor of Acta Matematica and professor of mathematics in [???] , and he had a number of young mathematicians in. the weather service. You see, for plotting charts and integrating the charts you need mathematicians. Yes, you need mathematicians, quite seriously — people who have a feeling of how to integrate a field of given data, like drawing equal lines on a Fourier map. You can’t leave this to a cleaner, after all. The military don’t mind wasting people, and then of course these mathematicians liked the job, so they were there, and it was quite a crowd, and so we started a little physics colloquium. This was very nice. It gave me some exercise, because it was about six miles away, and I walked there every weekend. It was very pleasant for me to have Blumenthal there, who had a very varied range of interests. He was a very good linguist, he knew Russian very well, he knew French and English, and German, of course. We became very great friends. He was Sommerfeld's first pupil. When Sommerfeld became lecturer in Gottingen, Blumenthal was in his first class, so you see there was another relation.

Young:

I gather that if it hadn’t been for the German Army and the Russian front, the weather station with the mathematicians and the great big X-ray table, there would have been no dynamical theory, is that correct?

Ewald:

Certainly not. While I was reaching the results in my dynamical theory, I finally managed to understand Darwin's paper, which is very simple and very beautiful. It's a wonderful paper, very penetrating. But I’ll tell you the difficulty we had. Considering the interaction of a single atomic plane with incident X-rays, you get a cross rating effect, so you not only get a reflection, which is a zero order spectrum, but you get H K orders all around it. Well. Darwin just ignored those. He only spoke of reflection, and this always kept me from taking this theory seriously. I didn’t see what became of these other directions.

Young:

I got through the Darwin theory long ago, so that I felt that I really understood it. That's why I want you to write up the dynamical theory in a form that's at least as easy to comprehend as the Darwin theory. Maybe I am biased, but I think the Darwin theory is fairly easy nowadays for a person to comprehend. It might not have been back in those days.

Ewald:

I wouldn't agree. I think it is, for this very reason, that is, that you don’t know what happens to these other spectra… or course, they are not in phase in general, so you can neglect them in most cases, but not, for instance, if you have simultaneous reflections.

Young:

No, of course not in that case.

Ewald:

But this is a general optical case. Think of integrals of an X-ray along the symmetry axis, where you are bound to get simultaneous reflections. You don't want to cut that out. Well, the Darwin theory would be quite helpless in that case. It would have to be modified and it would become too complicated, that it's probably easier to do my form of theory than Darwin’s.

Young:

I still say you've got to write it up.

Ewald:

He’s been promising to do that, I think, since 1916.

Young:

This morning he told me that after he completes giving this course at Leeds this year, then he’ll be inspired again, and hell write it up. This was the story after he gave the course in 1954, too.

Ewald:

I have not been able to solve the phase problem, and now I know that I shall not be able to. At times this was not so clear, and I was waiting for something to happen in that field.

Young:

This brings up something else, which I think — partly for myself, and partly beoause other people have told me this … it’s characteristic of your work that you refuse to publish until you feel the job is complete, and there is a certain elegance associated with everything that you publish. I think the dynamical theory is elegant, but it's not easy to read.

Ewald:

I don't think this is quite true of what I did in the Handbook, Handbuch der Physik that is. It is not a derivation. You tell about it in words, and that doesn't satisfy a lot of people, they want the whole story.

Young:

This drive for completeness, elegance, and general perfection in the work that you do has caused you not to publish things that later other people have published, and have become known for, I believe. I have in mind in particular the Paterson synthesis and the Beaumont effect. Is that correct?

Ewald:

The Beaumont effect I never foresaw. The Paterson method I had in 1925. I knew what the Fourier theories with the intensities as coefficients, that that was the [???] of the original, of the intensity. At that time I was trying to find methods for amp holding (?), and since these were not successful, I said, “What’s the good of it?” Furthermore, at that time the whole Fourier method of dealing with structure analysis was not generally used, and therefore it did not occur to me that it might be useful just to plot the experimental data in this different fashion. After all, Paterson's diagrams are nothing but re-plotting of the experimental data in a more suggestive way.

Young:

I think you've said in times past that you consider the dynamical theory as your life's work. What are the dates of publications on parts of this theory, and in what journals?

Ewald:

Der Journal der Physik, probably in 1918 or 1919.

Young:

And this is essentially the whole dynamical theory?

Ewald:

Yes.

Young:

How about the publications since? Are there publications since?

Ewald:

I took up this question when I left Stuttgart, and I told, you I retired, to my mother's house and was very happy, and there I began working again on the dynamical theory, because there is still an unsolved problem, which I may tell you about. That led to the publication in 1937 in [???] Kristallographie, which is the continuation of [tape defective here] several diffracting centers in each cell. So we used a quality composite crystal. If you have that, then you can deal in a general fashion with the case of having only two rays in the crystal, namely one primary, and one [tape defective here].

Young:

A transmitted ray?

Ewald:

Transmitted or reflected. Interview interrupted)

Young:

We were discussing the problem left in the dynamical theory.

Ewald:

The problem is this: either you have a simple crystal, and then you can have many diffracted (diffracting) rays, simultaneously diffracted, and then you can solve it, or you have a crystal which has a structure factor, and then you can only solve the case of having one incident and one reflected ray. And the general case is too complicated. for solution, and my attempt in the Kristallographie paper was was to deal with this general case.

Young:

For a simple crystal, you mean [???] point [???] scattering

Ewald:

A crystal without a structure pattern.

Young:

The reason you were interested in this was to take care of the ease of simultaneous reflection?

Ewald:

This is one reason, but a more important reason is that I wanted to build up a theory of optics in a crystal, which would comprise X-rays as well as ordinary optics — all range of wave lengths. If you want to do that, you must have a strict theory. The Fourier transformation of the electro-magnetic fields, that is strict, but not the theory of interaction.

Young:

When did you start on your Krystall und Roentgenstrahl?

Ewald:

This began when I gave a short course on X-ray diffraction. The University of Munich arranged a kind of course for G.I.s coming back from the war, and there was one course which I gave on X-ray diffraction. This was during the Easter vacation, I guess. I asked two students, would they write it down, and work it out, so that it could be published, and I wrote to Berliner, the editor of Naturwissenschaften, that by Whitsuntide I would send him a manuscript of a book which could be published. One of these students was Laporte, and the other was Heidenberg. They did a very satisfactory job. I had to do it a report on that, and this led from one page to the next and then to the next, and certainly I wasn't ready with the manuscript by Whitsuntide. I think this course was given probably in 1920. And it took me at least three years instead of three weeks — or five weeks, between Easter and Whitsun — to write the book, and then it came out in 1923.

Young:

So really from about the end of your dynamische Theoriem a sharp break, and then you are on Kristall und Roentgenstrahl.

Ewald:

Yes, that's right. But you know, in this whole book no discussion is given of intensity. This was quite characteristic then.

Young:

Now let me ask you to retell a little story that you told once before about Taman. Do you recall the story?

Ewald:

He was a great metallographer, and really one of the great scientists, a physicist, a chemist, and a metallurgist. He was professor in Gottingen, but he was born in East Prussia, or rather, a little bit further east. I think he came from [???] which I am not sure what state it belongs to now. And. he spoke with a strong accent, like these people have. He was shown Drucker's laboratory in Stuttgart, at a meeting of the [???] Bunsen-gesellschaft. Then he was asked his impression, and he said, “Well, what do you people do there? I just take a crystal, and look at the window across, and then I know how to orient it.”