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Oral History Transcript — Dr. Otto Frisch

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Interview with Dr. Otto Frisch
By Thomas S. Kuhn
At Cavendish Laboratory, Cambridge, England
May 8, 1963

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O. R. Frisch; May 8, 1963

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 teh 1920s. Also prominently mentioned are: Niels Henrik David Bohr, Albert Einstein, James Franck, Fritz London, Karl Mueller, Peter Pringsheim, Isidor Isaac Rabi, Otto Stern, Hans Therring, Otto Warburg; and Universitat Wien. Intervew conducted as part of the Archives for History of Quantum Physics (AHQP) project.

Transcript

Kuhn:

Now, I don't know where you would like to start.

Frisch:

Well, let's get the chronology right. I studied in Vienna from 1922 to '26, so the Schrodinger equation came just as 1 quit. That is about the one thing I remember of the new quantum theory. Professor Flamm of the Technical High School gave a talk in our seminar, or something, on the Schrodinger equation. And I have a vague recollection that I was confused and excited at the same time; it did seem to be something new and potent. There was a rather long silence after the talk and then Thirring spoke up and said, "Of the many things I haven't understood, I want to inquire about one which I haven't understood at all." Well, that's about my one recollection of this.

Of course I knew a little bit about excited states and so forth, and in fact my own doctor's thesis was concerned with getting a crude excitation function; namely, I worked with Przibram in Vienna, who had been interested in the discoloration of salts by radiation. He suggested I should reproduce what other people had found, namely discoloration by fast electrons; and then see how much I could lower the energy of electrons before the thing fails. And I managed to get not discoloration, but thermoluminescence down to something like 6 volts, but it was an extremely vague and subjective phenomenon. All I got was two experiments, where I did observe a little light flash after bombarding at 6 volts.

I did not get anything at 8 volts; I got it quite clearly at 10 volts. So that's the quality of the work I produced, And I did get my Ph.D. all the same, Doctor of Philosophy to be precise.

Kuhn:

Before you heard of the Schrodinger equation, had you been aware at all of the matrix mechanics?

Frisch:

No, no, not a bit. Not a bit. But then I was not at all a theoretician; I was basically just an experimenter; and although I was interested in mathematics, I never tied up mathematics and physics. Mathematics was just a pleasure.

Kuhn:

Was that a usual thing at Vienna? Could you have been a theoretician at Vienna in this period?

Frisch:

Oh, I think so; I an there were lectures given on theory; but these lectures were all very conservative and hide-bound. There were lectures on Maxwell's equations and working out all the classical problems. Thirring gave a big lecture course on theory which covered four years; one quarter of the whole theoretical physics. I think he never even touched anything that was controversial or new, or not understood.

Kuhn:

What about the Bohr atom, for example; how largely would that have figured?

Frisch:

Now, I can't remember. I knew about it; I knew the idea of excited states. Much of these ideas actually came to me informally because one of my colleagues, a year or two older, who did his doctorate about the same time, worked also on this phenomenon of discoloration and thermo-luminescence. But he had a much better understanding and made some real headway, for he had some clear ideas. But, for instance, as I recall it, he was the first to show to our knowledge that Stokes rule in luminescence had something to do with quantum theory. I dare say it must have been understood by other people elsewhere, but it was obviously so little talked about that he thought he had discovered it.

Kuhn:

I am perhaps leaping too far at conclusions, but this does sound, in terms of the developments that I've been particularly studying, like a fairly backward community. What sort of literature did one read as a matter of course?

Frisch:

Well, I don't know. I was never given any firm instructions, "You go to the library and read modern literature." It never occurred to me that I might. Maybe I was the backward one, because, as I say, my friend who worked on thermo-luminescence and so on obviously had a much better knowledge of what was going on and what the concepts were. And he had been, I think, studying some of the papers of the Pohl school, for instance, and did put the finger on the logical theory of what we now would call displaced atoms or excitons or whatever it was. In these data I think they were just called Farbzentren, color centers, in a general way. I was extremely eager, and probably more so than the average, because I was always rather an individual responding to my own thoughts and interests. M professor apparently thought that I was doing quite nicely building up the necessary gadgetry, and he never pushed me into doing more reading. I only got that when I got to Berlin afterwards.

Kuhn:

You speak of the four-year lecture course in theoretical physics. Was there really so very little mathematics included there? You spoke of yourself as liking mathematics, but never putting it together with physics; I'd suppose an awfully lot of putting together might have gone on in a four-year course.

Frisch:

It ought to have; well, they certainly did do things like, shall we say, the Hamiltonian equation, the Lagrangian equation and the Jacobi equation and things like that, plus classical physics. I remember that I found that very tough going because I couldn't see what it meant. The mathematics was reasonably simple and easy to follow, but I didn't know what it meant. And my own interest in mathematics was much more directed to what seems like number theory and topology, really pure abstract mathematics for the joy of it. I remember that when I eventually came up for exams, I chose physics and mathematics as my two subjects to be examined in. And in mathematics I nearly failed because it turned out that I had only the haziest notion of the theories about complex variables, functions of complex variables. I managed to guess the right answers on some of the questions, and so eventually got through; but I had never used that theory and didn't know anything much about it.

Kuhn:

Do you remember, were there any standard books you used along with the courses?

Frisch:

Not that I recall. You see in Vienna there was no system even remotely similar to supervision or anything like that here. The idea of the University was: you pay your fees, you visit so many lectures a week--10 lectures a week you have to go to, never mind which; you can choose them from music to archeology to whatever you like. At the end of your year you present a thesis and offer yourself to examination. You pass and you get a doctorate; how you do it is your own business.

Kuhn:

There weren't any sort of reference books recommended by the lecturers? Because I take it that if you were going to be examined in physics, you made a point of at least finding out what went on in the physics lectures.

Frisch:

Well, I did go to the physics lectures; I attended them, I took notes, I understood them reasonably well. Some of them I worked over with some care afterwards. But I hardly read any books. I got some books given to me, largely by Lise Meitner, who had some idea about what I ought to read; but on the whole I just stuck to the lectures. I had arguments with some of my friends about some physical problems; I think that's how I learned physics.

Kuhn:

How large a group was studying physics at Vienna?

Frisch:

Well, the classes in general physics were enormous, because all medical doctors had to go, and even the budding chemists, [pharmacists] the chaps who sell drugs, you know, had to go to these lectures. So they were enormous classes. The classes in theoretical physics were still quite large; I have no very clear recollection; but I recollect a large lecture room which must have held at least a hundred or two hundred people in which Thirring gave his lectures. Some of the more specialized classes were smaller; I think you had probably ten to twenty students or so in electrodynamics, or something like that. But it was also completely unsupervised, and hence almost completely random. For a time I went to a fantastic number of lectures. I went for five hours every running with the result that I always slept in the last lecture; I couldn't keep myself awake any longer, because it happened to be a very dull one too.

Kuhn:

At the time you were doing research in Vienna, how many other people were there ahead of you for degrees with whom you would have formed sort of the advanced student group?

Frisch:

I wouldn't say I formed anything like part of a group. I had just this one friend, and he was largely my friend because we were both keen on music. I knew one or two others; I would say that probably Przibram had a total of half a dozen people working for him at any one time; and there were several other factions. So I would guess there were about thirty people at any one time working for their doctorates. No, it must have been re; because the doctorate was very much more common; in Vienna practically everyone took a doctorate. The thesis work usually was only about a year, or a year and a half. I'm afraid I'm very unreliable on these recollections.

Kuhn:

On some of these questions there are ways to check, and anyway it gives us an impression. Who besides Przibram would one have done a thesis under? Thirring would have been another.

Frisch:

Oh Thirring would have given thesis (work). And then there were the experiment groups; in these days the atom splitting was just beginning. Well, it was more than beginning in fact. There was a great deal of controversy because the Vienna group under Stetter and Kirsch and Ortman and others, Pederson, a Swedish chap, were getting results which were in contrast to the results practically anywhere else. They used scintillation methods like most people used in those days, and it was afterwards generally believed that they were rather uncritical in check experiments. They used students to count scintillations; [for] they had the idea that if the student didn't know what it was all about, so much the better; they would have no bias. But unfortunately the students apparently felt there was a bias in the direction that the more they saw the more they were praised. So, in consequence, they tended to see scintillations where there weren't any. I don't know whether some of them actually cheated or whether it was a psychological effect, but the Vienna people discovered that practically all elements could be split by alpha ray bombardment, and this couldn't be confirmed either in England or in Germany, where the work. was repeated. But in Vienna everybody seemed to believe it, and I remember that Przibram, who was not himself engaged in that work, shook me by the hand in his hearty and fatherly way when I went off to Berlin in '27 and said, "You tell those Berlin chaps not to be quite so skeptical about our results." Something like that. About the same time they were beginning to build up electric. measurement methods; and when they eventually got those going, everything collapsed. They began to get correct results.

Kuhn:

How does it happen that you were not involved with the nuclear group at that time?

Frisch:

I honestly don't know. I have a vague recollection that somebody once asked me whether I would like to take part in the scintillation counting, and I said I was too busy. I had started my own thesis at the time, and I felt it occupied me fully, and I wasn't interested in the other work. Maybe if they had told what they were doing, I might have become interested. But since they wanted to use me as one more body to peer through a microscope for an hour a day, I wasn't interested.

Kuhn:

You might have been doing your thesis in that area, mightn't you?

Frisch:

Yes, I might have. So the question is why did I go to Przibram? I have an idea it had something to do with the friendship of Lise Meitner and Przibram about that time. She mentioned him to (???) and said, "Send him along to me, and I'll see what I can do for him." Not that she ever did anything to boost me; in fact she was always falling over backwards in her fear of being accused of nepotism.

Kuhn:

No, it was actually because of Lise Meitner that I wondered a little bit why you hadn't gone to the nuclear group, because it would have been a natural thing. She knew how much was going on there.

Frisch:

It's also possible--I was never aware of it; but now that you say it, it's quite possible—that she was skeptical about this work of Ortman and Stetter and so on; and didn't want me to get mixed up in it. And for that reason she suggested

Przibram.

Kuhn:

Now when you went to Berlin, you went where? To the Reichsanstalt?

Frisch:

I went to the Reichsanstalt, that's correct, and did some entirely empirical work trying to improve measurements of brightness of light intensity; I happened to get into the photometry division. I was seeing if one could constitute a unit of brightness to replace or standardize the candle power based on a fairly complex proposal by my boss, who was Geheimrat Karl (MuIler), a brilliant experimenter and not in the least bit interested in theory. It never came off; I made it work after a fashion; but it seemed a complicated thing; and it was all based on an idea by old Warburg. And when Warburg died and became replaced as the director of the Reichsanstalt, the project was immediately abandoned.

Kuhn:

You may just from that experience be able to help with one problem that's been on my mind and which I've never had a chance to look into. James Franck said he was quite sure--and this you see would be really before his time--that the whole origin of the standard black body radiation at the Reichsanstalt, which is where it starts really, had been in response to a desire to do just this stunt of replacing the candle; to try to find some way of standardizing luminescence. This would take it back into the 1890's, which is not an unnatural time, with the growth of electrical industry and electrical lighting, for the problem to have arisen.

Frisch:

Well, I have no first hand knowledge at all. When I got there, it seemed just a natural thing that the photometry division should be interested in black body radiation. Warburg's proposal [was as follows]: You've got one wave length; you measure the brightness in this wave length at two temperatures; and you also measure the ratio of the total radiations at these same two temperatures. If you do that accurately enough, you can determine both temperatures; or at least you can fix them. I mean, you can specify that the two ratios must have certain values, and you can always get back to those two temperatures.

Now, MüIler had made a competing proposal based on absolute measurement of the total radiation, which would have been a very rational and elegant thing to do; but at that time many people were skeptical about whether absolute radiation could be measured accurately enough. But I think that was certainly at a time when Planck's formula was fully established and was the basis for it all. Nobody had any idea of testing it; after all, I got there in 1927. And as far as I recall, when Planck got his formula, he talked to Rubens, who was not, I think, at the Reichsanstalt but at the physics department of the university, asking him to check whether this formula agreed with measurements.

Kuhn:

Well, there's a good deal of measurement made in advance of the production of the formula. Lummer and Pringsheim were both at the Reichsanstalt at that time.

Frisch:

Yes, they must have been. Yes, I see.

Kuhn:

Did you find at all that situations for physicists in Berlin were very different? You speak of having had one--

Frisch:

Well, Berlin seemed very much more alive. For one thing there was a colloquium there which had a front bench of Nobel Prize men practically. I mean, on the front bench there was Hertz, and Einstein, and Nernst and a few others, I don't know who, of similar caliber. There all the modern papers were brought up and discussed. So that practically all students watched physics growing, as it were, from first hand. The Reichsanstalt itself was a rather stodgy place, at least what I saw of it. However, right next door to the photometry section there was Bothe doing work on atom splitting and so on and just getting ready to discover the neutron, which, as you know, he never realized. Now that was much more lively, and there I learned a good deal more about modern physics although I still had a strong tendency to stick to classical problems which I could really, so to say, understand with my finger tips.

Kuhn:

When you say you learned re about the new developments, did you do this simply by sitting in and talking to people; or did you make any systematic attempts?

Frisch:

No, I made no systematic attempt. I have never been a good learner; I mean my idea of working has always been to listen to so thing and say, "Well, now, does it mean this?" Then I would sit down and try to work it out for myself. This is a wasteful way, and I sometimes regret that I wasn't pushed early into saying, "Let's look up things in books." But I could nearly always work out for myself what I wanted; as a result I have never become very good at quantum theory.

Kuhn:

Was there great excitement or great debates about the new developments? I realize I'm missing a year or part of a year in here somewhere. You took your degree in '26 and then in '27 went to Berlin?

Frisch:

That's right. I spent a year in industry in Vienna in a radios and things like that.

Kuhn:

Was this for lack of a job?

Frisch:

Well, actually at that time I wanted to go into industry. I thought it would be career. A friend of my father's happened to have an opening, and so in I went. I was quite happy there for a year, and then to my surprise I got an enquiry about whether I wanted to come to Berlin with a fellowship--a grant to work on this unit of light. I jumped on it because it was a chance to go abroad and a chance to see new places and new people and so on. And that's what got me back into science, even though there my job really wasn't frightfully scientific; it was largely a matter of comparing photo cells and building equipment which would measure down to 1/10 of a per cent, and so forth. I learned very much more science when I came to work with Stern after 1930. I stayed three years in Berlin.

There was a little episode in between which might just interest you in your context.

When my grant came to an end, in fact before it came to an end, I got interested in a problem to such an extent that I managed to get myself a place at Pringsheim's lab and worked there in my spare time on it. It got published even. The idea was that if, in the Zeeman effect, you separate out the various angular momentum states of the atom and if a photon is emitted, this photon must presumably take with it the angular momentum (when that) changes. That seems clear enough; [for] if you go in the direction of the field, you get polarization. But I said "What happens to the angular momentum if the photon goes of perpendicular to the magnetic field?" There you get linearly polarized light which according to our existing theories had no angular momentum. And that to me seemed a contradiction. I said, "This can't be; the photon must somehow keep the secret of its angular momentum even if it isn't displayed in Maxwell's equation." And so I thought I might be able to demonstrate that a photon may have a right-handed or a left-handed spin about an axis perpendicular to its propagation which doesn't show up in its polarization properties, but which might show up if you tried to reabsorb the photon again in a suitably quantized atom. I remember that I went with that idea to none less than London, Fritz London. Fritz London was in Berlin at that time, and so I. went to him and exposed this idea. And he thought it was sensible, and so was sympathetic and said, "Yes, do try that, by all means." It sounded to me as if he himself didn't know the answer.

It's of course quite possible that he was sure the answer would be negative, but he saw no harm in a young man trying the experiment to convince himself. So I went and did the experiment; and as one might have anticipated with better understanding, there was no difference. The photon didn't have any secret spins as it were. What I did was to have an emitter and an absorber, both of mercury vapor and both in a genetic field. If they were lined up in the axis of the field, then, when I turned the polarity of one field round, there was a big change in the reabsorption, because right-hand light was turning into left-hand light, you see. If I lined them up cross-wise to the magnetic field, there was no change in absorption if I changed one of the fields. And that, of course, disproved my idea.

After this happened and while I was writing up that result, I happened to come across Franck, who had just visited Berlin; and I told him about this experiment. Then Franck suggested in his very kindly, tentative way, never making anybody feel that he might be an idiot, that if a photon goes out from a circulating electron, you might imagine the photon as flying off, not from the center of the system, but as it were, a little way off [center]. If it were emitted off center by one wave length, it would just allow you to take away that much angular momentum by orbital motion, as it were--by being off center. And that of course is the answer, if you want to have a model answer; and I duly put it into a postscript to my paper and thanked Franck for it. And I told myself afterwards, "Now, if I had only met Franck, or somebody with his intelligence before I began, he would have stopped my wasting several months on an experiment like that."

Kuhn:

I'm very glad you told me that. Did you go to the colloquium regularly while you were in Berlin?

Frisch:

Yes, yes. That was one thing I really did; it was, of course, Lise Meitner who made me go at first. But I very quickly saw that this was extremely interesting; and I mean, I was present when the Raman effect was for the first time presented, and things like that. I was present when electron diffraction was discovered and presented.

Kuhn:

Do you remember discussions that went on in connection with those? On electron diffraction for example?

Frisch:

No. My recollection of discussions is merely that any real discussion was really over my head when it came into theory. Much of the theory seemed to me quite meaningless. Very often it was rather stamped on by the big shots in the front row. I mean, Einstein would get up and point out that this didn’t make sense. One typical item of course was Nernst rising to his feet and shaking his finger and saying, "I told you that 40 years ago:"

Kuhn:

Taking the electron diffraction as an example, clearly to some people this was magnificent confirmation of something they already believed. Surely to others this was something that was going to force them to believe something they knew they didn't want to believe, and there were probably some others who thought maybe the experiments were wrong or something of the sort.

Frisch:

I don't recall any such reaction. I think it was clearly expected that people who had really been in the swing, I mean people like Einstein and so on, would say, "Well, this is a very fine achievement; we can see that it really is the way that de Broglie has predicted it." A good many of. them already felt that this was confirmed by the success of the Schrodinger equation. But I don't remember any time anything was discovered and disbelieved to an appreciable extent. I mean, Raman's discovery was immediately accepted, again as something which had been expected. People only made fun slightly because the reporters said that Raman had put into his paper that this was a strict analog to the phenomenon for which Compton got his Nobel Prize, or something like that. It was some phrase like that, sort of hinting, "How about a Nobel Prize for me?"

I don't even remember whether the competing publication by Tamm, Mandelstam, et. al. in Russia became known immediately or whether that was something only discovered much later.

Kuhn:

Were you in at all on the debates that went on as to the interpretation of the new mechanics?

Frisch:

The interpretation of wave mechanics? No, hardly at all. I remember having lots and lots of arguments with a Polish fellow student of mine in Hamburg later, when I came to Stern. I seemed to have somehow soaked up the complementarity ideas; I really don't know where I got them, because by the time I came to Hamburg in '30, I was already a nice little liberal in that respect. I knew all the answers about localizing photons and what happens if you try to cut a photon in two and things like that. I had a good school there because there was this very bright and intelligent Polish chap, Jablonski; he's gone to America since. He didn't believe all this, and he therefore quizzed me very closely and then fought me tooth and nail. Once again we made music together; oddly enough that always [seems] to be the bond, with me.

But where I really got to understand these things, I don't knows I'm trying to associate a name with it; I'm quite sure it wasn't Miller. I knew Pringsheim slightly, and of course I worked with Pringsheim for a while on this experiment. Later when my grant came to an end, Pringsheim offered me a place and an experiment, which also was done--a much more straightforward thing. Incidentally it was an experiment which he set up and which he interpreted wrongly. And I never spotted it; it was later found out that what I had proved, or thought to have proved, wasn't at all that. Pringsheim thought we had proved that an excited state would have a different branching ratio in decaying depending on which way it was excited. That would have been a very startling thing indeed, in view of the idea of independence of states. But in fact what happened was that that state had a hyperfine structure; and, depending on which way you excited it, you got a different proportion of the hyperfine structure, giving reversal phenomena and things like that. It was all much more complicated. That paper is in the literature; it was published under our joint names, Pringsheim and myself.

Kuhn:

Still it doesn't sound as though the man who made that interpretation at that period was the man who was going to give you the whole argument about splitting photons and so forth.

Frisch:

I think I probably argued a good deal with Pringsheim during that time I was with him, and I think I must have learned quite a lot. I certainly had the best opportunity because the apparatus was set up in his private office, and so I saw a great deal of him.

Kuhn:

Do you have any notion what you were reading to get these ideas from?

Frisch:

Once again, I have no recollection of what I read. I think by that time I have learned to look at Zeitschrift fur Physik and things like that.

Kuhn:

Do you suppose for example that you wrestled hard at one point with the Heisenberg paper, the one with the uncertainty principle?

Frisch:

No, no.

Kuhn:

Or Bohr's Como speech, which is a good deal more obscure, which reaches for complementarity?

Frisch:

No. I think all this complementarity stuff I got very much later when I was in Copenhagen. I worked for five years in Copenhagen, and there of course we had Bohr on tap, and there was no great difficulty in turning the tap.

Kuhn:

Well, I thought you spoke of having it pretty well on tap when you went to Stern for the arguments--

Frisch:

When I went to Stern, I already had a good many of the things. I think I would sometimes find an argument in the literature which intrigued me. Also at Stern's there was a good colloquium where Minkowski, for instance, presented ideas like that. I still remember that within a few days of arriving in Hamburg I attended one of those colloquia and spoke up in the discussion. As a result I found afterwards Minkowski coming to me and talking to me and explaining things. Not a very close, but a very pleasant friendship ensued. I learned a good deal from Minkowski. You know he is now in Palomar.

Kuhn:

Actually at the moment he's in Berkeley. I talked with him there a couple of times last year. I think probably he's going to stay there now; he's been working on the radio telescope there. What about Stern himself?

Frisch:

Stern was very interested in fundamental questions, but usually from a thermodynamic angle. He was basically a thermodynamicist. At one time he even tried to give out a sort of informal training in thermodynamics, setting up little quiz problems and making us solve them. But that petered out again. One of his experiments I did there was apparently still a live issue at the time That was the attempt to show that space quantized atoms can jump into a different state of space quantization if the magnetic field changes. Nowadays of course one looks at this in terms of resonance, that is, since Rabi. But in those days nobody thought of resonance, and the question had apparently been originally put as "What happens if the atom in its [trajectory] comes into a magnetic field of a different direction? Does it stubbornly persist in its own direction and violate space quantization, or does it follow, or what?" And Stern had at one time a discussion with Einstein in which Einstein gave a wrong answer; I've forgotten what it was. At any rate he gave an answer which Stern felt sure was wrong, and it turned out Stern was indeed right. But that was of course for Stern a tremendous stimulus to do an experiment to prove that he was right and Einstein was wrong. So a great deal of work was spent on this experiment, first by an American visitor who didn't quite pull it off, and then by E. Segre and myself, who rebuilt this apparatus and eventually got it to work. That of course then caused Rabi to think a bit more about it, and he got a lot more out of thinking about the role of the hyperfine structure, which we had completely neglected. I think the resonance method developed out of it.

Kuhn:

I talked just a little bit with Stern. Unfortunately he is not as happy as I would like him to be about this idea of going around trying to collect recollections of people. He had had what I take was a bad experience. He had previously made some tape in Zurich, and then he listened back to it. And he felt that although there wasn't anything there he didn't mean, still it would be bound to be misunderstood by anybody who listened to it.

Frisch:

I'm hoping to see him in the near future. I'm going to Switzerland in about three weeks; and I think I'll call him; he's in Zurich right now. I've always wanted to ask him what people were really thinking at that time when they did the Stern-Gerlach experiment and how on earth they managed to expect two states only when they thought that there was an angular quantum of unity. But I really ought to read up on this first, before I quiz him.

Kuhn:

I'll tell you what he told me because I think what he told us is not entirely right, but I'd be very interested so see whether he says the same sort of things to you. I think if he takes the questions somewhat more seriously than when we'd ask him, he may give a rather different answer. I don't think by any means he told us a lie, or anything of the sort; but I think he may have responded too much off the top of his head. Quite recently he's published a little paper on a thermodynamic derivation, and clearly he is not now comfortable with where quantum mechanics came out. I wonder whether that went back to the days when you were with him, which was, after all, very close to the classic papers on the interpretation.

Frisch:

Well, in those days he never talked about the foundations of quantum theory as far as I recall. He was very much engaged, I think, in a live job; he had to make the molecular beam method work and squeeze out all the useful results there could be. You know of course of this magnificent achievement--that in 1925, I think, he published the first paper called, "Untersuchung zur Molekularstrahl-methode" in which he typed out the whole program. It all was done, and practically everything that he wrote down turned out to be useful, and it was all done by him and his students.

So he was so busy getting these concrete results straightened out and proving the diffraction of molecular beams, and so on.

He was, I think, rather put off when this peculiar phenomenon of dips in the diffraction curve occurred; I don't know if he even mentioned it to you. Well, it so happened, you see, when I came there, he had already done a good deal of work on the diffraction of helium beams on crystal surfaces with Este Estermann was just about to go off on sabbatical leave or something like that, and so I more or less took Estermann's place after I had been trained in turning the right stopcocks at the right time. One of the things which he and Ester had found was that the curves had constant irregularities. The diffraction spectrum, for instance, didn't have the Maxwellian shape, it had some dips in it; "Dellen," he used to call them. He used to sort of vacillate between thinking "Now this may be something very interesting, if we could only understand." Or else saying, "Oh, this is some minor disturbance due to perfections of the crystal;

Then in one period when he was away for a few weeks, I used the opportunity to upset the apparatus thoroughly and put everything out of adjustment to take measurements at all angles. And I managed to find phenomenologically a very nice, simple law; but I didn't understand what it meant. The explanation was finally given in England by a chap called Devonshire and another one whom I ought to know very much better. He used to be in Cambridge and later became first principal of Staffordshire College; now I can't think of his name; he's died since. Anyhow that was one result on the, side as it were which gave me very much pleasure of course; and Stern let me publish it by myself; because, he said, "Well, you did most of the significant work by yourself."

Kuhn:

You say yourself he was interested in fundamental problems, and his record of publication on fundamental problems is magnificent up to the time of the--

Frisch:

Exactly, then he felt this was side-tracking. I mean, it might be quite interesting; but he would much rather go ahead proving de Broglie diffraction for as many different molecules as one could.

Kuhn:

When I talk of fundamental problems, there's a sense in which from about 1928 many of those molecular beam experiments, which are wonderful and which give useful values and so on, are not directed to problems fundamental in the sense that some of his early experiments had been fundamental. Had he turned off that area?

Frisch:

Well, while I was there, he was still chiefly interested in problems which I would call fundamental; such as proving the de Broglie relation, measuring the magnetic moment of the proton. Again he was sure he was confirming the magnetic moment of the proton as given by, the Dirac theory; he was very much put out when he got a different value.

Kuhn:

That's is terribly interesting.

Frisch:

Well, put out is perhaps the wrong phrase, but he certainly tried very hard to find an error in our interpretation. Finally he rather reluctantly admitted that he had discovered something which he didn't mean to discover. He was put out in the way that Columbus was put out when he realized, if he did, that he hadn't come to India.

Kuhn:

That I'm particularly glad to know.

Frisch:

This business of the flipping around of the first space quantized atom is a pretty fundamental idea which he felt he wanted to test. He encouraged my doing an experiment which he also had mapped out but which I had thought of independently; that is to measure the recoil in the emission of optical light. But he was also very interested in purely instrumental means. For instance, he was bothered by the fact that there are so many kinds of atoms which you cannot record. So he was trying to develop an instrument which would record any kind of atom which can be ionized, which of course means any kind. And that was the last piece of work which he and Estermann did before they quit.

But of course these really fundamental problems were beginning to give out just at the time when he left, and he was being drawn more and more into more detailed problems. For instance, one of the very elegant things, I think, was a thermodynamic one measuring the dissociation energy of alkaline molecules by observing the number of atoms and molecules present in a vapor simply by the magnetic deflection; molecules don't get deflected. He certainly was never very interested after perhaps one or two attempts merely to measure a quantity. After having measured the g factor for one or two atoms, this was no longer interesting to him.

Kuhn:

Do you want to tell me a little bit about Copenhagen now, or should we have lunch and come back?

Frisch:

I think it wouldn't be a bad idea to have lunch.

Kuhn:

[The following addition to the talk with Frisch is dictated by Thomas S. Kuhn from memory: In practice we did not have a chance to resume the recorded discussion, but after lunch I did ask Frisch one or two questions about his time in Copenhagen. In particular I asked him whether by the time he was there, interest had shifted pretty much entirely to nuclear subjects. He said it had and that he felt Bohr thought the problems of quantum mechanics were pretty thoroughly settled. Frisch then added one anecdote that can be checked further. He said that he had been present at the birth of Bohr's idea of the compound nucleus. It had begun to emerge during a seminar when Bethe was presenting a paper. Bohr kept interrupting with question after question, obviously in a state of some agitation. Bethe would say from time to time, "But that is the supposition on which we are operating; please let me continue." Bohr would then subside briefly and shortly interrupt again with questions that the other members of the audience found it hard to discover a reason for. Frisch feels that what was happening was that Bohr was realizing during the course of Bethe's paper that it was absurd to think of the nucleus as bound together by individual inter-particle forces. It was not long after this that the idea of the compound nucleus was worked out.]