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Interview of Leon Rosenfeld by Charles Weiner on 1968 September 3, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/4848
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Origins of interest in nuclear physics when Gamow came to Gottingen and wrote his alpha radioactivity paper. Assisted Born with treatise on quantum mechanics. Work with Pauli on quantum electrodynamics. Half-year in Copenhagen (1930) working with Bohr, an arrangement which lasted until the war when Rosenfeld was called home. War years in Utrecht, Holland. Lived in England after war until 1958. Topics discussed include: 1931 Rome meeting; reaction at Copenhagen when Bohr received Rutherford’s letter announcing the discovery of the neutron; Heisenberg’s three papers on nuclear structure; a colloquium at Copenhagen on the Fermi experiments; attendance at the 1934 Kharkov Conference; large scale exodus of German physicists as Hitler came to power; Bohr’s assistance in the emigration of refugees. Shows how the physicists themselves recognized the significance of what they were doing, using as examples the discovery of the neutron, the compound nucleus, the neutrino idea, the positron, the Yukawa prediction and fission.
This is a tape-recorded interview with Professor Leon Rosenfeld in the Niels Bohr Institute in Copenhagen. We had decided that we would start in the late twenties and early thirties, and would talk about the beginning of your interest in nuclear physics. The reason we are skipping all the other material is that a good deal has already been covered by Tom Kuhn.
Well, it started right at the beginning of nuclear physics, if I may say so. It started when Gamow came to Gottingen. I was then working in Born’s Institute and, one day, I see entering the Institute a sort of Slavic giant hardly able to speak one word in German. This was Gamow, a name that was quite unknown then, of course. Waiting until Born would appear, we started immediately conversing, and naturally I asked him what he was doing, what he was working on, and he told me on alpha radio-activity. I thought that it was a hopeless problem. At that time we were hardly beginning to understand the atoms, or a way to treat the atoms. It was the beginning of quantum mechanics. It was in 1927, so quantum mechanics, wave mechanics, were already in existence. The Gottingen specialty was the statistical interpretation of the wave function by Born which had already been developed, and I was especially interested in those questions. In fact, either shortly afterwards, or perhaps it was already started, Born started writing his treatise on quantum mechanics which appeared under the title Elementary Quantum Mechanics, and he asked me right away to help him in that, to check. I read all the chapters in manuscript and checked all the calculations and so on.
Perhaps I had already started that work, but anyway, that was my background. I was very, very green. So this idea of tackling problems relating to the nucleus struck me as being a bit unrealistic, too ridiculous, if you like, at that time, because we knew so little about the nucleus. And, in fact, I could soon see how little even Gamow knew. Then he started explaining what he had been doing so far in Russia. His first attempt was quite wrong. In fact, he simply introduced a repulsive potential because he knew that the alpha particle had to come out, so it was repelled by the nucleus. He wanted to describe the nucleus expelling the alpha particle, so he had to have some repulsive force. But in his first attempt — and this is an indication of how fumbling people were at that time with quantum mechanics — he extended the repulsive potential right to the origin, right to the center of the nucleus. And therefore he got nothing, no resonance as we would call it now, nothing resembling a state with a finite lifetime. He just got a continuum and nothing more. But then he had the brilliant idea that just in order to get the resonance he had to have this composite potential, repulsive at long distances, as it was actually a Coulomb repulsion, but then becoming attractive and expressing the fact that if it were not for the finite strength of this attractive potential, the alpha particle would be bound to the nucleus. And once he had that schematic potential, the phenomenon followed.
However, he was guided more by physical intuition than by mathematics or even by principles of quantum mechanics. So he sought a solution which would directly express in its time dependence the decay. So he introduced — since one knew already from Schrodinger’s work that the time dependence was an exponential function which for a stationary state would have a purely imaginary exponent with a real energy e –iEt/h — he introduced right away a complex value for the energy, which would give him then for the real part the decay: e-λt, you see. And therefore he put the problem as a problem of finding solutions of the Schrodinger equation corresponding to imaginary eigen values of the energy, and further specified by a boundary condition which he expressed as the requirement that asymptotically they should correspond to a given outward flow of particles. Well, he found such a solution, and that gave him then the means to determine the value of the imaginary part, which is the decay constant, in terms of the mass and potential parameters. So he had solved the problem in that way. And one of the variables was the width of the attractive potential, which he identified with the nuclear radius. That gave him then the Geiger-Nuttall law expressing the dependence of the decay constant on the nuclear radius. This appeared to us — not only to me but also to Born — highly paradoxical, or heterodox, I should say. When Gamow told Born, Born invited him to give a colloquium. I think it was at that colloquium that Born heard the whole thing for the first time. Gamow had not told him beforehand.
With whom had he discussed the problem he was working on?
Well, I think he had worked quite alone in Russia at that time.
But he came...?
Oh, he came directly from Russia.
Yes, I know, but with many of these ideas already developed?
Oh, yes. He had practically the whole thing when he came here.
I see. So what you are describing is his explanation of it once he had presented it. You weren’t involved in day-to-day discussions. He had worked it out.
No. He had worked it out. At any rate, he had worked out the main idea. He told me of his first wrong attempt as something of the past, but had already then the right idea, if I remember rightly — it’s difficult to remember exactly all the details. It may be that he got the final solution or worked out the consequences in Gottingen. That is very probable. But he had certainly the main idea; he had it already when he came.
I see. And then he presented this at the colloquium.
Then he presented it at the colloquium. Born at the end was quite enthusiastic, and he said, “That’s a very great discovery.” Born said, “Mind you, this is a very general method for quite a lot of phenomena.” And he immediately pointed out another possible application of the same principles to what was called then the cold extraction of electrons from metals. You see, you have the Richardson effect when the metal is heated and the electrons come out. But even without heating, since the electrons are kept inside the metal by a potential — this is an approximation — and this well has a finite depth, if you apply a potential difference, with the metal potential it forms a finite barrier, and you have the same thing. You have the tunnel effect. Born immediately pointed this out, that it was quite a general effect, which was shortly afterwards called the tunnel effect, and susceptible to many applications. But then apart from that Born was quite critical of Gamow’s method of solution, because he said that solutions with complex eigen values did not belong to the principles of quantum mechanics — they were quite foreign to them — and there was no telling whether they were acceptable solutions or not. At that time one was groping in the dark. At that time Wilson or some Englishman had investigated the problem of the hydrogen molecule ion (two nuclei and one electron), and he thought that he had proved that there was no analytical solution of Schrödinger’s equation for that problem.
That was just about at that same time?
Yes, it was at the same time.
Who was at this colloquium? Was this a regular weekly colloquium?
It was a regular weekly colloquium, yes. Among the people who were at Gottingen at the time was Wigner. Wigner was then the special assistant of Hilbert. Hilbert wanted to be kept abreast of what was going on in physics, and he had a special assistant whose task it was to read the new papers in physics and explain them to him. That was Wigner’s job.
So he was there, and you? You were there in what capacity?
At that time, if I remember rightly, I was just a research student. Then later I became assistant of Born.
Had you already obtained your degree?
Yes, I had already obtained my degree. I had obtained my degree two years before. Then I spent one year in Paris working with Louis de Broglie, and then I came to Gottingen.
I see. Was that on a fellowship?
On a Belgian fellowship.
Well, you were telling about the other people at the colloquium.
Yes, and then Born strongly felt that it was a good problem and the solution was obviously correct, but it was not well founded. It was some kind of phony solution. And then he said: “And I know how to do it.” In the following weeks, very quickly, I would say, Born produced a solution, an alternative solution, of the problem based on the use of only stationary solutions and perturbation. His idea was to introduce fictitiously an infinite potential, so that inside he would have a spectrum (and even a discrete spectrum) of stationary states. Then there would be an outside region in which you would have a continuum overlapping with the discrete spectrum. Then you introduced as a perturbation, which was a bit hard because it was infinite, the potential just corresponding to the barrier, cutting off the infinitely high barrier and reducing it to a finite height; and then outside there could be a Coulomb potential. The potential could be anything in the outside region; that would not change the situation qualitatively at least. And then the effect of the perturbation is to couple the states that were stationary before inside with the continuum outside, and through this coupling the states are no longer stationary because there can be transitions to the continuum at the same energy, and the transition probabilities give you the decay constant. So that is a perfectly correct method of solution, perfectly correct in spite of this seemingly infinite perturbation; for one can see that the calculation of the transition probabilities is nevertheless quite justified. It leads to the same formula, or a similar one, as Gamow’s treatment.
What was Gamow’s reaction to this or was there any need for him to react?
Oh, his reaction was simply that he did not understand all the fuss. He had produced a solution which has a quite clear physical interpretation, and that was that. Why should his solution not be acceptable? Nowadays, of course, we look at things with greater wisdom, and we find that both methods are just equivalent, and both justified and acceptable.
Did Gamow wait to publish until Born came up with a different procedure?
No. Born’s policy with publications was very liberal. Anybody could publish whatever he liked, and Born did not care. So Born did not even ask Gamow, I think, whether he wanted to publish or what he wanted to publish. Gamow was perfectly free to do what he liked. And at that time we had the Zeitschrift fur Physik, and the editor, the old Scheel, had also the view that a journal was there to publish whatever was sent in to the journal on the responsibility of the author. So he did not care even to read or to control or to referee papers. Everything that came in was published.
How did that work out in terms of the quality of the papers?
Well, the quality was, as you can imagine, very uneven. But at that time such latitude was still possible because the number of publications was still rather small, and even so it was regarded as phenomenal that this Zeitschrift fur Physik filled meters and meters of shelves in contrast with the other journals in which the papers were scrutinized before being printed.
It wasn’t a case of the man who was in charge of a particular institution from which the paper was written taking responsibility?
Well, not really taking responsibility. But surely, though Born allowed or did not interfere, he would have objected to anything being said in the paper involving his responsibility. For instance, it was customary to say that one thanked Professor Born for the hospitality and so on. He would not have that sentence if he had not read the paper and agreed with it.
Once this was explained in the colloquium and Born’s only objection was in the approach to the solution, was it clear what the implications of the paper were?
Oh, yes. Everybody was enthusiastic. Then there was one man there who immediately became great friends with Gamow. That was Houtermans. The friendship was not only scientific. It was also due to the fact that they had human affinities. They were both a bit Bohemian and reckless more than the average — very tame compared with the youth of today — but then they were regarded as having very queer and curious manners. Well, Houtermans immediately started, inspired by Gamow’s paper, to consider a problem which played an essential part in the future development: “What happens if a particle, a proton, say, comes onto a nucleus? It has a chance of penetrating through the Coulomb barrier and being captured by the nucleus. That was the first idea of a nuclear reaction. However, at that time protons were the only particles one could consider, and the probabilities of such proton reactions, calculated by this approach, were very small because of the high Coulomb barrier. Therefore, Houtermans at that time did not think — nobody could — of such reactions being produced in a laboratory. So he rather discussed the implications for Stellar energy production. That was right there in the beginning — the theory of the nuclear origin of stellar energy. You see, that came immediately. And then it was this paper of Houtermans that gave Cockcroft the idea that after all it is not outside the technical possibilities to accelerate protons sufficiently in a laboratory, by a sufficiently strong potential difference, to really get them into a nucleus. But then Cockcroft had to fight for years with Rutherford before he could persuade Rutherford to accept the idea, you see, of working with something else than string and sealing wax.
But he apparently…
Then he succeeded.
The experimental success was in ‘32, but they had been working for several years.
At the end it was the other way around. Rutherford became impatient. When he had been converted to the idea that, after all, such a machine could be built — this was a trait of his character — he became impatient and he scolded Cockcroft and Walton because they were too slow.
You talked about the impact of this in a general way, but we had started off talking about you, how you reacted and how you got involved. What were you doing in Gottingen at this time?
At that time I had other preoccupations. First of all, I had to learn a great deal, because my previous education was so deficient in modern physics. I had to catch up.
Did you learn quantum mechanics there for the first time?
Yes, except that I had read Schrodinger’s papers and so on. So there I owe very much both to Born and to Wigner. It was the time also when Wigner started his group theory analysis of atomic and molecular spectra; and learned those techniques there — how you could build up solutions of the Schrodinger equation with the right angular momentum dependence and so on. And then with Born, of course, I learned quite a great deal in working with his book. As a matter of fact, Born got ill at that time, and so I had actually the responsibility for the last steps, you see — not for writing it, but having the book at least in proof. So that took most of my time then. Then it was especially Wigner, you see, who impressed me, who made me understand. He never said anything critical to anybody openly, but he had a gentle way of intimating to people that they had better do this or that. To me he made it quite clear.
I wanted, you see, to tackle very general problems — discuss the general expression for current or what not. When Dirac’s electron came up, I examined the new expression for the current, which looked very different, of course, from the Schrodinger’s expression. What was the relation between the two? And in doing that I found the separation between the part of the current which is a convection of charge and the part of the current which is due to the spin and which is a rotational current, what is called the Gordon decomposition of the current, because Gordon published it very soon afterwards. I was discouraged from publishing anything on that by Wigner. You see, when I showed him that, I was very proud, but he said, “And so what?” Then he said, “I don’t think it is meaningful to examine the current density. I think it would be interesting to discuss some real physical problem.” It was a broad hint that I had better treat such a real problem. At that time, in his book, Born discussed symmetry properties of the matrix elements, and I was daily discussing this with him, and so he told me that it would be interesting to study some application of that to a problem involving dissymmetry, where one could see what happens when there is not the spherical symmetry of the ordinary cases. And then he said one of the problems (that he knew very well, of course; he had worked himself on it in a classical way) is the optical activity, the natural optical activity of dissymmetric molecules. And that started me then on this optical problem, to treat the optical activity from the point of view of quantum theory, of quantum mechanics. I started that problem as an exercise in order to see what happens with asymmetrical systems, how the matrix elements look. It was interesting. But then people got to regard me as a supreme expert in the optical applications of quantum mechanics, which was quite unfounded, because I did not know anything in optics and very little in quantum mechanics.
You knew more than they did.
Then I got a letter from Jordan, who was then in Hamburg, and Minkowski had a problem about some astrophysical application of the Faraday effect, the rotation of the polarization by an external magnetic field. There the molecule itself may be symmetrical but the rotational asymmetry is introduced by an external magnetic field. Then Jordan said that Minkowski should like to know what quantum mechanics has to say about that problem. Minkowski had a suspicion that the fine structure, the multiplet structure, would be without effect on the Faraday effect, would not appear in the Faraday effect. And so I started on that.
What was his special interest? How was he going to relate it to the sorts of problems that he was working on?
That I don’t remember. He had a special application in view, but that I couldn’t tell you. It must have been some inference on the solar magnetic field or the spectral lines, or something.
Do you recall what year this was?
That was ‘27 - ‘28. I had not finished that problem when I went to Zurich. In Zurich I was immediately put by Pauli on quantum electrodynamics, which was quite new. Then after a while I got stuck in some difficulties in my calculations in quantum electrodynamics, and then Pauli told me: “Well, you must not keep like that. It is good in such a situation to do something else for a while and rest by tackling some other problem. Why not take up your Faraday effect calculations” (which I had in abeyance) “and finish it?” That was very good advice, and I finished that, and then came again to quantum electrodynamics and solved the other difficulty as well.
What about this advice? You say when you came there he put you on quantum electrodynamics. What type of freedom of choice did one have in actuality, not in theory? When you come into a lab of someone as forceful as Pauli, do you really have much choice?
Well, yes, but it was a proposal. It was not imposed. It was a proposal. You see, at the end of the academic year 1928, I had the choice to stay in Gottingen. Born would have liked for me to continue there. But on my part I wanted some change. So I wrote to Bohr. But Bohr replied that it was not convenient for me to come there for some reason that I’ve forgotten — he was not available or something. Then I wrote to Pauli as the next choice, and from Pauli I got a very encouraging answer saying, “Oh, yes. We have just completed with Heisenberg this paper on quantum electrodynamics.” It had not yet appeared at that time. I remember very vividly his words: “Das ist ein Gebiet das noch nicht abgedroschen ist” —“It is a domain that is not yet thrashed out.” Which is an indication of how far within one year quantum mechanics, the atomic part of it, had already been fully exploited — not fully but I mean in all essentials. So that people like Pauli felt that it was finished and you ought to do something new. So there was no compulsion at all. I was only too glad, you see, to come into a new domain. So the first day I came to the Institute in Zurich Pauli gave me the proofs of their first article to read and to study, and criticize; he added very kindly.
By the way, this was 1929. You were at Leyden a little while after Gottingen, weren’t you, as a lecturer?
No, not at all. My only visit to Leyden (that was just occasional) was just because at that time von Neumann’s papers came out in the Gottinger Nachrichten. Von Neumann came to Gottingen. He was in Berlin at that time, but he came very frequently to Gottingen because he was great friends with Wigner. Well, anyway, he published his papers then on the formalism of quantum mechanics, Hilbert space, and all the rest of it in the Gottinger Nachrichten. And of course we all studied those papers very intensively. And while Born was ill (that was in ‘28, I think, or the winter of ‘27) the lectures had to be given, and Heitler and myself were the two assistants, and we shared the task of giving the lectures. Heitler was interested in the more physical aspects, and I chose the more formal things — those new things of von Neumann. And there among the audience was the eldest daughter of Ehrenfest, the mathematician but interested also in physics. I suppose that it was through her that Ehrenfest got to know that I had studied these papers. So he wrote to me to ask whether I could explain clearly to him the contents of those papers of von Neumann’s. So I sent him a report on that, and he seemed pleased with it. Then he invited me to go to Leyden to give a colloquium. It was nothing more than that. So that was only an occasional visit. At that time my headquarters, if I may say so, were at Liège, the University of Liège in Belgium. They paid my stay in Gottingen. They sent me out. I actually got a Rockefeller fellowship, but then I did not make use of it because of a ridiculous incident that happened in between — namely, that I was called for military service in Belgium. Then I renounced this Rockefeller fellowship, and then it turned out that I was dispensed of military service for some administrative reason. So I was again available, but the fellowship was no longer available. Then I got an offer from Born of an assistantship in Gottingen. So that was that.
That’s where we were. You went to Pauli.
Yes. This was renewed one semester. So altogether I spent three semesters in Gottingen, one year and a half. Then I deserted Gottingen, so to speak, not because I did not enjoy it, but I felt I’d better see more of the world. And, as I said, then I landed in Zurich — again with money from Liège. Then it was again a fellowship from Liège.
From university funds?
From university funds, yes.
You described the initial problems at Zurich and then turning away, getting back to the Faraday effect, and after handling that adequately, going back again to quantum electrodynamics.
That was the point where we left. In the meantime, we had also left nuclear physics.
We’ll weave it in.
I had other experiences, first of all with Gamow, because we became very great friends in Gottingen, and also with Houtermans, but not so much with Houtermans. Gamow then told me everything he was doing, and we discussed it; so I kept always an interest, but I must say only an interest as an observer. I did not take part in the developments there, but in incidents which are certainly interesting for Gamow’s development. When we had completed his calculation, then he looked for corroborating evidence or experimental evidence, and we did that together — researching the volumes of the Philosophical Magazine in the library for the articles of Rutherford and his school. I always remember how surprised both of us were to find that those people in Cambridge that we hardly knew about had discovered a whole spectrum of nuclear states from the gamma radiation. They had classified the gamma radiations in the same way as the atomic radiation was classified—the terms and transitions between terms. That decided, I think, Gamow to go to Cambridge.
Prior to that he had no idea and you had no idea of the type of experimental work that was going on? You had no interest in it other than to corroborate a formulation.
We knew, of course, that those people studied radioactivity, that they found all kinds of bodies with alpha, beta and gamma radiation. We had a school knowledge of those properties certainly. But we felt that that was a domain too hazy and too undeveloped for fruitful theoretical work, you see.
What about the experimental work at Gottingen? Was there any tie at all with Born’s group and Franck’s?
Yes, but in a quite different direction. There were two experimental institutes. The one directed by Pohl was quite outside of our preoccupations. He studied crystals and especially the optical properties of crystals and all those very complicated things. The other was, on the contrary, very closely connected with the theoretical division: that was Franck’s institute. And, in fact, the theoretical institute was a part of it. You see, there was a huge building divided in two stories, and downstairs was Pohl — that was Pohl’s domain — and then upstairs was Franck and the theory together, you see. Franck was interested, of course, in collision phenomena, and that was the origin of Born’s interpretation of the Schrodinger wave function, which was, of course, quite clear here in Copenhagen at that time; but Born was not so closely connected with Copenhagen. So they say that he discovered the interpretation of the wave function without knowing all the work that was going on here about the comparisons between Schrödinger’s formalism and quantum mechanics. So for Born it was really an independent step. But that was directly in connection with Born’s attempt to apply quantum mechanics — Elsasser was his pupil then — to collision problems, because those collision problems were studied by Franck, and Born and Franck were good friends and talked and discussed these problems all the time.
But in the work that Gamow was doing — the realm of disintegrations and penetration and so forth — there was no way of talking with Franck about it, because this hadn’t been his line of experiment?
Your impulse was to go to the literature rather than to talk with your colleagues about it?
You felt that this was what turned Gamow to a greater interest in the Cavendish Laboratory?
But you didn’t get involved.
No, I was not directly involved. I was only interested, because then I was so neutral about everything from ignorance. I took part in all the discussions but not constructively. I think Wigner noticed that, and he got me out of it by forcing me to tackle a special problem. Born could have done it, but he didn’t. You see, he left everyone to find his problems for himself. But then after this egging from Wigner I was more aware of the need of finding a problem, and therefore I immediately took up the hint when Born said: “Oh, this ought to be investigated.” Then I said: “Well, I’ll try to do it.” Perhaps I would not have done it if it had not been for Wigner’s stimulation.
He was about your age and your position.
It was not a question of age. It was a question that I felt very much inferior to the Germans and English and American visitors of the same age, because the state of education in Belgium was very inferior in all those modern things.
Let me digress again. You mentioned visitors — American visitors. Do you recall who was there then?
Yes, I was much together with Witmer. Unfortunately Witmer is now doing numerology. Too bad. But I was very much together with him, not because I had any special affinity for him, I must say, but just because we were in the same pension and always talking together. We even wrote two notes together, which were not very good. But then there were other Americans. There was Mitchell. He was the son of the astronomer Mitchell, the specialist on eclipses, and he made a career in metal physics, think, or solid-state physics.
I don’t know.
He died a few years ago. There was another called Watson.
W. W. Watson?
I couldn’t tell you really. He was a spectroscopist.
Yes, he just retired from the department at Yale.
With that one I lost touch completely. Then there was an Englishman called Linfoot, who was then a mathematician. He did not come for physics. He came for the mathematical institute, but he was in the same pension and we discussed mathematics together, because I had a much better grounding in mathematics than in physics.
How about Condon? Did you see Condon?
No. He was in Gottingen, but before. It was before my time.
Yes, he was already back in 1928, I remember. He might have gone in 1926-‘27.
I did not see Condon, but there was much talk of the Franck-Condon principle.
What was your impression? You said that you felt inferior because of the deficiencies in your education. You felt inferior to the Americans, the British and the Germans. Did you see any distinctions that you recall among these groups — the Americans, English and Germans — in terms of their own preparation?
I would say offhand that the best prepared were the Germans. But the Americans were pretty good. The English I cannot say, because the only ones I met there were Linfoot, who was a mathematician, and then Atkinson, who was an experimental physicist but very knowledgeable in theoretical questions, too, but of course not really an expert. Then the third was Dirac.
Was it obvious then that he was in a class by himself?
Oh, of course. He had quite a fantastic personality.
Now let’s get back to Zurich.
Yes. Then in Zurich there was not much talk of nuclei at all in the beginning, the first year, in ‘29. The only thing that Pauli then did just about that time, I suppose, was the interpretation of the hyperfine structure, this splitting of the atomic to the nuclear magnetic moment. That was in discussions with Stern, his old friend from the Hamburg time. But that was hardly nuclear physics. Well, I guess it was nuclear physics. It taught us that the nuclei had also magnetic moments, but that was quite reasonable since we thought protons would behave like electrons, would have spins. We were sure there would be some structure of some kind with an angular momentum. Why not? So that was nothing very striking. And so nobody was especially interested in nuclei in Zurich at that time. The fashionable thing was then the theory of metals, under Sommerfeld’s impulsion and the outstanding problem that everyone was aiming at was the superconductivity. That was too hard a nut to crack. So I did also a bit of calculations of metals. Then Landau came and asked us, “What are you working at?” He did not know anything about metal theory. He listened to the kind of problems that we raised — for instance, the magnetic properties of metals. And then a week later he came with the solution — diamagnetism — he had done everything.
And this was the first time that his attention had been turned to it?
Yes. Landau was amazing. He could tackle any problem. In fact, he told me several times that he did not like to be called a physicist. He was a theoretician; that is a man trained to think about any problem, any problem whatsoever. To think rationally about any problem whatsoever: that was what he considered his profession. He was always a bit paradoxical in his presentation of things, but I think with a grain of salt he meant it. He said: “Today we are interested in physics, because those are the most fascinating problems. But there are other problems in biology, in sociology, in everything.” He said: “I never looked for any problems, but listening to conversations of colleagues, I heard about a problem which looked to me interesting and challenging, and then that was an occasion to think about it.”
When did he tell you this? When did he describe this?
At that time in 1930 or so.
He was conscious of his own style.
Did he have any special ability to determine the characteristics of a problem? Was he particularly perceptive in the way he would ask a question to get at the root of it or did he just listen?
Well, he was so quick, you see. First of all, he listened and perhaps he asked one or two questions to get hold of a problem. But then he was so quick that the man with the problem had nothing more to say. Let us leave Zurich then. There was not much nuclear physics there. Then we come to Copenhagen. You see, in 1928 I already wanted to come to Copenhagen, but Bohr told me, “Let us postpone that until next year.” It turned out that I remained three semesters in Zurich. That puts us into ‘29.
It puts you into ‘30.
I am sorry. I’m mixing up the dates.
I have a note here: 1929 to ‘30.
I missed Copenhagen the first time. After one year in Gottingen I wrote to Bohr, and then he told me it was not convenient, and that decided me to remain one more semester. And I wrote Pauli and agreed to come to Zurich, but there was still one semester which I spent in Gottingen.
That would be the fall semester of 1929?
Of ‘29. And at the end of this semester, Bohr wrote to me that they were organizing a small conference meant for the old pupils, for old visitors of Copenhagen, but he said: “Since you have expressed the wish to come to Copenhagen, and it’s only by accident that you have not come, then I also invite you to come for that occasion.” That was very kind. So that was my first visit to Copenhagen, but that was only for the conference — that was only for a week. That was my first meeting with Bohr. And at that conference there was hardly any talk of nuclear physics. The only thing that was on the verge of it was Mott’s calculation of the application of the Bose statistics to alpha particle scattering — you see, with the symmetry property. That was regarded as something also quite new and unfamiliar — that you had to take account of this summarization and that had a great influence on the angular distribution of the scattering compared with the classical — at least for sufficiently high energies.
Yes, that was the main focus.
Well, that was one of the items. You see, Bohr did not make any program for those meetings. He had a general introduction from himself, and then he asked everyone: “What do you want to tell us?” And so everyone came with his own work, obviously the last work that he was doing, and Mott was just doing that at the time. Mott being directly in touch with the Rutherford lab and having therefore this problem put to him by the people who were interested in collisions of alpha particles, you see.
But it didn’t dominate the meeting. It was just another subject that was discussed.
Yes. That was ‘29. That must have been in the summer of ‘29. Then I came to Zurich. It was the first semester, the winter semester, of ‘29. I was working in electrodynamics. But then Bohr wrote to me and invited me to come back to Copenhagen to help him with the writing of some paper that he wanted to finish. So I left Zurich and came here, worked with Bohr for some months, then went back to Zurich. So altogether I spent practically three semester in Zurich. Then I got my first appointment in Liège. The Liège people wanted to exploit me in the end. It was quite natural. But they were very liberal. They gave me a lectureship, but I was allowed or at least it was tolerated — I don’t think they were very glad — that I arranged for my lectures to take place during one part of the year, either the winter or the summer as it was more convenient, and I spent the other part of the year abroad. I spent still one semester on that condition in Zurich, and then the ties with Bohr became more and more intimate, and finally I spent half of every year here in Copenhagen and the other half in Liege.
Why did the ties develop in that way? Was it a personal affinity?
I suppose that Bohr must have found me congenial to his way of working. It is difficult for me to say exactly. He may have found me patient. What he told me explicitly is that he noticed that I was interested in the questions of principle that were then also for him the foremost questions. I put to him questions aiming at principles rather than details.
What was your reaction to the relationship? You felt that maybe he found it easy to work with you, but what about you working with him?
Of course it was very hard in the beginning. Of course, there was never any doubt in my mind that if I found it difficult all I had to do was to make the necessary effort to adapt myself and to understand him. So I just worked very hard. And then it was very easy in a way. He was very easy of access. There was no inhibition at all. One could ask him anything. And if one was not satisfied with the answer, one just told him so, and he tried to repeat it in another way. And so by asking sufficiently many questions and trying to approach a subject from many ways, it was possible to get hold of his meaning, although it was very difficult even for him to express it, and not because he had unclear ideas. I should say perhaps the contrary. He thinks of the implications of every statement so deeply and so quickly that he was always unhappy to make a statement without qualifying it, because he saw the complementary aspect, and he wanted to indicate that one had to take account of it. Now, for people who are unprepared for that kind of thing, it made it very hard; and it gave the wrong impression. It often gave the impression that he was hardly knowing what he was talking about.
You started this relationship here in ‘30. This was the beginning of a whole new period, and it kept up until the war, didn’t it?
Until the war, yes. Then I was recalled to Belgium.
Right. Is it fair to say, then, that you and Bohr got interested in nuclear subjects in the same way at the same time?
Yes. Then, of course, as soon as ‘31 the great development occurred in nuclear physics with the discovery of the neutron.
That was ‘32.
‘32, yes, but even in ‘31 one felt that things were coming to a climax. I’m sorry. I should present it a bit differently. In ‘31 things came to a crisis, because then, you see, one felt that one had a theoretical means in hand to discuss the properties of electrons, even relativistically, and those of protons as well; and then one saw that it was quite impossible to make anything of a system of protons and electrons close together within the confines of a nucleus. So that was the crisis. And then came the extra difficulties, the precise difficulties, the spin of the nitrogen, the beta decay problem, where the energy of the electrons was spread out, although the transition was characterized by a definite energy. That was Mott’s and Ellis’ work. So I may say that in ‘31 one got interested more and more in nuclear problems because one had developed this background — at least one assumed that protons would behave like electrons. At least there was a starting point, you see, for a theory. So long as one did not have those relativistic formulations, one could always say: “Well, the problem is so difficult, because surely in those small regions one must have large velocities, so it must be out of reach.” But when one had developed the relativistic theory, one could not make anything of it. The electrons inside the nuclei seemed to lose their spin. That was the 14N riddle. And they behaved in a completely aberrant way when they came out of the nucleus, losing part of their energy — one did not know where. So that was absolutely a critical stage. And that was felt very strongly in 1931.
That was the year of the conference in Rome.
Yes, the conference in Rome was a muddle in the sense that one could present all kinds of problems but no solution, no acceptable solution. One only saw the difficulties.
Let me ask a whole bunch of things about that that interest me. I did come across a letter inside just now, just five minutes before we started, from Fermi inviting Bohr to that meeting in Rome and saying that they would talk about “nuclear” problems. There were lots of things that Bohr could have chosen to talk about at that meeting, but he presented this conservation of energy problem. Now, had he been working on that paper before the meeting, or did he prepare it specially for the meeting?
He prepared it for the meeting, but those questions that he discussed in the paper were questions of daily discussion in the Institute.
Anyway? Whether or not there had been a meeting? The meeting didn’t precipitate that?
Oh, no. This Rome meeting did not precipitate anything. It was just a stock-taking, so to speak.
What was the general feeling? I have the proceedings, but when you see the individual papers, it doesn’t tell you what the total atmosphere was.
Well, about the atmosphere: I would not say there was any despondency, because it was felt this was quite a new domain. One was certain that the electromagnetic forces could not in any way explain the stability of nuclei, so there must be other forces coming into play of which one had no idea. So it was not really despondency; it was rather a feeling that one knew too little and that the problems were too big, and that surely one had to be prepared for the most extraordinary things.
Including a reexamination of the general basis of physical theory.
Well, one did not go so far, because one had this very strong evidence from the gamma spectra and also in fact from the beta transitions showing that one had to do with quantum transitions. So that Bohr’s postulates of quantum theory were at any rate verified. And one was convinced that any nuclear theory must be built on the basis of quantum mechanics, and then, as I said, relativistic quantum mechanics, because one has assemblies of particles very close together, and therefore with strong couplings and large velocities. And since at that time all the divergencies of quantum electrodynamics had already been disclosed, we knew that the theory that one had for those relativistic effects was certainly not a good theory, not a reliable theory, that it was itself in need of correction; so that the problems were mixed up. That was reflected in the general discussions.
As I said, it was more a stock-taking of the difficulties, but there was no suggestion at that time in what direction the solution would come. But everybody was convinced, if you like, that there would come a solution, but with no idea, of course, how it would look. Bohr at that time had the attitude that he always had in times of crisis, a completely open mind. He said he was prepared for anything, including renunciation of the conservation of energy in the single processes. Gamow had no feeling really for those questions of principle. He tried all kinds of makeshifts, with potentials, you see, and he introduced even, to Landau’s great anger, different potentials for the interaction of nuclei with protons and for their interaction with alpha particles, and special levels from which only neutrons would be emitted, others from which only protons would be emitted, and others again which would only emit alpha particles. Landau was quite furious about that. He said: “How would the nucleus recognize in which energy it is in order to choose which particle to emit?” Then he played about with nuclear spectra and connecting them with the zeroes of Bessel functions and so on — I mean all kinds of attempts at systematization which did not touch the deep problems really.
I don’t think Gamow attended the meeting. A paper was presented.
Gamow was not there.
But a paper was there.
The paper was read.
He showed me the postcard that all of you sent from the meeting to him with all of your names on it, and wishing that he had been there. But Landau was there? This comment of Landau’s reaction…
That was before, when the two were together in Copenhagen. Then the three of us were always together. Delbruck was also with us.
You were in a lively crowd there.
When you were at the meeting at that time did you get any impression that any particular group was really in this new field practically full time?
No, not yet. Not even Fermi. At that time Fermi had just organized the meeting more to learn what the problems were, you see, for himself. But he had not yet decided what to look for.
Was there much informal goings on there, socializing, discussions and so forth?
Oh, yes. It was quite lively. Of course, the Cambridge people, Blackett especially, had something to say. But it was just experimental data that could not be interpreted.
So the field in itself did not exist quite in the sense that people could consider themselves in it, because they didn’t know how to get into it.
No. It was just gropings in the dark here.
That was in September-October of ‘31, I guess. Then you’re back with your six months on and six months off type of affair. How would you characterize that next period in terms of what began to take place?
Well, then came the great breakdown, or breakup, I should say — that was the discovery of the neutron.
In February of ‘32.
That opened the field immediately. When Bohr told us that-that he had heard it from Rutherford — we gathered at that time not far from here at a pension, Miss Have’s. And I remember there was Gamow, Landau and several others, Delbruck surely, perhaps Casimir, but I’m not absolutely sure. One may check, because we have a photograph of the old guest book of Miss Have’s with the names.
Is that included in the film in Philadelphia?
No. In fact, it’s something that has not yet been done and should be done. I had a copy of those pages that I have prepared for sending to Kuhn, but unfortunately I haven’t done it.
It would be fascinating to have that list year by year.
I know why I have not done it. I recollect now. It is because the pages themselves, many of them, would be useless without some comment, you see, because it is not always legible and easy to find out. And it is this work that I have not yet done.
Let me suggest that you could put it on the tape, and we’ll transcribe it. In other words, you’ll say, “Photograph No. 1, or Page No. 1,” and just do it talking. It’s bound to be difficult anyway, you see, because it’s very difficult to put in full sentences. Put it on tape and then we’ll transcribe it. You can send it to me, and I’ll transcribe it, and give a copy to Kuhn. I can just see how useful that would be, because I wouldn’t have to do a lot of this business of: “Who was there? Who was there?” if we already have some sort of a list, which is not an exclusive list, but it tells you that at least these people were there. That’s very important.
Yes. We have also something more official in a way. Mrs. Schultz, whom you have met, held a record of all the visitors, when they came and when they left, and we have the record.
You have the book.
Yes, we have the book, but we have no copies.
I’m going to talk with her tomorrow, and maybe I’ll work on that book myself and see what I can do. Maybe I’ll copy some things out of it.
Well, anyhow, there was this session the same evening, and immediately, everyone started saying: “Now that we have the neutron, well, the 14N riddle is out of the way. Now we can at least localize the difficulty of the beta decay, which becomes a property of the neutron.” You see, the neutron sends out an electron, and it becomes a proton.
This was almost the immediate reaction.
Yes, it was the immediate reaction. It came as a liberation from our problems, and then it became also easier to understand how protons and neutrons, which have the same mass, would be held together by special forces which need not be relativistic, because the electron is somehow absorbed in the neutron, hidden in the neutron.
What were you thinking that the neutron was? Did you think it was a proton and an electron together or what?
Well, no. I think the most popular view at that time must have been that the electron which comes out in the beta decay is created, just as a photon can be created. You see, there’s a field and it is emitted.
This was Fermi’s view.
Then it was taken up by Fermi. On the other hand, Heisenberg wrote three papers at that time about the nuclear structure based on the idea of neutrons in which he seemingly describes the neutron as an agglomerate of a proton and an electron. But if you read carefully the papers and more so if you had heard Heisenberg at the time when he presented his theory here in the colloquium, then you would have known what is perhaps obvious, that he did not take that picture seriously. But it was for him a sort of help to figure out what kind of forces one could expect, you see, from the possibility of virtual exchange of an electron from a neutron to a proton, the neutron becoming a proton and the proton becoming a neutron. We had other examples of that with electromagnetic interactions. We knew then already that virtual exchanges of photons produced these interactions: the Coulomb-force by exchange of longitudinal photons, whereas exchange of ordinary photons produced the electro dynamic effect. So that we knew that the theory was strong enough to incorporate that kind of exchange forces, and that was the origin of Heisenberg’s proposal of the first form of exchange force, which is called by his name, the Heisenberg force. So this model of the composite neutron was only a help to guide the imagination, but it was not taken seriously. And very soon Majorana then noticed that this Heisenberg force was not sufficient to explain the saturation of the helium nucleus and he proposed another type of exchange force, operating with the new operator, the isospin, which had been introduced by Heisenberg along with the ordinary spin.
Some of this, I recall now, had been on our History of Science Congress program in Paris. Joan Bromberg discussed it.
Yes, she described that very well. Then there was the intervention of Ivanenko, who thinks that he has done it all.
Just put all the pieces together and assembled them.
Ivanenko, to be fair to him, has had this idea independently — that obviously you must now build up the nuclei out of protons and neutrons held together by a certain kind of force. But it must be said that everybody had this idea as soon as one heard of the discovery of the neutron. Then it was quite clear that they must be held together, the protons and neutrons. The binding of two massive particles was more easily conceived by theoreticians, because, being so heavy, they could be bound together by non relativistic forces. Then all the difficulties arising from the relativistic character of the electron were of another order and could be separated. That was the great step forward. That was felt by everyone immediately. And to say that Ivanenko had the idea that the neutron was an independent particle and Heisenberg still had the idea of the composite neutron — that is just pointless. You see, there was no real difference.
Yes. It’s good to know that this was in the air. Let me ask another question. I just learned by looking into the file of the Fermi letters that Bohr sent his manuscript of his talk to Fermi only after the discovery of the neutron, because he just delayed. Now, the thing that interests me is that in the published version of it he still talks about the same things. Originally, I guess, it was the Faraday lecture when he first mentioned this non conservation of energy and the problem we’re beset with. Does he mention it again at the Rome talk? I think he has that in there, too. The question is: If one felt a sense of despair in just stating the problem in October of 1931, and then comes along the discovery of the neutron, which, as you indicated, clarified so many things, you would think that in this paper there would be substantial changes.
No, certainly not, because he felt that it would not be reasonable to anticipate. It was, after all, a writing up of the talk he had given, so he could not possibly anticipate.
No, but if you changed your view, you could add a note.
The only thing he could have done would have been to add a footnote, you see: “Since the conference there has been this and that.”
But he didn’t do that.
He didn’t do it, but, you see, think it was justified not to do so, because the specific difficulty that he insisted on namely, this uncertainty with the energy — was not solved by the discovery of the neutron. It was solved by the hypothesis of the neutrino, which came later.
This explains something, because there are letters, for example, that he wrote to Rutherford in May after Rutherford had told him about Cockcroft-Walton; and in his response to that letter he said, “It’s wonderful. A whole new field has opened. And, by the way, I’m sending you a copy of my lecture where I talk about non conservation of energy.” This was still a live issue for him.
The issue was still there, but it was displaced. It was displaced from the problem of nuclear structure to a specific property of the neutron. And that was a great advance, because it allowed one to discuss many points of nuclear structure without coming into that awkward problem. It was possible to apply to an assembly of protons and neutrons the principles of quantum mechanics based on conservation of energy. After all, beta decay is a very, very improbable phenomenon, involving lifetimes immensely longer than the time necessary for a neutron to traverse the nucleus. So the two problems could be separated cleanly. That was the great advance.
Now, in April of that same year, ‘32, there was a conference here, the Easter conference, which he had organized even before the neutron was discovered, before he had heard of it; and I know that he invited Chadwick to the conference and said there was bound to be good discussion about that. Chadwick couldn’t come. I know all this from just an hour ago looking at the papers. Do you recall that conference, though? Because apparently the subject was intended to be nuclear physics. Was that part of the time that you were here?
I don’t think so. I don’t think I was at that conference.
It was April. Well, then I’ll have to talk with someone who was.
At any rate, I have no vivid recollection, so most probably I was not there.
Yes, April 7th to 13th, and he invited Chadwick to come. He had planned it much earlier, though. Anyway Chadwick didn’t come. What about the reaction to the next lively thing, the Cockcroft-Walton disintegration of light elements, artificial disintegration? Were you here when that word came? Do you recall how it came and what the discussion was? And, again, what the reaction was?
That was, of course, also greeted as it deserved. But I feel that it was greeted more as a technical achievement than a new physical insight, because, after all, all those reactions which took place had been expected from the simple Gamow theory. So the real importance, which is fantastic of course, of Cockcroft and Walton, was that they showed that it was technically feasible, and that the repetition and continuation of those experiments by the French, by Joliot, led to the discovery of the artificial radioactivity, of the building up of nuclei by proton capture. That in turn prompted Fermi to say: “Well, if I could put neutrons into the nuclei, then this should go much more easily. This could go even with slow neutrons because the neutrons have no barrier. Neutrons go directly into the nuclei.” And then came the Fermi development with this extremely ingenious idea of Fermi’s to slow down the neutrons by collisions in paraffin, in hydrogen-rich substances. Then came the discovery of this other family of radioactive nuclei on the other side of the stability curve, and, perhaps, more important still, the discovery of resonance neutron capture, because that was quite unexpected.
Well, that was a phenomenon which was not predicted by the usual theory which had been successful so far — the theory treating the interaction between the target nucleus and the incident nucleon as a potential, schematically a potential, without considering the possibility of interactions between the incident nucleon and the single constituent nucleons of the target, treating the target as a whole. That goes for proton reactions or alpha particle reactions because of the barrier. That’s good enough. There’s only a small chance, you see, of a particle going through the barrier. But one saw that neutrons behaved quite differently, because they went directly into the nucleus with their full energy, and then one saw experimentally that they could be captured practically with certainty, with a cross-section equal to the geometrical cross-section of the nucleus. So they stuck to the nucleus once they hit it. And that behavior was quite impossible to describe by any real potential. Now we know that it is possible to describe it if you take a complex potential, the optical model so-called; but at that time one did not think of that — fortunately, I think. This was again a crisis — how to interpret those capture reactions with neutrons. Again, there was a colloquium here in which those new facts were presented, the Fermi experiments.
I forget who brought those up. Wheeler told me the name of the person who reported on it.
I can’t remember who it was.
But this was presented. Had you had word of it before the colloquium?
Yes, the word went out. It may have been Placzek, because the first reports were written in Italian. There were small notes in Ricerca Scientifica which were written in Italian, so only very few of us could read them directly. That must have been one of the reasons at least why this colloquium was arranged, in order to have a general review. But it is not the only reason, because at that colloquium was also discussed the difficulty of theoretical interpretation. It was quite impossible with a real potential to obtain so many resonance states and to obtain resonance capture. There was another contradiction between capture and scattering. There was practically always capture and not that amount of scattering that one would expect from a potential. I still remember Bohr turning to the audience and saying: “Well, here we are with a process which is absolutely in contradiction with all the properties of the model of nuclear potential which we have used so far. Somebody must have a crazy idea to come out of it.” He then added: “Has any one of you a crazy idea?” And then the crazy idea came very soon afterwards from Bohr himself. He did not take the experiments with slow neutrons, which were difficult to interpret because they involved all of quantum mechanics, but the previous experiments made by Chadwick and others with fast neutrons, in which you also had an anomalous capture for any energy.
In those experiments you can treat the incident neutron practically as a classical particle: it is a very small wave-packet. Asking himself what one would expect classically led him at once to this idea of the incident particle spreading its energy among all the nuclear particles and formation of the compound state. Then this idea of the compound state was extrapolated towards lower energies to account for the resonances. And he could also explain why one finds sharp resonances for low energies and not for high energies: when one tries to build up the spectrum of a nucleus out of the possible vibration modes of the nucleons, then as one piles up the individual contributions from the nucleons, one gets a larger and larger level density. That is the mathematical problem of the “partitio numerorum,” which Bohr of course did not know anything about; but when he talked to his brother about this special problem of physics, then Harald of course could immediately say, “Well, we have a formula just giving this density of levels — that is, the number of ways in which you can express an integer as a sum of integers. And this goes exponentially.”
How long a time did it take him to work all this out?
Oh, a very short time, a few weeks.
Were you here then?
No, I was not here in this crucial period. I was here at the colloquium. That I remember. But I must have left soon afterwards, because I heard of this idea of the compound nucleus by a letter from him. I ought to look up and place this letter.
Do you think it’s in the collection?
No, I don’t think so, but unfortunately I have not yet classified my letters from that time, and therefore there is yet no microfilm of them.
Will you microfilm them?
Oh, I must do that. Anyway they are kept.
I ask by instinct, you know. It will be interesting to know those results. But you would describe this as the second crisis — the first one being 1931 — and the second one had quite different problems since the whole field itself had already started. But within that field there existed this second theoretical crisis, again based on experimental anomalies that just could not fit with the state of existing theory.
Let’s go back for a minute. During this period there were a couple of conferences. In one case you indicated it was a sort of a stating of the problems. Now, the next conference I know of was the one in the Soviet Union.
In Kharkov; yes, I was there.
It would be interesting to have your view of how that came about and also the state of Soviet physics as you saw it at that time, since supposedly they were to discuss nuclear physics.
It was not very exciting. In fact, the trip was more exciting to see Russia than for the physics. Of course, the Russian physicists were very active and keen all of them, headed by Landau and Gamow and those we already knew.
And Joffe. Was he there?
Joffe, of course. But there was very little nuclear physics. The only center was Kharkov, where they had a machine, a cyclotron. Whether it was working, I doubt. No, no, they were planning to build one.
They didn’t get one until just before the war. They didn’t even get it working.
No, no, it wasn’t working. No, I think the first cyclotron in Europe was the one here, the first that worked.
I can verify this again as of today from reading letters that Bohr was writing to Cockcroft and it was November, 1938. Cockcroft wrote to him saying, “We expected to be much further along by now, but we’re not getting too many good results. We hope to solve the problem.” Bohr wrote back and said, “We’re beginning to get good results. We have so-and-so.” And two days later Bohr couldn’t contain himself and wrote another letter saying: “We’ve got it.” And then he gave a whole list of the successful results. That was ‘38. Getting back to the Soviet trip, do you remember what the topics of discussion were? Ivanenko says he has the proceedings somewhere. I’ve never seen them.
They may have been in the Soviet journal, which was then written in German. But there were no proceedings in a special book. It was a bit dull, with little discussion, and nothing exciting came of it.
But you say it was interesting just to see the country.
Oh, yes. It was more interesting to see the conditions of work and the country as a whole. It was quite an experience then.
This was just before Gamow came out of there.
It was after he left. I do remember. You see, Joffe complained bitterly about Gamow’s flight, because it had made it impossible for any other Soviet scientist to get permission to go out.
He told you this at that meeting?
Yes. Gamow must have left shortly before.
I can check the date when he left. The doubt in my mind is about the date of the meeting.
The meeting was in May, because we witnessed the 1st of May parade in Leningrad.
I think that makes sense, because he only came to the United States in ‘34 after having been to Cambridge and to Copenhagen on his way. So there’s not much more to say on that particular meeting?
No, there was nothing very exciting.
Did you go to the London meeting? There was a London conference on theoretical physics.
Yes, I did.
Yes. But there again, you see, the meeting was not well organized. I mean it was not organized in such a way as to be very fruitful, because it was too large. There was a big crowd of people. Well, in fact, I suppose compared with the present crowds you see at meetings it was a small affair.
But compared to Rome?
Compared to Rome, there were more people than at Rome. Then I remember we had a meeting also at Copenhagen. I think it was in ‘35, was it? We could also check that. Anyhow we felt that it was becoming too big. The whole room downstairs was completely filled, whereas in the previous conferences that we had had here only the first benches were occupied or if people spread out they were spread out all over the place. But there they were tightly packed over the whole room. We felt that that was the beginning of a new period.
Yes, already then. And in London that was very much the case. Also, big towns like London or Paris are not really good for meetings, because people disappear, so to speak. You must have experienced that just now [in Paris].
There are too many distractions.
There are so many distractions, and people run about. I suppose it may have been fruitful for small groups coming together and discussing their problems, experimenters, new techniques and so on.
But it was divided into nuclear physics and solid-state physics.
To begin with. But even the nuclear physics part: I don’t recollect that there was any sensation or anything really very exciting, I should say. What about the neutrino?
Well, you see, the thing that was exciting was artificial radioactivity at that meeting, because Joliot presented the papers on it, and the only discussion that I remember in the proceedings as being of any spirited nature was the discussion of those developments. Of course, that was known before the meeting, but the meeting occurred within a few months of the experimental work on it. I just recall that.
Oh, yes. It may be that I have no vivid recollection of that because this discovery of Joliot had already been vividly discussed in Copenhagen before the meeting. I remember that the first I heard of Joliot’s discovery was in Liege. I was then in one of my Liege periods, and Joliot came to give a lecture. It was quite unexpected. He had just got the results, and he presented them quite fresh. It was before the publication in Nature. I had to leave for Copenhagen a few days afterwards, so I came with the news, you see, that nobody had yet heard. I was received with the greatest suspicion. Nobody would believe it, you see. Then I think a week later this description in Nature came, and then people started reading it, and then they started thinking: “Well, we could perhaps repeat the experiment. After all, only a counter is necessary.” Then, of course, the whole thing started. [Interval]
This is Part 2 of our tape, and we’re on the point now that has to do with the recognition of the physicists themselves of the significance of their work as they’re doing it or the significance it’s going to have in the field. You had some general comments on that.
Well, looking back on the history and trying to revive the impressions we had in the course of time, I have the impression that the protagonists were very conscious and they had a very clear idea of the relative importance of the steps they were taking. I would not say that the whole thing was in any way planned. On the contrary. In a sense one went from surprise to surprise. The discovery of the neutron was a great surprise. But, at any rate, it was a surprise that we were prepared to receive and to make use of. The first night that the news came through and that people thinking of those problems were together, all were prepared to develop the implications of the idea and in a way to make the best of it on the spur of the moment. Then came, of course, a period of hard work to build that up correctly. The next example was this question of the neutron reactions and the compound nucleus idea, which was taken up immediately. And everybody realized that it was a tremendous step, a major step, so to say. The next was the introduction of the neutrino idea. Sometimes one laughs mildly about Pauli’s shyness in first not publishing it but telling it and being extremely cautious. But this is true science. This cautiousness may be partly exaggerated hesitation, but it may also be just the conscientiousness of the theorist who is anxious to say: “This is an hypothesis which may explain experiments but which is not uniquely deducible from the experiments which it is invented to explain.” So it’s just a matter of conscientiousness, to specify the logical position. And then Fermi certainly jumped on it, and he had a very fine nose to smell the right things. And as soon, at least, as Fermi’s theory was available and embodied Pauli’s neutrino idea, then I think that everybody immediately accepted it. And when the first experiments attempting to detect neutrinos experimentally gave favorable results (that was long ago; when they first gave results that were at least in conformity with expectations — I mean the first collecting of recoil atoms and measuring of missing momentum), those experiments were greeted as nice experiments; but they did not create any excitement, because everybody was convinced beforehand that they would come out like that. At that time the idea of the necessity of this neutrino was so ingrained…
Would you say that was true of the positron?
With the positron, yes, it was true. Well, the first announcement of positive particles was again made by Joliot in correspondence with Bohr. You may perhaps find the letter from Joliot, although I think we have looked for it and it is missing. But anyhow Bohr looked at the photographs that accompanied the letter, and he did not believe it. He tried all kinds of interpretations and especially that it may have been an electron going backward and by chance hitting the other, just by fortuitous coincidence. And Joliot accepted that possibility. He could not counter it. So no harm was done, because if the experimenter cannot defend himself, then it’s too bad. He cannot claim priority afterwards. But the curious thing was that the connection with the Dirac positive particle, which he had proposed as a proton in his first paper…
Which Oppenheimer subsequently ruled out.
Yes, Pauli did it also, but did not publish it. But we knew perfectly well that according to Dirac’s theory, there could be a positive particle exactly similar to the electron, in all probability. But, curiously enough, that fact had no influence at all on Bohr. We pointed that out to him, you see. It might be the Dirac hole. But he said, “If this is a purely experimental point, it must be judged according to experimental criteria.”
The Dirac theory?
No, the photograph of Joliot — whether it was genuine or not.
But if he had believed in Dirac’s idea, then he would have been more likely to have been liberal in his interpretation of these results.
Probably. It’s anybody’s guess.
But since most people did believe it, this is another point.
And then Blackett made his experiments with a view to discovering Dirac’s particle, and he succeeded actually. But also for some reason that I don’t recollect now, his experiments were not convincing enough. Thin Anderson was the man who succeeded in convincing people that it was a genuine pair production. I think it was because of those lead screens which made the direction quite unique, and there was no possibility of discussion. That was it.
I know there were some letters…
There I would say Bohr was extremely critical of the experiments.
Of the Anderson ones.
Of all experiments. He was finally convinced by the Anderson experiments.
But you’re saying with the neutrino thing, the experiment wasn’t even the issue.
No, the neutrino was another thing. The neutrino was invented to get rid of this energy nonconservation and nothing else. And it tied up very well, as Fermi showed, with all the facts of beta decay. And then I suppose on the side of the theoreticians there was a general relief that this was a way of avoiding a great … It was felt, you see, that assuming the existence of this particle, which, after all, was rather normal — I mean it satisfied the Dirac equation with mass zero, or perhaps with a small mass one was not wedded to the idea that the mass was exactly zero, it could just be another particle. It was a cheaper way of getting out of the difficulty than abandoning such a fundamental principle as the conservation of energy. The same thing with the anti-proton. When the anti-proton was produced, one admired the splendid technique, the technical achievement that was necessary to make this experiment possible; and one was also glad of the new field of experimentation that was opening by studying directly reactions of particles with anti-particles. But the fact that the anti-proton was produced, when all the theoretical conditions predicted were realized, was not considered even as a triumph of theory. The theory by that time was so confirmed indirectly in other ways …
By other theoretical successes.
Yes, by other successes, that one felt it could not be otherwise.
Another challenge to see how this point holds up: the Yukawa prediction in 1935. Now, when Anderson and Neddermeyer and so forth originally came up with the mesotron, was there not a reaction that this in fact satisfied the Yukawa principles? Was there excitement about that?
This was ‘37-‘38.
Well, yes. There was a great degree of excitement but for another reason. You see, this idea of Yukawa was not unique like the others. The Dirac idea of a positive particle was, as we now see it, a consequence of the theory — almost inescapable. And with the neutrino, the same or still more so. It was just introduced for a single purpose, really, and it had no other effects because of its small mass. Nobody could dream of the present possibilities of detecting neutrinos. That was quite out of the scale of achievement that one was prepared to expect.
It had taken 25 years to do it.
Yes, but even so, 25 years is such a short span of time. Well, that’s another question. But with the meson, that was something different. Of course, the main point of Yukawa’s idea was that one had to have a massive particle transmitting the force, because the force is of short range. That was the really fundamental thing. But then immediately people came up and said: “Well, but this meson is alone, then it must have an integral spin. But a pair of mesons could also do the job and then they would have the spin one-half.” In other words, there were different possibilities, you see. And then: was there a single sort of meson? Was it charged? Was it neutral? Was it of spin zero or spin one and so on? So there were a number of possibilities. So when the discovery was made, then people were very excited because they hoped by seeing the particle and studying its properties, that could decide between the various theoretical possibilities that were open. Now, as it turned out, the situation was more complicated. But that’s another story.
I’ve never heard that particular interpretation of why the meson was interesting. Usually it is said: “It was interesting because it fulfilled the prediction that there was a particle.” But you’re saying if you could look at this thing, then you could choose among the theoretical alternatives and variations. I see then that this is different. Now I would like to go to fission without getting into the story that you say you’ve already talked about with Kuhn. What about the discovery of fission? How did this fit in? That I think is what started you off on this.
Fission again came as a tremendous novelty, but again one was prepared for something quite extraordinary, because one had discussed — and here especially with Meitner and Frisch and Jacobsen — all the time those so-called trans-uranic elements and the so-called isomeric sequences. And it was such a muddle. It was quite impossible to fit them into any rational scheme. So one felt that there was a great confusion of some sort, and one got more and more the impression that there must be some fundamentally wrong circumstance in the interpretation; but nobody thought of what was wrong, until the news came from Hahn and Strassmann who as chemists, you see, put their hand in the fire, that the particle or fragment that they studied could not be a trans-uranian; it must be barium. It could not be a heavy element. But that was not the end of the story yet. Then Frisch and Meitner, of course, concluded that it must mean that there is a splitting of the nucleus into fragments. Well, that was that. It was rather an inescapable conclusion. They tried to figure out why that was so. One could understand that the heaviest nuclei become unstable because the surface tension becomes too small and then that they may split. When Frisch presented to Bohr these conclusions, Bohr accepted the conclusions because it was an argument directly following from the experiments. But he did not understand why the nucleus would split. That seemed to be a mode of decay which at first sight seemed to involve an improbable play of circumstances and would therefore be very seldom. Instead of that, it must be the usual one, because obviously all those former experiments which had produced all those so-called trans-uranians must be fissions. They must all be fissions. So, you see, fission is a frequent phenomenon. Bohr could not understand that. And that took place on the way to America, on the boat. I accompanied him, you see, and when we met on the boat, he said: “I have in my pocket a paper which Frisch has given me which contains a tremendous new discovery, but I don’t yet understand it. We must look at it.” Then during the trip that took six days, I suppose, he got hold of the solution; and it turned out to be an extremely simple solution, on the basis of his own theory of the compound nucleus.
Of course the incident neutron is captured, and a compound nucleus is formed, and then the energy is statistically distributed among all the degrees of freedom of the system. And one of those degrees of freedom is an oscillation of great amplitude which then may produce a fission. Now according to classical statistics there is equi-partition of energy. That is, there is the same amount of energy in the middle on each degree of freedom. Therefore, the fission is just as probable as the other reactions — emission of one neutron, of two neutrons, an Alpha particle, of anything… Then there are factors correcting this, but they do not change the order of magnitude. And 50 one understands that fission is as probable a process as the others, roughly. That was the first great step. Now, Frisch tells us that when he gave Bohr this famous paper and explained to him what it was about, Bohr said: “Of course, how stupid we have been not to think of that.” But that is not contradictory to what I say. That is the first natural reaction: “Of course that wipes out all the paradoxes of these isometric sequences. It’s an experimental result which explains all the rest and which explains the instability of the fragments and their further decays and so on.” So that was the first reaction. Then the next step was to understand how this fission could be so frequent. That then he solved on the boat. Then the last step was to explain why there were elements fissile with slow neutrons and others which were not. And that he did in Princeton.
This is with Wheeler and so forth.
You were in Princeton too?
Yes, I was in Princeton. But, again, I was busy with other problems, and so I did not take part in the elaboration of the theory.
You’ve discussed this in more detail in the interview with Kuhn?
I discussed all this in more detail with Kuhn.
Let me get back then to this other question. We were talking about the early days of nuclear physics in the ‘30s, and we neglected to talk of another event which influenced things then. And that is the coming to power of Hitler and the large-scale exodus of physicists who for reasons of religion, genealogy, politics or protest left Germany, and later, countries that were German-dominated. So the question is, first of all, about the immediate reaction in response to this. Even beforehand, whether people felt that because of the political developments in Germany, conditions there were becoming unstable. In other words, even before Hitler, was there any feeling about the state of science in Germany either for political or economic or other reasons? Was there any feeling of people not wanting to stay there and wanting to leave?
Yes, several of them were clear-sighted enough or pessimistic enough and they looked around for positions abroad. Oldenberg was one of those who left very early. I don’t know what motivation he had. I knew him in Gottingen, but then I lost touch with him.
He’s still up in Cambridge, Massachusetts.
Yes, and so I don’t know. But I knew that he left very early. He took the opportunity of getting away. And then there were some others, perhaps, who left earlier. Well, that was mostly individuals, you see. Some physicists were politically interested, and those that were saw things coming. With a bit of political experience, no great shrewdness was necessary to predict what was coming — at least in the main features. Moreover, Hitler was quite explicit, and those who cared to read Mein Kampf knew of this. He explained everything that he wanted to do and he actually did it. Then some people said, “Oh, that’s a madman,” but if you observed the way in which he managed very shrewdly to build up the party in Germany and to seize every opportunity to make progress and then the demoralization among the population, it was just as simple as to predict any consequence of quantum mechanics. But many physicists took pride in keeping aloof from politics and neglecting that as a concern of the crowd. They were above all that, and the university would never be touched. Many things could happen, but their position would remain. They lived in that illusion. For them it was a terrible shock.
When it did happen, I know that Bohr was visiting, as he often did anyway. He was very conscious of what the problems would be. Do you recall any discussions here, or at meetings or at other international gatherings, of what the situation would be?
Yes. Bohr was not among the clear-sighted. His brother was much more clear-sighted. Harald, although he was a mathematician, and mathematicians have the reputation to live in the clouds, was quite different. He understood everything, and he scolded his brother because he was so naive and over-optimistic. Optimism is a character trait of Bohr, which is justified in science where he had control of things. But he had a tendency to be optimistic in every walk of life even when it was not at all justified. And that was very much the case with Hitler.
You mean he thought that it was impossible
Oh, he thought everything would settle down, and, after all, it wasn’t so bad and that sort of thing. But he was soon made awake by the reality, because the second character trait of Bohr is facing facts. And as soon as the facts are there, then he forgets everything he may have said before that’s contradictory to that. He takes the situation as it is, and he lives up to it and adapts himself to the new situation immediately and whole-heartedly. That is one of the main reasons of his success in physics, though it’s not the only one. But certainly that has contributed to his pursuing during all his life this tremendous transformation of physics, which he started in 1913; but if you compare the 1913 ideas with the present ones, it is a new world of ideas. And he has all the time followed without effort, not only followed but kept the lead of things. He was all the time considered as the leader, you see. Everyone turned to him for what next.
He wasn’t defending the past.
He was ready to break with it.
Oh, yes, immediately. On the contrary. He would always say: “How happy we are to be contradicted by new facts, because we get rid of one error, of one illusion that we have.”
So that his basic philosophy is deeper than just a character trait. This was his whole philosophical outlook, too.
Oh, yes. It was not opportunism. It was a very deep realism. You see: “The situation is there; we have to live with it.” Also the contradictions, the contradictions in the use of classical concepts. “Well, we have to live with it.” Einstein was trying to reduce the theory of light to quanta, to particles. Planck was trying to reduce it the other way and to get rid of the particles. Bohr said: “We have both aspects. We have to live with the situation. And it’s a matter of inventing the right language, that is, the right attitude of mind, to live with those complementary aspects without incurring contradiction.” That was his whole philosophy.
When did the facts become clear in the Hitler situation?
Oh, very soon.
You see, the sequence was this: that Hitler took power in January, and the first announcements took place in March, the beginning of March, and people started leaving their posts in April and May and so forth. Now, by that time, he was involved?
Then he left everything and started giving all his thought and all his effort in creating this committee, organizing it — not alone, I mean, but he was heart and soul in the matter.
There were two committees. There was the Academic Assistance Council in England, but there was a Danish group that I learned of today — a man by the name of Friis at the head of it. What was his background? Where was he from?
Well, I’m not too sure about that. I couldn’t tell you off-hand.
Is he alive, do you know?
Whether he is still alive, that I don’t know either.
His name is Olaf Friis.
Yes. Rozental knows him very well. Mrs. Rozental knows him very well. So from her I have heard many stories about Olaf Friis. In fact, she is a refugee, and he took her on as secretary — well, finding jobs for her to keep her going. He is an historian.
I didn’t know any of this.
That I know, but I have never seen him.
Well, you say that Bohr got involved. How was it? Through this committee? I know Harald Bohr was very active for the mathematicians and others, and he sent letters to people in the United States telling who was available. When did it affect the situation in physics and specifically here?
Well, here we got Frisch, Halban, Hevesy. Hevesy, in fact, organized a whole center here, and the new development of his tracer theory was done here.
How long was he here?
I don’t remember exactly when he came. It must have been about ‘34, I suppose, or ‘35.
But he stayed a while.
And then he stayed until ‘43, I suppose, when he was forced to flee to Sweden to escape persecution, and then he remained in Sweden.
He was one of the long-term people. Was there anyone else who stayed that long?
Well, we could not accommodate everybody. It’s a small country.
And, of course, strains were beginning to develop here, too, because there was the threat of war and so forth.
And so some came and went. Bethe and Peierls. There were many. I don’t remember them all. But those that stayed for longer times were, as I say, Frisch and Halban.
How about relationships with scientists in Germany, though, during this period up until the beginning of World War II?
Well, it depends. The real scientists were in emigration — I mean the best ones. Practically they all emigrated. And those that remained — we had no relations with them.
But up until ‘38. Some of the ones who later left were still there, like Debye and Bothe. I don’t know if he later left. I don’t know if he stayed or left, come to think of it, but I know that he was there.
No, Bothe didn’t leave. He was there. Heisenberg was there during the war.
Well, no relations — I mean there was no systematic attitude. There was very little in the way of relations except with Heisenberg, of course. What would you say? You did not dare to write openly on any of the issues. Or if you did, you risked to damage the person, and he would not dare to answer. So you had no relations. With Heisenberg it was a bit different because of the old times, but I mean the relations became a bit fresh, you see. That was unavoidable.
It’s still to be determined how much of the physics survived there. I think I’m on the trail of that in terms of knowing who was there after the war, knowing who left.
Well, there were very clever people left in Germany: Heisenberg, Weizsacker, Flugge, I mean as physicists. They knew certainly all that was knowledgeable about fission. There’s no doubt about it. Then what happened really to them? They were unable to do anything other than the bit of caricature that they produced there. I am unable to say. I mean I have no direct experience. I don’t know. I can tell you what Goudsmit thinks, but then you can read his book. I don’t know more. It is a bit of a mystery for me.
I can get it someday through letters and other things. Well, anyway generally this assistance to the displaced scholars took place — there’s not much that you have to add on that?
No, I don’t think so. I think in your paper you have put the case — more from the American point of view, but that was also the main point. The English were unable to absorb people. I think you are a bit mild in your article. I think it was due to the lack of imagination of the administrators of the universities.
There was one phrase I used in there where I said “lack of money or room or will,” and that certainly contributed. The system was so rigid; it did not have any degree of freedom. But one other question, and I think that will be the final one. I would like to know about the immediate period when World War II was obviously coming to an end and people could get back to other problems besides keeping themselves alive and being involved in war work or one thing or another: Were there particular problems in theory that seemed to be at the top of the agenda to be tackled? What were the things that appeared to be most important? There was the whole field of quantum electrodynamics, the whole field of nuclear theory and particle theory. But what were the kinds of things that seemed obvious — crisis type questions, as there were earlier? Were there any such crisis type questions?
I should say no, because there were the prospects of nuclear physics opening, and there one had the methods and the techniques also. So it was a field, the development of which was, so to speak, prepared. The elementary particles did not come up so soon. That was a bit later. But then the other great problem was the problem of quantum field theory, quantum electrodynamics and meson theory; and there at the beginning, you are asking — there one had the hope, probably exaggerated, certainly exaggerated as we now see, but even exaggerated at the time, that the new techniques of Tomonaga and Schwinger and Feynman would solve those problems, you see. So it was not a crisis, but rather the expectation of the liquidation of a crisis. Of course, that went wrong in a way. It was still great progress, of course, but not so great as some leading people expected at the time.
Did you see then what we now call particle physics developing as a separate field? Was it apparent from the European point of view? Where were you physically at the end of the war?
I was in Holland because in 1940 I happened to get an offer of a chair in Utrecht. That was before the German invasion. And so I spent all the war years in Utrecht, in Holland.
When did you get back here? I’m trying to find out what the problems were from the European point of view.
Then I went to England, because at that time, in ‘46, ‘47, the English made a great effort in physics. I suppose that the offer made me at that time was part of this, that they thought that they could develop things and so on.
Where did this come from?
Manchester. Blackett was there. But most of those undertakings were rather flops.
Well, look at those machines that they were building in Birmingham, in Glasgow, in Cambridge, even in Harwell. One cannot say that they were completely unsuccessful. The one that was partly successful was the one in Liverpool, but that was only due to a lucky accident — namely, that there was a man called LeCouteur who was a mathematician, not even an experimental physicist, but he was capable of calculating an orbit, a classical orbit, and he found out how one could extract a beam from a cyclotron. So for a while Liverpool had the monopoly of beams outside the cyclotron, and they did some nuclear physics experiments with that.
Well, the lack of success in other institutions — was this due to a lack of experience with a large-scale organization, the technology needed?
Yes. Well, there is much bad luck there. All the projects were aiming at energies which were just too small to get hold of the really new phenomena. For instance, the Liverpool machine was just too small to produce pi-mesons in great quantities. Otherwise, it could have been a boon of course. It could have been a meson factory.
Was there a reluctance to think big? Or was it a question of the countries ruined by war and not enough funds? These are real questions.
Well, they made a great effort. It was a very gallant effort, but somehow they did not make the right predictions. One cannot criticize them, I think, honestly.
It would be interesting to dig in and find out. Maybe it was just a shortage of experience.
Yes. I do not dare to say anything there, because I don’t know.
I’ll ask that in a neutral way of people who were in a position there and ask them what they thought it was. We may find that part of the problem was funding. Anyway, the reason I ask that is that it seems clear — the very fact that they built the machines there and in the United States and elsewhere — that there was a whole field they wanted to get into.
Oh, yes. The will was there. The tremendous success they had was Jodrell Bank of course.
Yes, that’s on a different level.
But, you know, the thinking of Jodrell Bank was based on an error in an estimate, an error by a factor of a million. They intended to look at the cosmic radiation with their radar. It was thought that they would get signals, reflected signals, from the cosmic rays. But the calculation was wrong by a factor of a million, just a slip of the pen, but then they saw the meteors. And so for a while they ran the place since they had the equipment, surplus equipment from the Army, so they ran the place for meteoric research. And then by chance they got hold of a radio star, a signal from one of those radio stars.
Cygnus A, was it?
Yes, surely the most powerful. And then, of course, they had a good case for expanding this thing and to build this monster.
They used this surplus equipment, but it’s interesting that the surplus equipment at Berkeley was used to build a high-energy machine — the surplus radar equipment. Alvarez used it.
That was the idea. In Manchester also it was high energies, cosmic rays. And then for a while Bristol was a great center with very little equipment because they had this connection with the emulsion factory nearby, Ilford, and so they could make emulsions, detecting particles much more accurately than the others, and that led to tremendous discoveries. In Manchester they were only partly successful. There was a huge machine with two magnets which never did anything, but with more modest equipment put in a corner of the laboratory, they found those two “V” particles. And then for two years they could not find any others, until arrangements were made to put those receptors in the laboratory at the Jungfraujoch, and on the first day they were there they got “V” particles in great number. I mention that because this illustrates the genius of Blackett. He was not deterred by flops. He says, “That’s a flop. We must try to make the best of it.” That was Jodrell Bank, you see. When he detected this miscalculation, he could have said, “Oh well, let’s drop the whole thing.” But he said, “Well, we have the equipment. Let us try to see what we can do with it.” With the “V” particle, it was still better, because when no other “V” particles came up, people started doubting, you see, and saying, “Well, those two may be just accidents, tracks coming together.” But he maintained that they were genuine: he examined every conceivable interpretation of the tracks as carefully as he could with all his experience, and concluded that all were excluded except that they were genuine particles coming from a nucleus. And he stood firm.
It was just a question of convincing others. He was convinced.
Oh, he was not even interested in convincing others. He said, “That’s a fact.” But then on the strength of this fact, and on the strength of his authority, of course, he managed to get the receiver installed at the Jungfraujoch, which was not a simple matter.
Now, how long did you stay in England?
When did you leave there?
That must be a whole other chapter. I might like to talk to you another time when I have more background on this whole period in England, because you seem in a particularly good position to have observed, having come from a different environment and then going there in the postwar period, seeing how it differed from your own experience. But I don’t have enough background to ask you about it now. I think we’ll stop here.