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Interview of Werner Heisenberg by Joan Bromberg on 1970 June 16, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/5027
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Interview focusses on Heisenberg's papers on nuclei published in the 1930s. Also prominently mentioned are: Niels Bohr, Enrico Fermi, Ettore Majorana, Wolfgang Pauli; Fermi Interaction, neutrinos, quantum electrodynamics, and quantum mechanics.
Theory did occupy our thoughts. There was, on the one hand, the theory of Gamow on the alpha objectivity of nuclei, which was more or less understood by that time. And then the discussions with Bohr about the problem whether energy is conserved in beta decay. You probably remember that this was a main point of discussion, and by means of these discussions on the beta decay, I remember that we gradually came to the conviction that beta decay cannot be compared to the alpha decay. In other words, that something very funny happens in beta decay which is different from what happens in alpha decay. In alpha decay things could be understood quite simply by saying that the alpha particle runs across a potential barrier, and so on, as Gamow had done. But in the case of the electron, that was different, and somehow we came to doubt whether an electron existed in the nucleus or did not exist in the nucleus. So that I should say that briefly before the neutron was discovered in 1932, as I remember, it was already a question whether one should really say that electrons existed in the atomic nucleus. There was, of course, not the slightest doubt that electrons came out from the nucleus in the form of the beta decay. But we had the impression that there was something funny about the beta decay, and this something funny, of course, is reflected in Bohr's idea that the energy is not conserved, and, at the same time, it suggested somehow that perhaps the electron is not really in the nucleus, it is only created when it comes out of the nucleus, or something like that.
So, I should so say that when I heard about the neutron, I felt it very plausible that one should say that a nucleus consists of protons and neutrons. But, at the same time, the main point was not this. The main point was whether one should say that electrons existed in the nucleus. And, of course, one could say a neutron consists of protons and electrons, or something like that, because a neutron decays into electrons. But, first of all, there was a problem of energy level conservation. Then, the neutrino of Pauli was just being discussed or being discovered, and therefore, somehow I felt that the nicest picture of the nucleus would be that there were no electrons at all in the nucleus, and the nucleus only consisted of neutrons and protons, and that the neutron somehow creates an electron when it decays into a proton. So that was more or less the background for these papers on the nuclear structure.
I agree: it was not really a model of the neutron. We had...you know the trouble is that afterwards when things are clear it is very difficult to imagine how they are when they are not clear. At that time we had an unclear feeling that the neutron somehow can be considered as consisting of proton and electron, but also somehow not because after all the neutron seemed to be very similar to the proton. And I had from the very beginning the idea that the neutron was a kind of a brother to the proton, and that there was a kind of symmetry between protons and neutrons. You remember back to my first paper, that was a main point: the fact that in the nucleus were an approximately equal number of protons and neutrons. This pointed to a kind of symmetry between proton and neutron, not a complete symmetry, but rather good symmetry. And, on account of this symmetry, it was not so nice to say that the neutron is a compound particle consisting of proton and electron while the proton is an elementary particle. And so...but still at that time we didn't know any way out yet, but still I felt that it was perhaps justified to forget about electron in the neutron or electron in the nucleus and to say somehow the electron is created when a neutron changes into proton.
But apparently already predating that the discussions on beta decay convinced you that…
Yes, well, as I said before, it has been in the state of very unclear thinking and not of clear thinking, but I had the idea that just from the empirical fact that the number of neutrons and protons was more or less equal in the nucleus, there should be some kind of symmetry between proton and neutron — now this symmetry apparently did not exist, at least at that time we thought it did not exist, because we knew that the neutron can decay to proton and electron while we did not know that the proton could decay into neutron and positron. But then when the positron was discovered, then, all of a sudden, one saw there is a chance for a real symmetry. And so that made the decisive difference.
But even before that was clear by the discovery of the positron, even at that time, we didn't believe too much in electrons being inside one of these particles. And rather already by Bohr's idea of the nonconservation of energy it was imagined as possible that the electron is not there, that it is actually created in the act of beta decay.
One of these questions I asked was where you got the idea of postulating a force between proton and neutron that would be like the chemical exchange force, and whether it was a difficult idea?
Well, this idea was really earlier than the discovery of the positron. It was in that state of the theory where we still imagined that somehow the neutron may be composed of proton and electron. And then I tried to make at least a picture where it is symmetrical between both and said, all right, if the neutron is composed of proton and electron, that electron is probably being bound very close to the proton in order to be a small particle, then it may be that the electron changes over from the neutron to the proton and thereby we have a kind of exchange force between these two things.
Now later on it turned out that this exchange force is by no means the only force: it is one of the four or eight different forms of force which you can imagine. But I felt it was some help at least to establish this symmetry which I wanted to be there. So my interest had already been before the positron was discovered that there should be a very strong symmetry between proton and neutron. And you remember that already, I think the in the first or the second paper, I introduced this isospin rule [group?], just to say that these two things are not just a pair of particles but [that?] spin only together. And the exchange force was still a kind of compromise between the old and the new ideas — to say, well, how can we imagine that the force should be symmetrical and still the charge is different in the two cases.
Do you remember at all if your paper seemed natural to those who read it and what parts were the surprise?
Well, I remember that a number of physicists didn't object much but a few, even very good physicists, objected strongly. First, for instance, my assistant — at that time, that was Professor Guido Beck, who is now in South America, I think — he protested very strongly, and said, well, it was a scandal because I really claimed that there were no electrons in the nucleus while one could see that there were electrons because electrons come out. And he said that was simply not logics and it was a shame that produce such ideas.
But even such very good physicists as, for instance, Weisskopf — he was very dissatisfied with the paper. Just briefly after that time I was on a skiing holiday, together with Weisskopf in ? and he explained to me that these two papers on the nucleus were very much below my standard, as he expressed it. And he said, well, your other papers have been so good but this is pure speculation and you did nothing. One can't actually believe that there are no electrons in the nucleus, that is not true because electrons are there, and so on. So, at these points, I had strong objection from several physicists.
What did Dirac give you in reaction?
I didn't see Dirac much in that time. And I think his reaction that was later, and then, you know, Dirac is different in his attitude from many other physicists. As a rule Dirac would not express any views on problems of other people. And, at that time, he was strongly engaged in the problem of the electron and the positron which, of course, was just being experimentally discovered, and he was not so much interested in the nucleus. I think he would just have said, if I had asked him well, I don't know anything about the nucleus — that's your business or so. So I have no reaction. At least I cannot remember any reaction of Dirac on the problem.
Were you in close contact with Fermi and Majorana and people like that in 1932?
Not in '32. I might have met...well, with Majorana, yes. I might have met Fermi occasionally in Copenhagen or at some conferences. I remember there was once a conference in Rome where I met Fermi. But I heard about Fermi's paper on beta decay only by its publication, so I don't think that Fermi even sent to me a copy of the paper before it was printed. I got a reprint and then. I saw the paper.
But Majorana, he was in Leipzig in my Institute at that time, so his papers, of course, they were just discussed frequently in our seminar and I have discussed personally with Majorana. So with Majorana it was different. But, of course, the Majorana paper, that was concerned with nuclear forces. And Majorana, he had the idea that one could have more general nuclear forces between proton and neutron than those which I had assumed, and as you know, he was interested to explain the saturation of nuclear forces by means of these different exchange forces. So the Majorana paper and the Fermi paper I would put on a different level. The Fermi paper — that was really published without any discussion with me, at least. I don't know with whom Fermi might have discussed it, but Fermi was a very... he stood entirely on his own feet, you know. He was always very independent of others. And so I saw Fermi's paper when it was published and found it extremely interesting.
And my next reaction to Fermi's paper was to ask if such a force exists as Fermi had introduced — electron, neutrino, and so on — what are the consequences for more general problems (for instance, in collisions between elementary particles at very high energies, and such things)? And that then later after a few years led to this idea of multiple production of particles. That was a paper which I wrote, I think, '35 or '36. And Fermi's paper was in '34 was it not?
In '36, yes, so it must have been one or two years after Fermi's paper, I should say.
I wondered, at one point, if your work on the derivation of exchange force from Fermi's theory preceded your work on predicting showers from Fermi's theory and had any genetic...
No, I would definitely say that my ideas on exchange force had nothing to do with Fermi's paper. The paper on the exchange force was more a copy of the old ideas which one had in the theory of chemical bonds . . .yes, chemical binding between two atoms.
I asked that question very badly. Of course, your paper on exchange force precedes by two years the Fermi paper, and what I meant to ask is this: As soon as you had read the Fermi paper you made the suggestion that you might be able to derive the exchange force from the Fermi interaction, and then I wondered if that led you into investigating the Fermi interaction and shower theory?
Well, that is quite possible, yes. I couldn't say yes or no. I just don't remember how it was, but I now know to what you refer, I mean this paper where I tried to use the Fermi interaction to connect this idea with the idea of the exchange force. But I don't know exactly how this was, I mean, all these things are mixed in my memory now so I wouldn't say that that was the logical consecution of papers, but there is some connection probably, yes.
Let me go back, please, to 1933, and from what I see in the papers and in your letters to Niels Bohr (which are in the Archive), although in July it seems as if you had not yet decided that the neutrino theory was a good one, but that, after that, you began to pick up the neutrino and become very interested in the connections between the atom and the forces that hold the atom together and that give you light radiation and the nucleus exchange forces. And I wondered, among other things, what it was that induced you to believe in the neutrino, and whether you were beginning to move in the direction of Fermi's theory before his paper itself came out?
Well, yes, to some extent, that is certainly so because the idea of the neutrino led the theory back to things which one could deal with mathematically. You know, when you just had the general idea that energy is not conserved and there is no thing like neutrinos, then you didn't know how you should write up any kind of equation or any kind of formula which describes the fact.
If, however, you say there is a neutrino, then you can imagine that you may write down an interaction force which says that electron and neutrino are produced if such-and-such happens. What actually then Fermi later on has done. So for my mind I would say the a neutrino hypothesis brought back again the discussion from a very speculative region to a normal region of quantum theoretical discussion. If you then do not speak about nonconservation of energy, you just say that everything is as it was, only there's a new particle. All right, then you can again write down something, and then of course the idea was, well, if we have then some kind of interaction which means production of and neutrino, then this interaction may act as a kind of glue between the protons and neutrons in the nucleus.
So I should put it this way: looked for the glue between the particles, because this glue was apparently not electromagnetism, not the electro-magnetic forces. It was what we nowadays simply call strong forces or nuclear forces. And I wanted some kind of interpretation for these forces and I thought the correct interpretation is that it is exchange of electrons and neutrinos. Now this was wrong, of course, as one knows later on, because it was exchange of pions. But at that time, of course, one had to speculate about it, and so it was natural to think of electron and neutrino.
In the letters that you wrote Bohr in April and in March of 1934, you outlined an analogy between the atom and the nucleus, and the Coulomb forces were analogous to the exchange force, and the created particle, the light analogous to , and then, later, in September 1934, you add the final fields, the radiation field of the atom and the Fermi field as being analogous...
By the Fermi field, do you mean the Fermi field from this beta decay theory of Fermi? Ja, ja.
And so it seems a complete suggestion that you might have been looking for such a field, like the field of Fermi's even before Fermi brought it to your attention.
Yes, I would say that this is probably correct, but I didn't dare simply to write down something as Fermi did. I always had the idea that one must invent something very new and very different from other things in order to describe this kind of force. But what Fermi actually had done is that he had simply written down an expression which says that the proton changes into a neutron or a neutron to proton by emitting electron-neutrino. So he had simply written down an expression which does a job and I didn't dare to just write down such a thing. I felt that there must be done something more about it than just that, and so I was then, of course, very enthusiastic when I saw that one can go on this straightforward way.
Do you remember anything about Bohr's reaction to Fermi's. It seems strange that already two months after the reception of the paper he still, in the Solvay speech, spoke of energy nonconservation.
Did Bohr still speak about energy nonconservation after the Fermi paper?
Well, I have to put it this way: there's a letter at the end of January to Fermi thanking him for a preprint. And Professor Rosenfeld says that he reported on that preprint and Bohr seemed very pleased with the theory in this letter and yet Bohr finished the Solvay report in early March of 1934 and there he still says that there is no grounds for making a judgment on the neutrino and that there is at least a possibility of energy nonconservation.
Yes, that's very interesting. I mean it shows again how unclear things are to begin with. One is not willing to change one's mind at once if one hears about a new idea. One sees, well, that might be so, and, of course, Pauli had made very strong arguments in favor of his neutrino. But, at the same time, the neutrino had never been seen. It was just a speculative object — of course, an object which now by means of Fermi's theory had come into a nice picture — but still it was not certain, and therefore, probably Bohr still liked his idea a little bit but not too much, and then, of course, Pauli was extremely angry on Bohr, saying, well, it is all foolishness things what you say, so Bohr didn't dare, but he just said very cautiously, why it's not quite so.
Was there any difference in the particular criteria that you brought to bear on the Fermi theory and that Bohr brought to bear? Was Bohr, for example, more interested in the experimental confirmation of the neutrino hypothesis?
Well, I would put it this way that I was probably convinced of the neutrino slightly earlier than Bohr but only because I spoke very much with Pauli, and Pauli, of course, made a strong impression on me. And then when I saw Fermi's paper I thought, well, that probably is the solution, and it took some more time for Bohr to come to this conviction. But experimentally there was just no confirmation to be obtained at that time. It was far away. You remember that the real proof of the neutrino was only twenty years later, or something like that, so there was no chance to have more than the things which one already had at that time.
Bohr gave a speech at the Rome Congress; again I learned from Dr. Rosenfeld that Fermi said to Rosenfeld afterwards something very roughly like "that's not the way physics is done." Do you remember, in any way, the attitude of Fermi toward nuclear physics and towards the whole problem of the nucleus before the discovery of the neutron — whether there was any contrast? Well, let me say this: before the discovery of the neutron, I have the feeling that you felt that the time was not right for doing nuclear physics.
I agree. That is probably exactly as it is, and that is probably true for any of us, that means including Fermi also. Nobody felt that one can really do something. There was one exception to it. That was the theory of Gamow for alpha decay, but then one had the feeling, well, this was a very lucky case where one can do something without really understanding the nucleus. But in order to come to a real theory of the nucleus, there is just no basis. And only the discovery of the neutron then changed the whole picture all of a sudden.
Now, let me change the subject to another one. And the question most broadly is to ask you — this whole concern of yours with fundamental length and with the limits of applicability in quantum mechanics shows itself especially from '36 on — whether there was any prehistory of that which connected it- self in some way with nuclear theory, whether for example your ability to separate the nuclear phenomena in an extreme way into those that could be treated by quantum mechanics and those that could be not, gave some impetus to your interest in the whole question of finding the domain of quantum mechanics?
Well, that's a difficult question. I would definitely say that, of course, nuclear physics with its very new aspects, new forces, strong forces, and so on, suggested that we have entered a new field of physics in which things are somewhat different from old quantum theory. And that became clear, certainly after '36 when this idea of multiple production had come in and one saw the air showers of many particles come, and all that. At the same time, we had so many discussions on principles at Copenhagen and it was a purely theoretical idea to say; if the constant "c", velocity of light, plays such an important role and the quantum of action plays such an important role, not only for the phenomena but also for the philosophy of all the phenomena, then there should be a third constant because we always have three things that are mentioned (mass, velocity, and length and so on).
So that it was natural to say there might be a third constant and that should be either length or mass, and then it's again natural that we lower [?] this constant or go beyond [?] this constant, say, then the phenomena will become very different from what they have been before, or what they are mentioned to be before. And so, from purely theoretical reasons I mention that there should be again such a situation as we had had in relativity and in quantum theory and now again we should have such a situation with respect to fundamental length or fundamental mass or something like that.
How far back in time did these discussions in Copenhagen go? Do you remember that?
I should say they have been all the time and I couldn't say when. I mean, of course, these things come and go away again, and then they take this form and that form. But they took a definite shape in the end of the thirties, I should say, around '37 or '38. I think my paper on the fundamental length was in '38, I suppose. But I couldn't say when it had started...when we have started these discussions.
Did the theory of showers have a very important effect in directing your attention again to fundamental length?
Yes, for me this theory of showers was very important, but you know that also mixed with several disappointments. Because first, I thought that when I had seen that the Fermi interaction would lead to such showers that the air showers had actually to be explained on this basis. And then the papers of Oppenheimer and Carlson and Bhabha and Heitler appeared which showed that actually the air showers ought to be explained as cascades. And then I didn't know...I remember that I was in the United States in 1939 and I gave a lecture on showers and said: "I agree with the cascade theory. That must be correct. At the same time I still feel that in the air showers one also sees the relics of this other shower phenomenon which I had studied in my theory, namely multiple production of particles at one step." And, of course, I again got rather strong objection. I remember a discussion in Chicago in some of this meetings in a kind of congress, summer school, or something like that, where Oppenheimer rather strongly criticized my paper, saying, "Well, about the air showers now we know where they are produced; they are produced by cascades. Of course, nobody can exclude the other kind of showers but also nobody can prove the other kind of showers. It's quite unsure." I had to agree to that because at that time one just didn't know which it was.
But, for my feelings, these other kind of showers were the more interesting ones because they showed that fundamentally there was some new feature coming in, that nature had the chance to close up the things at small distance. So for me it was always extremely important to find the point where nature can close up certain phenomena.
And I could see that the multiple production of particles, together with Dirac's idea of the anti-matter and the anti-particles, that this really gave such an entirely new aspect to the phenomena that one could see — that the particles are the smallest particles. In other words, that dividing particles has no meaning any more. And so this point was for me the decisive point and therefore I felt the cascade theory of showers was a disappointment in spite of the fact that they had...it was proved experimentally.
In June of 1936 there was a rather large conference at Copenhagen, and at this you suggested your first paper on showers, and, at the same time, Bohr had just gotten his theory of the compound nucleus, and I wonder if you have any memories of this conference. On the one hand, it would be interesting to know what people like Amaldi and Wick have to say to the compound nucleus model, and on the other hand, to know the reception to the shower theory.
I would say that Bohr's paper was generally received with approval, that is, most people said, well, it looks as if the interaction between particles in the nucleus is a rather strong interaction, and then it's a good idea to compare the nucleus with a sandbag or something like that. And also the Bohr theory could at once explain these very sharp resonances at very small velocities of the neutrons. So I think there was general approval, still mixed with the idea that, well, it may be that both theories are right to some extent — that is, on the one hand, the nucleus acts like a sandbag; and on the other hand, it may still be useful to consider the nucleus as a kind of shell structure with different quantum numbers.
But this paper of mine on the showers, I think, was...did meet much less approval, that is, most people said, well, that is rather wild speculation. Of course, mathematically, it's correct, what I said that the Fermi theory did lead to such an interaction, but, after all, the Fermi interaction may not be the only interaction. Other interactions will come in, and nobody knows whether showers are really to be produced in that way. I don't know whether at this conference the shower theory of Bhabha and Heitler was already known. It was not, I see. But it must have been very soon afterwards that that came. When was their first paper?
I'm sorry I didn't bring...I remember in '37 but I did not bring a reference to it. By one letter I see Bohr mentions that he was extremely excited about your shower theory. Do you remember his reactions?
I only would say that Bohr's reaction was rather positive on my shower theory. I'm not sure.
Of course, it's difficult to say because this letter was a letter to Dirac and Dirac had just published his little paper in Nature reintroducing energy nonconservation. And Bohr here was trying ...was sending Dirac back his reprint of his reply in Nature and was trying to demonstrate the fertility of the neutrino hypothesis had deduced your shower theory.
Oh, I see, as an example that the neutrino theory may be something useful. Well, by that time, certainly Bohr had already given up the nonconservation of energy, and he wanted simply to explain that to Dirac in a nice way. But I also feel that Bohr was very interested in the shower theory. He didn't feel that it was too wild a speculation. He always said, well, of course, we don't know too well. As I have said now already several times it is so difficult nowadays to go back into a state of physics in which things were so unclear. Every year very many ideas and new phenomena come up, and then things are again disproved. And you know it was all very much less defined than it is nowadays.
May I ask you a little bit about where you on Bohr's own work on the compound nucleus? The first question Bohr had already said in the article in 1938 which just preceded your article on fundamental length. acknowledge it. He mentions the fact. He argued the intermediate nucleus was present from the very beginning of Rutherford's experiments and had talked about it. And I wondered if this idea was very much in the air?
I couldn't say exactly. I do remember this talk between Bohr and Rutherford which I reported in my book also, when we went on our walk in the garden near Carlsberg. But whether Bohr had already written his paper or was just had in mind to write it, that I can't say. I know that Bohr had rather early come to the idea that the nucleus is something very different from an atom. You know in Leipzig we had always worked on the idea that the nucleus is built up of shells, the shells are not so well-ordered because there is no central point, but still the shells are there to some extent, and so on. And we had already written a few papers which had been quite successful in explaining properties of the nucleus by means of the shells.
But Bohr always hesitated to agree with it. I mean, he said, all right, you can try, it's interesting, and so on, but still, he said, the interactions are so strong, you know, should it not be quite different. And then when these resonances came up and when one saw that there were very sharp lines and very low energy, then he got the definite idea, now this model of the sandbag is definitely better because it was quite clear that those shells which we do not see at ? are not suited to explain sharp resonances. That was quite clear. So I would say that Bohr had the idea of the sandbag already rather early — perhaps in '34 or even earlier than that — but he didn't want to write anything about it before he had good experimental evidence. But when he wrote about the sharp neutron resonances, then he felt, oh, this is a good evidence for ?
That is extremely interesting because of, what shall I call, the popular account that comes to one's [?] ears at the Institute is that Bohr suddenly came to this idea when Møller had given a seminar on Bethe's explanation of Fermi's results. But from what you say, that's not at all the case, and Bohr was already playing around from a rather, from an old conviction of the great difference between the nucleus and the atom...
Yes, I would say that Bohr did play around with it already earlier than that. I remember now the paper of Bethe which you mention — that was, of course, a very interesting paper. But I would say that Bohr earlier than that...well, he had always emphasized in all discussions — that always means, I should say, at least since 1933 or so — that there is a decisive difference between the atom and the nucleus in the sense that in the atom there is a center, and this center, the nucleus, makes all the rounds with shells and so on. In the nucleus, you have no center. In the nucleus you have just matter. And now, of course, we could argue, well, still if there is no center, but still the nucleus is a kind of spherical ball, a spherical top.
But Bohr always felt that this is not too good, this picture is not really good, and he insisted on saying, well, the interaction between the single nucleons in the nucleus is more important than the interaction between the electrons is in an atom. Which is quite natural because electrons in an atom have only small forces compared to the force between nucleus and electron, while in the atomic nucleus the force from proton to proton, or ? is the strongest force there is.
This material particularly interested me because I don't think I've seen any other records on this. You may or may not remember this, but after the Copenhagen Conference in September '34, which was an extremely small in- formal one because Bohr's son had just died, apparently you gave a report on the nucleus and had some conversation which even spoke of writing a joint paper with Bohr on this. Do you remember at all what you might have been thinking at that time, of what kind of statement you might have thought of writing?
Unfortunately, no. I don't even recall that I did ever intend to write a paper together with Bohr and I don't recall what I said at that conference. Of course, I remember the year just on account of the death of Christian....
The paper you published in the was in some ways a re-working of the conference paper although I don't know to what extent. You finished the paper for the volume in January 1935. It was a review of nuclear problems in which you spoke, among other things, of relation between exchange forces, and in which you made a thorough- going analogy between the atom… of course, from what you say, Bohr would not have sympathized with this analogy...
Well, he did sympathize with the idea that there must be some new force between protons and neutrons, and this new force could be bound the together or could be correlated with the small particles (electron-neutrino, think). So far, he did sympathize. But he always emphasized the other point that the geometric structure of the atom and that of the nucleus are different things because in the atom you have the center and in the nucleus you have none.
Now I can't say for definite at what time Bohr first has emphasized this strongly but I think that's a time that is even not well defined because there would be a conversation. Bohr would perhaps have asked, well, but there's no nucleus in the nucleus, so what about the center of the nucleus? And then one would speak forth and back, and then it would be forgotten again, and then it would come up again — the theme — but only I do remember that Bohr always hesitated a little bit to make the comparison too narrow. If actually the magic numbers would have been discovered at that time, I don't know how Bohr would have reacted — that would be interesting — I mean the later stuff of Mayer - Goppert, and so on. But at that time Bohr felt only that those shells in the nucleus which we discussed in Leipzig were perhaps a convenient way of calculating things but they were not a very serious analogy [?]. He would say, well, actually, these shells were always mixed together through strong interactions.
Did the Bohr model make any great difference in the direction of the work you were doing at Leipzig after that?
Not very much, because we felt that, especially after this discussion that I mentioned in my book, even if Bohr is right, it then may be a difference between chose cases where a particle is rather strong and actually enters the nucleus or a particle which is always bound in a low shell at a low energy, and for such a particle the nuclear interaction between this and other particles doesn't play such a very important role. So later on, one has put this into the form given to it by Weisskopf by saying if particles are rather low-lying, have a low energy in reaching the atomic nucleus, then the number of transitions which they can make is limited by the Pauli principle because they can only… they must go above to high energy to interact, and therefore the local interaction is ? down and doesn't play such an important role, and therefore it may be that both models — the shell model and the sandbag model — are both true to some extent. They are not a complete contradiction.
I have now a kind of difficult question that perhaps is not really very easy to answer. I have formed, as I said, in these questions the tentative conclusion that you had, to some text, a method of working which was when you thought you could fit the domain of quantum mechanics — the domain in which quantum mechanical ideas would apply — you set about to solve those problems which could be solved, and that in a sense your work on the nuclear papers represented an application of that technique. Do you think there is any truth in all this kind of ....
Yes, I would agree with what you just said. I tried to apply quantum mechanics as far as possible in the nucleus, and in order to do so, we had, first of all, made assumptions about...
...it could be treated by means of old quantum mechanics and separated completely from other problems in nuclear physics which could not be treated by quantum theory. But I would put it this way. First of all, I actually did work on this line and therefore had introduced formally forces between protons and neutrons and so on, in order to be able to apply quantum theory. But this other thing that we tried to work on atomic shells in the nucleus — that was more a formal point. Actually, I felt that even if one took the Bohr point of view it might just as well be possible to use only quantum theory. So actually when I had this discussion with Bohr and Rutherford of which I wrote in my book, then I made that calculation in Leipzig in which
I tried to see whether from the point of view of quantum mechanics Bohr's view could be correct, that is, I studied how a proton which is hit into a nucleus, how that would react with other particles in the nucleus, and actually it came out that the range apparently of entering the nucleus is rather small. So that actually a particle is stopped more or less at once when it enters a nucleus, and that, of course, was very much in line with Bohr's ideas. .So I think there must be also letters from myself to Bohr, telling him that this has been the result and that fitted so nicely to his ideas. So that I remember definitely that I considered Bohr's ideas not as in any kind contradiction to the views of applying quantum theory.
Several times in letters you speak of Bohr's style and Bohr's ways as something somewhat unique. Can you say anything about Bohr's ways of doing physics and how it applies, for example, to his work in nuclear physics?
Bohr's way of doing physics was always based on using the correct concept, so the most important thing for Bohr was to find the correct concepts from which you could understand nature, and not so much to work quantitatively on the result of certain assumptions. So Bohr almost never made very strict calculations — I mean, mathematical deductions are rare in Bohr's papers. But what is always the central point in Bohr's papers, that is the discussion of the concepts from which he starts in order to explain the things. And in this way I should say that his style was very unique. His style could be compared with the style of Faraday, or Gibbs, or such people. So that the whole weight was not on the mathematical deduction, on the mathematical detail, but the whole weight was on the concepts from which you have to start.
Therefore, he always was very careful in discussing the meaning of words and also to show that the words have not a clear meaning, that one must be very careful about words and so on. And all that was, of course, very unusual in ordinary physics, in what other people did. For instance, Sommerfeld, who was my teacher here in Munich, he would always give definite problems to his pupils, that is, they have to solve mathematical problems connected with physics, and the whole weight was on the mathematical description that the mathematics was good, that the proof was good, and that the research was in agreement with experiments, and so on. But all that did not interest Bohr primarily. Bohr was primarily interested in the correct concepts.
Would that tend to explain why his work was, in some respects, rather different from yours — for example, different from your reaction to neutrons where you sit down and you get the mass defect curve, trying to see how the phenomenon comes out of the problem?
Yes, my reaction would be, all right, now we know that there are neutrons. Let us make an assumption, namely, nucleus being composed of neutrons and protons, and let us see how far this assumption leads. Can we get correct mass defects, and so on. But Bohr would not go so directly to a mathematical deduction.
I would also say that that was perhaps most strongly seen in the time of quantum mechanics because Bohr was not too unhappy about the many contradictions which we had in old quantum theory before quantum mechanics was there. But the idea that there should be a mathematical scheme in which all these things are then consistently connected, that was, for Bohr, not so necessary as it was for me. I think Bohr would have...well, he certainly agreed with pleasure that this could be done but it was not an absolute necessity for him, while I always felt that in order to avoid these hopeless contradictions the only way out is a mathematical scheme. So one had to find the mathematical scheme which then will connect all these different features.
That's extremely interesting. Perhaps the last thing I would like to ask you are two things I mentioned I would like to ask about. I would like to on' the one hand to understand something of the background of nuclear theory as against quantum electrodynamics and here ...
Oh yes, they were rather independent problems: quantum electro- dynamics on the one hand, and nuclear physics on the other hand. I would say quantum electrodynamics, that was in the thirties a subject which was, I should say, almost too sophisticated for Niels Bohr. I mean it was a subject which could be treated with very complicated mathematical methods, and it was not a problem in which you could do much with new concepts. And so Bohr...he listened to the talks of Dirac and also myself in the Copenhagen meetings, but Bohr was not too interested in those things. He wasn't interested, for instance, in the theory of showers and the cascade theory of showers because that was such a definite phenomenon which you could see the experiments. If I could put it that way, Bohr was interested in experimental situations, in phenomena, and he tried to get such a close view of all the phenomena, to acquire the phenomena in his mind, so strongly that he could then form the right concepts to start for the explanation. But he would not be too interested in complicated mathematical analysis like it was necessary in quantum electrodynamics.
But from your point of view, your work in nuclear theory seemed to be most closely connected with the work in quantum electrodynamics; it was connected with your concept of a particle...
Yes, I had always both these interests. I mean I was always very happy to discuss things with Bohr because Bohr would always have to make the concepts clear. On the other hand, I was also interested in these mathematical problems of quantum electrodynamics because I felt that finally you must write down a mathematical scheme which explains things. I always looked as a final aim to the mathematical scheme. And that was not perhaps what Bohr did. Bohr looked to a scheme of concepts, a number of concepts, and not a mathematical scheme — if one wants to make such a strong distinction. After all, we have spoken so much forth and back, but still I would say there is a slight difference in tendency between Bohr and myself.
I tried to put it this way in a talk which I had to give after the death of Max Born. I compared my three teachers which I have had, that was Sommerfeld, and Max Born, and Niels Bohr. And I would say: Sommerfeld he was most interested in solving problems which you could compare with experiments. He did not mind the concepts — I mean, the concepts somehow had to be all right, but he was not too much interested. And also he was not too much upset about contradictions, as in old quantum theory, there were contradictions. He wanted to do calculations and to get all correct results. And then there was Max Born who believed strongly in the existence of rigorous mathematical schemes. He was very much a mathematician, but he was not a one philosopher. He was interested...he always suggested that I must find the theory in quantum mechanics which replaces Newton's mechanics, but he did not see that in order to do that you have to derive or to apply new concepts.
And, finally, Bohr, he was a philosopher who looked for the concepts. And he said, well, first we must have our concepts right — that means actually we have to make our mind clear, and before we can do that we have no chance really to solve the problems.
So, I don't know whether I am clear enough to compare these three people.
Thank you very much. I have come to the end of my questions.