Carl Weizsäcker

Kuhn:

Could you tell me about your very early contact with Professor Heisenberg; in fact it might be well to start with how you got interested in the sciences because it was clearly a prior interest in science — in fact I think particularly in astronomy — that brought you so early to Heisenberg.

Weizsäcker:

Well, that is not really very closely connected with quantum theory. I don’t remember how I got interested in science; I just remember that there was a time when I suddenly discovered that I was very much interested in the stars. I may have been eight years old or so. But then I met Heisenberg here in Copenhagen in January ’26 [1927]. I’m not quite sure about the month. My mother came home from a party and said that she had met a very nice young man, a German who was working with Bohr — ‘that famous physicist, Professor Bohr’ — and that he played the piano so well, and they had some discussions about the German youth movement, and his name was Heisenberg. And at that time I had already read the popular paper in the sort of popular periodical on astronomy in which it was said that now the problems of atomic physics seemed to be solved, or in any case in a new state, mainly by the work of some people, one of whom had the name of Heisenberg.

So that’s how I was induced just to ask my parents to invite him. You might be interested, if you are interested in details, in some of the things I remember from that time. He came to our home, and of course I asked him many questions and I made my own statements which I think he thought not too mature. Then he invited me to the Institute; he was living in the Institute at that time. He showed me the library, he showed me as much of the experiments as were present at that time, and he played chess with me. This was very important to me because he was far better than I in every other thing, but in chess there was some possibility of being comparable to him. Then he showed me mathematical books, and I was a little bit afraid of the many things one was supposed to learn in mathematics before being able to understand physics. He said then, “Nature of course doesn’t calculate, but we must calculate in order to understand it,” and this I think was a very philosophical statement. It was evident that it was true, but at the same time it was strange to think how nature could manage to do things without calculation which we only can understand by calculation.

That was in the time, as I afterwards learned, when he was just working on the principle of uncertainty. But I don’t remember that we discussed these matters really when I was in Copenhagen. Then in April ‘27 we moved to Berlin. My father had been in the German delegation in Copenhagen and he moved to the German Foreign Office in Berlin. When I had been in Berlin for a fortnight or so — not more than that — Heisenberg wrote me a postcard and said I should come to Stettiner Bahnhof, the railroad station, and see him there on his way home from Copenhagen to Munich. He took a taxi and we went to Anhalter Bahnhof; in this taxi he told me that he now had found out that causality didn’t hold in atoms. That was very exciting and I felt somehow, I should say, that he himself was excited about the possibility of making such a statement.

Kuhn:

Are you quite sure that he himself put it that way? I don’t mean in those words, but did he talk about this as a violation of causality, that this was the fundamental problem?

Weizsäcker:

I think so. Of course it’s difficult to remember precisely what he said and what my interpretation was, but it is quite certain that I left him under the impression that the real thing he had found was that determinism was not true, or that the law of causality had broken down. I’m not sure how much I was able at that time, and how much he was able, to distinguish between the two things, because if you read his paper on the uncertainty principle he just says that in so many words: now it has been finally shown that the law of causality breaks down in physics. Since this is in his paper, this may have somehow influenced my expression of my memory at that time, but I know that this was the point which really struck me. After that, but that’s rather personal, I walked through the streets of Berlin thinking, “Now I should like to meet Einstein,” — whom I would certainly have recognized since his pictures had been in all the newspapers - - “and I would certainly ask him what he thinks about these views of Heisenberg!” But I never met him.

In any case, it is quite clear that in the moment I met Heisenberg, and then in April, he was feeling very strongly that what he had found here was not just a special result in physics, but was of basic importance, for, let’s call it philosophy. I remember that at that time we had some discussions about Kant’s philosophy; I had tried to read Kant and I didn’t like it. I wrote a letter to him saying that I didn’t like to go on reading Kant, and he wrote a letter saying that this was silly and that it was certainly worthwhile to study Kant thoroughly. This seems to indicate that he had spent some time studying Kant. I think that’s mainly what I remember from this first year. Then he went to Leipzig. I visited him in Leipzig, I think early in the summer of ‘28 — it may have been May or June. We discussed the question of my choice of field of studies, since I was to go to the University a year later and I wished to decide what to do. I think he really influenced me to choose physics.

I had found out easily that astronomy rested so much on physics that physics really was the thing I was interested in in science. But then I had also found out that I was deeply interested in philosophy, and I think Heisenberg at that time — well, he once wrote me a letter in which he just said, “Well now, there are beautiful philosophies existent but we can still use some more good physics; therefore you should study physics. But this was not the most serious argument; what he really said was that he felt that in the twentieth century it would be impossible to do philosophy without knowing physics, and that one could learn physics only by doing research in physics and this could only be done when one was young. I visited in June or May ‘28 and I remember that at that time he mentioned for the first time the name of Dirac — mentioned to me, at any rate, the name of Dirac.

Kuhn:

This was the second trip to see him in that same year?

Weizsäcker:

I don’t remember; I may have made several trips, but I remember this particular one. This is in connection with what you asked yesterday: how Heisenberg felt about the question of whether the problems were now finally solved or not. Yesterday we somehow got into what was not quite a clear situation with respect to the answer to this, because on the one hand I said that I had the impression he felt that the problems might have been mainly solved, and later on developed an understanding that it was not so. On the other hand, I remember, though this is going a bit ahead of the time I’m speaking of, that before ‘32 he always thought that nuclear physics would not be under the rule of quantum theory. If you remember what Weisskopf said just this morning [lecture by Weisskopf at the Bohr Commemoration Meeting], it’s quite interesting to see that he still finds it quite surprising that this same theory, quantum theory, should apply to two fields in nature as different as the atomic shell and the atomic nucleus. This reminded me of the attitude taken by Heisenberg, and I think by Bohr too, in the years before ‘32, that it just couldn’t be expected that the solution of the problems in the atomic shell would apply to the different conditions prevailing in the atomic nucleus.

Kuhn:

You were however already at the University and had specific reference to the problem of the electron in the nucleus which provided a reason for not expecting so.

Weizsäcker:

Yes, there was a very good reason for thinking so. This was in 1930 or ‘29. I don’t know what Heisenberg may have thought about the nucleus before, because before I wasn’t really interested just in the nucleus. But at that time I think two arguments combined, two reasons combined: the general one, which I mentioned before, is that you just don’t know why quantum mechanics should hold in the nucleus since it hadn’t been discovered in the nucleus. This of course would not have been sufficient because it might very well have been possible that it would hold in the nucleus. At that time, though, it seemed obvious that the nucleus was composed of electrons and protons and that the electron just couldn’t be present in the nucleus because according to uncertainty principles it would have had to be under an immense force.

In any case the evidence about mass defects tended to show that the energies in the nucleus are not comparable to the enormous energy which would have been necessary to keep an electron in the nucleus. Quite apart from the other problem, is this question connected with the Dirac: If an electron were in the nucleus it would be in an area definitely smaller than the Compton wave length which would mean that its motion would be strongly relativistic. Thus there was always the question whether relativity and quantum theory could be merged. I remember that Heisenberg said, “Well now, the problem of relativity and quantum theory and the electron has been solved by a young Englishman by the name of Dirac, and he is so clever it is just not .... es lohnt sich nicht, mit dem um die Wette zu arbeiten.” And I felt that Heisenberg somehow was just really unhappy to see that there was a man who was able to solve a problem which he hadn’t been able to do himself. This sort of thing!

Kuhn:

Did you get the impression that Heisenberg himself had been working on the relativistic problem? In his published papers from this period there’s just nothing at all.

Weizsäcker:

No, nothing at all. And I don’t think he had been really working on it, but if I try to interpret what I remember from that time it was rather that Heisenberg had felt that here something had to be found of perhaps quite a different nature, something in which he couldn’t work because he didn’t know how to start work.

Kuhn:

He made a very interesting statement to us the other day, and I want to quote it just to see how you react to it because I really was thinking of it more than of this nuclear problem when I raised the question with you yesterday about his attitude toward compatible and incompatible theory. I asked him whether there had been any thought when they were doing the matrix mechanics of trying to relativize and to find a relativistic matrix mechanics. He said that they’d never really tried because he felt, that he had hold of something here, a consistent system in Newtonian terms; but he thought it quite likely that a relativistic treatment would turn out to be something totally different, that it wouldn’t be just a matter of relativizing this, but that all sorts of other things would come into the system and things might behave in terms quite different.

Weizsäcker:

Yes, that sounds quite possible. Perhaps if we add this statement to what I told you about Dirac: perhaps one may say that Heisenberg was surprised to see that it was possible to invent a relativistic wave equation which seemed to be consistent. I mean, he might have expected that relativity and quantum theory were so different that it would need far greater steps than the step done by Dirac, not just the step to invent a relativistic equation.

Kuhn:

My impression is that that was not at all the usual attitude among physicists.

Weizsäcker:

Would you say? Well, if I try to interpret this feeling of physicists one may say that relativity brings in two aspects which are difficult to reconcile with quantum theory, and which are connected. One is that time in relativity seems to be treated in a manner which is quite different from the quantum theoretical manner. I think Heisenberg was quite aware, very early, of the fact that in quantum theory time is not to be compared with any space coordinate, but is, so to speak, far more basic. You can have all sorts of quantum mechanical systems without speaking of space and position, but you cannot have a quantum mechanical system without speaking of time. In relativity time enters in a manner which, at least apparently, is mathematically symmetrical to the manner in which space enters, and I think that this was an important point for Heisenberg to have thought that here was perhaps something quite different.

The other thing was that, in connection with the finite velocity of light, relativity brought in the necessity of considering actions with the finite velocity of propagation. That meant that the problems of the continuum would enter the game in a manner which was avoided in quantum theory. If you look back into the history of physics, especially of quantum theory, but wherever you go, I think you will always find that the continuum was the point where theories usually broke down. As soon as you treat a continuum as a dynamical entity and not just as a set of coordinates, that is as soon as you say that there is a body with an infinite number of degrees of freedom, in general theories break down. Planck’s quantum hypothesis just started by eliminating certain elements of continuity. It may very well be that Heisenberg at that time already felt, as he certainly felt later, that perhaps to unite quantum theory and relativity would mean to do another step against continuity. I remember at that time, or perhaps a bit later, hearing the argument from him that evidently there are three basic constants of nature, because there are three fundamental units in Newtonian mechanics, and two of the constants, h and c, had been found; the third one, perhaps the length, was still to be found and understood. This again was an indication that some theory was still missing. What I have just said was quite certainly his view a few years later; the only thing I don’t remember precisely is what he would have said about that in ‘28 or ’29.

Kuhn:

When he talked to you in the very beginning about the uncertainty principle, or in terms of what he said to you later about the earlier days, did he talk at all about his relations with Bohr over this principle?

Weizsäcker:

At that time he didn’t say anything, and later on —. Well, that’s a long story and perhaps complicated because it’s not quite coherent; in any case my memory is not quite coherent. I don’t think that Heisenberg ever tried to tell me the history of his discussions with Bohr in any historical sequence, but I remember that later on he said there was a really great tension between him and Bohr, early in ‘27, just when he had found his uncertainty principle. If I take Heisenberg’s interpretation, Bohr was at the same time not too happy to see that Heisenberg had found the solution to this problem, and, on the other hand, was not too happy about what Heisenberg said about the solution. So there was some sort of an ambivalent situation in which Bohr at the same time might have liked to have found it himself, but on the other hand didn’t think it was really the final solution and the correct statement of the solution.

Kuhn:

Do you have any notion what the focus of the disagreement about the substance of the paper was?

Weizsäcker:

In the substance there is certainly one thing that Heisenberg just stated: there is an uncertainty in momentum and an uncertainty in position, and they must be related in the manner expressed in the uncertainty principle. If this was so, it was evident that one could understand why both positions and momenta could be measured, but yet position and momentum could be expressed by operators which don’t commute. Heisenberg felt quite satisfied having discovered that this was so. Then he had devised his idea of a gamma ray microscope and he said that here it could be seen that the momentum transfer was such that no measurement could be made more precise than the principle. As far as I remember, Heisenberg told me that Bohr’s main criticism, or one of Bohr’s criticisms at least, was that Heisenberg had not at all proved that the transfer of momentum in the gamma ray microscope was uncertain at all; he only had proved that it had the right order of magnitude. And the real proof that it was necessarily uncertain I really think rests Bohr was quite right in that — on consideration of the duality of the nature of the photon. If the photon were just a corpuscle it would be quite possible to measure the momentum transfer, and this was somehow implicit in Heisenberg’s representation. But I think it is correct to say that it was really Bohr who pointed out that it was the complementarity of wave and corpuscle picture which made the uncertainty principle a real principle. This is what I remember of the content; I don’t know what you have heard about it.

Kuhn:

It fits quite well; this amplifies what I have gathered in certain respects. The people most involved, as one might expect but is disappointed to find, have clear recollections that there was tension but very little recollection of what it was about. I mean that this has been very much erased because of the pain that was involved at the time, I think. So in a sense this is really a more circumstantial account than any I’ve heard, but it fits quite well with certain of the hints that have come through in talking to other people.

Weizsäcker:

Another thing which I’m saying, not from having talked to Heisenberg but just from having read the two papers, is that, knowing Bohr’s style and way of expression of thought, I can very well imagine his not liking Heisenberg’s straightforward manner of making statements which he knew would be correct, but which he didn’t really deduce from understandable principles in an understandable manner. In this sense I can very well understand that Bohr should have felt quite unhappy about Heisenberg’s presentation, quite apart from all other reasons there may have been for disagreement. But I think the material point in it, as far as I remember from Heisenberg’s remarks, is the point that the particle and wave duality of all so-called particles was needed in order to make the principle consistent.

Kuhn:

Did you meet Bohr at all in this period before you went to the University?

Weizsäcker:

No, I never met him. When I was in Copenhagen there was no reason why I should meet a man like Bohr. I once visited the observatory and I met Stromgren, Elis Stromgren, the father of Bengt Stromgren, and I met Bengt too, who was very young at that time. But I didn’t meet Bohr. I met Bohr for the first time early in January in ‘32.

Kuhn:

We will not get to ‘32 yet. This I take it brings you really then up to the point at which you go to the University. You told me you were at Berlin for one semester, while Heisenberg was still away.

Weizsäcker:

Yes. I finished high school in February or March of ‘29. Then I went to the University of Berlin for one semester, where I tried to understand Schrodinger’s course on optics, which was a bit difficult for me. Heisenberg at that time was in America; I think it was precisely the year in which he gave the lectures in Chicago which later on were published as a book on the principles of quantum theory. Then he returned by Japan and India and we had arranged that I should be in Leipzig in the winter. I remember quite well that at some time in June or July, I cannot say for certain, but surely not later than August, I received a letter from him that I could go to the railway station in Leipzig on the 5th of November at 7:56 in the morning and he would arrive there; and so he did! That was not too difficult, of course, because he had already booked passage on a boat for his trip home from India.

Kuhn:

Let me ask you a somewhat personal question that occurs to me at this point. You were a good deal younger than he was, and he was also busy. It strikes me that you must have had a great deal closer connection with him before you got to the University than one would normally expect even a very bright and able prospective physicist to have had with a person in his position. I wondered a bit why this relationship had developed as warmly and as wonderfully as it had quite so early.

Weizsäcker:

Well, (the cordiality) is difficult to explain. I don’t know. But in any case I wouldn’t have hesitated to see him as often as he had time, and he liked it. He soon became a friend of our family, too; he had been playing the piano and he played in our home with my mother. There was a general friendship which developed rather soon.

Kuhn:

In the time before you actually went to Berlin and then to Leipzig, had you been doing a good deal more physics by yourself than was required?

Weizsäcker:

Not too much. In the last year in high school I was more interested I think in philosophy than in physics, and I had done a bit of work in philosophy — if it may be called philosophy. I thought that I would learn physics when I went to the University.

Kuhn:

And also learn mathematics when you got there?

Weizsäcker:

Yes. I never learned enough mathematics. Of courses I had learned some mathematics before and I never had difficulty in mathematics in school. I might say that my own decision to study physics, if it was the consequence of one event, was really the consequence of that quarter of an hour or half hour in the taxi between Anhalter and Stettiner Bahnhof. I realized that if that sort of thing can happen in physics, one couldn’t avoid knowing physics if one was interested in philosophical problems. I think that was really the starting point.

Kuhn:

You have some feeling then, I take it, that even when you were committed to being a student of physics that you might very well do philosophy later.

Weizsäcker:

Yes, I always felt that this would probably be so. I was very uncertain whether I would ever be able enough to be a professor of physics, whereas I never doubted that I would be able enough to be a professor of philosophy. I was quite surprised to see that I was able to do some physics at a reasonable level.

Kuhn:

At Leipzig, how early in your actual studies did the problems of quantum mechanics begin to enter into your education?

Weizsäcker:

You know that our German university system is extremely liberal, and so there was no difficulty for me in doing what I liked just as long as I was able to do it. In my first semester at Leipzig, which was my second semester of studies, I attended Heisenberg’s course on optics. Now since I hadn’t understood very much in Berlin with Schrodinger, it was like starting afresh. Heisenberg immediately gave me the task of writing it down since he wanted to have it written down for his students, so the first thing I had to do in Leipzig was to follow his course very closely and to write down what I understood of it. Then I went to see him and I showed it to him; he corrected all my many mistakes and in the end it was written, and perhaps mimeographed for the students in his seminar. This was the first thing that I did. Then in the next semester, the summer of ‘30, I entered his seminar “Uber die Struktur und Materie”; immediately he gave me a paper on the question of the conservation of energy in beta decay. I think that was the first thing I had to speak about in his seminar.

At that time it was a very exciting problem since Bohr thought that the principle of conservation of energy might break down in beta decay. For some strange reason, Bohr always wanted the principle of conservation of energy to break down and he never succeeded. At that time there were the experiments by Meitner and Orthmann and I think Ellis and somebody else in Cambridge, who found that the production of heat in radium-E corresponded to the average energy of the spectrum and not to the maximum energy, which was an argument in favor of the conservation principle’s breaking down. On the other hand, but this may have been a little bit later, it was found that the difference of the energy of the initial and the final nucleus corresponded to the upper limit of the spectrum, and that finally settled the question. There it was found that evidently energy was conserved. But I don’t remember whether this was already known in ’30. In any case that was the first problem he gave me, and I think it is somehow connected with the problem of how people felt at that time about the further developments. I think Heisenberg would not have been disappointed in any case to find the conservation of energy being violated. He would have felt, “Well now, that’s some new information.”

Heilbron:

Was there any discussion of this paper by Peierls and Landau? Where they try, among other things, to attach the beta decay problem to their —.

Weizsäcker:

Well, I don’t remember precisely. I thought the paper by Peierls and Landau had been just a little bit later.

Heilbron:

Just a bit — ‘31.

Weizsäcker:

In any case I don’t think it existed in ‘30. I remember Heisenberg’s writing to Landau, after having received the paper by Peierls and Landau, the Greek quotation from Archimedes, “Give me a place where I can stand;” and Landau replied that what Heisenberg had written there was evidently a misuse of the mathematical symbols sigma, delta, pi, and so on. [laughter] Heisenberg knows Greek quite well! His father was a professor of Greek. But this was connected with the problems of quantum electrodynamics and that sort of thing. I cannot tell you very much about these problems. Of course Heisenberg worked on electrodynamics precisely at that time, but that was too difficult for me. I didn’t understand it. Later on I tried to understand it; I even wrote a paper making use of Heisenberg and Pauli’s theory of quantum electrodynamics, but I think this was rather a paper written by Heisenberg and hindered by my efforts to understand what he did! I felt that Heisenberg was increasingly unhappy about the difficulties in quantum electrodynamics, the complication of mathematics and his inability to solve these problems. But I didn’t really understand what the problem was.

Kuhn:

Do you remember anything in particular that he may have said about these problems? About the likelihood, of their getting solved?

Weizsäcker:

I just remember the general question which I mentioned before, the problem of continuity, and, at that time at least, the problem of the self energy of the electron was not at all understood. It was still there from classical physics. Later on I know Heisenberg would have said an elementary length must be invented, but I’m not sure he used this term so early; but somehow he felt that there was something completely in disorder in the question of continuity.

Kuhn:

You talk about taking this seminar; were you also taking simultaneously the standard physics survey?

Weizsäcker:

Yes. Of course there was at first experimental physics. Debye gave wonderful lectures on experimental physics in which he used every possibility which was offered to smoke a cigar, such as Brownian motion, or anything of that sort. Then there was ordinary classical theoretical physics of which I just had hit on optics when I arrived in Leipzig, and later on mechanics and thermodynamics and so on, it was Heisenberg and Hund who gave these courses; I think they alternated one doing classical physics one semester and the other one modern physics, quantum theory, or relativity. Then exchanging. I think I had thermodynamics from Hund and mechanics from Heisenberg. So this was done in the regular manner and this was also how mathematics was done. Van der Waerden arrived in Leipzig at that time or a bit later, perhaps in ‘31 or so.

Kuhn:

I think he was perhaps there by ‘30, but I’m not perfectly sure.

Weizsäcker:

Van der Waerden was present in Heisenberg’s seminar from the beginning and he knew and understood everything; it was quite surprising. Here’s something that’s however not real science: Heisenberg and van der Waerden had a bet about Fermat’s theorem. I don’t know how it stood, but at that time the great Fermat theorem had been tested for all exponents below one hundred and one, but it had not been proved that the equation x¹⁰¹ + y¹⁰¹ = z¹⁰¹ was impossible. But van der Waerden said he could refute any counter-example within seven minutes. And Heisenberg said, “Well, now, I’m going to try it.” So Heisenberg had a week from one seminar to the next one, or from the tea after one seminar to the tea after the next one, to find out the counter-example; it was agreed that it should be correct if calculated with logarithms with four places behind the decimal point; so van der Waerden was not supposed to control [i.e. check] that.

Heisenberg worked hard and found an example which, according to his view, was stable against many different criteria known to mathematicians. Van der Waerden got it and was evidently a bit uncertain whether he would really achieve it; he seemed a little bit nervous, but he started calculating at the blackboard and it wasn’t more than three and a half minutes before he had shown that it was wrong. You see, Heisenberg has always liked number theory very much. He used to say that number theory is to mathematics what Bach is to music. I think when he was still in school he was very much interested in number theory. Van der Waerden was quite a surprising man because he knew mathematics — well, that was trivial — he knew theoretical physics, but he even knew chemistry. When chemical problems came up in Heisenberg’s seminar, van der Waerden was perhaps the best expert about them. Lots of people were there; I think Bloch was there from the beginning, Teller was there, and I soon developed a particular friendship with Teller. We had discussions about all sorts of things whenever we could. Teller at that time was just working on his doctor’s thesis on the hydrogen molecule-ion, which he did on a very obsolete calculating machine where he had to turn the wheel all the time. So it was certainly a good time in Leipzig. You know, of course, that all real scientific discoveries were defended on Tuesday night when ping pong was played.

Kuhn:

I’ve heard references to this, but I’ve never heard a good account of it.

Weizsäcker:

Well, that’s the most important thing that ever happened in Leipzig, but that again is not strictly science.

Kuhn:

It’s close enough.

Weizsäcker:

Heisenberg had his seminar on Tuesday afternoon at 3:00 and it lasted until 5:00 or so. Then there was tea, some cake was going around the table, and it was a very important thing to get the last piece of cake. It was not considered decent to take it the first time it went around, but it was very important to get it the second time. Then there was a colloquium with Debye; after the colloquium which lasted until 7:00, people went to have supper. Then about 9:00 the theoreticians, just the Heisenberg group, came back to the Institute. I don’t think Hund ever took part in the ping pong playing. I don’t remember him in any case, but I think all the younger people did. A matrix was drawn on the blackboard on which all the results of the different pairings were to be written down. One tried to fill the matrix completely in one evening, which was not always possible. There were some particular games played when there were discussions because there were more people there than could play ping pong at one time.

There were two tables in two separate rooms, but there might have been at least eight people, perhaps even sixteen, so the majority couldn’t play ping pong. Again tea was made, chess was played at times, and discussions were going on. The discussions were finally decided by a game of ping pong if they couldn’t be decided otherwise. This was very good for Heisenberg because he was not only the best physicist, but he was also the best ping pong player. So there was never any hope of getting the better of Heisenberg! And this is typical of Heisenberg — this is something I had not seen myself but it had happened when I arrived in Leipzig. Before 1929 Heisenberg was not the best ping pong player. Then he went on his trip around the world and he returned being the best player in the seminar, because during the time he spent on boats across the Atlantic and the Pacific he had tried very hard to learn ping pong very well. I think Heisenberg never could stand to do anything which people near to him knew better than he, so he somehow liked to be the best ping pong player.

Heilbron:

Now we know the true cause of the trip.

Kuhn:

At this point, what seemed to be the key problems being worked on? For those who would do it, quantum electrodynamics still seemed to offer real challenges, I take it.

Weizsäcker:

Yes. Heisenberg was still interested in ferromagnetism when I arrived, conductivity and ferromagnetism. It’s quite interesting to see that in the early times, when quantum mechanics was quite recent, it was not clear which sort of problems would be easily solved, and which might survive. For instance, it was not apparent that ordinary metal conductivity would be easier understood, or more or less easily — Bloch did a good deal at that time — while superconductivity would not be understood. I think this came as quite a surprise; one might have thought that ordinary conductivity would just be another type and superconductivity might be some quantum effect which one might understand quite well. Ferromagnetism was similar.

Kuhn:

Were there many attempts made to do superconductivity?

Weizsäcker:

I don’t remember real attempts, but I remember that it was discussed and Bloch certainly would have liked to understand it. I was not especially interested in solid state physics, so to this I just listened. Then there was ferromagnetism. Then nuclear physics came up — cosmic rays were considered interesting. I don’t remember precisely when Heisenberg really started working on cosmic rays. I think it was a little bit later. I feel that at that time Heisenberg had a list of interesting problems. He didn’t do anything in nuclear physics, for instance; Beck, who was in Leipzig at that time too, had written some papers on nuclear physics, but Heisenberg didn’t do anything in nuclear physics. None of us really did. Then suddenly in ‘32 when the neutron was discovered, Heisenberg immediately saw that this would help to apply quantum mechanics to the nucleus. So within a few months he wrote three papers on this question. I think that was very typical. Whenever something happened that showed that a particular problem now might be solved, Heisenberg was very quick to go to it. On the other hand quantum electrodynamics was constantly present the whole time.

Kuhn:

You said yesterday that you thought perhaps one of the reasons he had been able to move so quickly on with the discovery of the neutron was just his clear recognition of the problem of the electron in the nucleus earlier. Is it your impression that the time he described that problem to you was at the time of your own beta-decay paper?

Weizsäcker:

Yes, I think so. Certainly. When I had to give a report on beta-decay in his seminar, it was quite clear that the connection was like that. Electrons must be in the nucleus; electrons cannot be in the nucleus according to quantum mechanics; electrons come out of the continuous spectrum. Perhaps all these things are connected because they can only be in the nucleus in violating the rules of quantum mechanics. That was the connection.

Kuhn:

It was his view that one might simply need —.

Weizsäcker:

Something quite different, yes. I remember that, in any case, my reaction to Pauli’s idea of the neutrino, which I think was presented even before ‘32, was that I didn’t like the idea of the neutrino at all for the reason — I was probably influenced somehow by Heisenberg — that this would be a trivial explanation of facts from which we would hope to get so much information about basic problems. So I felt that this was just an attack upon profundity from the point of view of trivial solutions.

Heilbron:

But with the neutron one was happier?

Weizsäcker:

The neutron was quite different. The neutrino really is an invention to solve theoretical problems.

Heilbron:

I mean to say that one was happier with the neutrino after the discovery of the neutron.

Weizsäcker:

Yes, that’s quite clear. After the neutron had been discovered, the neutrino was in a far better situation. You see, before the discovery of the neutron there were other great problems in nuclear physics, for instance, the problem of statistics. It seemed that an electron in a nucleus lost its statistical properties because it was known that the odd-odd nuclei had statistical behavior which was opposite to what they were supposed to have if electrons had been present. Therefore, before the discovery of the neutron, there seemed to be a list of physical laws which broke down with electrons in the nucleus — statistics, conservation of energy, and I don’t remember what else. In any case, that was the impression. When the neutron was discovered, then the problem was changed into the question of how the neutron can emit an electron.

Kuhn:

Were there any people around Leipzig who were skeptical about the neutron, to whom it presented a problem?

Weizsäcker:

When the neutron was discovered, I can’t remember any skepticism. Of course you know that Bohr was skeptical about the positron for a very long time. I remember that very clearly; Bohr explained to us at length that this was most probably some misinterpretation of experiments. But skepticism about the neutron I can’t remember.

Kuhn:

Do you remember how that word first came? There were various signs of it before the Chadwick paper.

Weizsäcker:

Yes, of course. It began with Bothe; Bothe had some effects which he couldn’t really understand. Then the Joliots — I think it was just Joliot himself — had similar effects and it was quite mysterious. Then Chadwick’s paper came rather soon.

Kuhn:

There is a story that Majorana, when he first heard about some of the pre-Chadwick experiments, said, “The fool, he doesn’t know he’s got the neutron,” or something very like this. And of course Majorana, I take it, was at Leipzig at this point.

Weizsäcker:

I don’t remember whether he was at Leipzig exactly at that moment, but that’s quite possible.

Kuhn:

He may already have left by this time; I’m not sure he was even alive in ‘32. Around this time, anyway.

Weizsäcker:

I remember him very well, but I don’t remember the year. I should say that in Leipzig, as far as I remember in any case, the idea that there might be what we now call a neutron, a heavy neutral particle, was not considered very probably, or was not considered at all, before Chadwick’s paper. Now we know that Rutherford had the idea long ago, but that I didn’t know at that time. I think it came more or less as a surprise, but as a surprise which immediately showed that now things would be more easily understood in nuclear physics.

Kuhn:

What about the positron then, which comes so nearly at the same time?

Weizsäcker:

The positron is a bit later.

Kuhn:

Depending on which paper you take, though, it’s not very much later. The Anderson paper is still ‘32.

Weizsäcker:

Well now of course you say it’s not much later since you see it in the context of the great development of physics, but at that time one year meant very much to me! Especially ‘32 was a year full of discoveries. There was the neutron, there were the Cockroft-Walton experiments which were quite surprising to many people, showing that it was possible to do artificial transmutation, and then there was the positron. The positron, from our theoretical point of view, was mainly interesting because it showed that Dirac had not been such a fool as all theorists had thought he was in his theory of holes. I remember when Dirac invented his theory of holes; that’s a thing I remember quite clearly. I don’t know the year — ‘30 probably. In any case, Heisenberg received the paper or perhaps he heard about it before the paper was published. I don’t think Dirac sent any of his papers to Heisenberg before they went to print, while Heisenberg sent all his papers to Pauli.

In any case, I was told that when this paper was published, the Fermi group in Rome held some sort of ‘judicial meeting’ about Dirac who, in absentia, was doomed. I think they decided to ‘bastonare’ him because he had written such a nonsense. Heisenberg immediately, or very soon, said that if Dirac should be right that there are these negative states, that they are occupied and that there are holes in them, then the holes must have the same mass as the electron. His objection was that whatever else you may think about this horrible idea of having an infinite number of states occupied, in any case it couldn’t be the proton. Since there is no particle of the mass of the electron and positively charged, Dirac’s theory is wrong. Then Heisenberg was very much interested by Dirac’s idea and he wrote a paper on holes in metals — I think in metals. Since this is solid state I don’t remember the details, but I think it was metals, which he described to me, saying, “Well, now I have holes of my own.” That is to say that this was a reasonable application of the idea of holes moving like particles, while Dirac’s application is not reasonable, because it would be positive electrons which don’t exist. Then, when the positron was discovered it came as a great surprise and I think again it hit on the same note that caused Heisenberg to remark in ‘28 that Dirac was so clever you really can’t compete with him. Really by sheer theoretical arguing of the case, Dirac had predicted the positron, and his prediction had been blurred only by the fact that he didn’t dare to say it was a positron and tried to say it was a proton.

Kuhn:

Were there others besides Bohr who resisted the positron?

Weizsäcker:

I don’t remember. In any case Heisenberg didn’t. The discussions we had about the positron which I remember were in Heisenberg’s little hut in the Bavarian Alps where Bohr had been invited for skiing. This must have been in March or April of ‘33. At that time Bohr still said that the positron certainly wasn’t true. I remember that the others of us felt that Bohr might very well be mistaken because we all had a general feeling that Bohr had a particular tendency not to believe experimental results. Perhaps “experimental results” is putting it too strongly; for instance, in every day life up in the mountains when someone observed a certain phenomenon, say a certain color fringe in a cloud in the sky, and he tried to explain that by atmospheric phenomena, Bohr would say, “No, that’s just a contrast phenomenon; that’s just physiological,” or that sort of thing. He had a certain tendency not to believe that a phenomenon was objective. We sometimes wondered why. Or, for instance, a phenomenon which we call in German “Kugelblitz” — spherical lightning, spherical electrical discharge. In any case, there are reports about Kugelblitz. At that time I think there were about fifty reports about people who had seen it, printed reports and many oral reports, and Bohr just said, “A Kugelblitz is impossible and doesn’t exist.” When people say that such a Kugelblitz has entered through the window, gone through the room, and gone out of the window again, Bohr said that it was quite clear — they looked out the window on a dark night, there was a lightning flash, they were blinded by it, and then they looked around the room and saw the lightning wherever their eyes went, and that’s the whole story. This was an attitude of Bohr’s which we knew very well, so we didn’t take his opposition to the positron very seriously.

Kuhn:

Do you remember if he said anything about the Dirac hole theory in the course of discussing the positron?

Weizsäcker:

I can’t remember that. I just remember his opposition to the experimental fact, or to be fair to him, to the interpretation of experimental facts, which he didn’t deny of course, as indicating a new particle. Have you ever found this same trait in behavior?

Kuhn:

I myself, no, because I haven’t known him well enough. Rosenfeld when I very first met him talked to me about Bohr, and he put it not quite the same way but it clearly has something to do with the same thing. He said that not only did Bohr not take experiment terribly seriously, but equally that he didn’t take numerical results terribly seriously. If the logic of the situation made sense and held together, so that he was sure the experiment would come out ultimately, he was sure, the mathematics would come out ultimately.

Weizsäcker:

But that’s a somewhat different point. This was reliance on coherent argument. But the point I found, and I may be mistaken but I think it was our general impression, was rather that Bohr felt he knew a bit more about experiments than ordinary theorists knew. He always felt as if he had been doing some experiments himself and that he could not just be deceived by an experimentalist who said he had just discovered a new thing. I think that was rather his attitude.

Kuhn:

Yes, that’s very helpful.

Weizsäcker:

Then he had many stories — there was a famous new sort of “N-rays” which had been discovered and which then proved to be non-existent and he used all such stories to prove his point that you cannot trust people who say they have discovered something experimentally. But this was not in a clear connection with any theory which said that the experiment was impossible. It was perhaps so in the case of the Kugelblitz, I don’t know, but in general it was a case of general unbelief with respect to rash statements about experimental results. We had some other ideas about this attitude of Bohr’s. Of course we were all deeply admiring his profound ideas. Then we said, ‘Now, Bohr feels that according to his way of analyzing concepts, nearly everything may be possible in the end, because you never know whether a concept applies in the given situation or not. And that’s so difficult. Bohr shielded himself against the possible destructive consequences of this sort of thought by trying to rely very much on brute facts and not accepting new statements on phenomena and facts light-heartedly. Something of that sort.

Kuhn:

Yes. That’s very good. Now I asked about the Dirac theory because we have had one report, presumably just a little later than the meeting at the ski hut, in which people were apparently working on Bohr to make him recognize that the positron must exist. According to this report he finally said, “All right, probably it does, but it’s got nothing to do with Dirac’s holes.”

Weizsäcker:

That’s possible, but I don’t remember that. It sounds quite plausible, though, because that is what everyone would expect — that Dirac’s holes would have been particularly repulsive to Bohr since Dirac introduced such a huge mass of unobservable things. Heisenberg of course didn’t like it either, but I think Heisenberg rather had the idea that this might be an expression of something which might be expressed in a better manner later on. So this would not have been so difficult to swallow for Heisenberg as it was for Bohr.

Kuhn:

I’m about to get over more explicitly toward the issue of philosophical interpretation and measurement problems; this I think will open up a whole range of issues. Before we do, is there anything else you think of from this period?

Heilbron:

I’m particularly interested in the Landau-Peierls business, but we’ll get to that.

Weizsäcker:

Well, I must say that my memory of the Landau-Peierls business is hazy. I don’t remember very much; I think it came a bit too early for me really to understand what was at stake.

Kuhn:

Have you any recollection of the Bohr-Rosenfeld paper?

Weizsäcker:

Oh, yes. I think Bohr was still working on the proof sheets of the paper when I came to Copenhagen for half a year in ’33-’34.

Heilbron:

It wasn’t published until ’33. It was written I think in ‘32, but it wasn’t published until sometime in ‘33.

Weizsäcker:

Perhaps he was just working on it in ‘32; in any case I’m still in possession of some ‘fifth editions’ in proof sheets of the paper. Or rather, I think it’s combined of part which was fifth edition and part which was fourth. But I think I still have it in Hamburg. I may have got that in ‘32, however; I don’t remember precisely. In ‘32 I met Bohr three times and there was good possibility of learning about that. I met him first when Heisenberg brought me with him in January; then Bohr went to Heisenberg’s ski hut for the first time in ‘32 I think. I came here in September for the conference which was concluded by playing Goethe’s Faust... I think that was the greatest achievement in the field of poetry ever done by Bohr’s school! And I think the main merit is Delbruck’s. Well, that was the time of the Bohr-Rosenfeld paper. But again the Bohr-Rosenfeld paper was concerned with quantum electrodynamics and although I had been working a little bit in that field, my interest rather soon took the direction of nuclear physics, so I don’t remember details on the Bohr-Rosenfeld paper very clearly.

Kuhn:

I asked particularly because one or two people have said to me, “When we saw that paper we rather laughed that anybody should have put that much effort into a problem which after all one knew would come out.”

Weizsäcker:

Yes, I think that was a tendency which was present.

Kuhn:

Do you have any notion of how Heisenberg would have felt about that?

Weizsäcker:

I don’t remember positively. Heisenberg was always interested in this sort of discussion of thought-experiments, and I wouldn’t say that Heisenberg would have laughed at this effort. I don’t think so. But, on the other hand, I don’t think Heisenberg would ever have put corresponding effort into such a task. I think it was in connection with the Bohr-Rosenfeld paper that we developed the formula — do you know this one? Again it is not very serious — on the time it will take until a paper by Bohr is published. You see, Bohr always had a collaborator; at that time it was Rosenfeld. Now let’s assume that Bohr said t₁ is the time until the paper would be finished, and the collaborator said it would last a time, t₂, which evidently was much larger than t₁. The real time was then t₂/t₁ which means that the smaller the time Bohr indicated, the longer the real time would be!

Kuhn:

When you were a student at Leipzig, how much discussion was there of the sorts of issues that are involved in the uncertainty principle, complementarity, duality, and of measurement problems? To what extent was this an active enterprise at Leipzig?

Weizsäcker:

One might say it was no longer an active enterprise because from the point of view of physicists, the problem was settled. But I think all the physicists of the Leipzig group at that time, the theoretical physicists of the Heisenberg group, were really moved and interested by the philosophical implications and felt that this was extremely important and perhaps should be applied to other fields, and all sorts of things. I remember, for instance, having long discussions with Teller about all these things, and of course with Heisenberg, not so much with Bloch. Weisskopf was in Leipzig and he was interested in these things, but the one semester Weisskopf really spent in Leipzig I was in Gottingen, so I didn’t see much of Weisskopf at that time.

Kuhn:

Which semester actually were you in Gottingen?

Weizsäcker:

I was in Gottingen in the summer of ‘31, but then I decided to return to Leipzig. I thought I could learn more in Leipzig. A little bit later we had to defend our position: we also had to defend the position with respect to relativity — against philosophers. Official philosophy as taught in the universities I think has taken two different subsequent attitudes toward these things. The first attitude was that they were wrong and nonsense, and the second attitude was that they were true and trivial. At that time, to say it was wrong and nonsense was still the prevailing attitude, and in Leipzig we had several discussions with philosophers in which, no matter how differently we may have thought among ourselves, we were certainly a unified group in defending our new ideas against philosophers. Do you know the name of (Greta Hermann), who is now (Greta Henry)? She is German, trained as a mathematician, but she did a great deal in philosophy. She belongs to the school of Leonard Nelson, who was a well-known philosopher in Gottingen in the ‘20’s.

She left Germany in ‘33 and returned after ‘45. Now she is at the Padagogische Hochschule in Bremen and she is a highly intelligent person. I remember her in the following connection: when I was in Copenhagen in ‘33 - ‘34, Bohr gave me a paper sent to him by a certain Miss (Greta Hermann) whom he didn’t know at all. It was about causality in quantum mechanics, and Bohr said I should read it and perhaps answer it. The paper was wrong; it stated certain things which were not correct. So I wrote a letter to her, and later on, when I returned to Leipzig, she appeared there and stayed for a year. She told me that at the same time she had received my letter, she had received a letter from Heisenberg to whom she had sent the paper also. She was struck by the fact that Heisenberg and I had given the very same reply to her remarks, and she had felt that this is science. If two people, without having had any contact, gave the same answer, probably the answer was correct, and, in any case, it was science and not just talking about things you don’t understand. She decided to go to Leipzig and learn these things thoroughly. She had been in Gottingen, I think, before, but she came to Leipzig and stayed there. Now I see I’ve made a wrong statement in saying she left Leipzig in ‘33; evidently she left Germany a little bit later.

In any case, she was in Leipzig in ‘33 - ‘34, and we had many many discussions on philosophical problems with her. Since he had a very clear mind, a very well-washed brain, it was easy to discuss matters with her. Her tradition in philosophy was more or less Kantian, and she maintains that Kant’s views were not at all brought into any difficulty by modern physics, but only that it was necessary to interpret modern physics in the right way and Kant’s philosophy in the right way. I think she was probably right in that. This is just an example, in any case, of how we were induced to discuss these things by other people who learned about them. In our own group, not under the influence of other people, Heisenberg would probably have said, “Now these things are found, and they are true, and they show that things are quite different from what one had thought before. But now we must go on solving the problems of physics. That’s what we can really do, while the problems of philosophy which are raised by these results in physics are partly impossible to mankind. Partly it’s already clear.” And partly he might have said to me, “Well, you can do that when you are an old man.” But he never liked the idea very much that I did too much philosophy when I was supposed to learn physics.

Kuhn:

You said before that however you felt in private, the theoretical physicists united to argue with the philosophers. I take it that this means that there were some differences of opinion, differences in interest among the physicists, and I wondered what those would have been.

Weizsäcker:

Well, they were not really differences of opinion about the facts in physics, of course. I think that all of our group in Leipzig thought that complementarity was a wonderful idea, but of course there were differences as soon as one went into more detail. For instance, some of the younger physicists at that time were Marxists, and of course from Marxism one wild look differently at these things than, for instance, from no philosophy at all, or from logical positivism. Positivism, of course, was discussed very much and there was a general feeling at least that Mach was a great man, that Mach had great merit with respect to all these modern developments.

I should try to describe the intellectual attitude of the group of young people who at that time felt that it was very difficult not to be senile after having lived thirty years. I remember very well in ‘32, in a discussion between Heisenberg and Dirac, Dirac said, “Well now, Heisenberg, you are now past thirty and you are no longer a physicist,” or something of the sort. Dirac at that time, I think, was 29 1/2. Most of the other people were still younger. I feel that the general attitude was just an attitude of human modesty and an immense “Hochmut,” an immense feeling of superiority, as compared to old professors of theoretical physics, to every experimental physicist, to every philosopher, to politicians, and to whatever sorts of people you might find in the world, because we had understood the thing and they didn’t know what we were speaking about. In that sense, positivism — I think ‘logical positivism’ was not used so much at that time — was considered perhaps the ‘popular form’ of what we had understood better. One felt that positivist philosophers, when they turned up, were not far better than other philosophers, though perhaps a little bit better.

Kuhn:

I was fishing for one thing in particular when I asked the question, and now I will make it more explicit. Certainly part of what lies behind the Heisenberg-Bohr discussions in ‘27 over the uncertainty principle, must be Bohr’s feeling that there is a reality to the wave-corpuscle dualism which is simply not hinted at in the Heisenberg paper itself, and which grows with Bohr into complementarity. Another story I have heard a couple of times now, and which must stand for something, is that when Bohr read his Como paper in which one begins to see the complementarity and this difference emerging, Wigner is supposed to have said, “Well that paper will certainly not change anybody’s mind about the nature of quantum mechanics,” and I gather that in practice, initially, it didn’t. That is, that this whole element of dualism and complementarity which Bohr adds to the Heisenberg uncertainty principle was in fact at the start pretty much disregarded by most theoretical physicists. What I’m trying to some extent to find out, then — what has been a blank to me — is what goes on between this time, which is still early and before your own real involvement, and the time when you tell me that, in ‘32 at least, everybody felt that the complementarity was a great idea.

Weizsäcker:

Perhaps my statement was a bit rash, but I would say that as for the Heisenberg group, Heisenberg thought complementarity was a great idea and we had no reason to think in any way differently.

Kuhn:

I mean I didn’t mean to be doubting what you were saying, but I think there is clearly a transition between the end of ‘28 and sometime not late in ‘32, and I wonder how it came about.

Weizsäcker:

Yes, that is possible, but I may not really be the best witness in this respect because personally, as soon as I had seen Bohr for the first time, I knew that what had most deeply impressed me in Heisenberg was also present in Bohr even to a far higher extent than in Heisenberg. So I could never imagine at that time that one could consider Bohr’s contribution to be unimportant; to my mind it was really the one step through which these things came into their real meaning. Because I felt like that, and because the general attitude of the Heisenberg group was certainly not completely different from that, I might just not have realized that there were other people who did not say very much about it and felt, “Well, we can very well live without it.” I cannot remember any opposition to Bohr’s views. I remember, f or instance, Beck who is present here now, once said when we were in Copenhagen in ‘33 or ‘34 or so, “Think how Bohr would have behaved if he had had the task to invent special relativity. He would have said, ‘It is extremely difficult to know what we mean by time and by space and their connection,’ but he would never have said, as Einstein said, ‘Let’s find a new definition of equal times.’” I mean this sort of criticism was present of course — that with Bohr things became very profound, but they wouldn’t return to mathematical clarity. I think we all felt, on the other hand, that this was not necessary because we had so many good minds who were quite able to do that, so Bohr needn’t do it too.

Kuhn:

You told me yesterday something about this first meeting with Bohr, and I wish you’d tell me more, perhaps even going over that ground again, because I was much interested in what you said about your own feeling about the situation as you watched the two of them.

Weizsäcker:

The situation was that my parents had invited Heisenberg to do some skiing in Oslo where my father was then in the German embassy; we returned to Leipzig by way of Copenhagen and somehow Heisenberg asked Bohr whether he could bring me with him. Bohr wished to see Heisenberg on his way home. So we arrived there, or I arrived later — I don’t remember precisely — and the interesting thing was that this was a discussion in which Bohr intended to speak with Heisenberg about the philosophical implications of quantum theory. It was more or less announced in advance that this was the thing he wanted to discuss. I don’t remember what he said in that particular moment because in my memory it is connected with other things he said over the years, but I cannot remember that there was any argument or disagreement going on between Heisenberg and Bohr.

I think it was rather that Bohr liked to have Heisenberg’s ear present to hear what he said and to have some remarks from Heisenberg to induce him to go on. You know that attitude of Bohr’s very well. I was there, and in my memory it was a discussion which may have lasted three hours though it was probably less; I think it was at the villa and he was still living there. I remember formulating the impression at that time that Bohr was the first physicist I had met, which of course is a nonsensical statement since I had met excellent physicists before and I knew it, but it expresses my notion that Bohr was the first man who did physics in a manner in which I felt it ought to be done. The other impression which I formulated then was that this was because he was the first man I had met who really suffered by thinking, and that this meant taking things as seriously as they deserved to be taken. I had always felt that physicists, even Heisenberg with whom I had many battles about these sorts of things, were very precise in solving equations and in doing experiments, and completely imprecise in using concepts. With Bohr I felt that when he was not precise in using a concept it was because he knew that it was impossible to use it precisely, but he really tended to say clearly what he wished to do, what he thought.

This is more of an impression of the atmosphere, but this atmosphere is really what impressed me most. What he said to Heisenberg must certainly somehow have been connected with his famous traditional topics; for instance, that when returning to the same point in thought, you don’t return to the same point but to another point because you have gone away and in the meantime — what he used to explain by the Riemann surface. It is possible, though I don’t know, that at that time he already mentioned his ideas about life. He published them rather soon afterwards, so it’s rather probable that he should have mentioned them at that time. I got the impression from hearing Bohr that much was to be done, positively to be done, in following up his ideas; but at the same time I realized that this was probably not to be done precisely in what we call physics. Certainly many things in physics were to be expected at that time, but I felt that Bohr wasn’t speaking about physics very much; he was speaking about the general frame of possible human concepts, and perhaps biology would be a field, or perhaps the analysis of the foundations of mathematics. That was the time when Godel discovered his famous theorem that there are sentences undecideable in any formal system. Roughly at the same time I remember that Teller and I discussed these Godel things; I didn’t know about them, but Teller knew and told me. I felt that this might somehow be connected with Bohr’s ideas.

Kuhn:

How did Bohr feel about those theorems?

Weizsäcker:

I don’t remember that he ever said anything about them. Certainly he did not know very much about mathematical logic and about the foundations of mathematics, and I don’t think he was so much interested.

Heilbron:

Did he ever say much in detail about his Riemann surface ideas, or was it just this very general statement?

Weizsäcker:

I think it always remained rather general. I don’t remember that he spoke of the Riemann surface at that time; I think I first heard about his ideas on the Riemann surface from Heisenberg, because obviously, after having left Bohr, I discussed these things with Heisenberg and he probably told me a bit more about it. Once when I was in Copenhagen, in the winter of ‘33- ‘34, Bohr took me to his office and started talking about these things; then he drew the famous picture of the circle on which you go around in the Riemann surface. But he didn’t say much more which I can remember except that you start from one point and you return to it, but since you have gone around this singularity, around the pole, it’s another point to which you return; that this is closely connected with the fact that you cannot at the same time use a concept and analyze it. Then you come back into all the things he always said, but I don’t know any more specific statements except the one concerning this point.

I think it was in ‘33 in our ski hut that he discussed the problems of instinct and intelligence at length; he was extremely interested in that at the time. “It’s so completely wrong,” he said, “to think that intelligence, that the conscious intellect of man, is just another level of instinctive behavior; rather it’s just the opposite of instinctive behavior, because instinctive behavior can function only because the animal doesn’t know.” I remember that Heisenberg said, “Now tell us, Bohr, is there nothing of instinctive behavior in our own human condition?” And Bohr said, “Oh, yes, certainly, but only at places where we don’t know it,” which was quite consistent, of course.

Heilbron:

Like the salmon who finds his way home yet he doesn’t know how he does it.

Weizsäcker:

Precisely. Where we don’t know it, it’s instinctive; but where we know it, it’s more or less by definition not instinctive. All this was mixed up with much fun; for instance, we hadn’t shaved for a week in our ski knit, and in the end we had to do it before leaving. Bohr, when he was shaved, admired his smooth face in the mirror and said, “Now if you shaved a cat, don’t you think it would look intelligent too?” [laughter] You see, in just trying to analyze Bohr’s way of thought I was struck not so much by his general statements which at times, I think, suffered a bit from his never having really learned philosophy, but by the way in which he reacted to the slightest problems in everyday life. For example, when we were in our ski hut it was rather high in the Alps and Heisenberg and I had gone there a day or two before and brought all that was needed. Then when we were up there an immense amount of snow fell and we were quite uncertain whether it would be possible for Bohr to go up to the hut at all.

There was a larger house in the neighborhood with a telephone and from this place Heisenberg and Bohr spoke to each other when Bohr arrived in Munich. I was present in the house up there. Heisenberg said, “It’s very difficult to get up to our hut now since there’s so much snow and I don’t know what to do.” And Bohr said, “Well now, I am going to take a taxi,” which shows he had no idea about the real situation. Heisenberg said, “No, that’s impossible, but we are going to be down at the railway station and fetch you there.” So we did. I remember that going down there was extremely difficult because the path which we usually used was too fast [slippery]; there was so much snow that the first of the two people was always ‘pressing’ down [i.e. sinking in], while the second could just glide. Bohr realized the situation when we were there; we went back up in our own tracks and only because the tracks were there was it possible to go. We had to go up about a thousand meters or so. Bohr suddenly said, “Well, now, it’s very good that you should have come to fetch us here at the railway station.” I think he was with his son, Christian. “And it’s very unusual, too, because normally a mountain is a thing which we have to begin from below.” [Laughter] He couldn’t do anything without transforming it into a general theory of what a mountain was

Kuhn:

This story of the salmon, of Bohr’s attitude about instinct, is the one portion, of which I am aware, of the subjects he talked about, which I have had the greatest difficulty understanding. Can you make some sense of this?

Weizsäcker:

There are two different questions in your question; one is whether I can try to give an interpretation of Bohr’s ideas which would not go beyond what Bohr himself thought; the other is whether I can use it as if it were my own idea. The first thing is perhaps difficult to do precisely in Bohr’s words because Bohr never was too explicit about it, but I think I see rather clearly how it is connected with Bohr’s general ideas, which is probably the more important question. I think we can start with the very trivial thing; do you know the famous story about Der Tausendfuss — the ‘centipede’ who was asked in what order he moved his feet? In starting to think about it, he could no longer move them at all. Apply that to the salmon; the ability to reflect upon what you do yourself may be very important for typical human activities, especially for making decisions which are made in full awareness of what we have learned.

Certainly a human being can’t exist without that, but just this takes away the possibility of directly doing a certain thing. I think the idea of the salmon is very similar, for instance, to Bohr’s statement that there is complementarity between the use and the analysis of a concept. If you use a concept, if you just say a body is heavy and you use the concepts ‘body’ and ‘heavy’, then you don’t analyze them; you use them in a manner which is not instinctive in the strict sense because it is part already of an intellectual framework, but still it’s used in a direct manner. As soon as you start to try to find out what you mean by ‘heavy’ — evidently by gravity — and as you start to think about what a ‘body’ is, whether it really is a body, and so on, then you lose the meaning of the concept. You must lose it in order to be able to replace it by something better. This necessary loss precludes you from any direct use of the concept. I think again it fl the same thing as this famous story about the Danish candidate who wasn’t able to arrange his different “I’s”, his different egos.

Heilbron:

I don’t think I know this.

Weizsäcker:

Oh. Bohr quoted this many times. It’s from Danish literature but I never read it and I don’t remember the name of the author. Aage Petersen certainly knows. For instance, in Petersen’s radio talk on Bohr’s philosophy the story is quoted and Bohr quoted it in his last Lindau talk, too. The point is this: there is a young man who never manages to do practical things correctly, and in the end a friend of his provides some job for him. He must go to the railway station, or at least somehow go away, and the friend comes to see him but he has not yet managed to put his simple things into a bag. The friend, asks him, “What have you been doing?” “I have been sitting here.” “Well, what have you been doing sitting here? You should have been getting your things in order.” The young man replies, “Well, that’s what I say to myself, too, but who is the one to whom I say it and who is the one who says it? If I try to find out who is the one to whom I say it, and who is the one who says it, I find that the one who thinks about that is a third one. So what is the connection between the three? And then the one who thinks about the connection between the three is a fourth one, and you see there is no end to: it. So how can I put the things in my bag?”

Heilbron:

It’s mentioned but not told about in the radio broadcast; everybody knows about it.

Kuhn:

This helps, but it leaves open what has principally puzzled me about the salmon story. When someone asks — and lord knows people still do — ‘how does the salmon find his way back,’ and when they make reference to the possibility of a sixth sense, which Bohr immediately said was the wrong way of getting at it, clearly what they are asking, at least at one level, are questions like ‘is it the look of the ground, or is it salinity, or is it some knowledge of the ocean currents, or is it some knowledge of the gravitation?’ What are the clues on which instinct operates? I have some feeling from what I know of Bohr’s use of this story that there was no room left for such questions. There was some puzzle about how bats find their way; one assumes that what they do is done by instinct, but this does not eliminate the puzzle which was answered when, in practice, one found the radar. Yet I have the feeling in the case of the salmon, Bohr somehow or other was also saying, Those questions exist.”

Weizsäcker:

But now I think we must distinguish two things. We all know that Bohr had a way of interrupting people who asked him questions by again stating what he felt was important, and therefore it was very difficult to know whether he didn’t realize there was another question possible or what he would have said to the question because he really didn’t listen to the question. In that sense, probably the question which you formulate now is a question which was so far from the point which Bohr wished to clarify by the salmon story that he would have been impatient with one who asked a question about quite a different problem. But if I tried to answer the question in the way I think he might have answered it, if he had been patient enough to listen to it really, he would probably have said, “Well, now, there will certainly be some very good reasons which can explain that, but these reasons have two properties for which I don’t like to discuss them. Firstly, I don’t know them; and secondly, I don’t even know if they are in the realm of physics or if they are in the particular realm of biological wholeness which is complementary to a description of the salmon by physics. Now since that is open and undecided, why discuss this matter while I wish to emphasize that certainly, if the salmon knew, he couldn’t do it.”

Kuhn:

This has been very helpful; I don’t know how far we can usefully get pushing, but I would say that at least there is one further reaction. The second of these two — ‘that we’re not even sure that it’s a question in the realm of physics or whether it’s a question in that realm of biology which I think of as complementary to physics’ —is to some extent already coming back to the initial statement about the salmon, about this initial complementarity between consciousness and instinct.

Weizsäcker:

The difficulty here is a basic difficulty of Bohr’s thought; I don’t know exactly how he would have reacted to this question. That is, if we have different sorts of complementarity, that means complementarity between different pairs of concepts, the question is whether this complementarity really is, so to speak, materially the same thing, or whether the mutual exclusiveness between intellect and instinct is very closely connected with the complementarity between the physical description of organisms and their description by what he sometimes called ‘typical biological concepts’. As long as this is not settled, it will be difficult to decide about your question concerning the salmon. Probably Bohr would have said that materially these questions are different, but that even to discuss their difference was only possible after having understood their complementarity, or the complementarity inherent in both of them. Perhaps you may say it like that. Evidently Bohr has been quite uncertain about his own views on biology in his later years, as far as Aage Peterson told me yesterday and as far as I learned from Delbruck and perhaps others. Did you discuss that with him?

Kuhn:

I did not at all discuss that with him. I discussed it with some of the other people here and I certainly share that impression. I gather that Aage Bohr is not at all convinced that there were material changes. My impression from people here is that there very likely were. You told me one thing yesterday about something which you had taken from Heisenberg and used in a paper of your own that, when you corresponded with Bohr about it, turned out really to be a misunderstanding. I’d be glad to have you tell me that again, because I’m not sure I have it straight yet.

Weizsäcker:

One thing I can say is that you can find that in a correction which I added to my paper. The paper is on “Logik und Komplementaritat” in Naturwissenschaften, 1955, and is now printed again in the more recent editions of my book, Zum Weltbild der Physik, and there I have added half a page or so saying that I was mistaken in interpreting Bohr in a certain manner, and so you can find that printed there. If you like, I can tell you again.

Kuhn:

Why don’t you state the essentials again, but there’s no point in elaborating it.

Weizsäcker:

The point is just this. In a way it’s quite a profound point with Bohr. In my paper in ‘55 I said that the term ‘complementarity’ can be used in two different ways: one I call ‘parallel’ complementarity, the other I call ‘circular’. Parallel complementarity can be explained for instance, by position and momentum in which you just have a choice of two possibilities, like a balance where one arm goes up and the other goes down. Circular complementarity can be illustrated by the structure of the complementarity between the analysis of a concept and its use, where you cannot say that you have just a choice; if you analyze the concept you can get to a better-founded concept which you can also use, but then it’s another concept, so you move more in a circle than just moving a balance up and down.

I said then that Bohr in his description of the situation in quantum theory had used complementarity in both senses: position and momentum was one, and the other one was his complementarity between a description in space-time concepts and a description by causal concepts, because this evidently meant that the description in space-time concepts was a description by classical physics while the other was a description by the Schrodinger function which obeys a differential equation. This I had taken from Heisenberg’s 1931 book. I felt that was circular because you can define quantum theory only if you are already in possession of classical concepts, and on the other hand, classical physics is a limiting case of quantum theory, so this was some sort of a circle. Bohr then wrote to me that this was wrong, that I misunderstood him here, because a description by space-time concepts to him meant just a description of the concepts by time and position, and the description in the frame of causality to him meant a description according to conservation laws. Thus it reduced to the other complementarity of position and momentum, or time and energy. I learned an important thing which Pauli emphasized later on when I sent him my paper on Bohr’s complementarity. Pauli sent me a postcard on which he said, “Sie tun Bohr Gewalt an; Bohr ist nicht zirkular. And I think this is really an important point. I still feel that the use of the concept of a circular relationship between, say, the use of a concept and its analysis or between classical physics and quantum theory, is quite good; but I learned from this that Bohr didn’t like to consider it to be circular but he liked to consider it what I called parallel. So in this sense I learned really better Bohr’s intention. The first mistake was Heisenberg’s, but I think it was a very natural mistake.

Kuhn:

I’m very pleased to have that story straight.

Weizsäcker:

I think it’s a very important point. I mean, if you’re interested in Bohr’s philosophy, it’s very important to see, as Pauli said, Bohr is “nicht zirkular”. Bohr’s symbol really is the balance, not the circle; the symbol of his idea of complementarity is the balance. You have a choice but you don’t move around. Although, perhaps my idea of the circle is just induced by his Riemann surface. So in that sense you may say he’s circular, but in any case he didn’t like the systematic expression of these things as being circular.