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In footnotes or endnotes please cite AIP interviews like this:
Interview of Niels Bohr by Thomas S. Kuhn, Leon Rosenfeld, Aage Petersen, and Erik Rudinger on 1962 November 17,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Niels Bjerrum, Percy Williams Bridgman, Charles Galton Darwin, Paul Adrien Maurice Dirac, Albert Einstein, Ralph Fowler, Hans Marius Hansen, Werner Heisenberg, Georg von Hevesy, Harald Höffding, William James, James Jeans, Walter Kossel, Paul Langevin, Max Theodor Felix von Laue, Henry Gwyn Jeffreys Moseley, John William Nicholson, Wolfgang Pauli, Max Planck, Boris Podolsky, John William Strutt Rayleigh, Rosen, Carl Runge, Ernest Rutherford, Johannes Robert Rydberg, Frederick Soddy, Arnold Sommerfeld, Edmund Clifton Stoner, John Joseph Thomson; Universität Göttingen, Universität München, and University of Manchester.
[Immediately before this discussion, Professor Bohr had talked informally to Thomas Kuhn and Rüdinger about the philosophical conceptions with which the early parts of the interview deal. Some of the questions addressed to him derive from that earlier discussion.]
Now we were just speaking about a kind of philosophical attitude one took at the earlier dates, and I tried to explain to Professor Kuhn that in some way I took a great interest in philosophy in the years after my [high school] student examination. I came especially in close connection with Høffding. That was just a minor thing, but I pointed out to him that there were some errors — actually there were many errors — in his formal logic. He took that to heart, and there came out a new edition, where he says that he has got some various help from one of his students....
Do you remember the kind of errors?
No, but ... perhaps we shall find that edition of Høffding's, so we will see whether it says what kind of errors it was. They were really fundamental, not small things; but he was also not an expert in logic, so these things were just an incident. At that time I really thought to write something about philosophy, and that was about this analogy with multivalued functions. I felt that the various problems in psychology — which were called big philosophical problems, of the free will and such things — that one could really reduce them when one considered how one really went about them, and that was done on the analogy to multivalued functions. If you have square root of x, then you have two values. If you have a logarithm, you have even more. And the point is that if you try to say you have now two values, let us say of square root, then you can walk around in the plane, because, if you are in one point, you take one value, and there will be at the next point a value which is very far from it and one which is very close to it. If you, therefore, work in a continuous way, then you — I'm saying this a little badly, but it doesn't matter — then you can connect the value of such a function in a continuous way. But then it depends what you do. If in these functions, as the logarithm or the square root, they have a singular value at the origin, then if you go round from one point and go in a closed orbit and it doesn't go round the origin, you come back to the same [value]. That is, of course, the discovery of Cauchy. But when you go round the origin, then you come over to the other [value of the] function, and that is then a very nice way to do it, as Dirichlet [Riemann], of having a surface in several sheets and connect them in such a way that you just have the different values of the function on the different sheets. And the nice thing about it is that you use one word for the function, f(z). Now, the point is, what's the analogy? The analogy is this, that you say that the idea of yourself is singular in our consciousness — do you think it works; am I doing it sufficiently loud? Then you find — now it is really a formal way — that if you bring this idea in, then you leave a definite level of objectivity or subjectivity. For instance, when you have to do with the logarithm, then you can go around; you can change the function as much as you like; you can change it by 2π when you go one time round a singular point. But then you surely, in order to have it properly and be able to draw conclusions from it, will have to go all the way back again in order to be sure that the point is what you started on. — Now I'm saying it a little badly, but I will go on. -That is then the general scheme, and I felt so strongly that it was illuminating for the question of the free will, because if you go round, you speak about something else, unless you go really back again [the way you came]. That was the general scheme, you see. Will you ask something?
Yes. How did problems of this sort come to you in the first place? With whom did you talk about problems like the free will?
I don't know. It was in some way my life, you see. And I talked with somebody. But this was also extravagant, you see, so I think I did not learn something from other people, I just tried to show how close the analogy between our consciousness and, such functions were, and that was really very close, indeed, but as just a help. What I was prepared for was that when we use any kind of word, this word has a certain connection with a certain degree of objectivity, and that you had to go back again, all the same way, in order to show what you could do with it.
Did you write anything down about this analogy?
No, but I was very occupied with it. No, I did not write anything down, but I spoke to the various people who came here. That was what I spoke with Kramers about, you see. Of course it was a kind of luxury, but still it was also helping to [find out] what to do.
What would the f(z) correspond to, and what would one of the Riemannian sheets correspond to?
It was especially made for the question of the free will, where you have to go round and where you do not speak about the same thing, unless you move back again. You see the whole thing is very, very obscure. [Moves to blackboard.] If you have such a thing like this, and you go around here, then you certainly are treating things in an orderly manner, but you gradually get over into some other meaning of the words. Now, I say it very badly, but that was the kind of interest [I had]. We were later on very interested in the particle-wave problem. I felt also — but not to do anything with it — that it was more so that if one created a photon, then one had made a knot in existence, a knot which was of a very difficult kind to say, and only when that photon was absorbed, annihilated, that knot was untied. And that [view] I felt, was nothing, but [that] it had to be done in that kind of way was really something which was formidable. But now we know that these are solved by the non-commutation rules, and therefore, the non-commutation rules are certainly something great. But in order to understand what they mean — You cannot get over that problem of the particle and the wave. And, therefore, it is also so nice that this lies in the complementary description.
Did this first group of ideas about free will first come to you at the university before you started the work on surface tension?
I think it was in those years before I got so [busy].... I was not really a kind of day-dreamer. I was prepared to do some very hard work, and this surface tension was a very great amount of work. Whether it's good or not, that's something else. But in between I was just interested also as regards the problems of biology, just what the problems of teleology meant, and so on. Therefore, I meant only that it was a natural thing to me to get into a problem where one really could not say anything from the classical point of view, but where it was clear that one had to make a very large change and that one got hold of something which one really believed in.
Did you carry on your interest in these problems by reading books of philosophy?
No, not at all. Of course, I felt that philosophy — But that is my error, you see. It is not an error now, but it was an error in those days. — I felt ... that philosophers were very odd people who really were lost, because they have not the instinct that it is important to learn something and that we must be prepared really to learn something of very great importance.... There are all kinds of people, but I think it would be reasonable to say that no man who is called a philosopher really understands what one means by the complementary description. I don't know if it is true, you see, because one can tell [there are] all kinds of people, and time goes.... — I think, at any rate here, the thing is preposterous. I do not also know how the thing is here. — But if you take it on the whole, or a few years ago, they did not see that it was an objective description, and that it was the only possible objective description. So, therefore, the relationship between scientists and, philosophers was of a very curious kind. First of all I would say — and that is the difficulty — that it is hopeless to have any kind of understanding between scientists and philosophers directly. It has to go over the school. I don't know exactly how it is, but let us say, if you go back to ... the Copernican system, then some scientists they thought that it also was beautiful. But they were killed. Bruno was absolutely killed, and Galilei was forced to recant. But in the next generation, the school-children did not think it was so bad, and thereby a situation was created where it belonged to common knowledge or common preparation that one had to take that into account. I think it will be exactly the same with the complementary description. It may be it's already, but I do not know.
I think not yet.
No, I think not. It is so odd that even scientists like — What was the man we discussed things with in M.I.T.? There was this meeting at this beautiful house — Bridgman. Bridgman was also so that he did not know what it meant, ... but it was something that he did not like. I think he saw it as simply talk.
But Einstein was always apparently asking for a definition or a clarification, a precise formulation of what is the principle of complementarity. Could you give that?
He also got it, but he did not like it. Einstein didn't like it, you see.... When I met Einstein for the first time — now this is, of course, not to be taken literally, because Einstein was not so stupid — but that was in Berlin in 1920, and I said to him, what is he really after, what is it that he is trying to do? Does he think that, if he could prove they were particles, he could induce the German police to enforce a law to make it illegal to use diffraction gratings or, opposite, if he could maintain the wave picture, would he simply make it illegal to use photo-cells? That was, of course, in all friendliness, but it was the idea to say that this problem we cannot get over, and that means that actually we got something new in the quantum. That was the point. Otherwise, there was absolutely nothing to do to explain that on the classical lines. I have said it very badly, but I hope that you understand that it is just the idea of it, and then gradually it came out. But one had then first of all to get the waves into the particles, into the actual particles, and so on. There was a lot to do, but then it dissolved itself.
May I ask, already in school were you interested in fundamental mathematical concepts?
Yes, I was, but I suppose most boys feel that the main interest in the teaching about fundamental concepts is that they suddenly get the idea: how much do the teachers understand? And that is a very great question, — I say it so stupid, — it is a challenge. Perhaps it is entirely wrong, but I felt that it was really something to get into.
Did you get the idea of a Riemann surface in school, or was that first at the university?
No, it was at the university, and that was by a mathematician, Julius Petersen, but there was no question of having any kind, of discussion with him. He was old and did not want to discuss with students. He went away at once after the lecture.
Could I ask how the problem of free will was usually discussed then?
I don't know, and I am very sorry what I have started, on, but perhaps I will try to clear my thoughts another day. But the thing was that it was not a question. But ... everyone knew that it was a trouble, and that it did not fit in with classical physical ideas, and therefore, one wanted a broader scheme to put such questions in. I think it was also not too good, but I think it was an idea [I had] by myself which I really did not discuss, perhaps with my brother, but I felt just that it was a kind, of escape or solution.
How did you look upon the history of philosophy? What kind of contributions did you think people like Spinoza, Hume, and Kant had made?
That is difficult to answer, but I felt that these various questions were treated in an irrelevant manner.
No, I knew what views Berkeley had. I had seen a little in Høffding's writings, and I thought it was obvious that so could one do it, but it was not what one wanted.
Did you read the works of any of these philosophers?
I read some, but that was an interest by — oh, the whole thing is coming [back to me]: I was a close friend of Rubin, and, therefore, I read actually the work of William James. William James is really wonderful in the way that he makes it clear — I think I read the book, or a paragraph, called —. No, what is that called? — It is called "The Stream of Thoughts," where he in a most clear manner shows that it is quite impossible to analyze things in terms of — I don't know what one calls them, not atoms. I mean simply, if you have some things ... they are so connected that if you try to separate them from each other, it just has nothing to do with the actual situation. I think that we shall really go into these things, and I know something about William James. That is coming first up now. And that was because I spoke to people about other things, and then Rubin advised me to read something of William James, and I thought he was most wonderful.
When was this that you read William James?
That may be a little later, I don't know. I got so much to do, and it may be at the time I was working with surface tension, or it may be just a little later. I don't know.
But it would be before Manchester?
Oh yes, it was many years before.... [L. Rosenfeld remembers clearly that he and Bohr first encountered James together around 1932]. You see, the problem is so difficult, and it may be even irrelevant and immodest to speak so, but I was not interested in philosophy as one generally called it, but I was interested in this special scheme, and that was even not too good.
Did you often see Høffding?
Oh yes, I had very much to do with Høffding. He had some difficulties, and I came out here and tried to read him about poetry. That was Wildenvey, a Norwegian writer. And Høffding was really very interested [in complementarity], far more interested than any philosopher who has been called a philosopher, because he thought it was right. He had not too great an understanding of it, but he wrote an article about these things, which is far better than any other thing which has appeared in philosophy since. Perhaps it is wrong.
Oh, I think that is to go too far.
No, I think that it is not too far; it may be that it is not good.
Well, he wrote mainly about his own anticipations of these ideas. ...
It is an odd thing. First of all, it is not at all meant to be an objective description, and philosophers may be much, much better than I think they are, but actually, now it is thirty-five years since one really got the ... [answer]. But I speak about the time, let us say up till after the war [since when] there may be some better, but I do not know what their names are. Then the philosophers simply were critical, but Høffding was not critical. — I don't know. That is a very difficult thing, and that is also a thing we shall not go into, but it would be nice to ask you, how it really is with the philosophers. How are the philosophers in Berkeley? Do they take it for obvious that these things are right, or do they not? I think they do not, as far as I know them, but perhaps they do.
Almost none of them has the technical competence necessary to follow these ideas into the problems from which they have been developed, and, therefore, none of them is really in a position to deal with them in any depth.
Yes, but that is a very odd situation, because first of all it is really not a learned subject. We have really put it in very simple ways, and. I think they more give expression for some kind of dissatisfaction. They may be right — ... We will not speak in a wrong manner about it, but that is the point....
I don't understand how you can feel your view is so obvious and so simple when you must also be aware that this attitude of yours, for instance to the use of language, is very unusual and very difficult really to live oneself into.
First of all, I am an old man, you see, and, therefore, I don't know at all [how it seems] to young people or so. But I really think that by these few arguments — that the measuring apparatus are heavy bodies and thereby outside the description — one gets at once into the complementary description. And I do not — but perhaps I am wrong, perhaps I am unjust — I do not know why the people don't like it. Of course, we know with Einstein, and that, of course, is really the difficulty, ... that the philosopher says: When the atomic physicists do not agree, why should we trouble? That is also very understandable, but the whole thing with Einstein is so difficult to me because really Einstein had a lot of criticism, and he was shown at every single point, to my mind, that he was entirely wrong. But he did not like it. He simply took the view of old-fashioned philosophy, took the view of Kant; he took the view more than Planck. Planck was another man, you see. Planck really was religious and, therefore, for instance, he spoke to us that way, and came to Copenhagen and gave a fine talk saying — and that was after these things were cleared up — that we cannot say ... that we can have a description in space and time and in momentum and energy at the same time. But he said that a God-like eye could certainly know what was the energy and the momentum. And that was very difficult you see. And then I said to him when we came back from it — he was very nice — I said to him: You have spoken about such an eye; but it is not a question of what an eye, can see; it is a question of what you mean by knowing. And then he said, perhaps I was right, but that was too complicated for him. I said: No, it is not complicated at all, it is just whether one would like to go into it or not.
He said perhaps you were right, but it was too complicated for him?
Yes, well, he said perhaps I was right, I think he did, but that I am not sure of because that was to give the whole thing away. But he said that it was too complicated for him, and I said: It is not complicated, but it is impossible from the way you are speaking. If you speak that way, then it is impossible, but if you really speak in another way, then it is a solution.
When do you suppose that was?
I think that was in thirty, after the whole thing was cleared up. He was also old then, and he became still older, — I am not sure it [what follows] is true at all, because he was much better than he looked, but in some way it can be said that he used the last forty years of his life, not to say fifty, to try to get his discovery out of the world. It is not true, it is not fair even, because he was also satisfied with the things. But it came to that; that was how I felt it, and [about] Einstein similarly. Einstein did this wonderful thing and a very, very fine paper, that in 1917, but since that time he tried to get it out of the world. Now I hope that we are right, and I will also be very, very careful, and we are not taking it from the point of view of such a high-handed absolute way of speaking — that is not the meaning — but it actually was the case with Einstein, and it was terrible that he fell in that trap to work with Podolsky. That was very interesting how it came. It came because I knew it (from Ehrenfest). [Moves to board.] It all came from this box, you see — that is a very bad drawing of a box - and I proved, that if he really weighed the box and could tell about its energy, then he lost the time. He was also struck by it, and that was the solution. But then he liked it so little that he found out something completely immaterial, which he then spoke of, saying: This box can be used in two manners. It can be used in the way of weighing the box both before and after. But nobody prevents me from doing differently; nobody prevents me from opening the box and seeing how the clock stands. And, therefore, he felt that without interfering with the photon, which went out of the box, he could choose whether he wanted to measure its energy or what time it went away. That is certainly also a paradox, but so quantum physics is. These are two different strictly separate kinds of experiments, and they are complementary to each other, and there is nothing else to say about it. That is clear, I think. But then he mixed himself up with Podolsky in a problem of saying this is far more complicated, and then he made such an impression on philosophers in the world, and especially in America, that the whole thing from that time is standing in suspense. That is not true, but I mean [there is] something in it, at any rate for the following twenty years. It doesn't contain anything, what they have made. It was just a treatment of a problem of several articles, which has to be treated in configuration space, and the paradoxes are exactly the same as those in this box.
But don't you think that this paper contributed a great deal to purifying terminology, including your own?
Oh, first of all, that is right in that way that it pointed to a paradox which was not [yet] discovered. But it is even not true, because when Einstein brought his other thing out with this box, then I really knew the solution at once. I do not know if I was prepared, that it could be so bad, but as soon as one heard about it, one knew what the solution was. With the Podolsky thing one had also to think a little to see what the solution was. In that way, it is so. But that is something which is typical of quantum mechanics that one can not know all the paradoxes, but when they come up, one can solve them.
You started, I think, to say that Ehrenfest had told you something about how the Podolsky-Einstein collaboration came about.
No, that was not Podolsky-Einstein, that was really that Einstein had this idea that one could treat a box in two different manners. The treatment of the box is really complementary to other treatments. If you have the one, you cannot have the other. It does not bring anything new. But then he came to work with Podolsky, and then they sent out the paper, and I wrote an answer.
What did Rosen have to do with it?
Rosen, I don't know. Rosen is worse, from my point of view. Rosen even today believes it; Podolsky has given it up, as far as I know. You will have to make sure yourself, but that is the point. Rosen is a nice man, and so on, but he is not inclined to give it up. I have spoken with him in Haifa, in the Technion. That was very, very difficult, but I tried to because this problem is absolutely something which is no problem whatsoever. The whole idea is absolutely nothing when one really gets into it. You may think that I say it too strongly but it is true; there is absolutely no problem in it.
But you think one has to treat each of these problems separately? You don't think one can keep a general scheme of attacking such problems?
Oh, but one can, you see. That is the next thing. I don't know how it really is, but I can say a few words about it. [Returns to box diagram and adds an emerging photon.] You do it that way that you say you have something coming out here, then what can you do with that? You cannot describe at the same time when it comes out (when it is open or closed there) and what the energy is. That is impossible. But then one can, of course, treat it in many ways, and it is a question of transformation from the time to the energy. But you cannot transform the problem. That is what people do not generally see. I hope also to write a little bit about it; we must put a proper order in. You have a problem, and you can do a lot with mathematics, but you cannot change the real problems which come out of having it to do. So one can treat it in any nice way, but it is just complementary.
This paradox did not, it seems to me, have an effect only on the philosophers; it also provoked a great stir among physicists, didn't it?
Yes, but that is something else. ... Dirac came here after that. He said that now — Dirac had been very, very curious — he has said it several times, he said: Now we have to start all over again, because Einstein has proved that it does not work. Then I wrote certainly something against it, and I have even commented upon it in my things about Einstein. It is also true that physics is a very complicated thing. I do not mean that Dirac was then — we convinced Dirac, Rosenfeld and I, that it was nothing. Heisenberg also wrote against it, but he made an error, and, therefore, he gave it up.
He wrote against Einstein?
Yes, he wrote against Einstein, but that was nonsense.
Did Dirac think that somehow the whole scheme of quantum mechanics had to be given up?
Yes, at that time, but we convinced him that it was not so. Rosenfeld and I, we talked about a week here at Carlsberg.
So Dirac was really prepared to give up his own relations?
Yes, that is very nice. I don't know what he was prepared to, because it was (nothing), ... but he had found out that one could do similar things with spin, and then he felt it so odd that it could not work.
But what was it in Einstein's argument which impressed Dirac? He could not have felt it was too odd?
No, but first of all, I do not know how much he had read it, and it is very complicated. They did not carry it out, Rosen and Podolsky and Einstein, and, therefore, it is very complicated to get into it. And in some way it is a very interesting problem, because it is a problem which helped to keep one on the straight line, if I am right, you see. ...
Didn't it surprise you that of all people Dirac should comment?
No, because Dirac had done a similar thing.... There was a man in America who made some very bad experiments on the Compton effect and found that they were not correlated in the energy, and Dirac wrote to Nature that that was nice, because then one could get over the troubles of quantum mechanics. I wrote at once to Naturethat that was a misunderstanding. — We can look that up one of the next days. — So he did the same [again later]; it was very interesting. He is a complete logical genius. But that is the kind of errors he makes. In the Compton effect, you know, the point is this, that you actually have this correlation with the energy. I had written something about this, many years before, and I wrote just then back that that [possibility] was gone; now we knew it from wave mechanics. — It is very stupid what I say, but we will just take these problems up.
Let me take you, if I may, back to the very beginning again. Would you tell us just a bit more about your early relation with Høffding? Just what sort of a person was he?
He was a very fine person. First of all, he was an imposing person in the way of understanding, and he was the best kender, I think, of his time, of Spinoza and such things. — I think I must stop, but I can tell you a little bit of a story about him. He had so many sorrows, when he was old. He married when he was more than eighty, for the second time, and his wife really died in an asylum before he. I went out an evening in all these troubles, when his wife was in an asylum, to try to cheer him up, and brought out some poems of the Norwegian poet Wildenvey, a very philosophical kind of poet, to read for him, which I also did. Then we were sitting in our dining room, and having some tea, and then there is a statue in that room of Hebe, which carries the nectar to the gods. He suddenly said to me, "If I had realized how difficult it is really to get to know what the sentiment of Hebe is, whether she is mild or severe."— You see, that depends, just on what one likes to do in that kind of statue. — But he added, that he lived upstairs, and every morning when he came down, he looked up to Hebe to see whether she was satisfied with him or not. That seems a very odd story; it is a very beautiful story, because he took things very seriously.