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Oral History Transcript — Niels Bohr

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Interview with Niels Bohr
by Thomas S. Kuhn, Leon Rosenfeld, Aage Petersen, and Erik Rudinger at Professor Bohr's Office, Carlsberg, Copenhagen, Denmark
November 7, 1962
Oral history interviewee photo

Transcript

Session I | Session II | Session III | Session IV | Session V

Bohr:

Now I thought we should just speak a moment about the general problem because in that there are a lot of difficulties. When my first paper came out it was actually objected to in Göttingen. There was no interest for it, and, as I told you, there was even a general consent that it was a very sad thing that the literature about the spectra should be contaminated by a paper of that kind. The paper was just a playing around with numbers and there was nothing in it.

Rüdinger:

That was what your brother wrote to you?

Bohr:

Yes. My brother wrote to me, but it was clear that that was the general consent. You see, it's not the intention to speak about other things now, but I'll only say that I went to Göttingen in the summer of '14 -- the next summer. And there I gave a talk about the spectra from the point of view, or at any rate something like the points of view that I had put a half a year before in this paper in the Physical Society. There it really is done in a very nice style. Not to praise it too much, but it is done with a great amount of thought. And that was then received with (grace). There was by then no question about it. In the meantime, the question of the helium spectrum was cleared up the year before, and it fit it so well, you see. And from there I went to Munich and gave a talk at Sommerfeld's colloquium, and that was also in '14. That was before anybody had touched the problems. And that was also received by general consent, you see. So that was the question.

Kuhn:

It rea1ly came that quickly then that in spite of the first reactions in 1913, both in Munich and Göttingen by '14 --

Bohr:

Yes. Because at first there actually was nothing. That's what we'll come to. But now the question is how was it presented? And it is really (the question of the connection with the model of Nicholson) -- which was nonsense. It was a play with numbers. When one looks into it one sees even that he doesn't get an agreement with Planck's constant. He gets one line at 25, then he gets another line 23, I think; then the next thing is something else, you see. That is just a kind of (harmonic) series. But it didn't even fit. He says, "It fits good enough; we do not know the numbers." But he had not that idea that it should fit; that's the point. But I took the view to begin with -- only in the first paper -- that Nicholson seems to have got some agreement with something, so let us look for how it could be. Let us see how it was possible that Nicholson could get some agreement. And then I said that these are different things; they have not to do with the emission of lines but rather with dispersion, and so on. Also that was wrong, and that I knew then very soon afterwards. Due to politeness, and also to be sure, in the first paper I treated Nicholson as if it might be something. It was absolutely nothing -- it was just a real play with numbers. He looked up in the corona of such things some lines, and they didn't know what they were. He said that it was five electrons in a ring and that they oscillated and so on; and it made absolutely no sense whatsoever. And also in another paper from later on, Rutherford just says that it is a kind of terrible thing that such things should come in. So, I mean, that this was the temptation. But actually it takes very little time (out of the first paper). And now the question is, where do they get the idea in Göttingen that I got the factor of two wrong? In the expression for the Planck vibrator, you get E = τhω. But the kinetic energy is only half of it, and the kinetic energy is what enters as regards the phase space. Then it is given by this formula I gave to begin with, without saying what it meant but simply that it was the proper formula. That I knew, you see; that is the point. But I wanted to go on to show how it was to be used. I said something about the fact that it seems to be the mean value, and so on. That is impossible really to go into. But that was what the others objected to, you see. They didn't read farther in it than the first page where I say, "We are going later to see how it is actually." That is the form which directly can be connected with the correspondence argument.

Kuhn:

My own reading of the papers and the letters makes me wonder whether at the time the first paper was written it was really the parallel to kinetic energy that had brought you to the factor of two.

Bohr:

Yes, but the factor of two is (the true one), you see. What I did was, in my own calculations I used that formula, but with not a very close kind of argument. Then in this first letter I really give a strong argument for using that formula. But I don't begin with that. One has to have this formula. Then in the third (part), or whatever you call go closer into it from the point of view of the spectra. And now I understand how it's done. When I had such a formula like that, then it was all very, very rough -- just saying that it fitted something. Now it's becoming suddenly serious. And now, therefore, it was most important to show that this is a general way to get to the correspondence. And that was the whole point. ... Earlier one had some kind of considerations, but no great emphasis was put on anything. You see, one felt that we have the stationary states, and so on, and the stationary states must be some help. But then one saw that the quantum theory was a more connected thing and that we could write something up which had to do with the spectra and at the same time with the states. That was also the revelation that the frequencies were not those of the motion but came directly through the spectra. So in the whole thing a viewpoint is developed which is so much more embracing. Now I speak about it so loosely, but -- and this is not to praise -- but it's done very well in this part three. Because there one does show that whenever you oppose the classical idea, you get the correspondence. Now I might just say what it contains. [Bohr asks to see the paper.] Now when I have made myself free of it and then start speaking about the emission of line spectra, a lot of things come out. Not only the helium spectrum, but there is also that you have this large number of lines in the solar spectra. Why is it difficult to get them in laboratories? That is mainly due to the fact that it demands a very great amount of space for the orbits to be able to develop. Now that can be done in different ways. But then the whole thing of the spectra of zeta puppis -- that is that spectra where you first found the helium lines -- gave rise to thinking of the difference between spectra in the laboratory and spectra in the stars. ... And -- not to praise --that is done fairly well. And, therefore, one sees that in this paper there is developing a whole view about the spectra of stars. I actually wrote a paper on the spectra of stars. And that must have been in '14, or something like that. That was taken up then some few years later by Saha and done very well, but all that I had. ... All the problems present themselves, you see, when you first start. But that is only one thing, and there were an infinite number. Far example, one gets the whole thing of absorption, and that is the main thing really. Thus far, in the question of atoms, one had these very curious relations between absorption and emission. Now one had this view that absorption had to start with the atoms in the normal state, and the emission from an excited state, and that gave their difference. And one was saying also a good deal about the spectra of R. W. Wood. And then I even also said -- [begins looking for a particular statement in the paper before him.] You see Franck and Hertz's work was first in ‘14, I think. ... Now I do not know where to find it, but you can look. But I just also anticipate their results in saying that one shall only expect that an atom in the normal state can only be transferred to another stationary state -- to the next stationary state. But it doesn't matter; we will go through that properly some day.

Kuhn:

There are a number of references in your writings to Franck and Hertz, sir, and it occurs to me that you may be thinking of one that comes in 1916, a bit later. There you actually point out that it must be getting a negative ion current because of secondary emission and that they're really getting excitation potentials and not ionization potentials.

Bohr:

One had the Franck and Hertz, and they didn't fit, you see. And we actually had a most wonderful apparatus. I invented an apparatus for distinguishing between the two things by a number of grids. But things were very difficult in the war time. The main pressure, of course, was on the military things. And then, actually, this apparatus caught fire. We had made it up, and it was (enclosed) in cotton wool. You see, when one works with mercury vapor, one has to heat it up considerably in order to get enough vapor. And then it caught fire and fell to pieces. But this glass blower, he was then interned. Glass blowers are very odd people, and most glass blowers are very strong minded, you see, and so on. And he went around and scolded the English, you see. And then many people objected to it. And Rutherford tried to support him, but it was not so simple a thing in the war time. But it didn't work any longer and he was interned. He was the man who had made these very fine glass tubes for the proving that the alpha particles turn into helium. But what I'm speaking about is just one sentence here [in the paper] where I say that one must expect that one can only bring the atom in that way from a normal state to some other of the stationary states. It is not my meaning to say too much about it, but it is really very, very sober. But then there are in between -- and only in a few places -- some remarks on Nicholson where I just say that he may consider something quite different, and so on. But that was too polite, you see, and not right at all. And that was then given up in the next; that was the pain you see.

Kuhn:

Just as a matter of historical detail -- actually in the next paper you make a rather different suggestion, but I think you were not yet, even in the second paper, rejecting Nicholson.

Bohr:

No, I was not rejecting Nicholson, but I didn't use it for anything. No, but the whole thing was that in Göttingen there was a misunderstanding, but I felt that one had to be so polite and if possible see whether there could be something in it.

Kuhn:

Do you remember, sir, at what point you became fully convinced that there was nothing in it? Because, you know, there is that very interesting letter to Rutherford at the end of January in 1913 in which you indicate that you have been very much bothered by the apparent contradictions between your results and Nicholson's. So, apparently there was a period in which more than politeness was involved in taking that seriously.

Bohr:

Yes. But that is earlier, you see. And it may be difficult to understand, but it was so that when I saw -- oh, there it is, yes. [The letter.]

Kuhn:

I could read to you from this. This is a letter of January 31st, 1913, to Professor Rutherford. "I am now much more clear of the foundation of my considerations, and I think that I also now better understand the relation and the difference between my calculations and, for instance, such calculations as those published in recent papers of Nicholson on the spectra of stellar nebulae and the solar corona."

Bohr:

You see, first of all, that was the time when I learned about Nicholson.

Kuhn:

Those papers come out in the Monthly Notices of the Royal Astronomical Society. How did you hear of them?

Bohr:

I don't know, but I think also the work of Fowler on the helium lines was printed there. Or somebody wrote to me that Nicholson had done something; I don't know. I mean only that this is a kind of trap because we learned about this; it was only a very short time, you see. And I don't know how accurate I (went) and so on. It is very curious; partly that it has to do with oscillations in a ring, partly that it has to do with 25 units of angular momentum. And then there are some odd kinds of series. ... There were all such things. I thought there might be something in it, although, I do not know what I thought. And that has, therefore, nothing to do with the actual work. The actual work would have been simpler if I had just written about the things I had myself. But I didn't, you see. And then it also went out very soon. I feel that you are not quite satisfied. But this is an odd thing to explain; because it all has to do with a few weeks.

Kuhn:

I'm sorry if I radiate an aura of dissatisfaction. I entirely agree with what you are telling us about. And I am delighted to have you tell us this about the first paper. I could point myself, I think, to still other things that seem to me terribly important about it for what comes later. I still have the hope that you will be willing and will be able to remember things that will tell us how you got to that paper, some of the steps that went into getting it. And I suspect that it is useful in helping to remember that to look at some of the things which no longer seem quite so satisfying about it, because they are very often the things that tell what it was like just before the formulation.

Bohr:

Shall we now just look at the questions you have written? Now the first thing is the transfer to Manchester. And you say that my first real conversation with Rutherford seems to have occurred in the home of Lawrence Smith early in 1912. Of course, it is not 1912; it is really in the end of '11.

Rosenfeld:

Yes, it was after the Solvay meeting.

Bohr:

Yes, that was after the meeting and also I went up there after the discussion with a friend of mine in Oxford. The whole thing was very interesting in Cambridge, but it was absolutely useless. And I felt that Rutherford was such a wonderful man, and so on, you see. So that's just a small question. [Reads again from list of questions] "What can you tell us of the impression he then made on you and of the topics he discussed? In particular, what he said about the first Solvay Congress from which Rutherford had just returned? How did he then feel about the problem of the quantum?" Now, of course, Rutherford was, just as Thomson, most impressive -- and even more; you see, Rutherford had such a very strong personality and spoke very loudly and so on. And it was very interesting really to hear his remarks about the Solvay meeting. But I do not think that he really knew too much about the quantum. I think I knew much more, you see. ... In the former years I had been very interested in the quantum. But it was very interesting to hear what he said, and so on. But I do not remember that he said anything definite; he just said that it is odd. But I do not think he took it seriously, you see. Perhaps he did, but I meant only that it was nothing to work with for him.

Kuhn:

Did he talk about his own atom at that time?

Bohr:

No, I do not think he did. He did mention it in Oxford in May also. But these things were at that time not anything to speak about in a conversation, you see. We'll come back to it. Then you say, "What was the state of the Rutherford atom during the period of your stay at Manchester? Was it, for example, already clear that the nucleus was positively charged and that the surrounding negative charge was discrete electrons? Were the electrons thought to be arranged in rings or individual orbits, and what were the spatial relations of the orbits? How convinced were Rutherford and his group that the model was right?" That's also very difficult to answer, you see, because it was that one felt that the origin of the large angle scattering of alpha rays came from the center of the atom and was due to some kind of particle with a large mass and a large (charge). But I do not think that anyone thought about whether they were arranged in rings or whether they were single orbits or anything of the kind. It never was discussed because it just was odd. It was only me, you see, who just was clear (that it was really unstable).

Kuhn:

Rutherford's own writings on the subject are themselves quite unclear as to what he thinks, but I notice, for example, that in this paper that is published in the Phil. Mag. for October, 1912, which is on the origin of beta and gamma rays he says, for example, "The general evidence indicates strongly that the transformation of energy from the gamma ray form to the beta ray form, or vise-versa takes place in definite units which are characteristic for a given ring of electrons, but vary from one to the other." So here is at least one point where thinking in terms of rings does enter in.

Rosenfeld:

But was that not something he had taken over from Thomson?

Bohr:

Yes, first of all, it was Thomson who had these rings, and that was the reason that the ring came in. And I do not think a deeper meaning can be put to such a (saying).

Kuhn:

I think my question is being misunderstood. I’m not for a minute asking who invented the notion of rings. There had been a lot of work done early in the century on ring atoms, mainly Thomson's, but there are others also. But it's curious to know how uniform an idea there may have been at Manchester about the distribution and nature of the negative charge.

Bohr:

You see, I do not think there was any; I think that it was just talk. One just said rings instead of (other) things. It may be that you think I am evading things, but the point is that, at any rate, I have no knowledge of it. And it was really so that in a short time it was there. Then I was just interested in seeing, first of all, that it was unstable. Now, anybody may have seen that it was unstable; it was an obvious thing that it was unstable. The next thing was then that I learned that there were more substances than there were places in the periodic table, and thereby I came into this thing of the isotopes. But that was different from the other things. That was a most extraordinary thing that you should get every line in the spectra exactly the same in different elements. And I must say -- it may be wrong -- but I took it for granted that that was the explanation of a very large part of the study of the radioactive substances. And next it followed at once that all the properties change by the expulsion of the alpha and beta particles. And that was also a clean thing; it probably took only a few days to be clear about it. And I do not even want to say that nobody else thought about it, but only that that was what I thought about it. And thereby that was never again of any kind of uncertainty -- that was the point. But this has nothing to do with the actual building up of an atom. See, that is the first thing. And then it was very curious for me that Rutherford was, at any rate, not prepared to take the view. But I said to him, "I feel that will, in a few years, be considered as the basis for the Rutherford atom, because it is clear that it is a far more extensive and definite thing that you have got elements where all the properties are the same and which change in this regular way." But he said that one shouldn't jump to such conclusions based on extrapolation of evidence. But I feel that that's even wrong against Rutherford because if he had been more in a mood for listening he probably would have seen the point. But I do not know. He was a bit impatient, and he had so much to do and, he did not want to go into it, and so on. That's very, very difficult. Let’s see: [Reads again] "How much were the problems of the quantum known and discussed within the Manchester group? -- radiation, specific heat, photo-effect, and so on." I do not think that I know anything about that because they were not presented so. Due to the instability of these things I was myself thinking about the photo-effect and so on. But only for a very short time because then I came into other things. In the group in Manchester that was not a question that it was necessary to attack. I don't think that anybody, for instance, thought about such things as specific heat or photo-effect. They could have taken it up, but they didn't because they were all busy with their special piece of research work. So whether they thought about heat radiation, specific heat and so on, I don't know -- that was not discussed.

Kuhn:

Were they all pretty well convinced about the Rutherford model?

Bohr:

Well, that is the thing. In some way I suppose they were, but not more than Rutherford, you see. That was a very great problem. Rutherford, at any rate, didn't want -- I could have published it just as a suggestion. And I went (five) times to Rutherford and so on about it, but the whole thing went too quickly. And then when did van den Broek's work come?

Kuhn:

Late in 1912, I think.

Bohr:

Yes. Late in 1912; that was when I had gone, you see. And it was a very odd paper because it was actually nonsense. He had very many more elements because he just took one nucleus for every element and that couldn't be. But there was one point in it and that was saying that hydrogen was one and helium two, and so on. So I really think I have given a correct impression of everything in what I wrote about the Manchester times. But I cannot tell, you see, what the others thought. But I do not think they thought anything. It was, first of all, a smaller group -- not too small. And then each was so extremely occupied with some piece of research work. For instance, then you also say, "Did news of von Laue's discovery reach Manchester while you were there." No, that came through -- now, I don't know, when is that done?

Kuhn:

Well, the actual announcement was made June 8th and was published in the Proc. of the Munich Naturfor, which came out in early July. The journal probably would not have gotten to Manchester, but it was so exciting there that a letter might very easily have come.

Bohr:

But that was when I left, you see. I went back to Copenhagen in June. And then I think that the story was the following: that young Bragg was in Cambridge and saw how to (turn) the thing; it was a very great improvement on what Laue had. And then they started to do some work, and, also then they started in Manchester in '13. Moseley and Darwin did, a good deal of work to see how some of the lines were and how it was then with the dispersion. Darwin had a good deal of understanding of (wave) problems and that is a very interesting piece of work. But then first in '13 -- even first in the summer of '13 -- Moseley started on the other things. ...

Rosenfeld:

Was the old Schuster still about?

Bohr:

No, he was not about. And that would have been very interesting, you see, but I did not know anything about the spectra, you see.

Rosenfeld:

But you did not meet Schuster at that time?

Bohr:

No, I did not meet Schuster at all. He was (generally) away; he had a big place of his own. And it was very different, you see. There was just this hard working group on different problems, and there was very little of general talks. I had also very much to do, you see. In the time I was there I made this paper on the alpha particles. And with the alpha particles one sees just how much I had at that time. Then you say, "At Manchester even more than at Cambridge you must have encountered many new sorts of problems. How did you go about mastering them? Did you, that is, listen to lectures --" But that is not so simple. In Manchester there were no lectures; it was also late in the term. Rutherford was occupied by writing a big book, you see. He had tremendous working power, and that was also the reason that it went so, you see. And you mention also self-education, and so on. But there was not time for much self-education. I just first had these general ideas and worked on the scattering of the alpha particles. And then you ask about whether I had thought about that before, but I already had very close understanding of what was written about these things of J.J. Thomson. And I just felt that it (wasn't too correct).

Kuhn:

What was a working day for you like at Manchester? Did you go to the laboratory, did you hole up in your room somewhere, how did it go?

Bohr:

When I came to Manchester, I thought now how wonderful it would be to get into the technique of the radioactivity. Of course, also Rutherford suggested it. And I went a few weeks to the course they had -- that was Geiger and Marsden's. And they were very kind in showing things. But then I came into the other things and said to Rutherford, "I have so little time, so I'd better stop that." So that was what I did, you see. And then actually from then on, I worked at home. I lived at Hume Hall; it was a very interesting place. And there I had a room -- I think I had two rooms. I don't remember, but when you come next I'll have read about it in my letters to my wife. I was older than the others; I was a doctor, and I think I had a little sleeping room and a place to work. And then I actually didn't see the others too much because I just worked there. There was a very great amount of work on the alpha particles. I do not say it's good, but I say only that one had to find out how to do this. It is a rigorous calculation about the problems you meet in dispersion. ...

Kuhn:

I like the way you started telling us about a working day. It occurs to me to ask, would you then, do you suppose, spend the whole day at and around Hume Hall? How often would you go down to the laboratory? How often do you suppose you saw Rutherford and other people at the laboratory?

Bohr:

I suppose, but I'm not sure, that I came down for tea. But, you see, there was not so much to talk about. I knew how Rutherford looked at the atom, you see, and there was really not very much to talk about. I just had to concentrate very, very much on this paper and about the whole theory of dispersion. The next point is -- as you say we will come to that later -- whether dispersion is analogous to the question of electrons moving in an atom. That's something else, but I had to work it out. And the whole thing was so extremely short.

Kuhn:

I wonder then if there is anything further you want to say about Rutherford's role at the laboratory in the research?

Bohr:

You see, Rutherford was, of course, just as I have written, a very great person. He kept up his own research, and advised all the others. At the same time that he was really working on this big book. I think he worked most nights on that you see, and so on. And that was very impressive, and I also knew so much about Rutherford. I had read his earlier things and so on, so that was all a most impressive experience.

Rosenfeld:

Was there any social life in the evenings and so on at that time?

Bohr:

I don't know; we may have had some. Perhaps there was a tea in Rutherford's house, you see. But I'm not sure even whether it was that he invited, all or just a few, but I came sometimes to Rutherford's house. And Rutherford certainly was very nice, you see, and this question that he didn't actually believe in these things didn't play any part. You see, Rutherford a very interesting man. And when anyone came in connection with Rutherford he got some unforgettable experience about his ways and also much he could do. You see, there are a lot of stories about Rutherford. When he was in Montreal they wanted to show that these emanations could be condensed. Therefore, they got some money and ordered a Linde machine from Germany. That was not a very ordinary thing in those days because there were only a few laboratories which had the facilities of liquid air. He had that coming, you see. Then everything was prepared. I don't know if it's true, but the story is that when the ship came to Montreal, nobody in the laboratory was allowed to go to bed before it was set up and, the emanation condensed. Whether that was true or not, I don't know, but that was what was believed of Rutherford, you see. You may be interested in the various talks I have given about Rutherford. He came here several times, gave some talks about him in which I told stories. And Rutherford said. I was telling stories against him, and so on, but that was not the idea. Rutherford was, just as I have written, something quite extraordinary, you see; he was able to do anything, and so on. The little thing that he didn't believe in it, that made small difference because I thought that will all come out. Then it happened that some other people wrote about it and did not do it too well, and so on. But that was not clear, you see.

Rosenfeld:

Rutherford himself just thought that he had disproved Thomson's model -- that he had shown that the positive charge was --.

Bohr:

Yes, yes. Absolutely.

Rosenfeld:

Then he didn't want to follow up the consequences?

Bohr:

No, you see, but it was rather what could he do? It was really a theoretical piece of work, and it was then extreme luck that a man like me got into the hydrogen spectrum and so on. That could have lasted a very long time because the problem was not a (settled) one. And it could also have lasted a long time before the Moseley came out, and so on. And Moseley's thing -- that is presented in a wrong manner, you see, because then we knew the hydrogen, we knew, the helium. We knew then the whole beginning, you see; and that was then not a surprise that one gets the whole series.

Kuhn:

You knew, almost above all, the Whiddington relation.

Bohr:

Yes, but the Whiddington relation was also only something which was of importance for a time. We knew the Whiddington thing, but we did not know the actual thing. That was also not clear to Moseley. We did not know what, later on -- many years later, we came to know as the Pauli principle. But before the Pauli principle we found out how the electrons are distributed in the atom. ... Pauli was wonderful, but there is absolutely not a word that is new in the Pauli principle. That was all done by Stoner. I studied first of all the difference between the hydrogen spectrum and the other spectra. But then the other spectra had a fine structure, and in my paper that was a very bad thing, but I didn't know. And it was (probably even) not done. But Catalan had done a lot about the spectra. And that Stoner took into account, and then everything came in order. I was really only interested in how the periods start, but all the details were wrong, you see. ... Pauli took the Stoner paper as a revelation, but he also did some other work on the Zeeman effect. One has the Passhen-Back effect which shows how the anomalous Zeeman effect by large fields transforms into the normal Zeeman effect in an odd manner. And that Pauli was able to calculate through and just to show how it went, and with the same idea as Stoner. That was such a fine piece of work, so it was called the Pauli principle, but one could have really called it the Stoner principle. When we really get this in order then we will also come into these later things which are all much more interesting than the first. The interesting thing in the first is how to treat the hydrogen atom. Next one gets into other things, and we need not today then speak about dispersion because that we will go through in the paper of '16. But then there were a lot of things; partly the very fine contributions of Sommerfeld. Then I wrote a big paper here where I actually went properly into the correspondence principle, and there we had a lot of things. One had really what there is to have about those lines which can appear or not appear, and so on. Then the periodic table came, and it was further then that the Pauli principle came. But then first of all the work of Heisenberg came with the helium spectrum, and so on. That's a long story, but I hope we have some time for it. But now what can we do now?

Kuhn:

Would you say a bit more then about the paper on alpha particles? Did you really start on that very shortly after you got to Manchester?

Bohr:

Yes, because you see, I started on that when Darwin's paper came out.

Kuhn:

I think that paper came out in July; I'll have to check.

Bohr:

Oh, I think it came much earlier, but perhaps I saw it.

Rüdinger:

It came in June.

Bohr:

The first of June, yes. Now I'll find out when I came to Manchester I think it is in the middle of March. Then there was April, which I perhaps used for the Hevesy things. And then this paper came the first of June. Then, probably, I read it once and became clear. I left, at the latest, in the middle of July, so there was just time for it. So I do not think I saw it before it came out. And Darwin was very nice about it. He just said, it was right, you see. That paper was also not so good, you see because partly, of course, as you know, there is a paradox in it. But that matters not so much with the quantum theory. That I have written up very accurately in my paper from '48. But it was a long paper and probably it took that time. I gave it first to Rutherford on my wedding trip, and that was in the beginning of August. And I worked on it really in Cambridge, you see, on the way there because it was such a long thing. But I had it also in Manchester. But there I did this odd thing that one considers that the electrons are moving in some way and one uses dispersion. But that was only said to get some approach to the closest way to do it. And the dispersion is such a general thing that one can, at any rate, get some connection with dispersion that way. But then with Bethe later on we had the dispersion in the quantum theory, and, therefore, we knew how dispersion had to be treated.

Kuhn:

It is so striking to see your use of Whiddington formula and of the Planck condition to get those electron frequencies. ...

Bohr:

... But you see the whole thing is so rough, and it's very difficult to say more about it than there is. Partly from this paper, and partly from the work I had, before -- you see I had all this view in Manchester. -- I could just then roughly say how strongly electrons are bound. Of course, this is only the normal state of the atom where the electron is bound as strongly as possible. You see, I'm so sorry because most of that was wrong. And then even you see when I came to Copenhagen, I had not too much time. I was an assistant of Knudsen and worked the whole day with experiments on friction in gases at very low temperature. But then I went to Knudsen and said, "I had better stop." I went out in the country together with my wife, and we wrote a very long paper on these various things. And then when I came in, I think that I discussed it with some body, and perhaps I was told that there was something known about the spectra so I looked it up.

Kuhn:

You have no notion with whom you may have discussed this?

Bohr:

Oh, yes, that was Professor Hansen. Hansen had been in Gottingen, and he was the only one who had some interest in these things. I just told him what I had, and he said, "But how does it do with the spectral formulae?" And. I said I (would look it up,) and so on. That is probably the way it went. I didn't know anything of the spectral formulae. Then I looked it up in this book of Stark. And then I just saw at once that this is the way the spectrum comes. Then I went back to Hansen and said, "Now we've spoken about this Ritz combination principle, and so on; is it this?" And he said he didn't know. But I said, "I've got this view that the combination principle is only that you get the spectra lines out of the difference." And he said he wasn't sure, so I went back again.

Kuhn:

Do you suppose that it is really first at that point that you get the idea of stationary states, in contrast to the notion of the permanent state -- the state of closest binding for the electron you're dealing with?

Bohr:

Yes. (There you have it.) But still this is difficult because first of all the work of Nicholson is such (confusion). There I thought perhaps it is that he deals with other states. And we knew there were many states in the Planck theory, and so on. But that was all one did with it until one saw that the whole thing came out. And then, of course, one should have left Nicholson entirely, and I have also very nearly left it, but I just tried to say that there are some points --. You see, in this long letter there are just a few references to Nicholson, where I say that perhaps it is so, but, of course, it couldn't be.

Rosenfeld:

When was Bjerrum's work on the molecules?

Bohr:

Yes. When Bjerrum started on these things, I didn't know about it. And the problem is whether I knew about it before; I'm not so sure -- but that I can find out. He was away. I think I met Bjerrum when I came back in the middle of '12. Then he had not the other thing; at any rate, he didn't tell about it. Then I told him about just this question of the isotopes and of various things. He has sometimes said to me that he remembers that very well, you see. Actually I don't know, but when one got into the other thing, then, of course, one knew something about the Bjerrum things also. As Bjerrum did it, one doesn't distinguish between the motion of the system and the frequency of the spectrum. But one has to do it, you see. Now I do not know if I know it exactly, but if I could think a little -- no, I'm not prepared to do it because I'm too tired. The point is only that they come out accidentally, but they come out very, very differently from the way Bjerrum did it. They come out just due to the fact that it is the square; then one gets that the differences contain linear functions of the frequency. That was a wonderful piece of work. But that was brought in very beautiful order by (Heurlinger). But that we must be very careful about, and that we'll go into some other day.

Petersen:

Do you know if Hansen has ever written about your early conversations with him?

Bohr:

No. I don't think he has. I don't think he has. You see, Hansen was more just a listener, and actually it came so that I could come back to him saying that now I have looked at the spectra and found out how to account for them.

Petersen:

But he may have kept a diary.

Bohr:

Yes, but that I don't know. But probably not. You see, it was so that I was a bit older; I was a doctor before I went to England and then I was asked a little later to be the opponent at his dissertation.

Rüdinger:

What was he working on?

Bohr:

He was working on the anomalous Zeeman effect, but all on the way in which Voigt has put it. That was very beautiful because one gets very nicely in absorption these (???) and so on, but it doesn't touch at this point. ... He had been with Voigt for a year in Gottingen and was very well acquainted with the whole group, and so on. But that is earlier than my paper, and my paper wasn't believed in Gottingen.

Kuhn:

You have, of course, with this remark on Bjerrum’s work, put your finger on exactly the key transition in these earliest developments. And this is detaching the mechanical and the optical frequencies one from the other. Now, in that connection, I would be interested in trying to see more clearly just when and how that came in your own work.

You have, I think, now looked at that letter to Hevesy that Eric Rudinger first noticed.

Rüdinger:

This is the letter where you say that you put the energy equal to h times the mechanical frequency. I think we looked at it some days ago.

Bohr:

Yes, but that that’s an early thing.

Kuhn:

That was February, 1913.

Bohr:

Yes, but you see all these things were absolutely trivial. It was not trivial in Manchester. But it was in the (air) to try to use the Planck ideas in connection with such things. That was completely trivial. The point was just the detachment -- that was the only point in it. I think one found the idea that Planck's constant and the orbital motions are connected in a very close manner even in the Solvay Conference. There was Sommerfeld's work and especially Langevin. Langevin had calculated out the energy of the motion of atoms in a magnetic field and had also found out that one has still a semblance of the magneton. It was not accurate at all. And, you see, the whole thing is a very odd thing. I came in, you see, and saw that it is not so bad because actually one went into many things as soon as one had such a point. Therefore, also in the first paper there are a lot of really quite new things about absorption spectra and everything. You see, it's a very odd thing; I was taking things seriously, and at the same time I felt that this thing -- that we got into the hydrogen spectrum -- might have been an accident. But then (???) came into it, and, then it developed.

Kuhn:

The very fact that within a month you discovered the hydrogen spectrum, wrote the first paper, and sent it off to Rutherford indicates how very, very nearly you were already there. And that piece of work could not conceivably have been done in a month except by somebody who had all of the pieces already in his hands.

Bohr:

Yes, but that is not right, you see, because first of all we worked very quickly. And the point is that one had a general idea that one should have these many, many states. And you can do a lot in a fortnight, or a month, you see. And it was a revelation that one could, out of the hydrogen spectrum, read the actual happening. Of course, one could not perhaps have had so many words, and so on. But practically all the words are even new, you see -- if you look at it. Then there is in the beginning this thing about this formula -- it is not too simple and not really too easy to understand. But then one goes in, and one says now we have a view; then one says that the combination rules come from the transitions. And then one goes through everything again. Then one says this formula means that we have the correspondence, and that is then the basis for it.

Rüdinger:

But, Professor Bohr, still there is that interesting thing that at the beginning of the paper you consider the different stationary states when being formed from an electron falling in from the outside as corresponding to the emission of different numbers of quanta. And later on --

Bohr:

Yes, that is right, but that is taken too seriously, you see. It's not so, actually. One could say, if one has such a thing, it looked as if there were a different number, and so on, but that was not taken seriously you see. It was not taken seriously at all. There are some sentences about this which I actually agree are almost nonsense, but that didn't mean anything.

Kuhn:

Let me simply say that it is a characteristic, and not an invariable, experience for the historian that the sentences which, in retrospect, mean nothing and have been discarded are the ones that can tell you most about the process of the creation of the idea.

Bohr:

Yes. That is also just my point of view. Namely, I had the possibility of thinking of different stationary states, and then I just saw how the spectrum came out of the hydrogen formula. Then from there it is completely changed, you see. It mustn't be taken too --. [They look for and find the paper.]

Kuhn:

That initial statement that one emits tau quanta of frequency omega over 2 in going to an orbit whose mechanical frequency is omega is a perfectly natural thing to say on the parallel to Planck because if the Planck oscillator emits energy thw it has emitted tau quanta of frequency omega.

Bohr:

Yes. You see, as soon as we saw this, then Planck's formula fell into line. In Planck's formula the main thing is it can only change by one. (Therefore, if Planck has done a greater transition, it's only by one.) It is a system which has only one frequency; that was the point which brought it in order.

But probably I was thinking of all kinds of things, you see. First of all, I'm not sure if it is a (???) sentence because it is so hard for one to see what it means. But next it was just to say we get now the Planck formula. We just say in the beginning that we will see what happens. But, when writing this, I certainly knew the solution. That is the point. It was completely right that this quantum number is the quantum number of the states and not of the transition process.

Kuhn:

That formulation is to me more exciting than to you because it’s the first place in any of the literature that I know of where there is a real, categoric dissociation of the mechanical frequency and the optical frequency. ... Do you suppose that this dissociation between the mechanical and the optical frequencies was also in your own development a product of the recognition of the Balmer formula?

Bohr:

Absolutely. I remember looking at the Balmer formula --. You see, the thing is that you expect too much. [Bohr at the blackboard.] One has the Balmer formula. The Balmer formula is this: [k/n2 - k/m2]. Now one says that the spectrum is just due to transitions just like in the photo-effect. And then one tried to express the various things from that. Quite another thing in the writing is that all these points are not sufficiently cleared away in parts II and III. But that is because it would have been an enormous thing really to get into it. Therefore, my point really was that partly one had this formula and partly one understood the (Rydberg) connection between the other spectral lines. Not, of course, in all details, but only as regards the first states -- which are not seen, you see.

Kuhn:

At the very end of the first paper you suddenly speak of the angular momentum quantization having approached it earlier through something like energy quantization. Was that the first time, so far as you know, that you had tried to deal really with angular momentum in that way?

Bohr:

No, because, you see, angular momentum is really not a hidden thing. Angular momentum is just the relation, for a circular orbit, between the kinetic energy and the frequency, and that was known all the time, you see. Everybody had that in some paper. You can find it in Nicholson. That was what we all had, you see. So that's not a point. It really would have been very much more beautiful if it had all been left out. In some ways also it's nice that it is there because then I tried to do something with it. But most of it is sheer nonsense. But then it gets so beautifully in order because then (Moseley) comes then (Kossel) came -- have you read that of Kossel?

Kuhn:

I haven’t read it, but I know of it.

Bohr:

(Then I could contribute to it in Part II.) And that was (real) you see. But then gradually one felt that in this (race) of quantum numbers it was quite essential to take into account. That was not done by anybody. That is only this kind of thing that the orbit goes in and comes out and that is cleared out in my paper in Naturwiss. There I speak about the periodic table. And the point in that is true, but it is not right how many electrons we put in the various things. We did that just by symmetry and then it was not right.



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