Niels Bohr - Session II

Bohr:

There is the question of the transfer to Cambridge and the transfer to Manchester, and about Cambridge and Manchester at that time. Now, perhaps it will be disappointing to you, but there is not a lot to say about it, you see. It was that I got a stipend from the Carlsberg Foundation to go to England to work with J. J. Thomson. And, I think, in that my father was involved very much; I think it was arranged among themselves. But at that time the idea that a young man of some promise went from Denmark to another country was quite general. Now I cannot speak about it today, you see, if we are really to make it proper, but in my whole youth my brother played a very large part. He went to Göttingen. He was 1 1/2 years younger than I, but he had his exam and his doctor's dissertation before I had anything. I can just say that we have found out, yesterday, that there is a very large number of letters, but they are mostly private. It was that I was engaged at that time to my wife, and I really came back from Manchester to be married in the first of August in '12. I came to Manchester in the middle of March, and I went back to Denmark in what must have been something like the beginning of July. But I also went back for a moment to arrange things, you see. It was not a very simple thing to be married when I was in another country. We had to see about housing and such things, so I went back for a short visit — perhaps it was in May. That we will really find out. But that was the general thing.

Kuhn:

Was your brother's trip to Göttingen also done through the Carlsberg Foundation? And were many such fellowships given each year, do you suppose?

Bohr:

No, no there was nothing. First of all I don't know if my brother had a Carlsberg fellowship. But there were no fellowships; it was just that in singular cases one could just turn to them. So there was not at all any kind of regular stipend. And now we come to the first point, namely, whether I always had thought of a career in theoretical physics. There are a number of things which have been written about me without my knowledge really. And I don't know whether the people writing it have gotten ill, or how far along it is. But something has been written just about me and about my early days. This is without any (pretension) from my side. But (there was) actually interest in me. I had very much to do with my brother. He was in all respects more clever than I; he also was a great mathematician you know. But I was considered something of a different character because I was from first youth able to say something about philosophical questions. For example, you have in biological science the question of teleology. It is a very odd thing that actually a tree grows so that the branches are thick enough to carry the various things. They have a very big stem and smaller and smaller and so on. But I just understood very early that this was so, that one couldn't put the question, because if the branches were not able to carry the things, there was no tree. But it is a very essential point to see in teleology is. And my father was a physiologist, and he, at any rate, understood that something was be expected from me. But I was not set for a career in theoretical physics — that was just due to (chances). When I really got into the problems and had been at Cambridge and so on, then I had no time to do any experiments myself. I took very great interest in all the experiments the laboratory, but I had no time to do anything myself, with my own hands. But my first paper was a partly experimental study. Have you seen it?

Kuhn:

Yes. I have not read it, but I have looked through it.

Bohr:

And that was the kind, of thing that I thought I should do you see — both thinking about the theoretical aspect of it and about the experimental. But that became unpractical, you see. I also started when I came to Cambridge, on a little experimental study and also even at Manchester. I'll come to that in a moment. It was only a few weeks. But that was my attitude as a physicist. And so in some way, I think, that answers the first two questions.

Kuhn:

I find in talking to different people that there was immense variation from one country to another about the conception of what physics was, and, about the relation of theoretical and experimental physics. Was this a choice which one could consider; I mean, was there such a thing as experimental physics as opposed to theoretical physics when you were a student?

Bohr:

But, you see, it came from Germany that partly one had experimental studies. But that was technological — at any rate, more or less technological. It was the general course in physics where they made a lot of experiments and so on. And they called that experimental physics. And then there was in Göttingen, for instance, a (maiden) institute of Franck and Born; it was called the Institute for Theoretical Physics. Franck I don't think one could call a theoretical physicist really; he was just a very great experimenter. But they did it; they called the new things theoretical physics. And thereby from that time, also when the Institute was made here, we kept the name. It may not be practical, you see; we could perhaps much better have called it an institute for atomic physics. But it was thus called generally in the world, you see, and especially in Germany it was called theoretical physics.

Rosenfeld:

I think it was only in Germany because in England it is not even now a current word.

Kuhn:

In England it would be called applied mathematics. But it would still be thought of as somewhat separate.

Rosenfeld:

But it is not exactly —

Bohr:

It was all called physics in, England. But, of course, there was a great distinction between the work and, of course, the men. The most interesting man, was, of course, Lord Rayleigh, who wrote about the problem of sound. He wrote about it in a way which (cannot) be made better. That contains, one might say, the main source used for the quantum theory. Of course, it was not the point then how the quantum theory was, but I mean it was done so well that one there finds most of the formulas. Now, of course, it is so complicated again. But isn't it true?

Rosenfeld:

The Germans found the name, but the real thing was done by the English physicists. There was not only Lord Rayleigh, but also Maxwell and also Tait.

Kuhn:

And before that it was the French —

Bohr:

Oh, yes, of course; but, I mean, in England it was all mixed up. For instance, Lord Rayleigh was a very great experimenter; he used most of his time for experiments. These other things were a sort of a relaxation. He was able to do so tremendously. Maxwell was the same; Maxwell also came into the work of (???). But he actually did not do too much experimental work, but it was his interest. And in the general course of physics in Cambridge it's a marvelous tradition they have. Much of the tradition goes back to Maxwell. I mean Maxwell made all kinds of instruments for that general course which, when I came, Thomson gave. That is just to (map) it out now. I don't whether there is any thing in those first two points which we have not touched upon.

Kuhn:

On that second one —. It's still true, I think, when you are a student there, both Jeans and Larmor held appointments, as mathematicians, not in natural philosophy.

Bohr:

Absolutely

Kuhn:

Now did that result in a separation between them and Thomson’s group, or was that just a name?

Bohr:

You see first of all there was a very definite difference in their work because (Jeans and Larmor) were actually what you now call theoretical physicists, or mathematical physicists — whatever you like to call them. It would have been simple to call them (mechanical) physicists, but that is something else. The point is that they did not do experiments. We come to both of them in the next question. And so they were just interested in mathematical theory and nothing else, as far as I know. I think really that Larmor had never done any experiments. He was a very deep thinker. And we used to joke about the fact that he had very great gifts for making things difficult. I loved that in some ways because it is a way to think over things. He gave lectures, but they were Generally not followed. But I followed them And also in Copenhagen we had a teacher, Thiele, who had a similar tendency and gave lectures about the theory of numbers, or something like that. They were about odd kinds of numbers, and that was certainly difficult. But it was interesting for a young man who wanted to tackle things, you see, so that was really one of the few lectures I heard in Copenhagen. They were the only ones, I think. It's not so interesting, but I can tell you a story about him. He wasn't satisfied with a simple deduction of the formulas of spherical geometry (where one takes the two main lines by themselves). But he had a way to put the thing all together and then calculate with imaginary numbers. Then it was intended to come out with a certain expression equal to zero, and that meant that both the parts should be zero — A plus iB is zero and so A and B should be zero. That wouldn't work. He used the whole hour up, and he could hardly see anything — he was almost blind. Then he gave up on finding it. He said, "You must try yourself; it comes out." Now my brother and another young man who was interested, in mathematics started to look into it. And they could not possibly get it to come out. But then by chance they found a notebook which had been kept by someone who had taken the course in former times. Thiele had used five semesters in getting through this course, so that if one turned back five half then one came to the came lecture. And there it stood in the notebook: "At this point Thiele wanted to show that it could be done by a complex number, but it wouldn't come out." [General Laughter.] Oh, but he was really the most gifted man in Copenhagen. But he was almost blind and so on. And this was even a very, very simple thing compared with what he did in treating the theory of numbers, you see. And it probably never will come out, you see. Then when Thiele died, no more was heard about it. But that is only from my student days.

Rüdinger:

Now we were talking about the relation between the theoretical and the experimental physics — was it a natural thing for you when you wrote the first paper, which got the Gold Medal, to make these experiments yourself?

Bohr:

Yes, of course. I mean I made the experiments completely alone in the physiological laboratory. It is not my meaning to (pretend) about it, but it was a great amount of work. And if you read it, you see, you will see how it goes. And I made the apparatus also by myself.

Rosenfeld:

It was in your father's laboratory?

Bohr:

Yes.

Rosenfeld:

There was no physics laboratory?

Bohr:

There was a physics laboratory, but at that time there was various work. Christiansen was working on electricity of waterfalls. And that was very interesting, and something came out of it, but it was something he was doing quite alone. Later on there came some more money to the physical laboratory, but then there was practically nothing.

Kuhn:

Well, did this mean that most people who did theses in physics at Copenhagen did not do laboratory work? Was it unusual to experiment?

Bohr:

No, because first of all there was very little of physics even. Then each of them had something. There was Prytz’s laboratory which was an experimental one. Really also these people were very fine people. He had thought very much of measurement problems, and how to weigh various things with utmost accuracy. And he has written also a book about these things; and he had made very subtle remarks. I can tell you a little, but that's something else, because that played a part when I came to England. He invented the following experiment for class work, for showing things. He had a little lantern with a light in it — an ordinary candle . It was hanging by a thread, and then he cut the thread. Then it falls and the candle goes out — at once — as soon as it falls. Now this is a most interesting experiment — why does it go out? The candle only works when there is circulation of the when the fresh air comes in underneath. But if you drop it, then air, there is no longer any gravitation. And the current stops and it goes out. It was a very, very interesting experiment. It was a little ex- periment, but Rutherford got so interested in it when he heard about it that he wanted to try it once. But that was a very small part of what they had in Cambridge. They had at Cambridge these big (jars at) the mathematical tripos, and there was also physics. And there were any number of fine experiments from Maxwell's and Rayleigh days. And they were sometimes very beautiful. For instance, they had a round target up at one corner of the ceiling. And then you have a gun which you aim at the target. Then you connect the trigger of the gun with the target, so that when the target falls the gun fires. But the bullet falls with the same velocity, so it always hits. It was very, very beautiful. These things that I have told — they all belong to the nice things (from my memory).

Rosenfeld:

The fact that this Gold Medal subject was about the superficial tension of water — was it just the result of Christiansen's experiments?

Bohr:

Yes. Christiansen was just thinking of what was interesting. He had read a paper of Lord Rayleigh where Lord Rayleigh had developed the theory of jet vibration. I took it up, and one other did also. That was (Pio Pedersen). We got two very different answers. But I took it up also the theoretical side, you see. When the paper was actually finished — and it was intended that it should be published — I saw that the Transactions of the Royal Societywere the right thing. And after some time I got a letter from them written by Lamb, the mathematical physicist. Lamb said that there was something wrong with it. You see, I had considered the problem both as regards finite vibrations, but also as regards friction, just to see how great this effect was. Friction is difficult work. But there is a correction which is proportional to the square root of the coefficient of friction. And Lamb thought that in every case the correction would be as the square of (the decrease), you see. But that's not true. At any rate there was this correspondence with Lamb, (and they gave in and) it was published. But the whole point was to try to do the experiments in a way which fitted best with the theory. And there were a number of things to take into account. And. I had such a very long (jet), and so on. Pio Pedersen did splendid work, but, I mean, (it was not right). ... I calculated out the motion for a finite vibration and then it comes that there is a change in it. [Bohr illustrates at the blackboard.] These are very small things. But then, you see, I studied the vibration of a jet at a distance from where the jet emerges. And that meant that all such things went away. If you are very close to the orifice then, of course, you will have the inner part of the jet going quicker, and so on. Now, this is not so very interesting, but I think it may still be the best' examination of the surface tension, in water. At that time there was very great interest in such things; now there is no interest in it. I don't know why, but so it is. But as Rosenfeld asked, in that way Christiansen meant very much to me, at any rate, in setting this question in the Academy. I was very young, you see. I think I was a student of two years or perhaps one year. I answered it, you see; but it took one and a half years for doing it. And that time was used to the utmost. And also Pedersen's work was very beautiful but it was not too accurate in the theoretical aspect.

Rüdinger:

Was it expected by Prof. Christiansen that you should do the experiments as well as the theory?

Bohr:

No, I don’t think so. I don't think so. I don't think it was to be, you see. But that is also a possibility. But another thing was that he knew that I was interested in surface tension, you see. Christiansen again a help in that he proposed that when I went in for the examination, I should have a paper. He also thought there were many interesting things in the electron theory of metals, so he just gave me that. But out of that came something very different. I went into it not as — this is very sad to say — not as a student, and a lot of odd things came out. You see now all these things are very difficult to say so quickly because it's quite a long thing, but we can do that another time. We have time.

Rosenfeld:

May I just ask one thing about this Copenhagen time. About Knudsen — Martin Knudsen — was he older?

Bohr:

Yes, he was older. He was much older. I think he was at least 10 years older than I, but perhaps it was 20. You see, he died many years ago. I suppose he was 20 years older than I. And be made these very beautiful experiments.

Rosenfeld:

Was that at the Technical High School?

Bohr:

Yes, it was. Christiansen had, in his way, touched upon so many things. But he had only done a little with it. But Knudsen actually took it up.

Rosenfeld:

Oh, that was Christiansen's experiment?

Bohr:

Yes, but then Knudsen made something out of it, you see. He actually used these low pressures. But Christiansen had another thing which was very beautiful. He had two prisms of glass, and he pressed them together. Then one can alter the law by which the gas diffuses if the distance between them is smaller than the mean free path. So there was an interest in these things. But Knudsen was very different from Christiansen. Knudsen was something wonderful. But Knudsen actually was not known when I was a student. All his work was really coming out when I came back from England (or had come out while I was gone). And I had, of course, some insight in the calculations, and so on.

Kuhn:

We are on our way back to Cambridge, but I do want to ask you still this one question in my mind. Why was it Cambridge that you picked?

Bohr:

Oh, but first of all I had made this great study of the electron theory. I considered, first of all, Cambridge as the center of physics, and. Thomson as a most wonderful man. It was a disappointment that Thomson was not interested to learn that his calculations were not correct. That was also my fault. I had no great knowledge of English, and therefore, I did not know how to express myself. And I could only say this is incorrect. And he was not interested in the accusation that it was not correct, and so on. And actually it comes to this — if you read my paper carefully — that it is more difficult to say what is correct, you see. There were really so many blunders. But what I mean to say is that Thomson was a genius who actually showed the way for everybody. Thensome young man could make some things a little better. But that was not what interested Thomson. In the years after this he was interested in his own atomic model. did that in the same manner, you see. Things needed not to be very correct, and if it resembled it a little then it was so. So it is very difficult to say what was the interest in Cambridge about Thomson's model because there was among the students in Cambridge nobody who was really interested in theory. Not, at any rate, in that kind of theory. You see, I had a lot behind me, and that they didn't know. They didn't understand that. We will come gradually to talk about that. The one who came then when I was there and who was a few years younger than I was the very gifted young man, Willy Bragg. He was the son of William Brag. He was called Willy — his name was really Lawrence. And he took up — but that was also the next year — the problem of the Laue discovery. And he really showed how to do the things you see. I mean, Laue's was the same kind, of thing. Laue was correct in this, that it is clear that it was interference, but how to produce the interference and what to do, was not correct. And at any rate it was mixed up in a very odd way. And he thought some of the lines were due to coincidences, but they were due to regular reflections from the layers, and all that was cleared up by young Bragg. That was the reason then that the older Bragg took up the whole problem together with his son.

Kuhn:

Then it was really the younger Bragg who did the theoretical —.

Bohr:

No, no, not only. Because the older Bragg was marvelous, you see. He was really a great man. He had not done very much until then, but he has made these beautiful experiments on alpha rays. He found the range of alpha rays. And he was very interested in the quantum theory, and argued that light was particles. And he was quite definite about it. He was the only one in England who took any interest in quantum theory. That's also the next year, so we will come back to it. Jeans had a special interest which we will also come to. But then due to Laue's discovery and his son's work he just changed around and took a view that at any rate, as far as these phenomena could be understood, they were due to waves. I’m glad things go so smoothly today. But then the first point comes — what was the difference between Cambridge and Copenhagen? And there was, of course, a lot of difference. But in my case it was not so simple. I attended the lectures of Thomson and of Larmor, also Jeans. But Thomson thought that he ought to give me an experimental topic. The problem was a certain thing he had seen in gas discharges which seemed odd to him. There was absolutely nothing to be gotten out of it, and we will come to that in a moment. And I worked on it, but there was really nothing to get out of it. And then there were the other people. But in England people are not so simple. In general, English people are very good in some ways, but it takes time. And they thought, "Now comes this foreigner —." And I perhaps also spoke as though I understood something, and they thought that was nonsense. And I actually had no friends among the younger students. I had a friend in Owen, who was from Wales, but that was gradually — that was just a general interest in life, or so. For instance, Whiddington was himself very, very fine, but he thought that it was just a nuisance to have somebody there going around, and so on. He had some nice results already before I left.

Kuhn:

Did you talk about them?

Bohr:

No. Although I was really interested, I just heardabout them. So actually I felt rather unhappy in Cambridge. I had some friends, but they were friends of my father. There was Professor Dreyer in Oxford. He invited me to come over there and talk with him , and he said that I should talk to Rutherford. And after seeing Rutherford in Cambridge, I went up there in the autumn to stay with Professor Lawrence Smith. He was a very nice man; he was a kind of pupil of my father. He had been in Copenhagen (doing work). He invited Rutherford, and I talked with Rutherford. We talked about the Solvay Congress from which Rutherford just returned ... but we did not talk about Rutherford's own discovery. I felt that it had to be that way. But when I saw the actual reports [of the Solvay Congress], there was absolutely no work of [Rutherford's discovery in them]. And that we will come to because it has some connection with things. I told Rutherford that I would like to come up and work and also get to know something about radioactivity. And he said I should be welcome, but I had to settle with Thomson. He wouldn't take any student away from Thomson. So I said I would when I came back to Cambridge. — I'm not sure whether this occurred toward the end of 1911 or the beginning of the next year. But I think it was toward the end of '11. — I just said to Thomson that I had only a year now in England and should be glad also to know something about radioactivity. Thomson answered me as though he had not listened at all. He said he knew that nothing could come out of the thing he had given me and that he would give me something else. But I was not interested; I had come to be so deeply interested in other things. So I said that it had been a very nice time but that I wanted also to know something about radioactivity. So that is really all that there is to be said about the Cambridge days. I tried to read various things; I liked Larmor's book. I don't know whether you have read it. I think very, very few have read it; perhaps Rosenfeld, who reads most things, has read it.

Kuhn:

I certainly have not read it through, but I've looked at it.

Rosenfeld:

It's a wonderful book, yes.

Bohr:

Yes, but he really has this gift of making things difficult — very much so. But his treatment, I think, of the Zeeman effect is just wonderful. It's done, you know, in this way, that if you say that the system is symmetrical then in a magnetic field the whole thing will turn around the axis going through the symmetry center because the Coriolis forces due to this rotation are exactly the same as the magnetic forces.

Kuhn:

Do you remember other things that you read at that time? And what some of the subjects of the lectures that you attended were?

Bohr:

Oh, you see, there was a lot to do because I tried to get my dissertation which I brought along, translated. And there was a man who was later Bishop who was a tutor in the Cavendish. He took a very kind interest and so on. But Thomson did not take so much interest in it. And it is a very unhappy thing, but it was left aside when I got so much more to do, you see. We'll come to that; in Manchester it became a very, very busy time in the three months I was there. But in Cambridge I read, for instance, the whole Pickwick Papers, and I looked up every word. I thought that was a way to get into English. I looked up every word which I was not absolutely sure of how to express in Danish — even if I could tell what it meant. And then my brother came on a visit, and also my brother-in-law and so on. And so there was a lot to do, and then, of course, I (read) about these various things. Actually, I got a rest. I didn't learn much at Cambridge. But then I was prepared in Manchester and ready to throw myself into the work, you see. We can go to Manchester now perhaps.

Rüdinger:

I have one question. You attended some lectures of Larmor's didn't you, in Cambridge?

Bohr:

Yes. And they were very nice, but they were variational calculus — it was the elements of variational calculus; very interesting points. And I also had some discussion with Jeans; they were also not very fruitful because I do not think I expressed myself very clearly in English. Jeans was very nice in that when I came to Birmingham [ in '13] then he had read my papers and he took it up with great interest. We must get what Jeans has written about the spectra. Have you read that?

Rosenfeld:

Yes, a long time ago.

Kuhn:

I know of it, but I have not —.

Bohr:

Yes. I also had it, but I don't know where it is now. Perhaps it is at the Institute. We must get that or buy it and so on.

Kuhn:

Did Thomson also lecture at this time?

Bohr:

Yes. Oh, Thomson gave the general lectures once a week, or two times a week; I think it was once a week. That is always the Cavendish professor; so Rutherford also did until perhaps when he was old. But I'm not sure that he didn't do it all the time.

Kuhn:

Were there colloquia or seminars at which research problems were discussed?

Bohr:

There were some meetings at the Mathematical Society, or the (Philosophical) Society, as I think it was called. But I do not remember anything of interest then, you see. Later I came back and gave a number of talks myself. But actually the whole thing was very dead there. Thomson had been the great leader in physics, but he was probably tired. And then he gave one lecture most beautifully in the Trinity College about golf, where he very beautifully showed how it is. First you had, to have a ball, then one has a man and he can hit it, and so on. And then you try to calculate how great a force the man has, and one finds that the man can hit it several hundred meters, I think, or something like that. perhaps not. But actually a golfer can get it to fly many, many times as far. So what is the reason? It comes from the rotation, you see. And then he used two (strings) and had the ball in between, and then he showed how one can get the ball to go around a (pillar). But the whole thing is so interesting. And the point is that if you give the ball (under) spin, then you can for a long time take the gravitation away from it. And therefore, actually the path of the ball for a long stroke is this: it goes and then rises and falls again because the spin holds longer than the other things. That was a great pleasure to listen to. But that is really the main thing I remember from the Cambridge days.

Kuhn:

I would like to ask you one or two other questions before we let you go ahead to Manchester. There is the paper of J.J. Thomson from the Philosophical Magazinein April, 1912, which you cite both in the first the papers on the constitution of atoms and molecules, but also early in the alpha scattering paper. That work must have been going on just at around the time you were at Cambridge. I wondered whether you had gotten involved with the problems of Thomson’s atom model or with the sorts of problems that come up in the paper.

Bohr:

[Reads] This is "Ionization by moving electric particles."

Kuhn:

There is some reason to think that this whole way of thinking itself was familiar to you at the time.

Bohr:

Yes. Surely it was, you see. I'll have to read it, you see, to see what's in it. But the thing was that as regards the relationship between Thomson and Rutherford, the discovery of the nucleus was a very sad thing. Thomson had, in his work on ionization and on the scattering both of alpha particles and of electrons, analyzed the problems marvelously, you see. But only in the first approximation, as everything was in Thomson's [model]. ... But these things were done so well; he had even considered single scattering of the beta rays. You see I will be very glad to go through that with you. But Rutherford then showed that part of the scattering of alpha rays — which Thomson has also treated in a very interesting way by the compound scattering — was due to single scattering. And really a very large part of it was, but that depends on what you experiment with. Then Thomson didn't believe it. And that brought Thomson out of physics, you see. That was a very, very serious thing. Thomson was absolutely against it. He wrote various papers against me saying that this is nonsense, and that if you just change the forces you can get anything, therefore it's not true. He was very clever. This was a very great calamity, and that lasted for six years. When Rutherford came to Cambridge, Thomson wouldn't even believe in the isotopes; he didn't believe in Aston's experiments. Aston's work was so finely done. And so that is a very odd thing that it should happen. ...

Rosenfeld:

But what did he say himself of the neon, of those parabola?

Bohr:

I don't know. We can also come back to that, you see, because there was a discussion between Thomson and me in Birmingham, a year later. You see it was really me who got the idea of the isotopes; and I also them to make experiments in the proper manner at Cambridge, and then, of course, it came out. But that's another matter. Shall we go on to Manchester?

Kuhn:

I would be glad now to go on to Manchester, or more particularly if there are some things that you remember of those first conversations with Rutherford, when you first visited him, about the Solvay Congress —.

Bohr:

Now all of this today has been very general, but there are some points in it. And this about Thomson is another thing and one need not write it up. You see Thomson went out of physics, and he never came into it really again because he objected to the only progressive way. He was even very difficult to Rutherford sometimes. But that's something else, you see. I loved Thomson; Thomson had a very interesting mind. I visited with Thomson when I came —. And I shall later tell you some wonderful stories about (Dirac) which I heard from Thomson. That was later on, in '27 or so on. (Thomson has probably forgotten these things), but he was out of physics. It really brought him, with one stroke, out of physics. But that is clear; that is not such a difficult thing, you see. And in some way, for instance, but in quite a different manner this thing also brought Lorentz out of physics. Then Lorentz worked on the dikes in Holland. And he did the enormous work on the theory of the motion, and so on. And he was even so wonderful in the Solvay meetings. But I doubt whether he went in for anything. He may have understood what we were after with dispersion, but again that was a year before his death, and so on, so it's very difficult. He was also very old. He was just marvelous and friendly. Thomson was also very nice when he got over it; but he was simply difficult to Rutherford. He felt that the young people shouldn't say this and that. "I know so much more about it and have thought about it" and so on. It's very, very difficult to understand, but so it was often in England. For instance the man who went to Oxford, Townsend. He had been one of those who developed the whole theory of ionization which had been the main point at Cambridge. But first of all he got to hate Thomson; it was most unreason able you see. And next he even thought that such experiments as Franck's were nonsense. And he also went entirely out, but he probably would have gone out of physics anyhow, you see. So it's very difficult, so we'll leave this sort of thing.

Kuhn:

Was there no one then at Cambridge who took the Rutherford atom seriously? Were you the only one who responded well to it?

Bohr:

Yes, but you see I did not even respond to it. I just believed it. But when I came up there I took it up. And that was also because I lacked some information, and that information I got from Hevesy. If I tell for about half an hour about other things, then the whole thing is in disorder, but —.

Kuhn:

That can all be put in order later, by all means.

Bohr:

Actually it was not a success in Cambridge, although I learned a good deal of English and also of mathematics. And also, of course, I read what there was to read, and I certainly read these things. And then, with my own work, I just felt that now that Darwin has tried to bring it over to the Rutherford atom that has to be done properly. The errors which were still in Thomson, he has kept. He has kept the whole thing; there was absolutely nothing new in Darwin except calculations. And therefore that came so. But that was a few weeks, at any rate, after came up there. When I came up there I can say that Rutherford was very nice and arranged that I could take part in courses by Geiger, who was a great expert, and had actually done this work of the scattering of the heavy atoms. But it was so that just a few weeks later on I said to Rutherford that it would not work to go on making experiments and that I would, better like to concentrate on the theoretical things. So that went out very smoothly. But in Manchester I had certainly an important experience, and that was that I met Hevesy. And Hevesy was most extraordinary. Hevesy was, first of all, very (???),but he had such very fine manners. He knew how to be helpful to a foreigner, you see. In Manchester — I can just tell this, but perhaps it will not go in, you see — it was so that I came into the laboratory. And I think Marsden and Geiger were kind to me in these experiments, and so on, and showed me —. But people in England, for good or bad, they are different. And they thought now there has come someone who just wants to talk, and so on, or not to talk. So actually, they avoided me to begin with you see. But that is very difficult to understand because that was not the intention; it was just that they were busy, you see. But Hevesy just asked me about things, and therefore I learned, as I have written in the Rutherford, various things from him. I didn’t know anything about radioactivity; I had, of course, read in Cambridge a lot of Rutherford's work and so on. And. I read also even before I came to England. But the point was that Hevesy told me that there are more radioactive substances than there is room for them in the periodic table. And that I didn't know. But that was, to me, absolute evidence of this: that then one had the isotopes. And I'd like, therefore, just to speak a little bit about it because it's very different from what one thinks. It meant that if one has a system of a point charge with different masses, then the heavy particles move so slowly that one shall expect all of the physics to be exactly the same in the two cases. One must separate then between those things which come from the electron system and those things which come from the nuclei. From the nuclei come the radioactive things and all the things which have to do with the masses of the isotopes, but all the rest is the electrons. And this is a very interesting thing. And that was the first point. The next was that it immediately followed that one had the radioactive displacement law. But Hevesy sometimes said to me that I expressed myself in an odd manner. In discussing argon ... I said, "It's the wrong Argon," and, of course, he gradually knew what I meant, you see. I felt that it was a complete accident with the atomic weights, and, therefore, argon actually should have an atomic number one greater — or is it one less; we'll just find it out. But you see I got into it really — and now this is wrong actually — I got into it really deeply. I understood that the atomic weights were just accidents and that one could expect, within a certain limit, anything. And that is what I refer to, but I do it in a very simple manner. Have you read the Rutherford Lectures?

Kuhn:

Oh yes.

Bohr:

When I went to Cambridge, Thomson actually didn't believe in these things. He thought that there were some kind of chemical combinations and that you could fix the hydrogen atom onto the argon atoms, and that there was not this kind of thing. But we understood it so well, you see. And that was cleared up in Manchester; in the very beginning. That was the one point. The other point was just the displacement law. And you'll also be interested in this discussion in Cambridge where I (guessed) that something was not right, but I just took up the question of whether in hydrogen one could have what you now call tritium. And then I saw that it was a way to show this by its diffusion in palladium. Hydrogen and Tritium will behave similarly but the masses are so different that they will get separated out, you see. Now I'm quite sorry really to have gotten into this, but you see in the first weeks in Manchester I found these two things. And just from that moment I understood.

Kuhn:

This was really the first weeks there?

Bohr:

Yes. And I went to Rutherford, which I have written. But then Rutherford said (I should just as well say it correctly), so actually he thought that that was extrapolations. He thought that this meager evidence about the nuclear atom was not certain enough to draw such consequences. And I said to him that I was sure that it would be the final proof of his atom. And then it was that I got so much to do in Manchester. I then also worked in between just to see how it fitted in with the quantum because the point was that here we had an atom which was unstable — that any system of point charges is simply unstable. I don't know if you know how it is, but it is just this, that if you have point charges at rest — how ever you have them — then you can just expand the system. And, therefore, if you have one equilibrium you have any number, of any size. ... But if the charges have velocities then you have something corresponding to the radiation; but the radiation just gives rise to the dissipation of energy and that also doesn't give definite configurations. So was the next thing that comes out at the same time. But this was the point. Then gradually the whole thing got disturbed. I wanted to publish these things, but then I came into the other things about the spectra. And, therefore, I left it until that was done. And then the various things of Fajans and Soddy came out of it. Both Fajans and Soddy took. the absolutely opposite view about the atom. And you must read that. ... It was clear, and that was the point in the Rutherford atom, that we had something from which we could not proceed at all in any other way than by radical change. And that was the reason then that one took it up so seriously. But that solution came first just about half a year after I left Manchester. And. I had very much to do because I became then for a short time the assistant of Knudsen. Knudsen was then professor after Christiansen. And that was very nice, but it took all the time, you see. So I went to Knudsen and said that I would rather not, you see. And I was then out in the country actually and was writing the whole thing up. I had written a very large manuscript of all kinds of things. When I went back to Copenhagen, then I came into this (use) by means of the combination rule and so on, and as soon as one saw that it was clear. There could be no doubt, you see. There might have been doubt in many minds — but there was no doubt about it. But let us just see what more we can say about Manchester. The Manchester time was very interesting. In some ways it was similar to the Cambridge, you see. It takes a half a year to get to know an Englishman. You see, they were very different. In America the style is different. And now perhaps also the style is changing in England. But it was the custom in England that they would be polite and so on, but they wouldn't be interested to see anybody. For instance, I could tell you that when I was in Cambridge still, then I went Sundays to the dinner in Trinity College. And I sat there among the advanced students. The Fellows were at the high table; then the advanced students had another table; and then the students filled up the hall. We had one course more than the students, and the Fellows had one course more than we. And before we went from the hall, the students were away; and before the Fellows went away, then the room was empty and dark. But now, I was sitting there, and nobody spoke to me ever in many Sundays. But then they understood that I was not more eager to speak to them than they were to speak to me. And then we were friends you see, and then the whole thing was different. And that is a very, very general experience from that time in England. I don't know how it is now, perhaps that is still so, but perhaps it isn’t.

Rosenfeld:

It isn't, no. It's certainly not that way now.

Bohr:

Everything has changed and especially through the connection with America. Both countries have changed each other and so on. It is different. But at that time either one had some connection or he hadn’t, you see. My father was once upon a travel to South Africa with the British Association. And, there was an Englishman, and they went to the bathroom in the morning; and the Englishman wouldn't go away from the other, [i.e., let Bohr's father into the bathroom first. ] But my father kept standing, you see, because he felt he was also not afraid of the Englishman, and then they became very close friends. So there are small differences between people. And, also in Copenhagen you see I was in many ways concentrating on something and therefore I didn't come into the ordinary students' life. And I do not know, therefore, how the students' life was and whether there was much life at that time. But then, for instance, Darwin, when he understood that that was with the calculation of the thing, he appreciated it very much, you see. Although it was a disappointment to him because it was his first work in Cambridge, and it really contained nothing. And Thomson was very critical of it in Cambridge.

Kuhn:

What sort of criticism did Thomson make of it?

Bohr:

I don't know. But, at any rate, I think Thomson wouldn’t notice it — wouldn't give some prize for it, or something like that. I will try to get to know that. But that's only examples of how things were in England. On the other hand, things are wonderful in England; if you become friends with a man, then he doesn't know what not to do, you see. But Hevesy was really a fellow in that he had not that way of the English. He was a Hungarian nobleman, you see, who had great experience in treating people, and so on. And he just showed some interest in me, you see. I think Hevesy was a little younger than I, but he was almost the same age. But, of course, I knew so much more about general things than the others. Most people knew very little in Manchester. (And then I would always start by saying) that Rutherford was, of course, the simply wonderful leader in research. But one cannot expect everything. And he took such interest in all the experimental work, and also in many other things. But he was just not prepared for the idea that one could draw such conclusions and so on. And the whole thing was very short, you see; so that was the reason. He was very nice, and then when I made these things with the alpha Particles, he was very nice indeed. And he took great interest in it although he had then the busiest time of his life because he was working with Geiger on a new edition of the whole book of radioactivity. Anybody else would have had to isolate himself, but he also kept the laboratory and so on. But he was not prepared for everything, you see. So that was a very curious thing. But then I married in Denmark in the first of August, and we went to England to see Rutherford and to deliver the final paper about the alphas. And they were very, very nice. And it was just natural that a man would say, “0h, that is too much," and so on. But the point is that Hevesy meant that what I did in there was really some hind of contribution to atomic chemistry and that it changed the whole thing. And that wasn't known in Manchester, and so on; then, of course, it came so. But then there came various troubles later that we can tell about.

Kuhn:

Having talked this much with you about it this must have been part of the background for Hevesy's own work on the alpha part of the displacement law that comes out early in 1913.

Bohr:

Yes. Yes. That was the whole background. You see this thing is difficult because if one knows this then it is clear from facts. But that was the way he came into it and that was the reason why he thought that he ought to tell about it. It was that he was in Copenhagen then, an, that was the 10th anniversary in 1923. That issue of Naturwissenschaftencame to me as a present. This was the main paper, you see, but Kramers also wrote a paper. But this is difficult because it's not my meaning to be pretentious, you see. First of all, it was wonderful, and I got so much out of it, but it was just as it came.

Kuhn:

Is there any likelihood that conversations between Hevesy and others also transmitted this point of view to Soddy or Fajans?

Bohr:

Certainly. He transmitted it both to Soddy and Fajans, you see. But they needed only a little, you see, because they knew a lot about it. But Hevesy was collaborating with Fajans, and Fajans took it from him. I mean, also what he did himself, you see, because there were a lot of investigations, and they tried actually to get everything to fit and to see how various substances behave, and so on.

Kuhn:

They were in touch with Soddy also?

Bohr:

You see, that I don't know. I think it would be very nice if you could have Hevesy here. Now Hevesy is not well now. He was here last year; he was given a prize, as you may have heard about. But it would be nice if you saw him.

Kuhn:

In fact, I've briefly already seen him. But if he is well enough to able to consider coming here later, I would love to see the two of you sit down together and talk over that time at Manchester.

Bohr:

The only point that I feel very strongly is this: that, when I came into these things, I really understood what there was to understand about it. There was nobody else who took an interest in such things. came then very soon after. Partly there was this discussion in the laboratory, and partly Russell and (Rossi) did something themselves. But I explained to Russell that the two Ioniums are two substances which are isotopes. ... I said that that is obvious that the spectra are the same. It must be exactly the same in everything. That they didn't take it so accurately or perhaps they left it out and so on — if so, it was that they were not saying it, you see.

Kuhn:

But actually you think you had talked to Russell before those experiments?

Bohr:

No. When the experiments were under way they were troubling that they couldn't get the spectrum, and I said that it is obvious that you can't get any spectrum because they have to be the same. I would also like to speak to you about how it was then with Aston. But that is not the point because if I had not come into this problem of the isotopes then it would have made no difference when I eventually got to the hydrogen atom. But it was actually the reason that I felt we have now got some knowledge of the atom, and now we must see how it fits. But that was a great amount of work, and I actually tried many things.

Kuhn:

I should think this would be an excellent place to stop for the day. And I think when we meet again on next Wednesday at 10:00 we should really work again, if it's agreeable to you, sir, from substantially these same notes. And I'd like to know more about that first face to face meeting with Rutherford that took place at Lawrence Smith's and then the development of your own work at Manchester. And we have talked about some very important points in that period, but I should, think we could talk in vastly more detail and talk also more about your personal reactions in Manchester.

Bohr:

Yes, yes, yes., And then there was also this thing that Rutherford in some way wavered, you see, when my atom was out. Then I got some letters — and I have also written about them — from Moseley. Have you seen them?

Kuhn:

Yes. I have.

Bohr:

But these letters show no resemblance to the way that Moseley published the thing. Because there he speaks about Nicholson. And I think it was that Rutherford felt that now that we have got such wonderful results, we must not mix them up with theory. ... And he even spoke to me in one of his later years and said that he felt that I had had much more to do with the atomic number than van Broek. And. I said, "Of course, I had," you see, but that was now not a point to speak about.

Kuhn:

Did you get to know Moseley well at Manchester?

Bohr:

I got to know Moseley really partly in this discussion about whether the nickel or cobalt should be in the order of their atomic weights. Nickel and cobalt was one of the outstanding points. Mendelejeff had such deep understanding that he actually changed nickel and cobalt around in spite of their atomic weights. Moseley came to me and. Darwin and he asked what I thought about that. And I said, "There can be no doubt about it. It has to go according to the atomic number." And then he said, "We will try to see." And that was the beginning of his experiments.

Kuhn:

But that conversation would have been in your first Manchester visit?

Bohr:

No. No. That was much later. First of all the hydrogen spectrum was out, the helium spectrum was out, and then in the summer of 1913 then I had this conversation with him. And then he did it at tremendous speed, and then, of course, he died, so things were so difficult.

Kuhn:

Had you known him at all during the first time you were at Manchester?

Bohr:

No. I had known him, of course; everybody knew Moseley. Everyone knew who was there, and so on. But it was that time when I came over with the second paper to Rutherford; it was something like June '13. Then I heard this. But I had, never really talked with Moseley. You must understand; it was not so simple — the world was different. First of all, everybody was extremely busy — that is everybody around Rutherford was. Rutherford carried on this enormous research program, and he had people to work on all kinds of things. And, therefore, they were very busy. And then if a man had some odd views, it didn't interest them. But it was a help to me that Hevesy took it seriously and really advised me, as I have written there. And then I came into a simple view, and had absolutely certain the isotope and the displacement law. But that was there all the time. Then the alpha ray paper was a few weeks later, but then I didn't put everything in it. I just gave something which was obvious and also how I used Whiddington's results and so on. But I thought now that the paper with the displacement law would soon come out, and so on. Actually it came out half a year later, but it was that it took a little time — it took a half a year — to find the clue in the combination law...

Rosenfeld:

But Darwin for instance; what was his reaction to the alpha scattering paper?

Bohr:

The scattering work that was obvious . He thought that his own work didn't contain anything.

Rosenfeld:

And the use of the quantum that was made in that work, what about that?

Bohr:

No, but that was not so important. And next it was that when one knew that one could not possibly do anything except use the quantum theory —. There was very little interest in the quantum theory you see in Cambridge. Even Bragg's was also a bit later. There was only this interest that Jeans thought that he could get out of the dilemma.

Kuhn:

And still in 1911 Jeans thought that this was the way that it had to Work out?

Bohr:

Yes. Absolutely, yes.

Kuhn:

Still Rayleigh brings it to the Solvay in 1911.

Bohr:

In writing about the Solvay Congresses I have not written what my own views were because that I felt was difficult. But it was clear that it couldn't be done.

Rüdinger:

Do you remember anything from discussions with Jeans at Cambridge? You didn't convince him that there was a principle?

Bohr:

No. And I don't think also that he spoke about such things: I don't know what he spoke about — gas theory or something. And perhaps we went into that, but he was not interested. He was very curious; he went away at once when the lecture was over so as not to see anybody. But he was so nice, for my papers were out in the first of April of '13 and by the time of the meeting in the autumn of '13 he must have read it all. Then also perhaps he understood that I was not a complete fool. I do not know what he thought. Then he suddenly took it up as something definite. ... Jeans was not believed at all, but he was the first to express any interest outside the Manchester Group, and the Manchester group was very doubtful. If one reads how it is actually put in Moseley's paper, then you see that it doesn't resemble what we have spoken about. But now, this is another thing, you see. And we must always take care that all such things shall not come into the paper; at any rate, it must be done in the proper manner. It is just as it has to be; it is quite different. You see, it is very different from the ordinary work in physics. There one bases one's work on the (formulae), definitely. Then, of course, there can be new ideas, but then these ideas are, at any rate, clear from the beginning. When Heisenberg came here, and also Dirac, they did wonderful things, but every word in any of their papers was obvious to me. Not that I had made it myself, but it was obvious that this was what we were waiting for. But then, of course, there were different groups. And then another (???). But as long as one had then the same group then it was very different. Now it was also, of course, different in Copenhagen because it was the given subject. And I tried to be helpful all the day you see with anybody. So it was different. But a wonderful thing really was the solution. ... Then the solution — epistemologically — settled on the distinction between the wave and the ' particles. That was a point which was missed by Heisenberg. You have seen Heisenberg's first paper perhaps. And that was easy to miss, and so on. Now I don't know if you agree, but this complementary point of view is really obvious, you see — it's completely obvious. And that is, therefore, the solution of the thing. Bohm, for instance, writes that we can measure a lot of things, but he has misunderstood because one can measure anything. One can measure the moment of an electron and its position just by letting it fall on a photographic plate. Then we know where it is, and we also know what the momentum is there because we can measure the momentum before. But all of these things are of no interest; the point is that in any experiment then you must use the space time relations, or energy and momentum relations at least two times, otherwise they are not defined. And. then that is what the whole thing comes to. It's only that you disturb the classical relations in the' definition, and then that is all that we need and want. Then I thought perhaps on Wednesday we could go through some of the other parts where I was alone, you see. Then Sommerfeld comes in.

Kuhn:

I still hope you will be willing to return do more with the Manchester.

Bohr:

Yes. Oh, but I shall be very glad to. But I have now almost said the essential things, you see.

Kuhn:

There are still a lot of things I should like to ask you about that and about your own work there.

Bohr:

But I was you see — from my dissertation on — used to working quite alone. And with the dissertation I had no discussion with anybody. And Christiansen didn't care, you know. lie was not interested. He was afraid of influencing people. And so actually in my dissertation I have not any talks with anybody. And so it was also in Manchester. On the essential things I was quite alone.

Rosenfeld:

But did. Christiansen give you guidance in the literature and so on? For instance, when you started on this electron theory, you found all the literature yourself?

Bohr:

Yes, you see, I think it might have been. You see one had the name of Drude. Perhaps Christiansen said that Drude has written something about it, or perhaps he didn't. It was that the literature was clear.