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Oral History Transcript — Sir Nevill Mott

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Interview with Sir Nevill Mott
By Thomas S. Kuhn
At the Master's Lodge, Gonville and Caius College
May 13, 1963

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Nevill Mott; May 13, 1963

ABSTRACT: This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Hans Albrecht Bethe, Patrick Blackett, Niels Henrik David Bohr, Max Born, Paul Adrien Maurice Dirac, Arthur Stanley Eddington, Eileen Rutherford Fowler, Ralph Fowler, George Gamow, Douglas Rayner Hartree, Walter Heitler, Frederick Lindemann (Viscount Cherwell); Universitšt GŲttingen, Manchester University, and University of Cambridge.

Transcript

Kuhn:

I had not realized quite how far out Oxford had been.

Mott:

Oh, Oxford was completely dead until 1933 when Lindemann went across and collected the cream of Germany. It's the one good thing he did, that man.

Kuhn:

I'm not clear why there was so little in London.

Mott:

I suppose just bad appointments. I don't know why; Manchester was alive, but rather on other and different things. Of course it had been very alive when Rutherford and Bohr were both there.

Kuhn:

I take it that part of this problem for Manchester, for London is one that I raised with you in the question sheet (and one which I wish you would expand on) and that really has to do with the relations between natural philosophy as experimental physics on the one hand and applied mathematics as theoretical physics on the other. I get some impression that Cambridge was perhaps the only place in which there was — a fairly easy bridge back and forth between the two, and that physics in the other places meant predominantly experimental work and was therefore not in a particularly good position to follow what was going on in the more theoretical development. Does that make any sense?

Mott:

Well, you asked what other places were there. Oxford, for some reason which I don't — well, because of bad appointments. Lindemann was a politician, a courtier, and very soon became disinterested in physics? And that was it. He didn't build up the school or attract people to him. As I said, I don't think there was much in London, though it's a question of the time scale. English physics was revivified in the late 1930ís when all of Rutherford's young men went and founded schools in the provinces. Best thing that ever happened. But that didn't happen before because there was this big gap — [Interruption] — between J. J. Thomsonís bright period and Rutherford. Then there was the first war which killed some of Rutherford's bright men. It was only in the late 1930ís that there was a group of people to go out and find provincial schools. I think it's as simple as that. There were so very few people before to go — out and do it. If you haven't lived in that time you wouldn't perhaps appreciate the difference between the huge numbers of physicists running around in all directions now and the tiny numbers of first-class people, extremely small elite" in the late 1920's and early 1930's. In America the difference must be even greater, but there was the Gottingen school you know" the German group, plus the Cambridge group, Copenhagen, and then there was Fermi; but the numbers were quite extraordinarily small. In theoretical physics in Cambridge, if one or two people did anything seriously a year, that was good.

Kuhn:

How many people were there doing theoretical physics when you were a student; I mean by then it was already appreciably larger.

Mott:

I went to Manchester, and then I came back here in 1930.

Kuhn:

Iím thinking of your period as a student though.

Mott:

Well, we did the Mathematical Tripos; and specializing in theoretical physics, letís say quantum theory, I should say there were about six.

Kuhn:

Well, when you say six that were specializing in quantum mechanics, if you would rake in the classical problems also doing physics, what is now to me doing physics rather than mathematics but taking the Tripos, which would be more than six. There must have been some people taking classical problems.

Mott:

I can only speak of the people I remember who took theoretical physics and wanted to go on to research. There was a man called (Brainard) who went to see [R.H.] Fowler; and Fowler said: ďIím going to America; come back and see me next year.Ē Poor Brainard was so disgusted that he switched over to law and is now a very distinguished solicitor. Then there was a chap called (Erstle) who got better marks than either (Brainard) or me, but didnít go so far. Then there was a chap called (Fray). I remember him chiefly — he worked with me — because he got fined 20 pounds for playing tennis with nothing on. Then there was Gaunt, who wrote a paper on alpha particle scattering and I think was interned by the Japanese in Hong Kong during the war and died. Just canít remember who was with me, but nobody that went very far. Dirac, Hartree (a little older than me) were the figures. Oh, Wilson was an exact contemporary of mine.

Kuhn:

You said in a memorandum that you wrote me that you regretted, and you thought that a certain amount of harm was done by, the separation between the natural philosophy program and the mathematics program. I wondered whether you would tell me about the effects of that separation and the difference in preparation of the people doing the two fields when you were a student. Iíve got very little knowledge really of what one would take what subjects one could take for granted that a person would have prepared in each of these fields.

Mott:

Well, I would have prepared electromagnetic theory, relativity, dynamic, of course; quantum theory, the quantum mechanics hardly because it didn't yet exist. I would have done the Bohr orbits and the Sommerfeld theory, statistical mechanics but I would not have gone for my examinations to any lectures by Rutherford; and, we learned theory before we had learned the facts, which I think was a bad training. It did then, and I think still does, tend to produce people who are interested in mathematical techniques, rather than explaining facts. But it is a curious situation which has in a certain sense lasted. We continually try to break down the fact that the natural philosophy people, their prescribed course and their examinations, do not include any descriptive, atomic physics.

Kuhn:

Roughly, just what would you suppose they'd prepare. You've said what somebody taking the Tripos with an eye to physics would take. Now what happens if one stayed in natural philosophy!

Mott:

Well, this is what I mean; if one is taking the Mathematical Tripos, specializing in what you would call natural philosophy, well, we call it theoretical physics.

Kuhn:

Well, now suppose one is preparing experimental physics.

Mott:

Oh, well, then you would do laboratory work; you would go to descriptive lectures about the structure of the atom, about heat, about wave motion. But you would do, as well as the mathematical side, a lot of facts, particularly experimental evidence. You would get questions like, what is the evidence that the atom has a nucleus and that the nucleus has such and such properties. This sort of question would be quite absent on the Mathematical Tripos.

Kuhn:

How much mathematics would the people doing experimental physics then take?

Mott:

Not enough. You can't really do anything in three years. But you're talking of the situation at that time.

Kuhn:

Insofar as you can separate it.

Mott:

Yes. Of course, Dirac, you see, started as an electrical engineer, as you must know. The real geniuses are not much affected by their education. Well, what other points —?

Kuhn:

One thing, I'd be particularly glad if you could really sit back and talk about Fowler. He clearly seems to me, from what I've been able to determine, to be the person who brought quantum mechanics to England. I can get very little information about him. Of course, he's best known now for his statistical work. To a great extent I think his role in quantum mechanics is not sufficiently represented in his publications, but rather through his students. I've got very little notion what sort of a person he was, what sort of teacher he was, how well he really knew the developments that were taking place.

Mott:

Well, may I say first of all that Fowler — without the slightest disrespect for his memory because I really revere it — was not, and never would have claimed to be, a scientist of the highest caliber, a Dirac. Well, a Dirac in the time of his —. He was an extremely competent mathematician, but again, not good enough to make his mark in pure mathematics, so he turned to statistical mechanics and worked through it with his technique, the method of steepest descents which all his pupils learned. He was a very bad lecturer. Couldn't be worse. Didnít think it out; went quickly. He had a very powerful physique, like Rutherford himself. Bluff and loud voice. Vigorous, immensely vigorous. He was, I should say, convinced very early that this quantum mechanics was it. He got the hang of it enough to lecture on it, but perhaps not enough to give a very good or coherent lecture on it as I remember his lectures. But he could pick up and tell people what problems were arising out of the Cavendish and could be worked on in quantum mechanics. As far as I remember, he didn't put me onto the alpha particle scattering. I think I put myself onto that. It was a pretty obvious thing to do with the papers coming out on scattering theory, Faxen and Holtsmark and Born. But when I showed him the paper, he read it through and was quite sure this was right. He patted me on the back and sent it in for publication. And some of the other things that were done about stopping power by other people — he knew enough of quantum mechanics to tell the students, "Look, here is a problem, here is roughly how you should do it, have a shot. Then I think Wilson and Gurney, who were applying quantum mechanics to solid state at that time, were in close contact with Fowler. Whether he put them onto the problems or not, I donít know... But he was capable of it.

Kuhn:

You speak of him as having been a terrible lecturer. How was he with research students? How are face-to-face relations with him?

Mott:

Good. [He would say]: Yes, I don't understand this bit. It's badly written. I think you should do it this way but really, I suppose you better go and ask Dirac." Very forthright, knowing his limitations and in very close touch with what was going on in the Cavendish. In that sense he was a model of what a mathematical physicist attached to the Cavendish ought to be. Someone who knows what the experimental work is and where quantum mechanics can help it. So I think what you ought to say about him is that not only was he Rutherford's son-in-law, but he was professionally wedded to the Cavendish too; in that he perhaps was the first man we had who saw, what the job of a theoretical physicist could be — not entirely to follow his own nose, but to deliberately ask, ďHow can a mathematical-physics school add to and embellish the experimental work?" You see, he'd absolutely got that aspect of it right. He certainly wasn't a good lecturer. I think possibly he didnít take the trouble. He hadn't any great lucidity. It all came out rather quick. He was quite a character. I remember his going to sleep in a lecture, given by. Professor Jeffreys and waking up half way through and saying, I've been asleep, but I know that what Professor Jeffreys has been saying is nonsense." He scheduled the Kapitza Club. I think of him rather as a man like the portraits of Henry the VIII you can see in Trinity. Very broad and muscular with a loud voice, enjoying life to the fullest. Of course, he had a stroke through overwork; but that kind of full blooded man sometimes does. But then he was only half the man after that, but even half of Fowler was quite a chap. Do I make myself clear?

Kuhn:

Yes, indeed you do, and you've been very helpful. I particularly wanted an impression of Fowler, and I've had very great difficulty in getting it. This really gives me a far fuller sense of him than I've had at all before.

Mott:

His wife died having their fifth or fourth child, I think. She was a very pretty woman.

Kuhn:

Well, now I gather, from things in the correspondence in Copenhagen that she'd not been, at all well before that either or at least that she'd frequently been ill. There are a great many references to her health and her inability to come to Copenhagen — the hope that she will be well enough and so on.

Mott:

Well, I think she always had difficulty having children, but she was a very pretty girl. I don't think there could have been much wrong with her; she couldn't have been so attractive if she'd been ill, I feel. But I don't know anything about that. She was the sort of young woman who any young man even when she had had five children, would look around at very closely.

Kuhn:

Let me ask you about Dirac's role. We take it for granted and I take it that it was realized quite early here that he was terribly bright and able. Was, there resistance to some of his stuff here?

Mott:

I don't quite know what you mean, because who would have resisted? Fowler didnít, which is another of Fowler's virtues. He spotted Dirac' as being absolutely it. As I think I told you, I really learned quantum mechanics through reading for a whole term and trying to understand (frightful) papers on radiation theory.

Kuhn:

That's the emission and absorption of radiation?

Mott:

Yes, and that was quite an education.

Kuhn:

Thatís given me quite a time, too.

Mott:

You know, another thing about Cambridge in those days there was no mathematical institute, practically nowhere to sit in the Cavendish for us theoreticians and we used to work in our rooms, in our college rooms. A terribly isolated business. It never occurred to me to go and ask Dirac. Later on I did, but not at that state.

Kuhn:

You spoke in your memorandum of one point at which Dirac pointed out to you that you were getting off the track. Do you remember what that was?

Mott:

You remember that in this collision you sum a series of spherical harmonics. I had not arranged the phaseís right so as to get an out-going wave and no in-going wave; and he said, "Look, you haven't done this right.Ē And when I did and used Fowler's steepest descents method, well, Rutherford's scattering came out all right.

Kuhn:

Had you gone to him in the first place because you weren't getting the Rutherford formula?

Mott:

I didn't know whether I should get the Rutherford formula or not. I didn't know what one would get. Oh, no, I think. I sort of remember. I don't know if you know that formula, but it has a curious phase factor in the scattering amplitude and I remember getting this phase factor and saying, ďGood God, this has nothing to do with the Rutherford scattering. My answer can't be right." Yes, I must have expected to get the Rutherford formula. And I remember walking up from the Cavendish, where my lodging then was; anyhow it suddenly struck me that of course this is a phase factor and in the square of the amplitude means nothing; it's so obvious now. Then I realized that the answer was right and felt extremely pleased. Yes. I must have been expecting to get the Rutherford formula. That's it.

Kuhn:

On Dirac, of the work there is no question. He's also quite obviously an extra-ordinary human being and one of the shyest people I think I have met. That was his role and how did it develop as a teacher, as an influence on science at Cambridge?

Mott:

He always got on much better with foreigners than with us, I think. He always seemed to be quite a total rationalist. Curious. I think I, have to say his influence was not very great as a teacher. Now and then these extraordinary bombshells came, out; the spinning electron, the positron, more or less and that was it. And he always, of course, has given this lecture based on his book with admirable character. But he never was a man who would advise a student to examine the experimental evidence and see what it means. So his influence would be on, the side of the older mathematical development at Cambridge, which results from our educational system. Dirac is a man who would never, between his great discoveries, do any sort of bread and butter problem. He would not be interested at all.

Kuhn:

Has he, in fact, had many research students?

Mott:

Not many. Well, during most of my life I haven't been here, but I don't think he has many now. I donít think heís had many. I think you would get a better answer for that — oh, I see, you want someone who has known him all the timeÖ I canít think of anybody who really can be described as a student of Dirac, as you know who has got somewhere.

Kuhn:

Dirac's way of doing quantum mechanics strikes me as being quite special. In many ways, I like it better than any of the others, yet very few people use it. The notation is different. The really fundamental part played by transformation theory is different. Has that served to isolate him from the Cambridge community as it does — now speaking scientifically — in the U.S.? People pay much too little attention in the United States to that book. Everybody admires it and nobody uses it. Has that also been true here?

Mott:

A bit the same here, though, of course, everyone goes to his lectures here: I think you ought to ask someone in the field theory outfit, someone like Richard Eden, who would tell you that better. I have the impression that people do tend to use his notation a bit more here with its elegance. But Dirac, at present, you see, is not in the swim with the enormous numbers, of people who are pushing field theory forward. He seems to be working quite off-center, as he always did. You don't see him in the Cavendish, talking to that group. I think if you want to talk on the origins of quantum mechanics you shouldn't want to follow Dirac later than the positron; will you?

Kuhn:

No. Oh, I shall try to follow him also into the point where it became clear that the electrodynamics wasnít going to respond as well as other things had. But in any case, clearly not past '34 and in terms of my own preparation, I shall not be as well off at '34 to ask questions as I am at '30.

Mott:

Yes. Well, when did I go to Bristol? Well, so far as I'm personally concerned all Dirac's discoveries just sort of fell on me and there they were. I never heard him talk about them, or he hadn't been in the place chatting about them. They just came out of the sky.

Kuhn:

Did you hear most of them only when they appeared, or did you get some of them at the Kapitza Club or Delta Squared V?

Mott:

Oh, either the Kapitza Club, or Dirac would have told Fowler and Fowler would be spreading it all over the place. In the small group that we were, I think it would all have appeared before he published it. I can't be absolutely sure of that. I'm sure I never heard anything from him.

Kuhn:

What role did Hartree play in all of this? He seems on the one hand to be someone who was deeply interested; and on the other hand, one whose own special contributions were almost all of them very much on the applied side. I donít mean engineering applied, but applied mathematics in the American sense! The term perhaps as against the one in which it means theoretical physics.

Mott:

I can't very well remember where Hartree was in these years. I know he was in Copenhagen when I was. Hartree and Gamow were there.

Kuhn:

And I think immediately after that he came back here. He'd been here until he went to Copenhagen. I think he came back here and then went to Manchester.

Mott:

Well, I know my first year in Cambridge, which was 1930. In '31 had to give the quantum theory lecture in the Cavendish, the wave mechanics lecture in the Cavendish. That was a discipline because if you didnít do it well, the students just didnít come. So I don't suppose Hartree was there. Perhaps he t d gone to Manchester. Hartree was a good teacher, very interested in it. Hurtrec, I think, would have given good courses in quantum theory wherever he was. But as you say, his own contribution was just that single thing, really, the self-consistent field. Calculating one atom after another. He used to say, ďCharming, you know; I really like doing arithmetic." And all the rest of us absolutely hated it. I don't think you could say he did make much contribution except as a teacher and a teacher at the undergraduate level, really.

Kuhn:

You must have known him here before the two of you were in Copenhagen. Was he somebody who was a real participant in discussions and pretty deeply concerned with what was going on, or was his interest in arithmetic already dominant?

Mott:

No, I think he would participate in discussions, at the time when Schrodinger's equation first came along — and how one would do a many ... body problem. Yes, I think he did. Well, the truth of the matter is, I don't remember in my student days in Cambridge very many discussions. We were all just a little too isolated, though Fowler did do his best. But we hadn't the habit of talking all the time; that the later generation of physicists very wisely acquired. I remember when I went to Copenhagen — oh, Blackett was about then in Cambridge — I remember him saying to me, ďNow youíve come back from Copenhagen and you work in the laboratory and you talk to people. You never did before." Yes that's it. Copenhagen taught us the value of an institute life. In Copenhagen, Gamow and I used to talk every day, or rather he talked, and I chipped in. Do you know Gamow?

Kuhn:

Not well.

Mott:

He's a bit fat now, isn't he? He was a slim young man then, absolutely full of vigor and talk. Very amusing, and still is.

Kuhn:

I think vigor and talk are still very characteristic of him.

Mott:

Yes, perhaps they are. But I remember' Fowler's remark when he saw what Gamow had done: "By God, why didn't I think of that." And this is often so with quantum mechanics, and still is. As soon as you saw it, you kicked yourself for not seeing it before.

Kuhn:

Is it still true?

Mott:

Well, no, not in the remote, not in the fundamental particle field; I didnít mean that at all. Put in solid state there are still things happening every now and then which once they are down they look so simple that you wonder why in the devil you didnít think of it yourself.

Kuhn:

You spoke in, this first memorandum which you wrote us a year ago of people who had been around at the time you were but who did not really go on. This raises an issue about which I've gotten very little information. When physics changes its character as drastically as it did in the twenties, there are likely to be a bunch of people who learned it before the change began, but possibly also some who did it at the time, who might have gone on and done something if it had stayed the old way but who really pretty much drift out of it. If I may raise a questionable authority, C. P. Snow speaks of this phenomenon of people who really can't face up to retooling to the extent that it would be necessary to do. I wondered how much that was in evidence here and to what extent there were people who just rather got left out and never came back — not necessarily because of their intrinsic ability, and not necessarily because they got attracted out to law, but because the field had moved out from under themÖ

Mott:

Oh, Iím sure there were people who taught electromagnetic theory and the old quantum theory who never made the grade with the new quantum theory. I can't function two of my own teachers I remember who were good up to this point. I donít think either of them are alive now. Their names were Birtwistle and Cunningham. I don't think either of them is such that you would have heard the names.

Kuhn:

Oh, I know both names. Birtwistle, because he wrote one of the first books on quantum mechanics.

Mott:

On quantum theory. He never wrote on quantum mechanics, did he? Tsk. He had a book that came out in '28, I should say. Well, perhaps I'm wrong. Perhaps he did try and make the grade.

Kuhn:

It wasnít a very good book, and I don't think it lasted very long, but it was one of the first places you could read the new theory.

Mott:

Then I'm wrong about him. What was it he didn't believe? Perhaps he didn't believe relativity. Perhaps he did, my memory is bad.

Kuhn:

I donít think he believed the new interpretation, though Iím not sure of that. I haven't read the book. Bohr, I know, was quite upset by the book.

Mott:

Larmor was about and was always saying he didn't believe in something or other, relativity probably in spite of his contributions to it. Well, there must have been such people, but I can't think of anybody who had made useful contributions to the old quantum theory and who didn't do well with the new. In fact, who did make useful contributions to the old quantum theory in England? I don't know of anybody who did. Do you? Not at least at the more theoretical side which is the one I know best. That was almost exclusively the Germans and Copenhagen. No, it isnít really clear to me that anybody except Fowler, who came relatively late to it really knew what was going on.

Mott:

Fowler did know what was going on, and he had this vigor, you know. That was my chief recollection of him. So he would get a superficial understanding of things very quickly.

Kuhn:

You speak of doing quantum theory for the Tripos and saying of course this would still be the old quantum theory.

Mott:

I doubt if it would. Let me see, when did I take it? It must have been '26. There couldn't have been much new quantum theory then, could there? No, it must have been old quantum theory.

Kuhn:

But does this mean that you still used, say, Sommerfeld as a book you knew fairly well for preparation?

Mott:

Yes, yes.

Kuhn:

What else did you use? Did you use Born's Atommechanik?

Mott:

We used a book by Andrade. I don't think we used Born. Well, I forget.

Kuhn:

Would one in this period, be expected to handle German well enough to be able to use things in German?

Mott:

If you mean for the Tripos, no. But immediately afterwards it was obviously necessary; and as soon as I had done it. Tripos, I spent a vacation in Germany for that purpose.

Kuhn:

You wrote me of this reaction, which delights me, of disappointment with the Schrodinger equation because it made things look familiar again.

Mott:

Yes, you'll put that in your book wonít you? That was (Brainard), I think, before he went off to law, who said that.

Kuhn:

This was not a typical Cambridge reaction?

Mott:

Oh, I remember chuckling about it and probably repeated it to someone else, and we all agreed. Well, who is ďwe all?Ē Birtwistle? I canít tell you. I mean, look, itís 1927; Iím a final year undergraduate; and I donít know a typical Cambridge reaction at that time.

Kuhn:

I ask questions realizing that I can't hope to get full answers, and sometimes I get surprised. This, however, is a peculiar story. I don't mean that I doubt it; on the contrary Iím sure it's right; and I'm delighted by it; but what is odd about it, in terms of my experience elsewhere, is that except in Gottingen itself, it's terribly hard to discover people who were much on to the new quantum theory before the Schrodinger equation. People looked at it and had such trouble making out matrix mechanics that they really puzzled about it, couldn't do anything about it, then let it go. Then the Schrodinger equation came out, and people started to learn quantum theory. When they learned matrix mechanics they learned it through the Wave Equation. Well, we were in that period trying to understand the —. In advanced lectures we would have had something like the Kramers-Heisenberg dispersion theory; we would have seen the Heisenberg paper attempts to frame the matrix mechanics around it. I would have been extremely excited. Look here, here is' a mathematical technique which is an exciting one; it looks like getting round all these difficulties of the quantum theory which we were just being introduced to, and it was all most attractive.

Kuhn:

Would you have had matrices before you ran into them this way?

Mott:

No. I don't remember any course with lectures on matrices. There may have been one I think. One just picked it up out of books. There was Courant Hilbert, (Methoden der Mathematischen Physik). That was, all we could want in that.

Kuhn:

Would you, as a student at Cambridge, have run into quaternions at all?

Mott:

No. I didn't personally, maybe other people did. And then we saw the Schrodinger equation. Oh, dear; oh, dear this is just the old vibrating membrane again. [Laughter] You see what I mean. It was quite a disappointment. Tsk. Do you suppose this extra immediacy in response to matrix mechanics had something to do with the heavily mathematical nature of theoretical training?

Mott:

Yes. Here we were, coming up in the mathematics way. Here was physics presenting something which we thought vas obviously going to challenge us and that we could get on without having to learn und understand all this dreary stuff about the Zeeman effect and God knows what else — all this wilderness of facts. That was a typical, Cambridge attitude. So I think you could perhaps say that was so.

Kuhn:

You were still a student here when electron diffraction was done. Do you remember how that hit Cambridge?

Mott:

I donít. I know the first course I ever gave in quantum mechanics, which was in Manchester — no. I was going to say I based the whole course on it, but Iím not sure that I did now. No, thatís gone. Couldnít tell you.

Kuhn:

Let me then take you to Copenhagen and see whether there is some way in which I can get you to remember more about what the exciting topics were there at that point. You have mentioned in your memorandum that you had a lot of talks with Gamow, and it was there I guess that you picked up the tunnel effect. At the level of more obviously fundamental problems and worries, what was going on there, in this year?

Mott:

The Uncertainty Principle and the electronic spin. Was the electron spin an observable thing? How should one apply the Uncertainty Principle to the measurement of a half quantum of angular momentum? At least that's my memory of the things we mainly got Bohr, talking about. The whole Uncertainty Principle, and' what was observable and wasn't, was absolutely racing round the place then. As I was interested in the double scattering of electrons and polarized beams, that is what I remember talking to Bohr about, or Bohr talking to me.

Kuhn:

One thing I'm particularly curious about is to whether this particular problem of the Uncertainty Principle and its relation to spin was real a problem that emerged strongly for people only through Dirac's equation or whether it existed from the very start. I ask that because there's a sense in which one might have taken it for granted that, within limits, spin was a real property, reall , measureable until one got it out of a relativistic equation in which nothing like spin went in.

Mott:

That's a good question. When was Dirac's equation published?

Kuhn:

Fairly in '28.

Mott:

Iím trying to think what year I went to Copenhagen.

Kuhn:

You were there, I think in '28, '29.

Mott:

In the autumn of Ď28 I was there. Has to be.

Kuhn:

I'm fairly sure the equation was out while you were there.

Mott:

The equation was out. Well, I can only say that —. I donít think anybody had tried to think about polarized beams before then. At least I know of no case. I think youíre probably right that Bohr started to think about what the electron spice really was as a consequence of Diracís equation, and therefore applied all his probes, and the contradictions to it.

Kuhn:

It wouldnít have to have happened that way. Spin was an off property from the beginning, and the question could very clearly have existed earlier.

Mott:

Sure. We really had the Uncertainty Principle from when? í27. And we had Dirac in í28. Well, that didnít give us much time to apply the pre-Dirac spin; there were so many other things to apply; and whether Borh had started to think about it or not before then, Iíve no real evidence. Sorry, couldnít tell.

Kuhn:

That, in particular, may still turn up; thereís an immense amount of Bohr correspondence in Copenhagen for which thereís been time to explore for its richness, but not for the details that itís going to contain. You donít remember stumbling blocks in that discussion or moments of particular excitement in its development? Itís a very neat argument.

Mott:

No, I was fascinated by the way Bohr argues, but it wasnít my natural way of thinking really. I wanted to get the answers as to what would happen if you set up such and such an apparatus and made measurements. I remember almost feeling sometimes, ďLook, I wish Bohr would let one get on with it without examining everything.Ē You see what I mean?

Kuhn:

I think you were not alone in having that feeling. Did you carry away from that experience in Copenhagen anything that you think of now as having been important to you in your subsequent work?

Mott:

Oh, well, even if I reacted against it, I carried away just that. ďFor Godís sake, look at all the implications of what youíre doing. Itís extremely valuable, and that is the way you will find new paths for progress.Ē

Kuhn:

You wrote in your memorandum that there were great doubts in Copenhagen already then as to whether the quantum mechanics was going to apply to the nucleus.

Mott:

Well, I think we already saw that you couldnít put an electron into the nucleus, surely. Therefore, it obviously couldnít.

Kuhn:

Was that that clear-cut then? You see, it could have been, but on the one hand the natural presumption when you get something of this sort that is handling so many problems which one couldnít handle before is, ďItís going to handle these things, too,Ē particularly when you know as little about the nucleus in terms of structure as one did then.

Mott:

Well, do you examine that in the papers. My recollection is that I couldnít absolutely swear to it; but we did have discussions, Bohr, Gamow, Hartree and I, in which Bohr made it quite clear to us that we couldnít put an electron into the nucleus.

Kuhn:

Oh, I think thereís no question of that problem already existing then.

Mott:

Obviously, the first thing one would say when they got introduced to Gamow is, ďWhat can we do with the electrons?Ē Obviously, one would say at once that one (couldnít put it into the nucleus). Therefore, you need a new concept which Fermi brought. Your key people have died, havenít they? Fermi, of all people. Too bad.

Kuhn:

Fermi, of course, is one of the people I would have loved to have been able to talk with. He goes to the nucleus so quickly.

Mott:

Well, he did almost the first thing which went beyond quantum mechanics, in a way, namely, the beta decay. Thatís what you should have gotten. Still, that you can quite easily do anyway.

Kuhn:

That would have been a delight. But from the point of view of people lost -Ė Pauli, Schrodinger, —

Mott:

Of course, you had a good session with Heisenberg.

Kuhn:

Yes, he had a very good session with Heisenberg. Less good one less prolonged with Bohr, but a useful one.

Mott:

I expect Heisenberg, in that field, is your key man, really.

Kuhn:

Since Bohrís death, surely, yes. Oskar Klein has also been very useful. Heís got a fine memory, an uncommonly good memory and it often comes out with whole conversations. This is a trick memory in some way Ė most people, or course, simply canít do this. You said in the memorandum that you thought nobody had had problems with the Dirac equation Ė neither in understanding nor in accepting it. Iím a little skeptical. That is such a strange equation.

Mott:

Well, he must have read Darwinís paper just before Dirac and I had been studying that myself and it was sufficiently like Diracís. No, it didnít have four components, but having two it made one seem almost at one, ďWell, my God, this is how it should be done.Ē

Kuhn:

Had you also been studying the Pauli papers?

Mott:

What do you mean — the matrices? Oh yes, they were common knowledge.

Kuhn:

I wondered particularly of your speaking of the Darwin paper because those two came out independently and more or less at the same time.

Mott:

Well, I think Darwin had been thinking in terms, hadnít he, of differential equations, as I remember. Pauli was matrices, and Diracís was a differential equations, as I remember. Pauli was matrices, and Diracís was a differential equation, ultimately. And the point that it was linear in the time was obviously appealing at once.

Kuhn:

Were you bothered about the negative energy solutions?

Mott:

They are not really a feature of Diracís equation; they are just a feature of relativity theory whenever you have a square root.

Kuhn:

Yes, but theyíre a problem in quantum mechanics if they are not in classical relativity as Dirac himself emphasized but you canít avoid transitions. You can discard the negative energy solutions in relativity theory and you canít really do that in quantum mechanics.

Mott:

All I mean is that the trouble existed in the Klein-Gordon equation and it was nothing specific to Dirac. Dirac saw it very clearly. I think people like myself saw that here was an equation which one could at once apply to any scattering problem. I had some truck with Eddington — I mean Eddingtonís equation — and saw that it couldnít be interpreted, which Eddington never seemed to admit. Nice man, but curiously remote.

Kuhn:

Did you argue with him or discuss —

Mott:

Oh, personally, I remember going up to his observatory and going with him, yes, very well.

Kuhn:

He just couldnít see the point.

Mott:

He could see it, but he didnít seem to be bothered. ďYes, that is a point, I see, but —.Ē And I wondered very much whether one could apply that equation to any problem; of course that it had two Ďtimesí in it made it very difficult to apply. I remember being struck. Moller was the first man who had a really good field theoretical treatment of the two electron collision problem. I remember being very much impressed by that paper.

Kuhn:

Your work starts right off on scattering and stays for a good many years just on scattering. Were there other things you worked on at all at that time or did you really carry yourself all the way through the scattering problem before you picked up the next step?

Mott:

Well, except for the little paper with Ellis on beta decay, everything I published was on scattering, was it? I canít remember. Yes, I think it was.

Kuhn:

I think thatís right. I think this list which I put on the outline is actually, so far as I know, a complete list of your papers from the beginning until you get into nuclear problems, and that runs í28 to í31.

Mott:

Well, thatís not very long. Yes, I suppose so.

Kuhn:

No, the time in which to unlock as much as you did on the scattering problems, that isnít very long. Of course, it is a period in which there are all sorts of other things breaking quite fast and itís a matter of some interest to me to know to what extent you also involved yourself somewhat with those but simply didnít publish or something of the sort.

Mott:

Well, what things were there? There was, of course, molecular binding, and —.

Kuhn:

The whole set of issues that group theory was unlocking in this period —, and then there were all the radiation problems.

Mott:

As I remember, I did publish a paper on radiation. Didnít I publish a paper on the radiation emitted during scattering? Well, that would be how I came to it. Yes, I did. What things do you mean in group theory?

Kuhn:

Well, largely, of course, the applications are to atomic and molecular problems. Perhaps the newest ones to molecular problems — the sort of things that come outí points follow up on the —.

Mott:

Was that so early?

Kuhn:

Oh yes, I think Weylís book was published in í28, possibly itís early í29. The Heitler-London paper is í27, so thereís an awful lot going on on those problems as well. And, of course, there is all the work on solid state to which you come back later.

Mott:

Yes, Wilson was doing solid state. A curious thing is that to the best of my recollection, I never had a talk with Wilson while I was in Cambridge. Itís funny, that. He kept very much to himself, you know.

Kuhn:

And of course this is a period when the whole question of the electromagnetic field, quantizing the field, the set of problems that emerged somewhat with Diracís paper that you studied so carefully. Those donít drop from that point on.

Mott:

Oh, yes, one had the zero point energy coming up at once, didnít one? Well, youíre asking me whether I did think about other things. Iíve no great recollection of so doing.

Kuhn:

Iím not saying this to reflect in any way on the work you were doing, but I think one of the things that is of considerable interest is to know how a fairly drastic change of the sort that went on in í25 and í26 affects specialization — sort of begins to lead out to a whole number of new fields. Certainly by the early thirties this is quite clear. Now a lot of people immediately after this begin to jump around a lot. Youíre one of the first that pick out a really fruitful row of problems and simply push along on it for a while. I wondered how that came about because most people did not do that quite this early.

Mott:

You mean they jumped about and had a look at everything.

Kuhn:

At least at a fairly large number of topics for a bit and then settled down and became the experts on one of them.

Mott:

Yes, where would one get oneís problems? I think from my mathematical training. I probably knew very little physics at that time; that is to say, I was hardly aware of the problems in molecular physics. I certainly wasnít an expert on spectroscopy and had little idea of what there was to be done there. And I think the atmosphere of the lab where everything was being scattered by everything in nuclear physics was very compelling. Iím just thinking about what this period amounts to. There was one year in which I was at Cambridge with Fowler and then one year I was partly in Copenhagen and partly in Gottingen and then I went to Manchester. Gottingen was a terrible disappointment. The good people seemed to have gone. There was Heitler, I think, who didnít seem to want to meet people and I didnít meet many people at Gottingen. After Copenhagen I wished I had stayed at Copenhagen. But I donít remember joining, belonging to, the sort of society at Gottingen which is so well-known as described in books like that book of Jungkís Brighter Than a Thousand Suns. It seemed to be an authentic description, but it just wasnít happening at that time. I donít think, except for that bit in Copenhagen, I really got into the most stimulating groups which were trying everything. And then I went to Manchester where it was my job to explain quantum mechanics to Bragg and these people and I got interested in x-ray scattering problems a bit. Then I came back to Cambridge in í30 and I was at the Cavendish full of nuclear physics and that was it. So except for those four or five months in Copenhagen, I donít think Iíve been in the real fountainhead of quantum theory.

Kuhn:

At Gottingen, youíre reasonably sure that it was at that particular point that society didnít exist, rather than that they were less good at taking in people who were there only briefly?

Mott:

Well, Max Born was there and I suppose I must have written to Max Born to ask if I could come. But he completely ignored people, you know. I hardly saw him. I think Heitler was there. My German was moderate, perhaps I was too shy, but I didnít get into any garret where one was sitting around talking about quantum theory at all in Gottingen. Disappointing. May have been my fault. But Heisenberg and Pauli were not there. Theyíd already gone. Born is extraordinary, compared to the responsibility which a Cambridge professor feels that he make sure people are coming, whether they meet other people. Born just didnít try. Please donít feel I have anything against him. I got to know him very well later and like him, but at that time he didnít do that sort of thing. I wanted to go to Gottingen because I wanted to see Germany and I thought it was a good place. But certainly it wasnít, at least not for me.

Kuhn:

Did you, through or in spite of, this feeling of limited communication, get any sense of the attitudes toward what had happened and were different there?

Mott:

My contacts with the place were so small that I simply cannot tell you.

Kuhn:

Does anything occur to you, Sir, which I might have forgotten to ask you?

Mott:

Well, I donít think anything about that. Except for Dirac, quantum theory was not created in England — anything but. But compared to Gottingen in its heyday and Copenhagen and Munich, we didnít really have a school of quantum theory, because Dirac wasnít a school man. He worked by himself. But thanks to Fowlerís influence, quite a number of people were put on the road to seeing, ďLook quantum mechanics is something new. Weíve got to apply it all around,Ē just as Gamow did and Heitler did and other people in other countries.

Kuhn:

This business with Munich does surprise me a little bit. Both you and Dirac went to Copenhagen and Gottingen, but Munich was also a major center. I mean itís also true that your natural references on the continent are Copenhagen and Gottingen, no Copenhagen, Gottingen and Munich. I wonder why that —?

Mott:

Well, I think Heisenberg and Pauli had been there during their most productive days. They were, werenít they, or do I remember wrong?

Kuhn:

Yes, but they both started in Munich.

Kuhn:

They both started in Munich. The grand old man Sommerfeld was there.

Kuhn:

They were both sent up to Gottingen by Sommerfeld. Theyíd both done important work while they were at Munich.

Mott:

Yes. I donít know.

Kuhn:

Sommerfeld, of course, had written the Bible.

Mott:

Of course, being a collision theory man, I read Bornís paper, which made me think Gottingen was a good place. No, I donít think there is anything else.

Kuhn:

No, Iím not trying to detract from Gottingen, but I think it is true, from where I sit —

Mott:

— That Munich looks more the place?

Kuhn:

Well, not necessarily more the place, but certainly on a par, until the period when you get the new quantum mechanics. I mean that can do it, you see.

Mott:

Well, it was a new quantum mechanics breaking out in Gottingen with a terrible reputation. People like Oppenheimer went there just before I did and it was at its peak and it was felt that there was the place to go to. Bethe — where was Bethe then? He was probably in Munich. Of course, heís exactly a contemporary of mine, and I met him at a conference in Leipzig (one of Leipziger Vortrage), when I was there with Walker. Most impressive at that age. Then after Hitler he came to England before he went to America. Oh, he was impressive. He would go into a room and shut himself out for a few days and come out with a complete explanation of order-disorder transitions, or something. I mean he was a past master of applications of quantum mechanics, wasnít he?