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Oral History Transcript — Sir Nevill F. Mott

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Interview with Sir Nevill F. Mott
By Charles Weiner
At the University of Cambridge, Cambridge, England
May 21, 1971

Listen as Mott describes his background in wave mechanics and who he learned from.

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Nevill F. Mott; May 21, 1971

ABSTRACT: Background in wave mechanics, educational background; undergraduate at the University of Cambridge (circa 1925-1930); Niels Bohr Institute, Copenhagen (1928-1929); University of G6ttingen (1929); teaching mathematics at the University of Cambridge; Kapitza Club; work with George Gamow, Niels Bohr, Charlie Ellis, Harrie Massey.

Transcript

Weiner:

mp3

Well, we're starting off, I think the initial question on my long list was your own background in wave mechanics, where you learned it, and from whom.

Mott:

Yes. Iíll go back again by saying that both my father and my mother were research students in the Cavendish Laboratory under J. J. Thompson, and you will appreciate that I was brought up feeling that the Cavendish was the place to work in and that physics was an interesting subject. They were neither of them in physics after they married, or at least not for long, but this did make a difference. Well, I took mathematics at Cambridge and got a mathematical scholarship from my school, but I always thought that atomic physics was what I wanted to apply my mathematics to, and you will remember that in Ď26 and '27 it was a very exciting period. Schrodinger was writing his paper, and Heisenberg and Bohr. And I read these papers and had to learn German for the purpose. If you ask me why I did it, I mean, I had never thought of doing anything else. Since I can remember.

Then I was, as I said, in the mathematics faculty, but I used to come into the Cavendish, to hear lectures and hear people talk, and I suppose I learned quantum mechanics not from anyone, but from these papers. After all, my supervisor, when I was a research student, was R.H. Fowler, but I would say he was learning it at the same time. We could none of us really teach each other.

Weiner:

I noticed in the book, the university report, which he did give lectures on quantum theory and Rutherford I think gave lectures too in the twenties. Do you remember any of those? I don't know what the content was, but do you remember attending any of them?

Mott:

No, I donít. I donít think I learned quantum mechanics from any lectures. I think I learned it from the, original papers. But I may be wrong. I think if I had attended a lecture and got anything out of it, would remember, because I remember a lot of lecture courses.

Weiner:

But you did come to the Cavendish, and in addition to hearing Rutherford talking about the experimental work, did you have any opportunity to see the work in progress, or people who were doing it?

Mott:

In one location when I was an undergraduate, I did a practical course in the laboratory, and there Dr., what was his name, Sell, who had also taught my parents — In fact, he was very tickled by that — and yes, sure, I talked to experimental people, but I haven't any vivid recollection of it. But I certainly knew what was happening, even when I was an undergraduate.

Weiner:

This wasnít required, was it, the practical course?

Mott:

No. No, I did it because I wasn't sure whether I should switch to physics for my final year, and when I got the practical course I decided not to. I'm sorry. I talked to experimental people but I don't have any vivid recollections. I also talked, as soon as I was a research student, to Fowler, and there was Max Born's paper on scattering of electrons, which was really the first paper which clearly showed that Schroedinger's psi was a probability wave. At least I should think so. I think Max Born should get more credit for that than he does. Max Born was always very annoyed that all anybody remembered from that paper was the trivial matter of the Born approximations, but the profound physics was just taken for granted. And reading that paper, and knowing about the Rutherford scattering law was basic to physics, I asked myself, how can one prove this quantum mechanically? I'm absolutely sure nobody put me onto this. I thought this one up myself and did it, just a few months before somebody in Germany, Borgman, did it by a much more elegant method. I took it to Tyler and he said OK and we published it. So then, having done my first piece of research on the scattering, and Massey was doing experiments on scattering, paper by Massy and Bolard, we just got together.

Weiner:

Massey was in what department at that time?

Mott:

Well, he was in the Cavendish.

Weiner:

And you were a research student in mathematics.

Mott:

I think by the time I met Massey — let's see, when did that book come out?

Weiner:

That was '31.

Mott:

'33. That would have been — I probably was around there, a research student. My research student experience was peculiar. I went to see Fowler, taking my tripos (1) and, probably, I know Fowler said he was going to America, to come back a year later, so — but I had to find my own. Another man came out who'd taken the math tripos was so discouraged by this that he went off and took law, another tripos in law, and became the university solicitor. See, we were not coddled in those days. Then I was one year in Cambridge as a research student and one year I was in Copenhagen and Göttingen, and then I got a job in Manchester under Bragg.

Weiner:

About 1927?

Mott:

'29. I should think. 1930, I came back to Cambridge, and then I found I was still a research student and was expected to pay four pounds a term or something, which was worth money then, to Fowler, but Fowler, bless his heart, had gone to America for a year, and had left his research students to me. So I thought it was a bit silly, now that I had a job in Cambridge and Fowler was away and I was looking after Fowler's research students, and getting grants all the time that I should pay four pounds. So I withdrew from the Ph.D. and never took it.

Weiner:

On matter of principle, I think you're perfectly correct.

Mott:

Well, I thought it was so silly. I mean, I was a fellow of the college and a university lecturer. This was more useful to my career than any Ph.D.

Weiner:

I understand, at that time many people in a similar way did not think it necessary to get the Ph.D. Do you have any feeling for —

Mott:

Somebody else was asking me about that the other day. I really have no recollection. But certainly nobody thought it was extraordinary to withdraw from the Ph.D. I mean, there were other reasons. It wasn't only the four pounds. One would have to write a thesis, which again was not worth doing.

Weiner:

May I ask for a minute about Göttingen. What year were you there?

Mott:

Oh, well, let's see if I can remember. I took my degree, my B.A., in '27 — '28 — '28 to '29 I was the winter in Copenhagen and the summer in Göttingen.

Weiner:

So you would have missed Gamow then in Göttingen.

Mott:

I didn't see him in Göttingen, but I saw him in Copenhagen. He came to Copenhagen a number of times. He — have you had any Danish? He always wanted to buy him cigarettes. I don't think that's immaterial — he was quite a character. No, he and I were in Copenhagen together. Yes. Well at that time I hadn't really done anything except, of course, to do the Rutherford scattering formula by wave mechanics. Very jealous of — naturally, of Gamow. He had down his alpha decay and this seemed absolutely splendid. I remember Gamow saying, ďYou construct the alpha particleĒ— But anyhow, I didnít. It's there I got interested In the antisymmetry of wave functions due to a paper by Oppenheimer, and out of that came the collision (?) between two particles, equal particles, to which you've referred. I took an awful long time persuading Bohr that this was right.

Weiner:

What was his objection, do you recall?

Mott:

Canít remember that. He wasn't convinced. But this was again splendid ... I mean, Bohr said to me, "Do you think —?" Bohr was a wonderful, fantastic man to work with. Anyhow, in the end it was OK. In Copenhagen I did the relativistic scattering program of double scattering, and these collisions. Then I think I came back to Cambridge between Copenhagen and Göttingen, and Fowler had picked up this double scattering, and it was he who made me apply it to alpha particles, Instead of applying it to electrons. And then of course it got to Rutherford and Chadwick and Blackett and everywhere. And they proceeded to test this. That's how it happened. Werner: Did they test it was a wild theory to shoot down, or was it particularly useful for them at the time? I'm trying to determine their motivation and attitude.

Mott:

No, I donít think that they thought it was a theory to shoot down... Iím sure Blackett didnít. I think they just felt, look here. Itís something plumped right in the middle of other discussions — weíd better look at it. It was Chadwick who first got the answer that it was ok. He took it along to Rutherford, who said gruffly, "Well, quite good. If you ever get anything like this, go and tell me.Ē I donít think there was any question of shooting down. I think Rutherford — I donít think it would be at all fair to say Rutherford was anti-theory, except when he made jokes. The Gamow thing was fairly impressive.

Weiner:

I noticed on the Gamow thing, working in the archives, I find that Gamow was in very close contact with Cockcroft, which you might expect, that he wouldnít be writing to Rutherford if he wasnít writing to the person directly concerned with a problem at that moment. And Iím curious, itís been established that there was a certain amount of communication. In your case, do you think it was through Fowler and Chadwick that had something going. Then there may have been others on a more informal basis. Do you recall any of these?

Mott:

Well, I recall the Kapitza Club very well, and I think I must have talked about this thing at the Kapitza Club, but again I have no recollection of that.

Weiner:

We have the notebooks, on film, so we can check.

Mott:

You could check that. Well, then, I went to Manchester for a year, and that was a nice experience. Then I came back here for three years. Then I got very friendly with Charlie Ellis, and we published a paper, which practically discovered the neutrino, but we stopped short of saying that it existed. That was a great pity. I donít know if you know the paper?

Weiner:

Iím not familiar with it. What year was this? Between í30 and í31?

Mott:

— get that list of my publications; I want the old ones in the thirties — thanks so much. Iíll get you the reference.

Weiner:

At the Rome meeting in 1931 the neutrino was mentioned by Galsmith in his talk, not by name, but he referred to Pauliís suggestion, which he had heard the pervious summer, I guess, when Pauli was visiting. Iím curious — so this work with Ellis was of course after that. Ellis was also at the Rome meeting.

Mott:

Oh, Ellis certainly was. (Thank you very much.) Then we did one on conversion of gamma rays with Harold Taylor, who later became secretary general of the faculty, chief administrator here. This one, here it is — Itís the second one with C.B. Ellis there.

Weiner:

I see. ďEnergy Relations in the Beta Ray Type of Radioactivity Disintegration,Ē that was í33. May I look at this for a moment? During this time, you were in the mathematics faculty, though.

Mott:

The mathematics faculty had no buildings, you know. We were supposed to work in our college rooms. I donít know if you know this about Cambridge, but people most often have two rooms, one in college where they do their college supervision, and one in their department. In those days you didnít have one in your department. And I used to do about eight hours college teaching a week — a big stint, I always thought. Itís really what drove people away from Cambridge. As well as the ordinary university lecturing. I mean, the only university building I ever came into was the Cavendish.

Weiner:

I see. So you worked pretty much then on your own. Your affiliation was formally with the mathematics department. To whom did you lecture? Physics, Math? What level?

Mott:

Well, Iím sure I gave standard mathematics faculty lectures. Letís see. I was technically a mathematician. I also remember giving a course on quantum mechanics in the Cavendish. The first time I gave it, the members dropped from 30 to 4 before I finished. So I pulled my socks up, and the next time I gave it, they didnít.

Weiner:

Better than Oppenheimer — his course at Cal Tech dropped from twelve to one auditor.

Mott:

Yes. This happens. This is the challenge. You canít let it happen again.

Weiner:

So that was the relationship. On the advanced courses, you lectured in the Cavendish, and the people attending the course would be physicists.

Mott:

The thing you have to remember about Cambridge. It's still so, is that you can very well be a theoretical physicist in the math faculty, and in my day there was really awfully little distinction. There was no sharp barrier. The barrier now has become much sharper, which is a pity.

Weiner:

I noticed on the doctoral dissertations that some things in the mathematics department, a majority of them in a given year, would be on physics problems, which you don't ordinarily see.

Mott:

Yes, I mean, Fowler was on the mathematics faculty, but he's next to Rutherford in the Cavendish. That wouldnít happen now. The mathematics faculty are, suspicious of the Cavendish, in that, not so happy.

Weiner:

Did you say that Fowler had rooms in the Cavendish?

Mott:

Fowler had a room in the Cavendish. You know where Rutherford used to sit?

Weiner:

No, I don't know, Iíve been only up this far, I don't think Iíve been up farther.

Mott:

Rutherford used to sit where Martin Well now sits, in the old Cavendish.

Weiner:

Where the museum is, on that corridor, where the glass display cases are? Or maybe I'm mixing it up with this floor.

Mott:

Yes. This wasn't built then.

Weiner:

Well, then I havenít.

Mott:

After lunch Iíll show you.

Weiner:

I have a number of other questions but Iím worried about your time.

Mott:

Well, we'll go along for lunch in about five minutes. If you really want to do some other talking — you're staying till Monday?

Weiner:

Now, I've had a call from Copenhagen, and because of some little problem, I should get home. I've cut everything short.

Mott:

Oh. You're leaving here tomorrow?

Weiner:

Yes, Iím finishing up with the archives in the morning, and I'm supposed to catch a train at three.

Mott:

Tomorrow?

Weiner:

Tomorrow. To London. Maybe at lunch — let's use the five minutes now, if I can ask you a question — and we'll see, maybe it's possible to answer. About the 1931 Rome meeting, do you remember how it came that you were invited — there were five people from Cambridge who went. I don't remember clearly now, but I know Ell was one. Fowler of course was the other" I know Rutherford, Chadwick and so forth were not there. Do you have that wonderful photograph? By the way, there were motion pictures taken which we have in our archives in New York now — people walking around —

Mott:

Blackett was there. Thereís me. There's Blackett.

Weiner:

Was Aston there?

Mott:

Yes, he was. There he is.

Weiner:

Fowler, Mott, Blackett, Aston — right. Do you recall how the invitation came? What you thought it was going to be, and in fact what it was? I'm curious, for example, why Rutherford didn't go.

Mott:

Well, I expect Rutherford went to one too many things. He probably refused. But I wouldn't know that.

Weiner:

Fowler, Mott, Blackett, Aston — right. Do you recall how the invitation came? What you thought it was going to be, and in fact what it was? Iím curious, for example, why Rutherford didnít go.

Mott:

Yes, I'm almost sure I got the invitation directly. I expect the Italian authorities asked Fowler who should come from the Cavendish, and Fowler said me. But of course — that would probably be ft. Always liked to give the young men a chance, Fowler did.

Weiner:

Well, you had more than a chance, because you had an important paper on the program, and there were more participants than papers. I'd like to ask some impressions of the meeting, because it seems that the timing for a conference on nuclear physics — Itís not very clear to me why at that particular time — I think I know some of the reasons, because the Rome group was getting ready to do work in that field, but there was nothing — what was your impression of the conference? Was anything accomplished? Did it serve any important purpose?

Mott:

My impression is, no. Most conferences are useful places for private discussions. And what can you say of any conference? The first International Conference I ever went to at Leipzig, and I have a much more vivid recollection of this being useful, meeting people like Hans Bethe for the first time, than I do the Rome conference. What I remember about the Rome conference was that it was touristic occasion — first time I'd ever been to Roee. We saw Mussolini. We saw Tivoli and it was intoxicating.

Weiner:

I get the feeling from the papers of the meeting, subsequently published, that there was, not a sense of triumph about the subject matter but a sense of frustration. Your paper, for example, talks of principles of wave mechanics and so forth. Others talked of specific problems in hyperfine structure, electron spin — the general problem of what to do within the nucleus, when you can handle electrons and protons even at relativistic speeds, but somehow things didn't seem to be going well.

Mott:

What year was it?

Weiner:

This was '31. There was — Fowler offered a model, a very tentative model of the nucleus, and —

Mott:

— yes. No, I don't remember it as an exciting scientific occasion. I don't believe I ever was a nuclear physicist. The nearest I came to it was these papers with Ellis. Because I don't believe I ever got thinking really about the inside of a nucleus. The internal conversion of gamma rays, that was outside, really. Then when I went to Bristol, I never did any more nuclear. So Iím sorry, Iím not really the man to tell you much about the Rome Conference, because all I can remember about it is Mussolini and the Coliseum and going down to the Tivoli and doing that sort of thing.

Weiner:

I tell you very frankly that many of the participants even those who were directly concerned with nuclear physics, had the same reaction. I was just curious about it.

Mott:

Did you ever know Mrs. Ellis? She's dead, of course.

Weiner:

No.

Mott:

She was a Polish dancer. Curious girl. She was rather the life and soul of it.

Weiner:

Well how did the book with Massey come about? It ended up in 1933, and in the introduction, you said that you specifically would not deal with protons or with problems involving discussion of nuclear structure. The purpose of writing the book was to deal with relatively slow velocities, because most tests had treated only the fast particle scattering. Now, I'm especially interested in the decision to leave out the nuclear aspects of it.

Mott:

Well, in later editions that Massey has written, of course he put it in again. But Iím sure the decision was as I just said that neither of us felt we were nuclear Ďphysicistsí. We felt that there was a big subject here, the scattering of electrons; that you didn't have to — you could use the Born approximation, which, being a perturbation theory, appeared to many theorists, and things like the Lanzar (?) effect, and electron diffraction in molecules was a subject of concern In which we wanted to lay the foundations.

Weiner:

You certainly had enough to do without —

Mott:

As I said, there was Garnow on the nucleus, and we were involved with the edge of it, but we didnít feel we had any theoretical concern about nuclear.

Weiner:

You made a statement in the book, ďIt is quite possible that quantum mechanics is not applicable within the nucleus."

Mott:

Did we? Is that true, turned out to be true? If quantum mechanics is applicable in the nucleus, it is sufficiently different from Schroedinger's equation. I think that almost by definition everything that is true is quantum mechanics. But I think when we said that, we meant Schroedingerís equation or even Diracís equation, as written down when we wrote the book.

Weiner:

Well, that's all you had to work with.

Mott:

— not the whole story — that turned out to be true. But what that means is not really so much a modification of quantum mechanics. It was the type of exchange force you get between the neutron and prentam (?) for instance, and the neutrino.

Weiner:

Well, Iím curious about the time period involved in writing the book. This would help clarify it for me. The preface was signed In September of 1933. Do you remember when you first started the book and how long it took?

Mott:

I thinkÖ at allÖIt was in Cambridge, wasn't it?

Weiner:

By the time the book was published you were moving. You were just moving at that time I think to Bristol.

Mott:

No, I can't tell you that. Silly things stick in oneís mind. I remember I had a meeting in Germany. Somebody was asking how the book was getting on, saying they'd heard that only De Mucha Tyle (?) was written the German part of the book. I think my part was written before the Massey part. Surely that's right. Because Massey, as you know, even in the first edition, did most of the comparison with experiments. I suspect my part was written by, took about a year to write, and it was maybe six months before that preface or a year before that preface. But I couldnít tell you for certain.

Weiner:

Well, what I'm getting at here is that this would have been after the neutron was discovered but during the period when it wasn't clear how the neutron was going to affect nuclear theory. Or even how it would affect the entire subject of nuclear physics.

Mott:

I don't remember hearing about the kind of interaction between the proton and the neutron when we wrote the book. Iím afraid I won't be able to help you here. There's a paper by Mark who first treated the slow electron scattering. I think I used that paper in my attempts to write about scattering, phase shifts and all that. The motivation for the book, you know, was to apply these methods to lots of places. I think with all the speculation on relations among nuclear forces, all the speculations, I don't think I was involved very much. Iím not sure that Cambridge was really the place where it was done. Fermi and —

Weiner:

Heisenberg and Fermi and Bohr. People who were very deeply involved in the quantum electrodynamics that was developing.

Mott:

Yes. But I just wasnít, you see — partly because we didnít understand it or it wasnít being done in Cambridge. We werenít aware of it.

Weiner:

I remember in the preface you thanked Weisskopf for reading the proof. Where was he at the time? How did that come about? That must have been the final stage.

Mott:

I canít think of that...

Weiner:

It surprised me, because the name popped up —

Mott:

— surprises me, too. I canít remember where that happened. Iím sorry. I wonder if he remembers doing it. Iíve quite forgotten that. Of course, that I had nothing to do with, since it was Hassey's pages. He might have. It's the sort of thing. I don't [???] Massey, it was at the beginning of my actual career, was most In charge of. (?)

Weiner:

Iím just trying to look at that as a way of capping off this period and in particular, the special relationship that developed between your theoretical work and the experimental work.

Mott:

Well, Iím sure Fowler deserves the credit for that. He — there he was, Rutherfordís son-in-law, big in the Cavendish and everything which one of the young fellows did, which was useful, he would pick up and tell the people. I donít think it would ever have occurred to me to go to Rutherford and say, ďLook here, thereís something which Iíve discovered, hadnít you better test it?Ē Surely Fowler did that. [?] might have been Bohr.

Weiner:

Gamow was able to do it through Cockcroft, but I thought that was a difference channel of communication.

Mott:

Oh yes, but I think Gamow had a much more certain personality than I had at that age. I think he would — I mean, I think I would feel the kind of thing to do would be to publish it. I donít know where I got the idea that it ought to be distributed to the experimentalist. Perhaps itís hard to remember now how isolated one was in a way then. There were terribly few theoretical physicists around in England, and — there was Douglas Archie and Fowler and Dirac, and myself and A.H. Wilson. I never talked to Wilson. I didnít talk to anybody. At Copenhagen we liked to talk. The English tradition, Iím afraid — itís changing then but you just worked out things on your own. And the idea of theoreticians being part of a lab, I think that just sort of happened at the time of Gamowís visit, the beginning of quantum mechanics, but it wasnít formalized or, you had to find out for yourself that youíd better be part of a lab. I think that I learned gradually. I remember Charlie Ellis and also Blackett saying that should be so.

Weiner:

Well, I think another time that it was demonstrated here was when Farrell came into the Munn for a period. He was sort of a house theoretician.

Mott:

Yes. That would have been after the Hitler time, wouldnít it, about í35?

Weiner:

Yes. He was only here for a year.

Mott:

Well, after I was in Bristol, I learned my lesson then. We built up Bristol on that principle. (crosstalk) Come along and have lunch now.