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In footnotes or endnotes please cite AIP interviews like this:
Interview of John Cockcroft by Thomas S. Kuhn on 1963 May 2,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Part of the Archives for the History of Quantum Physics oral history collection, which includes tapes and transcripts of oral history interviews conducted with circa 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: Philip Ivar Dee, Ralph Fowler, Joseph Larmor, Ernest Rutherford, John Joseph Thomson, Ernest Thomas Sinton Walton, Webster; University of Cambridge, Cavendish Physical Society, and Kapitsa Club.
You began, Sir, as a student of engineering.
No, I began as a student of mathematics in Manchester in 1914 but I was only there for two terms and then I went to — after the war. When I came back I was a student of engineering. So I stopped the mathematics and went to engineering and then went back to mathematics at Cambridge.
What accounted for that switch?
You mean after the war?
Well, if I could even start a little earlier. How did you first get interested in mathematics; how was this whole direction toward science taken? This was not in your family to any extent, was it?
No, I think it was the influence of the school, the things I was good at school, physics and mathematics. It was a toss-up whether I went to mathematics or physics in Manchester. These were the two possible subjects. Probably it was rather chance which I went into at that time. Rutherford was at Manchester at that time and I might easily have gone there. I did go to the physics lectures in Manchester — Rutherford’s physics lectures — as part of the mathematics course.
I realize that choosing between mathematics and physics means a rather different thing in this country or did perhaps even more so in those years than it would in the United States.
That’s right, yes. Yes, that’s a characteristic difference I think, rather. It’s an unfortunate one in this country that you have to make this choice. In Cambridge it didn’t matter very much because there was a tremendous amount of applied mathematics and theoretical physics in the mathematical Tripos course. I spent half the time on what was essentially theoretical physics or tremendous numbers of lectures by people like Eddington and Milne, and one could go to on the theoretical physics side. One could deliberately bias one’s course to follow one’s own interest, which is what I did. I didn’t pay much attention to the purer mathematics topics apart from the minimum that I had to do.
That would not have been true at Manchester.
No, not so much during Manchester. That’s right, no.
What would the mathematics curriculum largely have consisted of there?
Well, the professor was Horace Lamb and he was a great hydro-dynamics man, you see. In the first year course it was mainly the ground work — the calculus and corresponding things of pure mathematics: matrix theory, geometry and all these things.
Was matrix theory part of the curriculum?
Oh, yes, oh, surely.
And this is matrices, not simply determinants.
That’s right, yes, yes.
That was not generally taught.
No, it may not have been general, but it was certainly part of the course in Manchester.
You can guess why I prick up my ears at this.
Yes, I can. It’s rather unusual. Of course, there are extant, if you’re interested, in the Cambridge University Report, so-called "Lecture Lists." You’ll find a bound volume and you’ll see what the "lecture lists" were in Cambridge in the 1920’s. You can see the first lectures on quantum mechanics appeared. Have you picked those up at all?
I haven’t really picked them up, but I’m very much aware of their existence. By and large at the moment we are least concerned with material that is already down. It’s the material that may vanish otherwise that we’re principally concentrating on. I did not realize that Cambridge where it, was fairly easy to go back and forth was atypical in this respect. Even applied mathematics at Manchester would not get you a lot of physics unless you went to the physics lectures also?
Well, you would go to the fairly elementary physics lectures (in that course). I could have gone to more advanced lectures if I had just known more about it. Elementary lectures were too elementary at that time. (I’d learned it all at school) and I should have gone to the more advanced lectures in physics.
What did a student electing at the undergraduate level to do mathematic or physics think of as a career?
Well, I think today students are, and have been for a long time more influenced by their school masters than anything else. The school masters know what they are good at and advise what to do. Some of them may change when they come to a university, but by and large, there is a tendency to follow the advice of the school masters and to continue the way they have been going in school in the last two years.
This more or less independent of what the long-term prospects in the field may be?
Yes, I think they’re aware of the general demand for them in the field of science and engineering and then the fact that the government report that is published says that there is a shortage of scientists in this and that field does have some influence in a general way.
But surely there wasn’t all that need for physicists and mathematicians in the early times.
No, I think in these days, 1914 until 1920, the question was where there were outlets for mathematicians and physicists. You could say that the mathematics was good for going into the civil service and physics was good for going into the universities or going into teaching in the grammar schools and perhaps of some use in industry and (knowing more use in industry) and then there will be that general consideration.
Well, what took you to engineering then when you came back from war?
Oh, well, that’s very simple. I thought that since I was a few years older, four or five years older, it was about time I got a job. There were more immediate prospects of getting a job in engineering. So that’s why I went back to Manchester and took engineering. Then after I had a little experience in industry, I was advised by my professor in engineering to go, to Cambridge and do what he’d done. Go on studying mathematics. It was very good advice.
Was this advice given with an eye to being a better engineer or with an eye now to going back to the sciences and to mathematics?
Well, not specifically. He thought that it would be a good general education to go and get some more mathematics. I hadn’t really done a great deal of mathematics. I just had the one year in the university and so he thought I should profit by doing more. I was interested also in physics and he gave me letters to Rutherford to take to Rutherford which I did as soon as I got to Cambridge. Rutherford suggested I do some physics after I had done mathematics.
You came to St. Johns when you came to Cambridge?
That’s right. Yes, yes.
I noticed that and wondered why it had been St. Johns? I asked this largely because; of course Dirac went to St. Johns.
Oh, well, it was the same professor, (Miles Walker) in Manchester who had been at St. Johns and who naturally suggested St. Johns. I had no connection with St. Johns. I knew it was a good mathematical college. It had three or four fellows interested in mathematics.
You had already a Master of Science degree, therefore you must enter in a different status from someone who had entered directly from school.
Yes, I was what they call an affiliated student which meant that I could omit the first year course. So I could go straightaway on to part two of the mathematics Tripos. That first year I did the usual standard things in the mathematical Tripos and the third year I was concentrating on what they call the “Schedule B” of the mathematics Tripos which now corresponds to part three; there you can take a lot of specialized subjects and I picked out in the Schedule B all the theoretical physics and applied mathematics topic that I was interested in, you see. So that was a very good opportunity to move back towards physics.
It was easier to learn physics while being a student of mathematics at Cambridge than it was at most places but I wonder whether the division between the physics natural philosophy curriculum and the mathematics curriculum was still pretty real. Did it divide the faculty? Did it divide the students? Did it mean that you knew virtually nothing about experimental work and experimental subjects in this training?
No, formerly it did. But actually what I did was to go into the experimental class in the afternoon and when I was not doing the mathematical Tripos work I went to the advanced physics experimental class. There’s no real difficulty about doing this if you wanted to do it. You had to exercise some initiative — no one would tell you to do this. Tutors wouldn’t advise you to do this but it’s possible to do it.
Did many other people do that?
Very few, very few. No, I think I was rather unusual in that sense.
Would you say that there was any difficulty for others in passing back and forth from the theoretical to the experimental?
I don’t think there was any formal problem, no real barriers. It was a question of taking the initiative yourself.
But realistically, some very good people may fail to take the initiative, their absorption in one or the other may be so total that they don’t do this; they may nevertheless be first-rate people. What I mean to ask, then, is in practice how did it seem to work out?
Well, I think you’ll find there were quite a few people who were working on the borderline. There was Nevill Mott, who was taking the mathematical Tripos at about the same time I was. He became a theoretical physicist Allen [H.] Wilson took the mathematical Tripos and became a very important expert on solid state physics. Well, there was Massey, who was on the borderline. You could say Bullard has also done a great deal of theoretical physics. So if you look around [there were] people who took more senior jobs and you’ll find that they were able to work on both sides of the barrier. But the average student wouldn’t do this.
I don’t know all of the people you just mentioned. Are they people who took the mathematical tripos and who then did experimental work in the way that you are?
Well, certainly Nevill Mott. I don’t know whether he ever did any experimental work, but he certainly became a theoretical physicist. I don’t know whether he ever did any experimental work. He may have done some.
Not, so far as I know, in his early career.
No, no, Allen Wilson, I doubt whether he did experimental work, but you would find him about on the Cavendish a good deal and at all the Cavendish Colloquia and so on. Bullard, I think, was the opposite. He was an experimental physicist, but he would go to probably a lot of the theoretical lectures. I don’t think at this level of interest there was a very sharp dividing line because we would be working in the Cavendish and at the same time finding out what was going on in the world of theoretical physics very well. There would be no distinction and in places like the Kapitza Club there would be mixed people.
What about the Cavendish Colloquium itself? Did that give both theoretical and experimental talks?
Oh, yes. It was primarily an experimental Colloquium, but we did have lectures by people like Heisenberg and Bohr and Schrödinger when they came over, and you would find that audience was a mixed one. Perhaps 80 percent experimental and 20 percent theoretical.
Was there always a larger group of experimentalists? Does that 80/20 division mean in part that there simply were more experimentalists?
Yes, I think that’s about right. Yes
It isn’t necessarily that they were better represented because a larger proportion of them went to the Colloquium.
No, I think that would be about the ratio then.
Then, how did it happen that having theory to the extent that you have, one identifies you with experimental work principally in your later career?
Well, that’s because I went to work in the Cavendish as what corresponded to a PhD student in 1924 and then I was theoretically a full-time experimentalist, you see. I suppose most of my time was spent on experimental physics at that time — probably at least 80 percent of the time — and only some time going to advanced lectures on the other side, the theoretical physics side.
Was the decision to do experimental work and to identify yourself with it taken consciously or was it just a matter of having taken a good opportunity and then seeing how it developed?
No, it was quite deliberate. See, when I went to Cambridge in 1922, I had this letter to Rutherford, and so I went to see him and asked if I could work in the Cavendish on the experimental side when I had taken my mathematical course. He agreed that I could do that if I got a first class degree. So as soon as I was through I went along and was taken on.
When he said first class degree, he meant a first class degree in theory.
Yes, that’s right, mathematic Tripos.
Because I take it many of the experimental physicist’s did not take math, but just took physics.
Yes, they would take the natural science courses.
Did Rutherford mean first class in math rather than first class in [experimental work]?
Oh, yes. He knew I was doing the mathematical Tripos. Oh, yes, surely.
Would he have recommended to you the mathematical Tripos rather than the natural philosophy Tripos as the best one for training to come back and do experimental work at the Cavendish?
Oh, I think he was very broadminded. I don’t think he would have (objected) either way in this. He relied a great deal on this son-in-law Fowler, as to his interpretation of what was going on in the theoretical world. He realized that it was important.
I’d be terribly grateful if you’d tell me something about Fowler. His name keeps coming up as it naturally would, but so far the people I’ve talked to have not been able to give me any very clear notion of him, the extent of his interests, the manner in which he related to other people on the faculty and to his students.
Well, Fowler, of course, was a very good classical mathematician and as you know, he wrote several of the major books on statistical mechanics. He was a very dynamic character, very good with getting on with students and a good lecturer. He was also very much interested in what was going on in physics. He was always present at the Physical Society meetings. Of course, he was very close to Rutherford, and almost Rutherford’s principal advisor on the side of theoretical physics. We always thought that Rutherford got his interpretation of what was going on through Fowler I think that in a way perhaps Fowler was a better mathematician than a theoretical physicist. He didn’t take the initiative on the theoretical physics side in a way that he might have done with this equipment. He had very fine mathematical equipment and he might have got on to the tunnel effect, but you see that he didn’t. He didn’t have perhaps quite enough physical intuition to come right out on top, I would say. Still, he was a very important influence. You know he married Rutherford’s daughter, of course.
Was there anyone else in Cambridge at the time you were here as a student who to any extent took the same sort of hand on modern physical problems, excluding relativity in which obviously Eddington [had influence]?
Eddington was there. Well, then there was Milne on the internal constitution of stars. He was a considerable influence, I would say. He was in rather a specialized field of astrophysics. We have the older group, you see, like Larmor but they’re more on the classical side. They were interested in Maxwell and Lorentz physics and that kind of thing.
Was Larmor still very much of a force? Did he still have many students?
Oh, I would go to his lectures. He would have three or four students. No, he wasn’t much of a force.
How about research students?
No, I don’t think he’d have many research students. No, he represented the physics of the 1900’s you know. He was a grand old man and it was worth going to hear him. Sometimes he would come out with some new ideas such as the transmission of electromagnetic waves by the ionosphere and he would come out with those in his lectures before they became generally known He was interesting to go to but he really did not represent the new age in theoretical physics at all.
And you’d say most of the research students would go to Fowler?
Yes, they would go to Fowler. Well, and to Geoffrey Taylor, too. But he was again more classical physics of the Rayleigh type and (we would naturally go to him.)
Was J. J. Thomson still —
J. J. Thomson was lecturing, yes. I used to go to J. J. Thomson’s lectures. There again it was really [for] the physics of the early 20th century that we went to J. J. Thomson. (Electrons and all this kind of thing; discharge in gases, and isotopes). It was very nice to have him there and to be able to go and listen to the great man.
Professor Bohr, in talking to us shortly before his death, talked about his immense admiration for Thomson, but also about his sense that Thomson had by 1920 or thereabouts pretty much cut himself off from the profession by the stands he had taken on the Bohr atom and on isotopes, initially, and so on. Was that felt here, did it have an influence here?
Well, I don’t think that by the early 1920’s we would expect to get anything new by going to J. J. Thomson lectures. It was rather looking backwards rather than forward. So I think to that extent it’s true, yes.
Did he express himself as against any particular ideas or as disturbed by the direction which physics had taken?
No, no, I don’t think so. No, but it was just that he wasn’t taking any interest in the newer developments, I would say.
Eddington, I take it, was a big figure in this period.
He was a big figure, yes. Naturally, we went to Eddington’s lectures, yes. He would turn up at Cavendish Physics Society meetings and took great interest in what was going on in the Cavendish so he was fully aware of everything, all the new developments. He did a very interesting monograph on the Cavendish Laboratory in 1931, which shows how well aware he was of everything going on.
He was an astronomer, rather than a mathematician for purposes of appointment, was he not?
Well, he was professor of astronomy.
Again, did that make any difference in terms of the expectation that students would be influenced by him? I have to remind you that my experience is with the American university system where a physicist can go to lectures in the astronomy department, but is just not very likely to.
Well, in this case, the lectures will be published in the lecture lists as additional lectures which students would go to; you just took your choice. If you wanted to go, you did go. It was sort of a great bill of fare. You picked out the areas you were interested in. You weren’t advised to go to them; it was purely personal choice. …You have to have sufficient margin of time or vitality to do these things. I guess that’s what it adds up to. If you have it you can go and do the extras.
When one’s margin of time and energy was not vast, did one then pick one’s courses or one’s lectures with an eye to examinations?
Well, yes. I think. There were two groups of examination papers, you see, in the mathematical Tripos: one which you more or less had to cover although you could neglect certain parts of it, and then the advanced papers; there you could be very selective. You could elect to take certain papers on the advanced side, what we call the Schedule B, and then you would select the advanced lectures to cover the advanced examination papers. That’s really what one did. I don’t think I was concerned about examination questions in relativity or very much with the interior constitution of stars, but the fact that we had Milne and Eddington there meant that one would go and listen to them.
I take it that the man who came in some ways principally first to share on the theoretical side Fowler’s interest and concern with the modern physics in the sense of the quantum was Dirac.
That’s right. Yes. Certainly, yes.
How quickly did his successful research begin to make a dent on the nature of physics at Cambridge? Clearly, people used the papers, but what of his role as a teacher, his role as somebody who would attract students’ interest? You speak of Fowler as a person of considerable magnetism, considerable influence on students. Did that happen at all with Dirac? He is a quiet person; one takes it that he’s rather isolated in a number of ways.
Yes, I should think his influence would be felt first among the research students through his taking part in colloquia and taking part in meetings of the Kapitza Club and so on. His interests would become known to them and help to stimulate them. Then when he became a university teacher — I’m not sure when that was, he got a fellowship in 1928 or ‘29 — he would then for the first time begin to give some lectures. Again, one could check up on the date of the lectures. Then it would begin to penetrate down to the lower level. One would start first of all with the research student level in a very informal way, I would say.
Did you know people who were beginning to be attracted to him? How did his influence work out in practice? I ask this again presupposing that this is a person who is unusual in his personality as well as in his talent.
Well, I suppose he became well-known to most of the research students and the Cavendish people (on this photograph). [Showing photograph.] All the younger people there would hear Dirac talk at the Cavendish Physical Society at weekly meetings and many of them would go to the Kapitza Club to hear him talk, and get to know him in that way.
Did people understand his talks?
Oh, I think so, yes. He’s quite a clear lecturer.
I’ve not heard him lecture.
Oh, yes, very precise.
Did one look forward to his talks with —
No, I think they just were taken as sort of run-of-the-mill colloquia that one went to as a matter of course.
What recollections do you have with respect to the development of quantum physics in your time here? That is, you were here, although I’m not sure about the year in relation to your own work level, during a very exciting period from 1922 on.
1924, well, yes, I suppose I really became aware of it by the time I got in the Cavendish in October, 1924, and there —
Do you suppose there was any significant awareness of it? Did it enter as a topic in any lectures you went to before that time?
Well, of course one was familiar in general with all quantum ideas through the general course of the lectures and the mathematical Tripos.
Where would the quantum enter in lectures that pointed toward the mathematical Tripos? Would one get problems in theory of spectra, Hamilton-Jacobi technique’s and so on in the mathematical Tripos?
Oh, yes, oh, certainly. We would get all that. Sommerfeld would be a standard textbook and things of this kind. Born’s books would always be a part of the library that one had at that time. But the newer things would come the other way, through the colloquia and. through the visits of people like Heisenberg, Schrödinger, and Bohr. I was brought right up to date by people of this kind who would visit and by looking at the current papers and getting people like Dirac to talk with the members of this group and about them. So I think we were very well aware of everything that was going on by the time I became a research student.
Heisenberg was here and gave a talk in the summer of '25. Were you at that by any chance?
Yes, I was there.
Do you remember how this paper was taken? It’s a very strange paper; at least it was a very strange paper in its published form. Couldn’t have been a more important one.
Well, I think that Heisenberg was recognized to be one of the leaders of the younger generation of theoretical physicists. I think by and large he would be quite well understood and accepted.
You don’t have any recollections yourself of the occasion.
Oh, no, no, my memory isn’t good enough for that. Except that I remember Heisenberg coming several times to Cambridge and talking.
Are there particular things that you may remember about the developments that immediately followed on that — the matrix theory as it developed with Born’s contributions, the coming of the Schrödinger equation?
Well, of course, we were all excited by the coming of the Schrödinger equation. That was one of the great landmarks, coinciding with these experimental proofs of (G. P. Thomson) and Davisson and Germer and so on. I think we were all very excited by Schrödinger theory which was certainly much easier to understand for the experimental physicists. It was so extremely elegant.
Had they worried at all earlier about the matrix approach? There isn’t a great deal of time between the two, of course.
No, I don’t think they had. I think that the technique was more difficult for the experimentalist to understand and the Schrödinger equation was much easier for them to understand. They were very familiar with all the wave ideas in classical physics, you see.
Of course, I think one of the things that initially appealed to a number of people about the Schrödinger’s equation or about Schrödinger papers was not only that it was familiar as a wave and a wave equation, but also the notion that it somehow was physically familiar because one had gotten rid of the discontinuity. One thought that now one was back to a fully causal classical description, in which one can trace continuous phenomena. Was that much of an issue as you recollect it here?
Not that I remember, no, no. No, I don’t remember that.
You sent us the very interesting memorandum [on the feasibility of producing reactions by artificially accelerated protons] that you had written for Rutherford and you said that that was in substantial part a product of the aftermath of conversations that you had had with Gamow when he was here in ‘28 .
Well, yes. I’ve been looking up the date of Gamow’s visit and what I have is papers sent to me by Gamow from Copenhagen which I think is about October, 1928 I didn’t have a record of his visit until I got this book.[Minute Book of the Kapitza Club] His visit was in January,1929. So where exactly that memorandum comes in that sequence, I really don’t know. The only other thing I know is by my lab notebooks. By the beginning of ‘29 I was starting to assemble equipment for this new line of work. So that’s —. Exactly when Gamow visited us, and how this fits into the date of the memorandum, I don’t know. The paper I got directly from Gamow was probably the first thing that triggered it off.
Was that a reprint or was that a manuscript?
No, that was a [???] [mimeographed?] edition of the paper which was subsequently published in Zeitschrift fur Physik. It was really an advance copy of that. It came from Copenhagen and I have this at my home now. But certainly I did have discussions with Gamow in January and exactly when I started work to produce that memorandum, either before or after, I don’t know.
Well, it may be possible to figure that out. The essential point here is the intervention of this theoretical idea.
The tunnel effect — that was important.
You did not concern yourself with accelerators until there was reason to think that the nuclear potential barrier was not what one had taken it to be previously. But I notice in either your own or Walton’s Nobel address there is a reference to his joining you, saying that he had previously been working on accelerators.
Well, he was working on a very primitive kind of linear accelerator of the Sloan Lawrence type, and also on a very primitive betatron by just trying out the ideas of the betatron. He published a paper on the theory of the betatron on focusing effect and things of this kind. This would be in late 1928. He was more or less working in parallel in the same lab for a few months, you see, before he decided to drop what he was doing and come and join me.
But he had been concerned with these problems independent of the tunnel effect.
That’s right, yes. He was put onto the betatron by Rutherford; I think that’s what the situation was. His notebooks would show exactly when he started work. But I think it was in the autumn of 1928.
Well, you see what puzzles me. It would be very neat to say that the announcement of the tunnel effect make’s it feasible to think about building artificial accelerators, accelerators to induce transmutation, and this is clearly the pattern that you yourself indicate for your own interest in the field. On the other hand, one suddenly discovers that Walton was doing accelerators without benefit of this new —.
Well, you see, Rutherford, if you look at his Royal Society presidential addresses, was asking for millions of volts to do this and was thinking of millions of volts at that time. It wasn’t until I saw Gamow’s paper that I began to think it was worthwhile to go into it because you couldn’t produce high voltages of millions of volts in the Cavendish environment. As long as you could do it with a few hundred thousand volts, then it was worth making a start. The techniques available, the Coolidge techniques showed that you could put three hundred kilo volts on a tube. So it became feasible. On the other hand, if Walton had made a betatron, why he could have made a small betatron for several million volts
In the background of the memorandum that you did for Rutherford, were there previous conversations in which you had shown some skepticism about the millions of volts possibility?
No, I don’t think so, no, no. No, we knew we could do the few hundred thousand volts, so I suppose it really started by reading the Gamow paper.
In this period, how closely did Rutherford himself follow the new ideas that were coming in and to what extent did they play a real role in his direction of the experimental work at the Cavendish?
Well, he would be told about them by Fowler, I think. He kept informed about them in that way. So when the students had ideas about doing work of this kind, he would be well enough informed to know that this was the sensible thing to do.
He himself, did not have any resistance, shall I say?
No, he was always extremely tolerant, I think, towards new ideas, even though perhaps he was skeptical about them. He was quite tolerant and allowed people to go ahead and try things out and helped them get funds and so on.
Do you remember examples of the sort of things he might have been skeptical about and nevertheless have supported?
For example all the developments of the new techniques in the lab. He didn’t know whether they were going to come off, or pay off, but at the same time he was quite willing to let them go ahead and see what came out. Blackett was working away on his cloud chamber, controlled cloud chamber, and I suppose Rutherford wouldn’t have any clear ideas as to what was coming out of it, but he knew Blackett was a good man and he was prepared to let him go ahead and see.
This is the inevitable uncertainty about the technical feasibility in the long run for a proposal for a more elaborate and finer piece of equipment. Would the same sort of liberalism still have held if the technical feasibility of the equipment was less at issue, than the soundness of the theoretical conception?
I don’t think Rutherford would be very critical on the question of whether it was theoretically sound or not. No, I shouldn’t have thought he would.
How did he react to the whole idea of the tunnel effect?
Well, I think he would be very interested because of the (discrepancy between the energy of the alpha particles and the height of the potential barrier). It certainly was quite a puzzle to him for some time. So I think the fact that that explained the entire classical radioactivity made a great impression on him.
But you don’t remember whether in fact he expressed great joy on hearing of this idea or whether on the contrary he said, “Yes, but it violates conservation of energy.”
Oh, I don’t think you can do that. No, I don’t think so. You’ve heard his Göttingen lecture, I suppose, in which he sounds quite enthusiastic about it. You’ve got that record, I suppose, have you?
I don’t have it. I’ve got a reference to it.
Do you have any particular recollections that you think may possibly be relevant to our work — anything that’s on your mind that you think might be helpful or anything that you think people with my present interests might do well to remember.
Well, I think the history of the neutron discovery is fairly well known by now. It’s an example of the random way in which these discoveries happen. First of all the Bothe and Becker discovery of the radiation from Berlin and then Webster being put on in the Cavendish to look for the effects in a cloud chamber and not finding it, you see. He would have found it if he’d had a bigger source. His source was too weak. This was picked up by Chadwick and then he did more refined experiments and actually got onto the idea, but it’s sort of a random chain of events. The main point was the fairly quick reaction time to the situation.
Those stages, as you say, are quite well recognized I wonder whether there are particular things you may remember about colloquia on the subject. People who kicked themselves for not having gotten there sooner. Particular things that may have happened with the equipment.
Yes, I suppose looking back one can always kick oneself for not having found these things when they were lying about waiting to be picked up. [Laughter] Certainly, yes. Well, there was a great example of the positive electron not being picked up. It could have been picked up. Many times there must have been electron tracks going through cloud chambers that didn’t have magnetic fields on them that could have shown up the positive electron.
Almost certainly there were some examples of the things going through when there was a magnetic field on them.
And it happened first of all to Anderson. How did people here feel about that? You know, there was a good deal of resistance to Anderson’s announcement.
That’s right, yes. There was a certain amount of skepticism here. Millikan came over and announced Anderson’s results in the Cavendish Physical Society, and Dee was there. He was sitting in the third row back or so. He got up and said that he wasn’t sure; some of these tracks looked as though they were curved both ways. [Laughter] Mrs. Millikan was sitting in the back row and she thought this was terrible that a young man should get up and say this to her distinguished husband. But it shows you that perhaps there was a slight atmosphere of skepticism until more work was done.
I know in Copenhagen the whole idea was very much resisted by Bohr.
Was it? Yes, yes.
I’m not altogether sure why in certain circles there was as much resistance as there was.
Didn’t they believe the Dirac theory at that time?
In Copenhagen, I think it may very well have been skepticism about the Dirac theory; in fact, the whole notion of the infinite cloud with a few holes in it was not well taken.
No, well, that could have accounted for it, yes.
I’ve at least had one story that when people finally did persuade Bohr that really now the experimental evidence for the positron was overwhelming, he said, “All right, but it hasn’t got anything to do with Dirac’s holes."
I never had a chance, unfortunately, to talk to him about that period. Individual episodes of this sort are not necessarily, when they come from memory long afterwards, to be taken altogether literally. But I must say that the notion that there was real resistance to the idea in Copenhagen comes from a great many people. I may add I don’t think this was a question of resistance to the Dirac wave equation, but to the whole theory of protons.
I don’t think you would have found that same resistance on theoretical grounds. Any skepticism would be purely on experimental grounds at Cambridge. Blackett was working so soon on the subject that cleared all that away.
But surely if there is skepticism of that sort, it’s not only that one thinks there are other possible ways of interpreting these tracks, but also that one somehow wants to feel that there are other ways, because the notion that there should be tracks like this seems so implausible. One won’t look so hard for other possible interpretations if the new entity is one that fits very well with current ideas.
Can you think that a positive electron should have seemed quite such an odd entity, such an implausible thing even to experimentalists?
No, I think it was just conservatism. I don’t think there was any opposition to it really. I think it was purely skepticism as to whether the experimental results were right or wrong. I don’t think people would resist the idea of a positive electron at all, among the experimental fraternity.
Well, sir, I’m very grateful to you.
Thanks, you’re welcome.
[End of interview]
Shortly after the preceding conversation, I asked Sir John to explain to me more precisely the difference between the function of the Cavendish colloquium and the Kapitza Club. He said that the Cavendish colloquium was devoted largely to reports on work being done in the Cavendish; its second function being the presentation of very occasional lectures by distinguished visitors like Millikan, Bohr, Heisenberg, and so on. The Kapitza Club, by contrast, was devoted largely to the consideration of work done outside of Cambridge. It was somewhat more theoretical in its orientation, and its meetings were for members only, so that discussion was usually far livelier. Sir John had indicated earlier, I think perhaps before the recorder was turned on, Cavendish colloquium.
[End of Note]