Rudolf Peierls - Session I

Peierls:

You want to start with the period from my Rockefeller Fellowship on. I’ve forgotten whether the earlier period was covered in the [quantum physics history] interview. Perhaps it was.

Weiner:

Yes, I think in detail in Berlin and then Munich and Leipzig with Pauli, and Zurich. Some of the specific papers were done in great detail. But I have a couple of questions to fill in. They come in naturally, though, in connection with your later interests. For the record, let’s say that today is August 11th, 1969. This is a tape-recorded interview with Sir Rudolf Peierls. This is Charles Weiner asking some of the questions and commenting occasionally. I’d like to start by talking about the period in the early ‘30s. With Pauli’s recommendation, you received a Rockefeller Fellowship, with the intention of going to Rome and then to Cambridge, much as Bethe had done.

Peierls:

Only the other way around.

Weiner:

He had gone to Cambridge

Peierls:

He had gone to Cambridge for the winter and Rome for the summer, and I thought I could improve on that.

Weiner:

I see. But did you have a year in each place in mind, or was it a one-year fellowship?

Peierls:

No, a one-year fellowship split halfway.

Weiner:

I see. I’m not clear then on the chronology, just to establish that. Because my notes say that in the winter of 1932 you went to Rome, and that in the spring of 1933 you were offered an assistantship in Hamburg.

Peierls:

I was, of course, looking for a job for the autumn of ‘33 when my Rockefeller Fellowship would be finished. I was offered that job in Hamburg, and they were anxious to have me come soon with the idea of my abandoning the second half of my Rockefeller year, and I had informally agreed to that when things changed, and that was scratched.

Weiner:

You had agreed to it in the autumn, then, of ‘32.

Peierls:

Well, sometime during the autumn.

Weiner:

That establishes the chronology. In this decision, how were you aware of what was going on in Germany? Was it through reading about it or through discussions with other colleagues?

Peierls:

Mainly through reading the newspapers, and, of course, occasionally talking to people who lived there, including my parents. One was worried how things would go for some time, of course, but the final point at which I made my decision was before Hitler came into power. It was when the government of Schleicher fell, who was a relatively reasonable person. He was replaced, under pressure from Hitler, by von Papen. It was then that we saw the red light and decided nothing good could possibly come out.

Weiner:

By this time you were married. You married in 1931.

Peierls:

That’s right.

Weiner:

And so the “we” infers you and your wife. Did you discuss this with others in Rome at the time?

Peierls:

We did, and the general reaction of the Italian colleagues was to say: “Oh, we understand how you feel, but you’re exaggerating. We’ve gone through similar things, and they’re not really so bad, and you’re pessimistic in deciding to go home.”

Weiner:

How about Fermi? Did he join in this?

Peierls:

I don’t remember that I discussed it with Fermi specifically. If I did, his reaction must have been the same as the others or else I would remember it.

Weiner:

What were your specific reasons for not wanting to go back, other than the uncertainty? Was it a question of whether you would actually be able to be employed?

Peierls:

No, that wasn’t the primary thing. That was part of it, of course, but generally the climate of opinion, the political climate, was one in which you couldn’t possibly do any sensible work. And since also the basic trend was anti-Semitism, you didn’t want to be in a place where you were not wanted, whether they actually threw you out or not.

Weiner:

This was before any of the laws.

Peierls:

Oh, sure.

Weiner:

It was at least six months or more.

Peierls:

I can’t remember the dates, but I think it must have been about March or February that we decided. No, maybe …

Weiner:

You see, that again mixes up the dates. Wouldn’t it have been the autumn?

Peierls:

No, it wasn’t the autumn. I think maybe it was around Christmas. I’m afraid I don’t remember the month, but it’s pinned down by the date when Schleicher’s government was replaced by von Papen.

Weiner:

Well, Hitler came to power on January 30th or 31st, ‘33.

Peierls:

Oh, well, then I’m wrong. Then it must have been December.

Weiner:

I see — it was before that. And then the laws relating to university positions came in around March.

Peierls:

Then we must be talking about Christmas, which actually makes sense because I remember then making a special trip to see my father, who was then in Switzerland, probably around Christmas, to try to persuade him to leave the country, too.

Weiner:

I see. Had he moved to Switzerland?

Peierls:

No, he was just on a vacation.

Weiner:

Did he move? He stayed?

Peierls:

He stayed, mainly because he was getting on; he was half retired. He didn’t know any foreign languages, and didn’t like to be dependent on other people, and he entirely agreed with me on the principle — he understood — but he said, “I’m too old to move.” A year later he said, “You were right. I should have gone then,” but by that time the financial restrictions got so much worse. “If I could leave on the conditions I could have had a year ago, I would go; but now it’s too miserable.” So he finally left late in ‘38 or early ‘39.

Weiner:

Did he go to England?

Peierls:

He stayed in England about a year, I think, until he got his visa to come to the United States, where he had a brother, who was quite well off and quite happy to support him.

Weiner:

Where in the United States was that?

Peierls:

Oh, just outside of New York, in Montclair.

Weiner:

Now, during 1932, when you had been offered the position at Hamburg, this was directly through Stern, who offered you the assistantship?

Peierls:

Yes. Well, it wasn’t for him to offer, but my correspondence was with him. And this was quite shortly before I turned it down. I mean it was only a few weeks between my saying “yes” and then changing my mind and saying “no.” What in fact happened was an informal correspondence with Stern. I had said I was willing to come, and then I got an official letter from Lenz, who was then the professor of theoretical physics, making an official offer, and at that point I said “no.”

Weiner:

Where would you have worked had you gone there? Stern had four people essentially in his laboratory — Frisch and three others. You wouldn’t have gone into his laboratory, but into the Department itself.

Peierls:

Yes. Well, I never got the details of the set-up there sorted out very clearly. We didn’t get far enough. But I think the job I was offered was essentially the job that Pauli had had when he was in Hamburg, which probably was the job of assistant. You know, in those days a job, unless it was a chair, was an assistant to somebody. And that was an assistant to Lenz, the professor of theoretical physics. And, of course, the fact that it was a job that Pauli had before made it more attractive.

Weiner:

Right — a sort of succession. Had you been in touch with any of the people there on any visits prior to that?

Peierls:

No.

Weiner:

And you hadn’t been in touch with, say, Frisch, who was there?

Peierls:

No.

Weiner:

You didn’t know him at that time?

Peierls:

I think not. I might just have met him at some conference or other. I think not.

Weiner:

And during this period, you had known Bethe from Munich. Had he been in touch with you regarding his life at Tubingen during that same year?

Peierls:

No. I’m trying to think, had met Bethe at a conference of the German Physical Society. Now I’m trying to remember what year that was. I think it must have been during the summer of 1932, maybe the year before, perhaps ‘31.

Weiner:

In ‘32 you were in Rome for the summer, unless you had come back.

Peierls:

No, I only went there in the autumn.

Weiner:

I see, right.

Peierls:

But it was either essentially on the way to Rome, or it was the year before. I could reconstruct that probably.

Weiner:

I ask for this reason because he, in Germany, was experiencing the changes that were going on and was reacting to them, and I have thought that if you had been in touch with him, this would have been a very good source of information.

Peierls:

If I did see him, it was certainly before we got close to the decision of not going back. Nobody liked what was going on in Germany, but then one always has grievances. I mean one doesn’t like some things that are happening in this country, but people in general don’t start thinking about moving out of the country. Some do.

Weiner:

Well, then, the decision was taken to stick with the original decision and go to Cambridge.

Peierls:

Right.

Weiner:

And you described in the interview with Dr. Heilbron how once you arrived in Cambridge you worked with Dirac or under Dirac. This was the intention.

Peierls:

Well, I intended to make contacts with whomever I could find. There wasn’t very much activity that summer in Cambridge, and I had occasional conversations with Dirac and with Fowler, but I was very much on my own.

Weiner:

But at that time you were conscious of the need to find a permanent position.

Peierls:

Well, obviously.

Weiner:

So the decision not to accept the Hamburg one and to stay on your fellowship was essentially one to emigrate.

Peierls:

Oh, yes.

Weiner:

It was very fortunate that you were on a fellowship at that time, because the rules of the Rockefeller Fellowships were that you had to have a permanent position to return to. The records still showed that you had a permanent position, which would have been at Zurich.

Peierls:

In Zurich. This was in order because Pauli had written a letter to the effect that I could come back to Zurich, but there was a sort of understanding that I would not insist on that right. There was no practical possibility of going back, although legally, in view of the statement he had made to the Rockefeller Foundation, one could have insisted on it.

Weiner:

It essentially was a post-doctoral with the idea of continuation of your training rather than just something to enrich you, so that you were supposed to return to your original institution.

Peierls:

Well, I don’t think that the rules were necessarily that you had to return to your original institution, as long as you had some permanent job to go on to. They obviously, particularly in the depression years of the ‘30s, were not interested in having people on their hands who would then say, “Right, but now where do I go?”

Weiner:

Right, stranded. And then it’s the responsibility of the foundation.

Peierls:

Not legally but morally. So that was the point of such a letter. And by acting on that basis, and accepting the sort of informal understanding with Pauli that I would not come back, it meant that I was on my own. I mean I obviously couldn’t go to the Foundation and say, “Now you help me find something else.”

Weiner:

Now, Cambridge was the base. I know the background of hoping for a fellowship to Trinity College and then applying for

Peierls:

No, that came up much later, the possibility of Trinity. There was no prospect of staying in Cambridge, no practical one, except there was then a fairly junior job advertised, which was a university demonstrator ship, I think; and people mentioned it to me. The English practice of always advertising jobs and people applying for them was unfamiliar to me, and therefore I naively thought that if a job was being advertised, they must be looking very hard for somebody who was willing to take it. My attention was drawn to that, and I immediately wrote off to Bethe, also drawing his attention to that opening, to some people’s surprise.

Weiner:

He was back in Munich at the time. He must have been, because he had been dismissed in April of ‘33.

Peierls:

I’d forgotten why he was, but anyway he was there. We both applied, and neither of us got the job.

Weiner:

And then later you applied …

Peierls:

I applied for a job in Manchester. Well, I applied for various other things which I didn’t get. But I applied in Manchester and got a very encouraging informal reaction from Bragg, who was then the head of the department in Manchester. However, he then wrote a rather embarrassed letter later saying that he found there was too much resistance in the University to giving jobs to foreign nationals when there was in fact unemployment amongst academic people in England, and that it just could not be done. They did manage to give a job to Bethe, but they only had one at this stage. So he then arranged for me to get a grant from what was then called the Academic Assistance Council, private money collected for refugees by a committee in Manchester. There was a big committee in London, but there was a separate enterprise in Manchester. They then supported me for two years.

Weiner:

What was the post? It was a fellowship essentially. Or at least the funds were similar to a fellowship. But was there any academic position involved?

Peierls:

I think it was just called a grant. I don’t even know that it was called a fellowship. I think the University gave me some title like research fellow.

Weiner:

But you knew then during these two years that it was temporary, and so you still were basically unemployed in terms of the long-range.

Peierls:

Yes. That’s right.

Weiner:

I can ask later when we’ve covered this period how this affected your whole outlook during the period. Did you have any children at the time?

Peierls:

Our first child was born in August ‘33. So in fact when we took the decision of not going back to Germany at all and face whatever was coming, we knew there would be a first child.

Weiner:

You went to Manchester then, I imagine, in the fall of ‘33, and Bragg was there and Hartree, Bethe came in October, I think. Is that right?

Peierls:

Yes, we must have come about simultaneously.

Weiner:

And he lived with you.

Peierls:

Yes.

Weiner:

One of the things that you mentioned [in the quantum history interview] was your efforts to obtain a permanent post, including one in India. It’s interesting that Nordheim and Heitler were also writing for the same position. We have some letters from the Bohr correspondence and from the Academic Assistance Council correspondence which shows that apparently Born went to India for a brief period.

Peierls:

But that was later.

Weiner:

It was about ‘34, I think.

Peierls:

Maybe it was, but it was only on a temporary basis. I don’t think he intended to stay. But I’m not sure.

Weiner:

Apparently he had no other choice. There were no other positions, and this was what he decided to do and what Rutherford recommended that he do.

Peierls:

That’s right. There was a rather amusing episode — I don’t know whether it was mentioned on the earlier occasion — about that job in India with the testimonial I got from Sommerfeld.

Weiner:

Oh, yes — which was a very strange way of recommending one for a position when the recommending letter is filled with references to other people?

Peierls:

Positively preferring another man and sending me a copy of it.

Weiner:

I just wanted to establish the uncertainty and so forth. But, getting back to the work that you did in Cambridge, apparently you had earlier held a critical view of Dirac’s theory of holes; and by working in Cambridge, perhaps with Dirac, you had come to a better understanding of it and defended him in a sense, because there’s a letter that you wrote making this statement … This is a letter to Pauli, who, of course, was highly critical of the theory. You said, “I am not as completely pessimistic as you are on the question of how far we can take the Dirac theory of holes and obtain single-valued answers.” You argue that there were difficulties with the theory, but the fault wasn’t with Dirac. It was the problem that the infinite self energy would be faced with the same limitations because the problem is fundamental with electrodynamics rather than with Dirac’s particular approach to it.

Peierls:

Yes, I don’t think I was ever critical of the Dirac theory as such. Since clearly one had to combine that with the electromagnetic theory, and the electromagnetic theory was in great trouble, this multiplied the difficulties, but I never held that against the Hole theory. I remember, in fact, very early on, when the Dirac Hole theory was new. I was in Copenhagen and talked to Bohr, who was then very much against this and thought it was inconsistent and quite impossible, that the use of infinities and so on was quite unjustified. And I defended the basic concept to Bohr and said I thought that while it was clear it was not a complete mathematical theory at that stage — because this question of subtracting infinities had to be put on a proper basis—that I saw no reason why it shouldn’t be put on a proper basis.

Weiner:

And this was the first opportunity you’d had to really work with Dirac so that he would clarify his ideas. Prior to that you’d had them only in terms of published papers.

Peierls:

You could never work with Dirac, because if you asked him to explain his theory, he would do it in exactly the words which were in his published paper. He had already made up his mind that this was the best way of presenting it, and he saw no reason to change his words. And generally when you tried to talk to Dirac, as I tried in those days, if you asked him a question, he would say “yes” or “no” if he had thought out the question and knew the answer — or if he hadn’t, he would say, “I don’t know.” But the sort of speculating and thinking aloud that often goes on when you talk with other people, that doesn’t work with Dirac.

Weiner:

This was generally true, or was there anyone that he may have done this with, though? — Someone whom he had a special relationship with?

Peierls:

I don’t think so. He has never had any students really. He has occasionally written joint papers. I wrote a joint paper with him and somebody else on one occasion, with him and Pryce. But that really was that we had each thought out certain aspects of the problem and decided to pool our ideas and publish them jointly. I can’t say that this developed in any way out of conversation.

Weiner:

So there was no conversation with him really. There was no dialogue.

Peierls:

No dialogue in physics.

Weiner:

Pauli was very critical of Dirac. How would you characterize his attitude on this? Apparently your letter was in response to a very critical letter that Pauli had written.

Peierls:

Yes. Well, Pauli was not critical of Dirac in general. I mean he had a high respect for him, of course. But he was doubtful whether there were not inconsistencies at one time — I mean at the time that he must have written this letter, which I don’t have any more. At that time Pauli was worried whether the logic of Dirac’s idea was really consistent and whether it could be incorporated into proper quantitative physical theory. It isn’t that he wanted to throw out the idea, but simply that he was worried about the implications and what trouble one would get into.

Weiner:

And so your better understanding of it wasn’t based on discussions with Dirac but rather, apparently, from turning your attention to it more than you had in the past.

Peierls:

Yes.

Weiner:

Did this appeal to you more than other things at the time? Was this a very exciting thing for you?

Peierls:

It certainly was an exciting thing. But I had all that time, both in Zurich and in Rome, been interested in the problems of quantum electrodynamics. I don’t think I was ever able to make any very constructive contribution, but that was the front line in physics in those days, and clearly one was thinking about it, and from time to time one thought one saw a new point. I don’t think, for example, the fact that I was in Cambridge and close to Dirac had anything to do with that.

Weiner:

You see, these are the kinds of things on the surface that would appear to be connected. But the advantage of being in Cambridge was that you had more time to think about it.

Peierls:

No, I don’t think so. I mean I was in Zurich essentially on full-time research, as I was in Rome. Cambridge made little difference as far as that goes.

Weiner:

Well, what else did you do at Cambridge? What else occupied your time?

Peierls:

I was still interested in some solid state problems. I can’t now remember how much progress I made on those during that summer, because I tend to confuse it with my second period in Cambridge two years later. But I think I must have been mainly busy on thinking about relativistic quantum mechanics. Oh, I remember. I wasted a lot of time. I’m sorry. I take that back — that was later — on a problem in nuclear theory. But that didn’t come out until later.

Weiner:

It will be interesting later to talk about what you mean by “wasted.”

Peierls:

I’ll tell you that.

Weiner:

There were some papers that were submitted during the Cambridge period, but apparently they had been written in Rome: this dispute with A. H. Wilson, for example, which was an outgrowth of your thesis.

Peierls:

Yes. That was written in Rome, but it was convenient, of course, to discuss that in Cambridge. I talked a lot with R. H. Fowler in those days and also with Wilson, of course; and I think we more or less sorted this out — the disagreement.

Weiner:

After your response to his criticism was published?

Peierls:

Yes. I mean I had at first, of course, tried to sort it out in correspondence with him before writing a paper contradicting him, but that didn’t work. And after a lot of abortive correspondence, I finally decided to publish a note on that.

Weiner:

Was this a pretty well-known disagreement at the time? Did it attract any attention?

Peierls:

I don’t think I can judge that. It was known to everybody in Cambridge, of course, and I think it was known to the people interested in the electron theory of metals at the time. But how widely that went, I can’t say.

Weiner:

Did Bloch in any way get involved in this?

Peierls:

No, except that he, of course, knew that he was right and that Wilson was wrong, and that was that.

Weiner:

I ask because you had referred to it in a casual way as a famous paper. But I’m sure you didn’t mean famous as prestigious, but one that caused …

Peierls:

Caused a stir, locally.

Weiner:

Sort of a stir within that community you mean.

Peierls:

Yes.

Weiner:

And was not an issue in later years. It was resolved pretty much then?

Peierls:

Yes. There was one little incident. This was after I left Cambridge. Apparently Brillouin, who was one of the experts in that field, visited Cambridge, gave a seminar. Fowler had then the habit of falling asleep at seminars, but he could control himself sufficiently not to open his eyes when he woke up but listening to the conversation for a while with his eyes closed and then asking a pertinent question, giving the impression therefore that he’d been listening all the time. Well, this misfired when Brillouin talked, because when he woke up, he said at the end of his talk, “Well, how is it now with that controversy between Wilson and Peierls? Who do you think is right?” And he had missed the fact by being asleep that Brillouin had devoted about a half an hour of his talk to an exposition of this, and knowing that since he was in Cambridge, Wilson was probably in the audience, he had chosen his words very carefully and tactfully to get around that embarrassment. And now with Fowler asking this straight question, he was forced to say definitely “yes” or “no.”

Weiner:

Was his response reasonably straightforward?

Peierls:

I suppose so. I wasn’t there.

Weiner:

What else did you study at Cambridge during this approximately six or eight months’ period?

Peierls:

Well, I was generally trying to catch up with what was going on in Cambridge, mainly on the theoretical side — Fowler’s work and that of his collaborators. I suppose that’s about all — I mean in addition to the things I’ve already mentioned.

Weiner:

Did you become aware of the work that Cockcroft was doing, for example, and the nuclear physics experimental work that had sort of taken a new turn just about the time that you had come?

Peierls:

No, I don’t think I had much contact with that. I mean I went no doubt to some seminar meetings where this was reported. But being the summer in Cambridge, there weren’t very many meetings.

Weiner:

But, of course, you were there longer than the summer — unless I misunderstand the dates again. You made your decision not to go to Hamburg about in December of ‘32, and you arrived in Cambridge …

Peierls:

About April [of 1933].

Weiner:

I see. It was close to December. So it wasn’t as long a period as I thought. It wasn’t six months certainly.

Peierls:

Well, it was pretty nearly. I mean I arrived in April, and I left at the end of September.

Weiner:

Yes. Were there any others who could be characterized as refugees or émigré’s, depending on the specific circumstances, which were coming in at that time?

Peierls:

Oh, yes. During that summer, of course, the movement started, and quite a lot of people … I don’t recall anybody whom I knew well who was permanently in Cambridge^[1] — I mean for a long period in Cambridge. But lots of the people who were coming to England either had decided to settle in England or were making inquiries, of course, turned up in Cambridge, and we saw a lot of them — including, for example, Simon, who I remember turned up during that summer just in the process of making his decision to leave Germany and go to Oxford.

Weiner:

Were you still in London at the time of the meeting in Albert Hall in the early fall when Einstein spoke? There was a large public meeting on the refugee problem?

Peierls:

I don’t know where I was. I certainly didn’t go to that meeting, because, for one thing, I don’t think it would have been proper for refugees to go for that.

Weiner:

This was for raising money.

Peierls:

For raising money, exactly. And I don’t think I would have regarded it as proper to go even if I had had notice of the meeting which I don’t know.

Weiner:

You may have already been gone. I’m not sure what month, but it was in the fall — October or November.

Peierls:

I know that Szilard was visiting a lot, and of course he was very busy on these problems.

Weiner:

What about the positron? You described in the earlier interview some of the reaction — well, of the earlier work, the work done at Cambridge preceding the positron. But once the discovery had been confirmed, was there much discussion of it, and did it have much impact? Was it considered a very exciting thing?

Peierls:

Well, it was exciting first of all because it was a new particle and it was rather surprising, because one of the objections made against Dirac’s view that there might be … Well, it was, I suppose, first Oppenheimer who pointed out that the best, most sensible interpretation of the negative energy states or holes was not protons, as Dirac originally wanted, but positrons. But then the first objection that came to everybody’s mind was: “Well, if such a thing existed, then the experimentalists would have seen it long ago.” So that, therefore, was a surprise in a sense. It was exciting in enriching physics, because in those days we didn’t discover new particles every year. Also, of course, it confirmed the basis of Dirac’s view of holes immediately. And then, of course, one started thinking about the implications for practical problems — for example, it was then fairly soon realized that ideas about the attenuation of gamma rays in going through matter had to be profoundly changed, because at high energies in heavy elements the positron production was the dominant process. And people started thinking about that. I clearly was interested in that. At the time I don’t think I made any contribution to that.

Weiner:

These discussions that you refer to would have been in the period still at Cambridge.

Peierls:

Oh, dear. I can’t even remember now when the positron was discovered.

Weiner:

It was ‘32, but it became effective and published and known in ‘33.

Peierls:

Yes. I probably had advance information through Blackett, who was in Cambridge then. But whether these discussions were then or later, I couldn’t say.

Weiner:

Did you have regular discussions on an organized or a semi- organized basis with other people in Cambridge?

Peierls:

Nothing organized.

Weiner:

Nothing, primarily because it was the summer.

Peierls:

Yes, and Cambridge isn’t very highly organized anyway or wasn’t in those days. There were always in Cambridge, of course, regular seminars. And whether they continued during that summer, I don’t know. I think probably the meetings of the so-called Kapitza Club continued.

Weiner:

Yes. Well, certainly Kapitza himself was still there unless it may have been that he took a trip back to Russia that summer.

Peierls:

He was away part of the summer, but I certainly met him then.

Weiner:

And you met Blackett.

Peierls:

Yes, certainly, and I met many other people. Yes, I think I probably also spent some time in Cambridge still thinking about the problem of magneto resistance, which had been a puzzle in the electron theory of metals, and on which I had published some stuff earlier. But the linear law which Kapitza had found for this magneto resistance was a puzzle, which didn’t get resolved till many many years later — a few years ago.

Weiner:

I know there were no papers as such published there. Any papers that came out during that period were essentially ones that had been started either in Zurich or in Rome.

Peierls:

That’s right.

Weiner:

These were sort of finishing them off. How would you characterize that period, though, in terms of what it did for you or the effect on your later period?

Peierls:

I don’t think that’s easy to say. I mean I was young and learning all the time, and I don’t think that the fact that I was in Cambridge had any very special significance for me — much less so than being in Rome with Fermi.

Weiner:

You had been there, of course, earlier — in the summer of 1928.

Peierls:

Just for a vacation.

Weiner:

But this effect of being exposed to a different scientific environment had already … You were quite sophisticated by the time you returned in 1933, because

Peierls:

Yes, except I hadn’t really appreciated the scientific environment of Cambridge. In ‘28 I was there for a summer vacation more or less as a tourist, and I spent one period of I suppose a week or two in Cambridge not seeing any physics at all — it was just one of those summer courses for foreign students, quite wide and general, mainly language and so on. Then I came back for a few days towards the end of my stay in England, and I met Dirac once. I called on Fowler, and I was then asked to give a talk in the Kapitza Club, to my great surprise, because I was really quite junior. But Fowler just said, “Here is somebody who knows the work of Felix Bloch and can tell us about it because we haven’t understood it.” So I found myself with my then very limited English and rather limited knowledge of physics addressing the Kapitza Club. But I cannot really say that at that time I got any sort of feeling for the atmosphere there.

Weiner:

But by the time you came back in 1933 you had been to the Soviet Union a couple of times, and you had been to Italy. And so you were rather sophisticated. Your international contacts in physics had been developed.

Peierls:

Yes, certainly.

Weiner:

Let’s talk about the international ties for a minute, before we go on to the details of the Manchester period. Apparently the invitation to go on to Russia came about (there were a couple of trips) through Landau’s being in Zurich.

Peierls:

No, I don’t think so. I think the first invitation, which was to a Soviet conference on physics to which they decided to invite quite a few foreigners, came through Frenkel, who was a very active theoretical physicist, and who had seen my papers and was interested. He may have consulted Pauli — I don’t know — but Pauli came to that conference, and I just got an invitation, and obviously was very surprised and pleased and decided to go.

Weiner:

Where did the funds come from for that?

Peierls:

The invitation covered all expenses inside the Soviet Union. You had to find your own fare to, in this case Odessa, where the conference was, and back again from Leningrad. Well, I had enough money myself from my parents to cover that sort of fare. In those days you traveled third class by train, sitting up all night, and it wasn’t very expensive.

Weiner:

This in 1930? You went to Leningrad in 1931 for two months.

Peierls:

Right; this was 1930, the conference.

Weiner:

How did that invitation to Leningrad come about? Was this also through Frenkel? He was there.

Peierls:

He was there. Well, I talked to several people. I can’t remember who really took the initiative. It may have been a young experimentalist, Dorfman, who talked to me and said wouldn’t I come to Leningrad. No doubt Frenkel would have had to give his approval.

Weiner:

And what was the subject of your lectures during the two months?

Peierls:

The electron theory of metals, which, in fact, I was persuaded by Dorfman to give in Russian, which, since I had only more or less started to learn Russian, must have been agonizing for the audience but very instructive for me.

Weiner:

Right, at their expense. What was your impression of the research interests on these two visits? For example, was it possible to characterize their interest as being dominated by solid state or … In other words, how was it different from what else you knew was going on in other countries in physics?

Peierls:

It wasn’t really different so far as I could judge for myself. You see, I would talk mainly with the theoreticians, except for giving an outline of electron theory of metals, which was meant as far as possible to appeal to experimentalists who were interested in that subject. That was the front line of physics then and one of the applications of quantum mechanics. The other was atomic spectroscopy, about which I didn’t know much in detail. I don’t think I had much contact with the experimentalists there, particularly as Joffe’s laboratory, to which I was attached, was mainly solid state. I wasn’t inclined in those days to make a picture of the whole set-up and ask, “What are they doing in other laboratories?”

Weiner:

Of course, the time you were there was very limited, and you were stationed pretty much in one place. How large a group attended the lectures on a consistent basis, and at what level were they?

Peierls:

I think they were at a level we would now call graduate students and higher. I mean it would be the kind of course I would give today to graduate students interested in solid state theory — both theoretical and experimental students — and anybody else who cared to come, any senior person who cared to come along. I occasionally still meet Soviet physicists who say they were at those lectures, and that was their first introduction to solid-state theory.

Weiner:

How about Gamow? Was he there at the time?

Peierls:

No, he was abroad.

Weiner:

This was his important year abroad, right.

Peierls:

More than a year, I think.

Weiner:

I guess it was two years till he returned. How about nuclear physics as a field? Was this at all of interest?

Peierls:

Not of interest to me at that time, because there wasn’t any nuclear physics really.

Weiner:

I was referring to interest in the Soviet Union.

Peierls:

Yes, there were some people already thinking about accelerators and doing experiments with alpha rays and so on, but I had no contact with them.

Weiner:

How about Ivanenko? Did you see him there?

Peierls:

Oh, yes.

Weiner:

And what was his concern then?

Peierls:

Well, he was always going for the most fundamental issues. Undue modesty was never one of his vices. In those days he was lively and very entertaining, very witty, and stimulating, although some of his ideas were quite wild.

Weiner:

At that time was he senior among the advanced students? When did he get his degree? I think he had probably just gotten his degree a little earlier.

Peierls:

Oh, yes. Well, all that theoretical group there — Landau, Gamow, Ivanenko, Bronstein (who was very good but died later — I think in a camp) — were sort of on one level. Frenkel, being a professor, was more senior. [Note: This was not quite accurate: Bronstein was several years junior to the others.]

Weiner:

But those were the most outstanding or the ones you remember most. Is this influenced by their subsequent careers, or was it obvious then?

Peierls:

That was obvious then. Of course, Landau and Gamow were both abroad at the time.

Weiner:

That was ‘31. While you were there in ‘31 you went to the conference — I guess you’d call it a theoretical physics conference. You published a paper later which was originally presented there. This is the Kharkov conference. I think it was November. And it was a paper on The Absorption Spectra of Solids. It was published in a Soviet journal in ‘32, and it was submitted from Zurich as a summary of a lecture delivered at the Theoretical Physics Conference at Kharkov in ‘31. It’s a topic which you later took up in greater detail for your Habilitation paper.

Peierls:

Yes, it was essentially a progress report on that work. Let’s see: that must have been the summer of ‘31.

Weiner:

It couldn’t have been, because the paper was submitted in November, and you were in the Soviet Union, I thought, in September, because at least you submitted a paper from Leningrad in 1931 in September. It could have been August — that’s true.

Peierls:

Right. Now, what happened was: I was in Leningrad in the spring of ‘31 for two months by invitation, when I gave some lectures and where, incidentally, I got married, and at that point my wife did not immediately get permission to leave the country, and so when I left at the end of that two months, it was not clear whether she’d ever be able to come out, or, for that matter, whether I’d ever be able to get back, because you needed a visa for every trip, which wasn’t a matter of routine. So, obviously, on summer vacation, as soon as I could get away, which was, I think, at the end of July, if I remember, I went back to Leningrad and stayed until I had to get back to my job in Zurich, which was presumably the end of September. And obviously I must have been to that Kharkov conference that summer.

Weiner:

Do you remember the conference itself? Do you remember anything that was discussed there other than your own paper?

Peierls:

I don’t even remember my own paper well. No, I remember being in Kharkov, though I wouldn’t be sure of the date except for this evidence. I don’t remember much about the conference I’m afraid.

Weiner:

The fact that you got married during the two months’ visit to Leningrad … This was rather a sudden thing, was it? Or had you …?

Peierls:

Well, not quite so sudden, because we met actually the previous year at the conference in Odessa and saw a good deal of each other then; and, in fact, there was after the conference a small party that went on. Well, the whole conference, all those who wanted, were taken on the boat across the Black Sea, and then a smaller party traveled through the Caucasus, and then in fact my future wife and I stayed a little longer there. And then we started to correspond. And so when we got married we’d seen each other for a total of two or three weeks maybe, but we’d corresponded in the middle for six months.

Weiner:

She was apparently a physicist, too, to be at the conference. At what stage was she in her work when you met?

Peierls:

She had essentially got what we’d call a bachelor’s degree, except she hadn’t got a degree because for reasons of her background and so on, she was not officially registered as a student. Her training was that of a first degree in physics, and she essentially then worked as what we’d call a research assistant, a technician. I mean she was taking readings at one time on some nuclear physics and at other periods in some geophysical work.

Weiner:

Then it was possible upon your return in the summer of ‘31 to bring her out with you.

Peierls:

Yes. At the last minute I think the papers came through two days before I had to leave or something like that.

Weiner:

What was your impression of the physics enterprise in the Soviet Union at the time? Did it look particularly hopeful? Was there great feelings of growth, or was it in fact…?

Peierls:

There were feelings of growth. Certainly there were a large number of enthusiastic people. On the theoretical side, I was in a better position to judge. There was then clearly evidence that a man like Landau was absolutely unique. Of course, I didn’t meet him there — I met him in Zurich. It was also very evident that science was being encouraged. Physics in particular was being officially encouraged. So while the standard of living in those days, of course, was very poor, physicists were in a good position relative to the rest of the population. You had the feeling they were privileged. I wasn’t then very interested in experimental facilities; there was no means of judging the adequacy of their laboratories. One noticed the vast amount of bureaucracy and paper work that ran through all their administration and that made some things more complicated than they would be elsewhere.

Weiner:

Was there any problem in emphasis on choice of research problem? By this I mean outside emphasis? Was some field encouraged particularly?

Peierls:

No.

Weiner:

So, in other words, the problems were physics problems which were generated internally within the physics community.

Peierls:

They had, of course, always the Marxist theory that science must be planned. There were official programs, plans for science for the next year or five years or whatever it may be. Now, that wasn’t during those early visits. This was a bit later when I was around while they were making one of their plans, and it was quite clear that what was really happening was that they did research like everybody else and wrote a plan to embody as far as it could be foreseen what they were intending to do. And the extra art was to write into their plans some degree of achievement that you foresaw, which would look respectable but at the same time should be pitched fairly low, so that you would at the end of the period be able to say that you had fulfilled your plan or maybe had exceeded it. But there was no impression at all that somebody from above was trying to influence them in what they wanted to do. No doubt on the experimental side there was a question of funds for equipment. About this one always has to argue with the authorities. But that I wasn’t in contact with.

Weiner:

Of course, that was true elsewhere, too, whether there’s a private or a public benefactor.

Peierls:

Of course. But I mean, in other words, did not have the impression in those days that in the way science operated there was any real difference. Well, the difference, as far as the governmental system goes, was the great enthusiasm of the young people and the respect for education at all levels. That’s traditionally Russian. It had nothing to do with their political system. It had always been so. And also a fact that was already clear in those days, although I may confuse those impressions with later ones, of the one thing that stuck out if you traveled in a bus or streetcar that people around you would not be reading magazines or newspapers — they would be reading textbooks; sometimes poetry or serious novels, but most of the time textbooks. Partly this was true perhaps because the magazines were so incredibly dull and not worth reading.

Weiner:

How many times in the ‘30s did you have an opportunity to go?

Peierls:

Well, let’s see. I was at the conference in ’30 — I was there twice in ‘31. And then I spent one summer there in ‘32, essentially a vacation, though I visited some institutes as well. And, yes, then I went on a walking tour through the Caucasus with Landau and a friend of his, and I think on the return visited Kharkov where Landau then had a job — spent a few days there. Now, that was ‘32. I think next must have been in ‘31. And the last time I went before the war was in ‘37 when there was a conference in Moscow and when already the chance of foreigners to go there was already deteriorating, when the mass arrest had started. This was heading for Stalinism.

Weiner:

When in ‘34 did you go? Was it after Kapitza had returned? He, I think, went in the summer for his traditional visit. Was it before the summer or after?

Peierls:

I have very little recollection. In fact, I’m not sure I did go in ‘34. I don’t know why that figure stuck in my mind.

Weiner:

One of the things that would be interesting would be to compare the changes, what signs you saw of any changes. ‘34 might have been a good year to observe that.

Peierls:

Oh, just a minute. I think we went in ‘35. I’m not sure we went in ‘34. [Note later: We did go in 1934.] We surely went in ‘35. No, sorry, that’s wrong. I was thinking of ‘37, but that was a conference. Well, generally, there were not any great changes I could observe, except that the standard of living was improving a little. I mean in the first few times I’d been there, there were acute economic difficulties, shortages, hunger even in the villages and so on. And so things got somewhat better in that way between ‘31 and ‘32 but ‘34 was worse again. And then the last time in ‘37, everybody’s fear of arrests and so on was very noticeable. I remember two remarks which perhaps characterize the climate there. It was said as a joke that those people with private telephones were trying to get it disconnected because they thought that people to be arrested were maybe picked from the telephone directory. Of course, it was quite an irrational remark, because as the telephone directory was published only every three or four years, getting your telephone disconnected wouldn’t get you out of the list very quickly. Another thing was that the physicists then were gloatingly showing each other a Russian edition that had just been published of an English book by I think a man called Newman who was an intelligence agent, who wrote up his experiences during the First World War, and it was republished in Russian, just published as a piece of literature. What amused them was the preface in which he said that the best period for a spy to operate, or for an intelligence agent to operate, is at the outbreak of a war or when there is just some general scare, because when everybody is suspecting everybody else of being a spy and there’s utter confusion, then your chances of getting away with things are best. This remark had obviously escaped the authorities who had approved the publication of the book, and people were delighted to show this to each other.

Weiner:

What was the occasion of the visit? You mentioned a conference. I do remember a conference in ‘37, but I forget what the subject was. Was it nuclear physics?

Peierls:

I think it was nuclear physics.

Weiner:

Do you remember very much about that? In particular, did many people come from other countries?

Peierls:

Yes, quite a few came. I remember Niels Bohr there. And I think I was involved in a discussion on beta decay, maybe in fact I was in the chair at that time — I don’t remember. But I have somewhere a reprint of the paper I published on that, on that occasion of the conference. The conference was a bit half-hearted, because all the local people really had their thoughts elsewhere.

Weiner:

Because of the general political climate. Was there any discussion of emigration? Of course, it would have been quite difficult to do. But had this come up? Had people approached you and talked with you about this?

Peierls:

No, nobody did. That, of course, was very difficult, very difficult to get away. The one person who was actually thinking about it but no doubt not talking about it was Gamow.

Weiner:

That was in an earlier period.

Peierls:

Sorry, this was earlier. He was already out. He was the only case of anybody who really took active steps which actually took the form of getting permission to go to the Solvay Conference with some very respected and senior people urging that he and his wife should be allowed to go to the Solvay Conference and promising that they would come back.

Weiner:

She was to be his secretary.

Peierls:

Ah, yes. And I think they had never had any intention of coming back.

Weiner:

Apparently not. I’ve talked with him about this. Well, I just wanted, while we were on that subject, to take it to the logical point.

Peierls:

Incidentally, it’s not part of our discussion now, but if you are interested in Soviet physics of that period, you might like to talk to my wife some time, who of course was there from the early days and who knew Gamow and all these people.

Weiner:

Well, I’ll get to meet her while I’m here during this week, and I may have an opportunity to talk to her more at length — then perhaps in England on a visit if I can’t do it this week. I will be in Oxford, as a matter of fact. Yes, I’m interested, because I’m interested in a comparative study of environments for new fields of physics in a particular time-period. I think it would be very interesting. I have already enough to make me think it would be worthwhile to follow it through. One question on this before we get to Manchester. Had you considered the Soviet Union as a place where you could seek employment during this period when you were an ‘émigré without a position?

Peierls:

Well, the thought went through my mind, but I was not enthusiastic.

Weiner:

What would be the negative factors?

Peierls:

Well, the political set-up.

Weiner:

In ‘33 this still would have been … In other words, even before the repressions, still the general political environment seemed to offer no particular refuge.

Peierls:

That’s right. I don’t know what would have been my position if I’d had a chance to go there and nowhere else. I mean I probably would still have preferred it to going back to Germany, Nazi Germany. But I was young and optimistic enough to believe that sooner or later I’d find a job somewhere.

Weiner:

Some of the people who came from Germany did go either because of desire or need or both, but didn’t last very long, of course. There was a purge of foreigners.

Peierls:

That’s right. I wouldn’t claim that I foresaw that, and that I didn’t go because I knew it wouldn’t last.

Weiner:

Well, now we’re back to Manchester, where you did some very interesting papers with Bethe, some on your own; and you began to get involved with nuclear physics — for example, the paper you did with Bethe on the neutrino, which was submitted in February of 1934. I imagine this is your first paper on nuclear physics, and it’s the first one to deal with something other than solid state or quantum theory.

Peierls:

Yes, I can’t recall the sequence. I mean I wrote a paper on vacuum polarization, which was to do with relativistic quantum theory; but I can’t remember whether it came before this or afterward, but that was not solid state. Also, I wrote another couple of papers with Bethe on the neutron-proton scattering and the deuteron. That was I think after the beta decay paper.

Weiner:

Well, on the neutrino paper — that’s exactly what it was titled — had either of you been in touch with Fermi and known of his theory of beta decay at that time?

Peierls:

I think his paper was published at that time. We weren’t in direct touch with him on that, because all that grew up after I left Rome, and I hadn’t been in touch with Fermi since then.

Weiner:

His paper was submitted in January ‘34, and so the question I ask is — was your paper in Nature, which was submitted in February 1934, intended to be a criticism of the neutrino hypothesis?

Peierls:

It wasn’t intended as a criticism. I mean this was a time when the neutrino hypothesis had more or less convinced people except that one had always a reservation: we won’t believe in this particle until some day we’ve seen some effect of it, which came much later. But it was in fact in that connection of seeing how easy it would be to see neutrino induced reactions that we went through this argument. That’s essentially what our paper was about — to say — if neutrinos can be emitted in a reaction, then they can also be absorbed in the inverse reaction; so let’s find out how likely that is. And it came out that the cross section was extremely small, so that in those days one found it just impractical to do the experiment.

Weiner:

I think it was specifically said that one could conclude that there was no practical possible way of observing.

Peierls:

Right. Well, this was before we knew about nuclear reactors and things like that. This wasn’t meant as a criticism. On the contrary, it was to remove a difficulty. If the cross section had been much larger, one would have seen the effect or should have seen the effect before. So this, in other words, said: “The fact that you haven’t seen it yet isn’t evidence against it.”

Weiner:

Was there anything in particular at Manchester other than Bethe that turned your attention to this subject?

Peierls:

No, again, this was what anybody who was reading the literature was excited about. I mean since Bethe and I were together we talked about it, but I would imagine I would have been just as interested without that contact. There was nobody else in Manchester with a special interest in this except E. J. Williams, to whom we no doubt talked about this. He was a very able theoretician. He was then working with Bragg, largely on solid state problems; but he had wide interests, and certainly would have been interested in that problem.

Weiner:

What was the nature of your collaboration with Bethe? It was, I’m sure, quite different than the paper that you did with Dirac.

Peierls:

Oh, yes, yes. No, well, we saw each other very frequently partly because he stayed in our house, so we had plenty of opportunities for talking, and we saw each other at the University as well. Bethe is certainly a man with whom you can have a dialogue.

Weiner:

Under the circumstances there, what other obligations did you have, what specific obligations?

Peierls:

No obligations. This was a research fellowship, or whatever it was called, but it was essentially in the nature of a research fellowship. So apart from an unspecified, unspecific obligation to do research, of course, and for that purpose to spend a reasonable amount of time in the Department, I had no obligation. On one occasion several of us together — Bethe and I, and I think some others — arranged a sort of introductory course to quantum mechanics, because then quantum mechanics was not yet taught as part of the regular physics course, and there were many people there who wanted to know something about this new theory, and we gave a course of lectures’ on that, but that wasn’t an obligation. It was a moral obligation, if you like.

Weiner:

Would the existing theoretical physics staff have been capable of preparing and presenting such a course?

Peierls:

There was no theoretical physics staff as such. Williams was a theoretician by interest and training. He was in the physics department. I suppose he could have given such a course, and for all I knew he participated but that I don’t remember. He was slightly older than we were and so had come to quantum mechanics from a more classical point of view. I don’t know how familiar he was with the formal side of quantum mechanics. Then separately there was in the mathematics department a group under Hartree called applied mathematics, which was really including theoretical physics. And Hartree certainly was a very distinguished man but a little afraid of the highbrow. I mean his work on approximate wave functions of atoms, which has been very important, was coming to the problem from a rather simpleminded point of view; and I think he was a little too modest to think that he could get into the foundations of the subject. He probably could, but he didn’t.

Weiner:

Was there any conscious feeling in England that you were aware of, regarding the need for more work in theoretical physics and also regarding what the relationship should be between theoretical and experimental?

Peierls:

No, I don’t think I was aware of any such need at the time. There were some very competent, outstanding theoretical physicists in England — not only Dirac, who was always rather limited in his influence on other people because of the way he thinks, but people like Mott, for example, whom I had seen a lot in Cambridge. In fact, I should have mentioned him. He was in Cambridge during that first summer we were there and extremely kind to us. He was perhaps more worried than we were about the fact that here we were with no job after the end of September and with a family coming and so on and did what he could to advise us and so on. He was certainly a first-rate man. Now, he then moved off to a job in Bristol, and a little later but probably still during my Manchester period, I had conversations with him in which he made it clear that he wouldn’t think it right to have any research students (I mean Ph.D. students) normally at Bristol in theory. The way he presented or expressed that was that in those days the prestige of Cambridge particularly on the science side was quite overwhelming, and so one could take it for granted that all really first-rate students would be at Cambridge. The ones he could train at Bristol were already maybe good but not the first category. And also it wasn’t clear then that there was any outlet outside the academic sphere for people with a Ph.D. in theoretical physics. It would be wrong to encourage people to go in for that, because what would they do?

Weiner:

I see. It was the pipeline question that came up then.

Peierls:

Yes. So certainly didn’t have any reaction myself that one ought to train more students.

Weiner:

And yet, of course, because you and Bethe were theorists and also men of good reputation it contributed to your being sought at Manchester and Bristol, where Bethe came later, and Birmingham and so forth, and Cambridge.

Peierls:

Yes. But, you see, this was a time of depression. Now, there were of course quite a few young theoreticians around — mostly trained at Cambridge in those days — and there weren’t enough jobs for them. I mean that there was a need for some theoreticians in the various universities was always clear. There was no feeling that there was an undersupply. In fact, I remember also conversations with Hulme, who was in Cambridge during my first stay in the summer of ‘33, who actually told me that he was getting out of theoretical physics because with so many refugees coming in and, as it were, breaking the market, he didn’t think his chances were too good. Now, this was not said in any spirit of complaint or resentment, but just stating facts.

Weiner:

Of course, the theorists among the refugees were perhaps more visible, but there’s no statistic that I know of yet that demonstrates that the majority of the refugees were theorists — unless you have some feeling for that. I’m probing actually.

Peierls:

I’d have to think about that. I think it’s partly that theoreticians are more easily assimilated. I mean they don’t have to get used to different kinds of apparatus or new ways of handling a workshop or things like that, and so it seemed easier to bring theoreticians in and give them jobs. And I think looking back I would now say that there probably was a shortage of indigenous theoreticians in England, and that’s why so many of the jobs are now held by former refugees. But if that is true, I wasn’t conscious of it at the time.

Weiner:

How about the relationship of theory to experimental work, given the Sommerfeld influence? There were certain kinds of tastes that both you and Bethe and perhaps others had developed. This wasn’t true of other places in Germany certainly. My impression is that it depended very much where you were in Germany.

Peierls:

Yes.

Weiner:

Was this the same story in England, or can you characterize it as different in England?

Peierls:

No. Well, it was a little different, but, if you like, even more so, because, as I have said, the best people were trained in Cambridge undoubtedly. Now, there they were not really under the influence of Dirac. They went to his lectures and profited enormously from that. But otherwise the person running the school of theoretical physics, such as it was, was R. H. Fowler. Now, Fowler was a very good man but in my opinion not a very good teacher, because he tended to have a large project and put students on little corners of that where they really weren’t familiar with the whole nature of what he was after but just looked at their little problem. And then after getting their Ph.D. they went out to a small place or something, they were very poorly placed to continue. I know many of them who simply went on writing papers on the thing they had started on and which by that time wasn’t really very interesting. So I remember I did realize at the time that it was unfortunate that the best place — I mean best from the point of view of supply of students — wasn’t really very well set up for training.

Weiner:

It was quite well set up on the experimental side, though.

Peierls:

Oh, yes. But, of course, you see, basically in Germany theoretical physics was regarded as a separate subject, and generally in a university you’d have a separate department of theoretical physics. In this country it’s usually a part of the physics department, and that happens sometimes in England, but basically in those days — that is to say at Cambridge, which would set the pace — theoretical physics was regarded as a branch of mathematics. And until fairly recently a student who went that way and who studied under the senior theoreticians in Cambridge would be in the mathematics faculty and would never see the inside of a laboratory officially.

Weiner:

And then I think you pointed out in the earlier interview that theorists tended to be attached to the colleges, and so they weren’t clustered. Certainly in the Cavendish you wouldn’t find them.

Peierls:

That’s right, though this became more noticeable later, because in 1933 the numbers were so small and the Cavendish so limited, that you would find all of them in the library — I mean nobody else had much space then. That became more of a factor later.

Weiner:

Now, getting back to the specific details of Manchester, did you have contact with Bragg’s group? I noticed that you were certainly influenced by some of the problems he was working on. And in some of your papers during the period you acknowledged either his comments or calling the problem to your attention.

Peierls:

Well, that, of course, applied particularly to the order-disorder problem on which Bethe and worked in Manchester and which I continued for a bit in Cambridge later. And there were other people there with whom I had contact: R. W. James, for example, who was an expert on X-ray scattering and X-ray diffraction and the more involved theory of that, like the width and the intensity of X-ray lines and so on. I got quite interested in that and had many discussions with James about that. I don’t think I ever wrote a paper about that. But certainly what I learned there has come in very handy later in presenting things— for example, in my book on solids I certainly used what I had gathered or the ways of reasoning I had constructed for myself in helping to understand what James was doing and so on.

Weiner:

But when you get involved in the nuclear physics work, you’re making some specific comments on counting experiments, something very definitely experimental.

Peierls:

I was in those days a frequent visitor to Cambridge, and this paper you mention about counting experiments resulted from a visit to Cambridge. I think it was written at Manchester, but I had been to Cambridge, and I had talked to Ellis, who was then involved in counting and getting lifetimes for some rather weak source. It may have been one of the carbon isotopes, but I wouldn’t be sure now which one it was. And he complained that it was difficult to evaluate the data in a way that would minimize his statistical error, and then I decided I knew how to do that, and I wrote it up.

Weiner:

There was sort of resistance to this. For example, Chadwick in Cambridge had established in early years a drill for counting and so forth, which is not being criticized in this paper. But was there a feeling that there was a separation and that there should be? In fact, the experimentalists had their procedures

Peierls:

No, it was the experimentalist, Ellis, who of course was using the normal procedures … But for most cases, you see, normally you have mostly substances … You have some sample sources, and you can go on counting at reasonable intensities for some time. And then it’s not critical what you do. And Ellis’s work was probably the first occasion where you wanted to get as much information out of a small sample as you possibly could, and therefore you got involved with the limits of these statistical procedures. Now, I don’t think Chadwick would have opposed that approach. I mean he hadn’t himself had occasion to use a sophisticated procedure, but he was too good a physicist to oppose a more efficient procedure when it was pointed out to him.

Weiner:

I wasn’t implying that was the case. I was just curious about the separations that did exist.

Peierls:

You have two things. First of all, there’s a feeling maybe on the part of the experimentalists that this is something they ought to be able to sort out for themselves, and also the fact that not many theoreticians would be interested in that kind of problem or would regard it as their job to think about. But that, I think, might happen whether you have close collaboration and close contact or not.

Weiner:

Other than the work you mentioned with James and the thinking on your part that went into it, was there any other collaboration at Manchester of this type — anything with Bragg, for example? — Anything with experimental work?

Peierls:

I can’t remember anything very specific. Of course, we heard a lot about what was going on in Bragg’s group and were interested in that, but I can’t recall any special items there.

Weiner:

Well, now, getting back to your own work; shortly after you wrote the paper with Bethe on the neutrino, which we’ve discussed, a month later — in February of l934 — you submitted a paper on the vacuum in Dirac’s theory of the positive electron, pointing out that the discovery of the positron lent support to Dirac’s theory of negative energy states.

Peierls:

Well, that was obvious.

Weiner:

Yes, but pointing out that, of course, the theory is still incomplete and that there’s a need to modify some fundamental concepts. Now, the paper had been started in Cambridge and finished in Manchester. Did these difficulties that you referred to in the paper discourage you from pursuing this subject?

Peierls:

No, because my view at the time was that those were basically the same difficulties that one had met in quantum electrodynamics with the electron’s self-energy, and therefore those were the problems on whose solution that progress in physics depended on at the time. I mean I was discouraged to the extent that I didn’t know the answers, and they were unsolved problems, but you couldn’t make progress beyond a certain point without having some new bright idea.

Weiner:

So it was a deep interest, but on the shelf essentially, because there was nothing more you could do with it at the moment?

Peierls:

No, I kept trying.

Weiner:

You did; I see.

Peierls:

I was always (I think already in Zurich but certainly around the Manchester time) interested in the possibility of avoiding … After all, most of these difficulties go back to the concept of a point electron, which already Lorentz knew in classical theory would give infinities; and Lorentz’s view certainly was that the real answer was that the electron must have a finite size. Only an extended electron is hard to reconcile with relativity. It’s always been at the back of my mind that one might get around that and might find a way of reconciling the requirements of relativity with the requirements of finite electrodynamics. And I had another shot at that in about 1947.

Weiner:

Perhaps it was the Solvay paper?

Peierls:

No, that was essentially just a progress report or report on how it looked. Actually I didn’t publish anything on that. It was in a Ph.D. thesis of a pupil of mine, McManus, who worked out a formalism which answered the problem in classical physics at any rate without quantum mechanics. That looked satisfactory as far as it went but didn’t answer the question how you bring quantum mechanics into that. And that again I tried later in the 1950s, but it didn’t work.

Weiner:

This has been continuing.

Peierls:

That’s right. This was, if you like, on the shelf, but it was the sort of thing you take down from the shelf occasionally from time to time and look at again to see whether you can’t do any more.

Weiner:

It was, though, at this time when you begin to see in your work a moving toward nuclear problems, because the possibility of results was greater in nuclear physics than in quantum mechanics?

Peierls:

That’s right. I mean you tend to work on a problem if you regard it as interesting on the one hand, and on the other hand you can see some possibility of progress — see that there might be something that could be done. It’s no good working on problems which aren’t interesting, where you don’t care about the answer, and it’s no good staring at problems which require some insight you don’t have.

Weiner:

And yet, of course, during this time you’re still touching on the theory of metals, mostly in a discussion type of way, answering a criticism, for example — the Wilson thing we talked of earlier.

Peierls:

That was minor.

Weiner:

I’m wondering if the others were minor, too — for example, a criticism made by Kretschmann essentially criticizing the Bloch theory.

Peierls:

That was very interesting, because what it was primarily was that here was a paper that seemed complete nonsense, and I had always believed that when you see that something is wrong, you ought to do something about it in the interests of young people who are trying to get to a subject. There’s been a change in physics because, if you look at the journals at, say, the turn of the century, they are full of little papers written by quite distinguished people pointing out that something somebody else said was wrong. And I think that’s very healthy, partly because it helps people who want to get into the subject to realize that something is wrong and they don’t have to puzzle it out for themselves; partly because it tends to improve the standards of published papers a little bit if you know that if you write something wrong it’s going to be pointed out. Nowadays the tendency is to say, “Oh, well, let’s forget this stuff, and nobody’s going to read that anyway — it’s so wrong.” It’s pretty hard on young people, particularly young people who are not in contact with a group where somebody will tell them: “Forget that; it’s not worth reading.” So anyway in that spirit I just wrote a note. In those days also Bethe did that, writing papers pointing out that something was wrong. I remember one controversy. Bethe had written with Sommerfeld an article in the Handbuch der Physik about the electron theory of metals. He was attacked by somebody whose work he hadn’t done justice to in the view of the author. And he wrote another paper about this in which he complained about this and made a particular complaint that all the references in Bethe’s article to his work were in small print, small type. I think it was on my advice or my suggestion that Bethe then replied to this man’s latest thing and persuaded the editor of the Zeitschrift fur Physik to put the whole paper in small type, with the exception of the last paragraph, which essentially said “the list of errors in Mr. So-and-So’s paper does not claim to be complete.”

Weiner:

That was published?

Peierls:

Yes. You can still find that paper in the literature, all in small print except for the last paragraph.

Weiner:

This is under Bethe’s name?

Peierls:

Yes.

Weiner:

He may have mentioned this.

Peierls:

Well, Kretschmann’s paper seemed like that, and he in particular claimed that the perturbation theory — the approximation used in Bloch’s treatment of the electro-conductivity of metals (and by everybody else since then) was not justified. Now, his arguments for saying that it wasn’t justified were very confused. However, when I replied, I decided … [Interruption] So I sat down in order to write this paper to give a decent proof that the approximation was in fact justified, going through all the criteria one had to look at, and to my great surprise discovered that it was not. So this silly paper by Kretschmann, silly in the sense that he was using quite wrong arguments, really started off quite an interesting investigation. I mean I was worried about this situation — that here one was using with apparently good results a method that couldn’t on first principles be justified. It was rather later talking with Landau — when was that? I can’t remember. Anyway Landau pointed out to me how one could show that the results were still valid in the circumstances, and this is a very ingenious reasoning by Landau, which I think he never published but which I put into a paper of mine somewhere, and later, in my book. It’s only the modern approach to transport problems which have been developed in the l950s which sorted that out. So to that extent what was in my irritation arising from a silly paper had got me into something quite interesting.

Weiner:

This approach that you and Bethe had of correcting things in the literature seems to me (and please correct me on this) to come out of Sommerfeld’s orientation. For example, Sommerfeld students seem to be given to writing review articles or summary articles which perform a critical function, analyzing the existing literature, sorting it out.

Peierls:

I think that’s right. I don’t think it was restricted to Sommerfeld’s school. I mean Pauli, whose approach was quite different from Sommerfeld’s in many other ways, certainly wouldn’t like to let a wrong statement go un-contradicted in a field in which he was interested. I think this was general.

Weiner:

I’m relating that, which can be attributed to many things — to a desire for truth or a combative personality or anything — to this other aspect. And that is to write a proper, full review article, which both you and Bethe did as Sommerfeld students — in fact, as did Sommerfeld for the Handbuch.

Peierls:

Again, so did Pauli. Pauli wrote two big things like that. One is his article on quantum mechanics in Handbuch der Physik, which is still today extremely readable, and of course earlier his big article on relativity, which I think is still the standard treatment. That’s in that same spirit. But the writing of review articles I think is still very common. Writing little letters or articles pointing out that somebody else has made a mistake is not too common today. In fact, some people look on it as being pedantic and fussy and look down on it: “Why should you worry about what other people have done wrong?” But that’s a development of the time rather than any particular school.

Weiner:

In some journals it’s specific publication policy not to publish criticisms unless you can demonstrate that you have some new knowledge that you’re reporting and in the course of that criticize something. This is Physical Review publication policy.

Peierls:

But you can still get into Physical Review Letters, items which…

Weiner:

But in Physical Review proper this has been the standard. Well, that was very interesting about the Kretschmann paper, what it led to. Because there was no way of telling here, just from the list, that it had that many implications. It seems a particularly productive time for you in Manchester, because in February there’s one paper submitted with Bethe; then there’s another one in March, one that you submit on the Dirac theory; and then in March, there are the corrections to the Kretschmann paper, the answer to it; and then in April a paper with Bethe on the neutrino. In contrast to the earlier paper you used the name of it without quotation marks. I’m curious about that. It appears that you are anxious to clear up your position on the existence of the neutrino, and by making the statement that “It is not impossible in principle to decide experimentally whether they exist,” you propose tests involving beta decay. Did this note and the things in it represent a change in the attitude on your part and in his toward the existence of the neutrino?

Peierls:

No. I think we liked the idea from the very beginning. I think I was probably still in Zurich when Pauli first came up with the idea, and I liked it from the start, and so from that point of view there was no change. But with any new idea in physics, you have to decide what the evidence is for and what the doubts are. It was part of that discussion. But there was no major change.

Weiner:

I see. Well, actually the papers were just one following the other. It was a continuation. Very soon after that in July you and Bethe have a paper on the quantum theory of the diplon, as the deuteron was first called. Whose idea was it to study the deuteron? How did you get onto that?

Peierls:

I think those arose because of a contact with Chadwick in Cambridge. Now, I don’t remember now whether I talked with Chadwick. Probably I did, or maybe it was Bethe — I’m not sure. Anyway what happened was Chadwick in conversation said (he had just then measured the photo disintegration of the deuteron), “I bet you can’t calculate what that should be.” I don’t think he told us that he had measured it. He just said, “This is a problem that I bet your theory isn’t capable of saying what it should be.” This was a challenge, and so Bethe and I started talking about this. How these conversations went — I mean who contributed what I cannot now recall — but it was in conversations that we decided here indeed was something that could be done, and since the experimentalist (Chadwick) was interested in the answer, obviously it was worth thinking about it. And we discovered that a certain amount could be said about it, although we did not get the complete answer.

Weiner:

But the desire to learn something about forces didn’t play a role in this?

Peierls:

Well, it was a desire to understand a little more about nuclear physics, which was then becoming a subject in which experimentation was possible. We saw that some questions could be answered. For example, Wigner had already made a very simple theory of the deuteron, taking for that purpose the range of the forces infinitely short (one didn’t then know what the range of force would be). We didn’t, I think, formulate that as in a way of saying, “Well, we must find out what the forces are.” It was clear there were new forces, and that was part of the problem of trying to think about the dynamics of the situation.

Weiner:

What was your reaction when, in fact, the force problems couldn’t be resolved by this kind of thing? Did that become a separate issue? I’m thinking of your later work where the force problem was a major issue and I’m trying to determine whether, because forces entered in the way you stated just now, this became the beginning of a separate issue.

Peierls:

Well, it was clear that there were new forces at work which we hadn’t met before. This was obvious. It was also clear that we had no knowledge a priori of what these forces ought to be. Then, of course, a little later came the idea of Yukawa, that these forces might be mediated by mesons and how to make a simple theory of that, which unfortunately proved inadequate. It was rather later that I caught onto that. In fact, I still remember — I abstracted Yukawa’s paper for one of the abstracting journals, and I must admit that I completely missed the point of the paper. I think I looked back at that abstract later, and I’d worded it sufficiently carefully so that I didn’t give away that it seemed to me a misplaced idea, but it did at that time. I hadn’t understood what the point was.

Weiner:

You were given the paper to abstract.

Peierls:

That’s right.

Weiner:

You were one of the abstracters who worked then for Physics Abstracts.

Peierls:

Yes.

Weiner:

And the paper was already translated into English at the time…

Peierls:

It was published in English.

Weiner:

Oh, yes, that one was. I saw the abstract just recently, and I was surprised to see under what category it was classified in Physics Abstracts. I forget exactly what it was. [Electricity and Magnetism]

Peierls:

I’ve forgotten.

Weiner:

Did the abstracter have anything to do with classification?

Peierls:

At that time, no. Later on there was a scheme where there was a set of categories, and you were supposed to indicate where it should go, but at that time it was left to the editor.

Weiner:

Now, one of the questions I wanted to ask was about the 1934 London conference, where there were two main themes, solid state and nuclear physics. [Pause in recording for lunch] From the beginning, how many people from Manchester went and then what were your impressions of the conference itself?

Peierls:

I’m sorry. I’ll have to rack my memory to remember anything about the London conference in ‘34.

Weiner:

It was an international conference on theoretical physics. There were two main subjects — nuclear physics and solid state. It was in London, but I think there were perhaps some side meetings held in either Oxford or Cambridge. There was a paper that you and Bethe presented there on the photo-electric disintegration of the diplon. This doesn’t bring anything to mind?

Peierls:

No, presumably this is what we had done as we reported on it.

Weiner:

Wasn’t this the international congress to which Madame Curie came, for example? I don’t know if she was there, but perhaps Irene Curie — where they reported on the radio activity results they were getting. There was some discussion. Bethe figures in the discussions of the proceedings on that. And Max Born had a paper.

Peierls:

It was on non-linear field theory probably at that time.

Weiner:

I know he had something on nuclear physics, because that’s the source of the story about his secretary transcribing the paper to be unclear physics.

Peierls:

I remember that. Now, I’m sorry. I don’t seem to recall anything about that conference. In fact, I don’t know whether I was there, because if a joint paper by Bethe and me was presented, it might have been presented by Bethe.

Weiner:

In any case, I know from the program and from talking to individuals that papers were presented but they weren’t there at all, whether it was a joint paper or not.

Peierls:

But you say that there are discussions which show that Bethe was there.

Weiner:

Well, there’s no point in pursuing it. That paper was in the summer, the one paper with Bethe, “The Quantum Theory of the Diplon,” was in July of 1934, and then, for example, in November you were working on scattering, “Scattering of Neutrons by Protons.” That paper was submitted then. Did you become involved in scattering questions because your theory of the diplon failed to provide any information on the nature of nuclear forces?

Peierls:

No, not directly. The reason was simply that it’s the same system, it’s the same Schrodinger equation, whether you have a neutron and a proton bound together or whether you have them scattering each other, and therefore it was natural to go on to see what you could learn about scattering. And there the first thing was, since we had mainly taken the view that the range of the forces was very small or maybe zero, that gave us very simple results about the scattering. This by now is quite trivial, and I think it wasn’t very deep even at the time. I mean anybody could have written it down. Of course we missed the point, which wasn’t known at that time, that the forces are spin dependant — that is, you get for the singlet state very much more scattering at low energies. We had no means of knowing that. We might have foreseen the possibility of such an effect, but we didn’t. What we did do was to look at the effect of the finite range and how far the results would be modified, and that is really the beginning of what became known later as the effective range approximation, which is much more general. This was, if you like, the first application of that.

Weiner:

This again is on the force question. There had been work by Heisenberg and Majorana on forces at that time. Had you been involved in any way in correspondence with either one? Majorana in Italy, when you were there, was not doing this?

Peierls:

No, he was not doing that. I knew him well at that time and liked to talk with him, but I don’t remember any specific things that were discussed and certainly his theory of the forces came later. Of course, Heisenberg did two quite different things about the forces at that time. To classify forces he was the first to think of the possibility of exchange forces. If one looks back, it’s very interesting, because the reasons why he postulated exchange forces of some kind and why from then on everybody took it for granted that there must be exchange forces and that they be responsible for saturation and so on; looking back, the reasons were quite inadequate. The joke is that nevertheless he was right, because the experiments later showed that there were exchange forces, but the same experiments also showed that the amount of exchange content in the forces was not enough to produce saturation of the forces, which was the main reason why he first developed the idea. The phenomenological analysis, just that there might be such forces, and its extension by Majorana pointing out that Heisenberg’s analysis hadn’t been complete, and in fact that the exchange Heisenberg had written down would saturate the deuteron and not the alpha particle and therefore would not give the right answers — well, this was the kind of thing that everybody accepts once he sees it; I mean that you should classify the forces that way, though how much in fact is due to one kind or the other, one didn’t know. At the same time Heisenberg also tried to make a theory of the forces, which was a precursor of the Yukawa theory, representing the forces as mediated by the weak inter-action, by the decay effects, which were the only new things known, which was a completely crazy theory.

Weiner:

This was known to you through what — through the literature?

Peierls:

Yes.

Weiner:

Any personal contact or discussion?

Peierls:

No contact with Heisenberg at the time, no, but no doubt with other people with whom one discussed these things.

Weiner:

One of the means of communicating during this period would have been the discussion groups that Blackett had in London.

Peierls:

It wasn’t specifically Blackett. It was a society — I can’t remember the name [The Physics Club], but it was a discussion group that met at regular intervals in London, sometimes in Cambridge, I think, but mostly in London, and I remember arguments at the time because there was a limited membership of that society, and at first when the number grew because so many refugees had come and so on, it was more or less taken for granted that they might come along. And then people objected to guests being introduced, at least frequently or in large numbers, and the rules were tightened up; and I couldn’t understand that at all. I mean once you had a discussion, why not let anybody who was interested come in? I mean the numbers of interested physicists in those days were not enough to give any worry about the capacity of the lecture hall or anything like that. It was some people who felt they wanted to have restricted meetings so that they could really talk freely and ask stupid questions, and they felt they might be embarrassed if there was a larger audience, which I could never understand. I fought against it. I was then a member of the society.

Weiner:

It was actually a membership society. I wasn’t aware of that.

Peierls:

Yes, and there were some complicated procedures for electing members, and the idea was that either there was a fixed number, or at least there had to be a resolution for bringing in new people.

Weiner:

But apparently this was created to fulfill a need in the existing structure. It certainly was not similar to any of the official groups and was something that cut across university boundaries.

Peierls:

Yes. It was a deliberate attempt to get people from different universities together, because there weren’t, particularly in some of the smaller universities, enough people to make a viable discussion group.

Weiner:

Who was the motivating force behind it?

Peierls:

I’m not sure who started it because it had been going when I arrived in England, but Blackett was certainly interested. G. P. Thomson, I think, played a large part. In fact, I think G. P. Thomson had already left Cambridge and moved to Imperial College. [Note: Thomson did not go directly to London but spent some time in Aberdeen — I’ve forgotten the dates.] It may be that one of the motives was that he felt cut off at Imperial College and he wanted to keep in touch. There may have been others.

Weiner:

Of the names I know of who were there were Nordheim, Heitler, Bethe, yourself, Blackett, Thomson, who else? These are the ones that occur to me. Do you recall any others?

Peierls:

Oh, that would take some time. There were certainly lots of people there — Maurice Pryce, Dirac occasionally came. Oh, it was a fairly large group.

Weiner:

Twenty-five?

Peierls:

Something like that.

Weiner:

Would it include theorists and experimentalists?

Peierls:

Yes.

Weiner:

And apparently they had no means of communicating, say, within the Cavendish and within Cambridge itself, which had certainly a large enough concentration of people.

Peierls:

I think it would not have been necessary for Cambridge. They had close enough contacts probably. And there were things like the weekly colloquium at Cambridge and the theoretical physics seminar and the Kapitza Club and so on to get people together. I think it was partly in the interests of people who were outside Cambridge — to keep them in touch — and partly, of course, for the Cambridge people also to get the comments of people who were outside, either former Cambridge people who had left or people who were sitting elsewhere. It was definitely an attempt to broaden the coverage as between universities, and for the smaller groups it was also probably the only opportunity of taking part in a discussion. I imagine, for example, at Imperial College Thomson wouldn’t have had many people with whom to talk about modern physics. No doubt some; he had his pupils and collaborators — but a rather small group.

Weiner:

And yet London, of course, being so central, would provide a good location to meet with people.

Peierls:

That’s right.

Weiner:

It was easier to get to London from Oxford than it was to get to Cambridge.

Peierls:

Well, there was nobody at Oxford then.

Weiner:

I was just giving that as an example.

Peierls:

Yes, but, for example, Manchester or Bristol. To come from all these places to get to Cambridge was rather awkward, and that’s why it’s easier to meet in London.

Weiner:

What would determine the agenda for discussion?

Peierls:

I don’t know whether there was a formal committee of officers running it, but there would no doubt have been a secretary at any one time whose job it was to send on notices and who would also, either on his own or in consultation with other people, invite somebody to give a talk.

Weiner:

And then there would be open discussion on the basis of that?

Peierls:

Yes. There was no question that once they got together, the discussion was quite open. There was only the question whether it would have been less open if there had been more people present.

Weiner:

Do you recall any particular highlights of the discussions, any subjects that come to mind?

Peierls:

No, I’m afraid not.

Weiner:

Did you feel that it was helpful to you, though, in two senses: in terms of the development of your own research interests, and secondly, in bringing you into contact with a larger number of English physicists than you would ordinarily see?

Peierls:

Oh, definitely.

Weiner:

I was going to ask you what other means of getting to know large numbers of English physicists were available to you?

Peierls:

Well, there were first of all occasional meetings of the Royal Society, which were always somewhat formal. They still had the fiction that the meeting consisted of papers which had been submitted to the Royal Society being read. After all, historically, the Proceedings of the Royal Society are a record of their meetings and should contain papers which have been read already at that time. Only a small fraction of the papers were actually read at meetings. Generally I find that reading a paper or talking about a paper which has been submitted for publication is not the best way of holding scientific meetings. But there is still that fiction. And, of course, also you had to be invited … If you were not a Fellow; you had to be introduced by somebody. But occasionally that could be done and you met people there. Apart from that, it was mainly just personal contact by traveling around. I mean when I wasn’t in Cambridge I used to make quite regular trips to Cambridge to see people and keep in touch. And then, of course, other people, while I was in Manchester, might come and visit Manchester; and you might talk with them and so on. That was the main thing.

Weiner:

How long after you had arrived in England did you feel that it was going to be your permanent home and that you were really a part of the British physics community? I’m saying this recognizing that for the first seven years you had no permanent job, but is it possible that even within that period you had a feeling that this was it?

Peierls:

Well, I certainly had a feeling that I wanted to stay. I liked England very much. In fact, I developed a strong liking for it already on my first visit in ‘28, although I didn’t really see much of the scientific community then, but of life in general. And so I don’t think I ever had any doubt that if I could manage to hang on, I wanted to stay; but, of course, I didn’t feel a very firm assurance that I would be able to. I was not convinced that it was going to be my permanent home. In fact, I still remember when our first daughter was born in 1933, in choosing a name for her, we were very conscious that it should be pronounceable in any language of any country where we might eventually live.

Weiner:

What was the name?

Peierls:

The name chosen was Gaby, perhaps not terribly successful, but that was the name. I remember that this argument was in my mind. I also remember one incident. There was a question of applying for a job somewhere out of the way. I don’t think it was in connection with that job in India, because I did apply for that. But it may have been when there was a job going in Ecuador and feeling that we might be interested in going there. Either it was that or some other place when I, on the whole, felt I did not want to apply. But I wrote for advice to Fowler. It must have been therefore when I was in Manchester. I said essentially, “Obviously you can’t promise me I’ll get a job, but I’d like to know that in your opinion it’s not unreasonable to think there is a finite chance and that it would not be foolish therefore to turn down a possibility elsewhere which I intensely dislike.” And his reply was encouraging. He said that certainly he couldn’t promise, but the chances seemed reasonable, that sooner or later something would turn up, and by this I felt reassured.

Weiner:

Had you ever considered going to America? Other people were going.

Peierls:

This never figured very largely. I don’t know why. I mean it was quite a natural idea. But somehow … I mean later on when I had many invitations and offers of jobs in America, of course I considered it. At that later time I came to the view that while I liked visiting America, on the whole I liked the life in Europe and particularly in England better.

Weiner:

But that was after the war.

Peierls:

That was after the war. But at that time somehow it was just never a serious proposition, although Bethe had of course gone off to America, and many other people. I remember already in Switzerland — it must have been in about ‘31 — one German émigré, Lande, was visiting; and he had gone to America presumably in 1930 or thereabouts.

Weiner:

As early as 1928 or ‘29, I think.

Peierls:

Of course, many people had gone for various reasons. But he explained to us — and I remember being surprised — that he had gone because he saw that things in Europe were heading the wrong way and nothing good could come of it and it was time to get out. This was in ‘31 when I hadn’t yet formed the view that things were going downhill so permanently in Germany. I wasn’t able to foresee it at that time. So I was surprised at his pessimism, and of course later on surprised about his vision — that he had seen it that early.

Weiner:

What was your impression of American physics, and from what sources did you gather this impression? This is at the time in the ‘30s.

Peierls:

I don’t know whether I had a very clear impression except that some of the young people that came over seemed a little bit raw and uncivilized, but I wasn’t generalizing then. And, as I said, it wasn’t at that time any conscious decision, to ask myself the question did I want to go to America and turn it down. I mean, for example, if at that time I had had a specific invitation from an American university, I would have considered it quite seriously. I don’t know what the answer would have been, probably not, because then probably I would have been led to the kind of reasoning that came up later. But it wasn’t like that. Somehow we didn’t think about it.

Weiner:

You had known, of course, of American work in the literature. One paper you criticized was by an American author — I guess somebody at Michigan, one of Randall’s students. Do you remember that? This is early.

Peierls:

You don’t mean Shankland?

Weiner:

No, this is later. Let me see when this was. Well, anyway, I don’t think it was very significant. Oh, yes, you did it with Eugene Guth criticizing a paper by Baker, who was one of Randall’s students at Michigan, for using the Fermi-Thomas method incorrectly in calculating atomic potential functions. That was 1930.

Peierls:

Yes. Well, that was improving on … I mean you noticed somebody had done something wrong and criticized it, but that the author happened to be an American was of no consequence.

Weiner:

I wasn’t trying to make a case out of it; I was just pointing out that you were aware of work all over. When Bethe went, after being so close physically and intellectually, did he write to you? Did he give you his impressions of America at the time? And were you able to get any feeling from him?

Peierls:

He didn’t write much, but he came back for visits. We saw a lot of him. He was quite happy obviously. His only remark I recall — but this is not particularly significant of America or American physics. He had lost a lot of weight. He was a man of tremendous appetite in those days, and when he came back he had lost a lot of weight, and we said, “Good heavens, what happened?” And he said, “Well, it’s so difficult, because for social reasons you have to have lunch in the faculty club” or cafeteria or whatever it was “and you can eat two lunches if you feel like it, but you can’t eat three — that would look too greedy.”

Weiner:

So he had to get along with two.

Peierls:

He was exaggerating. He was joking — no doubt. But that kind of thing clearly doesn’t say much about the scientific atmosphere. And I don’t have any recollection of anything very significant. In those days I think one was thinking of physics as physics and not American physics or German physics or English physics. Of course, the country where you lived had some influence on your life and on your standard of living and things like that, but you didn’t expect that physics would be any different anywhere else.

Weiner:

But one could get impressions of the quality of physics certainly. Let’s put it this way: If out of a particular laboratory in any specific country, there always came bad papers — then…

Peierls:

You would think of the laboratory but not of the country.

Weiner:

But if, for example, out of a particular country came all bad papers, then you could characterize the country. I’m not suggesting that you do this or that in fact it’s possible.

Peierls:

But one didn’t think of it that way. Of course there were countries where there was just no physics going on. The Zeitschrift fur Physik in those days contained always lots of papers from places like Rumania and Bulgaria where just seeing where the paper came from you decided it probably wasn’t worth reading. But beyond that extreme position you would think of places and not of countries. It’s true, however, that there wasn’t very much theoretical physics in the United States in those days. I mean there were some good theoreticians, but there were no very strong centers, and again one knew that there had been … I mean Willard Gibbs was somebody whose name one always thought of with respect. But it was then, as you know, common in those days for people who wanted to get a picture of modern quantum theory to arrange to spend a year in Europe. That was the center.

Weiner:

I’ve been doing some work which sheds some interesting light on this (I’ll discuss it with you later) about where Rockefeller Fellows chose to spend their fellowships. And amazingly the majority of them came to the U. S. And the question is now to find out why. There have been a very large number of them.

Peierls:

In physics?

Weiner:

Yes. This really changes one’s view of the thing, and you have to dig in. But I don’t want to take the time now to go into that. That’s something you can interview me about.

Peierls:

But you mean Europeans?

Weiner:

Europeans did this. Germans as well, certainly.

Weiner:

A couple of hours have gone by, and we’re not refreshed, but I think no problems will occur. When we left off, it was at a logical point to take up a new topic. I wanted to talk about the remaining period at Manchester just before you went to Cambridge, and in particular there’s an important paper on quantum theory of atomic nuclei in Reports on Progress in Physics, in the nuclear physics section of it. And there are a number of ideas that are advanced there, something that sounds like what later people would refer to as the compound nucleus. At least the phrase, the “rearrangement of constituents” of the nucleus is a very provocative one.

Peierls:

Could you remind me of the date of that article?

Weiner:

The date was 1935, and I don’t know when it was sent in.

Peierls:

Well, it didn’t take very long to publish then.

Weiner:

And I was curious first of all about why you wrote it. Were you invited to do this? Was this the way it was usually done in this Reports on Progress in Physics?

Peierls:

Yes.

Weiner:

Who invited it? Who was the one responsible?

Peierls:

Well, it was whoever then was papers secretary of the Physical Society. I don’t now remember who it was. It was probably backed up by a committee but they would not, of course, say who suggested the idea. I have an idea that this was part of a bigger article. I may confuse it with another review, but I have a feeling that somebody else — and I don’t remember who — was reporting on theoretical physics in general and that he asked me to contribute something on what then was known as the nucleus. I may confuse that with something else. We could easily check on that.

Weiner:

I’m looking for what may be a copy of it… Yes, this is it.

Peierls:

Yes. You see, Temple had been writing regularly on the quantum theory, and he then found the subject getting too big, that he couldn’t report on all parts of it; and then he asked therefore various other people to contribute pieces. So probably in that case my contact was not with the editor or the papers secretary but rather with Temple directly. I didn’t know Temple well in those days, and therefore that may have been through the intermediary of somebody else, but I don’t know.

Weiner:

I see. Well, on the paper itself, it’s clear throughout that the concern is with the nuclear force and its nature and it suggests a number of things. One of them is that you refer to Wigner’s suggestion for a combination of the two kinds of exchange forces. I’m curious to know how you knew about it, how you heard of that; and then were you expecting a simpler solution?

Peierls:

You mean simpler than just a combination?

Weiner:

Yes.

Peierls:

I don’t know what I was expecting. People had been talking about just very simple schemes of force, and this clearly was a complication, as in fact later one found that there seemed to be nothing but complications in the structure of the nuclear force. I don’t think I expected anything very strongly either way.

Weiner:

You did hear … This is an unpublished suggestion that Wigner made, about combining the Heisenberg and the Majorana exchange forces, and you refer to it as the most likely proposal of the various ones around to account for the evidence from deuteron scattering. Did you recall how you heard about that?

Peierls:

I don’t recall. Wigner was visiting Manchester on several occasions. Therefore I might have heard it directly from him. Alternatively, it’s the usual kind of hearsay that somebody who had been to a conference might have made that remark. I couldn’t recall exactly how it came to me.

Weiner:

Did you, in writing that paper or at that time, consider the electron-neutrino exchange as a serious possibility?

Peierls:

As a mechanism for the forces?

Weiner:

Yes.

Peierls:

No, it had always seemed to me a very unattractive idea, and I think that was the right hunch. Well, there was a strong argument — namely, that the electron-neutrino interaction is extremely weak, as we know, and therefore indirect effects of its second order effects, which could give rise to nuclear force, would give an extremely weak force.

Weiner:

We were talking about the electron-neutrino exchange possibilities.

Peierls:

Well, in one of Heisenberg’s papers this was discussed with apparently reasonable quantitative results. But the reason for that was simply that because of two-body intermediate stages involved, those particles having very small mass, you would get a dependence on distance with some fantastic power of the distance — I think the inverse 5th power or more—and therefore a system interacting with such singular forces would have an infinite binding energy; and therefore if you choose to believe in these forces down to some stupidly small distance, and cut them off there, you could get any finite answer. But that was really quite unreasonable, and I knew that, and so I wasn’t ever very interested in that possibility except it was a possibility provided there was some completely new factor we hadn’t yet discovered—why the effect should be so much stronger than it seemed, so much stronger than you would expect.

Weiner:

Well, were you prepared to take steps, and if so, which ones, to make the weak exchange force stronger?

Peierls:

I didn’t know how you would do that. I mean, as I’m saying, I didn’t believe in this. It would need some mysterious way of making it much stronger without making beta decay more likely.

Weiner:

And you weren’t willing to postulate …

Peierls:

And I wasn’t willing to play with that. I mean a complicated mechanism like that is sensible only if it fits in naturally with what you see. If you have to push it, then there’s no point.

Weiner:

Now, in the course of the paper, you suggested a second stage of rearrangement of constituents. What was the origin of that?

Peierls:

I’m not sure. You mean in this paper?

Weiner:

Yes. There’s a second section of the paper dealing with nuclear reactions, and you distinguish three stages for such a reaction, the second stage being one of rearrangement of constituents. And then you went on to account for the large neutron cross sections observed in, say, Fermi’s experiments. I don’t know if there’s a part here that’s underlined on this rearrangement business.

Peierls:

I don’t see it.

Weiner:

Are you in the second section of the paper?

Peierls:

But the second section is about interaction forces.

Weiner:

The point of this discussion is that you’re making something of the rearrangement, regardless of the source of the idea, and it brings to mind later discussions on the compound nucleus.

Peierls:

That’s right. Now, so far as I remember, the date of this was before Bohr’s theory of the compound nucleus reactions. But, of course, certainly I had no clear picture of the compound nucleus in Bohr’s sense. But what I had in mind, I think, was that if you have what’s called a transmutation there — in other words, a reaction where something other than the incident particle comes out; for example, a proton going in and an alpha particle coming out — then it is not simply a collision process in which one particle knocks out another, but you have to have a rearrangement in which the two protons and two neutrons get together to form an alpha particle and to come out together. And that was the kind of thing I had in mind. In other words, one was then thinking about the nuclear reactions more in the way in which what is now called direct interactions, where therefore only a limited number of particles take part. Even then you get these rearrangement problems. And also to get them at low energies, to get enough energy for anything to come out, you have to benefit from the full binding energy of the final nucleus. In other words, all the particles have to arrange themselves in the right order to form the stable nucleus for the final product. That was the kind of thing.

Weiner:

Had you been thinking about that for very long before you wrote this review?

Peierls:

Not so very long, I think, but it was what was in one’s mind when one thought about nuclear forces at that time.

Weiner:

In who else’s mind might it have been? Did you discuss it with anyone else or had you heard it expressed?

Peierls:

I don’t think it was discussed much, but from other people’s papers like Bethe’s and Wigner’s and so on, one knows that they’ve been thinking about these things at the time.

Weiner:

As far as the compound nucleus itself, when was it that you first heard of it? Do you recall?

Peierls:

I think probably in visits to Copenhagen. I was visiting Copenhagen fairly frequently in those days. And since Bohr took a very long time to get his papers written up for publication properly, but was always very fond of talking about his ideas, I’m sure … I think I must have in fact heard Bohr lecture on the subject somewhere — I can’t recall where — in which he presented these ideas. I think actually it was not in Copenhagen but it was somewhere when Bohr was visiting and I was present that this came up the first time.

Weiner:

I think his work on that, if recall, was around 1935 when he was developing these ideas.

Peierls:

Yes.

Weiner:

Well, that’s an important point. Throughout this period at Manchester there were papers we haven’t talked about—for example, one on the statistical theory of super lattices. And what it suggests to me is that you were getting some input from Bragg and Williams in discussions of this work, and I want to know what role this played in calling these problems to your attention. Did you feel you had a special responsibility as a resident theorist to comment on …?

Peierls:

Well, subconsciously maybe that played a part, but what really happened was that Bragg and Williams were then making a simple theory of super lattices; and Bethe and I, listening to that, decided it was just too crude and that one could do better. In fact, we were a little doubtful whether this even as a crude approach, was at all justified, which I think was too pessimistic. But certainly it left out important features of the problem, and we felt one could do better, and so Bethe and I discussed this, and I think the approach was sort of gradually developed in our conversations; but the final idea of the approximation which made it workable was Bethe’s. He then worked out an approximation which is now one of the standard techniques in the subject. And although I was not taking part in that work directly, we discussed it all the time, and I was very interested. And now which was the paper?

Weiner:

This one is where you extend Bethe’s method to a different kind of super lattices.

Peierls:

I thought that was written from Cambridge.

Weiner:

It’s 1935. I’m sorry, it was published in 1936, but it was received in November of ‘35.

Peierls:

Right. Well, I moved to Cambridge probably in August or September of ‘35, and my recollection is that the bulk of that work was done in Cambridge. I may be wrong. Maybe I’d started that in Manchester.

Weiner:

I have nothing here to refute that.

Peierls:

Well, the time looks a bit short to do it all in a couple of months in Cambridge, but I certainly did some of that in Cambridge. But certainly it was a result of the Manchester period undoubtedly.

Weiner:

And how did the move itself come about? You became a resident assistant at the Mond Laboratory in Cambridge, and this was, as you pointed out, either in August or September of ‘35.

Peierls:

I presume my job started on the first of October.

Weiner:

Was this a position that had been open and you sought, competed for?

Peierls:

No, no, no. It came out of the blue. What happened was that shortly before that Kapitza had been, as you know, detained in Russia and couldn’t return. And therefore the laboratory had money to spare, essentially Kapitza’s salary, and there was no senior person to appoint in Kapitza’s place. Cockcroft had already taken over responsibilities for running the lab; and they decided to use the funds to create two junior positions.

Weiner:

Temporary positions?

Peierls:

Yes — without tenure. Well, I don’t remember whether I had a letter or whether at some visit to Cambridge, Rutherford asked me would I be interested in such a position. Anyway it was Rutherford who essentially offered me the job.

Weiner:

At that time there was still hope that Kapitza would be able to return, I gather; so that would explain the temporary nature of it. I think that from other evidence I have that feeling.

Peierls:

I don’t think so, because that was a time when arrangements were already well on the way to ship his equipment.

Weiner:

Oh. My timing is off on that. Maybe it had become apparent by that time that he wasn’t coming back.

Peierls:

No, I think the reason for not making a permanent appointment was that in a place like that it wasn’t the practice to have any permanent staff except the director or the technicians and so on, probably not to compete with the universities or something or to take the responsibility, they made relatively junior and temporary appointments.

Weiner:

What was the reaction to Kapitza’s leaving? You came at the time when, as you just pointed out, it had been pretty firmly established that he was not going to come back. And it was just at the time when they had completed the installation of the large, expensive equipment.

Peierls:

Yes. Well, it was a source of regret, of course. I know that some people were commenting that this was sort of extremely uncivilized of the Soviet government to do that. I myself didn’t feel that, and some other people didn’t, because of course one deplored the rather restrictive attitude of the Soviet authorities to foreign travel of their scientists, which was then not as difficult as later but certainly very difficult, and Kapitza was in a privileged position in being probably the only person in the Soviet Union other than diplomats with a permanent passport under which he could go in and out. And he rather enjoyed that position and went back home every summer and got a lot of fun, I think, out of this special position. But there was no particular reason that he should be entitled to that any more than any other Soviet scientist. Generally travel restrictions were a bad thing, but, after all, he had been sent to England with a government grant to acquire knowledge that he would eventually bring back, and that some time they would put their foot down and say, “Well, now you better come back and work for us” didn’t seem all that unreasonable. From the point of view of Cambridge, it was, of course, a pity.

Weiner:

So many years had intervened that it became more difficult to understand, though. I imagine that by that time he was considered more a part of Cambridge than he was of anywhere else.

Peierls:

By whom? Cambridge, of course, regarded him as a fixture. But the Soviet government, of course, did not.

Weiner:

Did you know his wife there?

Peierls:

Oh, yes.

Weiner:

She was still there at that time, or had she already joined him?

Peierls:

I think she had already left. She stayed or came back, I think, to close down their house and so on and bring back the children. But I think by ‘35 when we moved to Cambridge she had already gone. I’m not sure. I had seen them so often at Cambridge that I can’t keep the occasions apart.

Weiner:

Now, the position at the Mond Lab was as you defined it. What were your duties?

Peierls:

No official duties again other than doing research. And, of course, there was an implied moral obligation to take an interest in the problems arising in the lab, but that’s never clearly specified in that kind of research.

Weiner:

During this period after Kapitza had left, since the lab had essentially been built for him, given the lack of that kind of leadership, was there a clear research program for the lab?

Peierls:

Oh, yes, because there was considerable staff. It was by that time a well-equipped low-temperature laboratory because Kapitza needed low temperatures for his work. He had in fact developed a helium liquefier for that purpose, and low temperature physics was a flourishing subject. It’s true that the lab might have been at that time lacking a little in leadership in the sense that there was no very senior and experienced low temperature expert present. And maybe for that reason they brought in more people — I was, to say, an assistant theoretician — in the hope that that would lead to some further ideas that might help stimulate the lab. Another experimentalist, a Canadian, was brought in, with again the hope of strengthening the lab, though he wasn’t a very senior person either. But there was then already David Schoenberg, who is now I think the most senior of that one laboratory and who was starting work on diamagnetism and so on, in which he’s maintained an interest. So it was quite an active laboratory — not outstanding in its leadership perhaps but perfectly civilized.

Weiner:

Certainly still flourishing.

Peierls:

Yes.

Weiner:

You again seemed to have been influenced by your environment, because the first paper there other than the one we discussed on nuclear physics (which you finished but it was something that had come from Manchester) was a paper in March ‘36 on magnetic transition curves of superconductors. I’m not sure that that was your first paper there.

Peierls:

It was one of the early ones, yes.

Weiner:

And a significant one. That’s about it according to your bibliography. That would be the first one then.

Peierls:

Yes.

Weiner:

Well, “The Statistical Theory of Super lattices”

Peierls:

That was a hangover from Manchester.

Weiner:

And then “The Magnetic Transition Curves.” But then within a month you’re on quantum theory again. It seems to have been a pattern that none of these subjects has been dropped for you. And it is fair to say that you’re sensitive to your environment?

Peierls:

Yes. Well, I think there’s a question of how one gets onto research on some particular problem. There are people like Dirac who think on their own and are probably very little influenced by outside opinions or facts, although he does, of course, follow the literature, and he’s aware of what other people are doing at least in the fields in which he is working. But I don’t proceed that way at all. I think investigation to me very often starts either with somebody asking me a question that interests me and where I get intrigued with what the answer might be, and no doubt this question on superconductivity, this paper on superconductivity, arose in that way — that I saw experiments, I think some probably being done in Cambridge.

Weiner:

Gorter and Mrs. Casimir.

Peierls:

Well, anyway, the people in the Mond Laboratory, of course, were following the work on superconductivity, it being in their field, and there was an intriguing question of what the magnetization curve of the spheres should actually look like, and I saw that there was a phenomenological way of describing this. It wasn’t a very deep paper. The actual microscopic detail of what goes on was later sorted out by Landau, but this was just a phenomenological way which I think was helpful at the time for thinking about it.

Weiner:

You mentioned being aware of what else was going on. It reminds me of what you said earlier about doing abstracts for Physics Abstracts. Did they send you a number of papers on a regular basis, or did you just elect to do certain ones?

Peierls:

Well, I did abstracting for a number of journals. The Science Abstracts, now Physics Abstracts, simply send you papers, and they pick out what they think is in your field, and if you agree, you write the abstract and if not, you turn it back and say, “You better send it to somebody else.” You had no general choice. At one time I was writing abstracts for Chemical Abstracts, which covered a lot of physics, too. They were in the United States. There was a different arrangement where I would take over a whole journal, in that case for a time it was Zeitschrift fur Physik, share it with an experimentalist, and we would between us sort out who would do what paper; or if it was impossible, find somebody else; but usually we could manage.

Weiner:

Did you do this for extra income?

Peierls:

Well, the extra income was very slight. They didn’t pay very much. It all helps, of course, and we were short of money. But it was partly a feeling of duty, that this was something that one has to do; partly because it does induce one to read some papers which probably one otherwise wouldn’t have read. It was a good thing from that point of view. And I found it a fairly easy thing to do, because in those days I was rather quick at getting the point of a paper and formulating in a few sentences what it was. So I liked doing it.

Weiner:

Did they seek you out for this, or was it through a friend?

Peierls:

To Science Abstracts I think I applied. I wrote to them and asked could I do some abstracting. You had to offer your services. With Chemical Abstracts — there I think it was the other man, an experimental physicist with rather chemical leanings who got involved in this and asked me to work with him.

Weiner:

Was there any collaboration in terms of the Science Abstracts itself, someone giving you some guidelines or discussing your abstracts?

Peierls:

No.

Weiner:

This was accepted pretty much as you did it?

Peierls:

That’s right. I imagine if an abstract was in some way unsatisfactory, either too long or not sufficiently informative or not clear, they might complain. I never had that. I mean they would, of course, send you some rules about how one should do the abstracting, and then they would then send you the papers, and that was that.

Weiner:

What touched this off immediately was the idea of being aware of what else was going on and would be of interest. You mentioned earlier, though, and that’s what made me realize that you had been involved in the first place, was the abstract on the Yukawa paper that you did. Had you heard of the paper at all prior to that time?

Peierls:

No.

Weiner:

Had any of your colleagues heard of it?

Peierls:

Not at that time. At least I wasn’t aware of it.

Weiner:

And when you did read it, what was your reaction?

Peierls:

Well, as I said, I missed the point, and therefore it seemed to me one of those speculations that might be right but hadn’t anything very special to recommend itself.

Weiner:

Had you heard of Yukawa prior to this?

Peierls:

No. Nobody had heard of Yukawa until this paper got known.

Weiner:

And was there anything coming out of Japan that would make you want to look at the Japanese journals when they did come out?

Peierls:

No. I mean we knew there were some good physicists in Japan and in fact for the resources of the country and so on, there was a surprisingly large number. They had then the reputation of being very good on detail and being very patient and doing long calculations that nobody else would have the courage to undertake. But the main reason why one didn’t read the Japanese literature was that any important papers would in general be published or at least summarized in the international or European or American journals.

Weiner:

Much in the way you were handling it for Science Abstracts.

Peierls:

Yes.

Weiner:

I assume that some of the Japanese journals were taken, though, in the libraries of the large physics institutions.

Peierls:

Oh, yes. But even in those days you couldn’t read every journal, and you generally assumed that if there were something important in a Japanese journal you would hear of it somehow. It would be published somewhere else or at least the reference would guide you to it. This might have been unfair, but that’s the impression one had.

Weiner:

So even though you read this particular article, you saw no special interest in it. You abstracted it, and as you explained, you said nothing wrong because of the language you used.

Peierls:

Right.

Weiner:

When did it become apparent that this was a significant paper?

Peierls:

Oh, when I heard other people talking about it and giving their impressions of what it meant. Then I realized what was going on.

Weiner:

How had they learned of it? Through your abstract?

Peierls:

Oh, no. They had read it independently.

Weiner:

They read the original paper.

Peierls:

Yes.

Weiner:

This is a case where one didn’t wait then for the…

Peierls:

Well, I’ve forgotten now in fact whether the original paper was in a Japanese journal or whether it was in some international journal. ^[2] It may have been. Well, it’s enough for one person to read a paper and draw others’ attention to it. Of course, if you knew there was something special, you would certainly look for it.

Weiner:

Do you have any feeling on the time of that, whether it was before the abstract had appeared in print?

Peierls:

It certainly was after the abstract had been written, of course, but whether it had by that time appeared or not, I don’t know.

Weiner:

Now, they knew of it and they saw the significance of it, but how did they and you evaluate the arguments?

Peierls:

Well, it was the idea of making a little more sense of the existence of strong short-range forces but at the cost of postulating the existence of some new particle; and that seemed to me a bit far-fetched. But when I saw the strength of his original argument, that such a force of finite range must be transmitted by some kind of field, it seemed inescapable that he must be right, in philosophy at least, and that such particles must in fact exist. That’s what gave it the strength and in due course they were found.

Weiner:

Was there any rush to verify this and in fact look for such a particle? Do you know firsthand of any group of experimentalists who in fact set out to look for it?

Peierls:

I don’t think anybody set out very directly to look because it was clear from Yukawa’s work that the mass of such a particle would be far too high to reach with any accelerator that you had dreamt of. So the only place you could look was in cosmic rays. And the cosmic ray experts were, of course, all the time trying to analyze the radiations and what they consisted of and whether any new particles were in there, so this didn’t give any particular spur to their efforts. That’s the kind of thing they were looking for anyway.

Weiner:

But since they’d been doing this for a number of years, and no such particle had been observed, did it raise any questions?

Peierls:

I don’t think so, because the clarity with which one understood what was going on in cosmic rays left much to be desired, and it wasn’t a situation where you said, “Oh, if this was there, they would have seen it long ago.” I mean one was still at the stage where one was wondering whether certain effects were induced by protons or electrons or gamma rays or something. So, clearly, if there had been any new kinds of particles mixed up with them, as indeed there were, one didn’t have the tools to sort that out.

Weiner:

How did this relate then to the whole force problem? Once it was discussed, did you feel that it was on a new level, a new plateau, in any sense?

Peierls:

It didn’t immediately help. One thing it did suggest was a particular law for the variation of the force with distance, the Yukawa law; but the details of that wasn’t of much concern at that time. It couldn’t tell you the range of the force, because that depended on the mass of the unknown particle. In fact, conversely, from what was estimated of the range, Yukawa guessed what the mass would be, and that was about right. It couldn’t tell you whether there was an exchange or ordinary force; if you wanted to decide that, you would have to know whether they were only charged mesons or also neutral ones. So, one didn’t know enough to make any practical use of it immediately.

Weiner:

That would account for the lack of a rush for experimental verification of it, even though the other reasons you gave were sufficient. This would make it understandable to me why theorists weren’t urging experimentalists to find some way to intensify their cosmic ray work, for example.

Peierls:

Well, how could you intensify it? The point is people were doing what they could. They had sufficient motivation to study cosmic rays.

Weiner:

Even if the particle were to be found, it wouldn’t have affected in any very great way the fundamental problems.

Peierls:

That’s right. No doubt it may have intensified the efforts of experimentalists to look at their data and see whether they perhaps can give evidence of such a particle.

Weiner:

I don’t know of any case yet.

Peierls:

No. I mean unless they came out with definitive evidence, they would have kept that to themselves.

Weiner:

I think this is a good point to break. Next time I want to remind us to get back to the remaining year at Cambridge and talk about a few of the things and then talk about the transition to Birmingham.