Rudolf Peierls

Walker:

As I said in my letter, my thesis will be on the history of the German nuclear research during the war and after the war, and although you were not directly involved, I think you are indirectly involved with these developments.

Peierls:

Yes.

Walker:

And I would like basically to ask you about two things in general, one about your own work on the critical mass of the bomb, and the second, I'd like to hear your impressions of simply what you knew about the German work while you were in England, your dealings with your German colleagues during the war and after the war, your general impressions.

Peierls:

Yes. Before we go into details, I have also taken an interest in the German atomic energy program during the war…and I have had some extremely interesting correspondence with Professor Rose — have you come across him?

Walker:

Yes, he's written an unpublished article which will be turned into a book which I've read.

Peierls:

Yes. And it's very interesting, because although he is not a physicist, he doesn't know anything in particular about physics, he has managed to discover details not clear to any of the physicists, particularly that Heisenberg completely misunderstood the physics of an atom bomb.

Walker:

I disagree with Professor Rose. Heisenberg spent extremely little time and effort on the questions of nuclear energy and nuclear explosives.

Peierls:

Yes, but still, he was in charge of —

Walker:

— yes, he was the administrative head. But he spent most of his time on the theory of cosmic radiation, and his attempt to expand quantum mechanics. There were many Germans working on these topics, but Heisenberg made two quick reports on the theory of a uranium machine, as he put it, and these in a sense became dogma for a few years because no one dared question him, until a few other German scientists did better work and finally convinced him to change their minds about proper reactor design.

Peierls:

Ah, OK, now in my experience, what interests me is not so much the question of reactors, how —

Walker:

— they're connected. I'll show you something which was very simple, Rose has claimed that Heisenberg was thinking of a slow neutron fizzing thing.

Peierls:

Ah, it wasn't that so much. I mean, the key thing is that Heisenberg had, at least at some time, changed his mind, at some points, he had an estimate of the critical size of 235 which was not far out, at other times he came to a different conclusion. But he believed that you could not get an explosive chain reaction in, such as the order of the critical mass, because the neutrons, the neutron energy would be increased and the cross section would go down. Now, this is of course completely crazy. Because as your material gets hot, the neutrons will not take the temperature of the medium unless you put a moderator in which would be completely crazy. And so there he seems to have completely misunderstood what he was doing.

Walker:

Well, I don't mean to defend Heisenberg, but that's a part of the report which is quoted by Rose, comes from Heisenberg's paper on nuclear reactors.

Peierls:

Yes.

Walker:

OK. It is within the section in which Heisenberg considers a homogeneous mixture of uranium and water.

Peierls:

Yes, yes, but I mean he does make statements, now I can't be precise about the article in which this is done, at least, and of course coming back to the sources, I'm taking the quotations from Rose —

Walker:

Yes, it's clear that Heisenberg said very different things at different times, and never considered the problem. Let me just show you something, I want to clearly show where he was confused and not simply say he was confused.

Peierls:

Right.

Walker:

You know Samuel Goudsmit's book?

Peierls:

Yes.

Walker:

Well, he wrote his book Alsos^[1] in which he claimed that the Germans don't understand how a bomb works, and they were just going to drop a pile which would fizz. Now, I've found from my own research that although Samuel Goudsmit carefully captured all the German scientific reports, he hadn't read them when he wrote his book. Rose champions Goudsmit's argument that Heisenberg did not understand how a bomb worked. One of the reasons is what you mentioned, but Rose's main point is the fact that Heisenberg was only thinking of a slow neutron bomb.

Peierls:

Well, he has changed his article, quite a bit, you've probably seen an early version.

Walker:

Well, this is a slide that Heisenberg used in a popular lecture in 1942.^[2] He only designed layer machines for nuclear reactors, and this slide represents a layered nuclear reactor machine, normal uranium, that's 238, 235, this is the moderator, and he's showing all the possible processes that can happen, high energy fission, resonance absorption, capture of moderator, and this is what a machine should do, the moderation of a neutron, and its path back, causing fission in 235. Now, this shows a mass of pure uranium - 235, and as you can see, he shows that after one collision the neutron will cause fission or after two or three. Now, the neutrons released by fission move at extremely high speeds, and each inelastic collision will only take away a small bit of energy, —

Peierls:

— right —

Walker:

So this shows a high speed nuclear chain reaction in U-235 which you would have to call explosive.

Peierls:

Right.

Walker:

Now, this has nothing to do with critical mass.

Peierls:

No, no.

Walker:

But this does show that, and you would expect this because it's not very difficult, it's simple, that in pure uranium 235 or pure plutonium, that it would be a high energy, a fast neutron reaction.

Peierls:

Right. That is true I think, Heisenberg says that in places, but he still says in some places that you need tons of 235 for an explosion.

Walker:

Yes.

Peierls:

And how could he imagine that you could get, what several tons of 235 for critical mass estimates? It was much smaller than that, I can't understand.

Walker:

Well, this is not clear because there aren’t any historical sources, there's a book by R.V. Jones, do you know that book?^[3]

Peierls:

Yes.

Walker:

And in the book Jones recounts what he overheard Heisenberg say, or as he read the transcript, Heisenberg was in England at Farm Hall. And he mentions that the argument that Heisenberg used which would produce a critical mass of several tons was a very crude argument. He said that basically —

Peierls:

Yes, I remember that, yes.

Walker:

It says that Heisenberg said that this was a quick calculation he had made in 1940. I believe that. Heisenberg looked at nuclear reactors and, the theory of nuclear reactors and simultaneously the theory of nuclear explosives — I mean, it's the same problem, in a sense. In 1939 and 1940 Heisenberg made a crude nuclear reactor design. He made, according to the evidence from Jones and others, a crude calculation of critical mass. And then he didn't think about the problem again, you see. Now, Heisenberg himself is a bit to blame, after the war it became clear to everyone that nuclear fission is very important, that this uranium question is very important, and the German scientists, Heisenberg and the others, were simultaneously accused by their Allied colleagues, your colleagues for example, of working for the Nazis, and by patriotic Germans of saying that they had failed Germany, that perhaps the Nazis shouldn't have won the war but Germany shouldn't have lost, and their response was a very careful one. They said, "We did very good work. We knew about all these things in theory, but due to technical difficulties, we didn't think it was feasible," and then, those were statements of fact, then they'd say, "Now of course we think it would have been horrible to do such things." After the war, Heisenberg gave the impression, "Of course I realized that these things were important. Of course we all worked very hard on them." But if you look at Heisenberg's papers, he spent very little time on it. That's one of the reasons he was surprised. [when he heard of Hiroshima.] He made a very crude estimate, maybe the first time, the only time he thought about the question. Remember at this time you and many others had left Germany, Heisenberg was, aside from Planck who was very old, and Sommerfeld who was also aged, only Heisenberg and his students like von Weizsaecker and others, were left in Germany and no one questioned him at all. So I agree that Heisenberg made a very rough crude estimate, the sort of thing which you'd expect a physics student to do on the board if you gave him the problem in class, but it's misleading, as Rose suggests, that Heisenberg had worked all the way through the war on this, because he simply didn't.

Peierls:

Yes.

Walker:

But the source, Rose cites, you mentioned about the critical mass, the cross section shrinking because of increasing temperature, now this hints at a bomb, but it's in a section dealing with this homogeneous reactor, and it's hard to say, this is an explicit calculation. See, that's the problem.

Peierls:

No, not a calculation, but I mean, he was quite sure that this would happen.

Walker:

Yes, but there's another problem, and that is that people have the impression that Heisenberg was the only person who determined what was done. There were other scientists. In the first three years of the war the German project was under the control of the Germany army. In February, 1942, when Heisenberg gave this lecture, there was a conference, all the German scientists — Hahn gave a talk on fission products — gave lectures. Afterward, the "Heereswaffenamt" produced a report giving an overview of the entire project. This mentioned Heisenberg's work, and other physicists independent of Heisenberg, and stated that the critical mass of a uranium-235 bomb and is probably between 10 to 100 kilograms, which is about the first estimates you had.

Peierls:

Yes, we were a bit lower, but we were, under estimating.

Walker:

So in a sense, see, that's the other problem, directing too much attention to Heisenberg.

Peierls:

Yes. But I mean, the crucial thing I think was that about that time, Heisenberg decided that to produce the right amount of the separated isotope was not feasible in Germany in wartime, and in a way he's right because even in America, it took longer than the war in Germany. However, one couldn't at that point say exactly how long it would take, nor how long the war would last.

Walker:

I think you have to look at, the different time periods.

Peierls:

Of course.

Walker:

First the Blitzkrieg period, in which up until say October, 1941, the German armies were still advancing in Russia, and every intelligent German thought that the war would soon be over.

Peierls:

Yes.

Walker:

And during this period, there was no rush to produce a "Wunderwaffe" [wonder weapon]. Now, during the next period when the Russians counterattacked at the beginning of 1942, the Germans were still confident of victory but they thought, this may take a few more years. Two or three years. In 1943, the war got much worse. The Americans and British started bombing Germany. Then the scientists began to get very pessimistic about the chances of the war. So it's only as of 1943 that they thought that such superweapons were needed, so 1943 was the time in which they were asked, for something which will be applicable in two or three years.

Peierls:

Yes.. And that of course —

Walker:

In this public lecture, Heisenberg said "Pure uranium-235 is undoubtedly an explosive of utterly unbelievable power," and then he went on to mention, that you can make plutonium in a reactor, which the Germans were aware of, it's a simple enough thing —

Peierls:

— yes —

Walker:

— and then he said, "And this new element 94 is probably also an explosive of utterly unbelievable power," so it is not true that he tried to keep these things quiet at all.

Peierls:

No, no, not at all.

Walker:

However, they were asked repeatedly in 1942 and early "43," this sounds very interesting, however, can we expect practical applications in two to three years?" And the scientists said, "No, no… probably not." And a good comparison here is with Werner von Braun and the rocket people. They were asked, "This sounds very interesting, how much time would you need before we could expect practical applications?" And they said, "Tomorrow, if we only have enough money and support." So it's a very different sort of thing. This moral question, which of course you personally played a part in, the moral question of nuclear weapons, I put it in a short form like this, the German scientists did not try to build nuclear weapons. However, they also did not try to avoid building them.

Peierls:

That's right.

Walker:

The question never came up.

Peierls:

Yes.

Walker:

I will show that these scientists did everything they could for the German war effort, they made a distinction, which they appreciated, which you probably didn't and still don't. That is, they say, "We're not working for the Nazis, we're working for Germany."

Peierls:

There's an interesting point, of course. Why did Heisenberg push ahead with the work on the nuclear reactor, although he knew that it wouldn't be practical? I think initially this was simply because that way he could keep a lot of good scientists out of the front lines. And at the end, when the war was nearly lost, was almost lost, finished, he went on because if they had managed to get the chain reaction going before the war was over, they would have added to the prestige of German science and that would be helpful after the war. Of course, if Fermi hadn't done it a few years earlier.

Walker:

Yes, you remember, as you are perhaps aware, at the very end, the German scientists were sure that if we've only gotten this far, this is proof that the Americans and British are behind us.

Peierls:

That's right.

Walker:

They were supremely arrogant.

Peierls:

Tremendous arrogance.

Walker:

Well, about the first part, about saving scientists, unfortunately I would be a little more cynical. First of all, the first few years of the war, Heisenberg wasn't in charge. A man named Diebner was in charge. Now, it's quite clear that saved many young scientists by giving them contracts to work with the army on nuclear physics. Heisenberg was in a position in the summer of 1942, he became the director of the Kaiser Wilhelm Institute in Berlin. From then on, he was the dominant person in the entire project, OK. Well, I would put it this way, — he did save some young scientists. However, he didn't save every young scientist who came to him. For example, he was doing war work, so as a matter of course he refused to take anyone who was not German.

Peierls:

Well, he was in a position that he couldn't have got away with it, because somebody else would have turned them down.

Walker:

Well, yes, but for other people — some individuals, Heisenberg continued working at it until he found something for them. If they could not work with him in Berlin, they would get an assistant position doing so called necessary work at the University of Freiburg. But for some people he simply said, "I'm sorry, I can't help you." And some young physicists applied to him and he said, "Well, my Institute needs people to work on the following topics, if you'd like to work on these topics, you can come. Otherwise I'm afraid I can't help you." So it's not simply that he set out saving every other physicist who came to him.

Peierls:

No, but still, by running his project, he would therefore have a large number of people there who would otherwise have been in the army.

Walker:

Yes, I would say, it was security, by running this project, all these scientists were doing something which was classified as being vital to the war effort.

Peierls:

Right.

Walker:

Thus they did not have to serve in the army, they received more food than most people, they received all the electricity they wanted and so on. See, that's another explanation of Heisenberg's position. He was the director of the entire Institute of Physics. The only work at the Institute which was war work was the nuclear reactor group. However, because of this one group, the entire Institute was given this status.

Peierls:

Yes.

Walker:

So in a real sense, this project was seen as interesting research, first of all. It was an interesting problem, obviously someday important. It was a very good source of security for the scientists involved. And, but I would say, certainly by 1941, none of the scientists involved had any hope of it affecting the outcome of the war.

Peierls:

Yes.

Walker:

And so that's the way I think you have to understand the rest of the war. They continued working on it, as it was interesting, and because they had good reason to continue working on it.

Peierls:

OK, let's get onto the things you really want to ask about.

Walker:

No, no, this is what I want to ask about.

Peierls:

You might also be interested, you probably haven't seen, I wrote for the Royal Society a biography, about Heisenberg together with Mott.^[4]

Walker:

Oh, I've heard about it, I would like very much a copy of it. Do you also have — yes. — I read your review of Heisenberg's "Der Teil und das Ganze."^[5]

Peierls:

Yes, well, that was a long time ago, that was before I was …?

Walker:

Yes, this was ‘77.

Peierls:

I have many personal things in there which might not particularly interest you.

Walker:

Oh, I'm interested in many things.

Peierls:

Which might not have relevancy to your thesis.

Walker:

Well, the core of my thesis is the nuclear reactor research, in a sense looking for critical mass estimates, is looking for something which wasn't there.

Peierls:

Right.

Walker:

And so basically, the German work during the war boils down to isotope separation and nuclear reactors.

Peierls:

Yes.

Walker:

But I'm not simply writing about the science, although that's the backbone, so to speak. I'm also interested in the connections between the scientists and science policy of Nazi Germany. I have researched in Heisenberg's Institute papers in Munich. And I found many things were very interesting. For example, the so called critical error by Bothe about graphite.

Peierls:

Yes.

Walker:

Well, after Bothe measured the cross-section of graphite, it appeared that graphite could not be used as a moderator.

Peierls:

Yes.

Walker:

And at the same time Heisenberg and a co-worker Doepel had looked at water and heavy water. Water also did not look suitable whereas heavy water looked very good.

Walker:

So they decided by 1941 to concentrate only on heavy water as a moderator. Now, course graphite can be used as a — moderator.

Peierls:

Yes, now, I suspect, but you probably know more from your sources, that Bothe did not do a wrong measurement, but that his graphite was not pure enough.

Walker:

Yes.

Peierls:

The point is, at the time, the — there are impurities in graphite which can increase the absorption cross section, impurities which you cannot detect any other way, or at least couldn't at that time detect in any other way, and Fermi started also with graphite, which had too much absorption, but he had the hunch that his graphite wasn't pure enough, and that if you kept at it by purifying it further, you would eventually bring the cross section down. An additional insight which Bothe did not have. He didn't do the measurements wrong. Would you agree with that?

Walker:

Yes, This overview report discusses Bothe's measurements, and it said quite clearly that in retrospect, Professor Bothe's measurements were probably inaccurate because the graphite most probably contained boron, which absorbs neutrons.

Peierls:

Right.

Walker:

However, in any case, it is not economically feasible at the present time to build a graphite producing plant. That's the key. The heavy water was coming from Norway and it was free because the Norwegian government had to pay for it, so when the scientists asked, "May we have heavy water from Norway?" they said, "Fine." When the scientists asked, "May we build a graphite producing plant in Germany? May we build a heavy water plant in Germany?" they said, "Are you crazy?" Because no one thought that it was going to affect the war. It would cost so much money that it was never considered. So that's a bit of a myth. After the war, in a sense, the Germans, German physics community really had been broken down into two groups, a group of people around Bothe and a group around Heisenberg, and Bothe has basically been the whipping boy, his measurement, was the one in error, but that's absurd. In fact this explains why Bothe — no one measured graphite again. It wasn't feasible.

Peierls:

— well, measuring it again would have given the same answer. You'd have to work very hard to purify it.

Walker:

Yes, and that was going to be expensive.

Peierls:

Well certainly to produce very large quantities of very pure graphite for reactors that would have been expensive. I think in America they had to develop technologies to do it.

Walker:

And the Americans didn't care how much it cost.

Peierls:

Right.

Walker:

I find, the more I research, the less it becomes myth, the more it's very reasonable why they did this and that.

Peierls:

Yes. Well, I'm glad because I mean, I knew Bothe and have a high opinion of him.

Walker:

After the war of course Bothe as far as I know never discussed the topic again. You can understand why not. Of course he won a Nobel Prize afterwards. But he did suffer a little bit from criticism. After the war in Heisenberg's circle, they spoke of their work, which of course was very good, just as good as the Americans' — there was a problem, but that was, in Heisenberg's words, "in another Institute." Which — was Bothe. I realize it's hard just to forget what you learned after the war, but if you could try it for a moment, what did you know about the German work while the war was still going on?

Peierls:

Rudolf Peierls talks about working with intelligence officers to find out what was going on scientifically in Germany during World War II.

I can tell you. It started in a very funny way, because quite early on, I was asked by the intelligence people, not directly, I never talked to them directly but through somebody else, what — what should they do to discover what was going on in Germany along these lines?

And I said, "Well, there are a number of people who would probably be connected with it, and therefore, if you have the possibility of finding out the movements of those people, whether they are away from their university, whether they have lots of visitors and so on, it might help."

So I wrote down a list, starting with the name of Heisenberg, and then an answer came back, "it's peculiar that you should have mentioned his name, Heisenberg, because he lectured in Cambridge in 1939 and we have no record that he ever left the country."

So I was horrified because I thought, if that was the level of our scientific intelligence, then God help us. But the fact is they improved because they put a very good man in charge. Then I decided I would do my own intelligence, from what I could get from publications. And I knew that each semester the physikalische Zeitschrift published a list of the physics lectures at every German university, and that was very informative, because you can't fake that, because it would attract local attention, to say someone was lecturing if he wasn't, and these lists showed that practically all the Germans, the German physicists, were in their normal places lecturing on their normal subjects. Very different from the picture that you would have got from a similar list in this country (England) or in the United States. Not only because of the atomic energy. And so, then, there were some exceptions. Heisenberg for example did not lecture. Then he — then in Leipzig some student wrote a paper, obviously a thesis — he thanked Hund for help and so on, not Heisenberg, although Heisenberg would have been interested in the subject. So Heisenberg either wasn't there or was too busy.

He gave a lecture, a very formal lecture at Vienna, I think — it was old stuff, nothing new to us. So I concluded Heisenberg was up to something.

Then another interesting point was, in the abstracting journal the Physikalische Berichte, there were people who wrote abstracts of papers on nuclear physics and isotope separation, including for example Wirtz.

So I could make a list of names of people who probably were involved. I didn't get them all. But the main impression was, there was some mild work going on, but no crash program.

Walker:

That was exactly correct.

Peierls:

That was correct, but we didn't dare rely on it.

Walker:

When the German project was founded they had a meeting and they invited of course, as is typical in Germany, only the Institute directors.

Peierls:

Yes.

Walker:

And they divided up topics, and for example Paul Harteck the physical chemist received isotope separation, Bothe received the topic of doing what was called preliminary nuclear measurements, cross-sections and so on. This was also to be done at Heisenberg's instance in Leipzig where Doepel was. Doepel did most of the work, and the Kaiser Wilhelm Institute of Physics in Berlin - Dahlem was to be cited for large scale experiments. And there was very little movement even from one Institute to another. They just set things up in each Institute, and went further. It was only toward the very end of the war, when it was harder and harder to get materials, that Bothe's group joined the Dahlem group for the last few experiments. Were you involved with the Farm Hall Business? No?

Peierls:

No.

Walker:

Did you go to America during the war?

Peierls:

Yes. When it was suddenly decided that they would not have a project in this country, but would join up with the Americans, then a lot of us went over to America. We spent six months in New York working with the isotope plant design people, Kelex Corporation, and then after six months went to Los Alamos.

Walker:

When did you come back?

Peierls:

When the war was over. December ‘45.

Walker:

Then to go backward a little bit, I read your article in the Cambridge Philosophical Society Proceedings.

Peierls:

Yes.

Walker:

I also read the papers you cited, Perrin's. It seemed to me that Perrin had suggested an interesting problem, and you were interested in looking at the problem carefully, and it was only afterwards (crosstalk)

Peierls:

— that's right –- In fact, I nearly got into this in quite a stupid way, because from Perrin's paper it came out that there was a critical mass, so that anything below that critical mass — just below that critical mass would be stable, anything a little bit above the critical mass would be a chain reaction. Now, I couldn't immediately see that this sort of discontinuity was right and I thought it had something to do with the approximation he made. He used the diffusion equation which is justified if the dimensions of the system are much greater than the mean free path, so any particle makes many collisions before it reaches the surface (crosstalk) are going to be — Well, if you think just of the thing without a moderator, from the beginning, this is not a good approximation. And therefore I thought, I'll improve the approximation.

Walker:

Make it more general.

Peierls:

Make it more general. And more realistic. And see whether this critical size effect is still there. Of course it was there, that critical size effect, and the calculation was right. I then saw that later. That led me to the calculation which gave a fairly good approximation to a critical size, provided you had a situation where the cross-sections were, practically alike, all the neutrons had the same cross-section and approximately the same velocity, which in fact is not bad for a bomb, for pure uranium 235.

Peierls:

And then at the same time, you got the time scale affairs, the chain reaction again neglecting the variations in velocity, and then in fact, I wondered whether it was sensible or reasonable to publish this paper because it might have some connection with a weapon and maybe there was something I shouldn't publish about it. So I talked to Frisch, who was in Birmingham at this time, and between us we decided that since Bohr had given a reasonable assessment that you could not make a bomb with uranium, thinking of natural uranium, there was no question of a weapon and therefore it was perfectly safe to publish this paper. And it was only then when Frisch came along and said, "Now, supposing somebody gave you a large quantity of separated uranium, what would happen?" Then (with my paper, my formula, one could make a reasonably good guess at the cross-section from Bohr's theory) we came out of it with an estimate of critical size which was surprisingly small because one had thought about tons. And then we said, "All right, now let's see what will happen if you get such a chain reaction" and since I also had some estimates on the time scale, and it was a question of the competition between the expansion of the material over the developing heat and the chain reaction, and we concluded that you would get a high efficiency, we didn't know how high, but it was clearly a tremendous value.

Walker:

That's important. As you expressed it yourself, intuitively you thought of tons.

Peierls:

Yes.

Walker:

Frisch I believe was the one who suggested that you make a simplifying assumption that almost every collision will cause fission.

Peierls:

Well, we did that between us, and it seemed to follow from the Bohr theory, now that isn't quite true.

Walker:

It's not quite true, but this is Goudsmit's argument that the Germans were thinking of a slow neutron device, but you can see that Heisenberg had the same idea in mind. It's what you would think, it's not, as you know, once you have made your formula, these aren't difficult problems.

Peierls:

No, no. I think the vital step was this, that to separate isotopes on a practical, on a macroscopic scale, seems a crazy idea. It seems like science fiction because nobody had separated isotopes except in microscopic quantities, or perhaps milligram quantities of very light elements where the mass ratio was much bigger and the difference was much bigger between the isotopes and so it was a much easier problem. So to do that with large amounts seemed quite crazy, and therefore, one didn't practically think about what would happen if we separated 235. Although Heisenberg obviously had that picture, then he thought of that as an academic thing, not something , not as something we practically have.

Walker:

Well, he thought, maybe someday.

Peierls:

Yes.

Walker:

And he also thought that it would make much more sense to try to build a working reactor, and then make plutonium, because that's easier to separate.

Peierls:

Yes.

Walker:

Well, he thought it would be much easier to separate chemically.

Peierls:

Yes, that is true. That's true. Probably. Probably plutonium is cheaper than 235, I don't know. But that of course in a way is more indirectly because you have first to build a reactor and operate it some time.

Walker:

That's another reason, that's another explanation. Heisenberg's group was involved with nuclear reactors, and other people, Paul Hartek, a man named Clusius, a man named Groth, were interested in isotope separation, and Heisenberg supported reactors much more than isotope separation. In fact, isotope separation work suffered through relative lack of support. By the end of 1941, although the Germans were trying many different isotope separation processes, they hadn't separated anything, nothing at all, and several scientists who were keen to push the reactor group first said, "Look — we haven't been able to separate any uranium. We know in theory that once we have a nuclear reactor we can make plutonium which in theory should be much easier to separate. Therefore we should go ahead with nuclear reactors." The other people continued working on isotope separation, but they received much less funding, much less support, so that by the beginning of 1942, they had basically decided that, eventually we should be able to make nuclear energy, and nuclear explosives, but it makes more sense to go the plutonium way. It's a matter of semantics what term you use. If you say, did the Germans try to build atomic bombs? Did they know how to build a bomb? The bomb is a technological apparatus after all. Then you have to say, no. Because they never tried to do what you did in Los Alamos. However, it's not because they didn't want to build weapons, and it's not because they were afraid of making such horrible things, it's just that they were only so far along.

Peierls:

Yes. That's certainly true.

Walker:

And in a sense, the moral question isn't relevant. Well, afterwards, after 1945, then it becomes relevant.

Peierls:

Yes. Well, of course, the story that they did not choose to do it, to make atomic weapons for moral reasons, was pushed by journalists including Kaempffert of the NEW YORK TIMES, one of its exponents, and of course this terrible book by Jungk.^[6]

Walker:

Well, Heisenberg and von Weizsaecker and others are a little to blame themselves, because they simultaneously didn't want people saying that they were so stupid that they didn't know how to build bombs. But they didn't want to shout to the world that they knew how to build bombs, nuclear weapons, and they were very vague, and people like Jungk were able to quote half of what they said and ignore the other half.

Peierls:

Well, it's basically a bad book because he starts from the postulate the reaction that everybody, anybody who worked on the atom bomb is a criminal, starting with Oppenheimer, and anybody who didn't —

Walker:

— is morally superior.

Peierls:

Morally superior, including even that Klaus Fuchs did what he did.

Walker:

That sort of made him better. He would have been a criminal if he hadn't done that.

Peierls:

Yes.

Walker:

In Goudsmit's book, he claimed that the Germans didn't know how to make plutonium. Now, that was false.

Peierls:

Yes.

Walker:

However, it was partially Heisenberg's own fault. They had just written what's called the Fiat Review of German Science, 14 volumes, careful explanation of what they were doing during the war. Now, there are two volumes, cosmic radiation and nuclear physics. About 400 pages. And you can read them all very carefully and you cannot find a single reference to plutonium. You cannot find a single reference to the properties of pure uranium-235 as an explosive. The Germans carefully did not mention any of this. And in Heisenberg's famous article which was in Die Naturwissenschaften and later in I think NATURE, Heisenberg did not mention any of this. He didn't want to mention this at all. They didn't want to publicize it. And as I said, if you read the original technical reports, you can see that they did know this. Goudsmit hadn't read the reports when he wrote his book. Of course he was under a great deal of strain, he'd lost his parents and so on, and then he received these reports coming out from Germany, giving the impression, exactly what Jungk would do in such clear form later, that the reason the Germans didn't try to do these things was because they were morally superior, and that's the impression Heisenberg gave. And so Goudsmit wrote his book claiming that the Germans were so stupid that they didn't know how to make plutonium. Heisenberg's response was interesting. He wrote to Goudsmit privately, pointing out that when you look at this paper and this paper, you'll see that's not quite right.

Peierls:

Well, he [Goudsmit] took a lot of that back. You know, the article, he sent me a copy, that he wrote, maybe it was in the obituary notice of Heisenberg, sorry, I don't — he wrote an article in which he said, "We've been too hard on Heisenberg," and the way he puts it is that, because he was a great physicist, we expect him to be great in every way and he was only human. I think, toning down the criticism he made before.

Peierls:

The article was written and I think published, shortly before Goudsmit died, and probably in ‘60 —

Walker:

The point is that Heisenberg wrote to Goudsmit, and Goudsmit admitted that he was wrong about plutonium, and then Heisenberg did not publicly but in a letter to Goudsmit asked him, "Well, maybe you should write a letter to the Bulletin of the Atomic Scientists saying that your estimate of our work wasn't quite right." Because Heisenberg even then, he didn't want to publicly say, "We knew all about plutonium, we knew how to build bombs".

Peierls:

Did the Germans, simply from the fact that neutrons are captured in 238, conclude that therefore there must be these two elements, or did they actually have experiments?

Walker:

They managed to produce small quantities of neptunium. They did not manage to produce any plutonium. However, they were certain that neptunium gave off beta particles.

Peierls:

Yes. That was an inference.

Walker:

Yes, that was an inference, plutonium was an inference. That's another aspect of the German uranium project, there was very little cooperation. Often what was being done in one Institute, the other Institutes didn't know about, and Fluegge was at Hahn's Institute, and Fluegge is the one who published an article in 1939 in Die Naturwissenschaften about the possibility of a uranium machine to produce high energy, and he continued working on resonance absorption, but actually had very little to do with Heisenberg's group. That's the problem with looking at the German group as one coordinated group. It wasn't even that. It was a group of 50 or 60 scientists doing different projects.

Peierls:

Yes.

Walker:

Could you tell me about your relations with your German colleagues in the fifties, from ‘46 say to 1960, I would almost call it a rehabilitation of German science.

Peierls:

Well, I mean, it took some time to get used to such contact again, and I was a little upset when one of the first people to be brought to this country officially to give lectures was —

Walker:

— was Heisenberg.

Peierls:

Was Heisenberg. It seemed to me that one should make a distinction and start the relations with people like von Laue or Planck, particularly von Laue who had never done anything that he didn't think was right, — you know the typical story that in Hitler time, von Laue never went out of doors without carrying a brief case and a parcel or two parcels, because if you carry something under each arm you cannot give the Hitler salute. That was typical. Then, in fact, I wrote a memorandum about relations with the German scientists, and sent a copy to Heisenberg while he was in this country, and of course he didn't agree, I didn't expect him to, and in fact, once he was in this country, I invited him to come to Birmingham but sent him this paper representing my opinion, and he agreed to come, and said we would talk about this, but then somehow —

Walker:

— it didn't work out.

Peierls:

An itinerary change or something, he couldn't manage it. Whether it was, he was glad not to manage it, I don't know. But I met him. I met him at conferences, of course. But I always tried to give everybody the benefit of the doubt, or if I met someone of whom I knew nothing, I assumed that they were reasonable people until I had proof to think otherwise. But of course, particularly just after the war, it was remarkable how many people were absolutely sure that they were against Hitler all the time. In fact, the remark was, what a genius this man Hitler was, how firmly he ruled Germany with his dozen supporters and everybody else so firmly opposed to him.

Walker:

Yes, the de-Nazification proceedings weren't very well done. There wasn't a very well worked out process. But the comment of an American major is very good on this point. He said, "It's interesting to come to Germany and find out that the Nazi period was a dream. I've been here interviewing everyone, and it turns out that there were no Nazis, they were just a dream. Every man, woman and child in Germany can prove that he was pro-Jewish and actively opposed to the Nazi regime. It's amazing, how could there be a Nazi Germany when every man, woman and child has 17 people who can prove that he was actively opposed to the regime at every moment and did everything within his power?" About Heisenberg's behavior, there's something very interesting which I found lately which again has never been discussed very much, only by another colleague of his, a Dutch physicist named Casimir, — Heisenberg was invited to come to Holland to give some talks in'43. His Dutch colleagues asked him to come. They thought if he came on an official visit, that would help their situation. They were barred from working, they were being oppressed, as all intellectuals were. And Heisenberg came just after the SS had taken all the Dutch Jews to Auschwitz. Heisenberg came on an official trip, gave scientific talks, and in informal conversation, tried to convince his colleagues that German, he didn't say Nazi, German domination of Europe might actually be the lesser evil, lesser than Bolshevism. Now, Casimir for example mentions this after the war. But also —

Peierls:

He doesn't mention it in his book, though.

Walker:

I thought he did. Well, recently there was an article in the popular Bild der Wissenschaft, a magazine in Germany, an interview with four of Heisenberg's students — it was an advertisement for Heisenberg's collected works — journalists asked questions of von Weizsaecker, Wirtz, Hund and Casimir, and Casimir mentioned it then.

Peierls:

I see.

Walker:

But also, a better source is, visits were very limited, very controlled, whether a German professor could go abroad during the war to talk, and afterwards, Heisenberg had to write a report to the education ministry. At first I thought that Heisenberg would just write what they wanted to hear, "they love Germans here," but actually he was very frank. Heisenberg said, [of the Holland visit] "Although scientific cooperation is possible, my colleagues sharply rejected my attempts to justify the German foreign policy. Perhaps if the war comes to a happy end…" meaning Germany winning, that these things will be calmed down. See, the point, is that Heisenberg was anti-Nazi but extremely pro-German.

Peierls:

He was a strong patriot.

Walker:

Well, I would even use the term "pan German." I would say he would have fitted in well during the First World War. The point is, the Nazi regime was able to use him very effectively as a propaganda tool, because he was the better side of Nazi Germany. He was the "good German" who wasn't a Nazi but also thought that Germany should control Europe and so on.

Peierls:

Yes.

Walker:

It boils down to, these scientists said, "We were working for Germany and not Nazi Germany," but their extreme pro-German stance contributed to the Nazi effort. And it's one thing Heisenberg never apologized for, was his extreme pro-German stance.

Peierls:

Yes, I know. He didn't see that there was anything to apologize for.

Walker:

Yes.

Peierls:

Yes, but you know, there's one story which Rose quotes, I think it was, from me, I mean, I haven't got it, it's second or third hand from me — that Heisenberg met Scherrer the Swiss physicist, rather late during the war, it was clear to both of them that Germany was losing the war, and then Heisenberg said, "It might have been so good, it might have been so nice."

Walker:

"It's a pity we lost."

Peierls:

Yes. And Scherrer blew up.

Walker:

Yes, well, Heisenberg also went to Denmark and told his colleagues, I talked to a man, Stefan Rozental.

Peierls:

— I was suggesting that you might go to him.

Walker:

— It turned out that Heisenberg said the same thing when he went to Denmark, you know, that Germany should win the war. Heisenberg gave many talks at what were called the “Deutsche Wissenschaftliche Institute”, basically centers of forced Germanization, in occupied countries with close ties to the SS. The Nazi government got some support from the scientists. They were very pleased when they heard that Heisenberg wanted to go to Holland. They personally said, "We think it would be a very good idea if you went." And another very pro-German academic was with him. They were very pleased to have them as ambassadors.

Peierls:

Another thing, after the war, postwar, there was a conference where there were discussions again about the war. There was a group of people, including Lisa Meitner, and talk came to the Nazi atrocities, and Heisenberg said, "Yes, but in any period of spiritual renewal, there are cruelties."

Walker:

Yes, it's hard — but you knew Heisenberg. I only knew him as an historian.

Peierls:

Well, it's complicated because, I mean, I was his student and I learned a lot from him.

Walker:

But physics isn't everything.

Peierls:

Oh no, but I mean also, personally. It was a pleasure to work with him.

Peierls:

Yes, and there, I think, he, I don't know if he was a Nazi but he certainly was much closer to being, Nazi ideas.

Walker:

Well, it's an open question, who was a Nazi? What is a Nazi?

Peierls:

Well, I'm not very interested in who joined the party, as you say.

Walker:

The question is, what does Nazi mean here?

Peierls:

Well, I think, you know this nice old comment, when God made a German he gave him three possible attributes, but in order not to go too far, nobody could have all three of these attributes at the same time. And these were to be intelligent, to be honest, and to be a Nazi. You could see, if you were intelligent and honest, you couldn't be a Nazi. If you were intelligent and a Nazi, you couldn't be honest.

Walker:

If you were an intelligent Nazi, you couldn't be honest, if you were an honest Nazi, you couldn't be intelligent.

Walker:

In 1942, Sommerfeld sat down to write his lectures in book form. It became very famous. He was going to publish them, and Heisenberg wrote to him saying that "a political adviser and close friend of mine, also a physicist, would like to call to your attention certain guidelines which are now in use, that is, we note, the publisher noticed that you mentioned Einstein's name four times in your lectures, and we wonder if you couldn't get by with mentioning him a little less often?" The guidelines were, special relativity could be attributed to Aryans and so on. No need to mention Einstein. Special relativity would have come about anyway. Now, general relativity is a little harder. "But you should make the point that this has nothing to do with world relativism, but is merely a calculating tool." Sommerfeld's reply, to the publisher, he wrote, "I omitted the two references of Einstein's name in the special relativity section and one in the general relativity section, but I must mention him once." After the war the names were quickly put back in, but you'll find that the first edition, it was put out in Berlin in 1943 — I think you'll find that Einstein is only mentioned once.

Walker:

The viewpoints are very relative. Today, among say Heisenberg's former students, contemporary Germans, if a German saved one Jew or if he stood up for the ideas of one Jew once, then this man was not an anti-Semite. However, among several Jewish scientists, Jews of today, if a German once did not stand up for a Jew, or once helped persecute a Jew, or once helped persecute the ideas of a Jew, then this man is an anti-Semite. And there's really no chance for compromise there. That's one reason why I was very pleased to talk to Professor Rozental and pleased to talk to you. I've been interviewing all these German scientists, and of course, when you talk to someone about their work in a sensitive area, they put their best side forward.

Peierls:

Of course.

Walker:

They themselves said "I was working for Germany, not for the Nazis." The physicists in occupied Holland couldn't appreciate that. I plan to do is explain, this is the German viewpoint and this is how they justified it, but this was not appreciated in Holland and —

Peierls:

I remember one German, one story of how a German physicist during the war visiting Holland, I think it was Becker, Richard Becker, on the whole a reasonable person, he visited people in their homes, old friends, and they told him about all the shortages in food, and he said, "Well, yes, well, you Dutch of course are only interested in food and material things, whereas we Germans are thinking of higher things."

Walker:

It's easy to think of higher things when you have food.

Peierls:

Right.

Walker:

Where were you from originally?

Peierls:

Berlin.

Walker:

Berlin, so your family was a German Jewish family?

Peierls:

Yes.

Walker:

And after the war of course you didn't want to go back to Germany.

Peierls:

Not for a long time. First of all I went back a couple of times to conferences, then, a few years ago, I spent a month in Munich. I mean, really it was clear always there were very many reasonable people there, and that in fact most of the people I met were, in fact hadn't been born under the Nazis or were in mature. Still, it's emotionally difficult. But eventually, I got used to it.

Walker:

In the fifties the Germans had their so called economic miracle. And suddenly the West German economy was doing better than the British economy. The German nuclear energy research in the postwar world is very fascinating. From ‘45 to ‘55 they weren't allowed to build nuclear reactors, but then from ‘55 to ‘60 they caught up with the rest of the world very quickly. In 1946 the Americans were saying that, we shouldn't help these people, they're Nazis. In 1950 they were saying, we need to help our German friends in the face of the Russian peril. The German scientists sent their young graduate students to the American secret laboratories, where they were trained, and they were helped a great deal by the Americans in applied nuclear research of course Britain had its own program, the Calder Hall complex and other things, when you were back in Britain. I'm talking about the second half of the fifties, during the first part of the fifties was hard times for them but the second part they were catching up and getting much more support from America than the British were. Was there resentment in England?

Peierls:

No, I wasn't aware that they were getting so much support.

Walker:

That's the only way that they could catch up.

Peierls:

Well, you have to distinguish of course academic nuclear physics and the nuclear industry, they're quite separate things.

Walker:

Different things, yes.

Peierls:

I wasn't very much in contact with nuclear power, after the war — I did a bit of consulting. Now, certainly, in academic physics, there was no special support. The German universities were of course supported more generously than the British ones because the economy was in better shape and they could afford more. I don't think there was much American money going there.

Walker:

Well, the Marshall Plan rebuilt the universities to begin with.

Peierls:

At that time, yes, but at that time, they just got — I believe they had been completely destroyed and they were rebuilt, but they were not in any particular way leading, and in fact it took quite some time before the level of German research was comparable to the economic progress. But -– Now, how much help they had on the nuclear power side, from the Americans, I don't know.

Walker:

Quite a bit. I've been looking into that whole — what I've seen is what's, at one time, some German politicians wanted West Germany to build their own nuclear weapons and station them. It was part of the controversy of re-arming West Germany.

Peierls:

There of course Heisenberg was very much on the other side.

Walker:

Yes. In a sense they'd learned their lesson, that nuclear bombs were tainted. However at the same time Heisenberg was very much an advocate of the so-called peaceful uses of nuclear energy. But there really is no such thing. The technology which you need for nuclear power uses enriched uranium or heavy water, and natural uranium plants, and if you have the things you need to make nuclear power, they can also be used to make nuclear weapons.

Peierls:

Yes, that's certainly true, but in fact nowadays any industrialized country that wants to make nuclear weapons can make them. But not everybody does.

Walker:

Yes, and the same, that, this phrase "peaceful uses of nuclear power" to the layman is very misleading. It tends to suggest that it has absolutely nothing to do with weapons, where unfortunately it always implies a potential.

Peierls:

Not directly. You have to work quite hard to get from that industrial program into a weapons program. Because on the one hand, the isotope separation plant, you would only want a moderate enrichment if you were making nuclear power, and to upgrade, say, a plant producing 3 percent enriched material to one producing 100 percent, it's a big step. It can be done, though, of course. The other thing is, if you have reactors which will no doubt produce plutonium, you first of all have to extract the plutonium — the fuel, much sooner from the reactor than you would if you had only a power program. Then you have to have a reprocessing plant, which is again, possible, but it's a biggish organization, difficult, actually, with a high rate of radioactivity — so, it's a big step for people to take or not take.

Walker:

Well, if India could do it, West Germany probably could.

Peierls:

Oh, certainly West Germany could, there's no question. I mean, West Germany could do it.

Walker:

Probably Argentina, Brazil, South Africa's developing a program.

Peierls:

Yes, probably. And Sweden could do it if they wanted to.

Walker:

It's true that in the late fifties von Weizsaecker and Heisenberg and others came out publicly against making nuclear weapons. I think this is a natural extension of the lesson they learned at the end of the war, that, it's the same thing they were saying in 1946, stressing their work on nuclear energy and not mentioning the fact that they worked on nuclear explosives.