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Oral History Transcript — Dr. H. G. Kuhn

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Interview with Dr. H. G. Kuhn
By John Bennett and Anna Shepherd
July 9, 1984

 

Transcript

Bennett:

Could you tell us a little bit about your work in the 1930s and particularly personalities that you were involved with?

Kuhn:

In the 1930s, we have to divide between the early thirties and the slightly later thirties. Until 1933, I worked in Germany, and my subject was atomic physics, especially atomic spectroscopy, and molecular spectroscopy, which was a field that had just undergone a very important change. One might say the twenties were a great time for atomic physics, and for quantum theory in the new form of quantum wave mechanics, and some of these new fields were strongly requested at Gottingen University, by very distinguished people. I worked there at first under James Franck.

Bennett:

What was he like to work with?

Kuhn:

James Franck, the difficulty in giving an answer is, I want to answer without being too enthusiastic. He was quite an exceptional person, and one can safely say, anyone who had any dealings with him would always have remembered him. Whenever he talked to you, or to anyone, he would talk to a person, and you feel this in his very expression, and with the subject alone in mind; but he was not only a very distinguished physicist, he had the Nobel Prize, but very interesting to talk to, and he was a man who liked talking to you and developing his own theories and ideas while talking to you. It gave you a wrong impression that you had taken a part in it. Well, you had, in a way, but not a very active part. And it was a very great time, a great privilege to work under him and then with him, until 1933. That was then the end of this period.

Bennett:

Could you talk a little about ‘33 and what was happening around that time?

Kuhn:

Well, 1933 was the year of takeover by Hitler. The crisis had started before then, but by 1933, it was clear that anyone who had any relations with Socialism, Communism, and especially anyone who was either Jewish or part Jewish, or had a wife who was part Jewish, just had to leave the country, unless he chose to stay there as a second class citizen. So there was no choice. One had to leave. In fact, at that time in 1933, my lectureship was taken away from me. This position is somewhat different in Germany compared to other countries. An important step in your career is the so-called venia legendi, the permission to lecture. This is the first step to an academic career, and this was taken away from me by order of the Ministry of Culture. This is why my wife and I left.

Bennett:

And other colleagues of yours also.

Kuhn:

Oh, yes indeed. It was, in the field of physics. There have been a great number of distinguished Jewish or part Jewish physicists. Well, I need only mention that even Niels Bohr was half-Jewish, and in Oxford, James Franck was of Jewish descent. It is the descent that matters; religion has nothing to do with it, of course. He was fully Jewish, so was Born, Max Born. Mathematics was one also of the great fields in Gottingen, even before the time, well before the time we’re talking about. There was Coutant who was Jewish, Levy, and various others, so these rules, this new legislation had a disastrous effect on Gottingen in particular, but of course also in other universities.

Bennett:

Why did you choose England or Oxford?

Kuhn:

At that time, it was extremely difficult to find anything anywhere, any academic position or chances for work, and merely to enter the country; it was the time of financial crisis all over the world, so the choice wasn’t really a free choice. My first idea was, where in past years scientists used to emigrate to was America. But in America, it was extremely difficult to set anything at that time, and I was fortunate in having some connections to England. I had met one or two English physicists, and in fact, this became very concrete soon, because Professor Lindemann, the later Lord Cherwell, had seen this coming. He was also interested in politics, and he travelled through Germany to see if he could find some people to come to England, some scientists, in particular physicists. At that time, physics in Oxford was only in the beginning stages, and he wanted to get people to Oxford or to other places, and he had connections especially with the chemical industries, and they were able to help with stipends. So one day, I saw him in James Frank’s flat, and we had a talk, and the result was an offer to come to Oxford. This appeared especially convenient, because in Oxford, the work going on in the laboratory was on a very small scale, but there was already some very good spectroscopy going on, on a small scale. There was one person, Derek Jackson, a young man, two years younger than I, and he had started research work in spectroscopy, and Professor Lindemann suggested could come and join him. This was even better, from one other point of view. Jackson also happened to be very rich and to be able to pay for all his equipment. So there was a prospect of working with somebody I didn’t know, but of whom I knew, and with means of buying equipment, which is of course important, and this is what happened. We then came to Oxford and I started out on joint work.

Bennett:

How did you come to work under Professor Simon?

Kuhn:

At that time, I didn’t know Professor Simon. We had heard that he would also come to Oxford, but I’d never met him in Germany because we worked in different fields. He worked on thermodynamics, low temperature physics. He was senior to me, and he had already a chair in Germany and was a well known scientist. It so happened that in fact we crossed the Channel on the same day, without meeting there, but we met soon and became very good friends. Before the war, again, we kept in touch. He was sometimes interested in our work, and I was interested in his, of course. But we did not work together at all, until the war came, and the problem of the atom bomb turned up.

Bennett:

Before we go on with that, could you tell us a little bit about the sort of work you were doing when you first came? What sort of work were you actually engaged on?

Kuhn:

In Oxford or in Gottingen?

Bennett:

Well, either, if you like.

Kuhn:

They are related to one another, yes. This was, in Gottingen before 1933, I worked on molecular and atomic spectra. Here I stressed particularly the atomic line, because that was Jackson’s interest, and also the equipment was so good, so we worked on methods of high resolution spectroscopy, to study properties of atoms, so-called hyperfine structures, and other problems. At the same time, I did some work for myself, also on spectroscopy, on some problems slightly connected to astrophysics, namely, on pressure broadening of spectral lines, that is, something quite different from the work I did with Jackson, and it fitted in very well, because he didn’t always confine his time to working in the laboratory. He had other interests -- horse racing, fox hunting, and social life even. But when we worked together on atomic spectra, we both worked hard, and got on very well with one another. We were very different from one another, but it became a very pleasant collaboration, for many years.

Bennett:

To what extent were you able to, or were you following the advances that were taking place that would ultimately lead to the discovery of fission, the bombardment of elements with neutrons and that sort of thing?

Kuhn:

Yes. These was a very great change that took place in nuclear physics -- I had talked about the great discoveries and advances in the twenties, in the theory, in quantum theory. But in the thirties, some discoveries were made that changed greatly the field of nuclear physics. It was mainly the discovery of the proton, one might say, and especially the discovery of the neutron. That was a discovery that solved a number of problems that had seemed quite insoluble in atomic physics, in nuclear physics. And out of this came then this discovery of nuclear fission, that happened, I think it was in ‘39, it was immediately before the outbreak of the war. That, of course, every physicist got excited about, something that somehow nobody had thought about. Not so long before, Rutherford had advanced that he thought that these enormous energies in the nucleus, there was no chance that they would ever be used in practice -- something, I’m not quoting him verbally, but something on that line. And it seemed quite out of the range of possibilities. And then this one discovery, of nuclear fission, suddenly opened up new possibilities.

Now, it’s a discovery, fundamentally interesting, that a nucleus could break into two about equal pieces, instead of some bits being shot out of it. But mainly, of practical importance. From the purely basic point of view, basic physics, one might say it’s not as important an event as the discovery of the neutron. But then, of course, it was -- everybody, I think almost every physicist who kept in touch with basic physics of atoms and nuclei realized that there was a possibility of partly terrifying, partly very useful applications. But it was only a possibility. One didn’t know enough to be sure that it would be possible at that time. This was just at the time of the outbreak of the war, when in different countries, people got together and said, “What do you think about this, do you think it is possible or not?” Until then, it became secret, because one realized it could have military possibilities. Nuclear physics at that time, in nuclear physics in England meant practically Cambridge and Rutherford’s Laboratory. In Oxford, of course there were also people who worked on, or were well informed in nuclear physics.

At one time (Hans) Halban came here but I think that was later, and of course, for a short time Schroedinger was here, and there were certainly physicists who realized possibilities, and one discussed it, but the vital measurements were only made, somewhat later, and of course, they were then kept secret, so one didn’t hear much about it, and they weren’t published. The main problem was what is called the critical size. One realized that if one had pure uranium 235, that is, the one isotope of uranium enriched, then it should be possible to have an explosion of many orders of magnitude greater than ordinary explosions. But it all depended on the initial size. One had to must have enough of this enriched uranium at the same time, but how much? One didn’t know whether it was the size of a tennis ball or a football, or of the whole room, and this of course was important, and this was measured and computed now frantically, in Germany, in England, in America. And what was surprising, in Germany they made serious mistakes. They were delicate measurements, quantitative measurements. But here, of course, England and America soon joined forces, as far as scientists are concerned, and then the development began.

Bennett:

Did you yourself feel at this stage that Germany was capable of utilizing this knowledge as well as America or Britain or anybody else, that one could mobilize its resources in this direction?

Kuhn:

Well, if you ask for my own reactions, there was one time when I was literally asked to join this work. Until that time, I didn’t know myself what the critical size was, had no idea, because it had become secret. When I joined the work, it was about 1939, or the beginning of 1940 -- it was already a concrete possibility, not a certainty, and when I was first faced with this, it was quite a shock to think of the possibilities. On the other hand, at first there was no reason to think that Germany should not be able to do this. One didn’t know how difficult it would be, quite. And there were certainly enough first class physicists in Germany, people like Heisenberg, and a number of others. So at first, one’s idea was, what’s going to happen if this becomes a secret weapon of Hitler? Hitler certainly wouldn’t have had any hesitation in using it. This I decided when I was asked then to join in with this, it seemed the right thing to do.

Bennett:

And presumably along with many other scientists, you were either only recently naturalized or still classified as an alien, so you were excluded from other kinds of war work. Is that true?

Kuhn:

Yes, this is true. At the Clarendon Laboratory under Lindemann, the Admiralty began straightaway taking an interest in physicists, and the Admiralty work began -- the Admiralty is a senior establishment and they were very, particular, nobody was allowed to have anything to do with any Admiralty-connected work such as radar, unless not only he was born in this country, even your parents had to be born in this country, so there was no question of any foreigner joining into that work. I don’t know if it has been the same in all laboratories but certainly with the Admiralty this was indeed the situation. At Oxford, the majority of the young physicists had been swallowed up by the Admiralty, and what was left was Professor Simon, coming from Germany, Nicholas Kurti coming from Hungary and Germany, (he was even in Paris at that time) and there was Arms, he was an American, and there was myself, coming from Germany, and so, we all started as a very strange team. There was also a Welshman, Llewellyn, that was about the closest to an Englishman. This is how this developed.

Bennett:

That really was mirrored in the other groups that were being recruited in other centers.

Kuhn:

In other centers, I imagine there must be something like this too, yes. It was particularly pronounced in this case because of the Admiralty connection. And you might say, why wasn’t one forbidden to work on that? But this was soon taken over by a small organization, and not by a body like the Admiralty, and there it became possible. Both Simon and Kurti and I, we had become naturalized. But Arms was still an American, he even traveled to America for an expedition there to discuss this problem with the Americans.

Bennett:

We should perhaps some -- May I ask you a question about what it was like working under Professor Simon and the sort of work you were doing?

Kuhn:

Oh, Professor Simon was certainly an almost ideal person to work with or to work under. He was personally very pleasant to get on with. I think if somebody didn’t get on with Professor Simon, there must be something wrong with him, and it is even more than that; a very good understanding had developed before the war between him and Lindemann, Lord Cherwell, and this was very important indeed. I always liked Cherwell very much as a director of the laboratory, but he was a little remote; you know, his strong interests he had in politics and other matters. And Simon was straight forward and objective, he just worked on his subject, and the two got on very well together, and their joint influence was also greater in the university. And then later, one had the feeling with Simon that everybody trusted him completely. He was not a man who had his own axe to grind ever, so it was very easy to work with him, indeed. Essentially he was a really unselfish man. Even before I had anything to do with him in the same work, I remember if I wanted to borrow an instrument from him, he was always very generous. I remember very well the case of one precious instrument, Jackson and I had borrowed from him; unfortunately, a wire fell down on it. This was under high tension, so there was a hissing noise, and his galvanometer was damaged, and if somebody takes this as lightly as Professor Simon took it, you know, it’s good to work with him. It was a very pleasant, happy collaboration in our group in Oxford and later also our contacts with other groups in industry, in Birmingham.

Bennett:

So did you have contact with Peierls and Frisch at this time?

Kuhn:

Not with Frisch at that time, but with Peierls. Our work here under Simon was concerned with the separation of the uranium isotopes, and many of the experiments were done by industry -- ICI metals -- in Birmingham. And we needed often theoreticians to help us in the mathematical side of it. Peierls was connected with it in that way, in working out the theories. These were theories of gas flow, mathematically rather complicated, not especially of any interest in physics, but you needed somebody who really was a good mathematician and physicist, as Peierls indeed was, and with him, under him, Fuchs worked. Often the problems were simply referred to Fuchs so we had also direct dealings with Fuchs, when there were certain parameters on these measurements that either had to be changed or not, but such details are important, even if they are small since the total work concerned is on such a large scale, and some little advance of perhaps 10 percent of efficiency is important. This was the kind of work.

Bennett:

Do you know any anecdotes of Fuchs?

Kuhn:

No. I don’t think he was a man who would produce anecdotes. He was essentially a quiet serious man, reticent. When I try to think back, I see ourselves sitting -- I don’t know if my wife was there too -- sitting at a table in a restaurant having lunch together, and he was so very silent, pleasant, quiet spoken, but not a very transparent man, and not a person easy to chat with. So I doubt if you get any Fuchs anecdotes from anybody. I only remember being quite shattered when, after the war, I heard somebody saying, “Have you heard there’s a vacancy in Harwell, the director of mathematics in Harwell?” And that Fuchs had been found to be a spy. I had not known him before he came here. If I had taken the trouble or had been able to ask people in Germany, it would have been very easy to find out that he came from a politically very strongly left oriented family. But in other direct contacts there was no problem.

Bennett:

The marvelous story about Professor Simon and the sieve, and what that led on to, perhaps you could take us through that.

Kuhn:

Yes, this is a story which is boosted a bit too much. What it perhaps tells one is this: the problem we were faced with was rather an unusual problem. One had to produce somehow membranes, let us call it, with very small apertures, where gasses had to diffuse through. The apertures had to be very small and a short in the direction of the flow. Well, if the apertures are very small, nothing much goes through, so you also had to have very many holes or apertures in that coil, so this was our problem; what to do about it. Everybody thinks of a sieve, and Simon came one day with a sieve from his kitchen, and said, “What about if you rolled this together?” And well, this wasn’t taken very seriously, but from then, one went on thinking, what one could do, and then in a very logical sequence, we came to something which was practically for the separation of isotopes quite useless, but would serve very well for research on the process of diffusion. That really would teach you something about the possibilities. And were these membranes that were made at first, industrially made, where you could really see the little holes, you could measure them, and they were useful then, in preliminary experiments. But the sieve was at the very beginning of it, and was never more than a joke.

Bennett:

What were you trying to achieve with these membranes? Could you just explain that further?

Kuhn:

Well, there are two kinds of gas flows. Where a gas of atmosphere pressure flows through a pipe, it behaves much like a fluid passing through a pipe, the molecules stay together, more or less. And the other kind of flow is what’s called the molecular flow, where more or less every molecule does something by itself and passes through. This has the effect that if you deal with a mixture of two gasses, say one was heavier with heavier molecules, the other with are lighter molecules, mixed together; when they flow through a pipe, they all move forward together, and The gas comes out in the same composition. But if you let them diffuse through very small holes, then the heavier molecules are just a little slower than the others. So statistically, there are more of the lighter ones coming through. That’s all there is to it, really. Of course, then the problem is to do this on a scale that still produces enough material, that was essentially the problem. No great high flying physics involved. But quantitatively, it becomes difficult, and especially difficult when you are asked to tell how far this flow through this say piece of cotton wool is molecular, how is it’s “body flow,” so to speak? Then you come to a field where in fact theory gives you little guidance, and this is what made our work here in Birmingham, in a way quite interesting. I found it interesting, just as a physicist, not in my line, but kinetic theory of gasses would be the heading.

Bennett:

How did that particular project develop, when you started out with that initial goal, how did that develop once you achieved the immediate goal of diffusion?

Kuhn:

Well, there was no definite goal you could ever achieve. You let it go better and better. With the real membranes, they improved in reality. You simply had to test them empirically. But then you did this, what you wanted to separate are two kinds of molecules containing uranium, uranium hexafluoride. They are terribly heavy, and their masses for different isotopes differ from one another by less than one percent, so to measure this difference experimentally, on thousands of specimens, would be just impossible. So what you do, you make a kind of model, using a gas mixture of two ordinary gasses of quite different atomic weight, which you separate in this way. And a sieve isn’t good enough, but something like a micro-sieve is needed and it’s action is not based on different sizes, but our differences of speed of molecule. But it is eventually empirical, and then you have to keep in touch with the industrial? Thus, I met people from different industries, and altogether, Id played a great part. So the study and the testing of these membranes was one of the things I worked on; really just one corner of the whole subject.

Bennett:

Did you end up doing practical work with hexafluoride gas?

Kuhn:

We as a group certainly did, yes. I myself hardly at all. But some others were. There were a few experiments like this. But mainly the membrane were tested by indirect methods. There almost like model experiments, in fact became so refined that we were quite confident; when we tried it out on the uranium hexafluoride, we could predict the separation to within perhaps 10 percent or so. But the early experiments were all so directly. But they were difficult and therefore confined to few specimens. On the other hand, making these membranes in huge quantities, you have to expect differences from one to the other, that was roughly the “bib job.” And this was also the picture we forward in America on our visit to there, and we compared experiences with the Americans.

Bennett:

That was in ‘43?

Kuhn:

Yes, ‘43 to ‘44.

Bennett:

Before we go on to that, could we talk a little bit about the MAUD Committee work itself, because by that time you were presumably out of the tube alloys team. But the events leading up to the MAUD Committee report, were you ever called to the meetings of the technical committee yourself?

Kuhn:

No. Simon would have gone to these -- the committees dealing with the design of the bombs and the whole system. No, I was kept informed, I read the reports, as far as I could find time, but this field of membrane production was a vital point, and could be separated in the work.

Bennett:

Overall did you have a picture of the totality of the work that was going on under the auspices of the MAUD Committee?

Kuhn:

Yes. Once you were in this work, there was no secrecy about part of it. And that included basically, at some time, the American work. If you had at that time asked me what was the critical size, I probably would not have known it, because why should I? Or the design of the bomb. In principle, I could probably have asked to look at them, but why should I? I knew basically what was happening, and that was all that mattered.

Bennett:

Here we are once again. What can you tell us of Niels Bohr?

Kuhn:

Niels Bohr. Although I have never worked with him, I am not a theoretical physicist, I have met him on several occasions. He was a fairly frequent visitor to Gottingen. Of course, with his friendship with Franck and Born. And I also met him, I heard him give a talk at a conference immediately after the war in Sweden. He was asked to give a review of the development of quantum theory. Well, if I had been Niels Bohr, I would have said, “I can’t talk all about my own work,” but he did in fact hardly mention himself in, this very valuable and interesting talk, and made it almost appear that he had not much to do with it. That was one of my recollections. Then I met him on the occasion of his visit to Oxford on the matter of the atom bomb, but he just came to my room, at that time I was still doing some other experiments I had only just been roped into the war work, and he would then just come and ask us what we were doing, and discuss it with you, sort of on your level. Well, rather on his level, making you feel as if you were on his level. He was such a very great man, at the same time such a straight and pleasant human personality. He never talked down to anybody.

Then again, I met him just before we went to America on this occasion in 1943, when we were in the office of this organization, then under Mr. Akers, in London, with his secretary, who was a marvel of efficiency, and there also Niels Bohr came in very unhappily and, “My watch has gone wrong, it’s broken, what do I do? “ And of course he was at once taken care of. But he was one of the very great men. If you ask how great is a physicist? One criterion would be, has he opened up a new development? And that only lets a very few people through. And with Bohr, one could say, he’s done it twice, at least. At first, in the years about 1914 when he developed his quantum theory of the atom. That meant the beginning of a great development of atomic physics, where he kept on being active right through. And then again in nuclear physics, when the structure of the nucleus became known. The neutron had been discovered. Then again, in the explanation of nuclear fission and all this, he played again a part in nuclear physics. He was really one of the very very great people, and certainly made very very little of it. I had a great admiration for him, and my distinguished teacher in Gottingen, James Franck, had an enormous admiration for him, referred really difficult problems in the last resort, his judgment. He was one of the very few outstanding people in physics in our lifetime.

Bennett:

Can you recall the visit of the Americans to this country, presumably a visit to you at Oxford, to see how you were getting along? That was Pegram and Urey, I believe.

Kuhn:

Yes, I remember this, but not very well. Pegram and Urey, these two I remember, especially Harold Urey, whom later I met very often. For one year he was a fellow of my college. But at that time, this was an interesting meeting. The only impression I can remember is that they were quite impressed by the state of affairs in this country. As I say, I had at that time very little of a hand in it. And I certainly remember that they were impressed by their visit, to see how things were tackled. But I had not anything directly to do with this.

Bennett:

Why don’t we go on to --? Well, tell us about the visit to the United States, how that came about, what you did there, and how long you stayed, what happened when you came back?

Kuhn:

Yes. Well, developments in this problem of tube alloy, as it’s called here at any rate, had gone on in the two countries with information being exchanged, but it was still felt that we ought to have direct contacts. And so quite a party of physicists and engineers traveled there. Of course, it was all very secret. I had to tell my wife I was traveling somewhere, I don’t know how long, I can’t tell you where. She guessed that it was America, but nothing else. And we started off in Southampton, or Liverpool, I forget which it was, and it was not a very pleasant journey, in a way. It was in the war, blackout, the ship had to be blacked out. It was in the autumn. It was very windy. I don’t suffer much from sea sickness, but most people do, and what happens then -- but this is perhaps a story you’ve been told by others, that Frisch gave us concepts on the boat, but -- tell me if you know the story already.

Bennett:

Please tell us.

Kuhn:

In order to improve his own seasickness, he played the piano, and it helped him and others overcome it. Every night, I think. It was quite a nice experience. Otherwise, we were interested in trying to figure out which way we were going. We had no idea where we would land in America, of course, on the coast, because America is quite long, and eventually, we weren’t told anything, the captain wasn’t allowed to tell us, but we noticed suddenly the course was changed. We turned right up north and then back again. And some people had the idea they would try to set up a little ????? quite primitive and make some measurements, to see where we were, and the captain became terribly alarmed about it, because we weren’t allowed. But then in the end we learned it was Newport News in the south. And I still remember being questioned, on entering the country, still on board ship. Several people appeared, officers of the different security organizations, and questioned us all. Well, we all passed without any difficulties. We traveled in fact officially as government officials, temporary government officials, and the only real difficulties were two, one was Peierls, because of his wife being Russian, and the other one was Charles Arms (?) who was an American, and had got stuck in this country on this work here, but eventually, we all landed. There were various impressions. At first it was most extraordinary to be in a town like New York, in this brightness, being used to blackout in England. I had of course taken my torch with me.

Well, you had to have your torch with you always. I felt really silly, having a torch, although as we were assured, all this brightness, it was really dim-out. Of course, the Americans had also a dim-out. It wasn’t quite as bright as usual, because there was still a danger of air attacks, and they had even alerts. They had alerts now and then just to try it out. It amused me very much. These alerts were well known to me from my connection with Birmingham, for instance, with IC methods. There, the alarm -- then it’s everywhere, there were notices saying, “When the klaxon sounds, hurry.” In America, it says, “When the alarm signal is given, be quiet, keep quiet. “ This was a difference of temperament. And it was interesting to meet people. Harold Urey was the scientific leader of the group, and we found it very useful to discuss our experiences. I, of course, only discussed or mainly discussed the experiences in the narrow field I was working on, on membranes and the separation by diffusion. But it was very useful. And it was interesting that we had no difficulty in entering any secret establishments there, apparently less than most Americans had, and it was a very pleasant experience. Although in the American population, there was some feeling, almost, on German -- but England was not always all that friendly. But you would never feel this with your colleagues. Of course, it was an interesting time.

Bennett:

When you came back, what were you able to report, in terms of their work vis-a-vis yours?

Kuhn:

I think, in this field, it was more the idea of us reporting to them, because I believe, I don’t know if I’m right, that our techniques of testing these membranes were more refined than theirs, or theirs were perhaps quicker, I don’t know, but it was interesting. It was also interesting to have met the people concerned. One thing I remember at that time -- it happened to be just at that time -- we heard the rumor, I remember it as a rumor or more than a rumor, that the Germans had given an order to France, an order for a large number of compressors, and we were terribly worried. What does that mean? We thought, they are now onto the same thing. Because we had remarkably little real knowledge about how far Germany had got. In fact, it turned out to be wrong. What happened, it was connected with the V-1 or V-2, and of course they were nowhere near as far as American and England had got. But altogether the atmosphere always was that of, we have to hurry with this job, otherwise it will be too late. That was very much -- I remember the first, at first when I was roped into this job, it was quite nerve-wracking, almost. You see, for instance, certain words you didn’t use. You weren’t allowed to use the word uranium. Uranium hexafluoride was “hex.” I don’t know what it was in America. Something else, probably.

Everything had its special name, and once there was one rather elderly gentleman, a chemist who was connected slightly, marginally drawn into this field, and he didn’t quite realize the details. He rang me once, “Oh, I want to ask you some question about this membranous material” we had asked him about, and I said to him, “What do you mean hex?” “What?” “Are you talking of hex?” “Oh, you mean uranium hexafluoride?” He was deaf and spoke very loudly, and I felt like sinking to the ground with embarrassment. And again, in America, then one had very much that one had to keep one’s written documents very secret. I could tell you a story, but it was not very relevant. On the first night in New York, I didn’t want to go to bed straightaway, so I got out, I wanted to get a book out of the library, and by chance it was an English book. It was a book by Harold Nicholson, PUBLIC FACES. I don’t know if you happen to know this book. It’s a nice little book. And I took it to bed with me and read it, and I was quite shocked, it was telling the story of the atom bomb! It’s an interesting little book to read. It’s a kind of farce of a young diplomat and his girl friend, talking about a bomb that has been developed, some rare minerals found. It was so completely like the atom bomb that I -- but eventually I did fall asleep.

Bennett:

When you came back, flew back, did you fly back?

Kuhn:

Yes, I flew back. I flew back alone. Out on the outward journey, I was on a ship with many others. On the return journey, it was through chance, I had fallen ill, and traveled at another date, through Canada. But that was undramatic. At that time, air transport across the Atlantic was not as hair raising and dramatic as it must have been in earlier days, the time when Niels Bohr was taken across.

Bennett:

You came back here to then carry on with what sort of work?

Kuhn:

Well, at that time of course still the war was continuing, much in the same way, until the bomb was dropped, more or less.

Bennett:

Even though a large contingent had gone to the States in early ‘43.

Kuhn:

Yes. Yes. But at one time, in the development of diffusion membranes, there was quite close connection, and we wished to continue this, and even when this connection had ceased, we in this country of course still wanted to carry on, because we still would, in the future, be able to separate the isotopes, and so I have never been informed of the latest stage of development of membranes in America. I don’t know what they are like. What they were here, I knew then. If you question me now too much, I would only roughly know the answer. But some work went on until the bomb was dropped.

Bennett:

How did you get news of the bomb being dropped?

Kuhn:

The actual bomb? Well, I had known of the test bomb. People like Simon would know about it. We all knew about it then. But then very soon after, the real bomb; it was in August, and my wife and I were taking a holiday at the seaside. We were in Wales, in a forsaken little place, really, one of the very miserable little Welsh poor farms, quite a way from anywhere -- and there, a newspaper was produced, something dropped, I forget the exact details, but something ghastly had happened, some superbomb had been dropped, and my wife showed it to me, and she said, “Is this what you’ve been on? “ I wasn’t allowed to say, of course. I shrugged my shoulders, and quickly ran to the next telephone, in the next town or village, and telephoned to Simon. I said, “Look here, am I allowed to say?” I knew of course it was the bomb that had been dropped, and I had to ask him, “Is it all right if we say now that we have been doing this?” And he assured me that he had in the meantime received permission. This must have been the first bomb.

Bennett:

What was your reaction to it?

Kuhn:

Well, it was of course a horrible thing, to read about such a disaster, and it was not quite unexpected, of course. I am sure I surprised at the second bomb being dropped so soon after, and this, I think, up to this day, remains hard to explain. And until now, people have been, wondering whether this was the right thing. In this context, I might mention something which I only heard only after that. The protest, rather, the … Wondered whether this was the right thing. In this context, I might mention something which I also heard only after that. The protest, rather, the advice that was given by a group of people guided by Franck, people working on this bomb project in America; it became known as the Franck Report. What happened was this. When Franck and his colleagues knew that the bomb was about to be dropped some time, they got together and discussed it, and sent a memorandum to the U.S. government to advise that the bomb should not be dropped on a town immediately, but that a warning bomb should be dropped in a deserted area, with observers being invited from all countries.

It also pointed out the importance of (?) a step for the future political relations in the world. It is now a well known, published document. One can easily get hold of this “Franck Report.” It is hard to say whether it was the right thing or not the right thing to have dropped the bomb immediately. There are certain arguments which are difficult to judge for us. The question was, if such a test bomb had been dropped, would it have impressed people sufficiently so that the Japanese, would have called off the war? This is by no means certain, because it was this test bomb, was, in a way, a bomb -- but it would not fully demonstrate the terrifying explosive effects. One could not immediately have foreseen how much worse it was than a bombardment of towns, a number of other towns. So this is certainly an argument. I would not be able to say that I can judge whether it should have been done or not. But I like to think of it that it is typical of James Franck. He was somebody who thought not only of physics, most of the time it was physics perhaps, but he also thought of these questions of human responsibility. Franck was not only a physicist, he was also a man whose advice was sought in other reports. When I visited Franck after the war, in America, when there were still lots of similar problems hydrogen bombs and what not, certain people like Teller turned up to ask his advice. Not about physics -- Teller didn’t have to come to Franck to get advice on physics -- but on problems of conscience also, and this, I found, has happened very often, that Franck was a person whose judgment was trusted so strongly on human decisions. But at that time, it is difficult to say whether it was right to have dropped the bombs or not.

Bennett:

But there’s no doubt in your mind that it was right to do the work that led to them, because of the uncertainty about who else was capable of doing that, at the time.

Kuhn:

Yes, at that time. You really had no idea of what the Germans had got, and the Germans were certainly trying something. We knew this. They had taken heavy water from Norway, and Heisenberg was working on it. Well, Heisenberg was one of the greatest physicists. He and a number of other physicists had, strangely enough, really misjudged the critical bomb size therefore and had mainly worked not on the direct application of a bomb, but on reactors. But there again, reactors mean producing plutonium. So I think at that time, from the knowledge that anybody on this side of the Channel had, I still think it was right to develop it.

Bennett:

And we have a last of question to ask, I think we should have a sort of a looking back one -- what is your feeling about the discovery of fission, in relation to the other science developments?

Kuhn:

Well, if you’re talking about science alone, fission has been one step in the particular field of one branch of nuclear physics only, but it’s not. I would not class it as, one of the most important experiments, from the point of view of learning about this world of physics. For instance, the discovery of the neutron is much more decisive, though it was connected with the other. And there are some important discoveries that probably never will be of practical applications. How do we know? Such as non-conservation of parity, it’s really hard to think of any practical application, and yet it is a very basic experience on the nature of forces, and of course there are astrophysical discoveries that are much more important than that of fission. After all, perhaps when fusion will be used properly for producing energy, fission will look even less important. But this is looking into an uncertain future.

Bennett:

Good.