Bernstein PP 2003

19

Click here to load reader

Transcript of Bernstein PP 2003

Page 1: Bernstein PP 2003

243

Phys. perspect. 5 (2003) 243–2611422–6944/03/030243–19DOI 10.1007/s00016-003-0177-8

The Drawing or Why History Is Not Mathematics*

Jeremy Bernstein**

The mystery of whether or not Werner Heisenberg gave Niels Bohr a drawing that Bohr came tobelieve was that of a German nuclear weapon, is discussed and resolved.

Key words: Hans Bethe; Aage Bohr; Niels Bohr; James Chadwick; Leslie Groves; Wer-ner Heisenberg; J. Hans D. Jensen; J. Robert Oppenheimer; Abraham Pais; RudolfPeierls; Robert Serber; Edward Teller; Victor Weisskopf; Copenhagen; German BombProject; Los Alamos; Manhattan Project; chain reaction; nuclear fission; nuclear reac-tor; moderator; plutonium; uranium.

The Drawing

Beginning in 1977, I did a series of interviews with the physicist Hans Bethe whichresulted in a three-part New Yorker profile that was published in December of 1979.1

Our interviews proceeded chronologically from Bethe’s birth in Strasbourg in 1906, hischildhood and early student days in Germany. Then we discussed the Hitler years.Bethe, who is half-Jewish, was forced to emigrate in 1933, finally coming to the UnitedStates in 1935, where he became a professor at Cornell University, which he never left.We then discussed the war years. Since Bethe did not obtain his American citizenshipuntil March of 1941, he was not allowed to work on classified military projects untilDecember of that year. He then worked on radar for a year with a summer – the sum-mer of 1942 – spent in Berkeley with a study group assembled by Robert Oppenheimerto examine the prospects for a nuclear weapon. Los Alamos did not get underway untilthe spring of 1943. Bethe was one of the early recruits. He later became the head of thetheoretical division.When I conducted these interviews I knew something – not a greatdeal – about the German attempts to make a bomb and I was curious as to what Bethe,and the other scientists at Los Alamos – many of whom were refugees from Hitler’sEurope – knew about the German work at the time they were doing their own. Inresponse to this question Bethe told me the following anecdote, many of the elementsof which were new to me then.

During the war, he said, Werner Heisenberg had visited Niels Bohr in Copenhagen(figure 1). Bethe did not give me a date and in my New Yorker article I said it was in

* I have discussed some aspects of this story earlier in my article,“What Did Heisenberg Tell Bohrabout the Bomb?” Scientific American 272 (May 1995), 72–77.

** Jeremy Bernstein is Professor Emeritus of Physics at the Stevens Institute of Technology anda former staff writer for the New Yorker.

02_Bernstein 8.8.2003 10:28 Uhr Seite 243

Page 2: Bernstein PP 2003

244

October of 1941. This, as it happens, is slightly wrong. It was mid-September. Bethethen told me that Heisenberg and Bohr had a conversation – or conversations – insome “back alleys” in Copenhagen. Heisenberg later recalled that the one importantconversation took place in the Faelled Park behind Bohr’s institute, but in his recentlyreleased letters Bohr distinctly recalls that it took place in his office at the physics insti-tute. However, a new letter from Heisenberg to his wife Elisabeth written at the timeof the visit describes three visits that Heisenberg made to Bohr’s home.* One conjec-tures that the essential conversation took place at the time of the second visit. Bethethen told me that during this conversation Heisenberg gave Bohr a drawing which,later, Bohr “transmitted” to the people at Los Alamos. He went on to say,“It was clear-ly a drawing of a reactor, but when we saw it our conclusion was that these Germanswere totally crazy – did they want to throw a reactor down on London?”2

I pointed out in my article that the Germans were not crazy at all. Since 1940, theyhad known that one of the uses of a reactor was to manufacture what we would latercall plutonium. They knew that this element was at least as fissionable as uranium andoffered great advantages in its chemical treatment. As it happened, the Germans werenever able to make a functioning reactor during the war so they never learned how dif-ficult it was to actually use plutonium in a bomb. But if the people at Los Alamos had

Jeremy Bernstein Phys. perspect.

Fig. 1. Werner Heisenberg (1901–1976) and Niels Bohr (1885–1962) in happier times in Copenhagenin 1934.

* This letter has not been published. I am grateful to Michael Frayn for disclosing its contents.

02_Bernstein 8.8.2003 10:28 Uhr Seite 244

Page 3: Bernstein PP 2003

realized – which they didn’t – that the Germans knew about plutonium their concernabout the German program – which was always considerable – might well haveapproached the panic level.

When I listen to the tape I made with Bethe on which he describes this incident Iam always struck by what I failed to ask him. I didn’t ask, for example, what he meantby “later.” As I subsequently found out, the Heisenberg visit was in the fall of 1941,while Los Alamos did not begin until the spring of 1943.Why did it take something liketwo years for Bohr’s transmission to take place and, indeed, just how did Bohr trans-mit this drawing? If I had asked these questions I would have saved myself, as it turnedout, a good deal of grief. In any event, I described this incident in the New Yorker andforgot about it. I don’t recall anyone showing any interest for nearly fifteen years. Thescene shifts to the early 1990s. By this time I was an adjunct professor at the Rocke-feller University where the late Abraham Pais was a professor. Pais had recently pub-lished his biography of Bohr which discusses the visit but not the drawing.3 This hadinspired the journalist Thomas Powers, who was working on his book Heisenberg’sWar,4 to call Pais to see if he knew something about the drawing. Powers had his rea-sons. His book presents a revisionist view of Heisenberg’s wartime activities whichclaims that Heisenberg had a deep knowledge of nuclear weapons which he concealedfrom his colleagues, thus helping to prevent the bomb from falling into the hands ofHitler. Heisenberg never claimed anything like this himself and few, if any, historiansof the period accept it, but that is not what concerns me here. What concerns me is thedrawing. Powers had taken my version – or, more exactly, Bethe’s version – of this storybut had added his own twist. To have given Bohr this drawing, he argued, was forHeisenberg a traitorous act – an act of betrayal and sabotage. He ends his chapter onthis with the dramatic denouement, “Bohr was convinced that Heisenberg’s crudesketch illustrated the working principle of the bomb he was trying to build for Ger-many. With this simple piece of paper Heisenberg had put his life in jeopardy.”5

Before he published his book Powers had contacted Aage Bohr, Bohr’s son, a physi-cist and also a Nobel Prize winner, who had been Bohr’s confidant during the war (fig-ure 2). Powers wanted to learn more about Heisenberg’s Copenhagen visit and, in par-ticular, to find out what Aage Bohr knew about the drawing. In due course he hadheard from Aage and what Aage had had to say had put Powers in something of aquandary. Aage said in no uncertain terms that the notion that Heisenberg gave Bohra drawing during his visit in 1941 was pure fiction. As he put it in a letter to Powersdated November 16, 1989,“Heisenberg certainly drew no sketch of a reactor during hisvisit in 1941. The operation of a reactor was not discussed at all.”6 Powers had contact-ed Pais thinking that, as Bohr’s biographer, he might know something about the draw-ing which he had not put into his book. Pais, as it happened, did not know anythingbeyond what he had read in my article so he asked me on one of my visits to Rocke-feller if I knew anything beyond what I had written. I had to admit that I did not but Isaid I would try to look into it. I then went home and said to myself, “Now what?”

The two obvious people to contact were Aage Bohr and Bethe to see if there wassome way to reconcile their accounts. Pais was returning to Copenhagen where hespent much of the year and he offered to speak with Aage and indeed to bring him acopy of the tape I had made with Bethe to see if this would help. In due course I got a

245The Drawing or Why History Is Not MathematicsVol. 5 (2003)

02_Bernstein 8.8.2003 10:28 Uhr Seite 245

Page 4: Bernstein PP 2003

246

letter from Pais saying that he had discussed the matter with Aage and played the tape,“None of which has changed Aage’s calm and firm opinion that there never was adrawing Heisenberg gave to Bohr.”7 This is something that Aage has now repeated tome through intermediaries a number of times over the years.* One of the things he hassaid is that if there had been a drawing which, in 1941, his father had thought repre-sented a plan for a German atomic bomb he would certainly have used his contacts inthe underground to promptly transmit this to the Allies. As we will see, Bohr did com-municate with the British by underground courier. Meanwhile Bethe wrote to say thatthere was absolutely no doubt in his mind that he had seen a drawing at Los Alamos.He added,“Whether the drawing was actually due to Heisenberg, or was made by Bohrfrom memory, I cannot tell.”8

Since this was getting me nowhere I tried to think of other people I knew who hadbeen at Los Alamos at that time. Two came to mind. There was Victor Weisskopf, agood friend of Oppenheimer’s who was, knowing Viki, bound to have been at the cen-ter of any interesting action. Weisskopf replied that he had no recollection at all of a

Jeremy Bernstein Phys. perspect.

Fig. 2 Aage Bohr (b. 1922) and Niels Bohr (1885–1962) inspecting a camera outdoors in 1953.Credit: Niels Bohr Archive; courtesy of American Institute of Physics Emilio Segrè Visual Archives.

* The principle intermediary has been Finn Aaserud, Director of the Niels Bohr Archive, Copen-hagen. I am grateful to him for his help and for permission from the Niels Bohr Archive to quotethe Bohr correspondence with James Chadwick.

02_Bernstein 8.8.2003 10:28 Uhr Seite 246

Page 5: Bernstein PP 2003

drawing but that I should ask Bethe – again a blank wall. I also wrote to Rudolf Peierls– Sir Rudolf Peierls – who with Otto Frisch, both refugees from Germany in England,had in 1940 made the first correct calculation of the so-called “critical mass” – the min-imum mass of uranium-235 – needed to make a bomb. It was this calculation that real-ly started the Allies’ effort to build a bomb. Both Frisch and Peierls had come to LosAlamos as part of the British delegation. In due course I heard from Peierls whooffered an ingenious solution that he thought might reconcile the accounts of Betheand Aage Bohr. Peierls conjectured that there was a drawing – he had never seen it atLos Alamos – which in fact Heisenberg had given to Bohr in his 1941 visit. But, he sug-gested, Bohr had kept this secret from his family, and Aage in particular, to protectthem from knowing about something that could gravely endanger them if discovered.As ingenious as this is, I soon came to the conclusion that it was extremely implausible.In the first place Bohr shared many confidences with his son during the war includinghis impressions of Heisenberg’s visit. This went on after the war when there was nolonger any danger of the information falling into the wrong hands. Moreover, the BohrArchive in Copenhagen has no record of such a drawing nor of any discussion about it.Once again I had hit a blank wall. But then I got an idea, something that I should havethought of in the first place. I would call Robert Serber.

Serber, who died in 1997, was at the time of his death a professor emeritus of physicsat Columbia University. I had known him for many years. Among other things he hadhelped me with the technical aspects of the German program which are discussed inmy book, Hitler’s Uranium Club.9 This is an annotated version of the transcripts madeof conversations among German nuclear physicists captured at the end of the war andinterred in a large mansion – Farm Hall – in England. Serber and I had studied Heisen-berg’s remarks on the physics of nuclear weapons to see how much he understood;rather little as it turned out. No one was more qualified than Serber (figure 3). In themid-1930s he had become a postdoctoral fellow with Oppenheimer. The two becameclose friends. Oppenheimer developed a great respect for Serber’s unassuming bril-liance, so it was no surprise when he was invited to join the 1942 study group in whichBethe was also a participant. Serber, like Bethe, was one of the early recruits to LosAlamos. Serber produced The Los Alamos Primer.10 These are the transcripts of aseries of introductory lectures he gave to new arrivals at Los Alamos to explain thephysics of nuclear weapons.They were declassified in 1965 and are still, for anyone withthe technical background, the best introduction to this subject. I called Serber and itbecame clear at once that I had struck a gold mine.

The Los Alamos Meeting

To understand what I learned we have to back up a little. When we left Bohr it was1941, and he was still in Copenhagen. He remained there until September of 1943,when he was warned that the Gestapo was going to arrest him. Denmark had beenoccupied by the Germans since 1940. Bohr escaped to Sweden with his family and onOctober 5 he was flown alone to Scotland. The same day he traveled to London andbegan a series of briefings and debriefings about the German and Allied programs. I

247The Drawing or Why History Is Not MathematicsVol. 5 (2003)

02_Bernstein 8.8.2003 10:28 Uhr Seite 247

Page 6: Bernstein PP 2003

248

will return to what little we know about the contents of these later. On November 29Bohr sailed to New York with Aage on the Aquitania, where he arrived a week later.He then went to Washington where he was introduced to a number of people connect-ed with the American program including General Leslie Groves who was in charge ofit. On December twenty-seventh he and Aage took the train to Chicago where theywere met by Groves who accompanied them on a two-day train trip to New Mexico.During this trip Bohr and Groves talked incessantly. It was more likely Bohr talkingand Groves trying to keep up. Bohr was never easy to understand in conversation. Itappears that during these conversations Bohr showed Groves the drawing. One doesnot know if this was a drawing Bohr had with him or if it was something he sketchedfor the occasion. I have not been able to learn exactly what Bohr told Groves about theprovenance of the drawing except that it certainly had something to do with Heisen-berg. By the time they reached Los Alamos on December 30th Groves was alarmed.He thought that Bohr was describing plans for a German nuclear weapon. By the endof 1943, the British knew a great deal about the German program.11 They were fairly

Jeremy Bernstein Phys. perspect.

Fig. 3 Robert Serber (1909–1997) at the blackboard in Berkeley. From Peace and War, by Robert Ser-ber with Robert P. Crease © 1998 Columbia University Press. Reprinted with the permission of the pub-lisher.

02_Bernstein 8.8.2003 10:28 Uhr Seite 248

Page 7: Bernstein PP 2003

sure – not certain – that the Germans did not have an industrial-size program like ourManhattan Project. They were also reasonably sure – not certain – that the Germansdid not have a plan to use radioactive materials as weapons. But until this was absolute-ly nailed down by having people on the ground one had to investigate all leads andhere was Bohr with the drawing of what he thought was a German nuclear weapon.Little wonder that Groves reacted decisively.

He persuaded – ordered is probably a better description – Oppenheimer to convenea group to listen to what Bohr had to say about the German program. This happenedthe very next day – December 31 – and Serber was part of the group. He told me thathe came a little late to the meeting and when he arrived Oppenheimer showed him thedrawing asking him what he thought it represented, indicating that it had something todo with Heisenberg. Serber realized at once that it was the drawing of a reactor and hetold me that he thought that it looked a little “silly.” I did not, unfortunately, ask himwhether he thought it was a somewhat silly looking drawing of a reactor or a silly look-ing drawing of a nuclear weapon. As I will explain in due course, both are true. In anyevent, Serber had some documents pertaining to this occasion of which he generouslyoffered to send me copies. When they arrived they turned out to be a two-page reportby Bethe and Edward Teller analyzing what must have been the drawing. Alas, nodrawing was submitted as part of the report and the original drawing seems to havedisappeared. Then there was a letter written by Oppenheimer to Groves dated Janu-ary 1, 1944, summarizing the meeting of the previous day and Bethe and Teller’sreport. It is a brief letter which is easier to present than summarize. Oppenheimerwrites:

Dear General Groves:

I am enclosing a memorandum written by Bethe and Teller after the conference yes-terday. Present at the conference were the Bakers [the code name for Niels andAage Bohr], Bethe, Teller, [Richard] Tolman, Weisskopf, Serber, [Robert] Bacher,and, for a small part of the time, as you know, Oppenheimer. The calculationsreferred to and described in the accompanying memorandum were carried out byBethe and Teller, but the fundamental physics was quite fully discussed and theresults and methods have been understood and agreed to by Baker.

I believe that it would be appropriate to emphasize that the completely negativefindings represented in the accompanying memorandum apply to the arrangementof materials suggested by Baker and take into account all the physical elementswhich appeared important to him. No complete assurance can be given that with anew idea or a new arrangement, something along these lines might not work. It is,however, true that many of us have given thought to the matter in the past, and thatneither then nor now has any possibility suggested itself, which had the least prom-ise. The purpose of the enclosed memorandum is to give you a formal assurance,together with the reasons therefore, that the arrangement suggested to you by Bakerwould be a quite useless military weapon.Very sincerely yours,J.R. Oppenheimer12

249The Drawing or Why History Is Not MathematicsVol. 5 (2003)

02_Bernstein 8.8.2003 10:28 Uhr Seite 249

Page 8: Bernstein PP 2003

250

In short, such a reactor cannot explode like a nuclear weapon. It might, of course, be anasty source of radiation.This letter made it clear what Bethe had meant by “later” and“transmitted” in the interview I had with him. “Later” was this meeting and “transmit-ted” was the arrival of Bohr at Los Alamos at the end of 1943. It also gives the list ofpeople who were there. I had already contacted several of them. Oppenheimer andBohr were dead as was Tolman who had died in 1948. This left Bacher and Teller. Imanaged to reach Bacher, who had headed the experimental division at Los Alamos,on the phone and he told me that while he remembered the meeting he did not remem-ber the drawing. I was amused by his reference to Groves in the present tense. “Youknow how he is when he gets riled up.” An assistant to Teller responded much later.She said, “Dr. Teller suffered a mild stroke last year and has difficulty in recalling cer-tain periods of time. He read your letter with interest; however he does not recall anyinformation with regard to your question concerning Bohr and Heisenberg.”13 None ofthis resolved the riddle of where the drawing came from. But I am now going todescribe what I think is the solution. I am not going to try to reproduce the chronolo-gy of the discoveries that have led me down this path – some quite recent – but ratherto outline the argument. My basic premise is that Aage Bohr is right. No drawing wasgiven to his father by Heisenberg in September of 1941, something that everyone whoknew Heisenberg said was so uncharacteristic of his cautious nature as to defy belief.If I accept this then I am obliged to explain how Bohr got this information. Fromwhom, and when? That is what I am now going to set out to do.

Reactor Physics and the German Project

The first clue I want to present is found in the report of Bethe and Teller.14 It is in thefirst sentence of the second paragraph which reads, “The proposed pile [this was theterm the Americans then used for “reactor”] consists of uranium sheets immersed intoheavy water.” Let us deconstruct this. In the rough sketches that Bethe has made in anattempt to reproduce what he saw in 1943, he always draws what looks like a cross-sec-tion of a cylindrical container. In short, the heavy water and the sheets of uranium wereapparently placed in a cylindrical container. But what about the heavy water itself?What is it, and what purpose does it serve? What it is, is easy to describe. A moleculeof ordinary water consists of two atoms of hydrogen bound to one atom of oxygen,H2O. But there is an isotope of hydrogen in which the nucleus consists of one positivelycharged proton – like ordinary hydrogen – and one electrically neutral particle – theneutron. This nucleus is called the deuteron, D, and heavy water consists of twodeuterons bound to one oxygen, D2O. It occurs at a frequency of about one part in 5000in ordinary water. But why is it important for reactors?

To answer this we must discuss briefly a few aspects of nuclear fission in uranium.In 1938, when Otto Hahn and Fritz Strassmann observed what Lise Meitner and hernephew Otto Frisch later identified as the fissioning of uranium, they had bombardednatural uranium – the kind you would get out of the ground – with “slow” neutrons.These are neutrons that move with speeds of a few kilometers a second. But naturaluranium is predominantly a mixture of two isotopes, U238 which is most of it, and U235

Jeremy Bernstein Phys. perspect.

02_Bernstein 8.8.2003 10:28 Uhr Seite 250

Page 9: Bernstein PP 2003

whose nucleus has three fewer neutrons and constitutes less than one percent, 0.7% tobe more accurate, of the content of natural uranium. In 1939, Bohr had the brilliantinsight that it was only the U235 that was fissionable by slow neutrons. The vast numberof U238 nuclei acted essentially as spectators. There is an energy threshold for neutronsabove which the U238 nuclei are fissionable, whereas the U235 nuclei are fissionable atall energies. Indeed, because of an oddity of quantum mechanics, the probability of fis-sioning a U235 nucleus is vastly enhanced the slower are the neutrons that bombard it.But when a U235 nucleus is split it emits a small number of neutrons – a little over twoon average – in addition to the fission fragments. But these neutrons are “fast.” Theymove about ten thousand times faster than the slow neutrons which move at a fewkilometers a second. In a reactor using natural uranium, as opposed to a uranium bombwhich uses basically only the U235 isotope, the fast neutrons must be slowed down towhere they become “thermal” – that is, move at speeds of a few kilometers a second.The fast neutrons have a low probability of fissioning a U238 nucleus because notenough of them have energies above the fission threshold for U238. This is where theheavy water comes in.

To moderate the speeds of the neutrons a “moderator” consisting of light nuclei isintroduced. One wants the nuclei to be light because when the neutrons bounce off ofthem one wants them to give up as much energy as possible to the target nucleus. Ifthey hit a brick wall – so to speak – all they would do is to bounce off in a new direc-tion with undiminished energy. The proton – the nucleus of hydrogen, the lightest ele-ment – would be ideal. But the problem is that the proton can capture the neutron toform a deuteron and thus be lost to the fission chain reaction. Light-water reactors arepossible if one uses uranium that has been enriched with U235.

The deuteron, which does not capture many neutrons,* is next best as a moderator,hence the heavy water. But heavy water must be painfully extracted from ordinarywater. The saga of the German attempts to do this and the Allied attempts to disruptthem has been the stuff of films.15 Suffice it to say here, that because of the difficulties,the Germans never had available more than a few tons of the stuff. We never ran intothis problem because our reactors used a highly purified form of graphite in which theneutrons bounce off the relatively light carbon nuclei. Early in the war, the Germansdecided that graphite would be too expensive to purify so they opted for heavy water.Thus the sentence in the Bethe-Teller memorandum tells us – as if we did not know italready – that the reactor being considered was German. Now we come to the “urani-um sheets.” Here we have a major clue which will lead us to another question.

To exploit this we must say something about the German nuclear program.16 In thefall of 1939, German Army Ordnance drafted a group of scientists to come to Berlin towork on nuclear energy with the idea of creating a weapon. The members of the group,which included Heisenberg and Hahn, referred to themselves as the Uranverein – theUranium Club. The group realized that there were two important things it had to do;design a reactor and find a way to separate U235 from U238. The reason for the first was

251The Drawing or Why History Is Not MathematicsVol. 5 (2003)

* I am grateful to Richard Garwin for pointing out that at a slow rate the deuteron can capturea neutron to become the nucleus of superheavy hydrogen, the triton.

02_Bernstein 8.8.2003 10:28 Uhr Seite 251

Page 10: Bernstein PP 2003

252

to see how to design a controlled nuclear chain reaction. After 1940, when what welater called plutonium was introduced as another possibility for fission, there was anadditional reason for the reactor; to make plutonium. The reason for the second – ura-nium separation – was that a bomb – whose explosive force must be released very rap-idly – would work on the fast unmoderated neutrons that will fission U235. Thus onewanted uranium that was enriched to the point where U238 would be almost entirelyweeded out. In the end, the Germans did not succeed in either of these objectivesalthough they made considerable progress.

Our concern is with the reactor. In 1941, the time of Heisenberg’s visit to Bohr, therewere two groups working on reactors. There was a group in Berlin and Heisenberg’speople in Leipzig. They employed different designs, but neither used metallic uranium.Making metallic uranium was the sole province of the Auer Gessellschaft and theywere not able to supply metallic uranium plates until the spring of 1943. Furthermore,none of Heisenberg’s reactor designs until 1943 resembled the design that Bethe andTeller analyzed. His typical design was a sphere inside of which were concentric spher-ical shells of powdered uranium embedded in various media such as heavy water orparaffin. In a report that Heisenberg published in the fall of 1941 – the same period hehad visited Copenhagen – there is a drawing showing a test reactor of this sphericaldesign.The Berlin people did propose plates of uranium embedded in heavy water, butwhile Heisenberg was aware of this project – he seems to have consulted for it – thiswas not what he was working on. The Berlin people did not have any metallic uraniumplates either in 1941. The obvious question is why would Heisenberg have given Bohra design for a reactor in 1941, which he did not work on seriously until 1943? The morelogical question to ask is did something happen in 1943 that was relevant to Bohr’sunderstanding of the German program? Here, I think that the answer is definitely yes.

Bohr and the Bomb

As I have mentioned, during the war Bohr had contacts with people outside of Den-mark. I want to focus on the communications he had with James Chadwick in 1943.Chadwick (figure 4) was the British physicist who had discovered the neutron. By 1943,he was deeply involved with the British program that was trying the develop nuclearweapons. He was able to communicate with Bohr by underground courier. In oneinstance the message, which had been reduced to microdots, was smuggled in the hol-lowed-out interior of a key. In another, a dentist made a cavity in a courier’s toothwhich he then put a filling over.The first message is from Chadwick to Bohr. It is datedthe 25th of January, 1943. It more or less speaks for itself. He writes:

I have heard in a roundabout way, that you have considered coming to this countryif the opportunity should offer. I need not tell you how delighted I myself should beto see you again, and I can say [to] you, there is no scientist in the world, who wouldbe more acceptable both to our university people and to the general public. I thinkyou would be very pleased by the warmth of the welcome you would receive. A fac-tor which may influence you in your decision is that you would work freely in sci-

Jeremy Bernstein Phys. perspect.

02_Bernstein 8.8.2003 10:28 Uhr Seite 252

Page 11: Bernstein PP 2003

entific matters. Indeed I have in my mind a particular problem in which your assis-tance would be of the greatest help. Darwin and Appleton are also interested in thisproblem and I know they too would be very glad to have your help and advice. [SirCharles Darwin, the grandson of the Charles Darwin, was a distinguished physicistwho was then working in the British nuclear weapons program. Sir Edward Apple-ton was its leader.] You will, I hope, appreciate that I cannot be specific in my ref-erence to this work, but I am sure it will interest you. I trust you will not misunder-stand my purpose in writing this letter. I have no desire to influence your decision,for you alone can weigh all the different circumstances, and I have implicit faith inyour judgment, whatever it should be. All I want to do, is to assure you that, if youdecide to come, you will have a very warm welcome and an opportunity of servicein the common cause. With my best wishes for the future and my deepest regards toMrs. Bohr.Yours sincerely,J. Chadwick 17

Bohr understood the meaning of Chadwick’s somewhat cryptic invitation to which heresponded, perhaps with a little less discretion than Chadwick might have wanted. This

253The Drawing or Why History Is Not MathematicsVol. 5 (2003)

Fig. 4 James Chadwick (1891–1974). Credit: American Institute of Physics Emilio Segrè VisualArchives.

02_Bernstein 8.8.2003 10:28 Uhr Seite 253

Page 12: Bernstein PP 2003

254

letter is undated but one would imagine that it was sent in February at the latest. Hewrites:

I can hardly express how deeply I appreciate your kind letter for which I thank youmost heartily. As you know, I am with my whole heart in the struggle for freedomand humanity and it is a great encouragement to feel, that my friends have not for-gotten me and are endeavouring to support my ardent wish to participate in thegreat common cause. However tempting it would be to me to follow your invitation,I find it after much deliberation to my great regret impossible for the present toleave this country. Not only I feel it to be my duty in our desperate situation to helpto resist the threat against the freedom of our institutions and to assist in the pro-tection of the exiled scientists, who have sought refuge here. Still neither such dutiesnor even the dangers of retaliation to my collaborators and relatives might perhapsnot carry sufficient weight to detain me here, if I felt, that I could be of real help inother ways, but I do not think that this is probable. Above all I have to the best ofmy judgment convinced myself, that in spite of all future prospects any immediateuse of the latest marvellous discoveries of atomic physics is impracticable. Howeverthere may, and perhaps in near future, come a moment where things look differentand where I, if not in other ways, might be able modestly to assist in the restoration,which is bound to come of international collaboration on human progress. At thatmoment, whether it will come before or after the cessation of hostilities, I shall makean effort to join my friends and I shall be most thankful for any support they mightbe able to give me for this purpose. I need not add, that I leave it to you to judge towhom you may convey the content of this letter. With my heartiest greetings andmost cordial wishes.18

In this communication Bohr expressed a view that he had been expressing since his dis-covery that only the rare isotope of uranium, U235, was fissionable at all energies. Heimmediately realized that it would be a monumental job to separate the isotopes ofuranium.They have identical chemical properties and their masses are so close that theeffects due to the mass difference are tiny. Bohr once remarked that, in his view, itwould take the resources of an entire nation to carry out this technology. In a sense hewas right. The most costly element of the most costly scientific project which had everbeen done – the making of the atomic bomb – was isotope separation. By 1945 therewere 75,000 people working on this at Oak Ridge, Tennessee. Curiously, Bohr neverthought of the plutonium alternative. Given his deep knowledge of fission he certainlycould have. One wonders whether his total aversion to the idea of nuclear weaponsprevented his thinking about them deeply enough.

Nonetheless, Bohr made an abrupt about face less than six months after he had writ-ten to Chadwick. He expressed this in a second communication to Chadwick. Unfor-tunately, it also is not dated. But we know its approximate date. We know this becauseChadwick referred to it in a meeting on September 10, 1943. He said that he had heardfrom Bohr within “within a month or so.”* Bohr begins this communication with the

Jeremy Bernstein Phys. perspect.

* I would like to thank David Cassidy for supplying this document.

02_Bernstein 8.8.2003 10:28 Uhr Seite 254

Page 13: Bernstein PP 2003

remarkable sentence, “In view of the rumours going around the world, that large scalepreparations are being made for the production of metallic Uranium and heavy waterto be used in atomic bombs, I wish to modify my statement as regards the impractica-bility of an immediate use of the discoveries in nuclear physics.”19 The phrase “rumoursgoing around the world” was apparently suggested to Bohr by Aage, probably to con-ceal specific sources. In any event, somehow in the summer of 1943, Bohr learnedenough about the German program so that he became seriously alarmed. But how?Enter the figure of J. Hans D. Jensen.

Jensen and the German Project

Jensen (figure 5), who died in 1973, was born in Hamburg in 1907. He was the son of agardener. He studied physics, mathematics, chemistry, and philosophy at the Universi-ties of Hamburg and Freiburg and got his Ph.D. in physics from the former institutionin 1932. In 1941, he was Professor of Theoretical Physics in the Technische Hochschule

255The Drawing or Why History Is Not MathematicsVol. 5 (2003)

Fig. 5 Hans D. Jensen (1907–1973). Credit: American Institute of Physics Emilio Segrè VisualArchives, W.F. Meggars Gallery of Nobel Laureates.

02_Bernstein 8.8.2003 10:28 Uhr Seite 255

Page 14: Bernstein PP 2003

256

in Hannover. After the war he became a professor at the University of Heidelbergfrom which he retired in 1969. Six years earlier he had been awarded the Nobel Prizein Physics for his theoretical work on the structure of nuclei. Jensen never wrote abouthis wartime experiences. He rarely spoke of them, although I will give one notableexception shortly. While he knew well many of the members of the Uranverein, he wasnever in the club. For example, while Heisenberg did tell Jensen about his work onreactors, he never told him that one of its purposes would be to make plutonium.Jensen had a reputation for very left-wing political opinions which may be why he wasnever part of the inner circle of the German program although he did work connectedwith it. Nonetheless, after Heisenberg returned from Copenhagen he encouragedJensen to make a visit possibly to repair the damage that Heisenberg thought he haddone with Bohr. In 1948, he wrote that in encouraging this visit he hoped that Jensenwould give Bohr a correct view – ein richtiges Bild – of their enterprise of building areactor.* With Heisenberg one is never quite sure of the agenda. Certainly ein richtigesBild would have included a discussion of plutonium. In any event, in the summer of1942 Jensen visited Copenhagen. The people there were not quite sure what to makeof him, or of his visit. There was some feeling that he might be a Nazi plant – which hecertainly wasn’t although he had been a member of the Nazi Party since 1937. Hejoined, he said, to preserve his academic career. His left-wing attitudes were well-known and it was even rumored that he was a Communist. But that he was able tomake two visits to occupied Denmark aroused suspicion. Bohr made it clear that hisnegative feelings about Heisenberg had not changed. Jensen described what he knewof Heisenberg’s work, but remember that in 1942, they were still using the sphericaldesign with a paraffin moderator and uranium metal powder. This was not the reactorwhose drawing Bohr brought to Los Alamos. But a year later Jensen was back inCopenhagen and he had new information.

One is not entirely sure where he got this information but there is a suggestive hint.On May 5, 1943, Heisenberg gave a lecture on atomic energy for the Aviation Acade-my. This was something that had been created by Hermann Göring for people such asengineers or military officers who had a special interest in aviation. Heisenberg’s lec-ture was one of a small series given by members of the Uranverein. By this time, andthis is somewhat ironic in view of Bohr’s communication with Chadwick, the Germanshad made it clear to everyone who might be of importance to the program, that atom-ic energy would not play any military role in the near future. In his lecture Heisenbergbarely mentions it except to say that it is another order of magnitude of difficulty com-pared to the applications he does discuss; namely, the reactor. Needless to say, he doesnot discuss plutonium. What he does discuss in some detail is the reactor he proposesto build the following summer. He is very specific about this. There is even a diagram.This is a reactor in a cylindrical container with a heavy-water moderator and metallicuranium plates. He says it will consist of one and a half metric tons – 3,300 pounds – ofheavy water and three metric tons – 6,600 pounds – of metallic uranium. The uranium

Jeremy Bernstein Phys. perspect.

* I am grateful to Cathryn Carson for showing me this communication and for discussions on thegeneral subject of the German program.

02_Bernstein 8.8.2003 10:28 Uhr Seite 256

Page 15: Bernstein PP 2003

is to be immersed in the heavy water in metallic plates. In the actual reactor which wasconstructed later that year there were less than three thousand pounds of uranium inthirty-eight metallic plates. It gave promising results but did not go critical; that is, pro-duce a self-sustaining chain reaction. There is no reason to think that Jensen was at thislecture, but there is every reason to believe that he knew about this reactor. Jensen wasworking on the extraction of heavy water and he was clearly in contact with Heisen-berg. Moreover, while this lecture was not open to the general public it was not regard-ed as supersecret. There are also clear reasons to be quite sure that on his second visitto Copenhagen in the summer of 1943, he told Bohr about it.

One reason to think so is because he said he did. Recently my colleague Mal Rud-erman, who is now a professor at Columbia University, told me that while he was onthe faculty at Berkeley Jensen came to dinner at his house.* During dinner Jensen toldthe assembled group that he was the one who had informed Bohr about the Germanprogram. Bohr says as much in the recently released letters that he wrote but did notsend to Heisenberg in 1957. In one of them Bohr writes:

As regards Jensen’s visit to Copenhagen in 1943, the war had already at that timetaken a course quite different from what you and [C.F. von] Weizsäcker expressedas your conviction in 1941 [that the Germans would rapidly win the war]. Jensendescribed the efforts to increase the production of heavy water in Norway and men-tioned in this connection that, for him and other German physicists, it was only amatter of an industrial application of atomic energy. At that time, however, I had tobe very cautious and sceptical, partly on the basis of rumours of new Germanweapons, partly because of my own difficult position due to the constant surveil-lance of the German police.20

Bohr’s Understanding of Reactor Design

This discussion with Jensen was what inspired Bohr to write his second communicationto Chadwick. I will not quote in detail from the rest of it since it is rather technical. Itdoes show that at that time Bohr really did not understand what a nuclear weapon was.He also did not seem to understand this reactor very well.** Bohr considered the casein which this reactor began to explode. In other words the chain reaction started to runaway. He asked the question what would stop it. He proposed the following answer. Hesaid that when the temperature of the reactor, which goes up because the chain reac-tion is producing energy, reaches a critical value it will blow off heavy water. When thishappens there will no longer be a moderator and the chain reaction will stop. By esti-mating this temperature Bohr concluded that the most power such an explosion couldgenerate is about a hundred times what a comparable TNT explosion would produce.

257The Drawing or Why History Is Not MathematicsVol. 5 (2003)

* I am grateful to Paula and Mal Ruderman for telling me about this incident.** I am very grateful to Freeman Dyson for several discussions of these technical questions. He

pointed out to me the flaw in Bohr’s analysis.

02_Bernstein 8.8.2003 10:28 Uhr Seite 257

Page 16: Bernstein PP 2003

258

He concludes:

Even if this is very great compared with that obtainable with ordinary chemicalexplosives, it would in view of the large scale bombing already achieved hardly beresponsible to rely on the effect of a single bomb of this type procurable only withan enormous effort. The situation, however, is of course quite different, if it is truethat enough heavy water can be made to manufacture a large number of eventualatomic-bombs, and although I am convinced that the arguments here outlined arefamiliar to experts, I hasten therefore to modify my statement [that atomic weaponswere not relevant to the present war].21

One can only wonder what Chadwick made of this communication. He surely knewthat heavy water has nothing to do with atomic bombs except that it can be used inreactors that make plutonium. There is no heavy water in a fission bomb and Bohr didnot know about plutonium. Furthermore, upon his arrival in Britain Bohr seemed to besending mixed messages. Chadwick got the impression that what Bohr was saying wasthat Heisenberg was not working on atomic weapons at this juncture.This was true, butBohr was nonetheless presenting his information as if it concerned the design of sucha weapon. This persisted until he got to Los Alamos where, as we have seen, he incitedGroves to call an emergency meeting. Bethe once told me that it was clear to him thatBohr, when he arrived at Los Alamos, was quite ignorant about nuclear weapons. Infact, Oppenheimer assigned Richard Feynman to Bohr to get him up to speed. He alsodid not understand the physics of this reactor very well. This is what Bethe and Tellersorted out. First let me explain briefly why Serber thought that, as a reactor design,what he saw in the drawing was “silly.” This will illuminate one of the problems withthe German program.

Remember that the drawing showed uranium plates embedded in heavy water.This is not an efficient design. The reason has to do with the uranium. Recall that themoderator – the heavy water – is slowing the neutrons down so they can cause fission.An ideal situation would be to have this slowing down take place without the neu-trons spending any time in the uranium. This is not possible. They diffuse in and outof the uranium during this process. But here is the problem. Uranium can captureneutrons. That is how plutonium is made. The probability of this capture depends onthe energy of the neutrons. At certain energies – “resonance” energies – the proba-bility becomes huge. Neutrons get eaten and are no longer available for fission. Soone wants a design that minimizes the time neutrons have to spend in the uraniumwhile they are slowing down. Plates are very bad. A fast neutron created in the inte-rior can travel a long way before getting out. During this time it is neither moderatednor can it cause fission, but it can get captured. Much better are chunks of uranium –spheres or cylinders or rods – which have the moderator all around them. This wasclearly understood by Enrico Fermi. His reactor that went critical on December 2,1942, had uranium lumps embedded in purified graphite. But this principle was alsounderstood by the Germans – at least some of them. As early as 1942, a young theo-retical physicist named Karl-Heinz Höcker, who was attached to the program, showedthat this lump design was vastly superior to the layers. Heisenberg was well aware ofthis analysis but chose to ignore it since the layer design was simpler to calculate with.

Jeremy Bernstein Phys. perspect.

02_Bernstein 8.8.2003 10:28 Uhr Seite 258

Page 17: Bernstein PP 2003

(There was a rival group in Berlin that used Höcker’s design which may have con-tributed to Heisenberg’s reluctance.) It was only his last reactor, which was capturedin 1945, that used metal cubes embedded in heavy water. This reactor was close togoing critical. But Bethe and Teller were not in the business of reactor design. Theywere analyzing the reactor design that Bohr had brought to them. Actually theyseemed to have improved on it.

For a long time I was completely baffled by the dimensions of the reactor Bethe andTeller analyzed. They assumed that it had forty tons of heavy water. This is a hugeamount – an order of magnitude larger than any corresponding number in the Germanprogram in which they struggled to get a few tons. Where did this number come from?Finally I asked Bethe (figure 6).* His answer really surprised me. “Partly from theCanadians,” he said. Starting in the early 1940s the Canadians had had a project tobuild a heavy-water reactor. There was a British-Canadian collaboration and some of

259The Drawing or Why History Is Not MathematicsVol. 5 (2003)

Fig. 6 Hans A. Bethe (b. 1906) at a meeting of the American Physical Society in 1957. Credit: Photo-graph by Bob Davis; courtesy of American Institute of Physics Emilio Segrè Visual Archives, PhysicsToday Collection.

* This conversation for which I am grateful took place on January 15, 2002.

02_Bernstein 8.8.2003 10:28 Uhr Seite 259

Page 18: Bernstein PP 2003

260

these people found their way to Los Alamos. The Canadians had made a theoreticalanalysis of how much heavy water was needed to make such a reactor work and hadcome up with something like forty to fifty tons if the reactor was to produce any sub-stantial amount of power.* Bethe and Teller knew about the Canadian reactoralthough they were not sure of its dimensions. They guessed. The model reactor thatthey analyzed had a “radius” of 2 meters which would hold forty tons of heavy water;their version of the Canadian reactor. Actually the first heavy-water reactor to go crit-ical was not Canadian but a small test reactor built at Argonne, Illinois. It went criticalon May 15, 1944, and used 6.5 tons of heavy water in a cylindrical aluminum tank. Thisis to be compared with the approximately 3,300 pounds that Heisenberg used in his lastreactor, which he was desperately trying to complete in the spring of 1945, before theAllies captured it. The total production of heavy water available to the Germans dur-ing the war was something like three tons.** Bethe and Teller wanted to know whattheir model reactor would do if the chain reaction ran away. They realized that Bohr’smechanism in which the heavy water is blown off is irrelevant.

What happens is this. Because of the characteristics of the reactor it takes a longtime – compared to the sorts of times that are involved in say a bomb – for the neutronpopulation to increase. It takes about a millisecond for it to double. In a bomb this issomething like a hundredth of a microsecond. A bomb that is made of U235 uses fastunmoderated neutrons.

During this millisecond the reactor expands since the fission fragments heat it up.But as it expands the uranium becomes less dense. A neutron has to go farther and far-ther to find a uranium nucleus to fission. Bethe and Teller estimated that if the reactordoubled its size this would decrease the density to the point where the chain reactionstops. They calculated how much energy would be released and pointed out that thiswould be less than an equivalent amount of TNT – two orders of magnitude smallerthan Bohr’s estimate. They also considered the effects of the radiation which is, as wenow know only too well, what really matters when a reactor blows up. This is what theycommunicated to Groves in their two-page report which put an end to any concernabout Heisenberg’s reactor as a nuclear weapon.

This then is the story of the drawing, at least as I see it. What are we to make of it?I readily admit that I have not given a mathematical proof that Heisenberg did not giveBohr a drawing in Copenhagen. If you insist, you can still believe that happened. His-tory is not mathematics. My appeal, however, is to plausibility, economy, and commonsense.

Jeremy Bernstein Phys. perspect.

* Freeman Dyson has pointed out to me that a reactor that produces a megawatt of power willheat up ten tons of water in an hour to the boiling point. Forty tons is not an excessive amountif the water is used as both a moderator and a coolant.

** The largest producer of heavy water in the world is Canada.To give some idea,a Canadian exper-iment to detect neutrinos produced in supernova explosions uses as a detector a tank that con-tains 1,000 tons of heavy water on loan from the Canadian Atomic Energy Commission.

02_Bernstein 8.8.2003 10:28 Uhr Seite 260

Page 19: Bernstein PP 2003

References

1 I published these as a book, Hans Bethe, Prophet of Energy (New York: Basic Books, 1980).2 Quoted in ibid., p. 77.3 Abraham Pais, Niels Bohr’s Times, In Physics, Philosophy, and Polity (Oxford: Clarendon Press,

1991).4 Thomas Powers, Heisenberg’s War: The Secret History of the German Bomb (New York: Alfred A.

Knopf, 1993).5 Ibid., p.128.6 Aage Bohr to Powers, November 16, 1989, quoted in ibid., p. 511, n. 31.7 Pais to the author, undated.8 Bethe to the author, undated.9 Jeremy Bernstein, Hitler’s Uranium Club: The Secret Recordings at Farm Hall (Woodbury, New

York: American Institute of Physics Press, 1996).10 Robert Serber, The Los Alamos Primer: The First Lectures on How To Build An Atomic Bomb

(Berkeley: University of California Press, 1992).11 F.H. Hinsley, et al., ed., British Intelligence in the Second World War, Vol. 3, Part 2 (London: HMSO

Publications Center, 1988); see especially Appendix 29 which was written in November 1944. It isastonishing what the British were able to find out by 1943 about this top secret German program.

12 Oppenheimer to Groves, January 1, 1944, from the files of Robert Serber.13 Joanne Smith to the author, April 3, 1997.14 Hans Bethe and Edward Teller, “Explosion of an Inhomogeneous Uranium-Heavy Water Pile,”

January 1, 1944, from the files of Robert Serber.15 For a particularly nice account, see Per F. Dahl, Heavy Water and the Wartime Race for Nuclear

Energy (Bristol and Philadelphia: Institute of Physics Publishing, 1999).16 I am very grateful to Mark Walker for several communications. I have also profited from his book,

German National Socialism and the quest for nuclear power 1939–1949 (Cambridge: CambridgeUniversity Press, 1989). I have also found useful David Irving’s The German Atomic Bomb: TheHistory of Nuclear Research in Nazi Germany, 2nd edition (New York: Da Capo, 1983). David Cas-sidy supplied me with a schematic showing all of Heisenberg’s reactor designs. A primary sourceis Werner Heisenberg, Collected Works. Series A/ Part II. Original Scientific Papers, ed. W. Blum,H.-P. Dürr, and H. Rechenberg (Berlin: Springer-Verlag, 1989).

17 Chadwick to Bohr, January 25, 1943, Niels Bohr Archive, Copenhagen; quoted in part in MargaretGowing, Britain and Atomic Energy 1939–1945 (London: Macmillan and New York: St Martin’sPress, 1964), p. 246. I am grateful to Finn Aaserud, Director of the Niels Bohr Archive, for supply-ing this letter to me.

18 Bohr to Chadwick, undated, probably February 1943, Niels Bohr Archive, Copenhagen; quoted inpart in Gowing, Britain and Atomic Energy (ref. 17), pp. 246–247. I am grateful to Finn Aaserud,Director of the Niels Bohr Archive, for supplying this letter to me and to the Niels Bohr Archivefor permission to quote it.

19 Bohr to Chadwick, undated, Niels Bohr Archive, Copenhagen; paraphrased in part in Gowing,Britain and Atomic Energy (ref. 17), p. 247. I am grateful to Finn Aaserud, Director of the Niels BohrArchive, for supplying this letter to me and to the Niels Bohr Archive for permission to quote it..

20 Bohr to Heisenberg, undated [1957], Document 9, in “Niels Bohr-Werner Heisenberg,” NaturensVerden, No. 8–9 (2001), pp. xxx–xxxi. I am grateful to Finn Aaserud, Director of the Niels BohrArchive, and to the Niels Bohr Archive for permission to quote this letter.

21 Bohr to Chadwick, undated (ref. 19).22 In contemplating what elements would be needed to believe now that a drawing was given to Bohr

in 1941 I kept thinking of Ionesco’s play The Bald Soprano. I commend the following: “In spite ofthe extraordinary coincidences which seem to be definite proofs, Donald and Elizabeth not beingparents of the same child are not Donald and Elizabeth.” See Eugène Ionesco, The Bald Sopranoand other Plays (New York: Grove Press, 1958), p. 19.

2 Fifth Avenue, 18-LNew York, NY 10011, USAe-mail: [email protected]

261The Drawing or Why History Is Not MathematicsVol. 5 (2003)

02_Bernstein 8.8.2003 10:28 Uhr Seite 261