What did Heisenberg Tell Bohr about the Bomb?

In September 1943 Niels Bohrlearned that the gestapo in Copen-hagen intended to arrest him. A few

weeks later, on the 29th, he, his wifeand several others hoping to escapefrom Denmark crawled in completedarkness to a beach outside Carlsberg.There they boarded a boat and crossedthe ̄ resund in secret to Sweden. On Oc-tober 6 the British ßew Bohr alone fromSweden to Scotland. Later that same dayhe traveled to London and in the eveningmet with Sir John Anderson, the phys-ical chemist in charge of the nascentBritish atomic bomb project. Andersongave the Danish physicist a briefing onthe Anglo-American program. Accord-ing to BohrÕs son Aage, who followedhis father to England a week later andwas his assistant throughout the war,Bohr was deeply surprisedÑshockedmay be a better descriptionÑby howfar the Anglo-American program hadalready progressed.

BohrÕs alarm very likely had twosources. First, during the 1930s, whennuclear physics was developing, Bohrhad said on several occasions that hethought any practical use of nuclear en-ergy was all but impossible. That view

was reinforced in the spring of 1939,when he realized an important detailconcerning the Þssion of uranium. InDecember 1938 the German physicalchemists Otto Hahn and Fritz Strass-mann had discovered that uraniumcould be Þssioned if it was bombardedwith neutrons. (HahnÕs former assis-tant Lise Meitner and her nephew OttoFrisch conjectured that the uraniumnucleus had actually been split in theexperiments and so coined the nameÒÞssionÓ for the process.) The experi-ments used natural uranium, 99 per-cent of which is in the isotope uranium238. About seven tenths of a percent isin the isotope uranium 235, whose nu-cleus contains three fewer neutrons.

Chemically, the isotopes are indistin-guishable. What Bohr realized was thatbecause of their structural diÝerences,only the very rare isotope uranium 235had Þssioned in the Hahn-Strassmannexperiments. He concluded, then, thatmaking a nuclear weapon would be al-most impossible because it would re-quire separating these isotopesÑadaunting task. In December 1939 hesaid in a lecture, ÒWith present techni-cal means it is, however, impossible topurify the rare uranium isotope in suÛ-cient quantity to realize the chain reac-tion.Ó One can therefore well under-stand why Bohr was shocked to learnfour years later that that was just whatthe Allies intended to do.

The second reason for BohrÕs alarmcan be traced back to a meeting he hadhad with the German physicist WernerHeisenberg in mid-September 1941, al-most two years before his escape toBritain. By 1941 the Germans had occu-pied Denmark for more than a year.During that period, they established aso-called German Cultural Institute inCopenhagen to generate German cul-tural propaganda. Among its activities,the institute organized scientiÞc meet-

ings. Heisenberg was one of several Ger-man scientists who came under its aus-pices to Copenhagen, in this case to ameeting of astronomers. He had knownBohr since 1922 and had spent a gooddeal of time at BohrÕs institute in Copen-hagen, where Bohr had acted as a kindof muse for the creation of quantumtheory. Now Heisenberg had returnedas a representative of a despised occu-pying power, touting the certainty of itsvictory, according to some accounts.

HeisenbergÕs Visit

Heisenberg spent a week in Copenha-gen and visited BohrÕs institute on

several occasions. During one of thesevisits, he and Bohr talked privately. Nei-ther man seems to have made any notes,so one cannot be entirely sure what wassaid. Also, Bohr was a poor listener, sothe two may well have talked past eachother. Nevertheless, Bohr came awayfrom the discussion with the distinctimpression that Heisenberg was work-ing on nuclear weapons. As Aage Bohrlater recalled, ÒHeisenberg brought upthe question of the military applicationsof atomic energy. My father was veryreticent and expressed his skepticismbecause of the great technical diÛcul-ties that had to be overcome, but he hadthe impression that Heisenberg thoughtthat the new possibilities could decidethe outcome of the war if the wardragged on.Ó Now, two years later, Bohrwas learning for the Þrst time of theAllied nuclear weapons program. Whathad the Germans done during those twoyears? No wonder Bohr was alarmed.

It would be fascinating to know in de-tail what was meant by Ònew possibili-ties,Ó but one can make an educatedguess. By the mid-1940s physicists onboth sides of the conßict realized thataside from Þssioning uranium, therewas an entirely separate route to mak-

92 SCIENTIFIC AMERICAN May 1995

What Did Heisenberg TellBohr about the Bomb?

In 1941 Werner Heisenberg and Niels Bohr met privately in Copen-hagen. Almost two years later at Los Alamos, Bohr showed a sketch of what he believed was Heisenberg’s design for a nuclear weapon

by Jeremy Bernstein

JEREMY BERNSTEIN is professor ofphysics at the Stevens Institute of Tech-nology and an adjunct professor at theRockefeller University. He also serves asa vice president of the board of trusteesof the Aspen Center for Physics. He haswritten some 50 technical papers, 12books and numerous magazine articles.He has worked as a staÝ writer at theNew Yorker magazine, taught nonÞctionwriting at Princeton University and wonseveral science writing awards. He is afellow of the American Physical Society,a Benjamin Franklin Fellow of the RoyalSociety of the Arts and a member of theFrench and American Alpine Clubs.

Copyright 1995 Scientific American, Inc.

ing a nuclear weaponÑthe use of whatlater came to be known as plutonium.That element is somewhat heavier thanuranium and has a diÝerent chemistry,but given its nuclear structure, it is atleast as Þssionable. Unlike uranium,though, plutonium does not exist natu-rally and must be manufactured in anuclear reactor by bombarding the re-actorÕs uranium fuel rods with neutrons.Once made, the plutonium can be sep-arated from its uranium matrix bychemical means.

From the moment this process wasunderstood, any reactor became, in acertain sense, a component of a nucle-ar weapon. There is no doubt whatsoev-er that Heisenberg knew this fact wellwhen he visited Bohr. He even gave lec-tures, whose texts have been preserved,describing such a possibility to highlyplaced German oÛcials. Is this what hewas trying to tell Bohr and, if so, why?There was such a lack of agreement be-tween the two men as to what exactlywas said that we will probably neverknow for sure.

As a corollary to this larger puzzlethere is a smaller one. There is evidencethat during the course of the Copenha-gen meeting, Heisenberg gave Bohr a

drawing. It is not clearwhether Heisenberg madethe drawing at the meet-ing or beforehand. Beingfamiliar with how theo-retical physicists commu-nicate, I would imaginehe drew the sketch onthe spot to help conveyan idea. In any case, un-der circumstances I willshortly describe, thisdrawing, or a replica,found its way to LosAlamos Laboratory in December1943, where it created a considerablestir: it appeared to contain direct infor-mation about how the Germans wereplanning to make nuclear weapons. Be-fore I describe how the drawing got toLos Alamos, let me tell how I learned ofits existence. There is a relation.

The Mysterious Sketch

Beginning in November 1977, I con-ducted a series of interviews with

the physicist Hans Bethe. Those sessionslasted on and oÝ for two years and re-sulted in a three-part proÞle for the New

Yorker magazine and a subsequent

book.The interviews, which I taped, followedthe chronology of BetheÕs life. Bethe,who was born in Strasbourg in 1906,emigrated to the U.S. in 1935 and hasbeen at Cornell University ever since.He became an American citizen in 1941,by which time, as he recalled, he wasÒdesperate to do somethingÑto makesome contribution to the war effort.ÓHe, like Bohr, was at Þrst certain thatnuclear weapons were entirely imprac-tical and went to work on the develop-ment of radar at the Massachusetts In-stitute of Technology.

In the summer of 1942 J. Robert Op-

SCIENTIFIC AMERICAN May 1995 93

WERNER HEISENBERG AND NIELS BOHR, shown above in 1934, had acurious meeting seven years later. Some speculate that the two physi-cistsÑthen from opposite sides of the warÑdiscussed nuclearweapons. When Bohr arrived at Los Alamos Laboratory in late 1943,he reportedly had a sketch of what he thought to be HeisenbergÕsplan to build a bomb. If so, the drawing most likely resembled the re-actor design shown at the right.

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penheimer convened a study group atthe University of California at Berkeleyto investigate nuclear weapons. By thistime Bethe was acknowledged as oneof the leading nuclear theorists in theworld, so Oppenheimer naturally askedhim to participate. On the way to Cali-fornia by train, Bethe stopped in Chica-go to pick up Edward Teller. There Be-the got the chance to see Enrico FermiÕsdeveloping nuclear reactor and, in hiswords, Òbecame convinced that theatomic bomb project was real, and thatit would probably work.Ó He spent thatsummer working on the theory of nu-clear weapons and in April 1943 wentto Los Alamos, which had just openedas a laboratory. Eventually he becamehead of its theory division.

Now to the drawing. On November29, 1943, Bohr and his son Aage sailedfrom Glasgow on the Aquitania for New

York City. They arrived on December 6.Bohr was assigned the code name ofNicholas Baker, and Aage became JamesBaker; they were also given bodyguards.On December 28, after having had high-level meetings in Washington, D.C.,with many oÛcialsÑincluding MajorGeneral Leslie R. Groves, the command-ing oÛcer in charge of the ManhattanProjectÑBohr departed for Los Alamos.On the 31st, presumably just after ar-riving at the laboratory, he met with aselect group of physicists. The principalpurpose of this meeting was for Bohrto tell the attending physicists what heknew about the German eÝort to makea nuclear weaponÑin particular whathe had learned from Heisenberg.

During one of my interviews with Be-the, he described this meeting, thoughnot in any detail, and told me aboutthe drawing. This is what he said to me(I have it on my tapes): ÒHeisenberggave Bohr a drawing. This drawing was

transmitted by Bohr later on to us atLos Alamos. This drawing was clearlythe drawing of a reactor. But our con-clusion was, when seeing it, these Ger-mans are totally crazy. Do they want tothrow a reactor down on London?ÓOnly after the war did the Los Alamosscientists learn that the Germans knewperfectly well, at least in principle, whatto do with a reactorÑuse it to makeplutonium. But Bohr was concernedthat one could actually use this reactoras some sort of weapon.

As far as I know, until I described thismatter in the New Yorker, no one hadever mentioned such a drawing in print.In fact, my article on Bethe was fre-quently cited as the source for this oddsidelight on the Bohr-Heisenberg rela-tionship. Hence, I found myself as akind of a footnote to a footnote to his-tory. My authority was shaken, though,at the start of 1994, during one of myperiodic visits to the Rockefeller Uni-versity in New York City, where I am anadjunct professor. Abraham Pais, a bi-ographer of both Einstein and Bohr anda professor of physics emeritus at theuniversity, called me into his oÛce. Ihave known Pais for 40 years but hadnot seen him in a while. This visit, then,

was his Þrst opportunity to tell meabout a call he had received severalmonths earlier.

It was from Thomas Powers, who atthat time was writing HeisenbergÕs War.

Powers had learned about the drawingfrom my book on Bethe. He was struckby the fact that at Þrst glance it seemedas if Heisenberg had given to Bohr, inthe middle of a war, a drawing of a high-ly classiÞed German military project.That was such an extraordinary thingfor Heisenberg to have done, if he diddo it, that Powers wanted to check thematter out. He therefore got in touchwith Aage Bohr in Copenhagen (his fa-ther had died in 1962). In a letter datedNovember 16, 1989, Aage Bohr wrote,ÒHeisenberg certainly drew no sketchof a reactor during his visit in 1941.The operation of a reactor was not dis-cussed at all.Ó

Stunned, Powers next contacted Be-the, who repeated to him exactly whathe had told me 10 years earlier. In aquandary, Powers had called Pais, andnow Pais was asking me. But Pais haddone his own investigation. He had spo-ken with Aage Bohr, who once again in-sisted that there had never been anysuch drawing. Pais had also checked thearchives in Copenhagen where all BohrÕsprivate papers and journals are stored.Nowhere, he told me, did he Þnd anymention of this drawing. Now it was myturn to be stunned. It is one thing to bea footnote to a footnote to history, butit is quite another to be a footnote to afootnote to incorrect history.

I promised Pais I would look into thematter myself, although, in truth, whenI left his oÛce I did not have the foggi-est idea of how I would go about it. Ob-viously, contacting Bethe again wouldnot get me much further. Nothing couldbe more direct than what he had told


HANS BETHE clearly recalls seeing a de-sign credited to Heisenberg at Los Ala-mos in 1943. He wrote to the authorabout this recollection last year (right ).At the left, Bethe sits in front of EnricoFermi, who designed the Þrst workingnuclear reactor in the U.S., in Chicago.FermiÕs success helped to convince Be-the that nuclear weapons could be made.

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me and repeated to Powers. I wouldneed witnesses independent of Betheand Aage Bohr. That much was clear.But who? Oppenheimer was dead. NielsBohr was dead. Groves was dead. Whoelse could have seen that drawing?

The Investigation

Ibegan, in fact, with less informationthan I have so far given the reader.

All Bethe had told me was that Bohrhad ÒtransmittedÓ a drawing to Los Ala-mos. He had not related any speciÞc de-tails about the December 31 meeting, soinitially I had no idea who might havebeen there. Indeed, I did not even havethe speciÞc date. All that I learned sub-sequently. But I did know physicistswho were at Los Alamos at the time andwho might have seen or heard about thedrawing. Two came to mind. One was

Victor Weisskopf, an old friend, whohad been close to Oppenheimer.

The other was Rudolf Peierls. Peierlsand Otto Frisch had in March 1940made the Þrst correct calculationÑinprincipleÑto determine the amount ofuranium 235, or the critical mass, need-ed to make a bomb. (The fact that thismass turned out to be pounds ratherthan tons is what really prompted theAllied eÝort.) Peierls, along with Frisch,was at Los Alamos as of early 1944. Ihave also known Peierls for many yearsand have frequently discussed with himthe history of nuclear weapons. So itwas quite natural for me to write him aswell. This I did in early February, andsoon after, both men answered.

Peierls replied that he had never seenthe Òfamous sketchÓ yet did not thinkthat either Bethe or Aage Bohr had delib-erately lied. He proposed that perhapsNiels Bohr had kept knowledge of thesensitive document from his family or

that perhaps Heisenberg had only shownthe sketch to Bohr, who might then haveredrawn it. He suggested I contact Betheabout this possibility. Weisskopf alsowrote proposing I contact Bethe oncemore, because he, too, had never seenor heard about the drawing.

Neither of these letters was what Ihad hoped to receive. Clearly, I had towrite Bethe to tell him what I hadlearned and to see if he could shed anyfurther light on the situation. But then Ihad an inspiration. I would call RobertSerber. Serber, a professor of physicsemeritus at Columbia University wholives in New York City, is also an oldfriend. After receiving his Ph.D. in 1934from the University of Wisconsin, hehad won one of Þve National ResearchCouncil Fellowships in physics andchose to go work with Oppenheimer atBerkeley. During the next few years, hehad become very close to Oppenheimer.

After a brief interlude at the Univer-sity of Illinois from 1938 until 1942,Serber returned to Berkeley to work onthe bomb with Oppenheimer. He wasthere in the summer of 1942 when Be-the and Teller arrived. By March 1943he had moved, with the Þrst batch ofscientists, to Los Alamos. One of his

early tasks was to give a series of intro-ductory lectures on bomb physics tothe arriving scientists. These lessonswere collected into what came to becalled The Los Alamos Primer, declas-siÞed in 1965 and Þrst published in itsentirety in 1992. If anyone knew aboutthe drawing, it would be Serber becausehe was in constant contact with Oppen-heimer throughout this period.

I called Serber, and immediately Iknew I had struck a gold mine. Not onlydid he remember the drawing vividly,but he also remembered the precise cir-cumstances under which he had seen it.He had been summoned to Oppenheim-erÕs oÛce on December 31, where ameeting was already in progress. Op-penheimer showed him a drawing withno explanation and asked him to iden-tify it. This was the kind of intellectualgame Oppenheimer liked to play. Serber

looked at it and said it was clearly thedrawing of a reactor. Oppenheimer re-plied that in fact it was a drawing ofHeisenbergÕs reactor and had been givento the assembled group by Bohr. Bohr,who was, as Serber recalled, standingnext to Oppenheimer, did not disagree.

That is what Serber told me. But healso said he had some written materialrelated to this meeting. A few days lat-er copies of two documents arrived: aletter from Oppenheimer to GeneralGroves sent the day after the meetingand a two-page memorandum writtenby Bethe and Teller on the explosivepossibilities of the reactor. Unfortunate-ly, although these documents were verysuggestive, they did not, at least when IÞrst read them, settle the issue com-pletely. The Bethe-Teller memorandumdid hold signiÞcant clues, but I will re-turn to them later. OppenheimerÕs let-ter made no mention of the drawing orof Heisenberg or of the Germans. But


ABRAHAM PAIS asked Aage Bohr, oneof BohrÕs sons, whether Heisenbergmight have given his father a drawingin 1941. Aage Bohr (left ) knew his fa-therÕs activities well, having served ashis assistant during the war. Pais wrotethe author last summer (right ) that AageBohr was certain Heisenberg had notmade a sketch for his father.

“[Nothing] has changedAage’s calm and firmopinion that there

never was a drawing.” —Pais

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the last sentence clearly implied thatBohr had spoken to Groves in Washing-ton about these matters. Perhaps some-thing in GrovesÕs own archives mightprove enlightening.

Meanwhile I had at last written to Be-the, and on March 2, I received his an-swer. It begins, ÒI am quite positivethere was a drawing. Niels Bohr present-ed it to us, and both Teller and I imme-diately said, ÔThis is a drawing of a re-actor, not of a bomb.Õ. . . Whether thedrawing was actually due Heisenberg,or was made by Bohr from memory, Icannot tell. But the meeting on 31 De-cember 1943 was especially called toshow us what Niels Bohr knew aboutthe GermansÕ idea of a bomb.Ó

Bethe oÝered a theory to explain themystery: ÒHeisenberg thought that themain step to a bomb was to get a reac-tor and to make plutonium. A reactor,

however, could also be used as a powersource. Niels Bohr was very ignorantabout the whole subject. Heisenbergprobably wanted to show Bohr that theGermans were not making a bomb butmerely a reactor. Bohr misunderstoodcompletely, and only on 31 December1943 was it Þnally explained to himthat this was not a bomb. That drawingmade a great impression on me. Again,I am surprised that Viki [Weisskopf]and Aage have forgotten about it. Whatdoes Serber say?Ó

I was able to write Bethe and tell himwhat Serber had said. I also wrote Tellerto ask for his recollections of the meet-ing. I was not sure I would get an an-swer and never have. But I had also writ-ten again to Weisskopf, sending himcopies of the memorandums from Ser-ber. On February 23, I received a typical-ly gracious Weisskopf letter, acknowl-edging that he had indeed seen thesketch but later forgotten about it.

I now had, I thought, enough materi-al to return to Pais. I played for him myBethe tape and gave him copies of allthe documents. He was about to returnto Copenhagen, where he spends abouthalf the year with his Danish wife. Hepromised me that he would speak toAage Bohr at an opportune moment.That happened late in June. By the 30thPais had written to tell me what hadhappened. He and Aage Bohr had met,discussed the letters and reviewed thetapes. Still, Aage Bohr felt certain thatHeisenberg never gave any such draw-ing to his father. So I wrote to AageBohr directly. In February of this yearhis assistant, Finn Aaserud, wrote,ÒAage Bohr maintains that it is entirelyimpossible that Bohr brought with himto the U.S. a drawing from the 1941meeting with Heisenberg and indeedthat the discussion at Los Alamos yourefer to had anything to do with the1941 encounter at all.Ó

Where does this leave us? I haveasked myself this question many timessince receiving PaisÕs letter last June. Iwas at a loss until recently, when I tookanother look at the memorandum thatBethe and Teller prepared for Oppen-

heimer and Bohr and eventually forGroves. It suddenly struck me that inthe Þrst sentence of the second para-graph of this report HeisenbergÕs im-print stands out like a sore thumb. Itreads, ÒThe proposed pile [reactor] con-sists of uranium sheets immersed intoheavy water.Ó In other words, Bethe andTeller were not considering any old re-actor design but rather a very particularone that Bohr had described to them.This design is actually the faulty reac-tor Heisenberg invented in late 1939and early 1940, which he clung to untilnearly the end of the war!

It is almost unthinkable that in the fewshort weeks from when Bohr learnedabout the Allied project to when he ar-rived at Los Alamos he would have pro-duced his own design possessing thesame ßaws as did HeisenbergÕs. He musthave gotten this idea from Heisenberg,

either verbally or in the form of a draw-ing. Where else could it have come from?

The Evidence

Let me explain. Any reactor requires fuel elements, the uranium, and

what is known as a moderator, a devicethat slows the speed of neutrons hittingthe fuel. Neutrons traveling near thespeed of sound are vastly more eÝectivein causing Þssion than are the rapidlymoving neutrons produced by the Þs-sioning itself. So the fuel elements in areactor are embedded in the moderator.But a designer must carefully choosefrom which material the moderatorshould be made and also how the fuelelements should be placed in it. Thelatter involves both art and science.

The trick is that the uranium itselfcan absorb neutrons without producingÞssion. This absorption becomes strong-er as the neutrons are slowing down. If


ROBERT SERBER not only rememberedseeing a drawing on December 31, 1943,but had kept several revealing docu-ments. He sent the author a letter (shownat right ) from J. Robert Oppenheimerto Major General Leslie R. Groves (bothshown at left ). In it, Oppenheimer as-sured Groves that the Òarrangement ofmaterialsÓ Baker (BohrÕs code name) haddescribed could not be used as a weapon.

“. . . the [device] suggest-ed . . .by Baker would be a quite useless military weapon.” –Oppenheimer

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the geometry of the fuel ele-ments is not well thought out,the uranium will absorb somany neutrons that a self-sustaining chain reaction willnever take place. In fact, themost eÛcient design involvesseparated lumps of uraniumembedded in a lattice withinthe moderator. How big theselumps should be, and howthey should be arranged, in-volves art. But the worst pos-sible solution is placing theuranium in sheets, or layers.

To return to the matter athand, note that Bethe and Tel-ler wrote, ÒThe proposed pileconsists of uranium sheets.ÓHeisenberg chose just such adesign because it involvedeasier calculations than didother schemes. Then there isthe question of the modera-tor. Bethe and Teller statedthat the sheets were to beÒimmersed into heavy water.ÓThis speciÞcation, once ex-plained, also has Heisenbergwritten all over it. The role ofthe moderator is, as I havementioned, to slow down theÞssioned neutrons. The bestmaterials for this purposeare the lightest because a col-lision between a neutron and an objecthaving a similar mass results in thegreatest energy loss. If the neutron col-lides with a heavier object it will bounceoÝ and change its direction but not itsspeed.

If mass were the only consideration,the ideal moderator would be hydrogen,whose nucleus is a single proton havinga mass sensibly the same as the neu-tronÕs. But, in reality, ordinary hydrogenfails as a moderator because it absorbsneutrons. In contrast, Òheavy hydrogen,Ówhich has an extra neutron in its nucle-us, does not absorb neutrons. Heavy hy-drogen is found in Òheavy water.Ó But inseawater, say, this heavy water is onlyabout one part in 5,000. So to use it asa moderator, it must be separated fromordinary waterÑan expensive and diÛ-cult process.

Carbon, on the other hand, is abun-dant and cheap, although somewhatless eÝective as a moderator. By late1940 Heisenberg had concluded thatonly carbon and heavy hydrogen shouldbe used as moderators. But in January1941, Walther Bothe, who was the lead-ing experimental nuclear physicist leftin Germany, began working with graph-ite, the form of carbon commonly usedin pencils. His experiments seemed toshow that graphite absorbed neutrons

too strongly to serve as an eÝectivemoderator. What Bothe did not realizewas that unless the graphite is puriÞedfar beyond any ordinary industrial re-quirement, it will contain boron impu-rities. Boron soaks up neutrons like asponge. One part boron in 500,000 ofgraphite can ruin that graphite as amoderator. All the same, because of Bo-theÕs experiment, Heisenberg and otherGerman physicists decided that heavywater was the only practical choice.

Needless to say, physicists who wereresponsible for the successful reactorprogram here made the same kinds ofcalculations. Like Heisenberg, they de-cided that a carbon reactor would needmore natural uranium than a heavy-wa-ter reactor. Fermi and his colleague LeoSzilard had also done experiments onneutron absorption by carbon. But Szi-lard was a fanatic about the purity ofthe graphite, and so their graphite, un-like BotheÕs, worked well as a modera-tor. Because carbon was so cheap com-pared with heavy water, they decidedthat it was the best moderator. FermiÕsreactor, which Þrst operated on Decem-ber 2, 1942, had a lattice of uraniumlumps embedded in carbon. All the Ger-man experimental reactorsÑnone ofwhich ever operatedÑused heavy-watermoderators. Look again at the sentence

in the Bethe-Teller memoran-dum: ÒThe proposed pile con-sists of uranium sheets im-mersed into heavy water.Ó Itis as if someone had writtenÒMade in GermanyÓ on thisdesign.

Putting everything togeth-er, there seems to be littledoubt that Heisenberg at-tempted to describe a nucle-ar device to Bohr. It seemsthat this device was his ver-sion of a reactor. He may, ormay not, have given Bohr adrawing, but Bohr clearly re-tained a visual memory ofthe design. Bohr, however,did not understand the diÝer-ence between a reactor and abomb at the time and as-sumed that Heisenberg wasdescribing a bomb.

So Aage Bohr may be quiteright when he says, as far ashis father was concerned,there was no discussion of areactor. He may also be rightthat Heisenberg never gaveBohr a drawing. None of theindividuals I have contactedare sure that the drawingthey saw was in HeisenbergÕshandÑonly that it was adrawing of HeisenbergÕs reac-

tor. This I think solves the puzzle, butit does not solve the mystery. What wasthe purpose of HeisenbergÕs visit in theÞrst place? Those who admire Heisen-berg have argued that it was to showBohr that the Germans were workingonly on a ÒpeacefulÓ reactor.

It also must be noted that when Hei-senberg visited Bohr, he clearly knewthat reactors could be used to manu-facture plutonium and that plutoniumcould fuel a nuclear weapon. Why, then,did he visit Bohr? What message was hetrying to convey? What was he trying topersuade Bohr to do, or not to do? Whatwas he trying to learn? That is the realmystery, one we may never solve.


FURTHER READING

HANS BETHE: PROPHET OF ENERGY. Jere-my Bernstein. Basic Books, 1980.

NIELS BOHRÕS TIMES: IN PHYSICS, PHILOS-OPHY AND POLITY. Abraham Pais. Ox-ford University Press, 1991.

THE LOS ALAMOS PRIMER: THE FIRST LEC-TURES ON HOW TO BUILD AN ATOMICBOMB. Robert Serber. Edited by RichardRhodes. University of California Press,1992.

HEISENBERGÕS WAR: THE SECRET HISTO-RY OF THE GERMAN BOMB. Thomas Pow-ers. Alfred A. Knopf, 1993.

MEMORANDUM from Bethe and Edward Teller discussesthe explosive possibilities of the nuclear reactor Bohr de-scribed to them when he arrived at Los Alamos. The docu-ment does not mention Heisenberg, yet the design it ana-lyzes matches one that the German invented in 1939 andsupported throughout the war.


Documents

What did Heisenberg Tell Bohr about the Bomb?