Did the German physicists make no atomic bombs during the Second World War because they wouldn’t or because they couldn’t? This is the question which Powers addresses in his extensive study of German atomic research: a question finally answered by the recent publication of the secretly recorded conversations between Heisenberg and the other German atomic physicists interned at Farm Hall, near Huntingdon, in the summer of 1945.
Heisenberg’s leading role among German physicists stems from the revolutionary mathematical theory which he formulated at the age of 24. Heisenberg was born in 1901 in Würzburg, the university town where Röntgen had discovered X-rays a few years earlier and where Heisenberg’s father was professor of Greek philology. Heisenberg shone at school, especially in mathematics and physics. True to the geneticist André Lwoff’s dictum, ‘L’art du chercheur, c’est d’abord de se trouver un bon patron,’ he began his career in physics as the pupil of Germany’s greatest teacher, Arnold Sommerfeld.
In the early Twenties, atomic physics was dominated by Niels Bohr’s model of electrons circling like planets around the sun-like nucleus, their concentric orbits governed by Newtonian mechanics combined with Max Planck’s quantum theory. Bohr’s theory accounted for the spectra of the simplest atom with only one electron, hydrogen, but it ran into difficulties with the spectra of larger atoms and left many other observations unexplained. Heisenberg broke away from Newtonian mechanics and substituted a new kind of ‘quantum mechanics’, based only on observable quantities, which correctly predicted many hitherto uninterpretable phenomena and was immediately acclaimed by most physicists as a tremendous advance. Refinements of his theory led him to the formulation of the Uncertainty Principle, which says that it is impossible to measure simultaneously the position and momentum of an atomic particle. Ironically, it was the philosophical implications of this principle, based largely on its misconceived application to the macroscopic world, that brought Heisenberg fame comparable to Einstein’s. For his discovery he was awarded the Nobel Prize for Physics for 1932. Quantum mechanics still forms the theoretical basis for much of present-day physics and chemistry: without it, for example, there would be no microchips, no computer technology and no electronics industry.
I first saw Heisenberg soon after he received the Nobel Prize, when he lectured in Vienna, where I was a chemistry student. Knowing nothing about him, I expected to see a portly professor: in came a slim young man who looked like one of us students and was quite without pomp. We were enormously impressed. I next encountered him in Cambridge in 1947, when he told my wife and me that he had never wanted to build an atomic bomb for Hitler. Fortunately, he did not say to us, as he did to a physicist in Oxford, a German refugee some of whose family had been murdered by the Nazis: ‘The Nazis should have been left in power longer, then they would have become quite decent.’ We believed Heisenberg until we read Samuel Goudsmit’s book Alsos, which argued that his talk of not wanting to build an atomic bomb was merely an excuse invented after the war to explain Germany’s atomic failure. (Shortly before his death, however, Goudsmit wrote to Heisenberg to apologise for having maligned him.) Powers quotes extensive documentary evidence to refute Goudsmit’s accusation, confirming the conclusion already reached in David Irving’s The German Atomic Bomb, that the German physicists wanted to build a reactor, but not a bomb. Their secretly recorded comments on Hiroshima have now provided further evidence of their reluctance; Heisenberg apparently regarded it as impracticable.
The story of the atomic bomb began in 1932 at the Cavendish Laboratory in Cambridge, with James Chadwick’s discovery of the neutron, a particle the size of the nucleus of the lightest atom, hydrogen, but without its positive charge, thus making it electrically neutral. Chadwick’s discovery led Enrico Fermi in Rome to irradiate many different elements with neutrons in the hope that absorption of neutrons by their atomic nuclei would generate new radioactive elements. On irradiating the heaviest known elements, uranium and thorium, Fermi did indeed find radioactivities not ascribable to any known elements; he attributed some of these to the formation of new elements heavier than uranium. In 1935, Lise Meitner, an Austrian physicist working at the Kaiser Wilhelm Institute in Berlin, persuaded her colleague Otto Hahn, a radiochemist, to join her in a further study of these ‘transuranic’ elements. They collaborated until Austria’s occupation by German troops in March 1938 robbed Lise Meitner of her protection from anti-Jewish persecution and caused her to flee to Sweden. Continuing the work in Berlin, Hahn and his associate Fritz Strassmann found, to their initial disbelief, that one of the elements formed on irradiating uranium could not be separated from barium, an atom only slightly more than half the weight of uranium. Just before Christmas 1938, Hahn wrote a letter to Meitner reporting this puzzling result. She and her nephew Otto Robert Frisch, who later became professor of what the local Cambridge paper called Unclear Physics, realised that the uranium nucleus had split in two with the release of a prodigious amount of energy, and first coined the term ‘nuclear fission’. In April 1939, Hans von Halban, Lev Kowarski and Frédéric Joliot in Paris discovered that fission of one uranium atom, induced by the absorption of one neutron, led to the emission of more than two neutrons.
Physicists realised at once that this observation opened the way to a nuclear chain reaction with the release of enormous energy. Two German physicists at Göttingen told the Ministry of Education in Berlin that uranium fission might be employed in an energy-producing reactor, while Paul Harteck at Hamburg wrote to the War Office that ‘it will probably make it possible to produce an explosive many orders of magnitude more powerful than the conventional ones ... That country which makes first use of it has an immeasurable advantage over the others.’ Harteck’s letter led to a conference at the Army Ordnance Department in September 1939, two weeks after Germany’s invasion of Poland. After attending this meeting Hahn declared: ‘If my work should lead to a nuclear weapon, I shall kill myself.’ Internment at Farm Hall fortunately robbed him of the means of carrying out his intention after his despair on hearing of Hiroshima.
At the same Army Ordnance meeting it was decided to co-opt Heisenberg, who later reached an understanding with his colleague Karl Friedrich von Weiszäcker to build an atomic pile (they called it a uranium machine) as a post-war power source and as a way of protecting young German physicists from the call-up. However, in May 1940 Weiszäcker realised that the plutonium generated in such a pile could easily be separated from the uranium and would be suitable as an explosive, and sent a report to that effect to the Army Ordnance Department. Heisenberg, on the other hand, felt sure that even though this was possible in principle, a bomb could not be built in time for the war: a view he consistently expressed to the authorities. An elderly Jewish physicist, Fritz Reiche, who managed to reach the United States from Berlin in March 1941, brought a message from the nuclear physicist Fritz Houtermans that ‘Heisenberg will not be able to withstand any longer the pressure from the government to go earnestly and very seriously into the making of the bomb, despite the fact that Heisenberg tries to delay the work as much as possible.’ In fact he did withstand the pressure, simply by insisting that it would, as he rightly believed, take too long.
In June 1942, the physicists’ lack of response to the critical state of the war appears to have induced Albert Speer, Hitler’s newly appointed minister for armaments and munitions, to call a meeting between high army and air force officers and nuclear physicists in order to urge the building of atomic weapons. Heisenberg introduced the subject with a general talk about nuclear research. According to Speer’s memoirs, Speer then asked him directly ‘how nuclear physics could be applied to the manufacture of atomic bombs’. Heisenberg was aware of Hitler’s order that all expensive projects which would take more than nine months to complete were to be cancelled, and replied that ‘the scientific solution had been found and that nothing stood in the way of building such a bomb, but the technical prerequisites for production would take years to develop, two years at the earliest, even if the programme was given maximum support.’ Heisenberg also reassured his audience that Germany need not fear an American atomic bomb before 1945 at the earliest. The physicists complained that their progress had been hampered by lack of money, but when Speer asked how much they needed immediately, Weiszäcker said 40.000 marks (about £4000), even then a very small sum. Twenty-two years later, Speer complained to Der Spiegel of the physicists’ paltry demands, which suggested that their work had only just begun. Heisenberg himself recorded a government decision to continue work on a nuclear reactor on a modest scale.
The question was: what with? A nuclear reactor using natural uranium metal or uranium oxide as fuel requires a moderator, which must be made of a substance in which neutrons are slowed down by being bounced backwards and forwards without being absorbed. The only suitable substances seemed to be either pure carbon, as in graphite, or heavy water – i.e. water in which hydrogen is replaced by its heavier isotope, deuterium. Fermi used graphite for the first reactor he built in Chicago. On the other hand, the German physicist Walther Bothe found that the probability of neutrons being captured, as opposed to bounced back by the nuclei of carbon atoms, was too high to make graphite a suitable moderator, however readily available. Bothe failed to realise that neutrons were being absorbed, not by the carbon atoms themselves, but by impurities in his graphite. On the basis of his erroneous result, the Germans decided to rely on heavy water. Their only source was a hydroelectric plant in Vermork, Norway, which used off-peak capacity for its production.
Early in 1940, the French managed to forestall a German bid for Vermork’s existing stocks of heavy water and diverted it to Joliot’s nuclear physics group in Paris. In June 1940, when Paris was threatened, Halban and Kowarski took the heavy water with them to Cambridge, where they continued their experiments on a pilot reactor, immediately underneath my office in the Old Anatomy School. I have often wondered how heavily they irradiated me with neutrons, but apparently it has done me no harm.
The Norwegians informed British intelligence of German demands for large quantities of heavy water – the British knew it could serve only one purpose. In 1942, an attempt by British paratroopers to attack the plant failed, with heavy loss of life. In February 1943, the Norwegian underground undertook a heroic and brilliantly executed raid which put the heavy-water plant out of action for several months without any loss of life. After it had been repaired, the US Air Force destroyed the plant beyond repair in November 1943. The Germans ordered the remaining stocks of heavy water to be shipped to Germany, but a member of the Norwegian Resistance sank the ferry carrying it across a fjord, with the loss of 23 Norwegian lives. Thanks, in part, to that sacrifice, the Germans never had enough heavy water to establish a self-sustaining chain reaction in a nuclear pile and could not even begin to make a plutonium bomb. Their experimental pile was constructed with uranium cubes, but these also became unobtainable after an Allied air-raid on the plant that made them. The Germans realised that heavy water could be replaced by ordinary water if their piles were made of uranium enriched in the fissionable isotope 235, but they never obtained any because all their enrichment experiments failed.
After the meeting at Harnack House, Heisenberg asked Speer and Erhard Milch, the General in Command of the Luftwaffe, separately and discreetly, what they believed would be the outcome of the war. They both hinted that the Germans were likely to lose, even though this was several months before they lost the battles of Stalingrad and El Alamein. That same summer Heisenberg apparently agreed with Karl Friedrich Bonhoeffer (the brother of Dietrich, whom Hitler later had executed for his part in the July 1944 plot) and other Göttingen physicists that it would be a catastrophe if Germany were to win the war; he did not express views of this kind when paying visits abroad, but he would have risked his life if any defeatist remark of his had been reported back to Germany. His reaction seems to have been to redouble his efforts to save German physicists: one of his senior colleagues, Walter Gerlach, actually obtained Goering’s authority to cancel the call-up papers of any physicist needed for research on their ‘uranium machine’. Gerlach deluded himself that after the war the victorious allies would need the German physicists to show them how to generate nuclear power.
In fact, the Farm Hall Transcripts and the documents quoted by Irving and Powers show how far behind the Germans were in 1945: they hadn’t even reached the point that Britain reached in 1941, before the Americans became involved. There was nothing in Germany to compare with the Peierls-Frisch Memorandum of March 1940, which contained a good estimate of the critical mass of the uranium bomb, suggested how it could be designed and predicted its likely effects, including its radioactive fallout. Nor is there anything to compare with the ‘Maud Reports’ of July 1941, which contained detailed technical recommendations, based on pilot experiments, for the manufacture of uranium 235 at a rate sufficient to produce five atomic bombs a month. It is worth remembering that up to June 1941, when Hitler attacked the Soviet Union, Britain had been fighting alone, and first possession of atomic weapons would have given them a strong chance of staving off subjugation by Hitler.
The Farm Hall transcripts reveal the German physicists’ bitter disappointment on realising that the Americans and British had left them far behind. ‘You are all second-raters,’ Hahn taunted his colleagues after hearing of Hiroshima. They did not realise that their enemies had succeeded in part thanks to the work of the many brilliant ‘non-Aryan’ German, Austrian and Italian physicists whom the Nazis had forced to emigrate. ‘I must honestly say that I would have sabotaged the war had I been in a position to do so,’ Hahn also remarked; and: ‘I thank God on my bended knees that we did not make a bomb.’ Heisenberg said that he had been ‘absolutely convinced of the possibility of our making a uranium engine, but I never thought that we would make a bomb, and at the bottom of my heart I was really glad it was to be an engine and not a bomb. I must admit that.’ Weiszäcker said:
I don’t think we should make excuses because we did not succeed, but we must admit that we did not want to, but even if we had put the same energy into it as the Americans and had wanted it as they did, it is quite certain that we should not have succeeded, as they would have smashed up our factories.
Later he remarked: ‘If I ask myself for which side I would have preferred to work, I would say for neither.’ He may have forgotten his memorandum to the Army Ordnance Department of May 1940. Walter Gerlach, administrative head of one of the uranium projects, feared that ‘when we get back to Germany we shall have a dreadful time. We shall be looked on as the ones who sabotaged everything.’ ‘I went to my downfall with open eyes,’ he said at another point, ‘but I thought I would try to save German physics and in this I succeeded.’ One thing that made me angry was to see Hahn referring to the ‘non-Aryan Miss Meitner’ as his ‘assistant’, when she had initiated their joint research on the irradiation of uranium with neutrons and had been the first to interpret his results. She had been a professor at the University of Berlin in her own right.
To return to the question I asked at the outset: it is true that the Germans were reluctant to build an atomic bomb – Hahn, because he found the idea abhorrent, and Heisenberg because he was sure that it could not be done in time – but they could not have done it even had they wanted to, partly thanks to effective Allied military action, partly because of their own scientific, technical and organisational failures, and mainly because even the Americans with their much greater resources did not have their first atomic bomb ready before July 1945.
As a patriot, Heisenberg had at first hoped for a German victory. He deluded himself that the Nazis would restore order and selfless purpose to Germany and eventually abandon their persecution of the Jews. By 1942 he must have begun to change his mind, because he joined the Wednesday Society, a debating club which included eight leading Germans later to be executed as conspirators in the July 1944 plot. Heisenberg held his breath, but escaped arrest.
British intelligence was well aware of the lack of German progress. For example, in January 1944 the Directorate of Tube Alloys (the code name for the atomic bomb) reported: ‘All the evidence available to us leads us to the conclusion that the Germans are not in fact carrying out large-scale work on any aspect of TA. We believe that ... the German work is now confined to academic and small-scale research.’ Leslie Groves, the American general in charge of the bomb, remained sceptical, however, and demanded that the Kaiser Wilhelm Institute in Berlin-Dahlem, together with Hahn and Heisenberg personally, be designated as prime targets for air raids. The Americans destroyed Hahn’s Institute for Chemistry, but missed Heisenberg’s Institute for Physics.
In some ways, a better title for Powers’s book might have been ‘War on Heisenberg’, given that a large part of it deals in minute detail with an American cloak-and-dagger plot to kidnap or assassinate him. It began with a plan to kidnap him on a lecture visit to Zürich in November 1942 which was later abandoned. It reached its culmination with the dispatch of Moe Berg, former baseball champion, but at the time a member of the OSS, with orders to shoot Heisenberg when he lectured in Zürich on 15 December 1944. Berg did indeed attend Heisenberg’s lecture with a loaded gun in his pocket and even got himself invited afterwards to the house of Paul Scherrer, the professor of experimental physics. At the end of the evening he walked back to Heisenberg’s hotel with him, but he kept his gun in his pocket, apparently because he had heard nothing to suggest that Heisenberg was building a bomb.
During their long talks, Heisenberg had told Scherrer about his and his colleagues’ work on a nuclear reactor. Scherrer, a prominent member of the European physics community, appears to have acted as a regular OSS informer; he promptly passed all this conversation on to them. At Farm Hall, Heisenberg said that he had his own informer at Scherrer’s institute, a man who told him what the Americans were doing. Judging by Heisenberg’s surprise and disbelief on hearing of Hiroshima, that informer cannot have kept him very well informed. The German SS also had their own informer at Scherrer’s party. He denounced Heisenberg for having admitted that the war was lost, even though Heisenberg had infuriated his hosts by adding that he wished Germany had won and denying all knowledge of the Holocaust. I suspected none of this hornets’ nest of spies and counterspies when I visited Scherrer in Zürich shortly after the war’s end.
Heisenberg was a patriot who saw Germany as a bulwark of European culture, often oblivious of the fact that the Nazis were bent on destroying that culture, even though they had attacked him viciously for continuing to teach the ‘Jewish’ theory of relativity. He had no sympathy for them, so the swastika on the dust cover of Powers’s book is misplaced On the other hand, he lacked the sensitivity to imagine what it would be like to be one of their victims, which turned his former mentor Niels Bohr against him for the rest of his life. In 1941, just as the German armies were approaching Moscow, Heisenberg saw Bohr on an officially sponsored visit to German-occupied Denmark. He told Bohr that a German victory over Russia would be a good thing. He regretted the destruction of Poland, but argued that it was to Germany’s credit that they hadn’t destroyed France as well, a remark which infuriated Bohr. He also advised Bohr, whose mother was Jewish, to make contact with officials at the German Embassy, since they might protect him, which to Bohr seemed tantamount to treason. Finally, Bohr gained the impression that Heisenberg was asking him to act as a go-between in securing an agreement between German and American physicists not to build any atomic bombs, a proposal which Bohr rejected because he saw it as an attempt by Heisenberg to discover what the Americans were doing.
On a visit to German-occupied Holland in 1943, Heisenberg told a Dutch colleague, Hendrick Casimir, that Germany’s historic mission had always been the defence of Western culture against the onslaught of the Eastern hordes, a matter in which neither France nor England had been determined enough; that a German-dominated Europe would therefore be a lesser evil, and that the Nazis might improve once the war was over. Casimir was as outraged. Memory of such remarks hindered restoration of Heisenberg’s old friendships after the war. Casimir later wrote:
A genius is someone who can create things that are initially beyond his own understanding. In that sense, Heisenberg was certainly a genius, and this goes rarely with a special gift for understanding the feelings and the way of thinking of others. Heisenberg did not have that gift. Perhaps his greatest shortcoming was that he was unable to grasp the full measure of the depravity of what was then the ruling group in Germany.
Powers’s book, with over 1500 notes and references to books, papers and interviews with all the survivors of the drama, is clearly the outcome of years of painstaking research. His evidence is readably presented, though sometimes inaccurate or repetitive or with irrelevant detail (who wants to know that two intelligence officers met in an office doorway two doors down the hall from the Director of the OSS?). On the technical side, the book is less informative than David Irving’s The German Atomic Bomb, but on the intelligence side it contains a great deal of interesting new material, such as a long list of the occasions when physicists in neutral or occupied countries passed on to the Allies information about German atomic work that had been given to them by disaffected German scientists. It is a great tragedy that this was met with disbelief, and construction of the atomic bombs pressed ahead in the constant panic and, as we now know, groundless fear that the Germans might make them first. We must be thankful, at least, that Germany was defeated with conventional arms two months before the atomic bombs were ready, even though the chief architect of the American bombs, Robert Oppenheimer, is reported to have regretted this.