In 1933 Werner Heisenberg was at his peak, one of Germany’s youngest and most brilliant physicists and with aspirations to be at the helm of German science. That year, at the age of 31, he received both the Max Planck Medal from the German Physical Society and the Nobel Prize. The great days of Planck and von Laue as research scientists were well past; Heisenberg’s were still ahead, and his dilemma was all the greater because he could have easily found a position abroad.
Heisenberg was — and still is — world-famous for his formulation of the Uncertainty Principle. The culmination of a series of papers completed over two years from 1925 to 1927, it was immediately hailed as a new direction for atomic physics. Rudolf Peierls described his contribution: ‘In 1925, at the age of 23, he wrote the paper that laid the foundations of quantum mechanics on which all subsequent generations have built.’ It was not just an extension of previous work but a new direction for atomic physics. Heisenberg’s early work, before the Uncertainty Principle, was crucial to quantum theory. Nearly three centuries of Newtonian physics had been contravened first by Max Planck with the idea of the quantum of energy, and then in 1913 by Niels Bohr, who realized that the quantum theory applied to matter as well as energy. But there were gaps and contradictions in their new theory. Newton’s laws concerning the motion of the heavenly bodies and gravity were derived from precise, directly observable measurements; the atom “was thought to be similar to the solar system, a central nucleus, like the sun, with orbiting electrons, like the planets, around it. It was assumed that similar laws applied to the infinitely minute subatomic particles with which theoretical physics was concerned.
By 1925 Heisenberg was ‘convinced that one needed a theory which abandoned the classical description of the electron orbits inside the atom, and instead concerned itself with quantities that were accessible to observation …’ That summer, laid low by an attack of hay fever, he escaped for a respite to the bare coast of Heligoland, and there he made the mathematical breakthrough that filled in crucial gaps in the existing quantum theory. ‘Heisenberg discarded the concept of orbits, which could not in principle be observed — and he proposed that the physicist should deal only with observable things …’ Max Born, Heisenberg’s professor in Göttingen, was quickly won over by Heisenberg’s new formulation, and with Pascual Jordan added another paper which completed the structure of matrix mechanics, as the new system came to be called.
In 1927 Heisenberg consolidated the theoretical structure of the new physics with the Uncertainty Principle, in which he said that the method of observation itself affected the results. An electron could not be seen; its presence and its movement could be deduced only by the effect it had on particles directed at it, which were deflected by the electron. The same “was true for particles in a cloud chamber — the track of these particles when they hit something else gave information about them. Thus the act of observation altered the behaviour of the particle observed. Certain variables are related to each other — position and momentum, or energy and time — and the more precisely one such variable is measured, the less precise can be the simultaneous measurement of its related variable.
Although the term ‘Uncertainty Principle’ is universally used, Heisenberg himself in his original publication in 1927 called it the ‘Inaccuracy Principle’, which perhaps would have made the concept easier to understand. Most physicists were convinced by its revolutionary concept, although Einstein could never accept it and Schrodinger always hoped that ‘some new development might avoid the need for this limitation of our concepts’.
Heisenberg’s set of laws helped to seal the ‘Copenhagen interpretation’ of atomic theory, and made him second only to Einstein — deservedly so, for his work has been the basis of much of modern physical science and its practical application in the electronics industry, notably radio and laser technology.
Born in 1901, Heisenberg was still young during the chaotic aftermath of the First World War, and the Youth Movement’s patriotic ideals and aims for a better future left a lasting impression on him. He became a youth leader and even after becoming a famous scientist still went hiking with his group. He studied at Munich under Arthur Sommerfeld, then joined the famous seminar at Göttingen led by James Franck and Max Born. He studied in Copenhagen with Niels Bohr, who immediately recognized his brilliance. The two worked profitably together, ‘work often taking the form of discussions on country hikes. His string of papers, each more original than the last, confirmed his reputation, which grew so quickly that in 1927, at 26, he was offered two professorships, in Leipzig and Zurich. He chose Leipzig and rapidly started building up his own seminar there.
Heisenberg was an attractive, outgoing young man; Born said he looked like a bright-eyed farm boy. He excelled not only at science but also in music, languages and mountain-climbing, and played ferociously competitive table tennis with his colleagues. As a chemistry student in Vienna, Max Perutz describes going to a lecture by Heisenberg, who had recently received his Nobel Prize. Perutz was expecting to see ‘a portly professor, in came a slim young man who looked like one of us students and was quite without pomp’.
After the dramatic exodus of his Jewish colleagues in 1933 Heisenberg did what he could to preserve the old atmosphere. But his sense of isolation was acute. ‘The immediate pre-war years, or rather what part of them I spent in Germany, struck me as a period of unspeakable loneliness, he wrote. The Third Reich’s policies alienated its scientists from the rest of the world. There could be no hope of change from within, and as Germany became increasingly cut off from international dialogue by growing distrust, Heisenberg missed his contacts with the scientific fraternity abroad.
He had visited Britain, the United States and Denmark, in each of which countries the atmosphere of personal and political freedom beckoned; emigration to the New World was, he wrote decades later, ‘a constant temptation. But he decided to remain, and build up his own group of young scientists in order to ensure German participation in scientific advances and to make sure that ‘uncontaminated science can make a comeback in Germany after the war’.
In 1935 Heisenberg was nominated as successor to his old teacher Sommerfeld in Munich, a tempting post in his early home town, to which he was very attached. But after virulent attacks by Stark, Lenard and the SS newspaper Das Schwarze Korps denouncing him and the theoretical physicists as ‘white Jews’, the authorities decided otherwise. The attacks grew so vicious that Heisenberg’s mother, who knew Himmler’s mother slightly, contacted her to ask for advice; on her recommendation Heisenberg wrote directly to Himmler to ask if Stark’s actions had the support of the SS, in which case he would resign. After a long interval he heard in 1938 that Stark’s attacks would stop, and he could stay. Even so, he did not move to Munich and he had to promise not to mention Einstein when teaching about relativity.
In 1939 he visited the United States because, as he later wrote, he wanted to see his friends there ‘before the war started’ and they might never meet again. He added another reason: If I was to help in Germany’s reconstruction after the collapse, I would badly need their help.’ This remark, made 25 years after the war, smacks of hindsight. According to Heisenberg, he was already sure that Germany would lose since the territory and resources controlled by Britain, the United States and the Soviet Union were overwhelming compared with Germany’s capabilities. But in 1939 the United States was determinedly neutral, and that August the Soviet Union signed a pact with Hitler; so his comments about the inevitable defeat of Germany lack conviction.
While lecturing that summer in Ann Arbor and Chicago, Heisenberg met Enrico Fermi, who had only recently left Fascist Italy. Fermi tried hard to persuade him to stay in the United States, arguing that new immigrants like himself were in ‘a larger, freer country where they could live without being ‘weighed down by the heavy ballast of their historical past. In Italy I was a great man; here I am again a young physicist and that is incomparably more exciting.’ Heisenberg understood his arguments and already knew that life under Hitler was hard; that the compromises he was forced to make with the regime would look bad to his scientific friends in the West; that if Germany lost he would have to take the consequences. The dilemma facing him was perhaps greater than for any other German scientist, since he was genuinely sought after in the West. Whatever one may think of his decision, his constancy was impressive.
On his return to Germany, Heisenberg was called up in September 1939, not to the Mountain Rifles, the regiment in which he had done his conscript service, but to the Army Ordnance department in Berlin. There the possible use of atomic energy was discussed and Hahn, the co-discoverer of atomic fission, declared, If my work should lead to a nuclear weapon, I shall kill myself It did, but he did not. At this time the Kaiser Wilhelm Institute for Physics became the centre for nuclear research — the ‘Uranium Club, as it became known — and Heisenberg was a regular visitor. Peter Debye, its head, went on a lecture tour to Switzerland and the United States and never came back, and eventually (in April 1942) Heisenberg took over as director. His colleagues abroad concluded he had sold out to the Nazis, and the fact that he was the head of an established group of German atomic physicists in Berlin became a major spur to the efforts of his former colleagues to develop a bomb for the Allies, in the belief that his team might be doing the same for Hitler.
Since then there has been endless conjecture about how close the Third Reich came to producing its own atomic bomb, and why it failed. The story is complex, and Heisenberg was intimately involved in it. His own account of the war years is ambiguous, and he remains a problematic figure for historians, who have continued to speculate about his war record.
In particular, Heisenberg’s visit to Copenhagen in the autumn of 1941 with his younger colleague, von Weizsäcker, ostensibly to give a lecture at the German embassy, but in fact to see his mentor and friend Niels Bohr, has aroused intense speculation ever since. In his play Copenhagen Michael Frayn makes Heisenberg say, ‘There are only two things the world remembers about me. One is the uncertainty principle, and the other is my mysterious visit to Niels Bohr in Copenhagen in 1941. Everyone understands certainty. Or thinks he does. No one understands my trip to Copenhagen.’ The play offers several intriguing variations on what could have happened during this fateful meeting.
After the lecture Heisenberg met Bohr at his house and took a walk with him. The two men remembered the occasion quite differently. Almost immediately the meeting went disastrously wrong. Both men knew that Bohr’s house was under surveillance, and Bohr was suspicious of the German physicist’s motives in making the visit. Meanwhile Heisenberg knew that any comment he made betraying Germany’s war interests would be relayed to Berlin, where it would be regarded as treason.
While Heisenberg had to be extremely careful about what he said, Bohr was better at talking than at listening. He never wrote down an account of the occasion. According to Heisenberg’s version, he asked if Bohr believed scientists would be justified in working on uranium research in wartime. Bohr, scientifically isolated in Denmark, had believed that practical application of nuclear fission was inconceivable in the immediate future. Aghast, he asked if Heisenberg believed the process could be used to make weapons; to which Heisenberg replied that he knew it was possible.
What were his motives in raising the subject? According to Heisenberg, Bohr was by then so shaken that he missed his main point, which was that enormous cost and technical effort would be needed to develop a bomb and that the physicists would have the terrible responsibility of advising their governments whether to go ahead or not. As Bohr understood it, Heisenberg was asking how far Allied research had got, and was proposing a common policy on both sides not to proceed with it. First Hitler had expelled Germany’s Jewish physicists, and now that they were working in America on nuclear weapons research his chief atomic scientist was proposing to Bohr that they should stop!
The meeting ended abruptly, and back at his hotel Heisenberg, dismayed, told von Weizsäcker that the encounter had failed. In 1947 he visited Bohr again in Copenhagen to try to unravel the misunderstanding, but this meeting also ended in disaster. Whatever his motives had been — to seek ‘absolution’ from Bohr for going ahead with the project; to find out how far Allied scientists had got with developing a bomb; to arrange a secret bargain with them not to go any further – from then on he was regarded with suspicion by scientists abroad.
Given the situation in which the two physicists met – Heisenberg, from the invading power, visiting Bohr, the ‘uncrowned king’ of occupied Denmark — a friendly meeting was virtually inconceivable, and Heisenberg was remarkably insensitive in not anticipating this. However, Just as tactless on other occasions is when Francis Simon recalled Heisenberg remarking to him after the war: ‘The Nazis should have been left in power longer, then they would have become quite decent – this after the discovery of the death camps, to a man who had lost relatives in them.
Meanwhile British and American Intelligence were intensely interested in the German team’s progress on atomic research throughout the war. There were clues that the Germans were trying to build an atomic bomb: the use of heavy water (needed to slow down neutrons), access to radioactive uranium ore in Czechoslovakia, But there were also contradictory clues: the relevant physicists mostly stayed at their separate universities rather than clustering in a single research centre. No one could be certain; no one wanted to take any risks; and no one could have the slightest doubt that if a German atomic bomb existed Hitler would use it.
Some have argued that Heisenberg and his colleagues held back from the project. It is true that he told Albert Speer, the Armaments Minister, in 1942, that Germany had no chance of developing an atomic bomb in wartime. Instead he and von Weizsäcker asked for relatively small funds for his team to work on an experimental nuclear reactor for atomic energy, which they then worked on for the duration of the war. This was indeed the case; the question is whether Heisenberg was hiding the fact that he believed a bomb programme was feasible, whether he actually hadn’t made the fundamental calculations needed to ‘start building a bomb’, or whether — as he reported – he had concluded that the project could not be managed in time to be used during the war.
British Intelligence was fairly sure as early as January 1944 that the Germans were not carrying out any large-scale work on an atomic bomb. Nonetheless, General Leslie Groves, the American in charge of the Manhattan Project, took no chances. At his instigation the United States Air Force bombed the Kaiser Wilhelm Institute for Physics in Berlin, where the small-scale atomic research was being carried out, and Heisenberg and Hahn were listed as personal targets. Hahn’s chemistry department was destroyed, though Hahn himself escaped injury, as did Heisenberg and his physics department.
As we now know, the German team came nowhere near to developing an atomic bomb. Its failure was partly due to the immense size and complexity of the effort required to produce it, as Heisenberg said in 1939. The United States succeeded, but its situation was very different from that of Germany in that it had huge economic resources, vast space, immunity from air attack and numerous enthusiastic and highly trained scientists (including many refugees from Europe). Germany, on the other hand, after the first two years of the war was subjected to extremely heavy air bombardment. Under these circumstances, and with the Axis armies retreating and space contracting after late 1942, it is hard to see how Germany could have mounted the massive enterprise needed. Furthermore, Hitler thought only in the short term where weapons were concerned, and ordered that no enterprise was to be started that would not deliver the product to the army in less than nine months. So a project that would take years “would never have been approved, and Heisenberg could tell Speer with a clear conscience that quicker production was impossible. Finally, the German scientists confidently believed that the Allies were nowhere near to producing nuclear weapons right up to the day the atomic bomb was dropped on Hiroshima — as we know from the Farm Hall Transcripts.
When Allied troops entered Germany in early 1945, a special intelligence force, the Alsos mission, was sent to identify the places and people (Heisenberg in particular) who would be working on a bomb if there was one. Its leading operator was Samuel Goudsmit, who, after searching the office of Heisenberg, his former colleague and friend, realized that the German team had got nowhere near to producing a bomb. Even so, American and British speculation about German research was still intense and at the end of the war the atomic physicists, Including Heisenberg, von Laue, Hahn and von Weizsäcker, were captured In the French sector by the British, smuggled out to England and Interned at Farm Hall, near Cambridge, formerly used by British Intelligence.
Whether the German physicists were held to prevent their capture by the Russians, or for the secrets they might reveal or both, every room was bugged and transcripts were secretly made of the recording cylinders. Not until 1992 were the files opened to the public and the English translation of the transcripts published.3 The main revelations came when the bomb was dropped on Hiroshima on 6 August. The interned scientists were thunderstruck and Heisenberg at first refused to believe it was an atomic bomb. The transcripts show they had had no inkling that the Americans could have made the massive industrial and technological investment needed to produce atomic weapons so quickly. Their next reaction was to reproach themselves and one another for their own failure to make one. Von Weizsäcker alone suggested that they had held back for moral reasons but he was quickly contradicted.
Heisenberg, speaking privately with Hahn that night, confided that ‘quite honestly I have never worked it out as I never believed one could get pure [uranium] 235 …’ He now revised his estimate of the critical mass needed to create a fast chain reaction, which would dictate the size of a bomb, and a week later gave his colleagues the correct calculations in an informal seminar. Rudolf Peierls, a former student of Heisenberg’s whose answer to the same question had launched the Allied bomb project in 1940, commented drily that ‘Heisenberg, though a brilliant theoretician, was always very casual about numbers …’ The German team had made other mistakes, notably in ruling out graphite as a moderator to slow down neutrons in a nuclear pile. In fact the graphite needed further purification, as Fermi and Szilard realized.
Heisenberg lived on for 30 years after the war, respected if no longer trusted — during a trip to the United States in 1949 half the guests invited to meet him at a reception organized by his former colleague Weisskopf stayed away rather than meet the man who had headed Hitler’s atomic research team. His postwar career “was outwardly successful. In Germany he opposed the development of atomic weapons while promoting the peaceful use of nuclear power, and helped to launch CERN, the European nuclear research centre. As director of the Max Planck Institute for Physics and Astrophysics he moved with it to Munich, his home city, in 1958 and stayed there until he retired in 1970.
When he had decided to stay in Germany in the 1930s he probably foresaw the risks – to his country from Hitler’s uncontrollable aggression and to himself from inevitable contamination by his association with the Nazi state. Heisenberg was a patriot as well as a physicist: ‘It would be [an] easy mistake to make, to think that one loved one’s country less because it happened to be in the wrong,’ as his dramatic counterpart comments in Copenhagen. He was caught by a relentless fate, and had his been a more profound or sensitive personality his predicament would have had the force of Greek tragedy.
Heisenberg’s reputation was clouded by his failure to explain his wartime record satisfactorily. But then, whatever account he and his colleagues gave of the war years (apart from von Laue and Hahn, who thanked God that they had failed to make a bomb), they were trapped in a dilemma: they did not want the Allies to think they had worked on a bomb for Hitler, but were equally reluctant to face condemnation by their countrymen, either as traitors or as incompetents, for having failed to produce one.