Heisenberg’s father was a classics and Greek professor at the University of Munich; academic achievement and intellectual interests ran in the family. Steeped in philosophy and initially drawn toward mathematics, an encounter with Arnold Sommerfeld, arguably the most successful physics professor of the 20th century, sealed his decision to study physics. As a young man after the turmoil of World War 1 and the short-lived postwar experiments in government in Germany, Heisenberg was deeply influenced by joining a youth movement that discussed science and philosophy, took long walks in the mountains, sang patriotic songs and talked about how to rebuild Germany after the war. It seems likely that this influence at an impressionable age left a lasting impression on his later decision to stay in Germany.
In 1922, after listening to a lecture in Munich by Niels Bohr, Heisenberg came under Bohr’s towering influence. Bohr became almost a father figure to Heisenberg. A gentle man who regarded spirited debate as the highest means of reaching the truth, Bohr sharpened Heisenberg’s drive to understand atomic physics and almost brought him to tears once by his “terrifying relentlessness” in seeking the nature of reality. Before Heisenberg went to Bohr’s institute in Copenhagen, though, he went to study with Max Born in Göttingen. Born who was to have as much influence as Sommerfeld in training some of the leading theoretical physicists of the time - among them Pauli, Dirac and Oppenheimer - was a physicist’s physicist, steeped in mathematical sophistication and an all-encompassing knowledge of the subject.
Born and Heisenberg decided to tackle the mess of spectral lines - light emitted from atoms - that had initiated the quantum revolution through Bohr’s theory of atomic structure but which had then turned into headscratchers. Bohr and Sommerfeld’s ‘old’ quantum theory explained the lines from simple atoms like hydrogen and helium, but higher elements proved recalcitrant. It was a more sophisticated, comprehensive theory of atomic structure that Heisenberg undertook as his challenge with Born. The story of how in the process he effectively invented modern quantum mechanics became part of the lore of physics. After suffering from a severe attack of hay fever in 1927, Heisenberg decided to spend some time recovering on the small island of Helgoland in the North Sea. The natural beauty of the island liberated his thoughts, and in the small hours of the morning one day, he suddenly saw that he could work out the rules for atomic structure by focusing purely on the numbers related to the spectral lines instead of trying to make sense of unobservable features like atomic trajectories or electron orbits. His epiphany illustrates the novelty and thrill of scientific discovery: he reported, “I had the feeling that, through the surface of atomic phenomena, I was looking at a strangely beautiful interior, and felt almost giddy at the thought that I now had to probe this wealth of mathematical structures nature had so generously spread out before me.” Heisenberg had undertaken one of the most significant steps toward understanding nature since Einstein.
Back in Göttingen, Born and Heisenberg’s collaborator Pascual Jordan realized that the numerical relationships their young colleague had found could be cast in the form of matrices; for all his brilliance, Heisenberg was untutored in the knowledge of 19th century mathematics that Born displayed. Through their work the three physicists laid out the first underpinnings of what became quantum mechanics. Called “matrix mechanics”, the scheme turned out to be too complicated to deal with actual physical problems, and led the Frenchman Louis de Broglie and the Austrian Erwin Schrödinger to replace it with wave mechanics and the Schrödinger equation which became the standard tools to calculate quantum phenomena. But Heisenberg had set the stage.
The work for which Heisenberg is best known - the rare scientific principle that enters the vocabulary of laymen and achieves the ultimate distinction of being indiscriminately used out of context - came about in 1927 when he was working in Copenhagen. After battling over some fundamental philosophical differences in interpreting quantum behavior, Bohr went off on a skiing trip. Walking at night in the park near Bohr’s institute, Heisenberg realized that there was a fundamental indeterminacy in our knowledge of the atomic world. Thus was born the uncertainty principle which says that you cannot know certain attributes of subatomic entities, like their positions and velocities, with simultaneous certainty.
The uncertainty principle, along with Gödel’s Incompleteness Theorems and the speed of light, puts absolute constraints on our knowledge of reality. It is, in some sense, another nail in the coffin of human hubris and our quest to achieve total knowledge of the world. And yet as the physicist Hans Bethe was to say to a lay audience half a century later, with these fundamental constraints the principle lets us calculate all kinds of properties of quantum systems to an unprecedented degree of accuracy. The uncertainty principle, Bethe said, eliciting laughter, should really be called the “certainty principle”. Heisenberg received the Nobel Prize in 1932 “for the creation of quantum mechanics”. His place in the history of science was more than assured.
In the 1930s, after quantum mechanics was made complete as a correct description of nature by Heisenberg, Dirac, Pauli, Schrödinger and others, Heisenberg acquired a professorship at Leipzig even as storm clouds gathered over Europe after Hitler became chancellor in 1933. He trained many outstanding future physicists, including Edward Teller, Rudolf Peierls and Felix Bloch, and included his students in his social life, playing Beethoven and Schubert for them; music was always a palliative to the uncertain ways of the world. His friendship with Wolfgang Pauli produced important results, and with Pauli he continued to make contributions to nuclear structure, providing important insights into nuclear forces and the interaction of light and matter (an early forerunner of what came to be known as quantum electrodynamics). Despite the dark times enveloping the continent, those seemed to be halcyon days: the accounts of the passionate discussions with Bohr, Pauli and others through the 1920s and 30s that Heisenberg recounts in two of his books (“Physics and Philosophy” and “Physics and Beyond”) are the ultimate testament to the life of the mind. He married a perceptive woman, Elisabeth, who was later to write a penetrating memoir of her life with Werner.
Many of Heisenberg’s students and colleagues were hit hard by the laws banning Jews from teaching and other government positions, leading to an exodus led by Einstein. In the light of attacks on ‘Jewish physics’ by anti-Semitic scientists like Phillip Lenard, Heisenberg faced a dilemma. On one hand, he knew that the attacks on relativity and quantum mechanics were nonsensical, but on the other he deeply felt an attachment to Germany that he felt went beyond the Nazis. Unlike others, Heisenberg did not go out of his way to protect his Jewish colleagues, although he kept the teaching of relativity alive in the face of constant attacks. In 1938, Heisenberg came under suspicion from Reinhard Heydrich, the sinister head of the Reich Security Office; he had been labeled as a ‘White Jew’ by Lenard and others while being considered as a successor to Arnold Sommerfeld in Munich. Heinrich Himmler, whose mother knew Heisenberg’s, interceded on Hesienberg’s behalf, and after some uncomfortable sessions with Heydrich and Himmler, Heisenberg was exonerated and warned to keep his personal opinions and professional views separate.
In the summer of 1939 Heisenberg visited the United States, lecturing at Columbia, Michigan and other universities. Many of these offered him positions and his exiled colleagues like Samuel Goudsmit pleaded with him to emigrate, but Heisenberg, who professed his love for Germany, still did not seem to understand that his beloved country had become indistinguishable from the Nazis. He sailed for Germany only a few weeks before Hitler attacked Poland and World War 2 began.
By that time, the German scientists Otto Hahn and Fritz Strassmann had discovered nuclear fission. The Manhattan Project kicked off in response to a letter written to President Roosevelt by Leo Szilard and Einstein; one of their overriding reasons for asking the president to take necessary action was precisely the fact that Germany’s premier physicist, Werner Heisenberg, may be working on an atomic bomb. Heisenberg was indeed asked to form a group of scientists that also included his friend Carl Friedrich von Weizsäcker, with a mandate to explore the practical uses of atomic energy. Their main interest was in what they called a “uranium machine” or a nuclear reactor. During their work the German physicists made some critical missteps, including rejecting graphite as a moderator; this led them to rely on heavy water, a scarce material produced in a mountain factory in Norway that was later sabotaged by British and Norwegian commandos. They also put off Hitler’s war minister, Albert Speer, by asking for too little money and projecting long odds for the project’s success. But two teams independently and competitively led by Kurt Diebner and Heisenberg did achieve neutron multiplication.
During the war, Heisenberg made a visit to his old mentor Niels Bohr that remains controversial to this day; it was memorialized by Michael Frayn’s play “Copenhagen.” Perhaps wishing to reconnect with Bohr, Heisenberg visited Copenhagen in September, 1941. Later, during a dinner at Bohr’s house, Heisenberg and Bohr went out for a walk. There Heisenberg tried to feel Bohr out on the Allied effort to build an atomic bomb (Bohr did not know the reality then and only found out about the Manhattan Project when he visited Los Alamos in December, 1943). Bohr was alarmed since he interpreted Heisenberg’s questions to mean that the Germans were working on a bomb, and terminated the conversation right away. Each man later gave his own justification; Heisenberg claimed that he wanted to find out if the effort to make a bomb, depending as it did on the enormous hurdles involved in separating the fissionable isotope of uranium from the non-fissionable one, was underway at all. He claimed that if Bohr had indicated that the Allies were finding it too hard to make one, he would have informed his government of the same thing. That seems self-serving and disingenuous, to say the least.
The fact remains that Heisenberg did not know how to make a bomb. He was aware that it would take a critical mass of uranium to make one, and even that there would be a new element - plutonium - that would be more feasible to separate from uranium than it was to separate uranium-235 from uranium-238. But all evidence indicates that he had failed to accurately calculate the critical mass of a few dozen pounds that would make the entire effort practical. Later he self-servingly claimed that his heart was just not in it and that he even deliberately misled Hitler and Albert Speer into thinking that the effort was not worth it. There is no evidence for the latter and scant evidence for the former. The truth might always remain a mystery. In the event, as the characters in Frayn’s assert, the ironic truth is that Heisenberg never succeeded in killing a single soul with an atomic bomb - an effort which was the whole impetus for the Manhattan Project - while the Allies successfully killed over a hundred and fifty thousand people using it.
As the war came to an end, Heisenberg set out on his bicycle, trying to scrounge what he could to protect his family and get to the American and British lines, risking his life as a potential deserter. He was finally captured by the Allies, along with other physicists like Otto Hahn, and housed in a palatial British estate called Farm Hall. Unknown to the German physicists, the entire house had been bugged and their conversations taped - Heisenberg naively told the others that the British were too gentlemanly to use the techniques of the Gestapo. When news of Hiroshima reached the scientists, there was disbelief and astonishment, although it seems to have come more from the incredulity that the Allies had separated uranium-235 and plutonium-239 than from them being able to calculate the critical mass.
After the war, Heisenberg embarked on his mission to rebuild German physics; at least he was spared that part of his original dream. He wrote insightful books on nuclear physics, philosophy and the halcyon days of the past, recounting his critical role in the creation of one of the most important intellectual frameworks the human mind has ever conceived. But he never made a scientific contribution comparable to his earlier ones. Among others, his former friend Wolfgang Pauli and Richard Feynman scathingly scoffed at his efforts to formulate a comprehensive, unified theory of physics. While his self-serving explanations of why he worked for the Nazis were met with scorn and disapproval, in another, more gentlemanly era, he continued to be received cordially and otherwise defended by old colleagues and students like Isidor Rabi and Edward Teller.
Werner Heisenberg died in 1976, aged 74. Because of his creation of quantum mechanics and the uncertainty principle, he remains one of the most important theoretical physicists in history. But it is his work for Nazi Germany that really makes him relevant for the current era, because that work raised critical questions that are more important than ever. Among them, the most important question is whether you can separate service to your country from service to its political regime. Heisenberg thought you could. Many of his colleagues emphasized that you could not; once the government of a country becomes evil enough, you cannot divorce working for your country from working for that government. In the end, the answer may remain as uncertain as his famed principle.
Thank you for a pleasant review of an important Contributor to Science, with the intractable Service Tension as a thoughtful closing.
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