Beyond Uncertainty: Heisenberg, Quantum Physics, and the Bomb
by David C. Cassidy
Reviewed by Michael D. Gordin
Unless you happen to be a historian of physics, probably the only thing you know about the life of Werner Heisenberg (1901-1976) is that in September 1941 he took a trip to Copenhagen to have a conversation with Niels Bohr, the wise man of quantum theory. Thanks to Michael Frayn's popular and award-winning play of 1998, Copenhagen, the controversial exchange the two had that evening in Nazi-occupied Denmark has become the most salient part of the public image of the mercurial German physicist.
This was not Heisenberg's first trip to Copenhagen. In the mid-1920s, he had served as Bohr's assistant at the latter's institute for theoretical physics in the Danish capital. It was a heady time: In 1925 Heisenberg formulated the first complete version of quantum mechanics (known as matrix mechanics), and then two years later he developed the eponymous cornerstone of his renown in the physics world: the Heisenberg uncertainty principle, which states, in one form, that you can know to arbitrary accuracy either the position of an electron or its momentum, but you have to choose -- you cannot simultaneously be certain of both. Those were the happy days of the Bohr-Heisenberg collaboration, a meeting of minds that lasted through 1933 and then began to sink gradually into mistrust.
Two things happened that year. First, Heisenberg was awarded the reserved 1932 Nobel Prize in Physics for his work on matrix mechanics; that same year the 1933 prize was awarded to Austrian Erwin Schrodinger, for his development of wave mechanics (an equivalent form of quantum mechanics developed almost simultaneously with it), and to Paul A. M. Dirac, the cryptic British physicist who also pushed quantum physics into maturity. The second event was less joyful: the appointment of Adolf Hitler as chancellor of Germany in late January and then his seizure of power in March 1933, beginning the catastrophic 13 years of the National Socialist Thousand-Year Reich. Heisenberg would remain trapped between the poles of physics and politics, and the challenge for historians is to make sense of his struggle between these two major forces in his life.
The two books under review here, David Cassidy's Beyond Uncertainty and Cathryn Carson's Heisenberg in the Atomic Age, represent important contributions to understanding Werner Heisenberg. Cassidy's book is an updated, abridged and slightly popularized revision of his classic 1992 biography of Heisenberg, Uncertainty, which remains the standard. The revised version (like the original) is not quite a full biography. It traces Heisenberg's life in detail from birth until the end of World War II in 1945, but only one chapter, occupying 20 pages out of more than 400, is devoted to the 30 years leading up to the physicist's death. These final three decades, when Heisenberg served as the single most important public spokesman for science in West Germany, comprise the subject of Carson's excellent scholarly volume, so her book and Cassidy's mostly differ in coverage. However, there are some areas of overlap.
That visit to Copenhagen is one such area, and a comparison illuminates the divergent strengths of the two books and what they reveal about one of the most fascinating and perplexing figures in 20th-century science. There is no disagreement about the bare structure of the visit. In September 1941, Heisenberg arrived in occupied Copenhagen to give a lecture on behalf of the Nazi cultural propaganda apparatus, but his real motive was to have a private conversation with Bohr. Worried about surveillance, the two men took a walk in the woods behind Bohr's institute, where Heisenberg broached the issue of nuclear weapons.
Did Heisenberg take this extremely dangerous (certainly life-threatening for someone who lived in Hitler's Germany) step of alerting Bohr about German interest in exploiting uranium for the war effort because he wanted his mentor's moral guidance about whether to pursue the project? Was he warning Bohr? Was he trying to initiate a no-first-use pact in which scientists on both the Axis and Allied sides would refuse to weaponize nuclear fission? We do not and cannot know the answers to these questions, but the controversy rages on. Was Heisenberg a loyal servant of the Nazi state, working on the physics of uranium mostly for a reactor (an "engine," as he called it) and not necessarily for a bomb (although that distinction itself is a matter of extensive debate)? Support for this interpretation can be found in his willingness to serve repeatedly as a cultural emissary in Nazi-occupied Europe and his refusal to move to another country. Was he a covert dissident, sabotaging the German project from within? Or was he simply a conflicted man, excited by the physics, loyal to "Germany" as he understood it, and caught up in events beyond his control?
Cassidy's approach to these fraught issues is characteristic of the entire book. Although he begins Beyond Uncertainty with the typical biographer's gambit of touring the family tree -- a revealing mix of upwardly mobile academics who clung to the privilege and respectability that scholarship afforded -- the book becomes a page-turner when we are thrown into Heisenberg's involvement in the suppression of the Bavarian Soviet Republic -- a brief attempt after World War I to establish a socialist state with Munich as its capital. This brutal period in the history of Munich, where Heisenberg had been living since age 9, left scars on the mind of the impressionable teenager. We then follow Heisenberg into the determinedly apolitical segment of the youth movement, where he was a devoted leader of slightly younger men, taking them on long hikes and discussing philosophy and music with them. Next comes an account of his stellar performance as a graduate student at the University of Munich, followed by the fast-paced intellectual drama that was the dawn of the quantum age. These episodes, however, take up only half the book.
Most of the narrative and some of Cassidy's most delicate historical excavations and nuanced interpretations concern the Nazi period. Why did Heisenberg stay in Germany? It was not an easy choice. As anti-Semitic fanatics fixated on the world's most famous Jew, Albert Einstein, relativistic and quantum physics came under attack, and Heisenberg stuck out his neck more than once to defend physics from those who would crush it in the name of ideology -- even to the point where he himself was attacked as a "white Jew" (an Aryan seduced to do Jewish bidding) in the newspaper of the SS; he risked his career and possibly his life attaining exoneration. At the same time, he made compromises: He agreed never to mention Einstein's name, as long as he could teach the physics. "In the end," Cassidy tells us, "Heisenberg came to regard his personal survival in Nazi Germany as tantamount to the survival of decent physics, and the continued survival of some elements of decent physics provided the grounds for Heisenberg's personal continuation of the struggle." Cassidy shows us both Heisenberg's occasional courage and his willingness to compromise, presenting us with the difficulties that surround any simplistic interpretation.
So too with regard to the visit to Copenhagen, and the entire uranium project: Living under a dictatorship places ordinary people under tremendous stress, and Heisenberg was an ordinary person (albeit a brilliant theoretical physicist). Cassidy presents Heisenberg's accounts of the Nazi period, many of which date from after the war (sometimes long after), and shows us where the contradictions lie. And they lie heavy on the ground, for Heisenberg seems to have said almost everything and its opposite if you look hard enough. In the end, Cassidy is sympathetic. Heisenberg was no covert saboteur of the Nazi bomb; rather, he was trying, however ineptly, to do what he thought was best for Germany in the long run -- alerting the Allies to the existence of the project, indicating how difficult it would be to make such a bomb, and hoping that both sides would refrain from the attempt.
Carson's approach is rather different. As its title suggests, Heisenberg in the Atomic Age concentrates on the years after 1945, although several chapters chronicle Heisenberg's public philosophical lectures before that date. Thus one might expect that the Copenhagen episode would be absent -- and so it is, after a fashion. Instead of trying to get to the core of who said what to whom, Carson shows us how the stories were built as "a postwar story." She presents each of Heisenberg's not entirely consistent accounts of the visit to Bohr, explaining who the audience was, what Heisenberg wanted to accomplish, and how audiences both in Germany and abroad reacted. Carson's approach is refreshing and surprisingly satisfying, given that we are never going to know what really happened. By examining how Heisenberg presented himself to his many publics, we can learn something about the man, but even more about the West Germany he inhabited, a nation that was trying to interpret its relation to the Nazi past just as much as its leading physicist was.
Carson's subtitle invokes the "public sphere," a term that conjures up the philosopher and social theorist Jurgen Habermas, author of The Structural Transformation of the Public Sphere (1962), which argued for the historical origins and cultural necessity of state-free intellectual intercourse. The usage is deliberate. Instead of adopting Habermas's views as an intellectual framework, Carson applies her rigorous historicizing impulse here as well. The Frankfurt philosopher published his book in a West German cultural milieu that was consumed with worries about the nature of public discourse and its horrific breakdown in the 1930s. Heisenberg, as one of the country's most prominent public intellectuals, participated in this world as much as Habermas. "Heisenberg's lived vision of the public sphere shaped West Germany's cultural and political settlement for science," says Carson. In the most detailed treatment of the topic available in English, Carson chronicles Heisenberg's deep involvement in science policy in the new, truncated Germany -- whether in civilian nuclear power, the Gottingen manifesto against German nuclear weapons, or state funding of basic research -- as part of the physicist's and the state's anxiety over a new, post-Nazi polity.
Carson also lavishes a great deal of attention on Heisenberg's public philosophizing. German public intellectuals like Heisenberg were expected to give frequent open lectures designed to foster civic cultivation and general education (the untranslatable notion of Bildung). For Cassidy, Heisenberg's philosophy was marginal to his life because he typically produced it only when called upon to do so in a public forum. Cassidy cites with approval Wolfgang Pauli's depiction (in a letter to Bohr) of his young friend Heisenberg as "very unphilosophical." As a sharp marker of the contrast between these two books, for Carson the fact that there was a specific occasion and audience for Heisenberg's philosophical writings is what generates their value as sources for Heisenberg's thought and for the debates over the public sphere in postwar West Germany. "When Heisenberg dealt well with philosophical problems," says Carson, "it was from taking up problems that forced him to think critically and putting science into synchrony with contemporary debate."
This tension between the public and the private runs like a scarlet thread throughout Heisenberg's life as presented in both works, and we see it once again in the Copenhagen visit. Was Heisenberg there for a private moral consultation or a public-minded venture of atomic diplomacy? Whether we can determine the private thoughts from the public acts lies at the heart of the biographer's dilemma.
The ultimate private-public act is writing autobiography. Heisenberg's 1969 autobiography, Der Teil und das Ganze (The Part and the Whole, translated into English as Physics and Beyond), is structured as a series of Platonic dialogues between Heisenberg and others, usually physicists. It is an important source for both Cassidy and Carson, but they use the work rather differently. Cassidy mines the book for anecdotes and reflections on the internal and external crises of the man and his culture, only occasionally correcting the physicist when the facts demand it. For Carson, on the other hand, the autobiography is itself a puzzle to be solved. She explores its construction, the revisions, the marketing of the original manuscript, and translations of the work to show us how, even when he claims to be most candid, Heisenberg is still hidden from our view -- hidden behind the very public sphere he so desperately wanted to preserve.
Michael D. Gordin is professor of history at Princeton University, where he teaches the history of modern science. He is the author of, among other books, Red Cloud at Dawn: Truman, Stalin, and the End of the Atomic Monopoly (Farrar, Straus and Giroux, 2009) and Five Days in August: How World War II Became a Nuclear War (Princeton University Press, 2007).