A Strange New Quantum Ethics
Jonothan Logan

Jonothan Logan, a theoretical physicist, first became acquainted with the records of the German fission project while working with Samuel Goudsmit, leader of the Allies’ nuclear investigating mission in World War II. More on Heisenberg and the atomic bomb can be found in Logan’s article "The Critical Mass" (May-June 1996 http://www.amsci.org/amsci/articles/96articles/Logan-s.html).

"Copenhagen Tames Complexity of Science" was the title of a recent review of Michael Frayn's latest play -- meant, no doubt, as a compliment. Audiences in New York, where the play opened in April after a long run in London, do seem dazzled by the heady counterpoint of history, quantum mechanics and postmodern epistemology electrifying the air between the play's characters -- Werner Heisenberg, Niels Bohr and Bohr's wife Margrethe. The play is quick, clever and artfully plotted. What's disturbing is that Copenhagen "tames" history, too, altering the facts and rearranging the moral landscape the real Bohr and Heisenberg inhabited.

The subject of the play is Heisenberg's famous September 1941 visit to Bohr in Nazi-occupied Denmark, an encounter the two later described very differently. Bohr, according to family members, perceived Heisenberg's visit as decidedly hostile, perhaps an attempt to pick his brain on the subject of fission or a probe for information on Allied research. Heisenberg maintained (to the Swiss-German journalist Robert Jungk) that he came simply to ask whether "as a physicist one had the moral right to work on the practical exploitation of atomic energy" and to offer reassurance that Germany was not building an atomic bomb. But Bohr, he said, misunderstood his good intentions and became alarmed. The two conflicting versions of the meeting encapsulate the 50-year-old controversy over Heisenberg's wartime work for Germany.

Jungk expanded Heisenberg's version into a full-blown legend of heroic resistance, Brighter Than a Thousand Suns (1958), which celebrated the supposed paradox that "German nuclear physicists, living under a saber-rattling dictatorship, obeyed the voice of conscience and attempted to prevent the construction of atom bombs, while their professional colleagues in the democracies, who had no coercion to fear, with very few exceptions, concentrated their whole energies on production of the new weapon." Jungk himself later disavowed this thesis when new information came to light. Bohr, as the historian Gerald Holton has discovered, composed (but never mailed) a letter to Heisenberg objecting in strong terms to the version of the Copenhagen visit Heisenberg had related to Jungk. Nevertheless, a still more highly embellished version of the resistance myth reemerged in Thomas Powers's 1993 "shadow history" of the German atomic bomb project, Heisenberg’s War.

That book, we learn from the play's postscript, was the inspiration for Copenhagen, and (the postscript's broad bibliography notwithstanding) effectively its sole source. The Powers book won little respect from historians, Frayn acknowledges, but for him this did not dim its fiction-enhanced intrigue: "There is material here for several more plays and films yet," he enthuses. [Two books that examine the background to Heisenberg's Copenhagen visit in detail are David Cassidy's Uncertainty (Freeman, 1992) and Heisenberg and the Nazi Atomic Bomb Project (University of California, 1998), by Paul Lawrence Rose.]

In Copenhagen, Heisenberg and Bohr are conceived as returning from beyond death to reenact their tense wartime meeting in a series of encounters, each "draft" a different imagining of Heisenberg's purpose. Now Heisenberg can try to convince Bohr of his good intentions; Bohr can question and respond, all in a style that recalls their brilliant collaboration at Bohr's institute in the 1920's, when together they explored the startling implications of the new quantum laws. Margrethe, throughout, serves as a one-woman chorus, skeptical of each of Heisenberg's drafts, distrusting his relentless need to impress and to win, trying to puncture the moral pretensions she detects. We see the virtuous version Heisenberg gave to Jungk, and the Powers variation -- that Heisenberg had possessed but withheld key knowledge of bomb physics: "I understood very clearly. I simply didn't tell the others." Margrethe suggests that his real object had been to obtain information about the Allies' fission work and to persuade Bohr to discourage them. Elsewhere she wonders if he came to impress them with his status, or just for a consoling glimmer of happier days.

The final and longest draft, staged to be the most compelling, is the playwright's favored invention. More complexly heroic than in the Powers design, Heisenberg here has succeeded in forestalling a Nazi atomic bomb, both by withholding information about plutonium from arms minister Albert Speer and by refusing to interest himself in the most basic physics of atomic explosives -- specifically, by not calculating the critical mass for a bomb. Had he but allowed himself to pause and do so, the final scene would have us believe (a nuclear explosion thunders offstage to dramatize the point), he would quickly have seen that a bomb could be built. This pseudoscientific fantasy is the play's central pivot.

The celebration of uncertainty is a continuous theme in the play. Human life, too, follows the quantum principle Bohr describes, "that there is no precisely determinable objective universe. . . . [It] exists only as a series of approximations." Thus, the play implies, no judgment of Heisenberg is possible, for all we can ever discover are the elusive, multiple refractions of his image in fallible memory. And this might be true in the ambiguous world Frayn has constructed to make it seem so, in which Bohr is unreliable and every disquieting revelation about Heisenberg is cancelled by a nimble riposte. But the real Heisenberg lived in a world of cause and effect and uneasy moral compromise, and -- the imperfect observability of the quantum universe notwithstanding – he left a trail of discoverable facts, facts that upend the playwright's favored interpretation and discredit his portrait of Heisenberg.

We know, to begin with, that Heisenberg did calculate the critical mass for a bomb. First, in a December 6, 1939 secret report for the German Army Weapons Department, he derived a formula that, with the nuclear parameter values then assumed, yields a mass in the hundreds of tons for the amount of "nearly pure" uranium 235 (U²³⁵) required for an exploding reactor (Heisenberg's model for a bomb at that point.) In 1940 Heisenberg's colleague Karl Wirtz heard him explain a further calculation to a circle of his collaborators; the details are known because Heisenberg explained them again on August 6, 7, and 9, 1945, while detained in England at Farm Hall. His simplified calculation used the random walk model Einstein had employed to derive mean diffusion distances in Brownian motion. Mistaken but plausible, it yielded a critical mass of tons, still vastly beyond what Germany could hope to produce. That Heisenberg had not formulated the full three-dimensional fission-diffusion equations is hardly the lapse Frayn's Bohr finds so inexplicable. The real Heisenberg did what working scientists do every day: He made a preliminary calculation, and when it yielded a mass so impracticably large, he saw no reason to invest weeks refining the estimate. That his calculation had hugely overestimated the critical mass he obviously didn't realize, or he would not have displayed the result to his colleagues and continued trusting it until news of the Hiroshima bomb forced him to rethink. The magnitude agreed with published work, and with prevailing assumptions when the war began, so why would Heisenberg have doubted it? But to acknowledge this is to recognize that there is neither mystery nor virtue in his miscalculation, merely embarrassment -- for Heisenberg, and for the play.

We also know something about Heisenberg's handling of the plutonium "secret." As proof that he had no wish to build weapons, Frayn's Heisenberg cites his withholding from Speer in June 1942 the possibility that reactor-generated plutonium (element 94) might be used to make bombs. "A striking omission," Frayn's Bohr admits. Whatever may have transpired between Heisenberg and Speer in June 1942, he was quite open about plutonium on February 26th 1942, when he spoke at a Berlin conference organized to garner support among the Nazi elite for the fission project. As the result of a farcical mix-up (the wrong invitations were mailed out) many of the invitees sent their deputies or regrets; but the audience for which Heisenberg composed his address that day would have included not only Albert Speer but Bormann, Göring, Himmler, Keitel, and Raeder as well. An emphatic point in the lecture Heisenberg prepared was that a reactor could be used not only to power the submarines of the Third Reich, but also to generate "...a new substance (element 94) ...which in all probability is an explosive with the same unimaginable effectiveness as pure uranium-235."

Also misleading is the play's representation that as late as June 1942, German scientists were still "slightly ahead of Fermi in Chicago." In fact, by the time of Heisenberg's encounter with Bohr, Allied scientists were much closer to a bomb than the Germans. At the time Heisenberg set out for Copenhagen in September, 1941, the scientist in charge of Germany's most promising uranium isotope separation project had just declared it a failure. Heisenberg's reactor experiments had yet to demonstrate any neutron multiplication. Not even microscopic quantities of U²³⁵ or plutonium had been isolated, so no experiments to determine the fission properties relevant to a bomb could be done. By early December the head of German Army Weapons research was contemplating cancellation of the German fission project. Allied scientists at this time had already measured the fission properties of U²³⁵ and plutonium. On October 9^th, Vannevar Bush informed President Roosevelt that an atomic bomb could probably be built, with an estimated critical mass of 25 pounds. By November, Columbia physicists had demonstrated isotope separation by gaseous diffusion. By distorting the true relative standing, the play suggests that if Heisenberg had succeeded then in convincing Bohr that the Allies could safely abandon the pursuit of atomic weapons, all humankind would have gained. In reality, the beneficiary would have been a triumphant Germany, whose tanks, bombers and well-trained soldiers seemed poised, in September, 1941, to complete the conquest of Europe.

Copenhagen, with its simultaneous, often incompatible readings of Heisenberg's mind, intends to confront the audience with the impossibility of true knowledge, of others or of oneself. Yet it deploys every resource of stagecraft to elevate one view as truer than the rest. From the opening of the play Heisenberg presents himself as an embattled figure: "I wonder if they suspect for one moment how painful it was to get permission for the trip. The humiliating appeals to the Party, the demeaning efforts to have strings pulled." Bohr tells how Heisenberg was subjected to "the most terrible attacks" as a "White Jew" for teaching Einstein's theories, and how the SS brought him in for interrogation. "He knows he's being watched, of course. He has to be careful about what he says."

In order to keep alive the image of a Heisenberg at odds with the immorality around him, Frayn conceals the true contours of Heisenberg's accommodation to the Nazi State. The audience is not told how, more than three years before the Copenhagen visit, Heisenberg had resolved his political problems. From Himmler, a family acquaintance, Heisenberg had sought "restitution of [his] honor and protection against further attacks," and had received it in the form of a July 21, 1938 official letter placing him thenceforth under Himmler's personal protection. (Part of the "restitution" Heisenberg requested was permission to publish an article in an official Nazi science journal.) The fantastic suggestion that Heisenberg, a committed patriot, was involved in anti-Nazi activities squares with nothing in a lifetime of political conformity, nor with his wife's statement that he "politely declined" when approached to join an anti-Hitler conspiracy. As for his visit to Copenhagen, the German Office of Cultural Propaganda had requested such a visit by Heisenberg and his colleague Carl Friedrich von Weizsäcker. This trip did require official clearance by the Nazi Party, as the play indicates, but this was easily arranged by Weizsäcker, whose father was State Secretary for Foreign Affairs. The propaganda office was sufficiently pleased with Heisenberg's performance to sponsor him on at least 10 such propaganda trips in the course of the war: to Nazi-occupied Budapest that same year, to Holland in October 1943 just after its Jewish population had been dispatched to Auschwitz, and to Nazi-occupied Poland (as a guest of his friend the Governor General) not long after the Germans, by murder and siege and starvation, had annihilated the Warsaw Ghetto.

By the play's elegiac conclusion, the audience has been led, through artful omission and misrepresentation of the historical record, to accept a thoroughly manipulated version of Heisenberg. This Heisenberg had discouraged pursuit of a bomb, had joined in anti-Nazi Resistance that later rescued the Danish Jews, had nobly saved the life of a condemned man, had narrowly escaped death at the hands of the SS on his perilous journey home through Germany's defeated ruins at war's end, and had "never managed to contribute to the death of one single solitary person." Bohr, by contrast, is charged with complicity in the human disaster of Hiroshima and Nagasaki.

"If people are to be measured strictly in terms of observable quantities . . .," Bohr begins, in a final turn of the play, "Then we should need a strange new quantum ethics," Heisenberg rejoins. "There'd be a place in heaven for me. And another one for the SS man I met on my way home." So fast and so far does Frayn take us, this somehow is not meant to shock. Losing sight of the moral horizon can make you feel giddy -- or sick.

A version of this article is available online at
http://www.americanscientist.org/bookshelf/Leads00/Logan.html