In 1941, Werner Heisenberg traveled from Nazi Germany to occupied Denmark to visit Niels Bohr. To this day, no one knows why the two physicists met. This mysterious event is the subject of Michael Frayn’s “Copenhagen,” a play that explores the resemblance of science to theater, and the drama that unfolds when physics and politics meet.

Theater and theory come from the same word — “thea,” a Greek noun which means “watching, seeing” — and the play’s three characters are observers in their personal and professional lives: The ghosts of Bohr (Jae Shin ’17), his wife Margrethe (Annie Saenger ’19) and Heisenberg (Noah Stetson ’18) form a triangle of observers, who watch each other as we watch them. Their investigation is simple, or seems to be: They want to know why Heisenberg visited his old friend and mentor at great personal risk. All three remember that the visit was made, and that it was awkward but cordial until Heisenberg and Bohr took a walk in the garden, ostensibly “for old times sake” but really to avoid surveillance microphones. They all remember that after 10 minutes, Bohr stormed back to the house and announced that Heisenberg was leaving. But no one remembers what happened during those ten minutes in the garden. “Copenhagen” is a series of dramatic experiments intended to determine that unknown quantity.

You probably know Heisenberg and Bohr from high school physics as pioneers in the field of quantum mechanics, a way of explaining the dynamics of subatomic particles. Bohr, founder of the Institute of Theoretical Physics at the University of Amsterdam, was a kind of scientific father to many of the men and several of the women who reinvented physics in the first half of the twentieth century, and to Heisenberg in particular. (Director Hannah Kazis-Taylor ’19 emphasizes this relationship by dressing Bohr in an old-fashioned grey double-breasted suit, and Heisenberg in a boyish blazer and slacks.) Bohr’s obsession, at least in Frayn’s play, is his theory of complementarity: that the nature of light or other radiation must be explained in terms of both particles (the quanta of quantum mechanics) and waves (the classical model for radiation). And true to his theory, Bohr is socially minded: He emphasizes the way he worked together with Heisenberg, and, as he fleshes out his moral objections to his protege’s work with the Nazis, evokes a moral sensibility that Heisenberg lacks.

If Bohr is the generous father, careful and intensely ethical, Heisenberg is the prodigal son, mercurial and ambitious. His famous theory of uncertainty states that you can’t know both the momentum and the position of a particle with absolute certainty; the more precisely you know where a particle is, the less precisely you know its momentum (its mass multiplied by its velocity), and vice versa. Like Bohr, Frayn’s Heisenberg embodies his science: you can never really know what Heisenberg is after, or even why he came to Copenhagen.

Bohr and Heisenberg are the play’s two protagonists, trying in the only way they know to figure out what happened that night. Locked in their own kind of complementarity (father/son; fast/slow; German/Jew), their struggle is not just epistemological, between two competing accounts, but also moral, as they weigh the choices they made during the war to help weaponize nuclear energy. At this moment four months before the Americans declared war, and four years before the Enola Gay dropped a nuclear bomb on Hiroshima, two scientists — experts in their field, perhaps, but not in politics — are elevated to the status of tragic heroes: Their choices will change the world. If those 10 minutes in a garden in Copenhagen hadn’t happened, history might have taken a drastically different course.

Margrethe is the one who wants to know what happened. “But why?” she asks in her first line, the very first words the audience hears. If Bohr and Heisenberg are tragic figures of a kind, Margrethe is the chorus, floating among and above the two men as they argue. She watches, asks questions, makes observations, jokes and corrections, and, on the production’s nearly empty stage, sometimes sits directly between the two men like a referee, or a judge. Margrethe, who typed all of Bohr’s papers, and who insisted that he explain his ideas in “plain” language, is our way into these characters and their physics. She asks them questions, gets them to explain, urges them to clarify.

As a playwright, Frayn does an admirable job of exploring the literary potential of science. But this comes with certain risks. As the Nobel Prize-winning physicist Paul Dirac put it to Robert Oppenheimer, “The aim of science is to make difficult things understandable in a simpler way; the aim of poetry is to state simple things in an incomprehensible way. The two are incompatible.” The more scientifically precise you are, the less poetic; the more poetic, the less scientific. The result is a strange kind of play — a sort of narrative theater, in which characters are less actors than storytellers or historians. Their activity is more forensic than dramatic.

Frayn seems to be more Bohr than Heisenberg: the play works out ideas until they’re quite explicit, repeating the experiment to get clearer results, in long, wordy monologues and dialogues. This is a formidable challenge for the actors, and one amplified by the spareness of the production, but they do an admirable job. Stetson plays Heisenberg with a kind of childish innocence, and intellectual ebullience, that infuse a long play with energy. Shin brings out Bohr’s reflectivity, his thoughtfulness, and, in a play that is so often about ideas, his feeling. And Saenger’s Margrethe is soulful and often quite funny. These characters emerge as people against “Copenhagen”’s backdrop of ideas.

Despite the play’s exhaustiveness, despite its language of bugged rooms and close observation, despite its repetition of experiments and questions, the characters’ search for knowledge ends in aporia. Neither Bohr nor Heisenberg can be certain about the choices he made, or why he made them. Margrethe can’t ever know what the ultimate consequences of their innovation will be. Heisenberg, the play’s animating genius, the destabilizing force who set the whole thing in motion with his inexplicable trip in 1941, becomes its poet and interpreter when he enjoins us, in the play’s final line, to rest in the “final core of uncertainty at the heart of things.”

“Copenhagen” is playing from April 6 to 8 in the Hopper Cabaret.

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