In September 1941, as Nazi Germany consolidated its hold on most of Europe, two of the century’s indispensable physicists met in Copenhagen, Denmark. Former colleagues, both of them key figures in the development of quantum theory, Niels Bohr and Werner Heisenberg had become estranged by the war, Bohr living on borrowed time in occupied Denmark, where his Jewish roots made him a potential target (he fled to neutral Sweden two years later), Heisenberg the head of Germany’s atomic research team.

What did they talk about, these two seekers after the mysteries of the atom? The question has intrigued historians ever since, and it intrigued playwright Michael Frayn, whose drama Copenhagen circles around it like particles around an elusive nucleus. Did Heisenberg help the Nazis in their quest to develop an atom bomb? Did Bohr, who later worked on the American nuclear program, try to talk him out of it—or fail to?

The play itself parallels Heisenberg’s famous Uncertainty Principle—the idea that in the subatomic universe, as Frayne has stated it, “there’s no way that we can ever know everything about the behavior of a physical object.” The playwright extends that principle to human behavior: “How we know why people do what they do, and even how one knows what one does oneself [is] a fundamental question [and] the heart of the play.”

Copenhagen takes place in a ghostworld afterlife, where time and space don’t exist—or rather, are as indeterminate as quantum theory tells us they are—and where the two men themselves (Bohr’s wife Margrethe is also there) try to reconstruct, with conflicting incomplete hypotheses, what went down in that uncertain encounter. Frayn’s absorbing puzzle of a play is onstage now at the Actors Theatre Playhouse, the Brattleboro area’s feisty community theater.

Through Sept. 20, West Chesterfield, NH, (877) 666-1855, ATPlayhouse.org.