Michael Frayn's Copenhagen has to be one of the most surprising international hit plays of our time. Theatrically unadorned, it consists entirely of two men, the German scientist Werner Heisenberg and his Danish mentor Niels Bohr, with Bohr's wife Margarethe sometimes chiming in, talking about nuclear physics and what might have been said in a ten-minute conversation between them in German-occupied Copenhagen in 1941. The potential implications of that conversation were enormous: it may have determined whether or not the Nazis developed atomic weapons. The moral and ethical questions raised are also supremely important. Still, Heisenberg and Bohr are not sufficiently well known historical figures that many people would be keenly interested in them, and a talky play about a difficult subject on a bare stage with virtually no action you'd think would be a tough sell.

The Playhouse has bet that the play's fame plus the cachet of Canadian superstar Brent Carver will bring its notoriously fickle audiences flocking. The company has put its money on the right horse when it comes to the quality of the material. Copenhagen is an intellectual mystery as intelligent, provocative and cerebrally challenging as advertised. And the performances are very strong. As Heisenberg, Carver takes some amazing risks in a characterization that seems at times simpleminded and eccentric but is absolutely original and tremendously effective. Victor Ertmanis and Susan Hogan as the Bohrs do terrific work as well. But a good part of the Saturday night audience at the show I saw was grumbling at intermission and coughing incessantly over the dialogue--never a good sign. This is a play that requires concentration and work from its audience.

The setting of the play is actually an afterlife of some sort. The characters are ghosts who replay and debate the meaning of Heisenberg's visit over and over. Why did he come? To try to enlist Bohr's help in the German nuclear program? Or to subtly sabotage that program? To try to organize physicists on both sides of the war to renounce the attempt to develop nuclear weapons? To try to help save the half-Jewish Bohr? Or to ask for Bohr's absolution? In the end whose hands were dirtier: the German whose work led to no one's death or the Dane who was instrumental in the nuclear deaths of tens of thousands of Japanese civilians? Heisenberg's most famous accomplishment, establishing the Uncertainty Principle in physics, becomes the play's primary metaphor for the essential elusiveness and unknowability of objective truth.

Ertmanis' Bohr is a solid, avuncular character who varies between obvious affection for his surrogate son and scientific peer, and frustration at Heisenberg's presumptuousness or naivety or whatever it is that the younger man brings with him. Margarethe has little to do in the first act but introduce the two men to the audience, but in the second Frayn lets her fierce moral indignation loose on Heisenberg and the political system to which he lends his science, and Hogan rises to the occasion with a powerful condemnation of the Nazis' deportation of Denmark's Jews.

And Carver. His Heisenberg is all hesitancy and distraction. A nervous, almost moronic smile plastered on his face, he glances obliquely every which way as long as he doesn't have to meet the others' eyes. There is so much he can't say, and not only because in 1941 he's being tailed and recorded by the Gestapo. He loves his country as much as he may dislike its political system. And it's not clear that even he is certain about his motives. Like Bohr, science is ultimately the most powerful force in his life. When he describes his discovery of Uncertainty he can barely speak of it. He struggles to hold himself together, as if he is still, after all these years, shattered by his revelation. Carver's own persona is so likeable that his making Heisenberg tentative and weird is really the only way to keep the play's sympathies properly balanced. Not always an easy performance to watch, it's nevertheless one I won't soon forget.

Glynis Leyshon's production mostly lets the actors do their thing. Alan Brodie's set consists of three chairs on a small revolve set upon a larger round platform within a circle of broken Gothic arches. Above the revolve hangs a chandelier in which a large bird appears to have built its nest. The semiotics of this design wasn't very clear to me. The cathedral metaphor appears a couple of times in the play, but what is most important is that no props, furniture or set pieces mediate between the actors and audience. I think the play could be equally effective with no set at all, especially as the characters are in a kind of limbo. This being a memory play Brodie's lighting is important, its changing colours and moods working nicely to help sculpt the emotion of particular moments. Jeff Toyne's sound design is less effective. Bach and Beethoven are heard whenever they are mentioned, along with various ominous sound effects, but at almost subliminally low volumes. That kind of uncertainty the play could do without.

Jerry Wasserman