`Copenhagen," Michael Frayn’s 1998 play about some of the issues involved in the development of the atomic bomb and its use in World War II, is playing at Bristol Riverside Theater through Sunday, March 30. Crucial to the play, and to the moral dilemma the work poses for the scientists involved, is the part played by two of the physicists responsible for developing the science that made the bomb possible, Niels Bohr and Werner Heisenberg. Bohr; his wife, Margrethe; and Heisenberg are the only characters in this thought-provoking and chilling drama.

Heisenberg came to Copenhagen in the 1920s to study with Bohr and stayed for a few years to work with him. They became close friends and, often by opposing each other’s ideas, were together responsible for precedent-breaking discoveries. They stayed closely in touch until Hitler’s ascendancy and the German occupation of Denmark made their personal and working relationship problematic.

Heisenberg came back to visit Bohr in 1941. This was a very awkward moment for both men, but Heisenberg got himself to Bohr’s house and Bohr let him in. To escape their conversation being bugged, the two men set out on a walk so they could talk more freely. (Before the war they had done much of their communicating while taking long walks.) They were back very quickly, and what they were talking about that so angered Bohr that he broke off the walk remains unknown. It is apparently still a matter of considerable speculation what Heisenberg said that so upset Bohr.

This development adds a wonderful layer of irony to "Copenhagen." Here are the two men who worked out the important physical principle of uncertainty, and at the same time their personal relationship is marked by a major uncertainty. A great deal has apparently been written speculating as to the reasons for Heisenberg’s 1941 trip to Copenhagen. Was he trying to get information that would help Nazi Germany develop the atomic bomb? It seems that serious physicists have devoted considerable energy to trying to unlock this mystery.

Indeed, Heisenberg was confronted with many complex choices, and much doubt remains about his role in the development of the atomic bomb. Did Germany not succeed in developing a bomb because Heisenberg withheld information? Or because he misunderstood the physics involved? Another ambiguity remains as to whether his acquaintance with various members of the German bureaucracy in Denmark helped make it possible for several thousand Danish Jews, about to be shipped east to be killed, to escape to Sweden. Bohr, who was half Jewish, himself escaped to Sweden and from there got to England. He ended up in Los Alamos, working on the Manhattan Project.

Bristol Riverside’s production does justice to the importance of the subject, without ever being pretentious. The visual qualities of this production add to its strength. When the play opens, all three characters, who are dead, stand framed in three doors set in simple wooden walls. As they step forward into the room and begin to try to unravel the mystery, the light changes from mysterious to normal. This is the only set, and Margrethe occasionally moves the chairs around a bit to suggest a different time and place, and perhaps at times to suppress her irritation. Although Margrethe has helped her husband by typing and retyping his drafts, she is far from being simply the self- effacing wife of a great man, and her frequent questions and comments are usually penetrating and often unpolitic.

Keith Baker, Bristol Riverside’s artistic director, takes on the role of Heisenberg. (He has also written the music.) Even Bristol Riverside regulars are not used to seeing Baker on the stage, and it’s a delight to see what an accomplished actor he is. Margrethe and Niels Bohr are played by Moira Wylie and Douglas Campbell, who are also married offstage. Campbell, now in his 80s, has had a distinguished career and is particularly known for his work with the Stratford Festival. His wife, who studied with Uta Hagen, also works as a director. The three of them do a spectacular job. This is a complicated script, and to grasp what’s going on you have to listen carefully to what’s being said.

Bristol Riverside has mounted a stunning production of this complex play. Edward Payson Call, who worked with Campbell at the Guthrie Theater, is the director. Scenery and lighting were designed by Greg Mitchell, costumes by Linda Bee Stockton, and sound by Brad Criswell.

"Copenhagen" is perhaps not everybody’s cup of tea. It requires the audience to pay attention, to concentrate, to think about what’s being said. And the implications of much of what’s being said can be terrifying. For those who want to be entertained without having to think, this may not be the best choice, and several people were seen leaving at intermission the night I was there. But for those who want to think about what they’re seeing and hearing and can bear to face up to the dangers this world faces, I cannot recommend this play and this production too highly.

"Copenhagen," through Sunday, March 30, Bristol Riverside Theater, 120 Radcliffe Street, Bristol. Drama by Michael Frayn. $34. 215-785-0100.

The Friday, March 21, performance will include a pre-show reception and talk with Dr. David Balamuth, Associate Dean of the college of Arts and Sciences at the University of Pennsylvania, speaking on the ideas of ethics in research and scientific responsibility. Reception at 7 p.m.; talk at 7:30 p.m.

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