Michael Frayn's Copenhagen focuses on 10 minutes in September 1941, when the German theoretical physicist Werner Heisenberg travelled to Nazi-occupied Denmark and visited his old collaborator, Niels Bohr.

With Bohr, Heisenberg had bolted together the famous "Copenhagen Interpretation" of quantum mechanics in 1927.

Despite its impossible paradoxes, quantum mechanics led irrevocably to the unleashing of the vast energies bound up within atomic nuclei, and the fission bombs which devastated Hiroshima and Nagasaki - capping the most monstrous atrocities of the second World War.

But the question of Heisenberg's motives in visiting his half-Jewish friend in 1941 has long divided historians. This, and Heisenberg's controversial wartime record, provides the backdrop to Frayn's three-handed play of ideas and memories. Heisenberg, Bohr and Bohr's wife, Margrethe, cautiously circle each other; retracing, over and over again, what was said and intended at the brief meeting which ended their friendship.

Inspired initially by Thomas Powers's book, Heisenberg's War, and David Cassidy's biography of Heisenberg, Uncertainty - which arrive at diametrically opposing views from the same evidence - Frayn's Copenhagen riffs elegantly, almost elegaically, on the physics as much as the competing loyalties between the three re-imagined characters.

Throughout its run at the National Theatre in London in 1998, critics heaped garlands upon it, while physicists - bemused that their speciality was suddenly de rigueur - offered minor corrections. Yet since the New York production, Frayn has been attacked for not sufficiently condemning Heisenberg's services to the Nazis, by such historians as Paul Lawrence Rose (Mitrani Professor of Jewish Studies and European History at Pennsylvania State University) and Gerald Holton, a Harvard professor of physics and history of science.

An exceedingly genial 68-year-old, Frayn is well able to battle with critics. A couple of weeks ago he affectionately mugged the poet and critic Tom Paulin in the Guardian, dismissing the latter's claim that, in Copenhagen, he had conflated Heisenberg's brand of German patriotism with that of Willy Brandt. Frayn wrote that this was like being "barked at by a lovable but confused sheepdog" whose "lovely woollen hair . . . has fallen over his eyes again".

Considering that, elsewhere, Frayn has graciously accepted criticisms of his under- emphasis of Nazi evil, what level of intellectual debate was he conducting with Paulin? He laughs: "Well, I've met him a few times and he seems an extremely lovable man, except that he tends to go up like a bomb sometimes. The way he goes on, he makes me out to be some kind of revisionist."

But the heat of such Holocaust-related debates is intense. Indeed, Rose targeted what he saw as Frayn's "subtle revisionism . . . more destructive than Irving's self- evidently ridiculous assertions - more destructive of the integrity of art, of science, and of history". This carries shades of our own recent "debate" over the late Francis Stuart and his sojourn in wartime Nazi Germany.

From Friday, Irish audiences will have the chance to see Rough Magic's production of Copenhagen. Director Lynne Parker says: "Working in the round, as we have often done, you get a very cinematic, naturalistic style of playing, which suits the play, because it has this sense of concentric circles of reality. With each one you enter, you get a different perspective on these monumental decisions placed on the shoulders of very fallible mortals."

The production is timely too in that, riled by the renewed controversy, Bohr's family has just Web-published several drafts of an unsent letter from Bohr to Heisenberg, written between 1957 (in reaction to Heisenberg's comments on the 1941 meeting in Robert Jungk's book, Brighter than a Thousand Suns) and 1962, shortly before Bohr's death. For Frayn, the drafts show "that the real Bohr remained angrier for much longer than my character". Bohr's drafts show him, inch by inch, toning down his anger and sharpening his central question to Heisenberg: "Which official state agency permission was given to talk to me about a question which was surrounded by such great secrecy, and held such great dangers?"

The balance of evidence suggests that Heisenberg was on some sort of official mission. Certainly, when he visited Bohr's Institute of Atomic Physics, he made loud, offensive remarks about the inevitability of a German victory. Then he visited Bohr with fellow-physicist Carl Friedrich von Weizsacker. Heisenberg began hesitantly, but something he said led Bohr to abort the conversation, evidently shaken by Heisenberg's suggestion that he was working on a German atom bomb.

Von Weizsacker, now 89 and living outside Munich, reacted quickly to the new Bohr letters, claiming that Heisenberg naïvely intended to communicate to physicists working with the allies (through Bohr) that the Germans would not build a bomb if the allies might agree likewise - a tall enough tale which contradicts scores of earlier accounts, including von Weizsacker's own.

So did Heisenberg go to spy? Or to extend some form of protection deal to Bohr? Frayn says: "In the middle of a war, to go across to your enemy and talk about an atomic bomb . . . I simply can't believe that Heisenberg would have been so naïve as to arrive on his own initiative without any authorisation."

Indeed, Heisenberg himself was under Gestapo surveillance. Despite winning a Nobel Prize in 1933, he had been characterised as "a White Jew" in an SS newspaper in 1937 for championing what Nazis regarded as the "degenerate" Jewish science of relativity. Heisenberg was eventually exonerated by no less than Himmler - through family contacts - in 1938.

That was the year fission was discovered in Berlin, and Heisenberg was put to work under the Wehrmacht Weapons Bureau in Berlin in 1939. He quickly submitted an optimistic report on the prospects of nuclear research, and headed a reactor group in Leipzig. By 1940, he knew a uranium-235 bomb was possible, and colleagues were working on producing weapons-grade plutonium from a reactor. In 1942, Heisenberg was appointed head of fission research in Berlin - the scarifying prospect of which drove the US effort at Los Alamos. Yet by the end of the war, the Germans had not even achieved a chain reaction.

SOME argue that Heisenberg sabotaged the nuclear bomb programme for moral reasons. Others claim that his incompetence as an experimental, rather than theoretical, physicist led him to failure. In Gitta Sereny's Albert Speer: His Battle with Truth, Speer made a much-contested claim that Heisenberg had told him that the nuclear programme was too costly and time-consuming, and in February 1942, the bomb project was suspended. But forever afterwards Heisenberg cautiously maintained that his team had thus been spared a moral decision.

There is even more puzzling evidence in the Farm Hall transcripts - secret recordings of German nuclear scientists detained in England immediately after the German defeat. When news of Hiroshima came through, Heisenberg is paraphrased as saying that German physicists had deliberately failed, because they did not want Hitler to win the war. Considering other transcripts, in which Heisenberg is recorded as making fundamental technical errors, perhaps Heisenberg was wise to the possibility of being recorded. Frayn says: "Yes, it came up in their conversations, but I don't think they could constantly have kept that to the forefront of their minds. You must remember that after the news of Hiroshima, they were distraught. It was a highly emotional moment. One project leader was sobbing."

In Copenhagen, Fraynalso posits the irony of how Heisenberg, who never produced an atom bomb, now stands on trial, rather than Bohr, who worked on bombs that killed more than 100,000 people. However, Frayn maintains that Bohr, in arriving at Los Alamos only in 1943, "contributed very little. He was way behind the advances that had gone on in his absence".

In the German-speaking world, Heisenberg is simply remembered as a great scientist. "The German press is always suspicious of people involved in the Nazi period," says Frayn, "but it's very interesting how people who knew Heisenberg, such as Professor Hans-Peter Durr, remember him as a very ordinary person - certainly as a brilliant physicist, but not at all the moral monster as he is frequently painted in the West."

Of course, in its underlying conceit, Copenhagen's contested approximations of events - exposing the limitations of biography, history and indeed memory - neatly parallel the bizarre mathematical and philosophical framework of quantum mechanics. Then there is Heisenberg's famous "uncertainty" principle, which states that the more you know about a sub-atomic particle's position, the less you know about its momentum - or, dare I say it, intention.

Copenhagen opens at the Project on Friday. Booking: 01-8819613