Letter casts doubt on 'Copenhagen' theory

One was a brilliant but otherworldly Danish physicist, the other a linchpin of Adolf Hitler's quest for the atomic bomb.

For more than 50 years their meeting in occupied Denmark in 1941 has been at the centre of historical debate over the fate of Germany's nuclear weapons programme, inspiring Michael Frayn's acclaimed West End play Copenhagen.

Now a letter released by an archive in New York has cast the event in a new light, contradicting claims that Werner Heisenberg, then the head of the German atomic programme, sabotaged Hitler's efforts to build a nuclear weapon from within.

Until now little of what happened at the meeting between Heisenberg and his Danish mentor, Niels Bohr, has been known. What was clear was that it ended abruptly and that almost all contact ceased between the two men, who had been close friends.

Bohr escaped from occupied Denmark in 1943 and made his way to England and then America, becoming central to the Allies' push for an atomic weapon. He died in 1962.

After the war, Heisenberg cited the meeting as evidence of his reluctance to help Hitler create the ultimate weapon of mass destruction. He had, he suggested, travelled to Copenhagen to share his qualms over nuclear weapons, becoming a sort of saboteur.

But was the German scientist motivated by an attempt to save mankind from the horrors of the bomb, or by a desire to prise information out of his mentor? The latest document, a letter written by Bohr to Heisenberg but never sent, suggests that Heisenberg was working as hard as he could on the German atomic weapons programme. "You said that there was no need to talk about details," Bohr said in extracts quoted by The New York Times, "since you were completely familiar with them and had spent the past two years working more or less exclusively on such preparations."

The newspaper quotes Bohr as saying that, under Heisenberg, "everything was being done in Germany to develop atomic weapons".

The fortunes of Heisenberg and Bohr had been entwined since the two men met in Gottingen in 1922 at a talk given by the Danish physicist. After a precocious intervention from the audience Heisenberg became a protégé, and went on to become Germany's leading atomic physicist.

The letter just published was contained in a New York archive and was not supposed to be made available to the public until 2012, but was released early with the consent of the Bohr family.

The reason the Danish physicist never sent it is not known, although he enjoyed a reputation for being mild-mannered, and might not have wanted to embarrass his old friend.

Although most historians regard the latest revelations as enough to discredit Heisenberg, the German scientist still has his defenders. Despite Bohr's huge intellectual abilities his communicative skills were never impressive and Heisenberg's defenders say that the two men could simply have misunderstood each other.