In September 1941, prominent nuclear physicists Niels Bohr and Werner Heisenberg met in Copenhagen, Denmark to discuss the possibility of building an atomic bomb.

At the time, Heisenberg was one of the premier scientists working to build an atomic bomb for the Nazis and Bohr later worked toward the same goal for the Allies in the United States.

Now, more than 60 years later, a debate sparked by the production of the Tony Award-winning play Copenhagen is raging between historians, trying to figure out why Heisenberg initiated the meeting with his former mentor. The historians are also attempting to work out the details of what the two scientists discussed.

Also, drafts of letters that Bohr wrote to Heisenberg, but never sent, were released in February on the Niels Bohr Web site (www.nba.nbi.dk).

The letters, according to the Web site, were written sometime after 1957, in response to a book about the atomic bomb called Brighter Than a Thousand Suns, by Robert Jungk, in which there were excerpts of letters Heisenberg had sent to the author.

One Penn State historian, Paul Rose, Mitrani professor of history and Jewish studies, has been researching Heisenberg's role in the Nazi attempts to harness nuclear weapons since 1983.

He published Heisenberg and the Nazi Atomic Bomb Project in 1998, providing evidence that Heisenberg was indeed trying to build a bomb for the Nazis and had incorrect calculations of the amount of Uranium235, or what is known as the critical mass, needed to build the bomb.

"Heisenberg was convinced you couldn't do it because they thought they needed tons of uranium," Rose said.

Two physicists working for Great Britain in 1940, Rose said in a speech he delivered to the Smithsonian Symposium in Washington, D.C., earlier this month, discovered the actual amount of Uranium needed to be only a few pounds.

Rose thinks Heisenberg's ego got in the way of him re-examining his calculations.

"Because he is arrogant, he is ignorant," Rose said. "Heisenberg could not have made a mistake in calculating the critical mass."

Warren Witzig, professor emeritus of nuclear engineering, worked on the Manhattan Project when he was 23.

He agreed with Rose that Heisenberg had miscalculated the critical mass, but added everyone did at the time. Witzig said there was intense competition to build the bomb.

"The Japanese weren't quitting," Witzig said. "Dropping the bomb saved in the order of a million lives."

Witzig thought Heisenberg said he was trying to stop the Nazi's attempts to build the bomb, because he wanted to save face for failing.

"I think he was hoping to put a better case on his activities during the war," Witzig said. Rose believes Heisenberg wanted to get information from Bohr about the allied bomb project and to figure out if he had missed any principle theories about nuclear physics

At the other end of the debate is playwright Michael Frayn, who wrote Copenhagen, which he said in the play's postscript, is based largely on a book called Heisenberg's War by journalist Thomas Powers.

Through his recreation of the 1941 meeting, Frayn suggested that Heisenberg may have sabotaged the Nazi atomic bomb project from within for moral reasons and that this is why he never succeeded in creating the weapon.

Frayn responded to criticism of his play by Rose and others in the March 28 edition of The New York Review of Books.

"My Heisenberg is saying that we do have to make assessments of intention in judging people's actions," Frayn said in the article.

He said people continue to respect Bohr in spite of his involvement with building the bombs used on Hiroshima and Nagasaki.

The Heisenberg character, Frayn said, would always be distrusted, even though he did not kill anyone.

"I suppose that is what sticks in people's throats — that my Heisenberg is allowed to make a case for himself," Faryn said in the article.

Rose believes Faryn's play is in part general revisionism of what actually happened.

"They think they've got a fact, but they change the meaning of it," Rose said about the many historians who wrote that Heisenberg was concerned about the moral implications of the bomb.

He said Heisenberg and the other scientists who worked on the Nazi atomic bomb project were morally bankrupt.

Rosebacks all of this up in his book, using secret reports from the Germans, Bohr's wartime correspondence with British intelligence and discussions of German scientists that took place in 1945, known as the Farm Hall transcripts.

Bohr's letters, which were released after Rose's book was published agree with his assertions.

"You (Heisenberg) spoke in a manner that could only give me the firm impression that, under your leadership, everything was being done in Germany to develop atomic weapons," Bohr said in one of the drafts to the letters he wrote to Heisenberg.