The Niels Bohr Archive in Copenhagen has posted on the Web (at http://www.nba.nbi.dk) 11 short documents, never before made public, relating to the controversial visit of Werner Heisenberg to Bohr in German occupied Denmark in 1941. Most of them are drafts of letters Bohr wrote to Heisenberg in 1957 or later, but never actually sent. Since Gerald Holton (Harvard) announced the existence of one of these unsent letters two years ago, there has been much speculation as to what its contents might reveal about what was actually said during Heisenberg’s enigmatic wartime visit to his former mentor. (See the articles by Holton and David Cassidy in Physics Today, July 2000.)

The Bohr family had intended to withhold these personal documents until 2012, the 50th anniversary of Bohr’s death. But as Finn Aaserud, director of the archive, puts it, the family has decided to release them now “to...

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