If one finds a difficulty in a calculation which is otherwise quite convincing, one should not push the difficulty away; one should rather try to make it the centre of the whole thing.

        ---Werner Heisenberg

The Historical Development of Quantum Theory, Vol. 3: The Formulation of Matrix Mechanics and Its Modifications, 1925-1926 by Jagdish Mehra and Helmut Rechenberg, Springer-Verlag, New York, 1982, p. 94.

This quote, where it is used as an epigraph on p. 172 in Operator Commutation Relations by Palle E. T. Jorgensen and Robert T. Moore, D. Reidel, Dordrecht / Boston / Lancaster, 1984, is cited incorrectly to p. 227 of the Mehra-Rechenberg work.

Of course Max Born cleaned up a lot of the math that Heisenberg didn't understand, and that is sort of what the Heisenberg quote is hinting at. And it is also the main thrust of vol. 3 of the Mehra-Rechenberg book set. It says that Heisenberg learned an important lesson from the work of Born and Jordan in 1926 (aside from not sleeping in Hilbert's classes!), and the lesson is the statement of Heisenberg which is quoted on p. 94. Mehra had taped and transcribed a large number of conversations he had over a period of years with Heisenberg, and they are cited with page numbers at many places throughout the book set, and this particular quote is cited as: Heisenberg, Conversations, pp. 163-164.

When I have advised my Ph.D. students in their thesis work, I must have used some version of this sentence from Heisenberg a large number of times, sometimes several times in one day.

Some interesting Heisenberg links:
     Heisenberg / Uncertainty Principle - Werner Heisenberg and the Uncertainty Principle
     Werner Heisenberg Winner of the 1932 Nobel Prize in Physics
     Copenhagen . Werner Heisenberg | PBS

I like to read papers and books by Werner Heisenberg, and am struck by how strong a turning point in his life and his thinking was the Fall of 1941.

I think that he never made peace with himself after the meeting with Bohr.

After I started to think about sources, I was struck by some newly released letters from Bohr that put the play Copenhagen (by Michael Frayn) in a different light. I also found it touching that much of the correspondence between Bohr and Heisenberg was in Danish.

There were quite a number of books about Heisenberg that came out before Michael Frayn's play was written. And there were some that came after. An interesting one was by Heisenberg's widow, Elizabeth. You can guess which part of the spectrum this book covered. It is worth reading all the same. The books in their totality cover quite a broad spectrum of interpretations, more or less the spectrum of possibilities from the play itself. My impression is that the transcripts from the 1945 Farm Hall internment of the German nuclear scientists were made public several years after Frayn had finished writing his play. But the Farm Hall transcripts weren't conclusive. I read them. However they certainly didn't throw a positive light on Heisenberg's motives. Others of the contemporaries wrote about this too, such as Lise Meitner, who had escaped to Sweden in the war years. Lise Meitner is a counter balance to the spin from Elizabeth Heisenberg's book. (I have an amazon list with some of these biographies.)

Werner Heisenberg's father had died when WH was 29, and Bohr became close to being a substitute father from then on. In the play, you hear the haunting sound of sea gulls in the background. The only prop in the entire play! Niels Bohr's oldest son had drowned in a sailing accident, and this sound effect from the play may be a reminder that Bohr was losing another son at the time of the 1941 meeting. Their personal bond was very strong. So that was probably also one reason why Bohr's letters were never sent, and weren't meant to be released. Well the Bohr family finally did decide to make them public in 2002; i.e., many years after Frayn's writing. I can't quite make out how well Frayn knew the Bohrs, but he has researched his subject to an impressive degree. I have a feeling that the play might well have gotten a different twist if Frayn had known about Bohr's letters. Bohr was always so extremely careful in his formulations, and I hesitate to attempt to summarize Bohr's letters; but I think you will find them interesting.

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