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Niels Bohr
Margarethe Bohr
Werner Heisenberg
The Moral Dilemma
The Bohr Letters
Heisenberg (photograph by P. Ehrenfest Jr., courtesy AIP Emilio Segrè Visual Archives, Weisskopf Collection)

Werner Heisenberg 1901-1976

Werner Heisenberg won the Nobel Prize in physics in 1932 for establishing the field of quantum mechanics. Heisenberg suggested that any theory of the atom must be based on observable phenomenon, such as the spectral lines emitted by atoms, and not pictorial constructs such as Bohr's nuclear model of the atom. For Heisenberg, this observable data could be culled to formulate a set of possible values for a hypothetical particle. These values could then be used to calculate, by mean of mathematical formulas, the probabilities of particular energy states and transitions among those states.
 
Quantum mechanics had a profound influence on the development of atomic and nuclear physics by providing a model for calculating such formulations as critical mass. Using mathematical laws of probability, nuclear physicists were able to determine how much fissionable material would be necessary to ensure the likelihood that enough neurons would collide with the material to cause fission, thus leading to the development of nuclear reactors and atomic bombs.

Heisenberg is best known for his uncertainty principle of 1927. The principle posits limits to the accuracy of knowledge about atomic behavior, since the means by which the researcher measures such phenomena -- short wave radiation bounced off a particle, for example -- alters the behavior itself.
 
The son of a university professor, Heisenberg studied theoretical physics under Arnold Sommerfeld at the University of Munich. During a 1922 lecture given by Niels Bohr at Gottingen, Heisenberg publicly questioned the mathematics of the Nobel Prize winner, earning Bohr's attention. Bohr invited Heisenberg for a hike, initiating their famous collaboration. Heisenberg received his doctorate in 1923, then went to work with Niels Bohr in Copenhagen. In 1927, Heisenberg returned to Germany to teach physics at the University of Leipzig.

Early in World War II, the deeply patriotic Heisenberg had conducted for the Germans chain reaction experiments with heavy water that led him to believe in the feasibility of a nuclear weapon. Heisenberg visited his mentor and former teacher, Niels Bohr, in Copenhagen in 1941, perhaps to ask his advice on the right course of action for the development of atomic energy. It is not certain what was said during this meeting, however Bohr came away with the impression that the Nazis were actively developing a nuclear bomb. Within a few days before Germany's surrender, Heisenberg was captured by Allied forces and incarcerated with other German atomic scientist at Farm Hall, a country estate near Cambridge, England. It was later determined that the Germans were never close to developing an atomic bomb. [For more on this topic, refer to the Journal of Chemical Education Webite.]
 
After World War II, Heisenberg directed the Max Planck Institute at Berlin, and then moved to the Max Planck Institute for Physics at Gottingen where he remained for the rest of his career.
 
In 1955, while working on a book about the development of atomic physics in Germany during the war, Robert Jungk queried Werner Heisenberg about his Copenhagen meeting with Niels Bohr. Heisenberg complied by sending a letter back recollecting what happened. An excerpted version of his letter was subsequently published in Jungk's book "Brighter Than a Thousand Suns."

Heisenberg's account of their meeting so upset Bohr that he dictated a letter to Heisenberg detailing his own, and very different, recollection of what had transpired. In the end, Bohr never sent his letter. In February of 2002, the Bohr family published Bohr's draft letter to Heisenberg on the Internet. The issue of what Heisenberg and Bohr said to one another in 1941 Copenhagen is still open for debate.
 
Werner Heisenberg died in Munich in 1976.
 
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