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Pre-war work on physics

In mid-1936, Heisenberg presented his theory of cosmic-ray showers in two papers. Four more papers appeared in the next two years.

In December 1938, the German chemists Otto Hahn and Fritz Strassmann sent a manuscript to The Natural Sciences reporting they had detected the element barium after bombarding uranium with neutrons, leading Hahn to conclude that a bursting of the uranium nucleus had occurred; simultaneously, Hahn communicated these results to his friend Lise Meitner, who had in July of that year fled, first to the Netherlands, then to Sweden. Meitner, and her nephew Otto Robert Frisch, correctly interpreted Hahn's and Strassmann's results as being nuclear fission. Frisch confirmed this experimentally on 13 January 1939.

In June and July 1939, Heisenberg traveled to the United States visiting Samuel Abraham Goudsmit at the University of Michigan in Ann Arbor. However, Heisenberg refused an invitation to emigrate to the United States. He did not see Goudsmit again until six years later, when Goudsmit was the chief scientific advisor to the American Operation Alsos at the close of World War II.