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German-US physicist Otto Stern (1888-1969, left) talking to Swiss physicist Paul Scherrer (1890-1969, right). Stern's main research was on molecular beams. In 1920 he worked with Walter Gerlach, using a molecular beam of silver atoms to test a prediction of quantum theory. The prediction was that the atoms had magnetic moments that were quantized into two orientations. The beam split into two as predicted and earned Stern the 1943 Nobel Prize for Physics. In the 1930s Scherrer specialised in nuclear physics and beginning in 1944 passed information to the United States about German science and German scientists, especially in relation to their efforts to develop a nuclear weapon. He later became the foremost proponent of Switzerland developing its own nuclear weapons. (KEYSTONE/SCIENCE PHOTO LIBRARY/AIP Emilio Segre Visual Archives, Segre Collection /SCIENCE PHOTO LIBRARY)