Splitting the Atom

Twentieth-century physics revealed enormous energy inside atoms. Ernest Rutherford described the atomic nucleus in 1911, and James Chadwick discovered the neutron in 1932. In December 1938 the German chemists Otto Hahn and Fritz Strassmann found that bombarding uranium with neutrons produced barium, a much lighter element. Lise Meitner, an Austrian-born physicist who had fled Nazi Germany because of her Jewish ancestry, and her nephew Otto Frisch explained that the uranium nucleus had split, releasing energy, and named the process 'fission'.

Scientists realized that a chain reaction might release vast energy. In August 1939 Albert Einstein signed a letter, drafted with Leó Szilárd, warning President Roosevelt that Germany might build such a weapon. On 2 December 1942 Enrico Fermi's team in Chicago achieved the first controlled nuclear chain reaction. Meitner was not included in Hahn's 1944 Nobel Prize, a decision widely criticized since.