Nuclear Fission Discovered

In late 1938, in a laboratory in Berlin-Dahlem, the German chemists Otto Hahn and Fritz Strassmann found that bombarding uranium with neutrons produced barium, a much lighter element. The result showed that the uranium nucleus had split into two smaller pieces, a process soon named nuclear fission. The discovery revealed a new source of energy on an unprecedented scale and, within a few years, made both nuclear power and the atomic bomb conceivable.

The background lay in the physics of the 1930s. After James Chadwick identified the neutron in 1932, scientists realized it could penetrate atomic nuclei without being repelled by electric charge. Enrico Fermi and his colleagues in Rome bombarded many elements with neutrons, including uranium, and believed they had created new, heavier "transuranic" elements. Several teams, including Hahn's in Berlin and Irène Joliot-Curie's in Paris, pursued the puzzling results, which seemed to show a bewildering mix of radioactive products.

Hahn and Strassmann's careful chemical analysis led them to an unexpected conclusion: some of the products behaved chemically like barium. Hahn, who had worked for years with the physicist Lise Meitner, wrote to her about the puzzle. Meitner, a Jewish-born Austrian, had fled Nazi Germany in mid-1938 and was in Sweden. Together with her nephew Otto Frisch, she worked out a physical explanation over the winter: the nucleus could behave like a liquid drop, stretching and dividing in two while releasing a great deal of energy. Frisch borrowed the term "fission" from biology. Their interpretation was published in early 1939, soon after Hahn and Strassmann's chemical findings.

Hahn alone received the 1944 Nobel Prize in Chemistry, awarded in 1945, and Meitner's omission is widely regarded as one of the Nobel committee's most notable oversights. Historians still debate how much of that was due to disciplinary boundaries, wartime isolation, and her exile, and how much to prejudice or Hahn's own choices.

The consequences came quickly. Physicists realized that fission released additional neutrons, which could in principle trigger a chain reaction. As war loomed in Europe, scientists, many of them refugees, alerted governments in the United States and Britain to the danger that Germany might exploit the discovery. These concerns fed into the Manhattan Project, which produced the first nuclear weapons in 1945. Fission also led to nuclear reactors and civilian power generation, leaving a double legacy of abundant energy and enduring questions about weapons, safety, and responsibility.