Technology & Medicine › Electric age › Nuclear energy (bombs and reactors)
Discovery of nuclear fission
December 1938 (Hahn, Strassmann; explained by Meitner, Frisch)· Invention or discovery· Tools, energy & materials
Well documentedSolid records, physical evidence or scientific consensus.
In December 1938 chemists in Berlin found that neutrons split uranium atoms in two; exiled physicists showed this releases enormous energy.
See it on the Technology & Medicine map →How it works
Otto Hahn and Fritz Strassmann bombarded uranium with neutrons and were puzzled to find barium, an element about half uranium's weight. Lise Meitner, an Austrian-born physicist who had fled Nazi Germany, and her nephew Otto Frisch explained it over Christmas 1938: the nucleus had split, turning a sliver of mass into energy, as Einstein's E = mc² predicts. Frisch named it 'fission', after cell division. Each split also frees extra neutrons that can split more atoms.
Why it mattered
Within months physicists saw that a chain reaction could power reactors or a bomb of unprecedented force, setting off the race toward nuclear weapons.
How we know
Hahn and Strassmann's paper in Naturwissenschaften (January 1939), Meitner and Frisch's explanation in Nature (February 1939), and letters between Hahn and Meitner.
Still unknown or debated
Hahn alone received the 1944 Nobel Prize in Chemistry. Meitner's exclusion is now widely seen as an injustice, shaped by her exile, her sex and disciplinary bias.
Connections (2)
▸Discovery of nuclear fission led to Nuclear energy (bombs and reactors)Well documented
The connection
Fission showed that splitting one uranium atom releases about 200 million electron volts, tens of millions of times more than a chemical reaction, and frees spare neutrons. Physicists quickly realized a self-sustaining chain reaction was possible. Fermi's team achieved one in Chicago in December 1942, just four years later.
When and how
Berlin and Sweden, December 1938; neutron release measured in Paris and New York, March 1939; Chicago Pile-1, December 2, 1942.
How we know
Published 1939 papers, including the Joliot-Curie team's and Szilárd and Fermi's neutron measurements, and Fermi's reactor logbooks document each step.
Debates and open questions
Leó Szilárd imagined a neutron chain reaction in 1933 and patented it, years before fission was known; whether others would soon have found fission anyway is debated.
▸Discovery of nuclear fission prompted the Manhattan ProjectWell documented
The connection
Within weeks of fission's announcement, physicists saw that a bomb might be possible. Fearing Germany would build one first, Leó Szilárd drafted a letter, signed by Albert Einstein, warning President Roosevelt in August 1939. Britain's 1941 MAUD report concluded a bomb needed only about 10 kilograms of uranium-235, pushing the US into a full program.
When and how
Einstein–Szilárd letter, August 2, 1939; Britain's MAUD Committee report, July 1941; full US commitment, December 1941 to mid-1942.
How we know
The Einstein letter and MAUD report are preserved and published; officials' memos show these reports drove US decisions.
Debates and open questions
How close Nazi Germany came to a bomb is still debated; most historians think its program was far behind, partly because of scientific and political misjudgments.