Lise Meitner helped explain a result that seemed impossible: a uranium nucleus could break into much smaller nuclei and release a large amount of energy. The explanation emerged in the winter of 1938–1939, after Nazi persecution had forced her to leave Germany. It combined chemical experiments in Berlin with physical reasoning developed in exile. The episode shows both the interdependence of scientific disciplines and the damage political exclusion could do to a scientist’s career.
A partnership across physics and chemistry
Born in Vienna in 1878, Meitner pursued physics when women faced substantial barriers to advanced education. She earned her doctorate at the University of Vienna in 1906 and moved to Berlin the following year. There she attended Max Planck’s lectures and began working with Otto Hahn, a chemist studying radioactive substances. Their partnership brought together different skills: Hahn could separate and identify materials chemically, while Meitner investigated the radiation and physical behavior associated with them.
The collaboration developed despite institutional restrictions on women. Early work took place under arrangements that reflected Meitner’s limited access to laboratory facilities. Over time she established an independent position and eventually led a physics section at the Kaiser Wilhelm Institute for Chemistry. The research involved tracking radioactive transformations through both chemical separation and radiation measurements. Identifying a new substance required more than observing an unusual signal; researchers also needed to understand how it behaved among known elements.
Exile and an unexpected chemical result
In March 1938, Germany annexed Austria. Meitner’s Austrian citizenship had previously offered some protection, but Nazi anti-Jewish rules now placed her position and safety in immediate danger. Leaving became difficult because her passport and permission to travel were contested. In July, colleagues helped her cross into the Netherlands. She subsequently reached Scandinavia and accepted a position in Stockholm. Exile separated her from the Berlin laboratory, although she continued discussing uranium research with Hahn by correspondence.
In Berlin, Hahn and the analytical chemist Fritz Strassmann continued bombarding uranium with neutrons and examining the products. At first they tried to interpret certain products as radium, an element relatively close to uranium in the periodic table. Repeated chemical tests instead indicated barium, a much lighter element. Hahn wrote to Meitner about this puzzling result in December 1938. The chemical identification was convincing, but ordinary forms of radioactive decay did not explain such a drastic change.
A nucleus imagined as a liquid drop
Meitner discussed Hahn’s letter with her nephew Otto Robert Frisch, a physicist then working in Copenhagen. Frisch joined her in Sweden over the Christmas holiday. They considered a model in which an atomic nucleus resembled a liquid drop. A large drop could become elongated and divide into two smaller drops. Applied to uranium, this suggested that neutron bombardment had produced a genuine division of the nucleus, rather than merely knocking away a small particle.
The two physicists also needed to explain the energy released. After division, the positively charged fragments would repel one another and move apart rapidly. Meitner used known relationships between nuclear masses to estimate that the combined fragments could have less mass than the original nucleus. That difference could appear as energy. Their estimate was about 200 million electron volts per division, far beyond the energy scale of ordinary chemical reactions. The calculation made the proposed mechanism physically plausible.
Naming a new nuclear process
Frisch returned to Copenhagen and tested the interpretation by looking for the energetic recoil of the fragments. His January 1939 experiment produced the expected signals. He adopted the word fission after discussing the biological term for cell division with William Arnold. Meitner and Frisch submitted a short joint explanation to Nature, followed by Frisch’s experimental report. The papers appeared in February 1939, rapidly changing the scientific community’s understanding of what uranium experiments had revealed.
The discovery therefore involved several connected achievements: Hahn and Strassmann’s chemical evidence, Meitner and Frisch’s interpretation, and further experimental confirmation. Separating those contributions helps explain why the history of fission cannot be reduced to a single dramatic moment. Meitner’s work also makes exile central to the story. A scientist removed from her established laboratory nevertheless remained able to evaluate its results and develop the physical explanation that made sense of them.