Advances in scientific understanding often do not build directly or smoothly in response to the data that are amassed, and in retrospect, after a major revision of theory, it may seem strange that a crucial hypothesis was long overlooked. A case in point is the discovery of a means by which the nuclei of atoms can be split. Between 1934, when a group of Italian physicists including Enrico Fermi first bombarded uranium with neutrons, and 1939, when exiled Austrian physicist Lise Meitner provided the crucial theoretical connection, scientists compiled increasing evidence that nuclear fission had been achieved, without, however, recognizing what they were witnessing.

Earlier, even before the neutron and proton composition of atomic nuclei had been experimentally demonstrated, some theoretical physicists had produced calculations indicating that in principle it should be possible to break atoms apart. But the neutron-bombardment experiments were not aimed at achieving such a result, and researchers were not even receptive to the possibility that it might happen in that context. A common view was that a neutron's breaking apart a uranium nucleus would be analogous to a pebble, thrown through a window, causing a house to collapse.

In Berlin, Meitner pursued research related to that of the Italians, discovering a puzzling group of radioactive substances produced by neutron bombardment of uranium. Fermi and others achieved numerous similar results. These products remained unidentified partly because precise chemical analyses were hampered by the minute quantities of the substances produced and the dangers of working with highly radioactive materials, but more significantly because of the expectation that they would all be elements close to uranium in nuclear composition. In 1938 Meitner escaped from Nazi Germany and undertook related research in Sweden, but her research partner Otto Hahn kept her informed of his continuing experimentation. Late in that year he wrote to her of a surprising result: one of the substances resulting from the neutron bombardment of uranium had been conclusively identified as barium, an element whose structure would have made it impossible to produce through any mechanism he envisaged as being involved in the experiments. Hahn even remarked that, despite the clear chemical evidence of what had occurred, it went "against all previous experiences of nuclear physics," but he also noted that together the number of protons and neutrons in the nuclei of barium and technetium, the accompanying product of the experiment, added up to the number of such particles that compose a uranium nucleus.

It was Meitner who finally recognized the significance of the data in relation to underlying theoretical considerations: the researchers had actually been splitting uranium atoms. Coining the term "nuclear fission," she quickly submitted her conclusion for publication in a paper coauthored with physicist Otto Frisch. When scientists in Europe and North America rushed to corroborate the findings, it became clear that the relevant evidence had been present for some time, lacking mainly the right conceptual link.