By 1939, the key idea had changed. Scientists were no longer asking only whether uranium could undergo an extraordinary nuclear transformation. They were asking whether one event could release neutrons that triggered more events, creating a chain reaction.
This seventh segment begins with an inherited world in which fission has been recognised and Europe is moving into war. That combination gives nuclear research a new urgency.
August 1939: a possibility reaches government
On August 2, 1939, Albert Einstein signed the famous letter prepared with the help of Leo Szilard warning President Franklin D. Roosevelt that recent work suggested a nuclear chain reaction in uranium might release vast amounts of energy and might make extremely powerful bombs possible. The letter did not deliver a weapon design. It delivered a change in perceived possibility.
One month later, Germany invaded Poland. The scientific question became entangled with a geopolitical one: if such a weapon were physically possible, which state might build it first?
From calculation to experimental programme
Research intensified on neutron multiplication, uranium isotopes, moderators and methods for producing a self-sustaining reaction. The difficulty was severe. Natural uranium is mostly uranium-238, while the rarer uranium-235 behaves differently under neutron bombardment. Neutrons can also be absorbed or lost before they sustain a chain.
At the same time, plutonium emerged as another possible fissile material. The problem was no longer a single scientific discovery. It became a network of questions about nuclear constants, materials purity, geometry, isotope separation, reactor design and chemical separation.
The Manhattan Project begins to take shape
By 1942, the American programme was moving from fragmented research into a concentrated wartime engineering effort. The U.S. Army Corps of Engineers created the Manhattan Engineer District. The scale and tempo changed sharply: sites had to be selected, industrial partners recruited, money committed and multiple technical routes pursued at once.
The project could not wait for perfect certainty. It had to make decisions while fundamental questions were still being answered.
December 2, 1942: the world answers in Chicago
Beneath the west stands of the University of Chicago’s Stagg Field, Enrico Fermi’s team assembled Chicago Pile-1 from graphite and uranium. On December 2, 1942, the pile achieved the first controlled, self-sustaining nuclear chain reaction.
The Department of Energy’s historical account records the moment at 3:25 p.m. Chicago time, when physicist George Weil withdrew the final control rod under Fermi’s direction and the reaction became self-sustaining.
Before Chicago Pile-1, a controlled chain reaction was a theoretical and experimental target. After it, it was an engineered fact.
What became possible next
CP-1 did not produce a bomb. Its importance was that it removed one enormous uncertainty. A neutron chain reaction could be deliberately assembled, controlled and sustained. That result provided a foundation for larger reactors and plutonium production.
The problem now changed scale. The next question was not simply whether nuclear physics worked. It was whether reactors, enrichment systems, chemical plants and entire new industrial communities could be built quickly enough to turn that physics into material.
Previous in the tube: Part 6: 1938–1939 — Fission Changes the Future.
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Continue to Part 8: 1942–1944 — Nuclear Physics Becomes Industry.