Manhattan Project & Atomic Age

In 1938, German chemists Otto Hahn and Fritz Strassmann split the uranium atom, releasing enormous energy in the process. ^[1] This is your VocaCast briefing on the Manhattan Project and the Atomic Age for Tuesday, April 21.

The physics that made the atomic bomb possible did not emerge suddenly. Years earlier, in 1934, scientists working at Enrico Fermi's Radium Institute in Rome discovered that bombarding uranium nuclei with neutrons appeared to create a new, heavier element. ^[1] That experiment planted the seed. But it took the work of Hahn and Strassmann — their careful chemistry showing that the uranium nucleus could actually split — to reveal what was truly happening at the atomic level. The splitting released enormous amounts of energy, just as Albert Einstein's special theory of relativity had established through the equivalence of mass and energy. ^[1] The theoretical framework already existed; now the experimental proof was undeniable.

The implications landed like an alarm bell across the physics community. Leo Szilard grasped the danger immediately and conceived the possibility of self-sustaining nuclear fission reactions — a chain reaction that could release devastating amounts of energy in succession. ^[2] Once you split one nucleus and release neutrons, those neutrons could split other nuclei, and the process could spiral exponentially.

Following the discovery of nuclear fission in uranium, two physicists at the University of Birmingham made a calculation that changed everything. ^[3] In March 1940, Otto Frisch and Rudolf Peierls, two Jewish emigré physicists, calculated that the critical mass of pure uranium-235 needed to trigger a chain reaction was remarkably small — as little as 1 to 10 kilograms. ^{[3] [4]} That same month, these two men composed a memorandum on the technical feasibility of an atomic weapon. ^[4] What had been theoretical was becoming alarmingly practical. The political dimension crystallized just as the science did. In 1939, Leo Szilard and Eugene Wigner persuaded Albert Einstein to send a letter to US President Franklin D.

Roosevelt warning him of the danger and advising him to establish an American nuclear research program. ^[5] The Einstein-Szilard letter was drafted in August 1939, warning specifically of Nazi Germany's atomic potential. ^[6] Britain moved first, initiating the world's first research project to design an atomic bomb in 1941. ^[3] But it was the United States that would mount the largest effort. The Manhattan Project was a secret US military engineering project undertaken during World War II to develop the world's first atomic bomb, lasting from 1942 to 1946. ^[7] General Leslie Groves was the man in charge of the Manhattan Project, overseeing the project throughout its duration. ^[8] The ambition was staggering in scale.

The project was conducted primarily at three top-secret towns, each with a precise function in the pipeline toward a working weapon. ^[9] Oak Ridge, Tennessee, handled uranium enrichment. Hanford, Washington, was where reactors transformed uranium into plutonium, an important nuclear fuel for the bombs. ^[9] And Los Alamos, New Mexico, was where bomb design and assembly took place. ^[9] At Los Alamos, J. Robert Oppenheimer, conducted the bulk of the remaining research and construction of the atomic bombs. ^[10] The pressure was immense.

Scientists knew that Germany was also pursuing atomic research, and the race to weaponize nuclear fission first felt existential. By mid-1945, the project reached its moment of truth.

The first nuclear device, nicknamed the Gadget, was successfully detonated in the New Mexico desert on July 16, 1945, during the Trinity Test. ^[11] The Trinity Test, conducted at the Alamogordo Bombing and Gunnery Range, was the world's first detonation of a nuclear weapon. ^[11] Witnesses reported a blinding flash, heat that could be felt miles away, and a mushroom cloud that rose into the atmosphere. The weapon worked. Less than three weeks later, the United States used its newly developed capability in combat. The atomic bombing of Hiroshima occurred on August 6, 1945, destroying much of the city and killing tens of thousands. ^[12] Three days later, on August 9, 1945, the atomic bombing of Nagasaki followed.

The use of the atomic bombs on Hiroshima and Nagasaki helped bring an end to the war in the Pacific and marked the beginning of the Atomic Age. ^{[12] [13]} The same physics that had captivated researchers in university labs now shaped the geopolitical landscape, and nations would spend decades grappling with the implications of weapons they could neither uninvent nor fully control.