Oppenheimer was appointed scientific director of the Manhattan Project in 1942, a choice that surprised those who knew him primarily as a theoretical physicist with a reputation for brilliance and difficulty. He proved unexpectedly gifted at the human management of a large, fractious, and brilliant scientific community under wartime pressure. His intellectual range — he could follow arguments in every division, from theoretical physics to chemistry to ordnance engineering — was essential to his authority. At the Trinity test on 16 July 1945, watching the first nuclear detonation from the control bunker, he quoted a line from the Bhagavad Gita: “Now I am become Death, the destroyer of worlds.” He subsequently became the most prominent scientific voice against nuclear proliferation, which led to the security hearing that ended his public career.

Feynman arrived at Los Alamos in 1943 as a twenty-four-year-old graduate student and made himself conspicuous by picking the locks on classified filing cabinets to demonstrate the security weaknesses of the facility — and by writing teasing notes inside. His formal contribution was to the theoretical calculations for implosion design, but his most lasting impact on Los Alamos was probably his practice of doing physics as if it were play. His wife Arline was dying of tuberculosis in a sanatorium in Albuquerque; he visited her every weekend. She died three weeks before Trinity.

Fermi achieved the first controlled self-sustaining nuclear chain reaction on 2 December 1942, beneath the stands of Stagg Field at the University of Chicago. The pile consisted of uranium and graphite blocks stacked by hand in a squash court. When the cadmium control rod was withdrawn and the reaction became self-sustaining, his colleague George Weil stood ready to drop it back in. Fermi watched the instruments, confirmed the calculations, and said: “The pile has gone critical.” A bottle of Chianti was opened. The telegram sent to Washington read: “The Italian navigator has just landed in the New World.”

Bohr escaped from Nazi-occupied Denmark in September 1943 in a fishing boat to Sweden, then flew to Britain in the bomb bay of a Mosquito aircraft, nearly dying when the pilot forgot to tell him to put on his oxygen mask. He arrived at Los Alamos in December 1943 under the alias Nicholas Baker, a cover that fooled no one who recognised his distinctive profile. He contributed to the theoretical work but was primarily concerned with the political consequences of the bomb, pressing Oppenheimer, Churchill, and Roosevelt — without success — to share nuclear information with the Soviet Union before the war ended, to prevent the arms race he foresaw with clarity.

Von Neumann’s contribution to the Manhattan Project was the mathematics of implosion: the precisely shaped explosive lenses required to compress a subcritical mass of plutonium to supercriticality fast enough to produce a nuclear explosion before the assembly blew itself apart. The problem required solving the hydrodynamic equations for the compression wave at speeds no one had previously attempted. He did much of the calculation by hand. He also contributed to the target selection committee and argued for dropping the bomb without prior warning on a city rather than on a military target or a demonstration site. He was not troubled by this position.

Heisenberg led the German nuclear weapons programme during the war — the programme that did not succeed. The question of whether he failed because he could not solve the critical mass calculation, or because he chose not to, has never been definitively resolved. The Farm Hall transcripts, recordings made covertly by British intelligence after the German physicists were interned in England following the German surrender, show Heisenberg learning of Hiroshima and immediately calculating — correctly — that a critical mass would require several kilograms, not tons. This suggests he had not previously made this calculation. He spent the rest of his life implying that he had, and that he had deliberately misled the Nazi regime.

Einstein did not work on the Manhattan Project — his pacifist politics and suspected security risk disqualified him in the eyes of military intelligence. His connection to the bomb is nonetheless definitive: his 1939 letter to Roosevelt, co-authored with Leo Szilárd, warned of the possibility of a German nuclear weapon and urged American research into uranium fission. This letter is generally credited as the proximate cause of the programme that became the Manhattan Project. Einstein later called signing it the greatest mistake of his life, though he also said he would sign it again under the same circumstances, since the possibility of a German bomb was real.

Meitner co-discovered nuclear fission with Otto Hahn in 1938, providing the theoretical explanation for the experimental results that Hahn had obtained but could not explain. She did not share the 1944 Nobel Prize in Chemistry awarded to Hahn for this work — an omission that remains one of the most discussed in Nobel Prize history. When approached to join the Manhattan Project, she refused, saying she had no desire to make a bomb. She spent the war in Sweden, following events at a distance, and was horrified by Hiroshima.

Rutherford discovered the nuclear atom in 1911, established the distinction between alpha, beta, and gamma radiation, and was the first to artificially transmute one element into another. He dismissed the possibility of useful nuclear power in 1933 — two years before his own student Leo Szilárd filed the patent for the nuclear chain reaction. He died in 1937, before any of his students’ most consequential work was done, but the Manhattan Project physicists were overwhelmingly the products of his laboratory and his approach to nuclear physics.

Chadwick discovered the neutron in 1932 — the particle without which nuclear fission is impossible. Without the neutron, there is no chain reaction and no bomb. He led the British Mission to Los Alamos and was one of the few participants who fully understood both the physics and the strategic implications from the outset. His discovery, made a decade before the Manhattan Project, was the necessary precondition for everything that followed.

Szilárd conceived the nuclear chain reaction in 1933 standing at a traffic light in London, patented it, and assigned the patent to the British Admiralty to keep it secret. He then co-authored the letter with Einstein that prompted Roosevelt to fund the programme. He subsequently circulated the Franck Report arguing against use of the bomb on civilian targets without warning, organised a petition signed by seventy Manhattan Project scientists against its use, and spent the remainder of his life working for nuclear disarmament. He may be the person most responsible for the bomb existing, and among those most responsible for the argument against it.

Compton directed the Metallurgical Laboratory in Chicago, under whose squash court Fermi achieved the first controlled nuclear chain reaction on 2 December 1942. He oversaw the transition from the experimental pile to industrial plutonium production, and served on the Interim Committee that advised Truman on use of the bomb. He supported the decision to drop without prior warning. The coded message he sent to Washington after the Trinity test — “The Italian navigator has just landed in the new world” — was his.

Bethe led the theoretical division at Los Alamos — the largest concentration of theoretical physicists ever assembled for a single purpose. He calculated whether the bomb would work. It did. He subsequently argued against the hydrogen bomb, against nuclear testing, and against ballistic missile defence systems, and was among the most persistent scientific voices for arms control for five decades after the war. He lived to ninety-eight, long enough to see most of his positions vindicated.

“In some sort of crude sense which no vulgarity, no humour, no overstatement can quite extinguish, the physicists have known sin; and this is a knowledge which they cannot lose.” Oppenheimer said this in 1947. His security clearance was revoked in 1954 following a politically motivated hearing in which former colleagues testified against him. He spent his remaining years at the Institute for Advanced Study in Princeton, thinking and writing, never returning to government service. President Kennedy awarded him the Enrico Fermi Award in 1963; it was presented by Lyndon Johnson, eight days after Kennedy’s assassination.

Bethe led the theoretical division at Los Alamos — the largest concentration of theoretical physicists ever assembled. He was responsible for the calculations that determined whether a fission weapon would actually work, and for the ongoing theoretical work that refined the design. His grasp of nuclear physics was encyclopaedic; Oppenheimer trusted him above all others with the central scientific questions. After the war he opposed the hydrogen bomb and became one of the most important scientific voices for nuclear arms control.

Compton directed the Metallurgical Laboratory in Chicago where Fermi built the first nuclear reactor. He was the administrative and scientific coordinator of the plutonium production programme, responsible for scaling Fermi’s laboratory chain reaction into the industrial reactors at Hanford that produced the plutonium for the Trinity test and Nagasaki bomb. He had won the Nobel Prize in Physics in 1927 for discovering what is now called Compton scattering — the change in wavelength of X-rays when scattered by electrons.

Szilárd conceived the nuclear chain reaction in 1933, standing at a London traffic light. He patented it, and assigned the patent to the British Admiralty to keep it secret. He co-authored the letter to Roosevelt — signed by Einstein — that initiated the American nuclear programme. He worked on the Chicago Pile but was excluded from Los Alamos. When the bomb was ready, he circulated a petition of sixty-eight scientists urging that it not be used on civilian targets without warning. The petition was suppressed. He spent the rest of his life working for arms control.

Chadwick discovered the neutron in 1932 — the particle without which a chain reaction is impossible, since it is the neutron that penetrates the nucleus without being deflected by its charge. Without the neutron, there is no bomb. He led the British Mission to Los Alamos, the team of British physicists who joined the project in 1943 under the Quebec Agreement. He had spent nearly four years interned in Germany during the First World War, where he built a primitive particle accelerator from toothpaste tubes and razor blades.

Groves was the Army Corps of Engineers general who built the Manhattan Project as an industrial and logistical enterprise. He had just completed the Pentagon — the largest office building in the world — when he was assigned to the project in 1942. He simultaneously built three complete secret cities (Oak Ridge, Hanford, Los Alamos), managed 130,000 employees, maintained total secrecy from Congress and from most of the Cabinet, and delivered the weapon on schedule. He and Oppenheimer disliked each other and worked together with extraordinary effectiveness. He later testified against Oppenheimer at the security hearing.

Brodie was the first strategic thinker to understand what nuclear weapons meant for military theory. Two months after Hiroshima, he wrote: “Thus far the chief purpose of a military establishment has been to win wars. From now on its chief purpose must be to avert them.” His 1946 collection The Absolute Weapon established the foundational concepts of nuclear deterrence before most military strategists had begun to think about the problem. He spent his career at RAND developing the theory of mutually assured destruction and critiquing its excesses. He was not at Los Alamos — he was the mind that thought about what Los Alamos had made.

Teller was present at Los Alamos but was already obsessed with fusion — the “Super,” as the hydrogen bomb was known — and contributed relatively little to the fission weapon that was actually built. After the war he campaigned for the hydrogen bomb over Oppenheimer’s opposition, and testified against Oppenheimer at the 1954 security hearing in terms that most physicists considered a betrayal. The Teller-Ulam design, worked out with Stanislaw Ulam in 1951, solved the staged compression problem that made thermonuclear weapons possible. Most of the physics community never forgave him for Oppenheimer.

Ulam was a Polish mathematician who arrived at Los Alamos via Harvard and the University of Wisconsin and became one of the project’s most original thinkers. He invented the Monte Carlo method — using random sampling to solve mathematical problems that cannot be solved analytically — while recovering from encephalitis in 1946. His insight that the key to thermonuclear ignition was radiation implosion rather than mechanical compression was the conceptual breakthrough that made the hydrogen bomb feasible. Teller provided the physics; Ulam provided the geometry.