9 Astonishing Facts About the Manhattan Project

During World War II, the brightest minds in America were gathered under the secretive title 'the Manhattan Project' to develop the most deadly weapon ever seen—an atomic bomb. Their complex yet captivating leader was J. Robert Oppenheimer, a pale-eyed, lean theoretical physicist who wore a porkpie hat and had a passion for Sanskrit.

The improbable tale of the atomic bomb and its conflicted mastermind will be portrayed on the big screen this July in Christopher Nolan’s film Oppenheimer. Refresh your knowledge of the Manhattan Project with these nine insights into the complex journey to the nuclear era.

1. A group of German scientists uncovered nuclear fission, making atomic weapons a very real possibility.

In 1938, physicists Otto Hahn and Fritz Strassmann discovered that when a neutron with sufficient energy strikes a 'fissile' atom, the atom fractures into pieces, releasing massive amounts of energy and more neutrons.

The energy released from a single split atom is not enough to cause an explosion. However, if a scientist managed to gather a large number of fissile atoms closely together and split one, the resulting neutrons would go on to split more fissile atoms. This chain reaction would rapidly escalate, leading to a tremendous energy surge, culminating in an explosion.

The timing was catastrophic for such a deadly potential. While many European physicists fled the Nazi regime, some German scientists had given Hitler a significant head start on developing a possible bomb.

The U.S. government recognized the urgent need to develop a nuclear weapon in response. The Army launched what would be known as the 'Manhattan Project' (named after its initial offices in Manhattan), setting up factories to produce raw materials and enlisting the country’s brightest physicists to relocate to a planned laboratory in New Mexico dubbed 'Project Y.' Major General Leslie Groves, tasked with assembling the scientific team, selected Oppenheimer to lead the lab in October 1942.

2. J. Robert Oppenheimer wasn’t the most obvious choice to lead the project.

Oppenheimer was a professor at the University of California, Berkeley, famous for his trademark porkpie hat and his constant chain-smoking. Despite his brilliance as a theorist, 'Oppie,' as he was affectionately called, had little to no experience in a laboratory setting. Though well-respected, some questioned his ability to manage the strong personalities on the project, which included as many as 20 current or future Nobel Prize winners.

Moreover, he didn’t exactly appear to be the right person to lead such a massive military operation. Due to his slight build and persistent cough (a result of smoking four packs of cigarettes a day), an army physical declared Oppenheimer 'permanently physically incapacitated' for service. He had to sign a waiver to begin his role, according to American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer by Kai Bird and Martin J. Sherwin (the basis for Nolan’s film).

The military was also wary of Oppenheimer’s left-wing activism: he had close ties to several Communist Party members. While the Soviet Union was an ally of America, the FBI labeled domestic communists as 'dangerous subversives.' Nevertheless, Groves remained adamant that Oppenheimer was the right man for the task.

3. The laboratory in Los Alamos, New Mexico, resembled a summer camp for physicists.

The small town in New Mexico was located near Oppenheimer’s summer ranch and offered enough seclusion for the Army to keep its operations under wraps. They transformed the desolate desert mesa into a sprawling laboratory, complete with housing for families and dorms for single workers.

A culture of working hard and playing hard took root. The typical six-day workweek often ended with a party, where Oppenheimer’s famed martinis were always served (reportedly stronger due to the site’s high altitude). The bachelors’ dorm preferred a more potent mixture: raw laboratory alcohol combined with pineapple juice and served over dry ice, according to Bird and Sherwin. At one point, 22-year-old future Nobel laureate Richard Feynman amused himself by cracking the combinations to locked filing cabinets throughout the site. The next morning, bewildered guards found the cabinets open but everything intact.

4. The main obstacle of the Manhattan Project was acquiring enough fissile atoms and bringing them close enough together to trigger a chain reaction.

The process of producing the 14 pounds of plutonium necessary for an atomic bomb required a massive industrial effort. In fact, the Nazi regime largely abandoned its bomb project because the task of generating that much fissile material seemed unattainable. The U.S. Army built several highly classified factories to produce uranium and convert it into plutonium.

Even after obtaining their baseball-sized piece of plutonium, the physicists at Los Alamos faced the problem of compressing the material to half its size in order to trigger the chain reaction. They surrounded it with precisely arranged TNT pieces that would detonate in perfect synchronization to force the plutonium inward.

5. The U.S. Army was obsessed with keeping the project a secret—even from their Soviet allies.

Oppenheimer and the other physicists, many of whom had connections to the Communist Party or labor unions, were kept under constant surveillance.

The government’s concerns turned out to be well-founded: before heading to Los Alamos, Oppenheimer was approached by a friend to pass on technical intelligence to Soviet agents via a backchannel. Though he refused, his failure to immediately report the conversation would later become a major scandal.

Despite all the security precautions, several members of the Manhattan Project ended up spying for the Soviet Union, including two high-ranking physicists at Los Alamos, Theodore Hall and Klaus Fuchs.

6. Physicists working on the Manhattan Project were deeply concerned about how their invention would impact global power dynamics.

The Manhattan Project had been presented to scientists as the U.S.'s only way to defeat a nuclear-capable Nazi Germany. By 1944, however, it was clear that Germany had no atomic bomb, and the Allies could defeat them without the need for nuclear weapons.

During a dinner that year, Groves surprised several physicists when he declared that the true target of the program was the Soviets. Many physicists believed that the Soviet Union had made tremendous sacrifices to prevent Hitler's victory. Additionally, they feared that excluding Russian scientists from the program would lead to a postwar arms race. Theodore Hall later admitted that this fear prompted him to spy for the Soviets.

7. The scientists objected when they learned that their government might use the atomic bomb against Japan.

Their argument was that the U.S. should use the atomic bomb as a bargaining chip to end the war with Japan, which had no nuclear program. They hoped that revealing the existence of the bomb would make its actual use unnecessary. However, their efforts, which included a petition in July 1945 signed by 70 scientists, never reached President Harry S. Truman.

The petition was delivered to Secretary of State James F. Byrnes, who chose not to present it to Truman. Byrnes also sat on the Interim Committee with Oppenheimer and other military and scientific leaders, appointed by the Secretary of War to advise the president on the bomb's use. When the committee recommended deploying the bomb immediately, Oppenheimer notably failed to communicate the strong opposition felt by many Manhattan Project scientists. At the time, Oppenheimer believed the physicists' job was to build the bomb, leaving the decision of its use to the government.

The scientists' concerns came too late. After years of intense, fast-paced work, they had no time for reflection. Now, with the bomb nearly complete, their voices were disregarded.

8. The Trinity test demonstrated the bomb's immense destructive power.

Many physicists believed they had successfully persuaded the government to inform the Soviets about the bomb and to use it merely as a bargaining tool against Japan.

On July 16, 1945, the military conducted the first successful atomic bomb detonation in the New Mexico desert—the beginning of nuclear weaponry history. The Trinity test deeply reinforced the apocalyptic reality of their creation. Oppenheimer famously recalled a Hindu verse: “Now I am become death, the destroyer of worlds.”

When atomic bombs were dropped on Hiroshima and Nagasaki just weeks later, the news sparked a mixture of triumph and sorrow among those at Los Alamos. Some, including Oppenheimer, grew increasingly troubled by the devastation their work had caused. Upon meeting Truman for the first time, Oppenheimer remarked, “Mr. President, I feel I have blood on my hands.”

9. After the U.S. dropped the atomic bombs, Oppenheimer became an advocate for nuclear disarmament.

The renowned “father of the atomic bomb” sought to prevent a postwar arms race by advocating for international control of atomic weapons and opposing the development of the hydrogen bomb.

The H-bomb utilizes the energy from fission reactions to trigger a nuclear fusion reaction, similar to the process that powers the sun, resulting in an explosion thousands of times more powerful than the atomic bombs dropped on Japan. Despite Oppenheimer’s objections, the bomb was developed and successfully detonated in 1952.

In 1954, the Atomic Energy Commission revoked Oppenheimer’s security clearance following a hearing about his left-wing affiliations prior to the war, which was orchestrated by his political adversaries due to his opposition to nuclear weapon proliferation.

Although the two atomic bombs dropped on Hiroshima and Nagasaki remain the only nuclear weapons ever used in warfare, the U.S. and five other nations conducted over 2,000 nuclear tests after 1945. In 1996, more than 70 countries signed the Comprehensive Nuclear-Test-Ban Treaty, which banned nuclear testing.