Who created the first nuclear weapon

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Leo Szilard was waiting to cross the road near Russell Square in London when the idea came to him. It was 12 September 1933. A little under 12 years later, the US dropped an atom bomb on Hiroshima, killing an estimated 135,000 people.

The path from Szilard’s idea to its deadly realisation is one of the most remarkable chapters in the history of science and technology. It features an extraordinary cast of characters, many of them refugees from Fascism who were morally opposed to the bomb but driven by the dreadful prospect of Nazi Germany getting there first.

Szilard himself was a Hungarian-born Jew who had fled Germany for the UK two months after Adolf Hitler became chancellor. He arrived in a country that was then at the forefront of nuclear physics. James Chadwick had just discovered the neutron and Cambridge physicists soon “split the atom”. They broke a lithium nucleus in two by bombarding it with protons, verifying Albert Einstein’s insight that mass and energy were one and the same, as expressed by the equation E = mc2.

Szilard’s eureka moment was based on this groundbreaking experiment. He reasoned that if you could find an atom that was split by neutrons and in the process emitted two or more neutrons, then a mass of this element would emit vast amounts of energy in a self-sustaining chain reaction.

Szilard pursued the idea with little success. It wasn’t until 1938 that the breakthrough came – ironically in the Nazi capital Berlin, where German physicists Otto Hahn and Fritz Strassman bombarded uranium atoms with neutrons. When they analysed the debris they were stunned to find traces of the much lighter element barium.

As luck would have it, Hahn and Strassman were opponents of the regime. Hahn wrote to the Austrian chemist Lise Meitner, who had worked with him in Berlin until she fled to Sweden after the Nazis occupied Vienna in 1938. Meitner wrote back explaining that the uranium nucleus was splitting into two roughly equal parts. She called the process “fission”.

The next piece of the puzzle came when Italian physicist Enrico Fermi, who had fled Fascism and was working at Columbia University in New York, discovered that uranium fission released the secondary neutrons that were needed to make the chain reaction happen. Szilard soon joined Fermi in New York.

Together they calculated that a kilogram of uranium would generate about as much energy as 20,000 tonnes of TNT. Szilard already saw the prospect of nuclear war. “There was very little doubt in my mind that the world was headed for grief,” he later recalled.

Others did have doubts, however. In 1939 the Danish physicist Niels Bohr – who was actively helping German scientists escape via Copenhagen – poured cold water on the idea. He pointed out that uranium-238, the isotope which makes up 99.3 per cent of natural uranium, would not emit secondary neutrons. Only a very rare isotope of uranium, uranium-235, would split in this way.

However, Szilard remained convinced that the chain reaction was possible, and feared that the Nazis knew it too. He consulted fellow Hungarian émigrés Eugene Wigner and Edward Teller. They agreed that Einstein would be the best person to alert President Roosevelt to the danger. Einstein’s famous letter was sent soon after the outbreak of war in Europe, but had little impact.

Things changed dramatically in 1940, when news filtered through that two German physicists working in the UK had proved Bohr wrong. Rudolf Peierls and Otto Frisch had worked out how to produce uranium-235 in large quantities, how it could be used to produce a bomb, and what the appalling consequences of dropping it would be. Peierls and Frisch – who Bohr had helped escape – were also horrified at the prospect of a Nazi bomb, and in March they wrote to the British government urging prompt action. Their “Memorandum on the Properties of a Radioactive ‘Super-Bomb‘” was more successful than Einstein’s letter to Roosevelt. It led to the initiation of the British bomb project, codenamed Tube Alloys.

The letter also galvanised the US into action. In April 1940 the government appointed the veteran physicist Arthur Compton to head a nuclear weapons programme, which eventually became the Manhattan Project. One of his first moves was to bring together various chain reaction research groups under one roof in Chicago. That summer the team began a series of experiments to make the chain reaction happen.

The bombing of Pearl Harbor in December 1941 added further impetus. A year later the Manhattan Project team was ready to attempt a chain reaction in a pile of uranium and graphite they had assembled in a squash court underneath a stand of the University of Chicago’s football field. On Wednesday, 2 December 1942, they did it.

Celebrations were muted. Once the reaction was confirmed, Szilard shook hands with Fermi and said: “This will go down as a black day in the history of mankind.”

Over the next four years the US, UK and Canada poured vast resources into the Manhattan Project. Tube Alloys continued for a while but was eventually absorbed into the US project. The Nazis initiated a nuclear weapons programme but made little progress.

On 16 July 1945 the US detonated the world’s first nuclear bomb in the New Mexico desert. The test was final, terrible proof that nuclear energy could be weaponised, and prompted Robert Oppenheimer to recall a passage from the Hindu scripture, Bhagavad Gita: “I am become death, the destroyer of worlds.”

The attacks on Japan started a worldwide arms race. Following 1945, the US developed massively destructive hydrogen bombs, which exploited nuclear fusion rather than fission. The Soviets developed and tested their own bomb in 1949. The world’s nuclear arsenal now stands at about 27,000 bombs.

J. Robert Oppenheimer (1904-1967) was an American theoretical physicist. During the Manhattan Project, Oppenheimer was director of the Los Alamos Laboratory and responsible for the research and design of an atomic bomb. He is often known as the “father of the atomic bomb."

By the time the Manhattan Project was launched in the fall of 1942, Oppenheimer was already considered an exceptional theoretical physicist and had become deeply involved in exploring the possibility of an atomic bomb. Throughout the previous year he had been doing research on fast neutrons, calculating how much material might be needed for a bomb and how efficient it might be.

Although Oppenheimer had little managerial experience and some troublesome past associations with Communist causes, General Leslie Groves recognized his exceptional scientific brilliance. Less than three years after Groves selected Oppenheimer to direct weapons development, the United States dropped two atomic bombs on Japan. As director of the Los Alamos Laboratory, Oppenheimer proved to be an extraordinary choice.

Oppenheimer was married to a botanist, Kitty. They had two children, Peter and Toni.

Early Life

Oppenheimer was born on April 22, 1904. Oppenheimer’s family was part of the Ethical Culture Society, an outgrowth of American Reform Judaism founded and led at the time by Dr. Felix Adler. The progressive society placed an emphasis on social justice, civic responsibility, and secular humanism. Dr. Adler also founded the Ethical Culture School, where Oppenheimer enrolled in September 1911. His academic prowess was apparent very early on, and by the age of 10, Oppenheimer was studying minerals, physics, and chemistry. His correspondence with the New York Mineralogical Club was so advanced that the Society invited him to deliver a lecture—not realizing that Robert was a twelve-year-old boy.

He graduated as valedictorian of his high school class in 1921, but fell ill with a near-fatal case of dysentery and was forced to postpone enrolling at Harvard. After being bedridden for months, his parents arranged for him to spend the summer of 1922 in New Mexico, a haven for health-seekers.

Robert stayed at a dude ranch 25 miles northeast of Santa Fe with high school teacher Herbert Smith as a companion and mentor. From there, he took five- or six-day horseback trips in the wilderness. This experience restored Oppenheimer’s health and instilled a deep love for the desert high country.

Oppenheimer enrolled at Harvard in September 1922. He graduated in three years, excelling in a wide variety of subjects. Although he majored in chemistry, Oppenheimer eventually realized his true passion was the study of physics.

In 1925, Oppenheimer began his graduate work in physics at Cavendish Laboratory in Cambridge, England. J. J. Thomson, who had been awarded the 1906 Nobel Prize in Physics for detecting the electron, agreed to take on Oppenheimer as a student. At Cavendish, Oppenheimer realized that his talent was for theoretical, not experimental, physics, and he accepted an invitation from Max Born, director of the Institute of Theoretical Physics at the University of Göttingen, to study with him in Germany.

Oppenheimer had the good fortune to be in Europe during a pivotal time in the world of physics, as European physicists were then developing the groundbreaking theory of quantum mechanics. Oppenheimer received his doctorate in 1927 and accepted professorships at the University of California, Berkeley, and the California Institute of Technology. At Berkeley, he became good friends with Ernest Lawrence, one of the world’s top experimental physicists and the inventor of the cyclotron. Lawrence named his second son after Robert.

Later Years

After the war Oppenheimer became an advisor of the Atomic Energy Commission, lobbying for international arms control. Beginning in 1947, Oppenheimer directed the Institute for Advanced Study in Princeton, New Jersey, where he convened great scientists. “What we don’t understand, we explain to each other.”

His security clearance was revoked in 1954 in a hearing during the Second Red Scare. Oppenheimer's old Communist sympathies were dredged up and his clearance was revoked a mere 32 hours before it was set to expire. Oppenheimer had made political enemies by arguing against the development of the hydrogen bomb, and revoking his clearance stripped him of political power. The scientific community was outraged at the treatment of Oppenheimer, and reviled Edward Teller, who testified against him at the hearing. For more information, please see Oppenheimer Security Hearing.