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Letter to the president -

Albert Einstein sent letters to President Franklin D. Roosevelt making him aware of the possibility of nuclear chain reactions, which could lead to the atomic bombs. He also warned about how Germany might work on a nuclear bomb project.

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The Nazi state hindered German progress -

And while Germany had been the first country to discover nuclear fission and begin research in 1939, the research lacked state support. This was due to the discrimination against Jews, which made many prominent scientists leave the country.

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Response to the letter -

President Roosevelt took Einstein's warning seriously, and soon convened a research committee, named the Advisory Committee on Uranium. It was due to his letters that the work on the nuclear weapons started in the US.

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Unrewarded involvement -

But while Einstein did warn the US about possible nuclear weapons, he was not actually part of the Manhattan Project. This is because he didn't get the security clearance required from the US Army Intelligence office. On top of that, they didn't allow scientists working on the project to consult with him.

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Coming to America -

The Manhattan Project saw the mass emigration of scientists from Germany and Eastern Europe to the US. Some of the scientists included Edward Teller (pictured), James Planck, and Niels Bohr.

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The UK and Canada supported the project -

The project was as led by the US with the support of the UK and Canada. But despite promises to share the research, the US didn't provide full details of the Manhattan Project to supporting nations.

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It became a big project -

The Manhattan Project grew big, and employed more than 130,000 people. It cost almost US$22 billion in today's money.

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The total cost of the project -

Building plants and producing the fissionable materials took more than 90% of the total expenditure. Less than 10% was for the development and production of the weapons.

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The research center -

Set up in 1943, the Los Alamos Laboratory was the project’s most significant research center. It was led by American research director J. Robert Oppenheimer.

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The first nuclear detonation test -

The first detonation of a nuclear weapon took place on July 16, 1945. Code named the Trinity Test, it took place in the Jornada del Muerto desert in New Mexico.

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The Trinity Test -

The name was inspired by the poetry of John Donne, whom Oppenheimer was reading at the time. The 25-kiloton blast created a 40,000-ft (12-km) high mushroom cloud.

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Top secret information -

The project was top secret because the US was worried enemy nations would learn about nuclear technology and weapons.

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Punishment at the cost of secrecy -

There were strict warnings about disclosing the secrets of the project. Punishments included prison sentences of up to 10 years or a US$10,000 fine.

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Women at work -

While the project had mostly male employees, there were still some female scientists involved, notably Leona Woods (pictured) and Maria Goeppert-Mayer. Also, hundreds of women were employed as secretaries and tasked with doing massive calculations by hand.

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Safety measures for employees -

To ensure the safety of employees at the lab sites, detectors were used to check the amount of radiation. Nose swabs were also required to detect the inhalation of plutonium dust.

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Security check -

Because the workforce employed was so huge, a special intelligence corps was set up to handle the security issues. Just like screening tests, lie detection tests also became normal practice.

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Spies all around -

Although they were allies during the war, the US didn't trust the Soviets. However, it was revealed after the war that some of the top scientists, including George Koval and Klaus Fuchs (pictured), were spying for the Soviets.

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Illegal activities -

It was later revealed that machinist David Greenglass was passing secrets on to a Soviet contact, Alexander Feklisov. Greenglass was Ethel Rosenberg’s brother, and he testified against her and her husband Julius. This led to their conviction and execution for committing espionage.

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Unaware of the details -

But even with many employees, not more than a few dozen people knew the full extent of the Manhattan Project. Only about 1,000 of them were aware that it involved atoms.

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The original name -

"Development of Substitute Materials" was the original name given to the project. However, they realized early on that it would be evocative of its real purpose.

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Why Manhattan? -

The project was named Manhattan because that's the home of Columbia University. This is where much of the theoretical research took place.

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Construction and testing sites -

The project took less than four years for its completion. The construction sites included Oak Ridge, Tennessee, Hanford, and Washington. In addition, the testing sites were in New Mexico.

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Atomic bombs -

Atomic bombs rely on the creation of a chain reaction that releases immense thermal energy. This happens when a neutron strikes the nucleus of an atom of the isotopes uranium-235 or plutonium-239 and splits the atom.

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Little Boy -

Little Boy was the code name for the bomb made of uranium–235. This was the type of bomb that exploded in Hiroshima, Japan, claiming 265,000 lives.

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Fat Man -

Fat Man was the second bomb developed by the project. It was even more powerful than Little Boy. Detonated over Nagasaki, the bomb destroyed 44% of the Japanese city.

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The Gadget -

The Gadget was a test version of the implosion system used in the Fat Man. This bomb was more robust than Little Boy and made from plutonium.

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Tickling the dragon’s tail -

"Tickling the dragon’s tail" was the experiment performed in which two scientists, Harry Daghlian and Louis Slotin, died because of acute radiation. Slotin is pictured wearing sunglasses, and Daghlian is seated in the back.

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The demon core -

After the deaths, workers began referring to the plutonium mass as "the demon core." Manual manipulation became forbidden. The core was later used to create the bomb tested at Bikini Atoll.

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The Manhattan Project today -

The Manhattan Project remains to this day a controversial subject. However, in 2015, the US Congress opened the Manhattan Project National Historical Park, made up of three units at Oak Ridge, Hanford, and Los Alamos. 

Sources: (History) (History Hit) 

See more: Movies with a nuclear storyline

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