J. Robert Oppenheimer is known as the “father of the atomic bomb” for his role in creating the first nuclear weapon during World War II. The theoretical physicist was director of the Manhattan Project’s secret Los Alamos Laboratory, which created the bombs that killed an estimated 100,000 to 200,000 people in Hiroshima and Nagasaki in August 1945.

Born in New York City in 1904, Oppenheimer obtained his Ph.D. in physics when he was only 23. After this, he taught physics at the California Institute of Technology and the University of California, Berkeley.

In 1942, General Leslie Groves Jr. asked Oppenheimer to lead the Manhattan Project despite concerns about Oppenheimer’s lack of managerial experience and Nobel Prizes (something many of the other possible candidates had). After the war, Oppenheimer served on the U.S. Atomic Energy Commission, where he argued for more oversight regarding the use of nuclear weapons and opposed the construction of the hydrogen bomb.

Here are some things you might not know about the famous physicist.

Oppenheimer’s most famous quote comes from the 1965 documentary The Decision to Drop the Bomb. In it, Oppenheimer recalls seeing the first nuclear bomb test on July 16, 1945:

“We knew the world would not be the same. A few people laughed, a few people cried, most people were silent. I remembered the line from the Hindu scripture the Bhagavad Gita: Vishnu is trying to persuade the Prince that he should do his duty, and to impress him, takes on his multi-armed form and says, ‘Now I am become Death, the destroyer of worlds.’ I suppose we all thought that, one way or another.”

Oppenheimer had a personal interest in Sanskrit, the sacred language of Hindu scripture. He studied the language while teaching at Berkeley, and read the Bhagavad Gita in Sanskrit. When he quoted the Bhagavad Gita in 1965, he was giving his own English translation of the text. There are other ways to translate this line, and its meaning—as well as what Oppenheimer was trying to convey by quoting it—is complex.

In 1942, Oppenheimer chose the Los Alamos Ranch School in New Mexico as the site for the Manhattan Project’s secret laboratory. It met many of the requirements needed for the project (for example, it was relatively isolated but still near a train track), and it was also in a part of the country Oppenheimer had fallen in love with when he was a teenager.

After graduating from high school, Oppenheimer was at first unable to enroll in Harvard University because he was sick with a severe case of dysentery. His parents sent him to a dude ranch in New Mexico to convalesce for the summer, and it was during his time there that he gained an appreciation for the desert and a love of horseback riding.

He returned to New Mexico multiple times over the next couple of decades, eventually selecting the desert Los Alamos ranch as the site for the Manhattan Project’s secret scientist town.

The Great Depression helped fuel an interest in workers’ rights and communism in the United States. During the late 1930s, Oppenheimer attended events supporting leftist causes, donated to the anti-fascist Republicans in the Spanish Civil War and subscribed to the leftist newspaper People’s World. He never officially joined the U.S. Communist Party, but many people in his life did—including his brother, Frank Oppenheimer; his girlfriend, Jean Tatlock; and his wife, Katherine “Kitty” Puening.

Even though the United States joined the side of the Soviet Union when it entered World War II, conservative U.S. officials were still suspicious of alleged communists. During the first Red Scare from 1917 to 1920, officials persecuted anyone suspected of communism, socialism, anarchism or any pro-worker activity. Leslie Groves Jr., the Army general who selected Oppenheimer to lead the Manhattan Project’s laboratory, was aware of Oppenheimer’s communist associations but didn’t consider them a major problem.

After the war, Oppenheimer’s opponents used these associations to smear him as a security threat.

In 1946, the United States formed the Atomic Energy Commission to oversee the country’s nuclear weapons program. Oppenheimer used his position on this commission to argue for more control of nuclear weapons and against the development of the hydrogen bomb, which the United States tested for the first time in 1952.

“He was opposed to pursuing the hydrogen bomb, the ‘superbomb,’ because that was 1,000 times more powerful than [the bombs dropped on] Hiroshima and Nagasaki,” says Cynthia C. Kelly, founder and president of the Atomic Heritage Foundation. Oppenheimer was worried about the potential destruction that an arms race to build bigger and bigger bombs would unleash.

Businessman Lewis Strauss, who became chairman of the Atomic Energy Commission in 1953, disliked Oppenheimer’s opposition to the hydrogen bomb, and held a security hearing to investigate Oppenheimer’s loyalty. This was at the height of the second Red Scare, when Senator Joseph McCarthy held hearings to expose supposed communists in the federal government.

With the help of the FBI, which illegally tapped Oppenheimer’s phone, the Atomic Energy Commission argued during the hearing that Oppenheimer’s association with communists made him a security threat. In 1954, the government revoked his security clearance, making him one of the many people to be blacklisted during that era.

With the revocation of his security clearance, Oppenheimer could no longer serve on the Atomic Energy Commission.

“Once his security clearance was denied in 1954, that ended his career as an advisor to the government of the United States,” says David A. Hollinger, a professor emeritus of history at the University of California, Berkeley and coeditor of Reappraising Oppenheimer: Centennial Studies and Reflections.

Even though President John F. Kennedy awarded Oppenheimer the Enrico Fermi Award for scientific achievement and leadership in 1963, Oppenheimer never regained his security clearance. He continued to speak and write about physics and nuclear technology until his death in 1967 at age 62.

It wasn’t until December 2022 that the U.S. Department of Energy vacated the decision to revoke Oppenheimer’s security clearance and officially acknowledged that his hearing had been unfair. This was a decision that scientists and historians had long supported and called on the U.S. government to make. (It may have been sped along by the fact that the movie Oppenheimer was scheduled to debut that summer.)