American physicist Julius Robert Oppenheimer (1904-1967) at Guest Lodge, Oak Ridge, Tennessee, USA, 1946. J. Robert Oppenheimer was best known for leading the Manhattan Project, which developed the first atomic bomb in 1945. He later opposed the development of the hydrogen bomb, and was stripped of his security clearances in 1954 amid anti-communist hysteria. This hugely limited his subsequent ability to impact US physics and politics. The decision was posthumously voided by the federal government in 2022 as a symbolic recognition that the process of revoking his clearances had been flawed.