Julius Robert Oppenheimer (/ˈɒpənˌhaɪmər/; April 22, 1904 – February 18, 1967) was an American theoretical physicist and director of the Los Alamos Laboratory during World War II. He is often credited as the “father of the atomic bomb” for his role in organizing the Manhattan Project, the research and development undertaking that created the first nuclear weapons.

Born to German Jewish immigrants in New York City, Oppenheimer earned a bachelor’s degree in chemistry from Harvard University in 1925 and a PhD in physics from the University of Göttingen in Germany in 1927. After research at other institutions, he joined the physics department at the University of California, Berkeley, where he became a full professor in 1936. He made significant contributions to theoretical physics, including achievements in quantum mechanics and nuclear physics such as the Born–Oppenheimer approximation for molecular wave functions, work on the theory of electrons and positrons, the Oppenheimer–Phillips process in nuclear fusion, and the first prediction of quantum tunneling. With his students, he also made contributions to the theory of neutron stars and black holes, quantum field theory, and the interactions of cosmic rays.

In 1942, Oppenheimer was recruited to work on the Manhattan Project, and in 1943 was appointed director of the project’s Los Alamos Laboratory in New Mexico, tasked with developing the first nuclear weapons, four years after the start of the German nuclear weapons program.[note 2] His leadership and scientific expertise were instrumental in the project’s success. On July 16, 1945, he was present at the Trinity test of the first atomic bomb. In August 1945, the weapons were used in the bombings of Hiroshima and Nagasaki in Japan, which remain the only use of nuclear weapons in an armed conflict.