Oppenheimer’s early success in research began in 1930 when he analysed Dirac’s relativistic quantum mechanics and theory of the electron (1928). He showed that a positively charged anti-particle with the same mass as the electron should exist, and this positron was first seen by C D Anderson in 1932. During the 1930s Oppenheimer built up a formidable team of young theoretical physicists around him, the first time that the subject had been studied intensely outside Europe. In 1939, working on stellar structure, he showed that any massive star, when its thermonuclear energy is exhausted, will collapse to form a black hole, which has mass but from which light cannot escape.

After 1942 as director of Los Alamos he concentrated on gathering scientists and generating an atmosphere of urgency, skilfully handling the interface between his military superior, General Groves, and the unorthodox research scientists under him.

Oppenheimer’s wife and brother were left-wing sympathizers and possibly communists, and he ran into difficulties in 1943 when Groves demanded the name of a communist agent who had approached Oppenheimer; after much delay he finally gave it. The first atomic test explosion took place in July 1945, and two atomic bombs ended the war with Japan a month later.

After the war Oppenheimer initially continued his important role in atomic energy, but he opposed the development of the hydrogen (fusion) bomb. In 1953 his political background and his support for the Super Program (the hydrogen bomb project) were questioned, and President Eisenhower removed his security clearance, ending his Government service. However, the Fermi Award was conferred on him by President Johnson in 1963, implying that doubts about his integrity had been resolved.