Minutes later, Oppenheimer's friend and colleague Isidor Rabi caught sight of him from a distance: 'I’ll never forget his walk I’ll never forget the way he stepped out of the car. As the roar engulfed the landscape and the mushroom cloud rose in the sky, Oppenheimer's expression relaxed into one of 'tremendous relief'.

It created a shockwave that was felt 160km (100 miles) away. With a force matching 21 kilotonnes of TNT, the detonation was the largest ever seen. The explosion, when it came, outshone the Sun.

grew tenser as the last seconds ticked off. In the final minutes of the countdown, as Bird and Sherwin report, an army general observed Oppenheimer's mood at close-quarters: 'Dr. That day in 1945 is one of several pivotal moments in Oppenheimer's life described by the historians Kai Bird and Martin J Sherwin in their 2005 biography American Prometheus, which provided the basis for the new movie biopic Oppenheimer, released 21 July in the US. He'd slept only four hours that night, kept awake by anxiety and his smoker's cough. At 5ft 10in (178cm), this made him extremely thin. He was always slender, but after three years as director of 'Project Y', the scientific arm of the 'Manhattan Engineer District' that had designed and built the bomb, his weight had dropped to just over 52kg (115lbs). Oppenheimer was a picture of nervous exhaustion. Around 10km (6 miles) away, the world's first atomic bomb test, codenamed 'Trinity', was set to proceed in the pale sands of the Jornada del Muerto desert, in New Mexico. It was the early hours of 16 July 1945, and Robert Oppenheimer was waiting in a control bunker for a moment that would change the world.

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