Thomas Powers in the New York Review of Books:
One of the many complexities of the character of J. Robert Oppenheimer is apparent in his response to the discovery of nuclear fission in January 1939. “The U business is unbelievable,” he wrote to a colleague once he had satisfied himself that uranium atoms really did split when bombarded with neutrons. “It is I think exciting, not in the rare way of positrons and mesotrons, but in a good honest practical way.” He meant that fission didn’t turn physics upside down and inside out like so many other discoveries of the first decades of the twentieth century. Fission was as practical as a hammer. The clincher for Oppenheimer was watching the dramatic green spikes on the oscilloscope of the Berkeley physicist Luis Alvarez when an atom split. “In less than fifteen minutes,” Alvarez wrote later,
he not only agreed that the reaction was authentic but also speculated that in the process extra neutrons would boil off that could be used to split more uranium atoms and thereby generate power or make bombs. It was amazing to see how rapidly his mind worked….
The speed of Oppenheimer’s mind would not have surprised those who knew him. At thirty-four Oppenheimer was famously brilliant.