In 1909, Ernest Rutherford, Hans Geiger and Ernest Marsden took a piece of radium and used it to fire charged particles at a sheet of gold foil. They wanted to test the then-dominant theory that atoms were simply clusters of electrons floating in little seas of positive electrical charge (the so-called ‘plum pudding’ model). What came next, said Rutherford, was ‘the most incredible event that has ever happened to me in my life’. Despite the airy thinness of the foil, a small fraction of the particles bounced straight back at the source – a result, Rutherford noted, ‘as incredible as...