Manjit Kumar at The Guardian:
With the light touch of a skilled storyteller, Rovelli reveals that Heisenberg had been wrestling with the inner workings of the quantum atom in which electrons travel around the nucleus only in certain orbits, at certain distances, with certain precise energies before magically “leaping” from one orbit to another. Among the unsolved questions he was grappling with on Helgoland were: why only these orbits? Why only certain orbital leaps? As he tried to overcome the failure of existing formulas to replicate the intensity of the light emitted as an electron leapt between orbits, Heisenberg made an astonishing leap of his own. He decided to focus only on those quantities that are observable – the light an atom emits when an electron jumps. It was a strange idea but one that, as Rovelli points out, made it possible to account for all the recalcitrant facts and to develop a mathematically coherent theory of the atomic world.
more here.