The Danish physicist who founded the quantum theory of the atom and, from his Copenhagen institute, presided over the field for a generation. His 1913 model showed why atoms don’t collapse: electrons occupy fixed orbits and jump between them in discrete leaps, each releasing or absorbing a precise wavelength of light, explaining the hydrogen spectrum and winning him the Nobel Prize. He developed complementarity — wave and particle descriptions both necessary yet mutually exclusive — and debated Einstein for decades over whether nature is fundamentally probabilistic. He escaped Nazi-occupied Denmark in 1943, advised the Allied bomb project, and spent his final years arguing, without success, for an open nuclear world.
In 1913 a young Dane did something physicists had thought impossible: he wrote down the rules of the atom. He said electrons cannot drift wherever they like around the nucleus. They are confined to fixed orbits, and they jump between them in sudden leaps, spitting out or swallowing exact packets of light. It sounded absurd, and it was incomplete, but it explained the precise colors that glowing atoms give off, colors no one had been able to explain. The model won Niels Bohr the Nobel Prize and made atomic structure intelligible in quantum terms. Three decades later the same man, having fled the Nazis and served as a consultant to the Allied bomb project, would spend his last years pleading with presidents to keep the nuclear world open rather than secret.