By the end of the 19th century, classical physics could not explain the spectrum of radiation emitted by a heated body. The Rayleigh-Jeans law predicted an energy that diverged to infinity at short wavelengths — what physicists called the "ultraviolet catastrophe" — turning black-body radiation into an experimental puzzle classical theory could not resolve.

Max Planck, professor of theoretical physics at the University of Berlin, worked on the problem throughout 1900. In October he proposed a formula that fit the experimental data; in December he presented the physical interpretation behind it. The paper he read at the German Physical Society meeting in Berlin on 14 December 1900 claimed that energy could only be emitted or absorbed in discrete quanta — E = hν, where ν is frequency and h is a new constant of nature. That constant, Planck's constant, has since become one of the universe's fundamental parameters.

Planck himself struggled to take his hypothesis seriously; he called it "an act of desperation" and spent years searching for a more classical explanation. But in 1905 Einstein explained the photoelectric effect by arguing that light itself comes in quanta — photons. In 1913 Bohr built his atomic model on the quantum concept. In the 1920s Heisenberg, Schrödinger and Dirac turned the idea into a complete theory.

Quantum physics today underlies modern technology from transistors to lasers, from MRI to quantum computers. The "act of desperation" for which Planck received the 1918 Nobel Prize in Physics opened the new reality of the 20th century — one in which the deterministic classical worldview had to give way to something stranger.