Quantum Theory Proposed

In 1900, the German physicist Max Planck proposed that energy is emitted and absorbed not in a smooth, continuous flow but in discrete units, which he called quanta. He introduced the idea almost reluctantly, as a mathematical device to solve a specific problem in the study of heat radiation. It nonetheless became the starting point of quantum theory, which reshaped physics and underlies much of modern technology, from lasers to semiconductors.

The problem Planck was tackling concerned so-called black-body radiation: the light and heat given off by an idealized object that absorbs all radiation falling on it. By the late nineteenth century, experimenters in Berlin had measured how the intensity of this radiation varies with wavelength and temperature. Classical physics could not account for the results. One formula worked reasonably well at short wavelengths, while another, later associated with Lord Rayleigh and James Jeans, worked at long wavelengths but predicted absurdly high energy at the short-wavelength end, a failure later nicknamed the "ultraviolet catastrophe."

Planck, a professor at the University of Berlin who specialized in thermodynamics, first found a formula that fitted the measurements and then sought a theoretical justification for it. He found that he could derive it by assuming that the tiny oscillators in the walls of the radiating body could only exchange energy in whole-number multiples of a basic amount. That amount was proportional to the frequency of the radiation, with a new constant of nature, now called Planck's constant, as the factor of proportionality. He presented his results to the German Physical Society in late 1900.

Historians still debate how far Planck himself believed in the physical reality of quanta at the time. He appears to have seen the step as a calculational trick and spent years hoping that a more conventional explanation might be found. The deeper implications were drawn out by others. In 1905 Albert Einstein used the idea to explain the photoelectric effect, suggesting that light itself consists of packets of energy. Niels Bohr applied quantization to the structure of the atom in 1913, and in the 1920s Werner Heisenberg, Erwin Schrödinger and others developed a full quantum mechanics.

The consequences were profound. Quantum theory overturned the classical picture of a predictable, continuous world, introducing probability, uncertainty and the strange behavior of matter at atomic scales. Planck received the Nobel Prize in Physics in 1918 for his discovery. His constant remains one of the fundamental quantities of physics, and in 2019 it was used to redefine the kilogram, a fitting legacy for an idea that began as a reluctant calculation.