History of quantum mechanics

The history of quantum mechanics is the roughly three-decade sequence, from 1900 to the late 1920s, in which physics moved from isolated puzzles about radiation and atoms to a complete and self-consistent theory of the microscopic world. It spans Planck's quantum hypothesis, the old quantum theory of Einstein and Bohr, and the mature formulations of Heisenberg, Schrodinger, Born, and Dirac, followed by the interpretation debates that continue today.

#The quantum hypothesis (1900)

Classical physics predicted that a hot object should radiate ever more energy at short wavelengths, a failure known as the ultraviolet catastrophe. In 1900 Max Planck resolved it by assuming that the oscillators in a black body emit and absorb energy only in discrete amounts proportional to frequency, with the proportionality set by a new constant h (Nobel Foundation, Planck). Planck regarded the step as a mathematical device, but it introduced the Planck constant and the word quantum into physics.

#The old quantum theory (1905 to 1924)

In 1905 Albert Einstein argued that light itself is quantized into what are now called photons, explaining the photoelectric effect and earning the 1921 Nobel Prize (Nobel Foundation, Einstein). In 1913 Niels Bohr proposed a model of the atom in which electrons occupy fixed orbits and emit or absorb light only when jumping between them, reproducing the spectral lines of hydrogen (Nobel Foundation, Bohr). In 1924 Louis de Broglie extended duality to matter, proposing that particles such as electrons have an associated wavelength, a hypothesis confirmed by electron diffraction and recognized in 1929 (Nobel Foundation, de Broglie). These ideas were productive but remained a patchwork of rules grafted onto classical mechanics.

#The two formulations (1925 to 1926)

The modern theory emerged in two forms within a year.

In 1925 Werner Heisenberg, working with Max Born and Pascual Jordan, developed matrix mechanics, which described observable quantities such as spectral intensities directly and abandoned any picture of electron orbits.

In 1926 Erwin Schrodinger published wave mechanics, built around a wave equation for a continuous wave function evolving in space and time. Physicists found the wave picture easier to use, and Schrodinger soon proved that the two formulations are mathematically equivalent.

Also in 1926, Max Born supplied the physical meaning of the wave function: its squared magnitude gives the probability of a measurement result, the rule at the heart of the theory. Paul Dirac then unified the approaches into a general formalism and, by 1928, produced a relativistic equation for the electron that predicted antimatter. Schrodinger and Dirac shared the 1933 Nobel Prize (Nobel Foundation, 1933).

#Timeline

#The 1927 Solvay conference and the interpretation debate

The fifth Solvay Conference, held in Brussels in 1927, brought the founders together and crystallized a disagreement that had been building. Bohr and Heisenberg advanced what became the Copenhagen interpretation, treating the probabilistic formalism as a complete account of what can be known. Einstein resisted, insisting that the theory was incomplete and that "God does not play dice." The dispute sharpened in 1935 with the Einstein, Podolsky, and Rosen paper on entanglement and with Schrodinger's cat thought experiment, both aimed at the measurement problem. These foundational questions were largely set aside for practical work in the following decades and were revived after John Bell's 1964 theorem made them experimentally testable (Stanford Encyclopedia of Philosophy).

#Legacy

By the end of the 1920s quantum mechanics was an operational theory whose predictions guided atomic, molecular, and solid-state physics. Its extension to fields and special relativity became quantum field theory. The interpretive questions raised at Solvay remain open, but the mathematical framework built in those years has stood unchanged.