Paul Adrien Maurice Dirac

Paul Adrien Maurice Dirac was born in Bristol in 1902. His father was a Swiss-born teacher of French; his early home was famously silent and severe, a childhood Dirac later described as training in speaking only when spoken to and only with exact precision. He studied electrical engineering at Bristol University from 1918, could not find an engineering job in the post-war slump, and took a free mathematics degree at Bristol to fill the time. In 1923 he went up to St John's College, Cambridge as a research student under Ralph Fowler. In 1925 Werner Heisenberg's first quantum paper arrived in Cambridge; Dirac rewrote it within weeks in his own algebraic formulation ("q-numbers") and was doing original quantum mechanics at twenty-three. Over 1925–1927 he produced the transformation theory connecting Heisenberg's matrix mechanics and Schrödinger's wave mechanics, the Dirac delta function, and the first formulation of quantum electrodynamics.

The central work came in 1928. Schrödinger's equation was non-relativistic and could not describe electrons at high energies; the Klein-Gordon equation was relativistic but produced negative probabilities. Dirac demanded a relativistic wave equation that was first-order in time (like Schrödinger's) and compatible with special relativity. The only solution was to introduce four-component spinors and a matrix algebra — the Dirac equation. Spin emerged automatically as a geometric consequence of relativistic covariance, not as a bolt-on assumption. The equation had twice as many solutions as expected, half of them at negative energy. In 1931 he interpreted the negative-energy solutions as antiparticles — predicting a positively charged twin of the electron. The positron was discovered in cosmic rays by Carl Anderson at Caltech the following year. Antimatter, which nobody had any evidence for, had been deduced from an equation. In 1938 he extended the classical Abraham-Lorentz radiation-reaction force to a fully Lorentz-covariant relativistic equation — now the **Abraham-Lorentz-Dirac (ALD) equation** — which inherits the classical pathologies but provides the correct relativistic form that any successor theory must reduce to.

He was Lucasian Professor at Cambridge 1932–1969 and shared the 1933 Nobel Prize in Physics with Erwin Schrödinger for "the discovery of new productive forms of atomic theory." His 1930 *Principles of Quantum Mechanics* is still in print and is the textbook every subsequent textbook is written against. Bra-ket notation, the Dirac delta, the Dirac sea, and Fermi-Dirac statistics (with Fermi, independently) are among his named legacies. He was famously laconic — his colleagues joked about the *dirac*, a unit equal to one word per hour — and resisted biographical attention. In 1971 he retired from Cambridge and took a chair at Florida State University in Tallahassee, where he worked on large-numbers cosmology and magnetic monopoles until his death in 1984 at eighty-two. He is buried in Tallahassee, with a memorial stone at Westminster Abbey beside Newton.