Alan Turing

By Lin Yue

In 1936, a twenty-four-year-old Cambridge mathematician asked what it means for something to be computable, and answered with an imaginary machine. Alan Turing's paper 'On Computable Numbers' described a device of infinite tape, a read-write head, and a table of rules, and proved that this humble contraption could simulate any conceivable computing process. The Turing machine was not built; it did not need to be. It defined the limits of computation itself, proving that some problems no machine could ever solve, and it gave the digital age its founding theorem: that a single universal machine can perform any computation any other machine can. Born in London in 1912, Turing was an awkward, brilliant boy who taught himself relativity and ran long distances to think. When war came, he went to Bletchley Park, where German Enigma-encrypted messages were strangling Britain's war effort. Turing designed the Bombe, an electromechanical machine that tested Enigma rotor settings at inhuman speed, turning cryptanalysis from artisanal guesswork into industrial process; historians estimate the work at Bletchley, to which his designs were central, shortened the war in Europe by as much as two years. He also designed early computers after the war, wrote the first chess program, and in 1950 published 'Computing Machinery and Intelligence,' which proposed the famous test: if a machine can converse so that you cannot tell it from a human, it may be said to think. In 1952 he was prosecuted for homosexuality, then a crime in Britain, subjected to chemical castration, and stripped of his security clearance. He died in 1954, at forty-one, from cyanide poisoning, an apple by his bed. The official verdict was suicide. Britain apologized in 2009; the Queen granted a royal pardon in 2013. His face is now on the fifty-pound note. The Bombe was only the most famous of Turing's wartime machines. At Bletchley Park's Hut 8, which he led, he also broke the German naval Enigma, the most complex variant, work that directly protected the Atlantic convoys feeding Britain. His statistical method, which he called Banburismus, turned codebreaking from intuition into mathematics, and his colleague Gordon Welchman improved the Bombe with the diagonal board that made it dramatically faster. After the war, Turing designed the ACE, the Automatic Computing Engine, at the National Physical Laboratory, one of the first detailed designs for a stored-program computer, though bureaucratic delays meant others built theirs first. At Manchester he worked on the Mark 1, wrote the first serious chess program, and then astonished biology with his 1952 paper on morphogenesis, showing how simple chemical reactions could generate the patterns of leopard spots and leaf arrangements, the founding paper of mathematical biology. The prosecution in 1952, for 'gross indecency' with another man, led to a year of estrogen injections, the chemical castration the court ordered as an alternative to prison. He lost his security clearance and, with it, the cryptographic work he loved. The apple, half-eaten by his bed, became the most mythologized detail of his death; whether it carried the cyanide remains debated, but the inquest ruled suicide. In 2017 Britain passed the 'Alan Turing law,' posthumously pardoning thousands convicted under the same statute.

Impact on civilization

Turing gave civilization two things it did not know it needed: the theory and the practice of computing. The Turing machine remains the definition of computability taught to every computer science student on earth; it is the reason we know, mathematically, what computers can and cannot do, and the universal machine he imagined is the direct ancestor of the stored-program computer. The Bombe made codebreaking an industrial science and arguably altered the outcome of the Second World War. His 1950 paper founded artificial intelligence as a field of inquiry before the field had a name. The arc of his life, genius destroyed by the state for his private life, then posthumously honored on the nation's currency, is also one of the starkest moral ledgers of the twentieth century: a civilization that nearly threw away the mind that had saved it. Turing's 1952 morphogenesis paper is still cited by biologists modeling pattern formation, a reminder that his genius was not confined to computing. The pardon movement he inspired changed British law for the living, not just the dead. And the fifty-pound note, issued in 2021 with his portrait, his birth date in binary, and a ticker-tape of his famous quotation about the ACE, put the most abstract mathematician of the century into every wallet in the country, a quiet national admission that the civilization he saved finally understood what it owed him.