Alan Turing

Alan Mathison Turing was born on 23 June 1912 in Maida Vale, London, while his father Julius, a civil servant in the Indian Civil Service, was on leave. His mother was Ethel Sara Stoney. Because his parents travelled between England and India, Alan and his older brother John stayed for long periods with a retired army couple in St Leonards-on-Sea.

From 1926 he attended Sherborne School, where not every teacher valued his interest in mathematics and science. His friend Christopher Morcom died in February 1930. Turing studied at King's College, Cambridge, from 1931 to 1934 and became a fellow there in 1935, partly on the strength of a dissertation on the central limit theorem.

In 1936 Turing described in 'On Computable Numbers, with an Application to the Entscheidungsproblem' an imaginary computing device, later called the Turing machine. He showed that Hilbert's decision problem has no general solution. Alonzo Church had reached a similar result shortly before; Turing completed his PhD under Church at Princeton in 1938.

From September 1939 he worked for the Government Code and Cypher School at Bletchley Park. He concentrated on the German naval Enigma and, as the first head of Hut 8, was involved in the British Bombe and in Banburismus, a statistical method. His work built on Polish results. In 1942–1943 he visited the United States to cooperate with the US Navy.

After the war he designed the Automatic Computing Engine at the National Physical Laboratory (1945–1946), one of the first detailed designs for a stored-program computer. In 1948 he moved to the University of Manchester, where he worked on the Manchester Mark 1. In 1950, in 'Computing Machinery and Intelligence', he proposed the imitation-game test later called the Turing test.

In 'The Chemical Basis of Morphogenesis' (1952) he argued that reacting and diffusing chemicals can form patterns such as spots and stripes (a reaction–diffusion model). Later researchers used the model to study patterns such as coat markings and the formation of digits, although its applicability remains a subject of research case by case.

Turing's first work arose from the foundational crisis of mathematics: Hilbert's programme (1928) asked for a general decision procedure, and Gödel's incompleteness theorems (1931) had shown limits to formal provability. Church and Turing independently gave a precise definition of computability, at a time when 'computers' were still people carrying out calculations.

The Second World War made cryptanalysis a large-scale secret industry, shielded by the Official Secrets Act. Afterwards Britain's Cold War climate treated men known to be homosexual as security risks, partly after the defection of Burgess and Maclean in 1951. Only in 1967 were homosexual acts between men in England decriminalised, under conditions.

The Turing machine and the idea of a universal machine that can imitate any other machine form a foundation of theoretical computer science. The Church–Turing thesis links his model to Church's lambda calculus as a description of what is computable. His designs and manuals also contributed to the early practice of programming.

At Bletchley Park, Turing's methods contributed to the decryption of German naval messages; the resulting intelligence, called Ultra, was helpful in the Battle of the Atlantic. Estimates of how much it shortened the war are uncertain and differ between historians. His contribution was part of the work of thousands of staff, including Gordon Welchman and Hugh Alexander.

With the Turing test (1950) he reduced the question whether machines can think to a behavioural experiment: an interrogator tries to tell a human from a machine in conversation. The test remains a subject of philosophical debate about what 'intelligence' means and whether behaviour is sufficient evidence for it.

In January 1952 Turing reported a burglary at his home in Wilmslow and in doing so acknowledged a sexual relationship with a man. He was charged with 'gross indecency' (Criminal Law Amendment Act 1885), pleaded guilty and was convicted on 31 March 1952. Instead of imprisonment he received a year's probation with hormone treatment, also called 'chemical castration'. His security clearance was withdrawn.

On 8 June 1954 his housekeeper found him dead at his home; he had died the day before from cyanide poisoning. A half-eaten apple lay beside his bed and was never tested. The inquest ruled the death a suicide. His mother and some later authors disputed this; the circumstances remain contested.

His wartime work stayed secret for decades. In 2009 Prime Minister Gordon Brown apologised on behalf of the government for Turing's treatment. In December 2013 Queen Elizabeth II granted him a pardon, and the 'Alan Turing law' (Policing and Crime Act 2017) retroactively pardons men cautioned or convicted under historical laws against homosexual acts.