Alan Turing

Alan Turing was an English mathematician and logician whose 1936 paper on computable numbers laid the theoretical foundations of computer science. During the Second World War he was a leading figure in the British effort to break German naval Enigma codes at Bletchley Park, and afterwards he helped design some of the first stored-program computers and posed the question of whether machines could think. In 1952 he was prosecuted for homosexual acts, and he died of cyanide poisoning two years later, aged 41.

Turing was born on 23 June 1912 in Maida Vale, London. His father was a member of the Indian Civil Service, and because his parents spent long periods in India, Alan and his elder brother John were largely brought up by a retired army couple in Hastings, on the south coast of England. He showed an unusual gift for mathematics and science from an early age, which did not always impress the classics-minded teachers at Sherborne School in Dorset, where he boarded from 1926. As a teenager he worked through the ideas of Albert Einstein on his own.

At Sherborne he formed a close attachment to a fellow pupil, Christopher Morcom, who shared his passion for science and astronomy. Morcom's sudden death from tuberculosis in February 1930 affected Turing deeply, and he later said he felt obliged to do the work Morcom could no longer do. Turing went up to King's College, Cambridge, in 1931, graduated with first-class honours in mathematics, and in 1935 was elected a fellow of King's for a dissertation proving a version of the central limit theorem in probability.

In 1936 Turing completed "On Computable Numbers, with an Application to the Entscheidungsproblem". The German mathematician David Hilbert had asked whether there was a mechanical procedure that could decide, for any statement in formal logic, whether it was provable. To answer, Turing first had to say exactly what a mechanical procedure was. He imagined a very simple device that reads and writes symbols on an endless tape, one square at a time, following a fixed table of rules. He argued that anything a person could calculate by following instructions could be calculated by such a machine, and he then showed that some questions, such as whether a given machine will ever stop, cannot be settled by any machine at all. Hilbert's problem therefore had no solution.

The American logician Alonzo Church had reached the same negative answer by a different route a few weeks earlier, but Turing's approach was more concrete and proved more influential. The paper also described a universal machine, one able to imitate any other machine if given a description of it as input. This is, in essence, the idea behind the modern general-purpose computer that holds its program in memory. Turing spent 1936 to 1938 at Princeton University in the United States, where he took a doctorate under Church, before returning to Cambridge.

From 1938 Turing worked part-time for the Government Code and Cypher School, and on 4 September 1939, the day after Britain declared war on Germany, he reported to its wartime base at Bletchley Park in Buckinghamshire. The German armed forces of Nazi Germany used Enigma machines whose settings changed daily. In July 1939 codebreakers from Poland, who had been reading some Enigma traffic for years, had shared their methods and replica Enigma machines with the British and French, including details of their own code-breaking device, the bomba. Turing designed a more general electromechanical device, the bombe, which tested possible machine settings against a guessed fragment of plain text and discarded those that produced contradictions. With an improvement suggested by Gordon Welchman, the bombe became the main tool for attacking Enigma, and the first was installed in March 1940.

Turing headed Hut 8, which worked on German naval Enigma, a particularly difficult variant. He worked out the indicator system used by the navy and developed a statistical technique, nicknamed Banburismus, that greatly reduced the work the bombes had to do. The intelligence produced, codenamed Ultra, helped the Allies route convoys away from U-boats in the Battle of the Atlantic. In October 1941 Turing and three colleagues wrote directly to Winston Churchill asking for more staff and machines, and Churchill ordered that they be given what they needed at once. Turing also devised a method used against the Lorenz teleprinter cipher, though he was not directly involved in building the Colossus computers used against it. The official historian Harry Hinsley later estimated that Ultra shortened the war in Europe by two years or more, a figure that is necessarily speculative. Turing was appointed OBE in 1946, but his wartime work stayed secret for decades.

After the war Turing joined the National Physical Laboratory, where in early 1946 he produced a detailed design for a stored-program computer, the Automatic Computing Engine. Delays and secrecy frustrated him, and a smaller Pilot ACE ran only in 1950, after he had left. In 1948 he moved to the University of Manchester, where he worked on software for the Manchester Mark 1, wrote the programming manual for its commercial successor, the Ferranti Mark 1, and with David Champernowne wrote one of the first chess programs, which he had to run by hand. He was elected a Fellow of the Royal Society in 1951.

In 1950 he published "Computing Machinery and Intelligence", which replaced the vague question of whether machines can think with a practical test: whether a machine could hold a typed conversation that a human judge could not reliably tell apart from a person's. This imitation game, now called the Turing test, remains a reference point in debates about artificial intelligence. In his last years Turing turned to biology. His 1952 paper on morphogenesis proposed that two chemicals reacting and spreading at different rates could produce regular patterns such as spots and stripes, an idea later supported in work on animal markings and limb development.

In January 1952 Turing's house in Wilmslow, Cheshire, was burgled, and when he reported the crime he told police of his relationship with a 19-year-old man, Arnold Murray. Sex between men was then a crime in the United Kingdom. Both were convicted of gross indecency in March 1952, and Turing accepted a year of hormone injections, a form of chemical castration, as an alternative to prison. He lost his security clearance and could no longer consult for GCHQ, at a time when, during the Cold War, homosexual men were treated as security risks, though he kept his university post.

On 8 June 1954 his housekeeper found him dead at home. The post-mortem found cyanide poisoning, and a half-eaten apple lay by his bed, although it was never tested. The inquest returned a verdict of suicide. His mother believed it was an accident caused by careless handling of chemicals in his home laboratory, and some later writers, notably Jack Copeland, have argued the evidence fits accidental poisoning better than suicide. The question remains open.

Turing's reputation grew steadily once the Bletchley Park story became public in the 1970s and after Andrew Hodges published his biography in 1983. In 2009 Prime Minister Gordon Brown made a public apology for his treatment, and in December 2013 Elizabeth II granted him a posthumous royal pardon. A 2017 law, informally called the Alan Turing law, extended pardons to other men convicted under the same laws. The Turing Award, often described as the highest honour in computing, is named after him, and since 2021 his portrait has appeared on the Bank of England's £50 note. Historians caution against crediting any one person with inventing the computer, but Turing's definition of computation and his universal machine remain at the centre of the subject.