Alan Turing

Alan Mathison Turing was born on June 23, 1912, in Maida Vale, London, the second son of Julius Mathison Turing, a civil servant in the Indian Civil Service, and Ethel Sara Turing ^[1]. Because his father's postings kept the parents in India for long stretches, Alan and his elder brother John spent much of their childhood in England in the care of family friends, an arrangement common among colonial service families of the period ^[2].

Signs of an unusual scientific mind appeared early. At Sherborne School in Dorset, which he entered in 1926, Turing showed little patience for the classical curriculum but pursued mathematics and science with intensity, sometimes working out advanced problems on his own before being formally taught the methods ^[1]. A famous story from his first term holds that a general strike halted rail service, so the thirteen year old cycled roughly sixty miles from Southampton to reach the school, staying overnight at an inn along the way ^[2].

At Sherborne he formed a close friendship with Christopher Morcom, a fellow pupil who shared his fascination with science. Morcom's sudden death from bovine tuberculosis in February 1930 affected Turing deeply, and his biographer Andrew Hodges traces some of Turing's later interest in the nature of mind and spirit to his grief over this loss ^[2].

Turing won a scholarship to King's College, Cambridge, in 1931, graduated with a first class degree in mathematics in 1934, and was elected a Fellow of King's in 1935 at the age of 22 on the strength of a dissertation proving a version of the central limit theorem ^[1]. Cambridge in the 1930s gave him both intellectual freedom and exposure to the foundational questions then agitating mathematics, particularly David Hilbert's Entscheidungsproblem, the question of whether a definite procedure could decide the truth of any mathematical statement.

His answer came in the 1936 paper "On Computable Numbers, with an Application to the Entscheidungsproblem", published in the Proceedings of the London Mathematical Society ^[3]. To attack the problem, Turing imagined a simple abstract device, now called a Turing machine, that reads and writes symbols on an unbounded tape according to a finite table of rules. He showed that a single universal machine could simulate any other such machine given its description, and he used this framework to prove that no general decision procedure of the kind Hilbert sought could exist ^[3]. The universal machine is widely regarded as the theoretical blueprint for the stored program computer, which is why any serious Alan Turing biography treats this paper as the hinge of his career.

Alonzo Church at Princeton had reached a related result by different means, and Turing spent 1936 to 1938 at Princeton completing a doctorate under Church's supervision ^[1]. He declined an offer to stay in the United States and returned to King's College in 1938, shortly before war made his particular talents urgently useful.

On September 4, 1939, the day after Britain declared war on Germany, Turing reported to the Government Code and Cypher School at Bletchley Park in Buckinghamshire ^[4]. His assignment was the German Enigma machine, an electromechanical cipher device whose settings changed daily and whose possible configurations ran into astronomical numbers. Polish cryptanalysts had achieved important breaks against earlier Enigma procedures and shared their methods with Britain and France in July 1939, giving the Bletchley team a crucial starting point ^[2].

Turing's central contribution was the design, with later refinements by Gordon Welchman, of the Bombe, an electromechanical machine that searched rapidly through Enigma settings by exploiting guessed fragments of plaintext known as cribs ^[4]. The first Bombe was installed in 1940, and dozens more followed. Turing took particular responsibility for the naval Enigma used by German U-boats, leading Hut 8 and developing statistical techniques, including a method he called Banburismus, to reduce the work the machines had to do ^[2]. Breaking the naval traffic fed directly into the Battle of the Atlantic, helping convoys evade the submarine packs that threatened Britain's supply lines.

Historians have debated how to quantify the effect of this intelligence, codenamed Ultra. Harry Hinsley, the official historian of British wartime intelligence, argued that it shortened the war in Europe by not less than two years ^[5]. Turing was appointed an Officer of the Order of the British Empire in 1946 for his war service, though the citation could not say what the service had been, since the Bletchley work remained classified into the 1970s ^[1]. Among Alan Turing facts that surprise modern readers, this enforced silence ranks high: the man who had done so much could tell almost no one.

After the war Turing joined the National Physical Laboratory in Teddington, where in 1945 and 1946 he produced a detailed design for the Automatic Computing Engine (ACE), one of the first complete specifications for a stored program electronic computer ^[6]. Frustrated by slow progress on construction, he left before a reduced version, the Pilot ACE, ran its first program in 1950. In 1948 he moved to the University of Manchester, where Frederic Williams and Tom Kilburn had just run the world's first stored program computer, the Manchester Baby. Turing became deputy director of the computing laboratory and wrote programming systems and documentation for the Ferranti Mark I, the machine's commercial successor ^[1].

Manchester also produced his most widely read paper. "Computing Machinery and Intelligence", published in the philosophy journal Mind in 1950, opened by asking whether machines can think and proposed replacing that vague question with a practical experiment ^[7]. In what he called the imitation game, an interrogator exchanges typed messages with a hidden human and a hidden machine and tries to tell them apart. The proposal, now known as the Turing test, remains a reference point in debates about artificial intelligence three quarters of a century later, and it belongs on any list of Alan Turing achievements alongside the universal machine and the Bombe.

His curiosity kept moving. In 1952 he published "The Chemical Basis of Morphogenesis", a mathematical account of how patterns such as animal coat markings could arise from the interaction of diffusing chemicals ^[8]. The paper founded a line of research in mathematical biology, and reaction diffusion systems of the type he described are still called Turing patterns.

Turing was homosexual at a time when sexual acts between men were criminal offences in Britain. Colleagues remembered him as unconventional and direct, an accomplished long distance runner who recorded a marathon time of 2 hours 46 minutes in 1946 and narrowly missed consideration for the 1948 British Olympic team because of injury ^[2]. During the war he was briefly engaged to Joan Clarke, a fellow cryptanalyst in Hut 8, but he ended the engagement after telling her of his homosexuality ^[2].

In January 1952 Turing reported a burglary at his Wilmslow home. The investigation revealed his relationship with Arnold Murray, a young Manchester man, and Turing was charged with gross indecency under the same Victorian statute once used against Oscar Wilde ^[1]. Convicted in March 1952, he was offered a choice between imprisonment and probation conditional on hormonal treatment. He accepted a year of oestrogen injections, a procedure often described as chemical castration, which caused distressing physical effects ^[2]. The conviction also cost him his security clearance, ending the consulting he had continued for Government Communications Headquarters, the postwar successor to Bletchley Park.

Turing was found dead at his home in Wilmslow, Cheshire, on June 8, 1954, having died the previous day, June 7, sixteen days before his forty second birthday ^[1]. The cause was cyanide poisoning, and a partly eaten apple lay beside his bed, though the apple itself was never tested. The inquest returned a verdict of suicide ^[2].

His mother, Ethel Turing, maintained that the death was an accident arising from his habit of conducting chemistry experiments with cyanide at home, and some later commentators have argued the evidence was consistent with that view ^[9]. Most historians accept the inquest finding while acknowledging the uncertainty. He was cremated at Woking Crematorium, and his ashes were scattered there, as his father's had been.

Recognition arrived slowly, then all at once. The Bletchley Park story began to emerge publicly in the 1970s, and Andrew Hodges's 1983 biography Alan Turing: The Enigma brought the full arc of the life to a wide readership ^[2]. In 2009 Prime Minister Gordon Brown issued a public apology for Turing's treatment, calling it appalling, and in December 2013 Queen Elizabeth II granted a posthumous royal pardon ^[10]. The subsequent legislation informally known as the Alan Turing Law extended pardons to thousands of men convicted under the same historical statutes.

His name is now embedded in the infrastructure of the field he founded. The Turing Award, given annually by the Association for Computing Machinery since 1966, is generally described as the highest honour in computer science ^[1]. The Alan Turing Institute, established in 2015, is the United Kingdom's national centre for data science and artificial intelligence. In 2019 the Bank of England announced that Turing would appear on the 50 pound note, and the note entered circulation on June 23, 2021, his birthday ^[10]. The 2014 film The Imitation Game, with Benedict Cumberbatch in the lead role, carried the story to a global audience.

The verdict of his own institution may serve as summary. In a 2013 statement on the pardon, the British government described his wartime work as vital and his later treatment as unjust ^[10]. The judgment matches the record: a mathematician who defined what computation means, helped defeat a dictatorship in secret, asked whether machines could think, and explained the leopard's spots, all within a working life of barely two decades.