Alan Turing

Alan Mathison Turing OBE FRS (23 June 1912 – 7 June 1954) was a British mathematician, computer scientist, logician, cryptanalyst, philosopher, and theoretical biologist. He is widely considered the father of theoretical computer science and artificial intelligence.

Turing's 1936 paper "On Computable Numbers" introduced the Turing machine, an abstract model of computation that formalised the concept of an algorithm and remains the foundation of computability theory. During the Second World War, he was central to breaking German Enigma ciphers at Bletchley Park, work estimated to have shortened the war by more than two years. His 1950 paper "Computing Machinery and Intelligence" proposed the Turing test as a criterion for machine intelligence, launching the philosophical foundations of the AI field.

Turing was prosecuted in 1952 for homosexual acts, which were then criminal in Britain. He accepted chemical castration as an alternative to imprisonment and died on 7 June 1954 from cyanide poisoning, ruled a suicide. He received a posthumous royal pardon in 2013 and appears on the Bank of England £50 note issued in 2021.

Early life and education

Alan Turing was born on 23 June 1912 in Maida Vale, London, the second son of Julius Mathison Turing, a civil servant in the Indian Civil Service, and Ethel Sara Turing (née Stoney). His parents spent much of his childhood in India, leaving Alan and his brother John in the care of foster families in England.

Turing attended Sherborne School in Dorset from 1926. His mathematical talent was evident early: he reportedly solved advanced problems without having studied elementary calculus. At Sherborne, he formed a close relationship with Christopher Morcom, a fellow student whose sudden death in 1930 from bovine tuberculosis deeply affected Turing and influenced his lifelong interest in the nature of mind and consciousness.

In 1931, Turing entered King's College, Cambridge, to read mathematics. He was elected a Fellow of King's College in 1935, at the age of 22, for his dissertation on the central limit theorem. From 1936 to 1938, he studied at Princeton University under Alonzo Church, receiving his PhD in 1938 with a dissertation on "Systems of Logic Based on Ordinals," which introduced the concept of oracle machines — an extension of Turing machines with access to an uncomputable oracle.

Computability and the Turing machine

In his 1936 paper "On Computable Numbers, with an Application to the Entscheidungsproblem," Turing defined a theoretical computing device — now called a Turing machine — consisting of:

An infinite tape divided into cells, each containing a symbol
A head that reads and writes symbols and moves left or right
A state register storing the machine's current state
A finite table of instructions (transition function)

Turing proved that a universal Turing machine could simulate any other Turing machine given its description, establishing the theoretical basis for general-purpose programmable computers. He also proved that the Entscheidungsproblem (decision problem) — whether there exists an algorithm to determine the truth or falsity of any mathematical statement — is undecidable. This result, arrived at independently and simultaneously by Church using his lambda calculus, is known as the Church–Turing thesis: any function that can be effectively computed can be computed by a Turing machine.

The paper is regarded as one of the most important in the history of mathematics and computer science.

Second World War: Bletchley Park

In September 1939, Turing joined the Government Code and Cypher School (GC&CS) at Bletchley Park. He became the leading figure in Hut 8, responsible for breaking German naval Enigma communications.

The Bombe

Building on earlier work by Polish cryptanalysts Marian Rejewski, Jerzy Różycki, and Henryk Zygalski, Turing designed the Bombe — an electromechanical device that dramatically accelerated the process of finding Enigma settings. The Bombe worked by testing candidate rotor positions against known or guessed plaintext (cribs), exploiting contradictions to eliminate impossible settings. By 1942, over 200 Bombes were in operation, and Hut 8 was routinely breaking naval Enigma, providing critical intelligence for the Battle of the Atlantic.

Contributions to Allied intelligence

Turing also contributed to:

Banburismus — a sequential statistical technique for reducing the work of the Bombes by eliminating unlikely rotor orders
Breaking the more complex Lorenz cipher used by German High Command (the "Tunny" machine), for which the Colossus computer was built — the world's first programmable electronic digital computer
Establishing a secure transatlantic speech encryption system, travelling to the United States in 1942–43 to liaise with US Navy cryptanalysts

Turing was appointed Officer of the Order of the British Empire (OBE) in 1946 for his war service, though the full nature of his contributions remained classified for decades.

Post-war computer science

ACE

In 1945, Turing joined the National Physical Laboratory (NPL) in London, where he designed the Automatic Computing Engine (ACE) — one of the first detailed designs for a stored-program electronic computer. His 1945 report described a machine far more ambitious than contemporaries: it included subroutines, a stack, and floating-point arithmetic. A simplified version, the Pilot ACE, ran its first program on 10 May 1950 and was one of the fastest computers in the world at the time.

Manchester computers

In 1948, Turing moved to the University of Manchester, where he became Deputy Director of the Royal Society Computing Machine Laboratory. He worked on the Manchester Mark 1, one of the earliest stored-program computers, and wrote the programming manual for the Ferranti Mark 1, the first commercially available general-purpose electronic computer (1951).

At Manchester, Turing also wrote some of the earliest computer programs for playing chess, implementing a paper algorithm ("Turochamp") that he tested by hand-simulating the machine's moves.

Artificial intelligence

Turing's 1950 paper "Computing Machinery and Intelligence," published in the journal Mind, is considered a founding document of the artificial intelligence field. The paper opens with the question "Can machines think?" and proposes replacing it with the imitation game — now known as the Turing test:

A human interrogator converses via text with both a human and a machine. If the interrogator cannot reliably distinguish the machine from the human, the machine is said to have passed the test.

Turing addressed nine objections to machine intelligence, including the "Lady Lovelace objection" (that machines can only do what they are programmed to do), the mathematical objection (Gödel's incompleteness), and the argument from consciousness. He predicted that by 2000, computers with 10⁹ bits of storage could fool 30% of interrogators in a five-minute test — a prediction that remained a benchmark for decades.

The paper also introduced the concept of machine learning: Turing argued that rather than programming intelligence directly, it might be more productive to build a "child machine" and teach it, analogous to educating a child:

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Mathematical biology

In his final major work, "The Chemical Basis of Morphogenesis" (1952), Turing proposed a mathematical model for biological pattern formation. He showed that a system of two chemical substances (morphogens) diffusing and reacting at different rates could spontaneously produce stable spatial patterns — such as stripes, spots, and spirals — from an initially uniform state. These Turing patterns provided the first mathematical explanation for phenomena like the markings on animal coats, the arrangement of leaves, and the structure of fingerprints.

The paper was largely ignored during Turing's lifetime but was vindicated by experimental evidence beginning in the 1990s. Turing patterns have since been observed in chemical systems (the Belousov–Zhabotinsky reaction), biological development (digit formation in vertebrate limbs, hair follicle spacing), and ecological systems. The field Turing founded is now known as mathematical biology or biological pattern formation.

Prosecution, death, and legacy

Prosecution

In January 1952, Turing began a relationship with Arnold Murray, a 19-year-old man he met outside a cinema in Manchester. After a burglary at Turing's house by an acquaintance of Murray's, Turing reported the crime to police and in the course of the investigation acknowledged his relationship with Murray. Both men were charged with "gross indecency" under Section 11 of the Criminal Law Amendment Act 1885 — the same law under which Oscar Wilde had been prosecuted in 1895.

Turing pleaded guilty and was given a choice between imprisonment and probation conditional on undergoing hormonal treatment (chemical castration) with diethylstilbestrol (DES), a synthetic oestrogen. He chose the latter. The treatment lasted approximately one year and caused breast tissue growth (gynecomastia) and other physical changes.

His security clearance was revoked, and he was barred from continuing cryptographic consultancy work for GCHQ.

Death

On 7 June 1954, Turing was found dead by his housekeeper. The cause of death was cyanide poisoning. An inquest determined it was suicide. A half-eaten apple was found beside his body, though it was never tested for cyanide. His mother and others have argued the death was accidental, noting his careless handling of chemicals, but the suicide ruling has not been officially overturned.

Posthumous recognition

Turing Award (established 1966) — the highest award in computer science, awarded annually by the Association for Computing Machinery (ACM). Often described as the "Nobel Prize of computing."
Royal pardon (24 December 2013) — granted by Queen Elizabeth II under the Royal Prerogative of Mercy
Alan Turing law (2017) — the Policing and Crime Act 2017 retroactively pardoned men cautioned or convicted under historical legislation that outlawed homosexual acts
Bank of England £50 note (2021) — Turing's portrait, alongside imagery of the Bombe and the Pilot ACE, appears on the polymer £50 note
Statue — a life-size bronze statue of Turing sits in Sackville Gardens, Manchester, depicting him holding an apple
GCHQ headquarters — the Turing Building at GCHQ's Cheltenham campus is named in his honour
Film — The Imitation Game (2014), starring Benedict Cumberbatch, dramatised Turing's wartime work and prosecution

Turing is widely regarded as one of the most important scientists of the twentieth century. His theoretical work on computability underpins all of modern computer science; his wartime cryptanalysis was decisive in Allied victory; his conception of machine intelligence anticipated the artificial intelligence field by decades; and his mathematical biology opened an entirely new branch of science.

References

Turing, A.M. (1936). "On Computable Numbers, with an Application to the Entscheidungsproblem". Proceedings of the London Mathematical Society. 2 (42): 230–265.
Turing, A.M. (1950). "Computing Machinery and Intelligence". Mind. 59 (236): 433–460.
Turing, A.M. (1952). "The Chemical Basis of Morphogenesis". Philosophical Transactions of the Royal Society of London. Series B. 237 (641): 37–72.
Hodges, A. (1983). Alan Turing: The Enigma. Simon & Schuster.
Copeland, B.J., ed. (2004). The Essential Turing. Oxford University Press.
Singh, S. (1999). The Code Book. Fourth Estate.