Alan Mathison Turing (23 June 1912–7 June 1954) was a British mathematician, logician, and codebreaker whose research helped establish the foundations of computer science and artificial intelligence. He developed a mathematical model of computation, contributed to Allied codebreaking during World War II, designed an early electronic computer, and investigated biological pattern formation. His career connected abstract questions about mechanical calculation with practical computing and the study of machine intelligence. (amturing.acm.org)
Early life and education
Turing was born in London to Julius Mathison Turing, an Indian Civil Service official, and Ethel Sara Turing. He attended Hazelhurst preparatory school and Sherborne School before entering King’s College, Cambridge, in 1931 to study mathematics. He graduated with distinction in 1934 and was elected a fellow of King’s in 1935. Lectures by Max Newman introduced him to foundational problems in mathematical logic, including whether mathematical reasoning could be reduced to a general mechanical procedure. (kings.cam.ac.uk)
From 1936 to 1938, Turing studied at Princeton University under Alonzo Church, receiving his doctorate in mathematics in 1938. His work belonged to an international effort to understand the scope and limits of formal methods, rather than to an established discipline of electronic computing. (universityarchives.princeton.edu)
Computability and universal machines
In his 1936 paper On Computable Numbers, with an Application to the Entscheidungsproblem, Turing described an idealized calculating device now called a Turing machine. It operates on symbols written on a tape, using a finite collection of internal states and explicit rules for reading, writing, and moving. The model made the informal idea of a step-by-step algorithm mathematically precise without depending on a particular physical mechanism. (cs.virginia.edu)
Turing also described a universal machine capable of simulating other machines when supplied with encoded descriptions of their rules. This separated a general calculating mechanism from the instructions determining its task, providing a theoretical foundation for programmable computers. Universality concerns what can be computed in principle; it does not guarantee that a computation can be completed with practical amounts of time or storage. (cs.virginia.edu)
The paper established that the Entscheidungsproblem, associated with David Hilbert, has no general algorithmic solution: no mechanical procedure can decide the validity of every statement of first-order logic. Church independently obtained a corresponding result using a different formalism. Their approaches underpin the Church–Turing thesis, which identifies effectively calculable procedures with computation by a Turing machine. Turing’s argument also established undecidability results closely related to the modern halting problem, although his original formulation differed from its familiar presentation. (cs.virginia.edu)
Wartime cryptanalysis
After the outbreak of war in 1939, Turing joined the Government Code and Cypher School at Bletchley Park. He worked on the analysis of messages encrypted with the German Enigma machine, making a major contribution to British wartime cryptography and codebreaking. These achievements formed part of a large collaborative operation and built on earlier breakthroughs by Polish cryptanalysts. (turingarchive.kings.cam.ac.uk)
Turing devised the central approach used by the British bombe, an electromechanical machine for searching possible Enigma settings. It exploited a “crib,” or suspected fragment of plaintext, to identify contradictions and eliminate incompatible settings. Gordon Welchman’s diagonal-board refinement substantially improved its effectiveness, while engineering teams constructed the machines. The bombe supported key recovery rather than independently translating intercepted ciphertext into readable messages. (tnmoc.org)
The success of this work depended on interception, mathematical analysis, machine operators, engineers, and intelligence assessment. Wartime secrecy restricted public knowledge of the operation for decades. Turing received an OBE in 1946 for his service. (tnmoc.org)
Electronic computing and machine intelligence
Turing joined the National Physical Laboratory in 1945 and developed plans for the Automatic Computing Engine (ACE). His proposal addressed the practical organization of an electronic programmable computer. He left before the simplified Pilot ACE was completed and moved to the University of Manchester in 1948, where he contributed to work on the Manchester Mark I. (npl.co.uk)
His 1950 paper Computing Machinery and Intelligence, published in Mind, replaced an unrestricted debate about whether machines think with an operational question involving an imitation game. In the arrangement commonly associated with the Turing test, an interrogator communicates through text and attempts to distinguish a machine from a human respondent. The paper examined objections involving consciousness, mathematical limitations, creativity, and other supposed barriers to machine intelligence. (courses.cs.umbc.edu)
Turing also considered learning machines, suggesting that a relatively simple “child” program might be educated rather than supplied with every feature of an adult mind in advance. This connected his discussion to what became machine learning, without specifying the architectures or training methods of later systems. (courses.cs.umbc.edu)
Mathematical biology
In 1952, Turing published The Chemical Basis of Morphogenesis. He investigated morphogenesis, the development of biological form, through interacting chemical substances that react and undergo diffusion. His analysis showed how a nearly uniform system could become unstable and develop spatial patterns. (dna.caltech.edu)
The proposed reaction–diffusion systems were simplified mathematical models, not complete descriptions of embryonic development. Their significance lay in demonstrating a mechanism by which local chemical interactions and transport could generate organized structure without an externally imposed spatial template. Turing explicitly acknowledged the idealized character of his assumptions. (dna.caltech.edu)
Prosecution, death, and commemoration
In 1952, Turing was convicted of gross indecency for a sexual relationship with another man and underwent compulsory hormone treatment as part of his punishment. He died on 7 June 1954; an inquest attributed his death to cyanide poisoning and returned a verdict of suicide. He had been elected a fellow of the Royal Society in 1951. (manchester.ac.uk)
The British government issued an apology for his treatment in 2009, and Queen Elizabeth II granted him a posthumous royal pardon on 24 December 2013. The Association for Computing Machinery established the A. M. Turing Award in 1966, naming it in his honor to recognize contributions of lasting importance to computing. (manchester.ac.uk)