Alan Mathison Turing was a British mathematician and pioneer of computing. He made seminal contributions to a number of fields: to mathematical logic, to cryptanalysis (code breaking), to computer architecture, to the philosophy of computation and artificial intelligence, and to mathematical biology. His significance is evidenced by the fact that the highest academic honour in computer science, the ACM's Turing Award, is named after him. Turing's top-secret wartime work as a codebreaker helped defeat Nazi Germany.
2012 marks Turing's centenary (he was born June 23, 1912).
Turing's first major scientific contribution was his proof, in 1936, that the decision problem for first-order logic was unsolvable. The solution to the problem itself was important, since it settled one of the main open problems in the foundation of mathematics. However, the method Turing used to solve the problem was perhaps even more significant. He introduced one of the first precise models of computability, now known as the Turing machine model of computation, which forms the basis for all modern computability and complexity theory, and the idea of a universal computer (essentially, a programming language interpreter).
During WWII, Turing worked at Bletchley Park, Britain's secret communications headquarters. There, he was responsible for the design of the Turing-Welchman bombe, a critical part of the effort to decode secret German communications. He broke the German naval Enigma cypher and later the Lorenz cypher, in the process inventing the statistical technique of sequential hypothesis testing. His work was invaluable to the Allied war effort, yet his contributions and the efforts at Bletchley Park as a whole were kept secret until long after Turing's death.
His experience with electronics at Bletchley Park prepared Turing for his post-war work at the National Physics Laboratory, where he designed one of the first stored-program computers, the ACE.
In philosophy, Turing is known, in addition to his analysis of the notion of computability, for his contributions to the analysis of consciousness and his work on the possibility of artificial intelligence (viz., the Turing Test).
His 1952 paper "The chemical basis of morphogenesis" is one of the classics of spatial modelling, describing how non-uniform patterns such as spirals can arise naturally.
He was open and unapologetic about his sexual orientation at a time when homosexuals were marginalized and persecuted. In 1952, Turing's homosexuality resulted in criminal prosecution for "gross indecency". He chose chemical castration in lieu of a prison term. Two years later, he died of cyanide poisoning in an apparent suicide. In 2009, Prime Minister Gordon Brown issued an official apology for the way the British government treated Turing.