Alan Turing
Alan Mathison Turing (23 June 1912 – 7 June 1954) was an English mathematician who did three things that each alone would fill a career: he defined what "computable" means, he broke the German Enigma cipher at Bletchley Park, and he wrote the founding question of artificial intelligence. He is often called the father of theoretical computer science.
The Turing machine (1936)
In 1936, at twenty-four, Turing published On Computable Numbers, answering one of mathematics' deepest open questions — the Entscheidungsproblem, whether a mechanical procedure could decide any mathematical truth. To answer it he invented an abstract device: a tape divided into cells, a read/write head, and a table of rules. He proved that this simple machine could compute anything computable — and that there exist problems no such machine can ever solve, including his own "halting problem" (no program can decide, in general, whether another program halts).
The universal Turing machine — one machine that could simulate any other by reading its description from the tape — is the mathematical form of the stored-program computer: the insight that instructions are data that machines can process. Every general-purpose computer, and every programming language from FORTRAN to AdaLogic BASIC, is Turing's machine made of matter. Turing completeness — a system's ability to express any computation the machine can — remains the benchmark of what counts as a real programming language.
Bletchley Park (1939–1945)
During the Second World War Turing worked at the Government Code and Cypher School at Bletchley Park, where he led the design of the Bombe — an electromechanical machine that attacked the German Enigma cipher by testing wheel settings against known plaintext. Enigma was broken routinely from 1941, and the intelligence it produced is credited with shortening the war by years and saving millions of lives. Turing also contributed to breaking the Tunny (Lorenz) cipher and worked on Delilah, a voice-scrambling system.
The work was so secret that Turing could never speak of it; his country decorated him in 1945 without being able to say why.
ACE and the thinking machine (1945–1954)
After the war Turing designed the Automatic Computing Engine at the National Physical Laboratory — one of the first detailed designs for a stored-program computer, and an unusually fast one. When NPL stalled on building it, he moved to Manchester, where he wrote programs for the Manchester Mark I, including some of the earliest experiments in machine chess and machine music.
In 1950 he published Computing Machinery and Intelligence, opening with the line "I propose to consider the question, Can machines think?" — and proposing the imitation game, now called the Turing test: if a machine's answers cannot be distinguished from a person's, the question of whether it "thinks" becomes unproductive to deny. The paper anticipated, and answered, nearly every objection raised in AI debates since — including the "Lovelace objection" Ada Lovelace raised a century earlier about machines' power to originate.
In his last years he turned to morphogenesis — the mathematics of how patterns like stripes and spirals emerge in living things — publishing a 1952 paper whose reaction–diffusion equations still anchor the field.
Persecution and aftermath
In 1952 Turing was prosecuted for homosexuality — then criminal in Britain — and given the choice of prison or hormonal treatment. He chose the treatment and continued working. He died on 7 June 1954, of cyanide poisoning, ruled suicide — though the evidence remains debated — two weeks before his forty-second birthday. An apple with a bite taken from it lay by his bed.
Britain pardoned him in 2013, and in 2017 the "Alan Turing law" posthumously pardoned thousands of men convicted under the same statutes. In 2021 his face went onto the £50 note — the scientist on Britain's most scrutinized banknote is the man the state once prosecuted.
The Turing Award, established by the ACM in 1966, is computing's Nobel-equivalent honor.