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.
He wrote the paper at Cambridge — and when it reached Princeton, Alonzo Church had already reached the same answer by a completely different route: his lambda calculus. The two definitions described exactly the same boundary of the computable, a convergence that convinced mathematicians the boundary was real. Turing sailed to Princeton that autumn to study under Church, finished his doctorate in 1938, then went home — and within a year was at war.
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 1946 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 Baby's successor, 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.
His chess program TUROCHAMP — written with David Champernowne in 1948 — was too complex for any machine then running, so Turing simulated it by hand, following the code's logic himself move by move. In 1952 it lost to a colleague in 29 moves; the man serving as the computer had invented the field anyway.
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.
See also
- History of Programming
- The first programmers · Manchester Baby
- Ada Lovelace — whose objection his test answered
- Charles Babbage · Analytical Engine
- John von Neumann · EDVAC report
References
- Turing, Alan M. "On Computable Numbers, with an Application to the Entscheidungsproblem." Proceedings of the London Mathematical Society s2-42 (1936).
- Hodges, Andrew. Alan Turing: The Enigma (Simon & Schuster, 1983).
- Copeland, B. Jack, et al. Colossus: The Secrets of Bletchley Park's Codebreaking Computers (Oxford UP, 2006).