A Large Reputation, Properly Sized
Alan Turing (1912-1954) is among the most frequently named figures in the history of computing, and he is sometimes described in ways that go beyond the evidence, such as being called the sole computer inventor. A more accurate picture is that he made several major contributions in different areas, some theoretical, some practical and some speculative, while working alongside and building on the work of many others.
The 1936 Paper
Turing’s most lasting theoretical contribution came in 1936, when he was a young mathematician at Cambridge. His paper “On Computable Numbers” addressed a question from mathematical logic: is there a mechanical procedure that can decide whether any given mathematical statement can be proved? To answer it, he needed an exact description of what a mechanical procedure is.
He described an imaginary machine, now called a Turing machine, that reads and writes symbols on an endless tape according to a small table of rules. Although it is a thought experiment, not a device anyone built, it captures the idea of computation in a form that can be analyzed mathematically. Turing used it to show that some problems cannot be solved by any such procedure.
The American logician Alonzo Church reached a related conclusion at about the same time using a different formalism, and other researchers contributed to the same line of work. The results are often discussed together. Turing’s description was particularly intuitive, and the Turing machine became a standard tool of theoretical computer science. He also described a “universal” machine that could imitate any other such machine given the right instructions, a notion that is conceptually close to the later stored-program concept, although the practical machines of the 1940s were designed for different reasons and by different people.
Wartime Codebreaking
During the Second World War, Turing worked at Bletchley Park, the British codebreaking center. He was one of many mathematicians, linguists, engineers and operators there, and the effort relied on large teams. He is particularly associated with work on German naval Enigma messages and with the electromechanical codebreaking machines known as bombes.
The bombes did not appear from nothing. Polish mathematicians had broken early versions of Enigma in the 1930s and shared their methods with Britain shortly before the war, and colleagues such as Gordon Welchman made important contributions to the design and use of the British machines. Turing’s part is best described as significant and shared.
It is a common error to credit Turing with building Colossus, the electronic codebreaking machines used against a different German cipher system. Those were designed by Tommy Flowers and others, and Turing was not their designer. The confusion arises partly because both efforts were at Bletchley Park, and partly because secrecy kept the details out of public view for decades. The overall wartime work also influenced later thinking about computer security and cryptography, though the connections were not widely known until the 1970s.
After the War
After the war, Turing worked on a detailed design for a stored-program machine, known as the ACE, at the National Physical Laboratory in London. Delays and disagreements meant that a smaller pilot version was completed only in the early 1950s, and Turing moved on before then. He later joined the University of Manchester, where he worked on programming one of the early stored-program machines and wrote about how to use it.
Turing also thought about what machines might one day do. In a 1950 paper titled “Computing Machinery and Intelligence,” he proposed replacing the vague question “Can machines think?” with a game in which a judge tries to tell a machine from a person through written conversation. This became known as the Turing test, though he called it the imitation game, and the paper is often cited as an early contribution to artificial intelligence research. Later in life, he also worked on mathematical models of biological pattern formation.
Personal Life and Death
Turing was prosecuted in Britain in 1952 for homosexual acts, which were then criminal offenses, and he was subjected to a court-ordered medical treatment. He died in 1954, and the inquest concluded that it was suicide, though some people have questioned aspects of that conclusion. In 2009 the British government issued a public apology for his treatment, and he received a royal pardon in 2013. His story is frequently cited in discussions of how prejudice cost society the work of talented people.
Placing Him Among Many
Turing’s legacy is real, but it is clearer when set beside others. Charles Babbage and Ada Lovelace imagined programmable machines a century earlier. Zuse, Atanasoff, Flowers, the ENIAC team, the Manchester and Cambridge groups, John von Neumann and many unnamed engineers and operators all contributed to practical computers. Turing supplied some of the sharpest theoretical ideas and took part in a vital wartime effort, but the field was shaped by teams, a pattern of collaborative invention that runs through the story of the computing pioneers as a whole.
His name is also attached to the Turing Award, given by the ACM since the 1960s, and it helps explain why he is so widely known. Honoring him is fitting, as long as the honor does not turn a complicated, collective history into a story about one genius.