Who was Alan Turing? The man behind modern computing

Alan Turing (1912–1954) was a British mathematician who was one of the founders of computer science and artificial intelligence. In 1936 he described an imaginary machine that captured what computing is. During World War 2 he was a leading codebreaker at Bletchley Park. In 1950 he asked whether machines could think, and proposed a test. His wartime work stayed secret until the 1970s, two decades after his death.

In short

• His 1936 "Turing machine" is the standard mathematical model of what computing is, and every general-purpose computer since works on its principle.
• At Bletchley Park he led the work on German naval Enigma and designed the bombe, as part of a large team that built on Polish breakthroughs.
• He was prosecuted in 1952 for a relationship with a man, died in 1954, and received a royal pardon in 2013.

What did Alan Turing do, in order?

Year	What happened
1912	Born on 23 June in Maida Vale, London
1936	Publishes "On Computable Numbers", describing the Turing machine
1939	Reports to Bletchley Park on 4 September, the day after Britain declares war
1946	Presents a design for the ACE, an early stored-program computer, at the National Physical Laboratory; appointed OBE
1948	Joins the University of Manchester, home of one of the first working computers
1950	Publishes "Computing Machinery and Intelligence", proposing the imitation game
1952	Publishes a paper on how patterns form in living things; prosecuted for "gross indecency"
1954	Dies on 7 June in Wilmslow, Cheshire, aged 41
2013	Granted a posthumous royal pardon on 24 December
2021	Appears on the Bank of England's £50 note, issued on his birthday

What is a Turing machine?

In 1936 mathematicians were asking whether there could be a mechanical method that decides, for any statement in mathematics, whether it can be proved. To answer, Turing first had to say exactly what "a mechanical method" means. His answer, in the paper On Computable Numbers, was an imaginary machine: an endless tape divided into squares, a head that reads and writes one symbol at a time, and a short table of rules saying what to do next.

Here is one at work. Its job is to add 1 to a number written in binary, here 1011, which is 11. The head starts on the rightmost digit, and it has two rules: if the square shows 1, write 0 and move one square left; if it shows 0 or is blank, write 1 and stop.

1. The head reads 1. It writes 0 and moves left. The tape reads 1010.
2. It reads 1 again. It writes 0 and moves left. The tape reads 1000.
3. It reads 0. It writes 1 and stops. The tape reads 1100.

1100 in binary is 12. Two rules, three steps, and the machine has done arithmetic without understanding numbers at all. Turing argued that anything a person could compute by following rules, such a machine could compute too. He then described a universal machine, one that reads another machine's rule table from its tape and imitates it. That is the idea behind every programmable computer: one machine that runs whatever program it is given.

The same paper settled the original question. Turing showed that no machine can decide, for every machine, whether it will keep producing output forever, a close cousin of what is now called the halting problem. So no general mechanical method for all of mathematics can exist. The American logician Alonzo Church reached the same answer by a different route the same year.

What did Turing do at Bletchley Park?

Bletchley Park was Britain's wartime codebreaking center, and the German Enigma machine was its central problem. Enigma scrambled each letter through rotors whose settings changed daily, giving an enormous number of possibilities. Polish mathematicians, led by Marian Rejewski, had broken earlier versions in the 1930s and built a machine called the bomba; in July 1939 they shared their work with Britain and France.

Turing designed the British bombe, an electromechanical machine that tested Enigma settings far faster than people could, by searching for settings consistent with a guessed fragment of the message. The mathematician Gordon Welchman added an improvement that made it far more effective. Turing also led Hut 8, the section attacking the German navy's Enigma, which was harder than the army and air force versions and mattered most for the Atlantic convoys.

He was one important figure in a collective effort. Thousands of people worked at Bletchley, many of them women, and breaking Enigma depended on Polish work, captured material, operators' mistakes and many other minds. His work there stayed secret for decades after the war.

Can machines think? The 1950 paper

In "Computing Machinery and Intelligence", published in the journal Mind in 1950, Turing set aside "can machines think?" as too vague to answer and replaced it with a game. An interrogator types questions to two hidden players, a person and a machine, and tries to tell which is which. If the machine can't reliably be told apart, Turing argued, we have as much reason to call it intelligent as we have for each other. This became known as the Turing test.

He also made a prediction, quoted in the Stanford Encyclopedia of Philosophy: that in about 50 years, computers would play the game so well that an average interrogator would have no more than a 70% chance of identifying them correctly after 5 minutes of questioning. The paper answers objections that still come up today, and it sketches "learning machines" that would be taught like a child rather than programmed in full, an early statement of the idea behind machine learning. The field of artificial intelligence got its name five years later, in 1955.

What happened to Alan Turing?

In 1952, after reporting a burglary at his home, Turing acknowledged a sexual relationship with a man, which was then a crime in Britain. He was prosecuted for gross indecency and, to avoid prison, accepted probation on condition of hormone treatment, often called chemical castration. He lost his security clearance and could no longer work for the government's codebreakers. He kept working, including on how patterns such as spots and stripes form in living things.

He died on 7 June 1954 of cyanide poisoning, and an inquest recorded suicide. That finding is the official one, but it is not unanimous: his family and some biographers have questioned or disputed it, noting among other things that he kept chemicals at home for experiments. What happened cannot now be known for certain.

The public apology came late. In 2009 the Prime Minister, Gordon Brown, apologized for the "appalling" way Turing had been treated. On 24 December 2013 he received a royal pardon under the Royal Prerogative of Mercy, announced by the UK government. A 2017 law, often called the Alan Turing law, extended pardons to thousands of other men convicted under the same laws. Since 23 June 2021, his face has been on the £50 note, beside a formula from his 1936 paper.

Why does Turing still matter?

Every computer, from a phone to the machines that train chatbots, is a universal machine in his sense: hardware that runs any program it is given. Every argument over whether an AI system "really" thinks still starts from the question he posed in 1950. For the story of what came after him, read the history of artificial intelligence, or for what is going on inside the machine itself, how computers work.

Check yourself

• Without looking back: what three parts make up a Turing machine, and what makes a machine universal?
• Why is it wrong to say Turing broke Enigma on his own?
• What did Turing replace the question "can machines think?" with, and why?

Sources

• On Computable Numbers, with an Application to the Entscheidungsproblem — Alan Turing, Proceedings of the London Mathematical Society, 1936
• The Turing Test — Stanford Encyclopedia of Philosophy, revised 2021
• Royal pardon for WW2 code-breaker Dr Alan Turing — UK Ministry of Justice, 2013
• Polymer £50 note — Bank of England, 2021
• Alan Turing: The Enigma — Andrew Hodges
• Alan Turing — Wikipedia