Alan Turing was a war hero prosecuted for being gay. He finally got a pardon.

December 24, 2013

Alan Turing (Science Museum, London/SSPL)

Alan Turing was one of the greatest minds of the 20th century. He laid the theoretical foundations for computer science, and his code-breaking efforts during World War II greatly aided the Allies's effort to defeat Adolf Hitler.

But Turing, a gay man, had the misfortune to live in an intolerant era. In 1952, he was prosecuted for "gross indecency" for having a sexual relationship with another man. He lost his security clearance and was forced to take hormone treatments. Two years later, he was found dead of cyanide poisoning. His death was ruled a suicide.

Now, the BBC reports, the computing pioneer has finally received a pardon from Queen Elizabeth. It only took the British government 61 years to recognize its mistake. Hopefully, it won't take the British government long to acknowledge that other gay men prosecuted under the same law also suffered an injustice.

There was no subject called computer science when Turing began his work in the 1930s, so he set to work laying its foundations. He first described what came to be called the Turing Machine in a famous 1936 paper. The Turing Machine consisted of a paper tape with characters written on it and a "head" that could read and alter the tape, one character at a time.

The Turing Machine was a thought experiment, not a practical computing device. But Turing used it to prove some of the fundamental theorems of computer science. He showed that given enough time and paper tape, the Turing Machine could perform every calculation that can be performed by any other sequential computing device, including those that would be invented far in the future. In other words, Turing proved that there's a fundamental equivalence among computing devices. Given enough time and storage space, every computer can perform exactly the same computations as every other computer.

Turing also showed that there are some problems that a Turing machine, or any computing device, cannot solve in any finite amount of time. For example, it's not possible to write a computer program that, given another computer program, can predict whether the second program will halt after a finite amount of time or continue running forever.

Today these fundamental insights about the nature of computation are taught in computer science departments around the world.


A German Enigma machine. (ell brown)

When the war came, Turing threw his considerable genius into defeating the Nazis. Turing was a key figure at Bletchley Park, Britain's wartime code-breaking lab. The Nazis used a sophisticated mechanical cipher machine called the Enigma to send coded messages. Turing and his colleagues at Bletchley Park built some of the most sophisticated mechanical computing devices yet seen to help break the Enigma code. And they succeeded, giving the Allies priceless intelligence about Germany's war plans.

Unfortunately, the work at Bletchley Park was so sensitive that the public didn't learn of Turing's contribution to the war effort until long after his death.

Some gay-rights advocates fault the British government for waiting so long to acknowledge the injustice it perpetrated against Turing 61 years ago. More importantly, they say, thousands of other British men were also prosecuted under the "gross indecency" statute, and some are still alive today. They hope that Turing's pardon will be the first step toward a pardon for all gay men who were prosecuted under Britain's homophobic laws.

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