Alan Turing (1912–1954) invented the idea of the computer before a single one had
been built. In 1936, aged just 24 and trying to settle a deep question in mathematical logic, he
dreamed up an imaginary machine — an endless paper tape and a head that reads, writes and shuffles
symbols by simple rules. That
Turing's machine wasn't built to compute — it was built to find the boundaries of
computation. With it he proved that some problems can never be solved by any program at all,
most famously the
During the Second World War, Turing led the team at Bletchley Park that cracked the German Enigma cipher, building electromechanical machines to chew through the code faster than any human. Historians estimate the effort shortened the war by years and saved millions of lives. He also asked the question that still haunts artificial intelligence — "Can machines think?" — and proposed the Turing test to settle it. Tragically, Britain repaid its wartime hero by prosecuting him in 1952 for being gay; he died two years later, an apple beside his bed. He received an official royal pardon only in 2013, and now stares out from the £50 note.