When hugely intelligent and accomplished people start worrying a lot about artificial intelligence , should we join them? The physicist Stephen Hawking recently told the BBC that “the development of full artificial intelligence could spell the end of the human race”. Earlier this autumn the entrepreneur Elon Musk called AI “our biggest existential threat” and compared the research under way to “summoning the demon”.

Why is this? Prof Hawking gives a concise summary of the big fear: that once there is true artificial intelligence — a full digital version of the human mind — it “would take off on its own, and redesign itself at an ever-increasing rate?.?.?.?Humans, who are limited by slow biological evolution, couldn’t compete, and would be superseded.”

AI does appear to be taking off: after decades of achingly slow progress, computers have in the past few years demonstrated superhuman ability, from recognising street signs in pictures and diagnosing cancer to discerning human emotions and playing video games. So how far off is the demon?