Paging Sarah Connor!

After decades as a sci-fi staple, artificial intelligence has leapt into the mainstream. Between Apple's Siri and Amazon's Alexa, IBM's Watson and Google Brain, machines that understand the world and respond productively suddenly seem imminent.

The combination of immense Internet-connected networks and machine-learning algorithms has yielded dramatic advances in machines' ability to understand spoken and visual communications, capabilities that fall under the heading "narrow" artificial intelligence. Can machines capable of autonomous reasoning—so-called general AI—be far behind? And at that point, what's to keep them from improving themselves until they have no need for humanity?

The prospect has unleashed a wave of anxiety. "I think the development of full artificial intelligence could spell the end of the human race," astrophysicist Stephen Hawking told the BBC. Tesla founder Elon Musk called AI "our biggest existential threat." Former Microsoft Chief Executive Bill Gates has voiced his agreement.