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What if there were two worlds, the real one and its digital reflection? The real one is strewn with sensors, picking up everything from movement to smell. The digital one, an edifice built of software, takes in all that information and automatically acts on it. If a door opens in the real world, so does its virtual equivalent. If the temperature in the room with the open door falls below a certain level, the digital world automatically turns on the heat.

This was the vision that David Gelernter, a professor of computer science at Yale University, put forward in his book "Mirror Worlds" in the early 1990s. "You will look into a computer screen and see reality," he predicted. "Some part of your world—the town you live in, the company