Picture a boy climbing on a train from Dusseldorf to Brussels, circa 2002. He switches on his handheld video game, and after a couple of seconds a message pops up on the screen: Someone else nearby is playing the same game, so would he like to join in? By hitting "yes," the boy sends a low-power radio signal that links his machine to the other. It's as if they're connected by a ca- ble. And the two children, who might not even see each other, play as if they're sitting in the same living room.
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