In the late 1950s far-sighted Isaac Asimov imagined a sunny planet called Solaria, on which a scant 20,000 humans dwelt on far-flung estates and visited one another only virtually, by materializing as “trimensional images”—avatars, in other words. “They live completely apart,” a helpful robot explained to a visiting earthling, “and never see one another except under the most extraordinary circumstances.”