Hartmut Neven believes in parallel universes. On a recent morning outside Google’s Los Angeles office, the 53-year-old computer scientist was lecturing me on how quantum mechanics—the physics of atoms and particles—backs the theory of a so-called multiverse. Neven points to the tape recorder between us. What we’re seeing is only one of the device’s “classical configurations,” he says. “But somewhere, not perceived by us right now, there are other versions.” According to Neven, this is true for not just tape recorders but all physical objects. “Even for systems like you and me,” he says. “There is a different configuration...