Multiplying Selves

“For any event, there is an infinite number of possible outcomes. Our choices determine which outcomes will follow. But there is a theory in quantum physics that all possibilities that can happen or could happen do happen in alternate quantum realities.” This summary of the many-worlds interpretation of quantum physics, a theory first proposed by the American physicist Hugh Everett in the 1950s, is given by a sentient android named Data serving as an officer on the Federation starship USS Enterprise. Technically it’s delivered by the actor Brent Spiner, performing as Data, at a pivotal moment in “Parallels,” an episode from the seventh and final season of Star Trek: The Next Generation. In that episode Data’s fellow officer Worf, a Klingon, returns to the Enterprise after some time spent at an intergalactic sports competition, only to discover that he has passed through a hole in space-time—there’s always a hole in space-time—and that the reality he now inhabits is different from the one he left, in ways that are subtle and profound, comic and tragic.Most strictly scientific formulations of the many-worlds interpretation hold that there is no practical way to move between branching...