Stephen Hawking says he’s obsessed with the idea of time travel these days. “To see how this might be possible, we need to look at time as physicists do—at the fourth dimension,” he writes. “Time flows like a river and it seems as if each of us is carried relentlessly along by time’s current. But time is like a river in another way. It flows at different speeds in different places and that is the key to traveling into the future.”