But what about wormholes? Doctor Who makes ample use of wormholes to jump between times and worlds. Are these convenient highways in space-time real?
Possibly. Einstein’s theory of general relativity proposes that huge objects with extreme gravity can cause distortions in space-time. In 1935, he and the physicist Nathan Rosen proposed a possible product of that kind of distortion: the Einstein-Rosen bridge, a “path” both backward and forward in space-time.
Physicists have often explained the path, now known as a wormhole, like this: Imagine space as a two-dimensional surface. Then, fold “space” in half. A path between what are now the top and bottom sheets provides a route between otherwise extremely distance points in the universe, or even points between universes. That's the wormhole.
Of course, folding space might not be as easy as the thought experiment. Plus, as far as we know, it’s not possible to actually travel along that hypothetical route, as Doctor Who and his companions do. The first problem is finding a wormhole. Wormholes are derived implications of broader theories, which means that they, too, are theoretical. While these convenient tunnels might exist, none has ever been spotted. Plus, time travel through theoretical wormholes faces lots of hypothetical problems. While some physicists have used quantum mechanics to propose that exotic matter could stabilize the holes long enough to make them traversable, these super-routes through space-time are generally thought to exist for only an instant.