How Time Travel Works, Theoretically, In Less Than 1000 Words
Science-fiction writers and readers have dreamed about traveling through time for hundreds of years, and while it may not be possible to step through a wormhole to the future or flip on a time machine, some researchers believe there are legitimate ways to travel through time. They're just not easy, safe, or even a little practical.
Two Black Holes
Traveling into the future is relatively easy—just wait. Going back in time even a few seconds, however, could require a terrifying sequence of events, like the collision of two black holes. In the vicinity of a black hole, space and time is so distorted that if two of them happened to high-five, a path could be traced around the two spatial anomalies that would take a traveler back ... to the beginning of their journey. According to astrophysicist Neil deGrasse Tyson, "it would be quite a ride for any material swirling in its vicinity," so it's probably not worth it just to go back where you started unless you have a death wish or some kind of Groundhog Day fetish.