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.
Wormholes
The idea of wormholes pops up time and again in science-fiction. Theoretically, if you slip into a wormhole, you'll exit at a different point in time and space, but that's easier said that done. It's not finding one that's the problem: Scientists believe space is absolutely lousy with the things. It's just that they're a billion-billion times smaller than an electron. Something that small is impossible to study, so while some researchers believe it's possible to enlarge those tiny wormholes to a size that someone could pass through, it's not clear how far one could travel or if anyone would survive the trip.
Light Speed
A better way to travel into the future is light speed. Thanks to physics weirdness, the faster you travel through space, the slower you travel through time. We're not talking about a sprint around the block—if you want to really make this kind of time travel work, you have to plan a long trip while moving at the speed of light, which is incredibly hard to do. Even astronauts orbiting Earth only approach about one-tenth of the speed of light. If you managed to build a craft that could handle it, though, you could theoretically land thousands of years into the future after tooling around space at light speed for just a few years.
The Time Doughnut
Science-fiction is full of time machines, from trendy cars to phone booths that are bigger on the inside, but while you can't just jam a flux capacitor in any old failed automobile, you can build a time machine that works as long as you have a big enough doughnut. Scientists hypothesize that a doughnut-shaped hole inside a sphere of normal matter can bend time. Using a forced gravitational field to create a time-like curve inside the sphere, whatever that means, the time traveler just has to run around the doughnut track inside the "machine." With every lap, they'll go further back into the past. The downside is that to build a time machine of this magnitude, you'd need a ton of cash and an entire team of scientists, so start writing those grant proposals now.
