From H.G. Wells to Marty McFly, time travel has made for quintessential science fiction.
But it turns out that time travel is possible in the real universe, too. To see why, watch this video, produced by Minute Physics:
The video starts out by redefining what time travel actually means. After all, as astrophysicist Carl Sagan famously joked, we’re all time travelers—at a rate of exactly one second per second. The key to what we normally think of as time travel is changing the rate at which we move through time. And to a certain extent, we do that in our everyday lives.
Ever since Albert Einstein developed his theories of special and general relativity, we’ve known that time and space are linked, and that moving through space actually slows down time. (In the video this is expressed by noting that if you walk around the block you will be three femtoseconds—a millionth of a billionth of a second—younger than someone who stayed in one place.) The passage of time is also affected by gravity, which is why clocks on satellites require different configurations than regular clocks.
But what about going back in time? That’s possible too, as long as the universe is spinning extremely fast. The spin creates spacetime “loops,” meaning that like circling the earth, one can travel through time and end up at back at one’s starting point.
Which, of course, brings us back to the mind-benders held so dear by science fiction writers, such as the Grandfather Paradox: What would happen if, somewhere along your time loop, you were to kill your own grandfather?