Look, we’re not totally ignorant about time. We know that the dimension of time is woven together with the three dimensions of space, creating a four-dimensional fabric for the Universe. We know that the passage of time is relative; depending on your frame of reference, you can slip forward into the future as gently as you please. (You just need to either go close to the speed of light or get cozy with a black hole, but those are just minor problems of engineering, not physics.)
But as far as we can tell, we can’t reverse the flow of time. All evidence indicates that travel into the past is forbidden in our Universe. Every time we try to concoct a time machine, some random rule of the Universe comes in and slaps our hand away from the temporal cookie jar.
And yet, we have no idea why. The reasons really seem random; there is nothing fundamental we can point to, no law or equation or concept that definitively explains why thou shalt not travel into the past. And that’s pretty frustrating. It’s obvious that the Universe is telling us something important… we just don’t know what it’s saying.
Go ahead, kill your grandfather
There are all sorts of philosophical debates for and against the possibility of time travel. Take, for example, the famous “grandfather paradox.” Let’s say you build a time machine and travel back in time. You find your own grandfather and shoot him dead (I don’t know why, but roll with me here). But wait… if your grandfather is dead, it means he can’t father your father, which means you never exist. So how did you go back in time to do the awful deed?
Perhaps, however, time travel into the past is, indeed, allowed, but your actions are constrained. Maybe the past already exists and is completely set in stone. What has happened has simply happened. If you had the ability to travel back in time and monkey around with the past, then the past should already encode those acts—nothing is new, because it’s literally in the past. So you can’t kill your grandfather because you never have, but you could be the stranger that sets him up on a blind date with grandma.
Maybe, like, time doesn’t even exist, dude. Maybe it’s a construct of our human consciousness as a way to organize and synchronize our sensory inputs. Maybe we’re imposing some deep, fundamental preconceived notion on a Universe that doesn’t care, and so this whole discussion is moot.
This is all part of very legit discussions of philosophy. But let’s see if physics can take a crack at it. After all, if we could (even theoretically) build a time machine, then that would settle a lot of late-night bar bets.
So can we?
Closed time-like curves
Physicists use a very particular language when trying to build time machines: the language of gravity, given to us by old Albert himself in the form of general relativity. That’s because the language of gravity as interpreted in GR is a story of the bending and warping of spacetime. GR is a theory of motion in our Universe and how that motion is tied to the underlying four-dimensional fabric of spacetime.
In GR, matter tells spacetime how to bend, and the bending of spacetime tells matter how to move.
To determine whether we can build a time machine, physicists want to know if it’s possible to construct a spacetime—to find a particular and peculiar arrangement of matter—that allows one to travel into the past.
The goal is to find “closed time-like curves,” or CTCs.
“Curve” means exactly what you think it does—a path through space and time. “Time like” means no cheating—at no point are you allowed to travel faster than light. “Closed” means that the curve meets up back with itself—imagine traveling in one direction, always moving forward, never exceeding light speed. Yet at the end of your journey, you find you’ve arrived in your own past.
That’s a time machine. That’s a CTC.
The weird thing is, CTCs exist! Over the decades we have managed to uncover many solutions of general relativity that allow for backward time travel:
- The (in)famous mathematician Kurt Godel (yes, that Kurt Godel) discovered if a universe is filled with uniform dust that was slowly rotating, you could find trajectories in that universe that wind up in their own past.
- You know wormholes, right? Those shortcuts through space? They can also act as time machines. The trick is to take one end of the wormhole and hold it still. Then take the other and accelerate it close to the speed of light. Keep it at that speed for however long you want. Now bring that end back to the original one. The two ends of the wormhole now no longer have synchronized clocks because of the time dilation effects of the near-lightspeed travel. Since one end is in the past of the other end, you can just hop on in and travel back in time.
- Let’s say you had an infinitely long cylinder (maybe you pick it up at your local home improvement store). Rotate that cylinder to nearly the speed of light. If you follow a careful, corkscrew path around the rotating cylinder, then, by golly, you’ll wind up in the past.
- The inside of a rotating black hole is a pretty interesting place, where the competing countercurrents of gravitational and centrifugal forces meet to open a throat in the center of a black hole, creating the possibility of CTCs.