Look, we’re not completely ignorant about time. We all know that the dimension of time is woven along with the three dimensions of house, making a four-dimensional material for the Universe. We all know that the passage of time is relative; relying in your body of reference, you’ll be able to slip ahead into the longer term as gently as you please. (You simply have to both go near the velocity of sunshine or get cozy with a black gap, however these are simply minor issues of engineering, not physics.)
However so far as we will inform, we will’t reverse the stream of time. All proof signifies that journey into the previous is forbidden in our Universe. Each time we attempt to concoct a time machine, some random rule of the Universe is available in and slaps our hand away from the temporal cookie jar.
And but, we don’t know why. The explanations actually appear random; there may be nothing basic we will level to, no regulation or equation or idea that definitively explains why thou shalt not journey into the previous. And that’s fairly irritating. It’s apparent that the Universe is telling us one thing essential… we simply don’t know what it’s saying.
Go forward, kill your grandfather
There are all types of philosophical debates for and in opposition to the potential for time journey. Take, for instance, the well-known “grandfather paradox.” Let’s say you construct a time machine and journey again in time. You discover your individual grandfather and shoot him lifeless (I don’t know why, however roll with me right here). However wait… in case your grandfather is lifeless, it means he can’t father your father, which suggests you by no means exist. So how did you return in time to do the terrible deed?
Maybe, nonetheless, time journey into the previous is, certainly, allowed, however your actions are constrained. Possibly the previous already exists and is totally set in stone. What has occurred has merely occurred. In the event you had the flexibility to journey again in time and monkey round with the previous, then the previous ought to already encode these acts—nothing is new, as a result of it’s actually previously. So you’ll be able to’t kill your grandfather since you by no means have, however you possibly can be the stranger that units him up on a blind date with grandma.
Possibly, like, time doesn’t even exist, dude. Possibly it’s a assemble of our human consciousness as a method to set up and synchronize our sensory inputs. Possibly we’re imposing some deep, basic preconceived notion on a Universe that doesn’t care, and so this entire dialogue is moot.
That is all a part of very legit discussions of philosophy. However let’s see if physics can take a crack at it. In spite of everything, if we might (even theoretically) construct a time machine, then that might settle a whole lot of late-night bar bets.
So can we?
Closed time-like curves
Physicists use a really explicit language when attempting to construct time machines: the language of gravity, given to us by previous Albert himself within the type of basic relativity. That’s as a result of the language of gravity as interpreted in GR is a narrative of the bending and warping of spacetime. GR is a concept of movement in our Universe and the way that movement is tied to the underlying four-dimensional material of spacetime.
In GR, matter tells spacetime methods to bend, and the bending of spacetime tells matter methods to transfer.
To find out whether or not we will construct a time machine, physicists wish to know if it’s doable to assemble a spacetime—to discover a explicit and peculiar association of matter—that enables one to journey into the previous.
The purpose is to search out “closed time-like curves,” or CTCs.
“Curve” means precisely what you assume it does—a path by means of house and time. “Time like” means no dishonest—at no level are you allowed to journey sooner than mild. “Closed” implies that the curve meets up again with itself—think about touring in a single route, all the time transferring ahead, by no means exceeding mild velocity. But on the finish of your journey, you discover you’ve arrived in your individual previous.
That’s a time machine. That’s a CTC.
The bizarre factor is, CTCs exist! Over the many years we now have managed to uncover many options of basic relativity that enable for backward time journey:
- The (in)well-known mathematician Kurt Gödel (sure, that Kurt Gödel) found if a universe is full of uniform mud that was slowly rotating, you possibly can discover trajectories in that universe that wind up in their very own previous.
- You understand wormholes, proper? These shortcuts by means of house? They’ll additionally act as time machines. The trick is to take one finish of the wormhole and maintain it nonetheless. Then take the opposite and speed up it near the velocity of sunshine. Maintain it at that velocity for nonetheless lengthy you need. Now convey that finish again to the unique one. The 2 ends of the wormhole now not have synchronized clocks due to the time dilation results of the near-lightspeed journey. Since one finish is previously of the opposite finish, you’ll be able to simply hop on in and journey again in time.
- Let’s say you had an infinitely lengthy cylinder (perhaps you choose it up at your native house enchancment retailer). Rotate that cylinder to almost the velocity of sunshine. In the event you observe a cautious, corkscrew path across the rotating cylinder, then, by golly, you’ll wind up previously.
- The within of a rotating black gap is a reasonably attention-grabbing place, the place the competing countercurrents of gravitational and centrifugal forces meet to open a throat within the middle of a black gap, creating the potential for CTCs.