Okay, galleons, I know it’s been a bit since I posted anything of even moderate significance, so I’ll rectify that now. And, in an effort to straight-up avoid boring you with my emotional/philosophical musings of the last few days (those are better suited for poetry, anyway, so that’s where I’ll hide them), we’re going to talk about something science-y and fun.
The future you have tomorrow won’t be the same future you had yesterday. ~from Rant by Chuck Palahniuk
Time travel is one of the core vertabra in the spine of science fiction (along with teleportation, artificial intelligence, and genetic engineering). We spend many moments in our life thinking about what we would do if we could go back and change the past. Ethical issues aside, science tells us that time travel is nothing more than a figment of our imaginations- an impossibility.
But is it impossible… or just very improbable?
Before we leap into the midst of modern scientific discourse on the subject of time travel, we need to have a quick primer on the nature of spacetime itself. I’m sure you are highly intelligent individuals, galleons, but time is a tricky beastie:
Yes, you best get used to a bit of the good Doctor infusing this post- it’s only fitting, after all.
So, time. What is time, anyway? Where does it come from? Can there be multiple time dimensions if there are multiple space dimensions? Is there a smallest unit of time? Is time a fundamental part of the universe or merely a useful construct to aid in our perception of it? Can time end? Can there be a universe without time?
Burning questions all. Physics has found answers to some of these questions, but not all. We’ll just cover the important bits here, but note that answering all the questions of time is a major goal in the land of science.
Let’s start with the trickiest part of time- its lack of universality. We tend to think of time as a definite. As something we all share, and that we all experience the same way. When it is 8:34 for me, it is 8:34 for you (supposing you are in the same time zone, of course).
Thing is… that’s not actually the case. In June 1905, Albert Einstein submitted an article to the German Annals of Physics that effectively shattered our concept of the universality of time. Welcome to Einstein’s theory of special relativity, ladies and gentlemen. The rules of the game now are dependent on how the world appears to individuals moving relative to one another (subsequently, I always like to cite special relativity as proof there is no such thing as objective truth, but that’s a philosophical diatribe for another day). “Time for you need not be the same as time for me.” (The Fabric of the Cosmos, Brian Greene)
The differences in time between observers in normal situations is so tiny as to be unmeasurable. But, if we look at something a bit less normal, such as relative movement near the event horizon of a black hole, the differences become much greater. But that is a complicated and long-winded bit of explaining I’d have to do to help you understand that. If you already understand, fantastic! If you don’t, just know that we each carry our own clock, our own way of measuring the world around us.
Each of us moves through spacetime differently based on motion relative to one another for a simple reason. We are always moving. Each and every one of us is in constant motion. But how? What if you are sitting completely still? Well, if you are sitting still, it’s true that you aren’t moving in space- but you are moving through time. In fact, all of your motion is currently through time. However, if you were to stand up and start running, you’d now be moving through both time and space. Ergo, some of your movement through time is shifted to movement through space. Like if you are in a car, and you are traveling north. If you are suddenly to start going north and west, you won’t move as far north, even if you travel the same distance as before. It’s a bit of simple math there, right? A little basic geometry. Spacetime works the same way. That is how time changes based on relative motion- motion through space impacts motion through time.
Now here’s where things get even trickier. Absolute time doesn’t exist, right? Take it on faith here that absolute space also doesn’t exist. However, absolute spacetime does exist.
Imagine spacetime as a loaf of bread (I adore this analogy). According to the rules of special relativity, you and I will observe time differently. We’ll say that’s the same as slicing the loaf of bread. So, you will be slicing the spacetime loaf perpendicularly. I’ll slice it along a 45º angle. Our slices won’t be the same, will they? We’ll have different events on each one (half of each of my slices will be on later slices of your corresponding loaf- in your “past,” if you will). However, if we reconstitute all your slices into one loaf or all mine into one loaf, we’d create the exact same loaf of bread. The loaf of spacetime that is absolute.
If you are confused, I apologize. I’m trying to condense the hell out of this information, so things might be getting lost.
Here’s what you need to know: Time is relative. Spacetime is absolute.
Time doesn’t flow in the way we envision it. The past is not lost. The past is. As is the present. As is what we think of as the future. It all exists in this spacetime loaf we were talking about earlier. Our consciousness creates the idea of flow, but time doesn’t actually flow with and around us. But even though it doesn’t flow, time does have an arrow- a direction to the way things unfold. Cause-and-effect. An egg doesn’t break and then fall off the table. That’s not how the laws of causality work. So time does have an arrow, even if it doesn’t flow.
As I said, time is a confusing concept (and I didn’t even touch on entropy). The science goes against what we perceive. Welcome to theoretical physics.
Now that I’ve scrambled your brains, let’s move on to the good stuff!
When it comes to time travel, there are actually a lot of ideas on the concept that have come and gone through the ages. Let’s examine them:
Here’s the big one. Probably the most popular idea regarding time travel involves the use of wormholes (such as those theorized to lurk in quantum foam or at the heart of black holes- by the by, did you know it was Oppenheimer who proved that black holes could indeed form? Before him, they were nothing but speculation). But it’s not as simple as science fiction would lead you to believe:
See, when it comes to wormholes, the prevailing scientific idea is that a wormhole in our universe could lead, not to another time in our own universe, but to a point in another universe. A parallel universe.
This is the multiverse, galleons. The concept that hundreds, thousands, millions of universes all exist side-by-side, differing slightly (or greatly) from our own. A wormhole would connect our universe to one of these other universes.
Theoretically, we could enter a wormhole in our universe and travel through it to a point in an alternate universe that is temporally “earlier” than what we deem the present in our universe. This would feel like time travel to us, though we’d actually have moved to a completely different universe.
However, a wormhole could also connect two slices on our universe’s spacetime loaf. Just as an Einstein-Rosen bridge (as the “tunnel” connecting two wormholes is called) could connect us to a point in another universe, so too could it connect us to a point earlier or later in the timeline of our universe.
Of course, we could never traverse such a path. The tidal forces at the heart of a black hole, where the supposed wormholes would reside, become infinite, and anyone unlucky enough to fall into it would be ripped apart (much as you would be stretched and killed by falling into the singularity of a wormhole-less black hole).
So, despite their overwhelming popularity in time travel fiction, I’m afraid wormholes aren’t a terribly good method of time travel. I mean, beyond the fact that you wouldn’t even survive the trip, you have no way of knowing where the wormhole leads until you traverse it. Isn’t there a better way?
Van Stockum’s Time Machine
In 1937, a man by the name of W. J. Van Stockum found a solution for time travel in Einstein’s equations. If we could build an infinite, spinning cylinder that spun at or near the speed of light, it would effectively drag the fabric of spacetime around with it.
By traveling around this cylinder, you could attain phenomenal speeds. In fact (though Van Stockum didn’t realize this at the time), by making a complete trip around the cylinder, you could actually arrive back at the start before you left.
Hello, time travel.
The faster the cylinder spun, the further back in time you would travel (though you couldn’t travel further back than the cylinder’s creation).
Sadly, this is in no way physically possible. We cannot make infinite objects.
Also, even if we somehow managed to create this infinite cylinder, the centrifugal forces would be so enormous that they’d rip the cylinder apart.
Gödel’s Rotating Universe
And then there was Kurt Gödel. He also found a solution to Einstein’s equations, but in Gödel’s version, it was the universe that was rotating. So, instead of traveling around a cylinder, you’d just have to take a rocket around the universe and end up at an earlier point in time.
So, in principle, you could travel between any two points in space and time. Now, Gödel’s universe has to be spinning above a certain speed, otherwise gravity will override the centrifugal forces and the universe will collapse in on itself… Our universe, for example, would have to rotate once every 70 billion years.
Good news, though! You’d only have to travel at just under the speed of light. Which makes it theoretically possible (discounting the fact that you’d die if you traveled at that speed…).
Oh, but here’s the bad news:
Our universe doesn’t spin.
Thorne Time Machine
Well, some time went by before another scientist stepped up to bat on the issue of time travel. When Carl Sagan was writing Contact, he needed a way for his heroine to make a two-way trip to a distant star. So, he asked Kip Thorne for advice.
And Thorne showed that it actually was possible to build a time machine. Well, so long as you could acquire some pretty bizarre types of matter and energy, such as “exotic negative matter” and “negative energy.”
Okay, that’s problematic.
See, negative matter is weird. It’s not antimatter, in case you were wondering. Negative matter possesses antigravity and would float up in Earth’s atmosphere. It is repelled by ordinary matter and other negative matter.
So you aren’t going to find negative matter in your backyard. If it even exists, it would be drifting about in deep space.
And as for negative energy? Actually, even though it’s extremely rare, that can happen. Ever heard of Casimir plates? Henry Casimir proved that there is a very small attractive force between two uncharged metal plates held in parallel. They are held so close to one another that virtual particles (particles that appear and annihilate according to Heisenberg’s uncertainty principle) can’t easily get between them- with more virtual particles outside the plates than between them, a small force is exerted on the plates, pushing them toward one another. Voilà. Negative energy.
So, theoretically, we could make these parallel plates and shape them into a sphere. Then we’d make another one. Then we’d string a wormhole between them. And one sphere (containing the mouth of a wormhole) would stay behind while the other was loaded onto a ship and traversed space. And so you’d be able to travel back and forth between two distant points in space with ease.
Of course, due to the relative nature of time, the mouth of the wormhole on the spaceship would be at a different point in time than the one left behind… so you’d not only travel through space, but through time as well.
This one’s possible… supposing negative matter even exists. And that we could manage to dig a wormhole out of quantum foam or something, expand it, and attach it to these spheres we’ve managed to construct.
This one’s not happening anytime soon.
Hawking and Misner Space
It’s Stephen Hawking’s turn to take a crack at this time travel thing. To do so, he started with an example of Misner space. Misner space is an idealized space that becomes the entire universe. So, take the room you are in now. Let’s make it Misner space- your room is now the entire universe. If you were to walk into the left-hand wall, you wouldn’t hit it. Instead, you’d pass through it… and would re-enter the room/universe through the right-hand wall.
Remember old video games where you’d try to run off the left side of the screen but would just run back onto the screen via the right side? That is Misner space.
So, back to your universe/room. Imagine the walls start closing in. Now, if you walked through the wall, you’re going to get a boost of speed from the speed of the wall. Keep walking through the wall again and again and you’ll keep gaining speed, eventually approaching the speed of light (and, like before, you’d eventually travel so fast you’d go back in time).
According to Hawking, the walls of this Misner space are mathematically the same as the mouths of a wormhole. So, we’re back to semi-familiar ground. Problem is, this Misner space is really unstable. Eventually, quantum radiation effects would build up until they became infinite. At that point, if you tried to enter the wormhole/walk through the wall, it would collapse and you’d die.
Hawking figured that was it for time travel- it was impossible. However, other physicists have since proven him wrong. In some wormhole solutions, collapse didn’t occur. Like Sergei Krasnikov said, “there is not a grain of evidence to suggest that the time machine must be unstable.”
Hawking has since had to go back on his original declaration that time travel was impossible, but he maintains that it is still highly unlikely and impractical.
This method, however? Pretty freakin’ impractical. All it does is show that time travel isn’t completely impossible according to the laws of physics.
Gott Time Machine
Are you totally discouraged yet? Let’s talk about a completely different approach to the question of time travel, then. J. Richard Gott proposed an idea for time travel centered around the idea of cosmic strings.
And just what is a cosmic string? Good question. They’re a remnant of the big bang predicted by multiple theories- they are thinner than an atomic nucleus, but with an enormous mass and they could possibly be millions of light-years long.
Well, Gott discovered that if you hurl two cosmic strings at one another, you can use the time just before they collide as a time machine. See, the space around the strings actually shrinks as the two strings approach one another, so if you travel around one, you travel less than 360º by the time you return to your starting place. So, you travel rapidly around this shrinking space, and you’ll eventually exceed the speed of light (according to a distant observer- because you don’t exceed light-speed according to your observations, special relativity is preserved). And, as we’ve already learned, this will lead you to travel into the past.
By now, I’m sure you know what’s coming.
Cosmic strings are super rare, if they even exist at all. Finding two to collide? That’s almost laughable. And even if you shaped one string into a large loop and used it to double-back on itself to create the same effect as the two colliding strings, Gott admits that, “a collapsing loop of string large enough to allow you to circle it once and go back in time one year would have to be more than half the mass-energy of an entire galaxy.”
Again, not happening.
Seems time travel is a total bust thus far. While physics doesn’t deem it completely impossible, the methods scientists have come up with are absolutely ridiculous.
But what if we did manage time travel? See, there’s another set of rules floating around that seem to be an obstacle on our way to traveling through time: paradoxes.
Grandfather Paradox: “The chief argument against the possibility of time travel is what theorists refer to as the ‘Grandfather Paradox’; this is the idea that if one could travel backward in time one could kill one’s own ancestor, eliminating the possibility said time traveler would ever be born–and thus could never have lived to travel back and commit the murder. In a world where billions believe their deity conceived a mortal child with a virgin human, it’s stunning how little imagination most people display.” ~from Rant by Chuck Palahniuk
According to the laws of physics, the problems brought up by the Grandfather Paradox aren’t possible. Time is. It can’t be ripped. It cannot simply stop. So, even if you went back and killed your parents before you were born, you wouldn’t simply wink out of existence. It’s just not possible.
Why? There are two theories. First, there’s the idea that you would be forced to act in a manner so that no paradoxes occur. This tampers with the idea of free will (which is pretty much bullshit, anyway), but it prevents time paradoxes. Of course, according to this camp, inanimate objects sent back in time could still change history, so… I don’t know if I can fully back this camp.
The second theory ties in with the idea of the multiverse. If you go back in time and alter history, what’s happened is that the river of time has forked and the universe has split. There are now two parallel universes where there was only one before. One universe maintains the original timeline, and one changes based on the the alteration you just made. This is the camp I tend to prefer, even though it means there’s an infinite amount of universes.
Information Paradox: You know this one as well. You travel into the future and read a book. You love it, so you bring it back to the present with you. While in the present, you meet the author of the book and give the book to them. They publish it under their name… but who wrote the original book? In this paradox, information has no origin.
Bilker’s Paradox: In this paradox, you know what the future will be. So you decide to prevent it by doing something that makes the future impossible. You know, like all the time traveling pre-apocalypse films. The hero has to do something to prevent the apocalypse that has already occurred in their time.
The Sexual Paradox: This one is a big plot point in Rant. This is the paradox wherein you father yourself- you travel back in time and impregnate your mother… with you. A biological impossibility.
So, physics has some ideas about how to negate time paradoxes, but that really doesn’t matter, seeing as time travel is still so goddamn impossible.
Guess we’ll just have to stick with science fiction for our time travel fix.
If you actually read all the way through this long-ass post, you just gained +25 to your geek cred. Congrats!
No, but really… sorry this is so long. I tried really hard to keep the length in check. I failed, obviously. I just get overly talkative when it comes to physics.