Lets have a talk about time travel and hard science.
First: I’m going to describe why your FTL methodology is either impossible under our current understanding of the laws of the universe or way too complicated to explain.
Second: I’m going to explain why it really, really doesn’t matter.
Lots of people have noted, in varying degrees of complexity, that special relativity, causality and FTL travel do not make for a scientifically rigorous world. They tend to use mathematics or descriptions of ‘light cones’ that are both illuminating and potentially misleading. The descriptions and understandings of why this is a problem vary due to an underlying paradox that I’m about to try to explain. Now, entire theoretical physics PhDs could be devoted to long-winded descriptions, Minkowski spacetime diagrams and musings upon the curvature of spacetime inside a wormhole.
So I’m going to fire a rocket-propelled ladder at a barn.
Yes, you heard me right.
This is a common thought experiment amongst first-year relativity students, so if you’ve ever had to sit through the ‘ladder-in-a-barn’ lecture you might want to breathe a deep sigh. Done? Ok.
Imagine a barn with two glorious wooden doors at either end and assorted farmyard equipment lining the walls. Inside the barn a ladder is lying on the floor (why it isn’t hanging on a wall is beyond me). It fits almost perfectly between the two doors when they’re closed.
So far so good.
If I take the ladder out and move it through the barn at a nice steady pace with both doors open then I can let the ladder get into the barn, close both doors, open both doors again and continue out of the barn without ever slowing the motion of the ladder.
Now, I’m going to take the ladder out of the barn and strap it to an insanely powerful rocket capable of accelerating it to an appreciable fraction of the speed of light. Don’t worry about friction or structural stability: This is theoretical physics: Everything is done in a vacuum using indestructible farmyard equipment (and sometimes spherical chickens). Once that’s done I’m going to open the doors of the barn and fire our relativistic rocket-propelled ladder towards the gap (Not really, my supply of relativistic rocket-propelled ladders is somewhat limited).
So now we have a ladder being fired through a barn at almost the speed of light.
Now I need to explain something.
Special relativity, fundamentally, is the idea that no matter where you are or how fast you’re going light always moves at the same speed. Again, for you pedants out there, we’re doing theoretical physics. That’s light in a vacuum.
This leads us to a problem, because if there is light shining down the length of our ladder it must take the same time for it to get from one end of the ladder to the other regardless of whether we’re standing on the ladder or standing by the barn.
Read that last paragraph again. It’s important. Read it? Good.
This means that if we’re by the barn looking at the ladder the ladder must be shorter, or else the light would get from one end of the ladder to the other too slowly according to the people on the ladder. But what about if we’re on the ladder? The same thing holds true. The barn has to be shorter, or again the light will get from one end to the other too slowly according to the people in the barn!
When I say the ladder must be shorter I don’t mean look shorter. It must actually be shorter. This is known as Lorentz contraction and it goes hand in hand with relativistic time dilation. I’m not going to get into any more specifics on those now though, instead we’re just going to move onto the core of this paradox.
From the point of view of the barn the ladder is now a lot shorter. This is great! We can now get it into the barn, close both doors and let it move for a (very short) while with both doors closed before we open the doors again!
But wait… From the point of view of the ladder the barn is a whole lot shorter… Now it won’t fit inside the barn at all… We’ll have to get it partway in, close the second door then immediately open it again, move the ladder through until it clears the first door, close the first door and open it again and then finish moving the ladder all the way out of the barn…
THIS IS NOT THE PARADOX.
Let me say that again.
THIS IS NOT THE PARADOX.
The reason it’s not a paradox is because, contrary to what you might believe, nothing happens at the same time. That moment when we closed the doors at the same time? Didn’t actually happen. What you saw were doors opening and closing as the ends of the ladder got close to them, but the actual timings of those events do not have to match up if the frame of reference is different. This is fundamentally because the information about those events can only move at the speed of light, and the speed of light has to be the same in all frames of reference. Information about the position of the front and back of the ladder reaches both doors at the same time from the point of view of the barn, but at different times from the point of view of the ladder, so the doors open in a different sequence.
Simultaneous events never actually occur. This is a bit of a mind-bending concept, so we’re going to take a few seconds. Experience them at your own rate.
… … …
That will do nicely.
Time for the paradox.
Imagine, now, that you have an FTL device that can send signals faster than the speed of light. This device lets you slave the second door to the first, so that the instant the first door opens an FTL message gets to the second door and the second door opens too. Similarly for closing the doors.
Now let’s stand by the barn and launch the rocket.
The ladder enters the barn, the two doors close, the two doors open, the ladder leaves. All is good.
And the same from the point of view of the ladder?
The ladder enters the barn. It goes pretty much all the way through. The back end of the ladder clears the first door. Both doors slam shut, the second set closing on a ladder moving at almost the speed of light.
The energy yielded by the collision is immense, blasting the doors off their hinges and shivering the ladder into splinters, almost all of which immediately combust. The barn is torn from the floor by the sheer forces involved, leaving behind some invincible farm implements and a flock of spherical chickens.
This is the paradox. Having an FTL device lets you set up situations where the events that occur differ depending upon your point of view. And again, I don’t mean it looks like it differs. One frame of reference results in a peaceful country evening in the vacuum. A different frame of reference results in flames, splinters, silent screaming and a bemused ovoid hen.
And it doesn’t matter if you introduce a delay to the FTL device, or what kind of FTL method you’re using. No matter how long you make the delay, if any signal moves faster than light I can make the ladder go fast enough to have the same problem: Effects differ upon your point of view. If I introduce the point of view of the near-luminal falcon divebombing my damn chickens things get even worse!!
Manipulating this paradox leads to all manner of weird and wacky interpretations of this problem, including bombs preventing their own detonation and people being seen arriving on planets before they left them, but the fundamental problem remains the same: Events can be seen differently depending upon your frame of reference. Going faster than light doesn’t make you time travel per se. It makes you potentially time travel for someone else while you see absolutely nothing wrong with the situation, and that is a far bigger problem.
Now, methods for getting around this problem hinge upon somehow fixing this. Preferred frames of reference don’t work, because the barn can be in the preferred frame while the ladder isn’t. Not allowing FTL to cause paradoxes might work, but then you can manipulate FTL devices to cause anti-paradox paradoxes that are even worse. Not allowing FTL travel inside the visible light sphere is a pretty good way around this issue, but if you jump back and forth enough you can still set up paradoxes (unless you travel via space-magic gateway, at which point you may as well be travelling to a different universe). Distorting space to somehow allow for the FTL devices might work, but it’s fearsomely complicated (requires general relativity, and I’ve never heard anybody say ‘You know what this story needs? A dash more general relativity’) and probably breaks some laws anyway.
So your FTL device breaks the universe as we like to think of it.
How do we get around this?? What possible way could we, as builders of worlds, use to avoid this terrible issue!?!
The answer is really simple.
Don’t worry about it.
Seriously. Unless the interaction of FTL systems and relativistic travel is going to be a major part of your story, don’t even bother trying to work your way though this problem. This is one situation in which expansive handwavy explanations are ill advised. I know that’s hard to accept, but trust me, trying to dig yourself out of the FTL-Relativity-Causality triangle is nigh on impossible. Just saying that you have FTL of some form and leaving it as a ‘thing that someone from the future fixed’ or an unmentioned fact of your world is generally better.
Trying to dig too much into the nuts and bolts of FTL travel is either going to turn into quite the treatise on relativity or this entire problem isn’t worth writing much about in the first place. The more you draw attention to it the more it’s going to be a niggling thorn in the side of readers who care, and the more it’s going to annoy the readers who really don’t.
Don’t write situations where it comes up as a plot point (Gates to other planets but nobody really goes to space). Create an FTL system where the entire concept is moot (Aliens offer to let you hitchhike occasionally, but they wont tell you how they do it). Make FTL happen, but never in a way where relativity comes into play. Make relativity a thing that never interacts with the FTL technology. Implicitly allow causality violations but set up the narrative so they never occur. If you need to then you can just ignore the whole topic; assume that everything does actually happen simultaneously and light just takes a while to get places. The vast majority of sci-fi writers who want to use FTL, be it for video games, literature, films or television, completely gloss over the fact that they’re breaking the universe. Those that don’t gloss over the fact give it very, very little time in their work, essentially just nodding to the fact that it’s a thing and then getting on with the more important job of writing a thoroughly entertaining piece of fiction. And you can do the same! You’re building the universe, after all. You get to choose what is important.
Now, this isn’t to say that you can just go wild with FTL devices. You need to have some sort of internally consistent framework for travel, be it that they’re fixed wormholes, or you jump into a hyperspace dimension, or even that you need to get in touch with friendly aliens. Make sure that you don’t change the rules whenever you feel like it: Even if the science is iffy and you’re just ignoring some laws of physics you can’t ignore the rules of a decent story, one of which is that people like to understand the rules of the story, even if they aren’t entirely the rules of the real world. The point is that no matter what your method of FTL travel, trying to fully explain it in an otherwise hard-science world won’t go all too well.
If anybody reads, plays or watches your work and decides that the lack of completely accurate treatment of FTL travel is the killer, then they’re either the kind of person who thinks that any kind of treatment of FTL travel is a killer or they’re about to revolutionise physics with a truly astounding proof that unfettered FTL is possible.
At which point it’s a thing that someone from the future fixed.