FTL

I swear to god this isn't another skeptical argument on my life it isn't just trust me just read the post

It must needs be remarked that extrasolar space aliens are cool as hell. Unfortunately, extrasolar space aliens would be so far away that it would basically “be really annoying” to try to visit them. Even if you hit 100-ε percent c, it would take kind of a long time to get anywhere, according to the aliens at least. This is well known.

Slightly lesser known but still pretty well known, arbitrary FTL implies arbitrary time travel,¹ and arbitrary time travel is logically impossible for obvious reasons. I have been lead to believe that there exist epicycles you could impose which would prevent causality violations, but all the ones I have heard seem pretty silly. Of the sort “what if physics allows you to move FTL in one direction but refuses to let you move FTL back” or “what if the universe screens out certain information from relativistic observers”. Those sorts of solutions do, as far as I can tell, resolve the problem, but they seem very silly.

at this point in writing the article I did slightly more research and found that someone had already written about what sorts of FTL travel would have to be disallowed to preserve causality in considerably more detail than I intend to. I make no claims that their post is correct (I have taken ill, and have a hard time keeping track of relativistic effects at the best of times), but you know, look at it.

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However we do not care about what is silly in our universe! We care about what is silly in Pathfinder! Perhaps Otolmens reaches out and crushes any mortals who attempt to cast Interplanetary Teleport in a way that would break causality, what is wrong with that?

I’ll tell you what is wrong with that, it means that if someone teleports from A to B, nobody near B who is moving at relativistic speeds relative to A and B can teleport to C. Which feels weird and unprincipled. Also I am pretty sure that if someone got up to relativistic speeds (relative to A and B) and teleported from A to B, they can prevent anyone not moving at relativistic speeds from teleporting from B to A. But don’t quote me on that.

Which brings us to the real problem! Relativity is hard to keep straight for many of us! Any solution that relies on the GM or glowfic author figuring out what set of FTL transmissions are and are not allowed is setting an unworkably high bar. On the other hand, relativity is objectively cool as hell, so I do not want to throw it out. Therefore I have a few suggestions:

Apparently All Good Ideas Are Taken

Allegedly there is a book or set of books called Lockstep where² everyone agrees to go into cryosleep so STL transit is experienced as FTL. To be clear that isn’t the people on the ships, that is everyone on all the planets who agreed to this setup. Clever, excellent work around, it seems kind of insane as a solution for a science fiction setting, but it seems perfectly viable for Pathfinder. Any time someone casts Interplanetary Teleport to move from A to B, the Contract of Creation or Otolmens or whatever sets the velocity of everything in the universe to precisely the velocity of A, except for whatever is supposed to be moving FTL. The “FTL” payload then moves at 100-ε percent c (experiencing ~no passage of time itself, due to relativity) until it reaches B. Motion unfreezes, and life goes on. At no point does any information travel FTL, so causality cannot be violated, and everyone is too paused to notice that everything is paused.

What About The Time Stop?

Yes yes you are a very attentive reader aren’t you? It is really more of a motion stop than a time stop, though, so stop calling it that. But yes the motion stop cannot propagate FTL any more than the payload can. The way I see it, either all “FTL” transits were predicted infinitely far in the past, and motion can be paused all at once³ everywhere in the universe based on that prediction, or prophecy is truly broken and we have to draw at least one spacetime diagram.

Say the motion stop propagates at c from the point of origin, A. If the motion stop ceases everywhere the moment A arrives at B (which does not allow FTL information transfer any more than synchronizing your watch with someone allows FTL information transfer), A ends up stopped for a long time, and B does not end up stopped at all, which results in uneven temporal progress. This is weird and probably undesirable. If the motion un-stop propagates from A at c, I believe that just works, although the initial motion stop does have to continue propagating out to the entire rest of the universe, not just up to B, to prevent uneven temporal progress. See the below low quality physics diagram.⁴

Blue here indicates a region of stopped time, and red indicates our “FTL” traveler, who can move at a downright lethargic 0.5c. If a spaceship near B wished to move at relativistic speeds to send information back in time, they could not, because the closest they could get their space line to the motion stop propagation line would be just above it.

the author of this article spent the next several hours trying to figure out if moving at 0.8c, nearly instantly decelerating to 0c for the duration of the motion stop, and then resuming moving at 0.8c at the end of the motion stop was problematic, among other things

And just to be sure, let us look at a relativistic spaceship, R, which moves away from A at 0.8c. There is, so far as I can tell, no significant material difference between R proceeding at 0.8c, decelerating very quickly to a relative 0c for the duration of the motion stop, and then accelerating back up to 0.8c very quickly as the motion stop ends.⁵

This does of course require truly enormous efforts to make everything add up to normal. Everything in the entire universe has to be motion stopped relative to A for potentially years. But that is fine, Pharasma created the universe. Relative to that, this is hardly anything at all.

Speaking of Gods?

Oh, right, yes. So an obvious concern with all this dimension hopping and gods who can interfere on many different planets is that it breaks causality. How can it be the case that when Iomedae uses up all Her intervention budget due to events on Golarion, that nearly instantly causes Her to stop intervening on other planets, which are presumably many lightyears away? Same technique. Any time information has to be communicated from one place to another FTL, a motion stop is imposed to allow that information to propagate at speeds less than or equal to c.

Now, the universe may be infinite, and if it is that is a problem because the motion stop has to be applied for an infinite period of time, every time. In some sense, this is no problem, because nobody experiences that time anyways. In another sense, that may be a problem, because I do not know what it means for motion to be paused for two periods of time as infinite as the universe is big, and then resumed. Potentially that does not make sense and is an enormous problem.

Luckily we can cheat and say Pharasma’s Creation is a finitely large region of space. The rest of the universe can feel free to progress while Creation is in motion stop if they so like. What I mean to say is without border patrol we couldn’t have the subjective experience of FTL travel, making FTL travel innately illiberal.

And Regions of Creation?

If you can Plane Shift from Golarion to Axis and then Plane Shift from Axis to Aucturn (the farthest planet from the Sun), that also probably⁶ implies FTL travel. We can solve this in a few ways. We can use a mapping from Axis to the Material that prevents FTL hijinks such that if you Plane Shift into Axis, walk one meter, and Plane Shift out, it places you in the Material one meter away from where you went in. There are other viable mappings left as an exercise to the reader.

We could, in addition to that mapping, scale everything up, such that the distance you can walk in a day on Axis corresponds to 200 miles instead of 20, because everything in Axis is 10x bigger that it seems

Or we could use the same method previously derived. Why should Aroden get all the fun?

And Another Solution

Just put all the solar systems in the same place. Just put everything in the same place. Right on top of each other. Have you read Unsong? Just like that. Every solar system’s crystal sphere, right in the same place, without allowing interactions between crystal spheres. The only way to interact with another crystal sphere from your own is to cast a spell like Interplanetary Teleport or Plane Shift. Then there is little motivation for FTL because everything is within a few light hours.

If a few hours is still too much lag for you, compress it even further. Castrovel is in the same place as Golarion, and so is Vesk Prime. Everyone’s perceptions of the sky are individually spoofed. Anyone who has ever been under the effects of Mind Blank already knows this.

1. And that’s real time travel, not MCU “well it branches the worldline at the point of arrival except when we forget” time travel.

2. Epistemic status: reddit comment I immediately lost track of

3. Simultaneous time isn’t real, so you cannot actually pause motion everywhere simultaneously from all reference frames. I am not sure what happens if you pause motion everywhere from the reference frame of A. Could that allow a relativistic observer to run into a motion-stopped non-relativistic observer if conditions were right? Leave your thoughts in the comments below!

4. I believe there is some pedagogical benefit to using low quality diagrams when explaining physics, math, engineering, etc. If there were not, explain why all my professors were like that.

5. Of course the deceleration and acceleration would occur over a non-zero period of time since instantaneous acceleration is impossible, so some fudging is probably necessary to prevent the the relativistic observer from tearing itself to shreds. I trust Otolmens to solve that on a case by case basis. Perhaps She can, instead of pausing and resuming the universe, just scan the existing universe, delete it, and program the behavior of 10^82 atoms for a replacement universe that behaves just right, post teleport.

6. The distance at which the planets in Golarion’s solar system are specified anywhere that I have looked.