So I was lying in bed this moring(summer break, yaaaay) when it suddenly occurred to me that most causality paradoxes are caused by the exchange of information, so if when moving FTL no information could be exchanged in or out, preventing any sort of causality paradox, then it further extended to stars to prevent that right, no information in or out of a star system of any construct or natural formation, large one floating in space. I should clarify that when I say cloaked I mean no information can enter or leave the area involved, whether that be a cluster of spaceships moving together, or a little pocket universe contained within a brown dwarf.

Let's say that all FTL spaceships and star systems are cloaked in an area. No FTL travel is allowed within the borders of the said constructs, or natural formations, presumably various solar systems.

Further let's say that FTL travel is confined to routes that cant form closed loops, and so can't lead to time travel.

Does this successfully prevent all Potential FTL paradoxes that could occur, most or none or just an indeterminable portion?

Note:This sprung from thoughts on routes taken, FTL routes in which causality paradoxes couldn't occur presuming the spaceships involved followed all of the constraints involved in the routes. Note:This is also a potential solution for dark matter, right? Note:This was something that @Willk pointed out, to a degree, assume that any faster than light spaceships are actively trying to avoid causality paradoxes

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    $\begingroup$ This is like a special rule that prohibits me from drawing a mustache on your face when you sleep when you are sleeping in a city. Except then you fall asleep on an international flight and I am on that flight with my Sharpie. By the time you are in the safety of Ulan Bator the mustache is done. There is nothing special about stars, or cities as regards information or mustaches. The only solution is to grow a mustache... IF YOU CAN!! $\endgroup$
    – Willk
    Jun 13, 2021 at 18:22
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    $\begingroup$ You are correct in that the analogy breaks down with growing the mustache. That was the coffee talking. There is no way to avoid the causality paradox. It is intrinsic to FTL travel. Even the allowed kind with Alcubierre drives. I am sorry. You could still grow the mustache though. Or press on ones have gotten pretty good. $\endgroup$
    – Willk
    Jun 13, 2021 at 18:35
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    $\begingroup$ @Willk The question isn't if causality paradoxes still occur, it's to the degree that this would prevent them. If causality paradoxes still occur, I was looking for an awnser. It's to what DEGREE it would prevent causality paradoxes. $\endgroup$ Jun 13, 2021 at 18:45
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    $\begingroup$ Doesnt that depend on to what degree interstellar travel is between places immune to causality paradoxes? Just like your drawn mustache immunity is complete if you never leave the city, but not good at all if you live on a boat in the mid Pacific. Intentional causality paradox could still happen easily - get newspaper, travel on impulse outside star system, warp to suburbs of distant system, travel on impulse into system with news before the newspaper has been printed. $\endgroup$
    – Willk
    Jun 13, 2021 at 18:56
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    $\begingroup$ @Willk Would you mind posting your comments in answer form? $\endgroup$ Jun 13, 2021 at 19:08

1 Answer 1


This is impossible

First, let me recommend the book Exultant, by Stephen Baxter. It digs right into these problems in a hard-science way. (It's also pretty good.) Most of what I'm about to say is based on that. If this is keeping you up at night, you'll probably enjoy the book.

You're going to run into at least two problems:

  1. FTL is necessarily time-travel

This is because there is no such thing as a universal "now."

  1. "Information," in the sense relevant to spacetime paradoxes, is an extremely broad category

I believe it includes all physical mechanisms, including gravity. If a star were to magically vanish in an instant, the resulting gravity wave counts as information. CMB counts as information; if your vessels absorb CMB, you've violated causality.

So, it's not clear what value there would be in having FTL, since avoiding a paradox would require you to have zero interactions with the spacetime environment along your route and at your destination: you can't observe, visit, or be observed. You aren't allowed to warp spacetime outside the cloak with your own mass. You also aren't permitted to block or interfere with other things: if your ship transits some distant star as seen from some distant planet, infinitesimally diminishing the starlight that reaches the planet, you've violated causality.

The effect of this cloak would seem to be, precisely: to prevent you from having any of the consequences of existing in the place where you say you exist.

@BMF said it:

This effectively sounds like "jumping" between causally-disconnected regions

Incidentally, this is almost exactly how they get around these problems in Exultant.

And none of this touches on the very many questions I'd have about the nature of this cloak. It would be a stupendous engineering feat to create this effect around a toothpick, to say nothing of a spaceship, a star, or multiple star systems.

And on the topic of dark matter: this definitely could not be a solution for dark matter, because the one thing we know about dark matter is something your cloak would absolutely need to prevent: gravitational interaction with spacetime.

Astronomers see galaxies spinning too fast to be accounted for by the matter that we observe around them. This makes astronomers think there must be other mass orbiting the galaxy. They invented the term "dark matter" to refer to this stuff they can't find that nevertheless influences the galaxy's rotation.

If something is cloaked such that it prevents paradoxes, that exact effect must be prevented.

The truth is that the cloak can't be a solution to anything, because "not solving anything" is a necessary part of the definition of not violating causality. If there is a feature of the universe that is explained by the existence of cloaks, then those cloaks are interacting with physical systems, which is the exchange of information, which means the cloak is not a cloak by your definition.

  • $\begingroup$ I'll read exultant, thanks. $\endgroup$ Jun 14, 2021 at 3:48
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    $\begingroup$ @Tom: just an amendment of your first point: "1. FTL is necessarily time-travel". FTL is only necessarily time travel when it's done with respect to multiple inertial reference frames. I think a frequently cited life-hack for hard-ish scifi FTL is to pick a SINGLE, special inertial reference frame in which FTL can be done; and then you're paradox-free! $\endgroup$
    – Qami
    Jun 25, 2021 at 16:00
  • $\begingroup$ @Qami That seems kind of silly though, since anything you'd use FTL to reach is almost certainly in a different frame. I mean, you're not gonna use your jump drive to run to the corner store for a pack of smokes. $\endgroup$
    – Tom
    Jun 26, 2021 at 15:24
  • $\begingroup$ Right; you'd use FTL to put yourself in the path of the target destination and then sublight engines to accelerate/decelerate to intercept. I'm pretty sure Larry Niven's Known Space world used this kind of method. $\endgroup$
    – Qami
    Jun 26, 2021 at 17:06

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