With our current understanding of space-time and SR (presumably, the understanding shared in the Star Trek universe), superluminal travel can lead to incidental time travel into the past. See this and this SE question for information, as well as this PBS video briefly explaining how FTL can be used as an instrument to set up causality-breaking trajectories.

In my experience with Star Trek, time travel is often limited to spatial anomalies and future Starfleet and whatnot, but with easy access FTL, time travel into the past should be a regular phenomenon. (Perhaps it is?)

Is this ever explained in the Star Trek universe? (TV shows, comics, books, etc.)

Just to be clear, I'm not hunting for physics inconsistencies in Star Trek (there are plenty to go around), I'm looking to see if this property of superluminal travel is ever mentioned, addressed, or even utilized (or explained away).

  • 1
    You might want to read this: memory-alpha.fandom.com/wiki/Slingshot_effect
    – Lorendiac
    Dec 27, 2019 at 0:40
  • This feels very opinion-based. We see time travel used with regularity by the various crews. Alas, each time it's used there seem to be a whole bunch of different rules and types of paradoxical behaviour that it's almost impossible to build anything approaching a *unified theory of Star Trek time travel
    – Valorum
    Dec 27, 2019 at 0:42
  • @Valorum does the question feel opinion-based because I assume Special Relativity applies to Star Trek?
    – BMF
    Dec 27, 2019 at 0:48
  • It's not really an answer, but the Voyager episode "Parallax" plays with similar concepts of effect preceeding cause in the region around a singularity. Not the phenomenon you describe, but at least some of the same words apply. Dec 27, 2019 at 2:55
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    @M.A.Golding causality isn't a rule established by science. It appears to be an emergent property of our universe. Many other conserved quantities can be identified by Noether's Theorem. Causality seems to be the result of observed symmetries.
    – BMF
    Dec 27, 2019 at 18:16

3 Answers 3


I don't know whether there's any explicit discussion of this; it seems unlikely in the TV shows because it wouldn't exactly make thrilling viewing, but I suppose there might be something in one or more of the many books or other non-canon material.

However, we do know from canon that warp engines have a maximum speed, not a maximum acceleration.

That implies that if two ships leave orbit from two different planets and go to warp 4, they're travelling at the same speed as one another regardless of the relative speed of the two planets, which eliminates the most obvious ways of winding up with causality violation as described in your links. It also implies the presence of a preferred reference frame, but this does not violate the principles of relativity provided that there is a physical mechanism responsible for the preferred frame; for example, the mechanics of sound in air has a preferred frame, the one in which the air itself is stationary.

Star Trek does establish that time travel via warp drive is possible, but the exact physics of that are deliberately left ambiguous. It does seem to require a deep gravity well, which suggests that the preferred reference frame in which warp drive speeds are measured might be connected to the gravitational field in some way, but that's entirely speculative.

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    Interesting. In modern science, there is no "absolute frame" from which one can globally measure velocity, acceleration, etc. But perhaps in the Star Trek universe, subspace is this frame. As you say, warp 4 is warp 4 for all observers. It makes for a good platform from which to handwave away the time travel issues of moving FTL in regular space. Clever...
    – BMF
    Dec 27, 2019 at 3:17
  • The Blish novelizations of the original series went into the physics a bit.
    – JdeBP
    Dec 27, 2019 at 11:33

In STOS never was relativity mentioned (except maybe implicitly in the Gary Seven episode) and communications with Earth were instantaneous. We also never really knew where in the galaxy they were although almost never in our own solar system. The one really scientifically accurate thing was respecting how large the galaxy was and how far from other galaxies ours (milky way) was -- in a couple of episodes, they made a big deal about inter-galactic travel and how long it would take.

Never was it mentioned that upon returning to Earth far more time would have passed so that relatives would be much older, etc. I think this was critical for the comfort of viewers -- too heavy for most people to deal with and hard to imagine crew signing up for a tour of duty like that.

  • Communication with Earth was not always instantaneous in the original series. There were numerous times where Kirk would report back to Earth and wait for a response - then have handled the situation before the reply came.
    – JRE
    Dec 27, 2019 at 6:53
  • @JRE: But were there not times where they were speaking to someone on Earth with zero lag? And if so, what accounted for the situation where replies had to be awaited? I don't think they mentioned that signals traveled only at light speed and indeed they used subspace communications which was implicitly superluminal.
    – releseabe
    Dec 27, 2019 at 7:01
  • Of course there were times they were in near instantaneous communucation with Earth. Subspace communucation is faster than warp speed. Still, distance does matter. The farther away you are, the longer it takes.
    – JRE
    Dec 27, 2019 at 7:03
  • @JRE: I can't recall a time when they mentioned distance from Earth, either how many light years or how long it would take to reach Earth although I could simply have forgotten.
    – releseabe
    Dec 27, 2019 at 7:25
  • @releseabe Many times they mentioned the names of the stars they were at, and some of them are real names of real stars that have real distances from Earth, although such distances were usually not known as accurately back when TOS was made as now. I think that in "The Enterprise Incident" it was said it would take 3 weeks for a subspace message to reach Starfleet Headquarters. Dec 27, 2019 at 18:04

One could argue that time travel is a regular phenomenon

However while I am probably misunderstanding the question Alcubierre drive

I dont think Star Trek warp is true FTL according to relativity

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    It doesn't matter how you achieve FTL, it is the superluminal communication of information that can lead to breaks in causality.
    – BMF
    Dec 27, 2019 at 1:06
  • I think this does a good job at explaining how to setup a causality-breaking FTL trajectory.
    – BMF
    Dec 27, 2019 at 1:13

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