Every so often, there's an episode of Star Trek involving time travel, and more often than not, the "rules" seem to change from episode to episode. For example, Future's End, Braxton explains that they're all caught in a causality loop, which they break out of at the end of the episode. Then, in Timeless, Kim sends back a transmission and alters the past (he does this multiple times, in fact, and the signals seem to not conflict). In the film First Contact, it's plausible that the Phoenix only flew because the Enterprise showed up.

It seems like there's not much consistency in the way that time travel works (the closest we get is Annorax's vague explanation in Year of Hell), but is there a plausible in-universe explanation as to why the "rules" seem to change so much?

  • I think the best in-universe explanation for things like this is that we (as the audience) simply don't understand it well enough. Clearly, in-universe, time doesn't behave like people expected in the 20th and 21st centuries.
    – Nerrolken
    Commented Apr 30, 2015 at 0:38
  • 2
    The closest you'll get is probably O'Brien's simultaneous "I hate temporal mechanics."
    – Izkata
    Commented Apr 30, 2015 at 1:01
  • there was a temporal cold war. Mr Spock had hoped to repair the timeline so there is some Vulcan-accessible consistency. Commented Apr 30, 2015 at 1:18
  • 3
    Wibbly-wobbly, timey-wimey
    – Joe L.
    Commented Apr 30, 2015 at 1:53
  • 2
    Trust in the Prophets.
    – Boelabaal
    Commented Apr 30, 2015 at 17:06

1 Answer 1


No Trek show or movie of which I'm aware explains just how timelines and parallel universes work in its own universe. It seems to be a mixture of theories that have presented themselves over the nearly 50 years the show has been around. What we do know includes:

  • The "main" timeline can be altered ("Assignment: Earth", "Yesterday's Enterprise," just to name a few)
    • People can cross back from a former version of the "main" timeline ("Yesterday's Enterprise," "Endgame," Abrams' Star Trek, etc.). These people remember the altered timeline.
    • People in a "protected" area during timeline modification can remain unaffected by the alteration ("City on the Edge of Forever," First Contact).
    • Other non-time-sensitives will not notice any change in the timeline (seen in the above examples). Some, such as Guinan, may be able to sense a change, but be unable to provide details. The effects of these changes on the Q Continuum are undocumented.
  • Parallel timelines of generally undefined origin can coexist with the "main" timeline ("Parallels," among other TNG episodes).
  • Parallel universes, such as the Mirror Universe, can coexist with all of the above without a defined point of divergence. They seem to operate independently of the "main" timeline except for crossover situations.
  • By the TNG era, the Federation has figured out some, most, or all of this, at least to the ability that individuals can recall. By the time of the Enterprise-J, perhaps even more is known. Little, if any, information about the discrepancies and theories has been shown on-screen.

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