This question started me thinking about the nature of the mirror universe.

Just what kind of universe is the mirror universe in Star Trek? I can think of three possibilities (and realize there may be more):

  • Is it an actual mirror universe, or is it a mirror image that was created with our universe with some kind of symmetry to us, so it'll always be a dark reflection of our universe?

  • Is it simply an alternate universe that happens to have a lot of similarities to our own?

  • Is it an alternate timeline that forked at some point along the way when one person made a decision and things split on a quantum level?

And if the last one, then are all the different trips to the mirror universe, from Enterprise through Deep Space Nine to the same alternate timeline, or to different ones? If they all went to the same one, why to just that one?

  • I suspect the Prophets had something to do with its creation, given they apparently tried to reconnect the universes in DS9 2x23, Crossover (IIRC, not apparent in that episode, but suspected and never confirmed by the characters later on)
    – Izkata
    Dec 21, 2011 at 0:30
  • 11
    It's a sexy universe! Dec 23, 2011 at 22:39
  • The out of universe answer is clearly number 1, but that's mostly because they don't want to hire new actors for mirror universe episodes. Oct 26, 2014 at 19:05
  • best argument I've seen that it's a a parallel universe was at the end of a webisode of the "Starship Farragut" or "Exeter" ( Unsure which ) where a time-travellign race sends the crew back to teh American revolution, were they save the fledgling revolution. After, the aleins tell them they have no effect on the main time stream, as another branch will be formed by their methiod of travel . . .then we see Washington writing about the success and how with strength of will the entire world might be liberated. he drives a dagger into an apple, mimicking the sword and globe of Evil Federation May 17, 2015 at 5:04
  • @Covertwalrus: Interesting, but that is not canon and it's hard to take it as being authoritative when it's produced decades after the original and is just one of many of the newer productions.
    – Tango
    May 18, 2015 at 13:27

3 Answers 3


I believe the answer is a mixture of two and three...it depends on your view of the nature of freewill. Would the same person make the same choices every time, given that all of the circumstances are the same? If you believe yes, then that implies #2 (because #3 is impossible). If not, then it's two and three together.

But clearly there is a point where histories diverged significantly. It's unclear exactly when this was, but it would seem to be no later than World War II. I cite the Enterprise Mirror Universe episode opening as evidence.

In the clip, we are treated to a militaristic view of our history, up until the point where the emblem of the Terran Empire appears superimposed over marching figures (apparently) from World War II (at any rate, the clip which immediately follows that is definitely from WWII). It's possible that the point of divergence is earlier, since later history still seems to hew reasonably closely to the primary timeline, at least until the events of TOS.

One other piece of evidence that supports this general time period is Archer's line in the Enterprise mirror episode that the Empire has been around for centuries. The line is spoken in 2155, which means, strictly speaking, that the Empire had to have been founded at least by 1955.

It's clear from the storyline that each trip is returning to the same universe and timeline...events which played out earlier influence later episodes. As for why...all I can guess is that either:

  1. There is only one other universe or timeline accessible in that way, or
  2. The mirror universe is the easiest one to access in that way.
  • But it is revealed that Enterprise takes place entirely on the Holodeck so that is just an in-universe fictional idea of the mirror universe.
    – Gaius
    May 22, 2015 at 18:28
  • 1
    @Gaius You've misinterpreted ENT4x22: "These ARe The Voyages..."the final episode of Enterprise. Riker and Troi were reviewing a historic holopogram, a record of past events. In the episode, Riker says, "Counselor Troi has suggested I might get a few insights by calling up an historic holoprogram."
    – T.J.L.
    Oct 29, 2015 at 20:57

It's a mixture of 1 and 3. Thanks to the Enterprise mirror universe episodes, we got to see a glimpse of the history of the mirror universe, and it forked centuries ago. Apparently Shakespeare's plays were more aggressive over there, but still very similar to what the main universe had.

It forked, but the other aspect was that it's a mirror to the main universe. Because it's a mirror, people are drawn into similar relationships in the mirror universe as they are in the main universe. This is why the NX-01 Enterprise's, the NCC-1701 Enterprise's, and Deep Space 9's crews had the same group of people working together in both universes. Given that the timeline forked centuries ago, it's exceedingly unlikely that the same individuals would be born centuries after the fork, much less end up working with the same people.

  • While I totally agree with your reasoning why the "mirror universe" as depicted is unlikely as an "ordinary split timeline", isn't the notion that it is actually a "mirror" just as unlikely? Consider that only very specific things are mirrored. Only certain traits were mirrored (or, let's rather say, altered ... as there is often no clear definition of what the "opposite" trait would be), but others (Spock's behaving logically, for instance, or the people who designed and built Mirror-Enterprise, Mirror-DS9 etc.) were just like in our universe. Apr 3, 2014 at 12:51

I am only guessing here but there are even today current theories of multiverses that some theorists say could include every possible universe with every possible outcome. This is by no means canon, but a real world theoretical explanation. If a nearly infinite set of universes exists, then practically you could travel to any universe with any possible event with any different number of outcomes. Therefore exact "mirror" universes would exist of every other universe.

I don't know of any canon explanations of these mirror universes that would explain how or why they always went to the same one or if the theory of multiverses I used to explain it is considered canon. But it seems that throughout the different sets of series that each series had it's own mirror universe, which could explained away by the different methods used to get to these mirror universes.

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