In the Star Trek: Deep Space Nine episode "Crossover" two members of the DS9 crew cross over to an alternate universe. It is the same universe that Kirk went to many years earlier in the TOS episode "Mirror, Mirror."

In the episode "Crossover" Quark is killed. So now there's no Quark in the mirror universe. Odo was also killed so now there's no Odo in the mirror universe.

I'm just using these as examples to show how quickly (within a day) changes occur between the mirror universes.

In addition circumstances would change. For example Sisko would not meet his wife on the beach and as a result Jake would never be born. O'Brien never meets his wife, never gets married, and has no children. Or maybe they meet different women to have children with. Or they could even meet the same women but have different offspring with them because they procreated at different times (different egg and sperm).

Other unions would form creating different offspring due to these different sets of circumstances. Some people might get killed or remain alive due to different circumstances in one universe or the other. All of this would result in creating different offspring.

Assuming that they're closely 'mirrored' during Kirk's time, how is it possible that 4 to 5 generations later many of the characters are still the same, in the same places, are associating with the same people, etc.?

Is there some sort of connection or mechanism between the two mirrors that moves them back into sync somehow with each other?

(Although outside of the actual personnel everything else seems to always be radically different between the universes with the exception of Spock whom we always know is the same in any universe.)

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    The implication is that there's a certain amount of historical inertia at play, keeping the timelines somewhat consistent.
    – Valorum
    May 21, 2015 at 21:06
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    “Sisko would never meet his wife on the beach and as a result Jake would never be born” — Mirror Sisko actually did marry Mirror Jennifer, although they never had a son. May 21, 2015 at 21:11
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    @JMFB - Where large events seems to want to occur, as if guided by an external force
    – Valorum
    May 21, 2015 at 21:15
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    I'm not sure it's established there is only ONE mirror universe. Perhaps it's simply the closest of current alternate realities, which implies it's most similar.
    – Agent_L
    May 21, 2015 at 23:25
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    @cde But doesn't that temporal shielding get pierced with a multiflux gamma tetryon ultrium antiproton pulse particle beam coming from a tertiary subspace flux-capacitor nacelle manifold?
    – JMFB
    May 22, 2015 at 17:02

3 Answers 3


(I'd prefer to make this a comment, but it's too long.)

I haven't seen "Crossover" and don't remember "Mirror, Mirror" very well, but there is a way to make this work-- at least well enough for a soft science fiction setting like "Star Trek".

Suppose there are many universes, and a mechanism that allows people to jump from one to another on rare occasions. And suppose that this mechanism tends to choose two universes that are nearby, where the cosmic definition of nearby is that some things in the physical vicinity (e.g. people, ships) are physically almost identical, but large and distant things (e.g. societies, races) can be wildly different. The bias also doesn't seem to apply to very small-scale things like brain patterns, so that although a "double" has the same face as person we know, he may not have the same personality. He might have a different name, too, but the Universal Translator can smooth that over. This is enough to explain either episode*.

To explain both episodes -- and the stated fact that both groups of people visited the same other universe -- requires something more. Just to recapitulate, "they went to the same place" means that when the DS9 officers found themselves in a strange version of DS9 (the station, not the series), they could consult the history books and read about something that happened a century earlier on board the Enterprise: some of the senior crew had acted goofy for several hours, showing mercy to enemies, unfamiliarity with shipboard disciplinary procedures, and surprise at the state of Spock's facial hair. Then they started acting normal again and reported having been on a version of the Enterprise weirdly bereft of torture and treachery, with a clean-shaven Spock and very few daggers. This corresponds exactly with the DS9 officers' memories of their own history, in which several members of the senior crew of the Enterprise had turned into disoriented, vicious, scheming psychopaths for a few hours, then recovered and told of-- you get the idea.

According to the "Nearby" rule, the DS9 jumpers should have found themselves in a self-consistent universe, in a station similar to their own, full of people who look like the ones they left behind, but with distant things different from what they remembered. It makes no sense that in such a universe, a distant group of people, a century earlier, should resemble a corresponding group from the travelers' home universe's history. It's not required by the "Nearby" rule, it can't apply to all groups of distant people (otherwise even one "didn't have a child" divergence would break the whole thing), and if it's not required then it's wildly improbable.

So let's add one more feature to the mechanism: laziness. It chooses two universes, A and B, that are similar to each other in a couple of different spots-- and those spots are where people can jump. It is pure coincidence that a universe that produced a twin of "our" Enterprise also produced a twin of "our" DS9, with no causal connection between them and everything in other places and times wildly divergent (although there may be other "twins" we don't know about). But that's just the kind of coincidence the Mechanism finds. This is an "in-universe" explanation that explains everything, but requires no explanation within either of the two universes.

(*) Don't ask me to explain how the uniforms can be so different, that just makes no sense at all.

  • I'll have to re-read your answer when I'm more alert. I'd like to ask you to watch the DS9 episode & see if you think your answer still fits. I watched DS9 for the 1st few seasons when it came out in the 90's & didn't like it very much so I didn't watch the last couple of seasons. I didn't really like Sisko. For me Kirk, Picard, & Janeway were amazing. Sisko was difficult for me to sit through. I'm rewatching them though & deciding to give it another chance. I'm finding I still feel in many ways the same as I did the first time. However this episode is a real gem & I encourage you to watch it.
    – JMFB
    May 22, 2015 at 7:25
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    @JMFB: Sisko is difficult to watch because Avery Brooks is an amazingly bad actor; deep velvety voice, handsome face, three facial expressions and basically nothing else. I don't know whether that, the writing, or the fact that I didn't see the entire series, is the reason I can't recall him ever delivering a really good line.
    – Beta
    May 22, 2015 at 12:37
  • I agree completely. He's a horrible actor, at least that's what I attribute it to. He's very mechanical almost like a robot reading lines. There were a few good lines but they're so few that I can't remember one now, which is telling.
    – JMFB
    May 22, 2015 at 16:01
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    If you add a concept of Universes constantly splitting this works really well. Initial accident creates a strong link between Universe A and Universe B, universes continue to split. Universe A-1678 experiences a new connection, finds similar universe in universe group B (due to previous link); finds B-4568. A-1678 and B-4568 now even more strongly linked
    – user20310
    Oct 4, 2015 at 19:48
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    I feel a short summary of this excellent answer is in order. Beta is just saying: "when you jump to a 'mirror universe', it's a "nearby" one - a very reasonable concept. But note that it is a nearby one AT THAT TIME. (In your universe.) that perfectly explains how the "nearby" universe was indeed "pretty similar' AT THAT TIME, IN EACH CASE." Quite right.
    – Fattie
    Mar 5, 2016 at 20:41

This is more into the philosophy behind it than anything direct from canon - but if you think about it, it was quite remarkable in the first recorded instance of a crossover resulting in our Kirk replacing a mirror-Kirk on a mirror-Enterprise crewed with mirror-crew belonging to a mirror-Federation.

He could have just as easily crossed over into a universe where the Shelliac were the dominant Alpha Quadrant race, or one where humans never evolved beyond proto-humans, or...

Or could he?

As you note - there are remarkable similarities in all instances of crossovers. We see Emperor Worf, but we don't really see what else was different about his life history - was mirror-Worf at mirror-Khitimer? Was he adopted by Terrans (probably not given the history of the mirror-Universe)? As for the Cardassians - mirror-Garak is the highest ranked member of his race - no indication of a Dukat or Dumar, or any number of Legats we see in our universe.

So - why is the mirror universe so closely parallel while still deviated?

Well, one thing to note - this is a parallel universe and not a divergent timeline (we are told that these are quite different at a few points in the series).

My guess would be, in-universe, there is something to do with relative subspace energy levels making crossover easier between these two universes as opposed to the multitude of other ones out there.

Out of universe - of course, they want to use the same actors and kind of play "what if" scenarios.

  • I like this answer & the one above, but neither addresses one of my primary concerns. I know I'm being specific, but what about offspring? One couple in 5 generations can spawn over 100 people, so killing Quark for example could eliminate 100 people 100 years later in the other universe. So from Kirks time till DS9 even if just a small percentage of people were killed in one universe & not the other, & a small percentage remained alive in one universe and not the other, the makeup of the people and where they're located would be radically different. How do you reconcile that? @Beta & HorusKol
    – JMFB
    May 22, 2015 at 7:33
  • I think a possible solution is given in Betas answer. "The Universe" chooses a parallel partner, where at all crossover locations and at all crossover times over the whole timeline of both universes (!) there are coincidentaly the same persons. If there are infinite other universes where all exist what is possible, it would finde one. Your problem is solved with having the (apparently) "same" person having different parents or ancestors in both universes. The only question is why the Universe tries to find a partner where the people look like, but everything else doesn't matter.
    – Hothie
    May 22, 2015 at 12:02
  • @JMFB: if you like I'll edit my answer to address offspring specifically.
    – Beta
    May 22, 2015 at 22:19
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    Another thought - what if each crossover is actually with similar but distinct universes? Each one is different to the last instance?
    – HorusKol
    May 23, 2015 at 2:58
  • @HorusKol hmm... Spock said there are infinite universes, right? I'll have to ponder that for awhile.
    – JMFB
    May 24, 2015 at 1:22

The exchange in "Mirror, Mirror" could only occur because the two universes were so similar. I've often wondered if the later MU episodes actually did occur in the same universe or merely one that had also been visited by a Jim Kirk from.......somewhere. I've also heard, but can offer no proof, that they had originally intended to imply that Kirk and company might not have returned to their original universe, but to one that was identical for all intents and purposes but that was abandoned to allow for the Marlena-Prime introduction.

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