That makes no sense. It's a fantasy notion, not a science-fictional one. There's also no evidence for it. Yes, we call it the "Mirror Universe" because Jerome Bixby called his episode "Mirror, Mirror," but there's nothing in the show itself to suggest that there's anything "mirror"-like about it or that it's simply a cartoony inversion of everything. It's never been referred to as anything but "the alternate universe."
Besides, we've seen plenty of canonical characters in the MU who weren't simply opposites of their regular selves. Even in the original episode, Mirror Spock wasn't too different from Regular Spock. In DS9, O'Brien, Jennifer Sisko, Garak, and others weren't too different, and certainly weren't just moral inversions of the characters we know.
Uhh... ever heard of the Nazis? Or Stalin? Or the Roman Empire? Or Atilla the Hun? There have been plenty of evil, brutal empires that have dominated much of the world at various times in our history. It's nonsense to say that it's impossible for an alternate timeline to produce a future where Earth has been taken over by a brutal dictatorship.
I agree with you that it's nonsense to say that, I don't know who said it, but it wasn't me. I said it's impossible for it to change the human nature into skewed counterparts of our own. Nazis weren't evil, most were just regular Germans, Russians weren't evil, even though Stalin was. You seem to imply that once Earth is taken over by a brutal dictatorship, every person will agree with it and enjoy spreading its brutality; good-hearted people like McCoy or explorers like Kirk will automatically turn into monsters. I can't accept that analogy.
The case with MU is completely different despite what you say above, it's simply not true. The whole point behind the episodes was that the writers wanted to show the characters in a different light to drive home a point. How can you say they are not too different?? Kirk committed genocides, Spock was all too happy to annihilate the entire Halkaan race, Mack's own Spock in this book commits incredible genocides in the name of twisted logic that our Spock would never even contemplate, Jennifer works for the Alliance to help eliminate humans, she performs kidnapping and blackmail, Kira (nonhuman) is a twisted lying psychotic megalomaniac, Phlox (nonhuman) does strange barbaric experiments on aliens, Odo (nonhuman) was incredibly cruel, Ezri (nonhuman) is an amoral ruthless mercenary, and the Ferengi (nonhumans) seemed to be changed for the better.
Here's a good quote from Wiki:
The characters in the Mirror Universe are generally the same as the characters in "normal" Star Trek continuity (for example, it has a James T. Kirk and a Spock), but their personalities are, on the whole, much more aggressive, mistrustful, and opportunistic. Whereas the Star Trek universe usually depicts an optimistic future which values peace and understanding, episodes set in the Mirror Universe show it to be marred by continual warfare, and compassion is seen as a liability. Uniforms are often more suggestive, such as women baring midriffs.
Though the Mirror Universe is much darker and more violent than the normal Star Trek universe, a few Mirror characters are more friendly or docile than their "normal universe" counterparts. For example, Mirror Quark puts himself at risk to help escaped slaves reach freedom, and while "normal" Brunt is selfish and power-hungry, Mirror Brunt is a kind and considerate Ferengi.
Humans in MU are vastly different than the ones in "our" universe (read evil, brutal etc); that can't be the result of a different time line.
Why not? It could have diverged a long time ago causing human history to have been more brutal on humanity and creating a every-man-for-himself mistrust-of-others type of mentality.
I agree with you that it could be the nature vs nurture thing, and they could be like that because of their surroundings, but how does that explain aliens who are also different? Small alien races are all changed, only major aliens seem unaffected, but I can't be 100% sure about Romulans, we don't see much of them.
What is the oldest reference we've gotten in the MU books? I know there was a reference to an Emperor George Bush, but I can't remember if there were any earlier than that. I know someone did refer to Shakespeare not being different, but that could be because things hadn't changed yet.
In
Dark Mirror, Illiad is a little different, Achilles kills Priam instead of letting him live, showing that even that one moment of mercy that original Achilles shows is nonexistent with these mirror humans.
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I think it's simply the case of book writers not understanding the original episode and dropping the ball.
Here's what the show's writers think from Memory Alpha:
Episode writers maintain that the mirror universe is not simply an alternate timeline, but instead a parallel universe where the patterns of events move in similar manners, but the intentions and characterizations are different, so the people of the mirror side will always remain (and always have been) skewed versions of their "normal" counterparts, so no true point of divergence can be traced, as the two realms move in a parallel manner.
From logical standpoint, the mirror universe must have been created with the Big Bang