Hm, a thought about the Niven Effect (the idea that time travel is possible but never gets invented, because if it does get invented, eventually someone makes a change so it doesn’t)… If time travel is possible, then there are bound to be some other technologies which are also possible, and related to time-travel tech, but which don’t actually involve time travel. In such a universe, would we expect those other techs to exist? Doubtless, often the change that ends up undoing time travel also undoes those related technologies. But then, those techs also have their own set of related techs, and so on. Maybe the only stable universe is one where no technological development at all happens, because any universe that starts down that road eventually ends up developing time travel, and eventually some time traveler will decide to go back far enough to undo all of it.
I’ve seen other reasons. Colonizing the past and hunting dinosaurs (guess the real reason they went extinct) are two from the novel Mastodonia that come immediately to mind.
The Game of Blood and Dust had a pair of aliens play a time travel based game with Earth. They each took turns changing history, with the goal of Dust being to manipulate human history into nuclear apocalypse, and the goal of Blood to keep humanity alive. Not that there was a moral distinction between the two.
"Best two out of three?
"Sure. This time I’ll be Blood, and you be Dust.
And I’m reading “Everlasting” (one of this year’s Hugo nominees) where time traveling is more about trying to make the present better than about avoiding a catastrophe.
Large scale time travel to make the universe better is a major plot point in Starplex. For example there’s artificial hyperspace gates near the worlds where intelligent life can evolve that only go active when entered from their side, making it far easier for spacefaring species to find each other. And it turns out that the reason the universe is so finely balanced between the mass needed for collapse or eternal expansion is that they’ve been sending stars back from the distant future to balance it out.
Nn/Mm
- City on the Edge of Forever - super-science arch sends people back in time, causing instantaneous changes off planet, but leaving people on the planet with the arch aware of the original history
- Tomorrow is Yesterday - breaking out of orbit of a superdense star sends you back in time. Try not to meddle, since we don’t know what will happen. P.S. Going fast to the Sun can send you back a little bit and then forward in time, so you can erase the changes you made the first time around. Convenient!
- Assignment: Earth: Time Travel (using a method that might be the same as in Tomorrow is Yesterday) is totally safe. Let’s do historical research by sending a starship to a delicate dangerous period of Earth’s history. What’s the worse that could happen. Regardless, the history books say that everything came out the way it should have
- All Our Yesterdays - these aliens have a “go to the past” machine. No worrries about changing the past - doesn’t seem to be worth discussing. But Vulcan psychology in the deep past is somehow affected by the “current” state of Vulcan society (or Spock’s making excuses)
- I’m forgetting one, clearly
Of course!
- The Naked Time - escape from a gravitational field causes clocks to go backwards a few days. Nobody worries about any effects on history. Was the Enterprise in two places at once? Who cares.
(I always figured 2, 3 and 5 were the same method (high gravity+high speed) - but the rules of how they work are different)
Also, travel into the past requires some kind of biological preparation to adapt them to that time or the person send backwards will die. It seems likely to be related to the effect that was making Spock act like ancient Vulcans, something about that time travel method entangles the traveler with the general nature of people of that time.
The only way I’ve ever been able to make Trek time travel (somewhat) make sense is to assume that time travel works drastically differently according to the method used, because the writers certainly made no effort to keep how it worked consistent.
Even more: the Guardian method can only view the past at one set rate, and apparently one set place in the past at a time. K&S end up in the exact place on earth as McCoy. What are the odds? infinite, unless the Guardian deliberately does that, And when the trip was done, they all came home (in their original uniforms, not the traditional accoutrements they were wearing.) Were they just sucked bodily out of the past, dressed by the machine, and pooted through the arch? Or was that they was the machine always worked?
Now we get speculative: since Edith Keeler’s non-death affected the history of earth, and the creation of the federation, what other affects existed we never saw? Were the Klingons the dominat power? The Romulans? Did Guinan, who was alive, notice the change, as she did in Yesterday’s Enterprise? (She should have.)
All Our Yesterdays is so weird that it could almost be retconned as a fake time travel method- everyone on the planet is in some Matrix-like environment, to while away the time before the planet is destroyed. The “biological treatment” is intended to help people accept the situation but it doesn’t work perfectly on humans or Vulcans
I still can’t believe this wasn’t the canon reason for the mirror universe.
The brutality of the Mirror universe goes back even further in Earth’s history. Humans are just more aggressive and had a darker path from the very beginning.
Slight tangent: I remember an article in one of the Best of Trek books (a fan magazine anthology) made the point of departure the Challenger explosion (this would have been written when that was still recent). The idea was the explosion scared the US away from spaceflight and that money went into Genetic Engineering leading to Khan who eventually took over the world and founded the Empire.
And yet, you still had the exact same sequence of the exact same sperms fertilizing the exact same eggs, over however many generations were needed to produce Kirk and all of the rest of the same people on the Enterprise’s bridge.
That’s one of my pet peeves with the way the whole alternate universes thing is portrayed, there is an equivalent of each person in each universe… WHY?, it’s like the writers assume that people are a constant across universes.
There’s a you in this universe, there is not in the other universes, in the first place odds are that there’s no body with your name, and if there is, is not “you” is another person completely different.
Not only are there exact parallel people in the (TOS) Mirror Mirror universe, but they stay parallel 90 years later,
It is a universe ruled by violence and assassination. One assumes Kirk gets assassinated by Spock the same day the episode ended (or Spock fails, more likely). Whatever happens should make big divergences from the timeline. But 90 years later all the DS9 character are still parallel. It’s like both universes have to follow the same basic path. Which is dum.
In a nod to Marvel, it’s most likely there are many evil parallel universi. All the shows that insist on having them go to their own universe.
I think they are “created” by the mind of whoever goes through. Sort of a quantum reality thing.
I can justify the original story because the closeness of the two universes is what made transfer between them by the transporter accident possible. (There are a near infinite number of universes, after all.) But that the similarity of the DS9 mirror universe makes no sense.
I never liked the mirror universe in DS9 but at least it was its own thing and didn’t interfere with the main plot. I think there are 2-3 lines in real episodes that reference it but no actual plot points.
Now it’s been established, the one off mirror universe episodes were far and away the best 2 episodes of Enterprise, again not affecting the main plot.
Disco having 2 main characters from the mirror universe… 
I read that article! Annoyingly it had an obvious error in it (It said that Pike was Emperor during the course of the episode - when the episode established Pike’s non-imperial fate quite clearly).
It’s a tangent timeline - at the place/time of transfer, conditions have to be incredibly similar, but both the past and future can be quit different) (fan wank)
It’s sad that a bunch of random people in ONE thread have put more thought into time travel than the original writers ever did.
It’s frustrating how little anyone cared about continuity back then, or even the laws of physics. A minimum of effort and they’d have avoided most of the problems being complained about here.
I think that in “Yesterdays” the fact that their sun was going to explode made worrying about changes in the timeline somewhat moot. It all ends anyway with the big boom.