Since the film has a time travel plot, all Abrams and company have to do is stay true to continuity in the 24th century scenes, show the temporal incursion, and then from there onward (including any sequels), we're in an altered timeline.
Time travel stories are absurd.
For the sake of argument, let's say that backwards time travel is possible, and that I was able to determine what the 'proper' timeline was. If I were trying to 'repair' damage to my timeline, I wouldn't travel back just far enough to
not quite fix things. I'd either travel back a short while and stop the villian from leaving at all, or I'd travel back far enough to intercept him and foil his plot before it began.
It's far better to steer clear of such stories in the first place.
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I agree with you and Captain Janeway: it is better to steer clear of time travel. I also agree with the OP that this is likely what we will see in STXI, an in-continuity Infinite Crisis type reboot.
As to why Spock doesn't simply go back and stop Nero from doing ANYTHING (since it seems clear he doesn't) for the sake of this outing, I would imagine that Spock is somehow unable to go back far enough to fix the damage. However, if you inject a little multiple universe theory in there, Old Spock didn't really travel back to his own past, he travelled back to another universe that at least until whenever Nero popped up, was identical to his, but then took a divergence.
What puzzles me is how Spock would notice this unless he travels back to the point where Nero instigates everything and is trapped there for years until Kirk grows up and he has a good shot at 'fixing things,' unable to time travel again. Perhaps his ship has a one-time-only function, or he stowed away on the Supersquid and escaped? Surely Vulcans aren't now immune to temporal shifts like certain El-Aurians?
The whole 'there's a universe where every possibility happens' thing has always irked me slightly but seems to be a valid concern with this apparent plot point. If you travel back to change the past, and succeed, then you displace your original future because you have shifted into one of the other universes where your desired outcome occurred. Okay. So let's say that, like in this case, you (or here Old Spock) are unhappy with the results. Instead of trying to 'correct' the timeline, why not simply figure out how to go back to the quantum universe where your 'past' is still intact (a la TNG's 'Parallels'), or is that harder than rewriting the future history of another timeline (the alternate reality you have found yourself in)? Or is that what you are in essence doing by trying to correct the timeline? Then, does the very act of time travelling with the intention of changing time somehow in and of itself displace you from your original quantum reality, as if the point to which you have travelled is a shatterpoint from which realities diverge?
But really if there's a universe for EVERY possibility, then EVERY MOMENT is a shatterpoint, and making just one decision renders you instantly displaced. So simply by virtue of being in the past where (presumably) in the original timeline you weren't (excepting predestination paradox) you have INSTANTLY displaced yourself from your own timeline universe. So in Star Trek IV, Kirk and company appear to have really traveled to their own past because --
they did. They gave Dr. Nichols the formula which he is credited to because they gave it to him originally. However, in the upcoming film, there presumably was no time-foolery in Kirk and co.'s past originally (based on the notion that the timeline is different due to Nero's interference), so simply by virtue of the act of time travelling Nero and Old Spock have travelled to an alternate timeline.
The point of this rant/intellectual exercise I think ultimately proves that the 'past' we see is another universe simply by virtue of the presence of Nero and Old Spock, given that Nero's appearance and destruction of the Kelvin did not originally occur and was the primary point of noticeable divergence for everything that subsequently occurs. As a TOS purist, that makes me a little more accepting of what is almost certain to come of this movie - a total reboot of the Trek universe into something only vaguely similar to what we have seen and a relegation of the old universe to Earth-2 status - and more able to judge the film on its own merits. In that regard at least, despite how successful the movie ultimately proves to be, I'd say Abrams
is successful.
And now my head hurts.
