Spock Prime is in a different reality now. He can't return to his own reality. He can travel in time in this new reality, but what's the point? Vulcan will never be
his Vulcan.
Sure, he could try to save Vulcan just because it's a good thing to do. But he knows about a lot of other catastrophes that are likely to happen, simply because this reality is so close to his own. He could stop the Doomsday Machine without bothering to travel through time at all.
That sucker ate up who knows how many planets before Kirk & the gang stopped it. Spock could remember the system where they encountered it and most likely figure out where it would be in this reality.
If Spock wants to do the greatest good for the greatest number, he shouldn't waste any energy on devising methods of time travel. He's got a full plate as it is.
Yes the film does use the many worlds theory, of that I have no doubt. But to me it was a cop-out on the part of those making the film.
It's not a cop out if they never take advantage of the alternate realities to shortcut the drama and the characters' angst. Sure, Spock's Vulcan is fine and dandy somewhere, but as long as he's never allowed to go there, and the audience is never allowed to see that it's all right, the emotional impact is still that the only Vulcan we're going to see has gone kablooey.
The nice thing about the many-worlds approach is that it's a nice way of avoiding the annoying aspects of time travel - the reset button, the illogic of the grandfather paradox, etc. Technically speaking, it's not even time travel, but rather space travel - between realities rather than star systems or galaxies. The new reality's timeline could be offset compared with the Prime reality by a few decades so that travelling directly from one to the other appears to involve time travel. But we don't know that's even the case.
What you call a cop-out is a simple solution to a seemingly unresolvable conflict: Creating something new, while retaining the full integrity of ALL established history (Canon).
Yeah, they hit upon the one and only way to give
Trek a fresh approach while mollifying the canonistas. Of course they get zero credit by the canonistas.
The problem with the "writers' intent" explanation is that (A) their theory of how Trek time-travel works is dicta, coming more from interviews than from anything in the film, and (B) it's incompatible with essentially every other Trek time-travel story we've ever seen.
Nobody knows "how time travel works," so there's no basis for criticizing their choice of one method over another. I prefer the Many Worlds method of time/space travel because it's an internally consistent theory that avoids paradoxes, actual real-life physicists treat it seriously, and
Trek's time travel is inconsistent and incoherent anyway and therefore can't be used as a standard for anything. I'm happy to see them jettison all that reset button nonsense.
One of my pet theories is that
Star Trek time travel has always worked under Many World's logic, it's just that they never realized it. Like so:
1. Starfleet crew accidentally travels to the past or future.
2. Either they step on the wrong butterfly in the past, or they see that the future isn't to their liking; either way, they know They Must Make Things Right.
3. After much ado, they Make Things Right and return to their own time.
What if they never travelled in their own universe at all, but rather to a parallel universe where of course some details of past or future would be different just because that's the way that universe is.
Under Many Worlds theory, every possible thing that could happen must happen in one of the infinity of parallel universes. The Borg
must take over some universe. If our intrepid crew happens to land in that universe, they will be understandably upset and try to change things, without realizing, this isn't their universe at all, and in fighting the Borg, they are fighting the laws of physics, and anyway there's probably ten trillion realities where the Borg have taken over, so what's one more or less?
When they return to their own reality and see that everything is all right, they might delude themselves that their actions made everything all right. But isn't it just as likely their own reality was never changed at all, and the reality they left behind hasn't changed either? They have no way of knowing whether they are accomplishing anything at all by their so-called "time travel."
Therefore, travelling back in time changes nothing, and merely spawns another reality.
I think that all possible realities have already been spawned, in which every conceivable thing that could happen, has/is/will happen. With an infinite number of realities already in existence, you can't create another unique reality through time travel; none are left to be created. I think that this is simply the structure of the cosmos.
I don't think it's possible to travel within your own timeline (due to the grandfather paradox). It is possible to jump realities and end up at another point in that reality's timeline. You might even be able to travel in another reality's timeline, since your grandfather isn't in that timeline and you can't kill him. If the reality happens to be one very close to your own, you might be fooled into thinking you're time travelling within your own reality.
Ironically, the reality you were born in - where there are places, people and things you might care about safeguarding or rescuing - is the only one you can't change. You can change all the others, but since there are infinite variations on those realities, why bother with any particular one? Save Vulcan and there are twenty quadrillion realities where it still goes kaboom. There's a message in there somewhere...
