Blackout: The second example in the picture gives the theory that most people here on the forums are taking. Each jump creates a new timeline, while the original just keeps going on minus one time traveller who left to another parallel line. That could result in many different timelines, all different form each other. I dont like the idea of this one because then any other timeline from the original one feels less legitimate, like i said. It also means that in our "Prime" timeline, Spock simply dissapears. That is how he ends. Picard goes "WTF whered spock go?" and thats all there is to it. That is depressing. That also makes his mission a lot less personal. He's on a mission to save an alternate universe Kirk, it will mean nothing whatsoever to his own universe (except that he abandoned it).
_________________________________________________
Tachyon Shield: This is the theory that I use too, except I disagree with you that it makes the mission less personal. Spock is literally responsible for this new timelines creation. Spock should consider himself the guardian of this new timeline since it's of his very own creation. If spock was to just leave things be it would mean the deaths of billions of people on Earth and Vulcan possibly even more. Their lives are just as important as the lives of those in his own original timeline.
Spock knows that things in this timeline should follow the path it should have followed because if it doesn't it could be disastrous. Imagine how the galaxy might turn out if Nero succesfully destroyed the Federation in the 23rd century.
Well, assuming that each time jump creates a new timeline, and Spock goes back in his own timeship that we have seen (and not on the same ship as nero), that means that Spock didnt actually help the timeline Nero ruined. He actually made yet another timeline after that one.
1. Original timeline. Nero jumps back. After that, spock jumps back on his own. Kirk and buddies do fine.
2. Timline created by Nero's first jump. No future-spock to be found, nero wins. Vulcan is destroyed, romulans control the galaxy.
3. Timeline created by Spock's jump after Nero's jump. A third timeline where spock fixes things. Movie timeline.
So spock doesnt really save the new timeline, either. He just creates a third timeline to....make up for it. I dont think that's good enough.
Plus I still dont like the idea of spock simply dissapearing from the original timeline. Poof.
Plus, if you use the multiple timeline idea, that suggest that every time time-travel has ever happened, it created a new timeline. Which means that even by the end of TOS, we werent in the prime timeline anymore. We havent been in the prime timeline in 40 years of television. In fact, we've been jumping from timeline to timeline ever since. In the original timeline, the enterprise just dissapeared during one of their various missions where they travelled through time, and kirk never even got to finish the 5 year mission. THATS the REAL prime timeline. Do you really want that?
In fact, looking at
this list, it means that today as of right now...star trek is in somewhere around its 50th timeline. We are 50 timelines away from the
actual Prime timeline. Star Trek XI is not
timeline 2. Star Trek XI is
timeline 52. I just...i dont like that.