Seriously, you're a Star Trek fan and you can't get how a timeline can fracture off into a parallel one if someone goes back and futzes with the past?
Let's use a videogame analogy. I play to level 24, and save that game. Now, I reload the game back at level 23, play again but do things different. I save that game with another filename. Now I have two different versions of events stemming from the same point. That's the idea in the film. Game A (old Trek) is one game session, and Game B (new Trek) is a different save file.
Heck, Enterprise made a Game C (mirror universe) by Zephran Cochrane shooting the Vulcans at first contact, so this is old news.
Let's use a videogame analogy. I play to level 24, and save that game. Now, I reload the game back at level 23, play again but do things different. I save that game with another filename. Now I have two different versions of events stemming from the same point. That's the idea in the film. Game A (old Trek) is one game session, and Game B (new Trek) is a different save file.
Heck, Enterprise made a Game C (mirror universe) by Zephran Cochrane shooting the Vulcans at first contact, so this is old news.