Everyone is confused with the changing of timeline and time travelling in Avengers: Endgame. Therefore, today cistheta has arrived with a theory that helps you to understand the concept of time travel in The Avengers: Endgame.
First you have to understand that, if you go back in time, do something. Whatever you did has no consequence in your timeline, so if you go back in time, kill baby Hitler, return to your own time, what happens is that you realize WWII still happened, Hitler lived and eventually died in his bunker just as you know from history books. There is no mention of you killing baby Hitler.
Returning in time and killing Hitler effectively created a different and unrelated timeline, one without Hitler, where events play out differently. It had no effect on your own timeline though and you can’t interact with it. That means you cannot change your own past, only create a “parallel universe” where something changes.
TIME TRAVELLING IN AVENGERS ENDGAME:
I’m not sure that the plot of Avengers Endgame really defines the mechanics of time travel that well. But in any case, the are some basic educated assumptions we can make about the time travel if we carefully consider the short explanation given by Bruce Banner, and the further extrapolation of the Supreme Sorceress when confronting him about the Time Stone.
First, we are told that the past cannot be changed. Sure, they can go back and mess up the past real good (they proceed to do so) but those changes don’t effect their own history. They don’t have to worry about paradoxes or causing themselves not to exist or whatever, because the past they are changing isn’t connected to their own history. It’s some kind of independent alternate history. I believe that Bruce Banner starts out assuming that this alternate history is mostly temporary and that besides being able to retrieve the stones from this alternate past and bring it back, there are no real persistent problems with changing things.
The Sorceress Supreme sets him straight on this matter. These alternate histories are in fact alternate parallel worlds which will branch off into different “worlds” or “time lines” when the heroes do something, such as taking the infinity stones. (Perhaps other actions as well, though that point isn’t clearly made.) So the heroes should be responsible and take care not to foul up these alternate history worlds by doing something foolish, like taking the infinity stones and not returning them, otherwise they will be responsible for the consequences they leave on these alternate histories, which are not just temporary copies, but in fact, persistent alternate worlds in their own right. Being permanently deprived of the infinity stones may prevent Thanos’s threat, but apparently there are other more nefarious cosmic threats just waiting to pounce on the universe left unprotected by these very important infinity stones. Banner must promise to return the stones once that are no longer needed before she will give hers over.
This pretty much establishes that each time someone travels to another time “location” any actions they make that change history (just being there is possibly an action in itself that changes history) branches into a new alternate timeline “world” which continues to exist. This complicates matters.
The movie does not approach this subject, but it must be considered that once alternate histories exist, it isn’t enough to specify the time and location to time travel to, but also which “branch” or alternate history one wants to go into. And also, once traveling into another branch under what circumstances might this create additional subbranches? The movie simplifies things by simply ignoring these complications.