Spoilers for the movie ENDGAME below the cut. Go see the flick first.
Several fans have voiced complaints about the plot holes caused by Avengers Endgame when they introduce Time Travel, especially questions about what happened to the original timeline? Now, as best I understand it, only two characters offer a statement as to how the rules of time travel work in this universe. One is Bruce Banner, the guy who cannot get his time machine to work properly, and the other is The Ancient One, who owns the Time Stone, and apparently understands its workings.
Banner’s statement is a flat statement that time travel paradoxes are impossible. You cannot erase yourself by going back into the past and killing your grandfather on his wedding day, because your personal past you remember remains your past, and when you are in the past the present you are changing is at that point your future, and hence can be changed.
In effect, this means that changes to the past have no effect on the present. If you steal a baseball glove from the past and bring it forward to your present two baseball gloves will be in your garage, the original one from the unaltered version of the past, and the one you took from the altered version of the past. The chain of events issuing from the altered version of the past exist in another timeline parallel to yours which you cannot enter.
The statement of the Ancient One is a little more confusing and creepy. She says that the Infinity Stones are what maintain the proper flow of reality and cause and effect, so that removing one from the past will create a divergent timeline. If the time traveler, however, returns the time stone to the very same point in the past from which it was taken, then no divergent timeline will ever have had been formed, and all events will continue in the main timeline as before.
If we take her at her word, then the changes to the past of something like stealing a baseball glove will not create a new timeline, but stealing one of the infinity stones will.
Now, this leads to two possible interpretations, neither one of which is explicitly stated by anyone, but the play of events only allows for one of these two. Either replacing the stolen Infinity Stones restores all events to the original timeline as if nothing had ever changed, or, the second option, is that the two timelines, the original and the divergent, are woven back together so that some editing of the original timeline is allowed. Now, there were some changes make to the timeline by the shenanigans in the past, such as Loki picking up the Tesseract and vanishing, Captain America fighting himself, Hank Pym running to the mail room, Tony Stark talking to his Dad, Thor’s Mom being told the date of her coming death, and so on.
None of theses necessarily create a time paradox, at least, so far as we know, with the exception of what happens to Loki in the divergent timeline versus what happens in the main timeline. Because in one of those two he is killed by Thanos, but I personally am hoping that the version where he has the Tesseract finds a way once again to fake his own death, and that will turn out to be the real reality.
At the end of the movie Cap returns all the stones to the exact time and place where they were taken from. Since, with such stones in his hand, he can do things like freeze time and cloud men’s minds and change reality and teleport, the heist of giving away the infinitely powerful stones is a lot less dramatic than the heist of people without infinite power trying to get them.
So the moment he puts the last stone back in place, presumably the Time Stone, what happens?
If all the events from the divergent time line are wiped out, then the version of Loki who escaped with the Tesseract retroactively has been erased. If, on the other hand, the events from both divergent and original are woven back into each other, the version of Loki we remember from all the previous movies still had the Tesseract this whole time.
So how did Captain America manage to live out his happy ending with Peggy Carter back in the 1970s, and yet somehow be on a bench on the Stark ranch in 2019? By marrying her, he clearly changes the past, and the daughter of hers who spoke to him at her funeral cannot ever have had existed, or else she is his daughter without knowing it, or something. Two Caps, and old and a young, one in ice and one married to Mrs Peggy Rogers live in the same time line.
Or there were and are two timelines, including one where he lived out his life until she died, and then as a sorrowing widower, he went back up into the musty attic into an old locked trunk, and got out his quantum realm suit and phial of Pym particles, suited up, and went through the quantum realm back to the moment five seconds after he left Banner and Falcon and Bucky, which to him had been decades ago. And then for some reason arrived fifty yards to the left, on a bench, without the suit.
But I like to think the Captain America used the time stone just before he put it back to bend the rules of time travel slightly and allow him both to live out his happy ending with Peggy, and to go back and say farewell to Bucky and Falcon. No matter what the answer is, time travel, by its very nature, is always going to be an inexplicable headache, no matter which rule of time travel a writer decides to follow. By its very nature, no answer about time travel is going to be completely satisfying.
But in movie terms, simply saying that changing the past does not change the present has fewer headaches and paradoxes involved in it that any of the other time travel films mentioned in passing in this movie, including Die Hard.
I will explain my theory as to why time travel is always annoying, and why time travel stories never quite make sense (not even mine) in another column.