Over the weekend, as people saw Avengers: Endgame, I became really surprised at the number of people who simply did not pay attention to the movie. The film actually slows down at more than one time to explain how time travel works in “reality” and how movies like Back to the Future, which rely to the “butterfly effect”, are bullshit. In fact, I believe that’s even spoken in the movie by Scott Lang.
In a long interview with the New York Times, Markus and McFeely explain how they spoke to actual physicists who said that the Back to the Future version of time travel that everyone is basing their Endgame nitpicks on, does not work:
It’s crucial to your film that in your formulation of time travel, changes to the past don’t alter our present. How did you decide this?
MARKUS We looked at a lot of time-travel stories and went, it doesn’t work that way.
McFEELY It was by necessity. If you have six MacGuffins and every time you go back it changes something, you’ve got Biff’s casino, exponentially. So we just couldn’t do that. We had physicists come in — more than one — who said, basically, “Back to the Future” is [wrong].
MARKUS Basically said what the Hulk says in that scene, which is, if you go to the past, then the present becomes your past and the past becomes your future. So there’s absolutely no reason it would change.
If you use that as a basis for any of your time travel questions, such as Cap’s ending, it answers them.