10 Crazy In-Universe Rules MCU Characters Must Follow
5. Changing The Past Doesn't Change The Future
Avengers: Endgame was a masterful exercise in fan service courtesy of its brilliant central time heist, even if the film's wacky time travel rules caused quite the headache for fans.
While classic time travel movies such as Back to the Future adhere to the logic that changing your past will effectively "rewrite" your present according to any changes made in the past, Endgame says that's a bunch of hooey.
As Bruce Banner (Mark Ruffalo) rather awkwardly explains, the MCU's time travel laws prevent the changing of the past. Rather, travelling back in time and making changes will simply create a new branching, alternate timeline, while the original doomed timeline stays in tact.
This is why they don't, as Rhodey (Don Cheadle) suggests, simply kill Thanos (Josh Brolin) as a baby, because it would only create a new timeline without rectifying Thanos' existing damage in the prime timeline.
And so, that's why they have to retrieve the Infinity Stones from the past, use them to un-snap half the universe, and then return them to their original time stream, ensuring everything remains in temporal tact.
It's certainly an interesting take on time travel and surely not what most expected from Endgame's timey-wimey shenanigans, even if it arguably just makes things even more of a headache for the heroes to figure out.