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It would seem like this debate was laid to rest in June 2019 or thereabouts, and yet I get the feeling people are still arguing about it. It's to say because there are so many sites that discussed it and came to different conclusions.

Here are a couple of videos that proposed explanations.

In May 2019 Screen Rant released this highly complex attempt to explain how Captain America (Steve Rogers) could go back in time and re-appear "magically" right after he left at the end of the movie.

Watch Why Avengers: Endgame's Time Travel Makes Perfect Sense from YouTube
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Why Avengers: Endgame's Time Travel Makes Perfect Sense

Okay, I'm not sure their explanation makes perfect sense at all. I have wondered for many months if Steve was supposed to be a time traveler from the start. For example, in Agent Carter we see his picture on her desk in the 1940s. That seems to clash with the idea that she married someone else. Also, we never actually see who Peggy married.

In June 2019 YouTuber The Marvelous Wave summarized the Russo brothers' explanation for how they decided time travel should work in the MCU.

Watch Russo Bros FINALLY Explain How TIME TRAVEL Works In AVENGERS ENDGAME from YouTube
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Russo Bros FINALLY Explain How TIME TRAVEL Works In AVENGERS ENDGAME

As far as the movie goes, it was their story to tell. And there is no indication whether Kevin Feige was involved in any of these time travel considerations.

We already know that MCU characters have fiddled with the timeline from Dr. Strange, because the Ancient One was the guardian of the Time Stone. Dr. Strange himself used it to defeat Dormammu (but he was "outside of time and space", or outside of our time and space).

Science fiction movies love to toggle the timeline and confuse audiences with alternative realties, alternate timelines, and parallel universes. To some people they are all one and the same thing. To other people they are distinct things.

The difference between a parallel universe and an alternate reality is the difference between an alternate reality & a parallel universe.

Or something like that.

Not being an expert in time travel myself, I try not to speculate too much about how it might work. Physicists concede that we already travel through time (the so-called "forward" direction), but Einstein's Theory of Relativity allows for the mathematical possibility of traveling backward in time. We just don't have any science that shows how to do it.

Scientists occasionally propose theories for time travel but they usually involve exotic energy.

Dr. Steven Hawking once said he threw a party for time travelers and announced it afterward as a test to determine if time travel is possible. Of course, if he was serious there was a considerable flaw in his logic. He would have to assume that any future time travelers:
  • [*=1]Learned about the invitation
    [*=2]Cared to accept the invitation
    [*=3]Had the ability to travel to his party
    [*=4]At least some time travelers would exist in the future and not in the past
If any of those conditions were false, then future (or past) time travelers would not attend his party. Hence, the fact a party for time travelers was ignored by time travelers doesn't mean there are/were/won't be any time travelers.

I would hope he was joking, though.

That said, I liked Professor Hulk's explanation about time travel in the movie. He said if you travel to the past then the past is in your future. If that is true and you travel to your own past then - in theory - you should be able to interact with yourself. But then you should remember doing so. Otherwise time travelers are careful NOT to interact with themselves or the universe somehow manages to preserve the temporal equilibrium (or something).

The Russos basically concluded that traveling into the past creates alternate timelines. The Marvelous Wave video speculates that Old Steve Rogers somehow reunited them. Maybe that works.

Time travel doesn't have to create all the paradoxes that people think it should. If you travel into the past and kill your grandfather, maybe you kill your grandfather after he helps conceive one of your parents. Maybe your parent was adopted. Maybe you kill the wrong guy.

It's not a foregone conclusion that attempting to kill your grandfather creates a paradox. Suppose you succeed in doing so without preventing yourself from being born, but before you traveled back in time you had grown up with your grandfather?

Well, that's a paradox only if changing the past prevents you from traveling through time. Otherwise it seems Einstein's theory doesn't preclude the possibility of altering the course of events through time - which eliminates the need for alternate timelines.

A time traveler might be incapable of interacting with the physical universe anywhere outside of his or her own time. Time is relative to mass and the time traveler will still have some mass. Does that create a "time bubble" that seals you in to your own personal time while you travel through the universe's past or future?

Traveling through time should be very dangerous. If you account for the fact that Earth is rotating, orbiting the sun, and following the sun as it orbits the Milky Way galaxy's center, and being carried by the galaxy through the universe on a 1-way journey - then going back into the past almost certainly guarantees you'll end up dead in space.

Presumably our time traveling civilizations are able to calculate where things will be and adjust for changes in location (which would be relative to the start of the time travel journey). Even if a permanent portal or tunnel (a time wormhole) is assumed, it would still have to traverse great distances to drop you at the point where you need to come out.

Another paradox that is occasionally used in SF stories is the idea that "same matter cannot exist in the same place at the same time". Anyone who has seen the movie Timecop should know that ends up badly.

And yet we have proven that quantum entanglement is a real thing. Two particles separated by vast distances can be attuned so that they are essentially identical in every way. One way to describe this effect (crudely, poorly, inaccurately, but in a visually comprehensible scope) is to say that two particles combine to become a sort of wave-like thing.

Hence, you might be able to exist next to your past or future self and touch that past or future self without any real harm coming to your selves.

All of which is to say that until someone succeeds in traveling through time and space we won't know what is or isn't possible within the physical parameters of the universe.

And thus I'm not sure there is really any need to explain the details.

We know that Disney/Marvel spun off a Loki series based on the error in the time travel adventure. So how does Steve Rogers correct that mistake? Does the Loki alternate timeline end with Captain America bringing him back to his imprisonment?

Ultimately the story unfolds as the author (or scriptwriter/director) say it unfolds. They can wave their hands and say this magic thing happens and that's that.

And just as the characters in the story (like Professor Hulk) might be wrong in their attempts to explain how time travel can and cannot work, so the story-tellers can be wrong. Which is not to say anyone can ever correct them and say, "no, it really happened this way".

Rather, they don't have to know how it all works despite their best attempts to explain it. All they have to do is say, "This is how the story unfolds and here is the best way we can explain it."

But you have to live with a little bit of doubt if the explanation doesn't convince you. The way to reconcile that seeming paradox is to assume that the story-teller is no better qualified to explain time travel than you are.

And thus it is a bit magical, but maybe that adds to the beauty of the story rather than detracts from it. At least for some.