Warning: Not even time travel will save you from "Avengers: Endgame" spoilers if you continue to read this post. That's just not how it works in the Marvel Cinematic Universe, or the real world.
In "Avengers: Endgame," the heroes who survived the events of "Avengers: Infinity War" attempt to undo the damage caused by Thanos and his Infinity Stones-powered "snap" in order to save their friends and the universe.
Unfortunately, by the time the remaining Avengers track down Thanos, the now retired Mad Titan had already destroyed the Infinity Stones and their one shot at bringing everybody who was wiped out in the Decimation back to life.
Or so they think.
Thanks to Scott Lang, aka Ant-Man, and his accidental imprisonment in the Quantum Realm, the team figures out that time travel is the key to setting everything right. Despite his initial refusal, Tony Stark's brilliant mind can't help but resolve the science and create the technology required to make time travel a reality. The Avengers' time heist to collect all the Infinity Stones from the past is a go.
But forget everything you learned from movies like "Back to the Future" or "Terminator" or even "Hot Tub Time Machine." The MCU has its own rules about time and time travel as explained by the Hulk and the Ancient One during "Endgame."
Even more spoilers for "Avengers: Endgame" ahead.
How is time travel possible?
As explained in the "Ant-Man" films, the Quantum Realm is a place where the normal rules of time and space don't apply.
Scott spent five years in the Quantum Realm and for him it was just five hours. Janet van Dyne, on the other hand, spent 30 years lost in the Quantum Realm and it seems she experienced it as a full 30 years. There is clearly plenty that remains unexplained about this dimension.
That said, it turns out individuals are able to enter the Quantum Realm at a specific place and time and travel through it to exit back to reality at a completely different place and time -- so long as they have the proper equipment.
In "Endgame," the key components are Pym particles, which allow the Avengers to become so small that they are able to enter the Quantum Realm, and the special time GPS invented by Tony, which navigates each time traveler to the when and where of their destination.
Does altering events in the past affect the future?
No. According to Hulk, whatever you do while in the past can't affect the events that have already happened in your timeline because that's not quite your past anymore. So things like the grandfather paradox or the butterfly effect are not really concerns in the MCU.
The Ancient One later helpfully creates a visual showing how going to the past and removing something like an Infinity Stone can cause timelines to branch out and create alternate, parallel futures. Think multiverse theory.
If stealing the Infinity Stones from a specific time creates alternate timelines, how are the Avengers able to return to their future?
That time GPS is really handy for navigating all possible time and space through the Quantum Realm, even between timelines.
Why does Captain America need to return all of the Infinity Stones?
In order to retain the integrity of 2012 Ancient One's timeline, 2023 Bruce Banner promised to return her Time Stone.
This is because according to their conversation (with handy visual guide as mentioned above), returning each Infinity Stone to the exact moment in the timeline that they are stolen would make it as if they had never left the timeline. Meaning, the branched out alternate timeline would cease to exist.
Returning each Stone is the only way to make sure that the 2023 Avengers don't accidentally doom an alternate timeline at the expense of saving theirs.
"Endgame" doesn't quite explain how returning each Stone would negate everything a 2023 Avenger did in the past they visited, but it does.
But doesn't altering events in the past have no effect on a future?
What about how the 2023 crew accidentally gives 2012 Loki the Tesseract and allows him to escape?
Theoretically all of that happened after 2023 Captain America swiped Loki's scepter with the Mind Stone. So Cap returning the Mind Stone should somehow negate that event. (Probably?)
Does 2014 Thanos traveling to 2023 with his entire army and engaging in that big battle have any repercussions?
Same rule as above, in theory. Since Nebula and Rhodey had already left that timeline with the Power Stone, the timeline with that specific 2014 Thanos would cease to exist once Cap returns the Stone to the temple on Morag.
But what about when 2023 Nebula knocked out 2014 Star-Lord? She did that before they grabbed the Power Stone.
Maybe he wakes up in time to still grab the Power Stone before Korath gets to it.
Would returning the Soul Stone to the correct time bring back Black Widow?
It appears the soul-for-a-Soul-Stone trade is permanent. Or at least irreversible even with the power of the Infinity Gauntlet.
This suggests just returning the Soul Stone also would not help undo Black Widow's sacrifice.
But it might not be the last fans see of her. In comics, the Soul Stone houses a pocket dimension called Soul World, where the souls of those sacrificed for the Stone reside.
The existence of Soul World was teased at the end of "Infinity War," where Thanos visits the soul of a much younger Gamora. If Widow's soul is also in Soul World, perhaps there is still a way to save her. Maybe Adam Warlock will be a factor.
So why didn't they just grab a Black Widow from a different point in time?
Probably because she wouldn't be the same Black Widow. And because as heroes they couldn't justify disrupting or harming another timeline by kidnapping their Widow. Or maybe because the movie was already three hours long.
Does Captain America choosing to live out his life in the past affect the future?
Nothing in the past 21 MCU films indicates that it was impossible that a 2023 Steve Rogers was secretly living his own life as the other events in the films were happening. Maybe he was always there, just waiting for that exact moment in 2023 to pop back into the Avengers' lives again as an old man chilling on a bench with a Vibranium shield.
Are you sure all of this makes sense?
Please direct all further questions to Kevin Feige c/o Marvel Studios.
This article is written by Tracy Brown from The Los Angeles Times and was legally licensed via the Tribune Content Agency through the NewsCred publisher network. Please direct all licensing questions to firstname.lastname@example.org.