BIG SPOILERS AHEAD, so tread carefully. Avengers: EndGame is the kind of movie that leaves you asking for more, and also ends us making you ask more questions. I am sure the biggest confusing aspect of the film that left people in doubts was the whole time travel aspect. Or rather, time travel through the quantum realm. I have to confess even I was a whole lot confused when I saw the film first. And that's because I have been thinking about it all wrong! Avengers: EndGame: Future Movies and TV Series About Thor, Black Widow, Black Panther That Marvel Has Set Up Through This Film (SPOILER ALERT).
Like War Machine (Don Cheadle) and Ant-Man (Paul Rudd), our concept of time travel in movies is based on movies like The Terminator, Back to the Future series, A Wrinkle in Time and even Hot Tub Time Machine. Which means, our first thought is that if we go in the past and change things there, it will change things in the future too, that is our previous present.
I am actually surprised that they didn't name-drop X-Men: Days of Future Past, a Marvel superhero film that again was about changing the past to save the future. However, Avengers: EndGame, through the character of Bruce Banner aka Professor Hulk (Mark Ruffalo), had already made it clear that the depiction of Time Travel in those films is totally wrong if science assumes it is possible. Did Avengers: EndGame Confirm a Major Fan Theory About Captain America in Age of Ultron? (SPOILER ALERT)
We are all Scott Lang and most of us really don't understand the technical jargons like quantum mechanics, Deutsch propositions, eigenvalues and inverted Möbius strips that the movie throws at us. So as a layman myself, let me explain the whole concept of time travel in Avengers: EndGame to you in very layman terms. Let's start off with the thing that set the ball rolling here:
The Quantum Realm
Albert Einstein, through his Theory of Relativity, has explained that travelling to the future is comparatively easy - that is, if you can travel close to the speed of light. While this is physically impossible for now, at least theoretically it is possible. It is travelling in the past that feel impossible even for Einstein to deduce.
That's where the Quantum Realm comes into play. As Lang tells us, rules of time and space work differently in this tiny universe. In Ant-Man & the Wasp, Janet van Dyne tells Scott Lang that there are time vortexes in the realm, and he better not get lost there. In EndGame when Ant-Man finally comes out of the realm (trapped there because Janet, Hope and Hank Pym got Snapped) after five years (thanks to a rat), for him, it was just five hours. Which means, he was trapped in one of the time vortexes before that rat came into the picture. So for Lang, he went inside the Realm, spent five hours there and came out in the future, five years later (for him).
When Scott Lang meets Captain America (Chris Evans) and Black Widow (Scarlett Johansson), he asks if there is any way that they could get inside the Quantum Realm and leave out of it at a point in the past and change things. They meet Tony Stark aka Iron Man who refuses to help them saying that it is impossible to do so and what happened to Lang was a fluke. Later, he himself figures out a GPS system that could navigate through the realm and make them reach a specific point in time and place. With limited Pym Particles (needed to go subatomic to travel in the Quantum Realm), they have the things they need for Time Travel. But how does going back in time and change things there doesn't affect their past? To understand how that, let's take the help of Professor Hulk.
War Machine offers the morbid option of killing baby Thanos, before he became a tyrant (bringing the baby Hitler joke there). The Hulk, while mortified at the suggestion, shoots the idea down saying that killing baby Thanos doesn't change their timeline in any way. All they would do is have the sin of killing a baby.
Banner explains that when you travel in the past through the present, your present becomes your past and the past that you travelled to becomes your present. So whatever changes you make there, doesn't affect your timeline, because that has now become your past and can't be affected. But you can do whatever you want to do in the past you travelled and bring all the things you need to the present you left, like the Infinity Stones. Avengers EndGame Movie Review: Marvel’s Superhero Film Is an Emotionally Wrenching, Enriching FanBoy Experience That Goes for Your Heart!
So doesn't bringing the Infinity Stones to the present from the past, change what happens there? Oh, yes, it does. But not the way you think! To understand that, let's recall the conversation the astral Bruce Banner had with The Ancient One (Tilda Swinton).
The Ancient One
The surviving Avengers figure out how to travel in the past and where to exit. They make batches of three and leave for their respective missions. The Hulk, Iron Man, Captain America and Ant-Man leave for 2012 during the climax of the first Avengers film. They figure out that Mind, Space and Time Stones were present in New York at that specific time. Thor (Chris Hemsworth) and Rocket Raccoon (Bradley Cooper) go to 2014 during the events of Thor: The Dark World to get Reality Stone. Nebula (Karen Gillan), War Machine, Black Widow and Hawkeye (Jeremy Renner) leave for Morag in 2014 before the beginning of the events of Guardians of the Galaxy where Power Stone is present (Hawkeye and Black Widow leave for Vormir from there to get the Soul Stone).
In 2012 New York, Bruce Banner meets The Ancient One (who was five years away from her death in the Doctor Strange film), who was protecting her New York sanctum from the alien attack. When Banner requests her to give the Time Stone, she refuses to do that as it would affect 'her' reality. Note the word, 'her'.
The Ancient One later makes a magical yellow line to present her argument, which she calls the main timeline which is surrounded by the six Infinity Stones that control the flow. So whenever any Avengers pick one stone from the past, that creates a new time that diverts from the main timeline, and because there is no Stone to protect it, there will be some dangerous consequences to that timeline. So if you remove all the six Stones from the past, it leads to six different timelines with all dangerous futures.
However, these newly created timelines or realities won't affect the main timeline that could be saved with the acquired Stones. But in the future, the fabric of time and reality that has been torn in the past could affect the main timeline at some time in future.
Banner assures her that they would once again travel back in Time and place the Stones back in the same position and time-slot from where they have been taken from. This will plug the leaks happened in the main timeline and thus prevent alternate realities from being made. The thought here still has its flaws, and even The Ancient One has her doubts. It's only when Banner tells her that Doctor Strange (Benedict Cumberbatch) gave Thanos (Josh Brolin) the Time Stone willingly (in Infinity War), that she realises Strange was up to something. The tweet below shows the timeline dynamics pictorially:
— Abu Thur Bin Udin Al-Asgardi (@m0ltres) April 30, 2019
Now that I assume the theory is explained, let's explain how it is shown practically.
The 2012 Avengers Redux
When The Hulk was talking to the Ancient One about meddling with Time and Reality, we see the practical consequences of that around the same place. When Iron Man, Captain America and Ant-Man try to steal Loki's sceptre (that has the Mind Stone) and the Tesseract (that has the Space Stone), they are inadvertently creating new realities. One of their plans doesn't go as planned thanks to 2012 Hulk and his hate for stairs, which makes the Tesseract land in a captured Loki's (Tom Hiddleston, the character is dead in the main timeline) hands. He uses it to escape from there, creating his own timeline (also, defeating the Avengers in this reality in the process). This leads to Cap and Iron Man improvise their mission and go back further in time to the '70s where they steal the Tesseract from the old SHIELD facility, as well as more Pym particles. This also creates a seventh timeline, that wasn't accounted for.
When all return (save for Black Widow, who dies to retrieve the Soul Stone) to their present timeline, they find it to be just as it is because, none of the things that they did in the past could affect them here. They only created new timelines.
Interestingly, Avengers: EndGame is not the first blockbuster movie to use this of Time Travel. In the 2009 Star Trek reboot movie, an elderly Spock (Leonard Nimoy) goes back in Time thanks to him getting trapped in a black hole, an era when his younger self (Zachary Quinto) also existed. Nothing that Spock did here, nor the fact that he meets his younger self, change his future in any way, because that future has become his past.
The 2014 version of Thanos, Nebula and Gamora
Another doubt that the people had is how the past version of Thanos manages to get into the main timeline where he is dead (thanks to Thor, who finally went for the head). This is the consequence of what Iron Man has been warning the Avengers about - if you mess with Time, it messes back with you. After capturing Nebula from the future, Thanos replaces her with the 2014 version of herself. This version brings her 'father' Thanos, 'sister' Gamora (who is also dead in the present timeline) to the current timeline to wreak havoc. This Thanos now happens to know what happened to his future self and seeks to have a complete world annihilation through the Stones, that are all present at the same place in the Avengers facility.
So with the arrival of the 2014's Thanos, Nebula and Gamora in the present, this results in the convergence of a newly created timeline to the main timeline. They have skipped nine years of their lives to do so (the present is set in 2023) but for Thanos, it hardly matters. So now we have two Nebulas, and the dead Gamora and Thanos back among the living.
Nebula killing Herself
In movies like Looper, when we see the future version of characters killing their past selves, this erases the future selves. After all, if your past is not there, then how does the future exist? That doesn't happen, though, when Nebula kills her past self. To understand that, we have to go back again to what Banner tells War Machine - killing anything in the past doesn't change your future in any way. So when past Nebula comes to 2023, we now have two Nebulas with different personalities and fates. Killing one won't erase the other.
The Return of the Snapped Avengers
When Iron Man asks the Hulk to snap back their dead comrades to life, he insists that nothing in the five years should be changed (which means his daughter, Morgan Stark shouldn't disappear). So when Doctor Strange, Spider-Man (Tom Holland), Black Panther (Chadwick Boseman), and co. return to 2023, they also have skipped five years in this timeline, like Ant-Man, though they haven't aged a bit. This leads to some very interesting consequences for these characters, which I hope will be explored in the upcoming Spider-Man: Far From Home.
The Old Captain America
Now comes the most confusing aspect - how did Captain America grow old by the end of the film?
As Banner promised The Ancient One, he arranges for the return of the Stones back from they have been picked through Captain America. The Cap doesn't return, though, to the present, but we, along with Banner, Falcon (Anthony Mackie) and Bucky (Sebastian Stan) see Cap as an old man sitting on the bench near their spot. We find that he has put the Stones back (I am curious how he put back Soul Stone in Vormir and Reality Stone back in Jane Foster) and then decided to travel down even further. He sought for that life he missed out on when he went into the ice in the '40s and later, we find out he married his old love, Peggy Carter (Hayley Atwell). Interestingly, Bucky knew of Cap's plan all along, which is why their last conversation, including a beautiful callback ("How Could I? You're taking all the stupid with you!'), was so intriguing.
So how did this happen? Did Cap create a new timeline just to have the life he missed back on? Isn't that kind of irresponsible from his end? He sure did travel back in time but he did not create a new timeline. In fact, now we have two Captain Americas living at the same point of time (the 2012 Avengers excursion and two Nebula fiasco proved that is possible). One Captain America that we have seen till now in MCU, who was in the ice and later fished out in 2012. Second Captain America married Peggy, but lived a life of obscurity in her house, never letting his identity out. Cap only comes out of obscurity to meet his friends, when his younger self goes back in time.
But wasn't Peggy married to someone after Cap went into the ice? Sure she did, but we never saw her husband. Also, when Captain goes back in the '70s with Iron Man, he stumbles in Peggy's office and found a picture of himself there, and not her husband. In Civil War, Peggy's niece Sharon Carter (Emily VanCamp) even tells Cap (who also snogs her quite inappropriately in the movie) that her aunt had secrets of her own. Was her time-travelling husband one of those secrets?
Also, let me clear one more thing here- the serum injected in Steve Rogers to make him Captain America doesn't stop his ageing or make him immortal. The reason why he was still young when he was finished out of ice, is that while serum didn't let him get killed, the ice allowed Cryogenics (the art of freezing and preserving bodies) to kick in and didn't allow Cap to age.
I am sure that you still might be having a few more doubts still. I am, myself, curious, as to how older Cap manages to get the (intact) shield without changing things in the past to pass it on to the Falcon. But then a movie like Avengers: EndGame, directed by Anthony and Joe Russo (hail to you, sires!) has to be enjoyed with some levy given to its loopholes. Though, if you think hard, you might solve those loopholes yourselves!