Warning: Spoilers for Avengers: Endgame are ahead.
Forget everything every movie about time travel has ever taught you about time travel — yes, including Back to the Future. In the words of Scott Lang, aka Ant-Man (aka Paul Rudd), Back to the Future has been lying to you all these years because that’s not how time travel works. Or, that’s not how time travel works in Avengers: Endgame, at least.
In order to save the world and undo Thanos’ (Josh Brolin) snap with the Infinity Gauntlet, the Avengers have to travel back in time to collect all the Infinity Stones before Thanos can. Sound insane? The rules that are clear are things like, they need Tony Stark's (Robert Downey, Jr.) time travel GPS bracelets to navigate the Quantum Realm; the whole scheme is called a “Time Heist” (all credit, Scott Lang); and the crew travels from the year 2023 (because Endgame has jumped five years into the future) back to 2012, 2013, and 2014, along with an unplanned stop in 1970, to retrieve the stones. But the way the movie uses time travel ends up making a few things hella confusing, and it just kind of forgets some of the details in the end. But hey, alls well that ends well, right?
However, if you’re still scratching your head as to what the time travel rules are in Avengers: Endgame, here we go:
How Does The Avengers' Time Travel Work?
Sorry to actually explain this in terms of Back to the Future, but it’s the easiest example to use. Basically, Back to the Future states that if you go back in time and alter the past, that subsequently changes your future — you know, how Marty McFly suddenly stars to disappear because his parents aren’t dancing? If they don’t dance, they can’t fall in love and have Marty, so there’s no way for Marty from the present to exist if his past has been changed. Altered events in the past have direct consequences for the future.
However, Endgame is basically saying that this example of time travel is wrong. As it’s explained in the movie (or, as people try to explain in the movie), time is actually more of a straight line. Or, as Professor Hulk (Mark Ruffalo) explains: “If you travel back into your own past, that destination becomes your future, and your former present becomes the past, which can’t now be changed by your new future.”
Sure, you can jump around in time and go back to the past, but that time jumping is still part of your forward-moving timeline. These events you experience aren't changed, they can't change your present, they just happen to be happening for you at a different time. (ABC’s Lost actually has a good explanation for the way this works, because those characters started jumping around in time, too.) Basically, just because Captain America (Chris Evans) is in the past, it’s still his present — he’s still moving forward, just now in a different year. Anything that happens to you is part of your story, which is always moving forward, even when you visit the past. Got it?
That’s why it’s totally okay for Tony Stark to have an entire conversation with his dad (Howard Stark played by John Slattery) or for Captain America to knock Captain America unconscious. It doesn’t affect their current realities. However, Loki (Tom Hiddleston) is a different story, but let’s put a pin in that for a second.
Why Does Removing The Infinity Stones Change The Past, Then?
In 2012, Hulk shows up to get the Time Stone from the Ancient One (Tilda Swinton) from Doctor Strange. She is unwilling to give it up to him because she knows it will have ramifications for her timeline and her universe. She explains that removing one stone throws everything into a cosmic unbalance, and in her time diagram a scary black void appears, clearly showing Hulk that this is B-A-D. If one of the stones is removed, and it could create a parallel universe or branch reality — and um, maybe a hint about future MCU shenanigans? Hulk is clearly not thinking about what happens if they accidentally create a parallel universe, and promises the Ancient One that the stones will be returned to their exact place and time when they were taken, to basically plug up the holes from which those parallel universes would spring. The cosmic balance will be restored. But it would appear that these branch realities happen anyway. Which brings us to...
So How Can There Be Two Nebulas?
To really drive this time travel point home, especially regarding how the past and future don’t necessarily affect one another, Present Nebula (Karen Gillan) kills Past Nebula and Present Nebula doesn't disappear. If we were following the Back to the Future rule, if Nebula were to kill her past self her present self should die. That doesn’t happen. The fact that Nebula in the past is dead does not affect Nebula’s present timeline whatsoever because her timeline is constantly moving forward.
If it did affect it, it’d be the “grandfather paradox” which is described as you going back in time to kill your grandfather before your parents are born, thus stopping your birth. But still, it doesn’t affect you since you have already happened. But then how could you kill your grandfather in the first place if you don’t exist? It’s a paradox and it is super confusing, I know. So Endgame kinda just glosses over all of that, and also makes it easy for us to try and wrap our brain around it because Past Nebula has technically traveled to the future, not the other way around, so everything for Nebula is actually happening at the same time. Let’s just accept this, and let’s move on.
How Does Gamora's Time Travel Work?
In the past, after 2018, Gamora (Zoe Saldana) is dead. But, she’s brought from before she died, to the future, when Thanos' army comes through the Quantum Realm portal. Her past self is now in the present day and it appears she’s going to say there. It could be argued that if she were sent back to 2014, her timeline would continue on the way it was and in 2018 Thanos would still kill her on Vormir, but since Past Thanos' present is jumping forward to 2023 to be killed by Tony Stark, that option is pretty moot.
The time jump into the future definitely complicates the time travel rules and Gamora’s timelines, since there are basically two Gamoras now — the one who died in Infinity War and the one who time traveled into the future and met Peter Quill (Chris Pratt) for the first time on the battlefield. This is all a little hinky, but it looks a lot like a branch reality coming to play in the main MCU timeline. The only question about this that really matters now is she joins Guardians of the Galaxy: Vol. 3 and relives all those times she fell in love with Quill.
So Is Loki Alive Now?
Yes, and no. In the current MCU reality, Loki is still dead. But the God of Mischief has a new show coming to Disney+. Awkward. The easy workaround for this? Just give Loki the Tesseract and tell him to go wild! In Endgame, the Avengers go back to 2012, they accidentally botch their attempt to get the Tesseract and in the chaos, Loki — who has just been captured by the Avengers in 2012 and is about to go back to Asgard with Thor (Chris Hemsworth) — grabs the Tesseract and disappears.
While it's possible that Cap returns the stones to before Loki grabbed the Tesseract, it's also possible that Loki's move created a parallel reality for himself because the timeline that he was on now no longer exists. Yes, Thanos still kills him in the future, that’s completely unavoidable. But, with him grabbing the Tesseract, he could have split his timeline (like the Ancient One was talking about) and headed in a new direction.
This would make a lot of sense, because this past version of Loki would be completely separate from everything else and be free to travel about the galaxy wherever his Disney+ show takes him. Supposedly the Loki show is going to be a prequel, and it technically still can be, just in a parallel reality.
How Does Captain America's Time Travel To Peggy & Back Work?
While Endgame spends a lot of time setting up time travel, and how it works in the universe, it appears to undo it all in the last 10 minutes of the movie. Because the Ancient One has scared everyone into returning the Infinity Stones to their rightful place, Captain America goes on one last time travel mission to bring them all back. While he succeeds in doing so, he doesn’t return to present day. Instead, he decides to have a dance with his best girl and spends like 50, 60 years living happily ever after with Peggy Carter (Hayley Atwell) instead.
When he shows up at the end of the movie, he’s now aged into an old man and passes his Captain America shield (from the past) on to Sam Wilson (Anthony Mackie) who now takes on the title. This is great, and honestly, I’m so happy for Steve and Peggy because they deserve their happily ever after! However, in doing this the timeline has definitely split into two because Steve has 2,000% altered Peggy’s future — you know, the one where she had a husband that wasn’t Steve and two kids before dying during Captain America: Civil War? Steve has actually done the grandfather paradox here because he has gone back and changed something in the past bringing into question if things are now different in the future. What about Peggy’s kids? It would stand to reason that, like there are now two Loki realities and two Gamora realities, there are now two Peggy Carter realities. In fact, the Russo Brothers confirmed to Entertainment Weekly that a second reality is exactly what's happening here.
"If Cap were to go back into the past and live there, he would create a branched reality. The question then becomes, how is he back in this reality to give the shield away?" Joe Russo told the mag. “Interesting question, right? Maybe there’s a story there. There’s a lot of layers built into this movie and we spent three years thinking through it, so it’s fun to talk about it and hopefully fill in holes for people so they understand what we’re thinking.”
As for how Steve totally pulls a Doc at the end of Back to the Future II and shows up at the exact time and place where Sam and Bucky are going to be, that probably has something to with the handful of Pym Particles Cap took during his trip to 1970. It's possible that rather than Cap changing our current timeline and magically knowing to sit by the lake at a certain time, Cap used one final Pym Particle to come back after Peggy passed away. Maybe.
But as all these branch realities shoot off, and characters travel between them, time travel in Endgame is very confusing. Seriously, Hot Tub Time Machine had more concise time travel rules. But branch realities have been introduced, Loki is in one of them, and with that, the MCU has officially already overused its time travel privileges.