The MCU is a franchise like no other with over a decades worth of movies all leading up to Avengers: Endgame, an event film that takes more twists, turns, and time jumps than you can throw a shield at – and although the movie does explain it’s own, relatively unique, rules of time travel multiple times, the exact logic and rules involved still leave a few questions to ponder.
The beginning of the movie sees the immediate aftermath of Thanos’ snap, and in it’s first of many big surprises Endgame shows the Avengers quickly track down Thanos and kill him. That huge shock is followed by another when the movie jumps forward five years and shows the lasting impact of losing half of the life in the universe.
Scott Lang, aka Ant-Man, is then freed from the Quantum Realm, thanks to the Disney owned Marvel and Pixar crossover that no one saw coming (*see image below). Once free Scott realises that the five years he has been missing only felt like five hours to him, and so he comes up with the idea of a ‘Time Heist’.
The movie then descends into all kinds of ridiculously awesome time travel shenanigans, that see the Avengers come up with a way to navigate the Quantum Realm, head back into their own past, and ‘borrow’ the Infinity Stones in the hopes of bringing back everyone that Thanos ‘snapped’ away.
Endgame’s specific time travel rules are then explained via two conversations, in the first Rhodey asks why they can’t just go back in time and kill baby Thanos, based on rules set up in movies like Back to the Future, Terminator, Time Cop, Time after Time, Wrinkle in Time, Somewhere in Time, Hot Tub Time Machine, and Bill and Ted’s Excellent Adventure.
The now talking ‘Professor’ Hulk explains that contrary to those movies; “if you travel to the past, that past becomes your future and your former present becomes the past, which can’t now be changed by your new future.”
This is essentially the Grandfather paradox, where contrary to the logic in movies like Back to the Future, if you go back in time and kill your grandfather, it wouldn’t mean that you were erased from existence, because you have to exist to kill your grandfather – it simply means that going forward in that timeline, you would never be born, but it would not erase you from your own original timeline.
Hulk’s explanation comes down to time being relative; take away time travel for a moment, and consider regular life; nothing you do now, tomorrow or at any point going forward will undo something that happened to you ten years ago. This logic still applies if you were suddenly able to time travel – you wouldn’t change your own past, memories, or experiences, you’d simply create a wholly new divergent timeline, where things happen differently, but to a different version of you, not the ‘you’ version of you (if that makes sense).
And so killing baby Thanos, like Rhodey suggests, wouldn’t change the Avengers past, it would instead create a new reality, with a separate set of events where Thanos couldn’t grow up to perform the snap, but it wouldn’t bring back those he had already snapped in the ‘Prime MCU’ universe, it would simply create a new one.
The explanation of how the MCU’s timeline alterations work is then further detailed during a conversation between the Ancient One and Bruce Banner. There the Ancient one tells Bruce; “The Infinity Stones create what you experience as the flow of time. Remove one of the stones and that flow splits. Now, this may benefit your reality, but my new one… not so much.”
And so, as removing an Infinity Stone from a specific point in time is a huge alteration, it completely splinters it from the original timeline. Bruce suggests that returning the stone at the point it was taken should solve the problem, and erase any divergent timeline by setting events back to exactly how they should happen. The Ancient One appears to agree, although she takes issue with the likelihood of Bruce’s survival and consequent ability to return the stone, and that is of course based around no the large alterations being made.
And so based on this theory, The Avengers decide that the only real solution is to travel back in time, steal the Infinity Stones from the past, bring them to the present, use them to perform a new snap which will bring everyone back, and then return the infinity stones to wherever they stole them from.
There are then a few things that we can take from these explanations; firstly a time traveller can’t simply go back and erase someone or something from their own past, instead any alterations create a new divergent timeline.
Then based on what Bruce and the Ancient One say, once the stones are returned that timeline can continue as normal. The thing to bear in mind however is that as soon as that initial time jump is made, because of the butterfly effect that timeline is now splintered from the original, and in most cases even returning the stones doesn’t ‘fix’ that timeline back to how things should be, instead it just allows it to continue on it’s new, slightly altered path.
It’s worth noting here that there are conflicting opinions on how the time travel works in Endgame between the movies’ directors, the Russo brothers, and it’s screenwriters Christopher Markus and Stephen McFeely. The Russo brothers explain that at the end of the movie when Captain America returns he has been living in an alternate reality, which fits in with the idea that you can’t go back an alter your own past without creating a divergent timeline. Markus and McFeely, however, state that old Captain America at the end of the movie has indeed been living in the Prime MCU the whole time – which goes against the idea of altering your own past and directly conflicts with the argument against simply going back and killing baby Thanos. And so although there’s no denying Markus and McFeely definitely have some weight in the conversation, the Russo brothers explanation is more in line with what the rest movie sets up.
And so we get two different types of divergent timelines, ones with only slight differences, where things should mostly follow a very similar path to the prime MCU that we’ve seen so far, once the stones are returned, and then others that are vastly different even once the stones are returned due to large alterations, like Captain America staying behind.
The first of the new timelines are in 2012 New York, where the Avengers realise there will be three different Infinity Stones at the same time. Iron Man and Ant-Man go for the Tesseract which houses the Space Stone, Captain America heads for Loki’s sceptre which contains the Mind Stone, and Hulk goes to the Sanctum Sanctorum for the Time Stone (which leads to his conversation with the Ancient One).
Captain America successfully takes Loki’s sceptre from Strike Team and S.H.I.E.L.D agent Sitwell, by whispering ‘Hail HYDRA’ – this splinters reality into one where they either believe Captain America is part of HYDRA or after discovering he isn’t, decide that Captain America is on to them… this is made even more complicated by the fact that Captain America from that reality, doesn’t know any of this. As this is a huge alteration it creates a distinctly different timeline where the events of Winter Soldier (and thus the rest of the MCU) could play out very differently.
In that same reality Tony’s loses the Tesseract after stealing it from his past self, and Loki takes it, escaping Thor’s custody with the space stone, another alteration that has a large impact on Thor: The Dark World, Thor Ragnarok, and presumably Avengers: Infinity War – and so this timeline is altered in a number of ways that can’t be undone by simply returning the stolen stones, and is thus a distinctly different and ‘uncloseable’ timeline.
After Loki takes the Tesseract, Tony and Steve head to 1970, for more Pym particles (which facilitate the time travel) and to find the Tesseract at an earlier point – there Steve sees Peggy Carter, and Tony meets his father.
The two successfully steal the Tesseract and some Pym Particles – but everything they do here can be undone, and so this timeline, can be easily closed off once the stone and particles are returned. It is worth noting that they are noticed, and the camp is put on alert for intruders, and so based on even the smallest difference having a knock-on effect, this could be looked at as an alternate, un-closeable timeline as well, but one that should follow a very similar path as the changes were minimal.
In 2013, Thor and Rocket head to Asgard for the Reality Stone/Aether, there Thor is reunited with his mother, and Rocket takes the Aether from Jane Foster – this is another not too damaging trip, as all they really do is take the Aether and Thor’s Hammer Mjolnir (both of which can be returned to just after they were taken), and so this one can be closed off relatively simply as well, but again technically based on the butterfly effect, everything could be different going forward thanks to Rocket being noticed and then chased by the Asgardian guards (although the movie does imply everything else will play out basically how we’ve seen it).
War Machine and Neubla head to 2014’s Morag to follow Peter Quill to the Power Stone – they knock him out and take the stone, and almost get away with another part of the heist that could be simply closed off.
The main problem here, however, is that Nebula’s neural network or (‘eye’-cloud) syncs up with her 2014 self, and 2014’s Thanos, Gamora and Nebula learn of the 2023’s Avengers plans to alter time. Again Thanos learns of his future success (and death) before any stone is taken, and so this timeline is another which is fundamentally altered regardless of taking and returning the stones.
The last of the time heist teams sees Black Widow and Hawkeye head to Vormir and take the Soul Stone, Black Widow sacrifices herself, and Hawkeye gets the Soul Stone – but again provided it’s a simple enough process to put it back, this timeline could be closed off and go on as normal once the stone was returned. It, however, an extension of the 2014 jump with War Machine and Gamora and so it’s tied to that one.
It’s also worth noting that Hawkeye technically created an altered timeline during his test jump, where he took the baseball glove, but never returned it.
And so at the end of everything, even after Steve returns the stones at the end of the movie, we are left with a number of timelines, some with seemingly inconsequential alterations, and others with massive differences. Primarily the timeline in which HYDRA either believe Cap is HYDRA or at the very least knows about them, and where Loki escaped using the space stone. And then the timeline in which Thanos learns of his future self and eventually travels to 2023 and dies, leaving that version of 2014 without a Thanos, Nebula or Gamora. We can also take the fact that the Gamora from 2014 is still around in the main MCU’s 2023 as extra evidence that Steve returning the stones doesn’t simply erase events in divergent timelines.
There is also the question on how the old Steve Rogers shows up at the end of the movie in what appears to the ‘prime MCU’ even though we know that Peggy got married to someone else, and was clearly shocked to see him having returned from the ice when she was older, and so his happily ever after with Peggy must of been in another divergent timeline created when he decided to stay.
Again this idea of Captain America growing old in an alternate reality and then returning to the main one has now been confirmed by the Russo brothers, but it has been argued against by Christopher Markus and Stephen McFeely.
It’s also worth pointing out that an important thing to remember with the Quantum Realm GPS watch ‘thingys’, is that the Avengers are able to jump not just backwards and forwards in time, but from one reality to another as well. And so once they have created an alternate timeline by altering the past, say for example ‘MCU – Timeline 1B’ is the one where Loki escapes, they are still able to travel back to their original reality, ‘MCU Timeline 1A’ despite their actions putting them in a wholly new timeline.
As confusing as it may seem once you start over thinking it, the explanation really is all there in the movie, and it basically all it comes down to the idea that every time a jump is made, based on theories like the butterfly effect, a new timeline is created – but realistically only a few of those matter, and have massive differences to the main MCU.
And whilst those massive differences will likely lead to the return of characters like Loki, and possibly anyone else who died in the main MCU, all of whom may have survived in the other timelines, the main thing to remember is that Endgame is a great movie, which combines everything the MCU is and should be, whilst setting it up for what it’s going to become.