WARNING: Major spoilers for Avengers: Endgame.
Avengers: Endgame brings time travel to the fore of the MCU and, as predicted, it's really rather complicated. Time travel isn't a totally new concept to the Marvel Cinematic Universe. Doctor Strange made use of the Time Stone in his solo movie to defeat the Dark Dimension's Dormammu by trapping him in a time loop, and Ant-Man and the Wasp teased that the Quantum Realm had "time vortexes". Avengers: Endgame takes that latter thread and runs with it.
After Thanos has destroyed all the Infinity Stones and been killed by Thor, the Avengers are lacking the means to undo the effects of the snap. A get around comes five years later in the form of Scott Lang, whose time in the Quantum Realm felt like only five hours; he rationalizes that, because time moves differently, it would be possible to leave and enter at different points across the timeline. A plan is formulated, leading to the entire second act of Avengers: Endgame (and a fair bit of the third) being concerned with time travel.
Time travel is hard. As something theoretically possible on a quantum mechanics level but physically impossible from a human vantage point (unless you count Planet of the Apes' use of relativity to move forward at a fast rate), it functions in fiction entirely on stated logic within the story at hand. There is no true rule to time travel, which means every movie can create new ideas. However, only a handful - The Terminator (just the first), Looper, Primer - follow their rules religiously. That's not a problem if the story a movie is telling is entertaining (see Back to the Future, which is impossible to track yet so exciting it doesn't matter), but it can still leave audiences scratching their heads.
Avengers: Endgame, as expected, doesn't make total sense. It pulls from multiple different ideas of time travel, using them all at once for different ends. This is all in service of the characters and their fight against Thanos - you can't get Captain America's incredibly satisfying ending without fudging things a bit - but it can mean making sense of what's going on a little tricky. And so here we'll explain every question you could possibly have about Avengers Endgame's time travel.
First, though, here's a glossary of the four different types of time travel that will be important in this discussion.
- Linear timeline (with time loops): whatever happened, happened. Going into the past can't change the past because it already happened, you simply enable it to happen. Example: Twelve Monkeys, The Terminator (just the first)
- Grandfather paradox: if you go back in time and kill your own grandfather, then you can't have existed to go back and kill him. Example: Surprisingly few, Donnie Darko comes close. Many others (such as Looper) subvert the idea.
- Butterfly effect: any small change in the past will have massive impacts on the future. Example: The Butterfly Effect
- Fluid timeline: a mixture of all of the above where what changes have an impact is decided based on in-story logic. Example: Back to the Future, Terminator 2: Judgement Day
How Avengers: Endgame's Time Travel Works
The justification of time travel is fundamental to Avengers: Endgame's second act, with characters repeatedly questioning its possiblity. Of course, Tony Stark's scientific explanation - its perils are rooted in quantum chromodynamics theory, Scott turned into a baby and old man because of the EPR paradox and Deutsch proposition, the wrist devices derive from the eigenvalue of a particle field accounting for spectral decomposition under his Mobius strip configuration - glances with genuine theory but one without any real application. Yes, Mobius strip time travel is a pre-existing idea (it's at the heart of the grandfather paradox), but it's not how the film presents time travel.
There are two scenes in Avengers: Endgame where time travel is explained. First is Banner ahead of Hawkeye's test mission. Responding to Rhodey's suggestion of killing Thanos in the crib (an adaptation of the "kill Hitler" time travel theory), Bruce dismisses most time travel examples from popular culture (Back to the Future especially, but Star Trek, Terminator, Time Cop, Time After Time, Quantum Leap, Somewhere In Time, Hot Tub Time Machine and Bill and Ted's Excellent Adventure are all mentioned), stating "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." The basic implication is that you can't change the past because you've existed in the future; no matter what you do, the end result is the same. Even if you were to try and kill baby Thanos, the future you have must be unchanged.
Later, when Bruce is attempting to get the Time Stone from the Ancient One, she talks to him and elaborates on his idea further: "The Infinity Stones create what you experience as the flow of time. Remove one of the stones and that flow splits." This suggests that, while the post-Decimation future the Avengers have come from will be there when they return, their actions in the past can impact the timeline to the point that new timelines are created (which, due to the lack of Stones, are much more fraught). While it's possible any change could do this, it's only explicitly stated that it occurs when an Infinity Stone is removed from the timeline, which Bruce proposes can be fixed if they "return each one to its own timeline at the moment it was taken so chronologically, in that reality, it never left."
In short, the Avengers can't change their own timeline as it already happened, so going into the past doesn't affect their own reality. However, removing the Infinity Stones from an earlier point does, creating darker timelines. To correct this, the Infinity Stones need to be returned to their original place in the timeline after use.
Now, both of these exposition characters are shown in Avengers: Endgame to not have full knowledge of the situation - Bruce is enlightened by the Ancient One, who is herself later corrected by Doctor Strange's plan involving giving up the Time Stone - but given these are the film's prime exposition beats regarding time travel, they can be assumed to be intended as accurate by the filmmakers. The ambiguity comes with what is a big enough change to alter the timeline: taking an Infinity Stone creates a branch, but returning it would essentially uncreate it; but what does that mean of the interactions the future Avengers have with their past selves, or the full-on diversion with Thanos?
What The Avengers' Time Travel Plan Is In Endgame (& How It Goes Wrong)
The Avengers intend to go back in time to takes the Infinity Stones to attach to their own, new Infinity Gauntlet. They split into four teams: Iron Man, Captain America, Ant-Man and Hulk to New York during The Avengers to get the Tesseract, Loki's Scepter and Mind Stone; Thor and Rocket to Asgard during Thor: The Dark World to remove the Aether from Jane Foster; War Machine and Nebula to get the Power Stone from the Morag at the start of Guardians of the Galaxy; and Hawkeye and Widow to retrieve the Soul Stone from Vormir.
There's a question of why they needed to go to the events of the movies - while the Battle of New York makes sense due to the three Infinity Stone argument, the Aether was in the Collector's museum since Thor: The Dark World and the Power Stone on Xandar following Guardians of the Galaxy, situations with fewer timeline implications. Of course, that would make for a much less interesting film and is argued away in-universe as requiring characters' knowledge of events.
The plan goes awry in multiple cases. Clint gets the Soul Stone but has to lose Natasha (which doesn't break the timeline). Thor talks to Frigga moments before her death, getting the goodbye he was robbed and a speech about being a hero, before taking Mjolnir. War Machine gets the orb, but Nebula is stopped from traveling back to the present by 2014 Thanos. Captain America gets the scepter after a tussle with his past self and Hulk the Time Stone (as well as knowledge of the timeline issues above) but Tony and Scott fail to secure the Tesseract, which Loki uses to teleport out; they instead go to 1970 to get it from Camp Lehigh in New Jersey, as well as more Pym Particles for the return journey.
Most Of The Infinity Stones (And Mjolnir) Create A Simple Time Loop
There's a lot to break down here, so let's first look at the most simple case. The major wrinkle of taking the Infinity Stones creating multiple timelines is addressed at the end of Avengers: Endgame, with Steve Rogers returning them - along with Mjolnir - back to their original time periods. Assuming Cap does this correctly (as implied), that means the actions surrounding the Reality Stone in Thor: The Dark World, the Space Stone in 1970, the Mind Stone in The Avengers, the Time Stone in 2012 and the Soul Stone in 2014 are all reverted to normal: he erases the timelines created by their removal. The question of how exactly Steve returned the Soul Stone, or his reaction to Red Skull being its guardian, is left to audiences' imaginations.
What that does leave is a pretty startling implication: this always happened. Rocket always stole the Aether from Jane, Howard Stark always bumped into grown-up Tony on the day of his son's birth, the Ancient One always talked to Banner four years before her death. It's a linear timeline and all of this was going on in the background of the MCU all along. While that's a retcon by anyone's reasoning, it is rather tight; none of the mentioned past-future interactions directly contradict the timeline.
Loki Created A New Timeline?
The first true break (or diversion) of the Marvel timeline in Avengers: Endgame is regarding the Space Stone in 2012. Tony and Scott intend to give 2012 Tony an arc reactor failure when he meets Secretary Pierce (something that presumably always happened), allowing them to remove the Tesseract briefcase from the equation. An angry 2012 Hulk complicates matters and the Space Stone is attained by Loki, who teleports out of New York.
Following the rules laid down by the Ancient One (and the fact that the film puts focus on the Loki moment), this a clear and intended break in the Marvel timeline that is not resolved by the time Avengers: Endgame comes to an end. In this reality, Loki escapes capture at the Battle of New York with the Tesseract.
The knock-on effects of this are serious: directly, Loki is still working for Thanos at this point so may give him the Space Stone years earlier; from a movie perspective, he isn't there for the events of Thor: The Dark World or Ragnarok, meaning Odin is never replaced and, possibly, Asgard may not be destroyed and Thor never loses Mjolnir; the Avengers also haven't completed their first mission, likely keeping them together longer and impacting solo movies up to and beyond Captain America: Civil War. The extent of all of this is speculation, sure, but the very immediate potential is massive. This would also mean that everything that happens subsequently in this time period is not part of the prime MCU universe: Steve Rogers didn't always fight his future self because he was never looking for Loki.
Practically, this is a get around of Loki's death at the start of Avengers: Infinity War. Thanos declared "no resurrections this time" and he was right, from a certain point of view. Loki is dead in the MCU going forward, yet a version of him is alive and well for new adventures at his most malicious in another timeline, which is a topic that will surely be explored in the Tom Hiddleston-starring Disney+ Loki show.
Thanos Breaks Avengers: Endgame's Time Travel Logic
The one Infinity Stone we've not accounted for yet is the Power Stone. Already, this was a weird case as Nebula and War Machine directly interfered with the opening of Guardians of the Galaxy, although it could be explained Star-Lord being knocked out always happened, meaning Steve returning the stone at the end maintained continuity.
But it's the impact of Nebula being in the past that breaks things. Due to her conscious network, the past and future Nebula are connected, which alerts a Guardians of the Galaxy-era Thanos to the Avengers' mission: he swiftly learns about his victory in Avengers: Infinity War, subsequent death and an attempt to undo it via time travel. And here's where it gets actually complicated: intercepting future Nebula, Thanos time travels the Sanctuary II to the post-snap world in a bid to stop the Avengers undoing his dead future self's work. His plan is now to reclaim the Infinity Stones, wipe out all life and build a new world with him as de facto God. In the end, though, Thanos from 2014 and all his forces are dusted by Iron Man's snap.
The temporal issues here are massive. In Guardians of the Galaxy, Thanos was sat in his throne letting minions do his bidding and yet to have a single Infinity Stone in his grasp. Avengers: Endgame gives him full knowledge of the future, takes him into the 2020s and kills him. With no Thanos or threat, the entire MCU after Guardians of the Galaxy is different, up to there not even being an Infinity War (at least not against the Mad Titan - we know the Collector is also coveting the Infinity Stones).
It's possible that this is all a result of Loki's Tesseract grab, which has already happened in the "present" of some of the future Avengers, but there's little to back that up. Conversely, it may be that Steve returning the Power Stone resets all of this timeline to make this Thanos essentially exist on a separate stream. But the most logical explanation using the film's rules is that something the Avengers changed in 2014 altered the past. This would be unrelated to an Infinity Stone, making the Ancient One's advice wrong, and mean all these smaller changes the Avengers were making in the past do have an impact, butterfly effect-style, leading to an infinite number of alternate timelines.
However, any of those assumptions go well beyond what's laid down explicitly by Avengers: Endgame, making big assumptions and attempting to consolidate multiple forms of time travel (linear timeline, butterfly effect, alternate dimensions) into one. The important thing is that this Thanos who learns beyond his time is defeated at the end.
How Can Nebula Kill Her Past Self?
A subset of the Thanos concern is Nebula. The notion of both versions being connected is actually rather logical by itself; think of it like taking a phone to the past - which one receives calls and messages? But when things are taken through to completion, it gets confusing.
During the present-day battle in Avengers: Endgame's finale, future Nebula convinces past Gamora to join her (a reversal of their arc in Guardians of the Galaxy Vol. 2), which leads to a showdown in which future Nebula kills her past counterpart. Common time travel logic would make this a grandfather paradox - Nebula can't exist to kill herself - but there's no slow fade away. This is further evidence that whatever is happening after Thanos comes to the future is from its own existent timeline, although any further exploration - or even an acknowledgment of what's happened from Gamora or Hawkeye - isn't forthcoming.
Captain America Goes Back To The Past (And Was Always There?)
The end of Avengers: Endgame has one final big time travel reveal. When Steve Rogers goes back in time to replace all the stones and, as the movie tells it, correcting every potential issue, instead of returning to the present, he uses the Pym particles to jump to the 1940s to be with Peggy. He gets that date, that dance, that life that he was forever robbed from by his duty. This is confirmed by an appearance of a much older Steve just after he leaves, saying he was happy with the life his chose and passing the shield to Falcon.
Thematically perfect and tear-inducingly delivered, this moment nevertheless creates even more complications thanks to, once again, borrowing from multiple forms of time travel. Captain America has inserted himself into the past, becoming Peggy Carter's husband.
It's notable that Marvel movies have been avoiding giving much of Peggy's post-Agent Carter background even as they teased Steve's eventual fate. The only proper mention of her husband came in Captain America: The Winter Soldier where, in an archived video at the Captain America Smithsonian exhibit, she explained how he was someone Steve saved during World War II; Agent Carter was canceled before the show could reveal his identity as promised and pictures on her bedside further showed only her children. It's distinctly possible that this ambiguity was intended to hide that it was really Steve Rogers all along.
This line of thinking comes with its own problems, many explainable. Steve and Peggy would have had to hide his return from the outside world, likely seeing him live under an alias. As spies, this would be well within their remit, and if the focus was on living a life together, a worthy sacrifice. It would also explain the now-incongruous fact that Peggy has a photo of young Steve on her desk; she's covering for him. It also doesn't make the Sharon Carter love story (too) disgusting as Steve wouldn't be her blood relative.
Of course, that would directly work against what happened to Guardians of the Galaxy-era Thanos; following that logic, Steve's return would have started a new chain of events that would surely butterfly into a totally different, possibly Avengers-less future. This solution here would that old Steve turning up straight after his disappearance isn't just him simply revealing himself but having traveled across from another reality using Pym particles for a proper goodbye.
Frankly, the time loop where Captain America was always in the past makes the most sense, with the Thanos problem a product of twisty time travel storytelling.
What About The MCU Timeline?
From all we've discussed, Avengers: Endgame leaves the MCU timeline pretty much as it is, with only one major branch thanks to Loki and the Tesseract and a closed loop with 2014 Thanos. And, for the most part, the implication is that the timeline of the MCU as has been told for the past decade hold true. No movies have been retconned or removed from continuity, just events added just off-screen.
But, as any Marvel fan knows, the MCU timeline isn't perfect. From Phase 1, which altered along with the original Avengers plan, there were always noticeable plot holes that make nailing down a proper order of events tricky: Spider-Man: Homecoming's "eight years later" claim is the most well-known, but complications run through the movies, from confusion over when Iron Man is set, to a very weird post-Civil War build-up to Infinity War.
Avengers: Endgame doesn't really concern itself with addressing any of these. Avengers: Infinity War already corrected the Homecoming mistake, and the movies revisited were all set in their release years. However, the ending does leave a question. Avengers: Endgame begins in 2018, the aftermath of Thanos' snap, then jumps forward five years to 2023. This is the baseline time for the Avengers, where the final battle occurs and when the snap victims return to. This means that, from an outside perspective, half the population disappeared for five years, then everything returned to normal... sort of. Half of the population will have mourned and aged five years, while another has been thrown back into a new world. This would be upsetting for families, friends, jobs and schools.
Indeed, while Peter Parker's return to Midtown High and reuniting with Ned is played as happy, this ignores that surely half of his class are now college age. This poses some major implications for Spider-Man: Far From Home - it surely must be a prequel set before Avengers: Endgame, otherwise the film will have to take place in a near-miss-dystopia 2023. It's further notable that 2020's MCU releases - Black Widow and The Eternals - are both prequels to the rest of the MCU, meaning the effects of the Infinity War don't need to be explored directly.
Time Travel's Future In The MCU After Avengers: Endgame
With Hank Pym returned, there's an unlimited amount of Pym particles, meaning the Avengers are technically able to travel into the Quantum Realm as much as they so wish. But is that something to actually expect in the MCU's future? The plot device was rather specific to Avengers: Endgame and an opportunity to revisit previous movies in a send-off Captain America and Iron Man, so it's unlikely to be immediately returned to in future Avengers movies. Time travel can be a narrative-breaking device (as we've already seen) and worth putting to bed.
However, there will still be several potential impacts of the time meddling. The most pressing is, of course, Loki. His Disney+ series will at the very least deal with the fractured timeline he created when stealing the Tesseract, although whether it's resolved (or how) is up in the air. It's also suggested that Star-Lord still hopes to find Gamora, hinting at his arc in Guardians of the Galaxy Vol. 3; it's also unclear where 2014 Gamora has gone, meaning there could still be a survivor of that confusing time loop.
Captain America's ending also leaves open a rather tantalizing possibility: Steve Rogers' eventual return. While he gets his happy life with Peggy, nothing says he doesn't get called on again to help fight another reality-threatening foe at some point down the line. Already, Chris Evans' portrayal of Captain America has reached classic status, and in 5-10 years, audiences would no doubt be excited to see him return (think the original Star Wars cast in the sequel trilogy).
For now, though, Avengers: Endgame's final moments seem intended to put a resolute end to all the temporal meddling. Thanos is defeated, the timeline is safe, Captain America got a happy ending. Just don't overthink it.
- Spider-Man: Far From Home (2019) release date: Jul 02, 2019