Most people walk out of Avengers: Endgame with similar awestruck reactions: They pulled it off! There are character arcs and consequences! It’s a comic book movie that made death feel permanent in a way that large scope superhero projects are usually incapable of!
Then, a few dozen hours later, there’s a normal moment of doubt: Did Endgame make any sense? I’m here to say: yes it does.
[Ed. note: this post contains major spoilers about Avengers: Endgame]
Time travel in Avengers: Endgame actually makes sense — it’s just incredibly convoluted. The short of it: Screenwriters Steven McFeely and Christopher Markus created a closed, time-travel system that allows the Avengers to mess with their own timelines without creating paradoxes. Some time travel movies spend their entire runtimes building on top of, then correcting, paradoxes. Endgame doesn’t have time for that nonsense. It’s a point made often in the film: we are NOT doing the Back to the Future movies.
If the movie left you a little stumped, here are the rules of Avengers: Endgame time travel and why they work together to make everything possible without sacrificing the rules.
Time travel in Endgame always goes through the Quantum Realm
An excited Ant-Man/Scott Lang (Paul Rudd) shows up at Avengers HQ five years after the snap. Only five hours passed by his watch.
Time works differently in the Quantum Realm, and so does space. The rules are different than in, say, Doctor Strange, which finds Stephen Strange wielding the Time Stone. The green Infinity Stone stored in the Eye of Agamotto just allows the user to slide through time, not to jump to different points in time. In order to move through time and space, the Avengers must shrink down to the Quantum Realm, then use the map Tony Stark developed (his “reverse mobius strip”) to send the traveler to a different part of the Quantum Realm, which will spit them out if time to a different physical location.
Anyone who has watched enough time travel movies knows the base idea of quantum theory: that an electron is capable of being two places at once at the quantum level. That could mean multiple dimensions, string theory, or the Back to the Future II multiple timelines. Those can all be in play if we accept this base, non-scientific definition of quantum mechanics.
The hand-mounted device that Tony Stark develops to navigate the Quantum Realm makes destinations out of probabilities, but the core, fictional quantum mechanics stay in place: there can be multiples of one thing existing at the same time e.g. a Captain America that fights a Captain America in 2012. Does real science back this up? Not in the superhero way, no. But the movie has its own playbook, too.
Time travelers experience their own, linear timeline
This one comes straight from Hulk, and it’s one of the core rules that Avengers: Endgame sets up with its time travel. When Rhodey and Scott Lang suggest they go into the past and stop Thanos from getting the six stones, Hulk replies: “I don’t know why everyone believes that, but that isn’t true. Think about it: 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 means we should not expect any Back to the Future or Looper-style time-updates in Endgame. In Looper, there’s a terrifying scene where a Rogue Looper is mutilated in modern time as bad guys cut pieces off of him in the past. That sends a shockwave to the future and suddenly, future guy is missing all his fingers. That could not happen based on Avengers: Endgame’s rules.
In the MCU, the past is cemented in time and forever exists to allow for a subject to jump through time. Captain America will always go into the ice and wake up later — that needs to have happened for us to be in this time travel situation in the first place. There are no fading photographs of Marty here. Reality is experiential for individuals, meaning a person’s perception of time is linear, regardless of how they jump around the timeline.
We are not dealing with a human perspective of time
There’s a very funny scene (that is also a great moment to take a bathroom break) in which Professor Hulk is testing time travel out on Scott Lang using the Quantum Portal in the van. Scott gets spit out as a teenager, then as an old man, then as a baby. A few scenes later, Tony explains that they accidentally traveled “time through Lang” instead of “Lang through time.”
This suggests that time and the individual experience of time are two entirely different things. Scott wasn’t trapped in the Quantum Realm for a reverse number of years, or until he grew old; the changes were instantaneous for him. This means that, in the Quantum Realm, the “flow” of time is unlike on our plane of existence, and is not locked to an individual’s experience.
So, to recap: The Quantum Realm is a middle ground that allows for travel through time and space. An individual experiences their own timeline linearly, but that’s not consequential to the timeline overall.
At the point in the movie where we’ve learned these three rules, time travel still doesn’t make a lot of sense within the movie: How can you not change the past if you are in the past?
It’s because we don’t have a reference for time that matters. Luckily, The Ancient One knows exactly what to tell us (and Hulk) back in 2012.
Together, all six Infinity Stones create a stable flow of time
Tilda Swinton’s The Ancient One, last seen in Doctor Strange, blasts Bruce Banner out of his Hulk body so they can have an Astral chat about why the brainiac beast can’t take the Time Stone out of the past to use it in the future. The Ancient One creates a thick, golden strand of light to represent the timeline, then gives this speech:
The Infinity Stones create what you experience as the flow of time. Remove one stone and that flow splits. Now, this may benefit your reality, but my new one, not so much. In this new branch reality, without our chief weapon against the forces of darkness, our world would be over-run and millions would suffer. Tell me, Doctor, can your science prevent all that?
Let’s use the Ancient One’s language here: there are not multiple “timelines” there are multiple “realities.” A branch reality can only split off the main timeline if an Infinity Stone is removed from that timeline and not replaced.
In order to delete all the branch timelines that the historical stone disruption will create, Banner makes a suggestion: “We can erase it. Once we’re done with the stones, we can return each one to its own timeline the moment it was taken so, chronologically, in that reality, it never left.”
Once again, “chronologically” and “that reality” are used separately because time doesn’t matter with regard to the human perspective. As long as six Infinity Stones simply exist in the same time together (regardless of location or being spread across the universe), they create that single reality. That reality isn’t a fixed one, either; it is all realities happening over each other, the quantum mechanics idea contained by some cosmic storytelling physics.
Everything happened and is happening
In a stable reality with a stable flow of time, all six Infinity Stones exist.
Given the rules laid out in the film, it seems impossible that alternate dimensions exist in the MCU (in the science-fiction sense, like there’s a doppleganger world). Instead, all possible realities are squished together in one flow of time, happening over, under, and simultaneously with each other. That’s the power all six Infinity Stones have over the universe.
If a stone is removed, a branch reality can be created, but that branch reality is unstable, dangerous, and is possibly unreachable from the Quantum Realm because it would be unmappable on Tony’s devices, which is based on a model of mapping Quantum Realm locations to places on the timeline. (People in the movie jump backwards and then to the present, no one is jumping to an undetermined future. Even Thanos needs to be lead from the past to the future) How would you map an alternate timeline that didn’t exist until you split it? You couldn’t.
The six stones create “what [we] experience as the flow of time.” The stones braid multiple human-perspective realities together into a single flowing existence. A character’s death removes them from the main flow of time (note that Frigga, Thor’s mom, still needed to be visited in the past before she died on Thor’s human-perspective timeline).
When Cap returns the stones across time, the rules disappear
When our heroes jump back in time, then bring the six Infinity Stones forward to 2023, the past splits into alternate dimensions, threatening the integrity of the flow of time. Luckily, the Avengers win, allowing Captain America to take the stones (and, in theory, Mjolnir) back to where they were pulled out of the timeline in the movie.
Once Cap misses his “timestamp” to return, remaining back in the 1940s, time travel is effectively impossible. Thanos destroys the stones in 2018 on “The Garden” planet, disrupting the uniform flow of time. Not only that, but the personal experiences of everyone left alive still happened (Thor still got a pep talk from his Mom, Tony still met his dad, Cap still went into the ice AND lived with Peggy during that time), but that includes their personal pasts. More importantly, our reality progressed without Infinity Stones to keep single dimensions happening over and in concert with each other.
Nothing in this movie’s main time-travel plot is about “undoing” except for the magical gauntlet snap. However, once the Infinity Stones are destroyed and we hear The Ancient One’s speech, we know that not having Infinity Stones potentially altered “what we perceive as the flow of time” It’s very possible that time travel could have only been discovered by Tony Stark after the Infinity Stones had been destroyed or that the Avengers’ plan wouldn’t have worked if the stones hadn’t already passed through their personal, human-based timeline. Now that there are no stones, all the Endgame-specific movie rules don’t necessarily apply to the rest of the MCU.
How the time travel logic plays out in Endgame
Since the rules only apply to things in this single movie, let’s apply these rules to track what actually happened in Endgame:
Did Steve Rogers really live out his days with Peggy? Yes. It looks like, from his age, that Steve just stayed with Peggy after returning the Stones.
Wait, didn’t we see photos of Peggy with her kids? Were those Steve’s kids? Yes, Peggy had kids. No, they were not Steve’s — they were her other husband’s, who Steve rescued during the war. This still happened, just not in our “reality” since the story we saw from Steve’s perspective is linear and includes Peggy’s photographs of her children.
So there are two Captain Americas alive in the 2010s? Yes. One is old and has had a full life with Peggy, one gets pulled out of the ice having lost Peggy. One inevitably becomes the other, but they both exist.
Do the splinter realities the Avengers create by taking the Stones exist? Presumably parts of them do. Let’s say Captain America returns the scepter (Mind Stone) to the exact place he stole it in 2012. The Hydra agents in that elevator still think Captain America is secretly Hydra. Did Captain America fight Captain America? No, that part was probably deleted. But it wasn’t deleted because of anything a human did, it was deleted because all six Infinity Stones work together so all realities flow in an orderly fashion.
Is Loki still alive? Yes. He used the space stone to teleport away in 2012, which we know is different from how things went for him at the end of The Avengers, when he was sent to Asgardian prison. Remember: we actually see Loki in that Asgardian prison in this movie as well, because both events happened. The one that is in our present reality is that Loki was free, alive, with the Tesseract after the Battle of New York. In the reality we all experienced in human-perspective time, Loki had to die for Thanos to obtain the Space Stone.
What year does Spider-Man: Far From Home take place in? Signs point to 2023, but it’s hard to say. Certainly not Tom Holland, who can’t be trusted with secrets. Did you notice his lines about being dusty and gone for five years were shot from the back so they could replace his dialogue later? That’s not time travel magic, just clever moviemaking.
Dave Gonzales is an entertainment writer and podcaster. Find him on Twitter @Da7e.