Because real-world time travel back and forth between the past and present is not (yet) possible, and the underlying science behind our working theories as to how it might one day be possible can be difficult to grasp, our ideas about it have largely been shaped by fictional depictions in books, television shows, and movies like Avengers: Endgame.
It can be interesting when fictional depictions of time travel attempt to ground some degree of their “science” in reality. But there’s a certain way our inherently imperfect, pop cultural understanding of it has taken on an existence of its own that makes it possible for us to accept (or at the very least understand) a couple of ways that time travel is “supposed” to work, without the story having to spend too much time explaining it.
Because present-day Thanos destroys the Infinity Stones within the first few minutes of Avengers: Endgame, time travel becomes a key part of the Avengers’ plan to resurrect everyone he dusted in Infinity War. After the remaining Avengers kill Thanos on his farm, they return to Earth. Five years pass as the universe descends into chaos and grief. But, when a random rat inadvertently allows Scott Lang to return from the Quantum Realm, he explains to the Avengers that for him, only five hours or so have passed, which leads him to reason that because time functions differently in the Quantum Realm they might be able to use it as a waypoint to travel into the past.
Vague as the explanations for the MCU’s take on quantum physics are, they’re not actually all that much of a distraction in Endgame because the movie presents the Quantum Realm as an implicit kind of deus ex machina that’s meant to make nearly anything possible. Spyridon Michalakis, the actual quantum physicist Marvel consulted during the production of Ant-Man and Ant-Man and the Wasp, has said so himself.
The messiness of the Avengers’ “time heist” has less to do with how the heroes get to the past, and more to do with the lengths the movie goes to explain why taking the Infinity Stones from the past in order to bring them to the future wouldn’t fundamentally alter major moments in the MCU we’ve seen play out in the previous films. As Professor Hulk prepares Clint Barton to undergo a round of tests to see if they can successfully send someone to a specific point in the past and pull them back, Ant-Man and War Machine ask the obvious question: Why they can’t simply go back in time to kill Thanos as a baby in order to change the events of the future, as characters in movies like Back to the Future and The Terminator have done? In response, an exasperated Hulk attempts to explain:
“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 future.”
This specific moment is where Endgame’s time heist conceit begins working against it, because while one movie centring its own take on time travel (no matter how convoluted) is perfectly fine, things get tricky when time travel flicks start throwing metaphorical stones in glass houses. By joking around about the larger, pop cultural concepts of fictional time travel, Avengers: Endgame, perhaps accidentally, invites us to compare it to these other films with similar plot devices. And the problem is that while those other stories’ science might not hold up either, they’re infinitely easier to digest and accept as part of their story than Endgame’s.
Back to the Future’s spin on time travel as a whole is, objectively speaking, riddled with enough plot holes that its premise doesn’t exactly work on paper. In moments when Marty McFly’s actions in the past begin to alter things in the future like his being born, he’s still fully able to exist in the past. As cool as watching Marty’s body begin to fade away is, the moment he began to change anything about the past that had a direct result on his future, the changes would be instantaneous and he wouldn’t be able to perceive that anything had changed because, from his perspective, nothing about the past would be “new.”
But, the Back to the Future franchise manages to get away with some of its logical inconsistencies by incorporating just enough of Einstein’s theory of relativity into its plot that when Doc takes a moment in Back to the Future II to explain how altering moments in the past can split timelines leading to alternate futures in different realities, it feels like a natural progression of the films’ logic, rather than an attempt at addressing earlier mistakes.
At times, the logic is at odds with itself, but because Back to the Future’s overall story is so tight, it’s relatively easy for the contradictory concepts to exist in your mind, because you understand that the movie needs a little bit of wiggle room in order for its narrative movie magic to work.
Doc’s original diagram of how timelines can split is so effective that Endgame ends up paying homage to it with a scene in which the Ancient One and the Hulk debate whether the Avengers’ plan might actually cause more harm than good. As the Ancient One explains how the six Infinity Stones as a unit are responsible for creating what we perceive as the flow of time, she also points out that removing any one of the stones would create new, potentially darker timelines where countless people would be endangered specifically because of any one of the Stones’ absence.
Banner ultimately convinces the Ancient One to give him the Time Stone by insisting that the Avengers’ plan to return all of the Stones to their rightful places in the past at the precise moment they were borrowed will work, and the movie quickly breezes past this before you really get a chance to contemplate just what all that entails.
Unlike the stationary (and very isolated) Power, Space, and Soul Stones, the remaining MacGuffins were all in the possession of people who Captain America (who ends up being tasked with returning the Stones at the end of the movie) would presumably have to interact with in some capacity, which would cause a rather significant deviation to the time stream. Endgame goes out of its way to imply that Steve’s mission is entirely possible and ultimately ends without the creation of a multiverse.
But subsequent interviews with the Russo Brothers and details featured in the latest Spider-Man: Far From Home trailer suggest that at some point during Endgame, someone did something that caused the timeline to split, and potentially a lot more than once.
All of this promises for curious things in the MCU’s future, but within the context of Endgame specifically, it ultimately ends up making it seem as if the screenwriters and directors felt the need to take potshots at other instances of time travel in pop culture because Endgame’s can’t stand on its own legs.