The wait is almost over. Marvel Studios' latest series Loki debuts on Disney+ on Wednesday, and while in true Marvel Cinematic Universe fashion there are a lot of things about the series that are a mystery, one thing that we know for sure is that time travel will be a major component. As MCU fans know all too well, time travel isn't a new concept to the franchise. After all, Avengers: Endgame was sort of built around the epic Time Heist that allowed the heroes to defeat Thanos — specifically the 2014 version of him — and set up Loki's time-altering escape to begin with. With time travel being so key to the series, let's break down what we know about how the concept works in the MCU. Grab your favorite Infinity Stone and strap in; this is going to get complicated.
When it comes to the mechanics of time travel in the MCU, there is surprisingly no single, simple explanation. Fundamentally, it comes down to a few things. For starters, we know how time travel does not work — Endgame laid that out pretty clearly — and we know roughly how it was portrayed on screen. We also know how Joe and Anthony Russo, directors of Endgame, have said time travel works but it differs from how writers Christopher Markus and Stephen McFeely have said it works. And then of course Steve Rogers might have broken the whole thing. Like I said, complicated.
First things first, we know that the MCU's time travel does not follow the so-called Back To the Future rules, meaning that if you interfere with the past, you aren't breaking the future. The primary time travel rule in Endgame is pretty straightforward: for the time traveler, going into the past is the future, thus making the "future" their past. The timeline as the traveler knows it is already set. It's their reality. Anything they do in the past will not change their own reality.
And, generally, that holds true. In Endgame, the heroes use the Quantum Realm to travel to various points in time, and the world as they know it remains intact when they return to the present even though they are changing things in the past. But that is where things get tricky. Doesn't interfering in the past do something? Per the Ancient One circa 2012, removing an Infinity Stone from its place in time causes a new reality fracture off with an unknown impact. To prevent that, the stones have to be returned to the exact moment they are taken.
Sounds pretty simple, but if you're still with me here, you know that's not exactly how it plays out because not all time travel in Endgame has to do entirely with Infinity Stones. 2014 Thanos, for example, uses the Quantum Realm to arrive in 2023 and dies there. That means there's a variant version of 2014 that is now without Thanos. The Gamora from that timeline is also absent, having stuck around in the present. Additionally, there's the matter of 2012 Loki swiping the Tesseract and vanishing. That Infinity Stone isn't going back to where it belongs so now we have a branching timeline there. The big one, though, is Steve Rogers' time travel adventure that sees him go back to the past after returning his stone, prompting him to stay and live a whole life with Peggy Carter in what may or may not be an alternate reality. That's where the conflicting takes from the Russos and Markus and McFeely come in.
For Markus and McFeely, what the Ancient One says is the standard. Only not replacing an Infinity Stone can create an alternate timeline. Anything else impacts the main timeline.
"That is our theory. We are not experts on time travel, but the Ancient One specifically states that when you take an Infinity Stone out of a timeline it creates a new timeline," Markus told Fandango in 2019. "So Steve going back and just being there would not create a new timeline. So I reject the 'Steve is in an alternate reality' theory. I do believe that there is simply a period in world history from about '48 to now where there are two Steve Rogers. And anyway, for a large chunk of that one of them is frozen in ice. So it's not like they'd be running into each other."
For the Russos? The Ancient One was only partially right. According to them, there are other ways to create alternate futures.
"It's not a time loop," according to Joe Russo during a 2019 Q&A. "Both Ancient One and Hulk were right. You can't change the future by simply going back to the past. But it's possible to create a different alternate future. It's not a butterfly effect. Every decision you made in the past could potentially create a new timeline. For example, the old Cap at the end movie, he lived his married life in a different universe from the main one. He had to make another jump back to the main universe at the end to give the shield to Sam."
Ultimately with such varying opinions on the creation of alternate realities, the only thing we really know about time travel in the MCU is this: it is possible and anything the time traveler does in the past does not impact their reality or their present. The jury's still out in terms of what happens if one travels to the future as we haven't seen that on-screen yet. It will be up to Loki to help define the impact time travel has more broadly by hopefully fleshing out how alternate realities are created and maybe, just maybe, even giving us some clarity on the whole Steve Rogers of it all.
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