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Avengers: Endgame and time travel: Why it all makes perfect sense

The biggest plot point leaves us with big questions, but when you really think about it, it’s pretty straightforward.

Marvel Studios

Avengers: Endgame isn't the first, and certainly won't be the last, onscreen presentation of time travel. But maybe in the future it'll be the past's best representation of the clearest dissection of time travel. 

Because Endgame does good work to make sure its rules of time travel make sense, both scientifically and within Marvel Cinematic Universe canon.

Those of you who've already seen Avengers: Endgame are already aware the major plot point of Endgame involves time travel -- that's how they bring back the "snapped". As my colleague Stephen Shankland explains, according to science, "Marvel's massive superhero movie bends the rules of the universe" with this movie. 

But it's called science fiction for a reason.

So...

Marvel Studios

"Should be fine if we strictly follow the rules of time travel," says Scott Lang (aka Ant-Man, played by Paul Rudd) as the surviving Avengers discuss the perils of changing what once was. "No talking to your past self, no betting on sporting events." Tony Stark (aka Iron Man, played by Robert Downey Jr.) tells Lang that movie (Back to the Future) is "horseshit."

Which begs the question: What *are* the rules? Well, as the Avengers educate Professor Hulk, there are more than a few pop culture, timey-wimey examples that have screwed up our notion of what time travel actually means.

According to the Avengers, here are just some of your favorite properties that didn't handle time travel properly:

The group didn't even mention Doctor Who, Looper, X-Men: Days of Future Past, The Time Machine, 12 Monkeys, or Men in Black 3. Time travel is one of the core tenets of sci-fi, but that doesn't mean it's easy (a) for a film or show to execute properly or (b) for people to understand.

Thankfully though, it is technically rooted in reality, again according to my colleague, quoting famed physicist Stephen Hawking: "General relativity can permit time travel," Hawking said. "Science fiction fans need not lose heart." Phew.

The time travel paradox

Why these other franchises are so off is relatively easy to explain: If you go back in time and affect the past or blatantly change it, your actions will change things in the future. As in Back to the Future, and as the Grandfather Paradox states, you could potentially prevent your own birth. Sort of like a Butterfly Effect sending ripples through your own life.

In a "typical" linear timeline of B2TF, Marty McFly's parents meet, have Marty, then grow old. Marty is born, grows up with them and then heads back to the 1950s.

But in the film, things get weird because time is a loop. We learn Marty will leave his present (Point D), to head back to the '50s and change the past (Point A). This action then affects his current present future (Point E to him, Point B to his parents) by erasing his birth in his own near-future past (Point C).

I made a perfectly simple chart so you can visualize it.

Caitlin Petrakovitz/CNET

As Lang says incredulously, "So Back to the Future is a buncha bullshit."

The best representation of how the Avengers approach time, and travel within it actually comes later in the movie, after they've weighed the merits of time travel in movies. Tilda Swinton's Ancient One explains it to Professor Hulk's astral form (which takes the form of human Bruce Banner): Removing one element (or an Infinity Stone, in their linear flow example), causes a split in the timeline creating a different path for the world.

The "time travel paradox" doesn't make sense, because that's not how time works -- in the real world or in Endgame. Changing a thing in the past that prevents a future thing, just means nothing happens. (But that's a boring movie, so I get it.)

Using the B2TF example, that means if Marty wasn't born at Point C, then he couldn't go back to Point A during his own current timeline of Point D, so he couldn't have had any effect on his parents getting together at Point B, and therefore wouldn't be able to make it to Point E to change anything.

See? It's simple.

As Prof. Hulk says: "I have no idea! We're talking about time travel here. It's either all a joke, or none of it is."

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Quantum tunnel, quantum bridge, quantum everything

"Have either of you guys studied quantum physics?" Lang asks Natasha (aka Black Widow, played by Scarlett Johansson) and Captain America (aka Steve Rogers, played by Chris Evans). "Only to make conversation," says Natasha.

With the word "quantum" still thrown in front of everything in this movie, it gets confusing deciphering what's actually real science and what's comic book science.

In the movie, Tony Stark describes the EPR Paradox, or the Einstein-Podolsky-Rosen paradox. Stark explains that it means Lang wasn't actually going anywhere inside the quantum tunnel: "Instead of Lang going through time, time is going through Lang," he tells Steve Rogers.

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Stark solves this with a "time-space GPS," a way for the team to travel through the quantum bridge without getting lost in it. None of any current science suggests that this is possible, writes Shankland. But then he leaves the door open a tiiiiiiiny bit, giving us hope that maybe that ugly brown van's quantum bridge was actually a wormhole (!!!).

With a form of so-called exotic matter -- something with negative energy density -- you could prop wormholes open, but nobody knows if this is even possible, except at the minutest quantum-mechanical scales.

What I'm hearing is that Hank Pym (the original Ant-Man, played by Michael Douglas) and co. created a wormhole, accidentally left Scott in it, and then the Avengers used the same wormhole and their GPS devices to save this single universe.

We're all cool with that right?

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The Avengers made use of the same wormhole Scott was accidentally left in. Right?

Marvel Studios

Lingering questions

Cool with it or not, that's what happened in Endgame, leaving some people with a lot of questions. Here are a few I've gotten since the movie's record-breaking opening

Can the Avengers only go back to their own pasts, or could they go back even further?

Honestly, it's unclear. Maybe they only have enough Pym Particles to go back a certain amount? But it's not like any team members needed to go back further -- all the Stones were spotted in the team's last decade-ish. 

How did Cap end up old when everyone stayed the same age?

TL;DR: 2023Cap took the long way back to the present.

After fighting with 2014Cap and remembering (yet again) what he gave up for the good of the universe, and after glimpsing Peggy Carter in 1970, 2023Cap (the one who swears) makes a defining decision.

As 2023Cap is using the quantum bridge to go back and replace the Stones and Mjolnir from exactly when they were stolen, he realizes he's done his service and that his future can be a different past. Cap drops off all the heisted items and instead of going forward with the final vial of Pym particles, he heads back to make that date with Peggy. (Or maybe a couple years later, after season two of Agent Carter.) 

Does this mean, as some have claimed, that he "stood by" while Hydra took over, or prevented Peggy from marrying her other love and having kids?

Not at all, first and foremost because 2023Cap would never stand by while injustices raged on. Cap knew Hydra and other threats would be taken care of because he took care of them. Literally, his past self dealt with Loki and Hydra and Thanos, and he knew it would happen because he'd already done it. (You'll recall this because your future self can't affect the past, as mentioned above.)

And second, Peggy's husband is *never directly named or shown.* That's huge and means it could easily be 2023Cap disguised and/or hiding if he ever needs to, or when his past self visits Peggy.

Fact is, Shankland answered this in his scientific strip-down of Endgame: 

Hollywood could have another big beef with physics: Time travel strips away any actual control any of us has on our own future, said Richard Muller, a physics professor at the University of California, Berkeley.

"If wormholes exist and we can go backward in time, it's a consequence in physics that we would be not be able to exercise free will. We would not be able to change our minds," he said.

It's this simple: 2023Cap went back in time because he was always meant to go back in time.

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2023Cap would never stand by while injustices raged on.

Marvel Studios

Wait, how does Nebula and Thanos dying work? And how is Gamora living now?

Yes, 2014Nebula and 2014Thanos are dead. D-E-D, dead. So is 2018Vision, 2018Heimdall and 2018Loki, 2023Natasha and 2014Gamora. So is this whole list of people.

While I agree it seems strange that 2014Nebula would be dead even as 2023Nebula survives, it's still for the same reason as Prof. Hulk and Nebula keep hammering: Your future self can't affect your past self -- apparently even to kill it.

We saw Loki though, where (when?) is he??

2012Loki though is alive and likely about to embark on his wild Disney Plus ride of a show. WHICH now has the option of diving deeper on the evil Loki, aka best Loki, and exploring the multiverse.

So all the timelines just coexist forever now?

Exactly. We now have a full-fledged multiverse in the MCU.

Probably.

Hold onto your butts.

If multiple timelines exist, does that mean somewhere in space/time there is a timeline where everyone is still dead and they didn't succeed and we're just not seeing it?

Yes. Well, maybe. In either case, aren't you glad we don't live there?

If you have more questions we can talk in circles about, leave a comment and let us know!