SPOILERS: Avengers Endgame Anti-theory

Pretty much this whole article is spoiler-filled. Don’t read it until you see the movie.

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ok, you saw the movie right?

I hate time travel plots. I hate time machines. I hates them so much. Once you have a time travel plot and a time machine you effectively lose control of the story. From that point onward, every single thing that threatens your characters and creates conflict has to be resolved by explaining why you didn’t use time travel to fix it.

Or you just pretend your story world doesn’t have time travel in between plots when you don’t need it. That’s what Star Trek does regularly. Better to have Trek with conflict than Trek with a time travel answer to every problem, even though it requires ignoring the canon.

But in my opinion that’s lazy storytelling. It is where you trap yourself when you work in a story world with time travel.

I have been kind of dreading the time travel in Avengers Endgame. Obviously they need to do some time traveling. And Marvel made the Dr. Strange movie rooted in time travel to establish something of how it might work. And two moments of time travel are critical to Avengers Infinity War. Using the Time Stone to move forward or backwards along a portion of the timeline is now thoroughly rooted in the lore of the MCU and there’s nothing anyone can do about it.

About half-way through Endgame a moment occurs that made me sit up straight in my chair. Because Endgame gives the Avengers a time machine, but I think the MCU takes it away again. And in fact, probably makes time travel practically impossible.

The Moment

Bruce Banner/The Hulk goes back in time using the Avengers’ time machine to 2012 New York City where he thinks he’s going to get the Time Stone from Dr. Strange. However he finds out that Strange isn’t on the scene yet and instead he confronts The Ancient One. She explains that one of the functions of the mysterious Infinity Stones is to ensure that there is a linear flow of time; that the timestream cannot branch. If all the Stones are present, linear time is enforced.

Branching timestreams are a prerequisite for time travel stories; otherwise they’re just pointless. If you cannot go back in time and change the past to affect the present there’s nothing useful about time travel (ok, you could get answers to questions that we don’t really know the truth about like “was Jesus a supernatural being” or “did T-Rex have feathers”) but the answers will only affect the present and the future, not the past.

Branching timestreams also conjure up the multiple-worlds problem. If the timestream branches, are you really changing your actual present and future, or just creating a new reality running in parallel with whatever you perceived as “reality”? Because if that’s all you’re really doing any grief you feel about your present and future isn’t truly going to be assuaged by changing the past — the source of your pain will still exist; you may just have access to a reality where the cause doesn’t. But you’ll know, won’t you?

Except if you use the eponymous Time Stone, which obviously can break its own rules. But it’s been held by various superpowerful entities that don’t use it to screw up the timeline so the Time Stone’s power is essentially moot.

The Eternal One’s metaphysics lesson is critical to keeping time travel out of the MCU. Let’s see why.

Why Do the Avengers Have a A Time Machine?

A short while after Thanos’ Snap to erase half of all life, he Snaps again to destroy the Infinity Stones because they’re “a temptation”. Guess what? Thanos gets rid of the system that enforces linear time on the cosmos.

That opens the door to allow a time machine to function. It takes the Avengers 5+ years to get the details worked out.

From the point in time forward of Thanos’ Snap to destroy the Stones, time travel becomes possible because the Infinity Stones are no longer enforcing linear time. Not just linear time from the Snap to destroy the Stones forward, but linear time as far back (apparently) as anyone chooses to go. If you have access to a time machine, and you exist in a moment where the Infinity Stones don’t, you can time travel.

Why Don’t the Avengers Still Have a Time Machine?

The plot of Endgame is that the Avengers travel back in time, and get all the Infinity Stones and bring them to the future. There, they create a new Infinity Gauntlet and Snap to restore all the people who got dusted by Thanos’ Snap.

The condition predicate for the Avenger’s Snap is that Tony Stark doesn’t want to lose his wife & daughter to some alternate reality so he requires the Snapper to use their willpower to just restore all the dusted people 5 years later in situ, not to stop them from ever having been dusted, or to stop Thanos from Snapping in the first place.

Then the Avengers intend to comply with the Ancient One’s requirement of Banner/Hulk to restore the Infinity Stones back into the original timeline to avoid a breakdown in linear time.

Thanos puts a monkeywrench into this tidy plan by time-jumping out of the past and into the present.

The Avengers defeat Thanos. He is dusted, so he never has to worry about this issue. But he’s dusted in his future. Which means he never assembled the Infinity Stones, and he never Snapped them out of existence.

Now there’s a paradox. If Thanos never Snapped half the life in the universe out of existence (and subsequently Snapped the Stones themselves out of existence) the Avengers would not have needed a time machine, and in fact, a time machine would not have worked because the Infinity Stones would have always been enforcing linear time.

There are at least two timelines in that period. One where Thanos goes to the future and never comes back, and one where he doesn’t. But the two timelines are intimately connected; Thanos must not go into the future to create the conditions precedent (his destruction of the Infinity stones) to allow the creation of the timeline where he does. The complexities of this paradox are probably going to give the Ancient One and Dr. Strange fits.

What Happens to Steve Rogers

Captain America should not be able to travel in time after Thanos’ defeat. There is a full suite of Infinity Stones in his present timeframe. Linear time should be locked. But since he does, we have to assume that it is possible to travel into any previous time where an Infinity Stone is missing even when the start point is in locked linear time.

If he jumps immediately to 1970, just after Stark leaves with the Tesseract, he’ll be locked into linear time because Rogers has got all the Stones with him when he arrives, and from that moment on, there will never be a time where the Stones aren’t enforcing linear time. From the Ancient One’s perspective, this is fine; but from Rogers’ it probably wasn’t. He’d have to spend 44 years getting himself to the right place on the right day to “put back” an Infinity Stone right after the Avengers took them so that the events of the timeline remained unchanged and Pepper marries Tony and they have a baby. That’s a logistical nightmare. And what if he dies or otherwise cannot complete the task?

So he has to replace the stones in the reverse chronological order they’re removed so that at each juncture he’s jumping back into a time without a full suite of Infinity Stones. But when he finally gets to 1970, he’s stuck. How does he get to 1945?

He cheats. He has to use the Time Stone. From the Ancient One’s perspective, it’s OK that he cheats since linear time is enforced — the presence of the Time Stone in the universe at any point prior to Banner/Hulk taking it in 2012 protects linear time. She’s got to be worried about the fact that from 1945 until 2012 there are two Time Stones in the universe, but she’s guarding one and Captain freakin’ America is guarding the other. And technically, if she really cares, she could use her Time Stone to go to 1945 and get Rogers’ Time Stone and bring it back to 2012 and get rid of the duplicate. She probably does that; she’s detail oriented.

Rogers has to ensure that nothing he does from 1945 onwards breaks that promise to Stark to protect Stark’s life outcome. So he cannot be “Captain America”. Maybe he marries Peggy Carter and they have a mutual agreement to never reveal his identity; or maybe after a brief interlude with Carter he goes off and finds another life. There’s some reason he doesn’t want Sam Wilson to know who his wife is/was. It’s a mystery.

There’s also two copies of Mjolnir. That’s actually relevant. Because Thor calls for it in Asgard on his time travel mission and takes it back to the future. You’d think he’d remember that the hammer vanished from his life, no? Well, not if there were already two Mjolnirs in that time frame so Thor never lost a hammer.

To Sum Up

Time Travel using the Quantum Realm works in the MCU. You can only start your trip between 2018 after Thanos’ Snap destroys the Stones to the moment just before the Avengers return with the stones in 2023. If you start from any other time, you can only go to a moment in time missing one or more Infinity Stones.

Tony Stark invented a working time machine. But the Avengers can’t use it again. Time Travel is going to be the province of Dr. Strange and the wizards, not an ongoing story problem for the MCU writers forever.