Avengers Endgame Time Travel Rules and Logic Explained

There's some seriously complicated business going on in Avengers: Endgame. Warning: risk of head explosion. Also spoilers.

Avengers: Endgame Time Travel Rules Explained

This article consists of nothing but massive Avengers: Endgame spoilers. You’ve been warned. We have a completely spoiler free review right here.

As a wise man once said, “No one should know too much about their own destiny.”

So if your destiny involves going to see Avengers: Endgame you should walk away right now. Seriously, turn around, you want no part of this. Really you probably shouldn’t have read the headline. We’re not 100% sure what you’re doing on the internet as a whole.

One of the most surprising things about Avengers: Endgame was the thing it didn’t do. We came out of Infinity War emotionally gutted, flabbergasted that they would end the film the way they did, but in the full knowledge that Spider-Man: Far From Home, Black Panther 2, and Guardians Of The Galaxy Vol. 3 were definitely going to happen.

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We saw Ant-Man & The Wasp and picked up all those great big whopping hints about time travel throughout the film. One of the Infinity Stones is a Time Stone for crying out loud. In short, we’d seen enough episodes of Star Trek: Voyager to know a gigantic reset button when we saw one. The remaining Avengers were going to go back in time, stop Thanos in the nick of time and the Snapture would never have happened.

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Why else would a global genocide not be mentioned on a single one of Marvel’s TV shows? Because it never happened.

Back to the Future is bullshit

Thanos Snaps

Only it did. They set everything up, then just turned right to the camera and said “Back to the Future is bullshit.” Nobody was going to prevent The Snap. They couldn’t even go back, find baby Thanos and (extremely graphic mime of garrotting a baby).

The reason why comes down to the time travel “rules” Endgame decided to run with. There are a number of different systems fictional time machines can run on. There’s the “Back to the Future” model, where changes in the past are instantly transferred into the present of the time travellers, resulting in photographs fading away and some really awkward questions about causality and why nobody’s memory changes. Then there’s the “What happens stays happened” rules, where any attempt to change the past will fail, or worse, bring about the events you wanted to prevent. That one tends to give you more plot consistency but makes the story as dull as dishwater by stripping any sense of agency for your characters.


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To understand the model of time travel Endgame’s using, it’s worth taking a quick look at the real-world science concepts they employ.

It’s not rocket science

Tony Stark

When Tony Stark is discussing the problems of time travel with Scott Lang he cites three major issues – that they’re operating at the Planck Length, and that they have to deal with the EPR Parodox and the Deutsch Proposition.

The Planck Length is an easy one – that’s 0.0000000000000000000000000000000016 centimetres. Now that’s a pretty meaningless number to a human brain, so to put in perspective – If Ant-Man shrank down and began measuring an atom in Planck lengths, counting at a rate of one Planck length a second, it would still take him over 10 million times the current age of the universe to finish the job.

read more: Marvel Movies Watch Order – An MCU Timeline Guide

That’s still pretty meaningless to a human brain, but trust us when we say it fits into your brain so much better than the next problem Stark raises, the EPR Paradox.


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Now this explanation of the EPR Paradox, or the Einstein-Podolsky-Rosen Paradox to give it its full name, is going to be wrong. Some of that may be our error despite having researched it as thoroughly as we can and then got smarter people to explain it to us simply, but it’s also going to be wrong because “the Quantum realm” is a place best described with higher mathematics, and not with a system of language designed to communicate which fruit is safe to eat.

Update: So it turns out the day we published this piece, it was announced scientists had observed the EPR Paradox for the first time.

But here we go: The EPR Paradox is to do with superposition, which you’ll know as the reason why a cat can be alive and dead at the same time if it’s in a box with a radioactive particle. The short, and again, probably wrong explanation is that some qualities of a subatomic particle don’t exist until they’re measured. Measuring the particle physically changes it.

That plays into the Heisenberg Uncertainty Principle, which states it’s possible to the measure the momentum of a particle, or its position, but you can’t measure both. If you measure the position, you change its momentum. If you measure its momentum, you change its position. Easy, right?

Now the EPR Paradox is the result of one of Einstein’s many attempts to gotcha the weirder parts of quantum physics, only to reveal how much weirder the universe really is.

The Paradox supposed that if a particle decayed into two other particles, and those particles travelled in opposite directions, you could measure one of those particles and use that information to deduce information about the other. In this way it would be technically possible to measure particles in a way that would breach the Uncertainty Principle.

read more: What’s Next for the Marvel Cinematic Universe after Avengers: Endgame?

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The only way the Uncertainty Principle remains in place is if these particles remain connected, “entangled” is the technical term, so that when a superposition collapses for one particle, it simultaneously collapses for the other, no matter how far away the particles are.

In short, information (the measurements of the particle) travels from one particle instantaneously, breaking one of the big no-no laws of the universe: faster-than-light travel.

The upshot of this, is Stark is saying that time travel and moving at the quantum scale is really confusing and hard to navigate, which I think is something we can all agree with at this point.

Finally there’s the Deutsch Proposition which while not being an actual thing, is a reference to the quantum physicist David Deutsch. Deutsch has had various achievements, but one of the things he’s best known for is his advocacy for the “Many Worlds” interpretation of quantum physics. Again the short, and probably wrong explanation for Many Worlds is that collapsing superpositions actually cause the world to break off into different timelines. The cat in the box is alive and dead at the same time, but when you open the box you create a world where the cat is dead, and a world where the cat is alive, which brings us rather neatly to the Many Worlds Interpretation’s sci-fi cousin, the branching timelines model of time travel.

Many worlds

Avengers: Endgame Cast

If you want a semblance of internal plot consistency and characters that don’t end up making bizarre decisions or suffer from freak coincidences to prevent paradoxes, your only real choice is the branching timelines school of time travel.

In this model if you want to go back in time and kill baby Hitler, nothing will stop you, but when you kill him the copy of Mein Kampf you inexplicably brought with you won’t fade into non-existence. Instead, the act of Hitler-murder will create a whole new timeline that branches off from yours.

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The Terminator films arguably use this model, the JJ Abrams Star Trek movies launch their entire continuity this way, and for a truly brain-melting version of this model check out Primer.

But what none of these films do is ask what happens to the people left behind in the pre-existing timeline that still has to live with the fact there was a Hitler after the time travellers disappear. Endgame takes this problem by the horns, points out that if you go back and kill baby Thanos, all the people killed by The Snap in this timeline will still be dead.

read more – Avengers: Endgame Ending Explained

Hence the need for a Time Heist, taking us through a delightful clip show of the MCU’s high points to collect each of the Infinity Stones.

Only over the course of the Time Heist several events in Marvel history are interfered with, and even after Captain America returns the Infinity Stones at the end of the film, we’re going to see a number of different realities branching out.

A brief tour of the Multiverse

The Avengers (2012)

We’ve counted, and over the course of Endgame the Avengers spawn four new timelines branching out of “our” MCU, Earth-199999 to give it its official designation, but to keep things simple we’ll call it Universe B (more on that in a bit).

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The offshoot universes are:

1. Alt-New York

This timeline separates from ours during the events of the first Avengers movie in 2012. The big details stay the same – Loki’s invasion is defeated, and we assume Captain America returns the Time Stone and the Mind Stone to their rightful places.

read more – Which Avengers: Endgame Deaths are Permanent?

The Space Stone however, has disappeared with Loki, after Tony Stark has a mysterious heart attack. Steve Rogers in this timeline will also find himself in an awkward position. On the one hand he’s discovered way ahead of time that Bucky might still be alive, while on the other hand the secret Hydra cell within SHIELD now has reason to believe he is one of their own. Captain America: The Winter Soldier is a very different movie here.

2. Alt-Morag

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This timeline is probably one of the more pleasant ones, all things considered. We’ve no way of knowing if the Peter Quill of this reality ever gets the Power Stone, or whether he, Groot, Rocket and Drax (no Gamora) somehow team up to defeat Ronan.

But what we do know is that this universe is one where one day Thanos and all of his armies one day completely disappeared, never to be seen again.

3. Alt-Asgard

This timeline is in almost every respect identical to Universe B, except that before she died Frigga had a touching conversation with her son from the future, and that Jane Foster was mysteriously attacked by a raccoon with a syringe.

4. Alt-Military Base

Another reality virtually indistinguishable from our own except that Howard Stark got some desperately needed parenting advice from a mysterious stranger.

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But aside from these four timelines, there’s also the implication that our own MCU, Universe B itself, is itself an offshoot from a hypothetical Universe A. After all if, as we’ve already seen, it’s impossible to change the past of your own timeline, how does Old Man Rogers manage to be sitting on that bench at the end of the film?

read more: Complete Schedule of Upcoming Marvel MCU Movies

The only solution that makes sense is that in the original timeline Steve Rogers A steps into the time machine, vanishes and is never seen again. That Steve Rogers eventually settles down in Universe B, our MCU, and lives a life of bliss as Peggy Carter’s secret husband, staying out of the way of all the significant events until his Universe B counterpart goes back in time, allowing Steve A to reveal himself.

The other time travel problem

However, if you really want to get your continuity in a twist, potential paradoxes are the least of the problems facing the MCU. Exactly half of the lifeforms in the entire universe have just travelled forward in time five years. As incredibly disorientating and potentially catastrophic as this is (unless you’re lucky enough for your best friend, the girl you have a crush on, your school bully and everyone else you know to be transported forward in time as well), those problems shrink compared to those of a multimedia megafranchise that’s supposedly based in the present day but which is now nominally set half a decade into the future. Anyone working at the MCU offices right now might do well to start reading about the UNIT Dating Controversy to discover just how messed up a time-continuum can really get…

Chris Farnell has done his own messing about with time travel (among other things) in his short story collection, Dirty Work and can be found arguing about this and more at @thebrainofchris.

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