Time Travel in "Schrodinger's Multiverse"

We finally got to witness the MCU's first attempt at time travel in Avengers Endgame, and a lot of people have mixed feelings about it. This radical theory presented here may prove useful in understanding the events of Endgame as well as the science behind the time travel itself.

While some people point out Smart Hulk's lack of expertise when it comes to time travel I believe he was on the right track. But before we get deep into this we need to understand the complexity of the multiverse, and time travel in the context of the multiverse. And for us to do that we need to establish certain in-universe scientific principles - reverse-engineered to support the movie's own definition of time travel - and then note the actions taken by our heroes.

I feel I should point out that I'm a theorist not a scientist, so please just glide over any scientific inaccuracies that may be ahead. This theory's goal is to satisfy Endgame's time travel shenanigans.

1. One might say the multiverse may be an infinite number of planes of existence,  superimposed on and out of phase with each other, each being slightly different than the next but all being similar to each other in some respect. At least for the most part. 

To borrow from quantum physics, the multiverse may simply be the superposition of the prime universe that forms infinite universes, superimposed on and out of phase with each other, reflecting the infinite number of paths available and the probabilities of each of those being chosen. I chose the words "superposition" and "superimposed" intentionally because those infinite universes are all supposedly occupying the same physical space and they are playing themselves out in real time but nobody can observe them "directly", hence the term "Schrodinger's Multiverse". We only see our own universe because it's the only universe we are in phase with, the same way you'll know the fate of the cat only after opening the box. So if the multiverse is a superposition, then one universe is just a prime universe existing in one of the infinite probabilities that make up the superposition. It's very important to have this definition of the multiverse because once we do we will realize that one moment exists within three variables not just two. Also, this version of the multiverse seems to fit well with the MCU judging from what we've seen in Endgame.

We've seen many live-action films and tv shows in our lives and most, if not all of them, deal with one or the other: time travel or multiverse travel. Keeping these separate keeps things simple. That's why time travel stories are easier to make and follow without having to worry about other universes and whatnot. Time travel in this instance will only need to satisfy 2 variables: where and when. But when infinite universes exist there arises another very important variable, at least in the MCU's case because of the use of the quantum realm,  that needs to be solved before one travels through time: which universe. 

2. The quantum realm exists out of phase with its universe and its phase shift properties allow both time travel and universe hopping.

The entire time heist in Endgame banks on their being able to manipulate the quantum realm to that effect. We see Scott Lang emerge from the quantum realm unscathed by the decimation. Time also moves differently in there than it does in the real world. Also, that quantum energy blast bestowed phase shifting abilities to Ghost when she was a child. These three facts show us that the quantum realm may also exist out of phase with its universe. It makes sense for time travel to be possible in the quantum realm. Another important point to note here is that time travel may only be possible if you first  phase out of the universe and into a place that allows time to pass you by, or get ahead of it. This brings me to my earlier point that in this reality time travel has 3 variables: coordinates, time and phase, because you need to phase into the correct universe when it's time to exit the quantum realm at a certain point in time.

3. Time travel without a destination anchor/phase randomly takes you to another universe.

Remember Smart Hulk's theory of time travel? The past can't be changed by time travellers. I believe this is why: TRAVELLING TO THE PAST WITH NO DESTINATION ANCHOR TAKES YOU TO A RANDOM DESTINATION UNIVERSE. In any other movie this theory would fail to apply but Endgame creates a specific case where it actually makes sense. They had no destination anchors in the past to make sure they phase in to the correct universe after exiting the quantum realm. They only had the where and when without the correct phase of their universe provided by the anchor. I'm of course talking about that quantum tunnel machine that sends them on their way and pulls them back to the correct universe. Both the time GPS and the quantum tunnel are made in the same universe which means they are in phase with each other. We've seen it in AntMan and Wasp. Without the quantum tunnel pulling him back Hank Pym would have been lost forever in the quantum realm. The same is also true for Scott Lang who was also lost in the quantum realm before the quantum tunnel was activated to pull him back. This fact allowed Past Thanos to follow the Revengers from his universe through the quantum realm into theirs. But our heroes weren't pulled into their past by a quantum tunnel that could have anchored them to their universe. They did travel to the past but who knows which universe they arrived in. So if the heroes couldn't change their past it's because:

4. THE TIME HEIST WAS NOT IN THEIR UNIVERSE but in at least 4 other random universes. They split into 3 groups, each with different time destinations to random universes; with the power and soul stone being obtained from one random universe, the mind stone from another, the space stone another, and the reality stone from another. The quantum tunnel would then ultimately pull all of them back to their original universe. No one is the wiser.

Why else would changing the past fail to bring any change in the future? Simply put: you're changing the wrong universe. That's why I said earlier on that Smart Hulk's explanation was on the right track. He may not have known everything - because how could he? -  and that may have ultimately led him to believe the past can't be changed. 

So knowing what we know about time travel in SCHRODINGER'S MULTIVERSE we start to really understand the consequences of the time heist, especially when you consider how many versions of Avengers did their own time heist and how many of them actually defeated Thanos, or whoever their Big Bad was. Because let's face it: not all Doctor Strange versions peaked into the future in an effort to get ahead of the Mad Titan. And even if they all did, some Avengers may have failed, died or been lost in the time heist; leaving their original universes unprotected from whatever is coming next. And even if they all succeeded in their time heist, some Avengers may have failed to defeat their own version of Past Thanos or whichever Bag Bad; leaving other universes where they did the time heist forever changed since no one would have returned the stones back to their original universe. AND, even if all the versions of the Avengers did defeat their Past Thanos some may have also failed to return the stones to their universes of origin since they may not even know that they didn't take them from their past. See where I'm going with this? 

5. THE MULTIVERSE IS IN CHAOS. The balance of power in the multiverse has shifted. Most of the stones were never returned to their universes of origin.

Universes are now forever changed because of this, just as the Ancient One said. The balance of power in the multiverse has shifted. Stability has been severely compromised. Not even the Avengers can fix this now. They don't even know what they did. And in extreme cases like this it's up to the mystical beings to fix this. So is it a coincidence that Doctor Strange 2 is in the multiverse of madness? Of course not. I could be wrong about this but we might see The Living Tribunal in this movie, at least in some capacity, restoring order in the multiverse or, in the very least, guiding others on how to limit the damages  on a universe to universe scale. 

But who knows. And one more question: is the MCU really a couple of movies or something else?