First of all, I want to say that I thoroughly enjoyed this movie. I watched it twice in one weekend which is something I don't normally do. Today, I will attempt to explain one of the key plot elements in the movie.
*****WARNING: SPOILERS AHEAD *****
Do not continue if you have not watched Endgame yet.
As many Marvel fans predicted, Endgame utilizes the element of time travel. Scott Lang manages to escape the quantum realm realizing that five years have gone by since entering, although, he claims it felt like only five hours. This experience prompts the idea of using the quantum realm to travel back in time to retrieve the Infinity Stones before Thanos collects them.
There is a scene where Bruce Banner (Professor Hulk now I guess) is explaining to the team that time travel does not work the way we have previously perceived. Scott and Rhody start naming off some time-travel related movies. It's at this point that I was a little distracted by the mention of different films that I miss part of what Banner was explaining. The gist of it is that whatever you do in the past will not affect their current situation. Therefore, killing a baby Thanos in the past will not change the reality of half of all living things disappearing.
After the movie, I spent some time thinking of the logic behind the time travel element. Keep in mind that what I am about to write is just my preliminary understanding of what I remember from the movie.
To aid in my explanation, I am creating some diagrams on Microsoft Paint. It will be messy, so please bear with me.
Let's start with the current timeline before any time-travel whatsoever. Our heroes lose the battle in Infinity War which took place in 2018. Three weeks later they kill Thanos after realizing he has destroyed all the Infinity Stones. Then time passes by, and the story picks up in the year 2023, five years later.
This timeline is what I will call the Blue Reality (using the term "reality" that the Ancient One mentions while talking to Hulk) as depicted by this blue line in my lovely Microsoft Paint drawing.
Again, bear with me as I continue to use the term "reality" along with an associated color. Blue Reality is now in 2023. It is in this reality where the remaining Avengers are learning to move forward from the effects of the snap. Scott Lang then tells them about the possibility of time travel through the quantum realm. They create the machine and suits and proceed.
In science, we learn that movement can happen in three dimensions. In our world, we can travel along the x, y, and z axes. When you walk from one place to another, your displacement from the starting point will have a different east-west (x-direction), north-south (y-direction), and a possible change of elevation (z-direction).
Time travel would then be the fourth dimension. In our space, after moving in the three dimensions of x, y, and z, there is no other way for someone to move. Therefore, if we could move through time, we would have to jump from one xyz plane to another xyz plane which is in a different moment in time. One reality to another reality.
My assumption for this hypothesis is that there are infinite realities that are at different points in time. Our space, our xyz plane is in one reality. There is then another reality that is behind us in its passage of time by a second. Another one is at its year 2017 on the timeline. Another is in its year 1900 and so on. All these realities are at different points in time. Moving through the fourth dimension, which is time, would mean being able to navigate to these different realities.
This movement between the realities, I think, is achieved through the use of the quantum realm. They are not "going back in time" in their reality (the Blue Reality). They are moving to other realities that are currently in different points in time, different years. The Avengers split into three teams. One travels to a reality that is currently in 2012 (New York), another to a reality in 2013 (Asgard), and another to a reality in 2014 (Vormir/Morag). In my diagram, I'll note the 2012 reality in orange, the 2013 reality in green, and 2014 in purple.
The time and mind stones are retrieved in the Orange Reality (2012), Rocket gets the reality stone in Green Reality (2013), and the soul and power stones are retrieved from Purple Reality (2014). Iron Man fails to get the space stone in Orange Reality, so he and Cap need to find it in another reality. They decide that they want to be in a reality where the year is 1970. So they jump once again (this reality is in red on the diagram).
Once most of the Avengers are back in Blue Reality (2023) with their respective stones, the Hulk proceeds in performing the snap that undoes Thanos's snap. During this time, Purple Reality's (2014) Nebula is transporting Purple Reality's Thanos and his armies to Blue Reality (2023).
In a later scene, Blue Reality's Nebula kills Purple Reality's Nebula. 2023 Nebula does not have to worry about disappearing or having any other adverse reaction to this because she killed a different reality's Nebula. This applies to Thanos and his armies. They are of the Purple Reality and, therefore, their deaths will not affect what Blue Reality's Thanos had already done. Blue Reality's Thanos already did the damage he needed to do in Blue Reality. Killing Purple Reality's Thanos would not change that. With Purple Reality's Thanos gone, it could be that Purple Reality gets to live the rest of their timeline without Thanos and his snap (lucky them).
This hypothesis can also explain why Blue Reality's Gamora (the one who fell in love with Peter Quill) will remain dead even though Black Widow already dies from the soul stone retrieval. Gamora dies from the retrieval of Blue Reality's soul stone while Black Widow dies from the retrieval of Purple Reality's soul stone.
Now let's talk about Captain America because this is where things got tricky. At the end of the film, Steve Rodgers takes the stones and returns them each to their original realities right at the moment they were taken from those realities. After that, we do not see him return to the platform. Instead, he is found sitting on a bench as a senior man which seems to imply that he traveled to a time in the past and just lived his life to get to this moment. We then see that he had traveled back to 1945 (or some time close to it) to marry Peggy.
Again, the assumption is that he traveled back to the past and just stayed until he caught up with the present (which is 2023). The problem is that we know that whatever you do in the past does not affect what has already happened. In Blue Reality (which, remember, is the reality we have seen taking place since the beginning of the MCU up until the Avengers start time traveling), Peggy married someone else as revealed in Captain America: Winter Soldier. Steve could not have married Blue Reality's Peggy. Instead, I think he traveled to another reality which was at 1945 during his time jump (let's call it Yellow Reality) and grew old there. He marries Yellow Reality's Peggy. I think once he was done living his life (maybe around the time Yellow Reality's Peggy dies), he jumps back to Blue Reality to meet with Sam and Bucky. This seems reasonable since Dr. Pym of Yellow Reality was probably still working on the Pym particles making travel through the quantum realm possible in Yellow Reality.
Anyway, this explanation is not perfect. There are probably places where realities converge and diverge, but I'm not a quantum physicist. For the most part, though, this explanation helps me wrap my head around the whole time travel thing. Feel free to leave comments and tell me what you think about the time travel in Endgame.
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