I get how the time travel worked in Endgame, I thought it was really cool. But how would it work going forward in time? Been thinking about it for ages and can’t figure it out

I have some thoughts on the possibilities of Avengers 4. This isn't a single theory that I'm convinced on, and I know it's pretty pointless trying to predict what'll happen, but it's been on my mind so I wanted to open it up for discussion. I'm new to Reddit and fairly new to the MCU so please be nice!

We know time travel is likely to be involved. I've been thinking of it in the sense of going from one point in time to another, but I realised that we don't know if the time stone can do this (unless it is disclosed in the comics - I haven't read enough, sorry). We do know that it allows its user to reverse time around them.

So, I'm wondering what would happen if Tony did reverse time back to some point in Avengers 1. Some people believe that he'll just destroy Thanos there, but I have two issues with that. For one, I think the rules of time travel mean that however Tony got hold of means of it (e.g. Doctor Strange 'sending' the stone to him) still needs to happen in the future. Presumably this would be soon after the end of Infinity War, which means the events of IW still need to happen (or maybe at least up until Strange sent future Tony the stone, if he did). Secondly, even if I'm wrong about that, changing things in A1 would likely affect every Earth-based movie after that, which I just don't think Marvel would do. In a sense it'd invalidate everything that happened in the following movies, which could be damaging.

Therefore, I think it's more likely for Tony to travel back and put things in place that would allow him to defeat Thanos at the right time in the future. There was a rumour that A4 is going to be set 5 years after IW. If it is true that he will reverse time, Tony would have to live the 5 years or so after the Battle of New York again - what if this is what that idea refers to? It's a reach, I know. But I also struggle to believe the IW ending will be unresolved for 5 years in the MCU. I'm not sure Tony would consciously go through all that again, considering the Avengers 1 ending and the events of Iron Man 3, unless he does and proves himself as perhaps the strongest Avenger in a psychological/emotional sense.

I don't think I'm the only one who felt weird about Civil War. It just felt avoidably catastrophic and tends to leave me a little angry, especially with its lack of resolution. What if Tony relives the timeline so far and changes things at that point? Rhodey implied that he regretted the accords situation, which could have been foreshadowing that Tony does too and is going to stop it from happening. This means he could fight with Steve in IW. The only good things that came from Civil War were Bucky's treatment in Wakanda and the subsequent events of Black Panther and Spiderman: Homecoming. Tony might somehow ensure things turn out the same in these ways. Maybe he would change things only after King T'Chaka's death.

Of course, none of this explains why Ant Man was there when they seemed to be shooting scenes set during A1, or gives any ideas of a solution to beat Thanos. I'm sorry if any of this is stupid, I'm probably going crazy because of A4, but please help me collect my thoughts on this.

As a big fan of time travel movies and stories, I was so stoked when Ant Man started talking Time Heist in Endgame. I was running through the theories in my head of how time travel could work as fast as they were spilling them out on screen. But I wasn't really prepared for the weirdness of how Endgame does it.

I left the theatre loving the movie, but completely baffled as to how their whole time travel system works, and whether or not they even bothered to stick to their own rules. Like many, I was thinking they played fast and loose with the rules, and ultimately made a jumble of their whole time travel concept.

But these are smart guys. They even said they took time to talk with physicists about how time travel theoretically ought to work. They've watched all the movies about time travel and seen all the complications and paradoxes and plot holes and messes. They knew the stakes for this movie. I just can't believe they would be lazy or sloppy about this.

Then a couple days ago it clicked, and the more I think about it, the more it makes sense.

We have two distinct moments in the movie where time travel is explained. First, there's Banner, who tries to explain that when you travel to the past, your past becomes your future, and your future becomes your past. It's played off almost as a gag, but according to McFeely and Markus, that's exactly what they were told by the guys they consulted with. So, that's the science.

Then you have Banner's moment with the Ancient One, which is more of the cosmic side of it, where she explains that the timestream is held together by the Infinity Stones, and that removing any one stone will splinter the stream and create chaos for anyone on it. Note, she doesn't say that making changes to the timestream creates chaotic splinters, but that removing a stone creates chaotic splinters. That's important, but more on it later.

There's a third moment that helps us understand all of this, though it isn't explicitly stated - the moment Tony solves time travel (the whole scene, really). Just before he figures it out, he has a visualization of a Mobius Strip, a shape that has often been used by theoretical physicists to try and explain time travel. There are different colored threads twisting and flowing across it. I thought they represented the Stones, given that they seemed to correspond to the Stones' colors. But individual characters also have their own "theme color," so what if those threads represent an individual's timeline? This is consistent with what Banner explains - each person has their own timeline. Each person's timeline can traverse reality, and intersect with the timelines of others - even, at times, create splinter lines for other individuals - but they cannot break reality.

Reality is a closed loop, with all the timelines of every individual and their experiences of reality held together in an ordered system, kept in order and together by the Infinity Stones. They are the power that prevents anyone's time traveling hijinks from disrupting reality and creating universe destroying paradoxes. They keep the Mobius Strip of time intact, so that people can move around on it. When Tony "solves time travel" you see a single thread loop across and come back and meet itself. What he just solved is how a person can move in time and come back to the moment from which they left - how to loop your own timeline to come back to the same place.

An example: Nebula. She runs into her past self, witnesses her past self killed, but she remains. Because seeing her past self killed is actually the future of her own timeline. However, by interacting with her past self, she is splitting her own timeline and creating a second version of herself within reality. That second version is killed. Her timeline is self-corrected.

This gets to the most irksome of the time travel mysteries at the end of the movie, which is how Steve Rogers could go back and live a full life with Peggy Carter, who already had a full life with a husband and kids. His interaction with her in the past created a new reality, a separate timeline for Peggy, where they live life together. But when he comes to the same time from which he departed, he's able to re-intersect with the timelines of his fellow Avengers.

TL;DR - Each person has their own timeline, all people's timelines can intersect and interact, even split off separate ones, but the Mobius Strip represents the closed loop of reality held in place by the Infinity Stones. Steve created a splinter timeline for Peggy that they occupied together until his timeline came back and intersected again with the Avengers in the modern day. All this is illustrated when Tony solves time travel.

EDIT: Right after posting this, I get sent this article from Polygon that essentially says what I've said here. Dag nabbit, Polygon, you beat me to it!

I have always struggled with how time traveling has been dealt with in the movies. In endgame, Tony Stark mentions the Planck scale and the Deutsch Proposition while calling wormholes Einstein Rose bridge, all very promising when dealing with time travel at a quantum level (accurate) AND smart hulk explain how when time traveling, the actions in the “past” do not change our future “If you travel to the past, that past becomes your future, and your former present becomes the past, which can't now be changed by your new future.” (Sic)

All great BUT SUDDENLY: CAPITAIN AMERICA

I believe it is safe to assume smart hulk is unable to bring him back to the present because cap never uses time travel in order to come back. Instead he spent his life in the past allowing himself to age normally reaching the same date when he travelled back in time and walk to that specific point where the avengers were waiting for him. Meaning he was on the same timeline THUS his actions would have changed the present where he left (two capitain America?) THAT IS NOT HOW TIME WORKS

If you were to travel back in time it only affects you as everyone you left in your former present they still continue to exist and move forwards. This must mean you are traveling to a different timeline that must previously exist. But if that is possible, why would that timeline be an exact copy of yours? Everything happened the same way? Is there a timeline for each second that existed and every time you decide to go back there is yet another timeline making it an infinite number of timelines?

More questions than answers but certainly the film made a mistake right?

Here is a fun article with a real physicist and spoiler alert, David Deutsch is now cannon in the MCU https://www.theringer.com/platform/amp/movies/2019/5/3/18527776/marvel-avengers-endgame-time-travel-david-deutsch-proposition-scott-aaronson

So, ever since endgame, I thought I understood time travel in the MCU, I thought it was one of the most clever ways of time travel, one which effectively counters any of the usual problems, but since reading Reddit, it appears I am either wrong, or no one really shares my interpretation, so I thought it would be fun / potentially interesting to share, it’s a bit of a ramble, so I’ve tried to sort it into a more friendly numbered list…

1. time travel in endgame

To establish how I interpret time travel to work, I must first establish how I interpret the multiverse, prior to Loki season one. I see the sacred timeline as an infinite bundle of individual timelines, which has been curated by the TVA to be identical, with any that aren’t being pruned. However, timelines may not necessarily move at the same speed. So while it may be 2024 in one timeline, an adjacent timeline may be 2012, because it moves slower. As there are potentially an infinite number of these identical universes, you will always be able to find a timeline which is moving at a different speed, and thus corresponds to the specific time you want to travel to. This means that it is impossible to change the future as all that does is change the future of that timeline, not the one that you’re originally from.

2. nexus events and the endgame time travel

Within the context of endgame style time travel, a nexus event is possible, this is what the ancient one describes in endgame, however I interpret this not as creating a branch, but just that specific timeline moving adjacent to the rest of the sacred timeline, likely resulting in the timeline being pruned. This can actually be seen when Loki escapes with the tesseract, this causes the timeline to move from the sacred timeline, and thus the universe is pruned. This does beg the question of where exactly Steve returned the time and mind stones, but the likely explanation is that, due to them needing to be returned to the exact same point that they left the universe, all the stones were returned to alternate universes than they originated from, moving at slightly different speeds, the stones work normally because the timelines are identical, unlike after Loki when different universes are sufficiently different to not work the same.

3. other forms of time travel

Outside of the endgame type time travel, we know of 2, maybe 3 depending on the canonicity of agents of shield, I will not be covering agents of shield, as I haven’t watched it. This is the time stone, and the time doors. The time stone clearly facilitates time travel within a specific universe, this can be seen in what if? but within the sacred timeline any use of the time stone is “supposed” to happen, any use of the time stone outside of what is supposed to happen, would likely result either in paradoxes so strong that the universe collapses, or the TVA stepping in to prune the branched timeline.

Now the time travel the TVA uses, requires an understanding of the TVA. The TVA clearly has time, despite the common misconception that it is outside time, it clearly has its own timeline due to the fact that you can’t stand up, walk five paces, turn around and see yourself still sitting, along with Loki’s time slipping in the second season. What the TVA is an outside observer and can therefore easily travel to any universe within the timeline, in order to prune it.

4. adjacent theory about the multiverse

This theory is based more on conjecture and is more something that I personally think is cool. When a timeline branches, moves away from the rest of the timelines, the difference will normally be so small that subsequent nexus events dont happen, thus the new, ever so slightly different universe kind of folds back into the rest. Following Loki when the TVA stops pruning timelines, a nexus event which leads to further small and inconsequential nexus events, is creating more universes following the events of the new timeline, effectively strengthening it, the more of these universes following the new timeline, the less likely it is to reintegrate back into the parent timeline, thus it becomes its own distinct universe within the multiverse. Since there are an infinite number of universes which can branch from the sacred timeline there’s effectively an infinite number of possible ways that a nexus event can happen, allowing for an infinite multiverse where events may be drastically different, such as the xgene entering the population and the xmen being formed instead of the avengers. This also accounts for Mr paradoxes plan in deadpool and wolverine, normally pruning a timeline destroys that specific universe, but mr paradox wanted to destroy the timeline, the collection of different universes following the same rough events, for this he needed the time ripper.

Now finally, I would argue that, despite what mr paradox says, earth 616 is not the sacred timeline, it hasn’t been the sacred timeline since avengers endgame, when Loki escaped, mr paradox is simply lying because he wants to convince deadpool not to cause a fuss, and knows tha deadpool, somehow knows about the idea of the sacred timeline. The sacred timeline no longer exists, what exists is an infinite multiverse overseen by Loki…

An idea I already have of it is what Hulk said in Avengers Endgame. Where if you leave your timeline to enter another. You are creating another parallel timeline where whatever you did changed the future of that parallel reality but not the one you originated from because the past in the reality of origin is set in stone. The new parallel reality can be twisted and altered because whatever has happened in your reality hasn't happened in the parallel reality. You are literally creating a timeline on your own that reacts how you configured it to be.

so if we use deke's multiverse theory and also what we know from avengers endgame, then I think that means that the timeline they travelled back to was not the one they travelled from. we know they've done this loads of times but I think each time they move to a different timeline and the versions of them from that timeline move to the next timeline. for example, in the original timeline, fitz and Simmons probably had different wedding rings, until deke picked those ones for them in the 2nd timeline, and in the 3rd timeline, and all the timelines after, he would've picked that ring for Simmons quicker because it looks like the ring his grandmother used to have. this raisises 2 questions I havent been able to answer, "Where did the versions of them from the new timeline go if the future they saw the world hadnt been cracked apart?" they probably wouldn't have been able to get back to the present either because Simmons smashed the piece of the time monolith and "Why did Robin still see the original timeline?" because you would've thought that she saw the new timeline that she had been living in, right?

anyways I hope this makes some semblance of sense