Headscratchers / Avengers Endgame - Timelines And Time Travel

Sooo... does time travel create an alternate timeline or not?

• If time travel creates an alternate timeline and can't affect your present, then there is no need to be cautious when time traveling. Go back, kill baby Thanos, take your sweet time getting the stones, come back to your present timeline, and undo the snap. There, you've saved at least two universes. No need for caution, you can even go back and yell, "Hey, I'm from the future and I'm here to save you, but saving you won't save me directly because of how time travel works, so can you help me out getting these stones?" And boom, you've got past versions of yourself helping you get the stones while you team up to casually stop whatever is happening in their point in history. On the other hand, if changing the past does affect the present, then snuffing baby Thanos is still the best and simplest option. It really feels like whatever version of time-travel would serve the plot at each scene or even each point in conversations was whatever they went with, even if it contradicted what was said literally seconds ago.
  • Yes and no, time is flexible in this case with both the guiding force of the infinity stones pushing it automatically back toward the prime timeline but also the possibility of splintering if too many changes are made. The issue with going back and killing baby Thanos is both moral and temporal since that's a pretty big change meaning it would almost certainly splinter the universe creating branching unstable timelines. They're cautious to begin with because tampering with time if they do fuck things up could make everything a lot worse if it carries over or not have any positive or negative impact if they spit the universe. The Infinity stones themselves are a different problem because they aren't just fancy power jewels, they're fundamental forces of the universe that are quasi-sentient in themselves. Sure they don't generally take an action but passively push the universe forward following the rules. Without them in the past time and reality would fall apart so simply taking them then keeping them is not an option. As for Thanos 2014 coming to the future, at that point, he's displaced from time and no longer properly integrated into the timeline. The infinity stones once back in place correct for this but the present leaves them with an extra dead Thanos as a result but the infinity stones smoothing over all the wrinkles in their quasi-sentient way allows for it and for Steven to live his life.
  • Time travel movies tend to use a lot of Applied Phlebotinum, but it's rare to see such a big-budget film using such a low grade of it. Banner has two conversations about time travel and explicitly contradicts himself - either that or somehow Infinity Stones and people affect the time stream differently if removed.
  • The conversation between Banner and the Ancient One addresses this point. Removing the Infinity Stones from the past would create a branching timeline, which is why the Ancient One objects to allowing the Time Stone to be removed, as the 2023 Avengers would be saving their own timeline at the expense of the one she would be left with. However, Banner counters that the branching can be undone by returning the Stones to the exact point in the past they were taken from. Presumably, the result would be a Close-Enough Timeline.
  • That...still doesn't make any sense, since it means you now have a single timeline, in which case killing baby Thanos is the best option. If you have branching timelines, then do whatever you like in the past, it has no consequences for you. Simple decency requires you to help people on your visit, but that's about it. If there is a way to undo the branching, then things work the same as they would in a story where there is only one timeline,e to begin with: kill baby Thanos and you're done. This story tries to have its cake and eat it and just doesn't make any sense as a result.
  • The original 2023 timeline will remain unaffected whether they returned the Time Stone to 2012 New York or not, and that alternate timeline will be created no matter what. However, if they don't return it, then the timeline that The Ancient One is left with is doomed when Dormammu attacks five years later as The Ancient One can foresee. Returning the Time Stone to the 2012 NYC timeline is an act of courtesy from one timeline to another - giving it back to the Sorcerers will allow the Dr. Strange of that universe to save that world using its powers. The only inconsistency is at the very end when Steve ages into the Prime Timeline despite drastically altering the past, though one could handwave it by saying that he used his Pym particles to return to Prime after Peggy's passing inane alternate reality.
  • Another way of saying the above would be that the main thing the Ancient One was concerned about wasn't about making alternate timelines, but rather that the future of those timelines would be very dark if the stones were gone. By making sure that the stones are returned before they are missed (not necessarily anywhere near the instant they were taken), Steve ensures that the timelines will at least stay fairly similar and that the stones will be available if they are needed.
  • Alternatively, one could say that this is a different Steve that came into this timeline. A Steve that had something similar happened but in a different timeline.
  • This is probably how it works - if you travel to the past and change history as you know it, you create a new branching timeline. But if you go to the past and avoid changing history as you know it, the original timeline is only slightly altered to become a Close-Enough Timeline. This is demonstrated when Banner explains to the Ancient One that returning the Infinity Stones will cause the branched-off stone-less timeline to disappear - and also in the end where Steve mentions how he's going to "clip off all the branches" by returning the stones. So going back and killing Thanos as a child wouldn't save the original timeline as that action would create a new branch. But going back, taking an infinity stone, and then returning it only slightly alters the original timeline so there's no branch. Also, presumably, when Steve goes back he lives his life with Peggy without altering history in any significant way - which is why he remains in the original timeline at the end.
  • So why doesn't Cap help people in the past. Sure, he creates an alternate timeline, and? It doesn't hurt people in his original timeline, as he simply disappears without a trace, but he makes the lives of the people in his current timeline a hell of a lot better, with basically all the problems from all the other movies nipped in the bud early.
  • It's at present ambiguous if he does or doesn't. That is he may well have created an alternate timeline by reuniting with Peggy, and thus went on to change a lot of things for the better before eventually returning to his original timeline near the end of his life. We may have to wait for official word on just what actually happened.
  • The Russos confirmed that Steve indeed went to live in an alternate timeline, and that the question that we should be asking is "How did he get back?" and not "Was old Steve present all along?"
  • Did Steve have a second Pym particle to use for his return trip? Did he need one? If so, all he had to do was wait until he reached the Civil War period (or what would have been the Civil War period, anyway) when Peggy died, and then come back.
  • He needed enough to make several jumps anyway, to cover all the places where he stones and Mjolnir needed to be returned. Perhaps he had enough for the extra jump as a contingency reserve, or perhaps he obtained more the same way he and Tony did in 1970.
  • The Ancient One states to Bruce that the Infinity Stones create or stabilize space-time, and their removal would be catastrophic beyond the obvious (Dr. Strange not having the Eye of Agamotto to defeat Dormammu, for instance). Indicating that, once the Stones are back in their proper places in spacetime, they can "guide" the timestream into a Close-Enough Timeline, allowing Steve to take The Slow Path to the present, which would normally not be allowed in a quantum mechanical multiverse version of time travel (Steve's very presence in the past would shunt him into an alternate reality, not the one he initially came from, even if he didn't interfere in anything).
  • They were limited in the number of time jumps they could make due to the Pym particle shortage. So unless baby Thanos happened to be conveniently close (physically and temporally) to at least one of the Infinity Stones, meaning that the murder mission and the Infinity Stone retrieval could be combined, then killing Thanos (and definitely creating the alternate timeline) would doom their mission. The target stone would have to be obtainable very soon after killing Thanos, because taking the "slow path" to a stone in an alternate timeline where they had no idea how their actions would change the paths of the Infinity Stones would be insanely risky. Considering that they probably have little-to-no idea where and when to even find baby Thanos (unless Nebula happens to know both his date and specific location of birth), let alone a certain location on most of the stones that far back, that would be unlikely to be a viable plan even if they were willing to throw caution to the wind and severely alter timelines.
  • But wait; if Steve created a linear Close-Enough Timeline, then does that mean the answer above is incorrect in claiming that the Russo brothers confirmed that Steve traveled to an alternate timeline? If so, then old Steve was there all along. So what did he change to make it only a "close enough" timeline and not the timeline? Where was he when young Steve visited Peggy on her sickbed or came to her funeral? If he changed time so that those events never occurred, does that mean the events of Winter Soldier or Civil War have no longer happened in the mainstream MCU timeline?
  • The true (meta) answer is simple: "Whatever the writers want it to be." The film tries to have it both ways: time travel seems to create alternate timelines if the change is big enough, but if this change is small or reversed no split occurs. Even slightly diving into this explanation quickly reveals that it makes no sense, but it does basically give the writers license to do whatever they wish.
  • That is not supported by the explanations given in the movie. Bruce says outright that they cannot change their own pasts, because they're not going into their own pasts. Neither he nor the Ancient One ever said that any particular degree of changes will make it split off or not. It's made clear that you are traveling to an alternate past — the reason they want to minimize the changes is that they don't want to dick over another timeline by just stealing the things and causing a mess, because they're not assholes. That is the point the Ancient One is making — not that removing the Time Stone would create another timeline, but that removing it would send her timeline on a darker path. It has nothing whatsoever to do with whether it "creates" an alternate timeline. The Russos' model is the only one that's consistent with how time travel is shown to behave in the movie.
    • They do say exactly that. The Ancient One specifically says removing the Time Stone splits the timeline and would create a new reality. Bruce responds that replacing the Time Stone at the moment it was taken would erase it. Much ado is later made about "trimming the branches." The implication being these are not alternate realities that would exist whether prime-timeline Avengers traveled to them or not. They are alternate timelines created by the Avengers, seemingly if the changes are big enough or not reversed. This (at least) makes sense, since in all their travels to the past they never encounter Zeppelins from Another World - meaning these are not alternate worlds (at least initially), but actually are their own pasts. An alternate world is only created if events are sufficiently messed with. As previously stated, this explanation does not hold up under even the slightest scrutiny but it's the one the movie is going with - probably because it gives the writers license to decide what's "real" and what isn't however they wish.
    • The Ancient One refers to herself and Bruce coming from different realities right from the start, before they say anything about taking the stones. Yes, she refers to her "new timeline," but that doesn't necessarily mean "new" starts from when the stone is taken — in fact, it's far simpler for it to mean the "new" timeline is just from them traveling back in time. I mean, Bruce makes it clear right from the start that they can't change their pasts; ergo, the past they're traveling to isn't their own. The simplest way, the way that makes the most sense, is just that traveling itself branches off the timelines. Otherwise, where's the threshold? Where does a universe decide that these changes are minor, so it's a Stable Time Loop, but this change is too much and creates an alternate timeline?
      So, again: The theory that every time they travel to is (or immediately becomes) a separate timeline is the simplest answer, that snips away questions of, "Why didn't Cap do this in the past?!" and you can get there without resorting to, "And the writers wanted a lazy way to get away with anything."
    • The stones are mentioned immediately, and talk of alternate realities comes after. Makes sense considering, "I need that stone," is the second thing Bruce says to her (after, "I'm looking for Dr. Strange.") The Ancient One also specifically says it is the removal of the Time Stone that creates a branching timeline, even literally drawing a picture demonstrating this (illustrating the timelines being united until the moment the Time Stone is taken). Bruce also doesn't say, "You can't change the past." What he says is, "Changing the past doesn't change the future." Meaning it creates an alternate timeline if the changes are great enough. Where's the threshold? Seems kind of arbitrary? Exactly. It's whatever the universe (i.e. the writers) says it is.
    • I'm not sure if "arbitrary" is quite the word, so much as some changes would clearly be more significant than others. Is Clint taking his son's baseball mitt from the test run likely to create an alternate timeline? Probably not, it's so unlikely to affect wider events (the kid probably looked for it for a few hours, got a scolding, and then got given a new one) that the timeline would just absorb it and nothing about the future would change. Stomp on the spider that bit Peter before it did so? Yeah probably more likely to cause a timeline branch, since that would domino into changing a lot of other events, without even touching on the infinity stones. Arbitrary would more apply if taking the mitt created an alternate timeline due to a zany and outlandish butterfly effect series of events, but stomping on the spider just meant that Peter wound up becoming Spider-Man when a different radioactive spider bit him the next day.
  • As for Steve, simplest answer: He still had everything he needed to go back to the main timeline whenever he wanted, he just wound up not using Banner's portal station. How? Probably had the other dimension's Tony build him a timeline jumper to push him back to his own timeline so that he could wander over and have the meeting with his friends on his own terms rather than popping out of the beacon old and having Bruce wail "AW NO I DID IT AGAIN!"
• Timey-Wimey Ball
• The simple answer to the main question: no. The time travel does not create any new timelines, it instead causes the user to jump to a parallel timeline running concurrently to theirs that already exists and allowing them to do stuff there. They aren't really changing the past because, in regards to the passage of time, they are currently in that universe's present-day; cause and effect goes from there, but it doesn't affect the time traveler's own universe. In other words: while in the main timeline those events already happened a while ago, those events are only just now happening in that other timeline, and is happening concurrently with the main timeline's present. The movie pretty much runs on the Dragon Ball school of time travel logic. That's why Loki's escape doesn't affect anything that happened in Phase Two or Three, and how Nebula can kill her other self and still be alive; because it's not actually her past self, but a completely different universe's Nebula.
• Going by Loki (2021), no, time travel does not create a new timeline, because if it did, the TVA would come along and "prune" it.
• As originally explained by Bruce, going to the past neither changes the present nor creates a new timeline. Even if you do stuff in the past that should result in a different future, it won't; everything will just Snap Back. The exception, as Bruce finds out, is if you remove an Infinity Stone, which does cause a branching timeline.

The effect of deaths on the timeline

• The 2014 versions of Thanos, Nebula, etc. are killed. Surely that ruins the timeline, right? In his linear timeline, Thanos has now died several years before the events of Avengers: Infinity War. How does that not have a massive ripple effect?
  • No, the rules of time travel for this movie are established early on and are specifically stated to be different from movies like Back to the Future. In fact, a proposed early plan to simply strangle baby Thanos is shot down for that reason. For instance, the Thanos that died at the end is not the same one that died at the beginning. He is an alternate Thanos who traveled in time from 2014 to 2023, something the Thanos that snapped at the end of Infinity War never did. The presence of Old Cap in the end however is the real headscratcher.
  • The timeline only branches off if you drastically alter history as you know it. Thanos' disappearance from 2014 creates a new branch where the events of Guardians of the Galaxy (2014) and subsequent films don't happen. However, Cap going back in time and, presumably, living his life with Peggy without disrupting history too much allows him to stay on his original timeline.
  • Perhaps it's similar to the time paradox duplicate situation in Bender's Big Score.
  • Thanos' army being wiped out by the Infinity Gauntlet could be a Cosmic Retcon similar to bringing back everyone to life: they are erased from 2023, but return to their original time period, presumably without memories of what happened, to ensure past events still occur the way they did. The only loose end is whether this includes 2014 Nebula, who was already shot when Tony snapped his fingers.
  • An interesting theory, but highly unlikely, given that Gamora from the alternate timeline is still around in the present.
  • There's no need to do that, as the plan is to return the stones to just after they were taken in the original timelines. So now in the original timelines, the stones are in their initial places and Thanos' army can't travel into the future. They're going back to before past Nebula and Thanos were killed.
    • Except the stones have nothing to do with the time travel, Nebula summons Thanos to the main timeline completely independent of any of the retrieval of them. The events that cause Thanos to start looking into everything happen before the stones are removed, within minutes of Nebula and the others arriving on Morag. Rhodey then returns with the stone, Nebula's held up by glitching and then is kidnapped and taken away, and past Nebula returns in her place from an entirely different location. If Cap shows up to return the stone to when it was taken if he's so much as a minute late Nebula's going to be gone already and there's nothing he can do. That timeline is losing its Thanos and his army for sure. Not that this is a bad thing... remember that at the end Bruce's concern is with causing a bunch of "really nasty alternate timelines" by removing the stones. Improved ones are probably fine.
  • Moreover, given that it was Stark who enacted the final finger-snap of the story, it's likely that his desire to have nothing about his present world change would cause the Gauntlet to override any anomalous repercussions of the Avengers' interactions with the past. 2014-Gamora's current presence doesn't change anything that's happened in the years since the dusting because she was dead already, so that version of her can stick around. But all the other changes - Thanos' and his minions' absence, Loki's escape, Frigga's foreknowledge that future-Thor's been grieving for her - were undone by Tony's desire not to disrupt history in any way that might endanger his life with his daughter. The Gauntlet's powers include the Time Stone's; paradoxes aren't an issue to it.
  • All the alternate timelines created (Loki escaping with the Tesseract, Quill getting knocked out, etc.) did still happen and still exist within their own specific universes (thus making for good spin-off material), but the Infinity Stones were returned anyway to prevent unforeseen complications. In the altered Quill timeline, Thanos and his army probably just... winked out of existence and never returned (which, depending on how soon in that timeline the Power Stone was brought back, would cause serious problems if Ronan still showed up to invade Earth, since the Guardians won't exist, especially without Gamora, but that's a matter for spin-offs to resolve, if ever). The Time Stone might only affect the time within the branch it's used in; it likely does not affect other timelines in the multiverse.
    • Presumably you mean Xandar, not Earth since Ronan never gives any indication of being aware of Earth or having a problem with it? Ah, but the only reason that Ronan even got the Power Stone in the first place is that Gamora showed up to try and steal it from Quill and set off the chain of events that resulted in it getting into Ronan's hands. If Quill didn't encounter Gamora, he probably wouldn't have wound up tussling with Groot and Rocket, he wouldn't have gotten sent to that particular prison and met Drax, Drax wouldn't have gone with him to Knowhere and contacted Ronan to get the stone. More likely what happened is Peter would have sulked around for a while, wound up getting pointed at the Collector anyway, and the Collector would have bought it off of him largely without incident (since without a larger crowd to distract him his servant probably wouldn't have been able to grab the thing).

Time in the Quantum Realm

• So Janet was trapped in the Quantum Realm for around 30 years and time for her was normal as she aged accordingly but in this movie it is stated that Scott's stay in the Quantum Realm felt like five hours instead of five years and because of that he hasn't aged. Shouldn't Janet be young since time is slower there?
  • Janet didn't get sucked into any time vortexes. She told Scott to not get sucked into any time vortexes either. He didn't listen.
  • Another difference between Scott and Janet's ordeal within the Quantum Realm is that Scott entered the Quantum Realm through the quantum tunnel device, and was still attached to that device when he's trapped because the people who were supposed to return him got Snapped. Janet entered the Quantum Realm without any special device or even preparation, and was wandering around the realm aimlessly until Scott managed to locate her position decades later. Presumably, going into the Quantum Realm through a quantum tunnel device alters the time perception of the user, as the Avengers could re-purpose the quantum tunnel into a time machine afterward.

Time travel is entirely conceptual but I know all about it somehow.

• How did Bruce know the rules of time travel weren't Back-to-the-Future style, before he'd even worked out whether or not time travel was possible?
  • He's an Omnidisciplinary Scientist. Just as astrophysicists in the real world could render the approximate appearance of a black hole before they ever actually observed one the year this film came out, Bruce has enough understanding of how the universe works through SCIENCE! to look at Tony's theoretical model of how time travel works and understand it.
  • It's a comic book movie, and this is endemic to scientists. They're ultimately a device to explain how the plot works and provide the occasional McGuffin and Deus ex Machina via Techno Babble. Tropes Are Not Bad, time travel is mindwarping enough without a convenient character to explain how it works this week.
  • He hadn't worked it out beforehand. When using the Quantum Van, they're flying blind—he literally says that he has no idea what he's doing. When Stark arrives with his quantum navigation model, that's when the tortuous rules of THIS specific form of time travel click for Banner, and that's when he understands what would work and what wouldn't.
  • Plus, knowing how doggedly both he and Stark tend to pursue problems once they get them into their heads, they may very well have tried to hash out Time Travel as a possible solution before, back when Stark was recuperating from his injuries and the arduous trip back to Earth. They would've kicked around any thread of possibility they could come up with, that might bring Parker, Betty, the Bartons and so many others back, but without Hank Pym's technology and Scott's direct experience of the Quantum Realm, they never managed to get past the point of raw speculation and dissing Hollywood Science.
  • Even before space travel we had a pretty good guess the moon was not made of cheese. We didn't know for sure but know whoever came up with it was just trying to entertain and not really thinking about it. Plus it's not like time travel is conceptual in the Marvel universe he knows the Time stone exist, quantum leaping through time though is a bit more conceptual.
  • Tony and Bruce can create some really informed hypotheses, but the big thing is they know something can go catastrophically wrong if they screw up. That in mind, they push for the most careful options to avoid causing worse problems.. and, well, Bruce is a little stunned that everyone else isn't just relying on Back to the Future for planning, they don't get the details right on that (remember Doc Brown explaining that once the past has changed, you can't go to the old future until it's corrected?).
  • That's how science can work. Just because science don't know if something is true doesn't mean that theorems can't be derived assuming X is true/false. That's half the point of theoretical science. Quantum Computation is only a theory but there are plenty of papers written on possible advances in technology assuming Quantum Computation is possible. P = NP isn't solved but the consequences of either P = NP or P != NP are pretty well understood. Bruce might not know if Time Travel is possible but assuming that Time Travel is possible, the science works out that you travel to alternate dimensions.

I See the Future, But...

• If The Ancient One knew that Stephen Strange was going to become a Sorcerer...why did she initially turn him away in his solo movie?
  • Probably she saw every variations of that particular moment, and saw that Strange would be humbled by that moment of rejection, thus molding into a better person and better sorcerer. It’s a bit like asking why Strange let Quill meltdown happen when he logically should have know it was coming. More than likely he saw an alternate path where preventing Quill meltdown would systematically result in Thanos victory through the ripple effect. Similarly, the ancient one, who can’t see the future beyond her own death, a limitation the time stone can apparently remove, saw that getting initially rejected would be counter-intuitively important for Strange's character growth.
  • Or she simply realized that Strange was too full of himself for his own good, and trolled him a little to give him an "It's not about you" reality check. Also, seeming too eager to accept a stranger as her successor might've offended her long-standing followers, at a time when holding her remaining adherents' loyalty was very important and precarious, given the whole Kaecilius situation. Not everything the Ancient One does has to be mystical; political savvy and being a good judge of character are pretty important if you're going to hold onto a position of leadership for thousands of years.
  • It's implied that the Time Stone lets you see general events and potential futures but not the exact, perfect future of what's definitely going to happen. It's possible that originally she only knew in a very general way that he was meant to be the greatest Sorcerer Supreme, or that he would be if certain things happened... but that when watching him after his accident (probably using astral projection) she decided that the way he was handling it meant he didn't actually have what it took and she'd been wrong about her assessment. Or just the way he was acting on their first meeting made her think twice. He was, after all, at his lowest point and she'd just had a very stark reminder of what giving power to someone not emotionally stable enough to deal with it properly could do.

Will the 2023 timeline unravel?

• The Ancient One told Banner that the Infinity Stones were a ‘stabilizing force’ that helped to control the forces of time. Since Thanos has now destroyed them, technically doesn’t it mean that the prime 2023 MCU universe will start collapsing in on itself? Does this now open the MCU to their own version of the ‘Incursions’ that we saw before the new Secret Wars in 2015?
  • If nothing else, it offers many future story possibilities. However, I think you may be misinterpreting the Ancient One's words. After all, if losing the Stones really had doomed the prime MCU then it seems like she would have told Bruce that instead. She seemed more concerned about losing them before the point in time where Thanos destroyed them and how it would effect future events rather than that the universe would die without them.
  • Thanos only destroyed the stones in the sense that he changed them into a form (atoms) that can't be manipulated by mortals. The cosmic forces they represent still exist, it's just the stones themselves that are gone. And the Ancient One needed the Time Stone to defend her timeline against a specific threat she knew was arriving soon (Dormammu) so the prime timeline losing access to the stones may not be as big a problem since he's already been dealt with.
  • Restoring the stones might be the subject of a future Dr. Strange or Captain Marvel movie.
  • It's possible that altering the universe to not need the infinity stones was part of Thanos' act of destroying them. We won't know for sure until later, though.

Time in the Quantum Realm II

• Scott goes in, spends five hours there, comes out and loses five years. Janet goes in, spends thirty years inside, and comes out looking like she's thirty years older. Explanation? Only thing I can work out is time is already wonky in the Quantum Realm so some parts pass in real time and some don't.
  • That's probably exactly the answer "Time is wonky in the Quantum Realm", maybe he did end up falling into a time vortex and that was the effect.
  • This is already reasonably answered in one of the folders above: "Another difference between Scott and Janet's ordeal within the quantum realm is that Scott entered quantum realm through the quantum tunnel device, and was still attached to that device when he's trapped because the people who were supposed to return him got Snapped. Janet entered the quantum realm without any special device or even preparation, and was wandering around the realm aimlessly until Scott managed to locate her position decades later. Presumably, going into the quantum realm through a quantum tunnel device alters the time perception of the user, as the Avengers could re-purpose the quantum tunnel into a time machine afterward."

Taking Mjolnir into the Future

• What exactly was the purpose behind taking Mojolnir from 2013? Yes, it was useful against a time travelling Thanos, but literally nobody was expecting something like that. And considering that there's nothing else out there that would want to end the Avengers in the present timeline, why would Thor bring it with him to the present and risk alternate dimensional collapse?
  • Most of all, Thor probably just wanted to have something that would keep reminding and proving to him that he is still worthy. He just needed it to be able to confront Thanos and keep going through the battle.
  • Thor had no reason to believe that Thanos would go into the future at that point - he just wanted to bring it with him. As for why, there's sentimental value, but fans have also speculated that he wanted to see if Steve Rodgers was worthy, since Steve had made it budge in Age of Ultron. By the film's own time travel logic, he could do so as long as Mjolnir was returned with the Aether.

Hulk's knowledge of time travel

• So Hulk explains that you can’t just go back in time and change the past to alter the future and such time traveling would create alternate timelines. The question is: how did Hulk know with such certainty that this is the case? At the time, he hadn't had any personal experience with time travel.
  • It's possible that Bruce and Tony worked out the rules for how time travel worked between scenes.
  • By that point in the film Tony had created a theoretical model of how time travel worked (the inverted mobius strip thing he was working on in his home) which Bruce would have looked over to prepare for the test. Being an Omnidisciplinary Scientist, it was then a simple matter to understand it and explain it to other people.

Steve only going back in time once

• How is it that Steve went back in time and brought all the stones and Mjolnir back to the timelines that belonged to them and not come back to the present at least once? The Reality Stone and Mjolnir come from 2013, the Power and Soul Stones come from 2014, the Space Stone comes from 1970, and the Time and Mind stones come from 2012 (plus Loki having the Space Stone of that timeline). By that logic, wouldn't Professor Hulk have to bring Steve back/send Steve off multiple times to get him to those different timelines before he goes back to the 40s and lives his life with Peggy?
  • Uh, no. Considering Tony and Steve use their Pym Particles to go from 2012 to 1970 on their own, they clearly don't need to return to the present for each trip. Steve just needs enough Pym Particles and he can take them all back in a single round trip.

Traveling to 2009?

• So, when 2014 Nebula brings 2014 Thanos to the present, we see this image◊ of the time destinations, which shows 3 dates - May 2012, January 2988, and November 2009. The first is obviously when The Battle of New York took place, and I'm assuming that the second is a remnant of what they originally planned to do for retrieving the Reality Stone (since Natalie Portman wasn't originally going to reprise her role), when Bor Burison fought Maleketh (since if you look closely, there appears to be a "BC" indicator next to the date), but who traveled to November 2009?
  • Probably no one. Maybe it's Random Number Generator that determined this time period.
  • According to the fanwiki, November 2009 is when Tony Stark first announced "I am Iron Man" and kicked off the modern age of Heroes (and Fury approached him about the Avengers Initiative). So presumably this was either meant as a bit of continuity porn fanservice, or Thanos has decided to go all Skynet and his backup plan is to kill Stark right at the beginning.
  • Though it's hard to tell exactly what year they chose, the first successful test of the time traveling suits and beacons send Clint back to a time when his family was not only alive, but his daughter Lila was noticeably younger. It's possible that this was the November 2009 trip.
    • Hawkeye's trip seems to be only shortly before the snap.

Another question concerning Cap's conventional "Time Travel"

• OK, he returns the Stones and Mjolnir to their places in history. OK, he decides to catch up with the past - and thus relive the lost decades with Peggy Carter. OK, he suddenly returns to the place where he was transported into the past - but as an old man who is approaching his 200's. Here's the question: was Cap really sure that he would relive his missing years and return to the quantum transporter without... you know... the risk of dying... in his really old years? Super Soldier Serum or not, but he's still mortal.
  • Scott Lang went through the Quantum transporter while 95 years old, and looked considerably older/less healthy than 200-something year old Steve Rogers did, and made it out fine, besides being turned into a baby. I'm sure elder Steve Rogers will be able to make it through the Quantum Realm just fine as he is.
  • I believe the OP is referring to Steve taking the "long path" back to the transporter's location—as in, living through all that time instead of skipping it. So, yes, there WAS a risk of him just up and dying of old age before getting to return there. However, it's also possible he returned there on a whim: Bucky seemed to have known of Steve's plan, and he looks satisfied and amused when Cap goes to the past, as if he already knew his friend would choose to stay there... but then he looks surprised and slightly shocked to see Steve back. The way I see it, Bucky and Steve discussed everything beforehand, even the part about bequeathing the role of Captain America to Sam, but when it came down to it Bucky thought Steve would just end up staying forever in the past, and he was genuinely surprised to see Steve lived long enough to complete his plan.
  • Presumably Steve would have stipulated in his will that the shield be delivered to the Quantum Transporter's site at the right moment, Back To The Future II-style, if he hadn't lived long enough to hand it over personally. But he was still hale enough to turn up himself, and Bucky is glad to see him, no matter how old he's become.
  • Steve didn't take the "long path." He was in an alternate past, and used his time thing to zap back to the main timeline after Peggy died. As for him not dying of old age on his own? He's a supersoldier, with a healing factor. He's a lot more resilient than someone even half his age. And if he did die? Well, then he dies. It's sad, but it's just a thing that could happen — a thing that he risks every day of his life.

Thanos-less timeline

• Am I correct in assuming that because Thanos left a timeline, that would create a new timeline? Does this mean that the people they couldn't save in the prime timeline are saved in the new timeline? Does this also mean that the "Win" involved the new timeline in addition to the prime timeline?
  • Yes, because there's no Thanos or his followers in that timeline, that means none of the events of Infinity War or Endgame happened there. Half of Asgardians weren't killed, nor did Loki or Vision or Black Widow or Iron Man die (Gamora didn't die either, but presumably everyone thinks she did, because she disappeared from that universe altogether). On a more negative side, without Gamora the Guardians of the Galaxy probably wouldn't have formed and Star-Lord is probably dead, though that at least means Ego was never even able to try and carry out his domination plan. The Stark and Banner of that universe might eventually build a time machine and Cap might eventually use it to travel back to the 1940s, but without the urgency they had in the prime universe it'll probably happen much later.
    • I'd argue that this new "Thanos-less" timeline would not exist in the first place. Going by the conversation between Bruce and the Ancient One, when Steve replaced the Power and the Soul Stones, the diverging events of the alternate 2014 timeline were negated, which means that everything that happened after the stones were removed, was negated. 2014 Thanos travelled to 2023 after the stones were taken by the Avengers in that timeline. In any event, with the TVA around, it seems like they said that the events of Infinity War and Endgame HAVE to happen, so if Thanos is not around prior to Infinity War, they'd either fix it or get rid of that timeline.

Cracking time travel in no time at all

• We all know how ridiculously intelligent Tony is, apparently being the second smartest person in the world after Shuri, but no amount of brain power on his part could have allowed him to solve time travel in seemingly one evening. Cap, Nat and Scott had visited him just earlier in the day requesting for his help in figuring out time travel, with Scott in particular postulating the whole concept to a skeptical Tony, the latter of whom constantly refuted the idea and wrote it off as absolutely ludicrous, which tells us that such a notion had never crossed his mind over the past 5 years. And yet, as soon as the sun sets, the man hunkers down, has some light snacks by his side and solves one of the universe's most impossibly perplexing conundrums before the end of a workday.
  • Who says it's all his own work? It's entirely possible—even likely, given his previous history with Aldrich Killian, Dr. Maya Hansen and Quentin Beck—that Scott in particular gave Tony the idea of hacking into and looking at Hank Pym's research (it's not like Pym himself was around to complain), and from there, Tony... extrapolated.

Why 1949 and not 1945?

• It always seemed a little odd that Steve Rogers would go back to live with Peggy in 1949 and not 1945 where he left off. If you got the chance to go back to your home time period after being a Fish out of Temporal Water, wouldn't you go back to the exact time you left so it's like you never Time Traveled in the first place? Y'know so that you don't miss anything or make loved ones worry, mourn and get depressed, or search for you never mind for four years. You also likely wouldn't risk leaving behind a lover and expect them not to move on to a new relationship during that gap. Granted, yes, if he did go back to 1945, the events of Agent Carter would have some serious explaining to do. Was the choice maybe based on something he learned after visiting elderly Peggy in 2014?
  • It is possible that he put 1949 by mistake, and knowing that he had no way of going back in time, he decided to stay in that year.

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