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Rest assured, my question is entirely Endgame spoiler-free unless you hover over the hidden quotes. Probably best not to read on.

Background:

Time travel is fickle, and Endgame's plot is centered entirely around it. Conventional time travel has a golden rule: change the past and you change the future. I am aware of what is said at the start of the film: (quoting Banner/Stark? from memory) "whatever happens in the past the future remains the same". This may explain how reality remains OK even after the un-snap with the vanished half suddenly coming back 5 years later, as the future goes on normally as if the vanished had really vanished for the last 5 years and only reappeared again without having to fill in the 5 year time gap.

My inquiry:

This begs the question, however: How is Nebula still alive and kicking at the end of the movie, after she kills (literally shooting a hole through) her past self? Logically speaking, shouldn't killing her own past self erase her from existence, completely, in the future?

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    I know... wibbly wobbly, timey wimey, stuff. – Mat Cauthon Apr 24 at 12:53
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    I haven't seen it as it isn't out here yet but surely the quote you provide is the answer to your question? It isn't the conventional means of time travel so trying to apply conventional time travel rules to it won't work. – TheLethalCarrot Apr 24 at 12:54
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    Not really. Just thinking about it makes me even more confused. The scene with the quote was really just a rush of words about the implications of time travel and I only caught that out of everything said. That happened way before said event (which happened in the last half hour) so I'm not sure if it was intended to answer that question. I'm hoping, though, someone will be able to dig up something from the film or in a future interview once it's released worldwide. – Mat Cauthon Apr 24 at 13:00
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    Your arm's off! No it isn't!! it's just a flesh wound! – djm Apr 24 at 14:01
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The "answer" (or at least the theory upon which the answer to your query is based) is in the scene you first quoted. Based upon my one viewing of the movie (spoilers, of course):

After Stark posits this theory that the Avengers' actions in the past would not change their collective present, the other characters in the room run through a plethora of movies that instead relied on your "golden rule." In those movies, the characters' actions in the past altered their present. At the end of this rapid-fire listing of movies, Ant Man says something like, "so Back to the Future was all bullshit?" The movie then moves forward without further analysis, having laid the groundwork exposition that the past cannot be changed. The implication of all this is that, even though present-Nebula shot past-Nebula, that act did not change present-Nebula's present condition. (For what it's worth, all but one of the other events in the movie appear to conform to the theory. The one that I think may not follow the rule (SERIOUSLY, this is a SPOILER from the end of the movie, so STOP READING NOW if you don't want to spoil it!): Captain America appears at the end as an old man, having gone back in time and apparently spending a lifetime with Peggy Carter. This could have altered her timeline, unless you accept that she was actually married to Cap in the original timeline. This is a possibility as past movies did not reveal the identity of her husband. If so, I would concede this follows the in-movie rule; otherwise, it would be (TV Trope warning) New Rules as the Plot Demands.)

The theory and the result of the event in your second spoiler-block does seem to defy logic. However, it is based upon one widely accepted theory concerning the potential impact of the plan they were discussing in the movie. See, for example (warning: hovering over the following link will reveal a spoiler), Prof. Miller's essay in The Philosophers' Magazine, June 25, 2017

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    @PaulD.Waite: I've edited the answer to name names. I originally avoided doing so out of an enhanced abundance of spoiler-avoidance caution, despite the spoiler tag. – Rob Apr 25 at 3:17
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    you changed your answer in the past, but it didn't affect the present! Either we've proved that the rule works in real life, or I'm very tired and need to go to bed. Yeah right right that would... probably be the one. I mean it essentially retcons the Agent Carter TV show out of existence, which seems mean. – Paul D. Waite Apr 25 at 3:29
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    I hate time travel. – Paul D. Waite Apr 25 at 3:30
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    @PaulD.Waite: I believe Agent Carter takes place in the late 1940s, while that scene of dancing is clearly intended to be 1950s. So no, the show has not been retconned out of existence. – Martha Apr 29 at 18:56
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    The Hollywood Reporter has an interesting take on the Peggy issue, as well as the most likely time frame into which Steve inserted himself (no pun intended): "The best bet is Steve returned to the timeline between 1947 (the end of the Agent Carter series) and 1953 (where she revealed she was married, per the Winter Soldier Smithsonian footage) to be with Peggy. At the end of Avengers: Endgame, a 1940s-style car passed through the scene, confirming the era." hollywoodreporter.com/heat-vision/… – Rob Apr 29 at 23:16
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There seem to be two parts to this.

Firstly, the quote you mention, whatever has happened in the past will always happen because they are changing their future not the past. They can’t change the past for themselves because they are in the future for themselves.

Secondly, the only way to change the timeline and spawn new ones as The Ancient One indicates is to remove an Infinity Stone from its timeline. As this didn’t happen with Nebula’s case it doesn’t affect her.

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    The Ancient One never says that removing a stone is the only way to spawn a new timeline - just that it's a particularly impactful way of doing so. Banner's comments earlier indicate that any change to the past will spawn an alternate timeline. – MartianInvader Apr 30 at 0:03
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Prime Nebula killed a different Nebula who was from different timeline. That's why she didn't die herself.

The movie made it clear that Back to the Future's time travel rules are wrong. The meaning of "whatever happens in the past, the future remains the same" is that whatever you change in past would be pushed to different timeline.

Remember, Prime Nebula's past never got killed because she didn't jump to future back in 2014. Otherwise, Thanos would never had snapped half the population of the universe, leading to Avengers not inventing time machine, leading to not bringing back past Nebula. This is a paradox. New timeline concepts are used just to avoid such kind of paradoxes.

  • Yep, definitely makes more sense. The media gives us their impression of what "time travel" is to them, and I watched Endgame with that impression in my head. – Mat Cauthon May 6 at 11:26
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Surprisingly this is actually how time travel is hypothesized to function. The many worlds interpretation of time travel is fairly simple. I don't have movie quotes as it's been out for only a week but pretend that what I put below is the sorceress supreme talking to Banner about the timeline:

"If you take the time stone and don't return it, the result will be that our reality will not have the time stone and therefore calamity will follow." - sorceress supreme

This is what keyed me in, especially given the explanation for magic given by the sorceress to the idea that the time travel is not time travel at all. Not really. It's actually travel to parallel universes completely indistinguishable other than that they are offset by a certain amount of time - this eliminates all paradoxes and would make Back To the Future complete horse dung as Ant Man put it. Whether this is a finite collection over some range of time or an infinite continuum is completely unknowable.

"We harness energy... drawn from other dimensions of the Multiverse... to cast spells... to conjure shields... and weapons... to make magic." - Sorceress Supreme

There are holes here of course. The existence of Captain America means that he is a Captain America from another universe where the snap occurred. He likely stayed behind when traveling to put the tesseract back. Now this means that if the year was say.... 1964 (the year before Robert Downey Jr. was born who plays Iron Man) then if the Avengers were to travel 46 years into the future they will hit the lucky jackpot of finding one universe that is not identical - the one where Loki stole the tesseract after the Hulk punched a door and that door punched Iron Man. However, this is not that same timeline or universe. It's a different universe where 55 years in the past a different universe had a snap occur and went world-hopping to defeat their Thanos and the universe the MCU exists in happens to be the universe that a Captain America and an Iron Man borrowed a tesseract from - note that this being the case proves that their victory against Thanos was inevitable so long as the other Captain America stays out of the way. I suspect that the collection of universes is incredibly muddied now that they did that. As the sorceress supreme pointed out - not putting the stones back risks the creation of parallel timelines that branch off. In other words the more they do that the more time travel actually resembles travel to parallel worlds and becomes useless.

This also explains Nebula. Nebula from the past is just Nebula from a parallel universe. It's phrased as time travel, but it really isn't and never will be in the pop culture sense.

Does the next thing need a spoiler?

TL;DR For once movies do something accurate to science.

An extra note:

Once the movie comes out on dvd and I can get exact dates I might get some help somewhere and build a universe map according to this theory showing whether this continuum can actually make sense or not. The problem is that supposedly Thanos snap is an unaltered timeline and so a continuous range of universi should then go back and spawn a continuous range of Loki stealing the tesseract universes - assuming no time travel occurs this will for sure give a definitive periodic wave-like appearance to the set of timelines. I'm curious where this structure in any way prevents or contradicts its own rules.

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    You put it very nicely indeed: Nebula from the past is just Nebula from a parallel universe. It definitely makes much more sense after reading all your answers - this just isn't the same to what we believe is the "correct" version of "time travel". – Mat Cauthon May 6 at 11:22
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    I've also fixed your spoiler spacers, but it's alright if you remove the spoiler quotes as my question itself already screams "enter at your own risk". Your call of course :) – Mat Cauthon May 6 at 11:24
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    While it's a fine theory and interesting answer, I am no fan of this scenario because it means nothing matters. There's no motivation to create pivotal change in your world, because you can simply rest assured there are many - infinity - alternate realities where your preferred outcome did happen. All your blood, sweat and tears only changes that number to infinity plus one. – Harper May 6 at 21:38
  • @Harper no it does not. Every universe is identical so the only time that one differs is when time travel changes something. And something matters because they are all identical so your actions in actuality have an influence on infinite universes because otherwise you wouldn't act in the other universes for that same reason. – The Great Duck May 6 at 23:28
  • @MatCauthon thank you. I prefer the blocks just so someone doesn't accidentally get hit with spoilers. – The Great Duck May 6 at 23:29

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