Random Time Travel Works In 'Avengers: Endgame' (11 items)

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  (#1) In The MCU, Changing Things In The Past Doesn't Change The Future

  Most of us, including Scott Lang and Rhodey, get our time travel knowledge from movies like Back to the Future and Timecop. In those movies, if you change something in the past, you also change the present. For instance, in Back to the Future, Marty McFly helps his dad be less of a dweeb in the 1950s. When Marty returns to the '80s, his family is living a nice upper-class life.

  That's not how time travel works in the MCU. When Rhodey suggests offing Thanos as a child in order to undo the snap, Bruce Banner says, "I don’t know why everyone believes that, but that isn’t true. Think about it: 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!"
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  (#2) If Something Changes In The Past, A New Parallel Universe Forms

  When time-traveling in the MCU, the things you do in the past won't affect the present you return to. For instance, if Rhodey went into the past and punted a young Thanos into the closest sun, he wouldn't change anything about his present - the Snap still would have happened. However, his actions would create a parallel universe with no Thanos. 

  When Bruce Banner asks the Ancient One for a crack at the Time Stone, she explains that changing something in the past doesn't change the future. It does, however, create a universe where the timeline branches off. The actions of the Avengers in Endgame don't change their timeline, but they do create new parallel universes that begin at the moment of each alteration. 

  Think of it like this: If you go back in time and stop the hit on Kennedy, nothing in your original timeline changes; instead, there's simply a new timeline in which Kennedy survives his trip to Dallas. That universe will play out wildly different than your original version, which still exists sans JFK.
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  (#3) The Avengers Can Take The Infinity Stones To The Future As Long As They Return Them To The Past

  Many of the MCU's rules for time travel are a little contradictory, if not flat-out made up. Which is fine, by the way - the Russo brothers aren't advocating for the advancement of time travel technology; they're telling a story.

  In simple terms, the Infinity Stones stabilize time itself. The Ancient One offers some elucidation, telling Bruce Banner that removing an Infinity Stone from the timeline interrupts the flow of time. However, the forces of good need the Infinity Stones to fight back against superhuman creatures like Dormammu.

  If the Ancient One is to be believed - and she usually knows what she's talking about - removing the Infinity Stones from their point in the timeline not only makes that timeline more susceptible to the forces of evil, but creates a new reality. That's why Cap has to return the Stones to their original points on the timeline. 
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  (#4) Several Parallel Universes Potentially Exist Because Of 'Endgame'

  By going back and changing minor pieces of the MCU timeline, the Avengers have likely created a series of parallel universes. While they haven't done anything in their timeline - aside from saving the universe and offing Thanos twice - they have altered other timelines, for better or worse. 

  In their trip to 2012, the crew created at least two alternate realities. Loki is now alive and in possession of the Tesseract, and HYDRA either believes Cap is one of them, or she thinks they're going to jumpstart the events of Winter Soldier. Either way, 2012 is rendered completely different. It's also possible that Loki now exists in the current MCU timeline in spite of his supposed demise at the hands of Thanos in Infinity War. 

  There's no way to know whether these new realities will affect the current MCU timeline, but now that time travel is officially on the table, anything is possible.
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  (#5) The Thanos That Perishes At The End Of The Movie Is From A Parallel Universe

  Partway through Endgame, a major problem arises: Thanos shows up to claim the Infinity Stones and create a new world in his image. But technically, it's not the same Thanos who snapped half the galaxy out of existence; this is a Thanos from a divergent timeline. 

  This Thanos is presumably the same version from Infinity War. He likely realized it was possible to travel to the future when he learned that War Machine and a new version of Nebula were trying to grab the Power Stone. This realization affected his behavior and subsequent decisions, changing the timeline and creating an alternate version of Thanos.
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  (#6) Time Travel Relies On The Quantum Realm

  To time travel in the MCU, you have to use either the Time Stone or the Quantum Realm. Barring some divergent timeline in which everyone has a Time Stone, the Avengers have to shrink down and travel through the Quantum Realm to go from point to point. 

  Unfortunately you can't just hop into the Quantum Realm and end up at the Battle of New York; you have to use Tony Stark's "reverse mobius strip" map of the QR, which spits you out at a specific point in time and allows you to return exactly when you left.
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  (#7) The MCU Timeline Is Secure

  With time travel now in the mix, it's reasonable to wonder if the entire MCU timeline has been irrevocably altered. Fortunately, that's not the case. All the moments you love from Phases 1 through 3 are preserved in amber. 

  Everything that happened in the past still occurred to make the characters who they are in Endgame. As Bruce Banner explains, their reality exists linearly, even if they aren't moving through time in a linear fashion. This means Thanos is always going to snap half the universe away, and the Avengers are always going to bring it back.
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  (#8) You Can Talk To Your Past Self Without Creating A Paradox

  One of the classic rules of time travel - largely popularized by Back to the Future - is that you shouldn't talk to an earlier version of yourself. This paradox can have dire consequences. In the MCU? Not so much. In Endgame, characters not only have conversations with their older selves, but in the case of both Captain America and Nebula, they also get into full-on brawls with their previous iterations. 

  Because Nebula and Cap aren't actually fighting previous versions of themselves, but rather alternate versions of their previous selves, no one disappears, or explodes, or does whatever else we've come to expect from a time travel paradox.
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  (#9) In The Quantum Realm, Time Can Travel Through You

  Characters can't just shrink into the Quantum Realm and start zipping through time. Without the proper means of maneuvering around the QR, you can end up trapped inside. As Scott Lang discovers, time travels through you rather than you traveling through time. That means you age at odd rates, growing older or younger each time you shrink down. 

  When Lang comes back as an old man, it doesn't mean he was stuck in the Quantum Realm for 50 years, it just means the Quantum Realm instantaneously aged him 50 years. Without a roadmap for how to traverse the realm and use it to your advantage, you won't be able to get anywhere.
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  (#10) Captain America Messes Everything Up

  Even though Endgame sets up a whole host of rules for time travel, Captain America breaks pretty much all of them at the end of the movie. After returning the Infinity Stones to their rightful places, he goes off to live his life - in the past. It's been theorized that his actions mean the Infinity Stones were still destroyed by Thanos, rendering time travel impossible. 

  Even if this isn't the case, Cap still may have altered the events of Winter Soldier. By marrying Peggy Carter, he creates a kind of time loop, and also removes her original husband from the equation. There are also questions about how Cap made it back to the Quantum platform as an old man. On top of that, shouldn't Cap be in an alternate universe, just as Thanos found himself in a new timeline when War Machine and Nebula traveled back to the events of Guardians of the Galaxy? 

  The existence of Old Man Rogers at the end throws the logic of the movie out the window - but maybe that's okay, because it gives the audience a moment of catharsis nearly a decade in the making.

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