Warning: This story contains spoilers for Avengers: Endgame.
Most people walking out of Avengers: Endgame, after they checked themselves after all that emotional blubbering, had two questions.
First, who was that random kid at Tony Stark's funeral? Easily answered.
The second one was much harder to grapple with. How did all the time travelling work, and how did Captain America end up on that bench as a much, much older man?
Now, a week after the $US1.48 billion (and counting) blockbuster dropped, the filmmakers have finally spoken about how it works.
Avengers: Endgame has broken box office records, reaching US$1.6 billion in just eight days. Photo / supplied
Thanks to a pair of interviews – directors Joe and Anthony Russo with EW and screenwriters Christopher Markus and Stephen McFeely with The New York Times – we now have a much better idea of how all the wibbly wobbly, timey wimey stuff works in the internal logic of the movie.
Essentially, Avengers: Endgame sets up its own time travelling rules – that changing the past doesn't actually change the past, only your future.
It's what Hulk was saying during the conversation about why killing baby Thanos wouldn't prevent the "snapture".
In Endgame, your experience of time is a continuum. If the past is point A and your present is point B, time travelling to the past doesn't mean you go back to point A. Instead, it's more like you're creating a point C because your past is now your present. So you can only affect point C or point D, your future.
Then, the conversation between Hulk and The Ancient One establishes that if you change something in the "past", it doesn't alter anything in your timeline between the change (ie. 2012) and your origin time point (ie. 2023).
"If (Captain America) were to go back into the past and live there, he would create a branched reality," said Joe Russo. Photo / supplied
What it does is create a separate timeline, a branch reality.
This is key, because the Russo brothers are now saying that's what Captain America did when he went back in time to live out his life with Peggy Carter. That life wasn't in the main Marvel Cinematic Universe timeline.
"If Cap were to go back into the past and live there, he would create a branched reality," Joe Russo told EW. "The question then becomes, how is he back in this reality to give the shield away."
Even for fans who had worked out the time travelling mechanics, Cap's presence at the end as an old man was a major plot hole. If he's an old man in the same space as Sam and Bucky, then how could he be in a different reality?
The implication from the Russos is he used his remaining Pym Particle to jump through not just time but also across dimensions to be in that spot at that moment, rather than having lived his life until then in the same reality.
Which answers all those questions about Cap going back in time and somehow not affecting everything that's already happened, such as Hydra filtrating SHIELD or Bucky being under the control of Hydra for decades, or, say, an alien invasion of New York.
In his new reality, maybe he stopped all those things.
Directors Anthony and Joe Russo said Captain America would have used his last Pym particle to return to the present. Photo / supplied
The Russos also confirmed that Bucky knew Cap's plan, given he said "I'll miss you" as Cap was stepping onto the quantum realm portal.
It also means there's another branch reality out there where Thanos doesn't exist after 2014, the opening moments of The Guardians of the Galaxy.
In comics, multiverses are a common storytelling device and if the MCU is going there, as it seems to be, it would explain why Elisabeth Olsen hinted the Scarlet Witch and Vision TV spin-off might have scenes set in the 1950s.
In the New York Times interview, writers McFeely and Markus revealed that it was always the plan for Tony Stark to die and for Captain America to finally get his dance with Peggy.
They also said early breakdowns of the story saw the heroes time travel to Asgard at the moment when the reality stone/Aether and space stone/tesseract were both there, and that Tony Stark would be in an invisibility suit but would end up fighting Heimdall who would still be able to see him because of his special powers.
