Early screenings of Avengers: Endgame led to directors Anthony and Joe Russo adding additional scenes breaking down the MCU's approach to time travel in an effort to "explain it as simply as possible."
"We actually met with a few professors to talk about quantum physics, and — look, time travel's not real. It doesn't exist," Joe Russo said during a live Q&A hosted by Periscope.
"So you basically, in a genre film, you create rules around which your movie is going to work. Back to the Future is such a ubiquitous film, that those rules have been used in a lot of other facets of pop culture. And we underestimated how ingrained they are into people's brains, those rules, and when we wanted to take a quantum physics approach to the movie, it's an alternate sort of concept about how time travel works."
The Russos and screenwriters Christopher Markus and Stephen McFeely then had to retrain the audience's minds. One scene of particular difficulty came when 2023 Nebula (Karen Gillan) shoots and kills her past self, 2014 Nebula, without consequence to her present self.
"We do these very secret screenings of the films early on with folks who work at Disney, and we started getting a lot of feedback, 'With Nebula, [when she] kills herself, she would disappear.' And sort of all the traditional tropes of Back to the Future," Russo continued.
"We said, 'No, we kind of explained it with in two sentences right here.' Everybody's like, 'I'm not getting what you're talking about.' So we actually had to go back and create those scenes with Smart Hulk, Scott Lang, and Rhodey, where they're talking through [time travel]."
The scene sees Rhodey (Don Cheadle) suggest killing Thanos (Josh Brolin) as a baby — theoretically preventing his 2018 snap that obliterates half of all life in the universe — but Hulk (Mark Ruffalo) explains changing the past doesn't change the future.
When Rhodey cites Terminator, Star Trek, and Hot Tub Time Machine as movies depicting the rules of time travel, Hulk breaks it down further: "If you travel to the past, that past becomes your future. And your former present becomes the past, which now can't be changed by your new future."
"It's funny, when we have these conversations sometimes with an audience when the movie's over when we're doing these secret screenings, we'll take some of the stuff that we actually say and we'll just write it into the scene," Russo added.
"So the way that Hulk is arguing with Cheadle about time travel in movies, is the things that we will say in a room to each other or when we're having a conversation with people after the movie. So we just used that to try and explain it as simply as possible."
Markus and McFeely earlier revealed at San Diego Comic-Con Scott Lang's (Paul Rudd) line about Back to the Future being "bullsh-t" was a "direct quote" from a quantum physicist consulted on their approach to time travel.
This led to Hulk meeting with the Ancient One (Tilda Swinton) in 2012, who could best explain how the removal of the Infinity Stones from their respective points in the timeline would affect coming events.
"The whole Hulk explanation, Hot Tub Time Machine, all that stuff, that was in large part because we showed it to some test audiences and every time Nebula shot Old Nebula, everyone said, 'Why doesn't she disappear?' 'Because we told you, it doesn't work that way!'" McFeely said.
"But we had to really underline it, so we go out of our way to just exposit the hell out of it."
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