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A quantum physicist told the Avengers: Endgame writers time travel depicted in Back to the Future is “bullsh-t,” a word of advice ultimately incorporated into a line of dialogue uttered by Ant-Man (Paul Rudd).

“’Back to the Future is bullsh-t,’” co-writer Stephen McFeely said at San Diego Comic-Con when asked what the Avengers scribes learned from a consultation with an expert. Co-writer Christopher Markus added it was a “direct quote from a physicist.”

“What other franchise could do what we were allowed to do? Go back to the other movies to get your six MacGuffins and bring them forward to solve your problem, and allow yourself to do all sorts of emotional triage as you do it — people get to meet their fathers and mothers and whatnot,” McFeely said.

“But if you operated by Back to the Future rules, every time you went back and came back, you would have a new Biff’s Casino,” McFeely said of Back to the Future Part II, where Marty McFly’s time travel hijinks inadvertently altered his present day reality for the worse.

“So you can imagine that if every time a new universe was created, we would get nowhere, and we wouldn’t know how to solve any one, because which one did what? It was not helpful to us and we were worried that maybe we couldn’t even do this idea.”

Markus and McFeely then consulted with the quantum physicist who helped them realize Rudd’s Scott Lang, and his familiarity with the Quantum Realm, was the key to Endgame’s time travel.

“[The physicist said], ’It probably doesn’t work that way, as much as I love those movies, it probably works this way. If it works, again, which it doesn’t,’” McFeely said.

“So the idea that a branch reality is a quantum travel — we got Scott Lang, maybe we can do something with this. There’s a lot of light bulbs going off when we did some homework.”

For explaining the Marvel Cinematic Universe’s rules of time travel to audiences, Markus and McFeely zeroed in on movie references and a scene of exposition in which 2012’s Ancient One (Tilda Swinton) demonstrates to Hulk (Mark Ruffalo) what happens when the Infinity Stones are removed from their respective places in the timeline.

“We had to boil our time rules down to something, and it came down to the stones. The stones are going to hold the universe in place, effectively. Take them out, things start happening,” Markus said.

“This is the pleasure of having this universe to deal with, we needed someone to explain it. ‘Who’s the most entertaining explainer in the Marvel Universe? Let’s get Tilda! She’s not dead back then, we can get her.’”

Touching back on Back to the Future, McFeely said, “It’s Doc Brown on the chalkboard, we called it that a lot. Turns out we had to do that a number of times.”

“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 [from 2014], everyone said, ‘Why doesn’t she disappear?’ ‘Because we told you, it doesn’t work that way!’” McFeely continued.

“But we had to really underline it, so we go out of our way to just exposit the hell out of it.”

Avengers: Endgame is available to own on Digital HD July 30 and on 4K Ultra HD and Blu-ray August 13.