Time travel is a huge component of Avengers: Endgame. In order to bring back those lost in Infinity War's Snap, the heroes have to make their way back to specific points in time to collect Infinity Stones, assemble their own gauntlet, and undo the damage. However, since time travel isn't exactly real, explaining how the time travel element of Endgame would work in-film is something that had to be addressed and it turns out, the way Endgame explains things is backed up by actual science -- and scientists.
On the commentary track of Avengers: Endgame, directors Joe and Anthony Russo along with co-writers Christopher Markus and Stephen McFeely break down a lot of little secrets and interesting tidbits about the Marvel Cinematic Universe epic and that includes revealing that when Hulk explains time travel in the film, it's the same explanation that quantum physicists gave the writers room, according to Markus.
"Speaking of adult in the room, his [Hulk's] explanation is essentially what we were told by genuine quantum physicist brought into the room to explain time travel to us," Markus said.
McFeely gave a little more detailed explanation of the wisdom gleaned from an actual physicist during the writers' San Diego Comic-Con presentation in July.
"'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 anyone, 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."
Upcoming Marvel Studios projects include Black Widow on May 1, 2020, The Falcon and The Winter Soldier in Fall of 2020, The Eternals on November 6, 2020, Shang-Chi and the Legend of the Ten Rings on February 12, 2021, WandaVision in Spring 2021, Loki in Spring 2021, Doctor Strange in the Multiverse of Madness on May 7, 2021, What If…? In Summer 2021, Hawkeye in Fall 2021, and Thor: Love and Thunder on November 5, 2021.