A physicist explains the science of Avengers: Endgame

Tony Stark and Bruce Banner are two of the finest minds in the Marvel Cinematic Universe — but even they get help from real-life scientists when it comes to figuring out mind-bending concepts like navigating the Quantum Realm and pulling off a “time heist” in Avengers: Endgame.

Dr. Clifford Johnson is one such scientist. A professor of physics at the University of Southern California, Johnson has advised on numerous Marvel projects, including Endgame, Avengers: Infinity War, Thor: Ragnarok, and Agent Carter. And he’s here to tell us why, in the words of Paul Rudd’s Ant-Man, “Back to the Future is bullsh—.”

Okay, not quite. Johnson is quick to say Back to the Future is “fun.” But when it comes to Endgame’s version of time travel, he explains that it draws on various theories, including string theory, to arrive at a version that differs from other pop culture takes. “What [the filmmakers] wanted to avoid was the obvious paradox of you go back and you change something, and then you see things become,” Johnson says. “They were just going for a different tone in the film.”

Johnson says his role and that of other advising scientists is to make the imagined science at a film’s core feel believable. For Endgame, that was where the notion of multiverses and the Infinity Stones creating branched timelines arose.

“My job is to help them tell the best story and give them the means to do so in a way that keeps you in the movie,” he says. “There are scenarios in various theories that try to describe the universe — and one of the things we discovered is that multiple universes come out of the equations. People have struggled with the idea as to whether that’s maybe telling us that’s a real thing. There are these multiple universes, but they’re sort of physical things right alongside each other, so you might imagine you could go from one to another. That’s all us using our imagination; there’s no evidence that’s true.”

If it was true, the version of time travel in Endgame is a more likely explanation for how the process might work — at least in terms of the rules. When the Avengers go back in time to recover the Infinity Stones, they discover that their past becomes their future and their present their past. In less circular terms, it means their lives continue in a linear, causal fashion no matter where they pop in on the timeline. This is how Tony (Robert Downey Jr.) can meet his father and resolve some of his daddy issues, and Captain America (Chris Evans) can return to Peggy (Hayley Atwell). Science seems to back this up in theory, according to Johnson.

“If we ever discover that time travel is a real thing — and it’s a big ‘if’ because there are all sorts of reasons why on paper it doesn’t look like it would work — my suspicion is that what the rules will be is that from an individual person’s experiences, the narrative will make sense,” he says.

According to Johnson, it’s possible using current theories to build equations that resemble time machines, but they typically neglect to include quantum physics (there’s that Q-word again). “These time machines can lead you to silly paradoxes,” he explains. “It would seem those machines can’t work because you create nonsensical things. How does the universe protect itself from some of that crazy stuff that those equations are suggesting?”

He adds, “If time travel ever turns out to be possible at all from whatever perspective, it will be in such a way that all the rules of the universe, including quantum physics, will constantly conspire as to not make you create unresolvable paradoxes. The universe will protect itself from nonsense. Because, for whatever reason, perhaps the most enduring rule of the universe is that it is comprehensible.”

For Johnson, he’s less concerned with getting precise, accurate science into a fictional film and more interested in showing the day-to-day work of scientists represented on screen — something he says Endgame does well.

“I always tell them Marvel has this long tradition of so many of these people being scientists, and so, let me help you at least make them sound believable and show them thinking things through, trying things out, sometimes failing — what happens in real science,” Johnson says. “It’s much more important to show that scientific process being done right than the actual science itself being correct to the letter.”

In Endgame, Tony, Bruce (Mark Ruffalo), and their teammates go through plenty of trial and error trying to wrap their heads around time travel. “They’re doing experiments, they’re getting it wrong, they’re trying to perfect it and eventually [they do],” Johnson says. “Normally with these kinds of things in movies, it’s just, ‘We need to do a bit of science magic here.’ There’s no process. The process was on display, and that was fun to see.”

Advising on films can range from a brief phone call where Johnson goes over some buzzwords to months of strictly confidential meetings where he can more directly offer concepts to inform storytelling and even help design tech and more. There’s a Science and Entertainment Exchange with a hotline for filmmakers to get in touch with scientists for this purpose.

Johnson adds that he wants audiences and storytellers alike to remember it’s not about fact-checking. “That’s the least interesting and useful part of the job,” he says. “Our job is to help the storytelling. The primary job isn’t to lecture an audience on science. It’s nice if we get an opportunity to excite people about science and show things like the scientific process — but our job is to help tell a good story.”

Presenting science as it is in Endgame is something of a personal mission of Johnson’s. “We spend so much time consuming entertainment. It seems a missed opportunity to not help sprinkle some of that in there. I like the whole business of putting science where people are not expecting it,” he muses. “It shows that science belongs everywhere, not just in the corner where it’s been put [as] that thing only geeks are interested in. It belongs to all of us. It’s part of our culture.”

Because of Endgame and advisors like Johnson, maybe we can all be a little closer to Black Widow’s quip about knowing quantum physics just “for conversation.” It’s certainly more likely than developing superpowers.

Related content:

• Avengers: Endgame explained: The logic behind the heroes’ master plan
• Avengers: Endgame directors answer Captain America mystery
• Avengers: Endgame writers on crafting the end of an era for the MCU