From Back to the Future to Terminator, time travel has been a theme Hollywood can’t get enough of–and one that they can hardly get right. Given the theoretical nature of quantum mechanics, there’s a lot of creative latitude filmmakers have taken with the idea of time travel. After all, if you can’t prove the plot is wrong, then it must be right, right? Not quite.
There are indeed mathematical equations and tests performed in particle accelerators that point to time travel being a possibility–so why don’t more filmmakers aim a little closer to scientific accuracy? Apparently, Avengers: Endgame directors Anthony Russo and Joseph Russo had the same question.
WARNING: MINOR SPOILERS AHEAD.
One of the key plot points in the final installment of the Avengers franchise revolves around the idea of time traveling. Fresh off of the destruction Thanos caused by snapping away half of the universe with his Infinity Stone-encrusted glove, Scott Lang (aka Ant-Man), hatches an idea to build a time machine, get the Infinity Stones from the past, bring them to the present, and snap everyone back into existence. While testing the machine that, of course, Tony Stark winds up constructing, a debate about time travel arises among the Avengers. James Rhodes (aka War Machine) suggests they just go back in time and kill baby Thanos. Challenged on his idea, he rattles off a list of time-traveling films and TV shows that he believes support his theory, including Star Trek, Terminator, Time Cop, Time After Time, Quantum Leap, A Wrinkle in Time, Somewhere in Time, Hot Tub Time Machine, and Bill and Ted’s Excellent Adventure. And it’s Bruce Banner who, in so many words, sets everyone straight with the Grandfather Paradox.
The Grandfather Paradox is a popular paradox in quantum mechanics that posits the idea that if you go back in time and kill your grandfather, you would be getting rid of your parents and ultimately you. The paradox is: how are you able to essentially kill yourself in the past yet exist in the future with your time machine? Or as Banner puts it: “If you travel to the past, that past becomes your future and your former present becomes the past which can’t now be changed by your new future.”
To which Lang replies, almost crestfallen, “So Back to the Future‘s a bunch of bullshit?”
Absolutely it is, according to quantum physicists Pieter Kok and Matthew Szydagis.
While both acknowledged their love of the movie, they’re equally quick to point out how it breaks the laws of physics (i.e., the McFly family slowly disappearing from the family portrait Marty took with him to the past).
“At every point along a time loop, the laws of physics should just be valid,” says Kok, a professor at the University of Sheffield. “There should not be any strange phenomenon. In the photograph, he sees his brother and sister fade–that’s not something that is physically realistic because it’s a photograph. There are pigments there–why would it disappear?”
What’s presented in Endgame is the Many Worlds Theory, which suggests that instead of changing something in the past that would alter the present, a change in the past would create a whole new universe. So the game plan for the Avengers is to return the Infinity Stones to their exact locations once they use them to fix their present with the intention of not spinning out a spiderweb of multiverses. The concept is actually one that famed British physicist David Deutsch used to possibly explain the Grandfather Paradox.
“There is no paradox if you killed your grandfather but that isn’t your grandfather, because it’s an exact copy of him in a parallel universe–that’s a classic solution to the problem,” says Szydagis, a professor at the University of Albany. “Another solution to the problem is that you can’t change the past but fulfill it. But that’s not very intellectually satisfying.”
What Szydagis does find intellectually satisfying, and what’s mentioned briefly in Endgame as well, involves the Möbius strip.
As Tony Stark is trying to figure out a GPS of sorts for time travel, there’s a holographic diagram that pops up in the shape of a Möbius strip, a surface in mathematics with one continuous side formed by joining two rectangular strips and twisting one end 180 degrees. “What makes this seemingly ordinary construct so fascinating,” states a diagram from Harvey Mudd College Department of Mathematics, “is that, while the original strip of paper clearly had two sides, the Möbius strip seems to have only one.” In some schools of thought, the Möbius strip has come to help explain space time. What it represents in time travel would be a scenario when you go back in time, two timelines would exist. But instead of running parallel to each other, they would exist simultaneously.
“I’ve seen, to the best of my knowledge, only one movie that does this and that’s Deja Vu with Denzel Washington,” Szydagis says. “The main character played by Denzel gets a phone call from a woman but she died several hours earlier. She’s both dead and alive–there are two timelines existing simultaneously on top of one universe.”
However, for the all the accuracy Endgame has in its jargon and understanding of the Grandfather Paradox, there’s still a fatal flaw in the film’s logic: You can’t travel in time farther back than when the time machine was created.
“With Einstein’s theory of relativity, a one-way ticket to the future is possible in multiple different ways. The problem with time travel to the past is that, as far as we can tell, it requires physical negative energy, which we’re not even sure exists,” Szydagis says. “We don’t even have a theory that predicts something like that that would have an anti-gravitational effect. It’s the key ingredient in any way we can come up with to building a real-life time machine.”
Kok also cites that a time machine would require an amount of energy that would essentially be equivalent to a wormhole on Earth (i.e., just not possible). But that restriction can be explained away by vast advancements in technology and funding in a film’s plot (we are talking about Tony Stark after all). But the fact that a time machine technically can’t transport you back to prehistoric times or even just a few years ago is something that pretty much invalidates 99% of time-traveling movies.
That said, all of the above is just speculative. Endgame may have fallen for the biggest no-no in time traveling there is, but the fact that the film was aware enough of all the time-traveling misfires before it is appreciated.
“Oftentimes travel is used as a bit of a cop-out. You have to do it really, really well not to make it a bit of a deus ex machina plot device,” Kok says. “But if the story is good enough, you don’t really care about plot holes.”