The wacky, high-octane sci-fi 2014 movie Edge of Tomorrow has it all: Tom Cruise, alien invasions, the U.S. military-industrial complex, and a shaky grasp of the concept of time.
But how realistic is its view of time travel? First, we need to dive into the movie a bit.
In a Groundhog Day scenario from hell, the battle-inexperienced Major Bill Cage (Tom Cruise) finds himself stuck in a time loop that reboots the same day again and again — but with aliens known as “Mimics” that are hellbent on destroying humanity. Each day, Cage dies on the battlefield between aliens and mankind, and each day he awakens in the same training camp surrounded by tough soldiers capable of deploying massive robotic “mech suits” to slaughter the invading alien army.
One of these soldiers is none other than Sergeant Rita Vrataski (Emily Blunt). With the help of particle physicist and biologist, Dr. Carter, Vrataski explains to Cage how he acquired his time loop skills from the very aliens they’re trying to kill.
Basically, an alien hive mind known as Omega is the brains behind this military invasion. Whenever another type of alien — known as Alpha — dies in battle, the Omega resets time, starting the invasion over again. When Alpha attacked Cage with a ghastly blue goo, he absorbed the same time rebooting abilities from Omega.
“How do I control it?” Cage asks.
“You have to die. Every day. Until the Omega’s destroyed,” Vrataski tells a dismayed Cage.
The rest of the movie shows how Cage uses the skills and knowledge he gleans during each reboot to eventually defeat the invading aliens.
By now, the time loop concept has become wildly familiar to Hollywood and has been featured in wide-ranging projects from Happy Death Day to Russian Doll. But at the time of Edge of Tomorrow’s release, the concept still felt pretty fresh. But this kind of time travel is more the domain of the screen than anything feasible in physics.
“The sort of loops that appear in Edge of Tomorrow are not physically possible, even given the most speculative physics,” Nikk Effingham, professor of philosophy at the University of Birmingham and an expert on the philosophy of time, tells Inverse.
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How Does Time Work?
As a movie that explores the concept of time loops, Edge of Tomorrow falls squarely in the time travel movie genre. So before we can explain the physics of time loops, we have to understand the differing perspectives on travel and, indeed, the nature of time itself. We like to think of time’s arrow proceeding in one direction — forward — but it’s not really that simple.
“Physics tells against the idea that time has a direction,” Effingham says.
John Friedman, physicist and professor emeritus at the University of Wisconsin-Milwaukee, tells Inverse that the “direction” of time we humans observe simply proceeds in a direction of increasing entropy — thermal energy or disorder — which has its origins in the “smooth initial conditions in the early Universe.”
The perception of time depends on where you stand. Effingham says it's possible to imagine that there are “time-reversed” people who are proceeding backward in time from our point of view while they’re traveling forward in time according to their perception. In other words: time is a finicky beast to wrap your head around.
“Edge of Tomorrow is sheer fantasy.”
Apart from physics, there are two general philosophical schools of thought on the nature of time: A-theory and B-theory. Both have differing takes on how we view the present, past, and future in relation to each other. A-theorists believe we can delineate events as taking place in past, present, or future, whereas B-theorists view temporal events in relation to each other.
“According to the A-theory, there are real facts about who is present, and who is past/future,” Kristie Miller, joint director of the Center for Time and philosophy professor at the University of Sydney, tells Inverse. “Likewise, the B-theorist thinks that all events exist, and that the present is just where you happen to be, and the past is just times earlier than that.”
Time travel in Edge of Tomorrow is logically possible whether you subscribe to an A-theory or B-theory of time — but the physics behind it is more debatable.
There are typically two different kinds of time travel stories we see in pop culture. In the first, you go back and change the past; this kind of movie violates what Seth Lloyd, a professor of mechanical engineering at MIT and co-founder of startup Turing, which devises time travel-inspired quantum computing applications to solve societal problems, calls the self-consistency principle. In the second kind of time travel story, you go back in time and your past actions are consistent with what we know happens in the future, so that kind of time travel logically works.
In Edge of Tomorrow, Cage is obviously trying to change the past, so this doesn’t really square with what we know about the physics of time travel.
“You can't actually go back into the past and then change it because you have to be consistent with what happened,” Lloyd says.
Is There A Scientific Basis For Time Loops?
Let’s get this out of the way first: the term “time loop” is a general fictional invention to describe the repetition of a segment of time and is not technically a scientific concept.
“We should just make a distinction between what we know about the physics of time travel in quantum mechanics and then just, you know, the notion of a loop in time,” Lloyd says.
It’s true that, on their surface, time loops seem to superficially resemble a physics concept known as a closed timelike curve.
“So when you say time loop, the thing that comes to mind is a closed timelike curve,” Nicole Yunger Halpern, author of Quantum Steampunk: The Physics of Yesterday's Tomorrow, tells Inverse.
“The entire Universe might form a time loop if it rotated in the correct way.”
Closed timelike curves are effectively trajectories through space-time that depart from a point in time and return to that same point in time through a closed loop via a mechanism like a wormhole. Yunger Halpern says closed timelike curves are compatible with Einstein’s theory of general relativity, which delineates the physical rules around space-time and gravity. It’s possible the creators of Edge of Tomorrow were inspired by the concept of closed timelike curves and tried to mimic a proposed physics concept with a fictional time loop conceit.
Lloyd suggests imagining entering the handle of a coffee mug and exiting through the same handle back to an earlier point in time. In theory, closed timelike curves would let you return to an earlier point in time and eventually wind back in the future — such as in other time travel movies like Looper. (Lloyd’s research group also simulated one kind of model of closed timelike curves.)
“There are some physicists who thought that certain kinds of time loops might be possible. For instance, Kurt Gödel suggested that the entire Universe might form a time loop if it rotated in the correct way,” Effingham says
Is Edge of Tomorrow Realistic — At All?
While Lloyd thinks time loops serve as fun narrative devices, he’s not sure about how they fit into our current understanding of physics.
“We don't know how to make them consistent with the laws of physics,” Lloyd says, adding, “the physics version of it doesn't really allow you to reset time.”
Lloyd says it doesn’t make sense that you would have different memories every time you go through the loop — as in Edge of Tomorrow. For her part, Yunger Halpern would logically expect the dead body of Tom Cruise’s character, Cage, to end up in the past — not a living version of him.
“The problem there is that if you die at the end of the loop, then you're not available to go through the loop again. So that's my sticking point,” Lloyd adds.
“It is not known whether the laws of physics allow time loops (closed timelike curves) on very small scales,” Friedman says. He says that incredibly tiny loops smaller than a proton — lasting 10^-43 seconds or with size 10^-33 centimeters — could theoretically be possible, but it’s unknown. He explains that macroscopic time loops to transport a human require negative energy — a difficult feat to achieve.
“I think its highly unlikely that there are macroscopic time loops,” Friedman adds.
Other experts are even more unequivocal.
“In the same way physics doesn’t allow for mind control and possession — even outside of time travel scenarios! — Edge of Tomorrow is sheer fantasy,” Effingham says.
Effingham says physics typically speculates about whether physical bodies can go back in time, but in Edge of Tomorrow, Cage is “psychically” time traveling and his future consciousness takes over his past body — a concept far beyond the bounds of physics.
Samuel Kuypers, a postdoctoral researcher in theoretical physics at the USI (Università della Svizzera italiana) who is researching a model of time travel, tells Inverse the only way Edge of Tomorrow makes sense from a physics perspective is if it’s “some kind of time travel where maybe his consciousness is the thing traveling back in time — maybe it's just pure data that's sent back on the closed timelike curve.”
So, if the aliens had some kind of superior technology that allowed them to send Cage’s memories back on a closed timelike curve, the movie can clear at least one logical hurdle. But Kuypers has an even more interesting theory: Edge of Tomorrow isn’t actually a triumphant action movie where the good guys save the day — at least not in every universe.
According to physicist David Deutsch’s model of closed timelike curves, we can avoid logical problems with time travel — like the grandfather paradox — by assuming there are parallel timelines, which physicists refer to as Everett’s “many worlds theory.”
“Time travel can only be made sense of because you travel to another timeline,” Kuypers says.
Applying this logic to Edge of Tomorrow, it naturally follows that Cage only succeeds to save the world from aliens in one timeline. In every other timeline, he dies in vain and humanity gets obliterated by blue-goo-dumping alien beings.
“My main observation about the movie is that we might have watched a tragedy without knowing that’s what we watched,” Kuypers says.
But whether you see Edge of Tomorrow as a tragic attempt to defeat aliens across time and space or a heroic Hollywood victory, one thing’s undeniable: It’s one hell of an entertaining reboot of the time loop genre.
