The Marvel Cinematic Universe really brought the term "Quantum Realm" to popularity. But How much were the movies true about the real quantum realm? Can we actually travel through it? Does time actually move slow there?


Movies like Ant-Man have suddenly shot this word into fame although the meaning used in the movie isn't exactly the same used by physicists. The Quantum realm is any universe where the laws of quantum mechanics are valid. This dictates that any system can be represented by a density matrix in a Hilbert space and that observables can be associated with hermitian operators all of which follow the born probability rule. This also dictates that the evolution needs to be a cptp map.

Too much scientific mumbo-jumbo? Well to put it simply, the quantum realm is a universe where, in general, everything is probabilistic. There's always a finite probability for the occurrence of a particular result in an event that may or may not be 0 or 1. This implies that one's position is probabilistic as well as momentum and any other quantity one can think of.

The Quantum realm is really nothing theoretical( it is but it can be applied in real life). It's the universe we live in. We generally don't observe these effects since in real life they're averaged out. These effects become more noticeable when we look into very small objects like atoms or electrons. Nevertheless, several macroscopic quantum effects like superconductivity and superfluidity also exist. Hence the entire universe we live in is the quantum realm.


So what we know right now as laws of physics are the laws of classical physics, which make no sense in the quantum realm/quantum physicist. Put a particle in a box. According to classical physics (and common sense), that particle should stay in that box forever. But under quantum mechanics, that particle can simply be outside the box the next time you look. In classical thinking, you can measure the momentum and position of something to an arbitrary degree of precision. Not so in the quantum world — the more you know about one, the less you know about the other. Is something a wave or a particle? According to the classical viewpoint, you can pick one and only one. But ask your friendly neighbourhood quantum mechanic, and they'll tell you something can be bothThe quantum world is hard to understand, but at some point, the rules of the subatomic give way to the rules of the macroscopic. Quantum mechanics is considered a theory of the microworld. The philosophical notions about quantum mechanics held by many philosophers and theoretical physicists are incompatible with the actual practice of the working scientist.


The next time someone compiles the most memorable Tony Stark quotes from the Marvel Cinematic Universe, this mouthful from Avengers: Endgame about the plausibility of time travel probably won’t make the list: “Quantum fluctuation messes with the Planck scale, which then triggers the Deutsch Proposition. Can we agree on that?”

Of course, comic books are not known for their scientific accuracy, and neither are the blockbusters they’re based on. The Planck part of the line, at least, does refer to real science. “The Planck scale is the universal limit, beyond which the currently known laws of physics break.” The rest, regrettably but perhaps predictably, is technobabble, as Deutsch himself concedes. “There’s nothing in my work that is generally known as the ‘Deutsch Proposition,’”.

Endgame was always bound to be about undoing that apocalypse. The question was what mechanism the moviemakers would use to explain away the Snapture without invalidating the events of the previous movie. Naturally, they picked time travel.

The genius of Endgame is that it’s in on the joke. When Lang (a.k.a. Ant-Man) emerges from the quantum realm five years after Infinity War—which to him felt like five hours—he suggests that the quantum realm could be the key to restoring all the life that was lost. “What if there was a way that we could enter the quantum realm at a certain point in time but then exit the quantum realm at another point in time?” he asks. “Like … like before Thanos.”

“Wait, are you talking about a time machine?” Rogers (a.k.a. Captain America) replies.

“No, no, of course not,” Lang protests. “No, not a time machine. This is more like, um … yeah, like a time machine. I know, it’s crazy. It’s crazy. But I can’t stop thinking about it.”

In a later scene, Lang lays out what he describes as the “rules of time travel,” which include “no talking to our past selves” and “no betting on sporting events.” If the Avengers follow those guidelines, he says, they should be able to pull off their “time heist.”

“I’m gonna stop you right there, Scott,” Stark responds. “Are you seriously telling me that your plan to save the universe is based on Back to the Future?”

Stark calls the concept “laughable” and “a pipe dream,” concluding, “That’s not how quantum physics works.”

“The accuracy bar for invocations of quantum mechanics in the film is unbelievably low, even by the usual standards of science in film,” Aaronson said via email. “In movies, I’ve seen, ‘quantum’ (and ‘entanglement’ and so on) just function as voodoo-words for whatever the writers want to have happened.”

The writers of Endgame wanted the good guys to take a mulligan without running afoul of the “grandfather paradox,” in which a time traveller’s actions in the past change the future in a way that would prevent the time travel from taking place in the first place. When Lang, James Rhodes (War Machine), and Clint Barton (Hawkeye) propose going back in time to kill Thanos as a baby or steal the Stones before Thanos gets them, they cite “basically any movie that deals with time travel” in support of their plan, specifically listing Star Trek, Terminator, Timecop, Time After Time, Quantum Leap, A Wrinkle in Time, Somewhere in Time, Hot Tub Time Machine, and Bill & Ted’s Excellent Adventure.

According to the constructs governing most of those movies, killing baby Thanos would prevent adult Thanos from ever existing, which in turn would remove any need for the Avengers to go back in time to kill baby Thanos. That’s when the viewer’s brain starts to smoke. But Endgame neatly sidesteps this pitfall. “Time doesn’t work that way,” Bruce Banner (the Hulk) declares, after explaining the paradox. “Changing the past doesn’t change the future.”

The film’s philosophy falls more along the lines of the many-worlds interpretation of quantum mechanics. In the present, post-snap Thanos has destroyed the Stones, preventing the Avengers from reassembling them and somehow reversing the snap. But if they enter the quantum realm, travel to previous places and times, abscond with the Stones, and bring them back to their own place and time, they can coordinate a snap of their own and save everyone. As the movie explains it, there’s only one catch: Each time they borrow a Stone, they’ll split off a branching reality in which the Stone would be missing, with potentially catastrophic effects for that newly created timeline. So once they save their own world, they have to return to the instants at which they made away with the Stones and put them back in their proper places, preventing reality from splintering.

“Quantum mechanics, as it’s been conventionally understood for the past 93 years, in no way whatsoever involves time travel into the past. Indeed, it’s sometimes been remarked that, for all of quantum mechanics’ revolutionary implications, time is ironically one of the few concepts that it didn’t change at all! In standard quantum mechanics (that is, without relativity), time is just a continuous parameter that flows at the same rate everywhere in the universe—exactly like it was for Isaac Newton.” Special relativity and general relativity, not quantum mechanics, were the developments in 20th-century physics that established that time might be mutable. The notion that five hours in the quantum realm might take five years in normal reality, Aaronson says, is “particularly ridiculous” given that time dilation stems from, for instance, travelling close to the speed of light or flying by a black hole.


It’s no surprise that Endgame is scientifically dubious; we are, after all, discussing a superhero movie that features Infinity Stones and Pym Particles. The scientific validity of its time travel almost wouldn’t be worth assessing if not for the fact that the script calls so much attention to how it handles a thorny staple of sci-fi storytelling. Although Endgame doesn’t exactly solve time travel—which would be quite a coup, considering actual scientists are still struggling with that one themselves—it does, at least, do its hand-waving in a novel and tongue-in-cheek way.

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