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?”
Most moviegoers must have greeted that delivery like Steve Rogers, Scott Lang, and Natasha Romanoff did: with blank stares and silence. But at least one viewer found the line to his liking. “I’m honored to have been mentioned by Tony Stark,” says quantum computing pioneer David Deutsch, a visiting professor of physics at the Centre for Quantum Computation at Oxford University’s Clarendon Laboratory, and the author of The Fabric of Reality and The Beginning of Infinity. “Since I now exist in the canonical Marvel universe, I have now officially become a comic book character.”
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; as Symmetry Magazine put it in 2016, “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,’” Deutsch says.
Beyond disavowing the Deutsch Proposition, Deutsch declines to comment on the movie’s treatment of time travel, perhaps not wanting to Marshall McLuhan–in–Annie-Hall a fellow scientist as esteemed as Stark. Nor can we contact Max Planck, the creator of quantum theory, without inventing a time-travel machine of our own. But Endgame’s invocation of quantum mechanics seems ripe for peer review. Although the record-breaking box-office earner cleverly positions itself as a more scientifically accurate alternative to the typical time-travel movie, an expert in quantum computing confirms that its story makes the same sort of logical leaps.
Before we summon our scientist, a brief recap. At the end of Avengers: Infinity War, Thanos assembled all six Infinity Stones and snapped his fingers, eliminating half of all life in the universe (and putting any remaining Mets fans out of their misery). 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.
Time travel has a reputation. For one thing, it’s basic: We’ve seen it so often before. For another, it’s notorious for not making sense; most time-travel movies unravel upon the slightest inspection, revealing massive inconsistencies, and the closer they come to plausibility, the more inscrutable they seem. 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.”
Scott Aaronson knows how quantum physics works about as well as anyone. Aaronson, a former MIT faculty member, is a professor of computer science at the University of Texas at Austin, and the founding director of UT Austin’s Quantum Information Center. He specializes in quantum computing and computational complexity theory, conducts quantum physics research, and recently drew attention to a debunking of an exaggerated report about scientists “reversing time” with a quantum computer. For our purposes, he’s a real-life Stark without the vast fortune and fancy suit.
When I first approached Aaronson, he hadn’t yet seen any of Endgame, but previous crossovers between Hollywood films and his field have taught him to be wary. “The accuracy bar for invocations of quantum mechanics in 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 happen.”
The writers of Endgame wanted the good guys to take a mulligan without running afoul of the “grandfather paradox,” in which a time traveler’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.
“So Back to the Future’s a bunch of bullshit?” a disillusioned Lang asks.
This is an inspired piece of screenwriting sleight of hand. We know those other movies are bullshit, Marvel seems to be saying. This movie is different from those movies, so this movie must not be bullshit. But another possibility exists: They can both be bullshit. After viewing Endgame’s time-travel scenes, Aaronson concludes that’s the case, saying, “As I predicted/feared, ‘the quantum realm’ is invoked here basically just as a magical amulet to enable time travel.”
As Aaronson explains, “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, traveling close to the speed of light or flying by a black hole.
That doesn’t mean the moviemakers completely pulled their plot out of America’s ass. “What’s true is that there have been decades of speculation about whether the merger of quantum mechanics with general relativity, into a still-unknown theory of quantum gravity, might allow for time travel into the past,” Aaronson says, although he doesn’t believe it will turn out to be possible. This is where David Deutsch comes in. In 1991, Deutsch published a paper called “Quantum Mechanics Near Closed Timelike Lines,” which has served as the basis of speculation about time travel and explains why Stark says his surname.
“Deutsch’s famous 1991 paper modeled a closed timeline curve as a resource that forces nature somehow to find a quantum-mechanical ‘fixed point’ of whatever evolution acts inside the CTC,” Aaronson says. The resolution of the grandfather paradox in Deutsch’s account, he continues, “would be that you’re born with probability 1/2, therefore you kill your grandfather with probability 1/2, therefore you’re born with probability 1/2, and so on—everything is consistent.”
When it comes to closed timeline curves, the allusion to Deutsch is as deep as Endgame goes. “I didn’t see anything about the plot of the movie that made any contact with (Deutsch’s) account,” Aaronson says, “beyond that they’re both somehow about time travel (and also, the buzzwords ‘eigenvalue’ and ‘spectral decomp,’ which relate to finding fixed points).” Stark utters both of those terms while messing with a simulation in the shape of a Möbius strip, which may represent overlapping timelines. Although they’re real terms—aside from the fact that Aaronson has never heard “decomposition” shortened to “decomp”—he notes that “in the context where they’re used here, they might as well have just talked about flux capacitors.”
Endgame spends more time deconstructing other movies’ versions of time travel than it does illuminating its own, which makes it tough to pin down some details. Although Endgame suggests that replacing the Stones should have knitted the divergent realities back together, some characters’ actions still seem certain to have created new timelines, and an elderly Captain America’s reappearance in his original reality remains largely unexplained. It’s likely, though, that some of this uncertainty is intentional: Marvel comics make ample use of the Multiverse, and more timelines potentially means more material for future stories, some of which are slated for forthcoming streaming service Disney+. And even if Endgame at times plays fast and loose with its own time-travel rules, it doesn’t do so in a way that cheapens the movie’s emotional moments. Some deaths aren’t undoable, and success is far from assured; if Doctor Strange can be believed, the eventual outcome is one of only 14,000,605 possible futures in which the universe has a happy ending.
It’s no surprise that Endgame is scientifically dubious; we are, after all, discussing a superhero movie that features Infinity Stones and Pym Particles. Its time travel’s scientific validity 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. Although Aaronson snarks that he’d like to travel back in time to make the movie’s jargon more consistent with quantum mechanics, he doesn’t want to be a wet blanket. “It’s fine in such a case just to suspend disbelief and enjoy the movie,” he says. Ultimately, Marvel’s most impressive feat of temporal manipulation in Endgame may not be attempting a plausible depiction of time travel, but making ticket-buyers treat this latest installment in its ongoing series as a genuine ending at all.
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