Is Back to the Future a bunch of BS, like Avengers: Endgame suggests?

In 2008, no one was sure if Iron Man, the first of Marvel Studios' live-action films, would work. Eleven years on, it seems an almost foregone conclusion. The Marvel Cinematic Universe has become such an integral part of the pop culture conversation it's difficult to imagine a time before it existed.

Traveling back, in our minds, to a time before that film feels nearly impossible. Almost as if the way back has been closed to us, locked behind some fixed point in the fabric of reality. Many of us, however, spent the weeks leading up to the release of Avengers: Endgame revisiting some or all of the MCU catalog. Watching old movies is a form of time travel, allowing us to relive the moments when we first saw them and remember how they made us feel.

**Spoiler Alert: There are spoilers for Avengers: Endgame below**

It's appropriate, then, that the concept of mutable time should become such a necessary notion in the Marvel cinematic narrative. First introduced in Doctor Strange (2016), time travel, at least within a fixed bubble, became a workable way to thwart the plans of a villain and restore order to the world. In last year's Avengers: Infinity War, Doctor Strange views more than fourteen million possible futures, seeking out any way to stop Thanos from accomplishing his goal of wiping out half of all life. Of those, only one was successful.

The idea comes full circle in Avengers: Endgame. The story begins shortly after the events of Infinity War. The world is ravaged. Those who have survived the vanishing are broken, but attempting to pick up the pieces of their shattered lives. There is, it seems, no reversing what Thanos has done. That is until Ant-Man resurfaces after five years in the quantum realm, thanks in no small part to a rattus ex machina. He's surprised to discover the world as it is and, after returning home to see his now-grown daughter, even more surprised to find that while only five hours passed for him in the quantum realm, five years have passed on Earth.

The consequences of the decimation fresh on his mind, Scott Lang makes his way to what remains of the Avengers headquarters to make the situation right the only way he knows how: quantum time travel. What follows is a hilarious sequence between Lang, Bruce Banner, and Rhodey regarding the reality of time travel as it exists in the MCU.

Rhodey (War Machine) asks the question on everyone's mind: why can't they just go back in time, find baby Thanos, and take care of the problem (complete with a cringe-worthy pantomime of infanticidal strangulation) before it arises? This solution is consistent with just about every time travel movie any of us have ever seen, but that's not how time travel works in the MCU, as Banner is quick to point out.

Banner, living now as a best-of-both-worlds hybridization of his human and Hulk forms, explains you can't change the present by changing the past. "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."

In just one sequence, Endgame succeeds in showing us that this won't be the time travel you know. The rules are different. The goal is not to prevent the snap from happening in the first place, but to retrieve the stones and undo what's been done, now, in the present. Because it has been done and there's no changing that. Lang, acting as the viewer's window into the world, says, "So Back to the Future's a bunch of bulls***?" It's a good question, one we intend to answer.

Is Back to the future a bunch of bullshit?

It's difficult to conjure a more iconic time travel story. If asked to imagine a time machine it's likely you think of one of three devices, the titular time machine of H.G. Wells, the Tardis, or Doc Brown's DeLorean. The doomed 1981 gull-winged sports car achieved a sort of immortality when it transported us to 1955, then to the future, and finally to the wild west.

It taught a generation about the grandfather paradox, a thought experiment which posits the consequences of traveling back in time and changing events such that you no longer exist. It set up the rules by which scores of genre stories have since been told. And it is, as Banner suggests one-hundred-percent grade-A bulls**t.

There are a number of problems with time travel as it's presented in Back to the Future, without even getting into the weird mother-son romantic endeavor. The trouble begins with the initial premise of the film, which is a pretty damning place for problems to start.

While the notion of time travel is not precluded by the Einsteinian model of physics, there does seem to be some basic protections against muddling with causality.

In a 1992 paper, Stephen Hawking said, "It seems that there is a Chronology Protection Agency which prevents the appearance of closed timelike curves and so makes the universe safe for historians." This has been borne out in experimentation.

In experiments designed to simulate time travel, particles are made to interact with the equivalent of an older version of themselves. In all experiments, the results indicated that no grandfather paradox was allowed. This is consistent with Hawking's statement regarding chronology protection. There seems to be some inherent physical rules which prevent any mucking with the arrow of time. In short, Marty couldn't have gone back and messed with events such that he ceased to exist.

There are other inconsistencies in Back to the Future. For instance, when parts of Marty's photograph begin to fade from existence, he still maintains memory of those events. The altering of the timeline seems to indicate that physical realities can be erased from existence if events are changed such that they never occur. But the continued existence of Marty's memories of these events stands in contrast to that truth. One of two things should be true: either they should fade both from the photograph and from his own mind, or neither should happen. Not one or the other.

But there's a larger problem with Marty's misadventures in time. According to the best models we have, he shouldn't have been able to go back at all. If the trilogy is to be consistent with what we know about the possibilities of time travel, only the second movie can have taken place.

So far as we can tell, time travel can only work in one direction: forward. And we're already doing it, both in the mundane and fantastical sense. Each of us are time travelers of a sort. Time, being the fourth dimension of space-time, after height, width, and depth, is something we traverse. When you commute to work, you're moving through both space and time. While you have some control over your movements through space, moving through time is determined by forces outside of yourself. We can only go forward, and at a standard rate.

But there is some wiggle room. The passage of time is not as uniform as it might seem. You can alter it with speed. Relativity tells us that the closer we approach the speed of light, the slower time moves on a subjective scale. As such, those flying in an airplane, or orbiting on the International Space Station, are experiencing time at slightly slower speeds. An astronaut who spends a year orbiting the Earth will have subjectively moved forward in time, albeit only a fraction of a second.

The effect can be seen more directly at the Large Hadron Collider where particles are being projected at near the speed of light. Those particles are experiencing time (in whatever amounts to experience for a particle) much more slowly than similar particles which are not being pushed through the collider. So, in a manner of speaking, we've already built time machines, many of them. Cars, planes, space ships and probes, anything that moves at anything approaching the speed of light has a measurable, if minuscule, relationship to how we move through time.

As yet, however, nothing we've built can move us backward, and it's likely nothing ever will.

While Back to the Future is, on its surface, a science fiction film, it probably belongs within the realm of fantasy.

How does Endgame measure up?

Given what we know about time travel and the rules established in Marvel's most recent film, Endgame stacks up surprisingly well. Banner (and the writers) were smart to cast aside the familiar tropes that accompany time travel adventures. In doing so they were not only able to distinguish themselves from what's been done before, but were also able to create a narrative that works, for the most part.

The time travel in Endgame is time travel in name alone. Yes, we see Earth's mightiest heroes revisiting their old haunts in a way that pays homage to a story more than ten years in the making, but they aren't really there. And that's the important distinction.

What Banner is really telling them is, you can't impact the future, or the present, because you aren't going to your past. The reality you will inhabit, once you've traveled through the quantum realm, is not the past you know. It only looks that way.

Time travel in the MCU borrows from one popular interpretation in science, namely that traveling to the past can only occur if the past you are visiting is parallel to your own. They can't erase what Thanos has done because they can't return to any moment within their own timeline prior to the one happening right now. Time only moves forward, even when visiting the past.

This notion is cemented when Banner has a conversation with The Ancient One. She makes clear that removing even one of the stones would have drastic consequences to reality, causing a rift similar to that demonstrated to Marty by Doc Brown in Back to the Future Part II. Once the Avengers return to their own time, they won't be aware of any negative consequences, but the world they've left will.

The worlds they're visiting aren't their own, which is why Cap is able to fight himself and Stark is able to visit with his father without messing with his own timeline.

The idea of multiple universes is one well-established in the Marvel canon. There are versions of characters existing in various states all over the multiverse. This was even the central idea in Sony's recent hit Into the Spider-Verse.

The writers of the MCU were able to skirt all of the problems inherent in time travel because it wasn't really time travel at all. But that didn't make it any less satisfying. More than that, it feels real, cleverly doing away with almost all of the troubling paradoxical questions which usually come with this type of story.

While the overall solution might have been predicted by fans before the release, they were able to present it in a way which was unexpected. And, while it's not without its own speed bumps (namely, the return of Cap in the final scene, after living an alternate life) the fact that the creators were able to approach a well-worn narrative vehicle from a fresh perspective, one that allows us to explore a whole new set of questions, is worth applauding.

Avengers: Endgame is in theaters now, and it's well worth your time.