Welcome to Trope School, where we pick apart all of your favorite movie tropes. Today’s class is all about Time Travel.

At 10:45 PM on May 7, 2005, in MIT’s Walker Memorial, a time traveler convention kicked off. Invitations had appeared in The New York Times, Wired, and Slashdot, making it the most widely advertised convention of its kind. The exact coordinates of the event were 42.360007 degrees north latitude by 71.087870 degrees west longitude.

Those spacetime coordinates (the when and the where) continue to enjoy prominent and widespread distribution because, naturally, if they were to become unavailable suddenly, then any time travelers after the point of discontinuation wouldn’t get the invite and therefore didn’t show up to the convention.

Despite the availability of a “landing pad” at the convention where travelers could safely arrive without smashing into someone, no time travelers attended.

Or have they not arrived yet? After all, the event is essentially infinite—anyone from anywhen can show up whenever they want, and it will be the same experience for the attendees at the convention. (Verb tenses start getting weird when discussing time travel.)

So, why didn’t anyone show up, or are we still just waiting on them? This question is similar to one asked by extraterrestrial enthusiasts all the time (Why haven’t they shown up?). We call it the Fermi paradox, but more on this later.

They haven’t shown up because, as best, we can tell, they can’t. Scientists have a few ideas about how they might (under certain circumstances), not least of which involves a black hole. That means MIT’s convention will remain a guestless moment in time, forever jilted, forever hoping . . .

So, if time travel isn’t possible, then why is it such a big deal, and why is it so complicated? It’s make-believe, after all.

Let’s take a look at the history of time travel and its paradoxes to see if we can’t find any answers to the questions posited by one of everyone’s favorite tropes: time travel.

How Did We Get Here?

Time travel didn’t begin its conceptual life in the tourism business. Its earliest incarnations usually involved one-way trips—usually forward, into the future. Buckle up: here come some facts . . .

We’re not exactly sure of the original composition of the Vishnu Purana from Hindu mythology (anytime between 1000 BCE to 275 BCE)—suffice it to say, a long time ago—but in it, King Raivata Kakudmi travels to heaven to meet Brahma. When he comes back home, it’s many days into the future.

In Buddhism, the Pali Canon (the “Word of the Buddha”), transcribed somewhere between the 4th and 5th centuries CE, mentions the flexibility of the flow of time. One of the Buddha’s chief disciples, Kumara Kassapa, explains that time in the heavens flows differently than it does here. (You know, kind of like Einstein’s Special Theory of Relativity . . . )

In fifteenth-century Japan, an ancient folk character finally gets the name Urashima Tarō in a piece of illustrated fiction from a popular genre called otogizōshi (basically, short stories). In this story, a fisherman named Urashima-no-ko visits an undersea palace. When he comes home three days later, 300 years have passed, no one believes his story, his family is all dead, and his home is long gone.

In these early aspects, time travel was the business of gods, the enlightened, or accursed fisherman. It wasn’t enjoyable, not in the way we think about it.

In the Western tradition, we see storytellers start talking about time travel in the late eighteenth century. First on that list is a French work of proto-science fiction called L’An 2440, rêve s’il en fût jamais (The Year 2440: A Dream If Ever There Was One) by Louis-Sébastien Mercier, then we get Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and then When the Sleeper Awakes (1899) by H.G. Wells. In most of these early examples, the only way to actually travel through time involved sleeping your way there.

We should clarify that this era really pre-dates the science fiction genre. Bellamy, for example, pre-dated both science fiction and Marxism and managed to anticipate both in Looking Backward, which is basically a Utopian thought experiment set in the year 2000 that predicted aspects of everyday life like credit cards and streaming music. Fascinating, but we’re hardly talking the page-turning, block-busting time travel tales that we all clicked through to see here.

We’re getting there . . .

In 1889, Henri Bergson writes Time and Free Will as his doctoral thesis. Bergson is a friend of another philosopher you may have heard of, Marcel Proust.

Next thing you know, time is the tool du jour for thinking and talking about how free will works. That’s neither here nor there for us, but it seeps into continental philosophy, changes how we think about time, and then sends us off to the races.

That brings us to The Time Machine, by H.G. Wells in 1895. Now we had a conveyance for traveling through time—sleeping pills and long-gone family members were no longer the names of the game. So, you can thank long-dead French philosophers and writers for your favorite installment in the Back to the Future franchise.

The Paradoxes

So, let’s set aside that we’re pretty sure time travel isn’t possible in this universe unless you can harness wormholes or travel faster than light. Never mind that the energy sources it would require to operate time travel equipment, even if you could figure it out, probably dwarf the output of our sun. And who cares that no one has shown up to our time travel conventions even though they would have, and we would never be having this conversation?

Never mind all that. Time travel is cool. Its nostalgia brought to life, regret made reversible, and mortality brought to its knees. Time travel represents mastery of the one enemy we can’t defeat: time. We all march inexorably toward death in one great, worldwide danse macabre—and we’re all generally pretty unhappy about it.

Time travel’s very improbability is its narrative draw, and since we wrapped our heads around it, we’ve made a lot of time travel movies. (See for yourself.)

But, this is a storytelling trope that involves some very tricky gymnastics. Most generic tropes are bendable—you can break their rules, and they’re still successful. With time travel, if you’re going to do it, you have to do it for real. If you’re making a parody or a comedy or something else that simplifies reality, then not to worry—do whatever you want.

But that isn’t the kind of time travel that we find truly compelling. We want to go back in time or to the future in a fantasy near enough to the operation of our reality that it gives us real escape—real nostalgia, real hope . . . not a silly ripoff.

That brings us to the paradoxes. Let’s take it from the top . . .

The Grandfather Paradox

The universe operates on conditions of causality. We’ve talked about this before, but to sum up, the universe began with one single, vital event, one bang . . . let’s call it the Big Bang. This bang immediately started banging out other bangs, which banged other bangs, and so on ad infinitum. It’s still going on today, but the bangs have calmed down a bit in scope.

But as everything bangs against everything else, it determines what will unfold. If you’re walking along with an ice cream cone, and someone jostles you violently, it determines that the force will knock the cone from your hand.

You didn’t choose for the ice cream to leap from your grasp. It just did. The action was determined by the surrounding environmental and spatiotemporal circumstances.

Now, multiply that times a million billion billion, apply it to every motion in every instant of existence, and suddenly everything determines what happens to everything else. (It’s a fascinating philosophy, and we highly recommend you read up on it, especially if you’re planning on making a time travel film.)

So, if as time has unfolded, every tiny little thing has exerted influences on every other tiny little thing, which ultimately causes influences on big things, then if you track back through that record of influences and change one, you’ve broken the flow, and things will now unfold in a different manner than they did the first time around.

This is the essence of the Temporal Paradox, where we find the grandfather paradox.

In this paradox, if you go back in time and change the delicate order of events that led to your parents conceiving you, you’ll never come to exist, so you would blink away. However, if you blink away, then you no longer exist in this timeline, so as it crawls back up “to date,” when you left to muck things up in the past, that doesn’t happen, so you never disturbed the past, so you were, in fact, born as originally determined, you did go back in time, you did mess things up, and again you weren’t born, but then, as time rolls back around . . . ad infinitum.

A classic example of this happens in 1985’s Back to the Future. In this beloved time travel romp, Marty McFly travels back in time by accident when his genius friend Emmett Brown puts him in a DeLorean that he’s converted into a time machine.

Hijinks ensue, and Marty ends up in the past, where he accidentally stumbles into the series of events that bring his parents together. The entire film is about Marty’s misadventures while trying to re-kindle the sparks between his parents that he never should have gone back in time to snuff out. As time starts to run out, Marty and his siblings disappear from their own timeline . . . because he has created a grandfather paradox.

According strictly to the rules, this wouldn’t really have been possible, as Marty’s presence alone is enough to have messed everything up, but that wouldn’t have been a very entertaining movie.

The Causal Loop

At its simplest, the causal loop occurs when a future event causes a past event, meaning that as the timeline from the past unfolds, it will inevitably lead to the event that goes back in time to cause the original even in the past. This might happen to a person, to an object, or simply to some data. It means, in our linear universe of cause-and-effect determinism that we can no longer determine a point of origin, and things will cycle forever.

The paradox, in and of itself, wouldn’t make for a very compelling story; however, the struggle to break free of it is where the real innovation in this paradox takes place. It can be hard to plausibly bust out of the loop, and sometimes you need a celestial crowbar, or a deus ex machina, to do the trick, as we see in 2014’s Interstellar.

Without dragging you through a full synopsis of this film (it really is worth your time), suffice it to say that a character discovers he can communicate with the past when he slips into a black hole. He would end up stuck there if he weren’t ejected from this place (the tesseract) by beings from the future.

The information he sends to the past allows his daughter to save humanity from the apocalypse that it faced in the original timeline, and the help from the future beings prevents the loop from trapping the primary character in a separate dimension.

The Fermi Paradox

In the summer of 1950, at the Los Alamos National Laboratory, Enrico Fermi, creator of the first nuclear reactor, was walking to lunch with fellow physicists Edward Teller (“father of the hydrogen bomb”), Herbert York, and Emil Konopinski.

The men were discussing recent UFO reports and the possibility of faster-than-light travel. The conversation wandered in new directions, as conversations do, but Fermi was stuck on the idea of the aliens. He blurted out something to the effect of But where is everybody?

In 1984, Herbert York wrote that Fermi “followed up with a series of calculations on the probability of Earth-like planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over.”

Fermi was talking about extraterrestrial beings. Basically, given the size of the universe (very big), the likely number of habitable planets (lots), and the likelihood of technological development (high), mathematically speaking, space should be pretty crowded.

But it isn’t.

The same argument applies to time travel, in a manner of speaking. If it were possible, then we should be bumping into time tourists, well . . . all the time. The fact that we don’t offers a number of possible explanations:

• Time travelers can’t just visit any point in time they want.
• Time travelers must conceal their identities lest the rest of us freak out.
• Time travel isn’t possible.

Occam’s Razor, or the Principle of Parsimony, tells us that all other things being equal, the simplest answer is usually the correct one. In this case, that seems to be that we simply just can’t travel through time in any conceivable fashion that we would recognize. Therefore, we don’t ever see any time travelers.

The Takeaway

So, what does all this science and philosophy mean for film? Are we to pack up our temporal toys and simply go home? Do we have a responsibility to verisimilitude to stop writing scrips about voyages back and forth through time? After all, none of this is possible, yet we’ve already established that this is a wildly popular film trope—popular enough to warrant this post, right?

Absolutely not. Verisimilitude, after all, just refers to the appearance of truth. We use the term when talking about movies, TV shows, books, video games, or anything else that we’ve created based on what we know about the real world. It means making things that look real.

In that regard, time travel has always been just a parlor trick. It’s a fantasy or a thought experiment used to pop our brains out of their day-to-day skulls and force us to think about the nature of the world around us.

So, when it comes to film, the emphasis here is on how you pull off the trick. We’ve laid out what seems to be the rules of the universe when it comes to time—time travel movies break those rules, to varying degrees of success. Time travel can be as simple as hopping into Mr. Wells’s machine, or it can be as complicated as crawling through storage sheds in Primer. (That one is such a mind-bender that you’ll probably need this illustration to make sense of what’s going on.)

The takeaway is to decide which set of rules you’re going to play by, how you’re going to break them, what those consequences will be, and how you will believably pull everything off in the end. Like Interstellar, you can let superior beings fix everything, or, like Primer, you can just keep creating new versions of a person that branch into different realities. The options are endless.

But, as we’ve detailed here, we all have a pretty good idea how time travel would or would not work, the point is that you can’t just say it works “Because I say so.”

You can, but you’ve just thrown all sophistication and verisimilitude out the window. If that’s the case, your movie better is pretty funny because that’s about all it will have going for it . . . after all, “time is the fire in which we burn.”