If you like the mechanism of storytelling, you probably like time-travel stories. I love them to no end. I’ve been marveled by time-travel ever since, after watching it for a good dozen times in a row, one day I deconstructed Back to the Future II and realized the entire movie contradicted itself. The kid I was when this happened stood there, shocked. How could a film so cleverly written and in which I invested so many hours, not make sense? Yet it didn’t, but instead of tarnishing my impression of the Back to the Future series forever, that realization made me love time-travel as a plot device even more.

As a writer, time-travel situations offer you the opportunity to bring your audience along on a trip through the structure of your story unlike any other premise. The plot is often laid out like a math equation, and instead of just waiting there to be taken on a linear ride, the audience is required to work their way out of a labyrinth where nothing can be incidental. Because like a math equation, a good time-travel story should be exact to avoid trapping the viewer in a labyrinth without exit. This is, however, a rule that a few have broken and still managed to deliver terrific movies. And there are also those who, despite being meticulous with their logic, applied it to not-so-exciting stories.

What I’m going to talk about here is not so much the stories, but the structural coherence of those stories. Going over many films, recent and old, I’m going to argue that there are essentially three ways to approach time-travel: the single timeline, the multiple timelines and a third option that I’m not going to introduce just yet. The first two approaches are mutually exclusive; even if that’s generally the first rule many time-travel films break. But for now, let’s go one by one.

Single Timeline

Basically, the single timeline or universe is a scenario where only one version of reality exists. This means that events are set and a time-traveler can’t change his present by jumping back in time, because any interaction in the past was already part of that past even before he decided to go back.

The quintessential example of this model is Terry Gilliam’s Twelve Monkeys, based on the 1962 French featurette La Jetée by Chris Marker. In both films, a prisoner haunted by the childhood memory of a man getting killed is sent back in time hoping to find a way to fix the state of civilization in his present. In Gilliam’s version, the scientists running the time-travel operation believe that a virus was released by a terrorist organization sometime during the 90s, which polluted the planet surface for years to come and forced humans to live underground. James Cole, the protagonist, eventually finds the true culprit of the virus release, but as he’s trying to stop him, police officers shoot him dead while letting the real criminal get away. Witnessing the shooting there is a young boy who turns out to be Cole himself as a kid, who will remember from then on the scene of his own death.

Twelve Monkeys works as a loop where its protagonist dies because he was sent back in time, but is sent back in time because he died. There lies the paradox of the single timeline, where cause and effect exchange places forming a seemingly impossible, yet logically coherent, series of events.

Cole sees himself getting shot, unaware that it's his own future (Twelve Monkeys, 1995).

The origins of the idea can be traced as far back as 1941 with the story By His Bootstraps, written by Robert A. Heinlein, although cinema took a bit longer to start exploring it seriously. Escape from the Planet of the Apes, the second sequel of Planet of the Apes, explained the whole setting of the original movie through the means of time-travel within a single universe. One of the most intriguing facts of the first Apes was that it established a post-apocalyptic Earth ruled by English-speaking monkeys… in the year 3978. That suggested that in barely two thousand years, apes were able to not only inherit a planet ravaged by nuclear war, but also evolve enough to develop a somewhat modern society and speech abilities. For a while, the way this came to be was left open to interpretation, but in Escape from the Planet of the Apes, a chimpanzee couple introduced in the original movie accidentally jump back in time and have a baby in the year 1973, shortly before being murdered by the authorities. The film’s last shot reveals the baby monkey in a cage crying the words ‘Mama? Mama?’, implying that he will spawn a new generation of intelligent apes sharing his hatred for humans, but also explaining their origin through a causal loop.

Planet of the Apes (1968) and Escape from the Planet of the Apes (1971), two memorable endings based on time-travel.

 Moving into the 80s, two films about warships utilized the single timeline scenario as well. The first was The Final Countdown, in which a modern American aircraft carrier enters a wormhole in the Pacific and turns up in 1941 right before the Japanese attack on Pearl Harbor. Much of the movie revolves around the dilemma of whether the carrier should prevent the attack and change the future or not. In the end, the ship gets transported back to the present right before the crucial moment, but losing a crewmate in the midst of things. The twist of the story is the identity of a mysterious character introduced early on, witness of the carrier’s disappearance and return, who is revealed to be an aged version of the lost crewmate decades after getting stranded in Pearl Harbor.

The Philadephia Experiment, made just four years later, uses a famous urban legend about an experiment on teleportation to tell yet another single universe time-travel story. A naval destroyer is sucked into a rip in the space-time continuum after two similar experiments are carried out 41 years apart. One of its sailors lands somewhere in Nevada in the year 1984, while the others struggle to survive in the limbo where the ship gets trapped. After bumping into the older self of one his mates, the protagonist learns that he successfully saved them all and fulfills the prediction by offering his help to the military team trying to close the time rip.

American F-14 vs Japanese Zeroes? With time-travel, it's possible (The Final Countdown, 1980).

Much of The Philadelphia Experiment dealt with a trip forward in time instead of backward, making it the opposite of The Final Countdown but also easier to handle in terms of paradoxes. In the mother of all modern time-travel stories, H.G. Wells’ The Time Machine, the main character travels all the way up to the year 802701 yet no causality problems arise. Steven Spielberg took advantage of the concept to criticize current preventive war politics when he adapted Philip K. Dick's Minority Report, a story where trio of psychics warn the police about imminent crimes. The measure works because changing a hypothetical future before it happens certainly doesn't put the present at risk, but the moment one introduces visits to the past pieces need to click in unexpected ways. Especially if the goal is to avoid dead ends in a single universe scenario.

One film that alludes directly to this is Barry Levinson’s Sphere, based on Michael Crichton’s novel, where a group of scientists explore a giant spaceship sunken in the Pacific Ocean and soon determine that it’s a human vehicle from the future. Inside the ship, they also find a giant sphere of unknown origin that grants the power of turning imagination into reality. While on their mission, one of them reasons that they will die before returning to the surface because the ship would never crash into the ocean if they live to share their findings. Surprisingly, they do survive their adventure and return to the surface, but they use the power of the sphere to erase their own memories and avoid the time paradox.

Better novel than film, Sphere (1998) was nevertheless a smart time-travel story as both.

A common feature of single timeline movies is that they get comfortable with their revolving plot and base the entirety of their payoff on the closure of some narrative loop. The set-up is so simple and effective that you can pretty much squeeze it into a five minute shortfilm and still get the same impact, as proved by Evan Borja’s animated thesis Otzi. 

But what if there were more loops within the loop? Spanish filmmaker Nacho Vigalondo answered this question with Timecrimes, a movie that takes the concept of a single universe to the extreme and builds a puzzle where the most outrageous turns respond to a beautifully interconnected series of actions.

In the film, a man named Hector finds a naked girl in the woods near his home. While trying to assist her, he is stabbed in the arm by a masked stranger and is forced to seek refuge in a nearby complex. There he meets a young scientist that suggests stepping into a cylindrical container to hide from the masked psychopath. The container, however, turns out to be a time machine and Hector is sent back a few hours into the past. Confused, Hector flees the complex in the scientist’s car but is run off the road by a van. He cuts his head in the accident, wrapping it with bandages in such way that he ends up looking exactly like the masked stranger that attacked him earlier. When the same girl he found naked comes to his aid, Hector realizes he must take her to the forest, strip her off her clothes, and attack his former self to recreate the chronology of the day as he remembers it to avoid a time paradox. But this is not even half of what’s waiting for him as the story moves forward and backward.

The bogeyman as a recurring character in time-travel films (Timecrimes, 2007; Triangle, 2009 and Looper, 2012).

What Vigalondo did with Timecrimes is notable in the sense that he twisted his characters’ journey into a nightmarish spiral like no other single timeline film did before. It explores the uncertainties of time travel at an abstract level, since Hector never really intended to time travel. He just gets swept into a surreal intrigue because someone somewhere turned on a time machine. Looking back at the chronological beginning of the narrative, Hector crawled out of the machine the moment it was activated, already having lived an entire adventure that would unfold only because of the machine itself.

 Multiple Timelines

Things get even messier once the single timeline approach is abandoned in favor of the multiple universes theory. According to this model, moving forward in time doesn’t differ from the single timeline logic, but jumping into the past always creates a parallel timeline where future events develop differently from the original reality.

Although just as challenging as the single timeline scenario, the problem with the multiple universe take is that very few of its movies withstand a serious scrutiny the way single timeline stories do. For different reasons, filmmakers opting for this alternative tend to break the rules they create, which inevitably leads to coherence issues and plot holes – starting with perhaps the most beloved time-travel saga in film history: the Back to the Future trilogy.

Arguably the best time machine concept ever invented, Back to the Future's DeLorean is a film history icon.

 In Back to the Future II, protagonists Doc and Marty end up stranded in an alternative 1985 where antagonist Biff Tannen controls their hometown. He got to that position by borrowing Doc’s DeLorean time machine in 2015 and giving his past self a magazine detailing the results of sporting events for the second half of the 20th century. Upon learning this, Marty suggests getting back to 2015 and stopping Biff before he can use the DeLorean. However, Doc warns him that it would be futile, since departing from an altered 1985 would result in arriving to an altered 2015 as well. He even draws a diagram to make sure the entire audience gets it, specifying that once Old Biff gave Young Biff the magazine at some point in the past, reality as they knew it changed into a new version stemming from an original timeline that no longer exists for them. But, if this is true, how could Old Biff get back to the original 2015 and return the DeLorean? In short, he couldn’t.

He would’ve arrived to an altered 2015 where he is king, and Doc and Marty would’ve never known what happened, let alone be able to travel back to 1985 and go through their mind-bending odyssey. Back to the Future II breaks the very rules it so carefully presents for the sake of a greater good. Without taking the license of letting Old Biff go back to the unaltered 2015, there would be no movie. Writers Robert Zemeckis and Bob Gale knew this, and they even planned a scene where Old Biff collapsed and vanished shortly after exiting the DeLorean, but eventually decided to cut it out since it may have confused the audience. And rightly so, because vanishing people is another cliché of time-travel films that never, ever makes sense.

What Doc explains, although logical, contradicts the triggering event that makes Back to the Future II (1990) possible.

Once Doc explains the nature of time-travel in Back to the Future II, one can interpret most time jumps through the series as triggers of alternative timelines. After all, the mere fact of you showing up in a past time where you weren’t supposed to exist is already enough of an alteration to affect everything else that comes afterwards. However, it’s actually impossible that anything you change during the trip can put your personal integrity at risk. Not even bumping into your parents and ruining their first date, like Marty does in the first Back to the Future.

Marty can't disappear if his parents don't kiss, because they are not really his parents.

 No matter how chilling it is to see Marty’s hand slowly disappearing as his father fails to kiss his mother during their 1955 high school dance; Marty can’t cease to exist. Yes, the lack of a kiss may result in his parents not getting married and giving him birth, but that unborn child would not be the Marty who’s witnessing the dance, just like the dancing parents are not really his parents. They are the alternative versions of his parents who may or may not raise an alternative version of himself in the alternative universe where this whole thing is happening. Marty, as the main character we follow through the movie, is nothing but an observer from another timeline who happened to abandon it when he jumped back in time earlier in the story. This is also why, at the end of the film, when he gets back to 1985 and discovers that his family, once lame, is now wealthy and happy, he can’t remember anything about it. Because he never lived the events that led to that, because that’s not really his family, but the family of another Marty whose place he’s going to take. If only the original trip back with the DeLorean didn’t remain as part of the newly engineered future, conveniently sending the genuine Marty to 1955, the duplicated Marty returning from the past would be stuck in a foreign reality where a copy of himself enjoys a better life with his family and the film’s closure would be far from comforting.

Marty runs over one of old Peabody's pines in 1955, affecting 1985. Tons of details like this fill Back to the Future. 

But Back to the Future is not the only classic blockbuster that downplays certain details for the sake of an emotionally satisfying ending. James Cameron’s Terminator 2 did pretty much the same.

At the end of that film, a friendly Terminator sent to protect teenager John Connor offers himself as a sacrifice to prevent the future war against the machines in which Connor will become the human resistance leader. The Terminator explains that in order to avoid the robot rebellion all evidence that he ever existed must be destroyed. As established through the events of Terminator and Terminator 2, Skynet, the rogue AI that eventually orchestrates the rise of the machines, is developed thanks to the technological remains left in the past by Terminators sent from the future by both Skynet itself and the resistance in hopes to change the outcome of their fight. This sounds like a single universe loop at first, but it can be interpreted in two ways that always lack a truly happy ending.

Can the Terminator undo the future and accomplish his original mission at the end of Terminator 2 (1991)? Not really.

 If the Terminator movies are, in fact, a single timeline scenario, then all the reasoning behind the Terminator’s actions in T2 can’t possibly stop the war, because that would create a predestination paradox. However, if we look at it from a multiple timeline perspective, then the Terminator does prevent the future war in the reality where he is. Unfortunately, his whole mission would be pointless as he would have never been able to help the original John Connor that sent him back in the first place. That future Connor would still exist in a former, inalterable timeline that the Terminator abandoned as soon he jumped back in time. That is, a timeline where war would rage on without being affected by all the constant deliveries of killer robots to the past.

If the flying DeLorean is cinema's most iconic time machine, the Terminator must be its most iconic time traveler.

Logic can sometimes get in the way of a good story, and that’s why many authors choose to take narrative licenses and forget their own formula. This is not necessarily a wrong choice, but it’s definitely a choice that requires a significant dose of ambiguity in the plot to work out. In fact, the ambiguity of the story will be somewhat proportional to the level of disregard for the rules of time-travel. Structurally tight films like Back to the Future and Terminator merely take a couple of minor liberties compared to a movie like, say, Rian Johnson’s Looper. Yet Johnson still has reasons to feel proud of his work.

Looper portrays a world where criminal organizations send people back in time to be killed by assassins known as loopers. A looper normally waits at a designated location for its victim to materialize before him; then shoots him or her. As part of the deal, each looper knows that his last victim will be himself, sent back with a pack of golden ingots strapped to his body as payment for his life. Trouble begins when looper Joe Simmons is faced with the task of killing his future self and fails. Up to here, everything is alright, but the movie’s logic falls apart as soon as we delve into the story from that point.

The film shows two versions of what happened the moment Joe was captured by the mob and sent back in time to fulfill his destiny as a looper. In the first version, Old Joe overpowers Young Joe and escapes before being shot. In the second version, presented as a flashback, Old Joe is killed by Young Joe. In terms of narrative order, this is how events unfold: one day in the future, Old Joe’s wife is murdered and he is taken by force from their home, put in restraints, sent back in time and killed by Young Joe. Awarded with his gold, Young Joe then travels to China, meets his wife and enjoys what’s left of his life until the day he’s captured and sent back to die. However, once the story comes to a full circle and Joe reaches the day his wife is murdered, he decides to break the loop by avoiding his predestined murder and hunting down a child who will become the Rainmaker, a mysterious figure that runs the mob responsible for his wife’s death.

The absurdity of this premise is obvious based on its relationship of cause and effect. In order to escape, Old Joe must lose his wife first, but in order to meet that wife, he must die and get his golden reward to pay for the China trip. Whether the story is set in a single or multiple universe doesn’t really make that much of a difference in this case, although part of Looper’s logical issues respond to the fact that the movie never settles for one model.

Are Kid Blue and Abe the same person? Looper is a movie with some deliberate questions without answers.

 On an earlier scene, a different looper escapes his murder too, but is caught by the mob again. The technique used to lure him back to them consists on torturing his younger self and, as the resulting scars show up on the older one’s skin, let him know that he’s in trouble. Why they don’t simply kill the young one and erase the old one from existence is a question for the ages. If anything, this shows that Looper wants to be a single universe experience at times, but constantly falls into all the reasoning traps behind the model.

Even the movie’s conclusion can be questioned. Old Joe, having finally found the Rainmaker as a kid, tries to kill him even if that means murdering his mother first. But Young Joe, present at the scene, realizes that the mother’s death will trigger a chain of events that may very well turn the traumatized child into the dreaded Rainmaker. Therefore, Young Joe kills himself, taking his older self out of the picture to save the kid’s mom and possibly thwart the Rainmaker’s genesis.

All these characters can't logically be together in the scene they share. Yet they are.

The problem is that the Rainmaker shouldn’t even exist in Old Joe’s future from the moment we are told that he is the result of his mother’s murder. That means that Old Joe needed to escape his own assassination in order to kill the mother, yet the set of events leading to Old Joe’s escape requires him to not escape in the first place. If he never escaped in order to meet his wife, how could the Rainmaker exist and be responsible for the wife’s death? Perhaps because someone else killed the mother. But if that’s even a possibility, then what does Young Joe achieve with his sacrifice? And I haven’t even mentioned the whole vanishing people thing.

A movie where diagrams do matter (Timecrimes, 2007) versus another one where they don't (Looper, 2012).

Looper, as a time-travel equation, is an absolute mess. But it’s also a fine time-travel movie, since most of that doesn’t matter. Unlike films such as Timecrimes, where most of the enjoyment comes from figuring out its intricacies, Looper uses time-travel as a backdrop to tell a character story about guilt and egoism. It openly gives up at trying to convey a logical time-travel scenario so its characters can find their own way. In this sense, Looper uses the same storytelling approach of Back to the Future, only that where Zemeckis chose to occasionally forget the rules to benefit the story, Johnson throws them out of the window from the beginning.

I mean, really.

Another common feature of multiple timelines films is their use of the ripple effect. Essentially, this example respects the main idea that there can be multiple realities and changes in the past, but the consequences of those changes overwrite the old reality leaving everything as seemingly one single timeline. In Looper’s case, this is true to the torture sequence, but contradicts the main character’s dual journey. In Back to the Future, it happens whenever the characters show a picture from another time changing its contents as they mess with the past, but it doesn’t explain why the character themselves remain the same. Other movies that take their chances with this idea include Frequency, Timecop, The Butterfly Effect or A Sound of Thunder. 

In Frequency, a New York firefighter and his son communicate through an old radio from periods thirty years apart during an aurora borealis. Timecop introduces a time-traveling police force that turns absurd from the moment their own changes erase any records of each mission for anyone but the cop involved. In The Butterfly Effect, a young man can travel back in time and rewrite his own past by reading his childhood diaries. And A Sound of Thunder, based on a short story by Ray Bradbury, deals with a company offering trips to prehistoric times that have unforeseeable consequences in the future. Of all these films, Frequency, The Butterfly Effect and Looper itself try to address the effects in the main character by suggesting the formation of new memories after each time change. However, no film goes as far as making the old memories disappear.

A different past makes a headline change and a hand vanish, from Back to the Future II (1990) and Frequency (2000).

Time-traveling characters are usually immune to their own time meddling because otherwise there would be no story. No matter how contradicting the fact that they can change their surrounding reality without changing themselves, this allows them to have control over the situation. All previous movies take advantage of this, but none takes it to the extremes of The Girl Who Leapt Through Time, where the protagonist literally jumps up to 99 times back into the past for reasons that range from saving a life to singing in a karaoke bar. Curiously, it also disregards entirely the existence of duplicates, allowing the title gal to go bananas with her trips without worrying about some other 98 versions of herself fooling around whenever she gets back in time.

A traffic sign as a splitting timeline in The Girl Who Leapt Through Time (2006), not based on a Stieg Larsson novel.

The other side of the coin would be Primer, the fastidiously thought-out thriller that holds the honor of being the most cryptic time-travel film in the history of science fiction. Written and directed by mathematician Shane Carruth, it tells the story of two engineers who accidentally build a time machine while experimenting with a weight reduction device. Once they figure out what they’ve done, they start using the machine to make money in the stock market, they duplicate themselves, then triplicate themselves, they keep secrets from each other and naturally worse comes to worst.

Primer’s impenetrability responds more to its editing choices than its plot, although it does feature a rather complex time-travel story. Protagonists Aaron and Abe first plan on using twin time-machines to make small six-hour trips into the past, but when Aaron discovers that Abe built yet another ‘failsafe’ machine in case he needed to go further back to undo undesirable events, stuff starts going off-rails. Both Aaron and Abe venture into the past behind each other’s back, with Aaron being ahead of the game and impersonating his past self to mislead Abe. He achieves this by drugging his duplicate and recording the day’s conversations, while both him and Abe try to deal with situations like the time machine being used by a third person, a murder attempt, and the health problems that time-traveling entails.

Primer’s time machine essentially works like the one from Timecrimes, but with a catch. A soon-to-be time-traveler turns on the device and then leaves, his duplicate materializes while the original remains hidden, and eventually the original enters the machine to become his duplicate in the past. The difference with other films is that the traveler must stay inside the machine for as long as he wants to travel back, and this time limit has to be carefully observed to avoid adverse physical effects. Knowing this, Primer sounds pretty much like another single universe scenario where things go ugly just because someone invented a time machine. Nevertheless, it’s a multiple timelines film with such a convoluted storyline that’s easy to miss the one instance that gives away its cards.

Primer's (2004) timelines: you're gonna need a glass of scotch on the rocks before you fully understand this one.

 At first, after each time Aaron and Abe turn on their machine, they lock themselves in a hotel room to prevent any paradoxes that might result from interactions with the outside world while they’re duplicated. For six hours, future and present versions of themselves coexist, just as long as it takes the present Aaron and Abe to enter the machine and start going back in time. However, during one of these hotel breaks, Aaron receives a phone call from his wife. Unsure if both his phone and his duplicate’s phone are ringing at the same time, he hesitates for a minute. But when he determines that as long as he doesn’t take the phone into the time machine it won’t be duplicated, he realizes that answering the call should be fine. The problem is that he forgets about it, the phone is duplicated and it rings again out in the street after Aaron and Abe have traveled back. “I think we broke symmetry,” says Aaron.

Can Aaron get his wife's call both in the hotel and the street?

Both characters reason that cell phones work as a network, with incoming calls bouncing from one device to another within an area and then stopping once they reach the right one. If this is true, when Aaron’s duplicated phone rang, his original phone back at the hotel didn’t. This would contradict Aaron’s immediate past when he answered the call at the hotel, changing the curse of events that led him to receive that call for a second time in the street. However, if he does get the call in the street, he should’ve never been able to receive it at the hotel in the first place. After all, aren’t they hiding there because they know their duplicates will be wandering the outside world simultaneously from the beginning?

It can be argued that the phone paradox is just pure speculation on the characters’ part, and that Aaron does not take the call in the street anyway. Maybe the hotel phone did ring as well. Moreover, the movie strongly suggests that the characters are experiencing alternative timelines. In a different scene, Aaron is unable to recreate a past conversation correctly after missing a shot his double scored the last time during a basketball game. As we know by now, only the multiple timelines model allows that sort of interference with established actions. But if the film is indeed a story told through multiple timelines, then why do the characters take so many precautions before each trip? No paradox can result from changes in their duplicates’ conduct. In fact, Aaron proves this the moment he drugs himself and pretends to be his past self in front of Abe without disappearing into thin air as a consequence. If this is the model we are supposed to follow, there’s no need for them to wait six hours in a hotel room or worry about ringing cell phones. Nothing would come out of the time machine after being turned on for the first time, because the time traveler would appear in an alternative timeline from which he cannot affect the original one. 

Abe sees how Aaron misses a shot he scored in an earlier timeline,  confirming that they live in multiple universes. 

The problem is, of course, that the characters can’t possibly know if that was the first time the machine was ever turned on. They can’t possibly know in which timeline they are or if a duplicate is coming whenever they activate their device. Their best bet is to always refer to causal logic and act accordingly, which represents the film’s appeal. Primer is a movie about characters facing an unknown situation and trying really hard to understand it in order to survive. The hotel breaks, the machine settings, the conversation recordings or the phone dilemma are good examples of what two educated people might do when dealing with time paradoxes, letting the uninitiated sympathize with them through an otherwise tumultuous journey.

Up to this point, every multiple timeline film listed here breaks its own logic to a greater or lesser degree. That doesn’t mean it’s impossible to come up with a bulletproof concept through that model, or that it hasn’t been done before. Harold Ramis accomplished it with fan favorite Groundhog Day, although he did so by creating his very own set of rules. In the film, meteorologist Phil Connors is forced to relive the same day over and over in the small town of Punxsutawney, Pennsylvania. No explanation is given as of why the phenomenon takes place, but for a countless amount of iterations, Connors experiences the exact same routine while he tries to better himself and win the love of his life. No particular element of the film contradicts another, yet the action definitely takes place in multiple universes.

Groundhog Day (1993), only a genius film could produce a screenshot this brilliant.

The concept has been around at least since Richard A. Lupoff wrote his short story 12:01 PM, first adapted as a short film and then into a feature. Besides Groundhog Day, a myriad of television shows have used the idea either as a recurring plot device or limited to individual episodes. Recently, the psychological thriller Triangle revived it from a disturbing spiritual perspective. Mainly set in a deserted cruise ship, the film tells the story of an abusive mother trapped in a looping set of events dooming her to forever relive the murder of her friends and death of her son at her own hands.

When not creating their own rules, multiple timeline movies can also choose to be vague enough to dodge any potential plot holes. This would be the case of Sound Of My Voice and Safety Not Guaranteed, two vastly different films with a surprise ending that admits a logically sound backstory. In Sound Of My Voice, two reporters infiltrate a cult led by a woman who claims to be from the future. As the investigation progresses, evidence suggests that she’s a fraud, until one last revelation turns the tables. Right before being handed to the police, the woman asks to be reunited with her mother, still a little girl in present time. Granted her request, she greets the girl with a secret handshake only taught to cult members as an access code to their hideout. The surprised girl asks the woman how she knew about the handshake, to which the woman replies that she learned it from her. In all fairness, Sound Of My Voice could be interpreted as a single timeline scenario, multiple timelines, or maybe not even a time-travel film at all, but it works as a story that remains vague enough to not trip itself.

Is this girl the mother of the first time traveler? Another tricky question from Sound Of My Voice (2011).

 In turn, Safety Not Guaranteed ends on a fantastically colorful note that confirms any suspicions the viewer may have, yet not clarifying what type of time-travel model the filmmakers were betting on. Throughout the story, a hotheaded reporter and his young assistants meet a mysterious man who placed a newspaper ad seeking for a companion to travel back in time with him. He intends to erase an accident from the past that killed his platonic love, but when the main characters find out that the girl in question is rather alive, the mental health of the time traveler is called into question. Confronted with these facts, the man not only denies lying, but also concludes that his future mission must have been a success since reality has already been changed. The fact that the time traveler is aware of two alternative scenarios in his sweetheart’s future suggests that he is dealing with multiple timelines. Why he can’t remember having already changed the past, however, sounds like a potential conflict of models. Yet the movie doesn’t offer enough information to fully understand the consequences of its time travel; it doesn’t need to; and it simply works because if this.

Investigative journalism in Safety Not Guaranteed (2012).

The Absence of Time

Remember when, a hundred paragraphs ago, I mentioned a third model different from the single and multiple timeline scenarios? I didn’t just do that to lure readers all the way to the end of this text, there is actually one. Introduced by writer Isaac Asimov in his novel The End of Eternity, currently set to be adapted by Kevin MacDonald, it requires the presence of an alternative dimension where time doesn’t exist. The model works as a dual timeline where inhabitants from one universe can rewrite another without being affected by it. In a sense, this is an approach that combines the rules of all the others and then adds a few new ones.

Quick recap: the single timeline, the multiple timeline and the Asimov model.

In the book, protagonist Andrew Harlan works for the Eternity, an organization and place outside time from where technicians called Eternals manipulate human history. Because of his job as an Executioner, responsible of creating the time changes, Harlan is tasked with a mission to secure the creation of Eternity by altering the past. He must bring a young colleague back in time with the scientific knowledge to invent the technology that makes time-travel possible. Problems arise when he falls in love with a woman that might be erased from existence if the Eternity keeps interfering with human history. Escaping together through time, they eventually come to a realization that affects both of them, Harlan’s organization and the whole reality as we know it. 

Easily one of the best time travel stories ever written, The End of Eternity can be compared to Charles Dickens’ A Christmas Carol in the way it presents characters as an audience witnessing time progression in a theatrical fashion. The difference is that Dickens’ protagonist had a passive role in the events, while Asimov’s characters are very much interactive. They can make changes in a single timeline scenario that would be self-contained if approached from within, only because the Eternity exists as a narrative license to avoid most causal paradoxes while still allowing the existence a beautiful human story wrapped in an elaborate science-fiction tale. If filmed, the director responsible of bringing Asimov's vision to life will have to squeeze a really meticulous story into a two to three hour script that, if written properly, could become one of the most celebrated science-fiction movies ever made.

But is The End of Eternity the only other possible approach to time-travel in fiction? Written narrative has always been years ahead of filmed one. And counting all possible mediums, there are as many time-travel stories as minutes in a century. For the purpose of this text, I deliberately focused mainly on feature films and left out dozens of novels, short stories and TV shows. I was tempted to include nice television examples such as 'The Odyssey of Flight 33' from The Twilight Zone, 'The Constant' from Lost and, my personal favorite, 'A Matter of Time' from StarGate SG-1, but that might be stuff for a future article. I didn’t list every time-travel movie out there either, but the ones that sufficed to explain my three-way approach to understanding time-travel as a plot device. Again, it may not be the only one, but it’s one that works. If you were unfamiliar with it, you may want to revisit some of your favorite time-travel films for a refreshing experience. And if you were one of those who say that time-travel stories can never make sense, you may want to rethink your assumption. As for me, I’m off to watch and read more time-travel stories, hoping to find yet a fourth, fifth or even sixth way to structure them.