When contemplating the idea of time travel, one must first consider the Grandfather Paradox. It goes something like this: Let’s say you traveled back in time and killed your grandfather when he was a child. If your grandfather died at a young age, that means his son (your father) was never born. If your father was never born, then you (his grandson) were never born either. If you were never born, you could not have traveled back in time. So, who really killed your grandfather?

The simple answer is this: no one. A person who was never born cannot travel back in time. But there’s the problem. If no one traveled back in time and your grandfather was not killed, he lives on to have a son and a grandson. And, by having a grandson—one who is hell-bent on becoming a time-traveling murderer—the paradox returns.

Writers and story-tellers have been obsessed with the idea of time travel and conundrums like this for over a century. There are many other theories like the Grandfather Paradox that consider what would happen if humans one day invent a time machine. Here is a breakdown of just a few theories presented in modern literature.

The Bootstrap Paradox

(An event in the future influences an event in the past)

Think of “cause and effect” as linear. Event A causes event B. Event B later causes event C, and everything is right with the world. In the Bootstrap Paradox, however, an event in the future causes an event to happen in the past. So, event A causes event B, which later causes event C. So far, so good. But then event C in the future becomes the conduit for the original event A in the past.

The Bootstrap Paradox presents itself in novels like Harry Potter and The Prisoner of Azkaban. During the narrative, Harry is attacked by dementors, but he is saved just in time by a patronus. Later in the text, Harry and Hermione travel back in time to the point when Harry is attacked. Together they watch the event unfold from a distance. The audience knows someone will eventually save Harry, since we watched him survive the dementors in the past. That someone turns out to be Harry himself, when he releases his own patronus. A cause in the future created an effect in the past.

But an event happening in the present for the first time has yet to be influenced by the future. In other words, if time is linear, when Harry fought the dementors on the first go around, he wasn’t saved by his future self, because his future-self did not exist yet. The only way Harry could have been saved in this situation is if time was non-linear, i.e. the past, present, and future exist simultaneously. This concept adds yet another enigma to rattle the minds of time-travel theorists.

The Oedipus Rex Prophecy

(Attempts to avoid an event from happening will be the very reason it happens)

This comes from the Greek tragedy of Oedipus, whose parents visited the Oracle of Delphi when he was born. The prophecy stated that baby Oedipus would one day kill his father and marry his mother. His father, to avoid dying by the hand of his own child, orders baby Oedipus to be taken away and killed, thus preventing the prophecy from becoming reality. It was because of this action—the botched killing of Oedipus—that leads to Oedipus killing his father years later. That is, the act of trying to avoid the future was the very action that caused the pre-ordained future to happen.

In The Time Traveler’s Wife (a novel I discussed in another article) Henry can travel forward or backward in time. Thus, Henry already knows what will happen in the future, but still he cannot avoid the future from happening. Early in the plot, Henry visits his past-self just a few minutes before a very embarrassing moment in his childhood. His future-self locks the bedroom door in attempt to prevent its happening. But it is this very action—the locking of the door—that causes the event to take place (the bedroom door was already locked, and future-Henry actually unlocked it, which allowed his father to enter the room and find Henry in a compromising position).

Other novels use the Oedipus Rex Prophecy as a warning siren. In H.G. Wells’s classic story The Time Machine (the book which is credited for first using this phrase), the narrator travels 800,000 years into the future, and he sees what has become of humanity. In this post-apocalyptic world, there are now two species: The Eloi, who live a carefree life aboveground, and the Morlocks who work ceaselessly underground to maintain the paradise the Eloi live in. This world where the super-elite relish in utopia while the lower-class endure extreme hardships is not far-fetched from humanity’s current trajectory.

Looking at the Oedipus Rex Prophecy, we have to consider whether or not humanity has free will to decide the path it takes. Are we doomed to become like the Eloi and Morlocks, or after having a glimpse of what will come, is there a chance to alter our future? It seems the words of Ebenezer Scrooge have never been so appropriate: “Are these the shadows of the things that will be, or are they shadows of things that may be?”

Schrödinger’s Experiment

(Two contradicting things exist at the same time)

Schrödinger explained it this way: Put a cat in a box with a lethal object and close the lid. Before you open the box again, it’s equally possible the cat is alive and the cat is dead. Applying this principal to time travel (although not Schrödinger’s intention), one can find two contradictory worlds existing at the same time. Some refer to this as the String Universe Theory or the Multiverse Theory.

Either way, enter Ray Bradbury’s canonized short story “A Sound of Thunder.” In this world, a person named Keith has recently won the presidential election. Meanwhile, Eckels, a futuristic big-game hunter, wants to go back to prehistoric time to kill a dinosaur. Time Safari Inc., a company which allows such possibilities, obliges him under one non-negotiable decree: Don’t touch anything. While in the pre-historic past, Eckels steps off the safety platform. His careless action triggers a literal butterfly effect, which sends ripples through the eons of time. When Eckels returns to the present day, Deutscher, not Keith, has won the presidential election.

But things are not that simple. Eckels has a memory of Keith being elected, so therefore Keith, in some other time and space, was elected. Eckels, it turns out, did not return to his universe, but one that is parallel to his. In this case, his original universe (where Keith is president), and the universe he is in now (where Deutscher is president) exist simultaneously. Similar details in Bradbury’s story imply that on other trips with Time Safari Inc. the natural timeline has been disrupted, creating many other universes parallel to the original.

The Simulation Theory

(The past is observable but not changeable)

Some authors believe we can travel back in time as only a visitor. We essentially ‘time travel’ to a place before the present, but we must relive the events exactly as they happened the first time. This theory does not allow for the past to be manipulated or influenced. Many would see this not as time travel, but rather as lucid memories, or as living in a computer-generated simulation, which contains a linear, unalterable timeline.

Think of it like playing a Mario Bros video game. Mario enters the world, trounces goombas, defeats Bowser, then leaves. Mario can still return to that world as many times as the player wants. The goombas and Bowser are non-playable characters (NPCs), meaning they cannot be controlled or altered by the player. They will always regenerate in exactly the same place and perform exactly the same movements. Even though Mario may not take the same exact steps every time, the end result is still the same.

Consider Slaughterhouse Five by Kurt Vonnegut Jr. Billy Pilgrim, a character based on Vonnegut’s own life, relives his memories in a nonlinear fashion. Some people would suggest he is not ‘time-traveling’ but rather experiencing Post-Traumatic Stress Disorder (PTSD). But let’s consider for a moment that PTSD is a kind of time travel, one where the subject is forced to relive—without being able to alter—a traumatic memory. This idea borders on the macabre, yes, but Billy Pilgrim is the most-plausible time-travel scenario, one where a person can visit the past again and again while changing nothing and feeling everything. Not the high-tech, sci-fi mechanisms audiences are used to, but still time travel nonetheless.

If you enjoyed this article, you might enjoy the following movies and the time-travel theories demonstrated within them.

Back to the Future (1985) – Grandfather, Bootstrap, Schrödinger
12 Monkeys (1995) – Bootstrap, Oedipus
The Butterfly Effect (2004) – Bootstrap, Oedipus, Schrödinger
Primer (2004) – Bootstrap, Oedipus, Schrödinger
The Jacket (2005) – Bootstrap, Oedipus
Looper (2012) – Bootstrap, Oedipus
About Time (2013) – Grandfather, Schrödinger, Simulation
Interstellar (2014) – Bootstrap, Schrödinger, Simulation
Dark (TV series 2017-2020) – Grandfather, Bootstrap, Oedipus, Schrödinger, Simulation