Time travel is a staple of the science fiction genre. But how exactly does it work? Because humans have never actually traveled through time, no one knows for sure, but writers have come up with a variety of theories. From the books, movies, and shows I’ve watched that incorporate time travel, I think these theories can be broken down into three main categories. And understanding these categories can help us enjoy sci-fi even more. *Warning: This post contains Avengers: End Game spoilers.*
Theory 1: Altering the past changes the present. In this theory, there is only one timeline, but events on it are not set in stone. If someone goes back in time and alters the past, the present is immediately changed. For example: After Fred kills Tim’s wife, Tim goes back in time and kills Fred when he is a child. As soon as Tim kills Fred, the present changes, and when Tim returns to his own time, he finds his wife waiting for him.
Back to the Future is one of the most well-known examples of this theory in action. The Flash also utilizes it during the first season finale. Over the course of that season, we learn that the main villain, The Reverse Flash, traveled back from our future to kill The Flash, but then got stuck in our time. After he causes a lot of issues, he is eventually defeated by Eddie Thawne, his ancestor. Eddie kills himself, which immediately changes the future, making it so that The Reverse Flash was never born. As a result, The Reverse Flash instantly disappears.
While I enjoy this theory of time travel, I always have trouble wrapping my head around the logic of how it works. Let’s look at the example I gave with Fred and Tim. After Tim killed Fred as a child, the whole timeline changed from that moment on, meaning that Tim’s wife was never killed. If that’s true, though, what prompted Tim to go back in time and kill Fred? Most writers seem to get around this by having the person who went back in time return to the moment they left, remembering the original timeline but not the new one, simply because they were the one who traveled through time. Thanks to suspension of disbelief, this works, but I can’t say I entirely understand it.
A variation of Theory 1 can be seen in X-Men: Days of Future Past. When Wolverine alters the past, it instantly changes the present. Instead of physically going back to his own time, though, only Wolverine’s consciousness is thrown forward to the moment he left. At the end of the movie, we are left to wonder if Wolverine remembers both timelines, or only the original.
Theory 2: Altering the past creates a new timeline. Ordinary timelines contain only one, straight line. With Theory 2, that is not the case. Instead, when a person goes back in time and changes something, the timeline splits, creating an offshoot that is entirely different from the original. If we stick with Fred and Tim, here’s how Theory 2 would play out. After Tim goes back in time and kills Fred, an offshoot timeline would be created. If Tim returned to his own time, he would find his wife alive, but would probably notice other differences in this new timeline as well. What he wouldn’t know is that the original timeline continued on without him from the moment he left. In that timeline, his wife is dead, he has vanished, but Fred is still alive.
My favorite example of Theory 2 can be found in Star Trek (2009). In the movie, a vengeful Romulan travels back in time and kills Jim Kirk’s father just as Jim is being born. This creates an entirely new, offshoot timeline but does no damage to the original timeline. We know this because “Spock Prime” follows the Romulan into the past and still retains all of his memories. A new timeline is created, but the original is left intact. I felt like this was an awesome plot device because it allowed the writers to tell a new story with the characters of Star Trek without angering fans by completely negating the original.
The Flash touches on Theory 2 as well, when Barry creates “Flashpoint,” an alternate timeline where he prevents his mother from being murdered. In The Flash, though, changing time is a bit of a taboo, and creating an offshoot timeline causes a slew of problems that don’t seem to happen in other stories.
This theory of time travel is used rather uniquely in Avengers: End Game. When Bruce Banner goes back in time and asks the Ancient One for the Time Stone, she refuses at first, citing this theory as her reason. By taking the Time Stone, Bruce’s future in the original timeline might be saved, but in the offshoot timeline, she would not have the time stone when she needed it, which would lead to terrible things. Because of this warning, Captain America returns all of the Infinity Stones to the moment right after they were taken, thereby preventing the creation of offshoot timelines.
The writers of End Game tweaked this theory a little bit to make it work for them, though. After Captain America returns all the Infinity Stones to their original time, he chooses to remain in the past and have a life with Peggy. If the writers followed Theory 2 to the letter, this would create an offshoot timeline, but it doesn’t. An elderly Captain America arrives in the original timeline at the end of the movie. I’ve concluded that the writers decided that only large changes create an an offshoot timeline, and since Steve was smart enough not to alter anything too big, he was able to remain on the original timeline.
Logically, I find Theory 2 a lot easier to follow than Theory 1, but I do find it a bit strange to have multiple timelines existing parallel to each other, with events occurring simultaneously. If many people had the ability to time travel, there could theoretically be an infinite number of timelines running parallel to each other at once, which could get pretty crazy.
Theory 3: “Altering” the past changes nothing but people’s perceptions. This theory seems to be much rarer than the previous two. I’ve only seen it successfully used once. Here’s how it works. The events on the timeline are all set in stone, including any “changes” someone made by going back in time. If we return again to Fred and Tim, here’s what would happen. Tim believes that he sees Fred kill his wife, so he goes back in time to an hour before it happened and does something to prevent Fred’s actions. As he does this, something occurs that makes it appear to the original Tim that Fred killed his wife, but he actually didn’t. In reality, Tim’s wife was never killed, and the two of them live happily ever after…at least for now. Another way this theory could play out would be Tim seeing Fred about to kill his wife, then going back in time and situating himself so that he is able to stop it before it happens.
As I said before, I’ve only seen this theory once. J.K. Rowling presents it beautifully in Harry Potter and the Prisoner of Azkaban. As Harry and Hermione make their way through the timeline the first time, they hear the executioner swing his blade and assume he has killed a hippogriff named Buckbeak, but they never actually see it happen. As it turns out, the executioner actually swung his blade at a pumpkin because he was frustrated that Buckbeak had escaped. But how did a securely tied hippogriff escape?
A few hours later, after a mysterious figure rescues Harry from a group of dementors, Dumbledore encourages Harry and Hermione to use a timeturner to go back in time. While hiding from their past selves, the two of them manage to help Buckbeak escape without being seen, causing the executioner to feel so frustrated that he slices open an innocent pumpkin. Harry also ends up saving his past self from the dementors after realizing that was who he saw in the first place.
Theory 3 is cool, but mind-blowing. For it to work, two conditions have to be met. First, the people who go back in time cannot be seen by their past selves, or if they are, they must remember seeing themselves the first time through. Since most people would freak out if they saw a different version of themselves running around, it works best for their past selves not to see them, or not to realize what they are seeing.
The second condition is that something/someone must prompt the individuals to go back in time in the first place. In Prisoner of Azkaban, Dumbledore fills that role. Though it is never explicitly stated, I believe that Dumbledore either saw or figured out who saved Buckbeak, which led him to tell Harry and Hermione to go back in time. Another way for this condition to be met would be if characters saw their future selves doing something, realized what was happening, and then chose to go back in time later to do what they saw themselves doing. Only a character who was smart and familiar with time travel would be able to pull off something like that, though.
As I hinted at in the title, there are some stories that present time travel in a way that does not fit neatly into one of these categories. One example of this is The Flash, which takes ideas from the first two theories but also throws in things like time wraiths and random details changing just because Barry created an alternate timeline, then went back and erased it. I’ve never been able to explain everything about time travel in The Flash, but I enjoy watching it anyway.
My favorite show that throws the time travel rule book out the window is Doctor Who. With this show, I believe the Tenth Doctor said it best: “People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint, it’s more like a big ball of wibbly-wobbly timey-wimey stuff.” In other words, don’t worry if you can’t figure out what caused what to happen; just sit back and enjoy the show. Upon reflection, that might be good advice when reading/watching any story that involves time travel. After all, understanding exactly what’s going on isn’t worth missing out on enjoying the story.
Do you know other theories on time travel? Let me know in the comments. And if you enjoyed this post, please subscribe to see more like it.
