The rules of time travel

In fiction writing

Photo by Daniele Levis Pelusi on Unsplash

Over the past few weeks, I’ve been researching how time travel works in fiction. Here are a few resources, tips, and things I’ve learned!

1) You make up the rules

Sure, physics provides lots of great inspiration – from the idea that time doesn’t exist to the idea that all possible worlds are happening in parallel. But fiction is written to entertain, so you can cherry-pick which details to focus on and which bits to ignore for entertainment’s sake.

Okay, so you don’t want to make the rules so ridiculous that your entire audience will poke holes in them – that would fail the test of what MD Presley calls the “credibility threshold” – but unless your audience is made up of quantum physicists, it’s okay to fudge a few things.

2) If time travel relates to the story’s core conflict, then you need to include the rules on page

(This is a key insight I learned from a conversation with the Mythcreants team. Check out their website and support them on Patreon here!)

Some stories use time travel as a plot device (time slip) to bring the protagonist into another era (e.g. A Connecticut Yankee in King Arthur’s Court). Here, the story is about a person experiencing another era; the time travel is not relevant to the story’s central conflict, and so the rules of time travel do not need explaining.

But if the time travel is relevant to the story’s core conflict, then you do need to explain the rules of time travel. Because if you don’t explain them, then readers won’t know the precise moves the antagonist and protagonist are allowed to make.

Here’s an example. Jamie is one of a handful of humans who exist in the year 3000 after a devastating war. He invents a time machine to figure out where humanity went wrong. Then he travels to the year 2500 and a bounty hunter tries to steal the time machine and change the course of history.*

But do Jamie’s actions in this 2500 have consequences for the same timeline that resulted in the year 3000 at the start of the story? Or does going back in time create alternative timelines and multiple new futures?

Without understanding what constrains the characters, it’s difficult to write a satisfying story, because stories rely on chains of cause and effect.

Equally, chess pieces being allowed to do anything they liked on a chessboard would not make for a satisfying game.

Like other kinds of worldbuilding, the rules of time travel are best explained in the first half of the book, so they don’t appear contrived to satisfy a particular plot outcome. (If you’re loosely following Save the Cat! then the “fun and games” part of the book is a good choice!)

3) Pick your kind of time travel

As hinted at above, you have several basic options for fictional time travel. These include …

i) A single unchangeable timeline

Here, time travel could involve either only being able to see events but not interact with them, much like a ghost. Or the time travel could end up being the cause of the events you tried to prevent (a causal loop).

Example: a billiard ball travels back in time and hits an earlier version of itself, which sends it moving into a wormhole in the future.

ii) Multiple timelines (and parallel worlds)

Here, you can go back in time, but if you change things, that will change the future in a parallel universe. As time goes by, parallel timelines are constantly splitting off from one another. There is a world in which everything possible has happened. This approach isn’t burdened by the various causality paradoxes that a single timeline has to deal with.

But for story purposes, it’s important to get clear on how many timelines are being considered in the story, and how they relate to one another. One way of doing this, which the science fiction line editor Katherine Kirk suggested, is to make a tube map, where each line is a separate timeline and each node a place where they intersect in some way.

There’s a great video here that explores different rules of time travel in films.

Now some books focused on a multiverse, perhaps drawing on the metaphor of the real world vs. every possible fictional universe, include the concept of a root timeline.

Example: In The Midnight Library, the protagonist, Nora, explores multiple possible lives she could have led, and experiences them first-hand for a time. But when Nora experiences those other worlds, she does so with the knowledge and experience she gained in her first life, on what is referred to as the “root timeline.”

iii) A single changeable timeline

Here, changing the past can change the future, but there are not multiple parallel futures – just one that will turn out differently. Now, it would make sense that very small changes can influence the future in big ways (the butterfly effect), but these are usually ignored for storytelling purposes. The Hitler paradox is often discussed here.

One possible rule for getting around this is to not permit time travel back beyond the instant when the time machine was first made.

Summary

This is just an introduction – there are so many different ways of dreaming up time travel in fiction! They all have different implications for causality and the imagination, but at the end of the day, if the time travel is relevant to the story’s core conflict, then you need to get some rules down on page so readers know what to expect and which paradoxes to ignore for entertainment’s sake.

*Okay, this is very loosely based on The Girl from Tomorrow, which was my fave television show when growing up, and I’m really showing my age now.