The line between fantasy and science fiction is always a blurry one. Sure, there are easy answers: The Lord of the Rings is fantasy, Star Trek is sci-fi. But some things don’t fit easily into one or the other. And then we have tropes which are simply assigned to one or the other, whether or not that makes any sense. For instance, superheroes are usually categorized as sci-fi, even though some superhero powers are indistinguishable from magic.

One of these Assigned Sci-Fi By Critics tropes is time travel. Some time travel books have science fiction vibes, others have fantasy vibes, and some honestly are just a framing tale for historical fiction. But in general, time travel is categorized as a science fiction trope.

Personally, I disagree. I think time travel could be any genre. I don’t think it’s particularly scientific, honestly. It’s one of those things, like telepathy or instant travel that only becomes scientific if you load it up with scientific-sounding explanations. I think that, to really satisfy me as a science fiction reader, a story needs to explain the rules of time travel at some point. That isn’t necessary in, say, a time travel romance. We can simply leave it unexplained, like the car the lovers drive in to meet each other. It isn’t part of the plot really, it doesn’t matter how it works.

Sci-fi readers want to know how it works. No matter how implausible it may be for it to happen at all, we want to know what the rules are and what we might expect. I don’t know about you, but I find it annoying when I think I’ve understood how time travel works in a story, only to have it work a different way where convenient.

So here are the three main ways time travel can work. This isn’t exhaustive, just one way of subdividing the possibilities.

1. You Can’t Change the Past

In this type of science fiction story, the timeline, once you enter into it, is immutable. Sometimes, things will happen to prevent you from altering the timeline. Other times, you can’t do anything at all, only stand and watch. And my personal favorite is when you can make influential changes, but when you get back you realize you caused all the problems to begin with. Nothing has changed, you simply participated in a great loop.

This, of course, gets really confusing when you think about it. Whose idea actually was it, if you got the idea from yourself? Could causality actually work that way? Probably not but if time travel works at all, why not, right?

Popular media using this method: Babylon 5, Pern, Doomsday Book.

2. You Can Change the Past, So Watch Out

In this system, you can make changes in the past–and that’s terrifying. You can date your own grandma. You can stop yourself from ever being born. If that happens, you can wind up with one of two terrifying realities: either you screw up your home timeline, so that when you get back, it’s a horrible dystopia and it’s all your fault . . . or you create a paradox which will destroy the whole space-time continuum. One of those things. If the latter is the danger, you will need a very smart person to tell you so because otherwise you’re just kind of blundering along and the first hint of consequences is when the universe ceases to exist. Ruh roh!

For dramatic effect, sometimes it takes time for changes you make to ripple their way through the system. I have trouble with this because it assumes a new dimension of time that happens faster than regular time but slower than immediately, either of which makes more sense. But when Marty McFly sees the picture of himself fading away, we know he’s screwed up and yet he still has time to fix the timeline before he’s erased. Lucky for him!

This may be the most common type of time travel. Back to the Future is the story that introduced most of us to the idea.

3. Many Timelines

What if, instead of creating a paradox, the changes you make introduce a new timeline? This is the “Star Trek compromise.” A lot of Trek watchers may not realize this, but when you time travel, you don’t get to go home again. Instead you return to a new timeline where your time travel journey has changed the past. You’ll still remember your past, but you return to a new reality. And no one else will remember the history you remember, because they’re the versions of themselves from this new timeline!

Kinda of disturbing when you think about it. The good news is, you can leave from a timeline without whales and come back and repopulate the whales and it’s fine, no paradoxes. The bad news is, the terrible dystopian timelines we sometimes see, that they “fix” by time traveling? Yeah those are still ongoing, it’s just that you created a new and better one to come home to.

This system is given interesting detail in This Is How You Lose the Time War, as the time travelers are conscious of operating across multiple timelines, switching them over to their preferred future one by one.

Others

Not everything fits tidily into these boxes. In some time travel stories, the authors clearly haven’t thought through the details. In others, there are specific rules for specific situations. For instance, in Doctor Who, you absolutely can change the past … but you can’t change a fixed point in time. It seems, sometimes at least, that a fixed point in time is something the Doctor and his companions know about, something that’s been part of their personal time stream up to that point. Which makes sense, those are the things that could create a paradox. If the Doctor prevents the eruption of Mt. Vesuvius, because he knows it will happen, then it won’t happen and he won’t know it will happen so he can’t prevent it!

Sometimes the Doctor can’t prevent fixed points in time. Other times, he or someone else can, but it will destroy the universe. Other times, he accidentally causes them himself! Basically dealer’s choice, here.

So, as a writer, what should you choose? I think you should do what you want. I don’t feel that any of these possibilities are more scientific than the others. But, if you want your book shelved with the sci-fi, you should pick a set of rules (not necessarily the ones I’ve mentioned) and stick to it. Sci-fi readers want to be able to follow along. We want to know what the stakes are and guess at the possibilities. You don’t have to tell us what they are, unless that makes it more interesting. Just don’t change the rules halfway, or we might throw the book against the wall!