The Three Theories of Time Travel

There are three different theories of time travel. Each one deals with how a change to the past will (or won’t) affect the future. You may find other theories but they all branch out of these three. Think of them as the three primary colors of Time Travel and every other color, or theory, as a mixture of the three. Go ahead and picture it, I won’t stop you.

1. Time is Mutable

This is the most straightforward one. You change the past. The future changes too. Simple. However, if you change something related to you, then you yourself will be affected.
Best case scenario: You get new memories.

Worst case scenario: You fade from existence.
Some Examples:

  • Back to the Future
  • Looper
  • Frequency
  • Chrono Trigger

2. Time is Immutable

Whatever happened has always happened. If you go back in time to try to prevent it then your actions will only cause it to happen.

  • Harry Potter and the Prisoner of Azkaban
  • Bill & Ted’s Excellent Adventure

Bonus. Fixed Points in Time

This is a mix of the previous two theories. Some small things can change, but bigger more important things cannot. You can go back in time and make a small profit, but you can’t kill Hitler.

  • Doctor Who
  • The Time Machine (2002 Film)

3. Multiple Timelines

Any change to history will cause a new timeline with a different future. You can tell the difference between this and Mutable Time by seeing if there are any changes to the Time Traveler. If he changes at all, then it’s Mutable, because his timeline had to merge with the new one. If he doesn’t change, it’s a sign of two timelines coexisting. Here, you have the free reign to change whatever you want, but good luck trying to get back to your original timeline.

  • Star Trek
  • The Legend of Zelda
  • Primer
  • The Man Who Folded Himself


Just so you know, despite Doc’s illustration of Multiple Timelines in Back to the Future Part II, it was precisely just that: An Illustration. It showed Marty how things have changed and was a good depiction of Multiple Timelines, but not how Mutable Time works in their universe. A more accurate depiction for them would be to just redraw the line from 1955 onwards, but who could figure that out? It would look like this.


There, perfectly understandable.
So there you have it, the three theories of time travel. Three primary colors or the three pieces of the Triforce or the three tubes of the Flux Capacitor. …No never mind, let’s just stick with colors.
Also, it isn’t always clear which theory some stories follow, because it might not show if there’s any change at all to history/The Time Traveler after a visit to the past, keeping in mind that changes can be delayed by a Ripple Effect, or the past might not be visited at all.

But if they did do those things, the story would totally fit into one of these theories, I swear.

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