Temporal Mutability - Television Tropes & Idioms

At least we're all okay, right guys?

Professor Hubert Farnsworth: You mustn't interfere with the past. Don't do anything that affects anything. Unless it turns out that you were supposed to do it; in which case, for the love of God, don't not do it!

Fry: Got it.

Professor Hubert Farnsworth: If, for example, you were to kill your grandfather, you would cease to exist.

Fry: [gasp] But existing is basically all I do!

— Futurama, "Roswell That Ends Well"

"When you're dealing with Time Travel, it's important to establish whose rules are in play: Is this Twelve Monkeys rules where you can't change shit, or Back To The Future rules where you can change shit but the timeline is kind of easygoing about it, or Terminator rules where you can change shit, but maybe you can't change shit and then you make a god-awful TV series and Christian Bale yells at someone?"

—'Ben 'Yahtzee' Croshaw, on time travel rules

AKA The Sliding Scale Of How Easy It Is For Time Travelers To Change The Past, And Why.

Time Travel is one of the richest concepts in Speculative Fiction; altering the past is easily one of the richest Time Travel plots.

Apparently, people (or at least SF writers) in general have a deep-seated dissatisfaction with the past—either their own recent past, or with the whole history of the world—because every time the subject of time travel comes up, characters inevitably start wondering whether they can use their Time Machine to change the past. Even if the characters have no intention of changing the past—even if the characters don't actually travel to the past at any point—some smartass will ask about the Grandfather Paradox, which will in turn lead to a discussion on the possibility (and morality) of altering the past:

Could you go back and save your brother from that fatal car crash? Could you punch your boss in the face, then go back and stop yourself? Could you prevent World War II by going back to 1930 and killing Adolf Hitler? (No.)

And if you could, should you?

Seeing as time travel is currently just a pipe dream, there's really no saying what would be possible when traveling to the past in Real Life. Writers are thus free to invent and follow whatever chronophysics they like, and as long as it's consistent the fans will usually accept it.

These settings tend to fall into one of the following categories (arranged here from least changeable to most changeable):

You Already Changed The Past: AKA Block time or Eternalism. Past, present, and future are an immutable whole. Consequently all time travel to the past results in the creation of a Stable Time Loop, by virtue of the fact that the past—including the interference of all those time travelers—already happened. Changing the past is out of the question—but there is the possibility that the history books don't tell the whole story. Even so, your attempt to travel back to 1930 and assassinate Hitler is almost certainly doomed.
Enforced Immutability: In theory, the past could be changed, but some force stymies anyone who tries. Maybe Time Police or Clock Roaches menace anyone who violates the Temporal Prime Directive, or maybe the past can only be visited via Intangible Time Travel.
Rubber Band History: Time is mostly immutable, like a wide river following a well-worn path. Travelers can make changes to the past, but these changes inevitably get smoothed over by the passing years. For example, it would be possible to travel back to 1930 and assassinate Hitler, but World War Two (or some equally bad conflict) would still happen anyway. Setting Right What Once Went Wrong works, but only in the short term; Making A Better World, unfortunately, doesn't work. Unless you were to apply a sufficiently large change, one that would stretch the rubber band until it snaps, freeing history to run in a different direction.
Temporal Balancing Act: There's no rubber band, so there's nothing to prevent you from making major, permanent changes to the past if you want to. But at the same time, it's possible for a conscientious time traveler like yourself to leave the past exactly as you found it. Or to change the past, then change your mind and go back again and un-change the past. Or to intentionally arrange a Stable Time Loop.
Temporal Chaos Theory: The Butterfly Effect is in full force. Simply by being in the past in the first place, you alter the past, both overtly and in ways too subtle to notice. And these changes inevitably snowball, eventually rendering the Present or Future (almost) completely unrecognizable. And sometimes, the universe hates you, so every change to the past only makes the present worse.
- It bears mentioning that over short enough time periods, settings that fall under Temporal Chaos Theory may not be distinct from those that fall under Temporal Balancing Act.

And in any setting where changing the past is possible, the alteration generally happens in one of two ways:

Overwriting the timeline: The old timeline ceases to exist, and is replaced by the new series of events resulting from your time travel (implying that Time itself exists in a sort of Meta-Time). The change to the timeline may be instantaneous, or it may cause a Delayed Ripple Effect, allowing you to race against San Dimas Time to Set Right What Once Went Wrong before you find yourself Ret Gone. You may find yourself the only person who realizes the past has changed. Overwriting the timeline is also prone to causing Temporal Paradoxes.
Branching timelines: Your time-traveling causes a new timeline to split off the original, and both timelines exist (if only temporarily) as Alternate Universes of each other. Depending on the setting, you may or may not be able to return to your native timeline after you've caused it to split. Thus, there's no danger of accidentally erasing yourself from existence—at worst, you'll prevent one alt-timeline's equivalent of you from existing. On the other hand you can't truly Set Right What Once Went Wrong, either: for every timeline that you fix, there's another that you don't.

And, fitting none of the above categories:

Timey Wimey Ball: The series says outright that time travel follows no rhyme or reason. Or, it starts off following the rules of one of the above categories, only to later contradict these rules (sometimes justified by stating that the original time travel expert was wrong, or that this new case is some kind of special exception to the general rules of time travel).

As an aside, it's interesting that no one ever seems to be nearly as concerned about time travelers altering the present or the future.

Examples:

Type 1: You Already Changed The Past

See article.
Beast Wars uses this form of time travel, with the titular conflict resulting in drastic changes to Planet Earth, allowing the events of the original Transformers to occur.
Animorphs is complicated, mostly due to including multiple methods of time travel. When using the "accidentally due to huge explosion" method, the result is a Stable Time Loop, but when the Sufficiently Advanced Aliens get involved, the timeline is mutable.

Type 2: Enforced Immutability

In his preface for The Great Divorce, C. S. Lewis cited as an inspiration a short story (whose author he could no longer remember) from an American SF magazine about a man who traveled into the past "and there, very properly, found raindrops that would pierce him like bullets and sandwiches that no strength could bite—because, of course, nothing in the past can be altered."
Star Trek had a Temporal Prime Directive.
Lost seems to be following this rule, in fact continually referring to "the rules" and expounding that history cannot be changed. Desmond's stories drift slightly into Rubber Band History, in that he can prevent individual instances of Charlie's death, but it will still happen as soon as he fails to intervene.
Perry Rhodan, sorta. Humanity gave the Time Police a good whooping, but time travellers seem to have strangely bad luck. There's one example where the bad guys send a team of time travellers to kill Rhodan shortly after his birth, but they crash land on Venus due to a navigation system SNAFU.

Type 3: Rubber Band History

See article.

Type 4: Temporal Balancing Act

Back To The Future used the Overwriting the Timeline version of this, with Delayed Ripple Effects.
Star Trek: Time works this way when the Enterprise encounters the Guardian Of Forever.
The End Of Eternity: Eternity is initially set up as Rubber Band History, but is revealed to be a Temporal Balancing Act at the end.
In Primer, Abe and Aaron are able to "preserve causality" by insulating themselves from the outside world prior to their trip back in time, but when they want to, they're able to manipulate past events to their own advantage. It's never specified whether they're Rewriting the Past or causing Branching Timelines.
- Presumably, they're rewriting the past. Remember the watch experiment? They were able to tell time was looping inside the box because when the box was started, and then later a watch was placed inside and the box closed and reopened immediately, the time elapsed on the watch was an even multiple of the external time past. The watch was circulating through a time loop a huge number of times, each internal loop presenting a finite probability that it would be the one that was reopened, and by building a bigger box and putting people in it (who can decide to exit at the external beginning of the loop), time travel happens.
- Primer may actually fall under Temporal Chaos Theory, as the characters never travel back further than a week—not enough time for wild divergences to manifest.

Type 5: Temporal Chaos Theory

See For Want Of A Nail.
See Butterfly Of Doom
The original Marvel UK comics run of The Transformers had this approach, with time-hopping characters from the future causing (both directly and indirectly) horrific effects to the timestream that result in the catastrophic "Time Wars" and numerous other paradoxes. The situation is eventually contained - barely - but the 'future' is no longer set, and indeed takes a wildly different path.

Branching Timelines

This is how time travel works in Dragon Ball Z, and the reason killing larval Cell in the main timeline didn't do anything.
This is how it used to work in Marvel Comics, particulary around the Stan Lee editorial era, flowing into the Wolfman era. It kind of stopped somewhere in the 90's, if this Troper has his ifs and buts right.

Timey Wimey Ball