| abstract | - Temporal Mutability -- 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? Nope. 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): 1.
* 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. 1.
* 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. 1.
* 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. 1.
* 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. 1.
* 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.
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* 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. Progress can be tracked with a Ripple Effect Indicator if one is available.
* Branching timelines: Your time-traveling causes a new timeline to split off the original, and both timelines exist (can be temporarily or permanently if it happens to be Another Dimension identical to your own but shifted in time) 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. No one says "But what if saving that guy somehow causes World War Three — because it didn't 'really' happen or it's not the 'correct' outcome, and humans are not supposed to change history?" See also this page, for a more in-depth discussion. Examples of Temporal Mutability include:
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