User:VidTheKid/Time Travel in Fiction

Apparently having an easier time understanding temporal mechanics than the average person (and apparently many TV writers) I believe I'm qualified to critique, or at least analyze and discuss, time travel as used in fiction, especially TV and movies. I'll talk here about individual works of fiction and how well they hold up to logic. But first, I'd like to describe in general the main types of time travel rules used in fiction.

Types of Time Travel
Most fiction involving time travel follows one of these types, but sometimes multiple types are used in the same fictional universe. In such cases, it may be explained by asserting that the particular type of temporal mechanics that will manifest are not inherent of the universe itself, but of the particular method by which time travel is achieved. For example, natural or long-term phenomena (such as a wormhole with one end 50 years in the future of the other) might tend towards single-history rules, whereas artificial or instant-transport methods (such as the conventional idea of a "time machine") often tend towards multiple-history rules.

Single History
In single-history time travel, the past can't be changed. Nor can the future be changed once known. This is because any attempts to change the past are already part of the past, and knowledge about the future already takes into account the fact that the future was known. For example, if you go back in time to kill your grandfather before he conceives your father, you won't succeed, because someone already tried (and failed) to kill him, and that person is you. Or maybe you do kill him, but were too late, as he had already conceived your father, and your grandmother kept his death a secret and arranged a fake marriage to hide the fact that your father was conceived out of wedlock, and never told anyone. Or maybe the man who you thought was your grandfather wasn't really your grandfather, or maybe your father was adopted. Maybe it turns out you were your grandfather. In any case, you may end up causing and/or participating in some noteworthy anecdote which your grandfather has told you dozens of times.

This is my favorite type of time travel. Examples can be seen in Bill & Ted's Excellent Adventure, Gargoyles, the TNG episode Time's Arrow, and Terminator (if you ignore the sequels).

Multiple Histories
In the multiple-history form of temporal mechanics, history diverges into different paths. While a quantum physicist would like to think that every possible history exists, in fiction it's much easier to portray history diverging at discrete points. Logic would indicate that the points of divergence must occur exactly when a person or information arrives from the future, as the very presence of the time traveler constitutes a change to history. In nearly every case, one history sends a traveler or information into the past, creating a "second" history. Although, since this "second" history usually does not make any attempt to alter its past, it actually causes the "first" history by inaction, though this fact is almost always ignored in portrayal. In this way, neither history is first or second, but the one with the more desirable outcome is usually considered to be the "main" or "real" history. Since humans are conditioned to think of events in terms of before and after, the characters in the more desirable history consider history to be changed forever, (if they're aware of the time travel at all) and believe the other history to "no longer" exist. The audience typically believes this as well, and presumably, so do the writers.

To use the grandfather example again, you might go back in time to kill your grandfather. Assuming you're successful, you cause a history in which your father is never born, and therefore you are never born. Since you don't exist in that history, there is no you to go back in time, causing the history in which your grandfather is not killed, and eventually, you are born.

This form can be seen in Dragon Ball Z, in a majority of Star Trek episodes involving time travel, and somewhat muddled in the Terminator sequels.

Convergent Multiple Histories
Occasionally, a situation is portrayed in which a history causes another history, which then causes itself. The first history then doesn't really ever exist, since no other history causes it. On the other hand, it might almost cause itself, with multiple equally plausible final outcomes, varied by quantum uncertainty. Or, in a more linear understanding, bits of information might pass from one "iteration" of the history to the next, with a cumulative effect, ultimately causing another history which causes itself by inaction. See Time and Again, Cause and Effect

Sloppy
Sloppy time travel rules are when a story portrays inconsistent results of time travel. Typically, this looks a lot like the multiple-history format, but doesn't hold up to logical scrutiny.

A very common symptom of sloppy rules is the active feedback concept. That's when you change something in the past, and some artifact you've brought from the future changes or disappears seconds later, but your memories of it aren't affected. Besides the issue of concrete records changing but not memories, there's the issue of timing. Why does a future artifact only change only after you make an obvious change in the past? It's an error to say that history actually changed at that moment in time. Maybe the two histories clearly diverged at that point, but they were already different, by the presence of you, the time traveler, and you were already going to make the change, so why didn't the future change "when" you first arrived in the past? After all, from that point onward, the future was going to be different. Some people might argue that because of uncertainty, multiple futures are possible at any one time, and the future artifacts only change when that change becomes absolutely certain. However, such a change can usually be used as a warning, and is usually reversible, so it can't possibly require absolute certainty. So then, future artifacts must therefore represent the most likely outcome, right? But then, if it changes, it must not be a very good forecast of the future at any time, or at the very least it doesn't factor itself into the equation. There's just no way to make it work logically. Under these rules, you could go back in time and kill your grandfather, and then you would disappear, just fade out a few seconds later, possibly as his heart stopped.

Another symptom of sloppy rules is the concept of a paradox being capable of destroying the universe. What would that be, some kind of massive explosion that occurs at a specific time? That really wouldn't resolve the paradox. Like active feedback, this is another faulty implementation of multiple histories. So you go back in time and kill your grandfather, and then suddenly the whole universe explodes. If the universe explodes, then there can be no you from the future to cause a paradox. You know what? That's another paradox! So what's the point in the universe exploding if it doesn't solve anything?

When enjoying fiction like Back to the Future and Quantum Leap, you kind of have to take things at face value, because the causality tends to fall apart under examination.

Discussion of Individual Works of Fiction
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