There are various ways that time-travel is described in stories and in physics.

• Some of these differ in how paradoxes are (or are not) resolved.
• Some differ in the physical limitations they impose.¹

There are various meta-studies and attempted catalogs of the entire field.

The world of recorded videos are very comprehensive in their collection of possible systems of time travel, particularly when fandoms interpret their own favorite or adjacent ouvres.

Sources

• New Rockstars: Time Travel Possible? Avengers Endgame vs Tenet (8 Types Explained)

Time travel fiction has been a recurring go-to for Hollywood films, from Avengers Endgame to Christopher Nolan's new film TENET. But which time travel movie gets it right?

In this episode of “BigQuestion”, Erik Voss and Hector Navarro dive deep into time travel in movies, TV, and literature to outline the 8 TYPES of time travel, from alternate timelines and paradox loops, to consciousness displacement and "anything goes" comedies. This is your definitive breakdown of the time travel Back to the Future, Avengers Endgame, Bill and Ted Face the Music, Terminator, Harry Potter, X-Men Days of Future Past, LOST, Game of Thrones, Hot Tub Time Machine, Interstellar, Looper, Rick and Morty, Groundhog Day, and more! Which time travel story has the worst plot holes? And which time travel story is the most accurate with what theoretical physicists and mathematicians have recently discovered to be possible?

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Tries to answer the Big Question:

What are the different versions of time travel? And which version gets it the most right according to real world physics?

Which refers also to....

• Minute Physics: Time Travel in Fiction Rundown

Time Travel’s Dirty Little Secret

But, in attempting to quantify the scientificness of different time travel models, they are overlooking a secret so small and dirty that it has been referred to, just now, as “Time Travel’s Dirty Little Secret”

Time Travel’s Dirty Little Secret is simply this:

Time Travel is a literary device.

It is not in any way a scientific concept.

Attempting to discuss the way that one time travel “model” is more or less “scientific” than another is like betting on which fish will win a mountain climbing contest. Time travel models are not “more or less” scientifically valid, they are each more or less “useful” in the telling of a specific story.

Models tend to differ in the way they deal with the potential for chronological parodoxes. The different approaches can be described as:

1. Ignore. Just don’t mention the paradoxes.
2. Explain. Have some plausible if hand wavy way of not needing to deal with time paradoxes at all.
3. Embrace. Have the plot itself gain tension from the need to somehow resolve a paradox.
4. Build. Systems with branching time don’t need to treat the time mechanism as something to be dramatically revealed, but instead can assume time branching is understood, and build the plot on top of this without needing to resolve paradoxes back to a single logical timeline.

In each case the details of how a time model works are entirely subservient to the needs of the story. I’m n so called “hard” science fiction, the mechanics will be completely consistent throughout. In “softer” science fiction or fantasy, there is less emphasis on ensuring consistency. Even then, inconsistencies must not be so marked as destroy the enjoyment of the story.

The original “Time Machine” by HG Wells, which he furiously jotted down in the back row of a dark cinema while watching 1991’s “HG Wells: Time Machine” had the first act resolution involve the lesson that - because he built the Time Machine I. Order to go back in time and save his fiancés life— any attempt to save her life would be ultimately fruitless. He would always find himself to be in a timeline where he invented the Time Machine and therefore in a timeline where his fiancé died. And many authors have been retelling that clever parable ever since.

Any story about altering some disastrous moment in history (such as the “Let’s Kill Baby Hitler” trope) is essentially a variation on Wells’s first act.

Note that a film franchise like “Back to the Future” employs all of the above techniques for dealing with time paradoxes:

1. Ignore: When old biff in 2015 stole the almanac and the car, went to 1955 and sparked off a whole new timeline, they were able to ignore the impossibility of him returning to his point of origin in 2015, so the movie could happen.

2. Explain - At other times they have vague or wand-wavy explanations - such as that Jennifer, on seeing her future self, may either cause an irreparable space-time rip, or she may simply faint.

3. Embrace. During parts of the emotional climax of the first movie, Marty begins to fade out of existence, showing that the paradox of his being there will resolve itself by him not existing, unless he can achieve his mission (reuniting his parents). Hence the resolution of the paradox was the explicit goal of the plot at that point in time.

4. Build. The second film, with its branching time model allowed for further building a new world on top of the existing, through channeling our future selves into specific branches of space time.

And none of that is what I came here to say.

What I came here to say is that Back to the future is a comedy of manners. We get a fish - called Marty - carrying modern senses and sensibilities (well, modern as of 1985) and we place him in a time where those senses and sensibilities are out of place. That’s a classic “comedy of manners” - the time travel device is simply the literary device used to Make It So.

A heck of a lot of stories are a “comedy of manners” — once you begin to look at the world through my newly patented “I see comedy of manners everywhere!” glasses. Mean girls, Heathers and the their ilk; Saltburn, Eddie the Eagle, Pulp Fiction, you name it. And at just $19.95 a pair, what discerning movie critic would be without these little modern wonders?

Oh, the answer, by the way, is salmon. Provided there is a suitable stream all the way to the top of the mountain.