Why is the multiverse approach the only way to make the story viable?
I'm happy with the paradox approach.
Neither way has been scientifically proven either way.
Not true. A paradox, by definition, is an insoluble contradiction, something that can't physically happen or be logically true, because it's saying two mutually conflicting things at once. (For instance, "This sentence is a lie." If it's a lie, then it's true, and if it's true, then it's a lie. There is no solution. It's simply an invalid premise.) If an equation results in a paradox, if it can't produce a definite, non-contradictory solution, then that rules it out as a valid equation. A paradox is not a solution, it's proof that you've formulated the problem wrong in the first place.
The idea of a moment in time being "erased" or "replaced" by a different version of itself is a contradiction in terms, because erasure or replacement means there's an earlier version and a later version of the same thing -- they exist at two separate points in time, before and after the change. But a single point in time cannot come after itself. The very idea of a single moment being "replaced" is a contradiction and an impossibility. It may look to the time traveler like it happens twice, but that's just because the time traveler looped back over their own worldline and experienced the same single moment twice, like rewinding a video. It's an illusion, not a physical reality. Or at least, it's only real in the time traveler's subjective frame of reference and doesn't outweigh the rest of the universe's frames of reference.
The only way there can be two different versions of the same single moment in time is if they exist simultaneously, by definition. Parallel timelines are the only way you can have more than one history in a way that's physically meaningful and possible. Anything else is just a fanciful conceit of fiction.
There is actually a well-developed scientific understanding of how time travel would work if it were physically achievable. The whole purpose of science is to let you extrapolate beyond direct experience and predict what would happen in new circumstances. Everything in the universe obeys the same fundamental laws, so if you understand how those laws apply in one set of conditions, you can predict how they would have to apply in another. The equations of General Relativity helped us understand how space and time work, how they relate to each other, and how two observers can perceive different measurements of time. So to say "we just don't know either way" is incorrect. We can make informed predictions about the possibilities, the same way we can about alien biospheres or starship drives. We know enough about the universe to know what's possible or impossible in broad strokes, even if we're just making informed conjectures about the details.
I find the paradox approach more interesting. Otherwise three is no issue with time travel. If every time you time travel you create a new universe then say Nero did everyone a favour by travelling through time and creating an extra universe? Everyone should have been thanking Nero instead of condemning him for being a genocidal lunatic.
Yes, exactly. Fiction favors the scientifically nonsensical premise of "rewriting" history because it gives more dramatically and emotionally satisfying results, with higher stakes. So say that. Don't say "science doesn't know the difference." Science knows perfectly well. Just say that stories don't have to be bound by science. A lot of human creation is about representing the impossible and nonsensical, like M.C. Escher's infinite staircases or impossible cubes. Art isn't limited to what's realistically possible. Unless it chooses to be, like hard science fiction or realist painting.
Still, there's the Ancient One's argument from
Avengers: Endgame: Even if your actions create a parallel timeline, the inhabitants of that timeline are still living people as much as the inhabitants of your own timeline, so if that timeline's history is altered in a way that causes mass death and destruction (such as Nero destroying Vulcan), then it would be incredibly callous to shrug that off as irrelevant. The stakes still matter even if your own history is unaffected. Plus, of course, the characters in the Kelvin films were the ones living in that altered timeline, so of course its events had life-and-death stakes to them.
Anyway, there can be dramatic merit in stories about time travel creating branching timelines, such as
Avengers: Endgame and
Loki, or stories about fixed timelines where nothing in the past can be changed, such as the movie
12 Monkeys or the time travel episodes of the
Gargoyles animated series. Time-paradox stories are such an overused trope that it can be refreshing to avoid them. I've taken a stab at fixed-timeline time travel plots in a few things I've written, including a couple of my Patreon stories (
index here). There are ideas worth exploring there, like how to avoid a seemingly inescapable fate in a fixed timeline, how to cope emotionally with the inability to alter a tragedy, etc. A lot of fiction is about characters trapped by destiny or by circumstances they can't change, going back to ancient narratives like the Oedipus and Achilles myths.
My favourite time travel story is I suppose the original H G Wells movie where every time he tried to save his fiance the incident changed so he could never be successful so a paradox would not occur.
"Original?" That was the fourth and most recent screen adaptation of the 1895 novel (after a live 1949 BBC version that wasn't preserved, the 1960 George Pal movie with Rod Taylor and Yvette Mimieux, and a dreadful 1978 TV movie with John Beck and Priscilla Barnes). And it's the only one that gave the time traveler a fiancee or had him attempt to save her life.
Also, he only tried to save his fiancee
once, and then abruptly jumped to the conclusion that every attempt would have to fail. Which is a complete failure of scientific thinking, since you can never assume a pattern based on a single result. I keep hearing people say he repeatedly tried to save her, and I wonder if the version I saw was missing a sequence, or if people just misremember it because the characters acted as if he'd made many attempts even though we only saw one.
