No, that is a huge coincidence. Yes, all possible universes exist, so there would be an infinite number of them, but the odds of randomly ending up in one, if it were random, are infinitesimal.
No, it's not. First of all, ending up in any other universe is equally infinitesimally likely. And secondly and more importantly, there'd be infinite universes where she indeed did end up elsewhere. We'd just look at the one relevant to this story, so this is just selection bias ^^
Selection bias doesn't come into it. Yes, if there are infinite Marikas randomly jumping into infinite universes, then yes, at least some are going to have a nonzero chance of leaping into a yuriworld very similar to her own world. But we aren't looking at this as one in an infinite series, we're looking at this as a single instance, and this instance is still exceptionally coincidental and improbable among them.
While there may be infinite universes like this one, there are not an equal number of universes of infinite types. Not all infinities are equal, and of all the universes that diverge significantly from ours, only a small fraction would be likely to return and intersect. This means that statistically, the odds of ending up in a universe that diverged and reconverged is much lower than the odds of ending up in a universe that has only barely diverged or (even more likely) has diverged significantly (and much earlier)
If it were purely random, the odds of ending up in just this sort of universe, one in which everyone she knows still exists despite massive historical differences, is absurdly small. Every person in the world is just one of millions of potential people that could come out of their parents joining. If your parents had conceived you in a slightly different position, a different sperm would unite with the egg, and you would be an entirely different person on the genetic level. But here we have a universe where not only is everyone in the world almost exactly the same person as in our world, but they got that way from -different parents- utilizing an entirely different means of biological reproduction.
This is all leaving aside whether or not it is actually even physically possible for the humanity of the 1900s to happen across a means to create a way for females to interbreed in a such a fashion that doesn't so drastically effect biodiversity among humans that it is literally impossible to end up with the same people, which I thinik is an open question. Also it may not be possible for the population of that period, after a major catastrophy, to rebound into a civilization with the same population and culture that exists in our world. I mean there's a whole litany of things that might make this story literally impossible, but, I mean....it's just a story. ;p
Now you can say that the author just picked this universe to show, and sure, that could be possible, but then there's no mystery to solve, and the author would be building this dramatic tension just to pull the rug out at the last minute with "Nope, it was just random!" That would be a...unique writing choice. What the author has done in this recent chapter is deliberately call attention to how unlikely this specific universe is, which makes it unlikely that it is accidental random swap. The fact that Yuriworld's Marika was into the occult and parallel universes also makes it likely there's some design behind this swap. I mean, if you're gonna jump into another universe without some kind of guidance, there are likely way more universes where the Earth isn't even here. She'd have been better served tying a noose or jumping off a bridge if she was -that- miserable.
TL:DR version: If the swap was random, this is massively coincidental and improbable/borderline impossible, and the author seems to be indicating that there is a design to it, which they wouldn't do if massive coincidence was an acceptable narrative to them.
last edited at Apr 20, 2019 5:12PM