There are a lot of stories about “paralell universes” where things are the same, but different. The premise is there are an infinite number of universes (falsely justified by quantum mechanics, but that’s another beef) and therefore anything that you could imagine must exist somewhere. I think this is an unreasonable idea (not that there’s any problem with silly fiction) and not to be taken seriously. I still enjoy this kind of fiction, but it seems the “infinite universes” excuse used to justify “this could really happen” falls apart under scrutiny. I would prefer no excuse at all to a faulty one.
First off, a parallel universe must (like ours) have consistent rules. If you want interaction between the universes, then they must all have the same rules. If physics is inconsistent (for the purposes of the argument, a reliable God is the same as a well enforced rule-set) then it can’t be a “quantum physics” justified alternate universe. In some fictional universes the fictional high level physics is abhorrent to low level baryonic matter. If you want it to make sense, it must be thorough. The excuse that “it’s another universe” actually allows for a startlingly narrow range of reasonable options.
Then there are the alien races. Life is insanely complex, and imagining an alien form of life opens huge vulnerabilities to outright absurdity incursion. We can easily imagine an impossible universe as follows. Imagine a race of creatures, call them the Infanticides, who kill all of their offspring. Such a race must die out in the first generation, it can not have a multi-generational culture. Let’s imagine a world where the Infanticides have have lived for a thousand generations. Well that was easy, now we’ve imagined a universe that can not possibly exist, even among an infinite sea of alternatives. Anything having to do with alien ethics, biology, society, or diet is especially vulnerable to this kind of paradox. Throwing together a silly alien civilization and then saying “It must exist somewhere!” is quite simply not true. In fact it’s very likely it must not exist.
Even if you avoid all of the previous pitfalls, a story can still make absurd claims. The root idea of the multi-universe excuse can be summarized as “Given an infinite number of universes, every conceivable universe will be manifest an infinite number of times.” This sounds good on the surface right? If you roll a (six sided) die an infinite number of times you will end up with an infinite number of 1’s, 2’s, 3’s, 4’s, 5’s, and 6’s. (I think the discrete analogy holds, in view of quantum descretization and associated exclusion principles. Let me know if I messed that one up.) However, you won’t get any 15’s, no matter how many times you roll the die. The impossible does not become possible just because you keep trying. Not only is the impossible still excluded, but it is totally within reason that an infinite number of imaginable universes will never be exhibited at all! This would be like rolling a die an infinite number of times and getting all ones. There is no extra-universal law (at all actually, a universe is a set of rules, so outside the rules…) which says that alternate universes must follow statistical distributions. Perhaps there are an infinite number of universes… all exactly like the one we are in now. Both the possible and the impossible need not exist, even with an infinite number of alternate universes.
I commiserate with the desire to latch on to “multiple universes” in order to justify story mechanics or plot. It seems like an easy way to explain broad literary license. The problem is, to a discerning reader, explaining your world as an alternate universe introduces a burden of expectation that few stories are able to bear. When I read fiction, I expect it to be inventive, entertaining, self-consistent, but ultimately imaginary. The excuse of a world that parallels our own is really no excuse at all; Please just write a good story and let it stand on its own.