What is this speculative fiction genre called?

Question

I'm thinking about the type of speculative fiction where the characters travel to another dimension or another universe, or someone/something from another universe arrives in this one. Examples:

• The Negative Zone in the Fantastic Four comics

• The Phantom Zone in pre-Crisis Superman comics

• Stephen King's "The Mist"

• Edwin Abbott's Flatland

In each case, the concept is that there's another world that's geometrically impossible to reach via conventional space travel, but instead is directly accessible from this one by breaking the barrier between universes.

I'm hesitant to just call it "fantasy", because the idea of a parallel universe has some tenuous basis in math and physics. But it doesn't seem to fit in any of other speculative fiction genre that I'm familiar with; "alternate history" and "surreal" don't seem to quite capture the nuance that the other world is spatially distinct and accessible from this one by means of unconventional geometry. "Multiverse" effectively describes the structure that I have in mind, but I haven't seen that used as a genre name.

Answer 1

I don't think it has a single widely-accepted name but I've fairly often seen the phrase "cross-world" used in the context of adventures where characters cross from one parallel universe or dimension to another. For example, Charles Stross' The Family Trade series is described as involving "cross-world culture shock" on page 84 of the book Unleashing the Strange: Twenty-First Century Science Fiction Literature. It's hard to google because you get a lot of unrelated use of the term like the "Cross World Travel Agency", but if you search "cross-world" (or 'crossworld') along with a term like "multiverse" or "parallel universe", you'll see a lot of people using it in this context. So if you were to speak of a "cross-world adventure" or a "cross-world travel story" to a science fiction fan they would probably understand you were referring to this kind of story.

For another option, you could call this "inter-universe travel", although the term gets a fairly small number of hits on google (but if you search for the term on google books, there are a decent number of published sources that have used it).

Answer 2

It is not a (sub)genre but a (popular) topic. Although indeed also inspired by the quantum-mechanical hypothesis it has far older origins, for example Plato already had thoughts on it.

Reference:
https://en.wikipedia.org/wiki/Parallel_universe_(fiction)