Along with time travel, the multiverse is a frequent plot device in science fiction. TV Tropes explains how it works like this:

A collection of distinct universes exists, said universes often being interconnected in a way that allows characters to travel to and from them. It might be as a tourist who just goes to look and tries not to change anything, or as a participant who goes in and interacts with the people in the other universe.

The multiverse, also known as the megaverse, omniverse, or outerverse can encompass an infinite number of possible and impossible “moments”, some of these linked by tidy and coherent timelines. But there may also be an equal or even greater number of incoherent, looped, knotted, or unconnected timeless moments that remain unchanging for all eternity.

Timelines need not progress forward, or even backward, but also what appears to be sideways through disjointed and unrelated frames of reference. The multiverse may have no apparent ultimate purpose or reason for existence, other than it just is.

In fiction, the multiverse can diversify story outcomes. If our heroine Rashida absolutely can’t solve the story problem in this universe, maybe she can solve it in another one and import the solution back. Ah, but there must be a catch that she finds out about later… And so forth.

The multiverse concept has a history in science too. But not an especially happy one. Claiming that things might be different in the universe next door has functioned as a way of sidestepping the implications of the way things are in this universe. For example, does it look like this universe is fine-tuned for life? Never mind, that’s just a coincidence. There are countless universes out there that have flopped … Really?

That said, writers can be as bit flexible in science fiction because it is an art form and is thus governed by the rules of art, not science. As a writer, you can bring in another universe — or a planet in this universe run by intelligent fire-breathing dragons — as long as everything you do after that follows the rules of good storytelling. Your story exists in its own universe (your imagination) and it succeeds or fails from there.

The science issues around claims of a multiverse are still worth heeding. The best science fiction is deeply rooted in science. See, for example, the short story “A Sound of Thunder” (1952) by Ray Bradbury (1920–2012). Exploring time travel, Bradbury introduced the basic concept of the butterfly effect to countless readers. (Seemingly tiny perturbations in our environment can result in huge changes later.)

Whether you want to read science fiction more deeply, write it, or choose it for a collection, here are some thoughts from two physicists and a film critic:

● Harvard physicist and philosopher Jacob Barandes speaks for many physicists when he grumbles that the multiverse is “unscientific nonsense”:

Unless and until we obtain experimental evidence in favor of a theory in which the laws of physics can vary from one region of the universe to another, any talk of other universes with different laws of physics is purely speculative, and should be treated as such.

Jacob Barandes, “The multiverse is unscientific nonsense,” IAI TV, November 13, 2023

True, but science fiction is by nature a form of speculation. However, good storytelling does require responsible speculation.

● Dartmouth College theoretical physicist Marcelo Gleiser isn’t quite so discouraging. He cautions though that a multiverse is in some ways like a religious belief rather than a science hypothesis:

But if we do live in a multiverse, can we ever know? Can the multiverse be observed? Is the multiverse a testable scientific hypothesis, or is it just idle speculation leading to a dangerous schism in the physics community? More to the point: Is the multiverse knowable? If other universes exist outside our cosmic horizon, we cannot ever exchange any signal with them. They are inaccessible to us and to our instruments. We could never see or visit them, nor be seen or visited by anybody who may live in them.

Marcelo Gleiser “The multiverse is cosmology’s unreachable frontier” Big Think, June 21, 2023

In a story, there is surely a way around this but it needs to be seen as a big If.

● Finally, journalist Peter Suderman points to a storytelling flaw that the multiverse backdrop risks, using The Flash (2023) as an illustration:

Multiverse movies make a fetish of choices and their infinite malleability, of the possibilities opened by exploring a vast array of alternate worlds. But ultimately, they undermine the very idea of choice in the context of storytelling by rendering every choice meaningless, since anything can be tweaked and twisted just by hopping to the right multiversal strand. In linear movies and in the real world, choices have power, partly because they give people options, but also because they have finality. They demonstrate character, will, and foresight; they are tests of who we are.

Peter Suderman, “The Flash Makes the Case Against the Multiverse,” Reason June 16, 2023

Eliminating finality in that way risks reducing interest in a story. If no choices are final, the characters are no longer like you or me. We can ignore all this, of course, if we want and just read or write the science fiction that suits us. But then the multiverses are no more than multiple sets and dressing rooms. Stories using the multiverse plot device might gain more subtlety if the complexities raised by the commentators above are heeded.

You may also wish to read: Madness: Why sci-fi multiverse stories often feel boring. In a multiverse, every plot development, however implausible, is permitted because we know it won’t affect our return to the expected climax. Every scene in the multiverse feels like waiting. The viewer will literally be waiting for the rest of the movie to happen while inside the multiverse.