Rick and Morty and Iris and Barry

The "Many Worlds" theory in quantum mechanics goes something like this: at the tiniest of scales, there may not be any particular reason why a particle goes left instead of right, so maybe it does both?  Reality splits, in one universe the particle went left, in the other it went right.  Each reality is valid and obeys all the rules of physics the same way.  There's a lot of problems with this theory on a practical level, but it opened up possibilities for fiction.  The Multiverse!  What If?!  Eternity Inc.!  And so forth.

Unfortunately, once you accept that anything that could happen did happen in some reality, it sucks the drama right out of the story.  For every universe where the protagonist succeeds, there's at least one where they failed.  In fact, one of the core premises of the Adult Swim cartoon "Rick & Morty" (particularly season 3) is that in an infinite multiverse everything has happened, so nothing you do matters.  There's always somewhere in the infinite multiverse where you didn't do that.  Nobody has a purpose, no one belongs anywhere, might as well just watch TV.  This is rather inimical to maintaining any sort of dramatic tension, even if you can get past the college sophomore level nihilism in general.  To make things worse, it's not just interdimensional travel that opens this can of worms, time travel deals in divergent realities all the, well, time.

Obviously, unless you want to write for Cartoon Network's late night lineup, you'll need a solution to this issue.  Before I go into some possibilities, it's useful to roughly classify the kinds of differences between universes in a Many Worlds multiverse.  I'm going to restrict myself to divergent realities, and leave out universe where the rules are outright different, since those aren't really involved in the stakes for time travel, nor are they usually examples of "how I could have done it differently."

In the next reality, Morty stands behind Summer.

At one extreme there are trivially different universes.  In one world I buttered my toast from left to right this morning, and in another I buttered it from right to left.  Once I finished eating the toast, there was no evidence of which way I did it, and no impact on anything.  You could travel into a trivially different universe and never realize you did it.  The butterfly has to flap its wings a little harder than that for any differences to mount up.

Next you have personally different universes.  The different choices noticeably affect a handful of people and those close to them, but rapidly fade once you get out of their lives.  Probably the best-known example of this would be the realities at the beginning and end of the first Back to the Future movie.  His life and that of his family is much different, but Marty's friends are still going up to the lake that weekend, and he probably doesn't have to learn a lot of new history in school.  This is where the drama-sapping is worst, because in principle both universes still exist in some form, Marty just can't visit his original universe with the technology available to him, but he lived in it and knows it really exists in some fashion.

Owlman is such a Rick.

From there, it's a matter of progressively greater divergences reaching back farther in time.  Did Superman start his career in 1938 or 1961 or 1986?  Did someone win a war in one reality that they lost in another?  Did the Age of Magic never end (assuming a reality where magic exists but is rare and weak in the present day)?  Drama can be preserved when traveling to such universes, because there's no reminders of what the protagonist might have done differently.  The protagonist may not even exist, or if they do it's in a radically different form.  At most, there's echoes of choices made, metaphors that help the protagonist come to terms with their own timeline.

So, that's the key.  Rule out dimension travel to all of the Many Worlds that are too similar to the main timeline.  It's one thing to know anything is possible in principle, but having it shoved in your face tends to engender a Rick Sanchez reaction, or even Owlman being overwhelmed by the cosmic angst in "Crisis on Two Earths."  If you can only ever get proof of the existence of significantly divergent realities, then it's easier to believe that what you do in your own reality matters: there is no evidence of a world where you didn't make that one horrible mistake.

There's two main classes of solution to this problem in a fictional setting: limitations of dimensional travel methods, or a limitation on the number of divergent realities that can exist.  The first keeps you from closing off later possibilities for storytelling, but the second makes it easier for the audience to trust that a story won't be completely invalidated later...assuming they can trust you to not whip out the retcon cannon every time you get bored or want a sales boost.

Limitations on travel.  It's hard, if not impossible, to travel to a nearby divergence for whatever reason.  This can be explained in terms of nodes in dimensional standing waves (you can only go to a calm spot in the wave), spiraling levels of energy (you can only move to nearby realities, but "vertically" nearby ones are strongly divergent and "horizontally" nearby ones are trivially divergent), problems with lack of finesse (anything that can punch through to another reality will punch pretty far past the boundary of your own reality), etc.

If you create a divergence by going back in time, you end up in the new reality, and you can't just dimension-travel back to your own timeline: it's too close.  You might even end up going to divergent versions of the other realities you normally visit, especially if their divergence points postdate your own time meddling.  Barry can't just go find the local version of Cisco and Vibe back to his original timeline every time he has a "Dammit, Barry!" moment.  He has to fix the reality he's in.

DC's multiverse could be explained by saying that there are only a countable number of divergent realities that can be reached via whatever travel method is being used, but there's still all the personally different realities "out there" but inaccessible.  Since you can always get back to Earth-2 or Earth-38, it would requite a model more like the "nodes in the wave" one than the "punch too hard" model, otherwise every time they visit Earth-2 it would be a noticeably different version.  There's only a handful of easily reached points represented by the numbered Earths, everything else exists in principle, but it's a "you can't get there from here" situation.

Since it's a limit of travel method, a writer could waive the limit for the purposes of a story, letting someone visit a personally divergent reality once or twice.  But if you do this too often, the drama-suck returns.

Buttery!

Limitations on reality.  One of the objections to the Many Worlds hypothesis is "where does all the energy come from for these extra universes?"  The idea of time-sharing or vibrational frequencies is often invoked, DC did so when they first had Flash travel to Earth-2.  Reality flickers among the different choices, and our consciousnesses pick out just the right ones (psychic dimensional travel often involving picking a different thread).  But surely there can't be infinite (or nigh-infinite) realities due to left-butter/right-butter choices?

Information entropy could be invoked to eliminate trivially different universes entirely.  Like virtual particles popping in and out of the quantum vacuum, reality creates ghostly trivial splits all the time, but the threads rejoin.  Once the toast has been eaten, the split seals.

Thing is, who's to say what a trivial difference is?  To the toast, it might matter a lot, after all.  Whether I turn left and die in a car wreck or turn right and live might not matter any more to the universe than how I ate my toast.

Any divergence below some critical level will eventually rejoin the "main" version of the timeline.  Only truly major decisions, those worthy of a protagonist, can permanently alter the course of a reality.  You may still have an infinite multiverse, but it's no longer one in which anything that could happen has happened.  A lot of stuff never happened in any corner of the multiverse, because everything collapsed onto one option.  You can't visit the "What If?" reality where you failed to stop the Collector from taking Marrina, because that reality ended up never solidifying...sure, events might have been very different had it existed, but it didn't manage to hit divergence "escape velocity".

As this is an ontological issue rather than a technological one, it's harder to get around.  Once you establish that a reality has been averted via heroic actions, it's difficult to bring it back without saying the protagonist failed after all.  Once you establish in the story that 99% of divergent realities are Vampire Dimensions, you sort of imply that every timeline will eventually be overrun with vampires, turning every story into existential survival horror, if only in the background.  And so forth.

The New 52 explicitly said "There are 52 alternate Earths, period.  Any time you want to visit an alternate Earth, it's one of these.  Don't waste any."  That didn't last very long, did it?  Marvel compacted ALL of their alternate Earths into a single Battleworld for the better part of a year, then launched a New Earth that unified elements of a few of the components and no chance to visit the old versions.  Yeah, not seeing that lasting very long either.  Rules about the nature of a shared universe have trouble surviving the whole sharing thing.

Still, that's a problem with the publishers and the Illusion Of Change.  It's not the fault of the multiversal scheme itself if no one has the willpower to stick with it more than a few years anymore.

Or maybe it's my fault, and buttering my toast the wrong way in 1986 was the last event needed to critically shift us into a reality where comics spiraled out of control....

He KNOWS something....

Dvandom, aka Dave Van Domelen, is an Assistant Professor of Physical Science at Amarillo College, maintainer of one of the two longest-running Transformers fansites in existence (neither he nor Ben Yee is entirely sure who was first), long time online reviewer of comics, author with brains of toast, occasional science advisor in fiction, and part of the development team for the upcoming City of Titans MMO.