To explain this core difference, we're gonna have to get a bit into higher mathematics, quantum mechanics, and the effects of choice in our perception of reality.

All my work is in the same metacanon. What this means is that each universe is somehow related to other universes in a way which does not result in annihilation. Sometimes the difference is parallel timelines, in which case the seperation is that of a multiverse. Sometimes differences are so different and architectural that they are seperated by a metaverse. The metaverse is itself contained to all works of the author, and there is a higher order of universe that could theoretically contain all fiction.

To get deeper into this, we have to start at the bottom.

We see reality as a single timeline, however we have free will. How can we have free will if time exists in a single line, however? It could simply be that our brains use quantum singularities to cause divergent timelines. Much like how a quantum event in the universe (such as a large star exploding) could cause multiple timelines, creatures of high sapience also create divergent timelines when they make choices. This results in an eventual spiderweb of cause and effect.

This is impossible. A good comparison of this is how a computer works. Even if there are multiple threads, if two threads attempt to work on the same item at the same time, whoever gets there first makes changes first, and locks the other thread.

Reality, in this way, works in quantums; moments in time. These moments are infinitely small, but they matter immensely. Even if two people make choices at the same time, in truth these moments probably happen slightly appart. So if A & B choose 1 or 2, and A gets their one quantum before B, the resultant timelines are A1B1, A1B2, A2B1, and A2B2. Of course, observation effects outcomes, so whoever's choice gets observed might become the dominant timeline.

Of course, it's also possible that simultaneous choices would combine into a manifold of correlated timelines. Because of quantum observation, an observed choice could take effect first. So even though A made their choice first, if they did it in a forest and nobody is around to hear, the universe might skip and result in them happening second. This would violate causality, however, and since we can't violate causality OR violate observation, we result in a manifold consistening of both choices occuring first. Thus on top of the original set [A1B1, A1B2, A2B1, A2B2] we also spawn the set [B1A1, B1A2, B2A1, B2A2].

Of course, there are only 26 letters in the English alphabet, and far more than 26 sapient entities in the universe. And though we are describing a simple situation where there is a choice between 1 and 2, there are many events in the universe which spawn many more than two timelines. A good example of this is the episode Remedial Chaos Theory from the show Community, in which a die throw results in six parallel timelines, and a single choice results in a seventh, divergent timeline. (Which ironically enforces the status quo.)

If we begin to chart realities, we can see how long causality chains would result in very different timelines. Imagine, instead of charting your personal timeline, we charted the timeline of the universe. From The Big Bang to whatever eventual conclusion, there are a near-infinite number of divergences. They are, however, all roughly the same universe.

Imagine that, simply based on choices, several fictional universes could be in the same multiverse. Let's take Aliens, Star Trek, Larry Niven's Known Space, and Blade Runner. They all use the same laws of reality, so there's nothing that really prevents them from meshing.

We could not, however, throw something like Harry Potter in there, however. Magic only works under different laws of reality, and those laws fundamentally change the nature of the universe. HP does exist in a relatively realistic setting, though, and we could easily see it existing under the same multiverse as Shadowrun, Conan the Barbarian, Star Wars, and Buffy the Vampire Slayer.

There are some concepts of reality that simply do not allow parallels, however. For example, LOST has inevitability as a central theme, and much of the show is about how it is ultimately impossible to change the timeline. Thus it seems that LOST appears in a monoverse - a universe with exactly one timeline.

There are some series which deal with the concept of multiverses, which thus make it impossible to mesh with other series. Both Marvel and DC have their own multiverses, which mean that they can only cross over via metaverse collisions.

Let's pull things back down before we get bigger. We've talked about universes and multiverses, as well as personal timelines. We've also briefly mentioned metaverses, where things get more complicated. But let's minimize before we maximize.

A universe is a single timeline in a multiverse. We observe it as a whole universe because we are three-dimensional creatures. Our universe began somewhere, a bunch of rules were set up, and it's going somewhere. Those rules that were set up, are going to end up in an eventual ending. Regardless of if that ending is a big crunch, or expansionistic noise, it's eventually gonna blur into a single point.

When you think about other people, you don't think about them as a chain of causality; you think of them as a person. A single point of human data. So, let us think about universes in this same way. And let us compact all those parallels into that single point. A good example of this is the DC Universe, which has multiple parallel timelines, but we can still recognize as a cohesive thing.

So, how would we compare the Marvel and DC universes? They use different rules, and have their own configurations. They're obviously similar in a lot of ways, just more abstract ways.

If a choice of what to eat for breakfast creates parallel universes, it's safe to say fundamental questions about the universe also create parallel universes. However, if these choices are extremely fundamental, they result in incompatible timelines.

Let's take, for example, whether magic exists or not. We've already talked about Harry Potter and Star Trek don't exist in the same multiverse. This choice, Magic0 or Magic1, is incredibly central to how a universe works. Remember when I used the notation A1B1 to describe universe timelines? We can describe multiverses similarly. By describing fundamental laws of the universe, or fundamental configurations, we create another line of reality.

So for example:

• DC Universe - Magic1-Quantum1-Gods1-Author0
• Marvel Universe - Magic1-Quantum1-Gods1-Author1
• Magical Reality - Magic1-Quantum1-Gods0-Author0
• Hard Science - Magic0-Quantum1-Gods0-Author0
• LOST - Magic1-Quantum0-Gods0-Author0

Because we can use the same notation, it is feasable perhaps that these universes could collide. This has occured with several Marvel-DC crossovers, as well as various other series. Although treated as non-canon, the truth of the metaverse is that everything is canon. Just perhaps in spinoff multiverses.

When two stars collide, sometimes if it's at the right angle with the right velocity, they can create a third astral body. Likewise, when two multiverses collide, they can create a third multiverse. And this is essentially what we, as authors who like to muck around with canon, do.

When DC and Marvel collide, the Amalgam Comics Universe is created. It's rarely used, and it's messed up, but it's still a thing.

When I do this kind of analysis, and I claim Shadowrun and Harry Potter take place in the same universe, I am Canon Welding a new multiverse.

And that's ultimately the fun of all this. Infinity within infinity, within infinity. Tickling the brain with endless possibility in ever way. None of it really matters, and it's all just for fun.