With all of the video games that promote choices, as well as comic books, and movies, and books that rely on this premise, I find that a lot of people I know haven't thought very hard about the mechanics of a multiverse and alternate realities. I'll warn you right now that at least some of this is going to verge on time travel territory and you know what I always say about even thinking about time travel, right? (You never say anything about time travel, says the reader).
If you think too much about time travel then your brain will explode.
True story.
Regardless of that, there are two main types of universes I'm going to talk about here. The first one is an adjacent universe. This one is easy to visualize (more or less). If our universe is expanding (it is) then what is it expanding into? The answer that seems obvious to me would be other universes and the space between them. It's like spreading out a bunch of coins on a table, where each coin represents a different universe. They may have different sizes, but each one is its own independent entity.
The fun thing here is that there is no reason to think that adjacent universes all act according to the same physical laws inside themselves.
What does that mean? Well, it means that there very well could be universes extremely similar to ours, except that the laws of those universes might allow for magic, or faster than light travel, or any other fantastical thing that we can only dream of.
That's the simple part to think about. Things get much more complicated when you add in alternate realities.
If adjacent universes are like coins spread flat on a table, then alternate universes would be like stacks of those coins on the same table. You might have a stack of pennies standing next to a stack of nickels or loonies or pesos. Each are very different from the other stacks (adjacent universes) but are very similar to each other. Of course, even within a stack of the same coin, each coin will bear slight differences from the others of their breed.
Alternate universes are generally stacked on top each other, but probably not in the same sense as the stacked coins. In Star Trek there is the idea (I'm sure it shows up in other places) that every piece of matter has a certain quantum frequency, that's part of why, despite the huge empty spaces between atoms and molecules things don't simple pass through each other. But if every alternate universe has a different quantum frequency then a limitless assortment of universes could inhabit the same physical space without interacting at all.
How does it feel to have a cow and an airplane standing in your chest at the same time?
I guess it's more common for alternate universes to be called parallel universes.
Well... here is pretty much THE episode for parallel universe exploration.
If we suppose that adjacent universes all form more or less the same way, how then are parallel universes created?
I think it would depend on who you ask, but from a storytelling perspective, it seems to come down to the choices of sentient beings.
There are two ways to think about this. In the first view, time is much like a tree, where history forms the trunk and main branches, and the outcomes of major events create different branches. This is the main model used in comic books and a lot of pop culture when characters time travel.
This is the route that X-Men takes with both Days of Future Past and Age of Apocalypse (although I have no idea if they'll do that with the upcoming movie version). But both stories involve one or more characters travelling back in time and changing events to create a new timeline, and by extension a new parallel universe. What these stories do not say is that while the characters think that they are changing the future, their original universe will continue to exist in its own bleak form. When they arrived in the past their presence simply caused a new split in the branches. So... Simple, right?
The other way to view time and parallel universes, is to liken it to a rope, or even a bunch of ropes, bound together with each thread representing a universe. The threads in one rope would be a group of parallel universes close enough in similarity as to be almost indistinguishable. These different universes are caused by every decision that every individual makes in a day. In the morning, I exist in one timeline, but when I choose to wear my yellow t-shirt instead of my red one, I create a split where I am actually wearing both.. but since the choice of a color of shirt rarely makes for a meaningful difference in a day or a life, then when I take my shirt off at night, both timelines fold back into one. (You can stop imagining me shirtless now. You're making me self-conscious)
But when say, the President of a nation declares war upon a neighbor instead of seeking peace, that creates a rift that would lead that rope of a timeline to split into two very different ropes that will never mend back into a single timeline.
That made at least some sense, right? I think it would be too easy to end up gibbering like a madman trying to explain all this.
But really, even though there is scientific/mathematical and heck, even religical proof for a multiverse (see string theory, for one) I am mostly coming at this from a story telling point of view.
So what does all of this mean? Whether fictional or real, this view of existence gives us a truly infinite multiverse of possibilities. And in an infinite existence, anything we can imagine can exist. The worlds of Star Wars, Star Trek, Lord of the Rings, and more could very well exist out there somewhere. And that makes me kind of happy.
