This section was written by Huginn.
Yes, at this moment, another version of you is reading this exact same impromptu essay.
Okay, so maybe it's not the same as using crafty cutlery to pass between worlds, but it's a start. Parallel universes are far from science fiction, though. There are millions, billions (infinitely many, even) versions of you. That's right, I'm talking to you. Not just me, not just the guy who lives two doors down, but you, too. Everyone. Not only that, but a very small fraction of them lives in worlds identical to our own. They tripped over a tricycle when they were three and busted their chin open just like you did. And that's just the beginning of the strangeness. How about worlds where there are only two dimensions instead of three? Or multiple dimensions of time? Some of the possibilities are simply beyond comprehension, but they could happen. Easily. It could've been us.
Of course, the problem is that the number of parallel universe theories is almost as big as the number of universes that those theories allow. However, they can be broken down into some pretty broad strokes, and knowing how they work might help you next time you end up the proud owner of a strange new blade (and down two fingers).
The first and simplest idea involves a basic assumption: that space is infinite and always expanding. Saying that space is infinite seems counter-intuitive to some people: after all, if there was indeed a big bang and the universe has been expanding ever since, then it can only expand so fast, so far. In practice, though, we can only see the universe expand by measuring how far we can see in the first place. Based on the speed of the light and the age of the universe, we can say that the universe (the one that matters to us and can affect us) is only what we can see, everything within a radius of about 78 billion light-years. Space doesn't end just 78 billion light-years away, though. Some people think it just wraps around, so that if you keep going in one direction, eventually you end up where you started. However, one theory speculates that instead, outside of the bubble of our observable universe, there's another universe, parallel to our own. In fact, if you say there's infinite space, there can't be just one parallel universe. There should be (you're going to love this phrase) infinitely many. In this case, the core idea behind the different universes is the arrangement of matter. Just as there are six ways you can seat three people in three different chairs, there are a lot of ways you can arrange all the matter in the universe, and each of those ways can be a distinct universe. Naturally, some of these differences can be small. That piece of paper on your desk could be just one inch to the right, and that would be a wholly different universe. However, the number of distinct universes is still finite (you can arrange the matter in the universe a huge number of ways, but it's still not infinite), so if space is still infinite (as we assumed it must be), eventually you get repeats, copies of other universes because the matter arrangement is the exact same. In such a copy of our universe, there would be a duplicate of you, me, and everyone else. Freaky, huh?
Oh, wait, it gets worse. Now let's think about something else changing. Instead of just rearranging matter like books on a shelf, let's talk about changing some fundamental properties of the books. Page thickness, print size if you change these values, then the books themselves look different. The same holds true for a universe. If you change certain physical constants (like the speed of light or the charge on an electron), the universe behaves in a new manner. You could make it so that atoms can't form without blowing themselves apart just as quickly, or better yet, you could add another dimension of time and throw out any semblance of causality to our existence. This second level of multiverse involves parallel multiverses (confused yet?) like the first level we talked about earlier. Put plainly, we might live in a multiverse where other universes are different arrangements of matter but have the same basic conditions, but then, outside of our multiverse, there might lie another multiverse with different conditions but with the same bubbles (distinct universes) inside it.
The third basic level of multiverse is something even casual viewers or readers of science fiction might be familiar with. In fact, odds are that if you're on this site, you read a book or three that use this very concept. We know from quantum theory that some processes or choices are inherently random. Why should one outcome happen, then, and not another, if they are truly random? The solution is to say that all outcomes happen, but the ones we don't perceive only occur in (you guessed it) parallel universes. In HDM, we see this mechanism in action through the disparate histories between worlds: in our world, John Calvin was a protestant who broke away from the Church, not a Pope, as in Lyra's world. This, however, confounds the idea of random mechanisms with free will and choice (That is, are the choices we make really random? From a purely mechanistic view, yes, because they come from electrochemical reactions in our brains, big systems of essentially quantum particles, but that's a wholly different topic.), so let's try a different approach. The classic example used is the rolling of a die. Each number, one through six, could land face up. We roll the die, and we get a one. According to this theory of ours, we just caused our universe to split into six, each of which having a different outcome for the roll of the die. This is the basic theory that most people are vaguely familiar with. Now we give it a twist. Instead of saying that each roll of the die creates a new branch in our universe, we go back to the first level of multiverse we discussed, where each universe is a different arrangement of matter. Naturally, one of these arrangements corresponds to the instant before we actually roll the die. Another corresponds to an instant during the die's roll, where the outcome is not yet known. Others still match the possible outcomes, different universes in which the die sits at rest on a one, two, three, four, five, or six. All of these are valid arrangements for matter in a universe, so we can picture the quantum nature of the choice, the decision, the outcome, as a path between universes. Time is just the morphing of one universe into another: we go from the pre-roll universe to the roll-in-progress universe to one of the six post-roll universes, and the only place quantum mechanics affects us is the particular post-roll universe we end up becoming.
The fourth and final basic level of multiverse involves something worse than all the physics I'm putting you through. Yes, it's bane of schoolchildren everywhere: math. We're all familiar with using math to describe how the universe works: e=mc2, F=ma, and so on. Well, why those laws? Who says e has to equal mc2? Why not e=4? It's the particular physical laws that describe our universe that we're looking at now. Each of our other levels has involved something changing to create other possibilities. Here, it's physics itself. It's the mathematical structure that defines our universe, not just its laws, but the whole thing. If the universe can be described by one equation, the greater multiverse, the only one that is all-encompassing, uses every equation that is valid. Instead of being nice, symmetric bubbles, these other universes could be in the shape of bow-ties or doughnuts. They could have anti-gravity as dominant instead of gravity. They could break the speed of light, if they even have light at all.
Okay, so let's recap, shall we? In short, the different universes we see in these categories arise from basic discrepancies in things we consider elemental to the nature of our own universe. In the Level I Multiverse, it's the arrangement of matter. Level II, the values of physical constants. Level III, the outcomes of random (quantum) events. Level IV, the very laws of physics themselves.
So, now you know about parallel universes. Don't you feel special? Wait, you want to go to one?
Ha. Ha-ha-ha. Ha-ha-ha-ha-ha. Ha-ha-ha-ha-ha. Ha-ha-ha-ha-ha. Ha-ha-ha. Ha. Ha. Ha.
Don't even get me started.