Fiction is, in and of itself, a fascinating thing. It exists in nonexistence, it thrives in nothing, and practically everybody has the ability to use it.
So how does it work?
The fiction universe is complicated enough with simple stories. We've all heard something like "In the Star Trek universe..." or "In The Lord of the Ring, it works like this..." but that is just looking at one universe, one faucet of the myriad of exists in the crystal of fiction. There are many universes, each one with its own laws and distinguishing features. There are even crossovers, which take two such universes and force them to collide. So how is all this possible?
The theory is thus: fiction universes are not separate at all, but each one part of the Multiverse.
To understand the Multiverse we must at least in part understand our universe. Our universe is governed by a set of rules and laws, usually referred to as the Laws of Physics. These lays tell us what can and can't happen, how things work, etc. In comparison, the Multiverse has none of these rules natively to itself. There is nothing to say what can and can't happen, whether things have to be done in a certain way, or even if anything exists. Instead, the Multiverse is broken up into microverses. Each microverse is, in and of itself, its own little universe. They have their own laws of physics and rules of nature, but not necessarily the same ones as ours or even each others. Gravity, for example, may be different in one microverse than the next.
A crossover therefore is the result of the collision of two or more microverses, creating a nanoverse. Nanoverses are, or at least tend to be, rather limited as they attempt to follow all the rules of both microverses. This can cause issues when there are conflicting laws and rules, however a compromise is generally accomplished at some point or another.
To avoid the problems that can arise from nanoverses, there are occasionally formed a macroverse. Macroverses are similar to the Multiverse because they don't follow any rules whatsoever. It is a place of possibility is the only substance and rules are made on whim. When one of these are formed, the rules are bent to adapt to the situation and characters that appear inside. This way the rules of both universes in a crossover can be followed, because there are no rules native to a macroverse. It is also worth noting that macroverses are highly unstable and often deteriorate into microverses as laws and rules began to become more consistent. They also have a bad tendency to 'spoil,' or begin to turn against it's inhabitants, deteriorating into microverses that classify as terrorverses and thence are useful for nothing but horror stories.
In short, the Multiverse is where all fiction resides. Microverses are formed, possibly from macroverses, as new fiction is created. Each microverse has its own unique Laws of Physics and Rules of Nature, often varying from each other. Nanoverses form from the collision of multiple microverses, which is caused by the formation of a crossover in the Multiverse, and allow the possibilities of both to exist. Similarly, macroverses allow anything and everything to happen as laws and rules are formed as time progresses. The Multiverse is a realm where all possibilities exists, and where anything can happen. This, in short, is the Physics of Fiction.