Quantum theory includes a concept called Superposition, which is the combination of all the possible states or positions of a system. This superposition concept is at the heart of things like the Schrödinger's cat paradox. In that thought experiment, the cat is both alive and dead simultaneously until you open the box and observe it. Once you open the box, the cat is retroactively defined as being alive or dead. It sounds a little crazy when dealing with things as large and easily observable as cats and boxes, but trust me that this superposition stuff happens all the time on the subatomic scale, which is where Quantum Mechanics presides.
MWI says that there's a very large number (probably infinite) of parallel universes, and more being created every moment. Every decision, every observation, creates more universes. Within the MWI framework such paradoxes as the Black Hole Information Paradox, the EPR Paradox, and Schrödinger's cat are not paradoxical.
Is the cat in the box alive or dead? Both, until you sneak a peek. Then our one universe turns into two universes. In universe A the cat is dead. In universe B the cat is alive. Did the cat notice anything? Well, in Universe B1 the cat is oblivious, but in universe B2 the cat is furious at having been locked in a box and claws up your couch to get revenge. In universe B2a you go out and buy a new couch, in universe B2b you just buy one of those couch-covers to hide the damage, and in universe B2c you're a slob who doesn't care about how your furniture looks. And so on. All these different universes were all identical till you opened the box.
On Life And Death
From the cat's perspective, however, the universes started branching before you opened that box. He's not dead, and he certainly never was undead or "both alive and dead". (He's alive, thank you very much, and he's gonna go take it out on your sofa.)
This leads to some interesting concepts. We may all actually have a form of Quantum Immortality, whereby our minds (or souls) can't actually die, they can only appear to die in other people's reality. We might live a in world where the cat died, but the cat automatically lives on in one or more worlds where it didn't die, and has no idea that it might be dead elsewhere. If this holds true, then you'll live forever, but you also die millions of times in universes you're unaware of.
A scientist could test this via an experiment known as Quantum Suicide, but if the theory proved wrong, you'd be dead, so scientists are a little reluctant to try it.
Communicating Between Worlds
It's possible, if Gravity is a classical force (and thus it's impossible to come up with a function theory of Quantum Gravity), that there may be a weak connection between the parallel worlds. If so, it may be possible, via artificial gravity or some similar means, to communicate with another reality.
Let's say I build a gravity machine that in theory can create some effect that should be observable in another reality. I state that I'm going to flip a coin. If it comes up heads, I'll wait 10 seconds, then pulse the machine once. If it comes up tails, I'll wait 20 seconds, then pulse the machine twice. I flip the coin. Regardless of how it lands, I should be able to see the effects of the opposite pulse pattern happening at the other time. Along with, perhaps, a few stutters caused by alternate realities where I was delayed or jumped the gun by a second or two. Having proven (to myselves, at least) that it works, I can then start doing some sort of morse code with my other mes.
Obviously, this won't work if MWI is BS. It also won't work if Gravity is quantized (if Gravity Particles exist).
MWI would also solve Temporal Paradox, and allow Time Travelers to escape the confines of the Chronology Protection Conjecture and Novikov Self-Consistency Principle. See Branching Time for more information.
Game and Story Use
- In theory, this means that any reasonably realistic "what if" style of Alternate History exists out there, as an actual parallel world. Getting too such an other world is tricky, or possibly impossible.
- But if it's not impossible, then eventually a world will exist where how to do so is figured out. The inhabitants of that reality will start to enter other realities. Every action taken towards this end will continue to create new branching realities. So, if I have to decide between which of a million realities to go visit, a million different me's will go visit those realities. This will muddy the explanations, and lead to Collapse of Causality in some of those realities, except for in the new universes it creates that are just like our universes except without the causality trouble. Is your head starting to hurt yet? I know mine is - except for in the other universe that's just like this one except my head doesn't hurt there. My head doesn't hurt there. Oh, wait, now it does.
- Via the suggested methods for communicating with your other selves, it's possible a Me Squad of the same scientist in multiple universes might pool their resources to collective invent a way to travel between realities.