I'm reading an excellent book called The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos by Brian Greene at the moment. Now I've read a lot of popular science books about physics and cosmology but none have so completely baked my noodle as this one!
It describes the many different theories around that involve parallel universes of some type or another (there are nine I think now) - some are not difficult to understand, some are mind-boggling in their scale but still make some sort of sense (string theory ones mainly, which still sound to me almost like mathematical fairy-tales) but the one I have reached now is really wild - the 'Many Worlds' interpretation of quantum mechanics.
I was familiar with the idea, which was invented by Hugh Everett III in the mid fifties (there is a great documentary out there following his son, E from the band Eels, finding out about his father and his ideas - highly recommend that). Basically what it says is that when a quantum event with various possible outcomes happens, rather than just one of the possible outcomes occurring and becoming 'reality', they all happen, each one forking off into another reality, another universe. Sort of.
Imagine a physicist in a lab with an experiment ready to measure the position of a single electron. Now according to the predictions of quantum theory this electron has (for example) 90% probability of being in one position and 10% probability of being in a second position. If the physicist performs the experiment enough times they will find those probabilities to match up extremely accurately with theory (quantum electrodynamics is the most successful theory in science in that respect). Here's where it gets weird.
'Many Worlds' theory says that both these possible outcomes occur, they just 'split' off into different realities, both as real as the other, neither one being the one 'real' universe. The physicist who finds the electron in one place is just as real as the physicist who finds it in the other. But if both outcomes occur, where does the probability factor go? Does it make any sense to say one outcome is more likely than the other when they both happen anyway? Everett suggested probability comes in after the event in the subjective experience of each person - but that just seems to me to mean the probabilities have no objective reality to them.
This is a really bizarre thing to get your head around when you start to think about it. From that example, I began thinking along these lines: say the physicist performs his experiment repeatedly, stopping when the electron appears in the more likely position, but repeating it again if it appears in the less likely one. Now remember - both outcomes occur - so each time the experiment is performed the electron will appear in the least likely position (regardless of how unlikely it is - say it's a 1 in a million chance) and a physicist will see that, and then perform it again. If you extend this on and on, you end up with a physicist who will have seen a huge number of one in a million chances happen, one after the other - and that's not an unlikely event, according to the theory that will happen.
How would this physicist react? His experiments will show that quantum theory, in his reality at least, doesn't match up with experiment. In theory, given a certain series of events (that will happen, remember) science in a reality that continually diverges along these lines might come up with a whole different quantum theory that does explain the results they see. Would that theory be wrong?
Keep going with this thought and you then ask the question - what if our reality is that one? One where a staggeringly unlikely set of events (according to one version of quantum theory) has and continues to happen and so we've come up with our own versions of theories to fit what we see. If all outcomes of quantum events occur, and someone in some version of reality sees them (regardless of what we think the probabilities are) then there will be countless realities with countless different results of series of experiments which lead to countless theories to explain them. We may be just one of them and our theories may no more be 'right' then any other out there in the 'Many Worlds' multiverse.
It twists your melon, doesn't it? I don't know if my thinking there fits with what the theory says, it quite probably doesn't, but it is always fun to go off on a train of thought like that :)
EDIT: I forgot to mention an absolutely excellent SF novel by Greg Egan called Quarantine. One of my favourites. He takes similar quantum ideas and suggests that our brains naturally have the function of unconsciously 'collapsing the waveform' which chooses one of a number of outcomes of an uncertain event to be real. Then he comes up with the idea that someone might be able to control that collapse - basically they can exist in and can in some way perceive all of the 'many worlds' and then pick which one they want to become 'reality'. It's a stunning book, and that is just the tip of the iceberg of amazing ideas in it. Get it, read it. ;)