Originally posted by Zort Boy
What is the theory on their exsitance and can actions in one influence events in another?
There are lots of them and they're very complicated. I don't know if I understand any of them at all. Check this out:
Two of the simpler ones seem to have the "universes" separated by space or time. In the former case, the idea (I think) is that in an infinite space matter will come to be arranged similarly - identical or almost identical - to in our own observable universe and so there are versions of ourselves far, far away right now - too far to interact with though. The prediction in the Wiki article has it as being probably about 10^118 meters away. The limit of the speed of light makes that too far to interact with in any reasonable amount of time.
Another version in which the Big Bang is later followed by a "Big Crunch" followed by a "Big Bang" followed by a "Big Crunch" etc. treats the "universes" as separated by TIME. If we could survive beyond the end of the universes we could possibly interact with the next one...far, far, in the future...
Basically the time it takes to interact in both situations is so long that it's practically impossible (longer than the age of the universe, for example).
Those are the ones that seem easiest to understand. There are more abstract ones too.
M-theory makes no intuitive sense but claims gravity can go from one of their kinds of universes to another one. This makes a third kind of "alternate universe" with another version of you all of which might be true! So we have some versions of you impossibly far away; other versions of you which will exist after this universe ends, and others who existed before this universes was born. All of these will take too long to conceivably interact with, but the M-theory alternate universes can be interacted with maybe with gravity...then there's something about universes being quantumly entanged with ours which proves some astronomical phenomenon or something...