[Brian Holtz] If two spacetime-disconnected regions are causally disconnected (such that none of the events in each has any possibility of influence on any events in the other), then it seems pure artifice to say the regions are in the same world. You could as easily say that all possible events in all possible worlds are in fact in the same world.
Is that true? Isn't Reichenbach's common cause a possibility? Something like: future light cones of a and b do overlap within the future light cone of c, and c can act on this overlap ... s.