But when physicists and philosophers talk about two different regions of spacetime being “two universes,” what they generally mean is that those regions are (1) very, very large; (2) causally isolated from each other; and hence (3) mutually unknowable by direct observation. The case for saying the two regions are separate universes is strengthened if (4) they have very different characters—if, for instance, one of them has three spatial dimensions whereas the other has seventeen dimensions. Finally—and here is the existentially titillating possibility—two regions of spacetime might be called
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