technomage
Paranormal Adept
Love the 5D skyscraper example. Great visual. I've heard arguments before regarding two seemingly separate three dimensional objects connected via larger dimensions, which would allow for the conveyance of information, at least, from one to the other instantaneously from a 3D perspective. Regardless, what you boiled down nicely is that anything of a higher dimension is going to consist of the first three dimensions as well (we may have higher dimension aspects of ourselves that we are unaware of), and therefore be subject to the rules of those three dimensions. Beyond that it's just a perspective issue and still makes it highly unlikely that any such beings have found us.
Now on the otherhand with multiple universes there is one way that I would accept the concept of visitors being able to happen upon us. If the universe we exist in were something of a "holographic" fractal there could be an uncountable number of similar and yet different iterations of our little location, and if one could just bounce from iteration to iteration there is a potentially infinite number of "Earths" to explore. The farther down the chain you go the greater the potential variations until you might reach a universe very different content-wise, but still following all of the same rules of physics. ( insert "Sliders" references here.)
I think you're talking about the Many Worlds interpretation of quantum mechanics, which I believe is different from what the Usual Suspect was talking about. I thought he was talking about something like Asimov's "The Gods Themselves" where there were two distinct and completely different universes with different physical laws that were connected in some way and could affect each other. In M-Theory, wouldn't a similar example be two universes in branes that touch somehow?
Maybe he was talking about a Star Trek: Mirror, Mirror scenario or that episode where Worf kept flipping from reality to reality or like in Stargate where the same sort of thing occurred, but that sounds like Many Worlds again. I'm not sure, but perhaps more clarification is needed from the Usual Suspect.