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As I understand the physics of space-time (and IANAP), the universe is not thought to be infinite, properly speaking. It is thought to be finite but unbounded. That is, there is a finite amount of space, but no boundary to it. You cannot reach the edge of the universe, beyond which would lie some metaphysical no-man's land, I guess. Space-time bends back on itself in other spatial dimensions of the manifold. In other words, think of peeling off the surface of a globe and laying it flat on a table. It would be a finite amount of 2-dimensional space on the table before you. Now take it and wrap it around the globe again; it is still a finite amount of space, but it has no boundaries because it has been wrapped back upon itself in three dimensions. Likewise, if you think of the three dimensions of space-time we are accustomed to experiencing being wrapped back upon themselves in a fourth dimension of the manifold, you could have a finite amount of space with no boundaries. Not that you could reach any such boundary anyway; you'd have to have been traveling the speed of light since the Big Bang just to catch up with the expansion. And you're certainly going to fit only a finite amount of matter into a finite amount of space.
Really interesting stuff. I will try to read more and see if general opinion agrees with this guy, but if it does I stand by what I said at the start: there is no way we can state that it is likely that other intelligent life forms exist :smile: