The question originally posed by Enrico Fermi, which simply asks why intelligent life hasn't found us if it is probable to be found in the universe (and therefore common).
Articles on KurzweilAI.net that refer to Fermi's ParadoxAnswering Fermi's Paradox By Hugo de Garis
Singularity Chat with Vernor Vinge and Ray Kurzweil By Vernor Vinge and Ray Kurzweil
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The Fermi Paradox