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D4TC human multiplanet scenario space planetary funeral pyre beacon SETI CETI alien communication
paradox Karl Schroeder D4TC alien SETI Fermi great silence silence extrasolar civilization
Karl Schroeder D4TC alien civilization extrasolar thalience science networks Ventus terraforming massive addressability SETI
D4TC human multiplanet scenario planetary SETI CETI communication alien space message genome