第5部分(第4/7 頁)

　〃Trivial；　really。　You　know　the　Drake　equation？〃

Norman　did。　It　was　one　of　the　famous　proposals　in　the　literature　on　extraterrestrial　life。　But　he　said；　〃Refresh　me。〃

Harry　sighed　irritably；　pulled　out　a　sheet　of　paper。　〃It's　a　probability　equation。〃　He　wrote：

p　=　fpnhflfifc

＇＇29＇＇　〃What　it　means；〃　Harry　Adams　said；　〃is　that　the　probability；　p；　that　intelligent　life　will　evolve　in　any　star　system　is　a　function　of　the　probability　that　the　star　will　have　planets；　the　number　of　habitable　planets；　the　probability　that　simple　life　will　evolve　on　a　habitable　planet；　the　probability　that　intelligent　life　will　evolve　from　simple　life；　and　the　probability　that　intelligent　life　will　attempt　interstellar　munication　within　five　billion　years。　That's　all　the　equation　says。〃

〃Uh…huh；〃　Norman　said。

〃But　the　point　is　that　we　have　no　facts；〃　Harry　said。　〃We　must　guess　at　every　single　one　of　these　probabilities。　And　it's　quite　easy　to　guess　one　way；　as　Ted　does；　and　conclude　there　are　probably　thousands　of　intelligent　civilizations。　It's　equally　easy　to　guess；　as　I　do；　that　there　is　probably　only　one　civilization。　Ours。〃　He　pushed　the　paper　away。　〃And　in　that　case；　whatever　is　down　there　is　not　from　an　alien　civilization。　So　we're　all　wasting　our　time　here。〃

〃Then　what　is　down　there？〃　Norman　said　again。

〃It　is　an　absurd　expression　of　romantic　hope；〃　Adams　said；　pushing　his　glasses　up　on　his　nose。　There　was　a　vehemence　about　him　that　troubled　Norman。　Six　years　earlier；　Harry　Adams　had　still　been　a　street　kid　whose　obscure　talent　had　carried　him　in　a　single　step　from　a　broken　home　in　the　slums　of　Philadelphia　to　the　manicured　green　lawns　of　Princeton。　In　those　days　Adams　had　been　playful；　amused　at　his　turn　of　fortune。　Why　was　he　so　h