pale blue dot -carl sagan- 第57部分

Our　species　has　discovered　a　way　to　municate　through　the　dark；　to　transcend　immense　distances。　No　means　of　munication　is　faster　or　cheaper　or　reaches　out　farther。　It's　called　radio。

After　billions　of　years　of　biological　evolution—on　their　planet　and　ours—an　alien　civilization　cannot　be　in　technological　lockstep　with　us。　There　have　been　humans　for　more　than　twenty　thousand　centuries；　but　we've　had　radio　only　for　about　one　century。　If　alien　civilizations　are　behind　us；　they're　likely　to　be　too　far　behind　to　have　radio。　And　if　they're　ahead　of　us；　they're　likely　to　be　far　ahead　of　us。　Think　of　the　technical　advances　on　our　world　over　just　the　last　few　centuries。　What　is　for　us　technologically　difficult　or　impossible；　what　might　seem　to　us　like　magic；　might　for　them　be　trivially　easy。　They　might　use　other；　very　advanced　means　to　municate　with　their　peers；　but　they　would　know　about　radio　as　an　approach　to　newly　emerging　civilizations。　Even　with　no　more　than　our　level　of　technology　at　the　transmitting　and　receiving　ends；　we　could　municate　today　across　much　of　the　Galaxy。　They　should　be　able　to　do　much　better。

If　they　exist。

But　our　fear　of　the　dark　rebels。　The　idea　of　alien　beings　troubles　us。　We　conjure　up　objections：

〃It's　too　expensive。〃　But；　in　its　fullest　modern　technological　expression；　it　costs　less　than　one　attack　helicopter　a　year。

〃We'll　never　understand　what　they're　saying。〃　But；　because　the　message　…is　transmitted　by　radio；　we　and　they　must　have　radio　physics；　radio　astronomy；　and　radio　technology　in　mon。　The　laws　of　Nature　are　the　same　everywhere；　so　science　itself　provides　a　means　and　language　of　munication　even　between　very　different　kinds　of　beings—provided　they　both　have　science。　Figuring　out　the　message；　if　we're　fortunate　enough　to　receive　one；　may　be　much　easier　than　acquiring　it。

〃It　would　be　demoralizing　to　learn　that　our　science　is　primitive。〃　But　by　the　standards　of　the　next　few　centuries；　at　least　some　of　our　present　science　will　be　considered　primitive；　extraterrestrials　or　no　extraterrestrials。　（So　will　some　of　our　present　politics；　ethics；　economics；　and　religion。）　To　go　beyond　present　science　is　one　of。　the　chief　goals　of　science。　Serious　students　are　not　monly　plunged　into　fits　of　despair　on　turning　the　pages　of　a　textbook　and　discovering　that　some　further　topic　is　known　to　the　author　but　not　yet　to　the　student。　Usually　the　students　struggle　a　little；　acquire　the　new　knowledge；　and；　following　an　ancient　human　tradition；　continue　to　turn　the　pages。

〃All　through　history　advanced　civilizations　have　ruined　civilizations　just　slightly　more　backward。〃　Certainly。　But　malevolent　aliens；　should　they　exist；　will　not　discover　our　existence　from　the　fact　that　we　listen。　The　search　programs　only　receive；　they　do　not　send。*

*　Surprisingly　many　people；　including　New　York　Times　editorialists；　are　concerned　that　once　extraterrestrials　know　where　we　are；　they　will　e　here　and　eat　us。　Put　aside　the　profound　biological　differences　that　must　exist　between　the　hypothetical　aliens　and　ourselves；　imagine　that　we　constitute　an　interstellar　gastronomic　delicacy。　Why　transport　large　numbers　of　us　to　alien　restaurants？　The　freightage　is　enormous。　Wouldn't　it　be　better　just　to　steal　a　few　humans；　sequence　our　amino　acids　or　whatever　else　is　the　source　of　our　delectability；　and　then　just　synthesize　the　identical　food　product　from　scratch？



THE　DEBATE　IS；　for　the　moment；　moot。　We　are　now；　on　an　unprecedented　scale；　listening　for　radio　signals　from　possible　other　civilizations　in　the　depths　of　space。　Alive　today　is　the　first　generation　of　scientists　to　interrogate　the　darkness。　Conceivably　it　might　also　be　the　last　generation　before　contact　is　made—and　this　the　last　moment　before　we　discover　that　someone　in　the　darkness　is　calling　out　to　us。

This　quest　is　called　the　Search　for　Extraterrestrial　Intelligence　（SETI）。　Let　me　describe　how　far　we've　e。

The　first　SETI　program　was　carried　out　by　Frank　Drake　at　the　National　Radio　Astronomy　Observatory　in　Greenbank；　West　Virginia；　in　1960。　He　listened　to　two　nearby　Sun…like　stars　for　two　weeks　at　one　particular　frequency。　（〃Nearby〃　is　a　relative　term：　The　nearest　was　12　light…years—70　trillion　miles—away。）

Almost　at　the　moment　Drake　pointed　the　radio　telescope　and　turned　the　system　on；　he　picked　up　a　very　strong　signal。　Was　it　a　message　from　alien　beings？　Then　it　went　away。　If　the　signal　disappears；　you　can't　scrutinize　it。　You　can't　see　if；　because　of　the　Earth's　rotation；　it　moves　with　the　sky。　If　it's　not　repeatable；　you've　learned　almost　nothing　from　it—it　might　be　terrestrial　radio　interference；　or　a　failure　of　your　amplifier　or　detector　。　。　。　or　an　alien　signal。　Unrepeatable　data；　no　matter　how　illustrious　the　scientist　reporting　them；　are　not　worth　much。

Weeks　later；　the　signal　was　detected　again。　It　turned　out　to　be　a　military　aircraft　broadcasting　on　an　unauthorized　frequency。　Drake　reported　negative　results。　But　in　science　a　negative　result　is　not　at　all　the　same　thing　as　a　failure。　His　great　achievement　was　to　show　that　modern　technology　is　fully　able　to　listen　for　signals　from　hypothetical　civilizations　on　the　planets　of　other　stars。

Since　then　there've　been　a　number　of　attempts；　often　on　time　borrowed　from　other　radio　telescope　observing　programs；　and　almost　never　for　longer　than　a　few　months。　There've　been　some　more　false　alarms；　at　Ohio　State；　in　Arecibo；　Puerto　Rico；　in　France；　Russia；　and　elsewhere；　but　nothing　that　could　pass　muster　with　the　world　scientific　munity。

Meanwhile；　the　technology　for　detection　has　been　getting　cheaper；　the　sensitivity　keeps　improving；　the　scientific　respectability　of　SETI　has　continued　to　grow；　and　even　NASA　and　Congress　have　bee　a　little　less　afraid　to　support　it。　Diverse；　plementary　search　strategies　are　possible　and　necessary。　It　was　clear　years　ago　that　if　the　trend　continued；　the　technology　for　a　prehensive　SETI　effort　would　eventually　fall　within　the　reach　even　of　private　organizations　（or　wealthy　individuals）；　and　sooner　or　later；　the　government　would　be　willing　to　support　a　major　program。　After　30　years　of　work；　for　some　of　us　it's　been　later　rather　than　sooner。　But　at　last　the　time　has　e。



THE　PLANETARY　SOCIETY…a　nonprofit　membership　organization　that　Bruce　Murray；　then　the　Director　of　JPL；　and　I　founded　in　1980—is　devoted　to　planetary　exploration　and　the　search　for　extraterrestrial　life。　Paul　Horowitz；　a　physicist　at　Harvard　University；　had　made　a　number　of　important　innovations　for　SETI　and　was　eager　to　try　them　out。　If　we　could　find　the　money　to　get　him　started；　we　thought　we　could　continue　to　support　the　program　by　donations　from　our　members。

In　1983　Ann　Druyan　and　I　suggested　to　the　filmmaker　Steven　Spielberg　that　this　was　an　ideal　project　for　him　to　support。　Breaking　with　Hollywood　tradition；　he　had　in　two　wildly　successful　movies　conveyed　the　idea　that　extraterrestrial　beings　might　not　be　hostile　and　dangerous。　Spielberg　agreed。　With　his　initial　support　through　The　Planetary　Society；　Project　META　began。

META　is　an　acronym　for　〃Megachannel　ExtraTerrestrial　Assay。〃　The　single　frequency　of　Drake's　first　system　grew　to　8。4　million。　But　each　channel；　each　〃station；〃　we　tune　to　has　an　exceptionally　narrow　frequency　range。　There　are　no　known　processes　out　among　the　stars　and　galaxies　that　can　generate　such　sharp　radio　〃lines。〃　If　we　pick　up　anything　falling　into　so　narrow　a　channel；　it　must；　we　think；　be　a　token　of　intelligence　and　technology。

What's　more；　the　Earth　turns—which　means　that　any　distant　radio　source　will　have　a　sizable　apparent　motion；　like　the　rising　and　setting　of　the　stars。　Just　as　the　steady　tone　of　a　car's　horn　dips　as　it　drives　by；　so　any　authentic　extraterrestrial　radio　source　will　exhibit　a　steady　drift　in　frequency　due　to　the　Earth's　rotation。　In　contrast；　any　source　of　radio　interference　at　the　Earth's　surface　will　be　rotating　at　the　same　speed　as　the　META　receiver。　META's　listening　frequencies　are　continuously　changed　to　pensate　for　the　Earth's　rotation；　so　that　any　narrow…band　Signals　from　the　sky　will　always　appear　in　a　single　channel。　But　any　radio　interference　down　here　on　Earth　will　give　itself　away　by　racing　through　adjacent　channels。

The　META　radio　telescope　at　Harvard；　Massachusetts；　is　26　meters　（84　feet）　in　diameter。　Each　day；　as　the　Earth　rotates　the　telescope　beneath　the　sky；　a　swath　of　stars　narrower　than　the　full　moon　is　swept　out　and　examined。　Next　day；　it's　an　adjacent　swath。　Over　a　year；　all　of　the　northern　sky　and　part　of　the　southern　is　observed。　An　identical　system；　also　sponsored　by　The　Planetary　Society；　is　in　operation　just　outside　Buenos　Aires；　Argentina；　to　examine　the　southern　sky。　So　together　the　two　META　systems　have　been　exploring　the　entire　sky。

The　radio　telescope；　gravitationally　glued　to　the　spinning　Earth；　looks　at　any　given　star　for　about　two　minutes。　Then　it's　on　to　the　next。　8。4　million　channels　sounds　like　a　lot；　but　remember；　each　channel　is　very　narrow。　All　of　them　together　constitute　only　a　few　parts　in　100；000　of　the　available　radio　spectrum。　So　we　have　to　park　our　8。4　million　channels　somewhere　in　the　radio　spectrum　for　each　year　of　observation；　near　some　frequency　that　an　alien　civilization；　knowing　nothin