pale blue dot -carl sagan-第47章

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mics；　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　nothing　about　us；　might　nevertheless　conclude　we're　listening　to。

Hydrogen　is　by　far　the　most　abundant　kind　of　atom　in　the　Universe。　It's　distributed　in　clouds　and　as　diffuse　gas　throughout　interstellar　space。　When　it　acquires　energy；　it　releases　some　of　it　by　giving　off　radio　waves　at　a　precise　frequency　of　1420。405751768　megahertz。　（One　hertz　means　the　crest　and　trough　of　a　wave　arriving　at　your　detection　instrument　each　second。　So　1420　megahertz　means　1。420　billion　waves　entering　your　detector　every　second。　Since　the　wavelength　of　light　is　just　the　speed　of　light　divided　by　the　frequency　of　the　wave；　1420　megahertz　corresponds　to　a　wavelength　of　21　centimeters。）　Radio　astronomers　anywhere　in　the　Galaxy　will　be　studying　the　Universe　at　1420　megahertz　and　can　anticipate　that　other　radio　astronomers；　no　matter　how　different　they　may　look；　will　do　the　same。

It's　as　if　someone　told　you　that　there's　only　one　station　on　your　home　radio　set's　frequency　band；　but　that　no　one　knows　its　frequency。　Oh　yes；　one　other　thing：　Your　set's　frequency　dial；　kith　its　thin　marker　you　adjust　by　turning　a　knob；　happens　to　reach　from　the　Earth　to　the　Moon。　To　search　systematically　through　this　vast　radio　spectrum；　patiently　turning　the　knob；　is　going　to　be　very　time…consuming。　Your　problem　is　to　set　the　dial　correctly　from　the　beginning；　to　choose　the　right　frequency。　If　you　can　correctly　guess　what　frequencies　that　extraterrestrials　are　broadcasting　to　us　on—the　〃magic〃　frequencies—then　you　can　save　yourself　much　time　and　trouble。　These　are　the　sorts　of　reasons　that　we　first　listened；　as　Drake　did；　at　frequencies　near　1420　megahertz；　the　hydrogen　〃magic〃　frequency。

Horowitz　and　I　have　published　detailed　results　from　five　years　of　full…time　searching　with　Project　META　and　two　years　of　follow…up。　We　can't　report　that　we　found　a　signal　from　alien　beings。　But　we　did　find　something　puzzling；　something　that　for　me　in　quiet　moments；　every　now　and　then；　raises　goose　bumps：

Of　course；　there's　a　background　level　of　radio　noise　from　Earth—radio　and　television　stations；　aircraft；　portable　telephones；　nearby　and　more　distant　spacecraft。　Also；　as　with　all　radio　receivers；　the　longer　you　wait；　the　more　likely　it　is　that　there'll　be　some　random　fluctuation　in　the　electronics　so　strong　that　it　generates　a　spurious　signal。　So　we　ignore　anything　that　isn't　much　louder　than　the　background。

Any　strong　narrow…band　signal　that　remains　in　a　single　channel　we　take　very　seriously。　As　it　logs　in　the　data；　META　automatically　tells　the　human　operators　to　pay　attention　to　certain　signals。　Over　five　years　we　made　some　60　trillion　observations　at　various　frequencies；　while　examining　the　entire　accessible　sky。　A　few　dozen　signals　survive　the　culling。　These　are　subjected　to　further　scrutiny；　and　almost　all　of　them　are　rejected…for　example；　because　an　error　has　been　found　by　fault…detection　microprocessors　that　examine　the　signal…detection　microprocessors。

What's　left—the　strongest　candidate　signals　after　three　surveys　of　the　sky—are　11　〃events。〃　They　satisfy　all　but　one　of　our　criteria　for　a　genuine　alien　signal。　But　the　one　failed　criterion　is　supremely　important：　Verifiability。　We've　never　been　able　to　find　any　of　them　again。　We　look　back　at　that　part　of　the　sky　three　minutes　later　and　there's　nothing　there。　We　look　again　the　following　day：　nothing。　Examine　it　a　year　later；　or　seven　years　later；　and　still　there's　nothing。

It　seems　unlikely　that　every　signal　we　get　from　alien　civilizations　would　turn　itself　off　a　couple　of　minutes　after　we　begin　listening；　and　never　repeat。　（How　would　they　know　we're　paying　attention？）　But；　just　possibly；　this　is　the　effect　of　twinkling。　Stars　twinkle　because　parcels　of　turbulent　air　are　moving　across　the　line　of　sight　between　the　star　and　us。　Sometimes　these　air　parcels　act　as　a　lens　and　cause　the　light　rays　from　a　given　star　to　converge　a　little；　making　it　momentarily　brighter。　Similarly；　astronomical　radio　sources　may　also　twinkle—owing　to　clouds　of　electrically　charged　（or　〃ionized〃）　gas　in　the　great　near…vacuum　between　the　stars。　We　observe　this　routinely　with　pulsars。

Imagine　a　radio　sig