pale blue dot -carl sagan-第35部分

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ut　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　signal　that's　a　little　below　the　strength　that　we　could　otherwise　detect　on　Earth。　Occasionally　the　signal　will　by　chance　be　temporarily　focused；　amplified；　and　brought　within　the　detectability　range　of　our　radio　telescopes。　The　interesting　thing　is　that　the　lifetimes　of　such　brightening；　predicted　from　the　physics　of　the　interstellar　gas；　are　a　few　minutes—and　the　chance　of　reacquiring　the　signal　is　small。　We　should　really　be　pointing　steadily　at　these　coordinates　in　the　sky；　watching　them　for　months。

Despite　the　fact　that　none　of　these　signals　repeats；　there's　an　additional　fact　about　them　that；　every　time　I　think　about　it；　sends　a　chill　down　my　spine：　8　of　the　11　best　candidate　signals　lie　in　or　near　the　plane　of　the　Milky　Way　Galaxy。　The　five　strongest　are　in　the　constellations　Cassiopeia；　Monoceros；　Hydra；　and　two　in　Sagittarius—in　the　approximate　direction　of　the　center　of　the　Galaxy。　The　Milky　Way　is　a　flat；　wheel…like　collection　of　gas　and　dust　and　stars。　Its　flatness　is　why　we　see　it　as　a　band　of　diffuse　light　across　the　night　sky。　That's　where　almost　all　the　stars　in　our　galaxy　are。　If　our　candidate　signals　really　were　radio　interference　from　Earth　or　some　undetected　glitch　in　the　detection　electronics；　we　shouldn't　see　them　preferentially　when　we're　pointing　at　the　Milky　Way。

But　maybe　we　had　an　especially　unlucky　and　misleading　run　of　statistics。　The　probability　that　this　correlation　with　the　galactic　plane　is　due　merely　to　chance　is　less　than　half　a　percent。　Imagine　a　wall…size　map　of　the　sky；　ranging　from　the　North　Star　at　the　top　to　the　fainter　stars　toward　which　the　Earth's　south　pole　points　at　the　bottom。　Snaking　across　this　wall　map　are　the　irregular　boundaries　of　the　Milky　Way。　Now　suppose　that　you　were　blindfolded　and　asked　to　throw　five　darts　at　random　at　the　map　（with　much　of　the　southern　sky；　inaccessible　from　Massachusetts；　declared　off　limits）。　You'd　have　to　throw　the　set　of　five　darts　more　than　200　times　before；　by　accident；　you　got　them　to　fall　as　closely　within　the　precincts　of　the　Milky　Way　as　the　five　strongest　META　signals　did。　Without　repeatable　signals；　though；　there's　no　way　we　can　conclude　that　we've　actually　found　extraterrestrial　intelligence。

Or　maybe　the　events　we've　found　are　caused　by　some　new　kind　of　astrophysical　phenomenon；　something　that　nobody　has　thought　of　yet；　by　which　not　civilizations；　but　stars　or　gas　clouds　（or　something）　that　do　lie　in　the　plane　of　the　Milky　Way　emit　strong　signals　in　bafflingly　narrow　frequency　bands。

Let's　permit　ourselves；　though；　a　moment　of　extravagant　speculation。　Let's　imagine　that　all　our　surviving　events　are　in　fact　due　to　radio　beacons　of　other　civilizations。　Then　we　can　estimate—from　how　little　time　we've　spent　watching　each　piece　of　sky—how　many　such　transmitters　there　are　in　the　entire　Milky　Way。　The　answer　is　something　approaching　a　million。　If　randomly　strewn　through　space；　the　nearest　of　them　would　be　a　few　hundred　light　years　away；　too　far　for　them　to　have　picked　up　our　own　TV　or　radar　signals　yet。　They　would　not　know　for　another　few　centuries　that　a　technical　civilization　has　emerged　on　Earth。　The　Galaxy　would　be　pulsing　with　life　and　intelligence；　but—unless　they're　busily　exploring　huge　numbers　of　obscure　star　systems—wholly　oblivious　of　what　has　been　happening　down　here　lately。　A　few　centuries　from　now；　after　they　do　hear　from　us；　things　might　get　very　interesting。　Fortunately；　we'd　have　many　generations　to　prepare。

If；　on　the　other　hand；　none　of　our　candidate　signals　is　an　authentic　alien　radio　beacon；　then　we're　forced　to　the　conclusion　that　very　few　civilizations　are　broadcasting；　maybe　none；　at　least　at　our　magic　frequencies　and　strongly　enough　for　us　to　hear：

Consider　a　civilization　like　our　own；　but　which　dedicated　all　its　available　power　（about　10　trillion　watts）　to　broadcasting　a　beacon　signal　at　one　of　our　magic　frequencies　and　to　all　directions　in　space。　The　META　results　would　then　imply　that　there　are　no　such　civilizations　out　to　25　light…years—a　volume　that　enpasses　perhaps　a　dozen　Sun…like　stars。　This　is　not　a　very　stringent　limit。　If；　in　contrast；　that　civilization　were　broadcasting　directly　at　our　position　in　space；　using　an　antenna　no　more　advanced　than　the　Arecibo　Observatory；　then　if　META　has　found　nothing；　it　follows　that　there　are　no　such　civilizations　anywhere　in　the　Milky　Way　Galaxy—out　of　400　billion　stars；　not　one。　But　even　assuming　they　would　want　to；　how　would　they　know　to　transmit　in　our　direction？

Now　consider；　at　the　opposite　technological　extreme；　a　very　advanced　civilization　omnidirectionally　and　extravagantly　broadcasting　at　a　power　level　10　trillion　times　greater　（1026　watts；　the　entire　energy　output　of　a　star　like　the　Sun）。　Then；　if　the　META　results　are　negative；　we　can　conclude　not　only　that　there　are　no　such　civilizations　in　the　Milky　Way；　but　none　out　to　70　million　light…years—none　in　M31；　the　nearest　galaxy　like　our　own；　none　in　M33；　or　the　Fornax　system；　or　M81；　or　the　Whirlpool　Nebula；　or　Centaurus　A；　or　the　Virgo　cluster　of　galaxies；　or　the　nearest　Seyfert　galaxies；　none　among　any　of　the　hundred　trillion　stars　in　thousands　of　nearby　galaxies。　Stake　through　its　heart　or　not；　the　geocentric　conceit　stirs　again。

Of　course；　it　might　be　a　token　not　of　intelligence　but　of　stupidity　to　pour　so　much　energy　into　interstellar　（and　intergalactic）　munication。　Perhaps　they　have　good　reasons　not　to　hail　all　ers。　Or　perhaps　they　don't　care　about　civilizations　as　backward　as　we　are。　But　still—not　one　civilization　in　a　hundred　trillion　stars　broadcasting　with　such　power　on　such　a　frequency？　If　the　META　results　are　negative；　we　have　set　an　instructive　limit—but　whether　on　the　abundance　of　very　advanced　civilizations　or　their　munications　strategy　we　have　no　way　of　knowing。　Even　if　META　has　found　nothing；　a　broad　middle　range　remains　open—of　abundant　civilizations；　more　advanced　than　we　and　broadcasting　omnidirectionally　at　magic　frequencies。　We　would　not　have　heard　from　them　yet。



ON　OCTOBER　12；　1992—auspiciously　or　otherwise　the　500th　anniversary　of　the　〃discovery〃　of　America　by　Christopher　Columbus—NASA　turned　on　its　new　SETI　program。　At　a　radio　telescope　in　the　Mojave　Desert；　a　search　was　initiated　intended　to　cover　the　entire　sky　systematically—like　META；　making　no　guesses　about　which　stars　are　more　likely；　but　greatly　expanding　the　frequency　coverage。　At　the　Arecibo　Observatory；　an　even　more　sensitive　NASA　study　began　that　concentrated　on　promising　nearby　star　systems。　When　fully　operational；　the　NASA　searches　would　have　been　able　to　detect　much　fainter　signals　than　META；　and　look　for　kinds　of　signals　that　META　could　not。

The　META　experience　reveals　a　thicket　of　background　static　and　radio　interference。　Quick　reobservation　and　confirmation　of　the　signal—specially　at　other；　independent　radio　telescopes—is　the　key　to　being　sure。　Horowitz　and　I　gave　NASA　scientists　the　coordinates　of　our　fleeting　and　enigmatic　events。　Perhaps　they　would　be　able　to　confirm　and　clarify　our　results。　The　NASA　program　was　also　developing　new　technology；　stimulating　ideas；　and　exciting　schoolchildren。　In　the　eyes　of　many　it　was　well　worth　the　10　million　a　year　being　spent　on　it。　But　almost　exactly　a　year　after　authorizing　it；　Congress　pulled　the　plug　on　NASA's　SETI　program。　It　cost　too　much；　they　said。　The　post…Cold　War　U。S。　defense　budget　is　some　30；000　times　larger。

The　chief　argument　of　the　principal　opponent　of　the　NASA　SETI　program—Senator　Richard　Bryan　of　Nevada—was　this　＇from　the　Congressional　Record　for　September　22；　1993＇：

So　far；　the　NASA　SETI　Program　has　found　nothing。　In　fact；　all　the　decades　of　SETI　research　have　found　no　confirmable　signs　of　extraterrestrial　life。

Even　with　the　current　NASA　version　of　SETI；　I　do　not　think　many　of　its　scientists　would　be　willing　to　guarantee　that　we　are　likely　to　see　any　tangible　results　in　the　＇foreseeable＇　future　。　。　。

Scientific　research　rarely；　if　ever；　offers　guarantees　of　success—and　I　understand　that—and　the　full　benefits　of　such　research　are　often　unknown　until　very　late　in　the　process。　And　I　accept　that；　as　well。

In　the　case