pale blue dot -carl sagan-第48章

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　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　of　SETI；　however；　the　chances　of　success　are　so　remote；　and　the　likely　benefits　of　the　program　are　so　limited；　that　there　is　little　justification　for　12　million　taxpayer　dollars　to　be　expended　for　this　program。

But　how；　before　we　have　found　extraterrestrial　intelligence；　can　we　〃guarantee〃　that　we　will　find　it？　How；　on　the　other　hand；　can　we　know　that　the　chances　of　success　are　〃remote〃？　And　if　we　find　extraterrestrial　intelligence；　are　the　benefits　really　likely　to　be　〃so　limited〃？　As　in　all　great　exploratory　ventures；　we　do　not　know　what　we　will　find　and　we　don't　know　the　probability　of　finding　it。　If　we　did；　we　would　not　have　to　look。

SETI　is　one　of　those　search　programs　irritating　to　those　who　want　well…defined　cost/benefit　ratios。　Whether　ETI　can　be　found；　how　long　it　would　take　to　find　it；　and　what　it　would　cost　to　do　so　are　all　unknown。　The　benefits　might　be　enormous；　but　we　can't　really　be　sure　of　that　either。　It　would　of　course　be　foolish　to　spend　a　major　fraction　of　the　national　treasure　on　such　ventures；　but　I　wonder　if　civilizations　cannot　be　calibrated　by　whether　they　pay　some　attention　to　trying　to　solve　the　great　problems。

Despite　these　setbacks；　a　dedicated　band　of　scientists　and　engineers；　centered　at　the　SETI　Institute　in　Palo　Alto；　California；　has　decided　to　go　ahead；　government　or　no　government。　NASA　has　given　them　permission　to　use　the　equipment　already　paid　for；　captains　of　the　electronics　industry　have　donated　a　few　million　dollars；　at　least　one　appropriate　radio　telescope　is　available；　and　the　initial　stages　of　this　grandest　of　all　SETI　programs　is　on　track。　If　it　can　demonstrate　that　a　useful　sky　survey　is　possible　without　being　swamped　by　background　noise—and　especially　if；　as　is　very　likely　from　the　META　experience；　there　are　unexplained　candidate　signals…perhaps　Congress　will　change　its　mind　once　more　and　fund　the　project。

Meanwhile；　Paul　Horowitz　has　e　up　with　a　new　program—different　from　META；　different　from　what　NASA　was　doing—called　BETA。　BETA　stands　for　〃Billion…channel　ExtraTerrestrial　Assay。〃　It　bines　narrow…band　sensitivity；　wide　frequency　coverage；　and　a　clever　way　to　verify　signals　as　they're　detected。　If　The　Planetary　Society　can　find　the　additional　support；　this　system—much　cheaper　than　the　former　NASA　program—should　be　on　the　air　soon。



WOULD　I　LIKE　TO　BELIEVE　that　with　META　we've　detected　transmissions　from　other　civilizations　out　there　in　the　dark；　sprinkled　through　the　vast　Milky　Way　Galaxy？　You　bet。　After　decades　of　wondering　and　studying　this　problem；　of　course　I　would。　To　me；　such　a　discovery　would　be　thrilling。　It　would　change　everything。　We　would　be　hearing　from　other　beings；　independently　evolved　over　billions　of　years；　viewing　the　Universe　perhaps　very　differently；　probably　much　smarter；　certainly　not　human。　How　much　do　they　know　that　we　don't？

For　me；　no　signals；　no　one　calling　out　to　us　is　a　depressing　prospect。　〃plete　silence；〃　said　Jean…Jacques　Rousseau　in　a　different　context；　〃induces　melancholy；　it　is　an　image　of　death。〃　But　I'm　with