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t　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