pale blue dot -carl sagan-第16部分

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anet；　it　may　be　that　there　are　other—perhaps　Triton…like—planets　far　beyond　Neptune　and　Pluto；　if　so；　Voyager　1　and　Voyager　2　are　still　within　the　Solar　System。　If　you　define　the　outer　limits　of　the　Solar　System　as　the　heliopause—where　the　interplanetary　particles　and　magnetic　fields　are　replaced　by　their　interstellar　counterparts—then　neither　Voyager　has　yet　left　the　Solar　System；　although　they　may　do　so　in　the　next　few　decades。*　But　if　your　definition　of　the　edge　of　the　Solar　System　is　the　distance　at　which　our　star　can　no　longer　hold　worlds　in　orbit　about　it；　then　the　Voyagers　will　not　leave　the　Solar　System　for　hundreds　of　centuries。

*　Radio　signals　that　both　Voyagers　detected　in　1992　are　thought　to　arise　from　the　collision　of　powerful　gusts　of　solar　wind　with　the　thin　gas　that　lies　between　the　stars。　From　the　immense　。power　of　the　signal　（over　10　trillion　watts）；　the　distance　to　the　heliopause　can　be　estimated：　about　100　times　the　Earth's　distance　from　the　Sun。　At　the　speed　it's　leaving　the　Solar　System；　Voyager　1　might　pierce　the　heliopause　and　enter　interstellar　space　around　the　year　2010。　If　its　radioactive　power　source　is　still　working；　news　of　the　crossing　will　be　radioed　back　to　the　stay…at…homes　on　Earth。　The　energy　released　by　the　collision　of　this　shock　wave　with　the　heliopause　makes　it　the　most　powerful　source　of　radio　emission　in　the　Solar　System。　It　makes　you　wonder　whether　even　stronger　shocks　in　other　planetary　systems　might　he　detectable　by　our　radio　telescope。

Weakly　grasped　by　the　Sun's　gravity；　in　every　direction　in　the　sky；　is　that　immense　horde　of　a　trillion　ets　or　more；　the　port　Cloud。　The　two　spacecraft　will　finish　their　passage　through

the　Oort　cloud　in　another　20；000　years　or　so。　Then；　at　last；　pleting　their　long　good…bye　to　the　Solar　System；　broken　free　of　the　gravitational　shackles　that　once　bound　them　to　the　Sun；　the　Voyagers　will　make　for　the　open　sea　of　interstellar　space。　only　then　will　Phase　Two　of　their　mission　begin。

Their　radio　transmitters　long　dead；　the　spacecraft　will　wander　for　ages　in　the　calm；　cold　interstellar　blackness—where　there　is　almost　nothing　to　erode　them。　Once　out　of　the　Solar　System；　they　will　remain　intact　for　a　billion　years　or　more；　as　they　circumnavigate　the　center　of　the　Milky　Way　galaxy。

We　do　not　know　whether　there　are　other　space…faring　civilizations　in　the　Milky　Way。　If　they　do　exist；　we　do　not　know　how　abundant　they　are；　much　less　where　they　are。　But　there　is　at　least　a　chance　that　sometime　in　the　remote　future　one　of　the　Voyagers　will　be　intercepted　and　examined　by　an　alien　craft。

Accordingly；　as　each　Voyager　left　Earth　for　the　planets　and　the　stars；　it　carried　with　it　a　golden　phonograph　record　encased　in　a　golden；　mirrored　jacket　containing；　among　other　things；　greetings　in　59　human　languages　and　one　whale　language；　a　12…minute　sound　essay　including　a　kiss；　a　baby's　cry；　and　an　EEG　record　of　the　meditations　of　a　young　woman　in　love；　116　encoded　pictures；　on　our　science；　our　civilization；　and　ourselves；　and　90　minutes　of　the　Earth's　greatest　hits—Eastern　and　Western；　classical　and　folk；　including　a　Navajo　night　chant；　a　Japanese　shakuhachi　piece；　a　Pygmy　girl's　initiation　song；　a　Peruvian　wedding　song；　a　3；000…year…old　position　for　the　ch'in　called　〃Flowing　Streams；〃　Bach；　Beethoven；　Mozart；　Stravinsky；　Louis　Armstrong；　Blind　Willie　Johnson；　and　Chuck　Berry's　〃Johnny　B。　Goode。〃

Space　is　nearly　empty。　There　is　virtually　no　chance　that　one　of　the　Voyagers　will　ever　enter　another　solar　system—and　this　is　true　even　if　every　star　in　the　sky　is　acpanied　by　planets。　The　instructions　on　the　record　jackets；　written　in　what　we　believe　to　be　readily　prehensible　scientific　hieroglyphics；　can　be　read；　and　the　contents　of　the　records　understood；　only　if　alien　beings；　somewhere　in　the　remote　future；　find　Voyager　in　the　depths　of　interstellar　space。　Since　both　Voyagers　will　circle　the　center　of　the　Milky　Way　Galaxy　essentially　forever；　there　is　plenty　of　time　for　the　records　to　be　found—if　there's　anyone　out　there　to　do　the　finding。

We　cannot　know　how　much　of　the　records　they　would　understand。　Surely　the　greetings　will　be　inprehensible；　but　their　intent　may　not　be。　（We　thought　it　would　be　impolite　not　to　say　hello。）　The　hypothetical　aliens　are　bound　to　be　very　different　from　us—independently　evolved　on　another　world。　Are　we　really　sure　they　could　understand　anything　at　all　of　our　message？　Every　time　I　feel　these　concerns　stirring；　though；　I　reassure　myself。　Whatever　the　inprehensibilities　of　the　Voyager　record；　any　alien　ship　that　finds　it　will　have　another　standard　by　which　to　judge　us。　Each　Voyager　is　itself　a　message。　In　their　exploratory　intent；　in　the　lofty　ambition　of　their　objectives；　in　their　utter　lack　of　intent　to　do　harm；　and　in　the　brilliance　of　their　design　and　performance；　these　robots　speak　eloquently　for　us。

But　being　much　more　advanced　scientists　and　engineers　than　we—otherwise　they　would　never　be　able　to　find　and　retrieve　the　small；　silent　spacecraft　in　interstellar　space—perhaps　the　aliens　would　have　no　difficulty　understanding　what　is　encoded　on　these　golden　records。　Perhaps　they　would　recognize　the　tentativeness　of　our　society；　the　mismatch　between　our　technology　and　our　wisdom。　Have　we　destroyed　ourselves　since　launching　Voyager；　they　might　wonder；　or　have　we　gone　on　to　greater　things？

Or　perhaps　the　records　will　never　be　intercepted。　Perhaps　no　one　in　five　billion　years　will　ever　e　upon　them。　Five　billion　years　is　a　long　time。　In　five　billion　years；　all　humans　will　have　bee　extinct　or　evolved　into　other　beings；　none　of　our　artifacts　will　have　survived　on　Earth；　the　continents　will　have　bee　unrecognizably　altered　or　destroyed；　and　the　evolution　of　the　Sun　will　have　burned　the　Earth　to　a　crisp　or　reduced　it　to　a　whirl　of　atoms。

Far　from　home；　untouched　by　these　remote　events；　the　Voyagers；　bearing　the　memories　of　a　world　that　is　no　more；　will　fly　on。




CHAPTER　10　　　SACRED　BLACK

　

　

Deep　sky　is；　of　all　visual　impressions；　the　nearest　akin　to　a　feeling。

—SAMUEL　TAYLOR　COLERIDGE；　NOTEBOOKS　（1805）

The　blue　of　a　cloudless　May　morning；　or　the　reds　and　oranges　of　a　sunset　at　sea；　have　roused　humans　to　wonder；　to　poetry；　and　to　science。　No　matter　where　on　Earth　we　live；　no　matter　what　our　language；　customs；　or　politics；　we　share　a　sky　in　mon。　Most　of　us　expect　that　azure　blue　and　would；　for　good　reason；　be　stunned　to　wake　up　one　sunrise　to　find　a　cloudless　sky　that　was　black　or　yellow　or　green。　（Inhabitants　of　Los　Angeles　and　Mexico　City　have　grown　accustomed　to　brown　skies；　and　those　of　London　and　Seattle　to　gray　ones—but　even　they　still　consider　blue　the　planetary　norm。）

And　yet　there　are　worlds　with　black　or　yellow　skies；　and　maybe　even　green。　The　color　of　the　sky　characterizes　the　world。　Plop　me　down　on　any　planet　in　the　Solar　System；　wsithout　sensing　the　gravity；　without　glimpsing　the　ground；　let　me　take　a　quick　look　at　the　Sun　and　sky；　and　I　can；　I　think；　pretty　well　tell　you　where　I　am。　That　familiar　shade　of　blue；　interrupted　here　and　there　by　fleecy　white　clouds；　is　a　signature　of　our　world。　The　French　have　an　expression；　sacre…bleu！；　which　translates　roughly　as　〃Good　heavens！〃*　Literally；　it　means　〃sacred　blue！〃　Indeed。　If　there　ever　is　a　true　flag　of　Earth；　this　should　be　its　color。

*　Like　〃gosh…darned〃　and　〃geez；〃　this　phrase　was　originally　a　euphemism　for　those　who　considered　Sacre…Dieu！；　〃Sacred　God！；〃　too　strong　an　oath；　the　Second　mandment　duly　considered；　to　be　uttered　aloud。

Birds　fly　through　it；　clouds　are　suspended　in　it；　humans　admire　and　routinely　traverse　it；　light　from　the　Sun　and　stars　flutters　through　it。　But　what　is　it？　What　is　it　made　of？　Where　does　it　end？　How　much　of　it　is　there？　Where　does　all　that　blue　e　from？　If　it's　a　monplace　for　all　humans；　if　it　typifies　our　world；　surely　we　should　know　something　about　it。　What　is　the　sky？

In　August　1957；　for　the　first　time；　a　human　being　rose　above　the　blue　and　looked　around—when　David　Simons；　a　retired　Air　Force　officer　and　a　physician；　became　the　highest　human　in　history。　Alone；　he　piloted　a　balloon　to　an　altitude　of　over　100；000　feet　（30　kilometers）　and　through　his　thick　windows　glimpsed　a　different　sky。　Now　a　professor　at　the　University　of　California　Medical　School　in　Irvine；　Dr。　Simons　recalls　it　was　a　dark；　deep　purple　overhead。　He　had　reached　the　transition　region　where　the　blue　of　ground　level　is　being　overtaken　by　the　perfect　black　of　space。

Since　Simons'　almost　forgotten　flight；　people　of　many　nations　have　flown　above　the　atmosphere。　It　is　now　clear　from　repeated　and　direct　human　（and　robotic）　experience　that　in　space　the　daytime　sky　is　black。　The　Sun　shines　brightly　on　your　ship。　The　Earth　below　you　is　brilliantly　illuminated。　But　the　sky　above　is　black　as　night。

Here　is　the　memorable　description　by　Yuri　Gagarin　of　what　he　saw　on　the　first　spaceflight　of　the　human　species；　aboard　Vostok　1；　on　April　12；　1961：

The　sky　is　pletely　black；　and　against　the　background　of　this　black　sky　the　stars　appear　somewhat　brighter　and　more　distinct。　The　Earth　has　a　very　characteristic；　very　beautiful　blue　halo；　which　is　seen　well　when　you　observe　the　horizon。　There　is　a　smooth　color　transition　from　tender　blue；　to　blue；　to　dark　blue　and　purple；　and　then　to　the　pletely　black　color　of　the　sky。　It　is　a　very　beautiful　transition。

Clearly；　the　daylit　sky—all　that　blue—　is　somehow　connected　with　the　air。　But　as　you　look　across　the　breakfast　table；　your　panion　is　not　（usually）　blue；　the　color　of　the　sky　must　be　a　property　not　of　a　little　air；　but　of　a　great　deal。　If　you　look　closely　at　the　Earth　from　space；　you　see　it　surrounded　by　a　thin　band　of　blue；　as　thick　as　the　lower　atmosphere；　indeed；　it　is　the　lower　atmosphere。　At　the　top　of　that　band　you　can　make　out　the　blue　sky　fading　into　the　blackness　of　space。　This　is　the　transition　zone　that　Simons　was　the　first　to　enter　and　Gagarin　the　first　to　observe　from　above。　In　routine　spaceflight；　you　start　at　the　bottom　of　the　blue；　penetrate　entirely　through　it　a　few　minutes　after　liftoff；　and　then　enter　that　boundless　realm　where　a　simple

breath　of　air　is　impossible　without　elaborate　life…support　systems。　Human　life　depends　for　its　very　existence　on　that　blue　sky。　We　are　right　to　consider　it　tender　and　sacred。

We　see　the　blue　in　daytime　because　sunlight　is　bouncing　off　the　air　around　and　above　us。　On　a　cloudless　night；　the　sky　is　black　because　there　is　no　sufficiently　intense　source　of　light　to　be　reflected　off　the　air。　Somehow；　the　air　preferentially　bounces　blue　light　down　to　us。　How？

The　visible　light　from　the　Sun　es　in　many　colors—violet；　blue；　green；　yellow；　orange；　red—corresponding　to　light　of　different　wavelengths。　（A　wavelength　is　the　distance　front　crest　to　crest　as　the　wave　travels　through　air　or　space。）　Violet　and　blue　light　waves　have　the　shortest　wavelengths；　orange　and　red　the　longest。　What　we　perceive　as　color　is　how　our　eyes　and　brains　read　the　wavelengths　of　light。　（We　Might　just　as　reasonably　translate　wavelengths　of　light　into；　say；　heard　tones　rather　than　seen　colors—but　that's　not　how　our　senses　evolved。）

When　all　those　rainbow　colors　of　the　spectrum　are　mixed　together；　as　in　sunlight；　they　seem　almost　white。　These　waves　travel　together　in　eight　minutes　across　the　intervening　93　million　miles　（150　million　kilometers）　of　space　from　the　Sun　to　the　Earth。　They　strike　the　atmosphere；　which　is　made　mostly　of　nitrogen　and　oxygen　molecules。　Some　waves　are　reflected　by　the　air　back　into　space。　Some　are　bounced　around　before　the　light　reaches　the　ground　and　they　can　be　detected　by　a　passing　eyeball。　（Also；　some　bounce　off　clouds　or　the　ground　back　into　space。）　This　bouncing　around　of　light　waves　in　the　atmosphere　is　called　〃scattering。〃

But　not　all　waves　are　equally　well　scattered　by　the　molecules　of　air。　Wavelengths　that　are　much　longer　than　the　size　of　the　molecules　are　scattered　less；　they　spill　over　the　molecules；　hardly　influenced　by　their　presence。　Wavelengths　that　are　closer　to　the　size　of　the　molecules　are　scattered　snore。　And　waves　have　trouble　ignoring　obstacles　as　big　as　they　are。　（You　can　see　this　in　water　waves　scattered　by　the　pilings　of　piers；　or　bathtub　waves　from　a　dripping　faucet　encountering　a　rubber　duck。）　The　shorter　wavelengths；　those　that　we　sense　as　violet　and　blue　light；　are　more　efficiently　scattered　than　the　longer　wavelengths；　those　that　we　sense　as　orange　and　red　light。　When　we　look　up　on　a　cloudless　day　and　admire　the　blue　sky；　we　are　witnessing　the　preferential　scattering　of　the　short　waves　in　sunlight。　This　is　called　Rayleigh　scattering；　after　the　English　physicist　who　offered　the　first　coherent　explanation　for　it。　Cigarette　smoke　is　blue　for　just　the　same　reason：　The　particles　that　make　it　up　are　about　as　small　as　the　wavelength　of　blue　light。

So　why　is　the　sunset　red？　The　red　of　the　sunset　is　what's　left　of　sunlight　after　the　air　scatters　the　blue　away。　Since　the　atmosphere　is　a　thin　shell　of　gravitationally　bound　gas　surrounding　the　solid　Earth；　sunlight　must　pass　through　a　longer　slant　path　of　air　at　sunset　（or　sunrise）　than　at　noon。　Since　the　violet　and　blue　waves　are　scattered　even　more　during　their　now…longer　path　through　the　air　than　when　the　Sun　is　overhead；　what　we　see　when　we　look　toward　the　Sun　is　the　residue—the　waves　of　sunlight　that　are　hardly　scattered　away　at　all；　especially　the　oranges　and　reds。　A　blue　sky　makes　a　red　sunset。　（The　noontime　Sun　seems　yellowish　partly　because　it　emits　slightly　more　yellow　light　than　other　colors；　and　partly　because；　even　with　the　Sun　overhead；　some　blue　light　is　scattered　out　of　the　sunbeams　by　the　Earth's　atmosphere。）

It　is　sometimes　said　that　scientists　are　unromantic；　that　their　passion　to　figure　out　robs　the　world　of　beauty　and　mystery。　But　is　it　not　stirring　to　understand　how　the　world　actually　works—that　white　light　is　made　of　colors；　that　color　is　the　way　we　perceive　the　wavelengths　of　light；　that　transparent　air　reflects　light；　that　in　so　doing　it　discriminates　among　the　waves；　and　that　the　sky　is　blue　for　the　same　reason　that　the　sunset　is　red？　It　does　no　harm　to　the　romance　of　the　sunset　to　know　a　little　bit　about　it。

Since　most　simple　molecules　are　about　the　same　size　（roughly　a　hundred　millionth　of　a　centimeter）；　the　blue　of　the　Earth's　sky　doesn't　much　depend　on　what　the　air　is　made　of—as　long　as　the　air　doesn't　absorb　the　light。　Oxygen　and　nitrogen　molecules　don't　absorb　visible　light；　they　only　bounce　it　away　in　some　other　direction。　Other　molecules；　though；　can　gobble　up　the　light。　Oxides　of　nitrogen—produced　in　automotive　engines　and　in　the　fires　of　industry—are　a　source　of　the　murky　brown　coloration　of　smog。　Oxides　of　nitrogen　（made　from　oxygen　and　nitrogen）　do　absorb　light。　Absorption；　as　well　as　scattering；　can　color　a　sky。



OTHER　WORLDS；　OTHER　SKIES：　Mercury；　the　Earth's　Moon；　and　most　satellites　of　the　other　planets　are　small　worlds；　because　of　their　feeble　gravities；　they　are　unable　to　retain　their　atmospheres—which　instead　trickle　of　into　space。　The　near…vacuum　of　space　then　reaches　the　ground。　Sunlight　strikes　their　surfaces　unimpeded；　neither　scattered　nor　absorbed　along　the　way。　The　skies　of　these　worlds　are　black；　even　at　noon。　This　has　been　witnessed　firsthand　so　far　by　only　12　humans；　the　lunar　landing　crews　of　Apollos　11；　12；　and　14…17。

A