A Short History of Nearly Everything- 第7部分

but　let’s　pretend　again　that　we　have　made　it　to　the　oort　cloud。　the　first　thing　you　mightnotice　is　how　very　peaceful　it　is　out　here。　we’re　a　long　way　from　anywhere　now—so　far　fromour　own　sun　that　it’s　not　even　the　brightest　star　in　the　sky。　it　is　a　remarkable　thought　that　thatdistant　tiny　twinkle　has　enough　gravity　to　hold　all　these　ets　in　orbit。　it’s　not　a　very　strongbond；　so　the　ets　drift　in　a　stately　manner；　moving　at　only　about　220　miles　an　hour。　fromtime　to　time　some　of　these　lonely　ets　are　nudged　out　of　their　normal　orbit　by　some　slightgravitational　perturbation—a　passing　star　perhaps。　sometimes　they　are　ejected　into　theemptiness　of　space；　never　to　be　seen　again；　but　sometimes　they　fall　into　a　long　orbit　aroundthe　sun。　about　three　or　four　of　these　a　year；　known　as　long…period　ets；　pass　through　theinner　solar　system。　just　occasionally　these　stray　visitors　smack　into　something　solid；　likeearth。　that’s　why　we’ve　e　out　here　now—because　the　et　we　have　e　to　see　hasjust　begun　a　long　fall　toward　the　center　of　the　solar　system。　it　is　headed　for；　of　all　places；manson；　iowa。　it　is　going　to　take　a　long　time　to　get　there—three　or　four　million　years　atleast—so　we’ll　leave　it　for　now；　and　return　to　it　much　later　in　the　story。

so　that’s　your　solar　system。　and　what　else　is　out　there；　beyond　the　solar　system？　well；nothing　and　a　great　deal；　depending　on　how　you　look　at　it。

in　the　short　term；　it’s　nothing。　the　most　perfect　vacuum　ever　created　by　humans　is　not　asempty　as　the　emptiness　of　interstellar　space。　and　there　is　a　great　deal　of　this　nothingness　untilyou　get　to　the　next　bit　of　something。　our　nearest　neighbor　in　the　cosmos；　proxima　centauri；which　is　part　of　the　three…star　cluster　known　as　alpha　centauri；　is　4。3　light…years　away；　a　sissyskip　in　galactic　terms；　but　that　is　still　a　hundred　million　times　farther　than　a　trip　to　the　moon。

to　reach　it　by　spaceship　would　take　at　least　twenty…five　thousand　years；　and　even　if　you　madethe　trip　you　still　wouldn’t　be　anywhere　except　at　a　lonely　clutch　of　stars　in　the　middle　of　avast　nowhere。　to　reach　the　next　landmark　of　consequence；　sirius；　would　involve　another　4。6light…years　of　travel。　and　so　it　would　go　if　you　tried　to　star…hop　your　way　across　the　cosmos。

just　reaching　the　center　of　our　own　galaxy　would　take　far　longer　than　we　have　existed　asbeings。

space；　let　me　repeat；　is　enormous。　the　average　distance　between　stars　out　there　is　20million　million　miles。　even　at　speeds　approaching　those　of　light；　these　are　fantasticallychallenging　distances　for　any　traveling　individual。　of　course；　it　is　possible　that　alien　beingstravel　billions　of　miles　to　amuse　themselves　by　planting　crop　circles　in　wiltshire　orfrightening　the　daylights　out　of　some　poor　guy　in　a　pickup　truck　on　a　lonely　road　in　arizona（they　must　have　teenagers；　after　all）；　but　it　does　seem　unlikely。

still；　statistically　the　probability　that　there　are　other　thinking　beings　out　there　is　good。

nobody　knows　how　many　stars　there　are　in　the　milky　way—estimates　range　from　100　billionor　so　to　perhaps　400　billion—and　the　milky　way　is　just　one　of　140　billion　or　so　othergalaxies；　many　of　them　even　larger　than　ours。　in　the　1960s；　a　professor　at　cornell　namedfrank　drake；　excited　by　such　whopping　numbers；　worked　out　a　famous　equation　designed　tocalculate　the　chances　of　advanced　life　in　the　cosmos　based　on　a　series　of　diminishingprobabilities。

under　drake’s　equation　you　divide　the　number　of　stars　in　a　selected　portion　of　the　universeby　the　number　of　stars　that　are　likely　to　have　planetary　systems；　divide　that　by　the　number　ofplanetary　systems　that　could　theoretically　support　life；　divide　that　by　the　number　on　whichlife；　having　arisen；　advances　to　a　state　of　intelligence；　and　so　on。　at　each　such　division；　thenumber　shrinks　colossally—yet　even　with　the　most　conservative　inputs　the　number　ofadvanced　civilizations　just　in　the　milky　way　always　works　out　to　be　somewhere　in　themillions。

what　an　interesting　and　exciting　thought。　we　may　be　only　one　of　millions　of　advancedcivilizations。　unfortunately；　space　being　spacious；　the　average　distance　between　any　two　ofthese　civilizations　is　reckoned　to　be　at　least　two　hundred　light…years；　which　is　a　great　dealmore　than　merely　saying　it　makes　it　sound。　it　means　for　a　start　that　even　if　these　beings　knowwe　are　here　and　are　somehow　able　to　see　us　in　their　telescopes；　they’re　watching　light　that　leftearth　two　hundred　years　ago。　so　they’re　not　seeing　you　and　me。　they’re　watching　the　frenchrevolution　and　thomas　jefferson　and　people　in　silk　stockings　and　powdered　wigs—peoplewho　don’t　know　what　an　atom　is；　or　a　gene；　and　who　make　their　electricity　by　rubbing　a　rodof　amber　with　a　piece　of　fur　and　think　that’s　quite　a　trick。　any　message　we　receive　from　themis　likely　to　begin　“dear　sire；”　and　congratulate　us　on　the　handsomeness　of　our　horses　and　ourmastery　of　whale　oil。　two　hundred　light…years　is　a　distance　so　far　beyond　us　as　to　be；　well；just　beyond　us。

so　even　if　we　are　not　really　alone；　in　all　practical　terms　we　are。　carl　sagan　calculated　thenumber　of　probable　planets　in　the　universe　at　large　at　10　billion　trillion—a　number　vastlybeyond　imagining。　but　what　is　equally　beyond　imagining　is　the　amount　of　space　throughwhich　they　are　lightly　scattered。　“if　we　were　randomly　inserted　into　the　universe；”　saganwrote；　“the　chances　that　you　would　be　on　or　near　a　planet　would　be　less　than　one　in　a　billiontrillion　trillion。”　（that’s　1033；　or　a　one　followed　by　thirty…three　zeroes。）　“worlds　are　precious。”

which　is　why　perhaps　it　is　good　news　that　in　february　1999　the　international　astronomicalunion　ruled　officially　that　pluto　is　a　planet。　the　universe　is　a　big　and　lonely　place。　we　can　dowith　all　the　neighbors　we　can　get。

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3　　THE　REVEREND　EVANS’S　UNIVERSE

小！说
when　the　skies　are　clear　and　the　moon　is　not　too　bright；　the　reverend　robert　evans；　aquiet　and　cheerful　man；　lugs　a　bulky　telescope　onto　the　back　deck　of　his　home　in　the　bluemountains　of　australia；　about　fifty　miles　west　of　sydney；　and　does　an　extraordinary　thing。　helooks　deep　into　the　past　and　finds　dying　stars。

looking　into　the　past　is　of　course　the　easy　part。　glance　at　the　night　sky　and　what　you　see　ishistory　and　lots　of　it—the　stars　not　as　they　are　now　but　as　they　were　when　their　light　leftthem。　for　all　we　know；　the　north　star；　our　faithful　panion；　might　actually　have　burnedout　last　january　or　in　1854　or　at　any　time　since　the　early　fourteenth　century　and　news　of　it　justhasn’t　reached　us　yet。　the　best　we　can　say—can　ever　say—is　that　it　was　still　burning　on　thisdate　680　years　ago。　stars　die　all　the　time。　what　bob　evans　does　better　than　anyone　else　whohas　ever　tried　is　spot　these　moments　of　celestial　farewell。

by　day；　evans　is　a　kindly　and　now　semiretired　minister　in　the　uniting　church　in　australia；who　does　a　bit　of　freelance　work　and　researches　the　history　of　nineteenth…century　religiousmovements。　but　by　night　he　is；　in　his　unassuming　way；　a　titan　of　the　skies。　he　huntssupernovae。

supernovae　occur　when　a　giant　star；　one　much　bigger　than　our　own　sun；　collapses　and　thenspectacularly　explodes；　releasing　in　an　instant　the　energy　of　a　hundred　billion　suns；　burningfor　a　time　brighter　than　all　the　stars　in　its　galaxy。　“it’s　like　a　trillion　hydrogen　bombs　going　offat　once；”　says　evans。　if　a　supernova　explosion　happened　within　five　hundred　light…years　of　us；we　would　be　goners；　according　to　evans—“it　would　wreck　the　show；”　as　he　cheerfully　puts　it。

but　the　universe　is　vast；　and　supernovae　are　normally　much　too　far　away　to　harm　us。　in　fact；most　are　so　unimaginably　distant　that　their　light　reaches　us　as　no　more　than　the　faintesttwinkle。　for　the　month　or　so　that　they　are　visible；　all　that　distinguishes　them　from　the　otherstars　in　the　sky　is　that　they　occupy　a　point　of　space　that　wasn’t　filled　before。　it　is　theseanomalous；　very　occasional　pricks　in　the　crowded　dome　of　the　night　sky　that　the　reverendevans　finds。

to　understand　what　a　feat　this　is；　imagine　a　standard　dining　room　table　covered　in　a　blacktablecloth　and　someone　throwing　a　handful　of　salt　across　it。　the　scattered　grains　can　bethought　of　as　a　galaxy。　now　imagine　fifteen　hundred　more　tables　like　the　first　one—enough　tofill　a　wal…mart　parking　lot；　say；　or　to　make　a　single　line　two　miles　long—each　with　a　randomarray　of　salt　across　it。　now　add　one　grain　of　salt　to　any　table　and　let　bob　evans　walk　amongthem。　at　a　glance　he　will　spot　it。　that　grain　of　salt　is　the　supernova。

evans’s　is　a　talent　so　exceptional　that　oliver　sacks；　in　an　anthropologist　on　mars；　devotesa　passage　to　him　in　a　chapter　on　autistic　savants—quickly　adding　that　“there　is　no　suggestionthat　he　is　autistic。”　evans；　who　has　not　met　sacks；　laughs　at　the　suggestion　that　he　might　beeither　autistic　or　a　savant；　but　he　is　powerless　to　explain　quite　where　his　talent　es　from。

“i　just　seem　to　have　a　knack　for　memorizing　star　fields；”　he　told　me；　with　a　franklyapologetic　look；　when　i　visited　him　and　his　wife；　elaine；　in　their　picture…book　bungalow　on　atranquil　edge　of　the　village　of　hazelbrook；　out　where　sydney　finally　ends　and　the　boundlessaustralian　bush　begins。　“i’m　not　particularly　good　at　other　things；”　he　added。　“i　don’tremember　names　well。”

“or　where　he’s　put　t