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ll　against　each　other；　seized　with　a　fit　of　。　。　。　inextinguishable　laughter。　

VOLTAIRE；　MICROMEGAS。　A　PHILOSOPHICAL　HISTORY　（1752）

In　the　seventeenth　century　there　was　still　some　hope　that；　even　if　the　Earth　was　not　the　center　of　the　Universe；　it　might　be　the　only　〃world。〃　But　Galileo's　telescope　revealed　that　〃the　Moon　certainly　does　not　possess　a　smooth　and　polished　surface〃　and　that　other　worlds　might　look　〃just　like　the　face　of　the　Earth　itself。〃　The　Moon　and　the　planets　showed　unmistakably　that　they　had　as　much　claim　to　being　worlds　as　the　Earth　does—with　mountains；　craters；　atmospheres；　polar　ice　caps；　clouds；　and；　in　the　case　of　Saturn；　a　dazzling；　unheard…of　set　of　circumferential　rings。　After　millennia　of　philosophical　debate；　the　issue　was　settled　decisively　in　favor　of　〃the　plurality　of　worlds。〃　They　might　be　profoundly　different　from　our　planet。　None　of　them　might　be　as　congenial　for　life。　But　the　Earth　was　hardly　the　only　one。

This　was　the　next　in　the　series　of　Great　Demotions；　downlifting　experiences；　demonstrations　of　our　apparent　insignificance；　wounds　that　science　has；　in　its　search　for　Galileo's　facts；　delivered　to　human　pride。



WELL；　SOME　HOPED；　even　if　the　Earth　isn't　at　the　center　of　the　Universe；　the　Sun　is。　The　Sun　is　our　Sun。　So　the　Earth　is　approximately　at　the　center　of　the　Universe。　Perhaps　some　of　our　pride　could　in　this　way　be　salvaged。　But　by　the　nineteenth　century；　observational　astronomy　had　made　it　clear　that　the　Sun　is　but　one　lonely　star　in　a　great　self…gravitating　assemblage　of　suns　called　the　Milky　Way　Galaxy。　Far　from　being　at　the　center　of　the　Galaxy；　our　Sun　with　its　entourage　of　dim　and　tiny　planets　lies　in　an　undistinguished　sector　of　an　obscure　spiral　arm。　We　are　thirty　thousand　light　years　from　the　Center。

Well；　our　Milky　Way　is　the　only　galaxy。　The　Milky　Way　Galaxy　is　one　of　billions；　perhaps　hundreds　of　billions　of　galaxies　notable　neither　in　mass　nor　in　brightness　nor　in　how　its　stars　are　configured　and　arrayed。　Some　modern　deep　sky　photographs　show　more　galaxies　beyond　the　Milky　Way　than　stars　within　the　Milky　Way。　Every　one　of　them　is　an　island　universe　containing　perhaps　a　hundred　billion　suns。　Such　an　image　is　a　profound　sermon　on　humility。

Well；　then；　at　least　our　Galaxy　is　at　the　center　of　the　Universe。　No；　this　is　wrong　too。　When　the　expansion　of　the　Universe　was　first　discovered；　many　people　naturally　gravitated　to　the　notion　that　the　Milky　Way　was　at　the　center　of　the　expansion；　and　all　the　other　galaxies　running　away　from　us。　We　now　recognize　that　astronomers　on　any　galaxy　would　see　all　the　others　running　away　。　from　them；　unless　they　were　very　careful；　they　would　all　conclude　that　they　were　at　the　center　of　the　Universe。　There　is；　in　fact；　no　center　to　the　expansion；　no　point　of　origin　of　the　Big　Bang；　at　least　not　in　ordinary　three…dimensional　space。

Well；　even　if　there　are　hundreds　of　billions　of　galaxies；　each　with　hundreds　of　billions　of　stars；　no　other　star　has　planets。　If　there　are　n　other　planets　beyond　our　Solar　System；　perhaps　there's　no　other　life　in　the　Universe。　Our　uniqueness　might　then　be　saved。　Sing　planets　are　small　and　feebly　shine　by　reflected　sunlight；　they're　hard　to　find。　Although　applicable　technology　is　improving　wit　breathtaking　speed；　even　a　giant　world　like　Jupiter；　orbiting　the　nearest　star；　Alpha　Centauri；　would　still　be　difficult　to　detect。　Iii　our　ignorance；　the　geocentrists　find　hope。

There　was　once　a　scientific　hypothesis—not　just　well　received　but　prevailing—that　supposed　our　solar　system　to　have　formed　through　the　near　collision　of　the　ancient　Sun　with　another　star；　the　gravitational　tidal　interaction　pulled　out　tendrils　of　sunstuff　that　quickly　condensed　into　planets。　Since　space　is　mainly　empty　and　near　stellar　collisions　most　rare；　it　was　concluded　that　few　other　planetary　systems　exist—perhaps　only　one；　around　that　other　star　that　long　ago　co…parented　the　worlds　of　our　solar　system。　Early　in　my　studies；　I　was　amazed　and　disappointed　that　such　a　view　had　ever　been　taken　seriously；　that　for　planets　of　other　stars；　absence　of　evidence　had　been　considered　evidence　of　absence。

Today　we　have　firm　evidence　for　at　least　three　planets　orbiting　an　extremely　dense　star；　the　pulsar　designated　B1257＋12；　about　which　I'll　say　more　later。　And　we've　found；　for　more　than　half　the　stars　with　masses　like　the　Sun's；　that　early　in　their　careers　they're　surrounded　by　great　disks　of　gas　aid　dust　out　of　which　planets　seem　to　form。　Other　planetary　systems　now　look　to　be　a　cosmic　monplace；　maybe　even　worlds　something　like　the　Earth。　We　should　be　able；　in　the　next　few　decades；　to　inventory　at　least　the　larger　planets；　if　they　exist；　of　hundreds　of　nearby　stars。

Well；　if　our　position　in　space　doesn't　reveal　our　special　role；　our　position　in　time　does：　We've　been　in　the　Universe　since　The　Beginning　（give　or　take　a　few　days）。　We've　been　given　special　responsibilities　by　the　Creator。　It　once　seemed　very　reasonable　to　think　of　the　Universe　as　beginning　just　a　little　before　our　collective　memory　is　obscured　by　the　passage　of　time　and　the　illiteracy　of　our　ancestors。　Generally　speaking；　that's　hundreds　or　thousands　of　years　ago。　Religions　that　purport　to　describe　the　origin　of　the　Universe　often　specify—implicitly　or　explicitly—a　date　of　origin　of　roughly　such　vintage；　a　birthday　for　the　world。

If　you　add　up　all　the　〃begats〃　in　Genesis；　for　example；　you　get　an　age　for　the　Earth：　6；000　years　old；　plus　or　minus　a　little。　The　universe　is　said　to　be　exactly　as　old　as　the　Earth。　This　is　still　the　standard　of　Jewish；　Christian；　and　Moslem　fundamentalists　and　is　clearly　reflected　in　the　Jewish　calendar。

But　so　young　a　Universe　raises　an　awkward　question：　How　is　it　that　there　are　astronomical　objects　more　than　6；000　light…years　away？　It　takes　light　a　year　to　travel　a　light…year；　10；000　years　to　travel　10；000　light…years；　and　so　on。　When　we　look　at　the　center　of　the　Milky　Way　Galaxy；　the　light　we　see　left　its　source　30；000　years　ago。　The　nearest　spiral　galaxy　like　our　own；　M31　in　the　constellation　Andromeda；　is　2　million　light…years　away；　so　we　are　seeing　it　as　it　was　when　the　light　from　it　set　out　on　its　long　journey　to　Earth—2　million　years　ago。　And　when　we　observe　distant　quasars　5　billion　light…years　away；　we　are　seeing　them　as　they　were　5　billion　years　ago；　before　the　Earth　was　formed。　（They　are；　almost　certainly；　very　different　today。）

If；　despite　this；　we　were　to　accept　the　literal　truth　of　such　religious　books；　how　could　we　reconcile　the　data？　The　only　plausible　conclusion；　I　think；　is　that　God　recently　made　all　the　photons　of　light　arriving　on　the　Earth　in　such　a　coherent　format　as　to　mislead　generations　of　astronomers　into　the　misapprehension　that　there　are　such　things　as　galaxies　and　quasars；　and　intentionally　driving　them　to　the　spurious　conclusion　that　the　Universe　is　vast　and　old。　This　is　such　a　malevolent　theology　I　still　have　difficulty　believing　that　anyone；　no　matter　how　devoted　to　the　divine　inspiration　of　any　religious　book；　could　seriously　entertain　it。

Beyond　this；　the　radioactive　dating　of　rocks；　the　abundance　of　impact　craters　on　many　worlds；　the　evolution　of　the　stars；　and　the　expansion　of　the　Universe　each　provides　pelling　and　independent　evidence　that　our　Universe　is　many　billions　of　years　old—despite　the　confident　assertions　of　revered　theologians　that　a　world　so　old　directly　contradicts　the　word　of　God；　and　that　at　any　rate　information　on　the　antiquity　of　the　world　is　inaccessible　except　to　faith。　These　lines　of　evidence；　as　well；　would　have　to　be　manufactured　by　a　deceptive　and　malicious　deity—unless　the　world　is　much　older　than　the　literalists　in　the　Judeo…Christian…Islamic　religion　suppose。　Of　course；　no　such　problem　arises　for　those　many　religious　people　who　treat　the　Bible　and　the　Qur'an　as　historical　and　moral　guides　and　great　literature；　but　who　recognize　that　the　perspective　of　these　scriptures　on　the　natural　world　reflects　the　rudimentary　science　of　the　time　in　which　they　were　written。

Ages　rolled　by　before　the　Earth　began。　More　ages　will　run　their　course　before　it　is　destroyed。　A　distinction　needs　to　be　drawn　between　how　old　the　Earth　is　（around　4。5　billion　years）　and　how　old　the　Universe　is　（about　15　billion　years　since　the　Big　Bang）。　The　immense　interval　of　time　between　the　origin　of　the　Universe　and　our　epoch　was　two…thirds　over　before　the　Earth　came　to　be。　Some　stars　and　planetary　systems　are　billions　of　years　younger；　others　billions　of　years　older。　But　in　Genesis；　chapter　1；　verse　1；　the　Universe　and　the　Earth　are　created　on　the　same　day。　The　Hindu…Buddhist…Jain　religion　tends　not　to　confound　the　two　events。

As　for　humans；　we're　lateers。　We　appear　in　the　last　instant　of　cosmic　time。　The　history　of　the　Universe　till　now　was　99。998　percent　over　before　our　species　arrived　on　the　scene。　In　that　vast　sweep　of　aeons；　we　could　not　have　assumed　any　special　responsibilities　for　our　planet；　or　life；　or　anything　else。　We　were　not　here。

Well；　if　we　can't　find　anything　special　about　our　position　or　our　epoch；　maybe　there's　something　special　about　our　motion。　Newton　and　all　the　other　great　classical　physicists　held　that　the　velocity　of　the　Earth　in　space　constituted　a　〃privileged　frame　of　reference。〃　That's　actually　what　it　was　called。　Albert　Einstein；　a　keen　critic　of　prejudice　and　privilege　all　his　life；　considered　this　〃absolute〃　physics　a　remnant　of　an　increasingly　discredited　Earth　chauvinism。　It　seemed　to　him　that　the　laws　of　Nature　must　be　the　same　no　matter　what　the　velocity　or　frame　of　reference　of　the　observer。　With　this　as　his　starting　point；　he　developed　the　Special　Theory　of　Relativity。　Its　consequences　are　bizarre；　counter　intuitive；　and　grossly　contradict　mon　sense—but　only　at　very　high　speeds。　Careful　and　repeated　observations　show　that　his　justly　celebrated　theory　is　an　accurate　description　of　how　the　world　is　made。　Our　monsense　intuitions　can　be　mistaken。　Our　preferences　don't　count。　We　do　not　live　in　a　privileged　reference　frame。

One　consequence　of　special　relativity　is　time　dilation—the　slowing　down　of　time　as　the　observer　approaches　light　speed。　You　can　still　find　claims　that　time　dilation　applies　to　watches　and　elementary　particles—and；　presumably；　to　circadian　and　other