千米小说网

Chapter 1 What is a black hole（第1页）

Chapter1Whatisablackhole?

Ablackhiohefravityissthatnothi，travelfastenoughtoesitsihoughtheywerefirstthefertileimaginatioicalphysicists，blackholeshaveifiedintheUheirhundredsandatedforintheirmillions。Althoughiheseobjeteractwith，andthusiheirsurroundingsinawaythatbehighlydetectable。Exactlywhattheiionisdependsonproximityrelativetotheblackhole:toodthereisfurtherafieldsomedramatidspectaomena>

&ermblackhole'wasfirstmentionedinprintinanartiEwingiingonasymposiumheldinTexasin1963，althoughsheionedwhoedtheexpression。In1967，AmeriphysiWheelerhandfravitationallypletelycollapsedstar'aopopularizetheterm，thoughtheceptofacollapsedstaredbyfellowAmerisRobertOppenheimerandHartlandSnyderba1939。Infact，themathematidationsofthemodernpictureofblackholesbeganratherearlierin1915，hysicistKarlSchwarzseimportaioein's（knownasthefieldequationsiheoryofRelativity）fortheisolatednmassinspace。

TwodecadeslaterintheUK，alittlebeforeOppenheimerandSnyder'swork，SirArthurEddingtonhadworkedoutsomeoftherelevaithetextofiingworkbytheIndianphysicistSubrahmanyandrasekharonenstostarswhehephysicalimplisofEddington'selytheassivestarswhentheyhaveusedupalltheirfueltoformblackholes，EddingtonhimselfproheRoyalAstronomicalSo1935asbeing‘absurd'。Despitetheapparentabsurdityofthenotion，blackholesareverymuchpartofphysicalrealitythroughalaxyandacrosstheUheradvancesweremadeiedStatesbyDavidFieinin1958，whoestablishedtheexistene-waysurfadingablackholewhosesigweshallstudyintheingchaptersisimmeehissurfa'tallowlightitselftobreakfreefromthepravitationalattrawithinandisthereasonwhyablackholeisblack。Tobegiandhowthisbehavihtariseweofirstuandaprofouhephysicalworld:thereisamaximumspeedatartiyobjetravel。

Howfastisfast?

Alawofthejuifyouwanttoescapeapredatoryouorunfast。Unlessyouhaveexalouflage，youwillonlysurviveifyouareswift。Themaximumspeedwithwhichamammalespleasantsituationdependsonplexbiochemicalrelatioweenmass，musgth，aabolism。ThemaximumspeedwithwhichthemostrapidlytravelliyintheUravelisthatexhibitedbyparticlesthathavenomassatall，suchasparticlesoflight（knohotons）。Thismaximumspeedbegivenverypreciselyas299，792，458metrespersed，equivalentto186，282milespersed，roagamillioerthanthespeedofsoundinair。IfIcouldtravelatthespeedoflight，Iwouldbeabletotravel

frommyhomeioAustraliaieenthofased，barelytimetoblink。LighttravellingfromourheSueightmiraveltous。Fromouroutermostpla'sajourafewhoursforaphototheSulight-minutesawayfromEarthau-hoursawayfromus。ThishastheiihatiftheSunstoppedshiningorifuurnedpurple，hdoutaboutsuchimportantinformatihtminutesorafewhoursrespectively。

&'snowsiderhowfastlighttravelfromevenmoreimmeantpointsioEarth。TheMilkyWay，theGalaxyinwhichourSolarSystemresides，isafewhuhousandlight-yearsacross。ThismeansthatlighttakesafewhuhousaravelfromoheGalaxytotheother。TheForheerofgalaxiestothelalaxies（ofwhichtheMilkyWayisasignifitmember）andishundredsofmillionsoflight-yearsawayfromus。Thus，anobserveroingastarinagalaxywithintheFornaxclusterlookingbackthtnowmight，ifequippedriateiion，seedinosaurslumberingarouh。However，itisonlythemind-boggliheUmakesthemotionoflightlooksluggishandtime-ing。Theroleofthespeedoflightasamandatoryupperlimithasanintriguiwheosiderhowtolauospace。

Escapevelocity

NowsupposesomedisasteroccurswiththeresultthattheentiremassoftheEarthwereshrunktoapoint，havingnospatialextentwhatsoever。Wecallsuobjegularity。Ithasmass'，amassiveobjectthatoccupieszerovolumeofspace。Ataverysmalldistanetreawayfromthissingularity，theescapevelouchlargerthanitwasat1，600km（andinfactwouldbeabout10%ofthespeedoflight）。Closertothesingularitystill，justureawaytheescapevelocitywouldbeequaltothespeedoflight。Atthisdistaitselfwouldnothavesuffitspeedtoescapethisgravitationalpull。Thisisthekeyideatouandhowblackholeswork。

Itiswuseularity'。Wedoattheendpointofaggravitationalcollapsethemattergoesdowritbutratherthatourclassicaltheravitybreaksdoweraquantumregime。Fromhereon，wewillusethetermsingularitytorefertothisultra-dee。

&horizon

Nowimagironautflyiandthatyouareapproagthissingularity。Whilestillatsomedista，youcouldalwaysthrioreverseafromit。Buttheclet，theharderadigbees。Eventuallyyoureachadistanwhichitisimpossibletoesatterhowpowerfulyihisisbecauseyouhavereachedtheeventhorizoically-definedsphericalsurface，whichisdefiheboundaryinsideofwhichtheescapevelocitywouldexceedthespeedoflight。Fht-experimehcollapsedtoapoint，thissurfacehereofradiusorewiththesingularityatitstre，easyenoughperhapsforourspacecrafttoavoid。However，theeventhorizonbeuchlargerwhentheblackholeisformedfromacollapsedstarratherthanacollapsedplahorizonhasanimportantphysisequence:ifyouareonthatsurfasideit，thelawsofphysicssimplywon'tallowyoutoescapebecausetodosoyouwouldobreaktheuniversalspeedlimit。Theeventhorizonisamandatorylevelofdemar:outsideityouhavefreedomtodetermii，andyourfutureremainsunalterablylockedwithin。

TheradiusofthissphericalsurfaamedinhonourofKarlSchwarzsentionedearlier。WhileasoldierinWorldWarI，Schwarzschildprovidedthefirstexactsolutioein'sfamousfieldequationsthatunderpiivity。TheSchwarzschildradiusiswrittenasRS=2GMc2whereMisthemassoftheblackhole，Gison'sgravitationalt，andcisthespeedoflight。Usingthisformula，theSchwarzschildradiusoftheEarthesouttobejusture。Similarly，theSchwarzschildradiusoftheSunisfoundtobe3km，meaningthatifthemassofourSuncouldallbesquashedintoasingularity，thenatjust3kmawayfromthispointtheescapevelocitywouldbeequaltothespeedoflight。AblaebilliontimesmoremassivethantheSun（i。e。havingamassof109solarmasses）wouldhaveaSchwarzschildradiusonebilliontimeslarger（theSchwarzschildradiusofapointmassthatisnsimplyscalesdirectlywithitsmass）。AsIdesChapter6，suchmammothblackholesarebelievedtobeattheanygalaxies。

Thisdesoftheeventhorizonbereasonablythoughtofwithionianphysideed，physititiesresemblingblackhiuriesbeforeEihersprofoundlygedofspadtime。Theprinkerswhoimagineddarkstars'thatresembleblackholeswereJohnMidPierre-Simoingbathe18thtury，andIwillnowexplainwhattheydid。

&heremarkablethingsaboutastronomyishowmudiscoverabouttheUniverseevenwheuetEarth。Forexample，nohumanbeiedtheSuhepreseheSuhelate19thturybyanalysirumofsunlight。Thisisparticularlyremarkableasthisstitutedthediscoveryoftheelemeself;itwasfoundontheSunldeteEarth。Evehe18thtury，someoftheideasbehindblackhined，andinparticulartheideaofwhatiscalledadarkstar。Thepersohefirstimaginativeleapwasverymuchaprodue。

JohnMichell

TheGeianerawas，inEnglaivepeaglishCivilWarwaslo，andEnglandhadbeealaivedomestiquillity（theriseofNapoleoillsomewayoff）。Likehisfatherbeforehim，theReverendJohnMichell（Figure1）receivedauyedudeheChurgland。AsareThorYorkshire，Michellwasabletouehisstificresearch，followinguphisisingeology，magism，gravity，light，andastronomy。Inonwithotherstistswithetime，suchastheastronomerWilliamHersdthephysirydish（ersonalfriend），Michellwasabletoridethewaveofthehinking。SirIsaahadrevolutiohewayinwhiiverseerulatinghislawofgravitationwhiedtheorbitsoftheplaheSolarSystemasbeihesameforcethatcausedhisfamousappletodropfromthetree。

1。JohnMiath。

&onianideasallowedtheUudiedusiidthisfreshbreedofstistswasabletodeploythisnovelworld-viewifields。Michellarticularlyedtousehihedistaarsbyusisofthelighttheyemitted。Hecameupwithvariousschemestodothis，byrelatingastar&#htscolour;healsosideredbinarystars（pairsravitationallybouher）aalmotionscouldgiveusefuldynamiichellalsoiarstendtoparticularareasofthesky，testingthisagainstarandomdistributionandinferringgravitationalg。heseideasracticableatthetime:fewbinarystarswereknown（thoughHerschelrodupressivecataloguesofvariousdoublestarsas）aioweenastar&#htscolourturobenotquiteasMichellhadthoughtitwas。heless，MichellwasstrainingtodoforthewiderUoheSolarSystem:allowastifial，anddynamialysisofobservationstoprovidenewinformationabouttheproperties，masses，anddistaheheavenlybodies。

&isightthatichellfollowedfromtheideathatparticlesoflightare，inMichell'swords，attrathesamemannerasallotherbodieseareated;thatis，byfthesameproportiontotheirvisiiae[bywhitmass]，ofwhichthereoreaso，gravitationbeing，asfarasweknow，orhaveaobelieve，anuniversallawofnature'。Suchparticlesemittedfrestarwould，hereasoned，besloweddowionalattraofthestar。ThusthestarlightreagEarthwouldbeslower。onhadshownthatlightslowsdowninglass，andthisexplaiheprincipleofrefra。Ifstarlightwasindeedsimilarlyslowed，Michellarguedthatitmightbepossibletodetectthisslowingbyexaminingstarlightthroughaprism。Theexperimentwastried，notbyMichell，butbytheAstronomerRoyal，theReverendDrNevilMaskelyne，wholookedforthediminishiabilityht。dishwrotetoMithatithadn'tworkedandthatthereisnotmuchlikelyhood[sidinganystarswhoselightissensiblydiminished'。Michellwasdismayed，butsuicalspesrequiredmugofimpoarlightaffectedbythegravitationalattraofthestarfromwhichitisemitted?Michell'tbesure。Buthewasboldenoughtomakeaiion。

Ifastarwassuffitlymassive，andgravityreallydidaffectstarlight，theionalforcecouldbesuffittoholdbacktheparticlesoflightpletelyahemfromleaving。Suobjectwouldbeadarkstar。Thislittle-kingioryinYorkshirehadthusbeepersontoceiveofablackhole。However，sofarMichell'srammethedistaarslayiwasmore，hishealthhadbeeandthishadstoppedhimusinghistelescope。dishwrotetohimagletter:ifyourhealthdoesnotallowyoutogooelesayatleastpermittheeasierandlesslaboriousemploymeheworld。'Thissingularexampleofajokefromdish（whowasnotoriouslybuttooaMichellhadceived。Weighingtheworld'meainwhichtweleadspheresateitherendofthebeamofatorsioractedbytwostationaryleadspheres。ThisallowsohestrengthofthegravitatioherebyioftheEarth。Noohisbefore。Michell'sideawasbrilliant，buthedidn'tlivetopletetheprojestead，Michell'sexperimenterformedbydishandisnowknownasdish'sexperiment。Thistransferofcredittodishismorethaedforbythenumerhsmadebydishwhieglectedtopublishatributedtosubsequentresear's'lawandb's'law）。

Pierre-SimonLaplace

OhersideoftheEnglishel，Pierre-SimoeranquilidyllaffordedbythepeacefulperiodoftheEe。LaplacelivedthroughtheFreion，thoughhiscareerprosperedasheihenewlyfouutdeFrahe恈olePolyteique。HeeveaperiodasMiheInteriorunderNapoleon，ashort-livedappoiheEmperorcametret。NapoleoLaplacewasafirst-ratemathematibutasanadministratorhewasworsethanaverage。erwroteofLaplacethathesoughtsubtletieseverywhere，lyproblems，andfinallycarriedthespiritof"infinitesimals"iration'。heradministratorsto，buttheworldhashadfewmathematisasprodudinsightfulasLaplace。Hemadepivotaltributiory，probability，mathematics，celestialmeiy，andphysics。Heworkedontopicsasdiverseascapillaryaets，inductivereasoning，solarsystemstability，thespeedofsouialequations，andspheriieoftheideashesideredwasdarkstars。

In1796LaplacepublishedhisExpositiondusystèmedumoeedpublic，thisbookdescribesthephysiwhiyisbased，thelawofgravityaiosintheSolarSystem，andthelawsofmotionandmeics。Theseideasareappliedtovariousphenomena，ingthetidesandthepreoftheequihebookalsosLaplace'sspesinoftheSolarSystem。Oicularpassageisofspecialrelevaoourstory。Laplacemadeacaleah-likebodywoulditsescapevelocitywasequaltothatoflight。Heshowed，quitecorrectly，thatthegravitatithonthesurfaceofastar，withdensityparabletothatofEarthbutwithadiameterofabout250timesthatoftheSun，wouldbesoievenlightwouldbeabletoescape。Thus，hereasobodiesintheUhereforebeiheystillbelurkiableinthedarknightsky，whileweimagiheonlythingsoutthere'werethebrightlumiwesee?TheHungarianastronomerFranzXavervohatLaplaceprovidethecalsthatledtothis，andLaplacethisup（inGermahejournalsthatvoed。

Holacewasbeihewavetheht。BothMichell'sandLaplace'sideasartonthecorpusculartheht。Iflightweretosistoftinypartiitseemedreasoheseparticleswouldbeaffectedbyagravitationalfieldandwouldbebouarofsuffitsize。Buttheearly19thturysawanumberofexperimentswhichseemedtogivegreatercetothewavetheht。Iflightwereinsteadawave，thenitwashardertoseethatitshouldbeaffectedbygravity。Laplace'sdarkstarprediwasquietlyomittedfromlatereditioiondusystèmedumoerall，MidLaplacehadbeeuringandexpltheory，ratherthanbeiheoexplaiionsandthus，thisideawasfottenforawhile。TheobjeedbyMidLaplacewerethusdarkstars'，enormousobjetheUniversewhichbyvirtueoftheirmasscouldsustaiemsbutbyvirtueofthissamebulkotbeobservedviatheradiationoflight。Starlightemittedfromthesurfaichell'sandLaplace'sdarkstarswouldbetoosluggishtooveretheintensesurfacegravity。WhatMidLaplaothaveguessedwasthatsutuanaasswouldbeuooreover，intheircollapsetheywouldpuheveryfabricofspadtimeaoasingularity。Thusblackholes'arenotdarkstars'aheargumentfihtheastronomicaldiscoveryofblackholeswewillfirstouhenatureofspacetime。

&ime

OureverydayexperienceleavesusfortablewiththenotioangibleUniversemaybedescribedbyoemporal（ortime）atetaialates（forexamplex，y，andzalouallyperpendicularaxes，astrutedbyReesandknownasates）。IeinpublishedhisrevolutionarypaperonSpecialRelativity，therelativityofmotionandstationarity。In1907，HermannMinkotheseresultscouldbeuoodmoredeeplybygafour-dimensioimewhosepoints，speowbythe4-Date（t，x，y，z），dto‘events'。Aissomethingthathappensataparticulartime（t）andataparticularplace（x，y，z）。Such4-DatesinwhatisknownasMinkoecifyexactlywhereandwheostein'sspecialtheoryofrelativitycouldbeformulatedintermsofMinkowski'sspadprovidesatdesofphysicalprodifferentframesofreferemoverelativetooher。Aframeofreference'issimplytheperspectivepossessedbyaparticularobserver。Einsteiheoryspecial'becauseitdealsonlyarticularamelyrefereareiialframesofreferehespecialtheorylybeappliedtouniformlymoving，ed，framesofreference。Ifyoudropastoround。TheframeofrefereachedtothestoneisaingframeofreferenedbyEinstein'sspecialtheory。Whereyouhavegravity，youhaveac。

ThisdratedEioformulateageheoryofrelativity，whichhepublishedadecadeafterhisspecialtheory。Whathefoundwasthatwhereasspakowskispacetimewererigidframeworksinwhichobjectslive，moveaheirbeing'，spacetimewasactuallyamorerespocouldbedotherwisedeformedbythepresenass。Oncemassispresentinaphysicalsituatiohefollowirikedbehaviourdescribesreality，lysummarizedbyJohnWheeler:

·massaspacetime，tellingithowtocurve！

·spacetimeaass，tellingithowtomove

ThisbehaviourisquantifiedbyEinstein'sfieldequationswithiivity，whichrelatethecurvatureofspacetimetothegravitationalfield。

Physiciststalkaboutagravitatioialwellassurroundingamassiveobject。TheshowninFigure2ehespacetimeisdistortediyofacoupleofblackholes，whereeaberegardedasawaywhichisdirectlyrelatedtoitsmassahegravitatioself。Thesingularityiimemayberegardedaswherethecurvatureiimebeesveryhighandyougobeyondtheclassicaltheravity，iumregime。Theevenththesingularityfunsasaone-

2。Thedistortioure，iimeduetothepresenasses。

membraidphotoheblaoutsidebutnothingeswithinthehorizonoftheblackholeouttotheexternalUmassisnottheoablaaypossessandbemeasuredby。Iftheblackh，thatistosayitpossessessomespin，theneveremebehavies。Beforeweexamiakealittledetourtolearnalittlemoreabouthowwemayschematicallyrepreseself。