霍金与黑洞学说【英文】Hawking

Outline:ThescientistInformationandblackholesHawkingradiationDetectionofblackholesBetsonblackholes3June2006HawkingandBlackHolesTheScientistBorn19421stclasshonoursfromOxford,after“notverymuchwork”SymptomsofALSduringOxfordyearsPhDandResearchFellowinCambridgeDiscoveredHawkingradiationin1974“ABriefHistoryofTime”publishedin1987NumeroushonorarydegreesandawardsOutspokenforworldpeace,welfareofthehandicapped,andothercurrentissuesAmyotrophicLateralSclerosis肌萎縮性脊髓側索硬化症AlsocalledLouGehrig’sdiseaseSymptoms:Difficultystanding,walking,orrunningClumsiness–FrequenttrippingorfallsDifficultywithfinehandmotionssuchasbuttoning,writing,turningakeyinalockAtrophyofhandmusclesAtrophyoftongueDifficultychewingfoodDifficultyswallowing(dysphagia)DifficultyspeakingMusclecramp4BlackHoleBlackholesrepresentthefinalvictoryofgravity.Ablackholeisblackbecausegravityissostrongthatlightcannotescape.TheescapevelocityatadistancerfromthecenterofanobjectwithmassMisr2GMvescapeTheescapevelocityincreaseswithmassanddecreaseswithradius.Ifvescapec,thenlightcannotescapeandwehaveablackhole.Lightrays5SpaceWarpsIfweimaginethespacetimeasa“rubbersheet”,thenanymasswouldproducewarpingsinit.Sinceblackholesproduceverystronggravity,spacetimeissignificantlywarped(curved)aroundthem.Weseestronglight-bendingandgravitationalredshift.6TheBlackHoleRadiusForanymass,thereisasmallestradiusbeyondwhichtheobjectbecomesablackhole.ThissmallestradiusrsiscalledtheSchwarzschildradius.Hawking:Thisdefinesthesizeofablackhole,anditdependsonthemassonly.AnythingsmallerthanitscorrespondingSchwarzschildradiusbecomesablackhole.2sc2GMr7DissectingaBlackHole•Anon-rotatingblackholeisparticularlysimple.•Thereisapointatthecentercalledthesingularity(奇點).Ithaszerosizeandinfinitedensity.Infact,itspropertiescannotbedescribedbycurrentlyknownphysics.•Theeventhorizon(穹界)isaspherecenteredatthesingularitywithradiusequaltotheSchwarzschildradiusoftheblackhole.•Whatisinsidetheeventhorizoncannotbeknownbyanyoneoutsidebecauseevenlightcannotescapeout.eventhorizonsingularity8EventHorizonandSingularity9No-hairTheoremHairheremeanssomethingcomplicated(e.g.differentstyles,colors,perms,etc…).Blackholeshavenohairbecausetheyaresimple.Onlythreethingscompletelycharacterizeablackhole(Hawking1972):–mass–angularmomentum–electriccharge10Undergeneralphysicalconditions,thesingularityisenclosedbytheeventhorizon.Informationwithintheeventhorizoncannotbetransmittedtotheexternalworld.Wesaythesingularityisconcealedordressed.Thosewhicharenotdressedarecallednakedsingularities.Mathematically,nakedsingularitiescanexist,butphysicalconsiderationssuggestcosmiccensorship:allsingularitiesareenclosed(RogerPenrose).Hawkingbetoncosmiccensorship(andconcededtooearlyin1997).CosmicCensorshipConjecture:NatureForbidsNakedSingularity11OnewaytrafficinNature?1.Thedisintegrationoftheeggwillneverhappeninthereversedirection(re-integration).2.Airmoleculesdiffusingoutofthebottlewillneverprogressinthereversedirection(infusion).TimeArrow12Thereisaveryimportantlawinphysics,whichgovernsthedirectionofanyprocessinaphysicalsystem.Thisiscalledthesecondlawofthermodynamics:Theentropyofanisolatedsystemneverdecrease.0SExamplethereverseisnotallowedbecomesSmallerentropyLargerentropySecondLawofThermodynamics13Ifwethrowcomplicatedobjects(withlowentropy)intoablackhole,wherehastheentropygone?Wherehastheinformationescapedfromtheblackhole?TheInformationParadox14FourLawsofBlackHoleThermodynamicsBardeen,CarterandHawking(1973)formulatedthefourlawsofblackholephysics,analogoustothefourlawsofthermodynamics.SecondLawThetotalsurfaceareaofblackholesisalwaysthesameorgreaterthanbefore.Whenwethrowmatterintoablackhole,orallowtwoblackholestomerge,thetotalareaoftheeventhorizonswillneverdecrease.15timespaceAreaTheoremThisimpliesthatthesurfaceareaofablackholeisameasureoftheentropy.Ifanobjecthasnonzeroentropy,thenithasatemperature,anditmustradiate!Atfirst,Hawkinghimselfcouldnotacceptthisimplication.1617GeneralRelativityandQuantumMechanicsGeneralrelativityandquantummechanicsaretwomajorachievementsof20thcenturyphysics.Generalrelativitydealswiththeverylarge.Quantummechanicsdealswiththeverysmall.Physicistsattemptedtounifythetwo.18HawkingRadiation(1974)WhenHawkingconsideredquantummechanics,manyofhisideasofblackholesneedtobechanged.Blackholesmayactuallyradiate!•Nearthehorizon,particle-anti-particlepairscanbecreatedsothatoneescapesandtheotherfallsin.•Radiationenergyfollowstheblackbodydistribution.19Inclassicalphysics,vacuummeansnothingexists.However,inquantummechanics,vacuumisactuallyaseaofvirtualparticles.Inquantummechanics,thereisaconceptcalledvacuumfluctuations.Althoughtheaverageenergyofspaceiszero,localfluctuationsofenergyareallowedbytheHeisenberguncertaintyprinciple.Energyfluctuationscreatepairsofparticlesandantiparticles(e.g.2photons).Apaircanexistmomentarilyandisthereforevirtual.Theyannihilatequickly.However,thevirtualparticlescanbecomereal,iftheintensecurvatureofthespacetimeoftheblackholeputsenergytothepair.VirtualParticles20Nearablackhole,th