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Chapter6Frravitationalphysics
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ThroughoutmostofthisbookwehavebeehegedgeravitationalphysicsreseargfromtestsonthesetrestothoseoheentireobservableUhischapterwewillsidersomeoftheissuesihetheoreticaldesofgravity.
&hasbeeheorythathasshapedofthegravitatioion.Thistheorytreatsspadtimeasasi，ahepropertiesofthatobjectbedetermihatexistswithinit.Ihopethatthereaderhas，bynow，beehephenomenalsuccessofEiheory.Itistrulyextraordinarythatasiheoryshouldbeabletoexplainsuchahysicalefestein’stheory，however，isuobethefinalwordiandingofgravity.Alothashappeheworldoftheoreticalphysice1915，andmuchofitsuggeststhatweshouldexpecttheretobeanevenmorefuheory.
Quantummedgravity
Notloeiheravity，atthebeginnihtury，theworldoftheoreticalphysicswasfed（on）bystistssuchasBohr，Heisenberg，Sger，andothers.Sion，andupuntilthispoint，physicshadbeenthoughttobedeterministic.Thatis，ifyouknowenoughinformationaboutthepositionandmotionsofallobjetheUhenyoushouldbeabletopredictthefuturewitharbitrarilyhighpre.Physicaltheoriesthatworkinthiswayareoasclassicaltheories.Einsteiytheoryisanexampleofaclassicaltheory.TherevolutionledbyBohr，Heisenberg，andSgerothertypeoftheory，quantummeiewquantumtheoryrobability，aediionofnatureinwhichitwasonlyeverpossibletocalculatetheoddsthateventseure，andwithinwhieverwhatthefutureholds.
Quantummeiastonishidescribedthe，andthebuildingblocksofallkraordinarypre.Later，PaulDirac，aprofessorattheUye，shothesenewideascouldbeusedtocreatequantumtheoriesofelectridmaghisledtowhatisaiclephysitummeicaldesofalloftheknowure，andtheforcesbetweentheWindingforwardtothepresentdaythereoseriousdoubtthatummeical.ThepredisoftheStandardModelhaveallbeehegjewelbeialverifi2012oftheexisteheHiggsboson（atheorizedparticlethatisanimportahemodernStandardModel，andwhichgivestheotherparticlestheirmass）.Itisquahatunderpinsmuistryandmaterialsgusthesemi-ductorsthatourputersaremadefrom;andthelasersaareusedtostructourDVDplayersandtelevisions.Quantummeicsisafactoflife.Itishownatureworks，anditbeesinglyimportaodescribethemicroscopicworld.
&eitsubiquityinallotherareasofphysics，theappliofthisapproachtogravityisstillood.WhiletheeleagicforbequantizediivelystraightforhilethematterintheUniversehaslongbeendescribedusingquantummeics，thequantumtheravityremaihisisprobablythesiunsolvedprobleminphysidhasbeefiftyyears.Itisextraordinarilydiiculttoapplythelogicthathasbeensosucotherareasofphysicstogravity，andsoourtbestdesofgravityisstillEinstein’sclassicaltheory.
Partoftheproblemwiththisstateofafairsisthattherearesituationsinwhiewouldexpectbothquantummedgravitytoberequired.Ohisisthetreofablackhole.Aswe’vediscussedalready，blawhearsuastrophiccollapse.Thematterthatwasinallypartofthestargetspressedbygravitytherdensities.Infact，agtoEiheory，thecollapseshouldueuntilallthematterissquashedtoasi.NtoquantumtheoryweshouldexpeewquaobeeapparentwhenwesiderverysmalldistancesdveryhigheI’vedescribedistherefimewherebothquantummedgravityareexpectedtoberequirediadesofthephysicsatplay.Butthereisheoryofquantumgravity，soitis（atpresent）impossibletoknowwhatshouldhappereofastarafterithascollapsed.Thisisobviouslyanunsatisfactorystateofafairs.Ifwewaodescribeeverythisiheumtheravity.
Thereasoinpatibilityofquantumtheravityaremany，aplicated.Firstthereisaceptualdifereein’sapproachtogravity，andhowforcesaretreatedinquantummeistein’stheravityisaresultofthecurvatureofspace-time.Thereiserpullsthiheapparentwayinwhichmassivebodiesmovetowardseachotherissimplyaresultofthecurvatureofspace-time.TheEarth，forexample，isowardstheSun，itissimplyinfreefall，followipathavailabletoitiime.Thisisnotthecaseforotherforces.Theelectricforple，isaresultoftheelectricfieldthatgetsgeedbychargedparticles.Theelectricfieldexistswithiime，butisnotthesameasspae，inanyseimearesimplythearenawithinwhichtheelectricforceplaysout.Thisideaofspadtimehavingtheirowexistendbeisoftheproblem，isbuiltintomostapproameics.TotrytousetheseapproachestravitytherefainsteretaughtbyEinstein.
Therearealsoodmathematiswhygravityandquantummeipatible.Theforemostoftheseisaprravityalizability.Whenquantummeicsisusedtodescribeaforce，theresultofthecalsthatweperformresultifinities.Ahisowesidertarticles.Toworkouttheforthem，quantummeically，wehavetoaddthetributionsfromallofthepossiblepositioieofthesefiguratiohepartievery，verywhichcasetheforesvery，verylarge.Summingoverallpossiblepositionsthengivesaresultthatisihisunreasonableanswerbecorrectedfor，iheelectricforce，byusingaprocessormalization.Thisprocessremovesthepartsoftheequationsthattributedtheiis，theinfiiallysubtratheiiohenasensibleanswer，whibetestedwithexperiments.Renormalization，however，doesn’tworkwithEinstein’sgravity.Theibesubtracted，asthereisnothingintheiionsthatlooksanythiermsthatbeeiumgravitycalsthereforeseemtogiveinfihingisobvi.
Therehavebeenmaofixtheseproblems.TheyrangefromgEiions（sothattheyappeartobemorerenormalizable）;togquantummeics（sothatitisnoloicles）;throughtogthewethinkofasspadtime（sothattheyarenotuous）.Itisnotpossibletogiveafairrepresentationofalloftheseapproacheshere，ointoahemiheyareveryplicatedideasandtheyareallwress.ThereareacoupleoftheoriesthatIfeelIshouldmentionthough’theseareStringTheoryandLoopQuantumGravity.Thesearebothextremelyboldandambitiousattemptstostrutumtheravity.Ifcorrect，thehopeisthatphysicistscouldusethesetheoriestodescribeereofblackholes.Theyare，however，quitedifereher.Theyprioritizedifereheproblem，andapproachthetedceptualdicultiesjustdesquitediferent>
StringTheorywasbornoutofparticlephysics.Itisbasedohebasistituentsofmatterarenotpoiicles，butareiinyone-dimensis.Thisisaradicalidea，andithasledtoalotmathsandphysideed，manyphysisiderittobeourbesthopeoffindingatheoryofquantumgravity.Thehypothesizedstringsareverysmall，sothatforthemostparttheyappeartousefectivelyaspoiiwetryandquahem，however，theirstriodifereheequationsthatgoverringsalsoaspectsthatlookalotliketheequationsthatgoverheravity.Ittherefravityis，tosomeextent，builtintTheory.Therearedrawbacks，however，asthecyoftheequationsthatgoverringsrequireustoaddbetweensixawoextradimensionsofspacetoourdesoftheUniverse.
&radimetobeact，sothatwedoheminoureverydaylives.heyhavetobethereforthetheorytobeself-t.Iiehesesmallextradimehepossibilitythatgravitycouldworkdiferentlyonverysmallscales.
LoopQuantumGravityisoftehemaiTheory.Thestartingpointforthistheoryistheideathatonverysmallscalesspace-timehasagranularstructure.Thatis，spadtimearehuousvariablesthatweusuallysiderthemtobe.Instead，space-timeisatomized.Quaheheloopsthatmakeupthisure.Thisisalsoaradicalidea，aobefavouredbyafiadosofGeybephasisoimeasthefualobjeterest’insteadofasabad.LoopQuantumGravityis，however，verymuchawress.ItremainstobeseeheapproachitemploysistheeTheory，orsomeotherasyetuheory，isapreferabledesofherworkisofuswouldbepreparedtomakeaheouteofthisdebate.
Partigravitationalfields
Quantizinggravityisfraughtwithdiculties.Solet’sturntoaioummeiagravitationalfield？HereweathesamewaythatEinsteindidinhisclassicaltheravity.Oncewehaveaclassicalspace-timeatourdisposal，hoillsiderereatthemattertasbeinggovernedbyquahismixedapproach，withmatterbeiummeidgravitybeiedclassically，isusuallyreferredtoassemi-classicalphysics.Itisalessambitiousprojefullquantumgravitybuthelessgivesusiinginsightsintohowquantumsystemsworkinthepresenceofgravity.
&hepiohissubjecthenHawking，whoin1974showedthatquantummeicsshouldleadtoblaittingradiation.Thisdiscoveryshockedthestifiunity，asagtoEiheeveresablackhole.Hawking’scalwasasemi-classie.Hetooktheclassicaldesforthespace-timearoundabladallowedquantummeicalparticlestoexistwithinit.Heshowed，usingafairlysimplequantummeicalcal，thatiftherewasnoradiatiopast，theexistradiatioure.Theonlypossibleexplanationforthiswasthattheradiationroducedbytheblackhole.Ofcourse，radiatiy，andinthissituationtheonlysourergyisthemasswithintheblaberthatmassisatypeyiheory）.SoHawkinghadshownthatblaaturallyshrink，byradiatingawaytheirmass，andthattheymusteveobe.
Hawkiwasnovel，andsparkedseveralnewfieldsravitationalphysiediatelyafterHawking’sdiscoveryitwasshown，byBillUnruh，thattheveryexisteibequestionedwhenwesiderthemwithiytheory.Particlephysicsisverymuainofquantummeiruhshowedthatifobserversareiion，withoiotheother，theirelypossibleforoecttheexisteumparticleswhiletheotherdetegatall.Thatis，whetherornotparticlesexistdepeionofthepersonwhtomeasurethe
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