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1、Chapter1ABriefIntroductionHerewemeetstringtheoryforthefirsttime.Weseehowitfitsintothehistoricaldevelopmentofphysics,andhowitaimstoprovideaunifiedde-scriptionofallfundamentalinteractions.1.1TheroadtounificationOverthecourseoftime,thedevelopmentofphysicshasbeenmarkedbyu
2、nifications:eventswhendifferentphenomenawererecognizedtoberelatedandtheorieswereadjustedtoreflectsuchrecognition.Oneofthemostsignificantoftheseunificationsoccurredinthe19thcentury.AftermanydiscoveriesintheareasofelectricityandmagnetismbyCoulomb,Amp`ere,andotherscientis
3、ts,theexperimentsandstudiesofMichaelFaradayshowedthatchangingmagneticfieldsgeneratedelectricfields.TheseresultsledJamesClerkMaxwelltoasetofequations,nowcalled“Maxwell’sequations”,thatunifyelectricityandmagnetismintoaconsistentwhole.Thiselegantandaestheticallypleasin
4、gunificationwasnotoptional.Separatetheoriesofelectricityandmagnetismwouldbeinconsistent.Anotherfundamentalunificationoftwotypesofphenomenaoccurredinthe1960’s.ToappreciatethescopeofthisunificationitisusefultorecallwhathadbeenunderstoodsincethetimeofMaxwell.AlbertEinst
5、ein’sSpecialTheoryofRelativityhadachievedastrikingconceptualunificationoftheseparatenotionsofspaceandtime.Differentfromaunificationofforces,themergingofspaceandtimeintoaspacetimecontinuumrepresentedanewrecognitionofthenatureofthearenawhere910CHAPTER1.INTRODUCTIONphys
6、icalphenomenatakeplace.Inaddition,QuantumTheory,asdevelopedbyErwinSchr¨odinger,WernerHeisenberg,PaulDiracandothers,hadbeendramaticallyverifiedtobethecorrectframeworktodescribemicroscopicphenomena.Finally,fourfundamentalforceshadbeenrecognizedtoexistinnature.Oneofth
7、emwastheforceofgravity,knownsinceantiquity,andfirstdescribedaccuratelybyIsaacNewton.Gravityunderwentaprofoundrefor-mulationinAlbertEinstein’stheoryofGeneralRelativity.Inthistheory,thespacetimearenaofSpecialRelativityacquiresalifeofitsown,andgrav-itationalforcesaris
8、efromthecurvatureofthisdynamicalspacetime.Thesecondfundamentalforcewastheelectromagneticforce,welldescribedbyMaxwell’sequations.Thethirdfundamentalforce
