e-book - stephen hawking - the nature of space and time

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1、1.ClassicalTheoryS.W.HawkingIntheselecturesRogerPenroseandIwillputforwardourrelatedbutratherdierentviewpointsonthenatureofspaceandtime.Weshallspeakalternatelyandshallgivethreelectureseach,followedbyadiscussiononourdierentapproaches.Ishouldemphasizethatt

2、hesewillbetechnicallectures.Weshallassumeabasicknowledgeofgeneralrelativityandquantumtheory.ThereisashortarticlebyRichardFeynmandescribinghisexperiencesataconferenceongeneralrelativity.IthinkitwastheWarsawconferencein1962.Itcommentedveryunfavorablyonthege

3、neralcompetenceofthepeoplethereandtherelevanceofwhattheyweredoing.Thatgeneralrelativitysoonacquiredamuchbetterreputation,andmoreinterest,isinaconsiderablemeasurebecauseofRoger'swork.Uptothen,generalrelativityhadbeenformulatedasamessysetofpartialdierentia

4、lequationsinasinglecoordinatesystem.Peopleweresopleasedwhentheyfoundasolutionthattheydidn'tcarethatitprobablyhadnophysicalsignicance.However,Rogerbroughtinmodernconceptslikespinorsandglobalmethods.Hewasthersttoshowthatonecoulddiscovergeneralpropertieswi

5、thoutsolvingtheequationsexactly.Itwashisrstsingularitytheoremthatintroducedmetothestudyofcausalstructureandinspiredmyclassicalworkonsingularitiesandblackholes.IthinkRogerandIprettymuchagreeontheclassicalwork.However,wedierinourapproachtoquantumgravityan

6、dindeedtoquantumtheoryitself.AlthoughI'mregardedasadangerousradicalbyparticlephysicistsforproposingthattheremaybelosshep-th/940919530Sep94ofquantumcoherenceI'mdenitelyaconservativecomparedtoRoger.Itakethepositivistviewpointthataphysicaltheoryisjustamathe

7、maticalmodelandthatitismeaninglesstoaskwhetheritcorrespondstoreality.Allthatonecanaskisthatitspredictionsshouldbeinagreementwithobservation.IthinkRogerisaPlatonistatheartbuthemustanswerforhimself.Althoughtherehavebeensuggestionsthatspacetimemayhaveadiscre

8、testructureIseenoreasontoabandonthecontinuumtheoriesthathavebeensosuccessful.Generalrelativityisabeautifultheorythatagreeswitheveryobservationthathasbeenmade.ItmayrequiremodicationsonthePlanckscalebutIdon'tthinktha