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1、ONTHEELECTRODYNAMICSOFMOVINGBODIESByA.EINSTEINJune30,1905ItisknownthatMaxwell’selectrodynamics—asusuallyunderstoodatthepresenttime—whenappliedtomovingbodies,leadstoasymmetrieswhichdonotappeartobeinherentinthephenomena.Take,forexample,therecipro-calelectrodynamicactionofamagnetan
2、daconductor.Theobservablephe-nomenonheredependsonlyontherelativemotionoftheconductorandthemagnet,whereasthecustomaryviewdrawsasharpdistinctionbetweenthetwocasesinwhicheithertheoneortheotherofthesebodiesisinmotion.Forifthemagnetisinmotionandtheconductoratrest,therearisesintheneig
3、hbour-hoodofthemagnetanelectricfieldwithacertaindefiniteenergy,producingacurrentattheplaceswherepartsoftheconductoraresituated.Butifthemagnetisstationaryandtheconductorinmotion,noelectricfieldarisesintheneighbourhoodofthemagnet.Intheconductor,however,wefindanelectro-motiveforce,towh
4、ichinitselfthereisnocorrespondingenergy,butwhichgivesrise—assumingequalityofrelativemotioninthetwocasesdiscussed—toelec-triccurrentsofthesamepathandintensityasthoseproducedbytheelectricforcesintheformercase.Examplesofthissort,togetherwiththeunsuccessfulattemptstodiscoveranymotio
5、noftheearthrelativelytothe“lightmedium,”suggestthatthephenomenaofelectrodynamicsaswellasofmechanicspossessnopropertiescorrespondingtotheideaofabsoluterest.Theysuggestratherthat,ashasalreadybeenshowntothefirstorderofsmallquantities,thesamelawsofelectrodynamicsandopticswillbevalidf
6、orallframesofreferenceforwhichtheequationsofmechanicsholdgood.1Wewillraisethisconjecture(thepurportofwhichwillhereafterbecalledthe“PrincipleofRelativity”)tothestatusofapostulate,andalsointroduceanotherpostulate,whichisonlyapparentlyirreconcilablewiththeformer,namely,thatlightisa
7、lwayspropagatedinemptyspacewithadefinitevelocitycwhichisindependentofthestateofmotionoftheemittingbody.ThesetwopostulatessufficefortheattainmentofasimpleandconsistenttheoryoftheelectrodynamicsofmovingbodiesbasedonMaxwell’stheoryforstationarybodies.Theintroductionofa“luminiferouseth
8、er”willprovetobesuperfluousinasmuchastheviewhere
