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1、UTTG-05-97WhatisQuantumFieldTheory,andWhatDidWeThinkItIs?∗StevenWeinberg∗∗PhysicsDepartment,UniversityofTexasatAustinAustin,TX78712Quantumfieldtheorywasoriginallythoughttobesimplythequantumtheoryoffields.Thatis,whenquantummechanicswasdevelopedphysicistsalreadyknewaboutvariousclassicalfields,notablythee
2、lectromagneticfield,sowhatelsewouldtheydobutquantizetheelectromagneticfieldinthesamewaythattheyquantizedthetheoryofsingleparticles?In1926,inoneoftheveryfirstpapersonquantummechanics,1Born,HeisenbergandJordanpresentedthequantumtheoryoftheelectromagneticfield.Forsimplicitytheyleftoutthepolarizationoftheph
3、oton,andtookspacetimetohaveonespaceandonetimedimension,butthatdidn’taffectthemainresults.(Responsetocommentfromaudience:Yes,theywerereallydoingstringtheory,sointhissensestringtheoryisearlierthanquantumfieldtheory.)Bornetal.gaveaformulafortheelectromagneticfieldasaFouriertransformandusedthecanonicalcomm
4、utationrelationstoidentifythecoefficientsinthisFouriertransformasoperatorsthatdestroyandcreatephotons,sothatwhenquantizedthisfieldtheorybecameatheoryofphotons.Photons,ofcourse,hadbeenaround(thoughnotunderthatname)sinceEinstein’sworkarXiv:hep-th/9702027v14Feb1997onthephotoelectriceffecttwodecadesearlier,
5、butthispapershowedthatphotonsareaninevitableconsequenceofquantummechanicsasappliedtoelectromagnetism.Thequantumtheoryofparticleslikeelectronswasbeingdevelopedatthesametime,andmaderelativisticbyDirac2in1928–1930.Forquitealongtimemanyphysiciststhoughtthattheworldconsistedofbothfields∗Talkpresentedatthe
6、conference“HistoricalandPhilosophicalReflectionsontheFoundationsofQuantumFieldTheory,”atBostonUniversity,March1996.Itwillbepublishedintheproceedingsofthisconference.∗∗ResearchsupportedinpartbytheRobertA.WelchFoundationandNSFGrantPHY9511632.E-mailaddress:weinberg@physics.utexas.edu1andparticles:theele
7、ctronisaparticle,describedbyarelativisticallyin-variantversionoftheSchr¨odingerwaveequation,andtheelectromagneticfieldisafield,eventhoughitalsobehaveslikeparticles.DiracIthinkneverreallychangedhismindab
