The Representation of Nature in Physics: A Reflection On Adolf Grünbaum's Early Writings Bas C.vanFraassen [presentedatasymposiuminhonor of Adolf Grünbaum, SantaBarbara2002; forthcominginJokic,A.(ed.)PhilosophyofPhysicsandPsychology:EssaysinHonor ofAdolfGrünbaum.Amherst,NY:Prometheus Books] [THISPAGETOBEDISCARDEDBEFORE PUBLICATION] 1.CompletenessCriteriaFor Science.............................................................................2 Determinismwithoutcausality? GrünbaumonBohm's mechanics...........................4 2.Appearancevs.Realityas aScientific Problem.........................................................5 ADistinction: PhenomenaandAppearances..............................................................6 TheAppearance-from-RealityCriterion.....................................................................7 ApparentrejectionoftheCriterion.............................................................................9 3.Grünbaum'sCritiqueofNon-RealistInterpretationsofQuantum Mechanics..........10 Whatisatheoreticallysignificantquantity?.............................................................10 Technicalsense(s)ofcomplementarity....................................................................11 Whatdoes ameasurementreveal?............................................................................13 Relationalproperties and invariants..........................................................................14 Relationalproperties: perspectivalorinteractional?.................................................15 4.Perspectives and measurementoutcomes.................................................................16 Differences betweenperspectiveandframeofreference.........................................19 Phenomenaandappearances:thedistinctioncontinued...........................................21 EinsteinandMinkowski...........................................................................................22 5.Does complementarityreallyhaveto dowithframesofreference?........................23 6.IstheAppearancefromRealityCriterionAbandonedinQuantumMechanics?.....24 Istherea'collapse'?...................................................................................................25 Mismatchesinspace.................................................................................................28 7.Theappearances yokeduntoaforbearingreality.....................................................30 8.Thestructureofappearance......................................................................................33 Appearancessystematicallyunlikethepostulatedreality........................................34 Relativestates andtheinvariants..............................................................................36 Appearance'kinematics'............................................................................................37 TheAppearancesdonot supervene..........................................................................39 TheFinalChallenge..................................................................................................40 GrünbaumSantaBarbara 0 The Representation of Nature in Physics: A Reflection On Adolf Grünbaum's Early Writings On The Quantum Theory Bas C.vanFraassen BeforeIturnto mymain topic,which Ishallexplorehereinthelightof Adolf Grünbaum'searlyworkinthephilosophyofquantummechanics, Iwanttosaysomething toexpress myowndebttoProfessorGrünbaum,bothforhis guidanceand forhis work. Oursymposiumincluded theopportunitytohearhis autobiographicalremarks, bothtouchinganddisturbingtothoseofus whohavesharedsomeofthelastcentury's painfulhistory.Theremarkableoutcome, atruesignofhopeinmyview,isthatAdolf Grünbaumhimselfcould haveemerged fromsuch achildhoodand youth,allbutlostto theoppressionandalienationofthatera,astheexampleofsocialconscienceand personalcharityforwhichIthankandadmirehim. For Ilearned as muchfromhis personalengagement—withusstudentsonapersonallevel,withsocialand moralissues, andwithphilosophyitself,whereheneveraimedtoturnastudentintoadisciple—as fromhisscholarlyachievements whichprovidedtheinitialbasis formyownreflections onthecharacterofphysicaltheory. AftergraduationmyfirstpositionwasatYaleUniversity,whereIwasfortunate toknowandlearn fromoneofGrünbaum'sownteachers, HenryMargenau. Itwaswith somecuriosity,ofcourse,thatatthispointIreadtwoofGrünbaum's earliest publications,bothcriticalresponsesto Margenau's philosophyofquantummechanics. Thefirst,"Realismand Neo-KantianisminProfessorMargenau'sPhilosophyofQuantum Mechanics,"includes astrongdefenseofascientificrealistpositionwithrespectto quantummechanics,butmainlyasagainstneo-Kantianand Idealistthemes thencurrent inattempts tointerpretthetheory.1Iwillconcentratehereonthesecond, "ComplementarityinQuantumPhysicsandits PhilosophicalGeneralization,"together withsomeofGrünbaum'searlyarticlesontheSpecial TheoryofRelativity(henceforth 1A.Grünbaum,"RealismandNeo-KantianisminProfessorMargenau'sPhilosophyofQuantum Mechanics,"PhilosophyofScience17(1950):26–34. GrünbaumSantaBarbara 1 "STR").2 Although Margenau'sinterpretationofquantummechanics andtheCopenhagen interpretationwerenotatallthesame,theyshared ananti-realistorientation.Theideaof complementaritywas generalizedquite farbeyondtheuseithadseeninilluminatingthe appearanceofincompatibleobservables,butthose generalizationswerethendrawnonto suggestsupportforthatorientationinwaysthatcriedoutforthephilosophicalcritique thatGrünbaumsupplied. Butthepaperalsoengages activelywiththequestionofhowwe shouldorcanunderstand theworlddescribedbyquantumtheory,acknowledgingthat theory's radicaldeparturesfromclassicalphysics.Thiswillalsobemyfocus here. While Iwillnotdisagreenearlyasmuch withGrünbaum'sviews asGrünbaumdisagreedwith Margenau's,Iwilldefend oneaspectoftheCopenhageninterpretationofquantumtheory thatIseeas radicalizingourunderstandingofphysicaltheory. 1. Completeness Criteria For Science Beforeweturnspecificallytoquantummechanics andtheCopenhageninterpretation I needtodraw onanother philosopherwhosewritingswere aparadigmanda guidefor AdolfGrünbaumas well asformyself:HansReichenbach.OnelessonthatItookaway fromthosewritings was thatscienceistobeunderstoodas anenterprisewitha distinctivecognitiveaimandwithloyaltytoadistinctiveempirical methodology. Reichenbachhimselfwas seriouslyconcernedwiththerequirementsuchan understandingplacesonus todisplaythecriteriaofsuccess thatanenterprisethus understoodmustaimtosatisfy—ifonlyas anideal,eveniftheperfectsuccess,that wouldconsist incompletelymeetingthosecriteria,isbeyondreachofsuch finitebeings 2A.Grünbaum,"ComplementarityinQuantumPhysicsanditsPhilosophicalGeneralization," JournalofPhilosophy54(1957):713–27;A.Grünbaum,"TheClockParadoxintheSpecial TheoryofRelativity,"PhilosophyofScience21(1954):249–53;A.Grünbaum,"ReplytoDr. Tornebohm'sCommentsonMyArticle,"PhilosophyofScience22(1955):233;A.Grünbaum, "ReplytoDr.Leaf,"PhilosophyofScience22(1955):53;A.Grünbaum,"Logicaland PhilosophicalFoundationsoftheSpecialTheoryofRelativity,"inA.DantoandS. Morgenbesser,eds.,PhilosophyofScience,399–434(NewYork:MeridianBooks,1960);andA. Grünbaum,"TheRelevanceofPhilosophytotheHistoryoftheSpecialTheoryofRelativity," JournalofPhilosophy59(1962):561–74. GrünbaumSantaBarbara 2 withfiniteresources as ourselves.Hisearlywritings onquantummechanicsincludeda strongdefenseofrelinquishingcertainearliercriteria,whichhadbeenheldsacrosanctin modernscience,butwhichrested,asheargued,on empiricallyvulnerable presuppositions. AsIlearnedfromReichenbachandGrünbaum,thesecond greatscientific revolutioninmoderntimestookplacearoundthepreviousturnofthecentury, withthe comingfirstof allofrelativity,andsecond,ofthequantum. Inheritedfrommodern scienceweretheclaimthatallphenomenainnaturederivefromanunderlying deterministicmechanics, andthephilosophicalconvictionthatascientificaccountis completeonlyifitisdeterministic. Supportingthatconvictionwasthephilosophical creed, currentamongneo-Kantians,thattheveryintelligibilityofnatureandthevery coherenceofexperiencerequiretheirpossibilityofbeingconceivableassetinarigidly deterministiccausalorder. Thatispreciselythecompletenesscriterion challenged most salientlyinReichenbach’searlywritings.Theprobabilisticresources ofclassical statisticalmechanicswerenewlyadaptedinsuch awaythat,asit seemedthen,no groundinginanunderlyingdeterministicmechanics waspossible. Reichenbachsided withavocalpartofthephysicscommunitythat explicitlyrejectedthetaskoffindingor postulatinghiddenmechanisms behindsuch apparentlystochasticprocessesas radioactivedecay. Natureis indeterministic,oratleastitcanbeor maybe—andifthatis so,determinismisamistakencompletenesscriterionfortheory. NowReichenbach,who didmuchtoprovide arationaleforthis rejectionof determinism,introduced anapparentlyweakerbut stillsubstantivenewcompleteness criterion:thecommoncauseprinciple.3Thisprincipleissatisfiedbythe causalmodelsof generaluseinsocialsciencesandfor manypurposesinthenaturalsciencesaswell. They aremodels inwhich allpervasive correlationsderivefromcommoncauses (ina technical,probabilisticallydefinablesense). Butthedemonstrationinthe60sandlater thatquantummechanics violatesBell’sinequalitiesshowsthateventhisthirdcriterion 3Seeforcomparisonmy"RationalBeliefandtheCommonCausePrinciple," inR.McLaughlin, ed.,What?Where?When?Why?EssaysinhonorofWesleySalmon,193–209(Dordrecht:Reidel, 1982),andreferencestherein.Undercertainconditionsthiscriterionactuallydemands determinism,asIshowthere. GrünbaumSantaBarbara 3 wasrejected,ineffect,bythenewphysics.4 Determinism without causality? Grünbaum on Bohm's mechanics AsGrünbaumdiscusses (1957,noteonpp.715–16)boththedenialofdeterminismand theanalysisofcausalityconnected withtheCopenhageninterpretationofquantum mechanicswaschallengedbyBohmianmechanics—proposedalternativelyasan interpretationofquantummechanicsand asarivaltheory.Bohmdescribes aworldof particleswhichhavepositionassolephysicallysignificantattribute,andwhosemotionis strictlydeterministic. Butthatmotionderivesfromthetotalquantumstate, whichimplies thattherearenonlocal correlationsamongtheseparticle motionsthatcannotbetraced backtoany"synchronizing"processintheircommonpast. Sothereareclearviolations theretooofthe common causeprinciple,theBell inequalities areofcourseviolated.This exampleshowsboththat adeterministicinterpretationispossibleandthat satisfactionof thecommon cause criterionisnotlogicallyimpliedbydeterminism. Bohm’s mechanics surprisinglypresentsuswithapictureofdeterminismwithoutcausalitysotospeak. CitingHeisenbergandReichenbach,GrünbaumemphasizesthatonBohm's reformulationofquantummechanics"thebehaviorexhibitedbythewaves [thegoverning wavefunctionforthemultiparticlesystem]wouldbe,classicallyspeaking,everybitas strangeasthatofa"particle" whosemotiondependsonthepresenceofaslitthrough whichitcouldnothavepassed."SinceBohm's mechanics hasbeen givensophisticated newformulations inrecentdecades,weknownowthatitcanaccommodatethe phenomena.But,asGrünbaumineffectpointedoutatthetime,Bohm's purportto give usaworldpictureclosetotheclassicaloneis certainlynotborneout.5 Inanycase,both thelessonthatasciencecanbeindeterministic,andthatthecommon causecriterionmay 4Seemy"TheCharybdisofRealism:EpistemologicalImplicationsofBell'sInequality," Synthese5(1982):25–38,reprintedinJ.CushingandE.McMullen,eds.,ThePhilosophical ConsequencesofQuantumMechanics(NotreDame,IN:UniversityofNotreDamePress,1989). 5SeeK.Bedard,"MaterialObjectsinBohm'sInterpretation,"PhilosophyofScience66(1999): 221–42;B.vanFraassen,"InterpretationofQM:ParallelsandChoices,"inL.Accardi,ed.,The InterpretationofQuantumTheory:WhereDoWeStand?,7-14(Rome:IstitutodellaEnciclopedia Italiana; NewYork:FordhamUniversityPress,1994). GrünbaumSantaBarbara 4 beviolatedbyphysicaltheory,sothatthesecannotbedefiningcriteriaofsuccessin science, remain. Howeverthatmaybe, I’llnowturntoafurthercompletenesscriterionthat seems compatiblewiththeserejections,andappearstobequitegenerallyacceptedatleast amongphilosophers and bythegeneralpublic. 2. Appearance versus Reality as a Scientific Problem. Thecompleteness criteriaofdeterminismandcausalityinvolvedemands forexplanation. Thesortofexplanationdemandedtherefromscienceisnotjustasupplyofmissing informationneeded forasimple,systematicaccountofthephenomena,butrequires connections deeperthan bruteorfactualregularity. Thusas anaspirantempiricistItend toseethosedemandsas placingaburdenof unwantedmetaphysics onthesciences.But now Iwanttosuggestthattherehasinfactbeenastilldeeper-goingdemand(criterionof success)uponmodernscience,which alsocametobechallengedpreciselyinthe developmentofthenew quantumtheory. Thatis thedemandthat,howeverdifferentthe appearances(tous) maybefromtherealitydepictedintheory,theorymust derivethe appearancesfromthatreality. Thesenseof"derive"is strong,andoncemoreconnected withsubstantivenotionsof explanation.Thisdemandcan alsobereadas again acriterion ofcompleteness:scienceisassertedtobeincompleteuntilandunlessitmeetsthat demand.Ineedtoexplainthisfurther,butletusatoncegiveitaname:the Appearance fromRealityCriterion.6 Beforeaskingfortheprecisesenseof"derive" involvedhere,letustakesome familiarexamplesinwhichsciencedoesofferussuchaderivation. Wecreditscience withadequateandsatisfactoryexplanationsofhowmanyfamiliarphenomenaare produced:howashis producedwhen weburn acigaretteorsomelogs,howmethaneis naturallyproducedinaswamp, andhowaflameisturnedyellowwhen asodiumsample is inserted.Copernicusalsoexplainedtheplanets'retrogrademotion,andthetheoryof soundaswavesinair explainstheDopplershift. In allthesecasesthefamiliarwitnessed 6Seemy"ScienceasRepresentationalandNon-representational,"forthcominginPhilosophyof Science2004(Supplement)forarelatedexplorationofthistheme.[AU:thiswillhavetobe updated] GrünbaumSantaBarbara 5 events arenotjustlocatedinthetheoreticalworldpicture,butshowntoderivefromthe theoreticallydescribedreality,althoughsuchtermsas"ash","cigarette,""swamp," "yellow,"and"retrograde"haveatbestaderivativestatus thereand atworstneedtobe treatedrathercavalierlyinthetheoreticalcontext. A distinction: Phenomena and appearances Letusintroduceadistinctionhere,thatmayatfirstblushseemtobea distinctionwithout adifferenceforsuchexamples. BythephenomenaImeantheobservablepartsofthe world,whetherobjects,events,orprocesses. Thesethesciences mustsave(intheancient phrase)—buttheseadmitofobjectiveandindeed purelytheoreticaldescription,which doesnotlinktheirrealitytocontextsofobservationortoactsofmeasurement. Bythe appearances Imeanthecontentsofobservationandofmeasurementoutcomes.The phrase"tosavethephenomena"is oftenrendered morecolloquiallyas"tosavethe appearances"and"appearances"is generallytakenas synonymouswith"phenomena," butofcoursebothareterms withapast. Onphilosophicallips theyareoftenloadedwith connotations thatlinkthemtomind andthought. So Iamintroducinganewtechnical distinctionandusehere,speciallyadaptingtheseterms toourpresentdiscussion, eschewingsuch connotations. Inthis newsensetheplanetarymotions—whateverthey areorarelike—arephenomena,butplanetary"retrograde" motionsare,paceCopernicus, (mere)appearances. GalileofamouslypromisedinTheAssayerthatthecolors,smells,andsounds in theexperiencedworldwouldbefullyexplainedbyaphysics amongwhosedescriptive parameters thosequalities werenotallowed. Descartes'posthumousworkTheWorld,or TreatiseonLight purportedtolaythefoundation ofaworldpictureentirelytransparentto thehumanunderstanding—althoughthetheorythatwas toprovideuswith thatworld picturewasbutabarelyenriched kinematics.Thecolorful,tasty,smelly, andnoisy appearances willbeshowntobeproducedas (yes!) interactionalevents,inwhichthe relataareinprinciplecompletelycharacterizedintermsofprimaryqualities, respectively,quantities ofspatialandtemporal extensionalone. Thesepromiseswerenot empty: thereweresolidachievementsbehindthem.Thoseachievementsaccumulated GrünbaumSantaBarbara 6 intoawesome riches bythelatenineteenth century. Describingthesuccess thatGalileo hadpromised,andusingournewterminologicaldistinction,wecansay: combustionof sodiumsamplesisanobservableprocess (phenomenon)thatcanbeexhaustively describedinphysicalterms,butthisdescriptioncanalsobeutilizedtoexplainhowthat processproducesa yellowappearancetothehumaneye, andofcourse,to acamera. The Appearance from Reality Criterion Givenallthatsuccess,itisnotsurprisingiftheappearancefromrealitycriterionshould begintopervadetheideals setforscience.Theorymust derivetheappearancesfromthat reality.By“derive”Ido notmean abarelogicaldeduction. Imean aconnectionofthe orderof explanationthroughnecessityand/orcausalmechanisms tobedisplayed. The derivationisrequiredtoshowandmakeintelligiblethestructureoftheappearances as beingproducedbytherealitybehindthemtotheir (possible)observers. Thedemandfor suchaderivationis notmetifscienceshouldsimplyissuesuccessfulpredictionsof measurementoutcomes,bymeansofsystematicrulesofcalculation,fromthestateof naturetheoreticallydescribed. Thatsortofsuccess doesnotipsofactoamounttoan explanationofwhyandhowtheappearances mustbethewaytheyare. Thestronger demandthatweshouldbeabletoseescienceasprovidingthatsortof derivation/explanationisacontinuingthemeinmuchscientificrealistwritingonthe sciences. I'llquotefromJarrett Leplin'sANovelDefenseofScientificRealism: Atheoryis notsimplyan empiricallaworgeneralizationtotheeffectthat certainobservablephenomenaoccur,butanexplanationoftheir occurrencethatprovides somemechanismtoproducethem,orsome deeperprinciplestowhichtheirproductionisreducible.7 Ontheotherhand, Iamnotcontrastingsuccessfulexplanationwithindeterministicor stochasticaccounts.8Anadequateexplanationofhowaneventisproduced neednotbe 7J.Leplin,ANovelDefenseofScientificRealism(NewYork:OxfordUniversityPress,1997),p. 15. 8Attemptstoprovidesuchaccountswhichcouldperhapsprovidesuchexplanationseventoday includeworkbyStephenL.Adlerandhiscolleagueson generalizedquantumdynamics,aswell GrünbaumSantaBarbara 7 deterministic. Forexample,ifweweretosaythat statisticalthermodynamicsexplains howburningacigaretteproducesash, andaddthattherearerandomsmallfluctuations thatmodifytheunderlyingmechanics,thatwould stillleaveuswithaderivation(inthe appropriatesense)ofthephenomenon. Notewell,though,thatwecan'tturn thisaround andconcludethatjustanyindeterministicaccount ofhowthings happenthencountsas suchanexplanatoryderivation! Iam alsonotcontrastingtheAppearancefromRealityCriterionwith instrumentalism. Fortheinstrumentalistdeniesthatatheoryis inanysenseastoryabout whattheworldislike. Thatis implausible,forastoryis usuallypreciselywhatatheory seemstobe,though whatitdescribes theworldto belikeis veryunlikehowitappearsto us. Atpreciselythatpointthecriterionapplies: thereisthenafelt gapinthestorythat mustbefilledsothattheappearancesclearlyderivefromwhatisreallygoingon. Buton theotherhand,again,we cannotturnthisaround. Theassertionthatthe appearances are producedinsomespecificway,withoutdisplayingthatway,doesnotbyitselfprovidea satisfactoryexplanation. Indeed,theidealistand(quasi-)instrumentalistaccounts,which Grünbaumdepicted asprevalentformsofeasyantirealismamongscientists,seemto me tofallunderthisheading.9 Foritiseasyenough,butentirelyuninformative,towavea handatsomerelationofthetheorizingand measuringagenttotheaspectsof naturethat aremeasured andrepresented,andclaimthatthisaccountsforhowthe appearancesdiffer fromwhatscienceostensiblydescribes. Ifleftas amereclaimthatdoesnot suffice. Supposethen,foramoment,thatwehaveasciencethat doesnotmanageto derivetheappearancefromtherealityitdescribes. Whatarethephilosophical alternatives? Thosewho acceptthetheorymaywellstillbelievethatitcorrectlydescribes whatthingsarereallylike. Ontheotherhand,theyhave thealternativeof rejectingthe criterion.Thenafterthis rejectiontheyhavestilltwofurtheralternatives. Fortheycan eitherdenythatthereisa gap"innature,"sotospeak(thatturnsouttobethe asearlierworkbye.g.,E.Nelson,DynamicalTheoriesofBrownianMotion(Princeton,NJ: PrincetonUniversityPress,1967).Suchaccounts,ifsuccessful,cansatisfytheAppearancefrom RealityCriterionnolessthandeterministictheories. 9A.Grünbaum(1957),pp.717–19. GrünbaumSantaBarbara 8 Copenhagenalternative1010),ortheycanadheretoacertain metaphysicaldoctrine.That doctrinewouldhavetheform:thereis asenseof"derive"(presumablyrelatedtonotions ofcausalityornecessityinnature)inwhichtheappearancesdoderivefrom the theoreticallydescribedreality, butthatconnectioninnatureisbeyondtheresourcesof theoryto makeexplicit. Toavoidtheemptiness of suchantirealistreactions as those criticallydiscussedbyGrünbaum,thatmetaphysicaldoctrinethenneedsto begiven somecontent.This gameisnotover;thereareexamplesstilltodayintheliteratureonthe interpretationofquantummechanics,which attempttodoso. Butwewillconcentrate hereonwhatmaybeofvalueintheCopenhagen approach,howevermuchitenmeshed itselfinsomeofthosephilosophicaltangles. Apparent rejection of the Criterion Thenewquantummechanicsdevelopedinthe1920swasperceived evenbysomeofthe physicistsmostcloselyinvolvedasnotbridgingtheexplanatorygapbetween realityand appearance.Some,as Iwilldiscuss,disagreethatthereisa gapthereforsciencetofill. Some—attimesthesameasthosewhodenythatthereis agap—strainedto bridgeitwith philosophicalaswellas putativelyphysicalexplanations. Letusfirstnotethebare facts ofthematter.Thevehicleforpredictioninquantummechanicsis,atheart, theBorn rule: If observable A is measured on a system in quantum state ψ, the expectation value of the outcome is < ψ, A ψ> Itakeitthatmeasurementoutcomesareprimeexamplesofwhatweshould classifyas appearances.Thequantumstatesarethenthetheoreticallydescribedreality. Atthispoint inmystoryitis ofcoursenotexcludedthatthoseappearances arecompletelydescribable interms ofquantumstates. Butis thatsoinfact? 10InviewofDonHoward'silluminatinghistoricalstudieswemustbeverycarefulnottoreadtoo muchintotheterm"Copenhagen";Iintendtokeepitsconnotationsminimal,andevensorealize thatmyreadingofBohrmaybecontentious.SeeD.Howard,"Bohr'sPhilosophyofQuantum Theory:ANewLook—‘WhoInventedtheCopenhagenInterpretation?’AStudyinMythology," PhilosophyofScience71,no.5(2004):669–82(Supplement). GrünbaumSantaBarbara 9
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