The Representation of Nature in Physics: A Reflection On Adolf Grünbaum's Early Writings

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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