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ConcernsaboutObserva/on,ClassicalandQuantum
J.Bu&erfield,Rome,1Dec2016
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Howdoesourbeingobserversaffectsthescopeandcontentofourknowledge:especiallyincosmology---bothclassicalandquantum?I:Thetopicwithoutregardtophysics:emphasizing:whatispossibility?II.Thetopicinclassicalphysicsandcosmology:emphasizing:whatischance?III.Thetopicinquantumphysicsandcosmology:emphasizing:howtounderstandquantumtheory?!
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I:TheObserver,regardlessofPhysics• 1)‘SubjecRvity’• 2)SelecRoneffectsonobservaRon• 3)ThelegacyofKant• 4)My(Realist!)verdict:Noworries!• 5)AnopenquesRon:whatisapossibility?II:TheObserverinaClassicalWorld• 1)Problemsofprinciplefor‘insiders’?• 2)SelecRoneffects:the‘A’word…• 3)ConfirmingamulRversetheory• 4)TheSrednicki-Hartleproposal:FrameworksIII:TheObserverinaQuantumWorld• 1)Thedreadedquantum!• 2)TheEvere&interpretaRonoftheuniversalstate
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I:TheObserver,regardlessofPhysics1)‘SubjecRvity’
Democritus:‘ByconvenRonhot,byconvenRoncold,butinrealityatomsandvoid’.…Andnaturalphilosophy/physics,fromGalileoonwards.Theneedto‘closethecircle’:torecoveradescripRonofexperiencefromscience’s(increasinglyarcane!)world-picture.Nowadays,peopleemphasizetwoobstacles:a) ‘qualia’:‘sciencecan’tgiveyouthesmellofchicken
soup’b) ‘indexicality’:RmelyacRonneeds‘I’,‘now’‘here’.
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2)SelecRoneffectsonobservaRonEddington’snetmetaphor.Bewareofbiassedsampling!
‘Whatismostprobabletohappenneednotbewhatismostprobabletobeobserved.’‘ObserversarelocatedonlyinplaceswithspecialproperRes.Asatrivialconsequence,probabiliRescondiRonedonthepresenceofobserverswilldiffergrosslyfromprobabiliResperunitvolume.’OurexpectaRonsshouldbecondiRonedonwhatwebelieveaboutourprocessofobservaRon.
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3)ThelegacyofKantMaybeourbeliefs,evenourknowledge,haveanineradicablehumancontribuRon?Variousmeaningsfor‘ineradicable’,‘human’:
Kant,Carnap,Kuhn…IanHacking(TheSocialConstruc4onofWhat?)
helpfullydisRnguishesthreedebates:Isthecontentofmaturescienceindependentof
thepathbywhichwefoundit?Doestheworldhaveinherentstructure?IsscienRficbeliefstablebecauseoftheworld,
ratherthansocialorganizaRonofscience?
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4)My(Realist!)Verdict:Noworries!InparRcular,asto3):Maybewe(thespeciesorcultureorindividual)contributeineradicablytoourknowledge.ButthatiscompaRblewithallof:
A) acorrespondencetheoryoftruth(Aristotle,Tarski);
B)ourhavinggoodreasontobelievetheories:whosetheoreRcalclaimsaretrueinacorrespondencesense.AnditisevencompaRblewith…
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C)OurformulaRngandconfirmingatheoryofthewholeworld(aTOE):itsgeneralproposiRons(oranelitesubsetofthem)arelawsofnature.TheideaofaTOE:a):Issupportedbytheamazingunityofnatureshownbye.g.thesecondscienRficrevoluRon1850-1950,andtheriseofprecisioncosmologysinceca.1965…b):RaisesWigner’squesRon.
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5)AnopenquesRon:whatisapossibility?HowmuchdoesstaRngthetruth(ineverydaylife,orscience,orphilosophy)requireaccepRngnon-actualpossibiliRes?Forexample:(a)Delibera4onanddecision.Howtounderstandwhatanagentthinksanddoes,purelyintermsoftheoneactualcourseofevents?(b)Chance.HowcanobjecRvetendenciesforvariouspossibiliResbejustafacondeparlerabouttheoneactualcourseofevents?(c)Con4ngency.Afalsetheorymighthavebeentrue,mighthaveevenbeenaTOE:e.g.classicalvacuumelectromagneRsm.
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Eachpossibilityisofcoursenon-actual:butreal,insomewidersensethan‘actual’.WhatEXACTLYdoessuchapossibilityconsistof?Thisisgenerallyagreedtobeaharder
quesRonthane.g.themind-bodyproblem!Leibniz’spossibleworlds,revivedinmodern
modalmetaphysics:DavidLewis’srealism.SotherearethreekindsofmulRverse:logical/metaphysical;cosmological;quantum/Evere&.
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Sekngasidecosmology…AsascienRficrealist,baskinginthesunlightoftheworld-pictureofmodernscience,Iworry‘only’that… ThedeepquesRonaboutthenatureof
possibilityisaggravatedbytheuseinclassicalthermalphysics,ofobjecRveprobabiliRes.HowshouldIunderstandthese?
Butturningtocosmology…
II:TheObserverinaClassicalWorld
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1)Problemsofprinciplefor‘insiders’?Cananagentinaworldformulate/believe/knowaproposiRonabouttheworldasawhole(thusincludingthemselves)?Ordoessomelogicalparadoxofself-referencelurkhere?Isay:‘Noworries’.But:1A)Possibilityagain!Non-actualiniRalcondiRonsoftheuniversecannotbetakenasshortfor,orsomesortofanalogueof,actualiniRalcondiRonsofanotheractualsystem:thereisnosuch!2A)Epistemicbadnewsforcosmologists:--ItcanbeveryhardtodeterminetheglobalstructureofageneralrelaRvisRcspaceRme...
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2)SelecRoneffects:the‘A’word…ThecurrentscienRficsituaRon:(i) Thedegreeoffine-tuningisolenstunning!(ii) fundamentaltheoryfacesgravedifficulRes.ThecurrentphilosophicalsituaRon:(i)BewareofessenRalismaboutexplanaRon!(ii)TheneedtocondiRonalizeonadescripRonofobservaRondependsonthenoRonof‘biassedsample’andso‘backgroundpopulaRon’;(cf.orthodoxstaRsRcs).
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ButneedtocondiRonalizedoesNOTdependon:a)indexicality,i.e.threatsof‘Doppelgangers’;b)theactualexistenceofotherobservaRons:inEddington’smetaphor,theremightneverbeanotherfishingtrip!ButtomakesenseofaprobabilityofacosmologicalparameteraccordingtotheoryT,pr(|T):---Doweneedtheactualexistenceofotherdomains,wheretakesothervalues?Isay:youonlyneedthis,ifyoufeelyoumustgroundT’sprobabiliResinactualfrequencies,acrossthecosmos.
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3)ConfirmingamulRversetheoryThepredicament:Theself-samemechanismthatgivesrisestosubtlefeaturesoftheCMBcan,underappropriatecircumstances,giverisetoamulRverse:asetofnon-interacRngdomains.So:HowcanwepossiblyconfirmamulRversetheorywithouteverhavingdirectevidenceabouttheotherdomains?Iendorse(followingAzhar)aclarifyingschemethatcombinesproposalsofAguirre,Tegmark,Hartle,SrednickiandHertog.
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Threeproblems:measure,condiRonalizaRon,andtypicality.(1):Measure.Whataretheelementsofthesamplespace:Domains?Regionsofequalvolume?Givenasamplespace:whichmeasure?(2)CondiRonalizaRon.ToallowforselecRoneffects:howshouldwecharacterizeourobservaRonalsituaRon?(3):Typicality.Howmuch‘underthetails’canobservaRonbe,withoutourinferringthetheoryisdisconfirmed?
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4)TheSrednicki-Hartleproposal:FrameworksSHdefineaframeworkasaconjuncRonof:i)acosmologicaltheory/modelT:takenassolvingthemeasureproblem,sowrite:P(/T);ii)acondiRonalizaRonscheme(‘selecRonproposiRon’)C;soconsider:P(/T,C)iii)axerographicdistribuRonoverthedomainswithnon-zeromeasureaccordingtoP(/T,C).Soconsider:P(/T,C,)
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SoP(D/T,C,)isthe`first-person’likelihoodofseeingdataD.Srednicki-HartleproposeaBayesianframework,soastocomputedegreesofconfirmaRonoftheframework:P(T,C,/D).TheyandAzharshow,intoymodelsT(e.g.withfinitelymanydomains),howvariouscondiRonalizaRonschemesC,andtypicalityassumpRons,fareinthelightofvariousdataD.
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III:TheObserverinaQuantumWorld1)Thedreadedquantum!
Themeasurementproblemprompted(speciallyintheearlydays)avarietyofradicalproposalstogivethe‘observer’---maybeinanodyneform,e.g.asanorthobasis!---afundamentalrole.
Asarealist,Idislikethemall!IstandwithJSBell.ButIurgeweshouldgobeyondtheusualsuspects,viz.• Dynamicalcollapse(GRW,Pearle,Penrose…),• Pilot-wavetheory(deBroglie,Bohm,ValenRni…)• Manyworlds(Evere&,Hartle,Deutsch,Wallace…)
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2)TheEvere&interpretaRonofTheidea:agenericbranch(be&er‘history’)ofcontainsaclassicalmulRversealaPartII:i.e.ofthetypedescribedbyHartle-Srednickiframeworks.Merits:1)providestheprobabiliRes,eginaconsistent-historiesapproach;2)Addingappropriatedetails,itgivesprobabiliResforwhatweobserve;3)SymmetriescanhelpuscalculateprobabiliRes,egbycoarse-graining&bytheuncalculableprobabiliRescancellingout.
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TechnicalquesRonsabound!Forexample:A) Whatis?A.1)TheNoBoundaryProposal:sumoverallthe4-manifoldsandmetricscuppingthegiven3-metric.A.2)DefiniRonandtractability?ThetransiRonfromtheEuclideantotheLorentzianregime?
(
3g) = ⌃4g exp[�I(4g)/~]
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A.3)ContrastirresponsiblepopularizaRon!SomesaycreaRonexnihiloisscienRficallyunproblemaRc,viz.‘fluctuaRonfromvacuum’.No!‘Vacuum’means,not‘nothing’,but‘lowestenergystateofthegivensystem’.‘QuantumfluctuaRon’meanshavingnon-zeroamplitudeformorethanone(typicallyclassical)alternaRve.
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B)HowtorelatetoinflaRon,soastogetclassicalmulRverse(s)?B.1)Worryinglymanymodelsoftheinflaton.B.2)DifferentmechanismsofeternalinflaRon.B.3)Variousquantum-classicaltransiRons:i)Differentcomponentsofthestateii)Thestateisspeciale.g.WKBiii)Insomeregime,relevantcommutatorsiv)PreferredquanRRes:(surelynotbydecoherence:thereisnoenvironment).
! 0
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TheanswerstoquesRonsinA)andB)mustensurethatparametersvaryacrossdomainsoftheensuingclassicalmulRverse.C)Hartle,HawkingandHertogrelatetheNBproposaltoinflaRon,usingconsistent-histories.Thegeneralmeritsrecur.Besides,insimplemodelsegminisuperspacewithquadraRcpotenRal:i)HistorieslabelledbyandexhibitinflaRon;ii)Our‘rareness’makesprobabiliResforwhatweobservevolume-weighted,enhancinginflaRon(high).
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AndbroadconceptualquesRonsabound…D)IfthebranchingiseffecRve,ieapproximate,thedefiniRonofeachclassicalmulRverse---andsoe.g.howmanydomainsithas---isineradicablyvague.Isthatacceptable?E)Howshouldweunderstandprobability?The‘usualquesRontoEvere&’ismuchsharperfor....
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Millegrazie!Mercibeaucoup!
Dankje!
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References
Azhar&JNB,ScienRficrealismandprimordialcosmologyh&p://philsci-archive.pi&.edu/12192/1606.04071Azhar,TesRngtypicality,PhysicalReviewDphilsci-archive.pi&.edu/119951506.05308Azhar,SpectraofcondiRonalizaRon,PhysicalReviewDphilsci-archive.pi&.edu/11996;arxiv:1601.05938JNBUnderdeterminaRonincosmologyStudHistPhilModPhysarxiv:1406.4747philsci-archive.pi&.edu/9866/
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Hartle,Hawking,Hertog,Classicaluniversesofno-boundaryquantumstate,PhysicalReviewD0803.1663Hartle,Hertog,ReplicaRonregulatesvolume-weighRng,PhysicalReviewD0906.0042Hartle,Hertog,Theobserverstrikesback,1503.07205Hartle,Hertog,Onebubbletorulethemall,1604.03580Hartle,Srednicki,Arewetypical?PhysicalReviewD0704.2630Srednicki,Hartle,Scienceinaverylargeuniverse,PhysicalReviewD0906.0042