Introduction
ChristianWüthrich,BaptisteLeBihan,andNickHuggett
Abstract
Quantumgravityoffersafertilegroundforphilosophicalwork,particularly throughitssuggestionthatspacetimemaynotbefundamentalbutmerelya derivativestructure.Assuch,theoriesofquantumgravitystandinalongtraditionofphysicaltheorieswithdeepimplicationsforthenatureofspaceandtime, andindeedthefundamentalstructureofourmaterialworld.ThisIntroduction summarizesthecontributionstothiscollectionbystructuringthemaround threethemes.Thefirstgroupofchaptersanalysesvariousaspectsofthesearch oflostspacetimeinquantumgravity.Thesecondgroupstudiesmetaphysical andepistemologicalaspectsoftheemergenceinplayinquantumgravity.The thirdgroupwidenstheinvestigationstoseveralkeymethodologicalchallenges arisinginthecontextofquantumgravity.
Contemporaryphysicshasmuchtoteachusaboutthenatureofspaceandtime, ashasbecomeobviouswiththeadventofrelativitytheoryatthelatest.Whatis lessobviousisthattherelativisticrevolutionmayonlyhavebeenthefirststep inalongerprocessofdeconstructingourpre-theoreticalcategoriesofspaceand time.Infact,attemptstounifythelessonsofgeneralrelativity(GR)withthe othergreatrevolutionofthetwentiethcenturyinphysics,quantumphysics,into atheoryofquantumgravity(QG)suggestaratherstrangeidea:that spaceand timeasweknowthemdonotfundamentallyexist,butinsteademergefromanonspatiotemporalstructure.Thus,thephysicsunderconstruction,whichwehope willonedayprovideamoreunifiedandfundamentalviewofreality,couldlead toanovelunderstandingofthenatureofspaceandtime,radicallyopposedto everythingpreviouslybelieved.Thisvolumeistheresultofourconvictionthat thisstunningconsequence,difficultasitmaybetoconceptualize,couldgenuinely changethewayinwhichmanydiscussionsinphilosophyareconducted—say,on theexistenceofspace,theflowoftime,ortheboundariesofspaceandtime,to namebutafewexamples.Hencethetitleofthiscollection: PhilosophyBeyond Spacetime. ChristianWüthrich,BaptisteLeBihan,andNickHuggett, Introduction In: PhilosophyBeyondSpacetime:Implicationsfrom QuantumGravity.Editedby:ChristianWüthrich,BaptisteLeBihan,andNickHuggett,OxfordUniversityPress. ©ChristianWüthrich,BaptisteLeBihan,andNickHuggett2021.DOI:10.1093/oso/9780198844143.003.0001
Thecurrentsituationincontemporaryphysicsresultsfromalonghistorical journey.Aswehavealreadymentioned,tworevolutionstookplaceinphysics duringthetwentiethcentury:specialandthengeneralrelativity,andquantum physics.Thefirstgaveanewunderstandingofgravityasafeatureofthegeometry ofspaceandtime,withdramaticphilosophicalconsequences:therelativityof simultaneity,thedynamicalrelationbetweenspacetimeandmatter,thepossibility oftemporalanomalies,andtheexistenceofsingularitiesinspacetime,forexample. Theseconddealswiththematterthatinhabitsspacetime,andposesequallydeep philosophicalchallenges:thenon-localconnectionbetweenentangledsystems, theappearanceofaclassicalworld,theveryexistenceofparticles.Whilebeingof enormousempiricalpowerandaccuracy,bothhaveraiseddeepandunresolved philosophicalandfoundationalquestions—astheever-expandingliteratureonthe subjectsattests.
Thereisasyetnogenerallyacceptedunifiedtheoryofquantummatterand gravity,capableofdescribingallphenomenathatneedtobeunderstood.Quantummattercannoteasilybedescribedasinteractingwithclassicalrelativistic spacetime.Butitseemsasifmatterandgeometrymustbecloserinnaturethan theyareinthetwoseparatetheories.Byitself,thisobservationgivesusreasonto thinkthatasuccessfultheoryofQGwillraisesimilarlyimportantchallengesfor ourconceptionsofspace,time,andmatter—perhapsabolishingthemaltogether asfundamentalentities.Althoughsuchatheorydoesnotyetexist,wemay neverthelessseethesilhouettesofcomingchangesinthefragmentarytheories thatdoexist,andcometounderstandnewpossibilitiesforepistemologyand metaphysics.Butjustasimportant,thereisgoodreasontothinkthatsomeof theproblemsinfindingatheoryofQGarethemselvesconceptual,inneedof philosophicalanalysisbyphilosophersandphysicists—justasrelativityrequired Einstein’sreconceptualizingofthenatureofspace,time,andmotion,andas quantummechanicswasdrivenbythecompetingpicturesofSchrödingerand Heisenberg.
Thisvolumeisoneofthefruitsofathree-yearresearchproject, SpaceandTime afterQuantumGravity(2015–18),fundedbytheJohnTempletonFoundation.The projectinvolvedbothphysicistsworkingonthephysicsofQGandphilosophers workingonthistopicorrelatednotionsincontemporaryphilosophy.Wehave selectedafewoutstandingworks,thefruitofintensediscussionsoverseveral years,andwehavedividedthemintotwovolumes.One—BeyondSpacetime:The FoundationsofQuantumGravity(CambridgeUniversityPress,2020)—dealsmore withphilosophicalquestionsarisinginthetechnicaldevelopmentofdifferent approachestoQG.Theothervolume—thisone—ismoredirectlyconcernedwith theimplicationsofQGforquestionstraditionallyseenasmorephilosophicalin nature.Ofcourse,thisdistinctionisgradualatbestandoftenblurred—notthe leastinfoundationalresearchinQG.Severalofthechaptersofeithercollection
couldequallywellhavebeenincludedineithervolume.Toafirstdegreeof approximation,thecompanionvolume,whichrequiresmoretechnicalskillsin physics,addressesawiderrangeofphysicists,whilewehopetoreachmany philosophersbeyondthenarrowconfinesoftechnicallydemandingphilosophy ofphysicswiththepresentvolume.
Thekeyaimofthisvolumeistoexpandknowledgeandunderstandingof thephilosophyofQGbythephilosophicalcommunity.Itemphasizeshow debatesinmetaphysics—regardingtime,emergence,composition,orgrounding, forexample—shedlightontheconceptualquestionsofQG;andconversely, howquantumtheoriesofspaceandtimecallintoquestionphilosophicalviews groundedinclassicalspacetime.Furthermore,thephilosophyofQGraises methodologicalquestions,forinstanceconcerningtherelationbetweenphysics andmetaphysics.Theessaysinthisvolumehavebeenchosentodemonstratetoa widerangeofphilosophersthesignificanceofthesubject,aswellasmakingnovel contributionstoit.
Theessaysareorganizedaroundthreemainsubjects:(i)thepossibleemergence ofspacetimeinvariousapproachestoQG,(ii)philosophical(especiallymetaphysicalandepistemological)discussionsofthenatureofthisrelationofemergence, and(iii)afinalsectiondevotedtomethodologicalaspectsofthephilosophyofQG. TheremainderofthisIntroductionsketchesthesetopicsandthecontributions.
1.1SearchingforSpacetime
ThefirstseriesofchaptersexploresvariousapproachestoQGandexamineshow spacetimemightemergeinthesespecificapproaches.Thefirstchaptercategorizes thewayinwhichspacetimecanbesaidtoemergefromamorefundamental butlessspatiotemporalstructureintoqualitativelydifferentlevelsandexemplifies theminthecontextofloopquantumgravity(LQG).Thefirstthreechaptersallaim toclarify,andperhapstosomeextentdownplay,claimsofemergenceoffamiliar spacetimestructuresinthiscontext.Thelastchapter,incontrast,proposesthat theclassicalnotionofspacetimewouldhavetobegeneralizedifnatureturnsout tobesupersymmetric.
Thefirstchapterby DanieleOriti proposesalevelsviewoftheemergence ofspacetimeinQG.AccordingtoOriti,theemergenceofspacetimecomesin degrees,where‘degree’or‘level’isnottobeunderstoodontologicallyasmetaphysicallayersorasequentialsuccession.Rather,levelsareintendedtoindicate abroadeningoftheperspectiveontheproblem,suchthatateachstep,novel conceptual,methodological,epistemological,orontologicalissuesshowupasthe complexityandtherichnessofthephysicsincrease.Heidentifiesfourdistinct levelsofemergenceofspacetimeandofthegravitationalfield,andoffersthe
helpfulanalogyoftheemergenceofhydrodynamicpropertiesof(super)fluids fromitsatomicconstitution.
Thefirstlevel,Level0,coversclassicalGRanddirectquantizationsofits geometricdegreesoffreedom.Alreadyattheclassicallevel,thereisasensein whichspaceandtimedissolve,asthegeneralcovarianceofclassicalGRimplies that,generically,thereexistsamultitudeofnotionsofspaceandtime.Moreover, intheHamiltonianformulationofGRrequiredforcanonicalquantization,thereis a‘problemoftime’inthattimeanddynamicsappeartovanishaltogether.Evenso, thefundamentaldegreesoffreedomarestillspatiotemporal,orgeometric.These difficultiesareexacerbatedatthequantumlevel,wheresuperpositionsofexact geometricconfigurationsarepermitted.Inthehydrodynamicalanalogy,Level0 wouldcorrespondtotheconstructionofmacroscopichydrodynamicobservables asfunctionsofdifferentones,withtheirquantizationaddingdifficulties.FollowingCarloRovelli,Oritibelievesthatthesechallengescanberesolvedbydeploying arelationalstrategy.
Level1complicatesthispicturebyaddingnew,distinctkindsofdegreesof freedom,whichareneitherspatiotemporalnorgeometricalinanydirectsense. Typically,thefundamentaldegreesoffreedomarecombinatorialoralgebraic,and shownocontinuumstructure.ManytheoriesofQG,suchasLQG,stringtheory, causalsets,andcausaldynamicaltriangulations,arenaturallyinterpretedtoat leastascendtothislevel.WhiletheexplicationofemergenceatLevel0involves theclassicallimit,toseespacetimeemergeatLevel1,thecontinuumlimit—to becarefullydistinguishedfromtheclassicallimit—mustbetaken(usuallyvia coarsegrainingorrenormalization).Unliketheotherlevels,Level1includesan ontologicalaspectinthatthepostulatednewkindsofdegreesoffreedomform anovelontologicalcategory.Thislevelisdirectlyanalogoustothemovefrom macroscopichydrodynamicstothegrainyworldoftheatomswhichconstitute thefluid.
Levels2and3donotproceedtoanovel,qualitativelydistinctkindofdegrees offreedom.Instead,theyinvolvedifferentconceptualperspectivesonthesame basicontology.Level2startsfromtherealizationthatthecontinuumlimitisoften notunique,butleadstodistinctmacroscopicphasesseparatedbypossiblephase transitions.Notallsuchmacroscopicphaseswillbegeometricorspatiotemporal. Whatneedstobeshowntoresolvethislevelisthatthereexistsaspatiotemporal phaseinsomeapproximationandsomelimit.Inthecaseofsuchnon-trivial macroscopicphasediagrams,spacetime,arguesOriti,canbesaidtobeemergent inamoreradicalsensecomparedtoLevel1.Inthehydrodynamicanalogy,one andthesamesystemcancondenseintodifferentmacroscopicphasessuchassolid, fluid,andgaseous,andwillonlyobeyhydrodynamiclawsinoneofthem.
Finally,theexistenceofdistinctmacroscopicphasesleadstothepossibility thatthesystemundergoesaphasetransition.Thus,asystemmaytransition
fromanon-spatiotemporaltoaspatiotemporalorgeometricphase,sustaining a‘geometrogenesis’.Thisiswhatmayoccurinquantummodelsofthebigbang whereonecandistinguishan‘earlier’,non-spatiotemporalphasefroma‘later’, approximatelyspatiotemporalone.Whilethistransitionoughttoberegardedas aphysicalprocess,thedifficultiesinvolvedinconceptualizingthesetransitionsas physicalyetnon-temporalprocessesjustifyregardingthemasLevel3transitions. Incomparison,appreciatingthephysicalnatureofphasetransitionsinthehydrodynamicanalogyseemsmorestraightforward.
Oriti’scategorizationofqualitativelydifferentkindsofspacetimeemergence givesusefulguidancetoafieldwhichhassofaronlystartedtoappreciatethe systematicallydifferentkindsofissuesinvolvedatthedifferentlevelofemergence.
JeremyButterfield’schapteraddressesthenatureandsignificanceof‘dualities’ inphysics.Thesesymmetriesattractedagreatdealofinterestamongstphysicists whentheywerediscoveredinstringtheoryinthe1990s,sparkingthe‘second stringrevolution’.Morerecentlytheyhavebeenthesubjectofintensescrutiny byphilosophers,fortheirimplicationsfortheinterpretationofstringtheory (forageneraloverviewaimedatphilosophers,seeLeBihanandRead2018). Forinstance,Huggett(2017)arguesthatdualsshouldbeunderstoodasfully equivalentdescriptions,sothatanyapparentdifferencesarenon-factual.Ifso, theT-dualitybetweentheorieswithdifferentradiifortheuniversemeansthat fundamentallyspacehasnodefiniteradius,and,developingalineofthought proposedbyBrandenbergerandVafa(1989),theobserveddefiniteradiusmust beemergent.
Butterfieldarguesthatsuchreasoningshouldberesisted.Drawingonworkby (andwith)SebastianDeHaro(e.g.2020),hefirstgivesaformalaccountof‘duality’: broadlyspeaking,twotheoriesaredualwhentheyaredifferentformalrepresentationsofacommon‘baretheory’.Forinstance,onecanrepresentthesamesystem ofmovingbodiesinframesofreferencewithdifferentorigins,orientations,and statesofrest.Thereisanisomorphismbetweenthetworepresentationsthatallows onetoseehowtheyaredescribingacommonsetofquantitiesinthesameway. However,thisformalmappingsaysnothingbyitselfaboutwhetheroneofthe representationsisrightaboutabsoluterest,orwhetherthereisinfactnostandard ofabsoluterest.
Suchquestionsareamatteroftheinterpretationofphysicaltheory,inthiscase thetwoduals.Dotheyautomaticallysaythesamethings,ordisagreesubstantively?
Butterfield(andDeHaro)thereforeinvokeaformaltheoryofinterpretation,in whichreferenceishandledbyactualworldlyextension,andsensebyextension acrosspossibleworlds.Ofcourse,inthisframework,whetherthedualssaythe samethingsdependsontheirintendedsense:Newton’s‘space’hasanabsolute standardofrest,whichNewtonhypothesizedtobethatof‘thecentreoftheworld’. Ofcourse,inthatsenseof‘space’,onecouldmeaningfullydisagreewithhim,even
thoughGalileansymmetrymeansthatnoexperimentcansettletheissue.Similarly forT-duals:onecouldinprincipleunderstandtheirdifferenceaseithermerely representationalorasreal.ForButterfield,untilwehaveatheorythatexplicitly unifiesthem,revealingtheunderlyingbaretheory,wemustunderstandthemas disagreeingabouttheactualworld.Thoseadvocatingforemergencewilldisagree, arguingthatonecaninsuitablecircumstancesinferthatdualsagreefactually,even absentanexplicitunderlyingtheory(e.g.HuggettandWüthrich2020).
Inherchapter, AlyssaNey askswhetherthereisevidencefortheemergenceofspacetime—specificallymetrical—structurefromquantumentanglement entropy:thisisanideathathasreceivedrecentattention(forinstance,Caoetal. 2017).Astartingpointforthiswayofthinking(thoughrelatedideasgoback further)istheRyu-Takayanagiconjecturethat‘holographically’relatestheentanglementontheboundaryofanti-deSitterspacetimetoanareainthebulk,using theAdS-CFTduality.(Specifically,theentropyarisingfromtheentanglement betweentheconformalfieldsintworegionsontheboundaryisdirectlyrelatedto theareaoftheminimalsurfaceinthebulkthatseparatesthem.)Itisquitestriking thattwothingsasdisparateasquantumentanglement—whichatrootmeasures theabilitytofactorizevectorsinHilbertspace—andmetricalquantitiescanbe relatedinthisway.ItisaninsightthatledCaoandco-authorstodescribehowto reconstructaspatialmetricfromanabstractquantumsystem.
Suchanideastronglysuggestsemergence.Thequantumstructure,fromwhich thespatialstructureissupposedlyrecovered,isseeminglynon-spatial,aswe noted.However,asNeyexplains(drawingontheearlierworkofothers),that conclusionishasty,forthemerecorrespondencedoesnotentailemergence. Amongstotherpossibilities,which,ifeither,ofthetwosidesismorefundamental? ThecorrespondencealonedoesnottellusthatitistheHilbertspaceside.If,Ney argues,onelooksatthederivationoftheRyu-Takayanagiconjecture,thereasoning seems,ifanything,toshowthatitreflectsthewayspatialstructureconstrains thequantumentanglement.(And,sheclaims,similarlyinearlierderivationsof theHawking-Bekensteinentropythatalsorelateentropyandarea:e.g.Bombelli etal.1986.)Inthatwayoflookingatthings,spaceseemstobeatthesamelevel asentanglement.Thus,sheconcludes,thesederivationsofspacefromHilbert spacedonotsupporttheclaimofemergence.Ofcourse,thatisnottosaythat theyareincorrect,orthattheremaynotbeothermotivationsforseekingto derivespaceinthisway:forinstance,onemighthopethatbystartingfrom suchasolidlyquantumfoundation,onemighteventuallyrecoveremergentGR, therebyprovidingaroutetoafulltheoryofQG.ButNey’sclaimisthatsuch motivationsarefuture-orientedspeculations,notatpresentwellsupportedby metric-entanglementcorrespondence.
Accordingtowhat TusharMenon calls‘Earman’sprinciple’,dynamicaland spacetimesymmetriesoughttocoincideinatheory.Thus,ifthedynamicsof
matterisLorentzcovariant,asisthecaseforthestandardmodel,thenthe appropriatespacetimevenueforthetheoryisonewhichisLorentzsymmetric, i.e.Minkowskispacetime.Ifitturnedouttobethecasethatmatterenjoysan additional,andhithertoundetected,symmetry,thespacetimeshouldfollowsuit andbegeneralizedaccordingly.
Inhiscontribution,Menonexploresthepossibilitythatmatterissupersymmetric,whichisthecaseifthetheory’sLagrangedensityremainsinvariantunder transformationsmappingbosonstofermionsandviceversa.Asitturnsout, suchtransformations‘mix’withspacetimetranslationsinthesensethatrepeated applicationsresultinanettranslationinspacetime.Menonconcludesfromthis thatsupersymmetryisinherentlyspatiotemporal,givingusallthemorereason toextendthespacetimearenaforsupersymmetricphysics.Suchasuperspace consistsina‘supermanifold’—constructedfromcommutingandanticommuting ‘supernumbers’—endowedwitha‘super-Minkowskimetric’.Therewardforformulatingsupersymmetricphysicsinsuperspaceisthattheresultingequationsof motionaremanifestlysupersymmetric,allowingthetheorytowearitssymmetry onitssleeve.
Asitturnsout,thelightpostulateofspecialrelativityisviolatedinsucha supertheory:thespeedoflightisnolongerinvariantinallsuperspacecoordinate systems.Menonconcludesfromthisthatweshouldbehesitanttoreadoffthe operationalinformationconcerningthebehaviourofmeasuringdevicessuchas rodsandclocksmadefromsupersymmetricfieldsfromthestructureofsuperspace.Maybeso,butthismightalternativelybecountedasastrikeagainstthe extensionofEarman’sprincipletothesupersymmetriccontext.
1.2TheMetaphysicsofSpacetimeEmergence
Thatspaceandtimeorspacetimemightemergefromanon-spatiotemporal structureseemsfranklypuzzling—anddifficulttoarticulateconceptually.Hence thesecondseriesofselectedchapters,devotedtothephilosophicalanalysisofthe claimthatspacetimeisemergent.Theseriescomprisesfourchapterswrittenby DavidYates,DavidJ.Chalmers,AlastairWilson,andJenannIsmael,respectively. Theydiscusstheprospectsofanalysingthedependenceofspacetimeonanonspatiotemporalstructureviaa relationoffunctionalrealization (animportant milestoneinadebatewhichisgainingmomentum—cf.theforthcomingspecial issueof Synthese onthetopic,editedbyKarenCrowther,NielsLinnemann, andChristianWüthrich),a relationofcausation,oreventhepossibilityto fully eliminatespacetimefromthefurnitureoftheworld.
Inhiscontribution, DavidYates examinestheproblemofempiricalincoherenceinQG:ifspacetimeisnotpartofthefundamentalontologyofphysics,how
isitpossibleforfundamentalphysicaltheoriestobejustifiedbyobservationsof spatiotemporallylocatedthingslikerods,pointers,andclocks?Asasolutiontothe problem,hedistinguishesbetweenviewsthatacceptandrejecttherealityofaspatiotemporalstructureoverandabovethemorefundamentalnon-spatiotemporal structure.Drawinglessonsfromthephilosophyofmind,hearguesthatweshould adoptaformofrealismaboutthedependentspatiotemporalstructureinorderto solvetheproblemofempiricalincoherence.
Thisdependentspacetime,accordingtoYates,caneitherbegroundedin orcausedbythemorefundamentalnon-spatiotemporalstructure.Thus,Yates seesspacetimeasasetoffunctionalrolesimplementedbyamorefundamental structure,aviewcalled‘spacetimefunctionalism’,whichwasdevelopedinthe contextofQGbyLamandWüthrich(2018).Thespatiotemporallylocatedthings thatactasevidenceforphysicaltheoriesaretherebyregardedasbeingrealentities causedby,orgroundedin,morefundamentalnon-spatiotemporalentities.Yates therebymakesacaseforaspecificsortofspacetimefunctionalismthattakes theexistenceofspatiotemporalroles,ontologicallyspeaking,veryseriously— againstanothersortofspacetimefunctionalism,morelinguisticinnature,that regardsspatiotemporalrolesasmerelinguisticrolessomepredicatesoccupyin thearchitectureofthetheory.TheviewisatoddswithLinnemann(forthcoming) whoarguesthatspacetimefunctionalismasageneralviewdoesnothelpwiththe problemofempiricalincoherence.
DavidJ.Chalmers focusesfirstontheemergenceofspaceratherthanof spacetimewithafocusonphilosophyofmindandthepossibilityofidentifying whathecalls‘Edenicspace’—namely,spaceasweimmediatelyfinditinthe manifestimageoftheworld—withrealphysicalspace.Hismainclaimisthatwe shouldbefunctionalistsaboutthemanifestimage,andthatweshouldnotexpect theEdenicspaceofthemanifestimagetofaithfullymirrorthestructureofthe physicalspace.Rather,weshouldconstruespaceasthestructuretriggering(our experienceof)themanifestspatialimage.Inotherwords,Chalmersarguesagainst whathedubs‘spatialprimitivism’,namelytheviewthatspaceisjustasitappearsin themanifestimage.Hefavoursinstead spatialfunctionalism—theviewthatspace iswhatcausestheEdenicspaceofthemanifestimage.
Themovefromspatialprimitivismtospatialfunctionalismhasmajorfallouts, notonlyforthestatusofthephysicalspaceinwhichwelivebutalsoforother sortsofmoreabstractspacesinvolvedinvirtualrealityexperiences.Indeed,he arguesthattherejectionofspatialprimitivisminfavourofspatialfunctionalism hasastraightforwardconsequence:virtualspacesshouldberegardedasreal spaces,functionallyimplementedbyanontologytowhichwehavenodirect epistemicaccessviaourexperienceofthevirtualreality.Thenhemovesonto analysingtheinterpretationofspacetimeinthecontextofGRandoftherelationof emergenceexistingbetweentherelativisticspacetimeandthemorefundamental
non-spatiotemporalstructureofQG.Likewiseandinlinewithotherworks,he discussesthestrategyoffindingspatiotemporalfunctionalrolesinthegeneral theoryofrelativitybeforeturningtoquantumtheoriesofgravitytoidentifythe realizersoftheseroles.Thereagain,hedefendstheviewthatthefunctionalist strategyjustifiesrealismaboutthespacetimeofGR.Drawingfromdiscussions abouttheexistenceofanexplanatorygapbetweenanon-spatiotemporaltheoryof QGandatheoryofGR,Chalmersthenfocusesonthepossibleexistenceofanother explanatorygap,thistimebetweenGRandthemanifestimage;hearguesthat spacetimefunctionalismmayclosethisgap.Inbrief,accordingtohim,weshould freeourontologyfromEdenicspaceassumptions;butweshouldnonethelessbe realistsaboutafunctionalphysicalspace,identifiedwiththestructurethattriggers ourexperienceofEdenicspace.
AlastairWilson alsoexaminesspacetimefunctionalismasawaytoanalyse thedependenceofspacetimeonanon-spatiotemporalstructure.Hebeginswith thefollowingquestion:shouldweconstruethedependenceofspacetimeonthe non-spatiotemporalstructureascausalornon-causal?AsWilsonpointsout, distinguishingbetweencausalandnon-causalrelations—alsocalled‘grounding relations’—requiresademarcationcriterion.Thiscriterionissupposedtoallow ustodecide,inthefaceofarelationofdependency,whetherthisrelationiscausal ornot.Anintuitivedemarcationcriterionisrelatedtotime.Causationwould refertocross-temporaldependencyrelations—thetworelataoftherelationbeing locatedatdifferenttimes—andgroundingtosynchronicdependencyrelations— thetworelatabeinglocatedattheverysametime.AsWilsonnotes,ifthistemporal demarcationprincipleweretoberight,thenitwouldfollowthattheemergenceof spacetimecannotbecausal(andnotgroundingeither),sincetimedoesn’texistat thefundamentallevel—therebypreventingusfromidentifyingthedependenceof spacetimeonthenon-spatiotemporalstructuretoacausalrelation.
ButWilsontakesanotherroadas,drawingonpreviouswork(Wilson2018), herejectsthetemporalcriterionandproposesanalternativemediationcriterion: causalrelationsaredependencerelationsmediatedbyalawofnature,while groundingrelationsaremediatedbyconstitutiveprinciples—whatitistobea particularthingorkindofthing.Thissuggeststhatwecanuseourunderstanding ofwhatitistobealawofnaturetograspthedistinctionbetweencausation andgrounding.Ofcourse,asWilsonacknowledges,thisamountstoreplacing questionsaboutthenatureofdependencyrelationswithquestionsoverthenature ofthosemediatingprinciples.Forinstance,ifweconstruecausalrelationsas relationsmediatedbyalawofnature,thenweneedtounderstandwhatalawof natureis—withoutappealingtocausation,atriskofviciouscircularity—which mightormightnotbeproblematic,dependingonwhichofthetwonotionsone takestobemorefundamental.Nonetheless,itisanimportantresultthatthe differencebetweengroundingandcausationcanbeilluminatedthiswayaswe
canthenanalyse,insomeapproaches,emergentspacetimeasbeingcausedbya non-spatiotemporalstructure.
AsWilsonnotes,thisideamightsurprisethereaderas,atfirstsight,itseems atoddswiththecommonassumptionfoundintheliteraturethatspacetime doesnotemergeviaacausalprocess,causationbeingtootightlyrelatedtothe existenceofspaceandtime(apossibleexceptioncouldbefoundincosmological modelsbasedonQG;seee.g.HuggettandWüthrich2018).Usually,thisrelation ofdependencyisconsideredtobebothnon-temporalandnon-causal,either becauseitisaprimitiverelationofgrounding(i.e.nottobespecifiedfurther viaanotherrelation)orbecauseitisarelationofmereologicalcomposition. ButWilson’snewcriterionsuggeststhatspacetimecouldliterallybe causedby anon-spatiotemporalstructure,inparticularinLQG.Wilsonthenshowsthat thereisawaytoavoidthissurprisingconsequencebyusingthefunctionalist machinery.Spacetimefunctionalismoffersanewwaytointerpretthesituationin LQG,spacetimebeingfunctionallyrealized—aconstitutiveprinciple—andhence groundedin,ratherthancausedby,thenon-spatiotemporalstructure,according tothemediationcriterion.Wilsoncloseswithadiscussionofthemany-instant landscapeviewdefendedbyGomes(2017).Accordingtothisview,theworldis a‘timeless’statespaceofspatialfieldconfigurations.Wilsonarguesagainthat theexistenceofspacetimeinthisapproachshouldberegardedasmediatedby afunctionalprincipleandso,becauseofhisdemarcationprinciple,asgrounded inthenon-spatiotemporalstructure.
Inherchapter, JenannIsmael arguesforanexpansiveviewofwhatthe‘emergenceofspacetime’mightamountto,whichopensanunnoticedandperhaps surprisingroutetorecoveringspacetime.Itistypicallyassumedthatconscious experiencewillberecoveredfromfairlyhigh-levelphysics,sothatreductionfrom anylowerphysicscan(withasighofrelief)ignorethemind—tobetakencare oflater.IsmaelcitesMaudlin(2007)asanexampleofthisposition.Sincethe physicswhichissupposedtoexplainthemindisspatiotemporal,thecorollary isthatphysicalspaceitselfwillhavetoberecoveredfromanynon-spatiotemporal theory—asprogrammesinQGtypicallyassume.
However,ofcourseallthatneedberecovered,strictlyspeaking,istheconscious experienceofspatiotemporality.Ifoneassumesthatthatisgivenimmediatelyin experience,thentheresultisthesame—physicalspacetimemustemerge.But, Ismaelargues,spacetimeisnotatallimmediatelyexperienced.Empiricalstudies supporttheview—whichhasantecedentsinBerkeley,Poincaré,andMach(on whomshefocuses)—andprovideevidencethattheconceptofspacetimeisa constructfromexperience.Morespecifically,differentsensorymodalitiespresent differentperspectivesonthephysicalworld,eachinitsown‘sensorymanifold’. Butthemindunifiesthesethroughsensoryandmotorinteractionwiththeworld, findingthatarepresentationinwhichtheyareviewedasdifferenttakesonasingle
spatiotemporalworldsimplifiestheirrelationsgreatly.(Thesignificantplasticity inthisunificationshowsthatitisnotafixedgiven.)
Theupshotofcourseisthatstrictlyspeaking,atheorywithoutspacetimeisnot requiredtodeliverspacetimeitself,butonlytoexplainthesensorymodalities,to recoverthesensorymanifolds.Thenthebrainitselfwilltakecareofspacetime, whichwouldberecognizedasamereappearance.Ismaelnotesthatsuchan accountwouldfitafunctionalistmodel:thefunctionsofthemodalitiescouldbe foundtobeplayedbythenon-spatiotemporal.Shedoesnotnecessarilyadvocate suchastrategy,butarguesthatitshouldberecognizedtounderstandtheepistemologyandgoalsofspacetimeemergence.Atfirstglancethestrategyappears radical,butIsmaelpointsoutthatitmaynotbesofarfromwhathasalreadybeen suggested.Forinstance,Rovelli(1991)suggestsastrategyofrecoveringrodsand clocksinsteadofspacetime:itisnotsuchagreatleaptoconsiderinsteadrecovering ‘informationgatheringandutilisingmachines’,cashedoutfunctionallyinnonspatiotemporalterms(seealsoBaron2020).
1.3MethodologicalIssues
Thelastseriesofchaptersreflectsthediversityofworksinthegrowingcommunity ofphilosophyofQG.IfacentralissueinQGistheconceptualarticulationofthe potentialemergenceofspacetime,manyotherissuesinQGarisealongtheway. Withthislastsection,wewanttoinvitephilosopherstodelvedeeperintothose otherquestionsrelatedtotheconstructionofatheoryofQG—andthustodraw theirattentiontoanumberofpointsthatcouldhaveimportantrepercussionsfor manyphilosophicalquestions.
ThefirstchapterbyRichardHealeydealswiththeperhapsmostfamous probleminthefoundationsofquantummechanics, themeasurementproblem. However,itdoessointhecontextofonespecificapproachtoquantumgravity, LQG,raisingthedifficultquestionofwhetherornotweshouldsolvethemeasurementprobleminordertomakeprogresswithQG.Thesecondchapterby KerryMcKenziefocusesontheconnectionbetweenphysicsandphilosophy,and onthepossibilityofobtainingmetaphysicalknowledgefromthecurrentstate ofphysicswherewelack,onemustnote,afinaltheoryapplyingtoalldomains ofobservation.ThefinalchapterbyAdamKoberinskiexaminesadeeppuzzle aboutvacuumenergy,sometimescalledthe‘vacuumcatastrophe’or‘cosmological constantproblem’.Theproblemarisesfromaclashbetweenthevalueofthe vacuumenergywecalculateviaquantumfieldtheory—whichmustbeextremely high—andthevaluewegetfromcosmologicaldatawhenweobservetheway theenergyisdistributedintheobservableuniverse,suggestinganextremely lowvalue.
Thesethreeissues,althoughquitedifferent,mayservetoshowjusthowfertile researchinthephilosophicalfoundationsofQGisforverydifferentquestions, traditionalandnew,technicalandfundamental.
RichardHealey’scontributiondiscussescritically,butsympathetically,Carlo Rovelli’srelationalinterpretation(1996)asawaytosolvethequantummeasurementprobleminthecontextof(covariant)LQG.AlthoughRovelliwasofcourse oneofthemainarchitectsofthistheory,Healeyshowsthattherearesignificant tensionsbetweenthetwo.Healeybelievesthatthesecanbeovercomeinhisown pragmatistapproachtoquantumphysics.
Rovelli’srelationalinterpretationofquantummechanicsdepartsfromvon Neumann’snotionofameasurementresultinginacorrelationbetweentheinitial stateofthe‘system’andthefinalstateofthe‘measurementdevice’.InRovelli’s interpretation,thereisnoplaceforanabsoluteandobjectivestateofasystem; instead,allstatesarerelationalinthatasystemisinastateonlyrelativetoanother system,whichmaybeanobserverwhohasorgainsknowledgeaboutthefirst system.Consequently,Rovellirejectstheideathatthereissuchathingasthe completedescriptionofthetotalstateoftheworld.
ApplyingtherelationalinterpretationtocovariantLQG,RovelliandVidotto (2015)proposetounderstanditstransitionsbetweenspinnetworkstatesasphysicalprocessesenclosedbetweeninteractionsbetweensystemswhoseboundaries areultimatelyconventional.Itisthese processes whichareultimatelynothingbut spacetimeregions.Theresulting‘relationalloopquantumgravity’,Healeyargues, strugglestoaccommodatetheconceptofanobservercapableofregisteringthe outcomesofmeasurementinteractionsasitdealsexclusivelyinspacetimeregions (andtheirconventionalboundaries).Sinceobserversarenotmerespacetime regions,moreworkisrequiredtoshowhowthey(andthusvonNeumann measurements)canbemodelledinthecontextofrelationalLQG.
Asoneofthemoralstobedrawn,Healeyconcludesthatthecharacterizationof observingormeasuringsystemsneedstobeestablishedasemergentinLQG.In thissense,herequirestheemergenceofspacetimeforthequantummeasurement toberesolved.Ashenotes,thisisindisagreementwithWüthrich(2017),who arguesinsteadthattheemergenceofspacetimeinQGrequirestheresolutionof themeasurementproblem,nottheotherwayaround.
Thesecondchapterofthisseries,by KerryMcKenzie,beginswiththeobservationthatmanyphilosophersdreamofafinalphysicaltheoryofeverything thatwillanswersomeofthedeepestandmostintriguingmetaphysicalquestions. Forinstance,istimereallyflowingorisitjustaperceptualartefactoftheway weexperienceastaticfour-dimensionalworld?Isthereasortofmodalglue connectingeventsinasystematicwayandexplainingwhytheworldseemsto obeysomelawsofnature?Wedonothavesuchafinal,absolutelyfundamentaltheory,butthissituationdoesnotpreventmetaphysiciansfromengaging
inmetaphysicalactivitybyusingconstraintsfromempiricallywell-confirmed physics.
McKenziearguesthatthiskindofnaturalizedmetaphysicsisproblematic.Ifwe stepbackandlookattheoreticalphysicsasawhole,wecanseethattheempirically well-confirmedtheoriesonwhichitisbasedshouldideallybereplacedbyanother morefundamental,andperhapsdefinitive,theory.UntilwefindthisHolyGrail— assumingthatitexists—wearestuckinafarfromidealepistemologicalsituation. Indeed,itseemsdifficulttoreliablydrawmetaphysicalknowledgefromthecurrent stateofphysics.Whyshouldwetrustourcurrentlymostfundamentalphysical theoriestogiveusempiricalaccesstothefundamentalstructureoftheworld? Afterall,weknowthatthosetheoriesarenotabsolutelyfundamental.
Aftercharacterizingherpreferredsenseof‘naturalisticmetaphysics’,McKenzie showsthatclaimsinmetaphysicshadtochangetheirtruthvalueasscience,and especiallyphysics,progressed.Withregardtotheclassicdebatebetweenscientific realistsandanti-realists,shearguesthatfriendsofscientificrealismmayappeal tothenotionofapproximatetruthandarguethatcertainstructuresarepreserved bythechangeintheoryand,therefore,canbeseentojustifycertainclaimsabout theworld.McKenziegoesontoarguethatthisnotionofapproximatetruthisof littleuseindealingwithmoremetaphysicalissues.Indeed,metaphysicalquestions (say,regardingtheexistenceofpastandfutureentities)areoftenontological orsubstantivequestionsthataredifficulttoanswerintermsofapproximation. McKenzieconcludesthatthevalueofengaginginmetaphysicalspeculationbased onourcurrentlymostfundamentalphysicaltheoriesisunclear.
Webelievethatthissituationoffersapowerfulmotivationtoshiftourattention fromthewell-establishedtheoriesofphysicstoQG.Indeed,McKenzie’schallenge tonaturalizedmetaphysicsdoesnotnecessarilyobligeustowaitforthedevelopmentandempiricalconfirmationofafinaltheory.LeBihan(2020),forexample, arguesinresponsetoMcKenziethatwecanobtainsubstantialmetaphysical knowledgefromspeculativephysicsbyexaminingthefieldofQGasawholeand lookingforfeaturespresentinalloralmostallapproachestoQG.
Thefinalchapter,by AdamKoberinski,engageswithatopicattheinterface ofQGandcosmology.Thenotorious‘cosmologicalconstantproblem’concerns whatsomephysicistshavelabelledthe‘worstpredictioninphysicsofalltimes’: theoreticalexpectationsforthevalueofthecosmologicalconstant Λ basedon considerationsfromparticlephysicsovershootobservationallimitsanywhere from50to120ordersofmagnitude(dependingonachoiceof‘cutoff’).The problemarisesbecausequantumfieldtheoryseemstosuggestthatemptyspace hasanenormousenergydensity,which,accordingtoGR,oughttomanifestitself inthegeometryofspacetime,couplingtotheEinsteinequationasacosmological constantterm,inflatingthelattertogargantuanproportionsinconsistentwith observations.
