einstein

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GravityinRever:theTaleofAlbertEinstein's"GreatestBlunder"

NaturalHistory,12/01/03

byNeildeGrasTyson

restingonthebacksofturtles,matterand

,justwhenyou'd

gottenfamiliar,ifnotreallycomfortable,withtheideaofabigbang,alongcomes

riousanduniversalpressurepervadesallofspace

andactsagainstthecosmicgravitythathastriedtodragtheuniverbacktogetherever

fthat,"negativegravity"hasforcedtheexpansionofthe

univertoaccelerateexponentially,andcosmicgravityislosingthetug-of-war.

Fortheandsimilarlymind-warpingideasintwentieth-centuryphysics,justblame

AlbertEinstein.

Einsteinhardlyevertfootinthelaboratory;hedidn'ttestphenomenaoruelaborate

theoristwhoperfectedthe"thoughtexperiment,"inwhichyou

engagenaturethroughyourimagination,inventingasituationoramodelandthen

workingouttheconquencesofsomephysicalprinciple.

If--aswasthecaforEinstein--aphysicist'smodelisintendedtoreprenttheentire

univer,thenmanipulatingthemodelshouldbetantamounttomanipulatingtheuniver

ersandexperimentalistscanthengooutandlookforthephenomena

odelisflawed,orifthetheoristsmakeamistakeintheir

calculations,theobrverswilldetectamismatchbetweenthemodel'spredictionsand

'sthefirstcuetotryagain,eitherby

adjustingtheoldmodelorbycreatinganewone.

Oneofthemostpowerfulandfar-reachingtheoreticalmodelverdevidisEinstein's

theoryofgeneralrelativity,publishedin1916as"TheFoundationoftheGeneralTheory

ofRelativity"andrefinedin1917in"CosmologicalConsiderationsintheGeneral

TheoryofRelativity."Together,thepapersoutlinetherelevantmathematicaldetailsof

ewyears,

laboratoryscientistsdevievermorepreciexperimentstotestthetheory,onlyto

extendtheenvelopeofitsaccuracy.

Mostscientificmodelsareonlyhallbaked,andhavesomewiggleroomforthe

eliocentricuniver

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conceivedbythesixteenth-centuryastronomerNicolausCopernicus,forexample,

it-the-Sunpartwascorrect,butthe

theorbitllipticalmadethe

Copernicansystemmoreaccurate.

Yet,inthecaofEinstein'srelativity,thefoundingprinciplesoftheentiretheory

inhad,ineffect,builta

houofcards,withonlytwoorthreesimplepostulatesholdinguptheentirestructure.

(Indeed,onlearningofa1931booktitled100AuthorsAgainstEinstein,heresponded,

"Whyonehundred?IfIamincorrect,onewouldhavebeenenough.")

Thatunassailablestructure--thefactthatthetheoryisfullybaked--isthesourceofoneof

in's1917refinementofhis

equationsofgravityincludedanewterm--denotedbytheGreekletterlambda--inwhich

elambdarvedtooppo

gravitywithinEinstein'smodel,itcouldkeeptheuniverinbalance,resistinggravity's

in'suniverwas

indeedbalanced,but,astheRussianphysicistAlexsandrFriedmannshowed

mathematicallyin1922,itwasinaprecariousstate--likeaballatthetopofahill,ready

er,giving

somethinganamedoesnotmakeitreal,andEinsteinknewofnocounterpartinthe

physicalunivertothelambdainhiquations.

Einstein'sgeneraltheoryofrelativity--calledGRbyverballylazycognoscenti--radically

dofttlingfor

SirIsaacNewton'sviewofgravityas"actionatadistance"(aconclusionthat

discomfitedNewtonhimlf),GRregardsgravityastheresponofamasstothelocal

rwords,concentrations

ofmasscaudistortions--dimples,

distortionsguidethemovingmassalongstraight-linegeodesics,whichlooklikethe

chibald"Wheeler,aphysicistat

PrincetonUniversity,putitbestwhenhesummedupEinstein'sconceptthisway:

"Mattertellsspacehowtocurve;spacetellsmatterhowtomove."

Ineffect,GRaccountsfortwooppositephenomena:goodol'gravity,suchasthe

attractionbetweentheEarthandaballthrownintotheairorbetweentheSunandthe

Earth;andamysterious,repulsivepressureassociatedwiththevacuumofspace-time

againstgravity,lambdaprervedwhatEinsteinandeveryotherphysicist

ofhisdayhadstronglybelievedin:itwas,but

nvokeanunstableconditionasthenaturalstateofaphysical

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systemviolatesscientificcredo:youcannotasrtthattheentireuniverisaspecial

geveren,heard,or

,inspiteofbeingdeeply

uneasywithlambda,Einsteinincludeditinhiquations.

Twelveyearslater,in1929,discoveredthatthe

univerisnotstaticafterall:convincingevidenceshowedthatthemoredistantagalaxy,

rwords,theuniverisgrowing.

Embarrasdbylambda,andexasperatedbyhavingthusblownthechancetopredictthe

expandinguniverhimlf,Einsteindiscardedlambda,callingitsintroductionhislife's

"greatestblunder."

Thatwasn'ttheendofthestory,onoverthedecades,theoreticians

wouldexhumelambda--morecommonlyknownasthe"cosmologicalconstant"--from

,sixty-nineyearslater,in1998,science

exhumedlambdaonelasttime,hat

yeartwoteamsofastrophysicists--oneledbySaulPerlmutterofLawrenceBerkeley

NationalLaboratoryinBerkeley,California;theotherbyBrianSchmidtofMount

StromloandSidingSpringsObrvatoriesinCanberra,Australia--madethesame

ofthemostdistantsupernovaverobrved,they

said,appearednoticeablydimmerthanexpected--adisturbingfinding,giventhe

iliationrequiredthat

eitherthodistantsupernovasactedquitedifferentlyfromtheirnearerbrethren,orel

theywereasmuchas15percentfartherawaythantheprevailingcosmologicalmodels

hadplacedthem.

Notonlywasthecosmoxpanding,butarepulsivepressurewithinthevacuumofspace

inghadtobedrivingtheuniver

ythingthat"naturally"accountedforthe

accelerationwaslambda,ysicistsdusteditoffand

putitbackinEinstein'soriginalequationsforgeneralrelativity,thestateoftheuniver

matchedthestateofEinstein'quations.

Toanastrophysicist,thesupernovasudinPerlmutter'sandSchmidt'sstudiesareworth

arexplodesthesameway,ignitingasimilar

amountoffuel,releasingasimilarlytitanicamountofenergyinasimilarperiodoftime,

heexplodingstarsrveasa

kindofyardstick,or"standardcandle,"forcalculatingcosmicdistancestothegalaxiesin

whichtheyexplode,outtothefarthestreachesoftheuniver.

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Standardcandlessimplifycalculationsimmenly:sincethesupernovasallhavethe

samewattage,uring

theirbrightness(asimpletask),

theluminositiesofthesupernovaswerenotallthesame,brightnessalonewouldnotbe

onecouldbeahigh-wattagebulbfarawayoralow-wattagebulbcloup.

re'sacondwaytomeasurethedistancetogalaxies:theirspeedof

recessionfromourMilkyWay,arecessionthat'spartandparceloftheoverallcosmic

lewasthefirsttoshow,theexpansionoftheunivermakesdistant

uringagalaxy'sspeed

ofrecession(anotherstraightforwardtask),youcandeduceitsdistancefromEarth.

Ifthotwowell-testedmethodsgivedifferentdistancesforthesameobject,something

thesupernovasarebadstandardcandles,orourmodelfortherate

ofcosmicexpansionasmeasuredbygalaxyspeedsiswrong.

Well,edoutthatthesupernovasaresplendid

standardcandles,survivingthecarefulscrutinyofmanyskepticalinvestigators.

Astrophysicistswereleftwithauniverthatixpandingfasterthantheyhadever

tgalaxiesturnedouttobeevenfartherawaythantheirrecession

rewasnoeasywaytoexplaintheextraexpansion

withoutinvokinglambda,thecosmologicalconstant.

Here,then,wasthefirstdirectevidencethatarepulsivepressurepermeatedtheuniver,

'

cosmologistscouldestimateitsnumericalvalue,becautheycouldcalculatetheeffectit

washaving:thedifferencebetweenwhattheyhadexpectedtheexpansiontobeandwhat

itactuallywas.

Thatvalueforlambdasuddenlysignifiedaphysicalreality,whichnowneededaname.

"Darkenergy"carriedtheday,t

accuratemeasurementsdonetodatehaveshowndarkenergytobethemostprominent

thingintown.

Theshapeofourfour-dimensionalunivercomesfromtherelationbetweentheamount

ofmatterandenergythatinhabitsthecosmosandtherateatwhichthecosmosis

nientmathematicalmeasureofthatshapeisusuallywrittenasthe

uppercaGreekletteromega([OMEGA]).Ifyoutakethematter-energydensityofthe

univer,anddivideitbythematter-energydensityrequiredtojustbarelyhaltthe

expansion(knownasthe"critical"density),yougetomega.

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Becaubothmassandenergycauspace-timetowarp,orcurve,omegaeffectively

aislessthanone,theactualmass-energyfalls

belowthecriticalvalue,andtheuniverexpandsforeverineverydirectionforallof

ca,theshapeoftheuniverisanalogoustotheshapeofasaddle,in

aiqualtoone,theuniverexpands

forever,butonlybarelyso;inthatcatheshapeisfiat,prervingallthegeometric

aexceedsone,parallel

linesconverge,andtheunivercurvesbackonitlf,ultimatelyrecollapsingintothe

fireballwhenceitcame.

AtnotimesinceHubblediscoveredtheexpandinguniverhasanyteamofobrvers

upallthemassand

energytheycouldmeasure,darkmatterincluded,thebiggestvaluesfromthebest

hat'slessthanone,asfarasobrverswere

concerned,theuniverwas"open"forthebusinessofexpansion,ridingaone-way

saddleintothefuture.

Meanwhile,beginningin1979,,aphysicistatMIT,andothersadvancedan

f,Guth

mental

ard

one.

Scarcelyatheoristintheworldhadaproblemwiththatrequirement,becauithelped

as,

however,anotherlittleproblem:theupdatepredictedthreetimesasmuchmassand

rred,thetheoristssaidtheobrversjustweren't

lookinghardenough.

Attheendofthetallies,visiblematteralonecouldaccountforverylittleofthecritical

utthemysteriousdarkmatter?Nobodyknowswhatdarkmatteris,but

dedthat

,erverswereataloss."Guys,"

theyprotested,"there'snothingeloutthere."Andthetheoristsanswered,"Justkeep

looking."

Bothcampsweresuretheothercampwaswrong--untilthediscoveryofdarkenergy.

Thatsinglecomponentraidthemass-energydensityoftheunivertothecriticallevel.

Yes,ifyoudothemath,theuniverholdsthreetimesasmuchdarkenergyasanything

el.

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Askepticallot,thecommunityofastrophysicistsdecidedtheywouldfeelbetteraboutthe

kinsonMicrowaveAnisotropy

Probe(WMAP)SAsatellite,

launchedin2001,wasthelatestandbestefforttomeasureandmapthecosmic

microwavebackground,thebigbang'sblueprintfortheamountanddistributionofmatter

hysicistscannowsaywithconfidencethatomegais

indeedequaltoone:thematter-energydensityoftheuniverweknowandloveiqual

ulation?Thecosmosholds73percentdarkenergy,23

percentdarkmatter,andameasly4percentordinarymatter,thestuffyouandIaremade

of.

Forthefirsttimeever,,intheirown

way,sone,justasthetheoristsdemandedoftheuniver,even

thoughyoucan'tgettherebyaddingupallthematter--darkorotherwi--astheyhad

'snomorematterrunningaroundthecosmostodaythanhad

hadforeenthedominatingprenceof

cosmicdarkenergy,norhadanybodyimagineditasthegreatreconcilerofdifferences.

Sowhatisthisstuff?Aswithdarkmatter,stanybodyhascome

toareasonableguessistopresumethatdarkenergyisaquantumeffect--wherebythe

vacuumofspace,insteadofbeingempty,actuallyetheswithparticlesandtheir

pinandoutofexistenceinpairs,anddon'tlastlong

ransientexistenceiscapturedintheirmoniker:virtual

particles.

Buttheremarkablelegacyofquantummechanics--thephysicsofthesmall--demandsthat

irofvirtualparticlexertsalittlebitof

unately,whenyou

estimatetheamountofrepulsive"vacuumpressure"thatarisfromtheabbreviatedlives

ofvirtualparticles,theresultismorethan[.120]timesbiggerthanthevalueofthe

y

bethemostembarrassingcalculationevermade,thebiggestmismatchbetweentheory

andobrvationinthehistoryofscience.

I'dsayastrophysicistsremainclueless--butit'ergyisnot

adrift,bitsoneofthesafesthomeswecanimagine:

Einstein''erdarkenergyturnsoutto

be,wealreadyknowhowtomeasureitandhowtocalculateitffectsonthecosmos.

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Withoutadoubt,Einstein'sgreatestblunderwashavingdeclaredthatlambdawashis

greatestblunder.

Aremarkablefeatureoflambdaandtheacceleratinguniveristhattherepulsiveforce

arisfromwithinthevacuum,acuumgrows,

lambda'while,thedensity

eaterrepulsivepressurecomes

morevacuum,drivingitxponentialgrowth--theendlessaccelerationofthecosmic

expansion.

Asaconquence,anythingnotgravitationallyboundtotheneighborhoodoftheMilky

Waywillmoveawayfromusatever-increasingspeed,embeddedwithintheexpanding

esnowvisiblewilldisappearbeyondanunreachablehorizon.

Inatrillionorsoyears,anyonealiveinourowngalaxymayknownothingofother

--orouralienMilkyWaybrethren's--obrvableuniverwillmerely

thestarrynightwilllieanendlessvoid,

withoutform:"darknessuponthefaceofthedeep."

Darkenergy,afundamentalpropertyofthecosmos,will,intheend,underminethe

contemporary

astrophysicistsacrossthegalaxykeepremarkablerecords,orburyanawesometime

capsule,futureastrophysicistswillknownothingofexternalgalaxies--theprincipalform

ergywilldenythemaccesstoentire

chaptersfromthebookoftheuniver.

Here,then,ismyrecurringnightmare:Arewe,too,missingsomebasicpiecesofthe

univerthatoncewas?Whatpartofourcosmicsagahasbeenerad?Whatremains

abntfromouttheoriesandequationsthatoughttobethere,leavingusgropingfor

answerswemayneverfind?

rectoroftheHayden

PlanetariuminNewYorkCity.