Breakthrough of the Year 2017英文文字文檔

BreakthroughoftheYear2017

Cosmicconvergence

Themergeroftwoneutronstarscaptivatedthousandsofobserversandfulfilledmultipleastrophysicalpredictions

On17August,scientistsaroundtheworldwitnessedsomethingneverseenbefore:Onehundredandthirtymillionlight-yearsaway,twoneutronstarsspiraledintoeachotherinaspectacularexplosionthatwasstudiedbyobservatoriesrangingfromgammaraydetectorstoradiotelescopes.Theblastconfirmedseveralkeyastrophysicalmodels,revealedabirthplaceofmanyheavyelements,andtestedthegeneraltheoryofrelativityasneverbefore.Thatfirstobservationofaneutron-starmerger,andthescientificbountyitrevealed,isScience’s2021BreakthroughoftheYear.

Especiallyremarkablewasthewaytheeventwasspotted:bydetectingtheinfinitesimalripplesinspaceitself,calledgravitationalwaves,thatthespiralingneutronstarsradiatedbeforetheymerged.Scientistsfirstdetectedsuchwavesjust27monthsago,whentheLaserInterferometerGravitational-WaveObservatory(LIGO)sensedaspacetremorfromtwomassiveblackholesspiralingtogetherinaninvisiblecataclysm.ThediscoveryofgravitationalwaveswasScience’s2021BreakthroughoftheYear.

Ifthatobservationsoundedtheclarionofdiscovery,thisyear’sproducedascientificsymphony.Thedifferencecomesdowntomatter.Ablackhole,theghostlygravitationalfieldthatremainswhenahugestarcollapsestoapoint,containsnomattertoheatupandradiate.Aneutronstar,incontrast,isaballofnearlypureneutrons,thedenseststuffthereis.Whereasthecollidingblackholesemittednothingbutgravitationalenergy,theneutron-starsmashupputonalightshowthatwasstudiedbymorethan70observatories.“Theamountofinformationwehavebeenabletoextractwithoneeventblowsmymind,〞saysLauraCadonati,aphysicistattheGeorgiaInstituteofTechnologyinAtlantaanddeputyspokespersonfortheLIGOteam.

ThegravitationalwavesfromthetwirlingneutronstarsticklednotonlytheenormousLIGOdetectorsinHanford,Washington,andLivingston,Louisiana,butalsotheFrench-ItalianVirgodetectornearPisa,Italy,which,aftera5-yearupgrade,hadstartedrecordingdatajust17daysearlier.Researchersimmediatelyknewtheywerewitnessingthedeathspiraloftwoneutronstars.Unlikeblack-holemergers,whichproducesecondslongpulsesoflow-frequencygravitationalwaves,thelighterneutronstarsproducedatelltalehigherfrequencyhumthatincreasedinfrequencyandstrengthover100seconds.

Thatcrescendocuedthefireworks.Twosecondslater,NASA’sorbitingFermiGamma-raySpaceTelescopedetectedapulseofgammarayscalledashortgammarayburst.Then,othertelescopestookaim.Becausethegravitationalwaveswerespottedbythreewidelyspaceddetectors,researcherscouldtriangulatetheneutronstarpair’slocationinthesky.Within11hours,severalteamsofopticalandinfraredastronomershadfoundanewbeaconontheedgeofthegalaxyNGC4993.Overseveraldays,thesourcefadedfrombrightbluetodimmerred.Then,after11days,itbegantoglowinx-raysandradiowaves.Theexplosionwaseasilythemoststudiedeventinthehistoryofastronomy,with3674researchersfrom953institutionscollaboratingonasinglepapersummarizingthemergeranditsaftermath.

Theobservationsbolsteredthe25-year-oldhypothesisthatneutron-starmergersproduceshortgammaraybursts.Andthereddishafterglowfitthemodelofaso-calledkilonova,inwhichneutron-richmatterflungintospacebycollidingneutronstarshostsachainofnuclearinteractionsknownasther-process.Theprocessisthoughttoproducehalftheelementsheavierthaniron,andtheheaviestoneswouldsoakupbluelight,tintingtheglowingradioactivecloudred.“It’sbeensuperexcitingtoseesomethingthatwasjustanideacometolife,〞saysDanielKasenoftheUniversityofCalifornia,Berkeley,whohasmodeledkilonovas.“Allthisstuffwasdonebasicallywitheyes-closedtheory.〞TheobservationevenbolsteredAlbertEinstein’sgeneraltheoryofrelativitybyconfirmingthatgravitationalwavestravelatthesamespeedaslightandnotmoreslowly,assomealternativetheorieshadpredicted.

Butthemergeralsoposespuzzlesthathavewhettedastrophysicists’appetitesformoredata.Forexample,thegammarayburstwassurprisinglyfeeble,saysVickyKalogera,anastrophysicistandLIGOteammemberatNorthwesternUniversityinEvanston,Illinois.Suchburstsarethoughttooriginatewhennarrowjetsofmaterialshootoutofaneutron-starmergeratnear–light-speed,likesearchbeams.ThesimplestexplanationisthatthejetmaynothavepointedstraightatEarth.However,it’spossiblethatastrophysicists’modelisn’tquiterightandthatneutron-starmergersproduceonlymutedgammaraybursts,Kalogerasays.Toresolvetheissue,astrophysicistsneedtoseemoremergers.

Theywouldalsoliketoseethegravitationalwavesrightuptothepointatwhichtheneutronstarsspiralintoeachother.Inthisfirstobservation,theLIGOandVirgodetectorstrackedthestarswhirlingaroundeachotheratanacceleratingpace,sendingouthigherandhigherfrequencygravitationalwaves.Butatabout500cyclespersecond,thewaves’frequencyclimbedoutofLIGO’ssensitivityrange,andthedetectorscouldn’tobservethefinalfewrevolutionsleadinguptothemerger.

Thosefinalrevolutionscouldprovideinsightsintothenatureofneutronstars,orbsofpurenuclearmatterslightlymoremassivethanthesunbutjust20to30kilometerswide.Astrophysicistswanttoknowhowstifforsquishyneutronstarmatteris—apropertyencapsulatedintheso-calledequationofstate.Inprinciple,thegravitationalwavescanrevealthatinformation:Thestifferthematteris,thelargertheneutronstarswillbe,andtheearliertheywillteareachotherapartastheyspiraltogether,alteringthesignal.“Ifwewanttodeterminetheequationofstate,weneedtoseethewholeevent,〞saysJamesLattimer,anuclearastrophysicistattheStateUniversityofNewYorkinStonyBrook.ResearchersplantoincreaseLIGO’ssensitivityathighfrequencies—forinstance,bymanipulatingthelaserlightcirculatinginthemassivedetectors—butdoingsomaytakeafewyears.

Scientistsalsohopetoseenewtypesofevents,suchasmergersofaneutronstarandablackhole,whichtheorysuggestsarerare.SupernovaexplosionsofindividualstarsinourMilkyWaygalaxyshouldalsoproducedetectablegravitationalwaves,whichcouldhelpastrophysicistsfigureoutexactlyhowthestarsblowup.Spinni