Astronomi

Hvorfra kommer energien til tyngdekraftsbølger fra?

Hvorfra kommer energien til tyngdekraftsbølger fra?


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Så vidt jeg forstår, blev de hændelser, der blev opdaget af LIGO, omdannet til ca. 4% af den samlede masse af fusionerende binære sorte huller til tyngdekraftsbølger.

Hvor kommer denne energi fra, dvs. hvad omdannes nøjagtigt til tyngdekraftsbølger?

Er det simpelthen de sammenføjende objekters kinetiske energi (disse objekters hastighed inden fusion er enorme, op til 60% af c, hvis jeg husker korrekt), så betyder det at udsende tyngdekraftsbølger får dem til at kredser langsommere, men bevarer deres oprindelige masser? Eller mister de kompakte genstande virkelig "ægte" masse, hvilket betyder at de bliver lettere, og i tilfælde af BH'er ændres deres radius i overensstemmelse hermed?

Lad os som et eksempel antage to BH'er, begge med 50 solmasser, der kredser hinanden langt nok (sig 1 lysår), så GW'er eller kinetisk energi ikke har nogen betydning for disse indledende massemålinger. Under fusionen skal de udstråle omkring 5 solmasser i GW'er. Ville det resulterende sorte hul have en masse på 95 eller 100 solmasser?


Udstråling af tyngdekraftsbølger gør en ubehagelig binær bane tættere og hurtigere. (Rob Jefferies)

Kilden til energien for både øget kinetisk energi og tyngdekraftsstrålingen er den samme: tyngdepotentialenergi. (PM 2Ring)

To sorte huller i en afstand af 1 lysår har en enorm mængde potentiel energi, ca. 10 ^ 48 Joule potentiel energi. Når de spiralformes, udstråles en betydelig mængde af denne energi som tyngdekraftsbølger

Dette er virkelig tabt masse. Massen af ​​det resulterende sorte hul er mindre end summen af ​​de to sammensmeltede sorte huller, men på intet tidspunkt bliver noget sort hul i sig selv mindre.


Som Rob med rette påpegede, reducerer emissionen af ​​tyngdekraftsbølger orbitalenergien og resulterer i en inspiration. Denne reduktion i total energi reducerer også massen af ​​den endelige BH, da $ E = mc ^ 2 $. Hovedparten af ​​gravitationsbølgeenergien udsendes (og energi = masse tabt) i den sidste kvidring, når adskillelsen nærmer sig Schwarzschild-radiusen.

For at kvantificere dette, lad os bare lave en simpel beregning af energibudget, startende fra to BH'er med samme masse af masse $ M_ bullet $, der kredser om hinanden i afstand $ d $ på en cirkulær bane. Derefter er kredsløbsenergien $$ E _ { mathrm {orbit}} = - frac {GM ^ 2_ bullet} {2d} = -M_ bullet c ^ 2 frac {R_s} {4d} $$ hvor $ R_s = 2GM / c ^ 2 $ Schwarzschild-radius for hver BH, og vi har antaget, at $ d gg R_s $ således, at kredsløbet er Keplerian. Den samlede startenergi gives derefter af resten af ​​massenergier plus orbitalenergien som $$ E _ { mathrm {total}} = M_ bullet c ^ 2 left [2- frac {R_s} {4d} right] . $$ Efter sammensmeltning fremkommer en rest af massen $ M_ {r} $. Energiunderskuddet er forskellen mellem den indledende og den endelige energi begynder {ligning} delta E = M_ kugle c ^ 2 venstre [2- frac {R_s} {4d} højre] - frac {M_rc ^ 2 } { sqrt {1-v ^ 2 / c ^ 2}}, end {ligning} hvor $ v $ er hastigheden på restværdien til centrum for masseforældrene. Denne energi er gået tabt ved gravitationsbølgestråling. Hvis dette svarer til et bestemt beløb $ mu $ hvilemasse, så fra $ delta E = mu c ^ 2 $ finder vi $$ M_r = sqrt {1-v ^ 2 / c ^ 2} left [ 2M_ bullet - mu - M_ bullet frac {R_s} {4d} right]. $$ Nu for $ v = 0 $ og $ R_s ll d $ er masseunderskuddet $ delta m equiv 2M_ bullet-M_r $ identisk med $ mu $: den udstrålede energi svarer til masseunderskuddet; det sidste hul har 95 $ M_ odot $ hvis $ M_ bullet = 50M_ odot $ og $ mu = 5M_ odot $. Især gravitationsbølgeenergien kan ikke kun tages fra orbitalenergien som antydet af et andet svar.

Masseunderskuddet er endnu større end den udstrålede energi, hvis resten har gennemgået et betydeligt hastighedsspark, således at $ v neq0 $ (forårsaget af asymmetrisk tyngdekraftsbølgestråling).


Hvorfra kommer energien til tyngdekraftsbølger fra? - Astronomi

På sproget i Albert Einsteins generelle relativitetsteori er gravitationsstråling eller tyngdebølger (GW'er) `` krusninger i geometrien af ​​rum og tid. '' En mindre abstrus måde at beskrive gravitationsstråling på er ved at trække en analogi til det elektromagnetiske spektrum. (lys, infrarød, radio, mikroovn, røntgen osv.). Ligesom disse repræsenterer former for fri stråling eller bølger forbundet med elektricitet og magnetisme, så repræsenterer GW stråling forbundet med tyngdekraften. Einstein forudsagde faktisk deres eksistens i 1916, samme år som hans artikel om generel relativitetsteori blev offentliggjort. Han beregnede endda strålingen udsendt fra et binært stjernesystem (den stærkeste kilde, der var kendt på det tidspunkt) og konkluderede, at strålingen var så svag, at den havde `` en ubetydelig praktisk effekt. '' I det næste halve århundrede forblev gravitationsstråling en teoretisk teori. nysgerrighed, der ikke havde nogen praktisk astrofysisk betydning. I de sidste to årtier har astrofysikere opdaget flere nye potentielle kilder og er kommet til at tro, at det ikke kun kan være muligt at opdage tyngdekraftsbølger direkte, men også at deres emission endda kan være den dominerende proces i udviklingen af ​​nogle astrofysiske objekter.

GENERATION OG DETEKTION

Elektromagnetisme og tyngdekraft er de eneste to grundlæggende, langvarige kræfter i naturen. Ligesom accelererede elektriske ladninger genererer elektromagnetisk stråling, genererer accelererede `` tyngdekraftsladninger '', dvs. masser, gravitationsstråling. Simpelthen analogt med elektromagnetisme er det ikke overraskende, at tyngdebølger forudsiges af generel relativitet og enhver anden levedygtig teori om tyngdekraften. Desuden, hvis tyngdekraften skal overholde lovene i Einsteins specielle relativitetsteori, skal tyngdekraftsstråling bevæge sig med lysets hastighed.

Fordi alle elektriske ladninger har masse, kan man forvente, at tyngdekraftstråling er lige så rigelig som elektromagnetisk stråling, men dette er ikke tilfældet. Overvej systemet angivet i fig. 1, der viser to partikler med samme masse M og modsat elektrisk ladning & # 177Q oscillerende i modsatte ender af en fjeder med en længde L. Forholdet mellem magt PG udsendt i tyngdekraftstråling til magten PEM udsendes i elektromagnetisk stråling af dette system

PG / PEM = (GM 2 / Q 2 )(L / ) 2

hvor er bølgelængden af ​​strålingen. Hvis partiklerne er elektroner (som er ansvarlige for det meste af den elektromagnetiske stråling, vi observerer), er den første faktor alene 10-43, hvilket illustrerer den utrolige svaghed ved tyngdekraften. Det andet udtryk i den foregående ligning er proportionalt med kvadratet af forholdet mellem massernes hastighed og lysets hastighed og er altid mindre end 1. Fra dette eksempel er det klart, at store, hurtigt bevægelige masser er de bedste kilder til GW'er.

Elektromagnetisk stråling detekteres af en lang række instrumenter, som alle fungerer på samme princip: elektromagnetiske bølger udøver kraft på elektriske ladninger. Ligeledes kan tyngdekraftsstråling detekteres af den kraft, den udøver på masserne. Hvis en GW er hændende på fjedersystemet i fig. 1, vil masserne blive drevet i svingning. Svingningernes amplitude er imidlertid meget lille igen på grund af tyngdekraftens svaghed. Faktisk er forholdet mellem den energi, der absorberes fra en tyngdekraftsbølge og den energi, der absorberes fra en elektromagnetisk bølge med samme styrke, givet nøjagtigt det samme forhold som i ligningen.

Som det er karakteristisk for elektromagnetiske bølger, har GW'er to mulige polarisationer og udøver kun kraft på stof i retninger vinkelret på bølgens forplantningsretning. Energien i bølgen falder omvendt med kvadratet for afstanden fra kilden. For en polariseret GW er kraftlinjerne i planet vinkelret på udbredelsesretningen illustreret i fig. 2. De resulterende accelerationer af fire testmasser, A, B, Cog D, er angivet med pile. En halv cyklus senere vendes retningerne for kraftlinjerne og accelerationerne. En gengivelse af den anden mulige polarisering opnås ved at dreje figuren med 45 & # 176. Den primitive gravitationsbølgedetektor i fig. 1 opnås ved simpelthen at forbinde masser EN og B (eller C og D) ved en fjeder.

KILDER

I betragtning af den iboende svaghed ved tyngdekraften er laboratoriekilder til tyngdekraftstråling ikke eksisterende. For eksempel spindes en 1-ton stålstang så hurtigt, at den er ved at blive revet fra hinanden med centrifugalkraft, udstråler mindre end 10-30 W. (Dette problem blev overvejet af Einstein i 1918.) Derimod er eksisterende detektorer kun følsom nok til at opdage en sådan kilde (i en afstand af en bølgelængde), hvis den udsender mere end 106 W. Nuværende håb om direkte detektering af GW'er er fastgjort på astrofysiske kilder, hvor massive kroppe gennemgår enorme accelerationer. Kortvarige binære stjernesystemer udsender kraftigt (10 25 -10 29 W) ved frekvenser fra 10-4 -10 -3 mHz, men der er i øjeblikket ingen detektorer, der er i stand til at detektere sådanne kilder, selv i nærheden. Selvom det ikke kan påvises direkte, er energitabet fra den kortvarige binære pulsar PSR 1913 + 16 på grund af tyngdekraftsstråling målt ved nøjagtige tidsobservationer af pulsarbanen. Disse målinger stemmer overens med forudsigelserne af generel relativitet inden for 1%, et resultat, der er en vigtig bekræftelse af eksistensen af ​​GW'er. En meget stærkere kilde er tyngdekollaps til et sort hul, hvorunder en stor del af massen af ​​en hel stjerne kan accelereres til hastigheder, der nærmer sig lysets hastighed. Det forventes, at så meget som 10 49 W GW vil blive udsendt fra en sådan kilde i form af en puls med varighed 0,001 s. Det er også blevet formodet at massive sorte huller (10 5 -10 9 M ) er placeret i kernerne i mange galakser og kvasarer. Under deres dannelse kunne disse objekter udsende så meget som 10 54 J energi i form af GW'er ved frekvenser mellem 10-1 og 10-5 Hz. Flere flere eksotiske kilder til gravitationsstråling med lav frekvens er blevet foreslået, for eksempel kvantegravitationssvingninger i det tidlige univers, en kvantekromodynamisk faseovergang fra frie kvarker til nukleoner og oscillerende kosmiske strenge. Perioden af ​​GW'er fra disse kilder spænder fra dage til år og er uden for området for laboratoriedetektorer.

DETEKTORER

De første forsøg på at opdage gravitationsbølger blev foretaget i 1960'erne af Joseph Weber, der akustisk ophængte en massiv (1400 kg) aluminiumscylinder og overvågede niveauet af excitation af dens laveste vibrationsfunktion. Resonant ved 1660 Hz blev denne detektor designet til at være følsom over for udbruddet af tyngdekraftstråling forudsagt at ledsage dannelsen af ​​et sort hul i solmasse. Hans påstand om påvisning af GW'er fra 1969 blev ikke bekræftet, og det er nu enighed om mening inden for det videnskabelige samfund, at GW'er endnu ikke er blevet observeret direkte. Flere kryogenkølede cylindre er nu i drift med en følsomhed, der er tilstrækkelig til at detektere de GW'er, der udsendes fra tyngdekollaps, der forekommer hvor som helst i Mælkevejsgalaksen. Ingen er blevet opdaget. Laserinterferometre, der registrerer den relative forskydning af to spejle (adskilt af hundreder af meter) af et Michelson-interferometer, er potentielt mere følsomme detektorer, der reagerer på et bredere frekvensområde. Det er håbet, at flere af disse instrumenter vil være i drift i 1990'erne med en følsomhed, der er tilstrækkelig til at opdage tyngdekollaps i galakser uden for vores egne. Ved lavere frekvenser (10-3 Hz) er radarstrækning af rumfartøjer og overvågning af faste jordvibrationer blevet anvendt, men disse metoder er endnu ikke følsomme nok til at opdage sådanne begivenheder som dannelsen af ​​massive sorte huller i fjerne galakser. Virkningen af ​​GW'er på ankomsttiderne for pulsarer er blevet brugt til at placere øvre grænser for en baggrund af GW'er med perioder på få år. Disse grænser har allerede givet vigtige begrænsninger for nogle modeller af kosmiske strenge.

GRAVITATIONAL BØLGESTRONOMI

Observationer af den binære pulsar PSR 1913 + 16 har fastslået eksistensen af ​​tyngdekraftstråling, men som en astrofysisk kilde til GW'er er dette system det mindst interessante objekt, man kan forestille sig, det vil sige topunktspartikler i kredsløb omkring hinanden. Den vigtigste motivation for forskere i marken er ikke blot at observere GW'er direkte og derved bekræfte deres eksistens, men snarere at være i stand til at bruge dem til at undersøge dybt ind i regionerne med stærke tyngdefelter og tæt stof, der kan blokere for andre former for stråling. Kun når GW'er detekteres fra et eller flere af de spektakulære astrofysiske fænomener nævnt ovenfor, betragtes tyngdekraftsbølge-astronomi som en legitim gren af ​​astrofysik.


Gravitationsbølger: Teoretisk fortolkning

Generel relativitetsteori siger, at tyngdekraften udtrykkes som en rum-tid-krumning, og den forudsiger eksistensen af ​​tyngdekraftsbølger.

Gravitationsbølger formerer sig gravitationsfelter produceret af bevægelse af massive genstande. De kaldes ofte krusninger af rum-tid krumning. Gravitationsfelter produceret af massive partikler styrer bevægelsen af ​​stof eller lys i rumtid på en måde svarende til, hvordan elektriske felter produceret af ladede partikler styrer, hvordan andre ladede partikler bevæger sig. Det er vigtigt at opdage tyngdekraftsbølger, fordi de ville bringe nye oplysninger om fjerne galakser, som elektromagnetiske bølger ikke kan. Det kunne også direkte bevise generel relativitet.

Hvordan stof udsender tyngdebølger

Ifølge generel relativitet udsender de, når nogen genstande med masse accelererer, tyngdebølger. Dette er analogt med de elektromagnetiske bølger, der udsendes af accelererende ladede partikler.

Overvej f.eks. En gravitationsbølge, der formerer sig i z-retning i kartesiske koordinater. Bølgen deformerer kun rumtid i retningen vinkelret på dens udbredelsesretning. Deformation af rumtid betyder krympning og strækning af den fysiske længde mellem to punkter. Gravitationsbølger påvirker rumtid på en sådan måde, at visse deformationsmønstre opstår med jævne mellemrum.

Hvis rummet på tidspunktet t strækkes i x-retningen ved den maksimale amplitude, vil rummet på tidspunktet t + T / 2 (hvor T er perioden), halvdelen af ​​perioden senere, være krympet af den maksimale amplitude og derefter strækker sig igen til sit maksimale ved tid t + T, en hel periode senere. Teorien siger også, at i y-retningen deformeres rummet i den modsatte fase af x-retningen. Således, hvis rummet strækkes ud i x-retningen, krympes rummet i y-retningen.

Strækker sig og krymper

Den relative længdeskift på to punkter som følge af tyngdekraftsbølgen udtrykkes som

Kilder til detekterbar tyngdekraftsbølge

Binært system

Tyngdekraftsbølgenes størrelse afhænger af afstanden fra kilden og det andet derivat af massefordelingen (massetider acceleration i tilfælde af en partikel). Man skal således overveje meget massive genstande, der bevæger sig voldsomt som kandidaterne til kilderne til detekterbare tyngdekraftsbølger. De mest lovende kandidater er kompakte binære filer bestående af enten to neutronstjerner, to sorte huller eller en neutronstjerne og et sort hul. De er små og tunge, hvilket gør det muligt for dem at kredse tættere på afstanden og ved en høj orbitalfrekvens, hvilket betyder, at det andet derivat af systemets massefordeling er stort. Derfor udsender systemet stærke tyngdekraftsbølger. Kompakte binære filer har et andet interessant aspekt forbundet med gravitationsstråling (forklaret i et senere afsnit).

Roterende neutronstjerner

Hvis ikke-aksesymmetrisk langs rotationsaksen udsender en roterende neutronstjer tyngdebølger. Hvis det er symmetrisk, holder det andet derivat af massefordelingen konstant på nul i systemet, hvilket ikke fører til nogen tyngdekraftsbølgeemission.

Supernovaer

Supernovaer er en god tyngdekraftkilde. De er kompakte og har store accelerationer. Svarende til roterende neutronstjerner, hvis en supernovas eksplosion har aksial symmetri, vil der ikke blive udsendt tyngdebølger til den konstante massefordeling. Indledende tæthed og temperaturudsving og andre faktorer kan dirigere asymmetrisk kollaps. Hvis en supernova, der er usædvanlig lys, observeres i tyngdekraftsbølgen, vil vi være i stand til at teste en forudsigelse af generel relativitet, der siger, at tyngdekraftens hastighed er den samme hastighed som lys.

Stokastisk baggrund

Dette kommer fra densitetsudsving i universets tidlige fase. Måling af baggrunden vil fortælle os om karakteren af ​​planke-størrelse universet og give spor til at teste de forskellige kosmologiske modeller. Selvom stokastisk baggrund er interessant, er den så svag, at moderne teknologi langt fra er i stand til at nå denne opgave.

Forholdet mellem kildens bevægelse og perioden og amplituden af ​​den deraf følgende tyngdekraftsbølge

Størrelsen af ​​tyngdekraftsbølger er proportional med det andet afledte af massefordelingen i det emitterende system og omvendt proportionalt med adskillelsen af ​​kilden og observationspunktet. Det næste spørgsmål, der opstår, er, hvordan perioden for en tyngdekraftsbølge er relateret til kildens bevægelse. Hvis binærerne er i en cirkulær bane, har de resulterende tyngdekraftsbølger en frekvens, der er dobbelt så stor som det binære system - dvs. tyngdekraftsbølgens periode er halvdelen af ​​kredsløbets periode.

Størrelsen ændres med retning. Imidlertid udtrykkes et vinkelmæssigt estimat af signalstyrken i den følgende formel.

For at se, hvorfor dette er proportionalt med det andet derivat af systemets massefordeling, skal du huske, at derivatet er proportionalt med og omvendt proportionalt med.

Dette fører til ovenstående ligning. Derfor, hvis frekvensen af ​​kredsløbsperioden øges, vil den resulterende tyngdekraftsbølge også øge dens frekvens og amplitude. Og dette opnås ved selv-energitab af tyngdekraftsstråling.

Gravitationsstråling og tab af selvenergi af binært system

Ifølge generel relativitet tager tyngdekraftsstråling energi fra kilden. Energitabet resulterer i mindre adskillelse af binærerne og derfor kortere orbitalfrekvens. Dette kommer af det faktum, at hastigheden for ændring i et binært systems omløbstid er negativt proportional med hastigheden for ændring i tyngdepotentialenergien i det binære system:

Når systemet udstråler gravitationsenergi, reducerer det sin omløbstid. Derfor har den resulterende gravitationsbølge en bølgeform med stigende frekvens og amplitude. Desuden er strålingsenergien proportional med kvadratet af amplituden af ​​den tilknyttede tyngdebølge. Dette betyder, at strålingseffekten øges hurtigere og hurtigere, da systemet mister sin energi på grund af tyngdekraftsstråling, hvilket resulterer i en større ændringshastighed i kredsløbets periode. Faktisk har den sidste fase af det binære system bestående af to neutronstjerner en så stor strålingsenergi, at neutronstjernerne mister al deres potentielle energi og til sidst kolliderer. Bølgeformen som følge af kollisionen vil være meget ru. Når man ser på bølgeformen, kan vi også få oplysninger om mekanismen for astronomiske begivenheder.

Kollision mellem to neutronstjerner

Binær neutronstjernefusion, beregnet af Maximilan Ruffert

Black Hole - Neutronstjerne binære fusioner

Hvorfor er det vigtigt at måle tyngdekraftsbølgen?

Gravitationsbølger er mest unikke, fordi de formerer sig uden at interagere med stof. Dette giver os mulighed for at få ny information om universet, som elektromagnetiske bølger ikke leverer. Amplituden og frekvensen af ​​gravitationsbølger beskriver frekvensen og massen af ​​den udsendende kilde. Formen på den sidste fase af et binært system kan give noget nyt indblik i astronomi. Stokastisk baggrund ville afsløre massefordelingen af ​​det tidlige univers på planke-skalaen og udviklingen af ​​det tidlige univers.


Gravitationsbølger

Gravitationsbølger stammer stort set på samme måde som elektromagnetiske bølger gør. Hvis du fremskynder en elektrisk ladning, som en elektron, producerer den en elektromagnetisk bølge. Energien til denne bølge kommer fra elektronens kinetiske energi.

På samme måde, hvis du fremskynder en masse, udsender den en tyngdekraftsbølge, og energien til den bølge kommer fra massernes kinetiske energi.
Da ovennævnte elektron også har en masse, udsender den faktisk tyngdekraftsbølger såvel som elektromagnetiske. Gravitationsbølger er dog meget meget svage, og de elektromagnetiske bølger tegner sig for langt størstedelen af ​​den mistede kinetiske energi.

Gravitationsbølger stammer stort set på samme måde som elektromagnetiske bølger gør. Hvis du fremskynder en elektrisk ladning, som en elektron, producerer den en elektromagnetisk bølge. Energien til denne bølge kommer fra elektronens kinetiske energi.

På samme måde, hvis du fremskynder en masse, udsender den en tyngdekraftsbølge, og energien til den bølge kommer fra massernes kinetiske energi.
Da ovennævnte elektron også har en masse, udsender den faktisk tyngdekraftsbølger såvel som elektromagnetiske. Gravitationsbølger er dog meget meget svage, og de elektromagnetiske bølger tegner sig for langt størstedelen af ​​den mistede kinetiske energi.


Ep. 71: Gravitationsbølger

Da han sammensatte sine relativitetsteorier, fremsatte Einstein en række forudsigelser. Nogle blev bekræftet få år senere, men forskere arbejder stadig på at bekræfte andre. Og en af ​​de mest fascinerende er begrebet gravitationsbølger. Når massive genstande bevæger sig i rummet, sender de krusninger ud over universet, der faktisk forvrænger materiens form. Eksperimenter er på plads og i gang med at opdage disse tyngdekraftsbølger, når de fejer forbi Jorden.

Viste noter

Udskrift: Gravitationsbølger

Fraser Cain: Pamela, du kom tilbage til Illinois?

Dr. Pamela Gay: Jeg er tilbage midt i landet. Det var dejligt at kunne optage ansigt til ansigt med dig i sidste uge.

Fraser: Det var virkelig sjovt, og AAS-mødet var også et oprør, må jeg sige. Selvom jeg ikke tror, ​​jeg nogensinde har skrevet så meget i mit liv, som jeg gjorde i løbet af de 4 dage. Jeg tror, ​​jeg udsendte mere end tredive artikler på fire dage.

Pamela: Du lægger mere ud end Phil og jeg tilsammen.

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Fraser: OKAY. Så vi vil sandsynligvis være klar til at gøre det samme.

Pamela: Og vi vil have endnu et møde undtagen denne gang midt i landet for alle andre midt i landet.

Fraser: Lige mødet var super sjovt. Det var dejligt at se alle de mennesker, der var kørt derned. Nogle mennesker kom ganske forskellige måder. Det var bare dejligt at se alle og tale. Det var meget sjovt.

Pamela: Vi kan ikke takke George nok for at hjælpe med at sætte alt sammen for os.

Fraser: Det var godt. Og til sidst havde vi omkring tres mennesker, var der?

Pamela: Ja, det var tres.

Fraser: Det var en hel skare. Det sidste lille stykke administration er, at vi sandsynligvis sender dette show lidt senere på mandage. Vi har et par flere ting, som vi skal gøre i disse dage for at bringe hele showet sammen, og det tager lidt mere tid. Så før vi prøvede at poste søndag aften og have det klar til mandag.

Nu bliver det senere og måske endda om aftenen mandag. Så ingen løfter, men jeg tror, ​​det vil stadig være mandage, så jeg tror, ​​at dagen stadig er den samme.

Da han først sammensatte sine relativitetsteorier, fremsatte Einstein en række forudsigelser. Nogle blev bekræftet få år senere, men forskere arbejder stadig på at bekræfte andre.

En af de mest fascinerende er begrebet gravitationsbølger. â € Når objekter bevæger sig i rummet som sorte huller, sender de krusninger ud over universet, der faktisk kan forvrænge formen på materienâ €. Eksperimenter er på plads og i gang med at opdage disse tyngdekraftsbølger, når de fejer forbi jorden. Så Pamela, hvad forudsagde Einstein?

Pamela: Ligesom at rulle en klippe rundt på et elastisk ark. Ikke at jeg ville vide, hvorfor du ville gøre dette, men til undervisningsformål gør os lærere det meget. Hvis du ruller en klippe rundt på et elastisk ark, kan du faktisk ende med at få bølger på det elastiske ark.

Det han regnede med var, at hvis du tager en genstand, en planet, en stjerne, et sort hul, og du rullede det rundt i stoffets rum og tid, kan du også opbygge bølger i rum og tid. Disse bølger, når de formerer sig gennem rummet, kan faktisk få objekter til at trække sig sammen og udvide sig, når bølgerne passerer gennem dem.

Fraser: Så bølgerne, som han forudsagde, at der krøllede på dette ark gummi, kunne bare omfatte et vakuum. Men hvis der tilfældigvis var ting på det gummilag som planeter eller stjerner eller noget andet, ville de også blive fordrejet. Så det er ikke som det underliggende rum nedenunder er forskelligt fra stjernerne og planeterne, og så videre bliver det bare forvrænget.

Pamela: Og dette er faktisk lidt nyt for vores tankegang. Når vi taler om ekspansionen i universet, forstår vi, at ting, der er gravitationelt bundet sammen, vil de forblive i samme størrelse.

Men dette rum, som de er indlejret i, udvides på grund af Hubble Constant og på grund af mørk energi og på grund af mange andre ting. Med gravitationsbølger ekspanderer alt og trækker sig sammen. Hvis en gravitationsbølge passerer igennem mig, udvider jeg mig og trækker mig sammen.

Sådan fungerer det, er at for hver en meter lang genstand får du en udvidelse eller sammentrækning, der faktisk er mindre end en proton som en hundredtusindedel af bredden af ​​en proton. Det er et af de små stykker af et atom.

Så jeg er ikke rigtig bekymret for at blive mærkbart udvidet eller kontraheret af tyngdekraftsbølger. Men når du begynder at se på objekter i planetstørrelse, så begynder du at være i stand til faktisk at få målbare udvidelser og sammentrækninger fra disse tyngdekraftsbølger.

Fraser: Men ekspansionen og sammentrækningen ville være svær at opfatte, fordi alt omkring dig bliver udvidet og indgået på samme tid, ikke?

Pamela: Men bølgen bevæger sig faktisk med lysets hastighed. Så det er ikke som om hele planeten oplever bølgen på én gang. Hvis du har en høj hastighed nok til at måle dette, så kan jeg se mig udvide og trække mig sammen, og så kommer du langt over i Vancouver tusind plus miles væk, vil du udvide og trække mig sammen senere på tid, hvis det er det retning bølgen passerer ind. Eller du vil udvide dig og trække dig sammen først, og så gør jeg det senere.

Så når vi prøver at lede efter tyngdekraftsbølger, ser vi faktisk efter dette ”det sker der nu sker det her” ændring, når vi foretager målingerne.

Fraser: Vidste Einstein hvorfor dette ville ske? Var dette bare noget, der kom ud af hans beregninger?

Pamela: Det kom godt ud af hans beregninger, men det giver faktisk mening, når du begynder at tænke på tyngdekraften som en geometrisk effekt. Der er to forskellige måder at se på tyngdekraften på.

Du kan se det enten som en standardkraft, hvor du kaster bosoner overalt, og bosoner, der vandrer fra objekt til objekt, er det, der overfører kraften. Dette er en standardmodel til at se på ting, som alle kræfter formidles af bosoner.

Fraser: Så du har partikler, der lynlåser frem og tilbage, hvilket skaber kraften.

Pamela: Ret. Eller i det mindste at bære styrken.

Fraser: Bærer styrken.

Pamela: Der er en anden måde at se på tyngdekraften på. Den anden måde at se på tyngdekraften er at se det som en geometrisk egenskab for, hvordan universet er sammensat.

Når solen sidder midt i vores solsystem, vrider det faktisk plads og tid rundt for at skabe denne tredimensionelle skål. Hvis vi var i stand til at se gitteret af rum og tid udefra, ville det gitter blive tættere omkring solen, og ting ville falde ned i den tættere del af gitteret, da de blev tiltrukket af solen, og planeterne rullede bare rundt inde denne skål.

På denne anden måde at se på universet, hvor virkelig rum og tid er et stof, der kan deformeres, som kan strækkes, og som kan presses sammen.

I den opfattelse, når du først får en massiv genstand, noget der deformerer stoffet omkring det, kan det bevægelige objekt skabe bølger, ligesom et objekt i bevægelse på en normal elastisk overflade kan skabe bølger. Her kommer det bare ud af at se på rummet geometrisk og se masser som skaber deformiteter i rumets geometri.

Fraser: Hvor langt kunne disse bølger sprede sig?

Pamela: Det er den rigtig seje ting. Der er intet der stopper dem. Hvis jeg prøver at skinne en lommelygte fra mig til dig, stopper den lommelygtestråle af jorden. Det stopper ved at sprede sig i luften. Det bliver stoppet af mange ting. Faktisk bliver lys fra de fjerneste galakser påvirket til venstre og højre. Det bliver tyngdekraftigt bøjet af mellemliggende genstande. Dens farve påvirkes af støv og stof, den går igennem. Dens farve ændres endda undertiden af ​​virkningerne af tyngdekraftsgenstande, som den passerer nær.

Alle disse forskellige effekter ændrer lyset mellem os og de fjerneste objekter i universet. Nu er en gravitationsbølge bare ligeglad. Det vil bare blæse gennem universet og udvide og sammentrække alt, hvad der kommer i dets vej. Men i sig selv vil det slet ikke ændres.

Fraser: Så det er ligeglad med støv. Det er ligeglad med, om det gennemgår et vakuum eller det går gennem planeter. Uanset om det går gennem et område, der har høj tyngdekraft, kan det bare passere gennem et sort hul?

Fraser: Og ikke engang bemærke og ikke blive suget ind i et sort hul?

Pamela: I modsætning til lys og stof er tyngdekraftsbølger det eneste, der bare blindt kan passere gennem noget.

Pamela: Så det giver os et værktøj til at finde ud af om begivenheder i udkanten af ​​universet, som vi ellers måske ikke har nogen måde at vide om.

Fraser: Okay, så det lyder som et godt værktøj, så hvorfor har vi ikke været i stand til at bruge det hidtil?

Pamela: Det er den hele â € œEn to meter lang genstand vil kun blive deformeret ca. hundrede tusind af bredden af ​​et proton.â €

Pamela: Ja, det er rigtig lille. Det er virkelig, virkelig svært at opdage, og problemet er, at der er så mange ting, der vil forstyrre vores opdagelser. Vi prøver, vi prøver virkelig, men disse er stædige og undvigende væsner. Vi har dog opdaget dem indirekte.

Fraser: Okay, lad os tale om det dengang. Hvad er de måder, vi har opdaget det ud i rummet? Jeg tror, ​​jeg ved, hvor det går hen.

Pamela: Nå, bølger bærer energi. De har fart med sig. This is why when you get hit with a large ocean wave it actually knocks you on your butt sometimes. It’s because the energy in that wave is being transferred to you.

Now that energy had to come from somewhere and the energy in gravitational waves has to come from the systems that they are in. When we look at really high math systems they contain a couple of different objects. You can’t have a gravitational wave if you just have this lone black hole hanging out spinning by itself. It has to be interacting with something that creates an asymmetry.

Fraser: Okay. And would that be because like I know that we talked about finding extra solar planets around stars, we can see the star because it is being yanked back and forth by the planet. We can see its motion this is sort of the way the wave lengths of its light is changing as the star is being pulled away and towards us.

And in the case of two compact objects, the two of them are going to be orbiting one another right? They will be moving in space or in some common point of gravity.

Pamela: It’s the motion that is so important here. If you can imagine you have a perfectly still cup of coffee and you very carefully drop cream in and you want to stir it in. The most effective way to stir it in is to take your flat spoon and move it away so that you muck up the fluid and get it moving the most.

Now if you take your pencil and put your pencil in end first and roll it between your hands like you’re trying to start a fire so that the pencil is rotating about its center axis but isn’t moving left, right, forward, backward, any of those things. It’s just rotating about that axis, that pencil is going to do nothing to mix up your coffee unless it really has a rough surface.

Fraser: And so we want the situation where you have two very massive objects moving very quickly in space.

Pamela: Around and around, moving the fabric of space and time around.

Fraser: Okay. So we can have two neutron stars or two black holes or two white dwarves or some combination…

Pamela: They’re really just low energy ones. As the earth goes around the sun, we’re creating gravitational waves with an energy of about three hundred watts. But that’s kind of boring and small and the sun is giving off like ten to the twenty-sixth watt that’s a one followed by twenty-six zeroes worth of watts compared to something my garage flood light gives off. So you can’t really detect that.

Fraser: So what are the objects that astronomers have seen so far?

Pamela: Back in the seventies a grad student and his advisor came across an object that contained two neutron stars, one of which was a pulsar. Pulsars are fast rotating neutron stars that we can measure the rotation cycles because for reasons that we think are associated with their magnetic field they have a hot spot that flashes past us and beams light at us as many as several hundred times a second in some cases.

We can use these flashes, these pulses that make these neutrons stars’ pulsars to measure the very careful, very small details of the dynamics of the system because those pulses are going to get Doppler shifted. They are going to get sped up and slowed down depending on whether the pulsar is moving toward us or away from us in its orbit. We can measure changes in the orbit by watching this over long periods of time.

What the graduate student and his advisor were able to observe was the period was actually decaying in this system. The two neutron stars were getting closer and closer to one another over time and the only way that’s going to happen is if the system is somehow radiating energy. The rate at which the stars were getting closer and closer to one another matched with what you would expect if they were radiating energy in the form of gravitational waves.

Fraser: So regular energy gets converted into gravitational wave energy that gets radiated out and causes sort of a loss of energy in the system.

Pamela: Nemlig. We have these stars that are orbiting, shaking up. They are rippling the fabric of space and time.

Just as it takes energy for you or I or ripple our sheets as we are trying to spread them out over our bed, it takes energy for these high mass objects to ripple the fabric of space and time and so they’re losing energy and that energy is propagating through space causing objects to contract and expand and hopefully someday be observed here on earth directly.

Right now, all we’ve observed is these systems losing energy and we have assumed that energy is getting lost to gravitational waves.

Fraser: Right, but can we see that distortion of the gravity waves that would be changing the size or the shape of the pulsars?

Pamela: We hope. We haven’t done it yet but we’re trying.

This is where there are these two neat gravitational twin observatories that act as a single observatory, truth be told, called LIGO. Laser Interferometer Gravitational-Wave Observatory. It’s actually a pair of different facilities one located out on the west coast and one located out on the east coast in Washington state and Louisiana state respectively here in the United States.

Each facility consists of a pair of twin arms off at right angles to one another. Inside of these arms there are tubes that have no air in them. They are complete vacuums and they’re shooting laser beams back and forth down these tubes. At the ends of the tubes the lasers can interfere with one another. When laser beams are allowed to interfere in specific ways you get nice little diffraction patterns.

You can very precisely measure the distance between where the laser is emitted, where it reflects, and where it eventually ends up getting detected by looking at these interference patterns. You hope to use the fact that you have two beams so one on one axis hopefully is going to get contracted and the other beam on the other axis is hopefully going to get expanded at the same moment and we’ll be able to measure this.

In a certain period of time corresponding to the amount of time that it takes to get from Hanford, Washington to Livingston, Louisiana or visa versa, we’ll see the exact same thing happen on the other coast of the United States.

Fraser: Oh, I get it. The beams are at a right angle and if the length of one of these arms gets shortened or lengthened just a slight little bit, it’s going to throw off the precision of the two lasers, how they’re interacting with each other in a measurable way.

So you might get the one gravity wave passing over the one facility and then a fraction of a second later it’s going to hit the second facility and in theory they should see the exact same length change in the one facility as they see in the other facility.

Pamela: Now the problem is, you also have things like UPS trucks. You have things like slight ground tremors, or all sorts of things all over the planet that are constantly causing bumps and jitters and skitters and all sorts of little motions in the system. All of these motions can wipe out the actual gravitational waves.

Fraser: I guess that’s why they have to have the two facilities. If you just have one, then any little fluctuation, any bump, I’m sure me jumping up and down over the facility would probably mess up their measurement enough to make it look like a gravitational wave. But by having the two, then they can try and see one change and then the second. So have they turned up anything yet?

Fraser: Is that just because it doesn’t exist, or does that mean they haven’t turned up anything because it’s not sensitive enough?

Pamela: It’s a complicated issue. They’ve been working on LIGO for a long time. It has a new set of instruments. They’ve been working very hard to tune everything to get everything lined up and find things. But, it’s hard work and they’re still working on tuning the system.

They’re still trying to figure out how to calibrate for everything that’s happening here on the planet Earth that’s mucking with their system. And there’re going to get there, hopefully. It’s been a long ride. When I was at Michigan State University as an undergrad one of my classmates was actually a summer intern down in Louisiana working on LIGO. They’ve been working to do this for a lot of years.

Fraser: How many events and what kinds of events were they hoping to see by now?

Pamela: It depends on where you look. Right now they’re estimating that once everything is fully operational, they should be seeing something a couple times a year. It also depends on what the universe decides to throw at us.

Fraser: Right and what kind of thing would they be seeing?

Pamela: Well, for instance when two black holes merge. That’s going to send out an amazing, whopping amount of gravitational waves. In some cases super novas that are asymmetric can send out gravitational waves. If you have an asymmetrical disk outside of a super massive black hole, you may also be able to see that.

All sorts of different things produce specific sets of gravitational waves. The shape of the waves the distribution of the waves is very specific to the type of object you are observing. Hopefully, some of these will become apparent.

There are also theorists out there that are constantly tuning their models. That’s the other thing that is fun to watch. The theorists come up with an idea “If this happens, we should see†, and then they lay out their plan and exactly what types of things will produce what size, or what type of black hole. We’re still working on honing in on that as well. Or what size, or what type of gravitational wave we’re still on homing in on that.

Fraser: But even when they don’t see something that’s still very interesting because that just means that this prediction might be incorrect or that prediction might be incorrect and that just lets them continue to focus their prediction.

At least they’ve got some kind of instrument that is checking that they can compare their theories against which the string theorists should be so lucky, right?

Pamela: Yeah, we are in the position where there have been a few events where I’ve seen press releases that have said, “LIGO did not detect something. That means the gravitational waves from this specific event that were also detected in light, could not have been bigger than†and then they give error bars.

So now we know at least the gravitational waves are smaller than a certain amount. And that’s still new information that we didn’t have prior to LIGO being constructed.

Fraser: But once they did make very specific predictions on how strong these should be.

Pamela: The problem is figuring out the physics of how exactly do things merge. What are the time scales? What are the fluid dynamics of your combining two different objects that are spinning, orbiting each other, or there might be a disk of material around them. It starts to get complicated when you’re not dealing with non-spinning clean black holes.

Fraser: Okay. So we’ve learned about LIGO and we’re still waiting on the result from them, are there any other missions or any other experiments in the works?

Pamela: Well there are some other ground-based observatories. But the greatest hope that we have rests in a mission called LISA.

LISA is going to be the space-based version of that and LISA in fact stands for Laser Interferometer Space Antenna.

Fraser: So the Hubble Space Telescope is the optical detector of gravitational waves?

Pamela: Basically, except there’s not going to be a lot of optical detectors involved. All they need to be able to see is their own laser.

Fraser: So it’s kind of the same thing as LIGO. You’re going to have space craft separated by a long distance pointing lasers at each other hoping to see that contraction and expansion.

Pamela: It’s a really neat system. If you’ve ever played with Tinker Toys, LISA really looks like a giant Tinker Toy construction where you have these three little disks (and they’re not all that little), on orbit separated by huge amounts and each disk is connected to the other two disks in this equilateral triangle with laser beams.

What they’re looking for is any gravitational cause that will change the separation of these three spacecraft from one another. Now unfortunately, even though it’s now in space, it’s not as simple as you might think. There will still be stuff interfering with out ability to detect gravitational waves. This is not going to be an easy task no matter what.

Once you get into space you have to start worrying about solar particles, drift in the spacecraft over time where their orbits aren’t quite as precise as we thought. The Earth’s gravitational field isn’t perfectly symmetric as you fly over different parts of the planet the gravitational pull is going differ from place to place. All of these different things have to be completely accounted for in trying to make sense of the data that we get out of LISA.

Fraser: Now what will be the sensitivity of LISA?

Pamela: LISA is actually going to be so sensitive that it may be able to, although it’s not designed for this, actually be able to detect a continuum of gravitational waves that started back when the universe had just formed.

What’s neat about gravitational waves is, if they were given off during the “Big Bang†during the whole period of inflation and during the three hundred thousand years leading up to the formation of the cosmic microwave background we can detect them.

We can’t detect anything before the cosmic microwave background in light because the universe was opaque. We couldn’t see through it. But, gravitational waves don’t care about the universe being opaque. They just go.

And so it’s possible once LISA is in orbit we may see this whole range of twittering in different frequencies of gravitational waves that are a signature of how our universe was formed and that’s just kind of cool.

Fraser: It sounds like gravity waves offer astronomers a whole new way of seeing. It’s not just the whole visible spectrum, it’s a brand new way to be able to see the universe and in many cases, I guess they could look at something in infrared or visible and then also look at it with gravity waves.

Pamela: What’s so cool about this is if you’ve ever looked at a pond on a really still day, you can tell when the motorboat goes by because you see the waves. You can tell when there’s a bunch of rocks between you and the motorboat because of the patterns in the waves. You can actually start to guess what’s going on out in the middle of the lake without actually having to look up and looking at the middle of the lake by looking at the waves that are hitting the shore.

Well, we can look out across the universe and see what’s going on in other parts of the cosmos without actually being able to see what’s going on in other parts of the cosmos with normal light. It’s a whole new tool.

Fraser: I talked to a researcher one time and he said it was like you’re listening. That it is such a completely different way. If you’re using your telescope, you’re seeing and if you’re using gravity waves, you’re listening.

Pamela: It’s a new sense. It’s something that’s going to be really fun to play with as we begin to develop the technology to do this efficiently.

Fraser: Well, let’s hope that we make a detection in the next few years and this whole new science opens up.

Pamela: The real thing to hope for is added budget for NASA. LISA’s not doing so well in the budget. We hope to be able to launch it somewhere around 2015 but it’s still in the formulation phase.

The money isn’t there to start building a spacecraft yet so hope that NASA all around gets more funding so projects like LISA will have the funding they need to allow astronomers to explore the universe in a whole new way.

Fraser: All right, so NASA if you’re listening, make sure you fund LISA. Let’s see if that helps. All right, well thanks Pamela. We’ll talk to you next week.

Pamela: Sounds good Fraser, talk to you later.

Dette udskrift matcher ikke nøjagtigt lydfilen. Det er redigeret for klarhedens skyld.
Transcription and editing by Cindy Leonard


What are gravitational waves?

Researchers at the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever detection of gravitational waves on September 14, 2015. Here, a technician works on some of the optics for a LIGO detector. Image via LIGO.

Gravitational waves are ripples in the structure of spacetime. Much as a ship traveling across the surface of a calm sea leaves a wake behind it, so moving objects in the universe create gravitational waves. The “ships” in the case of gravitational waves are extremely violent and cataclysmic events far off in the cosmos: black hole mergers, neutron star collisions, supernovae. All of these generate waves in the structure of spacetime, stretching and squeezing it as the ripples travel across the universe.

Because gravitational waves are extremely weak as observed from our earthly vantage point, the technology to detect them has become available only in recent years. Like all waves, gravitational waves diminish in size with distance, shrinking to faint echoes of those distant “shipwrecks” – those distant violent events in the cosmos – by the time they reach us. From our location, many light-years from a black hole merger or a neutron star collision, the waves compress and stretch space, and everything in it, by a thousandth of the diameter of a proton as they pass through the Earth. That’s a billionth of a billionth of a meter. We require very advanced technology indeed to see that change. It’s like seeing the distance between the sun and its closest neighbor among the stars – Alpha Centauri, 4.3 light years distant – change by the thickness of a human hair.

It was Albert Einstein who, in his General Theory of Relativity of 1915, first postulated the existence of gravitational waves. His suggestion that gravity travels in waves seemed logical: every type of light on the electromagnetic spectrum, from ultraviolet to visible to radio, travels in waves. Sound travels in waves. Why should gravity not be propagated in the same way? Einstein calculated that extremely violent events in the cosmos would cause space to ring like a bell. This was distinct from the idea of the static, unchanging gravitational fields that are generated by any object that has mass, like a star or a planet.

However, for decades after 1915, Einstein himself was unconvinced of the existence of gravitational waves. In 1936, he and colleague Nathan Rosen published a paper entitled Do Gravitational Waves Exist? which, initially, was rejected by one journal because of a mathematical error.

It was the error that had caused the authors to conclude that gravitational waves don’t exist. When Einstein had corrected the error, the paper’s conclusion became exactly the opposite! Although the evidence now pointed to their existence, Einstein remained unconvinced, and believed that even if gravitational waves did exist, they would be so very weak that humans could never develop the technology to detect them.

Albert Einstein in 1912. His general theory of relavity is fundamental to modern cosmology. It was Albert Einstein who, in his General Theory of Relativity of 1915, first postulated the existence of gravitational waves.

It should be noted that Einstein was not the only theorist who worked on gravitational waves. Important contributions were made by other famous scientists, among them Robert Oppenheimer, Roger Penrose, Karl Schwarzschild, Arthur Eddington, Kip Thorne and Richard Feynman. But it was Feynman who, in January 1957, finally convinced the doubters that not only do gravitational waves do exist, but they can carry energy as well, explaining this by using something he called his Sticky Bead argument.

Feynman’s work directly paved the way for today’s gravitational wave detectors.

Yet it would be another 50 years before the first gravitational waves were detected. Developing the concepts and the technology to do so took decades of hard work by many scientists. Finally, LIGO, the Laser Interferometry Gravitational-wave Observatory situated at two sites in the United States, started observing in 2002. It took several upgrades to LIGO, between 2002 and 2015, to give it the sensitivity to make its historic first detection.

The first detection, of two black holes merging some 1.3 billion light years distant, came in September 2015 and was announced to the world in February 2016 after months of work verifying that the signal, which had lasted a mere tenth of a second in perfect agreement with Einstein’s predictions, was real. Incredibly, LIGO had not yet begun its official observing run when the detection came: after its latest in a series of upgrades to improve its range and sensitivity, LIGO had been turned on for engineering tests. The black hole merger was detected almost immediately the detector was operational.

Another key prediction of Einstein was that gravitational waves would travel at the speed of light. By measuring the difference in time between when the gravitational wave signal arrived at the two LIGO observatories – in Hanford, Washington, and Livingston, Louisiana, separated by nearly 2,000 miles (3,000 km) – scientists were able to determine that Einstein’s prediction was completely correct. Gravitational waves do indeed propagate at the speed of light.

LIGO was joined in 2018 by the European Virgo detector in Italy, which has greatly improved the ability of scientists to pinpoint the location on the sky where the gravitational waves originated. Since then, LIGO/Virgo have detected some 50 black hole mergers, but also eight neutron star collisions and six neutron star-black hole collisions. Some of these may end up being to due to so-called “terrestrial interference”: vibrations from passing traffic and even distant ocean waves can cause false positives.

On January 14, 2020, LIGO also detected an event of completely unknown origin, which does not fit any models or predictions, perhaps, excitingly, pointing to the existence of a hitherto-unknown cosmic phenomenon.

Very soon the Japanese KAGRA observatory will join Virgo and LIGO in the detection of gravitational waves. In the 2030s, the European Space Agency will launch LISA, a space-based gravitational wave detector, which should enable the detection of low-frequency gravitational waves emanating from supermassive black holes and from supernova explosions. China has started work on building three gravitational wave observatories, its avowed intent to become the world leader in Earth- and space-based gravitational wave detection.

All of the gravitational wave events detected so far agree perfectly with Einstein’s predictions and with computer simulations derived from his calculations. Einstein would surely have been amazed that he was wrong, that human intellect and ingenuity has indeed triumphed and created the technology he thought impossible. He would also probably have regretted doubting his own work in predicting the existence of gravitational waves. But he would also, surely, have been happy that the detection of gravitational waves is also a confirmation of his theory of Relativity. There are now few places left to run for those who doubt Einstein’s greatest triumph.

Gravitational-wave astronomy is a completely new science and one which promises to unlock many of the universe’s mysteries. It’s no exaggeration to say that a revolution in our view of the universe is underway. In the future, it might even be possible to detect gravitational waves from the Big Bang itself, to hear the sound of Creation ringing out across billions of years.

If you would like to keep up to date with the latest gravitational wave events, the University of Birmingham in the U.K. has created this page, which is a database of LIGO and Virgo detections during their current observing run. The database is also available as a free app for Android/Apple phones, downloadable from their respective stores.

Computer simulation of two merging black holes producing gravitational waves. Image via Werner Benger/ Wikimedia Commons.

Bottom line: First postulated by Albert Einstein in 1916 but not observed directly until September 2015, gravitational waves are ripples in spacetime.


Where Did Those Gravitational Waves Come From? There's a Map

With today’s historic and incredibly exciting announcement of the first ever detection of gravitational waves came the news that these waves were generated by two merging black holes approximately 1.3 billion light-years away — an astrophysical event that is mind-blowing in its own right. So the next question that comes to mind is, unsurprisingly, in which direction did this black hole merger occur?

As it turns out, scientists of the Laser Interferometer Gravitational-Wave Observatory (LIGO) already have an answer, albeit a very general one.

As previously reported, LIGO is composed of two stations — one in Washington and the other nearly 2,000 miles away in Louisiana. The reason for having 2 stations is logical: should a gravitational wave pass through our volume of space (yes, these spacetime ripples pass through the Earth and os unimpeded), it must be detected by both stations to be confirmed as being &ldquoreal&rdquo and not a false positive caused by some kind of local disturbance near one of the stations. Secondly, having 2 stations allows LIGO scientists to triangulate the signal to derive a very general idea as to where these waves are coming from.

Thursday’s grand announcement pointed out that the Livingston station (Louisiana) &ldquoheard&rdquo the gravitational wave "chirp" 7 milliseconds before the Hanford station (Washington) on Sept. 14, 2015. As gravitational waves travel at the speed of light, this timing difference confirmed that the two detections were indeed the same event. Scientists were immediately able to deduce the direction the gravitational waves were traveling.

Now, the LIGO collaboration has released a map of the Southern Hemisphere skies, giving us a glimpse at the promising future of gravitational wave astronomy. In the map, contours have been added that represent the different probabilities for where the signal originated. The outermost purple line represents a 90 percent certainty that the signal’s source (the colliding black holes) is contained within that area. The innermost white contour line highlights a possible source region to a 10 percent certainty.

The band of stars through the middle of the image is the edge-on disk of the Milky Way and the Large Magellanic Cloud and Small Magellanic Cloud (two small nearby galaxies) can also be seen in the bottom portion of the image. It is worth noting that, although there is some uncertainty in the black hole’s distance, its location is far beyond our own galaxy and Local Group of galaxies.

As more gravitational wave detectors go online and their observations added to LIGO’s detections, better precision of the locations of gravitational wave sources will be pinpointed, making for highly detailed gravitational wave maps of the cosmos. So though it may not be precise, this is the first map of its kind where inside its contours two black holes merged 1.3 billion years ago.


What causes gravitational waves?

  • when a star explodes asymmetrically (called a supernova)
  • when two big stars orbit each other
  • when two black holes orbit each other and merge

An artist’s animation of gravitational waves created by the merger of two black holes. Credit: LIGO/T. Pyle

But these types of objects that create gravitational waves are far away. And sometimes, these events only cause small, weak gravitational waves. The waves are then very weak by the time they reach Earth. This makes gravitational waves hard to detect.


Teaching the Activity

Each student will need a copy of the student worksheet. Students will need to view the four graphs listed above to complete the worksheet. The student worksheet provides space for the students to record their answers but not for the calculations that the questions will require. Teachers should encourage students to use additional paper to methodically record their manipulations of the equations. Such documentation will facilitate the discovery of computational mistakes. "Showing your work" is a life skill in professional science.

The student worksheet is self-contained and the teachers guide provides solutions and some additional insights for teachers. After watching Einstein's Messengers the students will be ready to undertake the activity. The teacher will need to decide how the students should do the work. If the students work individually or in small groups, the activity will probably require the majority of a 90-minute class period or most of two 50-minute periods. The amount of class time could be reduced by assigning a portion of the activity as homework.

Another key decision for the teacher is the amount of review and practice to provide on the algebra techniques required by the questions in the student worksheet. Several of the worksheet items involve manipulations of fractional exponents. Students might attack the worksheet with greater confidence and accuracy if they have reviewed exponent operations first.


Indhold

Ordinary gravitational waves' frequencies are very low and much harder to detect, while higher frequencies occur in more dramatic events and thus have become the first to be observed.

In addition to a merger of black holes, a binary neutron star merger has been directly detected: a gamma-ray burst (GRB) was detected by the orbiting Fermi gamma-ray burst monitor on 2017 August 17 12:41:06 UTC, triggering an automated notice worldwide. Six minutes later a single detector at Hanford LIGO, a gravitational-wave observatory, registered a gravitational-wave candidate occurring 2 seconds before the gamma-ray burst. This set of observations is consistent with a binary neutron star merger, [7] as evidenced by a multi-messenger transient event which was signalled by gravitational-wave, and electromagnetic (gamma-ray burst, optical, and infrared)-spectrum sightings.

High frequency Edit

In 2015, the LIGO project was the first to directly observe gravitational waves using laser interferometers. [8] [9] The LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity. [10] [11] [12] These observations demonstrated the existence of binary stellar-mass black hole systems, and were the first direct detection of gravitational waves and the first observation of a binary black hole merger. [13] This finding has been characterized as revolutionary to science, because of the verification of our ability to use gravitational-wave astronomy to progress in our search and exploration of dark matter and the big bang.

There are several current scientific collaborations for observing gravitational waves. There is a worldwide network of ground-based detectors, these are kilometre-scale laser interferometers including: the Laser Interferometer Gravitational-Wave Observatory (LIGO), a joint project between MIT, Caltech and the scientists of the LIGO Scientific Collaboration with detectors in Livingston, Louisiana and Hanford, Washington Virgo, at the European Gravitational Observatory, Cascina, Italy GEO600 in Sarstedt, Germany, and the Kamioka Gravitational Wave Detector (KAGRA), operated by the University of Tokyo in the Kamioka Observatory, Japan. LIGO and Virgo are currently being upgraded to their advanced configurations. Advanced LIGO began observations in 2015, detecting gravitational waves even though not having reached its design sensitivity yet. The more advanced KAGRA started observation on February 25, 2020. GEO600 is currently operational, but its sensitivity makes it unlikely to make an observation its primary purpose is to trial technology.

Low frequency Edit

An alternative means of observation is using pulsar timing arrays (PTAs). There are three consortia, the European Pulsar Timing Array (EPTA), the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and the Parkes Pulsar Timing Array (PPTA), which co-operate as the International Pulsar Timing Array. These use existing radio telescopes, but since they are sensitive to frequencies in the nanohertz range, many years of observation are needed to detect a signal and detector sensitivity improves gradually. Current bounds are approaching those expected for astrophysical sources. [14]

Intermediate frequencies Edit

Further in the future, there is the possibility of space-borne detectors. The European Space Agency has selected a gravitational-wave mission for its L3 mission, due to launch 2034, the current concept is the evolved Laser Interferometer Space Antenna (eLISA). [15] Also in development is the Japanese Deci-hertz Interferometer Gravitational wave Observatory (DECIGO).

Astronomy has traditionally relied on electromagnetic radiation. Originating with the visible band, as technology advanced, it became possible to observe other parts of the electromagnetic spectrum, from radio to gamma rays. Each new frequency band gave a new perspective on the Universe and heralded new discoveries. [16] During the 20th century, indirect and later direct measurements of high-energy, massive, particles provided an additional window into the cosmos. Late in the 20th century, the detection of solar neutrinos founded the field of neutrino astronomy, giving an insight into previously inaccessible phenomena, such as the inner workings of the Sun. [17] [18] The observation of gravitational waves provides a further means of making astrophysical observations.

Russell Hulse and Joseph Taylor were awarded the 1993 Nobel Prize in Physics for showing that the orbital decay of a pair of neutron stars, one of them a pulsar, fits general relativity's predictions of gravitational radiation. [19] Subsequently, many other binary pulsars (including one double pulsar system) have been observed, all fitting gravitational-wave predictions. [20] In 2017, the Nobel Prize in Physics was awarded to Rainer Weiss, Kip Thorne and Barry Barish for their role in the first detection of gravitational waves. [21] [22] [23]

Gravitational waves provide complementary information to that provided by other means. By combining observations of a single event made using different means, it is possible to gain a more complete understanding of the source's properties. This is known as multi-messenger astronomy. Gravitational waves can also be used to observe systems that are invisible (or almost impossible to detect) to measure by any other means. For example, they provide a unique method of measuring the properties of black holes.

Gravitational waves can be emitted by many systems, but, to produce detectable signals, the source must consist of extremely massive objects moving at a significant fraction of the speed of light. The main source is a binary of two compact objects. Example systems include:

  • Compact binaries made up of two closely orbiting stellar-mass objects, such as white dwarfs, neutron stars or black holes. Wider binaries, which have lower orbital frequencies, are a source for detectors like LISA. [24][25] Closer binaries produce a signal for ground-based detectors like LIGO. [26] Ground-based detectors could potentially detect binaries containing an intermediate mass black hole of several hundred solar masses. [27][28] binaries, consisting of two black holes with masses of 10 5 –10 9 solar masses. Supermassive black holes are found at the centre of galaxies. When galaxies merge, it is expected that their central supermassive black holes merge too. [29] These are potentially the loudest gravitational-wave signals. The most massive binaries are a source for PTAs. [30] Less massive binaries (about a million solar masses) are a source for space-borne detectors like LISA. [31] systems of a stellar-mass compact object orbiting a supermassive black hole. [32] These are sources for detectors like LISA. [31] Systems with highly eccentric orbits produce a burst of gravitational radiation as they pass through the point of closest approach [33] systems with near-circular orbits, which are expected towards the end of the inspiral, emit continuously within LISA's frequency band. [34] Extreme-mass-ratio inspirals can be observed over many orbits. This makes them excellent probes of the background spacetime geometry, allowing for precision tests of general relativity. [35]

In addition to binaries, there are other potential sources:

    generate high-frequency bursts of gravitational waves that could be detected with LIGO or Virgo. [36]
  • Rotating neutron stars are a source of continuous high-frequency waves if they possess axial asymmetry. [37][38]
  • Early universe processes, such as inflation or a phase transition. [39] could also emit gravitational radiation if they do exist. [40] Discovery of these gravitational waves would confirm the existence of cosmic strings.

Gravitational waves interact only weakly with matter. This is what makes them difficult to detect. It also means that they can travel freely through the Universe, and are not absorbed or scattered like electromagnetic radiation. It is therefore possible to see to the center of dense systems, like the cores of supernovae or the Galactic Centre. It is also possible to see further back in time than with electromagnetic radiation, as the early universe was opaque to light prior to recombination, but transparent to gravitational waves. [41]

The ability of gravitational waves to move freely through matter also means that gravitational-wave detectors, unlike telescopes, are not pointed to observe a single field of view but observe the entire sky. Detectors are more sensitive in some directions than others, which is one reason why it is beneficial to have a network of detectors. [42] Directionalization is also poor, due to the small number of detectors.

In cosmic inflation Edit

Cosmic inflation, a hypothesized period when the universe rapidly expanded during the first 10 −36 seconds after the Big Bang, would have given rise to gravitational waves that would have left a characteristic imprint in the polarization of the CMB radiation. [43] [44]

It is possible to calculate the properties of the primordial gravitational waves from measurements of the patterns in the microwave radiation, and use those calculations to learn about the early universe. [ how? ]

As a young area of research, gravitational-wave astronomy is still in development however, there is consensus within the astrophysics community that this field will evolve to become an established component of 21st century multi-messenger astronomy. [45]

Gravitational-wave observations complement observations in the electromagnetic spectrum. [46] [45] These waves also promise to yield information in ways not possible via detection and analysis of electromagnetic waves. Electromagnetic waves can be absorbed and re-radiated in ways that make extracting information about the source difficult. Gravitational waves, however, only interact weakly with matter, meaning that they are not scattered or absorbed. This should allow astronomers to view the center of a supernova, stellar nebulae, and even colliding galactic cores in new ways.

Ground-based detectors have yielded new information about the inspiral phase and mergers of binary systems of two stellar mass black holes, and merger of two neutron stars. They could also detect signals from core-collapse supernovae, and from periodic sources such as pulsars with small deformations. If there is truth to speculation about certain kinds of phase transitions or kink bursts from long cosmic strings in the very early universe (at cosmic times around 10 −25 seconds), these could also be detectable. [47] Space-based detectors like LISA should detect objects such as binaries consisting of two white dwarfs, and AM CVn stars (a white dwarf accreting matter from its binary partner, a low-mass helium star), and also observe the mergers of supermassive black holes and the inspiral of smaller objects (between one and a thousand solar masses) into such black holes. LISA should also be able to listen to the same kind of sources from the early universe as ground-based detectors, but at even lower frequencies and with greatly increased sensitivity. [48]

Detecting emitted gravitational waves is a difficult endeavor. It involves ultra-stable high-quality lasers and detectors calibrated with a sensitivity of at least 2·10 −22 Hz −1/2 as shown at the ground-based detector, GEO600. [49] It has also been proposed that even from large astronomical events, such as supernova explosions, these waves are likely to degrade to vibrations as small as an atomic diameter. [50]


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