Sterrekunde

Op watter swaartekrag word die swaartekraglens van 'n liggaam waarneembaar?

Op watter swaartekrag word die swaartekraglens van 'n liggaam waarneembaar?


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Ek wonder of ons weet hoeveel swaartekrag 'n swaartekraglens rondom 'n hemelliggaam waarneembaar word (sigbaar in die sin dat ons sterre op verkeerde plekke, sterre agter die liggaam ens. Sien, soortgelyk aan hierdie neutronster, maar natuurlik deur ver nie so sterk nie, wil ek graag weet waar die grens lê). Sulke sigbare gravitasielense vind plaas rondom neutronsterre, swart gate en wit dwerge, of hoe? Wat van rooi en bruin dwerge? Hulle is ook redelik dig en het 'n hoë oppervlakte-swaartekrag (maar natuurlik nie so hoog soos die ander genoemde liggame nie). Buig hulle die ruimte rondom hulle sigbaar?


Die artikel ''n Praktiese relatiwistiese model vir mikro-tweede astrometrie in die ruimte' (Klioner 2003; versigtig, baie swaar wiskunde ...) beskryf die raamwerk wat ontwikkel is om die verwerking van die ESA te ondersteun. Gaia astrometriese ruimtemissie. Die doel van hierdie missie is om die posisie (astrometrie) van sterre en ander voorwerpe te meet tot ongeveer 20 mikrosekondes (een miljoenste van 'n boogsekonde, wat 1 / 3600ste graad is). Om dit te kan doen, moet u rekening hou met al die bronne van foute en afbuiging, soos lig wat naby massiewe voorwerpe buig, tot 'n vlak wat ver benede u verwagte akkuraatheid is (hierdie vlak van sistematiese fout is gekies om 1 mikrosekonde te wees, vandaar daardie deel van die titel).

Afdeling 6 van die referaat bespreek die buiging van gravitasie-lig en Tabel 1 gee die grootte van die effek vir verskillende liggame in die sonnestelsel. Die kolom onder leiding van $ delta_ {pN} $ gee die grootte van die effek in mikro-sekondes ($ mu $as). Soos verwag, het die son die grootste effek op $ 1,75 times10 ^ 6 , mu $as of 1,75 ", met Jupiter tweede ens. Die volgende twee kolomme gee die hoekskeiding wat 'n ligstraal na die liggaam moet beweeg om 'n $ 1 mu as $ of $ 10 mu as $ effek. So byvoorbeeld, enige ligstraal van 'n ver voorwerp wat binne gaan $ 11,3 ^ circ $ van Jupiter sal ten minste a veroorsaak $ 10 mu as $ effek. Die kleinste liggame wat 'n groter effek het as $ 1 mu as $ is die dwergplaneet Ceres (massa $ 8,958 keer 10 ^ {20} $ kg), en die mane Dione ($ 1,05 keer 10 ^ {21} $ kg) en Umbriel ($ 1,27 keer 10 ^ {21} $ kg) - hierdie massas is 'n paar duisend keer kleiner as die aarde byvoorbeeld.

Nou is die vlak van presisie wat benodig word Gaia is baie meer presies as en die afbuigings is tienduisende keer kleiner as wat u sou sien in 'n optiese beeld met 'n gewone teleskoop, bv. Eddington se bevestiging van algemene relatiwiteit tydens die 1919 sonsverduistering (space.com-verhaal). Die Klioner (2003) -artikel bevat 'n formule om die grootte van die effek van die ligdefleksie te bereken as 'n funksie van die digtheid, maar u moet weet watter vlak van afbuiging u belangstel om 'n antwoord te kry. Die formule (nommer 35 op bladsy 11 van die vraestel) vir die radius $ L $ van 'n liggaam wat 'n ligte afbuiging lewer as $ delta $ is: $$ L geq links ( frac { rho} {1 , textrm {g cm} ^ {- 3}} regs) ^ {- 1/2} links ( frac { delta} { 1 , mu textrm {as}} regs) ^ {1/2} keer 624 , textrm {km} $$ waar $ rho $ is die gemiddelde digtheid van die liggaam. Nadat u 'n mate van afbuiging / lensing gekies het wat u wil hê ($ delta $), kan u geskikte digthede kies (bv. asteroïdes) $ sim2 , textrm {g cm} ^ {- 3} $, Aarde en ander rotsagtige planete is $ sim5.5 , textrm {g cm} ^ {- 3} $)


Op watter swaartekrag word die swaartekraglens van 'n liggaam waarneembaar? - Sterrekunde

Ons weet wat lense is. Ons het dit in 'n bril, kameras, teleskope en ons oë. Hierdie optiese lense is van deursigtige materiaal en fokus lig. Sterrekundiges gebruik dit egter nou ook swaartekraglense om sterrestelsels, donker materie en buitesolêre planete op te spoor. Wat is 'n gravitasielens en hoe werk dit?

Materie verdraai die ruimte en buig lig
Die verhaal begin met Albert Einstein. Sy Algemene Relatiwiteitsteorie sê dat materie die ruimte daaromheen verdraai. Daarom kan u sien dat lig buig as dit deur die ruimte beweeg om 'n massiewe liggaam soos 'n ster. Maar hoe sou dit in die praktyk werk?

Die teorie voorspel dat lig van sterre agter die son 'n spesifieke hoeveelheid buig as dit na ons toe kom. Ongelukkig maak die son se glans dit onmoontlik om waar te neem. Behalwe tydens 'n totale sonsverduistering.

In 1919 het die Britse sterrekundige Arthur Eddington (1882-1944) en sy medewerkers die teorie tydens 'n verduistering getoets. Hulle het ses maande voor die verduistering die sterre in die naghemel afgeneem. Dit was dieselfde sterre wat hulle tydens die donkerte van die verduistering sou sien, sodat hulle kon bepaal of die sterposisies blyk te verander - en indien wel, met hoeveel. Die resultate bevestig Einstein se voorspellings, en beide Einstein en Eddington het internasionale bekendes geword.

Gravitasie lens
Einstein besef dat 'n ander teoreties waarneembare effek van massiewe voorwerpe wat lig buig, a was swaartekraglens. Hoe massiewer die voorwerp is, hoe sterker is die swaartekragveld, en daarom sal die ligstrale meer gebuig word.

Op sy eenvoudigste werk dit so. Stel u voor 'n ver voorwerp soos die sterrestelsel in hierdie diagram. Tussen ons en die sterrestelsel is 'n massiewe sterrestelsel waar die swaartekrag soos 'n lens optree om die lig rondom dit te buig. Die wit lyne wys lig wat ons nie van die aarde af sal sien nie. Die oranje lyne toon die ligpaaie wat deur die lens gebuig word en wat ons sal sien. Daar is twee afsonderlike paaie, dus sal ons twee verskillende beelde van die sterrestelsel sien.

Die eerste bekende swaartekraglens
Ander wetenskaplikes as Einstein het ook oor die teorie van swaartekraglense geskryf, maar almal was dit eens dat ons dit nie sou kon sien nie. En met die toerusting wat in die eerste deel van die twintigste eeu beskikbaar was, sou dit waar gewees het.

Die eerste gravitasielens is eers in 1979 ontdek. Dit was die kwasar Q0957 + 561. Kwasars is massiewe voorwerpe wat groot hoeveelhede energie uitgee en deur 'n teleskoop ietwat steragtig lyk.

In die geval van Q0957 + 561 het hulle 'n paar soortgelyke kwasars gevind. Na 'n bietjie studie was dit duidelik dat dit nie 'n tweeling was nie, maar twee beelde van dieselfde voorwerp. Die sterrestelsel YKOW G1, in die siglyn, het die kwasar se lig langs twee verskillende paaie gebuig en die dubbele beeld regstreeks in die middel van hierdie prent sigbaar gemaak. Die Twin Quasar, soos dit die bynaam gekry het, is net minder as nege miljard ligjare van die aarde af, en die lensstelsel is ongeveer vier miljard ligjare. Ons kon die kwasar nie sien sonder die lenseffek nie.

Verskillende soorte beelde
'N Swaartekraglens is nie so eenvoudig soos 'n optiese lens nie. Dit het nie een fokuspunt nie. Daarbenewens kan die lensvoorwerp iets soos 'n groep sterrestelsels wees wat geometries taamlik deurmekaar is. Die beeld hou ook verband met die manier waarop die voorwerp, die lens en die waarnemer opgestel word. As alles perfek in lyn is, sal die verre voorwerp soos 'n ring rondom die lensvoorwerp lyk. Dit word 'n Einstein-ring. Niemand het nog 'n perfekte een gesien nie - maar LRG 3-757 kom naby. Die swaartekrag van 'n helderrooi sterrestelsel (LRG) het die lig van die meer blou sterrestelsel verdraai.

Dit is meer gebruiklik om veelvoudige verwronge beelde te kry, hetsy as boë of interessant, as 'n Einstein kruis. Hier is 'n Einstein-kruis gevorm deur die sterrestelsel G2237 + 0305 wat 'n kwasar van agt miljard ligjare weg is.

Soms is die effek net om die agtergrondvoorwerp helderder te maak, maar meer algemeen is die ringe, boë en veelvuldige beelde. Die kopbeeld is 'n voorbeeld van veelvuldige beelde. Daar is 'n string van vyf afsonderlike beelde wat soos 'n slang lyk. Dit is die gevolg van die tros Abell 370 wat die lig vanuit 'n melkweg verdraai.

Die gebruik van gravitasie lens
Die ontdekking van swaartekraglense het Einstein se teorie ondersteun, maar sterrekundiges bestudeer dit nie om die rede of omdat hulle mooi foto's maak nie. Dit het belangrike waarnemingsinstrumente geword met 'n aantal gebruike. Hier is 'n paar voorbeelde.

1. Gravitasie-lense stel ons in staat om dieper die heelal in te sien as wat anders moontlik is. Die verste - en dus jongste - sterrestelsels waarvan ons weet, is op hierdie manier ontdek. Aangesien ons voorwerpe slegs kan sien as hul lig ons bereik, kyk ons ​​ook terug in die tyd as ons verder wegkyk.

2. Donker materie is materie wat nie met lig of enige vorm van elektromagnetiese straling saamwerk nie. Dit kan egter opgespoor word deur die swaartekrag-effekte daarvan. Sy bydrae tot lenseffekte help sterrekundiges om donker materie in kaart te bring.

3. Een tipe waarneming staan ​​bekend as mikrolensering en dit word gebruik om buitesolêre planete op te spoor. Die onderstebo punt is dat die metode planete met lae massa rondom verre sterre kan vind. Die nadeel is dat dit plaasvind as 'n gebeurtenis wat voortspruit uit 'n spesifieke belyning van voorwerpe. Dit beteken dat dit later moeilik of onmoontlik is om op te volg.

OPMERKING: Die beelde in hierdie artikel is afkomstig van die Hubble-ruimteteleskoop.

Inhoud kopiereg en kopie 2021 deur Mona Evans. Alle regte voorbehou.
Hierdie inhoud is geskryf deur Mona Evans. As u hierdie inhoud op enige manier wil gebruik, het u skriftelike toestemming nodig. Kontak Mona Evans vir meer inligting.


Die meetkunde van die ruimte


https://commons.wikimedia.org/wiki/File:Fireworks_in_Jaén_(cropped).jpg
Einstein se teorie verbind gravitasie-effekte met 'n kromming van ruimtetyd. Soos ons pas gesien het, ontstaan ​​die bekende effekte van swaartekrag as gevolg van kromming in die ruimtetydvelle van die ruimtetyd. Dat projektiele paraboliese bane volg en planete in elliptiese wentelbane beweeg, word byna presies herwin uit die kromming van hierdie ruimtetydvelle. Hierdie mees voor die hand liggende manifestasies van kromming verg tyd. Die kromming in die suiwer ruimte-ruimte-velle van ruimtetyd, dit wil sê die kromming van ons suiwer ruimtelike meetkunde, is byna onmerkbaar onder normale omstandighede.
Die effek is nietemin daar. Einstein se teorie sê wel dat die meetkunde van die ruimte krom word in die omgewing van baie massiewe voorwerpe. Dit geld vir die ruimte wat ons ken wat naby beide die groot aarde en die son is. Die afwyking van vlakheid in hierdie ruimtes is egter so gering dat geen gewone meting dit kan opspoor nie.

Om hierdie rede het ons al duisende jare geglo dat ons ruimte presies Euklidies is, terwyl dit net amper so is. Hierdie effekte is die moeite werd om na te streef. Alhoewel hulle klein is in die swak swaartekrag van die son en die aarde, word dit meer uitgesproke namate ons liggame van 'n meer intense swaartekrag nader of die enorme afstande van die kosmologie in ag neem. Dat die kromming selfs in geringe mate in ons deel van die ruimte daar is, is van groot fundamentele belang. Dit wys dat die ou idee dat die ruimte Euklidies moet wees empiries verkeerd is.

Om 'n idee te kry van hoe naby ons plaaslike meetkunde aan Euclidië is, kan ons die versteuring daarvan as gevolg van die son se skatting skat. Beskou 'n groot sirkel rondom die son wat ongeveer saamval met die aarde se baan. Euklidiese meetkunde vertel ons dat die omtrek van hierdie sirkel 2π x radius van die baan is.

Stel jou voor dat ons nou die son een kilometer tegelyk nader en 'n nuwe sirkel teken wat op elke stap op die son staan. Die Euklidiese resultaat vertel ons dat die omtrek van die sirkel vir elke kilometer wat ons nader aan die son kom, met 2π myl verminder word.
Dit is die Euklidiese resultaat. Vanweë die teenwoordigheid van die son is die ruimte rondom die son nie juis Euklidies nie. Volgens die algemene relatiwiteit verloor die sirkel vir elke kilometer wat ons nader aan die son kom, nie 2π myl in omtrek nie, maar verloor hy slegs (0,99999999) x2π myl.

Wat doen hierdie effense versteuring van die meetkunde van die ruimte aan die reguit lyne van die ruimtelike meetkunde? Dit laat hulle effens afwyk van wat u anders sou verwag.

Oorweeg twee punte A en B in die omgewing van die son om die effek te sien. Ons stel ons eers voor dat die ruimte in die omgewing van die son 'n plat, Euklidiese meetkunde het. 'N Reguit lyn tussen hulle is die kortste afstand en sal so ingestel wees:

As ons nou die meetkunde van die ruimte rondom die son vervang deur die meetkunde wat deur algemene relatiwiteit voorspel word, sal daar 'n verandering wees in die kortste afstand tussen A en B. Ons kan verwag wat die verandering sal wees. Wanneer ons die son nader, krimp die sirkels rondom die son nie meer so vinnig as wat Euclid verwag het nie. Dus kan 'n lyn van A na B effens korter word as dit sirkels effens verder van die son af volg. Die effek is dat 'n lyn, effens weg van die son afgewyk, nou die kortste afstand tussen A en B in die ruimte sal wees. Dit sal so lyk:

Dit verg 'n bietjie meer moeite om te sien dat dit net die korreksie is wat nodig is as ons die Euklidiese meetkunde van die ruimte vervang met die wat deur algemene relatiwiteit vereis word. Die besonderhede is in:
Bylaag: Geodesika van die ruimte naby die son

Die buiging is baie klein. Soos ons hieronder sal sien, blyk dit egter een van die vroegste effekte te wees wat werklik gemeet is.

Pasop: hoewel hierdie figuur 'n bietjie lyk soos die ruimtetyddiagram vir 'n liggaam in vrye val bo die oppervlak van die aarde, is dit nie dieselfde nie. Die lyn AB is hier in 'n gewone driedimensionele ruimte.


Hoeveel donker materie is nodig om lig te verdraai?

Ek verstaan ​​dat donker materie slegs 'n swaartekrag-eienskap het. Dit wissel nie direk met gewone materie of EM-straling nie. Die bewyse vir donker materie kom van waarnemings wat die swaartekragwet weerspreek. (Rotasiesnelheid van sterrestelsels wat ver weg is, is konstant ongeag die afstand vanaf die galaktiese middelpunt). Hierdie en ander gegewens het aangedui dat donker materie slegs deur swaartekrag in wisselwerking is.

Aangesien aangetoonde swaartekraglens van swart gate naby swart gate aangetoon word, word verwag dat groot swaartekragvervormings van donker materie ook lig kan verdraai. Is dit gesien?

Kan die toenemende rooi verskuiwing van lig met afstand verklaar word deur die kumulatiewe effek van donker materie wat 'n swaartekrag uitoefen wat van ons af weggerig word?

'N Ander manier om hieroor na te dink, is om te oorweeg dat ons waarneembare heelal 'n vaste dimensie het. Daar was nie genoeg tyd vir lig om ons van sterre buite daardie dimensie te bereik nie. As die werklike heelal egter veel groter was as die waarneembare heelal, en dit alles gevul was met donker materie, sou ons nie die swaartekrageffek daarvan sien op die lig wat uitgestraal is vanaf die grens wat ons kan sien nie? (solank die donker materie nie homogeen was nie) Die volgende uiteensetting dui daarop dat donker materie wat homogeen in die kosmos versprei is, nie swaartekrag-effekte kan hê nie. http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/sphshell2.html

Basies is die krag van enige sferies-simmetriese massaverdeling op 'n massa binne sy radius nul.

Dit beteken dus dat donker materie nie homogeen in die heelal is nie. Die digtheid van donker materie is ook bereken.


Sommige astronomiese aspekte van gravitasie

Soos hierbo gesê, laat studies oor swaartekrag toe om die massas en digthede van hemelliggame te skat en maak dit dit moontlik om die fisiese samestellings van sterre en planete te ondersoek. Omdat gravitasie 'n baie swak krag is, kom die kenmerkende effekte daarvan egter slegs voor as die massas buitengewoon groot is. Die idee dat lig swaartekrag kan trek, is deur Michell voorgestel en deur die Franse wiskundige en sterrekundige Pierre-Simon Laplace ondersoek. Voorspellings deur klassieke fisika en algemene relatiwiteit dat lig wat naby die son kan aflei, word hierbo beskryf. Daar is nog twee gevolge vir sterrekunde. Lig van 'n ver voorwerp kan naby ander voorwerpe as die son beweeg en daardeur afgewyk word. Hulle kan veral deur 'n massiewe sterrestelsel afgewyk word. As een of ander voorwerp agter 'n massiewe sterrestelsel is, soos gesien vanaf die aarde, kan geboë lig die aarde met meer as een pad bereik. As die gewig van die sterrestelsel werk soos 'n lens wat langs verskillende paaie fokus, kan dit veroorsaak dat die voorwerp meerdere voorbeelde van sulke blykbare dubbele voorwerpe gevind het.

Beide Michell en Laplace het daarop gewys dat die aantrekkingskrag van 'n baie digte voorwerp op lig so groot kan wees dat die lig nooit uit die voorwerp kan ontsnap en dit onsigbaar maak nie. So 'n verskynsel is 'n swart gat. Die relativistiese teorie oor swart gate is die afgelope paar jaar deeglik ontwikkel, en sterrekundiges het uitgebreide waarnemings daaroor gedoen. Een moontlike klas swart gate bestaan ​​uit baie groot sterre wat al hul kernenergie opgebruik het, sodat hulle nie meer deur stralingsdruk omhoog gehou word nie en in swart gate ineengestort het (minder massiewe sterre kan in neutronsterre ineenstort). Daar word vermoed dat supermassiewe swart gate met massas miljoene tot miljarde keer dié van die son in die sentrums van die meeste sterrestelsels bestaan.

Swart gate, waaruit geen straling kan ontsnap nie, kan nie aan hul eie lig gesien word nie, maar daar is waarneembare sekondêre effekte. As 'n swart gat een komponent van 'n dubbele ster was, kan die baanbeweging van die paar en die massa van die onsigbare deel afgelei word van die ossillerende beweging van 'n sigbare metgesel. Omdat swart gate materie aantrek, sal enige gas in die omgewing van 'n voorwerp van hierdie soort daarin val en voordat dit in die gat verdwyn, 'n hoë snelheid en gevolglik 'n hoë temperatuur verkry. Die gas kan warm genoeg word om X-strale en gammastrale rondom die gat te produseer. So 'n meganisme is die oorsprong van ten minste 'n aantal kragtige X-straal- en radio-astronomiese bronne, insluitend dié in die sentrums van sterrestelsels en kwasars. In die geval van die massiewe sterrestelsel M87 is die supermassiewe swart gat in sy middel, wat 'n massa van 6,5 miljard keer die son het, direk waargeneem.


FOCAL Beyond Stars

Om maniere te vind om die swaartekraglens van die son te benut, dwing ons om die son se korona in ag te neem, 'n probleem wat Eshleman (Stanford) en Slava Turyshev (JPL) binnekort aangespreek het. Ons wil 'n ruimtetuig nie net tot 550 AE kry nie, maar ook verder as om koronale vervorming te vermy, en gebruik die feit dat ons nie te doen het met 'n fokuspunt nie, maar met 'n fokuspunt. Laat ek een van Claudio Maccone se referate hieroor aanhaal (aanhaling aan die einde van hierdie berig):

& # 8230 'n Eenvoudige, maar baie belangrike gevolg van die bespreking hierbo, is dat alle punte op die reguit lyn buite hierdie minimale fokusafstand ook brandpunte is, omdat die ligstrale wat verder as die minimum afstand verbygaan, kleiner afbuighoeke het en dus kom saam op 'n nog groter afstand van die son af.

Ons het dus die vermoë om ver buite 550 AU te beweeg, en het eintlik geen ander keuse as om dit te doen nie. Die Corona van die Son skep wat Maccone 'n 'uiteenlopende lenseffek' noem en staan ​​die konvergerende effek wat ons met 'n gravitasielens assosieer, teë. Die resultaat is dat die minimum afstand wat die FOCAL-vaartuig moet bereik (hier parafeer ek die papier) hoër is vir laer frekwensies (van die bron elektromagnetiese golwe wat die sonkorona oorsteek) en laer vir hoër frekwensies. Die fokus is dus by 500 GHz ongeveer 650 AU. Die fokus is op 160 GHz op 763 AU.

Maar is ons beperk tot die gebruik van die son en, as ons 'radiobruggies' bou soos gister bespreek, dan is daar nabygeleë sterre as swaartekraglense? Dit blyk dat ook planete vir hierdie doel gebruik kan word. In sy studie van 2011 oor hierdie idee, wat in Acta Astronautica, Maccone produseer die nodige vergelykings, en let op dat die verhouding van 'n planeet se radius in kwadraat tot sy massa ons die afstand kan bereken wat 'n ruimtetuig moet bereik om voordeel te trek uit die planetêre lens. Daaruit het ons gedefinieer wat hy die planeet s'n noem fokuspunt.

Beeld: Die volledige BELT van brandpunte tussen 550 en 17.000 AU vanaf die son, soos geskep deur die swaartekrag-lenseffek van die son en alle planete, hier getoon op skaal. Die ontdekking van hierdie gordel fokale sfere is die belangrikste resultaat wat in hierdie artikel aangebied word, tesame met die berekening van die relevante antenna-winste. Krediet: C. Maccone.


Inhoud

In September 1905 publiseer Albert Einstein sy teorie van spesiale relatiwiteit, wat Newton se bewegingswette met elektrodinamika (die wisselwerking tussen voorwerpe met elektriese lading) versoen. Spesiale relatiwiteit het 'n nuwe raamwerk vir die hele fisika ingestel deur nuwe konsepte van ruimte en tyd voor te stel. Sommige fisieke teorieë wat destyds aanvaar is, was nie in ooreenstemming met die raamwerk nie. 'N Belangrike voorbeeld hiervan was Newton se swaartekragteorie, wat die onderlinge aantrekkingskrag van liggame as gevolg van hul massa ervaar.

Verskeie natuurkundiges, waaronder Einstein, het gesoek na 'n teorie wat Newton se swaartekragwet en spesiale relatiwiteit sou versoen. Slegs Einstein se teorie was in ooreenstemming met eksperimente en waarnemings. Om die basiese idees van die teorie te verstaan, is dit insiggewend om Einstein se denke tussen 1907 en 1915 te volg, vanaf sy eenvoudige denke-eksperiment waarby 'n waarnemer in vrye val betrokke was tot sy volledig geometriese teorie van swaartekrag. [1]

Gelykwaardigheidsbeginsel Wysig

'N Persoon in 'n hysbak wat vry val, ervaar gewigloosheidsvoorwerpe wat óf bewegingloos dryf óf met konstante spoed dryf. Aangesien alles in die hysbak saamval, kan geen swaartekrag-effek waargeneem word nie. Op hierdie manier kan die ervarings van 'n waarnemer in vrye val onderskei word van dié van 'n waarnemer in die diep ruimte, ver van enige beduidende bron van swaartekrag. Sulke waarnemers is die bevoorregte ("traagheid") waarnemers wat Einstein in sy teorie van spesiale relatiwiteit beskryf het: waarnemers vir wie lig teen konstante spoed langs reguit lyne beweeg. [2]

Einstein het veronderstel dat die soortgelyke ervarings van gewiglose waarnemers en traagheidswaarnemers in spesiale relatiwiteit 'n fundamentele eienskap van swaartekrag verteenwoordig, en hy het dit die hoeksteen gemaak van sy teorie van algemene relatiwiteit, geformaliseer in sy ekwivalensiebeginsel. In beginsel lui die beginsel dat 'n persoon in 'n hysbak wat vry val, nie kan sê dat hy in vrye val is nie. Elke eksperiment in so 'n vryval-omgewing het dieselfde resultate as vir 'n waarnemer in rus of eenvormig in die diep ruimte, ver van alle swaartekragbronne af. [3]

Swaartekrag en versnelling

Die meeste gevolge van swaartekrag verdwyn in vrye val, maar effekte wat dieselfde lyk as die van swaartekrag, kan wees geproduseer deur 'n versnelde verwysingsraamwerk. 'N Waarnemer in 'n geslote kamer kan nie weet watter van die volgende dinge waar is nie:

  • Voorwerpe val op die vloer omdat die kamer op die oppervlak van die aarde rus en die voorwerpe deur swaartekrag afgetrek word.
  • Voorwerpe val op die vloer omdat die kamer aan boord van 'n vuurpyl in die ruimte is wat versnel met 9,81 m / s 2, die standaard swaartekrag op aarde, en ver van enige bron van swaartekrag is. Die voorwerpe word met dieselfde 'traagheidskrag' na die vloer getrek wat die bestuurder van 'n vinnige motor agter in hul sitplek druk.

Omgekeerd moet enige effek waargeneem in 'n versnelde verwysingsraamwerk ook waargeneem word in 'n gravitasieveld van ooreenstemmende sterkte. Met hierdie beginsel kon Einstein in 1907 verskeie nuwe effekte van swaartekrag voorspel, soos uiteengesit in die volgende afdeling.

'N Waarnemer in 'n versnelde verwysingsraamwerk moet bekendstel wat fisici fiktiewe kragte noem om die versnelling wat die waarnemer en voorwerpe rondom hulle ervaar, te verantwoord. In die voorbeeld van die bestuurder wat in hul sitplek gedruk word, is die krag wat deur die bestuurder gevoel word, een voorbeeld, die krag wat 'n mens kan voel terwyl hy die arms op en uit trek as hy probeer om soos 'n top te draai. Einstein se meesterlike insig was dat die konstante, bekende trek van die Aarde se gravitasieveld fundamenteel dieselfde is as hierdie fiktiewe kragte. [4] Die skynbare omvang van die fiktiewe kragte blyk altyd eweredig te wees met die massa van enige voorwerp waarop hulle inwerk - die bestuurdersstoel oefen byvoorbeeld net genoeg krag uit om die bestuurder in dieselfde tempo as die motor te versnel. Analoog het Einstein voorgestel dat 'n voorwerp in 'n gravitasieveld 'n swaartekrag moet voel wat eweredig is aan sy massa, soos vervat in die gravitasiewet van Newton. [5]

Fisiese gevolge Redigeer

In 1907 was Einstein nog agt jaar weg van die voltooiing van die algemene relatiwiteitsteorie. Nietemin kon hy 'n aantal nuwe, toetsbare voorspellings maak wat gebaseer was op sy vertrekpunt vir die ontwikkeling van sy nuwe teorie: die ekwivalensiebeginsel. [6]

Die eerste nuwe effek is die gravitasiefrekwensieverskuiwing van lig. Beskou twee waarnemers aan boord van 'n vinnige vuurpylskip. Aan boord van so 'n skip is daar 'n natuurlike konsep van 'op' en 'af': die rigting waarin die skip versnel, is 'op', en onverbonde voorwerpe versnel in die teenoorgestelde rigting en val 'afwaarts'. Neem aan dat een van die waarnemers 'hoër op' is as die ander. Wanneer die laer waarnemer 'n ligsein na die hoër waarnemer stuur, veroorsaak die versnelling dat die lig rooi verskuif word, soos bereken kan word uit die spesiale relatiwiteit. Die tweede waarnemer meet 'n laer frekwensie vir die lig as die eerste. Omgekeerd is die lig wat van die hoër waarnemer na die laer gestuur word, blou verskuif, dit wil sê na hoër frekwensies. [7] Einstein het aangevoer dat sulke frekwensieverskuiwings ook in 'n gravitasieveld waargeneem moet word. Dit word geïllustreer in die figuur links, wat 'n liggolf toon wat geleidelik rooi verskuif word terwyl dit opwaarts teen die swaartekragversnelling werk. Hierdie effek is eksperimenteel bevestig, soos hieronder beskryf.

Hierdie gravitasiefrekwensieverskuiwing stem ooreen met 'n gravitasietydverwyding: Aangesien die 'hoër' waarnemer dieselfde liggolf meet om 'n laer frekwensie te hê as die 'laer' waarnemer, moet die tyd vinniger verloop vir die hoër waarnemer. Tyd loop dus stadiger vir waarnemers wat laer in 'n gravitasieveld is.

Dit is belangrik om te beklemtoon dat daar vir elke waarnemer geen waarneembare veranderinge in die tydvloei is vir gebeure of prosesse wat in sy of haar verwysingsraamwerk rus nie. Eiers van vyf minute soos vasgestel deur die horlosie van elke waarnemer, het dieselfde konsekwentheid as wat daar een jaar op elke klok verbygaan, en elke waarnemer verouder volgens die hoeveelheid elke klok, kortom, stem ooreen met alle prosesse wat in sy onmiddellike omgewing plaasvind. Dit is eers as die horlosies tussen verskillende waarnemers vergelyk word, dat 'n mens vir die laer waarnemer stadiger loop as vir die hoër. [8] Hierdie effek is min, maar dit is ook eksperimenteel bevestig in veelvuldige eksperimente, soos hieronder beskryf.

Op 'n soortgelyke manier het Einstein die swaartekrag-afbuiging van lig voorspel: in 'n swaartekragveld word lig afwaarts afgebuig. Kwantitatief was sy resultate met twee faktore af, die korrekte afleiding vereis 'n meer volledige formulering van die teorie van algemene relatiwiteit, nie net die ekwivalensiebeginsel nie. [9]

Gety-effekte Wysig

Die ekwivalensie tussen gravitasie- en traagheidseffekte vorm nie 'n volledige teorie oor swaartekrag nie. Wat die verklaring van swaartekrag naby ons eie ligging op die aardoppervlak betref, en let op dat ons verwysingsraamwerk nie in vrye val is nie, sodat fiktiewe kragte te verwagte is, 'n geskikte verduideliking. Maar 'n vry valende verwysingsraamwerk aan die een kant van die aarde kan nie verklaar waarom die mense aan die ander kant van die aarde 'n swaartekrag in die teenoorgestelde rigting ervaar nie.

'N Meer basiese manifestasie van dieselfde effek behels twee liggame wat langs mekaar na die aarde val. In 'n verwysingsraamwerk wat in vrye val langs hierdie liggame is, lyk dit asof hulle gewigloos sweef - maar nie presies nie. Hierdie liggame val nie presies in dieselfde rigting nie, maar na 'n enkele punt in die ruimte: die Aarde se swaartepunt. Gevolglik is daar 'n komponent van elke liggaam se beweging na die ander (sien die figuur). In 'n klein omgewing soos 'n hysbak wat vry val, is hierdie relatiewe versnelling min, terwyl die valskermspringers weerskante van die aarde groot is. Sulke kragverskille is ook verantwoordelik vir die getye in die Aarde se oseane, dus word die term "gety-effek" vir hierdie verskynsel gebruik.

Die ekwivalensie tussen traagheid en swaartekrag kan nie gety-effekte verklaar nie - dit kan nie variasies in die gravitasieveld verklaar nie. [10] Daarvoor is 'n teorie nodig wat die manier beskryf waarop materie (soos die groot massa van die Aarde) die traagheidsomgewing rondom beïnvloed.

Van versnelling tot meetkunde Redigeer

In die ondersoek van die ekwivalensie van swaartekrag en versnelling sowel as die rol van getykragte, het Einstein verskeie analogieë met die meetkunde van oppervlaktes ontdek. 'N Voorbeeld is die oorgang van 'n traagheidsverwysingsraamwerk (waarin vrye deeltjies langs reguit paaie teen konstante snelhede beweeg) na 'n draaiende verwysingsraamwerk (waarin ekstra terme wat ooreenstem met fiktiewe kragte moet ingestel word om deeltjiebeweging te verklaar): hierdie is analoog aan die oorgang van 'n Cartesiese koördinaatstelsel (waarin die koördinaatlyne reguitlyne is) na 'n geboë koördinaatstelsel (waar koördinaatlyne nie reguit hoef te wees nie).

'N Dieper analogie hou verband met getykragte met 'n eienskap van genoemde oppervlaktes kromming. Vir gravitasievelde bepaal die afwesigheid of teenwoordigheid van getykragte of die invloed van swaartekrag geëlimineer kan word deur 'n vry valende verwysingsraamwerk te kies. Net so bepaal die afwesigheid of aanwesigheid van kromming of 'n oppervlak gelyk is aan 'n vlak. In die somer van 1912, geïnspireer deur hierdie analogieë, het Einstein na 'n geometriese formulering van swaartekrag gesoek. [11]

Die elementêre voorwerpe van meetkunde - punte, lyne, driehoeke - word tradisioneel in drie-dimensionele ruimte of op tweedimensionele oppervlaktes gedefinieer. In 1907 stel Hermann Minkowski, Einstein se voormalige wiskundeprofessor aan die Switserse Federale Polytechnic, die Minkowski-ruimte bekend, 'n meetkundige formulering van Einstein se spesiale relatiwiteitsteorie waar die meetkunde nie net ruimte nie, maar ook tyd insluit. Die basiese entiteit van hierdie nuwe meetkunde is vier-dimensionele ruimtetyd. Die wentelbane van bewegende liggame is krommes in die ruimtetyd. Die wentelbane van liggame wat met konstante snelheid beweeg sonder om van rigting te verander, stem ooreen met reguit lyne. [12]

Die meetkunde van algemene geboë oppervlaktes is in die vroeë 19de eeu deur Carl Friedrich Gauss ontwikkel. Hierdie meetkunde is op sy beurt veralgemeen na hoër-dimensionele ruimtes in die Riemanniaanse meetkunde wat deur Bernhard Riemann in die 1850's ingestel is. Met behulp van die Riemanniaanse meetkunde het Einstein 'n geometriese beskrywing van swaartekrag geformuleer waarin Minkowski se ruimtetyd vervang word met verwronge, geboë ruimtetyd, net soos geboë oppervlaktes 'n veralgemening van gewone plat oppervlaktes is. Inbou van diagramme word gebruik om geboë ruimtetyd in opvoedkundige kontekste te illustreer. [13] [14]

After he had realized the validity of this geometric analogy, it took Einstein a further three years to find the missing cornerstone of his theory: the equations describing how matter influences spacetime's curvature. Having formulated what are now known as Einstein's equations (or, more precisely, his field equations of gravity), he presented his new theory of gravity at several sessions of the Prussian Academy of Sciences in late 1915, culminating in his final presentation on November 25, 1915. [15]

Paraphrasing John Wheeler, Einstein's geometric theory of gravity can be summarized thus: spacetime tells matter how to move matter tells spacetime how to curve. [16] What this means is addressed in the following three sections, which explore the motion of so-called test particles, examine which properties of matter serve as a source for gravity, and, finally, introduce Einstein's equations, which relate these matter properties to the curvature of spacetime.

Probing the gravitational field Edit

In order to map a body's gravitational influence, it is useful to think about what physicists call probe or test particles: particles that are influenced by gravity, but are so small and light that we can neglect their own gravitational effect. In the absence of gravity and other external forces, a test particle moves along a straight line at a constant speed. In the language of spacetime, this is equivalent to saying that such test particles move along straight world lines in spacetime. In the presence of gravity, spacetime is non-Euclidean, or curved, and in curved spacetime straight world lines may not exist. Instead, test particles move along lines called geodesics, which are "as straight as possible", that is, they follow the shortest path between starting and ending points, taking the curvature into consideration.

A simple analogy is the following: In geodesy, the science of measuring Earth's size and shape, a geodesic (from Greek "geo", Earth, and "daiein", to divide) is the shortest route between two points on the Earth's surface. Approximately, such a route is a segment of a great circle, such as a line of longitude or the equator. These paths are certainly not straight, simply because they must follow the curvature of the Earth's surface. But they are as straight as is possible subject to this constraint.

The properties of geodesics differ from those of straight lines. For example, on a plane, parallel lines never meet, but this is not so for geodesics on the surface of the Earth: for example, lines of longitude are parallel at the equator, but intersect at the poles. Analogously, the world lines of test particles in free fall are spacetime geodesics, the straightest possible lines in spacetime. But still there are crucial differences between them and the truly straight lines that can be traced out in the gravity-free spacetime of special relativity. In special relativity, parallel geodesics remain parallel. In a gravitational field with tidal effects, this will not, in general, be the case. If, for example, two bodies are initially at rest relative to each other, but are then dropped in the Earth's gravitational field, they will move towards each other as they fall towards the Earth's center. [17]

Compared with planets and other astronomical bodies, the objects of everyday life (people, cars, houses, even mountains) have little mass. Where such objects are concerned, the laws governing the behavior of test particles are sufficient to describe what happens. Notably, in order to deflect a test particle from its geodesic path, an external force must be applied. A chair someone is sitting on applies an external upwards force preventing the person from falling freely towards the center of the Earth and thus following a geodesic, which they would otherwise be doing without matter in between them and the center of the Earth. In this way, general relativity explains the daily experience of gravity on the surface of the Earth nie as the downwards pull of a gravitational force, but as the upwards push of external forces. These forces deflect all bodies resting on the Earth's surface from the geodesics they would otherwise follow. [18] For matter objects whose own gravitational influence cannot be neglected, the laws of motion are somewhat more complicated than for test particles, although it remains true that spacetime tells matter how to move. [19]

Sources of gravity Edit

In Newton's description of gravity, the gravitational force is caused by matter. More precisely, it is caused by a specific property of material objects: their mass. In Einstein's theory and related theories of gravitation, curvature at every point in spacetime is also caused by whatever matter is present. Here, too, mass is a key property in determining the gravitational influence of matter. But in a relativistic theory of gravity, mass cannot be the only source of gravity. Relativity links mass with energy, and energy with momentum.

The equivalence between mass and energy, as expressed by the formula E = mc 2 , is the most famous consequence of special relativity. In relativity, mass and energy are two different ways of describing one physical quantity. If a physical system has energy, it also has the corresponding mass, and vice versa. In particular, all properties of a body that are associated with energy, such as its temperature or the binding energy of systems such as nuclei or molecules, contribute to that body's mass, and hence act as sources of gravity. [20]

In special relativity, energy is closely connected to momentum. Just as space and time are, in that theory, different aspects of a more comprehensive entity called spacetime, energy and momentum are merely different aspects of a unified, four-dimensional quantity that physicists call four-momentum. In consequence, if energy is a source of gravity, momentum must be a source as well. The same is true for quantities that are directly related to energy and momentum, namely internal pressure and tension. Taken together, in general relativity it is mass, energy, momentum, pressure and tension that serve as sources of gravity: they are how matter tells spacetime how to curve. In the theory's mathematical formulation, all these quantities are but aspects of a more general physical quantity called the energy–momentum tensor. [21]

Einstein's equations Edit

Einstein's equations are the centerpiece of general relativity. They provide a precise formulation of the relationship between spacetime geometry and the properties of matter, using the language of mathematics. More concretely, they are formulated using the concepts of Riemannian geometry, in which the geometric properties of a space (or a spacetime) are described by a quantity called a metric. The metric encodes the information needed to compute the fundamental geometric notions of distance and angle in a curved space (or spacetime).

A spherical surface like that of the Earth provides a simple example. The location of any point on the surface can be described by two coordinates: the geographic latitude and longitude. Unlike the Cartesian coordinates of the plane, coordinate differences are not the same as distances on the surface, as shown in the diagram on the right: for someone at the equator, moving 30 degrees of longitude westward (magenta line) corresponds to a distance of roughly 3,300 kilometers (2,100 mi), while for someone at a latitude of 55 degrees, moving 30 degrees of longitude westward (blue line) covers a distance of merely 1,900 kilometers (1,200 mi). Coordinates therefore do not provide enough information to describe the geometry of a spherical surface, or indeed the geometry of any more complicated space or spacetime. That information is precisely what is encoded in the metric, which is a function defined at each point of the surface (or space, or spacetime) and relates coordinate differences to differences in distance. All other quantities that are of interest in geometry, such as the length of any given curve, or the angle at which two curves meet, can be computed from this metric function. [22]

The metric function and its rate of change from point to point can be used to define a geometrical quantity called the Riemann curvature tensor, which describes exactly how the Riemannian manifold, the spacetime in the theory of relativity, is curved at each point. As has already been mentioned, the matter content of the spacetime defines another quantity, the energy–momentum tensor T, and the principle that "spacetime tells matter how to move, and matter tells spacetime how to curve" means that these quantities must be related to each other. Einstein formulated this relation by using the Riemann curvature tensor and the metric to define another geometrical quantity G, now called the Einstein tensor, which describes some aspects of the way spacetime is curved. Einstein's equation then states that

i.e., up to a constant multiple, the quantity G (which measures curvature) is equated with the quantity T (which measures matter content). Here, G is the gravitational constant of Newtonian gravity, and c is the speed of light from special relativity.

This equation is often referred to in the plural as Einstein's equations, since the quantities G en T are each determined by several functions of the coordinates of spacetime, and the equations equate each of these component functions. [23] A solution of these equations describes a particular geometry of spacetime for example, the Schwarzschild solution describes the geometry around a spherical, non-rotating mass such as a star or a black hole, whereas the Kerr solution describes a rotating black hole. Still other solutions can describe a gravitational wave or, in the case of the Friedmann–Lemaître–Robertson–Walker solution, an expanding universe. The simplest solution is the uncurved Minkowski spacetime, the spacetime described by special relativity. [24]

No scientific theory is self-evidently true each is a model that must be checked by experiment. Newton's law of gravity was accepted because it accounted for the motion of planets and moons in the Solar System with considerable accuracy. As the precision of experimental measurements gradually improved, some discrepancies with Newton's predictions were observed, and these were accounted for in the general theory of relativity. Similarly, the predictions of general relativity must also be checked with experiment, and Einstein himself devised three tests now known as the classical tests of the theory:

  • Newtonian gravity predicts that the orbit which a single planet traces around a perfectly spherical star should be an ellipse. Einstein's theory predicts a more complicated curve: the planet behaves as if it were travelling around an ellipse, but at the same time, the ellipse as a whole is rotating slowly around the star. In the diagram on the right, the ellipse predicted by Newtonian gravity is shown in red, and part of the orbit predicted by Einstein in blue. For a planet orbiting the Sun, this deviation from Newton's orbits is known as the anomalous perihelion shift. The first measurement of this effect, for the planet Mercury, dates back to 1859. The most accurate results for Mercury and for other planets to date are based on measurements which were undertaken between 1966 and 1990, using radio telescopes. [25] General relativity predicts the correct anomalous perihelion shift for all planets where this can be measured accurately (Mercury, Venus and the Earth).
  • According to general relativity, light does not travel along straight lines when it propagates in a gravitational field. Instead, it is deflected in the presence of massive bodies. In particular, starlight is deflected as it passes near the Sun, leading to apparent shifts of up 1.75 arc seconds in the stars' positions in the sky (an arc second is equal to 1/3600 of a degree). In the framework of Newtonian gravity, a heuristic argument can be made that leads to light deflection by half that amount. The different predictions can be tested by observing stars that are close to the Sun during a solar eclipse. In this way, a British expedition to West Africa in 1919, directed by Arthur Eddington, confirmed that Einstein's prediction was correct, and the Newtonian predictions wrong, via observation of the May 1919 eclipse. Eddington's results were not very accurate subsequent observations of the deflection of the light of distant quasars by the Sun, which utilize highly accurate techniques of radio astronomy, have confirmed Eddington's results with significantly better precision (the first such measurements date from 1967, the most recent comprehensive analysis from 2004). [26] was first measured in a laboratory setting in 1959 by Pound and Rebka. It is also seen in astrophysical measurements, notably for light escaping the white dwarfSirius B. The related gravitational time dilation effect has been measured by transporting atomic clocks to altitudes of between tens and tens of thousands of kilometers (first by Hafele and Keating in 1971 most accurately to date by Gravity Probe A launched in 1976). [27]

Of these tests, only the perihelion advance of Mercury was known prior to Einstein's final publication of general relativity in 1916. The subsequent experimental confirmation of his other predictions, especially the first measurements of the deflection of light by the sun in 1919, catapulted Einstein to international stardom. [28] These three experiments justified adopting general relativity over Newton's theory and, incidentally, over a number of alternatives to general relativity that had been proposed.

Further tests of general relativity include precision measurements of the Shapiro effect or gravitational time delay for light, measured in 2002 by the Cassini space probe. One set of tests focuses on effects predicted by general relativity for the behavior of gyroscopes travelling through space. One of these effects, geodetic precession, has been tested with the Lunar Laser Ranging Experiment (high-precision measurements of the orbit of the Moon). Another, which is related to rotating masses, is called frame-dragging. The geodetic and frame-dragging effects were both tested by the Gravity Probe B satellite experiment launched in 2004, with results confirming relativity to within 0.5% and 15%, respectively, as of December 2008. [29]

By cosmic standards, gravity throughout the solar system is weak. Since the differences between the predictions of Einstein's and Newton's theories are most pronounced when gravity is strong, physicists have long been interested in testing various relativistic effects in a setting with comparatively strong gravitational fields. This has become possible thanks to precision observations of binary pulsars. In such a star system, two highly compact neutron stars orbit each other. At least one of them is a pulsar – an astronomical object that emits a tight beam of radiowaves. These beams strike the Earth at very regular intervals, similarly to the way that the rotating beam of a lighthouse means that an observer sees the lighthouse blink, and can be observed as a highly regular series of pulses. General relativity predicts specific deviations from the regularity of these radio pulses. For instance, at times when the radio waves pass close to the other neutron star, they should be deflected by the star's gravitational field. The observed pulse patterns are impressively close to those predicted by general relativity. [30]

One particular set of observations is related to eminently useful practical applications, namely to satellite navigation systems such as the Global Positioning System that are used for both precise positioning and timekeeping. Such systems rely on two sets of atomic clocks: clocks aboard satellites orbiting the Earth, and reference clocks stationed on the Earth's surface. General relativity predicts that these two sets of clocks should tick at slightly different rates, due to their different motions (an effect already predicted by special relativity) and their different positions within the Earth's gravitational field. In order to ensure the system's accuracy, either the satellite clocks are slowed down by a relativistic factor, or that same factor is made part of the evaluation algorithm. In turn, tests of the system's accuracy (especially the very thorough measurements that are part of the definition of universal coordinated time) are testament to the validity of the relativistic predictions. [31]

A number of other tests have probed the validity of various versions of the equivalence principle strictly speaking, all measurements of gravitational time dilation are tests of the weak version of that principle, not of general relativity itself. So far, general relativity has passed all observational tests. [32]

Models based on general relativity play an important role in astrophysics the success of these models is further testament to the theory's validity.

Gravitational lensing Edit

Since light is deflected in a gravitational field, it is possible for the light of a distant object to reach an observer along two or more paths. For instance, light of a very distant object such as a quasar can pass along one side of a massive galaxy and be deflected slightly so as to reach an observer on Earth, while light passing along the opposite side of that same galaxy is deflected as well, reaching the same observer from a slightly different direction. As a result, that particular observer will see one astronomical object in two different places in the night sky. This kind of focussing is well known when it comes to optical lenses, and hence the corresponding gravitational effect is called gravitational lensing. [33]

Observational astronomy uses lensing effects as an important tool to infer properties of the lensing object. Even in cases where that object is not directly visible, the shape of a lensed image provides information about the mass distribution responsible for the light deflection. In particular, gravitational lensing provides one way to measure the distribution of dark matter, which does not give off light and can be observed only by its gravitational effects. One particularly interesting application are large-scale observations, where the lensing masses are spread out over a significant fraction of the observable universe, and can be used to obtain information about the large-scale properties and evolution of our cosmos. [34]

Gravitational waves Edit

Gravitational waves, a direct consequence of Einstein's theory, are distortions of geometry that propagate at the speed of light, and can be thought of as ripples in spacetime. They should not be confused with the gravity waves of fluid dynamics, which are a different concept.

In February 2016, the Advanced LIGO team announced that they had directly observed gravitational waves from a black hole merger. [35]

Indirectly, the effect of gravitational waves had been detected in observations of specific binary stars. Such pairs of stars orbit each other and, as they do so, gradually lose energy by emitting gravitational waves. For ordinary stars like the Sun, this energy loss would be too small to be detectable, but this energy loss was observed in 1974 in a binary pulsar called PSR1913+16. In such a system, one of the orbiting stars is a pulsar. This has two consequences: a pulsar is an extremely dense object known as a neutron star, for which gravitational wave emission is much stronger than for ordinary stars. Also, a pulsar emits a narrow beam of electromagnetic radiation from its magnetic poles. As the pulsar rotates, its beam sweeps over the Earth, where it is seen as a regular series of radio pulses, just as a ship at sea observes regular flashes of light from the rotating light in a lighthouse. This regular pattern of radio pulses functions as a highly accurate "clock". It can be used to time the double star's orbital period, and it reacts sensitively to distortions of spacetime in its immediate neighborhood.

The discoverers of PSR1913+16, Russell Hulse and Joseph Taylor, were awarded the Nobel Prize in Physics in 1993. Since then, several other binary pulsars have been found. The most useful are those in which both stars are pulsars, since they provide accurate tests of general relativity. [36]

Currently, a number of land-based gravitational wave detectors are in operation, and a mission to launch a space-based detector, LISA, is currently under development, with a precursor mission (LISA Pathfinder) which was launched in 2015. Gravitational wave observations can be used to obtain information about compact objects such as neutron stars and black holes, and also to probe the state of the early universe fractions of a second after the Big Bang. [37]

Black holes Edit

When mass is concentrated into a sufficiently compact region of space, general relativity predicts the formation of a black hole – a region of space with a gravitational effect so strong that not even light can escape. Certain types of black holes are thought to be the final state in the evolution of massive stars. On the other hand, supermassive black holes with the mass of millions or billions of Suns are assumed to reside in the cores of most galaxies, and they play a key role in current models of how galaxies have formed over the past billions of years. [38]

Matter falling onto a compact object is one of the most efficient mechanisms for releasing energy in the form of radiation, and matter falling onto black holes is thought to be responsible for some of the brightest astronomical phenomena imaginable. Notable examples of great interest to astronomers are quasars and other types of active galactic nuclei. Under the right conditions, falling matter accumulating around a black hole can lead to the formation of jets, in which focused beams of matter are flung away into space at speeds near that of light. [39]

There are several properties that make black holes the most promising sources of gravitational waves. One reason is that black holes are the most compact objects that can orbit each other as part of a binary system as a result, the gravitational waves emitted by such a system are especially strong. Another reason follows from what are called black-hole uniqueness theorems: over time, black holes retain only a minimal set of distinguishing features (these theorems have become known as "no-hair" theorems), regardless of the starting geometric shape. For instance, in the long term, the collapse of a hypothetical matter cube will not result in a cube-shaped black hole. Instead, the resulting black hole will be indistinguishable from a black hole formed by the collapse of a spherical mass. In its transition to a spherical shape, the black hole formed by the collapse of a more complicated shape will emit gravitational waves. [40]

Cosmology Edit

One of the most important aspects of general relativity is that it can be applied to the universe as a whole. A key point is that, on large scales, our universe appears to be constructed along very simple lines: all current observations suggest that, on average, the structure of the cosmos should be approximately the same, regardless of an observer's location or direction of observation: the universe is approximately homogeneous and isotropic. Such comparatively simple universes can be described by simple solutions of Einstein's equations. The current cosmological models of the universe are obtained by combining these simple solutions to general relativity with theories describing the properties of the universe's matter content, namely thermodynamics, nuclear- and particle physics. According to these models, our present universe emerged from an extremely dense high-temperature state – the Big Bang – roughly 14 billion years ago and has been expanding ever since. [41]

Einstein's equations can be generalized by adding a term called the cosmological constant. When this term is present, empty space itself acts as a source of attractive (or, less commonly, repulsive) gravity. Einstein originally introduced this term in his pioneering 1917 paper on cosmology, with a very specific motivation: contemporary cosmological thought held the universe to be static, and the additional term was required for constructing static model universes within the framework of general relativity. When it became apparent that the universe is not static, but expanding, Einstein was quick to discard this additional term. Since the end of the 1990s, however, astronomical evidence indicating an accelerating expansion consistent with a cosmological constant – or, equivalently, with a particular and ubiquitous kind of dark energy – has steadily been accumulating. [42]

General relativity is very successful in providing a framework for accurate models which describe an impressive array of physical phenomena. On the other hand, there are many interesting open questions, and in particular, the theory as a whole is almost certainly incomplete. [43]

In contrast to all other modern theories of fundamental interactions, general relativity is a classical theory: it does not include the effects of quantum physics. The quest for a quantum version of general relativity addresses one of the most fundamental open questions in physics. While there are promising candidates for such a theory of quantum gravity, notably string theory and loop quantum gravity, there is at present no consistent and complete theory. It has long been hoped that a theory of quantum gravity would also eliminate another problematic feature of general relativity: the presence of spacetime singularities. These singularities are boundaries ("sharp edges") of spacetime at which geometry becomes ill-defined, with the consequence that general relativity itself loses its predictive power. Furthermore, there are so-called singularity theorems which predict that such singularities must exist within the universe if the laws of general relativity were to hold without any quantum modifications. The best-known examples are the singularities associated with the model universes that describe black holes and the beginning of the universe. [44]

Other attempts to modify general relativity have been made in the context of cosmology. In the modern cosmological models, most energy in the universe is in forms that have never been detected directly, namely dark energy and dark matter. There have been several controversial proposals to remove the need for these enigmatic forms of matter and energy, by modifying the laws governing gravity and the dynamics of cosmic expansion, for example modified Newtonian dynamics. [45]

Beyond the challenges of quantum effects and cosmology, research on general relativity is rich with possibilities for further exploration: mathematical relativists explore the nature of singularities and the fundamental properties of Einstein's equations, [46] and ever more comprehensive computer simulations of specific spacetimes (such as those describing merging black holes) are run. [47] More than one hundred years after the theory was first published, research is more active than ever. [48]


Perfect scale

The “gravitational lens” works like a weighing scale, with the light deflection of the background star being analogous to the movement of the needle on the scale. That’s because gravitational strength depends on mass – the bigger the mass, the bigger the effect of gravitational lensing. Consequently, after spending a further year and a half on careful analysis of the acquired data, we were able to directly obtain the mass of Stein 2051 B from the measured deflection of the background star. Stein 2051 B turned out to be 68% the mass of the sun.

While Eddington measured an already incredibly small angle of 1.7 arcseconds – roughly corresponding to the diameter of a human hair seen from 10 metres distance – the measured shift of the background star aligned with Stein 2051 B was 1,000 times smaller, up to 0.002 arcseconds. This reflects the fact that the space curvature is quite small.

In fact, the bending of light in curved space is quite similar to a ball rolling along the surface of Earth. While the Earth’s surface looks flat to us at first sight as we stand on it, the rolling ball follows its small curvature rather than moving strictly in a straight line. After rolling just about 6cm, its direction will have changed by 0.002 arcseconds.

Despite the huge success of Eddington’s observations of light bending by the sun, Einstein was sceptical about the prospects for observing this for other stars. In 1936, he concluded: “Of course, there is no hope of observing this phenomenon directly.” What he could not have predicted were the technological advances of the decades to come, such as the advent of fast computing engines and digital cameras.

The gravity of a luminous red galaxy has gravitationally distorted the light from a much more distant blue galaxy. NASA/ESA

The bending of light by stars is known as “gravitational microlensing”. Unlike the arc-like shapes of galaxies resulting from gravitational lensing (see image above), this weak phenomenon does not lead to observable image distortions. Crucially, it depends on a close alignment between background and foreground stars, which is quite rare. In principle, a foreground star creates two images of the background star, differing in luminosity. Their combined light can then lead to an apparent brightening of the background star as the intervening foreground star passes near the line of sight.

This effect, known as “photometric microlensing”, has been observed lots of times before. However, the measured positional shift of the star passing by Stein 2051 B marks the first ever observation of “astrometric microlensing”.

This latter effect holds the potential to shed new light on how stars evolve by surveying stellar remnants (white dwarfs, neutron stars and black holes) in our neighbourhood – along with brown dwarfs (“failed” stars not massive enough to sustain the nuclear fusion of hydrogen). These otherwise escape detection due to being faint or invisible, but gravitational lensing relies solely on their mass rather than their light.

By the end of its mission in 2019, ESA’s Gaia satellite will have found astrometric microlensing signatures that will provide reliable mass measurements for more than a thousand bodies, turning astrometric microlensing from a most curious effect into a useful astrophysical tool.

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Could something missing in the universe be revealed by ripples in spacetime?

For all its planets and stars and black holes and mind-blowing phenomena, the universe seems to be missing something, but that something might just be hiding.

Every strange and fascinating thing out there is supposed to belong in the universe. So what has gone unseen? New research suggests that gravitational waves could help figure out more about the mysterious dark energy thought to be lurking in the void. It is possible that gravitational waves—ripples in spacetime—could illuminate dark energy. These ripples encounter supermassive black holes or enormous galaxies as they traverse space.

More Astronomy

Because it has been proven that gravitational waves (which are possibly everywhere in galaxy IC 10, above) are are bent when they pass through or near these objects, dark energy might also have an effect on them.

“Gravitational waves can be used probe the nature of dark energy,” Jose Maria Ezquiaga, who coauthored a paper recently published in Physical Review Letters, told SYFY WIRE. “If the dark energy is in its essence a modification of gravity, this will affect the way in which the gravitational waves propagate. This is in some sense similar to the use of light to probe the nature of some material. In other words, gravitational waves can be used as probes of the components of the universe.

Dark energy is allegedly behind the universe’s expansion, but the problem is that its origin remains unknown. There are scientists who do not even think it exists. If it really is dark energy that is causing the accelerated expansion of the universe, gravitational waves, which emerge from black holes and neutron stars colliding, may tell us something as they trek through the darkness. If, as Ezquiaga said, dark energy is a strange way that gravity can be modified, it should affect gravitational waves.

The galaxies and black holes that ripples in spacetime run into have a tremendous amount of gravity. That level of gravity will bend the trajectory of a gravitational wave. When enormous globs of mass distort surrounding space, as described in Einstein’s general theory of relativity, they create a gravitational field that magnifies light behind them and makes them more observable. This is gravitational lensing. It is often taken advantage of by telescopes like Hubble to study faraway galaxies that are otherwise beyond what our technology can see. However, light is not the only thing that gravitational lensing can bend.

“If gravity is modified, then these modifications are a good place to look,” Ezquiaga said. “If a gravitational wave crosses these mediums, it can generate waves associated with the additional components of gravity. In many theories these are scalar waves, which differ from the gravitational waves in their polarization properties.”

When gravitational waves venture close to an object massive enough to be capable of lensing, they are supposed to either release an “echo” or get scrambled. This is where scalar waves come in. Scalar waves, which may or may not exist in the realm of physics depending on who you ask, are electromagnetic waves believed to run lengthwise. When gravitational waves come close to an object with intense gravity, the difference in speed between them and the scalar waves that are generated is what determines whether the gravitational waves echo or end up emitting a scrambled signal.

If there is enough difference in speed between the two types of waves, it will cause the gravitational wave to split in two, sending out an echo. This can also occur if scalar waves are generated in an expanse of space that is large enough. If there is not enough difference in speed and the delay is shorter than the time it takes for the gravitational wave to pass by a massive object, things get scrambled. Searching for these echoes in gravitational wave data might tell us what it is encountering, and whether it does come face to face with dark energy.

Ezquiaga believes that how we look for dark energy in the future depends on what evidence we find of gravity being modified.

“If some of these modifications are found, then the properties of the signal can serve to constrain the possible modifications of gravity,” he said. “For example, information about the time delay between the echoes or the polarization content of the signal will be very important. If no such modification is found, we can discard some theories. These constraints will become stronger as more gravitational waves are detected.”

Even though neither we nor our most powerful telescopes can see it, dark energy may not stay in the dark forever.


Woordelys

black holes: objects having such large gravitational fields that things can fall in, but nothing, not even light, can escape

general relativity: Einstein’s theory that describes all types of relative motion including accelerated motion and the effects of gravity

gravitational waves: mass-created distortions in space that propagate at the speed of light and that are predicted by general relativity

escape velocity: takeoff velocity when kinetic energy just cancels gravitational potential energy

event horizon: the distance from the object at which the escape velocity is exactly the speed of light

neutron stars: literally a star composed of neutrons

Schwarzschild radius: the radius of the event horizon

thought experiment: mental analysis of certain carefully and clearly defined situations to develop an idea

quasars: the moderately distant galaxies that emit as much or more energy than a normal galaxy

Quantum gravity: the theory that deals with particle exchange of gravitons as the mechanism for the force


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