Zwarte gaten: luisteren naar de grens van Einsteins theorie
Zwarte gaten slokken alles op dat te dichtbij komt, zelfs licht ontsnapt niet aan hun greep. We kunnen ze daarom niet zien, maar we kunnen ze wel horen. Op 14 september 2015 ving een detector in de Verenigde Staten voor het eerst zo’n geluid op, een fractie van een seconde, veroorzaakt door twee zwarte gaten die miljarden lichtjaren verderop met elkaar botsten en versmolten tot één nieuw zwart gat, dat nog even natrilde. Die trilling bevestigde precies wat Einstein een eeuw eerder had voorspeld. Maar wat als de volgende trilling dat niet doet? Wij onderzochten waar we dat het eerst zouden kunnen horen.
Twee zwarte gaten die botsen en trillingen uitsturen.
Waarom Einstein waarschijnlijk geen gelijk had
Einsteins algemene relativiteit beschrijft zwaartekracht niet als een kracht, maar als een kromming van ruimte en tijd zelf. Deze theorie bracht een revolutie teweeg in hoe we naar het universum kijken, van vallende appels tot de grootste kosmische schalen. In meer dan een eeuw heeft de theorie elke test overleefd die ze onderging, van het bestaan van zwarte gaten en zwaartekrachtsgolven tot kleine afwijkingen in de baan van Mercurius.
Ondanks dit overweldigende bewijs, zijn we steeds zekerder dat algemene relativiteit niet het laatste woord is. De theorie voorspelt singulariteiten: punten waar de kromming van de ruimtetijd oneindig groot wordt, waar de theorie tegen haar eigen grenzen aanloopt. Daarbovenop kan Einsteins theorie niet verenigd worden met kwantummechanica, onze beschrijving van het universum op de kleinste schaal. Daarom is de vraag niet langer óf Einstein gelijk heeft, maar waar de eerste afwijking zichtbaar zou kunnen worden.
En laten zwarte gaten daar nu een perfecte kandidaat voor zijn.
Luisteren naar de vorm van een zwart gat
Sla je een bel aan, dan gaat die trillen. Hoe hard je erop slaat bepaalt hoe luid het geluid is, maar de karakteristieke tonen worden bepaald door de eigenschappen van de bel. Door te luisteren naar die tonen, kun je dus de vorm van de bel bepalen. Exact hetzelfde gebeurt met zwarte gaten: als iets het zwarte gat treft, dan zendt het een karakteristieke set aan frequencies uit, die ons vertellen hoe het zwarte gat eruitziet. We kunnen letterlijk horen wat de eigenschappen van het zwarte gat zijn.
Hoe kleiner de bel, hoe hoger de toon. Hetzelfde geldt voor zwarte gaten.
Dat is precies wat zwarte gaten zo geschikt maakt om Einstein te testen: deze trillingen volgen exact uit zijn vergelijkingen. Elke noot ligt vast zodra je de massa en de spin van het zwarte gat kent, althans als Einstein gelijk heeft. Wijkt de gemeten trilling ook maar een fractie af van wat de vergelijkingen voorspellen, dan wijst dat naar iets wat Einstein niet zag aankomen. Het is dus een muzikale vingerafdruk van zwaartekracht zelf, en zo een directe manier om een theorie op de proef te stellen is zeldzaam.
Herhaalt de geschiedenis zich?
Begin twintigste eeuw ontdekten natuurkundigen iets vreemds: elk atoom gloeit op zijn eigen unieke manier. Verhit waterstof, en het licht dat vrijkomt heeft altijd exact dezelfde kleuren. Bij helium of andere elementen gebeurt hetzelfde: telkens een andere, maar altijd identieke vingerafdruk. Die ontdekking leek een detail, maar zette de deur open naar de kwantumrevolutie: een volledig nieuwe kijk op hoe de natuur in elkaar zit.
Zwarte gaten trillen op een verrassend gelijkaardige manier, met hun eigen unieke trillingen. Niemand weet zeker waar deze ontdekking naartoe leidt, maar de vorige keer dat we zoiets fundamenteel ontdekten, veranderde het dus voorgoed hoe we naar de natuur kijken.
Kleine stapjes, geen revolutie
Als je vermoedt dat Einstein ergens misging, is de verleiding groot om meteen met een nieuwe theorie te komen. Wij deden het omgekeerde: we gooiden Einsteins theorie niet overboord, maar pasten ze heel voorzichtig aan: met kleine, gecontroleerde correcties in plaats van een volledige herschrijving. Denk hierbij aan het bijstellen van een landkaart in plaats van deze weg te gooien. Je wilt weten waar er ergens een foutje in zit, niet de hele kaart opnieuw tekenen.
Met die kleine bijstellingen hebben we berekend hoe de trillingen van zwarte gaten zouden veranderen als Einstein geen gelijk had. Het resultaat vertelt ons hoe het getril specifiek zal veranderen. De vraag die volgt is dan: bij welke zwarte gaten, en in welk deel van het getril, zouden we die afwijking het eerst kunnen betrappen?
Klein en gevoelig
Het antwoord: kleine, snel roterende zwarte gaten zijn bijzonder gevoelig voor zulke afwijkingen. Niet elk zwart gat is dus even interessant. Voor kleine, snel roterende zwarte gaten veranderen de karakteristieke trillingen het sterkst wanneer zwaartekracht afwijkt van Einsteins voorspellingen. Dat maakt ze bijzonder interessante kandidaten om te beluisteren.
We hebben die voorspelling ook getoetst aan metingen van detectoren. Voorlopig doorstaat Einsteins theorie ook deze extra test. Dat betekent echter niet dat de zoektocht voorbij is. Integendeel, dankzij dit onderzoek weten we nu exact waar we moeten zoeken naar barstjes in Einsteins theorie, en hoe die barstjes eruit zouden zien.
Die kennis is exact wat de volgende generatie detectoren nodig heeft. Die nieuwe zwaartekrachtsgolfdetectoren zouden wel eens gevoelig genoeg kunnen zijn om zo een afwijking voor het eerst écht te ontdekken. Dan kunnen we gericht zoeken en met veel grotere precisie dan de huidige generatie kan.
Zwarte gaten in onze eigen achtertuin
Een van die nieuwe instrumenten, de Einstein Telescope, ET voor de vrienden, zou weleens vlak bij ons kunnen verrijzen: in de grensstreek tussen België, Nederland en Duitsland. Zeker is het nog niet, maar de kans is reëel.
Schematische voorstelling van de Einstein Telescope. Credits: NIKHEF/Marco Kraan
Tien jaar geleden hoorden we voor het eerst dat Einstein gelijk had. Voorlopig klinkt het heelal nog steeds zoals Einstein het berekende. Maar we zijn nog lang niet klaar met luisteren. Misschien staat met ET de volgende test wel letterlijk in onze achtertuin, en horen we dan eindelijk iets dat Einstein nooit voorspeld heeft.
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