Gravitational waves have been detected for the first time, rippling from the merger of two mighty black holes 1.3 billion light years away in a distant galaxy. This history-making discovery comes exactly 100 years after Albert Einstein first predicted their existence.
“Ladies and gentlemen, we have detected gravitational waves – we did it!” announced David Reitze, the Executive Director of LIGO, the Laser Interferometer Gravitational-wave Observatory, which is the gravitational wave detector that made this incredible discovery possible, to a packed press conference in Washington DC.
Gravitational waves are a prediction of Albert Einstein’s General Theory of Relativity. Imagine space as a rubber sheet, pulled taut. If you place a heavy ball onto the sheet, the sheet dips, creating a gravitational well. This is, in effect, what all objects with mass in the Universe do. Place a marble on the slope of that dip in the rubber sheet, and it will roll down to the heavier ball at the bottom of the well – in other words, the heavier ball’s gravity pulls it down.
However, imagine that instead of a marble, you drop in another heavy ball. The two balls crash together with enough force to cause the rubber sheet to ripple. This is analogous to the gravitational waves that ripple through space following the merger of two massive black holes and you can think of the waves as gravitational ‘radiation’ emitted by the force of the merger.
On 14 September 2015 LIGO detected the merger of two black holes, one with a mass 29 times greater than the Sun and the other with a mass 36 times the Sun. When they merged, the formed a black hole with 62 solar masses and the remaining three solar masses was turned into energy and released as gravitational waves. This burst of gravitational waves lasted just 20 milliseconds, but in that short time packed in a power equivalent to fifty times the energy output of all the stars in the Universe.
“This the first time that this kind of system has ever been seen,” says Reitze. “It’s proof that binary black holes exist.”
So how did LIGO do it? It is composed of two individual detectors in the United States, one in Livingston in Louisiana and the other in Hanford in Washington State, and its research spans almost a thousand scientists worldwide. Each LIGO detector features two four-kilometre long arms in an L-shaped arrangement. A beam-splitter divides a laser beam into two and each beam is then directed down the two arms and are reflected back and forth up to 400 times before the beams re-merge. Crucially these two beams are kept out of phase so that when they recombine they cancel each other out and no signal is detected. However, when a gravitational wave passes through the detector, it stretches and squeezes the space within the arms so that it takes slightly longer than normal for the laser beams to return to the beam-splitter and recombine. This has the effect of putting the two beams somewhat back into phase with each other, so that they don’t totally cancel each other out. As a result a signal is detected, indicating the presence of a gravitational wave. The trouble is, the detectors must be incredibly sensitive to detect the gravitational wave above sources of noise such as earth tremors, passing vehicles and so on.
Having two detectors helps rule out interference – local noise will only be seen in one detector, whereas a signal seen by both the Louisiana and Washington detectors must have an astrophysical origin. The gravitational waves that rippled through the LIGO detectors created a signal equivalent to the waves distorting spacetime within LIGO’s arms by 10^–21 metres, or one one-thousandth of the diameter of a proton. The signal was seen by the Livingston detector just seven milliseconds before the gravitational waves passed through the Hanford detector.
The signal seen by the two detectors was identical and matched perfectly what numerical simulations based on the General Theory of Relativity predicted. Study of the signal shows the wave’s frequency and amplitude increasing as the black holes spiralled into one another. The frequency betrays the masses of the two black holes, while the amplitude gives an indication of the distance the waves have travelled – the closer we are, the greater the amplitude. Its exact location could not be pinpointed by just two detectors, but later this year VIRGO, the European Gravitational Observatory, will come online with a sensitivity almost equal to LIGO and, with three detectors working, it will be possible to triangulate the positions of gravitational wave bursts.
Furthermore, there is a new generation of gravitational wave detectors on the horizon. In December, the European Space Agency launched the LISA Pathfinder spacecraft, which is a technological precursor for the full LISA (Laser Interferometer Space Antenna) that is planned to launch in 2034. However, LIGO’s discovery of gravitational waves does not make LISA redundant. In space, far away from interfering noise, LISA will be able to detect gravitational waves coming from much smaller systems, such as binary neutron stars. Also on the horizon is the Einstein Observatory, which is being planned by a consortium of European science organisations and will be able to detect gravitational waves coming from binary star systems and to much greater precision than LIGO.
Details of the discovery of gravitational waves are published in the journal Physical Review Letters.