Ten years after the first ever detection of a gravitational wave, researchers from LIGO, Virgo, and KAGRA (LVK) have confirmed a crucial insight into the surface of black holes. “Hawking would have enjoyed this.”
In an article in the journal Physical Review Letters, researchers from the LVK experiments analyze a collision between black holes that was observed on January 15th this year using the two LIGO detectors in the US. At a distance of 1.4 billion light-years from Earth, two black holes with a mass of 30-40 solar masses merged with a tremendous bang, shaking spacetime itself.
Gravitational waves are vibrations of space-time that arise when extremely compact objects such as black holes or neutron stars collide or merge. This follows from Einstein’s theory of relativity. He predicted the waves at the beginning of the last century, but doubted whether they could be measured.
Challenge
Detecting gravitational waves is indeed a challenge. Lengths vary by less than one ten-thousandth of the diameter of a hydrogen nucleus when such a distortion passes. This can only be measured with kilometer-long mirror systems and lasers.
On September 14th, 2015, such a vibration was actually observed for the first time with the LIGO detectors in the US, which had just been put into operation at that time. The detection GW150914, in which the European Virgo detector, co-run by Nikhef, also played a major role, was global news in 2016. Since then, new techniques have been introduced in the laser setups, further reducing noise.
Wealth of information
The current detectors in the US, Europe, and Japan together detect such a collision somewhere in the universe about once every three days. A passing wave causes the lengths of the installations to vary slightly. The precise shape of the movement provides a wealth of physical information about the colliding black holes.
According to astronomers, black holes are created by the collapse of burnt-out stars. These are extremely compact masses in the universe that distort space-time so strongly that no light can escape from these pits in space itself. Black holes therefore have an edge that acts as a spherical horizon; what happens behind that surface is invisible. Anything that crosses the horizon disappears forever.
Larger surface
The conclusion of the new study is that the surface area of the newly formed black hole is, with 99.999 percent certainty, significantly larger than the two original surfaces combined. Before the collision, the combined surface area of the holes was 240,000 square kilometers (roughly the size of the United Kingdom), and after the collision, it was 400,000 square kilometers (the size of Sweden).
This result neatly ties in with a statement made by British theorist Stephen Hawking in 1971 that the horizon of a black hole can only increase. Thanks to the work of Hawking and his colleague Jacob Bekenstein, the surface area of a black hole is considered a measure of the entropy of the system, i.e. the degree of disorder. According to the basic laws of physics, this can only increase. This fact is an important element in attempts to unify relativity theory and quantum theory.
Testing the theorem
In 2016, Hawking called theorist Kip Thorne of Caltech, one of the founding fathers of the LIGO detector, to ask whether the observations could test his famous surface area theorem. Hawking died in 2018 and never received that confirmation. “But if Stephen were still alive, he would have enjoyed what has now been observed,” says Thorne, who received a Nobel Prize in 2017 for his work on gravitational waves.
The new study on observation GW250114 (code for a gravitational wave observed on January 14th, 2025) shows how accurately current detectors can now measure the signals. The collision is very similar to the first one detected in 2015: a merger of two black holes with masses of 30 to 40 solar masses, approximately 1.3 billion light-years away.
Ringdown
Ten years ago, the detection of a gravitational wave was a scientific breakthrough in itself. Since then, measurement techniques have improved significantly, and laser setups can now also record the shape of passing space vibrations with great precision. “We hear them loud and clear,” say the authors of the study.
In particular, the so-called ringdown of a vibrating black hole created by the collision of two smaller black holes, the fading of the waves after the impact, provides detailed information about this. Among other things, mass and rotation can be deduced from this.
Journey
Professor of relativity Chris Van Den Broeck of Nikhef and Utrecht University, who has long been associated with the LVK collaboration, says that event GW250114 shows how feasible it has become to directly measure fundamental properties of black holes. “This is the beginning of an amazing journey of discovery.”
He says he is delighted with what LIGO, Virgo, and KAGRA have already shown. At the same time, he says he is looking forward to even better measurement techniques, for example with the Einstein Telescope, a proposed underground gravitational wave observatory with ten kilometer long arms. It may be built in the border region of the Netherlands, Belgium, and Germany.
Nikhef
Next week, physicists all around the world will celebrate the tenth anniversary of the first observation of a gravitational wave. The 2015 observation opened a new window on the universe, which until then could only be studied through light and radio waves. Nikhef in Amsterdam will also be celebrating the birth of this field of study.
