Hawking was right: black hole surfaces can only grow, not shrink
Ten years after the first gravitational wave detection, there is a new breakthrough
Ten years after the first ever detection of a gravitational wave, researchers from LIGO, Virgo, and KAGRA have confirmed a crucial insight into the surface of black holes. In , researchers from the three LIGO experiments analyse a collision between black holes that was observed on January 15 this year using the 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. 鈥淗awking would have enjoyed this.鈥
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, which predicted the waves at the beginning of the last century, but doubted whether they could be measured.
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.
Reducing noise
On August 14, 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.
Colliding black holes
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.
Significantly larger
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).
Stephen Hawking
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 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.
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.
Breakthrough in 2015
The new study on observation GW250114 (code for a gravitational wave on January 14, 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.
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.
Fundamental properties
Professor of relativity Chris Van Den Broeck of Nikhef and Utrecht 木瓜福利影视, 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." Anuradha Samajdar (GRASP) and Tanja Hinderer (ITP) are also enthusiastic about what LIGO, Virgo, and KAGRA have already shown. The researchers are looking forward to even better measurement techniques, for example with the Einstein Telescope, a proposed gravitational observatory with arms ten kilometers underground. It may be built in the border region of the Netherlands, Belgium, and Germany.