Almost exactly ten years ago, on September 14, 20915, the first detection of gravitational waves – the vibrations of the space -time were made by the fusion of two black holes. Now the Ligo gravitational wave detectors in the United States have again captured the signals of such an event. Thanks to the unprecedented clarity and dissolution of this gravitational wave signal, the GW250114 event made it possible to confirm two basic theories of black holes. The first comes from the British physicist Stephen Hawking and states that the area surrounded by the event horizon can never become smaller with such a fusion. The second theory set up by New Zealand mathematicians Roy Kerr says that black holes can be clearly described by only two parameters: their spin and its mass. Both theories have now been clearly proven for the first time.
Since commissioning the two league gravitational shaft detectors in the USA and later the European system Virgo in Italy and Kagra in Japan, astrophysicists have already captured the gravitational waves of around 220 cosmic collision events. When two massive objects move and then merge spirally, this releases energy, which is also manifested as a shock of space -time. The detectors register these weak, only a few minutes of waves because the interference pattern of two laser rays running in kilometers of tubes changes through these subtle compression and expansion. Most of the gravitational wave events previously detected comes from the fusion of two distant Stellar black holes. Some events also went back to the fusion of two neutron stars or unequal collisions of neutron star and black hole. Thanks to ever new progress in the technology of the gravitational wave detectors, the weak signals are now increasingly standing out from the background noise and, based on their wave features, reveals more details about the event.
Merging with overtones
This can also be seen in the breakthrough that is currently reported by the Ligo-Virgo-Kagra Collaboration. The GW250114 gravitational wave event was captured on January 14, 2025 by the two detectors of the Ligo gravitational wave observatory in the USA. The author of the signal was two around 32 sun mass heavy black holes at around 1.3 billion light years away. “This new pair of black holes is almost a twin of the historical first wave of gravitational wave from 2015,” says Maximiliano ISI from Columbia University in New York. “But our instruments are now so much better, so that today we could analyze the signal in a way that was not yet possible ten years ago.” Ligo, for example, detects the space for space with a signal-to-rush ratio of 80 to one-four times better than in the first gravitational wave event from 2015. “GW250114 is the loudest event-comparable to a loud call towards the initial whisper,” explains Geraint Pratten Pratten from University of Birmingham.
Thanks to this clarity and with the help of a method developed by ISI and his team a few years ago, the physicists have been able to precisely measure not only the spacetime waves of the approximate black holes, but also the back of the black hole resulting from the fusion. The signals of this subsequent sound only last a few milliseconds and can be recognized by special “overtones” of the signal. “Ten milliseconds are very short, but our instruments are now so good that this is sufficient to analyze the swinging of the resulting black hole,” explains ISI. “With the new detection, we have an extraordinarily detailed view of the signal – both before and after the fusion.” This revealed, among other things, that the resulting black hole is around 63 solar masses and turns 100 times a second.
Hawking area theorem and Kerr metric confirmed
The features that can be read on GW250114 have now made it possible for the team of researchers of the Ligo-Virgo-Kagra Collaboration to check two basic theories for black holes and to confirm using the observations. The first theory comes from the British physicist Stephen Hawking – a pioneer of researching black holes in 2018. In 1971 he had postulated that the area of two colliding holes surrounded by the event horizon had to be larger or at least the same after their merger. It can never be smaller with the resulting black hole. The reason for this is that the area of the black hole is proportional to its entropy – and this must increase according to the laws of thermodynamics. The GW250114 gravitational wave event now allowed ISI and his colleagues to measure these areas more precisely than before. This was the case: Before the merger, the two event horizons of the black holes enclosed an area of around 240,000 square kilometers – such as the area of Great Britain. The black hole resulting from the merger had an area of around 400,000 square kilometers. “The data confirms this prediction of Stephen Hawking,” said Pratten.
The second confirmed by the new data comes from the New Zealand mathematician Roy Kerr. In 1963, based on Einstein’s field equations, this developed a physical-mathematical system that describes the space-time geometry in the vicinity of a rotating black hole. Among other things, the Kerr metric explains how space-time and light are distorted by the black hole, but also allows a more precise measurement of black holes. A key message of the Kerr equations is that a black hole is a physically simple object that can be characterized by only two parameters: its mass and its rotation (spin). The vibrations of the resulting black hole, which are now captured for the first time in this clarity, reveal these parameters and thus also allowed the Kerr metric to check. “For the first time we were able to pick two overtones from the signal of the black hole and confirm that they behave exactly as Kerr predicted,” says Gregorio Carullo from the University of Birmingham. “This provides the unprecedented evidence that black holes in Kerr meters follow.”
Ten years after the first detection of a gravitational wave event, two important theories on the peculiarities of black holes were confirmed. “This field was based on purely mathematical and theoretical speculation for so long,” says Isi. “But now we are able to observe these amazing processes in action. This underlines the great progress in this field – and will pass on.” It is already planned to build another Ligo detector in Indiana. There are also plans for an even larger detector, Cosmic Explorer, whose measuring arms are supposed to be 40 kilometers long-the measuring lasers run through two four kilometers long tunnels. In Europe, a “Einstein Telescope” “Einstein Telescope” planned for the future is to get ten -kilometers long measurement arms.
Source: Ligo-Virgo-Kagra Collaboration, Physical Review Letters, DOI: 10.1103/KW5G-D732
