But the final reverberations as the newly formed black hole settled into its new state, aka The Ringdown, were significantly muffled from that first event, and scientists were unable to distinguish the ring from the original collision and ring. GW250114 Ligo’s improved sensitivity meant that scientists could measure the frequency and duration of the merged black hole’s ring much more accurately. The resulting analysis echoes results from 2019 that confirmed the “hairless” theorem.
Audio comparison of gravitational wave signals from 2015 and 2025. Credit: Ligo/Jaunava/Kagra
With the latest development, physicists have obtained “an extremely detailed view of the signal both before and after the black hole merger,” said co-author Maximiliano Isi of Columbia University, who led a 2021 study using the same method on 2015 data to observe Hawking’s area theorem. As with the hair theorem, the clearer signal from GW250114 further strengthens that previous result. The data from GW250114 revealed that the combined surface area of the two original black holes was about 240,000 square kilometers, about the size of the United Kingdom. After the merger, the new black hole was about 400,000 square kilometers, about the size of Sweden.
“Although it’s a very simple statement – ‘Areas can only increase’ – it has enormous implications,” ISI said. Notably, Hawking and Jacob Bekenstein later showed that the area of a black hole is proportional to its entropy, which must also increase by the second law of thermodynamics. This is a key element in ongoing attempts to develop a theory of quantum gravity. “It’s really profound that the size of a black hole’s event horizon behaves like entropy,” ISI told the Local. “This means that some aspects of black holes can be used to mathematically probe the true nature of space and time.”
CALTECH PHYSICIST KIP THORNE, a longtime friend of Hawking, recalled that when Ligo detected its first gravitational wave signature, Hawking called and asked if the collaboration could test his theorem. Hawking died in 2018. “If [he] had been alive, he would have enjoyed seeing the rise of merging black holes,” Thorne told The Local.
Physical Review Letters, 2025. DOI: 10.1103/kw5g-d732 (About Dois).
