Black hole merger provides strongest evidence yet for Hawking area law

Black hole merger provides strongest evidence yet for Hawking area law
by Clarence Oxford
Los Angeles CA (SPX) Sep 19, 2025
In a study published in Physical Review Letters, the LIGO - Virgo - KAGRA Collaboration reported its clearest gravitational wave signal yet from a black hole merger, offering the most precise confirmation to date of Stephen Hawking's area theorem. The signal, designated GW250114, was recorded in early 2025 during LIGO's latest observing run, using detectors nearly four times as sensitive as those from a decade ago.

"This specific collision involved two black holes that looked pretty much identical to the first two we saw," said Maximiliano Isi, assistant professor at Columbia University and coauthor of the study. Each was about 30 times the mass of the Sun. "Intrinsically, the signal is equally loud, but our detectors are just so much more high fidelity now."

Isi previously led a 2021 study that attempted to test the theorem with the first-ever detected signal from 2015, but the weaker data limited the analysis. The improved clarity of GW250114 enabled the researchers to isolate the post-merger "ringdown" - the oscillations of spacetime as the remnant black hole stabilized.

"The ringdown is what happens when a black hole is perturbed, just as a bell rings when you strike it," explained Katerina Chatziioannou, assistant professor at Caltech and coauthor. By identifying the distinct tones in the ringdown, the team confirmed the remnant behaved as predicted by the Kerr metric, the mathematical description of rotating black holes in Einstein's general relativity.

The study also validated Hawking's 1971 prediction that the event horizon's surface area must increase following a merger. "Even though it's a very simple statement, 'areas can only increase,' it has immense implications," Isi said. The result reinforces the view of black holes as thermodynamic objects, mirroring the second law of thermodynamics and linking black hole entropy to surface area.

Upgrades to LIGO and its partner detectors now yield signals at a far higher rate. "Ten years ago, we were observing signals about once per month," said Chatziioannou. "Today, we are observing signals about once every three days." This growing dataset promises sharper insights into black holes, astrophysics, and the underlying structure of the universe.

"This is really important as a tool in astrophysics and cosmology," said Robert Wald, theoretical physicist at the University of Chicago and coauthor. "The observatory, I think, is the key thing."

Research Report:GW250114: Testing Hawking's Area Law and the Kerr Nature of Black Holes