In a new study published in Physical Review Letters, the LIGO–Virgo–KAGRA Collaboration has used the sharpest to date gravitational wave signal from two merging black holes to precisely test Hawking's area theorem and the remnant black hole's nature. The paper describes GW250114, a signal captured by LIGO during its current observing run in early 2025 — almost ten years after it detected a disturbance from a similar merger for the first time in history. This time, the detectors were nearly four times as sensitive.
"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, associate research scientist at the Flatiron Institute, and coauthor of the study. Both mergers were of black holes 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 led a 2021 study that used the 2015 signal to test Hawking's area theorem with a similar technique: splicing it to isolate certain frequencies, or tones. Isi and colleagues identified the tones associated with the remnant, which allowed them to infer the area of its event horizon — the region of the black hole where no light can escape — showing that it was possible to test Hawking's prediction. But the poorer signal limited their analyses.
With GW250114, a clearer signal with improved instrumentation allowed the authors to isolate the "ringing" in spacetime that emanated from the remnant as it settled. "The ringdown is what happens when a black hole is perturbed, just as a bell rings when you strike it," said Katerina Chatziioannou, assistant professor at Caltech and coauthor. By "hearing" the modes in the ringdown, the team worked out the properties of the black hole and confirmed that they were consistent with the Kerr metric — an exact solution to Einstein's field equations of general relativity for a rotating black hole, described by mathematician Roy Kerr over 60 years ago. "Two black holes with the same mass and spin are mathematically identical," said Isi.
"It's very unique to black holes."
The ringdown also confirmed that the remnant's surface area increased, as Hawking predicted in 1971. "Even though it's a very simple statement,
'areas can only increase,' it has immense implications," said Isi. Hawking's theorem — the second law of black hole mechanics — mirrors the second law of thermodynamics, which says that entropy, or disorder, can only increase. The theorem led to the realization that black holes are thermodynamic objects, a paradigm shift cemented by Hawking's discovery that they have entropy and emit radiation due to quantum effects near the event horizon.
"It tells us that general relativity knows something about the quantum nature of these objects and that the information, or entropy, contained in a black hole is proportional to its area," Isi added.
The discovery follows a series of recent upgrades to improve LIGO's sensitivity and range. "Ten years ago, we were observing signals about once per month," said Chatziioannou. "Today, we are observing signals about once every three days." As the technology continues to improve, scientists will obtain even crisper signals of these ripples in spacetime — and from them, a better understanding of the universe.
"This is really important as a tool in astrophysics and cosmology," said Robert Wald, a theoretical physicist and professor at the University of Chicago who contributed to the study. "The observatory, I think, is the key thing."
