September 10, 2025• Physics 18, 160
The clearest black hole merger signal ever measured has allowed researchers to test the Kerr nature of black holes and validate Stephen Hawking’s black hole area theorem.
L. Reading-Ikkanda/Simons Foundation
Figure 1: Sketch of the gravitational-wave signal captured by the LIGO detectors. The signal encodes the various stages of the black hole merger, from the inspiral of two initial black holes to the ringdown of the remnant. With the highest signal-to-noise ever obtained for a black hole merger, the LVK Collaboration was able to identify both the fundamental and the first overtone modes of the ringdown signal.
L. Reading-Ikkanda/Simons Foundation
Figure 1: Sketch of the gravitational-wave signal captured by the LIGO detectors. The signal encodes the various stages of the black hole merger, from the inspiral of two initial black holes to the ringdown of the remnant. With the highest signal-to-noise ever obtained for a black hole merger, the LVK Collaboration was able to identify both the fundamental and the first overtone modes of the ringdown signal.×
Gravitational-wave astronomy is moving at breakneck speed. Just over a decade ago, the direct detection of gravitational waves was considered an elusive goal—perpetually said to be “five-to-ten years away.” Then came the 2015 breakthrough: the first observed merger of two black holes, known as GW150914 [1]. Detections have since become routine, with a catalog of black hole mergers now numbering in the hundreds. There is even evidence for a gravitational-wave background at nanohertz frequencies, plausibly sourced by a population of supermassive black hole binaries throughout the Universe. Now the LIGO detectors have captured the clearest merger signal ever recorded, GW250114 [2]. From such a signal, the LIGO-Virgo-KAGRA (LVK) Collaboration was able to draw two spectacular conclusions. First, it confirmed that the nature of the merging objects is consistent with that of Kerr (spinning) black holes. Second, it convincingly verified the area theorem formulated by Stephen Hawking in 1971, which asserts that the total surface area of black hole horizons cannot decrease in time [3]. These results are milestones that show how gravitational-wave observations are now able to probe some of the most fundamental laws of nature.
Today, three operational detectors—each based on kilometer-scale interferometers—enable the detection of gravitational waves: LIGO (in the US), Virgo (in Italy), and KAGRA (in Japan). The new detection, GW250114, which occurred on January 14 of this year, rang clearly in the two LIGO detectors as they were operating in their stable “science mode,” collecting data with high sensitivity for long periods of time (Virgo and KAGRA weren’t taking data at that point). The merger was similar to the first detection, GW150914, in terms of the distance from Earth, of the masses of the merging black holes, and of its strain, which is the stretching and compressing of distances induced by the passage of the gravitational wave. The strain for both events was vanishingly small (10–21), or about 1 m divided by the diameter of the Milky Way. But thanks to the remarkable progress in detector sensitivity, the signal-to-noise ratio increased from 26 to 80, a threefold improvement between the 2015 and the 2025 detections. GW250114 now stands as the sharpest merger signal observed to date, which allowed the LVK Collaboration to push the frontiers of testing theories of strong-field gravity and black holes.
When two black holes collide, the newborn remnant isn’t stationary but rings like a struck bell. In this “ringdown” phase, the black hole emits a gravitational-wave signal that encodes information about only two parameters: the remnant’s mass and its spin. The ringdown signal is expected to be composed of a set of “quasinormal modes.” The mode with the lowest frequency is the long-lived, fundamental mode. At higher frequencies are the more rapidly decaying overtones, which—loosely speaking—are equivalent to the higher harmonics of a plucked string. If the signal contains one or more overtones, researchers can check whether the remnant is a bona fide Kerr black hole, that is, a spinning black hole as predicted by general relativity.
Signals from a merger also offer the chance to test fundamental principles known as the laws of black hole mechanics, which could hold clues about the quantum nature of gravity and the relationship between quantum mechanics and classical thermodynamics. The second of these laws, formulated by Stephen Hawking, is the area theorem, which asserts that the total area of a black hole event horizon cannot decrease in time (at least, on timescales shorter than those of black hole evaporation, which are expected to exceed the age of the Universe). Gravitational waves allow us to test the area law by independently measuring, from the inspiral (premerger) signal, the masses and spins of the two merging black holes and, from the ringdown (postmerger) signal, the mass and spin of the remnant. Through data analysis, researchers can convert the masses and spins of the black holes into areas and check whether the final black hole area exceeds the sum of the areas of the initial black holes.
L. Reading-Ikkanda/Simons Foundation
Figure 2: By independently measuring the areas of the two merging black holes and that of the remnant black hole, the LVK Collaboration has verified Hawking’s area law.
L. Reading-Ikkanda/Simons Foundation
Figure 2: By independently measuring the areas of the two merging black holes and that of the remnant black hole, the LVK Collaboration has verified Hawking’s area law.×
Attempts to harness gravitational-wave signals in similar tests aren’t new. Indeed, the first claim of a validation of Hawking’s area theorem was published in 2021 [4]. The result was based on a reanalysis of GW150914 data, which concluded that the signal might contain the first overtone of the fundamental. That claim triggered years of back-and-forth discussions about the robustness of the underlying data analysis, leaving the field in a stalemate (See Synopsis: To Hear or Not to Hear Overtones in Black Hole Mergers and [5]). Now, the whopping loud GW250114 signal should clear lingering doubts (Fig. 1). With an exceptional signal-to-noise ratio, such a signal allowed for a robust identification of the dominant ringdown tone and its first overtone. The frequency and damping rates of each mode were found to be consistent with those expected for the Kerr spectrum of the black hole remnant. Using independent analyses of the pre- and postmerger signals (and neglecting data from the intractable merger phase), the researchers then derived the areas of the initial and final black holes, finding that, with high credibility, GW250114 complied with Hawking’s area law (Fig. 2).
The result is broadly important for the whole field. First, it closes a potential loophole on the extraction of overtones from the signal: It shows that, with a sufficiently high signal-to-noise, the presence of overtones is unambiguous—not interpretable as an artifact of fitting choices. Second, it sharpens thermodynamic tests by eliminating circular arguments that use conclusions on the postmerger phase to interpret the premerger one. In the case of GW250114, the inspiral data inform the determination of the areas of the two initial black holes, while the ringdown data inform only the determination of the final black hole’s area. Thanks to the independent analyses of the pre- and postmerger stages (for the inspiral, an analysis based on post-Newtonian or effective-one-body dynamics; for the ringdown, one based on the Kerr “quasinormal-mode spectrum”), the approach offers a genuine test of the area law or of its underlying assumptions.
GW250114 sets a new standard for the field. To claim overtone detections, future analyses of loud mergers will have to demonstrate similar robustness to model assumptions (in particular, to the choice of the merger start time). Ground-based interferometers have moved from detecting a few black hole binary mergers to precision tests of gravity, probing subtle features including quadrupole modes, overtones, and Hawking’s area law. With the expanding sample of detections from the next two runs (planned for this decade) and from next-generation upgrades, I can’t wait to see what’s next.
ReferencesB. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), “Observation of gravitational waves from a binary black hole merger,” Phys. Rev. Lett. 116, 061102 (2016).A. G. Abac et al. (LIGO Scientific, Virgo, and KAGRA Collaborations), “GW250114: Testing Hawking’s area law and the Kerr nature of black holes,” Phys. Rev. Lett. 135, 111403 (2025).S. W. Hawking, “Gravitational radiation from colliding black holes,” Phys. Rev. Lett. 26, 1344 (1971).M. Isi et al., “Testing the black-hole area law with GW150914,” Phys. Rev. Lett. 127, 011103 (2021).R. Cotesta et al., “Analysis of ringdown overtones in GW150914,” Phys. Rev. Lett. 129, 111102 (2022).About the Author
Chiara M. F. Mingarelli is an assistant professor of physics at Yale University. Her research connects gravitational-wave signals from supermassive black hole binaries with galaxy surveys, with a focus on pulsar timing arrays and on the anisotropy of the nanohertz gravitational-wave background. She has developed multimessenger strategies that link electromagnetic observations of active galactic nuclei to the gravitational-wave background and to signals from individual supermassive black hole binaries, creating detection protocols for pulsar timing data. Mingarelli is a member of NANOGrav and of the International Pulsar Timing Array and is a cochair of NASA’s Gravitational Wave Science Interest Group. She earned her PhD at the University of Birmingham, UK, and held positions at Caltech, the Max Planck Institute for Radio Astronomy, Germany, and the Flatiron Institute, New York.
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