Clearest black hole merger signal yet allows probe of Hawking's law

On September 14, 2015, researchers using a pair of giant detectors in the US detected gravitational waves for the first time — a century after Albert Einstein predicted their existence  in his general theory of relativity. For their contributions to building these detectors, called the Laser Interferometer Gravitational-wave Observatories (LIGO), Rainer Weiss, Kip Thorne, and Barry Barish were awarded the Nobel Prize for physics in 2017.

According to the general theory of relativity, when any sufficiently massive object accelerates through spacetime, it will set off ripples of gravitational energy through its fabric. These gravitational waves can travel uninterrupted across billions of lightyears at the speed of light. The louder waves are produced by the most fierce cosmic events, including colliding neutron stars and black holes.

This year, as researchers worldwide celebrated the 10th anniversary of the first detection of gravitational waves, they also announced another groundbreaking discovery. A network of detectors — LIGO in the US, Virgo in Italy, and KAGRA in Japan — had detected a clearer gravitational wave signal from a pair of merging black holes. The event was named GW250114 as it was detected on January 14, 2025.

Notably, the researchers said this was the clearest gravitational wave signal detected to date, allowing them to use it to test some of the more elusive predictions of fundamental physics theories.

Their results were published in Physical Review Letters in September.

Black-hole hunter

The twin LIGO detectors first detected GW250114. Each LIGO consists of two 4-km-long arms arranged in an L-shape. The arms have a vacuum. At the elbow, a highly stable laser beam is split into two beams and sent down  the perpendicular arms, bouncing back and forth between mirrors about 300 times.

When no gravitational wave is passing through the detector, the two beams travel exactly the same distance and cancel each other out when they recombine at a photodetector at the elbow. But when a gravitational wave is passing through, it distorts spacetime there in minute ways, slightly stretching one arm while compressing the other, changing the distance each beam travels by a small fraction. This causes the laser light waves to shift out of phase and produce a measurable flicker of light at the photodetector.

Virgo and KAGRA work on similar principles. When a gravitational wave is detected, the teams operating these three detectors share their data and run joint analyses.

“We look for signals in the data from our detectors with several methods. Some are model-agnostic and others are model-independent,” study coauthor, Virgo team member, and Gran Sasso Science Institute doctoral student Jacopo Tissino said.

Model-agnostic methods try to identify excess energy that appears simultaneously across all detectors, without making any assumptions about the nature of the signal. In contrast, model-dependent methods search  the data specifically for signals that align with theoretical expectations for black-hole mergers.

The GW250114 signal, which came from about 1.3 billion lightyears away, was detected using both methods.

Cosmic bell

The team found that the new signal was similar to the one detected in 2015.

“They are both pairs of nearly identical black holes, with small or no spin, masses just over 30-times that of the sun each, and revolving around each other in an orbit that’s close to a circle,” Mr. Tissino said.

Thanks to advances that increased the detectors’ sensitivity, the new signal was also much clearer. Per Mr. Tissino, these advances include lower laser noise, cleaner mirror surfaces, and lower measurement uncertainty.

As the clearest gravitational signal ever detected, GW250114 allowed the researchers to draw important conclusions about fundamental physics. Notably, they analysed the frequencies of gravitational waves emitted by the merger to present the most compelling observational evidence to date of the black-hole area theorem, which Stephen Hawking proposed in 1971.

The theorem states that the total surface area of black holes should never decrease, referring to the sum of the areas of the event horizons.

To this end, researchers independently analysed the signals from the early stages of the merger, when the black holes were relatively far apart, and from a later post-collision stage, when the merged black holes were settling into a single entity.

“With these two analyses, we could extract the areas of the initial two black holes and of the remnant left after the collision, and directly compare them to confirm that there was an increase as predicted,” Mr. Tissino said.

Growing catalogue

After the merger, the researchers also ‘listened’ to the new black hole’s vibrations and identified two distinct modes of ringing. These frequencies indicated that the resulting black hole behaved like a rotating black hole. Such black holes are expected to emit gravitational waves at specific frequencies and these waves are expected to fade at a certain rate.

As a result, the new study was also able to empirically verify a solution  that New Zealander mathematician Roy Kerr had proposed for rotating black holes in 1963.

Mr. Tissino said that for signals like GW250114, the main sources of error are well-understood and can be controlled. Researchers carefully selected data from different points of time before and after the merger and tested different assumptions, like whether the black holes’ orbits were circular or eccentric. In the process, they checked potential issues in the detectors’ calibration and confirmed they didn’t affect their analyses.

The continuing detection of merging black holes is helping astrophysicists build a steadily growing catalogue that’s helping them fine-tune their understanding of black hole formation and test more and more intricate predictions.

As the authors wrote in their paper, “The gravitational-wave signal GW250114 is a milestone in the decade-long history of gravitational-wave science. … The next decade of gravitational-wave science is bound to enhance our view of these highly dynamical, relativistic systems.”

Shreejaya Karantha is a freelance science writer.