Ringing black hole confirms Einstein and Hawking’s predictions

A decade ago, scientists first detected ripples in the fabric of space-time, called gravitational waves, from the collision of two black holes. Now, thanks to improved technology and a bit of luck, a newly detected black hole merger is providing the clearest evidence yet of how black holes work — and, in the process, offering long-sought confirmation of fundamental predictions by Albert Einstein and Stephen Hawking.

The new measurements were made by the Laser Interferometer Gravitational-Wave Observatory (LIGO), with analyses led by astrophysicists Maximiliano Isi and Will Farr of the Flatiron Institute’s Center for Computational Astrophysics in New York City. The results reveal insights into the properties of black holes and the fundamental nature of space-time, hinting at how quantum physics and Einstein’s general relativity fit together.

“This is the clearest view yet of the nature of black holes,” says Isi, who is also an assistant professor at Columbia University. “We’ve found some of the strongest evidence yet that astrophysical black holes are the black holes predicted from Albert Einstein’s theory of general relativity.”

The results were reported in a paper published September 10 in Physical Review Letters by the LIGO-Virgo-KAGRA Collaboration.

For massive stars, black holes are the final stage in their evolution. Black holes are so dense that even light cannot escape their gravity. When two black holes collide, the event distorts space itself, creating ripples in space-time that fan out across the universe, like sound waves ringing out from a struck bell.

Those space-deforming ripples, called gravitational waves, can tell scientists a great deal about the objects that created them. Just as a large iron bell makes different sounds than a smaller aluminum bell, the “sound” a black hole merger makes is specific to the properties of the black holes involved.

Scientists can detect gravitational waves with special instruments at observatories such as LIGO in the United States, Virgo in Italy and KAGRA in Japan. These instruments carefully measure how long it takes a laser to travel a given path.  As gravitational waves stretch and compress space-time, the length of the instrument, and thus the light’s travel time, changes minutely. By measuring those tiny changes with great precision, scientists can use them to determine the black holes’ characteristics.

The newly reported gravitational waves were found to be created by a merger that formed a black hole with the mass of 63 suns and spinning at 100 revolutions per second. The findings come 10 years after LIGO made the first black hole merger detection. Since that landmark discovery, improvements in equipment and techniques have enabled scientists to get a much clearer look at these space-shaking events.

“The new pair of black holes are almost twins to the historic first detection in 2015,” Isi says. “But the instruments are much better, so we’re able to analyze the signal in ways that just weren't possible 10 years ago.”

With these new signals, Isi and his colleagues got a complete look at the collision from the moment the black holes first careened into each other until the final reverberations as the merged black hole settled into its new state, which happened only milliseconds after first contact.

Previously, the final reverberations were difficult to capture, as by that point, the ringing of the black hole would be very faint. As a result, scientists couldn’t separate the ringing of the collision from that of the final black hole itself.

In 2021, Isi led a study showcasing a cutting-edge method that he, Farr and others developed to isolate certain frequencies — or ‘tones’ — using data from the 2015 black hole merger. This method proved powerful, but the 2015 measurements weren’t clear enough to confirm key predictions about black holes. With the new, more precise measurements, though, Isi and his colleagues were more confident they had successfully isolated the milliseconds-long signal of the final, settled black hole. This enabled more unambiguous tests of the nature of black holes.

“Ten milliseconds sounds really short, but our instruments are so much better now that this is enough time for us to really analyze the ringing of the final black hole,” Isi says. “With this new detection, we have an exquisitely detailed view of the signal both before and after the black hole merger.”

The new observations allowed scientists to test a key conjecture dating back decades that black holes are fundamentally simple objects. In 1963, physicist Roy Kerr used Einstein’s general relativity to mathematically describe black holes with one equation. The equation showed that astrophysical black holes can be described by just two characteristics: spin and mass. With the new, higher-quality data, the scientists were able to measure the frequency and duration of the ringing of the merged black hole more precisely than ever before. This allowed them to see that, indeed, the merged black hole is a simple object, described by just its mass and spin.

The observations were also used to test a foundational idea proposed by Stephen Hawking called Hawking’s area theorem. It states that the size of a black hole’s event horizon — the line past which nothing, not even light, can return — can only ever grow. Testing whether this theorem applies requires exceptional measurements of black holes before and after their merger. Following the first black hole merger detection in 2015, Hawking wondered if the merger signature could be used to confirm his theorem. At the time, no one thought it was possible.

By 2019, a year after Hawking’s death, methods had improved enough that a first tentative confirmation came using techniques developed by Isi, Farr, and colleagues. With four times better resolution, the new data gives scientists much more confidence that Hawking’s theorem is correct.

In confirming Hawking’s theorem, the results also hint at connections to the second law of thermodynamics. This law states that a property that measures a system’s disorder, known as entropy, must increase, or at least remain constant, over time. Understanding the thermodynamics of black holes could lead to advances in other areas of physics, including quantum gravity, which aims to merge general relativity with quantum physics.

“It’s really profound that the size of a black hole’s event horizon behaves like entropy,” Isi says. “It has very deep theoretical implications and means that some aspects of black holes can be used to mathematically probe the true nature of space and time.”

Many suspect that future black hole merger detections will only reveal more about the nature of these objects. In the next decade, detectors are expected to become 10 times more sensitive than today, allowing for more rigorous tests of black hole characteristics.

“Listening to the tones emitted by these black holes is our best hope for learning about the properties of the extreme space-times they produce,” says Farr, who is also a professor at Stony Brook University. “And as we build more and better gravitational wave detectors, the precision will continue to improve.”

“For so long this field has been pure mathematical and theoretical speculation,” Isi says. “But now we’re in a position of actually seeing these amazing processes in action, which highlights how much progress there’s been — and will continue to be — in this field.”