When
two black holes collide and merge, they release gravitational waves. These
waves can be detected by sensitive instruments on Earth, allowing scientists to
determine the mass and spin of the black holes. The clearest black hole merger
signal yet, named GW250114 and recorded by LIGO in January 2025, offers new
insights into these mysterious objects. Credit: Maggie Chiang for Simons
Foundation
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."
Source: Ringing black hole confirms Einstein and Hawking's predictions
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