Astronomers have detected a collision between two black holes in unprecedented detail, offering the clearest view yet into the nature of these cosmic oddities and confirming long-held predictions made by legendary physicists Albert Einstein and Stephen Hawking.
The event, dubbed GW250114, became known in January when researchers spotted it with the Laser Interferometer Gravitational-Wave Observatory (LIGO) — a set of two identical instruments located in Livingston, Louisiana, and Hanford, Washington.
The instruments detected gravitational waves, faint ripples in space-time produced by the two black holes slamming into each other.
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Searching for gravitational waves, phenomena predicted in 1915 as part of Einstein's theory of relativity, is the only way to identify black hole collisions from Earth.
Einstein believed that the waves would be too weak to ever be picked up by human technology, but in September 2015, LIGO recorded them for the very first time, later netting a Nobel Prize for three scientists who made key contributions to the development of this "black hole telescope."
The newly detected black holes were each around 30 to 35 times the mass of the sun, and they were spinning very slowly, said Maximiliano Isi, an assistant professor of astronomy at Columbia University and an astrophysicist at the Flatiron Institute's Center for Computational Astrophysics in New York City.
Isi led a new study for the LIGO-Virgo-KAGRA Collaboration on the GW250114 data, which published Wednesday in the journal Physical Review Letters.
"The black holes were about 1 billion light years away, and they were orbiting around each other in almost a perfect circle," Isi said.
"The resulting black hole was around 63 times the mass of the sun, and it was spinning at 100 revolutions per second."
These characteristics make the merger an almost exact replica of that first, groundbreaking detection from 10 years ago, according to Isi.
"But now, because the instruments have improved so much since then, we can see these two black holes with much greater clarity, as they approached each other and merged into a single one," he added.
Isi said the observation gives scientists a totally new view into "the dynamics of space and time."
Einstein and a two-toned ring
LIGO, which also has two smaller sister instruments — Virgo in Italy and KAGRA in Japan — is managed by a global scientific community of about 1600 researchers.
It works by detecting tiny stretches in space caused by the gravitational waves that amount to "a change in distance that is 1000 times smaller than the radius of the nucleus of an atom," as Isi puts it.
Scientists have used it to observe over 300 black hole mergers so far.
Earlier this year, the instrument detected the most massive black hole collision to date between two black holes approximately 100 and 140 times the mass of the sun.
Since it debuted, some of LIGO's key components — including its lasers and mirrors — have been upgraded to increase accuracy and reduce background noise.
This improved performance made its new observation over three times more precise than the inaugural one a decade ago.
That unprecedented clarity allowed astronomers to use GW250114 to confirm predictions about black holes made decades ago by prominent physicists.
The first prediction, devised by New Zealand mathematician Roy Kerr in 1963, builds upon Einstein's theory of general relativity, and states that black holes should be paradoxically simple objects, described by a single equation.
"Yes, black holes are very mysterious, complex and have important implications to the evolution of the universe," Isi said, "but mathematically we think they should be fully described by just two numbers. Everything there is to know about them should come from how big the black hole is — or what its mass is — and how fast it's rotating."
To test this theory, the researchers used a unique feature of black hole collisions: a "ringing" or vibration — like a bell that's been struck — that the final black hole produces.
"If you have a bell and you strike it with a hammer, it will ring," Isi noted. "The pitch and duration of the sound, the characteristics of the sound, tell you something about what the bell is made of. With black holes something similar happens — they ring in gravitational waves."
This ringing includes information about the structure of the black hole and the space around it, Isi added.
Although the phenomenon was faintly observed before, GW250114 returned a signal with "two modes … a fundamental mode and an overtone" with much more clarity.
"We identified two components of this ringing, and that allowed us to test that this black hole really is consistent with being described by just two numbers, mass and rotation," he said.
"And this is fundamental to our understanding of how space and time works — that these black holes should be featureless, in some way. It's the first time we are able to see this so compellingly."
Hawking's surface area theorem
The second prediction confirmed by GW250114 is one made in 1971 by British physicist Stephen Hawking, which states that when two black holes merge, the resulting surface area must be equal to or greater than that of the original black holes.
"It's a profound, but very simple theorem that says the total surface area of a black hole can never decrease — it can only get bigger or stay the same," Isi said.
Although previous LIGO observations offered tentative confirmations of the theorem, the clarity of this new signal gives researchers unparalleled confidence, Isi said.
"Because we're able to identify the portion of the signal that comes from the black holes early on, as they are separated from each other, we can infer their areas from that," he explained.
Then we can look at the very final portion of the signal that comes from the final black hole, and measure its own area."
Just like Kerr's equation, Hawking's theorem also uses Einstein's work as its foundation: "Einstein's theories are like the operating system for all of this," Isi explained.
Kip Thorne, one of the three recipients of the Nobel Prize for LIGO contributions, said Hawking called him as soon as he learned of the 2015 gravitational wave detection to ask if LIGO would be able to test his theorem.
"If Hawking were alive, he would have revelled in seeing the area of the merged black holes increase," Thorne said of the esteemed physicist, who died in 2018, in a statement about the new findings.
It's remarkable how this seminal, theoretical work is being confirmed decades later with advanced instruments, Isi said.
And confirming Hawking's equation, he added, could have implications for a very sought-after goal in physics — combining the seemingly incompatible theory of general relativity, which describes gravity, with quantum mechanics, which relates to the subatomic world.
"LIGO has created an entire new branch of astronomy. It has revolutionized what we think about compact objects, black holes in particular," he said.
"Before LIGO turned on, people weren't even sure that black holes could merge and crash and form in this way."
A long-awaited milestone
Gravitational waves are very weak, and the titanic task of detecting them is often described as looking for a needle in a haystack, according to Emanuele Berti, a professor of physics and astronomy at Johns Hopkins University who was not involved with the study.
He described the LIGO detectors as "hearing aids" that help with this process.
"A large group of scientists spent the last ten years improving those hearing aids, and now we can 'hear' the signals with much higher clarity," he said in an email.
"We can now test fundamental principles of gravity that we could not test ten years ago."
Among these principles, he added, is the idea that black holes are the simplest macroscopic objects in the universe.
The level of detail in the "ringing" produced by the GW250114 collision means scientists can say with confidence that the final object is consistent with the black holes predicted by Einstein's general relativity, which Berti says is "terribly exciting."
Leor Barack, a professor of mathematical physics at the University of Southampton in England who was also not part of the study, noted that among the more than 300 black-hole merger events recorded by LIGO, the latest one stands out as "particularly spectacular," and calls the new study a long-awaited analysis.
Scientists were able to extract two of the "pure tones" of the remnant black hole as it settled into its final form, Barack added.
"This included, for the first time, a clear extraction of the first 'overtone,' a fainter harmonious sound of the ringing hole, in addition to the primary tone," he said.
"This kind of test is the most precise to date, by a long margin."
The study represents a significant milestone in gravitational wave astronomy, said Macarena Lagos, an assistant professor at the Institute of Astrophysics of the Universidad Andrés Bello in Chile. Lagos also was not involved with the work.
She agreed that the detection of a second tone in the "ringing" black hole is particularly significant, adding that GW250114 demonstrates the success of LIGO's ongoing improvements and shows that gravitational wave detections can test fundamental physics in ways never before possible.
"While current tests of gravity still have broad uncertainties, this work lays the groundwork for future detections" of even better quality expected in the coming years, Lagos said in an email.
"These future observations promise to provide more precise tests of our understanding of spacetime and gravity."
