LIGO’s discovery of gravitational waves—ripples in space-time from powerful cosmic events—hit astrophysics more like a tidal wave than a ripple. At the dawn of its tenth anniversary, the multinational collaboration has set another scientific milestone, this time solving not one but two mysteries in black hole physics.
A paper published today in Physical Review Letters describes how the LIGO-VIRGO-KAGRA (LVK) Collaboration captured the sharpest-ever gravitational wave signal from a black hole merger. Further analysis of GW250114, the signal in question, validates two major predictions made by Stephen Hawking and Roy Kerr in 1971 and 1963, respectively.
First, we’re more certain than ever that when black holes merge, the resulting black hole is wider than both parts combined. Second, we only need to know two metrics to describe gravitational disturbances in black holes: mass and spin.
“It’s a beautiful, landmark result,” Arthur Kosowsky, a theoretical physicist at the University of Pittsburgh who wasn’t involved in the new work, told Gizmodo in an email. The latest results are “a confirmation of both the fundamental nature of a spinning black hole and also a remarkable test of strong-field general relativity,” he added.
The latest results come almost ten years after GW150914, the first gravitational wave signal ever detected, which LIGO observed in 2015. In 2021, physicists used the 2015 signal to test Hawking’s theorem. The team assessed a confidence level of 95% for this test, but with the new, cleaner result, that has jumped to an impressive 99.999%—as close as one gets to the “truth” in modern science.
“A decade ago we couldn’t be certain that black holes ever collide in our universe,” Steve Fairhurst, LIGO spokesperson and a physicist at Cardiff University in the United Kingdom, told Gizmodo. “Now we observe several black hole mergers per week. With the 300 gravitational-wave candidates observed to date, we’re beginning to provide a census of the population of black holes in the universe.”
Black holes are ringing
Black holes lose a lot of mass during a merger. The violent conflagration can also speed up a black hole’s spin, decreasing its area. Hawking and Jacob Bernstein’s black hole area theorem posits that, despite these factors, the product of a merger will still generate a bigger black hole.
In the merger that produced GW250114, the initial black holes exhibited surface areas around 92,665 square miles (240,000 square kilometers), whereas the final black hole’s surface area measured about 154,441 square miles (400,000 square kilometers). To put the final product in perspective, it weighs about 63 times the mass of our Sun and spins at 100 revolutions per second, according to the study.
“Musical” software developed by LIGO members—including Gregorio Carullo, an astrophysicist at the University of Birmingham in the United Kingdom—enabled the team to make such precise measurements. The tool essentially let them “hear” each black hole as it merged into a larger one, at sensitivity levels four times higher than a decade ago.
“Black holes are black, so it’s very difficult to ‘look’ into them,” Carullo told Gizmodo in a video call. Gravitational wave experiments offer an easy workaround, since everything controlled by gravity technically produces gravitational waves. Massive, cantankerous black holes are especially loud—and we’re getting better at tuning into these signals, he explained.
“When black holes collide, they emit these characteristic sounds that are specific and peculiar to that black hole,” Carullo said. “If we can hear these sounds, or notes, [that] depend on just the mass and the spin…you can extract the mass and the spin of the black hole.”
The fact that this is possible at all is what makes black holes so extraordinary, he added. “People think of black holes as something scary, but actually, it’s the simplest thing you can imagine.”
A merger of knowledge
Excitingly, gravitational wave astronomy is still “in its infancy,” Fairhurst said. LIGO’s Nobel-winning discovery was huge, no doubt, but no end goals exist for this project. If anything, the discovery of GW250114 marked the beginning of a new chapter in astronomy.
“In the coming years, we will continue to see the sensitivity of the detectors improve, providing ever more and higher-fidelity observations,” Fairhurst said. “At some stage, we are likely to observe something unexpected—either a signal that is hard to explain astrophysically, one that doesn’t exactly match the predictions of general relativity, or a signal from an unexpected source.”
In a release, Kip Thorne—one of three physicists who fathered LIGO—recalled Hawking asking him, shortly after LIGO’s historic gravitational wave detection in 2015, whether the instrument could test his area theorem. Hawking, unfortunately, passed away three years before LIGO finally did just that.
The anecdote, along with the story of how LIGO arrived at GW250114, shows how generations of breakthroughs—both theoretical and experimental—converge to expand our understanding of the universe. And that’s something to be very excited about.
Leave a Reply