Twin black hole collisions put Einstein’s general relativity to its most extreme test
Two colossal black hole collisions, detected just a month apart in late 2024, are reshaping how scientists interpret the most extreme cosmic events in the universe. These twin mergers not only provide fresh insight into how black holes form and evolve but also confirm, with unmatched precision, the predictions of Albert Einstein’s general theory of relativity. The findings may also help researchers uncover new, undiscovered particles that could extract energy from black holes.
In a study published October 28 in The Astrophysical Journal Letters, the international LIGO-Virgo-KAGRA Collaboration announced the detection of two remarkable gravitational wave signals from black holes with unusual spin patterns recorded in October and November of last year.
Ripples in Space and Time Reveal Cosmic Collisions
Gravitational waves are tiny ripples in space-time that occur when massive celestial objects crash or merge. The strongest signals come from the collision of black holes. The first event, GW241011 (October 11, 2024), happened about 700 million light years from Earth when two black holes — about 20 and 6 times the mass of our sun — merged. The larger one was identified as one of the fastest-spinning black holes ever observed.
Roughly a month later, a second event, GW241110 (November 10, 2024), was detected some 2.4 billion light years away. This merger involved black holes weighing roughly 17 and 8 solar masses. Unlike most black holes that spin in the same direction as their orbit, the main black hole in GW241110 spun in the opposite direction, marking the first observation of such a configuration.
“Each new detection provides important insights about the universe, reminding us that each observed merger is both an astrophysical discovery but also an invaluable laboratory for probing the fundamental laws of physics,” said co-author Carl-Johan Haster, assistant professor of astrophysics at the University of Nevada, Las Vegas (UNLV). “Binaries like these had been predicted given earlier observations, but this is the first direct evidence for their existence.”
Revealing the Secret Lives of Merging Black Holes
Einstein first predicted the existence of gravitational waves in 1916 as part of his general theory of relativity. Their existence was indirectly confirmed in the 1970s, but scientists did not directly observe them until 2015 when the LIGO observatory detected waves created by a black hole merger.
Today, the LIGO-Virgo-KAGRA network operates as a global system of advanced detectors. The team is currently in its fourth observation campaign, known as O4, which began in May 2023 and will continue through mid-November 2025. To date, about 300 black hole mergers have been detected, including candidates found during this ongoing run.
The recent detection of GW241011 and GW241110 demonstrates how far gravitational-wave astronomy has advanced in uncovering the inner workings of black hole systems. Both events suggest that some of these black holes could be “second-generation,” formed from the remnants of earlier mergers.
“GW241011 and GW241110 are among the most novel events among the several hundred that the LIGO-Virgo-KAGRA network has observed,” said Stephen Fairhurst, professor at Cardiff University and spokesperson for the LIGO Scientific Collaboration. “With both events having one black hole that is both significantly more massive than the other and rapidly spinning, they provide tantalizing evidence that these black holes were formed from previous black hole mergers.”
Researchers noted several intriguing patterns, including large differences in mass between the paired black holes — the larger being nearly twice as massive as its companion — and unusual spin directions. These traits suggest that the black holes formed through a process called hierarchical merger, in which black holes in densely populated regions such as star clusters collide multiple times over their lifetimes.
“These two binary black hole mergers offer us some of the most exciting insights yet about the earlier lives of black holes,” said Thomas Callister, co-author and assistant professor at Williams College. “They teach us that some black holes exist not just as isolated partners but likely as members of a dense and dynamic crowd. Moving forward, the hope is that these events and other observations will teach us more and more about the astrophysical environments that host these crowds.”
Testing Einstein’s Theory Under Extreme Conditions
The extraordinary precision of GW241011’s detection gave researchers an opportunity to test Einstein’s general relativity in one of the most extreme environments ever measured. Because this event was captured so clearly, scientists could compare the results with predictions from Einstein’s equations and Roy Kerr’s solution describing rotating black holes.
The rapid spin of GW241011 slightly distorted its shape, leaving a unique fingerprint in the gravitational waves. Analysis of the data showed an exceptional match to Kerr’s model, confirming Einstein’s predictions with record accuracy.
The significant difference in the masses of the colliding black holes also produced a “higher harmonic,” a kind of overtone similar to those heard in musical instruments. This rare feature, seen clearly for only the third time, provides another successful test of Einstein’s theory.
“The strength of GW241011, combined with the extreme properties of its black hole components provide unprecedented means for testing our understanding of black holes themselves,” says Haster. “We now know that black holes are shaped like Einstein and Kerr predicted, and general relativity can add two more checkmarks in its list of many successes. This discovery also means that we’re more sensitive than ever to any new physics that might lie beyond Einstein’s theory.”
Searching for Clues to New Particles
Rapidly rotating black holes like those observed in this study now have yet another application — in particle physics. Scientists can use them to test whether certain hypothesized light-weight elementary particles exist and how massive they are.
These particles, called ultralight bosons, are predicted by some theories that go beyond the Standard Model of particle physics, which describes and classifies all known elementary particles. If ultralight bosons exist, they can extract rotational energy from black holes. How much energy is extracted and how much the rotation of the black holes slows down over time depends on the mass of these particles, which is still unknown.
The observation that the massive black hole in the binary system that emitted GW241011 continues to rotate rapidly even millions or billions of years after it formed rules out a wide range of ultralight boson masses.
“Planned upgrades to the LIGO, Virgo, and KAGRA detectors will enable further observations of similar systems, enabling us to better understand both the fundamental physics governing these black hole binaries and the astrophysical mechanisms that lead to their formation,” said Fairhurst.
Joe Giaime, site head for the LIGO Livingston Observatory, noted that LIGO scientists and engineers have made improvements to the detectors in recent years, which has resulted in precision measurements of merger waveforms that allow for the kind of subtle observations that were needed for GW241011 and GW241110.
“Better sensitivity not only allows LIGO to detect many more signals, but also permits deeper understanding of the ones we detect,” he said.
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