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An animation of two black holes merging into one. In a new study, scientists used the clearest gravitational wave signal ever detected to “listen” to a distant black hole merger and put Einstein’s gravity to its toughest test yet. . | Credit: SXS
Scientists have used the strongest gravitational wave signal ever recorded to put Albert Einstein’s more than 100-year-old theory of gravity to its toughest test yet — and once again, it passed.
The signal, called GW250114, came from the merger of two black holes — each about 30 times the mass of the Sun — about 1.3 billion light-years from Earth. The event caused ripples in spacetime, called gravitational waves, that passed over Earth on January 14, 2025 and were detected by the US-based Laser Gravitational-Wave Interferometer Observatory (LIGO).
Scientists say the event is very similar to the one that led to the first direct detection of gravitational waves in 2015. That suggests the black holes in both mergers were similar in size and distance from Earth.
However, this new signal was recorded with about three times the clarity of that breakthrough discovery in 2015, allowing scientists to test Einstein’s general theory. relativity more rigorous than ever.
“It was clearly the biggest event,” Keefe Mitmanpostdoctoral researcher at the Cornell Center for Astrophysics and Planetary Sciences and co-author of the new paper, told Live Science. “This event provided more information than anything we’ve seen before about certain tests of general relativity.”
The exceptional clarity of the signal comes from a decade of constant upgrades to the detectors, Mitman said. These improvements reduced noise from sources that once interfered with cosmic signals, including seismic vibrations and even passing trucks. As a result, the detectors were sensitive enough to the tiny distortions in spacetime—changes 700 trillion times smaller than the width of a human hair—caused by the recently detected black hole merger.
The findings are detailed in a study published Jan. 29 in the journal Physical Review Letters.
The “ring” of a black hole
Because the newly detected signal was so clear, Mitman and his colleagues were able to zoom in on a transient stage after the merger known as “ringdown.” During this phase, the newly formed black hole vibrates briefly—much like a struck bell—emitting gravitational waves in distinct patterns, or “tones,” that encode key properties of the black hole, including its mass and spin.
In GW250114, the researchers detected the two primary tones predicted for such a merger. Each tone gave an independent measurement of the black hole’s mass and spin—and both matched, effectively verifying general relativitythe team reported in the study.
For the first time, scientists have also confidently identified a more subtle, short-lived “tone” that appears at the very beginning of the ringing—another feature long predicted by general relativity.
“This event made it very, very apparent that indeed this prediction of general relativity was present in the signal, which was really interesting,” Mitman told Live Science.
If the measurements did not agree, he added in a statement“We would have had a lot of work to do as physicists to try to explain what was going on and what the true theory of gravity would be in our universe.”
Previous analyzes of the same event, published September 2025confirmed another major prediction rooted in general relativity that Stephen Hawking proposed more than 50 years ago. Hawking predicted that the surface of a black hole is its size event horizon — can never shrink, even if enormous amounts of energy escape during a fusion in the form of gravitational waves.
The two LIGO gravitational wave observatories in Washington and Louisiana are separated by a distance of about 3030 km. This allows scientists to measure millisecond level differences in gravitational wave signals. | Credit: Virgo/CCO 1.0 collaboration
In GW250114, scientists estimated that the two original black holes had a combined area of about 93,000 square miles (240,000 square kilometers)—about the size of Oregon. After the merger, the resulting black hole had an area of about 155,000 square miles (400,000 square km)—closer to the size of California—which is consistent with Hawking’s prediction.
The golden age
Despite the repeated success of general relativity in describing large-scale cosmic phenomena, physicists are suspicious of the theory may not be the full description of gravity in our universe. For example, it cannot explain dark matter or dark energy, which are needed to hold galaxies and their clusters together and to explain the accelerated expansion of the universe, respectively. Nor does it reconcile cleanly with quantum mechanicsthe framework that governs nature at the smallest scales.
Scientists hope that gravitational waves from the energetic mergers of black holes may one day show subtle deviations from Einstein’s predictions that could reveal new physics.
The withdrawal phase holds particular promise for such tests, Mitman said. Many “beyond Einstein” theories predict slightly different vibrational patterns during the retreat phase—so measuring more than one tone, as his team did with GW250114, can help scientists put constraints on any possible deviations from general relativity.
If a discrepancy were found, researchers could compare the data with predictions from alternative theories of gravity to determine which, if any, matches reality.
“There must be a way to resolve this paradox so that our theory of gravity is consistent with the theory of quantum mechanics,” Mitman said in the statement.
State-of-the-art detectors, including the proposed Einstein Telescope in Europe and Cosmic Explorer in the US, will be 10 times more sensitive than current installations. In addition to detecting more events like GW250114, these detectors will be able to observe lower-frequency gravitational waves that correspond to more massive black holes, allowing scientists to probe entirely new classes of these cosmic giants.
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The researchers are also looking at the European Laser Interferometer Space Antenna (LISA), which expects to observe gravitational waves from supermassive black holes at the centers of galaxies. Planned launch in 2035, LISA is expected to detect a wave of events and could reveal dozens of distinct tones in a single black hole merger event, Mitman said.
“We’re living in a regime where we don’t have enough data, and we’re kind of twiddling our thumbs waiting for more data to come in,” Mitman said. “Once LISA is online, we will be overwhelmed.”
If funding for gravitational wave science continues, he added, “we’ll see more and more of these golden events and really start to learn wonderful things about the nature of gravity in our universe.”