A record-breaking gravitational wave signal let scientists
"listen" to a distant black hole merger and put Einstein's gravity to
its toughest test yet.
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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. (Image credit: SXS) |
Scientists have used the loudest 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 through space-time, called
gravitational waves, which washed over Earth on Jan. 14, 2025, and were
detected by the U.S.-based Laser Interferometer Gravitational-Wave Observatory
(LIGO).
Scientists say the event closely resembles the one that
resulted in 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 roughly three
times the clarity of that groundbreaking 2015 discovery, allowing scientists to
test Einstein's theory of general relativity more rigorously than ever before.
"It was very clearly the loudest event," KeefeMitman, a postdoctoral researcher at the Cornell Center for Astrophysics and
Planetary Science and co-author of the new paper, told Live Science. "This
one event provided more information than everything we've seen before regarding
certain tests of general relativity."
The signal's exceptional clarity stems from a decade of
steady upgrades to the detectors, Mitman said. Those 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 minuscule distortions in space-time — 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.
A black hole's "ring"
Because the recently detected signal was so clear, Mitman
and his colleagues could zoom in on a fleeting stage after the merger known as
the "ringdown." During this phase, the newly formed black hole
briefly vibrates — 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, researchers detected the two primary tones
predicted for such a merger. Each tone yielded an independent measurement of
the black hole's mass and spin — and both matched, effectively verifying
general relativity, the team reported in the study.
For the first time, scientists also confidently identified a
more subtle, short-lived "overtone" that appears right at the start
of the ringing — another feature long predicted by general relativity.
"This event made it very, very obvious that, indeed,
this prediction of general relativity was present in the signal, which was
really exciting," Mitman told Live Science.
Had the measurements disagreed, he added in a statement,
"we would have had a lot of work to do as physicists to try to explain
what's going on and what the true theory of gravity would be in our
universe."
Earlier analyses of the same event, published in September
2025, confirmed another major prediction rooted in general relativity that
Stephen Hawking proposed more than 50 years ago. Hawking predicted that a black
hole's surface area — the size of its event horizon — can never shrink, even
though enormous amounts of energy escape during a merger as gravitational
waves.
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The two LIGO gravitational wave observatories in Washington
and Louisiana are separated by a distance of roughly 1,880 miles (3030 km).
This allows scientists to measure millisecond-level differences in
gravitational wave signals. (Image credit: The Virgo collaboration/CCO 1.0) |
In GW250114, scientists estimated that the two original
black holes had a combined surface area of about 93,000 square miles (240,000
square kilometers) — roughly the size of Oregon. After the merger, the
resulting black hole had a surface 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 general relativity's repeated success at describing
large-scale cosmic phenomena, physicists suspect the theory cannot be the
complete 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 universe's accelerating expansion,
respectively. Nor does it reconcile cleanly with quantum mechanics, the
framework that governs nature at the smallest scales.
Scientists hope gravitational waves from energetic black
hole mergers might someday show subtle deviations from Einstein's predictions,
which could potentially reveal new physics.
The ringdown phase is especially promising for such tests,
Mitman said. Many "beyond-Einstein" theories predict slightly
different vibration patterns during the ringdown phase — so measuring more than
one tone, as his team did with GW250114, can help scientists place constraints
on any possible deviations from general relativity.
If a discrepancy were to be found, researchers could compare
the data with predictions from alternative theories of gravity to determine
which, if any, matches reality.
"There has to be some way to resolve this paradox to
make our theory of gravity consistent with our theory of quantum
mechanics," Mitman said in the statement.
Next-generation detectors, including the proposed Einstein
Telescope in Europe and the U.S.-based Cosmic Explorer, will be 10 times more
sensitive than current facilities. In addition to detecting more events like
GW250114, these detectors will be able to observe lower-frequency gravitational
waves, which correspond to more massive black holes, thereby allowing
scientists to probe entirely new classes of these cosmic behemoths.
Researchers are also looking ahead to the European Laser
Interferometer Space Antenna (LISA), which is expected to observe gravitational
waves from supermassive black holes at the centers of galaxies. Planned for
launch in 2035, LISA is expected to detect a flood of events and could reveal
dozens of distinct tones within a single black hole merger event, Mitman said.
"We're living in the regime where we don't have enough
data, and we're kind of just twiddling our thumbs waiting for more data to come
in," Mitman said. "Once LISA is online, we'll be overwhelmed."
If funding for gravitational-wave science continues, he
added, "we're going to see more and more of these golden events and really
start to learn wonderful things about the nature of gravity in our
universe."