Black holes are some of the most mysterious objects in the universe, but they aren’t always silent. When two black holes are close enough to each other, they spiral towards one another, eventually crashing in an enormous explosion and forming a single, larger black hole from the combination.
During this process they emit gravitational waves, ripples in the fabric of space and time that reach us here on Earth. These travel to us and change the distance between your nose and your ear, but by much, less than the a single atom! We are able to detect them with huge, sophisticated gravitational wave detectors – such as with the Laser Interferometer Gravitational Wave Observatory (LIGO) in the United States.
The “loudest” black hole merger event on record was detected last year. Known as GW250114, this cataclysmic collision has now revealed an exceptionally clear view of the newly formed black hole, revealing subtle signatures tied to its event horizon.
Using GW250114, my colleagues and I have decoded a previously hidden part of the signal, the so-called direct wave, which reveals how rotating black holes drag spacetime itself around them when they spin. Our research is published today in Nature.
Frame dragging
Gravitational waves carry information from the region just outside the newly formed black hole’s event horizon. This is a boundary beyond which nothing, not even light, can escape.
According to Einstein’s theory of general relativity, there is some weird stuff happening in this region. The theory predicts that a rotating black hole does not simply sit in space. Instead, it produces “frame dragging” – an effect in which the spacetime around the black hole is whirled around with it.
Close enough to the horizon, it is impossible for anything to remain still. It’s like a whirlpool: anything drifting too close is forced to turn with the water. Around a spinning black hole, it is not water being dragged around, but spacetime itself.
Direct waves
The direct wave is gravitational radiation that comes from right outside the event horizon, where everything that is falling into the black hole experiences frame dragging.
A black hole’s event horizon is not a physical surface like the surface of a planet or star. It is a boundary in spacetime. But general relativity predicts that this boundary has measurable properties, including how fast it rotates and how strongly gravity behaves there.
LVK Collaboration / Wikimedia, CC BY
The existence of the direct wave is predicted by theory, but until now it had never been detected. The wave allows us to study how fast the new black hole is spinning, and also the strength of gravity at the surface at the event horizon.
GW250114 provided a perfect case to hunt for this phenomenon, because it was so loud. Even so, the direct-wave component is hidden among other waves created by the two original black holes whirling in to collide. So our work used new techniques to reveal it, carefully separating this feature from the louder parts of the gravitational wave signal.
A signal from the boundaries of our knowledge
Detecting the direct wave opens up a new source of information about black holes and their event horizons.
For decades, the event horizon has been central to theoretical physics, but direct information from near the horizon has been difficult to access. It’s difficult for us to observe light that comes this close to a black hole – so gravitational waves are our only way in. And the direct waves are specifically the part of the signal that get us closest to the horizon.
Our work also opens a path toward future tests of Einstein’s theory of general relativity. If Einstein’s theory is correct, the direct waves, horizon rotation and surface gravity should all fit together in a precise way.
NASA’s Goddard Space Flight Center/Jeremy Schnittman and Brian P. Powell
Black holes sit at the boundary of what we currently understand. We have two big theories of physics: general relativity, which describes the large scale of gravity and spacetime, and quantum mechanics which describes matter and energy at the smallest scales.
Both theories are extraordinarily successful, underpinning technologies such as GPS, semiconductors, lasers and emerging quantum computers. Yet at a fundamental level, they do not fully agree.
Black holes are one of the places where this conflict may become visible. Near the event horizon, gravity is extreme, and questions about spacetime, information and quantum physics cannot be avoided. By studying black holes with gravitational waves, scientists may find cracks in our current theories and clues toward a deeper one.
