Black hole collision may have revealed event horizon signal for the first time

Analysis of gravitational waves generated by the merger of two black holes may have revealed, for the first time, a direct signature of the region near the event horizon, where rotation drags space-time itself.

Black holes are among the most mysterious objects in the Universe, but they are not always silent. When two black holes get close enough, they spiral towards each other, until they end up colliding in a colossal way and forming, through fusion, a single larger black hole.

During this process, they emit gravitational waves — ripples in the fabric of space and time that reach us on Earth. These waves travel here and change the distance between your nose and your ear, albeit by a tiny variation, smaller than the size of a single atom.

We can detect them with huge and sophisticated gravitational wave detectors, such as the Laser Interferometric Gravitational Wave Observatory (LIGO), in the United States, explains Neil Luresearcher at the Australian National University, in an article in .

A black hole merger with the most intense signal ever recorded was detected last year. Known as GW250114this cataclysmic collision has now revealed an exceptionally clear view of the newly formed black hole, showing subtle signatures associated with its event horizon.

Starting with GW250114, Lu and the researchers decoded a previously hidden part of the signal, the so-called direct wave, which reveals how rotating black holes drag space-time itself with them. The was published last week in the magazine Nature.

Reference dragging

Gravitational waves carry information from the region immediately outside the event horizon of the newly formed black hole. This horizon is a border beyond which nothing, not even light, can escape.

According to Einstein’s theory of general relativity, there are strange phenomena happening in this region. The theory predicts that a rotating black hole is not limited to being in space. Rather, it produces an effect known as “frame dragging”, in which the space-time around the black hole is dragged by its rotation.

Close enough to the horizon, it is impossible for anything to remain still. It’s like a whirlpool: anything that gets too close is forced to swirl with the water. Around a rotating black hole, however, it is not water that is dragged, but space-time itself.

Direct waves

The direct wave is gravitational radiation that comes from the region immediately outside the event horizon, where everything that falls into the black hole suffers the effect of frame dragging.

O event horizon of a black hole is not a physical surface, like the surface of a planet or a star. It’s a frontier in space-time. But general relativity predicts that this boundary has measurable properties, including the speed at which it rotates and the strength of gravity in that region.

The existence of the direct wave is predicted by theory, but has never been detected until now. This wave makes it possible to study the rotation speed of the new black hole and also the surface gravity at the event horizon.

GW250114 offered a perfect case to look for this phenomenon, as it produced such an intense signal. Still, the direct wave component is hidden among other waves created by the two original black holes as they spiraled until they collided.

So the work of Lu’s team turned to new techniques to reveal it, carefully separating this feature from the stronger parts of the gravitational wave signal.

A signal from the limits of our knowledge

Detection of the direct wave opens up a new source of information about black holes and their event horizons.

For decades, the event horizon has occupied a central place in theoretical physics, but it has been difficult to obtain direct information from its surroundings. It is difficult to observe light coming from a region so close to a black hole, so gravitational waves are our only route. And direct waves are, in particular, the part of the signal that brings us closest to the horizon.

The work of Lu and his colleagues also paves the way for future tests of Einstein’s theory of general relativity. If the theory is correct, the direct waves, the rotation of the horizon and the surface gravity should fit together precisely.

Black holes lie at the limits of what we currently understand. We have two major theories of physics: general relativity, which describes gravity and space-time on a large scale, and quantum mechanics, which describes matter and energy on the smallest scales.

Both theories are extraordinarily successful and underlie emerging technologies such as GPS, semiconductors, lasers, and quantum computers. Still, on a fundamental level, they are not entirely compatible.

Black holes are one of the places where this conflict can become visible. Near the event horizon, gravity is extreme, and questions about spacetime, information, and quantum physics become inevitable.

By studying black holes through gravitational waves, scientists will be able to find cracks in current theories and clues to a deeper theory.