If, in space, no one can hear you scream - it seems that you can actually hear the sound of a crash when two black holes collide.
Using the loudest gravitational wave ever heard, two Australian scientists and colleagues have been the first to witness the previously elusive “event horizon” - at the actual moment of collision, right before all light and sound are swallowed by the newly formed black hole for eternity.
The discovery and the novel way to analyze the data, published today in Nature , provide a new observational window onto the region closest to a black hole’s event horizon where quantum physics and theories of general relativity intersect.
The study, led by Dr Ling Sun and PhD candidate, Mr Neil Lu, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) and the Australian National University, as well as colleagues in Canada, the US, and Spain, opens the doors for astrophysicists globally being able to observe the previously elusive event horizon of a black hole.
“We measured the last sound the black holes made when they crashed. Hidden within that signal is a small component, called direct waves, that had not previously been well understood,” said Mr Lu. “Our new analysis allows us to decipher this component and extract unique information from close to the event horizon.”
The event horizon is the boundary around a black hole beyond which nothing can escape, not even light. At this boundary, the speed required to break free from the black hole’s gravity equals the speed of light. Since nothing in the universe travels faster than light, anything that crosses the event horizon is permanently trapped.
The scientists studied the gravitational-wave signal GW250114, recorded last year, the loudest yet, using the two Laser Interferometer Gravitational Wave Observatories in the United States.
According to Dr Sun, black holes are extreme objects that sit at the intersection of general relativity and quantum theory.
“We studied GW250114, the loudest binary black hole signal observed to date, about three times louder than the first gravitational-wave signal detected a decade ago,” said Dr Sun. “Our analysis shows that this exceptionally loud signal can be used as a powerful probe of the remnant black hole’s horizon, allowing us to measure its two fundamental properties: rotation frequency and surface gravity.”
“These measurements mark a first step towards future tests of general relativity with direct waves,” Mr Lu added.
The new analytical technique developed by Mr Lu, Dr Sun, and their collaborators means that astrophysicists will be able to study the strength of extreme gravity at the black hole’s horizon, and phenomena like “frame dragging” where black holes literally drag along the fabric of spacetime nearby, creating an environment where nothing is able to remain stationary relative to a distant observer like ourselves.
Nature
Observational study
Not applicable
GW250114 reveals signatures of post-merger black-hole horizon'
24-Jun-2026