In space, someone can hear you scream, in extreme enough circumstances: by using gravitational waves from the collision of two black holes, scientists have heard signals from closer to the event horizon than ever before.

Mr Neil Lu and Dr Ling Sun led a team that developed a novel way to analyze the data from the actual moment of collision, as the two black holes form one. They were able to reveal important properties of the newly formed black hole’s event horizon: its rotation frequency and surface gravity.

“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, from the ANU Centre for Gravitational Astrophysics (CGA) and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).

“Our new analysis allows us to decipher this component and extract unique information from close to the event horizon.”

The study is published in Nature by the team, which was led by PhD candidate, Mr Lu, and Dr Sun, also from CGA and OzGrav, and included colleagues in Canada, the US, and Spain. 

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.

In the region closest to a black hole’s event horizon, quantum physics and theories of general relativity intersect, which makes the new analysis such an important new observational technique.

The team analysed gravitational waves – ripples of spacetime itself. Unlike light waves, which are visible to telescopes, gravitational waves allow us to detect invisible things, such as black hole collisions. 

They studied the gravitational-wave signal from colliding black holes, GW250114, recorded by the two Laser Interferometer Gravitational Wave Observatories (LIGO) in the United States in 2025. GW250114 is the loudest event yet detected – about three times louder than the first gravitational-wave signal detected a decade ago.

“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 how fast the horizon is rotating and how strong gravity is there.”

“These measurements mark a first step towards future tests of general relativity with direct waves,” Mr Lu said.

The new technique developed by the team will enable astrophysicists to study the strength of extreme gravity at the black hole’s horizon, Dr Sun said. 

“This analysis will give insights into phenomena like frame dragging, where a spinning black hole drags the fabric of spacetime around with it; in the most extreme region near the black hole, spacetime is dragged so strongly that nothing can remain stationary relative to a distant observer like ourselves.”

“The thrill is that gravitational waves are bringing us closer than ever to the black-hole horizon – a region that once seemed beyond direct observational reach,” she said.

Find out more about gravitational waves at the LIGO site.