The loudest crash of gravitational waves ever heard has offered us insight into event horizons, the boundaries beyond which nothing can escape the grips of black holes.
The gravitational wave signal GW250114 was picked up in January 2025 by LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and KAGRA ( Kamioka Gravitational Wave Detector). The signal was created when two black holes with around 32 times the mass of the sun collided and set the very fabric of space rippling.
Now, a team of researchers assessed this signal and found a feature in the gravitational waves represents the collective event horizon of the involved black holes at the very moment of that collision.
"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," research co-leader Neil Lu, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), said in a statement. "Our new analysis allows us to decipher this component and extract unique information from close to the event horizon."
The team's research presents the intriguing possibility that scientists could use gravitational waves to study these mysterious black hole boundaries.
Event horizons and the point of no return
The concept of an event horizon first emerged through solutions to the equations of Albert Einstein's 1915 theory of gravity, general relativity. These solutions were developed by Karl Schwarzschild while serving with the German army on the Eastern Front in the First World War.
Schwarzschild found a point around a body with mass at which the escape velocity, the speed needed to escape the gravitational grip of that body, exceeds the speed of light. Also known as the Schwarzschild radius, the size of that boundary depends on the mass of the body. So the Schwarzschild radius for the sun would be about 1.86 miles (3 kilometers) from its center of mass; for the Earth, it would be just 0.35 inches (9 millimeters) from our planet's center of mass. That's the case with all planets and stars; the Schwarzschild radius is well within the bodies of those objects.
However, for a black hole, the Schwarzschild radius is far from the center of mass, acting as a light-trapping outer boundary: the event horizon. To escape the gravitational grip of a black hole from this point, matter would have to accelerate to a speed faster than the speed of light, which Einstein's theory of special relativity tells us would require infinite energy. Nothing in the universe travels faster than light; thus, nothing escapes the event horizon.
To understand why that shrouds a black hole in mystery, consider how no signal can travel faster than light. That means the event horizon is a one-way barrier for information. A black hole can swallow it, but the event horizon prevents it from spitting information out. We can never observe the interior of a black hole.
It's little wonder scientists are so keen to study event horizons and what happens there. They don't only want to understand the physics of matter engaged on a one-way trip into the maw of a black hole, but the effect on the very fabric of space itself these cosmic titans have.
The immense gravitational influence of black holes means that, as they spin, they drag the very fabric of space along with them, a phenomenon called "frame-dragging" or the Lense-Thirring effect. This introduces another rule about event horizons — not only does nothing escape this boundary, nothing there sits still either. This research brings scientists one step closer to understanding those rules in greater detail than ever before.
"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," team co-leader Ling Sun of OzGrav said. "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."
The results could also shed more light on the behavior of gravity in the most extreme environment in the universe, at the very edge of a black hole.
"These measurements mark a first step towards future tests of general relativity with direct waves," Lu said.
The research was published on Wednesday (June 24) in the journal Nature.
