Scientists May Have Detected The First Signature of a Black Hole’s Event Horizon : ScienceAlert

Everyone knows that no information can escape a black hole.

But a gravitational wave rippling outward from a massive collision between two hefty black holes may have brought us right to the brink – carrying the very first signature ever received of an event horizon.

Scientists had theorized that a gravitational wave known as a direct wave could carry information about the event horizon’s properties.

Now they’ve finally identified such a wave.

If it sounds like something out of science fiction to you, you are not alone.

“The event horizon is not something we can see directly with light, because by definition nothing escapes from inside it. But gravitational waves give us a different pathway,” theoretical physicist Sizheng Ma of the Perimeter Institute in Canada told ScienceAlert.

“When two black holes orbit each other and merge, this violent process disturbs spacetime itself in the region very close to the final black hole’s horizon. Some of those spacetime vibrations can travel outward as gravitational waves and eventually reach our detectors.

“We often find it thrilling that something which once felt almost like science fiction, namely using observations to learn about black-hole horizons, has become something we can actually do.”

The event horizon is not the black hole itself, but the boundary that separates the visible Universe from the region beyond a black hole’s grasp.

This boundary is the ‘point of no return’ for a black hole, beyond which the object’s gravity is so strong that not even light in a vacuum is fast enough to achieve escape velocity. The event horizon doesn’t emit, reflect, or scatter light. Anything that crosses it can no longer send light back to us.

As a result, neither the event horizon nor anything beyond it can be observed directly. Everything we know about event horizons comes from indirect observations of their effects on the space around them.

This brings us to gravitational waves: gravitational ripples in spacetime produced when massive objects such as black holes collide and merge, which we can detect here on Earth.

This signal is complex. There’s the final inspiral as the two black holes enter the last stages of approach before the collision; then, in the aftermath, the newly formed black hole rings like a bell.

The waves of this ringdown are called quasinormal modes, determined by the black hole’s mass and spin – and this is how scientists can tease out these properties from a gravitational wave event.

Quasinormal modes, however, are primarily linked to the light ring outside the event horizon, not the horizon itself.

Recent theoretical work has proposed a more direct probe of the event horizon: the direct wave, which should be tangled up in the quasinormal modes.

As the two black holes finish merging, this theory proposes, orbital motion switches from being governed by two black holes to being dominated by the newly formed single object.

The black hole’s extreme gravity literally drags spacetime around with its rotation; gravity redshifts and suppresses outgoing signals; and a single wave is emitted, oscillating with nearly twice the horizon’s rotation frequency – that’s the direct wave.

“As everything gets closer to the horizon of a rotating black hole, they are dragged into extremely rapid motion around it. But at the same time, the signal they send to us fades away very quickly because of the black hole’s strong gravity,” Ma explained.

“So what we see is a final, fast, rapidly dimming swirl near the horizon.”

Now, it’s impossible to overemphasize exactly how subtle gravitational wave signals are. By the time they reach Earth, they stretch and squeeze spacetime by less than the width of an atomic nucleus.

So it took an unusually hefty gravitational wave event for Ma and his colleagues to find the signal they were looking for. That event was GW250114 – the clearest gravitational wave signal received to date.

At first, as they teased the signal from the data, the researchers were cautious. Although the theory was sound, the complexity of gravitational-wave data meant there was always a risk of a false positive.

“Our initial reaction was mixed,” Ma said.

“But after the preliminary checks, the data behaved remarkably well – in fact, just as the theory predicted. The event was unusually loud and clean, and the way the signal evolved matched the expected direct-wave signature calculated from our theoretical model.

“That was the moment when the mood shifted from ‘This might be interesting’ to ‘Oh wow, this might actually be real.'”

The result still needs further testing against other gravitational-wave signals. The theoretical work will also undergo tweaking and refinement, now that scientists have an observational result to measure it against.

But if validated, the team’s breakthrough offers a whole new way to study black holes.

For example, that direct-wave signal can be analyzed to measure how fast the event horizon is rotating, and how quickly gravity causes information to fade away.

“For a long time, we could describe black hole event horizons beautifully in general relativity, but had very limited ways to probe them observationally,” Ma explains.

“This new component in gravitational waves is changing that. This result opens a pathway to studying the near-horizon region more directly, and in the future, with more events and more sensitive detectors, it could help us perform sharper tests of general relativity and build a deeper understanding of black-hole physics.”

And, if confirmed, this result would mark the closest scientists have come to probing the immediate vicinity of a black hole’s event horizon.

Related: Physicists Simulated a Black Hole in The Lab, And It Then Began to Glow

What’s more, this research could change how we study and understand some of the most mysterious objects in the Universe.

“We are getting closer than ever to the black-hole horizon,” Ma said.

“Black holes used to feel like objects we could only understand indirectly, through their effects on things around them. But with gravitational waves, we can listen to the final moments of a merger and look for signatures from the region right next to the horizon.

“That sense of reaching a place that was once completely out of observational reach is really thrilling.”

The findings have been published in Nature.