Physicists have coaxed a black hole's most famous glow out of a strand of optical fiber and, for the first time, watched that light react back on the simulated black hole that produced it.

The result gives researchers a rare, hands-on look at Hawking radiation ‪—‬ the faint thermal emission that Stephen Hawking predicted should leak out of black holes ‪—‬ and offers a first clue about the tiny push that could, in principle, make a real black hole slowly evaporate, the research team said in a new study.

Working with a tabletop experiment in optical fibers, the international team detected both the radiation and its long-sought "back reaction" — the way the radiation feeds energy back and reshapes the object that created it.

According to the new study, published July 1 in the journal Nature, the light behaved exactly as Hawking predicted it should: like the glow of a warm object, with a definite temperature and a spectrum that fades away steadily toward higher frequencies. It did so even in a regime where the usual textbook description of a black hole should break down.

Where three great theories collide

Hawking radiation is famous because it sits at the crossroads of physics' biggest ideas.

"Jacob Bekenstein predicted that black holes have an entropy and a temperature, and Hawking calculated the thermal radiation of the black hole," study co-author Ulf Leonhardt, a physicist at the Weizmann Institute of Science in Israel, told Live Science via email. "In Hawking-Bekenstein radiation, quantum physics, general relativity and thermodynamics come together — subjects that are normally in conflict with each other."

The conflict runs deep: General relativity pictures space and time as smooth and continuous, while quantum mechanics describes a world of discrete, unpredictable jumps ‪—‬ and no one has managed to fully reconcile the two.

That combination is exactly what makes Hawking radiation so hard to study. Astronomers have never seen Hawking radiation from a real black hole and probably never will; the glow is far too faint to pick out across the cosmos. So physicists have turned to laboratory stand-ins that obey the same equations, building black hole analogues out of flowing water, ultracold atoms and, as in this study, light.

Building a black hole from light

The trick behind every black hole analogue is a moving medium. "Imagine a swimmer in the sea with a current faster than he can swim," Leonhardt explained. "He is swept away. This is what happens beyond the [event] horizon, and this is why normally nothing can escape the black hole."

A black hole's event horizon is the boundary where that current — space itself, in real life — starts moving faster than anything can travel. To recreate it, the team needed a material that appears to rush along at the speed of light. Their solution was elegant: use light to make the "material."

"In optics we need a material that appears to move at the speed of light," Leonhardt said. "For this we use light itself — in nonlinear optics, light acts like a material."

In practice, the researchers fired an intense, ultrashort "pump" pulse into a thin photonic-crystal fiber — a strand of glass threaded with a pattern of tiny air channels running along its length, which lets researchers fine-tune how light moves through it. As it traveled, the pulse slightly changed how the glass bent light, creating a moving speed bump that raced along with it. A second, much weaker "probe" pulse then ran into this moving front. Where the probe could no longer keep up, an artificial horizon formed — and the black hole analogue was born.

Catching the glow and its pushback

The payoff came in the ultraviolet. According to theory, Hawking radiation is created in pairs: One partner escapes, while the other, carrying "negative" energy, is the mirror image that would fall into a real black hole. In the fiber, that partner showed up as ultraviolet light.

"We counted photons in the ultraviolet that correspond to the Hawking partners beyond the horizon," Leonhardt explained. "They have a wavelength around 233 nanometers. This was our signal."

Just as important as seeing the glow was understanding how it was made. For years, researchers assumed the fiber built up its Hawking radiation through a cascade — a chain of separate steps in which the light is converted first into one intermediate form, and then another, each feeding the next before the radiation finally emerges. The team found that, instead, a single, direct interaction does the job, with the pump and probe light producing the Hawking pair in one clean step. It is a much simpler picture that the researchers said may carry over to other analogues and perhaps even to real black holes.

Because energy has to come from somewhere, making Hawking radiation should nudge the source that created it. For a real black hole, that nudge is how it loses mass and, over unimaginable timescales, evaporates entirely — the process Hawking described in his landmark 1974 paper. No experiment had ever captured that recoil.

Here, the team saw it. Producing the radiation shifted a small fraction of the pump pulse's own light to a slightly different color, leaving a telltale lopsided pattern in the spectrum. That asymmetry, absent in earlier experiments, is the fingerprint of the back reaction, or recoil — the black hole analogue quietly paying the energetic price for its own glow.

The road to a quantum experiment

The result also speaks to one of the thorniest puzzles in black hole physics: the trans-Planckian problem. Trace Hawking's radiation back to where it was born and the calculation runs into territory no physicist can vouch for — the Planck scale, the vanishingly small size at which space and time are thought to lose their familiar meaning and all known physics gives out. Hawking's prediction, in other words, appears to rest on a foundation that may not exist.

"Any light getting away from the horizon is stretched out enormously," Leonhardt said. "So it must come from waves smaller than the tiniest scale in nature, where the physics is unknown. Would that still give Hawking radiation? That was the question, and we have answered it in our experiment." Remarkably, the glow stayed perfectly thermal even in this extreme regime.

The team's next step is concrete. So far, they have used ordinary laser light, which reproduces the spectrum of Hawking radiation but not its deepest quantum weirdness. Next, the team plans to "go quantum," Leonhardt said. "We will explore how to get into the quantum regime and observe quantum features such as entanglement" — the ghostly link that should tie each escaping Hawking particle to its lost partner.

See how much you know about black holes with our black hole quiz!

Scientists have found evidence that gravitational waves from a spectacular black hole collision carry signals from the very edge of the newly formed black hole. If confirmed by future observations, the discovery could provide an entirely new way to investigate what happens in the immediate vicinity of a black hole without ever observing it directly.

In a new study, the researchers analyzed an exceptionally strong gravitational wave event known as GW250114. They identified a "direct wave," a subtle feature of the total gravitational wave signal predicted by theory but never previously detected in real data. The signal appears to contain information from extremely close to the black hole's event horizon, the boundary beyond which nothing, not even light, can escape.

The findings, published June 24 in the journal Nature, suggest that gravitational wave observatories may eventually allow astronomers to probe regions that have remained inaccessible since black holes were first predicted by Albert Einstein's theory of general relativity.

Listening to the edge of a black hole

Although astronomers have photographed the glowing material surrounding some supermassive black holes and have detected dozens of black hole mergers through gravitational waves, the event horizon itself has remained frustratingly difficult to study.

Unlike ordinary light, gravitational waves are tiny ripples in space-time produced when massive objects accelerate. They pass almost undisturbed through the universe, carrying information about violent cosmic events that would otherwise remain hidden.

According to study co-author Sizheng Ma, a postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada, the newly identified signal offers a rare glimpse of what happens immediately after two black holes collide.

"When two black holes merge, they violently shake space-time itself," Ma told Live Science. "For a brief moment, the region very close to the newly formed black hole's horizon is swept into a fast, fading swirl."

Ma explained that the direct wave is the portion of the gravitational wave signal produced near the horizon and carries the imprint of that motion outward through space.

"That is why it is so interesting," he said. "It may let us 'listen' to what happens near the horizon, a region we cannot see directly with light."

A remarkable black hole collision

The team focused on GW250114, a black hole merger detected on Jan. 14, 2025, by the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in Hanford, Washington, and Livingston, Louisiana.

"Our earlier theoretical work predicted that black hole mergers should produce a direct-wave signal from the near-horizon region," Ma said. "The big question was whether this effect could actually be seen in real data."

GW250114 turned out to provide exactly the conditions needed to test that prediction.

"It was strong enough, clean enough, and close enough to the theoretical situation where this signal should be visible," he said.

To search for the elusive feature, the researchers first removed the best-understood part of the gravitational wave signal, which comes from the newly formed black hole settling down after the merger. Then, they examined the remaining data to determine whether it consisted only of detector noise or contained another physical signal.

"What we found was striking," Ma said. "The remaining signal followed the expected rhythm and fading pattern of a wave shaped by the region very close to the final black hole's horizon."

The team concluded that the leftover signal matches the behavior expected for a direct wave predicted by previous theoretical studies.

A new way to explore extreme gravity

The researchers stressed that their findings do not reveal what lies inside a black hole. Instead, they're providing a new observational tool for investigating the region immediately outside the event horizon.

"The gravitational wave data appear to carry an imprint from very close to the newly formed black hole's horizon — the famous point of no return," Ma said.

He explained that the measurements are consistent with space-time near the horizon being rapidly dragged around by the spinning black hole while the signal fades because of the intense gravitational field.

"For us, the exciting message is that gravitational waves may be giving us a new way to study the edge of a black hole using real observational data," Ma said.

Ma believes the method could eventually become useful for exploring ideas such as quantum gravity — which seeks to unite Einstein's theory of gravity with quantum mechanics — or the black hole information paradox, the longstanding puzzle of whether information that falls into a black hole is truly lost. However, it cannot test those questions directly yet.

"If quantum effects, or any deviations from the standard black-hole picture, leave a measurable imprint there, then direct waves could, in principle, help us search for them in the future," he said.

More observations will be needed

The researchers cautioned that the discovery is based on a single gravitational wave event. While GW250114 provided exceptionally favorable conditions, much stronger evidence will come only if similar signals are found in many additional black hole mergers.

"There are two main directions," Ma said. "The first is theory."

Current models capture the essential physics but remain simplified, and more realistic descriptions of black hole mergers will be needed.

"The second is observation," he added. "This result comes from one exceptionally loud and clean event, so the strongest confirmation would come from seeing the same kind of pattern in other black hole mergers."

As gravitational wave observatories continue to improve and detect increasing numbers of mergers, researchers hope to determine whether direct waves are a universal feature of black hole collisions.

"If the pattern appears repeatedly in the way general relativity predicts," Ma said, "direct waves could become a new way to study black hole horizons or the regions very close to them, and to test Einstein's theory in one of the most extreme environments in the universe."

If future observations confirm the team's results, scientists may have gained something they have sought for decades: a direct observational window into the very edge of a black hole.

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