Einstein was right, again: the first 'traces' of the event horizon captured

An international group of researchers has for the first time extracted direct traces of the event horizon of a black hole from gravitational waves, by analyzing the final moments of the merger GW250114. The study, published in Nature, provides a new verification of Einstein's general relativity in the most extreme gravity regime known.

The signal comes from the most intense gravitational wave ever recorded, captured by the LIGO Observatory in January 2025, already known for confirming Stephen Hawking's area theorem. By isolating the last burst of radiation, what the authors call the direct wave, the team was able to read information from a region closer to the horizon than ever before.

The event horizon is the "point of no return" beyond which nothing can escape, not even light. Its physics is described by two quantities: the rotation frequency and the surface gravity. The direct wave carries the imprint of both, oscillating at a frequency tied to the black hole's rotation and damping at a rate dictated by the surface gravity.

A new confirmation for Einstein

That gradual fading of the signal is the result of gravitational redshift: as the source approaches the horizon, the waves are "stretched" and lose energy until they fade away. It is the shape of this damping, along with the oscillation frequency, that provides coordinates of the horizon which have until now remained inaccessible to instruments.

To explain the phenomenon, the lead author Sizheng Ma of the Perimeter Institute for Theoretical Physics uses the image of a spoon stirring a glass of water. The final stage of the merger generates a vortex in spacetime that propagates as gravitational waves, traveling in all directions at the speed of light. If the "spoon" acts close enough to the horizon, it becomes possible to decode the physics of that region. "Normally this concept appears in science fiction, but now we can really touch the area around the horizon with gravitational data," Ma stated.

The collected data fully align with theoretical predictions for a Kerr black hole, which is rotating. The detection of the direct wave has a signal-to-noise ratio of 15.8 in the Hanford LIGO detector and 17.1 in the Livingston detector, values sufficient to speak of observational evidence and not mere statistical fluctuation.

"We have once again demonstrated that Einstein was right," Ma commented.

Among the information revealed is also how a rotating black hole drags the space around it, an effect known as frame dragging. "It’s like pressing a glass on a table and spinning it, so that the tablecloth wraps around the base," explained Maximiliano Isi, an astrophysicist at Columbia University. This method thus opens a direct observational channel to measure this dragging in the ergospheres of black holes.

The authors caution that further analysis will be needed to understand how much one can truly reconstruct of the horizon with this technique. The long-term goal is ambitious: to identify tiny variations related to quantum fluctuations and, through them, to search for any deviations from general relativity. "In this way, we can probe the region near the horizon in search of new physics," Ma concluded.