Gravitational waves may reveal secrets from black hole event horizons

A significant discovery may have provided the first evidence of gravitational waves carrying signals from the immediate vicinity of a black hole's event horizon. The signals were detected during the collision of two massive black holes, offering a rare glimpse into the chaotic aftermath of such cosmic events.

Researchers focused on the black hole merger designated GW250114, observed on January 14, 2025, by the LIGO detectors. According to Sizheng Ma, a postdoctoral researcher at the Perimeter Institute for Theoretical Physics in Canada and a co-author of the study, the newly identified signal provides a unique insight into what happens directly after two black holes merge. "When two black holes merge, they violently shake spacetime," Ma explained, as quoted by Live Science. "For a brief moment, the region very close to the event horizon of the newly formed black hole is swirled by a fast, fading vortex."

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When two black holes merge, they violently shake spacetime. For a brief moment, the region very close to the event horizon of the newly formed black hole is swirled by a fast, fading vortex.

This phenomenon, known as a "ringdown" signal, is the part of the gravitational wave signal generated in the close proximity of the event horizon. It carries an imprint of this otherwise unobservable region. "That's why it's so interesting," Ma added. "It allows us to 'hear' what's happening near the event horizon, a region we can never see in the optical spectrum."

The team's previous theoretical work predicted that black hole mergers would produce such a ringdown signal. The GW250114 event provided the ideal conditions to test this prediction, being sufficiently strong, clear, and close to the theoretical scenario where the signal should be visible. If further observations confirm this finding, it could revolutionize our understanding of black holes by providing a direct observational method for studying these enigmatic objects.

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That's why it's so interesting. It allows us to 'hear' what's happening near the event horizon, a region we can never see in the optical spectrum.