A ‘direct wave’ from colliding black holes reveals signature of a whirlp…

A ‘direct wave’ from colliding black holes reveals signature of a whirlpool in spacetime

Astronomers have detected a previously hidden component of a gravitational wave signal, providing the first observational evidence of a direct wave emanating from the immediate vicinity of a black hole's event horizon. The discovery, based on the event known as GW250114, offers a new method to measure the properties of the "surface of no return" and confirms the behavior of spacetime as a swirling whirlpool.

The signal was recorded on January 14, 2025, by the Laser Interferometer Gravitational-Wave Observatory (LIGO) interferometers in Hanford, Washington, and Livingston, Louisiana. The detection occurred near the end of the O4b subrun, a period of observation that began on April 10, 2024, and ended on January 28, 2025. While partners Virgo and KAGRA were not operating at the time, the LIGO detectors registered nearly identical signals shortly after 08:22:03 UTC.

GW250114 is described as the clearest gravitational wave signal received to date. It achieved a signal-to-noise ratio (SNR) of 80, a significant increase over the 42 SNR of GW230814 and the 26 SNR of the first-ever detection, GW150914, from a decade prior. This clarity allowed researchers to decode the direct wave, which is typically tangled within other waves created during a merger.

Decoding the Event Horizon

According to general relativity, a rotating black hole does not remain static but produces frame dragging, an effect where the surrounding spacetime is whirled along with the black hole's rotation. The direct wave is gravitational radiation originating from right outside the event horizon, where infalling objects are forced to orbit at the horizon's rotation frequency, ΩH. These signals decay at a rate set by the surface gravity, κ, due to gravitational redshift.

Theoretical work predicted that this component would oscillate near 2ΩH. Observational evidence from GW250114 showed a 90% credible matched-filter signal-to-noise ratio of 17.1 in the Livingston detector and 15.8 in the Hanford detector. These measurements are in full agreement with theoretical predictions for a Kerr black hole.

The event involved two black holes with masses of 33.6+1.2−0.8 M☉ and 32.2+0.8−1.3 M☉. The resulting merged mass was 62.7+1.0−1.1 M☉, with 3.1±2.2 M☉c2 of energy released. Both initial black holes had low spin — at most approximately 0.25 of the maximum possible spin — while the merged remnant had a spin of 0.68+0.01−0.01 of the maximum.

Confirming Theoretical Laws

The high SNR of the signal enabled the empirical confirmation of Stephen Hawking's 1971 area theorem. This theorem states that the total surface area of two merged black holes must increase or remain the same, even if the black holes lose energy through gravitational waves. In the case of GW250114, the two original black holes had a combined surface area of about 240,000 square kilometers, while the final black hole was approximately 400,000 square kilometers.

Researchers also identified the first overtone of the Kerr solution for a rotating black hole with a 4.1σ level of significance. Because the newly formed black hole vibrates like a struck bell, a stage called ringdown, scientists can use the pitch and decay of these harmonics to measure mass and spin. While the 2015 signal was too faint to distinguish more than one harmonic, GW250114 allowed for the measurement of two tones.

Implications for Quantum Gravity

The ability to probe the near-horizon region directly may help resolve the conflict between general relativity, which describes large-scale gravity, and quantum mechanics, which governs the smallest scales. By analyzing these direct waves, scientists hope to find "cracks" in current theories or signatures of quantum gravity. Future research will involve testing the direct-wave signal against other gravitational-wave events and refining theoretical models to determine if any future mergers deviate from the classical predictions of general relativity.