The LIGO detectors caught the signal GW250114—a gravitational wave from the merger of two black holes. After the main burst, a decaying 'tail' remained — direct waves from the newborn hole's horizon. It was expected that their damping would reveal the gravitational strength at the horizon's surface. But the measured value came out smaller.
The authors built a model: the wave source is matter falling into the hole, stretched and slowed near the horizon. The gravity there is immense, but due to a 'shielding' effect and the finite duration of the fall, a weakened damping rate reaches the observer. For GW250114, the calculation gave a damping of about 0.4 inverse milliseconds — roughly 60% of the expected gravitational strength.
A check on another event, GW231226, confirmed the model. So a direct measurement of the gravitational strength at the horizon remains out of reach: we only see its reflection, transformed by the geometry of spacetime.
The gravitational strength at the horizon of a stellar-mass black hole is so immense that even light cannot escape — hence the name 'event horizon'.
In the movie 'Interstellar', huge waves are shown on Miller's planet near the black hole Gargantua — an artistic depiction of tidal forces, which also influence the damping of gravitational waves.