The Event Horizon of a black hole indeed represents a boundary beyond which no information can escape, including gravitational waves. However, when we talk about the detection of gravitational waves from merging black holes using instruments like the Laser Interferometer Gravitational-Wave Observatory (LIGO), we are not directly observing the black holes themselves or their event horizons. Instead, we are detecting the effects of the gravitational waves generated by the black holes' motion.
Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects. As black holes merge, they create intense gravitational waves that propagate outward at the speed of light. These waves carry information about the merger process itself, such as the masses and spins of the black holes involved, the timing and duration of the merger, and the resulting properties of the final black hole.
LIGO and other gravitational wave detectors work by measuring tiny changes in the distances between multiple test masses caused by passing gravitational waves. These changes in distance are much smaller than the sizes of the event horizons of the black holes involved, so the information extracted from the gravitational waves doesn't violate the concept of the event horizon blocking information.
When two black holes merge, the gravitational waves they produce propagate through space, gradually weakening as they travel. By the time they reach LIGO detectors on Earth, the gravitational waves have become extremely weak. However, LIGO's instruments are designed with high sensitivity to detect these tiny fluctuations in spacetime caused by the gravitational waves. The detectors are capable of measuring displacements on the order of 10^-19 meters, allowing them to capture the gravitational wave signals.
While LIGO cannot directly observe behind the event horizons or obtain information about the black holes themselves, the detection of gravitational waves provides valuable insights into the astrophysical processes associated with black hole mergers. It allows scientists to study the properties of black holes, test predictions of Einstein's theory of general relativity, and deepen our understanding of the universe.