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I know that one of the defining features of a black hole is that all matter, including light, cannot escape a black hole. I was wondering if gravitational waves can. If this is true perhaps we could use a form of gravity sonar to map the insides of black holes.

So is it possible for gravitational waves to escape the gravitational pull of a black hole?

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The answer to your question is a definite no. Nothing, no particles, no information, can ever escape a black hole.

Electromagnetic radiation cannot escape a black hole, because it travels at the speed of light. Similarly, gravitational radiation cannot escape a black hole either, because it too travels at the speed of light.

How does gravity escape a black hole?

The event horizon is contrary to popular belief, not a physical thing, but a boundary, where the escape velocity equals the speed of light, and inside the horizon it exceeds it. This is the reason why nothing escapes a black hole. In other words, all worldlines inside the event horizon lead to the singularity. This includes the worldlines of the gravitational waves you mention.

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  • $\begingroup$ In the conventional understanding, the horizon is a door to the future. Once you cross is, you can't return just like you can't return from after midnight back to yesterday. The popular "speed of light" explanation is based on the fact that light moves with the speed of time, so moving faster than light is the same as moving back in time, which is impossible. And for this very reason, the "inside" of a black hole has no physical meaning. The existence or non-existence of anything beyond the horizon makes absolutely no difference to the entire external universe. $\endgroup$
    – safesphere
    Feb 27 at 6:22
  • $\begingroup$ @safesphere thank you so much! $\endgroup$ Feb 27 at 6:23

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