If the speed of gravity is the speed of light, how can someone outside of a black hole detect its change of mass? (lemmy.ml)

submitted 19 hours ago by TheGuyTM3@lemmy.ml to c/asklemmy@lemmy.ml

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I clarify my question: beyond the event horizon of a black hole, according to general relativity, the space-time flows faster than the speed of light. If it is the case, then, no information can be transmitted from here.

But then, if i drop an apple, say, in the black hole.

The black hole would then gain mass, and i could theorically mesure that gain with the event horizon radius variation and the attraction, meaning that the information of its mass and attraction change went from the center to get out of the event horizon.

In other words, that gravity information would have been faster than light?

How is that possible and where did i not understand something? (Just daydreamed about this stuff so maybe my question in itself is idiotic, sorry physicists)

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[-] Libra@lemmy.ml 27 points 18 hours ago

Because light is affected by gravity but gravity isn't. Gravity—the curvature of space-time—can't stop changes in the curvature of spacetime from propagating outward. But also that information isn't coming from inside the event horizon, it's coming from spacetime around it.

[-] TauZero@mander.xyz 2 points 2 hours ago

Gravity—the curvature of space-time—can’t stop changes in the curvature of spacetime from propagating outward.

This is false. If it were true, you could build a device to communicate from inside a black hole event horizon. By waving around a heavy ball you would create gravitational waves that a sensitive enough LIGO outside the black hole could detect. But this is impossible. You would create gravitational waves, yes, but they would fall towards the center singularity same as you, and will never penetrate and escape the event horizon.

this post was submitted on 15 May 2025

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