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)

  • 𒉀TheGuyTM3𒉁@lemmy.mlOP
    5·
    14 hours ago

    It’s a bit of a nitpick, but I’m not sure I’d use “flow” here. That makes it sound like it’s moving, while the Schwarzchild solution is actually static with respect to a properly chosen time coordinate (and without checking, probably the spinning and charged solutions as well). The spacetime is curved in such a way all timelike (c or slower) trajectories go inwards.

    Oh that makes sense. I though it was space time itself which was moving, bringing with it the objects on it. (probably had seen some illustration representing it like that)

    But yes the gravitational waves take is interesting, it burn my mind trying to imagine how to “trap” spacetime in itself.

    • CanadaPlus@lemmy.sdf.org
      2·
      1 hour ago

      It’s “just” curvature, both through space and time. The Einstein field equation literally has energy and momentum on one side, and a type of curvature measure on the other.

      The trick there is that curved 2D spaces can already be pretty weird, and it gets exponentially crazier in dimensions 3 and 4. This makes it both capable of doing surprisingly a lot, like putting Earth in a fixed, repeating orbit without much local distortion, and difficult to visualise even by analogy. Interestingly, dimensions 5 and higher aren’t any worse, which is actually a pattern that repeats across a lot of geometry.

      In slightly more mathy detail:

      A curved 2D surface can be completely characterised by the Gaussian curvature at each point, which is a single real number (aka a scalar). In dimension 3, you need to use the Ricci curvature, which is a 3x3 matrix/tensor, so 9 scalars, and in dimension 4 it’s the Riemann curvature tensor which is 4x4x4x4. There’s symmetries that you can use to get that down to 6 and 20 scalars respectively, but that’s still a lot more parameters on every point than we’re used to.

      But yes the gravitational waves take is interesting, it burn my mind trying to imagine how to “trap” spacetime in itself.

      There is a bit of nuance there, which is why I said “as I understand it”. Gravitational waves are classically defined in terms of perturbations of flat spacetime, and a black hole is nowhere near close to flat. It’s possible there’s been work showing how to define them in that context, but I’m not a specialist and I couldn’t name it.

      If this were electromagnetism I’d just use the superposition principle, but GR is not linear. In fact, there’s chaotic dynamics that can happen in black holes related to the Mixmaster universe model. It’s also possible (to my limited knowledge) that there isn’t nice propagating waves at all so much as just adjustments to the crazy bending everything is already doing.