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)
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.
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.
PBS Space Time actually made an episode about that: https://youtu.be/cDQZXvplXKA
This is a very interesting channel, and it approaches lots of my questions, thank you!
They’re one of the best physics-explaining channels I’ve found
In this specific case, the gravity of the apple just never went away. It exerts one apple mass before it starts to fall, while it’s falling, and whenever it crosses the event horizon from your point of reference.
Nothing is actually being “transmitted” per general relativity here. If someone were to jump in with some kind of gravitational wave generating device and send you a secret message from inside, that would be a straighter example. As I understand it that would not work; the waves follow null geodesics (light-type trajectories) and would be trapped with everything else. Which is weird, because spacetime is trapping itself, but deep physics is under no obligation to act normal.
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.
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.
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.
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.
Cause black holes don’t gravitate, but the event horizon does. Black holes dont really have volume in a way thats meaningful to us, but they do have a surface area, and thats what you can interact with.
You’re correct anything past an event horizon cannot interact with us, for all intents and purposes it doesnt exist. So all the interactions you associate with a black hole are interactions with the event horizon itself.
slightly off topic but the first time I got “stuck” in a black hole on SpaceEngine was scary as fuck
we fly your mom to it and see how much it wobbles from her orbit.
You only know the total mass, charge, and angular momentum of the black hole—you don’t know how those properties are distributed inside the event horizon. You see the apple approach the horizon and the horizon expands to encompass the apple-black hole system, but that information isn’t coming from the singularity at the center—it’s coming from the horizon.
Exactly, that information isn’t coming from inside the black hole. In fact it is the lack of information that tells us such things. We know the ratio between the lack of information (the event horizon) and the mass of everything inside.
The change in mass, electric charge, angular momentum are the only things that are detectable after the apple crosses the black hole - no hair theorem. As far as we know, no quantum information is recoverable. We don’t actually, like, see the apple ever cross the event horizon it’s a whole weird thing. The event horizon conceals all events from the interior of the black hole from the rest of the universe - we never see anything about the center of the black hole or any part inside it, ever. It’s all cut off.
The additional mass that changes how much additional force there is due to gravity is communicated at the speed of light, no faster.
There are interesting theories about black hole holography, universe holography brane vs bulk stuff, whatever.
According to our current understanding of physics and technology, you can’t.
Maybe that will change in in the future, but for now, sorry.
I have no idea. But maybe the gravitational location would appear to asymptotically approach the event horizon similar to how light from an object would appear to just approach the horizon and then stay there.
the fastest way to detect the event horizon is by shooting light past it, so no, not faster
