# I Extract information from a black hole by manipulating it

#### Creedence

Summary
Extract information from a black hole by manipulating the event horizon
I just wandering about the following hypotetical process to get some information from the inside of a black hole.
1. Drop a satellite into the black hole on a nearly tangential orbit.
2. After it crossed the event horizon drop a large mass after it (for example a small black hole).
3. This mass causes perturbations on the event horizon and at a certain part of the satellite's orbit may get outside.

Is it possible? Thanks for the answers,
Robert

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No.

#### Ibix

No. Crossing an event horizon is forever. It is literally the boundary between places where "somewhere other than the singularity" is included in your future lightcone and places where all routes lead to the singularity. You can certainly manipulate the 3d shape of an event horizon, but that can't change the fate of something already inside the horizon.

#### Creedence

No. Crossing an event horizon is forever.
In a static case. But in the case of two, rapidly moving black holes?
In the case of a rapidly orbiting binary black hole the common event horizon moves very fast. It may move faster than a massive object.

#### Ibix

See edited post above.

#### Ibix

So saying the satellite crossed the event horizon is another way of saying it will end up in the singularity. Trying to have it escape again, therefore, is trying to have it both fall into the singularity and not fall into the singularity. The scenario is self-contradictory.

#### Creedence

So saying the satellite crossed the event horizon is another way of saying it will end up in the singularity. Trying to have it escape again, therefore, is trying to have it both fall into the singularity and not fall into the singularity. The scenario is self-contradictory.
I think you suppose that the black hole is stationary. But in this case it is perturbed and changing in time. Some points get inside then leave.

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Some points get inside then leave.
No, they don't.

Why are you asking us a question if you don't believe our answers?

#### Orodruin

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No, no, and no. It seems you do not understand the meaning of "event horizon".

#### Ibix

I think you suppose that the black hole is stationary. But in this case it is perturbed and changing in time. Some points get inside then leave.
I am not supposing anything. The event horizon is simply not defined the way you appear to think. It is the boundary between regions of spacetime where you can ever escape and regions where you can never escape. As I already said, the 3d shape of the surface (for some definition of 3d) may change, but it is defined as the surface of a region of spacetime (not just space!) from which you can never escape. If you escape, you weren't in a region where you can't escape. So you weren't under the horizon.

The maths is complicated but the concept here really is simple: there are regions in spacetime (remember this is spacetime, so I am taking into account its dynamical structure) from which you cannot escape. The event horizon is defined as the boundary of these regions. If the evolution of spacetime means that you will be able to escape, you are not in a region from which you cannot escape and you have not crossed the event horizon. If you have crossed the horizon then you are in a region from which you cannot escape (even taking into account the future structure of spacetime) and that's all there is to say.

#### Ibix

Some points get inside then leave.
Just to add - I can see why you think this for a non-axi-symmetric event horizon. But it is wrong. You can certainly define paths that enter and leave a horizon (symmetric or otherwise), but they are not timelike. And the sensible interpretation of "point" in relativity is "timelike line". They cannot leave. Your intuition that they ought to be able to fails you in strongly curved spacetime.

#### Creedence

No, no, and no. It seems you do not understand the meaning of "event horizon".
I know what is an event horizon (or apparent horizon in this case). My fault was that I assumed that the innermost stable photon orbit is in the EH. Thanks for the answers.

"Extract information from a black hole by manipulating it"

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