Does BH Event Horizon Reveal Information on Mass, Angular Momentum, and Charge?

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In summary: If by "natural born" you mean a BH that has always existed and was never formed by the collapse of a massive object, then there is a mathematical solution of the Einstein Field Equations that describes such a BH (it's called the "maximally extended" Schwarzschild solution), but as far as I know, nobody considers that solution to be physically reasonable. So any actual BH will have been formed by the collapse of a massive object.
  • #1
sweet springs
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Hi.

Metric and electromagnetic field outside BH are determined by mass, angular momentum and charge of BH. Does this imply that information of the values of the three parameters come from BH through event horizon? Is it possible?

Regards.
 
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  • #2
sweet springs said:
Metric and electromagnetic field outside BH are determined by mass, angular momentum and charge of BH. Does this imply that information of the values of the three parameters come from BH through event horizon?

No. That information comes from the object that originally collapsed to form the BH. More specifically, at any event outside the BH's horizon, the observed fields at that event are determined entirely by the sources of those fields that are in the past light cone of that event, and those sources will therefore have to be in the collapsing object, before it falls below the event horizon.

This is actually not limited to BHs; it's true in general in relativity, as a consequence of the general principle that information cannot travel faster than light.
 
  • #3
Thanks. According to your explanation, I can assume that all the black holes should have their history of collapse. There cannot be natural born BH. Is this assumption right?

Regards.
 
  • #4
sweet springs said:
Thanks. According to your explanation, I can assume that all the black holes should have their history of collapse. There cannot be natural born BH. Is this assumption right?

Regards.

What is a "natural born BH" ?
 
  • #5
sweet springs said:
According to your explanation, I can assume that all the black holes should have their history of collapse. There cannot be natural born BH. Is this assumption right?

If by "natural born" you mean a BH that has always existed and was never formed by the collapse of a massive object, then there is a mathematical solution of the Einstein Field Equations that describes such a BH (it's called the "maximally extended" Schwarzschild solution), but as far as I know, nobody considers that solution to be physically reasonable. So any actual BH will have been formed by the collapse of a massive object.

Btw, one of the reasons the maximally extended solution isn't considered physically reasonable is that the answer I gave to your question doesn't work, because in that solution there are no "sources" anywhere--the entire spacetime is vacuum. What allows that solution to still have a "mass", even though it is vacuum everywhere, is the presence of a "white hole" singularity in the past, as well as the black hole singularity in the future. But as far as I know, nobody thinks a "white hole" is physically reasonable either, so that's another reason why the maximally extended solution is not considered physically reasonable.
 
  • #6
Thanks phinds and PeterDonis for taking me up to the higher level of understanding. Regards.
 

1. What is a black hole event horizon?

A black hole event horizon is the point of no return for matter or light that falls into a black hole. It is the boundary where the gravitational pull of the black hole becomes so strong that even light cannot escape.

2. How does the event horizon reveal information about a black hole's properties?

The event horizon can reveal information about a black hole's mass, angular momentum, and charge through the effects they have on the curvature of space-time near the event horizon. These properties can affect the size and shape of the event horizon, providing clues about the black hole's characteristics.

3. Can we directly observe the event horizon of a black hole?

No, we cannot directly observe the event horizon of a black hole as it is a boundary beyond which no information can escape. However, we can indirectly study its effects on the surrounding environment, such as its gravitational influence on nearby objects and the emission of radiation from the accretion disk.

4. How do scientists measure the properties of a black hole using the event horizon?

Scientists use various techniques, such as studying the motion of nearby stars or analyzing the radiation emitted from the accretion disk, to infer the properties of a black hole. By observing the effects of the event horizon on these phenomena, scientists can make estimations about the black hole's mass, angular momentum, and charge.

5. Does the event horizon provide information about the interior of a black hole?

No, the event horizon does not provide information about the interior of a black hole. It is a boundary that marks the point of no return, and beyond it, our current understanding of physics breaks down. The interior of a black hole remains a mystery, and we can only make theoretical predictions based on the laws of physics.

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