Can Extra Dimensions Solve the Central Singularity of Black Holes?

In summary, the size of a black hole refers to the size of its event horizon, which is the point of no return. The singularity at the center of a black hole is a point-like zero-dimensional object, but it is not the only part of the black hole. The event horizon and everything inside it make up the entire black hole. It is currently unknown what exactly happens inside a black hole, as classical general relativity does not account for quantum effects. The size of a black hole can vary, but smaller ones will evaporate faster due to Hawking Radiation. Efforts are being made to resolve the singularities in black holes through theories like AdS/CFT and LQG.
  • #1
jaydnul
558
15
I was watching a documentary about the universe and it claimed that black holes were sometimes as small as 2 kilometers across. Now before this, my general understanding of a black whole was that it had no physical extent in space, that it was just a 1 dimensional singularity, and the black planet looking thing was just where the point of no return started. So if I were sucked into a black hole, would I eventually run into a very dense mass 1 kilometers across or would it just be empty space all the way down to the singularity?
 
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  • #2
The 2 km refers to the outside dimension. No one really knows what happens inside a black hole. General Relativity and Quantum theory don't work together - new theory is needed.
 
  • #3
Jd0g33 said:
I was watching a documentary about the universe and it claimed that black holes were sometimes as small as 2 kilometers across. Now before this, my general understanding of a black hole was that it had no physical extent in space, that it was just a 1 dimensional singularity, and the black planet looking thing was just where the point of no return started. So if I were sucked into a black hole, would I eventually run into a very dense mass 1 kilometers across or would it just be empty space all the way down to the singularity?

When people refer to the size of a black hole, they mean the size of the event horizon, the thing you're calling "the point of no return". Classical general relativity does predict that there is a point-like zero-dimensional singularity inside the event horizon at the center, but
- That's just a part of the black hole, not the whole thing. The whole thing is the event horizon and everything inside it.
- We can't very well look inside the black hole to see what's at its center, but it's unlikely that there's really a pointlike singularity. At very small distance scales we have to pay attention to quantum mechanical effects; general relativity doesn't consider these, so its predictions cannot be completely trusted when very large masses are concentrated into truly infinitesimal volumes on the way to becoming a size-zero singularity.

A smallish correction: You said "1 dimensional" above but I assume you meant "zero-dimensional"; a one-dimensional singularity would be a line not a point.
 
  • #4
Nugatory said:
A smallish correction: You said "1 dimensional" above but I assume you meant "zero-dimensional"; a one-dimensional singularity would be a line not a point.

Ya that's what I meant :). Thanks for the clarification guys
 
  • #5
Kerr black holes have 1D singularities (ring).
 
  • #6
WannabeNewton said:
Kerr black holes have 1D singularities (ring).

so a Kerr BH singularity is not literally a 2D disk, but rather just the 1D curved line that comprises the disk's circumference? since we can't peek inside the EH and look and see the "singularity" directly, do there exist any predictions about the range of diameters of the disk formed by a Kerr 1D singularity? if so, I'm assuming it would depend on the BH mass and spin rate? also, when discussing Kerr BH's, why do we still refer to the "final destination" as a singularity when, if it is truly a 1D ring, it is not composed of a single point, but infinitely many points?
 
  • #7
Jd0g33 said:
black holes were sometimes as small as 2 kilometers across.

If you look at the way they are formed. They begin with a massive implosion of very large red giant Stars leaving only a small Black Hole as its remnant.
 
  • #8
Jd0g33 said:
I was watching a documentary about the universe and it claimed that black holes were sometimes as small as 2 kilometers across.

There is nothing that inherently limits the size of a black hole. They could exist much smaller than 2KM across (the EH diameter), but the smaller they are the quicker they evaporate due to Hawking Radiation so really small ones wouldn't last long.
 
  • #9
The singularities in a Kerr black hole are coordinate singularities. They can be resolved by changing the coordinate system. There is a strong belief a similar solution is possible to eliminate the singularity in a Schwarzschild black hole, although it remains to be demonstrated.
 
  • #10
Chronos said:
The singularities in a Kerr black hole are coordinate singularities. They can be resolved by changing the coordinate system

What coordinate transform gets rid of the singularity?
 
  • #12
Chronos said:

? This paper seems to be discussing only the coordinate singularities at the horizons, not the physical singularities. It's been obvious for a long time that these are mere coordinate singularities, but the physical singularities cannot be removed by any coordinate transform.
 
  • #13
Agreed. Coordinate transforms do not resolve point singularities as they suffer from infinite curvature, which produces other infinities. Efforts to remove this infinite curvature are being attempted in LQG and AdS/CFT. The Kerr central singularity is rather unique in that there are trajectories that need not pass through a region of infinite curvature. The lure of adding extra dimensions to resolve this singularity, such as in AdS/CFT, is undeniably tempting. Here is a paper some may find of interest: 'Infinities as a measure of our ignorance', http://arxiv.org/abs/1305.2358.
 
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1. What is the geometry of a black hole?

The geometry of a black hole is described by the Schwarzschild metric, which is a mathematical representation of the curvature of spacetime around a non-rotating black hole. This geometry is characterized by a singularity at the center, surrounded by an event horizon and a photon sphere.

2. How does the geometry of a black hole differ from that of a regular object?

The geometry of a black hole is significantly different from that of a regular object due to its extreme mass and density. While regular objects have a well-defined surface, black holes have a singularity at their center, which is a point of infinite density and zero volume. This results in an extremely curved spacetime around the black hole.

3. Can the geometry of a black hole change?

Yes, the geometry of a black hole can change due to factors such as its mass, spin, and the matter and energy it absorbs. As a black hole grows in mass, its event horizon and photon sphere will also increase in size, altering its overall geometry.

4. How does the geometry of a black hole affect the behavior of matter and light around it?

The extreme curvature of spacetime around a black hole can cause matter and light to behave in unusual ways. For example, matter and light that come too close to a black hole's event horizon can be pulled into the singularity, while light can also be bent and distorted by the strong gravitational pull of the black hole.

5. Is the geometry of a black hole the same in all directions?

Yes, the geometry of a non-rotating black hole is spherically symmetric, meaning it is the same in all directions. This is due to the fact that the black hole has no preferred direction of rotation. However, the geometry may differ for rotating black holes, which have a more complex shape known as a Kerr metric.

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