What is the structure of a black hole's singularity?

[Low-Q]

I have a couple of questions I cannot find a good answer to in the internet, so I ask you guys.

I have heard about singularity as a infinite small point with infinite gravity - I talk about what I assume is the center of black holes. The scientist are talking about quantum gravity - the description of something they don't understand.

Black holes are said to vary in size - at least the event horizon vary in size. How can the event horizon vary in size if the center of the black hols is a infinitely small point with infinite gravity (At least what I have learned)?

This make me think that the center isn't infinitely small at all, but has a structure. The theory arise from two things:
1. The "impossible" smallness and infinite gravity of a point doesn't make sense.
2. The star that collapsed into a black hole is probably not a perfect sphere with even density at any given radius.

I assume that point 2 determine the final shape of a collapsed star. If the star is big enough it ends with a black hole that has an internal structure rather than an infinitely small point. That is my theory.

Does anyone supports this theory? I cannot find anything about alternative theories about black holes and singularity.

Br.

Vidar

 

[jedishrfu]

Don't confuse the event horizon with the black hole.

The event horizon is where even light can no longer escape the black hole. Everything that falls into the black hole reaches the singularity. However Quantum Mechanics provides a loop hole that allows black holes to evaporate over time where particles are created at the event horizon that may escape into space while its counterpart falls into the black hole again.

For more info on the black hole and its event horizon:

http://en.wikipedia.org/wiki/Black_hole

 

[Low-Q]

Don't confuse the event horizon with the black hole.

The event horizon is where even light can no longer escape the black hole. Everything that falls into the black hole reaches the singularity. However Quantum Mechanics provides a loop hole that allows black holes to evaporate over time where particles are created at the event horizon that may escape into space while its counterpart falls into the black hole again.

For more info on the black hole and its event horizon:

http://en.wikipedia.org/wiki/Black_hole

What you describe is the Hawking radiation. The co-particle that escapes is energy. The black hole is black because light cannot escape it, and the "visual" size of a black hole is determined by the event horizon. However, matter that falls towards a black hole would probably shine so bright that no one would actualy see a black spot in the sky.
However, my question boils down to wether the singularity is a point of infinite gravity or not.
I don't think it is. I therfor ask if someone has similar thought about the subject.

Br.

Vidar

 

[phinds]

... my question boils down to wether the singularity is a point of infinite gravity or not.

Mathematically, the solution to the equations that describe black hole say that there is a dimensionless point of infinite density at the "singularity", BUT ... that is believed to be just an artifact of our not having a theory of quantum gravity and that when we DO figure out what quantum gravity looks like, the singularity will be better understood and most likely will not be a point of infinite density.

EDIT: Hm ... I see that what I just said is fully explained in the link jedishrfu provided. Did you even bother to read it?

 

[Drakkith]

The event horizon is the point where the curvature of space becomes so great that it curves back towards the singularity, leading to the effect that once you enter this region of space you cannot get out. There is no path that leads away from the singularity. However, space closer to the singularity is even more curved. The more massive the black hole is, the further away from the singularity the event horizon develops. Remember that even though the gravity at the singularity is infinite, the mass is not, and it is this mass that determines the overall curvature of space around the singularity. More mass equals more curvature. If you were to take the 10 solar masses of a black hole and compact it into an area of space 1 meter across (instead of having a singularity) you would still get an event horizon equal to a 10 solar mass black hole with a singularity.

 

[WannabeNewton]

Nothing special happens to space-time curvature at the event horizon whatsoever. Also, and I can't stress this enough, the singularity is not some point in space at the "center" of a black hole. It is a space-like hypersurface i.e. it is an instant of time (time being the Schwarzschild time coordinate).

 

[Low-Q]

OK. Thanks.

@phinds, I did read the article (eventually), and it is close to what I had in mind. Thanks for the reminder ;-)

Br.

Vidar

 

[Drakkith]

Nothing special happens to space-time curvature at the event horizon whatsoever. Also, and I can't stress this enough, the singularity is not some point in space at the "center" of a black hole. It is a space-like hypersurface i.e. it is an instant of time (time being the Schwarzschild time coordinate).

Can you elaborate? I've never heard this before.

 

[WannabeNewton]

There are many ways to see it and pretty much every good GR textbook will explain it in detail. A pictorial way is to look at the singularity curve in a Kruskal diagram. A more direct way is to just note that $\nabla^{\mu}r$ becomes time-like inside the event horizon hence $r = 0$, to which $\nabla^{\mu}r$ is normal, is a space-like hypersurface.

 

[Drakkith]

There are many ways to see it and pretty much every good GR textbook will explain it in detail. A pictorial way is to look at the singularity curve in a Kruskal diagram. A more direct way is to just note that $\nabla^{\mu}r$ becomes time-like inside the event horizon hence $r = 0$, to which $\nabla^{\mu}r$ is normal, is a space-like hypersurface.

I'm afraid I don't know what any of that means. I've never even taken calculus.

 

[jedishrfu]

Wikipedia has a fairly succinct description of the event horizon and quoted below:

http://en.wikipedia.org/wiki/Event_horizon

Interacting with an event horizon
-----------------------------------------------

A misconception concerning event horizons, especially black hole event horizons, is that they represent an immutable surface that destroys objects that approach them. In practice, all event horizons appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view (as the horizon-crossing event's light cone never intersects the observer's world line). Attempting to make an object near the horizon remain stationary with respect to an observer requires applying a force whose magnitude increases unbounded (becoming infinite) the closer it gets.

For the case of a horizon perceived by a uniformly accelerating observer in empty space, the horizon seems to remain a fixed distance from the observer no matter how its surroundings move. Varying the observer's acceleration may cause the horizon to appear to move over time, or may prevent an event horizon from existing, depending on the acceleration function chosen. The observer never touches the horizon and never passes a location where it appeared to be.

For the case of a horizon perceived by an occupant of a de Sitter Universe, the horizon always appears to be a fixed distance away for a non-accelerating observer. It is never contacted, even by an accelerating observer.

For the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope (or rod) to contact the horizon, in practice this cannot be done. The proper distance to the horizon is finite,[7] so the length of rope needed would be finite as well, but if the rope were lowered slowly (so that each point on the rope was approximately at rest in Schwarzschild coordinates), the proper acceleration (G-force) experienced by points on the rope closer and closer to the horizon would approach infinity, so the rope would be torn apart. If the rope is lowered quickly (perhaps even in freefall), then indeed the observer at the bottom of the rope can touch and even cross the event horizon. But once this happens it is impossible to pull the bottom of rope back out of the event horizon, since if the rope is pulled taut, the forces along the rope increase without bound as they approach the event horizon and at some point the rope must break. Furthermore, the break must occur not at the event horizon, but at a point where the second observer can observe it.

An observer crossing a black hole event horizon can calculate the moment they have crossed it, but will not actually see or feel anything special happen at that moment. In terms of visual appearance, observers who fall into the hole perceive the black region constituting the horizon as lying at some apparent distance below them, and never experience crossing this visual horizon.

Other objects that had entered the horizon along the same radial path but at an earlier time would appear below the observer but still above the visual position of the horizon, and if they had fallen in recently enough the observer could exchange messages with them before either one was destroyed by the gravitational singularity.[9] Increasing tidal forces (and eventual impact with the hole's singularity) are the only locally noticeable effects.

 

[PAllen]

There are many ways to see it and pretty much every good GR textbook will explain it in detail. A pictorial way is to look at the singularity curve in a Kruskal diagram. A more direct way is to just note that $\nabla^{\mu}r$ becomes time-like inside the event horizon hence $r = 0$, to which $\nabla^{\mu}r$ is normal, is a space-like hypersurface.

For the SC BH, it's kind of a strange hypersurface. On approach to the singularity, you have 2-sphere x line (3-cylinder? not sure what the correct geometric term is for this), with the area of the 2-spheres going to zero. The line is the extra killing field (that was timelike outside the horizon, but spacelike inside). The distance along the axis of two infall geodesics approaches infinity. Poetically, we can say the singularity is an infinitely stretched, collapsed 3-cylinder.