# Singularity - black holes

1. Nov 15, 2013

### 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 dont 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

2. Nov 15, 2013

### Staff: Mentor

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.

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

3. Nov 15, 2013

### Low-Q

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

4. Nov 15, 2013

### phinds

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?

5. Nov 15, 2013

### Staff: Mentor

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.

6. Nov 15, 2013

### 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).

7. Nov 15, 2013

### 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

8. Nov 15, 2013

### Staff: Mentor

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

9. Nov 15, 2013

### 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.

10. Nov 15, 2013

### Staff: Mentor

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

11. Nov 15, 2013

### Staff: Mentor

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

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

12. Mar 11, 2014

### PAllen

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.