# Does the event horizon of a black hole really represent the surface of the star/mass?

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1. Jan 7, 2018

### Philippos

Does the event horizon of a black hole really represents the surface of the "star" (or mass) itself?
What I mean to say is: That the event horizon is the (let's say it this way) sphere surface where the scape velocity is => than the speed of light. So it is not necessary for event horizon to be exactly situated where the surface of the mass/"gravastar" really is.
For example: Let's say that there's a black hole of 2cm radius and has a 5,98 * 10^24 kg. There, the surface of the black hole is 2,5cm away from the center of it. But the "imaginary surface" where the scape velocity is bigger than the speed of light doesn't have to be there./?

In conclusion my question is: Is it necessary for the event horizon to be situated in the black hole surface?
It's a question that I have in my head since I knew what the event horizon was. I would really thank if you could help me with this.

Thank you.

Philippos.

Last edited by a moderator: Jan 8, 2018
2. Jan 7, 2018

### Orodruin

Staff Emeritus
There is no such thing as the "surface" of the mass.

3. Jan 7, 2018

### Philippos

Sure, I mean the surface of the "object", the black hole. Just like the Earth or the Sun have a surface. I'm sure the black hole has a surface because it's just a lot of mass in a relatively tiny volume.

Sorry if I didn't express myself the right way.

4. Jan 7, 2018

### Orodruin

Staff Emeritus
As I said, there is no such surface.

5. Jan 7, 2018

### Philippos

Could you please explain me why then.
Thanks.

6. Jan 7, 2018

### PeroK

A black hole is not what you think it is. All the matter collapses and gets destroyed at the singularity. There is no object "like the Sun or Earth" inside the event horizon.

7. Jan 7, 2018

### Chronos

The event horizon is merely the theoretical distance from the center of mass of a massive body to the poinr at which its escape velocity equals the speed of light. That does not constitute a physical surface any more than the distance from a star at which ice can form constitutes the surface of a glacier.

8. Jan 7, 2018

### PeroK

I think he's asking about the surface of the remnants of the star inside the horizon.

9. Jan 7, 2018

### Staff: Mentor

All of the mass collapses into a very small size...mathematically a point.
https://en.wikipedia.org/wiki/Gravitational_singularity
https://en.wikipedia.org/wiki/Black_hole#Singularity
I'm not certain if scientists believe the black hole singularity actually exists or if there is some small but non-zero volume containing all of the mass of the black hole...

10. Jan 7, 2018

### phinds

@Philippos , the center of a black hole is called a "singularity". That word is not meant to imply a point, it is simply a short-hand for "the place where the math model breaks down and we don't know WHAT is/was happening". The only way we have to model it is that the mass is in some way engulfed in the "singularity" and there simply is no point in trying to talk about it in conventional terms. The hope is that if/when we develop a viable theory of quantum gravity we will have an explanation for what a black hole singularity really is.

11. Jan 7, 2018

### Philippos

So... we basically now nothing of what's going on "behind" the event horizon. Not even if there is some kind of volume or if the mass transforms into energy.
The conclusion is that we don't know yet. Am I right?

12. Jan 7, 2018

### Orodruin

Staff Emeritus
We have a pretty good idea because nothing suggests that our theory (GR) should break down right behind the horizon (it is expected to happen closer to the singularity). However, it is not very appropriate to view the event horizon as any sort of surface. It is a surface that locally travels at the speed of light, yet it remains stationary. The problem with visualising this is that generally our minds are very well trained in Euclidean geometry, but not so much in space-time geometry.

That being said, there is no way you can figure out what goes on beyond the horizon. No signal can reach you from there (if we make the assumption that you don't want to jump into the black hole yourself).

13. Jan 7, 2018

### Staff: Mentor

Not quite. It means we don't know if it collapses to a single point or something a little bit larger we don't know about yet. But we do know it must collapse into something much denser than a neutron star because the forces that keep a neutron a neutron are not strong enough to prevent further gravitational collapse.

It also may be useful to recognize that for a very large black hole, the average density inside the event horizon is pretty low (like, density of water, low) so whatever is in the middle must be very much smaller than the size of the event horizon.

14. Jan 8, 2018

### Philippos

Thank you, that answers my question.
I'm really excited to see the photo of the black hole! They say it will be taken this year.

15. Jan 8, 2018

### Ilythiiri

If i positioned a network of LIGO Mk.X's in a sphere around BH and measure passing gravity waves, would i get any information about BH interior?

16. Jan 9, 2018

### Staff: Mentor

No, because information has to be carried by something and this something cannot escape the black hole.

I think the topic has been sufficiently discussed. Thread closed.