# Radius of a black hole

1. Jul 4, 2007

### delta_simon

Does a black hole have a radius or is it zero?
I dont mean the event horizon, but the matter that have been squized down forming the black hole.

2. Jul 4, 2007

### mgb_phys

Classically the radius would be zero. But thats why there is an event horizon - so we don't have to ask questions like this.

3. Jul 4, 2007

### smallphi

The classical GR black hole solution does not apply close to the singularity so we don't know what's going on there.

4. Jul 4, 2007

### delta_simon

Why would the radius classically be zero? I mean it´s just matter squezed tight together? Would the gravity eliminate all neutrons into a single point?:grumpy:

5. Jul 4, 2007

### mgb_phys

Because in classical mechanics there is no limit to how small you can squeeze the particles. Basically all bets are off inside the event horizon.

6. Jul 18, 2007

Still, any point of mass must have some sort of area, and most spacial bodies are spherical in essence, so it would have a radius, or atleast a cross section of its center would.

7. Jul 18, 2007

### cesiumfrog

If you believe GR, then the centre settles down to exactly zero radius. Anything less extreme would violate classical GR. (Hence, we suspect GR is wrong in that regime.)

8. Jul 18, 2007

### mgb_phys

When you get that small you are in Quantum mechanics territory, anything that seems reasonable or obvious when talking about larger objects is wrong.

Getting GR ( the study of gravity and large classical objects ) and QM ( the study of small objects that aren't there if you don't observe them) to match up has been the main bit of physics for the last 80years. A singluarity is where they meet - so don't expect to understand it.

9. Jul 19, 2007

Still, even in this case there is an area for all objects with mass. And objects with area and happen to be circular have a radius in their cross section.

10. Jul 19, 2007

### ice109

reread his post. you cannot assume anything that you intuitively know to be true at this scale. so no not all objects with mass necessarily have an area

11. Jul 19, 2007

### ice109

reread his post. you cannot assume anything that you intuitively know to be true at this scale. so no not all objects with mass necessarily have an area

12. Jul 19, 2007

### mgb_phys

To quote one of the famous QM inventors (I can't rememebr which)
"The langauge developed by a 2m tall african ape to tell his friends where the ripe fruit is - is not necessarily ideal for understanding the properties of sub-atomic particles."

13. Jul 19, 2007

### ice109

reread his post. you cannot assume anything that you intuitively know to be true at this scale. so no not all objects with mass necessarily have an area