Black Hole Radius: Facts & Theory

In summary, according to the author, a black hole has a radius, but it's uncertain what happens near the singularity. Classical gravity doesn't apply close to the singularity, so we don't know what happens there. Even in classical mechanics, there is still an area for all objects with mass. Objects with area are circular, which has a radius.
  • #1
delta_simon
6
0
Does a black hole have a radius or is it zero?
I don't mean the event horizon, but the matter that have been squized down forming the black hole.
 
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  • #2
Classically the radius would be zero. But that's why there is an event horizon - so we don't have to ask questions like this.
 
  • #3
The classical GR black hole solution does not apply close to the singularity so we don't know what's going on there.
 
  • #4
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?
 
  • #5
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
Still, any point of mass must have some sort of area, and most spatial bodies are spherical in essence, so it would have a radius, or atleast a cross section of its center would.
 
  • #7
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
madphysics said:
Still, any point of mass must have some sort of area
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
mgb_phys said:
When you get that small you are in Quantum mechanics territory, anything that seems reasonable or obvious when talking about larger objects is wrong.

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
madphysics said:
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.

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
madphysics said:
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.

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
To quote one of the famous QM inventors (I can't rememebr which)
"The language 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
madphysics said:
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.

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
 

FAQ: Black Hole Radius: Facts & Theory

1. What is the radius of a black hole?

The radius of a black hole refers to the distance from the center of the black hole to its event horizon, which is the point of no return for anything that enters the black hole. This radius is known as the Schwarzschild radius and is calculated based on the mass of the black hole.

2. Can the radius of a black hole change?

Yes, the radius of a black hole can change over time. As matter falls into a black hole, it increases the mass of the black hole and therefore increases its radius. This process is known as accretion and can also occur when black holes merge with other black holes.

3. How is the radius of a black hole measured?

The radius of a black hole cannot be directly measured since light cannot escape from its event horizon. Instead, scientists use indirect methods such as observing the effects of a black hole's gravity on surrounding objects or analyzing the radiation emitted by matter falling into the black hole.

4. Is the radius of a black hole infinite?

No, the radius of a black hole is not infinite. While the event horizon of a black hole may seem like an infinite distance from the outside, it is actually a finite distance from the center of the black hole. The exact value of this radius depends on the mass of the black hole.

5. How does the radius of a black hole relate to its temperature?

The radius of a black hole does not directly affect its temperature. However, the temperature of a black hole can be calculated based on its radius and mass using the Hawking radiation formula. The smaller the radius of a black hole, the higher its temperature will be.

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