Black Hole Radius: Facts & Theory

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    Black hole Hole Radius
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Discussion Overview

The discussion revolves around the nature of a black hole's radius, specifically questioning whether it has a physical radius or if it is effectively zero. Participants explore concepts related to the event horizon, singularity, and the implications of general relativity (GR) and quantum mechanics (QM) on this topic.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants assert that classically, the radius of a black hole would be zero, while others question this by suggesting that it is merely matter squeezed tightly together.
  • There is a discussion about the limitations of classical GR solutions near the singularity, indicating uncertainty about the conditions in that region.
  • One participant argues that any point of mass must have some area, proposing that spatial bodies are generally spherical and thus possess a radius or cross-section.
  • Another viewpoint suggests that if one accepts GR, the center of a black hole would settle to exactly zero radius, implying that any deviation would contradict classical GR.
  • Several participants highlight the transition from classical mechanics to quantum mechanics at small scales, suggesting that conventional intuitions about mass and area may not apply in the context of singularities.
  • There is a reiteration that assumptions about mass and area may not hold true at the quantum level, emphasizing the complexity of reconciling GR and QM.
  • A quote referencing the limitations of human language in describing sub-atomic phenomena is introduced, underscoring the challenges in understanding black holes and singularities.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of a black hole's radius, with no consensus reached on whether it has a physical radius or if it is zero. The discussion remains unresolved, with differing interpretations of classical and quantum perspectives.

Contextual Notes

The discussion highlights the unresolved nature of the mathematical and conceptual frameworks surrounding black holes, particularly the transition between classical and quantum theories. Participants note the dependence on definitions and the limitations of current understanding in the context of singularities.

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

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