Black Hole Radius: Facts & Theory

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    Black hole Hole Radius
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SUMMARY

The discussion centers on the concept of black hole radius, specifically distinguishing between the event horizon and the singularity. It is established that classically, the radius of a black hole is considered zero, but this does not account for the complexities introduced by General Relativity (GR) and Quantum Mechanics (QM). The conversation highlights that within the event horizon, traditional physics breaks down, and the nature of mass and area becomes ambiguous. Ultimately, while GR suggests a singularity with zero radius, the interplay between GR and QM remains a significant challenge in modern physics.

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  • Understanding of General Relativity (GR) and its implications on gravity.
  • Familiarity with Quantum Mechanics (QM) and its principles regarding sub-atomic particles.
  • Knowledge of black hole physics, including event horizons and singularities.
  • Basic grasp of classical mechanics and its limitations at quantum scales.
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  • Research the implications of General Relativity on black hole singularities.
  • Explore Quantum Mechanics and its role in understanding mass at sub-atomic levels.
  • Investigate theories that attempt to unify GR and QM, such as string theory.
  • Study the concept of event horizons and their significance in black hole physics.
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Astrophysicists, theoretical physicists, and students of advanced physics seeking to deepen their understanding of black hole mechanics and the intersection of General Relativity and Quantum Mechanics.

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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