Find the mass of a black hole from just its radius?

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Discussion Overview

The discussion centers around the question of how to determine the mass of a black hole when only its radius is known. Participants explore various equations and methods related to the Schwarzschild radius and its implications for calculating mass.

Discussion Character

  • Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant suggests using the original Schwarzschild formula to find the mass from the radius.
  • Another participant proposes that the same formula can be rearranged to find mass using the Schwarzschild radius.
  • A participant expresses frustration at not obtaining the correct mass for a solar mass black hole and seeks alternative suggestions.
  • Several participants request clarification on the calculations performed by others.
  • One participant asserts that the formulas provided by others are incorrect and suggests starting with the Schwarzschild radius formula to isolate mass.
  • A participant shares a link to a calculator that can compute the mass based on the radius.

Areas of Agreement / Disagreement

There is no consensus on the correct method to calculate the mass from the radius, with multiple competing views and suggestions presented throughout the discussion.

Contextual Notes

Some participants' calculations and methods are not yielding the expected results, indicating potential misunderstandings or misapplications of the formulas involved.

Lamdbaenergy
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I know the original schwarzschild formula for finding the radius of the black hole, but what equation can be used to find its mass if you ONLY know its radius?
 
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Can't you use the same formula? Only instead of using mass to find the Schwartzschild radius, use the Schwartzschild radius to find the mass.
 
Bandersnatch said:
Can't you use the same formula? Only instead of using mass to find the Schwartzschild radius, use the Schwartzschild radius to find the mass.

Nope, got nowhere near the right mass. I tried for a solar mass black hole, too. Any other suggestions?
 
Can you show what you did exactly?
 
Bandersnatch said:
Can you show what you did exactly?

2GR/c^2, then I tried R^2c^2/2G
 
Neither of those is correct. Start with the Schwartzschild radius formula ##r_s=\frac{2Gm}{c^2}## and isolate the mass on one side.
 

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