Radius of a photon's circular orbit around a mass

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Discussion Overview

The discussion centers on the radius of a photon's circular orbit around a mass M, exploring concepts related to both Schwarzschild and Kerr metrics in general relativity. Participants examine theoretical implications and specific conditions under which these orbits exist.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant proposes that the radius of a photon's circular orbit might be the Schwarzschild radius.
  • Another participant references a Wikipedia article on the photon sphere, suggesting it as a source of information.
  • A question is raised regarding the Kerr metric and whether it allows for closed non-equatorial photon orbits, with an implication that such orbits would not be circular.
  • Additional resources in the form of PDFs are shared to aid in understanding the Kerr metric and its implications for photon orbits.
  • One participant expresses interest in the topic, indicating engagement with the discussion.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of photon orbits in the Kerr metric, and multiple competing views remain regarding the existence of non-equatorial orbits.

Contextual Notes

The discussion does not resolve the mathematical details regarding the conditions for photon orbits, nor does it clarify the implications of the Kerr metric fully.

birulami
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What is the radius of a photon's circular orbit around a mass M? My guess would be the Scharzschild radius. Is that true?
 
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For Kerr, the Wikipedia article only mentions the two (co-rotating and counter-rotating) circular equatorial photon orbits. Does Kerr have closed non-equatorial photon orbits? (If so, they would presumably be noncircular.)
 
Wow, that's very interesting! Thanks
 

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