Photon Orbit Around a Black Hole: Conservation Law Effects

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Discussion Overview

The discussion revolves around the nature of photon orbits around black holes, specifically addressing the implications of conservation laws and the types of orbits that photons can take. It touches on theoretical aspects of general relativity and the behavior of light in strong gravitational fields.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that photons can only take circular orbits around black holes due to the conservation of angular momentum and the constancy of the speed of light.
  • Another participant asserts that there are no elliptical orbits for any objects around a black hole, emphasizing the need for general relativity to describe such scenarios.
  • A question is posed regarding whether a photon can "sling shot" around a black hole and experience blueshift, raising considerations about the dynamics of light in gravitational fields.
  • There is a clarification that while parabolic and hyperbolic trajectories are possible, elliptical orbits are not applicable in the context of black holes.
  • One participant challenges the notion that only circular orbits exist, indicating that this is a simplification that does not account for other possible trajectories.

Areas of Agreement / Disagreement

Participants express disagreement regarding the types of orbits available to photons around black holes, with some asserting the exclusivity of circular orbits and others pointing out the existence of other trajectories like parabolas and hyperbolas. The discussion remains unresolved on the specifics of photon orbits.

Contextual Notes

The discussion highlights the limitations of applying Newtonian concepts to black hole dynamics and the necessity of general relativity for accurate descriptions. There are unresolved aspects regarding the implications of conservation laws on photon trajectories.

Ontophobe
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Hi. Now that I understand the conservation law of angular momentum, and given the fact that c is constant, I infer that if a photon were to go into orbit around a black hole, it could only take a circular orbit. No elliptical orbits for photons, because if it were to follow an elliptical orbit, it would have to speed up and slow down in order to cover the same amount of space per unit of time. Is this correct?

Related question: Can a photon "sling shot" itself around a black hole and blueshift itself in the process? (since accelerating itself isn't an option for the photon)
 
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There are no elliptical orbits for anything around a black hole. Black holes need to be described using general relativity. Already in the simplest case of the Schwarzschild metric, orbits are not ellipses.

Light trajectories around a black hole are not necessarily circular.
 
So, parabolas, hyperbolas, and circles are kosher, but no ellipses. Correct?
 
Ontophobe said:
So, parabolas, hyperbolas, and circles are kosher, but no ellipses. Correct?
Incorrect. This is also something which applies to Newtonian gravity - except circles, you can always find circular orbits simply because it is a very particular special case.
 

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