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Insights Orbital Precession in the Schwarzschild and Kerr Metrics - Comments

  1. Aug 11, 2016 #1
  2. jcsd
  3. Sep 3, 2016 #2

    john baez

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    Nice post! There's a typo:

    I guess you need something like two pound signs to do math here, not a dollar sign. Also, there are a number of bad line breaks - Wordpress blogs are unforgiving when you hit the carriage return.

    What I'd really love to see is a study of orbits in a Kerr metric that describes a 'super-extremal' black hole, one with ##J^2 > M^2##. These could get pretty weird!
  4. Sep 4, 2016 #3
    Thanks for the Post!

    Could someone explain where the conclusion about the photon orbits come from, i.e. that they exist for vanishing denominators of ##L## and ##\Gamma##?
  5. Sep 4, 2016 #4
    Good idea! I went away and commented out the horizon calculations in my own minimal simulator (I started a thread for it but struggling to find it ATM!), and tried a fairly complex bound 3+1D orbit with a = 1.1 and a = 1.5. Both produced "valid" orbits. It seems on the face of it that the equations of motion do not care about the existence of horizons ;)

    For info, the equations I use are from this paper for the Kerr-deSitter spacetime, and previously the seminal Wilkins paper. Both are full 3+1D solutions based on Hamilton-Jacobi analysis.
  6. Sep 4, 2016 #5

    john baez

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    Thanks! You may know about this article; it's not about the geodesics just the geometry of the Kerr solution, with a reasonable amount of detail on the 'ring singularity' and the closed timelike curves it gives rise to - those were my main concerns this weekend:
    I just noticed this:
    You may know all this stuff, but it seems nice.
  7. Sep 5, 2016 #6
    The first one is new to me (reading now, thanks), but I have seen the latter, very nice derivations for 2+1D and 3+1D. I wonder why the authors stopped short of providing equations for ##\frac {d t} {d \tau}## and ##\frac {d \phi} {d \tau}## though.

    This is an issue ATM because I think I have found a disagreement between Wilkins and Kraniotis et. al. in the ##\Theta## potential . . . my eyes say so and so does Maxima.

    [EDIT] pardon this but I've just showed in Maxima that they are equal after all. Since the Kerr-deSitter ##\Theta## potential is simpler, I shall use it for Kerr (##\Lambda = 0##) simulations in preference to the form in Wilkins.
    Last edited: Sep 5, 2016
  8. Sep 18, 2016 #7
    Very nicely, clearly, and succinctly presented - thank you for this :)
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