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Einstein equation point mass solution

  1. Dec 11, 2009 #1
    I'm just getting a taste of computational GR, and I have a question regarding the metric for the single point mass solution for the einstein equation.

    The metric in spherical coordinates for a point mass at [tex]r=0[/tex] is

    [tex]\eta=\left(\begin{matrix}-\left(1-\frac{2GM}{c^2r}\right) & 0 & 0 & 0 \\ 0 & \frac{1}{1-\frac{2GM}{c^2r}} & 0 & 0 \\ 0 & 0 & r^2 & 0 \\ 0 & 0 & 0 & r^2\sin^2\theta \end{matrix}\right)[/tex]

    As expected, it is symmetrical wrt angle, and I recognize [tex]\eta_{00}[/tex] (time entry) as the negativa square of the gravitational time dilation constant.

    The spatial [tex]\eta_{11}[/tex] entry for radius goes to infinity as r approaches the swartzschild radius. At this radius, photons can orbit the point mass.

    Is the a way to compute the curvature of space and of space-time a radius r away from the sphere without computing the entire Riemann tensor?
  2. jcsd
  3. Dec 11, 2009 #2


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    Science Advisor

    Not really, though. At r=2M, photons directed radially outward stay at constant radial coordinate, but I wouldn't call this an orbit. The photon sphere, where photons truly do move in circular orbits, is at r=3M.
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