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photon sphere sepehr-e foton Fr.: sphère de photons A surface where if a photon is emitted from one of its points the photon follows
a closed orbit and returns periodically to its departure point.
Such a surface exists only near sufficiently → G is the → gravitational constant, M is the
mass, c is the → speed of light, and R
is the → _{S}Schwarzschild radius.
For a rotating, → Kerr black hole, the situation is much more
complex due to the → Lense-Thirring effect.
In that case circular paths exist for radii whose values depend on
the rotation direction. More specifically, in the equatorial plane there are two
possible circular light paths: a smaller one in the direction of the rotation,
and a larger one in the opposite direction. |