Figure 22: Apparent shape of a Kerr black
hole for an observer at radius in the equatorial plane. (For the Schwarzschild analogue, see
Figure 14.) The pictures show the celestial sphere of
an observer whose 4-velocity is perpendicular
to a hypersurface . (If the observer is moving one has to correct for aberration.) The
dashed circle is the celestial equator, , and the crossing axes indicate the direction towards
the center, .
Past-oriented light rays go to the
horizon if their initial direction is in the black disk and to
infinity otherwise. Thus, the black disk shows the part of the sky that is not
illuminated by light sources at a large radius. The boundary
of this disk corresponds to light rays that
asymptotically approach a spherical light ray in the region
of Figure 21. For an
observer in the equatorial plane at infinity, the apparent shape of
a Kerr black hole was correctly
calculated and depicted by Bardeen [16] (cf. [54],
p. 358). Earlier work by
Godfrey [142] contains a
mathematical error.
|