The initial 4-dimensional construction of Kerr spacetime uses spherical coordinates on R3 and a coordinate t on the real line R. For large values, t can be considered as time and the spherical coordinate r as distance to the center.
The figure shows a slice t=const. In it, the radius r is drawn as
er, so r=-
is at the center of the picture.
Each of the two z-axes runs from - to +.
The ring singularity (that is, its t=const slice) is the equator of the central r=0
sphere, so it does not obstruct passage between r<0 and r>0. Though it appears
smaller in this picture, the r<0 side is just as extensive as the r>0 side.
This is the most general version of the Kerr black hole. In it, the inner and outer horizons separate spacetime into three regions I,II,III whose relativistic characters are quite different. Both horizons lie in the r>0 side, where they conceal the ring singularity from distant observers.
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