I have read this:
In General Relativity the black hole solutions which have so far been found form a four parameter family called the generalized Kerr-Newman family of black holes. The four parameters are mass M, angular momentum J, charge Q, and the cosmological constant Λ .
A black hole is a region of spacetime exhibiting gravitational acceleration so strong, that nothing, not even light can escape from it.
There is no consensus whether the mass of the black hole lies in the singularity or the event horizon. For an outside observer, the mass of the black hole may lie on the event horizon.
The singultarity of a rotating Kerr black hole is not a point, but a ring in the plane of rotation, though still zero volume.
Some physicists even believe that singularities are not real.
Though, none of the answers on this site explain what is exactly rotating in the black hole. Is it the singularity, like a point particle (with the mass of a black hole), then it is like the spin of an elementary particle.
Or is it the event horizon, that is rotating, since to an outside observer, all the infalling mass is frozen on the horizon.
Some answers say that the gravitational field (potential) of the black hole (its stress energy) is determined by the (though the stress energy is determined by mass, angular momentum, charge, cosmological constant) event horizon (its size).
Some answers say that a singularity of a black hole is not made of any matter, since it is not a place in space, rather a place in time.
Like Luboš Motl's comment to this question:
So there is a contradiction, because none of the answers specifically state what is actually rotating in a rotating Kerr black hole.
- What is actually rotating in a rotating Kerr black hole, is it the singularity or the event horizon?