Kerr spacetime is the unique explicitly defined model of the gravitational field of a rotating star. The spacetime is fully revealed only when the star collapses, leaving a black hole--otherwise the bulk of the star blocks exploration. The qualitative character of Kerr spacetime depends on its mass and its rate of rotation, the most interesting case being when the rotation is slow. (If the rotation stops completely, Kerr spacetime reduces to Schwarzschild spacetime.)
The existence of black holes in our universe is generally accepted--by now it would be hard for astronomers to run the universe without them.Everyone knows that no light can escape from a black hole, but convincing evidence for their existence is provided their effect on their visible neighbors, as when an observable star behaves like one of a binary pair but no companion is visible.
Suppose that, travelling our spacecraft, we approach an isolated, slowly rotating black hole.It can then be observed as a black disk against the stars of the background sky. Explorers familiar with the Schwarzschild black holes will refuse to cross its boundary horizon. First of all, return trips through a horizon are never possible, and in the Schwarzschild case, there is a more immediate objection: after the passage, any material object will, in a fraction of a second, be devoured by a singularity in spacetime.
If we dare to penetrate the horizon of this Kerr black hole we will find ... another horizon. Behind this, the singularity in spacetime now appears, not as a central focus, but as a ring-- a circle of infinite gravitational forces. Fortunately, this ring singularity is not quite as dangerous as the Schwarzschild one--it is possible to avoid it and enter a new region of spacetime, by passing through either of two "throats" bounded by the ring (see The Big Picture).
In the new region, escape from the ring singularity is easy because the gravitational effect of the black hole is reversed--it now repels rather than attracts. As distance increases, this negative gravity weakens, just as on the positive side, until its effect becomes negligible.
A quick departure may be prudent, but will prevent discovery of something strange: the ring singularity is the outer equator of a spatial solid torus that is, quite simply, a time machine.. Travelling within it, one can reach arbitrarily far back into the past of any entity inside the double horizons. In principle you can arrange a bridge game, with all four players being you yourself, at different ages. But there is no way to meet Julius Caesar or your (predeparture) childhood self since these lie on the other side of two impassable horizons.
This rough description is reasonably accurate within its limits, but its apparent completeness is deceptive. Kerr spacetime is vaster--and more symmetrical. Outside the horizons, it turns out that the model described above lacks a distant past, and, on the negative gravity side, a distant future. Harder to imagine are the deficiencies of the spacetime region between the two horizons. This region definitely does not resemble the Newtonian 3-spacebetween two bounding spheres, furnished with a clock to tell time. In it, space and time are turbulently mixed. Pebbles dropped experimentally there can simply vanish in finite time--and new objects can magically appear.
The complete model of Kerr spacetime built in Chapter 3 adds two more horizons to each such interhorizon region (there will be many regions)--and shows that Kerr spacetime is organized symmetrically around the spatial 2-spheres at which these horizons intersect.