### 3.3 Horizon-penetrating solutions

We noted in Section 3.1.1 that the time-independent maximal slicing of Schwarzschild with isotropic coordinates covers only the exterior of the black hole. This is because the time independence of this gauge requires that the lapse vanish on the horizon. It is possible to evolve into the black hole’s interior when starting from initial data constructed in this gauge, but it requires a choice for the lapse that yields a time-dependent solution [94]. The time dependence of such a solution is purely gauge, of course, since the spacetime is static.

It is possible to cover all, or part, of the interior of a single black hole with a time-independent slicing. However, doing so seems to require that we give up the maximal-slicing condition. To cover the interior of the black hole, we need a slicing that passes smoothly through the event horizon. A convenient way to generate such solutions is to begin with the metric in standard ingoing-null coordinates. If we want to consider a rotating and charged black hole, then we use the Kerr–Newman geometry in Kerr coordinates:

where and are defined by Eq. (79), and is the ingoing-null coordinate. This metric is regular at , where and are the locations of the event horizon and the Cauchy horizon, respectively.

This metric can be put into a form suitable for producing time-independent Cauchy initial data by making coordinate transformations of the general form

where and are suitably chosen functions of the radial coordinate . There are a few particularly significant solutions for the general Kerr–Newman geometry, and I will outline these below, listing the nonzero components of the metric in the standard 3 + 1 format.

#### 3.3.1 Kerr–Schild coordinates

A spherical coordinate version of the standard Kerr–Schild coordinate system is obtained from (83) by using the coordinate choice (cf. Refs. [7442])

The nonzero components of the lapse, shift, and 3-metric are then given by:
Cartesian coordinate components can be obtained from these via the standard Kerr–Schild coordinate transformations [80]
This yields the implicit definition of from
with and on the disk described by and .

#### 3.3.2 Harmonic coordinates

Harmonic time slicing is integral to some hyperbolic formulations of general relativity, and a time-independent harmonic slicing of the Kerr–Newman geometry does exist [1742]. The harmonic time slicing condition is , which can be written

This equation is satisfied by using the coordinate choice
The nonzero components of the lapse, shift, and 3-metric are then given by:
Cartesian coordinate components can be obtained from these via the standard Kerr–Schild coordinate transformations (92) and (93). However, for the harmonic slicing, the hypersurface is spacelike only outside the Cauchy horizon at .

Fully harmonic coordinates () can be defined when Cartesian spatial coordinates are used by employing a variation of the standard Kerr–Schild coordinate transformations [42]

This yields the implicit definition of from
Fully harmonic coordinates are useful because applying a boost to a harmonically sliced black hole yields a solution that satisfies (94) only if the black hole is written in fully harmonic coordinates. In this case, the boosted solution also satisfies the fully harmonic coordinate conditions.

#### 3.3.3 Generalized Painlevé–Gullstrand coordinates

The Painlevé–Gullstrand gauge choice for the Schwarzschild geometry has been rediscovered many times because of its simple form (cf. Refs. [8659896568]). It is another time-independent solution, but the 3-geometry is completely flat (not simply conformally flat). The lapse is one in this gauge, and all of the information regarding the curvature of spacetime is contained in the shift. The Painlevé–Gullstrand gauge also has an intuitive physical interpretation [75]. An observer starting at rest at infinity and freely falling will trace out a world line that is everywhere orthogonal to the hypersurfaces in Painlevé–Gullstrand coordinates.

A generalization of the Painlevé–Gullstrand gauge derived by Doran [48] includes the Kerr spacetime, and the extension of this solution to the full Kerr–Newman spacetime is trivial. In the limit that and vanish, this solution reduces to the Painlevé–Gullstrand gauge. The coordinate transformation is written most easily as

The nonzero components of the lapse, shift, and 3-metric are then given by:
Notice that the lapse remains one, but the 3-geometry is no longer flat when the black hole is spinning. Cartesian coordinate components can be obtained from these via the standard Kerr–Schild coordinate transformations (92) and (93). Like the Kerr–Schild time slicing, a slice of the generalized Painlevé–Gullstrand gauge remains spacelike for all .