This was mentioned in the enhanced version of "The Constant" as being
one of the equations on Daniel's chalkboard.
http://en.wikipedia.org/wiki/Kerr_metric
"In general relativity, the Kerr metric (or Kerr vacuum) describes the
geometry of spacetime around a rotating massive body. According to
this metric, such rotating bodies should exhibit frame dragging, an
unusual prediction of general relativity; measurement of this frame
dragging effect is a major goal of the Gravity Probe B experiment.
Roughly speaking, this effect predicts that objects coming close to a
rotating mass will be entrained to participate in its rotation, not
because of any applied force or torque that can be felt, but rather
because the curvature of spacetime associated with rotating bodies. At
close enough distances, all objects -- even light itself -- must rotate
with the body; the region where this holds is called the ergosphere.
The Kerr metric is often used to describe rotating black holes, which
exhibit even more exotic phenomena. Such black holes have two event
horizons where the metric appears to have a singularity. The outer
horizon encloses the ergosphere and has an oblate spheroid shape, a
flattened sphere similar to a discus. The inner horizon is spherical
and marks the 'radius of no return'; objects passing through this
radius can never again communicate with the world outside that radius.
Objects between these two horizons must co-rotate with the rotating
body, as noted above; this feature can be used to extract energy from
a rotating black hole, up to its invariant mass energy, Mc2. Even
stranger phenomena can be observed within the innermost region of this
spacetime, such as some forms of time travel. For example, the Kerr
metric permits closed, time-like loops in which a band of travellers
returns to the same place after moving for a finite time by their own
clock; however, they return to the same place and time, as seen by an
outside observer...
Kerr black holes as wormholes
Although the Kerr solution appears to be singular at the roots of =C4 =3D
0, these are actually coordinate singularities, and, with an
appropriate choice of new coordinates, the Kerr solution can be
smoothly extended through the values of r corresponding to these
roots. The larger of these roots determines the location of the event
horizon, and the smaller determines the location of a Cauchy horizon.
A (future-directed, time-like) curve can start in the exterior and
pass through the event horizon. Once having passed thrugh the event
horizon, the r coordinate now behaves like a time coordinate, so it
must decrease until the curve p***** through the Cauchy horizon.
The region beyond the Cauchy horizon has several surprising features.
The r coordinate again behaves like a spatial coordinate and can vary
freely. The interior region has a reflection symmetry, so that a
(future-directed time-like) curve may continue along a symmetric path,
which continues through a second Cauchy horizon, through a second
event horizon, and out into a new exterior region which is isometric
to the original exterior region of the Kerr solution. The curve could
then escape to infinity in the new region or enter the future event
horizon of the new exterior region and repeat the process. This second
exterior is sometimes thought of as another universe. On the other
hand, in the Kerr solution, the singularity at r =3D 0 is a ring, and
the curve may pass through the center of this ring. The region beyond
permits closed, time-like curves. Since the trajectory of observers
and particles in general relativity are described by time-like curves,
it is possible for observers in this region to return to their past.
While it is expected that the exterior region of the Kerr solution is
stable, and that all rotating black holes will eventually approach a
Kerr metric, the interior region of the solution appears to be
unstable, much like a pencil balanced on its point (Penrose 1968)."
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