As a rotating black hole twist the spacetime around it like the ripples in a whirlpool,
the twisting and warping of spacetime itself
begins influencing the particles and objects within it.
Far from the black hole,
these forces gently swirl objects around the black hole.
The closer you approach the event horizon of a rotating black hole,
the more extreme this interaction becomes eventually pulling anything
falling past the event horizon into complete lockstep with the black hole.
Let's have a top-down look at a non-rotating Schwarzschild black hole.
In this diagram, there is a central non-rotating black hole
and each point here represents some source of light.
When a light source is far from the black hole,
the light propagates outwards in all directions.
As the sources approach the event horizon,
the light sphere begins to distort towards the black hole center.
When one of these light sources crosses the event horizon,
the light becomes trapped inside.
Now, let's impart some spin on this black hole changing it
from a Schwarzschild black hole into a rotating Kerr black hole.
Since the Kerr black hole just drag spacetime,
light sources far from the black hole begin to
see a shift in the direction their light spheres propagate.
But there's an even more interesting change when a black hole is rotating.
Not only does the event horizon shrink from
the non-rotating Schwarzschild radius down to
about half of its normal size for maximum rotation,
but particles falling directly inward begin spiraling around
the black hole even though there are no forces acting on them.
There's a special distance from a rotating black hole
that defines a region called the ergosphere.
The outer boundary of the ergosphere is called
the stationary limit and outside of the stationary limit,
a spacecraft can park with respect to the black hole.
But within the stationary limit,
no spacecraft can ever appear at rest to a distant observer.
Even spacecraft entering the ergosphere
orbiting the opposite direction to the rotation of
the black hole will eventually be pulled by
the spiraling spacetime into a co-rotating trajectory.
Although the word sphere is part of ergosphere,
the ergosphere is not actually spherical but rather an ellipsoid.
While the event horizon is still spherical,
the ergosphere envelops the event horizon
only touching at the spin axis of the event horizon.
It's good to remind ourselves that the ergosphere and
the event horizon are boundaries and not objects,
so they don't interact with each other in the same way that particles interact with them.
I'll emphasize now that a clever spaceship captain can still escape from the ergosphere,
and can in fact steal rotational energy from the black hole.
The word ergosphere comes from the Greek root ergon, which means work.
The ergosphere is so-named because it's theoretically possible to
extract the energy from the black hole's rotation with some clever tricks.
For example, from within the ergosphere,
you could throw a ship's garbage against the rotation of the black hole,
accelerating the ship forward and in the spiraled spacetime,
end up with more kinetic energy than you started out with.
In a case like this, you're stealing energy from a black hole's rotation.
Roger Penrose first described this process of
stealing energy from a rotating black hole in 1971,
which is why we call it the Penrose process.
Without going into detail, within the ergosphere,
it's possible for the energy of a particle to become negative,
a consequence of the change in coordinate system at the stationary limit.
Ultimately, what this means is
a super-advanced civilization could survive around a rotating black hole,
extracting a surplus of energy using
the Penrose process until a black hole's rotational energy has been sapped.
They could also do the reverse,
storing energy as the angular momentum of a black hole and extracting it at a later time.
It might seem far-fetched to you to be talking about
spiralling spacetime and frame dragging,
but it's possible to measure the gravitational effects
of a rotating body without a black hole at all.
In fact, a space probe aptly called Gravity Probe B was launched back in
2004 to investigate just how strong the frame dragging effects are here on earth.
Gravity Probe B carried
four incredibly precise gyroscopes in order to measure these effects.
At the time of their construction,
these gyroscopes were the most spherical objects ever made,
differing from perfectly round by
no more than 40 atoms on a sphere roughly the size of a ping pong ball.
Since the effects are quite a bit weaker
around a planet like Earth compared to black holes,
it took four years of operation before NASA
reported agreement with Einstein's theory of general relativity.
We've been hiding a few details of the Kerr black hole behind the veil as it were.
The event horizon of a Kerr black hole should really be called its outer horizon because
the mathematics tell us that there must be another inner horizon hidden inside.
The outer horizon is basically the same as the event horizon;
it's the boundary from which nothing can escape.
Even if you've fallen through the outer horizon,
it's still possible to receive information from beyond
the event horizon right up until you fall through
the inner horizon often called the Cauchy horizon.
The Cauchy horizon marks the boundary within a black hole where
information from the entire history of the universe is compressed.
An observer approaching the Cauchy horizon would see more and more of the history of
the universe essentially being battered by
the extreme energies that are compressed within that region.
Crossing the Cauchy horizon would be perilous enough simply
due to the incredible energy densities one would need to survive.
But there's yet another mathematical danger lurking within the Cauchy horizon,
the Kerr black hole's ring singularity.
Unlike the point-like singularities we've been discussing for Schwarzschild black holes,
the singularity of a rotating Kerr black hole is a ring instead of a point.
The Cauchy horizon may be the universe's last stand at preventing
observers from violating cosmic censorship and glimpsing the singularity.
