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Black Holes are among the simplest objects in the universe. They are simpler than stars,
much simpler than planets, and vastly simpler than human beings.
Black holes are what is created when matter is compressed into a very small place. They
are General Relativity's most extreme prediction.
They are commonly created from the deaths of stars many times the size of our sun, usually
forming from the collapsed core of the super giant star, after it explodes.
At the heart of a black hole is a singularity; an infinitesimal point in space where the
pull of gravity is infinitely strong and the space-time infinitely curved. At the singularity
space and time no longer exist as we know them.
So what would happen if we traveled into one? To answer this question, let's look at a simple
black hole. One with mass, but doesn't spin or have any electric charge.
This animation shows our descent. At the lower left is a graph showing our trajectory.
The green region is a safe zone where our spacecraft orbit around the black hole is
stable and we can still get out.
The yellow region is a risky zone where our orbit is unstable. Here, a short burst of
our maneuvering thrusters will send us either into the black hole, or off into outer space.
The orange region is a danger zone, where there are no possible orbits for our space
craft, stable, or unstable. To remain in orbit in this zone we must constantly keep firing
our rockets.
The closer to the horizon we get, the harder we must fire our rockets to keep from falling
in.
The red line is the horizon. From here there is no escape.
At the bottom right of the animation is a clock, which records the time left until we
arrive at the central singularity. The clock records our proper time, the time we experience
- the time on our wrist watch. In this animation the clock slows down, not because time is
slowing down, but because it is more interesting to run the movie slower nearer the singularity,
so that we can see more clearly what happens there.
As we begin our approach, we can immediately see the distortion of light around the edges
of the black hole. This is the gravity well bending light - lensing the light from the
background stars into long circular arcs.
The event horizon is depicted as a red grid. This marks the boundary beyond which nothing
can escape - not even light.
Notice that we can see both the north and the south poles of the black hole simultaneously.
Since the black hole bends the light around it, we can see all around and into the back
of the black hole at the same time we see the front. This horizon is called the schwarzschild
radius.
About 3 schwarzschild radii marks the location of the innermost stable orbit - the green
area in our trajectory. Here circular orbits are stable, beyond they are unstable. Any
material accreting around this black hole finds its inner most edge here. Anything slightly
closer falls into the black hole. So long as we remain in this region (the green region)
- we can still get out.
At about 2 schwarzschild radii we reach the risky zone, an area of unstable orbits. Any
slip up here, such as a small firing of our thrusters would randomly send us either headlong
into the black hole or flying away from it.
Also in this region we arrive at a special location known as the photon sphere - the
distance where light rays can remain in orbit. This is the closest to the black hole that
anything can get and remain in orbit. For our rocket to stay here our thrusters would
have to expend an infinite amount of energy.
As we fall through the horizon at one schwarzschild radius something quite unexpected happens.
Instead of falling through the red grid that marks the horizon it stands off ahead of us.
The horizon splits into two distinct entities, the horizon and the anti-horizon. Light from
both of them hit us as we pass. The true horizon becomes visible only after we fall through.
The anti-horizon continues to remain ahead of us and we never fall through it.
It is a common misconception that if you fall inside the horizon of a black hole you will
be engulfed in blackness. What happens instead, the universe appears brighter and brighter
as you approach the horizon, tending to infinite brightness at the horizon. By this time in
our journey however, none of us would be able to sense any of this.
Once inside, space is falling faster than light, carrying us inexorably inward. Here
we are now at .8 schwarzschild radii. As we fall in a typical sized black hole such as
ours, the tidal forces are weak enough that we can fall deep inside the horizon before
we are torn apart. The gravity at our feet is stronger than the gravity at our heads
we feel this difference as a tidal force, which pulls us apart vertically. At the same
time we are crushed in the horizontal direction, like a rubber band being pulled.
Finally, as we approach the singularity, and we look up one more time we see that the intense
gravity of the black hole has concentrated the view of the outside universe into a thin
band around our waste. The views above and below are dim and red shifted, while the view
around our waste is bright and blue shifted.
We never actually get to see the center of the black hole because all light is rushing
towards it, and none away from it.
Sadly, we will never reach our final destination of the central singularity. Our journey ends
just short of our goal. Approximately one tenth of a second before we reach the singularity
we would be torn apart by the tidal forces. This happens regardless of the mass of the
black hole. In all black holes of any size our journey will end at roughly the same spot.
We will never reach the point of infinite curvature, where space and time as we know
them come to an end.