So, there you
stand at home plate, gripping the bat tightly, waiting for the
pitch. Here it comes, the pitcher winds up and releases. You
keep your eye on the ball, just as you've been taught. When the
time is right, you swing, and you connect solidly. As you watch
the ball soar higher and higher, towards the fence, you think
it's going into orbit.
Instead, it falls into the center fielder's glove. You're out.
Want to know why?
Gravity. You see, that ball was traveling about 40 meters per
second. Escape velocity for the Earth's gravity is 11100 meters
per second. Not even close.
Now, imagine an object that is so dense, it's gravity so strong,
that escape velocity is more than 299,792,458 meters per second.
That's the speed of light. If Einstein was right and nothing can
travel faster than the speed of light, then nothing could reach
escape velocity here.
This is a black hole.
Today's scientists believe that a black hole is the end product
in the lifecycle of a giant star. If this star is three or four
times as massive as our own sun, even after it has exhausted all
its fuel, then it can collapse under its own gravity. Just like
a crab burying itself at the beach and pulling the sand down
over itself, the collapsing star pulls in everything around it
Because this dying star has such huge mass, it becomes too
strong for even neutrons to resist. It eventually collapses down
to one incredibly dense point, called a singularity. This
singularity is surrounded by an event region in which the
gravity is so strong that nothing, not even light, can escape.
Anything that crosses a black hole's event horizon is crushed
into an incredibly dense singularity. With the addition of each
bit of matter consumed by a black hole, its event horizon
continues to expand. The only limit to this expansion is the
amount of available matter. It is theoretically possible to
consume millions or billions of stars.
In fact, some scientists theorize that rotating black holes
(also know as Kerr black holes) which contain billions of dead
stars lie at the centers of galaxies.
Black holes are still just a theory, but a very good theory. Now
that astronomers have acquired evidence that theoretical white
dwarfs and neutron stars really exist, the case for black holes
has been strengthened. Since not even light can escape a black
hole, then it should be invisible. However, the effects of its
massive gravity can be detected.
The idea of black holes was first theorized in the late
eighteenth century by English geologist John Mitchell and French
astronomer Pierre Simon Laplace. At one time, scientists called
them "gravitationally collapsed objects." Russian scientists
suggested calling them "collapsars," but it wasn't until 1969
when Princeton physicist, John Wheeler coined the term black
hole. Black holes have continued to hold public interest and are
a popular fixture of science fiction books and movies.