What is a black hole?
A black hole is a region of space time from
which nothing can escape, not even light. Imagine throwing a tennis
ball into the air. The harder you throw the tennis ball the faster
it is traveling when it leaves your hand and consequently the higher
the ball will go before turning back. If you throw it hard enough
it will never return, the gravitational attraction will not be able
to pull it back down. The velocity the ball must have to escape
is known as the escape velocity, and for the earth it is about 7
miles a second. As a body is crushed into a smaller and smaller
volume the gravitational attraction increases and thus the escape
velocity gets bigger. Things have to be thrown harder and harder
to escape. Eventually a point is reached where even light, which
travels at 186,000 miles a second, is not traveling fast enough
to escape. At this point nothing can get out as nothing can travel
faster than light (Einstein's theory of relativity).....this is
a black hole.
How do they form?
Black holes are thought to form from stars
or other massive objects if and when they collapse from their own
gravity to form an object whose density is infinite. In other words
a singularity. During most of the stars lifetime nuclear fusion
at the core generates electromagnetic radiation, including photons,
the particles of light. This radiation exerts an outward pressure
that exactly balances the inward pull of gravity caused by the stars
mass. However as the nuclear fuel is exhausted the outward forces
of radiation diminish, allowing the gravitation to compress the
star inward. The contradiction of the core causes its temperature
to rise and allows remaining nuclear material to be used as fuel.
The star is saved from further collapse...but only for a while.
Eventually all possible nuclear fuel is used
up and the core collapses. How far it collapses, into what kind
of object, and at what rate is determined by the stars final mass
and the remaining outward pressure that the burnt up nuclear residue
(largely iron) can muster. If the star is sufficiently massive or
compressible, it may collapse to a black hole. If it is less massive
or made of stiffer material its fate is different, it may become
a white dwarf or a neutron star.
By definition a black hole is a region where
matter collapses to infinite density, and where, as a result, the
curvature of spacetime is extreme. Moreover the intense gravitational
field of the black hole presents any light or other electromagnetic
radiation from escaping. But where lies the point of no return?
at which any matter or energy is doomed to disappear from the visible
If you were to imagine the simplest three
dimensional geometry for a black hole, that is a sphere, the black
holes surface is known as the event horizon. Behind the horizon
the inward pull of gravity is overwhelming and no information from
the black holes interior can escape into the outer universe.