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A black hole is a
region of spacetime from which nothing can escape, even light.
To see why this happens,
imagine throwing a tennis ball into the air. The harder you throw the
tennis ball, the faster it is travelling when it leaves your hand and
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 is about 7 miles a second.
As a body is crushed
into a smaller and smaller volume, the gravitational attraction increases,
and hence the escape velocity gets bigger. Things have to be thrown harder
and harder to escape. Eventually a point is reached when even light, which
travels at 186 thousand miles a second, is not travelling fast enough
to escape. At this point, nothing can get out as nothing can travel faster
than light. This is a black hole.
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