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The Question
(Submitted June 30, 1997)
How is it possible for black holes to emit matter even if their gravity field
is so intense?
The Answer
Black holes emit matter because their gravity field is so intense.
Specifically, black holes emit particles by a process known as
Hawking radiation.
What is ordinarily considered empty space is full of 'virtual pairs' which
are particle-antiparticle pairs which pop into existence, separate a very
short distance, come back together, and disappear a very short time
later. This happens so quickly that the Universe doesn't notice that for a
short while there was extra mass-energy. The law of conservation of
energy only holds over sufficiently long periods of time, and can be
briefly violated.
In the neighborhood of a black hole, the virtual pair can pop into
existence, and when they separate, one can go so deeply into the black
hole that its falling releases enough energy that the other particle can
continue to exist, outside the hole, with the total energy of the virtual
pair being zero.
It takes a huge gravitational field to release such a large amount of
energy when the particle falls such a very short distance. Such huge
fields are found only around black holes.
David Palmer
for Ask a High-Energy Astronomer
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