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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 an Astrophysicist

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