A short history of time 7

 

Chapter 7

Black holes ain’t so black

 

According to Hawking’s theory, a black hole ought to emit particles and radiation as if it were a hot body with a temperature which depends only on the black hole’s mass: the higher the mass, the lower the temperature.

 But how is it possible that a black hole appears to emit particles when we know that nothing can escape from within its event horizon (=the boundary of the black hole)?

 The answer that quantum theory tells us is that the particles do not come from within the black holes but from the “empty” space just outside the black hole’s event horizon, its boundary. So it’s not actually empty. There must be quantum fluctuations, which one can think them as pairs of particles of light or gravity that appear together at some time, move apart, and then come together again and annihilate each other.

These particles are virtual and cannot be observed directly by a particle detector but their indirect effects can be measured. 

So black holes must radiate like hot bodies if our other ideas about general relativity and quantum mechanics are correct.

 The existence of radiation from black holes seems to imply that gravitational collapse is not as final and irreversible as we once thought.

If a body falls into a black hole, energy equivalent its mass will return to the universe.

 When the mass of the black hole becomes very small, it will more likely disappear from our part of the universe and with it any singularity there might be inside it.

This was the first indication that quantum mechanics might remove the singularities that were predicted by general relativity.