The Hawking Radiation theory states that virtual particle-antiparticle pairs are sometimes created outside the event horizon of a black hole. Three things can happen to a pair of particles just outside the event horizon:

• Both particles are pulled into the black hole.
• Both particles escape from the black hole.
• One particle escapes while the other is pulled into the black hole.

For the third possibility, the particle that has escaped becomes real and can therefore be observed from Earth. The particle that was pulled into the black hole remains virtual and must restore its conservation of energy by giving itself a negative mass-energy. The black hole absorbs this negative mass-energy and as a result, loses mass and appears to shrink. The rate of power emission is proportional to the inverse square of the black hole's mass (High-Energy Astrophysics Learning Center).