As a result of quantum fluctuations, particle-antiparticle pairs can appear from the vacuum of space near the event horizon of a black hole in which the net energy of the particles is zero due to the matter and antimatter nature of the particles. In normal conditions, particle-antiparticle pairs that appear due to the quantum fluctuations annihilate each other. But if one member of the pair falls beyond the event horizon of the black hole, the other particle will not have a sister with which to annihilate. The member that does not fall into the black hole is then emitted from the black hole as hawking radiation. The particle that falls into the black hole effectively has negative energy, meaning that it will subtract from the overall mass-energy content of the black hole. Thus, after enough time has passed, the black hole will evaporate from these negative particles, while seeming to emit positive ones. Hawking radiation reduces the mass and the energy of the black hole and is therefore also known as black hole evaporation. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish.