Hawking's insight was based on a phenomenon of quantum physics known as virtual particles, and their behaviour near the event horizon. Even in empty space, subatomic "virtual" particles and antiparticles come briefly into existence, then mutually annihilate and vanish again. Close to a black hole, this manifests as pairs of photons. One of these photons might be pulled beyond the event horizon, leaving the other to escape into the wider universe. Careful analysis showed that if this happened, quantum effects would cause a "partner wave" carrying negative energy to be created and also pass into the black hole, reducing the black hole's total mass, or energy. In effect to an observer it would appear as if the gravitational force had somehow allowed the black hole's energy to be reduced and the energy of the wider universe to be increased. Hence black holes must gradually lose energy and evaporate over time. Considering the thermal properties of black holes, and conservation laws affecting this process, Hawking calculated that the visible outcome would be a very low level of exact black-body radiation - electromagnetic radiation produced as if emitted by a black body with a temperature inversely proportional to the mass of the black hole.