Hawking showed there are several ways to understand how energy can escape a black hole. First, separation of matter-antimatter pairs occurs just beyond the event horizon. This would produce radiation not from within the black hole itself, but from virtual particles getting boosted to higher energy states by the black hole’s gravitation. Another way to look at the process is that vacuum fluctuations could cause a particle-antiparticle pair to appear close to the event horizon. One particle could fall into the black hole, while the other would not. To fill the “hole” in energy left by the lone particle, energy could tunnel its way out of the black hole and across the event horizon, producing the observed radiation. This would cause the black hole to lose mass, and an observer would see radiation being emitted from the black hole.
If black holes can lose mass, then it logically follows that at least some of them could eventually disappear. This process is called black-hole evaporation. When particles escape from a black hole, the black hole loses not only energy, but also mass, because the two are interchangeable equals, as governed by Einstein’s famous E=mc2 equation. For the simplest kind of a black hole, a non-charged, nonrotating Schwarzschild black hole, physicists can estimate the amount of Hawking radiation that should be produced. A one-solar-mass black hole would produce a tiny output of energy — only about 10–28 watts. That’s pretty close to being absolutely black!