According to British physicist Stephen Hawking (1948–), energy can slowly leak out of a black hole. This leakage, called Hawking radiation, occurs because the event horizon (boundary) of a black hole is not a perfectly smooth surface, but “shimmers” at a subatomic level owing to quantum mechanical effects. At these quantum mechanical scales, space can be thought of as being filled with so-called virtual particles, which cannot be detected themselves but can be observed by their effects on other objects. Virtual particles come in two “halves,” and if a virtual particle is produced just inside the event horizon, there is a tiny chance that one “half” might fall deeper into the black hole, while the other “half” would tunnel through the shimmering event horizon and leak back into the universe.