Editing A Brief History of Time (section)

=== Chapter 7: Black Holes Ain't So Black ===
Hawking discusses an aspect of black holes' behavior that he discovered in the 1970s. According to earlier theories, black holes can only become larger because nothing which enters a black hole can come out. This was similar to [[entropy]], a measure of disorder which, per the [[second law of thermodynamics]], always increases. Hawking and his student [[Jacob Bekenstein]] suggested that the area of a black hole's event horizon is a measure of its entropy. 

But if a black hole has entropy, it must have a temperature, and must emit radiation. In 1974, Hawking published a new theory which argued that black holes can emit radiation. He imagined what might happen if a pair of [[virtual particles]] appeared near the edge of a black hole. Virtual particles briefly 'borrow' energy from [[Vacuum energy|the vacuum]], then [[Annihilation|annihilate]] each other, returning the borrowed energy and ceasing to exist. However, at the edge of a black hole, one virtual particle might be trapped by the black hole while the other escapes. Thus, the particle takes energy from the black hole instead of from the vacuum, and escape from the black hole as [[Hawking radiation]]. According to Hawking, black holes must very slowly shrink over time and eventually "evaporate" because of this radiation.