Friday Science: Hawking 7 (Black Holes Evaporating)

Friday reviews of Stephen Hawking's A Brief History of Time so far.
Chapter 1: Heliocentric
Chapter 2: Spacetime
Chapter 3: Expansion of the Universe
Chapter 4: Uncertainty Principle
Chapter 5: Elementary Particles and the Forces of Nature
Chapter 6: Black Holes

Chapter 7: Black Holes Ain't So Black
Here we get quite a bit of Stephen Hawking's distinctive work. Some points of interest:

• Black holes are defined as the set of events from which it is not possible to escape, which basically begins the black hole at the event horizon.
• The paths of light at the event horizon must be parallel to each other but never meet. That also means a black hole can never decrease in area.
• This non-decreasing property is similar to entropy. Jacob Bekenstein in fact suggested that the area of the event horizon was a measure of the entropy of the black hole.
• Entropy has to do with the second law of thermodynamics. The entropy of an isolated system always increases. That is, disorder increases.
• If a black hole has entropy, it should have a temperature and it ought to emit radiation. But a black hole can't omit anything.
• So space isn't really empty. Particles and antiparticles emerge and annihilate. Near the edge of the event horizon, some get separated before they annihilated and go into the black hole. This gives the appearance of a black hole emitting a particle. 
• Meanwhile, a flow of negative energy into the black hole would reduce its mass. The universe is too young, but this process could eventually disintegrate a black hole into nothing.
• There may be some primordial black holes (very small). Some of them might be disintegrating about now. Some scientists are looking for final bursts of their disappearance.