Where Is Everybody?

With so many stars in the galaxy, it might be assumed there must be advanced alien civilizations out there somewhere, but Enrico Fermi pointed out that if this were the case we should have been visited long ago, yet there is no evidence that this has happened. This puzzle is known as the Fermi paradox. Several explanations have been proposed. One possibility is that advanced civilizations do arise, but rapidly disappear due to their self-destructive tendencies. Another possibility is the zoo hypothesis—the aliens are here, but do not reveal themselves. There is also the rare Earth hypothesis which claims the Earth has unique features required for the evolution of intelligent life, so we really are the first advanced civilization in the galaxy.