In 1950, the physicist Enrico Fermi posed the following question: if intelligent life is abundant in the Galaxy, then why do we have no evidence of these other civilizations?

Stated in another way (Michael Hart 1975): we expect that intelligent beings would finally master interstellar space travel, and would eventually explore and colonize the Galaxy. This process might take 10 million years BUT this is much less time than the age of the Galaxy or the age of the Earth.

This appears to conflict with the simple observation that no intelligent beings from outer space have yet visited Earth.

Possible explanations:

• We are alone (uncertainties in the Drake equation).
• Physical problems make space travel infeasible.
• There is little interest in communication.
• There has not been enough time yet in the history of the Galaxy for the widespread development of intelligent life.