The Fermi Paradox

First, some background:

1. Our galaxy, the Milky Way galaxy contains between 100 and 400 billion stars and there are about 100 billion galaxies.
2. The universe is believed to be 13.8 billion years old.
3. Our sun and solar system are about 4.6 billion years old.
4. It is believed that most stars or many stars are orbited by groups of planets, just as our sun is. A few hundred of the larger ones have already been detected.

Now, the Fermi Paradox:

At Los Alamos National Laboratory in 1950, physicists Enrico Fermi, Emil Konopinski, Edward Teller, and Herbert York were walking to lunch when the conversation turned to a recent spate of reports of UFO sightings. Fermi used some straightforward math to show that if technological civilizations were common and moderately long-lived, then even containing hundreds of billions of stars, our galaxy ought to be fully inhabited. Since there are probably lots of inhabitable planets orbiting stars that are cosiderably older than our sun, there would likely be many extraterrestrial civilizations far more advanced than we are. Fermi theorized than even limited to travelling at sub light speed one or more of these civilizations would have had ample time to colonize the galaxy by now and we would have seen evidence of their presence. We haven't. That's the paradox.

A few possible explanations:

1. They exist, but not in great numbers
Something so special happened in the formation of the Earth and evolution of life on earth, that it just doesn't happen very often. Only one out of every...say 10000 stars has a civilization capable of being heard by us on the radio and none is close enough. The Earth only gets visited once in a blue moon and the last time was so long ago, that either man wasn't here yet, man was here but hadn't invented writing, or the written record that was made has such a primitive description of the contact that it seems like a crazy fantasy to those who come across it now.

2. We are mistakenly assuming that our primitive technology is capable of hearing them.
We are trying to listen for their signals with radio telescopes, but radio is just a phase which lasts a few centuries and is then usually replaced by a better technology. If you brought someone from the 1960s into the lobby of Microsoft headquarters today with a walkie talkie, he could listen all he wanted, but he wouldn't hear the e-mail, IMs, or land line phones in use in the building. He might incorrectly conclude that no one was there.

3. The great filter
Although evolution is common, there is some step in the evolution of intelligent life or the growth of civilizations and their technology that is extraordinary difficult to get through and almost no one ever does. If this is true, is the great filter behind us or ahead?

4. The cosmic zoo
Local supercivilizations know we're here, but they're protecting our natural development by refraining from communicating with us until we develop more.

5. One powerful species kills the competition
One of the first civilizations to arise and develop an advanced technology now kills any potential competitors who arise. Not wanting to waste their efforts, they wait to see if you will destroy yourself, but once you succeed in attaining a certain level, they come and destroy you.

6. The boondocks
Much of the galaxy is settled, but not this particular area.

7. We're too stupid or too far behind
Would the ants on an ant hill understand the meaning of a superhighway nearby? When Columbus visited America for the first time, did he walk up to an ant hill, greet the ants, and tell them why he had come?

Many other explanations have been suggested.