The story goes that, one day back on the 1940's, a group of atomic scientists, including the famous Enrico Fermi, were sitting around talking, when the subject turned to extraterrestrial life. Fermi is supposed to have then asked, "So? Where is everybody?"

What he meant was: If there are all these billions of planets in the universe that are capable of supporting life, and millions of intelligent species out there, then how come none has visited earth?

This has come to be known as The Fermi Paradox

The famous Drake Equation is said to create the Fermi Paradox because of the large number of galactic technological civilizations that the equation predicts when calculated using the most optimistic figures for each variable. However, Frank Drake's own current solution to the Drake Equation estimates 10,000 communicative civilizations in the Milky Way, not the billions that some Fermi Paradox proponents insist.

What assumptions lie within the Fermi Paradox argument, and how accurate are they?

1. The Paradox assumes the extreme that if there is any other intelligent life in the universe, then there must be large numbers, even billions, of similar or superior intelligent races in the galaxy or that a single race must have spread throughout the galaxy already. Who says? That's an awfully high figure. To insist on these high numbers almost seems to be a "straw man" argument. I mean, take the unlikely extreme and then argue against it, because the extreme is easy to criticize. In reality, any number given by anyone regarding intelligent life in the universe is an estimate or a speculation, nothing more. The real number could be anything between zero and infinity. An insistence on large numbers of intelligent galactic civilizations is not supportable. Many astronomers now believe that most planetary systems may consist of gas giants like Jupiter that may not be capable of supporting life. That does not, however, mean that the other extreme must be true, that there are no other systems capable of supporting life.
2. The Fermi Paradox also makes the assumption that if there is other intelligent life in the universe, it must be technologically advanced beyond ourselves and that it must have been advanced for quite some time. Again, who says? There are so many variables involved in the existence of another planetary system with intelligent life that we can't make any such assumption as to say that they must be more advanced than we are. This assumption is based solely on the theoretical age of the universe and there is little solid reason to believe that it is an inescapable assumption. We may be the most advanced race in this part of the galaxy. The first SETI signal we receive may be the extraterrestrial equivalent of I Love Lucy.
3. The Fermi Paradox also makes the assumption that these alien intelligences would have made their presence known to us by radio communications or by interstellar travel. Sez who? There may be other forms of communication than the narrow band of the electromagnetic spectrum that we call "radio". And even if not, we have been sending out signals for only a little over half a century. This means that, even at the speed of light, they've only reached as far as the stars within a circle of a little over fifty light-years. Besides, we have only been listening for such signals for about forty years, and only sporadically, at that. There was a big gap between Frank Drake's first attempt in 1960 and the next try, in the 1970's, and other gaps later. NASA's attempt lasted only a year. There's a lot of sky to be covered, and any purposeful signal must have been sent at a certain time for it to be reaching us now. If it was sent 2000 years ago from a planet that is 100 light years away, then we missed it by 1900 years. We didn't have the technology to receive it. They may have given up after 100 years or so and may have since perished or gone on to other forms of communication that are beyond our technology. If on the other hand, they just began broadcasting 100 years ago, but are on a world 200 light years away, we won't receive it for another 100 years. Finally, they may be like Fermi, and may have given up without even trying, saying "If there's anyone out there, why haven't we heard from them?" Another thing that we cannot do is to try to predict alien behavior.