The Fermi Paradox

I Stumbled on this about an hour ago and it got me thinking. As you may know,

The Drake equation states that:

where:

N is the number of civilizations in our galaxy with which communication might be possible;

and

R^* is the average rate of star formation per year in our galaxy

f_p is the fraction of those stars that have planets

n_e is the average number of planets that can potentially support life per star that has planets

f_ℓ is the fraction of the above that actually go on to develop life at some point

f_i is the fraction of the above that actually go on to develop intelligent life

f_c is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L is the length of time such civilizations release detectable signals into space.^[2]

-Wikipedia

By almost all accounts, this equation makes sense. By almost all accounts, we have not had contact with aliens or detected their presence. We know, with varying degrees of accuracy, values for R*, f_p, and n_e. This leaves the remaining four variables as reasons for our apparent isolation.

1. I find the first variable unlikely as the culprit. The materials that form the basis of terrestrial life form in conditions that might be found on almost any earth-like planet in its infancy. This has been shown scientifically and I've read the article but I'm not in a thorough mood. Looking it up is left as an exercise for the reader. Additionally, these materials are hardly the only possible building blocks for what might be considered "life:" Anything that replicates itself and passes on this ability to replicate. One possibility in this area is that life on earth is extremely different from other life. In some interpretations, like this short story by Terry Bisson, life elsewhere doesn't even have a physical manifestation.
   
2. The fraction of the above that actually go on to develop intelligent life. This is more plausible. We assume, just as we assumed that the earth was at the center of the universe, that human-level intelligence or greater is a logical evolutionary step. It may not be. If it is, what has prevented other equally intelligent life from forming on Earth itself? Natural selection is built on whoever replicates and protects himself (and herself, for sexual reproduction) better. There is no evidence that at the hunter-gatherer stage we were really much better than anything else at surviving. Or maybe the move to agriculture is actually the stepping stone here. The point is that we may not actually be, as we like to believe, the pinnacle of evolution. Another assumption is that intelligence in life is always characterized as it is in humans. Who's to say that dolphins don't have the mental capacity to build spacecraft, but choose not to leave their current, happier lifestyle. Curiosity or imperialism might not go hand-in-hand with intelligence.
   
3. The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. Pretty straightforward. The idea that intelligent life always emits radio waves or similar is covered by the question of whether or not intelligence is characterized in all species as it is in humans.
4. The length of time such civilizations release detectable signals into space. This is my favorite. If this were the cause of our inability to communicate with other intelligent life, it would imply that there is a very short window of time in the progression of life in which sufficiently advanced technology is developed to hint at a civilization's technological level. On one hand, the end of this period could be caused by advancement into a state with no physical manifestation or some other state where they communicate in a completely different way from humans. On the other hand, the course of civilization might require research that inevitably destroys the civilization itself. One fairly childish and syfy-channel-movie-esque example might be that research required for long-range space travel or transmission inevitably culminates in the destruction of a planet or solar system.

Anyway, that's just my take on it. I strongly encourage you to look it up for yourself.

Some other reading that's at least sort of relevant:
the short story "The Monkeys Thought 'Twas all in fun," by Orson Scott Card
The Wikipedia page on The Fermi Paradox
EDIT: Here's an interesting article that's sort of related.