Drake Equation and the Fermi Paradox

The Drake Equation is basically a means of doing rough calculations for the amount of intelligent life in the universe.

Basically it takes the form:

X stars in the Galaxy, of which
Y % have planets, of which
Z % can support life, on which
A % intelligent life has arisen, with
Then with the manipulation of the numbers you arrive a figure that gives you how close on average the nearest intelligent lifeforms are.

There are various mathematical expressions for this formula, and there are variations on how many terms the equations include.

The problem, of course, is that some of the variables are easy to pick (e.g., stars in the Galaxy), some are under study (e.g., how many stars have terrestrial-like planets), and others are just flat-out guesses (e.g., duration of civilisation, where we are currently running an experiment to test this here on Terra of Sol).

One of the problems that the Drake Equation produces is that if you take reasonable numbers for everything up to the average duration of technological civilisations, then you are left with three possibilities:

1 - If such civilisations last a long time, "They" should be _here_ (leading either the Flying Saucer hypothesis -- they are here and we are seeing them, or the Zoo Hypothesis -- they are here and are hiding.

2 - If such civilisations last a long time, and "They" are not "here" then it becomes necessary to explain why each and every technological civilisation has consistently chosen not to build starships (since the first civilisation to build starships would spread across the entire Galaxy on a timescale that is short relative to the age of the Galaxy), perhaps because they lose interest in spaceflight and building starships.

3 - Such civilisations do not last a long time, and blow themselves up or otherwise fall apart pretty quickly.

Thus the Drake Equation produces what is called the Fermi Paradox (i.e. "Where are They?"), in that the implications of #3 and #2 are not terribly encouraging to some folks, but the two flavours of #1 are kind of hard to understand.

Additional Information:
Drake Equation Abstract

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