From the Wiki:
Quote:
The Drake equation states that:
N = R* x fp x ne x fl x fi x fc x L
where:
N = the number of civilizations in our galaxy with which communication might be possible;
and
R* = the average rate of star formation per year in our galaxy
fp = the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
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The factor L is the only one that represents a factor of time, and as I indicated, specifies how long a civilization can send signals... but not
when. A new variable, S, needs to be introduced that compares the period of time during which a civilization exists
simultaneously with our own, in order for such signals to have a chance of being intercepted.