I posted this in January 2005:
Drakes formula allows some kind of estimate as to the number of intelligent societies there might be "out there".
The following is from a great book by A.K. Dewdney: Yes, We Have no Neutrons.
The formula is N = R* x Fp x Ne x Fl x Fi x Fc x L
For which:
R* = number of new stars that form in our galaxy each year
Fp = fraction of stars having planetary systems
Ne = average number of life-supporting planets per star
Fl = fraction of those planets on which life develops
Fi = fraction of life forms that become intelligent
Fc = fraction of intelligent beings that develop radio
L = average lifetime of a communicating society
The formula has appeared in several popular science magazines with the values set to:
N = 10 x 1 x 1 x 1 x 0.01 x 0.1 x L
So, N = 0.01 x L
The only numbers in the formula which anything other than a guess can be made are R* and L. Based on current observations most set R* at 10. Everything else in the formula would be a wild guess, except for L. More is known about L than any other part of the formula, since we are a communication society. Since we receive more and more of our communication from satellites, cable, and the internet, we are broadcasting less and less away from the earth. In the near future we will likely go dark as a significant source of radio/broadcast signals capable of being detected from space. If we say that our source of signals is about 100 years, drop the 100 back into the formula and you get 1. That must be us.