How many of these planets will evolve intelligent life?
Tough question, but if we really believe the evidence for natural selection
and survival of the fittest, most scientists would put this number at 100
percent -- that intelligent life is a natural outcome of evolution. Of course,
here we have only one example, earth. f(i) = 1.
How many of these intelligent species will develop technology
and use it to communicate? If we look at the earth, we see humans doing
it, but we also see whales and dolphins, who may also possess a moderate
level of intelligence but never developed technology. We'll set this number
to .5 as a first guess. f(c) = 0.5.
N = R * f(p) * n(e) * f(l) * f(i) * f(c) * L
N = 20 * 0.5 * 1 * 0.2 * 1 * 0.5 * L
Now we get to the hardest number to determine. "L" is the number
of years that a technologically adept and communicative civilization lasts.
We've only been in this phase of our evolution for about 50 years. Do advanced
civilizations blow themselves up after discovering the technology to do
so? Or do they get together and solve their problems before this happens?
For now, let's not assign a number to L. Let's plug in the other numbers
and see what we get.