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. 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. N = R * f(p) * n(e) * f(l) * f(i) * f(c) * L N = 20 * 0.5 * 1 * 0.2 * 1 * 0.5 * L
 Do advanced civilizations use their technology to solve their problems or do they destroy themselves? On earth we've survived the first 50 years.
 Multiplying all the numbers gives us N = L. In other words, the number of intelligent communicating civilizations in the galaxy equals the number of years such a civilization lasts! The figure about which we know the least bears a great significance in our calculations. Most scientists hope that if a civilization can overcome its initial tendency to destroy itself with its own technology, then that civilization is likely to last for a very long time. Let's hope those scientists are right. In any case, there should be at least 50 (the number of years WE'VE been around communicating) and if a communicative civilization lasts for millions of years, there may possibly be millions of civilizations we can look for.
 "SERENDIP: The Search for Life"