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# The Green Bank Formula Help (page 4)

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By — McGraw-Hill Professional
Updated on Sep 17, 2011

## The Complete Formula

The Drake formula in its entirety consists of the product of all the preceding factors and generates a number N . This is the number of technologically advanced civilizations that we should expect to find in the Milky Way galaxy:

N = R f p n e f l f i f c L

Let’s calculate N . Here are the values we have suggested for the variables in the Drake formula:

R = 10

f p = 0.2

n e = 0.5

f l = 0.1

f i f c = 0.01

L = 10,000

This yields a final estimate of N = 10. That is, based on the guesses made here as to the values of the variables, we can imagine that there are 9 technologically advanced civilizations in the Milky Way besides our own.

If these worlds are more or less evenly spaced in the spiral arms of the galaxy, it is unlikely that we will ever communicate with them by radio because the latency will be too great (unless, by coincidence, one of the other civilizations is within a few light-years of us). However, we have the capacity to hear signals from one of these societies even if we can never send a reply and expect it to be heard within an individual human lifetime, and even if what we hear represents the distant past (an extreme form of time-shifting communication). Thus SETI continues. The “odds” are slim. The potential rewards are enormous, even if it is nothing more or less than the joy of knowing that we are not alone in the Cosmos.

Practice problems of this concept can be found at: The Search for Extraterrestrial Life Practice Problems

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