Life on other planets: The number N of civilizations per galaxy that might be able to communicate with us can be expressed in the formula
Here S is the number of stars per galaxy, about 2 × 1011. The remaining variables, together with their pessimistic and optimistic estimates,9 are given in the table on the following page.
a. Calculate the most pessimistic and most optimistic values for N.
b. The single variable with the greatest range of values is L, the fraction of a star’s life during
which a technological society, once established, can survive. The pessimistic estimate corresponds to 100 years, the optimistic estimate to 1,000,000 years. Using pessimistic estimates for other variables, what value ofL will give 1 communicating civilization per galaxy?
c. Is this formula helpful in determining whether there is life on other planets? Explain your reasoning.