Intelligent Life in the Universe and Intelligence in the Universe

Uncertainty in Estimates of the Number of Extraterrestrial Civilizations

“…Wall, 1979). Since a typical estimate of N is 10 6 (Shklovskii and Sagan, 1966), this would imply that the accuracy of our estimate is 0.1%, whereas--as we have already seen--the uncertainty is very much greater. This fact underlines the need for a formalism which will lead to a more realistic estimate of the uncertainty.…”

“…Although one may present an assessment of the uncertainty by means of the width of the distribution function (as measured, for instance, by the standard deviation), the distribution-function representation is a more flexible way of characterizing uncertainty. For instance, we may note that Sagan (Shklovskii and Sagan, 1966), in his discussion of L, considers two possibilities: an "optimistic" one that L -10 9 , to which he ascribes a probability of order .01; and the alternative (probability 0.99) "pessimistic" possibility Shat L -102. He forms from these lifetimes the mean lifetime < L ) -10 and uses this in the Drake formula, as is quite appropriate if one is simply trying to determine the expectation value of N. If, however, one is interested in estimating the probability that N falls in some range of values (for instance, N in a range of very small values, such as would follow from the short lifetime L 10 2 ), it is preferable that one retain the two alternatives explicitly.…”

“…The preceding contributions to this chapter have been concerned with various estimates of N, which denotes the number of extant advanced tec' :;cal civilizations in our galaxy possessing both the interest and capability for interstellar communication (Shklovskii and Sagan, 1966). This number is estimated on the basis of a formula first presented by F.D.…”

Broadcasting but not receiving: density dependence considerations for SETI signals

“…In many representations of the Drake equation, f s is expanded to several terms to represent the various probabilities in the emergence of intelligent, communicating life so for example, in Shklovskii and Sagan [3], f s is expressed as…”

“…A common assumption is if Drake's Equation, N, is greater than one, or perhaps in the range of millions for our galaxy [3], then we should expect to eventually receive signals from another CC (or vice versa). Assuming that signals can be detected irrespective of their distance from their origin, this is a reasonable estimate.…”