## 8 Multiple Lifetimes

In actual fact, civilizations will not all have the same life span. The one in Drake equation is the average lifetime. And if variations in lifespan are be taken into account, there will be variations in the type of numbers that the equation throws up. A number less than one, for example, will indicate the probability of a civilization appearing. If the number is low, there will be no civilization present for long periods of time.

The Drake Equation gives the number of civilizations in existence simply by inserting the average lifetime of a Galactic civilization as the final factor. But it does not indicate the variation in current lifetimes, the number of civilizations that have occurred in the past and gone extinct, or the effect of multiple lifetimes and the effect on distances. These variables are implicit in the initial assumptions but rarely addressed explicitly.

Having observed the different fortunes of civilizations on Earth, we can assume that the lifetimes of civilizations are unlikely to be equal to each other. To use one lifetime in the calculation is unrealistic. The effect of different distributions of lifetimes is examined in Multiple Lifetimes.