## The Life Span of a Civilization

The final term is perhaps the most interesting and speculative of them all: How long do civilizations capable of communication last? To estimate how many civilizations there are in the Galaxy at any given time, we must know how long they last. Once lit, how long does the candle of civilization (on average) burn? Consider our own civilization, one with a recorded history of no more than 5,500 years and that has been highly technological for less than a century.

Planets and stars have long life spans, but we have only a little over 5,000 years of experience with civilization. And what is most disquieting is that we have reached a point of technological development that allows us, at the touch of a few thermonuclear buttons, to destroy our civilization here and now.

What is the average life span of a civilization? A thousand, five thousand, ten thousand, a million years?

Even if, over time, there have been many, many civilizations in our Galaxy, how many are there now?

Traveling to the location of another civilization in the Galaxy is a highly impractical proposition. In the best-case scenario, civilizations in the Galaxy are hundreds of light-years apart, and getting even to the nearest star would take us (using current technology) tens of thousands of years. The civilization might be long gone by the time we got thereāto say nothing of the mother of all jet lags incurred by the voyagers.

Close Encounter

### Close Encounter

If we approach any variable in the Drake Equation pessimistically, assigning it a low value, we end up with the possibility that there are very few technological civilizations in the Galaxy. But if we plug into the equation the most optimistic estimates for all the variables (such as life will always develop, intelligence will always evolve, technology will always arise), we end up essentially equating the number of technological civilizations in the Galaxy with the average lifetime of such a civilization.

If the average technological civilization survives 10,000 years, we can expect to find 10,000 civilizations in the Galaxy at any time. If 5,000 years is the norm, 5,000 civilizations may be expected. If a civilization usually lasts millions of years, then, assuming high values throughout the rest of the equation, we might expect to find a million civilizations in the Milky Way.

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