To how many technological civilisations is our galaxy home at this moment? It would be nice to know, so we could estimate our chances of ever coming into contact with somebody out there. In 1961, astronomer Francis Drake suggested a number of parameters relevant to this issue, and summarised them in an equation that bears his name to this day. One of the parameters is the mean life-span of a technological civilisation.
In issue 2008:2 of Skeptic Magazine that reached me today, Michael Shermer has an interesting paper where he states that of Drake’s parameters, the mean life-span is actually one of the few that can be given an estimate from empirical evidence. Shermer calculates the mean length of historical civilisations on Earth and arrives at a figure of 420.5 years. This is in my opinion all backward. Shermer has mixed up the uses of the word “civilisation”. Says he:
“… I compiled the lengths of 60 civilisations (the number of years from inception to demise), including: Sumeria, Mesopotamia, Babylonia, the eight dynasties of Egypt, the six civilisations of Greece, the Roman Republic and Empire, and others in the ancient world, plus various civilisations since the fall of Rome, including the nine dynasties (and two Republics) of China, four in Africa, three in India, two in Japan, six in Central and South America, and six modern states of Europe and America.”
What Shermer has collected is the lengths of political blocks in the chronologies of areas with continuous complex societies. As he starts his list, “Sumeria, Mesopotamia, Babylonia”, he is simply dealing with phases in an unbroken sequence of civilisation that continues to this day in Mesopotamia. Likewise with the dynasties of Egypt and China. They weren’t independent new starts from a repeatedly cleaned slate, they were simply phases in the lives of cultures that are still with us today. (Strangely, Shermer quotes Thomas R. McDonough of the Planetary Society on this very point in an endnote, but makes no mention of it in his text.)
Extraterrestrials working on the Drake equation won’t be interested in the political details of small parts of Earth’s surface over time. They want to know the likelihood of being able to catch a transmission from somewhere in our solar system. So in fact, Earth’s world history offers us only a single data point to judge what Drake’s mean life-span might be like. If by “civilisation” we mean an agricultural society with cities, then we’re at about 11,000 years and counting. If instead, and more reasonably, we mean a society with radio broadcast technology, then we’re less than 107 years into our window of interstellar visibility, counting from the first trans-Atlantic transmission.
Anyway, the search for extraterrestrial intelligence by means of radio astronomy appears quixotic to me. Let’s say somebody in a far-off solar system is transmitting in our direction. Would we even be able to separate such a little ghost of a whisper from the roar of that person’s sun?
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