The Drake equation is the prevaling estimate for estimating the number of extra-terrestrial civilizations in our galaxy. It is discussed, somewhat critically, in an article by Michael Shermer in Scientific American (July 15, 2002). See http://www.sciam.com/article.cfm?articleID=000800AC-8550-1D2B-97CA809EC588EEDF&catID=2
There are many other discussions available. This one has an interesting discussion of one parameter of the equation, L, which estimates the age of "communicating civilization" on a given planet. Estimates for L are, he points out, mere quixotic guesswork, with "conservative" estimates set at 50,000 years.
The article's own estimate for L may be undermined by the writer's own numbers. Specifically, using 60 earth civilizations as a basis for estimating L, he estimates L at either 420 years (the average for all civilizations) or 304 years (the average civilization value since the fall of Rome). Based on this he uses the Drake equation to conclude that there should be between 3.36 and 2.44 civilizations in our galaxy. That estimate is obviously belied by the fact that he can identify 60 here.
It is more reasonable, however, to assert that he has missed the point and cast his definition of civilization too narrowly. Eath civilization, by any reasonable definition, started between 5000 and 10000 years ago, and the individual civilizations in Shermer's collection are simply manifestations of that overall emergent civilization, with most building on a base provided by prior civilizations and competitors. Using 5000 years as L we reach a very different conclusion than Shermer does, with about 50 such planets in our galaxy. Given the number of candidate signals Seti@Home is identifying (an impressive number) based on a very limited search of sky, frequency, and bandwidth, we might easily surpass that number.
If however, we take the more restrictive assumption that a "communicating civilization" has at least radio broadcast facilities, than the examplar of our own civiilization gives us barely 100 years to work with as an estimate. If L=100, than the Drake equation yields a bit less than one civilization per galaxy (.8). If so, we're it, and the only way we can realistically raise the estimate is to extend our civilization into the future. The good news is that every 125 years of longevity suggests that there is another civilization out there. Not a bad way to think of things.
Reversing the flow is more interesting however. The more civilizations we find, the better our own odds for longevity.
I won't go nuts here The Drake equation is interesting and worth exploring further.
| -- Last edited September 18, 2015 |
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