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The Meaning of L

This month's Scientific American column by my favorite skeptic, Michael Shermer, is on Drake's Equation, and specifically on estimating the lifespan of civilizations. Drake's Equation for estimating the number of technological civilizations that reside in our galaxy -- "in science there is arguably no more suppositional formula," writes Shermer -- is written as:

N = R fp ne fl fi fc L
Check the links above for a full description of the equation. For our purposes, we're concerned only with L, the lifetime of a communicating civilization, and this is Shermer's focus as well:
Mars Society president Robert Zubrin says that "the biggest uncertainty revolves around the value of L; we have very little data to estimate this number, and the value we pick for it strongly influences the results of the calculation." Estimates of L reflect this uncertainty, ranging from 10 years to 10 million years, with a mean of about 50,000 years...

I find this inconsistency in the estimation of L perplexing because it is the one component in the Drake equation for which we have copious empirical data from the history of civilization on Earth. To compute my own value of L, I compiled the durations of 60 civilizations (years from inception to demise or the present)...

The 60 civilizations in my database endured a total of 25,234 years, so L = 420.6 years. For more modern and technological societies, L became shorter, with the 28 civilizations since the fall of Rome averaging only 304.5 years. Plugging these figures into the Drake equation goes a long way toward explaining why ET has yet to drop by or phone in. Where L = 420.6 years, N = 3.36 civilizations in our galaxy; where L = 304.5 years, N = 2.44 civilizations in our galaxy. No wonder the galactic airways have been so quiet!

I admire Shermer greatly, but I think his reasoning here is faulty. Looking at the context of the equation, by "lifetime of a communication civilization," I believe Drake was trying to ask this question: "Once an intelligent species acquires radio technology, how much time will pass before it permanently ceases radio transmissions for any reason?"

I don't think it's particularly instructive to look at civilizations on Earth for the answer to the question of L. Shermer himself notes 60 civilizations that have fallen, yet technological usage and progress has continued mostly unabated -- especially if one looks at such usage and progress on a planet-wide geographic scale, on a Long View time scale, or both.

It seems to me that we will only be able to reliably estimate L once we have empirical evidence that extraterrestrial intelligence has existed or does exist.

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