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Skeptic

In science there is arguably no more suppositional formula thanthat proposed in 1961 by radio astronomer Frank Drake for es-timating the number of technological civilizations that reside inour galaxy: N = R fp ne fl fi fc L

In this equation, N is the number of communicative civi-lizations, R is the rate of formation of suitable stars, fp is the frac-tion of those stars with planets, ne is the number of Earth-likeplanets per solar system, fl is the fraction of planets with life, fi

is the fraction of planets with in-telligent life, fc is the fraction ofplanets with communicating tech-nology, and L is the lifetime ofcommunicating civilizations.

Although we have a fairly goodidea of the rate of stellar formation,a dearth of data for the other com-ponents means that calculations are

often reduced to the creative speculations of quixotic astronomers.Most SETI (search for extraterrestrial intelligence) scientists

are realistic about the limitations of their field; still, I was puz-zled to encounter numerous caveats about L, such as this onefrom SETI Institute astronomer Seth Shostak: “The lack of pre-cision in determining these parameters pales in comparison withour ignorance of L.” Similarly, Mars Society president RobertZubrin says that “the biggest uncertainty revolves around thevalue of L; we have very little data to estimate this number, andthe value we pick for it strongly influences the results of the cal-culation.” Estimates of L reflect this uncertainty, ranging from10 years to 10 million years, with a mean of about 50,000 years.

Using a conservative Drake equation calculation, where L= 50,000 years (and R = 10, fp = 0.5, ne = 0.2, fl = 0.2, fi = 0.2,fc = 0.2), then N = 400 civilizations, or one per 4,300 light-years.Using Zubrin’s optimistic (and modified) Drake equation, whereL = 50,000 years, then N = five million galactic civilizations, orone per 185 light-years. (Zubrin’s calculation assumes that 10percent of all 400 billion stars are suitable G- and K-type starsthat are not part of multiples, with almost all having planets,that 10 percent of these contain an active biosphere and that 50percent of those are as old as Earth.) Estimates of N range wild-

ly between these figures, from Planetary Society scientist ThomasR. McDonough’s 4,000 to Carl Sagan’s one million.

I find this inconsistency in the estimation of L perplexing be-cause it is the one component in the Drake equation for whichwe have copious empirical data from the history of civilizationon Earth. To compute my own value of L, I compiled the dura-tions of 60 civilizations (years from inception to demise or thepresent), including Sumeria, Mesopotamia, Babylonia, the eightdynasties of Egypt, the six civilizations of Greece, the RomanRepublic and Empire, and others in the ancient world, plus var-ious civilizations since the fall of Rome, such as the nine dynas-ties (and two republics) of China, four in Africa, three in India,two in Japan, six in Central and South America, and six mod-ern states of Europe and America.

The 60 civilizations in my database endured a total of25,234 years, so L = 420.6 years. For more modern and tech-nological societies, L became shorter, with the 28 civilizationssince the fall of Rome averaging only 304.5 years. Plugging thesefigures into the Drake equation goes a long way toward ex-plaining 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 thegalactic airways have been so quiet!

I am an unalloyed enthusiast for the SETI program, but his-tory tells us that civilizations may rise and fall in cycles too briefto allow enough to flourish at any one time to traverse (or com-municate across) the vast and empty expanses between the stars.We evolved in small hunter-gatherer communities of 100 to 200individuals; it may be that our species, and perhaps extraterres-trial species as well (assuming evolution operates in a like man-ner elsewhere), is simply not well equipped to survive for longperiods in large populations.

Whatever the quantity of L, and whether N is less than 10 ormore than 10 million, we must ensure L does not fall to zero onour planet, the only source of civilization we have known.

Michael Shermer is publisher of Skeptic magazine(www.skeptic.com) and author of In Darwin’s Shadow: The Life and Science of Alfred Russel Wallace.

Why ET Hasn’t CalledThe lifetime of civilizations in the Drake equation for estimating extraterrestrial intelligences is greatly exaggerated By MICHAEL SHERMER

Species maysimply not be

equipped tosurvive for longperiods in large

populations.

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