Defense Intelligence Reference Document An Introduction To The Statistical Drake Equation

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these factors together, we find Ns fp ne fl fi fc ~ 1 x 109 , a billion planets on which
technical civilizations have arisen at least once. But that is very different from saying
that there are a billion planets on which technical civilizations now exist. For this we
must also estimate fL.
What percentage of the lifetime of a planet is marked by a technical civilization? The
Earth has harbored a technical civilization characterized by radio astronomy for only a
few decades out of a lifetime of a few billion years. So far, then, for our planet fL is less
than 1/108 , a millionth of a percent. And it is hardly out of the question that we might
destroy ourselves tomorrow. Suppose this were a typical case, and the destruction so
complete that no other technical civilization - of the human or any other species - were
able to emerge in the five or so billion years remaining before the Sun dies. Then Ns fp
ne fl fi fc fL ~ 10, and, at a given time there would be only a tiny smattering, a handful,
a pitiful few technical civilizations in the galaxy, the steady state number maintained as
emerging societies replace those recently self-immolated. The number N might be even
as small as 1 if civilizations tend to destroy themselves soon after reaching a
technological phase; there might be no one for us to talk with but ourselves. And that
we do but poorly. Civilizations would take billions of years of tortuous evolution, and
then snuff themselves out in an instant of unforgivable neglect.
But consider the alternative, the prospect that at least some civilizations learn to live
with technology; that the contradictions posed by the vagaries of past brain evolution
are consciously resolved and do not lead to self destruction; or that, even if major
disturbances occur, they are reveres in the subsequent billions of years of biological
evolution. Such societies might live to a prosperous old age, their lifetimes measured
perhaps on geological or stellar evolutionary time scales. If 1 percent of civilizations can
survive technological adolescence, take the proper fork at this critical historical branch
point and achieve maturity, then fL ~ 1/100, N ~ 107 , and the number of extant
civilizations in the galaxy is in the millions. Thus, for all our concern about the possible
unreliability of our estimates of the early factors in the Drake equation, which involve
astronomy, organic chemistry and evolutionary biology, the principal uncertainty comes
to economics and politics and what, on Earth, we call human nature. It seems fairly
clear that if self-destruction is not the overwhelmingly preponderant fate of galactic
civilizations, then the sky is softly humming with messages from the stars.
These estimates are stirring. They suggest that the receipt of a message from space is,
even before we decode it, a profoundly hopeful sign. It means that someone has
learned to live with high technology; that it is possible to survive technological
adolescence. This alone, quite apart from the contents of the message, provides a
powerful justification for the search for other civilizations.
4. The Drake Equation is Over-Simplified
In the nearly fifty years (1961-2009) elapsed since Frank Drake proposed his equation,
a number of scientists and writers tried to find out which numerical values of its seven
independent variables are more realistic in agreement with our present-day knowledge.
Thus there is a considerable amount of literature about the Drake equation nowadays,
and, as one can easily imagine, the results obtained by the various authors largely
differ from one another. In other words, the value of N, that various authors obtained
by different assumptions about the astronomy, the biology and the sociology implied by
the Drake equation, may range from a few tens (in the pessimist's view) to some
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