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Search for ExtraTerrestrial Intelligence
From the SETI notebook of James Brown
Drakes Equation
One of the first things asked when I say that I am evolved in the
search for extraterrestrial life is "what makes you think there
is any one else in the universe ?". That's a very good question
and one that deserves an answer. It is also one that has not
been successfully answered over the whole of human experience.
Carl Sagan participated in a conference of people interested in
the question of Communication with Extraterrestrial Intelligence
(CETI) that was held in the USSR back in 1971. In his
introduction address he said something that gets the whole thing
in perspective. He said "..the cetaceans (whales) are
undoubtedly another intelligent species inhabiting our planet,
and it has been argued that if we cannot communicate with them we
should not be able to communicate with extraterrestrial
civilizations". Without much reflection it would seem that it
should be much easier to communicate with whales than ET's.
After all we share the same planet, both breath oxygen, eat
plants that depend on the sun for there energy source. But since
that meeting, 15 years ago, we are no closer to talking with
whales than we ever were.
The conference had as its agenda the task of estimating the
possibility of communication with extraterrestrial intelligence
and since they were all capable scientific people they felt it
necessary to express there doubts and optimism in the form of
mathematics. Dr. Frank Drake was at the same conference and had
sometime earlier created a mathematical expression of the
possibility that could be used as a framework for this and future
discussions.
Drakes equation became a way for experts to tackle the problem
piecemeal, concentration in there own specialty without trying to
understand the question as a whole. For example the evolution of
technical civilizations was taken by an anthropologist, the rate
of star formation in the universe by a cosmologist, etc.
The equation is:
N = R Fp Ne F1 Fi Fc L
Where:
N = Number of civilizations to look for
R = Rate of star formation
Fp= Fraction of stars with planetary systems
Ne= Ecologically suitable planets.
F1= Fraction of planets on which life occurs.
Fi= Fraction of planets with life AND intelligent life
Fc= Fraction with communication capabilities like ours
L = Life time of such a civilization
It was generally agreed on that the terms most easily estimated
were the rate of star formation and the number of stars with
planets. The most difficult term was "L" the life time of an
intelligent civilization because we only have experience with one
such civilization and it appears intent on terminating itself at
the earliest possible moment.
AS you would expect none of these things can be estimated with
anymore than a good guess but here is what the results seem to be
based on everything we now know about the problem. An optimist,
like Dr Sagan would say that there is at least 300,000
civilizations that are capable of sending signals and that we
could detect in our galaxy alone. A pessimist, and there were
several even in that conference, would say that there can be as
few as 100 possible civilizations down to and including zero.
So what is the true number of civilizations? We shall never know
because even when we find the first, we shall never know when we
have found the last.
So what is the true number of civilizations?
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