CPS 100 Lecture Notes - Lecture 4: Green Bank, West Virginia, Star Count, Project Ozma
The Drake Equation
Within the limits of our existing technology, any practical search for distant
intelligent life must necessarily be a search for some manifestation of a distant
technology.
"What do we need to know about to discover life in space?"
How can we estimate the number of technological civilizations that might exist
among the stars? While working as a radio astronomer at the National Radio
Astronomy Observatory in Green Bank, West Virginia, Dr. Frank Drake conceived
an approach to bound the terms involved in estimating the number of technological
civilizations that may exist in our galaxy. The Drake Equation, as it has become
known, was first presented by Drake in 1961 and identifies specific factors thought
to play a role in the development of such civilizations. Although there is no unique
solution to this equation, it is a generally accepted tool used by the scientific
community to examine these factors.
-- Frank Drake, 1961
N = R* ∗ fp∗ ne∗ fl∗ fi∗ fc∗ L
Where:
N = The number of civilizations in the Milky Way Galaxy whose electromagnetic
emissions are detectable.
R* = The rate of formation of stars suitable for the development of intelligent life.
fp = The fraction of those stars with planetary systems.
ne = The number of planets, per solar system, with an environment suitable for life.
fl = The fraction of suitable planets on which life actually appears.
fi = The fraction of life bearing planets on which intelligent life emerges.
fc = The fraction of civilizations that develop a technology that releases detectable
signs of their existence into space.
L = The length of time such civilizations release detectable signals into space.
Within the limits of our existing technology, any practical search for distant
intelligent life must necessarily be a search for some manifestation of a distant
technology. In each of its last four decadal reviews, the National Research Council
has emphasized the relevance and importance of searching for evidence of the
electromagnetic signature of distant civilizations.
Besides illuminating the factors involved in such a search, the Drake Equation is a
simple, effective tool for stimulating intellectual curiosity about the universe around
us, for helping us to understand that life as we know it is the end product of a
natural, cosmic evolution, and for making us realize how much we are a part of that
universe. A key goal of the SETI Institute is to further high quality research that will
yield additional information related to any of the factors of this fascinating equation.
In November 2006, I was a participant in a panel discussion Defining the Drake
Equation at the Windycon Science Fiction Convention. My co-panelists were Seth
Shostak of the SETI (Search for Extraterrestrial Intelligence) Institute; Bill Higgins,
a physicist at Fermi National Accelerator Laboratory (Fermilab); and Bill
Thomasson. You can see a picture of our panel at MidAmerican Fan Photo Archive
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Windycon 33 Saturday Panels. I have decided to turn the preparation that I did for
that panel, and notes taken during the panel discussion, into a tutorial on the Drake
Equation.
Drake Equation History
The year is 1960 and Frank Drake of the National Radio Astronomy Observatory
(NRAO) in Green Bank, West Virginia undertakes the first attempt to find
extraterrestrial civilizations. Dubbed Project Ozma, for a period of 6 hours a day for
four months the NRAO radio telescope listens for radio signals of intelligent origin.
None are found.
Within a year a meeting is hosted in Green Bank to explore the issue of
extraterrestrial intelligence. Frank Drake needed to come up with an agenda for the
meeting in order to provide some structure to the discussion. To serve as an
agenda, he devises the Drake Equation. Sometimes known as the Sagan-Drake
Equation in the past, the meeting was attended by approximately a dozen
interested parties.
Drake Equation Overview
The Drake Equation is an attempt to encapsulate all the variables that would be
relevant to establishing the number of intelligent civilizations that existed in the
Milky Way galaxy and which were broadcasting radio signals at this particular point
in time. The Drake Equation is composed of seven terms. The first six are used to
compute the rate at which intelligent civilizations are being created and the final
term identifies how long each lasts on average as a broadcasting civilization. It is
worth stressing that the Drake Equation applies only to intelligent civilizations in the
Milky Way galaxy. It does not apply to civilizations in other galaxies because they
are too distant to be able to detect their radio signals.
The Drake Equation is:
N = R * fp * ne * fl * fi * fc * L
where:
N = The number of broadcasting civilizations.
R = Average rate of formation of suitable stars (stars/year) in the Milky Way galaxy
fp = Fraction of stars that form planets
ne = Average number of habitable planets per star
fl = Fraction of habitable planets (ne) where life emerges
fi = Fraction of habitable planets with life where intelligent evolves
fc = Fraction of planets with intelligent life capable of interstellar communication
L = Years a civilization remains detectable
According to the Wikipedia entry for the Drake Equation, the following values were
those used in the original formulation of the Drake Equation:
R = 10
fp = 0.5
ne = 2.0
fl = 1.0
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