Space Topics: Search for Extraterrestrial Intelligence
The Search for Extraterrestrial Intelligence: A Short History
Part 7: The Birth of the Drake Equation
The Green Bank meeting was also remarkable because it featured the first
use of the famous formula that came to be known as the "Drake Equation".
When Drake came up with this formula, he had no notion that it would become
a staple of SETI theorists for decades to come. In fact, he thought of it
as an organizational tool - a way to order the different issues to be discussed
at the Green Bank conference, and bring them to bear on the central question
of intelligent life in the universe.
The grand question of the number of communicating civilizations in our galaxy
could, in Drake's view, be reduced to seven smaller issues:
The rate of star formation in our galaxy at the time our Solar System was
formed (R*);
The fraction of stars that have planets around them (fp);
The number of planets per star that are capable of sustaining life (ne);
The fraction of planets in ne where life evolves (fl);
The fraction of fl where intelligent life evolves (fi);
The fraction of fi that communicate (fc);
The lifetime of a communicating civilization (L);
Denoting the number of communicating civilization in our galaxy by N, and
multiplying the different elements, we get the famous Drake Equation:
N= R* fp ne fl fi fc L
Star Birth
Image taken by the Hubble Space Telescope of star formation in nebula NGC 604 in galaxy M33. The formation of stars was, and still is, one of the only components of the Drake Equation for which empirical evidence is available.
Credit: Space Telescope Science Institute, Hui Yang (U. IL), and NASA
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The equation served its purpose well at the Green Bank conference. It provided
a framework that enabled the different researchers, who had very different
backgrounds and specialties, draw upon their specialized knowledge, and at
the same time contribute to the general question of the meeting.
Soon, however, to Drake's surprise, it became much more than that. The short
mathematical formula proved irresistible to SETI promoters: it reduced a
huge and almost unmanageable speculative question to a neat series of seemingly
specific questions. While the larger question seemed too large and speculative,
its seven components appeared to lend themselves to scientific inquiry. No
less important, posed as a formula, the question seemed mathematical and
quantitative. What better way of gaining scientific respectability than formulating
a mathematical equation?
The Green Bank meeting weighed in on each of the elements in the equation,
and came up with generally optimistic estimates. The rate of star formation
(R*) was the only element in the equation about which some reliable information
existed, and the conference settled on a conservative estimate of about one
star per year. Otto Struve was the resident expert on extrasolar planets
(fp), and he suggested that planets orbiting distant stars were, in fact,
very common. Su-Shu Huang gave an optimistic assessment about the likelihood
of planets having life supporting environments (ne), and Calvin and Sagan
suggested that on suitable planets life would ultimately emerge (fl). Lilly
gave an optimistic assessment on the likelihood of intelligence emerging
on a life-bearing planet (fi), based on his work on dolphins. If at least
two intelligent species emerged on Earth, doesn't that suggest that intelligence
is common? Lilly's views, however, were highly controversial, and often dismissed
by mainstream biologists. Even the sympathetic audience at Green Bank was
quick to note that dolphins were not a technological species, and would be
unlikely to send radio beams into space.
The final two elements in the equation were in the field of social science:
how likely are intelligent beings to communicate with other civilizations
(fc), and how long do civilizations last (L)? Significantly, there were
no social scientists at Green Bank. But while lamenting their absence, Morrison
also pointed out that even specialists were unlikely to have the answers
for such grand questions. In their absence, he suggested that based on
Earthly experience civilizations were likely to develop advanced technology,
and that curiosity and the urge to communicate appear to be universal. Furthermore,
he suggested, if civilizations are able to overcome the dangers of nuclear
self-destruction, they can probably sustain themselves for very long periods
of time.
In summarizing their discussions, the conference members concluded that
the number of communicating planets could range from fewer than 1000 to more
than a billion. Most of them thought the higher number a more likely estimate.
On this basis they called for a vigorous radio search for extraterrestrial
intelligence, using a 300-foot dish, very large computers, and patience to
search for at least 30 years.
--Amir Alexander
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