Article/Document:

Prof Says Beings From Outer Space Have Visited Earth (Carl Sagan)

Associated Press, November 26, 1962

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Summary: 

LOS ANGELES. (AP) - Some of the best scientific minds in the
country were stumped when a slender, dark-haired young man
chalked on the blackboard this equation:

N equals R FP NE FL FI FC L.

The speaker was Dr. Carl Sagan, a 28 year-old assistant
professor of astronomy at Harvard University.

His audience consisted of several hundred members of the
American Rocket Society, gathered for his luncheon address.

The equation was his way of expressing the mathematical
probability that intelligent beings from outer space have
visited earth.

Sagan soberly explained that in his equation N Stands for the
number of advanced technical civilizations in the universe
possessing the capability of interstellar communication.

R is the mean rate of star formation averaged over the lifetime
of the galaxy.

FP is the fraction of stars with planetary systems.

NE is the mean number of planets in each system with
environments favorable for the origin of life.

FL is the fraction of such planets on which life does develop.

FI is the fraction of such inhabited planets on which
intelligent life with manipulative abilities rises during the
lifetime of the local sun.

FC is the fraction of planets populated by intelligent beings on
which advanced technical civilizations rises.

And L is the lifetime of this technical civilization.

Sagan said information in his formula is based on current
estimates by astronomers. In making calculations, he assigned
each symbol an arbitrary numerical value.

As expressed in numbers, Sagan said, the formula means that at
least 1 million of the 100 billion stars in our Milky Way galaxy
have planets which have developed civilizations capable of
travel between the stars.

"Let's say that each of these civilizations sends out one
interstellar expedition per year," he said.

"That means that every star, such as our sun, would be visited
at least once every million years. In some systems where these
beings found life, they would make more frequent visits.
There's a strong probability, then, that they have visited
earth every few thousand years.

"It is not out of the question that artifacts of these visits
still exist or even that some kind of base is maintained,
possibly automatically, within the solar system, to provide
continuity for successive expeditions.

"Because of weathering and the possibility of detection and
interference by the inhabitants of earth it would be preferable
not to erect such a base on the earth's surface. The moon seems
one reasonable alternative.

"Forthcoming photographic reconnaissance of the moon from space
vehicles - particularly of the back - might bear these
possibilities in mind."

At a news conference Sagan predicted man himself would be
capable of interstellar flight at close to the speed of light
"within a century or two."

Asked if he believed in flying saucers, he said: "I do believe
there are objects which have hot be identified."

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