E T I @
is an acronym for the Search for Extraterrestrial
Intelligence. It is the science of using
telescopes, radio, and optical, to search the skies
for signals from alien civilizations.
of SETI began in 1959 with the publication of
a paper in the British journal Nature by Giuseppe
Cocconi and Philip Morrison. The paper discussed
the possibility of the existence of alien civilizations
and how we might be able to detect them. Their
conclusion was that the easiest method of detection
would be radio waves.
were chosen because they are capable of traveling
the vast distances between stars and can be generated
with reasonable amounts of power. We have been
sending radio waves out into space for more than
sixty years. All of our radio, TV, satellite,
and radar signals are currently spreading out
throughout the galaxy. Perhaps they've already
been detected by
same time as Cocconi and Morrison's paper was
published a young astronomer named Frank Drake
was putting together plans for the first search.
The search, named Project Ozma, was conducted
in 1960. Over a two week period the stars Tau
Ceti and Epsilon Eridani were scanned for alien
signals. No signals were found but the search
30 years since the initial Ozma search many others
have been carried out with more sensitive equipment,
over much longer time frames, observing thousands
of other stars. So far no alien signals have been
detected but we've really only begun to scratch
the surface. There are an estimated 100 billion
stars in the Milky Way galaxy alone! To complicate
matters further there are millions of frequencies
that a signal could be received on. It may be
that we just haven't looked in the right place
at the right time yet.
Equation was developed by Frank Drake in 1961
as a way to focus on the factors which determine
how many intelligent, communicating civilizations
there are in our galaxy.
r a k e ' s E q u a t i o n
= N* fp ne fl fi fc fL
The equation can really be looked at
as a number of questions:
represents the number of stars in the Milky
How many stars are in the Milky Way Galaxy?
Current estimates are 100 billion.
is the fraction of stars that have planets
What percentage of stars have planetary systems?
Current estimates range from 20% to 50%.
is the number of planets per star that are
capable of sustaining life
For each star that does have a planetary system,
how many planets are capable of sustaining
Current estimates range from 1 to 5.
is the fraction of planets in ne where
On what percentage of the planets that are
capable of sustaining life does life actually
Current estimates range from 100% (where life
can evolve it will) down to close to 0%.
is the fraction of fl where intelligent
On the planets where life does evolve, what
percentage evolves intelligent life?
Estimates range from 100% (intelligence is
such a survival advantage that it will certainly
evolve) down to near 0%.
is the fraction of fi that communicate
What percentage of intelligent races have
the means and the desire to communicate?
10% to 20%
is fraction of the planet's life during which
the communicating civilizations live
For each civilization that does communicate,
for what fraction of the planet's life does
the civilization survive?
This is the toughest of the questions. If
we take Earth as
an example, the expected lifetime of our Sun
and the Earth is roughly 10 billion years.
So far we've been communicating with radio
waves for less than 100 years. How long will
our civilization survive? Will we destroy
ourselves in a few years like some predict
or will we overcome our problems and survive
for millennia? If we were destroyed tomorrow
the answer to this question would be 1/100,000,000th.
If we survive for 10,000 years the answer
will be 1/1,000,000th.
all of these variables are multiplied together
when come up with:
the number of communicating civilizations in
value of the Drake Equation is not in the answer
itself, but the questions that are prompted
when attempting to come up with an answer. Obviously
there is a tremendous amount of guess work involved
when filling in the variables. As we learn more
from astronomy, biology, and other sciences,
we'll be able to better estimate the answers
to the above questions.
r y T h e E
q u a t i o n
each variable choose what you think is the best
answer from the combo box. After you've chosen
all your answers press the calculate button and
see how many communicating civilizations you think
there are in the galaxy.
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