Friday, November 17, 2006

SETI Message




ASTROBIOLOGY: ON THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE



Over 30 years ago, Carl Sagan (1934-1996) wrote the following:

"Civilizations hundreds or thousands or millions of years beyond
us should have sciences and technologies so far beyond our
present capabilities as to be indistinguishable from magic. It
is not that what they can do violates the laws of physics; it is
that we will not understand how they are able to use the laws of
physics to do what they do. It is possible that we are so
backward and so uninteresting to such civilizations as not to be
worthy of contact, or at least of much contact. There may be a
few specialists in primitive planetary societies who receive
master's or doctor's degrees in studying Earth or listening to
our raspy radio and television traffic. There may be amateurs --
Boy Scouts, radio hams, and the equivalent -- who may be
interested in developments on Earth. But a civilization a
million years in our future is unlikely, I believe, to be very
interested in us. There are all those other civilizations a
million years in our future for them to talk to."

[*Note #1] ...

... T.L. Wilson (Max Planck Institute for Radio Astronomy, Bonn,
DE) presents a review of some current considerations concerning
the search for extraterrestrial intelligence, the author making
the following points:

1) The author points out that N.S. Kardashev, in 1964,
classified possible extraterrestrial civilizations according to
the energy at their disposal, the scheme permitting a
determination of whether, in a context of communication, we
would be dealing with a civilization like our own (type I), a
rather advanced civilization (type II), or a vastly more
advanced civilization (type III). The transmission power of a
type I civilization is equal to the power expendable by all the
technological activity on Earth. For a specific direction, this
can be achieved by coupling the output of a 1 megawatt
transmitter operating at 10 centimeters to a 100-meter-diameter
telescope. The transmission power of a type II civilization is
the entire output of the Sun, which is equal to 10^(14) times a
type I transmission. The transmission power of a type III
civilization is equal to the power from our entire Galaxy, or
10^(11) times a type II signal.

2) The author points out that humanity has sufficient resources
at present to broadcast messages comparable to a type I
civilization in a specific direction, although in practice the
types of transmission are based on isotropic radiators. A type
II transmission might be transmitted by an extraterrestrial
civilization that had captured all of the power from its central
star. Such extraterrestrial civilizations are often referred to
as "Dyson civilizations". Type III civilizations have captured
the power of an entire galaxy.

3) The author points out that F.D. Drake, in 1965, proposed what
is now called the "Drake equation" as an attempt to quantify
estimates of the number of extraterrestrial civilizations. The
equation takes the form N = RanbcdL, where (N) is the number of
extraterrestrial civilizations in a galaxy communicating at any
given time, (R) is the average rate of galactic star formation,

(a) is the fraction of stars accompanied by planets, (n) is the
number of planets per star system with conditions needed to
support life,

(b) is the fraction of habitable planets on which life actually
arises,

(c) is the fraction of the life-bearing planets that develop
intelligent life, (

d) is the fraction of intelligent species that develop
communication technologies, and (L) is the "life-span" of the
communicating technological culture.

4) The author points out that stars are concentrated in
galaxies, and there are more than 20 galaxies within 3 million
light years of our own Galaxy. In principle, we should be able
to receive a message from type II or type III extraterrestrial
civilizations in any of these galaxies with technology currently
available. With an average of 10^(10) Sun-like stars per galaxy,
we could detect messages from extraterrestrial civilizations
even if the product of the last 5 terms in the Drake equation
were less than 1 part in 10^(8). The author suggests these
considerations provide a rationale for all-sky untargeted
searches: With the possibility of at least modest numbers of
perhaps readily detectable extraterrestrial civilizations
(especially of type II or type III), the extra sensitivity
conferred by targeted searches would not be an absolute
requirement for success. However, the fact remains that no
confirmed transmissions in the centimeter-wavelength range have
been received, from which it has been claimed that type II and
type III extraterrestrial civilizations do not exist at the
present epoch. The author suggests this claim is overstated: it
may be valid for a sizeable part of our Galaxy, but only if the
extraterrestrial civilizations are broadcasting in the
centimeter-wavelength range without interruption -- and if they
wish their signals to be detectable.

5) The author points out that there is an advantage in
transmitting signals at short wavelengths, and this explains the
interest in optical searches for extraterrestrial intelligence.
The author suggests the following example illustrates the
advantages of optical searches in regard to effective radiated
power: An extraterrestrial civilization orbiting a Sun-like star
could use a laser to illuminate a 1-meter optical telescope
through narrow-band optical filters. The extraterrestrial
civilization could then produce a short pulse lasting 1
microsecond or less, and this would produce a flash 300,000
times as bright as their Sun. Even without optical filtering,
the flash would still be 30 times as bright as their Sun, and
this factor would rise to 3000 if the diameter of the telescope
were increased to 10 meters (the diameter of our current Keck
telescopes). Because of the short pulse length, such optical
signals would not be found in conventional optical astronomical
surveys.

6) The author points out that if extraterrestrial civilizations
exist, they are not making their presence obvious. This in
itself suggests that type III and perhaps type II civilizations
are at best extremely rare. There are, however, many possible
reasons why we have not made contact with extraterrestrial
civilizations:

a) They may simply be very few.

b) There may be a number of extraterrestrial civilizations, but
these may be sending messages in optical or near-infrared ranges
that we have to explore comprehensively.

c) There may be extraterrestrial civilizations, but these may
not be interested in communicating and choose to keep themselves
hidden. This is more speculative, since it depends on the
cultural aspects of extraterrestrial civilizations. From
searches so far, the lack of contact demonstrates that
transmissions, if any, involve weak or intermittent signals (or
both).

7) The author suggests there seems to be no hope for
faster-than-light travel, so actual visits from extraterrestrial
civilization are unlikely. Even with the most efficient
propulsion systems, the energy needed to reach stars at 10 light
years in 20 years would be the equivalent of the present world
consumption for 1000 years. Such expenditure of energy would
hardly deter a type III extraterrestrial civilization, but even
then, broadcasts make more energetic sense than personal
appearances. There have been suggestions that extraterrestrial
civilizations might populate space with self-replicating
machines in space probes. This would allow colonization of large
regions of space in relatively short intervals of time, but it
seems vastly more complex than communicating by means of
electromagnetic radiation.


-----------
T.L. Wilson: The search for extraterrestrial intelligence.
(Nature 22 Feb 01 409:1110)
QY: T.L. Wilson: twilson@as.arizona.edu
-----------
Text Notes:
... ... *Note #1: Carl Sagan: _The Cosmic Connection: An
Extraterrestrial Perspective_, Doubleday, New York 1973, Dell,
New York 1975, p.222.
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