2. ASTROBIOLOGY: ON THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
Nearly 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.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 11May01
For more information: http://scienceweek.com/swfr.htm
-------------------
Related Background:
PROSPECTS FOR THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
The conjured image is poignant: intelligent life sprinkled
throughout our Galaxy, each sprinkle separated from the others by
1000 light years, each sprinkle searching for the others with
radio transmitters and receivers, small robotic spacecraft sent
beeping into empty space between the stars, the beeping like a
faint bleating in the dark as the sprinkles search for each
other. Of course, the conjured image may be wrong: there may be
intelligent life dense in the Galaxy; or we may be alone. It does
not matter. For the human species on this planet Earth, the quest
is part of our destiny, part of what we do as a species, and it
will go on as long as we remain civilized.
... ... J.C. Tarter and C.F. Chyba (SETI Institute, US) present a
review of current and future efforts in the search for
extraterrestrial intelligence, the authors making the following
points:
1) During the past 40 years, researchers have conducted
searches for radio signals from an extraterrestrial technology,
sent spacecraft to all but one of the planets in our Solar
System, and expanded our knowledge of the conditions in which
living systems can survive. The public perception is that we have
looked extensively for signs of life elsewhere. But in reality,
we have hardly begun to search. Assuming our current,
comparatively robust space program continues, by 2050 we may
finally know whether there is, or ever was, life elsewhere in our
Solar System. At a minimum, we will have thoroughly explored the
most likely candidates, a task not yet accomplished. We will have
discovered whether life exists on Jupiter's moon Europa, or on
Mars. And we will have undertaken the systematic exobiological
exploration of planetary systems around other stars, seeking
traces of life in the spectra of planetary atmospheres. These
surveys will be complemented by expanded searches for intelligent
signals.
2) The authors point out that although the current language
is that of a "search for extraterrestrial intelligence" (SETI),
what is being sought is evidence of extraterrestrial
technologies. Until now, researchers have concentrated on only
one specific technology -- radio transmissions at wavelengths
with weak natural backgrounds and little absorption. No verified
evidence of a distant technology has been found, but the null
result may have more to do with limitations in range and
sensitivity than with actual lack of civilization. The most
distant star probed directly is still less than 1 percent of the
distance across our Galaxy.
3) The authors conclude: "If by 2050 we have found no
evidence of an extraterrestrial technology, it may be because
technical intelligence almost never evolves, or because technical
civilizations rapidly bring about their own destruction, or
because we have not yet conducted an adequate search using the
right strategy. If humankind is still here in 2050 and still
capable of doing SETI searches, it will mean that our technology
has not yet been our own undoing -- a hopeful sign for life
generally. By then we may begin considering the active
transmission of a signal for someone else to find, at which point
we will have to tackle the difficult questions of who will speak
for Earth and what they will say."
-----------
J.C. Tarter and C.F. Chyba: Is there life elsewhere in the
Universe?
(Scientific American December 1999)
QY: Jill C. Tarter, SETI Institute, Mountain View, Calif. US.
-------------------
Summary by SCIENCE-WEEK http://scienceweek.com 11Feb00
For more information: http://scienceweek.com/swfr.htm
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