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.

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T.L. Wilson: The search for extraterrestrial intelligence.
(Nature 22 Feb 01 409:1110)
QY: T.L. Wilson: twilson@as.arizona.edu
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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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