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ScienceWeek
ASTROBIOLOGY: ON COMMUNICATION WITH EXTRATERRESTRIALS
The following points are made by Woodruff T. Sullivan III (Nature 2004 431:27):
1) Although the Search for Extraterrestrial Intelligence (SETI) has yet to detect a signal, the efforts continue because so little of the possible parameter space has been searched so far. These projects have almost all followed the dominant paradigm --launched 45 years ago by Cocconi and Morrison(1) -- of using radio telescopes to look for signs of extraterrestrial life. This focus on electromagnetic waves (primarily at radio wavelengths, but also at optical ones) was based on various arguments for their efficiency as a means of interstellar communication. However, Rose and Wright(2) have made the case that if speedy delivery is not required, long messages are in fact more efficiently sent in the form of material objects -- effectively messages in a bottle. Although the suggestion itself is not new(3,4), it had never before been backed up by quantitative analysis.
2) A fundamental problem in searching for extraterrestrial intelligence is to guess the communications set-up of the extraterrestrials who might be trying to contact us. In which direction should we look for their transmitter? At which frequencies? How might the message be coded? How often is it broadcast? (For this discussion I am assuming that the signals are intentional, setting aside the a priori equally likely possibility that the first signal found could be merely leakage arising from their normal activities.) Conventional wisdom holds that they would set up a beam of electromagnetic waves, just as we could do with, for example, the 305-meter Arecibo radio telescope in Puerto Rico, Earth's most powerful radio transmitter, or a pulsed laser on the 10-meter Keck optical telescope in Hawaii. Rose and Wright(2) conclude, however, that the better choice would be to send packages laced with information.
3) Unless the messages are short or the extraterrestrials are nearby, this "write" strategy requires less energy per bit of transmitted information than the "radiate" strategy does. Cone-shaped beams of radiation necessarily grow in size as they travel outwards, meaning that the great majority of the energy is wasted, even if some of it hits the intended target. A package, on the other hand, is not "diluted" as it travels across space, presuming that it's correctly aimed at its desired destination. For short messages, however, electromagnetic waves win out because of the overheads involved in launching, shielding and then decelerating a package, no matter how small it is. For a two-way conversation with extraterrestrials, the light-speed of electromagnetic waves is far superior.
4) As an example of a large message, consider all of the written and electronic information now existing on Earth: it's estimated(5) to amount to about one exabyte (10^(18) bytes). Rose and Wright(2) calculate that, using scanning tunnelling microscopy, these bits could be inscribed (in nanometer squares) within one gram of material! But this precious package would still require a cocoon of 10,000 kilograms to accelerate it from our planet to a speed of 0.1% of the speed of light, protect it from radiation damage along a 10,000-light-year route, and then decelerate it upon arrival.
References:
1. Cocconi, G. & Morrison, P. Nature 184, 844-846 (1959)
2. Rose, C. & Wright, G. Nature 431, 47-49 (2004)
3. Bracewell, R. Nature 187, 670-671 (1960)
4. Papagiannis, M. Q. J. R. Astron. Soc. 19, 277-281 (1978)
5. Murphy, C. Atlantic 277, No. 5, 20-22 (1996)
Nature http://www.nature.com/nature
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ASTROBIOLOGY: ON INTELLIGENT LIFE IN THE UNIVERSE
The following points are made by J. Cohen and I. Stewart ((Nature 22 Feb 01 409:1119):
1) The authors point out that it is possible to imagine the existence of forms of life very different from those found on Earth, occupying habitats that are unsuitable for our kind of life. Some of those aliens might be technological, because technology is an autocatalytic process, and it follows that some aliens might possess technology well in advance of our own, including interstellar transportation. So much is clear, but this train of logic begs the obvious question of where these intelligent non-humanoid aliens might be.
2) The authors point out that the subject area of this discussion is often called "astrobiology", although in science fiction circles (where the topic has arguably been thought through more carefully than it has been in academic circles) the term "xenobiology" is favored. The authors suggest the difference is significant: Astrobiology is a mixture of astronomy and biology, and the tendency is to assume that the field must be assembled from contemporary astronomy and biology; in contrast, xenobiology is the biology of the strange, and the name inevitably involves the idea of extending contemporary biology into new and alien realms.
3) The authors ask: Upon what science should xenobiology be based? The authors suggest that the history of science indicates that any discussion of alien life will be misleading if it is based on the presumption that contemporary science is the ultimate in human understanding. Consider the position of science a century ago. We believed then that we inhabited a newtonian clockwork Universe with absolute space and absolute time; that time was independent of space; that both were of infinite extent; and that the Universe had always existed, always would exist, and was essentially static. We knew about the biological cell, but we had a strong feeling that life possessed properties that could not be reduced to conventional physics; we had barely begun to appreciate the role of natural selection in evolution; and we had no idea about genetics beyond mendelian numerical patterns. Our technology was equally primitive: cars were inferior to the horse, and there was no radio, television, computers, biotechnology or mobile phones. Space travel was the stuff of fantasy. If the past is any guide, then almost everything we now think we know will be substantially qualified or proven wrong within the next 25 years, let alone another century. Biology, in particular, will not persist in its current primitive form. At present, biology is at a stage roughly analogous to physics when Newton (1642-1727) discovered his law of gravity. "There is an awfully long way to go."
4) The authors point out that evolution on Earth has been in progress for at least 3.8 billion years. "This is deep time --too deep for scenarios expressed in human terms to make much sense. A hundred years is the blink of an eye compared with the time that humans have existed on Earth. The lifespan of the human race is similarly short when compared with the time that life has existed on Earth. It is ridiculous to imagine that somehow, in a single century of human development, we have suddenly worked out the truth about life. After all, we do not really understand how a light switch works at a fundamental level, let alone a mitochondrion."
Nature http://www.nature.com/nature
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PROSPECTS FOR THE SEARCH FOR EXTRATERRESTRIAL INTELLIGENCE
Notes by ScienceWeek:
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."
Scientific American 1999 December
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