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SETI: Search For Extra-Terrestrial Intelligence

v1.0.8 / 01 sep 13 / greg goebel / public domain

* Interstellar flight is a common theme in science-fiction stories, but in reality the obstacles to such expeditions are enormous. An alternative approach to interstellar exploration is to survey the sky for transmissions from a civilization on a distant planet, but such a "Search for Extra-Terrestrial Intelligence (SETI)" effort is faced with obstacles as well. This document outlines the history, current efforts, and future prospects for SETI.

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[1] OVERVIEW
[2] RADIO SETI EXPERIMENTS
[3] OPTICAL SETI EXPERIMENTS
[4] WHERE ARE THEY? / THE INTERSTELLAR INTERNET
[5] COMMENTS, SOURCES, & REVISION HISTORY

[1] OVERVIEW

* Visiting another civilization on a distant world would be fascinating, but at present such a trip is beyond our capabilities. However, it is perfectly within our capabilities to develop a communications system using a powerful transmitter and a sensitive receiver, and using it to search the sky for alien worlds whose citizens have a similar inclination.

SETI is still no trivial task. Our Galaxy is 100,000 light-years across, and contains 100,000 thousand million stars. Searching the entire sky for some faraway and faint signal is an exhausting exercise. Some simplifying assumptions are useful to reduce the size of the task:

About 10% of the stars in our Galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of our Sun. These stars would be useful primary targets for interstellar listening. However, we only know of one planet where life exists, our own. There is no way to know if any of the simplifying assumptions are correct, and so as a second priority the entire sky must be searched.

Searching the entire sky is difficult enough. To find a radio transmission from an alien civilization, we also have to search through most of the useful radio spectrum, since there is no way to know what frequencies aliens might be using. Trying to transmit a powerful signal over a wide range of wavelengths is impractical, and so it is likely that such a signal would be transmitted on a relatively narrow band. This means that a wide range of frequencies must be searched at every spatial coordinate of the sky.

There is also the problem of knowing what to listen for, since we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it encoded. Narrow-bandwidth signals that are stronger than background noise and constant in intensity are obviously interesting, and if they have a regular and complex pulse pattern are likely to be artificial. However, while studies have been performed on how to send a signal that could be easily decoded, there is no way to know if the assumptions of those studies are valid, and deciphering the information from an alien signal could be very difficult.

There is yet another problem in listening for interstellar radio signals. Cosmic and receiver noise sources impose a threshold to power of signals that we can detect. For us to detect an alien civilization 100 light-years away that is broadcasting "omnidirectionally" -- in all directions -- the aliens would have to be using a transmitter power equivalent to several thousand times the entire current power-generating capacity of the entire Earth. It is much more effective in terms of communication to generate a narrow-beam signal whose "effective radiated power" is very high along the path of the beam, but negligible everywhere else. This makes the transmitter power perfectly reasonable, but the problem then becomes one of having the good luck to be in the path of the beam.

Such a beam might be very hard to detect, not only because it is very narrow but because it could be blocked by interstellar dust clouds or garbled by "multipath effects" -- the same phenomenon that used to cause "ghosted" TV images in the days of analog TV broadcast . Such ghosts occurred when TV transmissions were bounced off a mountain or other large object, while also arriving at our TV antenna by a shorter, direct route, with the TV picking up two signals separated by a delay and superimposing them with a slight offset. Similarly, interstellar narrowbeam communications could be bent or "refracted" by cosmic gas clouds to produce multipath effects that could obscure the signal. If interstellar signals are being transmitted on narrow beams, there is nothing we can do at this end to deal with this problem other than be alert.

* Modern SETI efforts began with a paper written by physicists Guiseppe Cocconi and Philip Morrison and published in the science press in 1959. Cocconi and Morrison suggested that the microwave frequencies between 1 and 10 gigahertz would be best suited for interstellar communications. Below 1 gigahertz, "synchrotron radiation" emitted from electrons moving in galactic magnetic fields tends to drown out other radio sources. Above 10 gigahertz, radio noise from water and oxygen atoms in our atmosphere also tends to become a source of interference. Even if alien worlds have substantially different atmospheres, quantum noise effects make it difficult to build a receiver that can pick up signals above 100 gigahertz.

The low end of this "microwave window" is particularly attractive for communications, because it is in general easier to generate and receive signals at lower frequencies. Lower frequencies are also desireable because of the "Doppler shifting" of a narrow-band signal due to planetary motions. Doppler shifting is a change in the frequency of a signal due to the motion of the source of that signal. If the source is approaching, the signal will be shifted up in frequency, while if the source is moving away, the signal will be shifted down in frequency. The rotation of a planet and its orbit around a star causes a Doppler shift in the frequency of any signal generated from that planet, and over the course of a day the signal can drift in frequency far out of its intended bandwidth. The problem gets worse with higher frequencies -- the wavelengths are shorter and the shift has more effect -- and so lower frequencies are preferred.

Cocconi and Morrison suggested that the frequency of 1.420 gigahertz was particularly interesting. This is the frequency emitted by neutral hydrogen. Radio astronomers often search the sky on this frequency to map the great hydrogen clouds in our Galaxy. Transmitting a communications signal near this "marker" frequency would improve the chances of its detection by accident. This frequency is sometimes called the "watering hole" by SETI enthusiasts.

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[2] RADIO SETI EXPERIMENTS

* In 1960, Cornell University astronomer Frank Drake performed the first modern SETI experiment, named "Project Ozma", after the Queen of Oz in Frank L. Baum's fantasy books. Drake used a 25-meter-diameter radio telescope at Green Bank, West Virginia, to examine the stars Tau Ceti and Epsilon Eridani near the 1.420 gigahertz marker frequency. A 400 kilohertz band was scanned around the marker frequency, using a single-channel receiver with a bandwidth of 100 hertz. The information was stored on tape for off-line analysis. Nothing of great interest was found.

The first SETI conference took place at Green Bank in 1961. The Soviets took a strong interest in SETI during the 1960s, and beginning in 1964 performed a number of searches with omnidirectional antennas in hopes of picking up powerful radio signals. In 1966, American astronomer and pop-science figure Dr. Carl Sagan and Soviet astronomer Iosif S. Shkolovskii published in collaboration a landmark book in the field, INTELLIGENT LIFE IN THE UNIVERSE.

In 1971, the US National Aeronautics & Space Administration (NASA) funded a SETI study that involved Drake, Bernard Oliver of Hewlett-Packard Corporation, and others. The report that resulted proposed the construction of an Earth-based radio telescope array with 1,500 dishes, known as "Project Cyclops". The price tag for the Cyclops array was $10 billion USD, and unsurprisingly Cyclops was not built.

However, in 1973 a SETI observation program was begun at Ohio State University using the "Big Ear" radio telescope, an ingenious home-grown instrument that consisted of two fencelike arrays arranged on the ends of a metal-covered flat court. The transmitter part of the array was parabolic, staring across the court to a flat rectangular reflector array that could hinge up and down.

The Big Ear could be aimed in altitude but not in azimuth, but it was intended as a survey instrument and that wasn't a major limitation. It was originally used to map wideband radio sources through the sky, but due to funding cutbacks that mission had to be abandoned; the SETI search, inspired by the debate about the Cyclops array, could be performed much more cheaply. During its SETI search, the Big Ear picked up a startling narrowband signal on 15 August 1977, known as the "WOW!" signal for the note one of the staff scribbled on the computer printout. The signal was not repeated and there are strong suspicions it was from some local source, possibly a military satellite. Searches continued into 1985; the Big Ear was finally dismantled in 1998.

In 1974, an attempt was made to send a message to other worlds. A 1,679 bit message was transmitted from the 305-meter Arecibo radio observatory dug into the ground in Puerto Rico toward the globular star cluster M13, about 25,000 light-years away. The pattern of 1s and 0s defined a 23 by 73 pixel "bitmap" image that included numbers, stick figures, chemical formulas, and a crude image of the Arecibo radio telescope itself. The 23 by 73 grid was chosen because as prime numbers, they are the only integer values that can be multiplied together to get 1,679, and hopefully alien listeners would be able to factor the number of pixels and recognize that they represented a grid. Given the distance to M13 and the cryptic appearance of the bitmap, the experiment amounted mostly to a publicity stunt.

In 1979, Sagan, Bruce Murray, and Louis Friedman founded the US Planetary Society, partly as a vehicle for SETI studies. That same year, the University of California at Berkeley launched a SETI project named "Search for Extraterrestrial Radio from Nearby Developed Populations (SERENDIP)", run by astronomer Daniel Werthimer. SERENDIP's search, as its named implied, was opportunistic, using auxiliary receivers on radio telescopes which were performing observations having nothing to do with SETI. As long as they were searching the sky, however, Werthimer figured they might as well check to see if there was an ET civilization there as well.

* In the early 1980s, Harvard University physicist Paul Horowitz took the next step and proposed the design of a spectrum analyzer specifically intended to search for SETI transmissions. Traditional desktop spectrum analyzers were of little usefulness for this job, since they sampled frequencies using banks of analog filters and so were restricted in the number of channels they could acquire. However, modern integrated-circuit digital signal processing (DSP) technology could be used to build "autocorrelation" receivers to check far more channels.

This work led in 1981 to a portable spectrum analyzer named "Suitcase SETI" that had a capacity of 131,000 narrowband channels. After field tests that lasted into 1982, Suitcase SETI was put into use in 1983 with the 25-meter Harvard/Smithsonian radio telescope at Harvard, Massachusetts. This project was named "Sentinel", and continued into 1985.

Even 131,000 channels weren't enough to search the sky in detail at any fast rate, and so Suitcase SETI was followed in 1985 by "Project META (Megachannel Extra-Terrestrial Array)". The META spectrum analyzer had a capacity of 8 million channels and a channel resolution of 0.5 hertz. The project was led by Horowitz with the help of the Planetary Society, and was partly funded by moviemaker Steven Spielberg. A second such effort, META II, was begun in Argentina in 1990 to search the southern sky. META II is still in operation, after an equipment upgrade in 1996.

The next year, in 1986, UC Berkeley initiated their second opportunistic SETI effort, SERENDIP II, and has continued with two more SERENDIP efforts to the present day. The current effort, SERENDIP IV, began in 1997, using the Arecibo dish. A "Southern SERENDIP" effort was begun in 1998 in Australia using the 64-meter radio telescope at Parkes, plus systems derived from the other SERENDIP efforts.

* In 1992, the US government finally funded an operational SETI program, in the form of the NASA "Microwave Observing Program (MOP)". MOP was planned as a long-term effort, performing a "Targeted Search" of 800 specific nearby stars, along with a general "Sky Survey" to scan the sky. MOP was to be performed by radio dishes associated with the NASA Deep Space Network, as well as a 43-meter dish at Green Bank and the big Arecibo dish. The signals were to be analyzed by spectrum analyzers, each with a capacity of 15 million channels. These spectrum analyzers could be ganged to obtain greater capacity. Those used in the Targeted Search had a bandwidth of 1 hertz per channel, while those used in the Sky Survey had a bandwidth of 30 hertz per channel.

MOP drew the attention of the US Congress, where the program was strongly ridiculed, and was canceled a year after its start. SETI advocates did not give up, and in 1995 the nonprofit "SETI Institute" of Mountain View, California, resurrected the work under the name of "Project Phoenix", backed by private sources of funding. Project Phoenix, under the direction of Dr. Jill Tarter, who had worked on MOP when she was at NASA, was a continuation of the Targeted Search program, studying 710 Sunlike stars within 150 light-years of the Earth. Phoenix used the 64-meter Parkes radio telescope in Australia, the 43-meter telescope at Green Banks, and the Arecibo dish, searching 70 million channels across a bandwidth of 1,800 MHz. The search was said to be capable of picking up any transmitter about as powerful as an airport radar within 200 light-years. Phoenix was completed in March 2004, with negative results.

The Planetary Society followed up the META project with a more ambitious effort, named "BETA (Billion-Channel Extraterrestrial Array)". This was a dedicated DSP box with 200 processors and 3 gigabytes of RAM. BETA was about a trillion times more powerful than the signal processing equipment used in Project Ozma. BETA actually only scanned 250 million channels, with a range of 0.5 hertz per channel, searching through the microwave range from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation in each hop. BETA was turned on in 1995, originally using a radio telescope at Oak Ridge, Tennessee; the project abruptly ended in 1999 when the radio telescope being used for the search was wrecked by a windstorm.

* The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to set up a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept was originally named the "One Hectare Telescope (1HT)", because it is to cover 100 meters on a side, but it is now known as the "Allen Telescope Array (ATA)", named after donor Paul Allen, a co-founder of Microsoft. The first ten dishes were set up in 2002, leading to initial operational capability with 42 dishes in late 2007. Ultimately, the array will consist of 350 dishes -- though due to funding shortfalls, use of and work on the array has been intermittent as of late.

The SETI Institute has been funding the construction of the ATA, while UC Berkeley is building the telescope and will provide operational funding. Berkeley astronomers will use the ATA to pursue other deep space radio observations. The ATA is intended to support a large number of simultaneous observations through a technique known as "multibeaming", in which DSP technology is used to sort out signals from the multiple dishes. The DSP system planned for the ATA is extremely ambitious. It is being seen as a prototype of sorts for an even more ambitious system, the "Square Kilometer Array (SKA)", which of course will be a kilometer on a side and will be a powerful tool for radio astronomy and SETI studies.

* Private citizens are also involved in SETI. A private SETI organization, the "SETI League", was established in 1994, and in 1996 the group began their own coordinated search program, "Project Argus". It can be thought of as something like an amateur radio organization with a focus on interstellar communications. The stations are based on old-style TV dish receiver antennas running about 3 to 5 meters in diameter; receiver sensitivity is about the same as that of the Big Ear telescope. The ultimate goal is to have 5,000 stations all over the world; at last count, there were over 140.

The SETI Institute got private citizens involved with SETI through a program called "SETI@home" began in May 1999 and ran through 2001. SETI@home allowed hobbyists to get involved with SETI research by simply downloading screen saver software over the Internet. The software performed signal analysis on a downloaded 350 kilobyte "work unit" of SERENDIP IV SETI radio survey data, and then reports the results back over the Internet. Millions of computer users in hundreds of countries signed up for SETI@home and provided hundreds of thousands of hours of computer processing time. The project was widely praised in the computer press as an interesting exercise in home-grown "distributed computing". A follow-on project was initiated after the first effort, and now SETI@home is an ongoing thing.

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[3] OPTICAL SETI EXPERIMENTS

* While most SETI sky searches have studied the radio spectrum, some SETI researchers have considered the possibility that alien civilizations might be using powerful lasers for interstellar communications at optical wavelengths. The idea was first suggested in a paper published in the British journal NATURE in 1961, and in 1983 Charles Townes -- one of the inventors of the laser -- published a detailed study of the idea in the US journal PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Most SETI researchers were cool to the idea. The 1971 Cyclops study discounted the possibility of optical SETI, reasoning that construction of a laser system that could outshine the bright central sun of a remote star system would be too difficult. More recently some SETI advocates, such as Frank Drake, suggested that such a judgement was too conservative.

There are two problems with optical SETI, one easy to deal with, the other troublesome. The first problem is that lasers are highly "monochromatic", that is, they only emit light on one frequency, making it difficult to figure out what frequency to look for. However, according to Fourier analysis, emitting light in narrow pulses results in a broad spectrum of emission, with the range becoming wider as the pulse width becomes narrower, and an interstellar communications system could use pulsed lasers.

The other problem is that while radio transmissions can be broadcast in all directions, lasers are highly directional. That means that a laser beam could be easily blocked by clouds of interstellar dust and, more to the point, we could only pick it up if we happened to cross its line of fire. Since it is unlikely an alien civilization would focus an interstellar laser communications beam on Earth deliberately, we would have to cross such a beam by accident.

However, as discussed earlier, the power requirements for omnidirectional interstellar radio broadcasts are tremendous, and narrow-beam radio communications are technically more plausible. As SETI researchers have adjusted to the idea that interstellar radio communications may be over narrow beams, the idea of hunting for interstellar laser beams has become no more troublesome.

* In the 1980s, two Soviet researchers conducted a short optical SETI search, but turned up nothing. During much of the 1990s, the optical SETI cause was kept alive through searches by a dedicated amateur named Stuart Kingsley, a Briton living in the US state of Ohio.

SETI old-timers have warmed to the concept of optical SETI. Paul Horowitz of Harvard and researchers with the SETI institute have conducted simple optical SETI searches using a telescope and a photon pulse detection system, and are considering further searches. Horowitz said: "Everyone's been mesmerized by radio, but we've done that experiment a lot and we're a little tired of it."

Optical SETI enthusiasts have conducted paper studies of the effectiveness of using contemporary high-energy lasers and a ten-meter focus mirror as an interstellar beacon. The analysis shows that an infrared pulse from a laser, whose light output is not bound by the inverse-square law of light emitted from a hot body like the Sun, would appear thousands of times brighter than the Sun to a distant civilization in the beam's line of fire. The Cyclops study proved incorrect in suggesting a laser beam would be inherently hard to see.

Such a system could be made to automatically steer itself through a target list, sending a pulse to each target at a rate, say, of once a second. This would allow targeting of all Sun-like stars within a distance of 100 light-years. The studies have also described an automatic laser pulse detector system with a low-cost, two-meter mirror made of carbon composite materials, focusing on an array of light detectors. This automatic detector system could perform sky surveys to detect laser flashes from civilizations attempting to contact us.

* Several optical SETI experiments have been conducted or are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter optical telescope. This telescope was being used for a more conventional star survey, and the optical SETI survey "piggybacked" on that effort.

Between October 1998 and November 1999, the survey inspected about 2,500 stars. Nothing that resembled an intentional laser signal was detected, but efforts continue. The Harvard-Smithsonian group has worked with Princeton to mount a similar detector system on Princeton's 91-centimeter telescope. The Harvard and Princeton telescopes were "ganged" to track the same targets at the same time, with the intent being to detect the same signal in both locations as a means of reducing errors from detector noise.

The Harvard-Smithsonian group has built a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter telescope. This optical SETI survey telescope is set up at the Harvard observatory site in Oak Ridge, Tennessee, and features a high-resolution, high-speed photomultiplier camera system with a custom processor to catch short transients of light.

The University of California, Berkeley, home of SERENDIP and SETI@home, is also conducting optical SETI searches. One is being directed by Geoffrey Marcy, the well-known extrasolar planet hunter, and involves examination of records of spectra taken during extrasolar planet hunts for a continuous, instead of pulsed, laser signal. The other Berkeley optical SETI effort is more like that being pursued by the Harvard-Smithsonian group and is being directed by Dan Werthimer of Berkeley, who built the laser detector for the Harvard-Smithsonian group. The Berkeley survey uses a 76-centimeter automated telescope and an older laser detector built by Werthimer.

* Incidentally, there have also been SETI searches based on the notion that space probes from alien civilizations have arrived in the Solar System and are monitoring our activities. It seems like a long-shot proposition, but if an alien civilization could build an automated probe that could maintain itself for tens of thousands of years, it would be no more and likely less difficult to send it to a distant star system on a scout mission than it would be to broadcast energetic signals to distant stars.

The first "Search for Extraterrestrial Artifacts (SETA)" search was performed in 1979 by physicist Robert Freitas and colleagues, scanning astronomical photography plates of the Earth-Moon "libration points", stable positions defined by the Earth and Moon that make good "parking places" for observational spacecraft. Nothing was found; the same group performed a more exhaustive search along the same lines in 1982 and came up negative again. No doubt the negative results were no surprise; the search was clearly conducted on the basis that it was worth the effort to check.

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[4] WHERE ARE THEY? / THE INTERSTELLAR INTERNET

* SETI experiments performed so far have not found anything that resembles an interstellar communications signal. Says Frank Drake of the SETI Institute: "All we know for sure is that the sky is not littered with powerful microwave transmitters."

The great Italian-American physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. While faster-than-light, or "superluminal", flight is ruled out by contemporary physics, no law of physics absolutely rules out interstellar flight at "subluminal" speeds, though the physical requirements are formidable. Assuming that stars are on the average about ten light-years apart; that an interstellar mission can be conducted at 10% of the speed of light; and that on the average -- factoring in failures -- an interstellar colony would be able to launch two successful interstellar missions 400 years after its establishment, then the "doubling time" of the interstellar colonies created by this advanced civilization would be 500 years. This would allow colonization of the entire Galaxy in five million years. Even conservatively limiting an interstellar mission to 1% of the speed of light and assuming it takes a millennium for a society to get to the point where it can mount two interstellar missions, this still means the Galaxy would be completely populated in 20 million years. That is a short interval on a cosmic time scale.

Given the lack of observable signals, as well as the lack of any persuasive evidence that extra-terrestrials have ever visited this planet, Fermi's argument suggests that there is no such interstellar civilization. This depressing argument is called the "Fermi paradox". Some critics have suggested that after all the failed efforts to find signals from other civilizations, that the exercise has ceased to have any scientific value that it ever had, and SETI enthusiasts need to concede they are pursuing a lost cause.

* However, the fact that SETI searches have not come up with anything very interesting so far is not cause for despair. As discussed above, trying to find another civilization in space is a difficult proposition, and we have only searched a small fraction of the entire "parameter space" of targets, frequencies, power levels, and so on. The negative results do place limits on the proximity of certain "classes" of alien civilizations, as specified in a scheme proposed by Soviet SETI researcher Nikolai S. Kardashev in the early 1960s and later extended by Carl Sagan. In this scheme, a "Type I" civilization is one capable of using all the sunlight falling on the surface of an Earthlike planet for an interstellar signal; a "Type II" civilization is capable of harnessing the power of an entire star; and a "Type III" civilization is capable of making use of an entire Galaxy. Intermediate civilizations can be numerically defined on a logarithmic scale.

Assuming that an alien civilization is actually transmitting a signal that we could pick up, the searches so far rule out a Type I civilization within a spherical radius of 1,000 light-years, though there may be many civilizations comparable to our own within a few hundred light-years that remain undetected.

A similar analysis using the same assumption shows that there is no detectable Type II civilization in our Galaxy. In the early days of SETI, researchers assumed that such advanced civilizations were very common in our Galaxy. It is discouraging that this does not seem to be so. However, it is important to emphasize that our SETI hunts have been based on assumptions on communications frequencies and technologies that may be laughable to alien societies, if they have the concept of humor. The lack of results do not say that alien civilizations don't exist. They only say that if they do, our most optimistic assumptions for getting in touch with them have proven unrealistic.

* There is another issue that hints as to why we don't see evidence of a large number of alien societies. That issue is time.

Our Sun is not a first-generation star. All first-generation stars are either very small and dim, or have exploded, or have burned out. This first generation synthesized the heavy elements needed to create planets and lifeforms. Later generations of stars, including our Sun, have been born and have died or will die in their turn.

Our Galaxy is more than 10 billion years old. Intelligent life and technological societies may have arisen and died out many times during this ten billion years, with civilizations falling after they exhaust their planetary resources or suffer some natural catastrophe. Assuming that an intelligent species survives for ten million years, that means that only 0.1% of all societies that have arisen during the history of our Galaxy are in existence now.

Science writer Timothy Ferris has suggested that since galactic societies may well be transitory, then if there is in fact an interstellar communications network, it consists mostly of automated systems that store the cumulative knowledge of vanished civilizations and communicate that knowledge through the Galaxy. Ferris calls this the "Interstellar Internet", with the various automated systems acting as network "servers". Ferris suspects that if such an Interstellar Internet exists, communications between servers are mostly through narrow-band, highly directional radio or laser links. Intercepting such signals is, as discussed earlier, very difficult. However, the network probably still maintains some broadcast nodes in hopes of making contact with new civilizations. The Interstellar Internet may be out there, waiting for us to figure out how to hook up with it.

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[5] COMMENTS, SOURCES, & REVISION HISTORY

* SETI is a very appealing idea. It is well within the capabilities of current technology, and compared to space missions can be performed at relatively low cost. It seems contrary that governments are willing to fund space projects costing tens of billions of dollars and whose benefits are debatable, but find the idea that searching for signals from other civilizations silly and not worth a fraction of that effort. On the other hand, given that SETI can be performed at low cost, it is debatable that governments need to be involved with SETI research, or that it would be good for the field it they were.

A coherent, long-term SETI program can be pursued on a modest scale with private funding. A large-scale government-funded program would suffer from fluctuating support -- and besides, as ground-based telescope builders have demonstrated over the last few decades, limited budgets can often lead to surprising ingenuity that is lost when more money is available.

SETI may prove to be a dead end, and any SETI effort has to accept that fact up front. That's the question, anyway: is there someone else out there, or are we the only ones? Either way, as has been said, the implications are staggering.

* Sources include:

Various SETI websites were consulted as well, and the History Channel TV program THE 20TH CENTURY WITH MIKE WALLACE also ran a program on extraterrestrial life that discussed the SETI effort. The TV show did not provide many details and was out of date, but it did have interviews with Paul Horowitz, Frank Drake, Jill Tarter, Louis Friedman, and of course the late Carl Sagan that were interesting from a personality point of view. I am afraid, however, I can never see a video of Sagan without thinking: "billyuns and billyuns!"

The show was also amusing because it had segments dealing with the UFO community at Roswell, New Mexico, and elsewhere. The last segment of the program discussed the relationship between the SETI community and the UFO community, showing it as fundamentally antagonistic, with the SETI people fearing the UFO groups were undermining the credibility of SETI efforts, and the UFO groups resenting being treated as cranks by the SETI people.

* Revision history:

   v1.0   / 01 jul 00 
   v1.1   / 01 oct 00 / Comments on Fermi paradox, narrowbeam radio.
   v1.0.2 / 01 jan 02 / Review & polish.
   v1.0.3 / 01 jan 04 / Review & polish.
   v1.0.4 / 01 jan 06 / Review & polish.
   v1.0.5 / 01 dec 07 / Review & polish.
   v1.0.6 / 01 nov 09 / Review & polish.
   v1.0.7 / 01 oct 11 / Review & polish.
   v1.0.8 / 01 sep 13 / Review & polish.

This document started life as half of an earlier document titled INTERSTELLAR EXPLORATION & SETI, originally released in June 1999. I included both topics in the same document because neither seemed to be elaborate enough to justify a stand-alone treatment. However, I kept on picking up new details. The original document started to grow, and so I changed my mind and split it into separate documents on interstellar exploration and SETI. The interstellar exploration document ended up as part of the SPACEFLIGHT PROPULSION document.

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