SETI (pronounced ['sɛti], to rhyme with "Betty") stands for Search for Extra-Terrestrial Intelligence.
Interstellar travel allowing direct discovery and contact with intelligent aliens is a common theme in science fiction stories, but so far the obstacles to such journeys have been insurmountable. An alternative approach to achieving such a "first contact" is to survey the sky in hopes of finding transmissions from a civilization on a distant planet. However, such an effort has many obstacles.
- 1 Overview
- 2 Radio SETI experiments
- 3 Optical SETI experiments
- 4 Probe SETI and SETA Experiments
- 5 Where are they? / The interstellar Internet
- 6 Criticism of SETI
- 7 See also
- 8 External links
However, it is perfectly within reach to develop a communications system using a powerful transmitter and a sensitive receiver, and use it to search the sky for extraterrestrial worlds whose citizens have a similar inclination as terrestrials. A basic assumption of SETI is the that of "Mediocrity" : the idea that humanity is not exotic in the Cosmos but in a sense "typical" or "medium" when compared with other intelligent species. This would mean that humanity have enough similarity with other intelligent beings that communications will be mutally desirable and understandable. If this basic assumption of Mediocrity is correct and other intelligent species are present in any number in the galaxy at our technological level or above, then communications between the two worlds should be inevitable.
SETI is still no trivial task. The Milky Way galaxy is 100,000 light years across, and contains a hundred billion stars. Searching the entire sky for some far-away and faint signal is an exhausting exercise.
Some further simplifying assumptions beyond Mediocrity are useful to reduce the size of the task. One is to assume that the vast majority of life-forms in the galaxy are based on carbon chemistries, as are all life-forms on Earth. While it is possible that life could be based around atoms other than carbon, carbon is well known for the unusually wide variety of molecules that can be formed around it.
The presence of liquid water is also a useful assumption, as it is a common molecule and provides an excellent environment for the formation of complicated carbon-based molecules that could eventually lead to the emergence of life.
A third assumption is to focus on Sun-like stars. Very big stars have relatively short lifetimes, meaning that intelligent life would not likely have time to evolve on planets orbiting them. Very small stars provide so little heat and warmth that only planets in very close orbits around them would not be frozen solid, and in such close orbits these planets would be tidally "locked" to the star, with one side of the planet perpetually baked and the other perpetually frozen.
About 10% of the stars in the Milky Way galaxy are Sun-like, and there are about a thousand such stars within 100 light-years of the Sun. These stars would be useful primary targets for interstellar listening. However, we know of only 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 bad enough. To find a radio transmission from an alien civilization, we also have to search through most of the useful radio spectrum, as 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, as we have no idea how a signal sent by aliens might be modulated, and how the data transmitted by it might be 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", that is, 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, ruining their own planet by doing so.
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 places the transmitter power within reasonable ranges, the problem being now of having the good luck to coincide with the path of the beam, with the possibility approaching to zero as distance increases.
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 causes "ghosted" TV images. Such ghosts occur when TV transmissions are 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.
Similarly, interstellar narrow-beam communications could be bent or "refracted" by interstellar 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 to be alert.
Modern SETI efforts began with a paper written by physicists Giuseppe 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 tends to also 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 frequency. The lower frequencies are also desirable 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, 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 "water hole" by SETI enthusiasts.
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 L. Frank 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 performed a number of searches with omnidirectional antennas in the hope of picking up powerful radio signals beginning in 1964. TV-Host/American astronomer Carl Sagan and Soviet astronomer Iosif Shklovskii together wrote the pioneering book in the field, Intelligent Life in the Universe which appeared in 1966.
In 1971, the U.S. National Aeronautics and 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, not surprisingly, Cyclops was not built.
In 1974, a largely symbolic attempt was made to send a message to other worlds. To celebrate a substantial upgrading of the 305 metre Arecibo Radio Telescope in Puerto Rico, a coded message of 1,679 bits was transmitted towards the Globular Cluster M13, some 25,000 light years away.
The pattern of 0s and 1s contained in the message defined a 23 × 73 grid which when plotted revealed some data about our location in the Solar System, a stylised figure of a human being, chemical formulae and an outline of the radio telescope itself. The 23 by 73 grid was chosen because both 23 and 73 are prime numbers and it was thought that this could aid any hypothetical alien listener to recognize the grid representation.
Given the limitations of the speed of light, no reply would be possible for 50,000 years and hence has been dismissed by some as a publicity stunt. A controversy arose because the transmission raised the serious question of whether a small group should be allowed to speak for Earth.
SERENDIP, Sentinel, META, and BETA
In 1979 the University of California, Berkeley launched a SETI project named "Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations (SERENDIP)" . In 1980, Carl Sagan, Bruce Murray, and Louis Friedman founded the U.S. Planetary Society, partly as a vehicle for SETI studies.
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, as 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", for "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. Also in 1985, Ohio State University began their own SETI program, named Project "Big Ear", which later received Planetary Society funding. The next year, in 1986, UC Berkeley initiated their second SETI effort, SERENDIP II, and has continued with two more SERENDIP efforts to the present day.
The Planetary Society is now pursuing a follow-on to the META project named "BETA", for "Billion-Channel Extraterrestrial Array". This is a dedicated DSP box with 200 processors and 3 gigabytes of RAM. BETA is about a trillion times more powerful than the signal processing equipment used in Project Ozma. BETA actually scans only 250 million channels, with a range of 0.5 hertz per channel. It scans through the microwave range from 1.400 to 1.720 gigahertz in eight hops, with two seconds of observation in each hop.
MOP and Project Phoenix
In 1992, the U.S. 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 U.S. Congress, where the program was strongly ridiculed, and was cancelled 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, previously Project Scientist for the NASA project, is a continuation of the Targeted Search program, studying roughly 1,000 nearby Sunlike stars. Seth Shostak also worked on Project Phoenix. From 1995 through March 2004, Phoenix conducted observing campaigns at the 64-meter Parkes radio telescope in Australia, the 140 Foot Telescope of the National Radio Astronomy Observatory in West Virginia, USA, and the Arecibo Observatory in Puerto Rico. The project observed the equivalent of 800 stars over the available channels in the frequency range from 1200 to 3000 MHz. The search was sensitive enough to pick up transmitters with power output equivalent to airport radars to a distance of about 200 light years.
Allen Telescope Array
The SETI Institute is now collaborating with the Radio Astronomy Laboratory at UC Berkeley to develop a specialized radio telescope array for SETI studies, something like a mini-Cyclops array. The new array concept is named the "Allen Telescope Array" (ATA) (formerly, One Hectare Telescope [1HT]) after the project's benefactor Paul Allen. Its sensitivity will be equivalent to a single large dish more than 100 meters on a side. The array is being constructed at the Hat Creek Observatory in rural northern California. 
The array will consist of 350 or more Gregorian radio dishes, each 6.1 meters (20 feet) in diameter. These dishes will essentially be commercially available satellite television dishes. The ATA is expected to be completed by 2007 at a very modest cost of $25 million USD. The SETI Institute will provide money for building the ATA while UC Berkeley will design the telescope and 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.
Another interesting UC Berkeley effort called SETI@home was conceived in 1995 by David Gedye and began in May 1999, and heavily sponsored by The Planetary Society. The existence of the SETI@home project means that any individual can become involved with SETI research by simply downloading screensaver software over the Internet. The software performs 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.
Over 5 million computer users in more than a hundred countries have signed up for SETI@home and have collectively contributed over 19 billion hours of computer processing time. The project is widely praised in the computer press as an effective exercise in home-grown distributed computing. As of June 22, 2004 the next generation of SETI@home was released to the public. It is based on the Berkeley Open Infrastructure for Network Computing (BOINC), which is being developed out of the UC Berkeley. SETI@home classic will soon retire as the next generation of distributive computing advances.
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. Now some SETI advocates, such as Frank Drake, have suggested that such a judgement was too conservative.
There are two problems with optical SETI, one of which is easy to deal with, the second of which is troublesome. The first problem is that lasers are highly "monochromatic", that is, they emit light only on one frequency, making it troublesome 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 frequencies becoming higher 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. This means that a laser beam could be easily blocked by clouds of interstellar dust, and more to the point, we could pick it up only if we happened to cross its line of fire. As 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 Stuart Kingsley, a British dedicated amateur living in the US state of Ohio.
Now the 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 says: "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 are now in progress. A Harvard-Smithsonian group that includes Paul Horowitz designed a laser detector and mounted it on Harvard's 155 centimeter (61 inch) optical telescope. This telescope is currently being used for a more conventional star survey, and the optical SETI survey is "piggybacking" 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 is now working with Princeton to mount a similar detector system on Princeton's 91-centimeter (36-inch) telescope. The Harvard and Princeton telescopes will be "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 is now building a dedicated all-sky optical survey system along the lines of that described above, featuring a 1.8-meter (72-inch) telescope. The new optical SETI survey telescope is being set up at the Oak Ridge Observatory in Harvard, Massachusetts.
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, rather than 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 (30-inch) automated telescope and an older laser detector built by Wertheimer.
Probe SETI and SETA Experiments
The possibility of using interstellar messenger probes in the search for extraterrestrial intelligence was first suggested by Ronald N. Bracewell in 1960, and the technical feasibility of this approach was demonstrated by the British Interplanetary Society's starship study Project Daedalus in 1978. Starting in 1979, Robert Freitas advanced arguments    for the proposition that physical space-probes are a superior mode of interstellar communication to radio signals. Subsequently, in a September 2004 paper featured on the cover of Nature Magazine , Christopher Rose and Gregory Wright showed that inscribing a message in matter and transporting it to the destination is vastly more energy-efficient than communication using electromagnetic waves if the message can tolerate delivery delay beyond light transit time   . Thus, a solarcentric Search for Extraterrestrial Artifacts (SETA)  would seem to be favored over the more traditional radio or optical searches.
Much like the "preferred frequency" concept in SETI radio beacon theory, the Earth-Moon or Sun-Earth libration orbits  might therefore constitute the most universally convenient parking places for automated extraterrestrial spacecraft exploring arbitrary stellar systems. A viable long-term SETI program may be founded upon a search for these objects.
In 1979 Freitas and Valdes  conducted a photographic search of the vicinity of the Earth-Moon triangular libration points L4 and L5, and of the solar-synchronized positions in the associated halo orbits, seeking possible orbiting extraterrestrial interstellar probes, but found nothing to a detection limit of about 14th magnitude. The authors conducted a second more comprehensive photographic search for probes in 1982  that examined the five Earth-Moon Lagrangian positions and included the solar-synchronized positions in the stable L4/L5 libration orbits, the potentially stable nonplanar orbits near LI/L2, Earth-Moon L3, and also L2 in the Sun-Earth system. Again no extraterrestrial probes were found to limiting magnitudes of 17-19th magnitude near L3/L4/L5, 10-18th magnitude for L1/L2, and 14-16th magnitude for Sun-Earth L2.
In June 1983, Valdes and Freitas  used the 26-m radiotelescope at Hat Creek Radio Observatory to search for the tritium hyperfine line at 1516 MHz from 108 assorted astronomical objects, with emphasis on 53 nearby stars including all visible stars within a 20 light-year radius. The tritium frequency was deemed highly attractive for SETI work because (1) the isotope is cosmically rare, (2) the tritium hyperfine line is centered in the SETI waterhole region of the terrestrial microwave window, and (3) in addition to beacon signals, tritium hyperfine emission may occur as a byproduct of extensive nuclear fusion energy production by extraterrestrial civilizations. The wideband- and narrowband-channel observations achieved sensitivities of 5-14 x 10-21 W/m2/channel and 0.7-2 x 10-24 W/m2/channel, respectively, but no detections were made.
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 physicist Enrico Fermi suggested in the 1950s that if there was an interstellar civilization, its presence would be obvious once we bothered to look. This is known as the Fermi paradox.
The paradox can be illustrated as follows:
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 a speed of 10% of the speed of light; and that it takes four centuries for an interstellar colony to grow to the point where it can launch a pair of interstellar missions, 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 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 very 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 argument, depressing for many SETI enthusiasts, is called the "Fermi paradox".
The fact that radio-based SETI searches have not come up with anything very interesting so far is not cause to rule out the existence of contactable alien intelligence. As the previous sections of this document show, trying to find another civilization in space is a difficult proposition, and we have searched only 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 called the Kardashev scale 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 have remained 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. It is possible that intelligent species abandon radio when new technologies are discovered, making the length of time a world is transmitting on conventional radio extremely short. 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 provides another possible explanation 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. 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 could be only 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 could maintain 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 link up with it.
Another theory which has been proposed to explain the apparent lack of interstellar communication is the suggestion that the galaxy may contain predatory (or otherwise aggressive) species. Those species smart enough to maintain radio silence are those that survive such predation. Yet another theory proposed by physicist Arnon Dar, and described in the PBS Nova show 'Death Star', proposes that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate.
Criticism of SETI
SETI has occasionally been the target of criticism by those who suggest that it is a form of pseudoscience. In particular, critics allege that no observed phenomena suggest the existence of extraterrestrial intelligence, and furthermore that the assertion of the existence of extraterrestrial intelligence has no good Popperian criteria for falsifiability . Author Michael Crichton, in a 2003 lecture at CalTech, stated that "The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion." 
In response, SETI advocates note, among other things, that the existence of intelligent life on Earth is a plausible reason to expect it elsewhere, and that individual SETI projects have clearly defined "stop" conditions.
- Extraterrestrial life
- Drake equation
- First contact
- Wow! signal
- Communication with Extraterrestrial Intelligence
- European Darwin Project
- Terrestrial Planet Finder
- Where Is Everybody? : an essay by Arthur C. Clarke on SETI
- Indistinguishable From Magic
- SETI Institute
- SETI FAQ
- SETI@home, the shared computing project
- The Columbus Optical SETI Observatory, Dr. Stuart A, Kingsley
- Detectability of Extraterrestrial Technological Activities by Guillermo A. Lemarchand
- First Contact Within 20 Years: Shostak – From SpaceDaily.com, 22 Jul 2004 (based on calculations to be published in Acta Astronautica)
- SETI: Searching for Life – Article series in Sky & Telescope magazine
- Searching for Good Science: The Cancellation of NASA's SETI Program (PDF) – By Stephen J. Garber, NASA History Office. Journal of the British Interplanetary Society, Vol. 52, pp. 3-12, 1999. Provides more details on the elimination of SETI funding by the US Congress in 1993.
- Open SETI Initiative – Gerry Zeitlin's site concerned with reforming SETI's approach
- NASA JPL Terrestrial planet finder
- NOAO sky survey program
- The Search For Extraterrestrial Intelligence (NASA SP-419, 1977)
- ET Might Write, Not Radiate
- IT & SETI: The Role of Computer Technology in the Search for Extraterrestrial Intelligence
- So You Want to "Get Into" SETI?
- Mark Elowitz's Web site on Exobiology and SETI
- Let's Build an Extraterrestrial
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