Realized Interstellar Radio Message projects
In 1974, a largely symbolic attempt was made at the Arecibo Observatory to send a message to other worlds. It was sent towards the globular star cluster M13, which is 25,000 light years from Earth. The first Interstellar Radio Message (IRM), the "Arecibo Message", was transmitted in Nov, 1974 from Arecibo Radar Telescope. Further IRMs Cosmic Call, Teen Age Message, Cosmic Call 2, and A Message From Earth were transmitted in 1999, 2001, 2003 and 2008 from Evpatoria Planetary Radar.
 Paper projects
A large number of paper projects also exist. For example, directed by Douglas Vakoch at the SETI Institute in Mountain View CA, the Interstellar Message Composition Project is charged with designing messages that could presumably be sent to extraterrestrials that convey basic scientific or mathematical principles, as well as human altruism. Vackoch's idea is to send a message of reciprocal altruism because hopefully any extraterrestrials would reciprocate with a reply back.
Vakoch has founded "Encoding Altruism", a workshop that started in 2003 in Paris that brings together anthropologists, philosophers, physicists, astronomers, theologians, musicians, and artists to address the challenge of communicating with extraterrestrials in a language and syntax that would be intelligible to an alien civilization. Vakoch's most recent research is highlighted through Greater Good Science Center, University of California, Berkeley.
 Optical 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 by R. N. Schwartz and Charles Hard Townes in a 1961 paper published in the journal Nature titled "Interstellar and Interplanetary Communication by Optical Masers". In 1983, 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 agreed with 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 star of a remote star system would be too difficult.
Some SETI advocates, such as Frank Drake, have suggested that such a judgment was too conservative; early 21st century humans have no means of knowing how a superior technology is communicating or would communicate, and negative results may simply mean humans are making the wrong searches.
There are two problems with optical SETI. 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 the uncertainty principle, emitting light in narrow pulses results in a broad spectrum of emission; the spread in frequency becomes higher as the pulse width becomes narrower, making it easier to detect an emission.
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 Earth would have to cross its direct line of fire by chance to receive it.
Optical SETI supporters 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, focused into a narrow beam by such a mirror, 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 constant rate. 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 contact.
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 dedicated British researcher living in the US state of Ohio.
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 University 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, an 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 at Leuschner Observatory and an older laser detector built by Werthimer.
 Gamma-ray bursts
Gamma-ray bursts (GRBs) are candidates for extraterrestrial communication. These high-energy bursts are observed about once per day and originate throughout the observable universe. SETI currently omits gamma ray frequencies in their monitoring and analysis because they are absorbed by the Earth's atmosphere and difficult to detect with ground-based receivers. In addition, the wide burst bandwidths pose a serious analysis challenge for modern digital signal processing systems. Still, the continued mysteries surrounding gamma-ray bursts have encouraged hypotheses invoking extraterrestrials. John A. Ball from the MIT Haystack Observatory suggests that an advanced civilization that has reached a technological singularity would be capable of transmitting a two-millisecond pulse encoding 1×1018 bits of information. This is "comparable to the estimated total information content of Earth's biosystem-genes and memes and including all libraries and computer media."
 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 (see Bracewell probe), 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. See Voyager Golden Record.
In recognition that any sufficiently advanced interstellar probe in the vicinity of Earth could easily monitor our terrestrial Internet, Invitation to ETI was established by Prof. Allen Tough in 1996, as a Web-based SETI experiment inviting such spacefaring probes to establish contact with humanity. The project's 100 Signatories includes prominent physical, biological, and social scientists, as well as artists, educators, entertainers, philosophers and futurists. Prof. H. Paul Shuch, executive director emeritus of The SETI League, serves as the project's Principal Investigator.
In a 2004 paper, C. Rose and G. Wright showed that inscribing a message in matter and transporting it to an interstellar destination can be enormously more energy efficient than communication using electromagnetic waves if delays larger than light transit time can be tolerated. That said, for simple messages such as "hello," radio SETI could be far more efficient . If energy requirement is used as a proxy for technical difficulty, then a solarcentric Search for Extraterrestrial Artifacts (SETA)  may be a useful supplement to 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 L1/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.
 Fermi paradox
Italian physicist Enrico Fermi suggested in the 1950s that if technologically advanced civilizations are common in the universe, then they should be detectable in one way or another. (According to those who were there, Fermi either asked "Where are they?" or "Where is everybody?")
The Fermi paradox can be stated more completely as follows:
The size and age of the universe incline us to believe that many technologically advanced civilizations must exist. However, this belief seems logically inconsistent with our lack of observational evidence to support it. Either (1) the initial assumption is incorrect and technologically advanced intelligent life is much rarer than we believe, or (2) our current observations are incomplete and we simply have not detected them yet, or (3) our search methodologies are flawed and we are not searching for the correct indicators.
Possible explanations for the paradox suggest, for example, that while simple life may well be abundant in the universe, intelligent life may be exceedingly rare. In 2000, Peter Ward, professor of Biology and of Earth and Space Sciences at the University of Washington authored a book claiming the Rare Earth hypothesis. In short, the theory claims that the emergence of complex multicellular life (metazoa) on Earth required an extremely unlikely combination of astrophysical and geological events and circumstances. This hypothesis contradicts the principle of mediocrity, which SETI takes as an assumption.
Another suggestion, made by astrophysicist Ray Norris in 2000 (and subsequently by Allen Tough) was that gamma-ray burst events are sufficiently frequent to sterilize vast swaths of galactic real-estate. This idea was subsequently popularized by physicist Arnon Dar, and described in the show Death Star on PBS Nova.
Science writer Timothy Ferris has posited that since galactic societies are most likely only transitory, an obvious solution is an interstellar communications network, or a type of library consisting mostly of automated systems. They would 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".
If such an Interstellar Internet exists, the hypothesis states, 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.
Although somewhat dated in terms of "information culture" arguments, not to mention the obvious technological problems of a system that could work effectively for billions of years and requires multiple lifeforms agreeing on certain basics of communications technologies, this hypothesis is actually testable (see below).
Another scenario is that it is not the civilisation itself that is transitory, rather it is the interest in exploring space that is transitory. A civilisation with technology that has advanced a mere century beyond ours might be capable of biological and cybernetic projects way beyond what we can envision today. The attraction of manufacturing their own lifeforms and living in virtual worlds of their own design may dwarf any of the inclinations to explore space that we would expect them to have.
An alternate hypothesis is that evolutionary pressures in many environments favor species which rapidly consume available resources once they achieve dominance. By the time they have achieved sufficient technology to come to the notice of other civilizations, they are already well on their way to exhausting the resources of their host planet. Therefore the time period available for communication is finite, and very small compared with planetary timescales. Once a planet's finite resources are exhausted no further species on that planet can develop advanced technology.
 Post detection disclosure protocol
The International Academy of Astronautics (IAA) has a long-standing SETI Permanent Study Group (SPSG, formerly called the IAA SETI Committee), which addresses matters of SETI science, technology, and international policy. The SPSG meets in conjunction with the International Astronautical Congress (IAC) held annually at different locations around the world, and sponsors two SETI Symposia at each IAC. In 2005, the IAA established the SETI: Post-Detection Science and Technology Taskgroup (Chairman, Professor Paul Davies) "to act as a Standing Committee to be available to be called on at any time to advise and consult on questions stemming from the discovery of a putative signal of extraterrestrial intelligent (ETI) origin." It will use, in part, the Rio Scale to evaluate the importance of releasing the information to the public.
When awarded the 2009 TED Prize SETI Institute's Jill Tarter outlined the organisation's "post detection protocol". During NASA's funding of the project, an administrator would be first informed with the intention of informing the United States executive government. The current protocol for SETI Institute is to first internally investigate the signal, seeking independent verification and confirmation. During the process, the organisation's private financiers would be secretly informed. Once a signal has been verified, a telegram would be sent via the Central Bureau for Astronomical Telegrams. Following this process, Tarter says that the organisation hold a press conference with the aim of broadcasting to the public. SETI Institute's Seth Shostak has claimed that knowledge of the discovery would likely leak as early as the verification process.
However the protocols mentioned apply only to radio SETI rather than for METI (Active SETI) The intention for METI is covered under the SETI charter "Declaration of Principles Concerning Sending Communications with Extraterrestrial Intelligence".
The SETI Institute does not officially recognise the Wow! signal as of extraterrestrial origin (as it was unable to be verified). The SETI Institute has also publicly denied that the candidate signal Radio source SHGb02+14a is of extraterrestrial origin though full details of the signal, such as its exact location have never been disclosed to the public. Although other volunteering projects such as Zooniverse credit users for discoveries, there is currently no crediting or early notification by SETI@Home following the discovery of a signal.
Some people, including Steven M. Greer, have expressed cynicism that the general public might not be informed in the event of a genuine discovery of extraterrestrial intelligence due to significant vested interests. Some, such as Bruce Jakosky have also argued that the official disclosure of extraterrestrial life may have far reaching and as yet undetermined implications for society, particularly for the world's religions.
As various SETI projects have progressed, some have criticized early claims by researchers as being too "euphoric" or "optimistic." For example, Peter Schenkel, while remaining a supporter of SETI projects, has written that "[i]n light of new findings and insights, it seems appropriate to put excessive euphoria to rest and to take a more down-to-earth view ... We should quietly admit that the early estimates — that there may be a million, a hundred thousand, or ten thousand advanced extraterrestrial civilizations in our galaxy — may no longer be tenable." Clive Trotman presents some sobering but realistic calculations emphasizing the timeframe dimension.
SETI has also 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.
In response, SETI advocates note, among other things, that the Drake Equation was never a hypothesis, and so never intended to be testable, nor to be "solved"; it was merely a clever representation of the agenda for the world's first scientific SETI meeting in 1961, and it serves as a tool in formulating testable hypotheses. Further, they note 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. Many detractors have not considered the collection and processing of data, the first order of business, and the refining of those data streams, in the case of SETI through algorithm optimization. To justify SETI projects does not require an acceptance of the Drake equation. Science proceeds through hypothesis. If one were to only take what was at face value observable, many scientific phenomena never would have been discovered.
The search for extraterrestrial intelligence is not an assertion that extraterrestrial intelligence exists or are visiting earth, and conflating the two can be seen as a straw man argument. There is an effort to distinguish the SETI projects from UFOlogy, the study of UFOs, which many consider to be pseudoscience. In Skeptical Inquirer, Mark Moldwin argued that the important differences between the two projects were the acceptance of SETI by the mainstream scientific community and that "[t]he methodology of SETI leads to useful scientific results even in the absence of discovery of alien life."
Some in the UFO community, such as nuclear physicist Stanton Friedman, say there is no basis for the search and it is therefore unscientific. Friedman has challenged SETI specialists to debate the issues, with no takers so far. Examples of objections to SETI include questioning energy requirements as well as why advanced civilizations would use radio.
 Active SETI
Active SETI, also known as messaging to extraterrestrial intelligence (METI), consists of sending signals into space in the hope that they will be picked up by an alien intelligence. Physicist Stephen Hawking, in his book A Brief History of Time, suggests that "alerting" extraterrestrial intelligences of our existence is foolhardy, citing mankind's history of treating his fellow man harshly in meetings of civilizations with a significant technology gap. He suggests, in view of this history, that we "lay low".
The concern over SETI was raised by the science journal Nature in an editorial in October 2006, which commented on a recent meeting of the International Academy of Astronautics SETI study group. The editor said, "It is not obvious that all extraterrestrial civilizations will be benign, or that contact with even a benign one would not have serious repercussions" (Nature Vol 443 12 Oct 06 p 606). Astronomer and science fiction author David Brin has expressed similar concerns.
Richard Carrigan, a particle physicist at the Fermi National Accelerator Laboratory near Chicago, Illinois, suggested that passive SETI could also be dangerous in the style of computer viruses. Computer security expert Bruce Schneier dismissed this possibility as a "bizarre movie-plot threat".
To lend a quantitative basis to discussions of the risks of transmitting deliberate messages from Earth, the SETI Permanent Study Group of the International Academy of Astronautics adopted in 2007 a new analytical tool, the San Marino Scale. Developed by Prof. Ivan Almar and Prof. H. Paul Shuch, the scale evaluates the significance of transmissions from Earth as a function of signal intensity and information content. Its adoption suggests that not all such transmissions are equal, and each must be evaluated separately before establishing blanket international policy regarding active SETI.
 Results of extraterrestrial contact for humanity
A part of the search for extraterrestrial intelligence involves the analysis of the implications of extraterrestrial contact for humanity, culturally, scientifically, technologically, and socially. Numerous contact scenarios have been created by scientists who are involved in the search for extraterrestrial intelligence in order to better contextualize what may eventually occur when humanity is contacted by an extraterrestrial species. These studies reveal that the result of extraterrestrial contact will be strongly governed by the benevolence or malevolence of an extraterrestrial civilization, how advanced it is technologically, and whether or not such a species sends robotic probes to contact humanity, as opposed to radio signals from a centralized source, as well as biological similarities and differences between humanity and the extraterrestrial species.
Malevolent civilizations, as speculated by Dr. Michio Kaku (2009) and Robert Freitas (1978) independently, may possess resources which can destroy humanity with little effort on the part of the extraterrestrial civilization and with little chance at resistance. Supporting the view of aggression, Seth Shostak, a senior astronomer at the SETI Institute, speculates that the finite quantity of resources in the galaxy and the "explorer" nature of any civilization would cause it to be aggressive in the same way that human explorers have been historically.
However, these views have been disputed by, among others, the late Carl Sagan, who speculated that any technologically advanced extraterrestrial civilization would be advanced ethically as well, and would not be aggressive, and that humanity would, due to the extraterrestrial civilization's technological prowess, reciprocate this non-aggression. Corroborating this view, a study conducted by James W. Deardorff states that only a small percentage of extraterrestrial civilizations may be aggressive. Such civilizations may use various methods in order to help humanity, such as immediate actions to avert catastrophe (i.e. creating computer glitches in nuclear-weapons systems on the eve of global nuclear conflagration) or long-term mitigation of risks which may destroy humanity before true technological cooperation.
Extraterrestrial civilizations may also have specific implications for various aspects of humanity against a backdrop of these broad contact scenarios. In particular, religion may undergo various degrees of change, from a reinterpretation of religious texts by theologians to accommodate the new discovery to a complete reinvention of religion, with extraterrestrials bringing humanity into an all-encompassing cosmic faith.
In addition to the religious problems which may arise, Michaud (2006) and Othman (2011) speculate that extraterrestrial contact may cause problems for global foreign relations, causing global political divisiveness over the involvement of radio astronomers worldwide in post-detection processes and over which bodies represent humanity as a whole in the wake of contact, as well as how, with what content, and whether a message should be sent in reply to what has been received from extraterrestrial intelligence. On a larger scale, Harrison (2000) has speculated that, as humanity builds relations with an extraterrestrial civilization, humanity may be given an invitation to a "Galactic Club" with numerous other civilizations.
As well as political implications, extraterrestrial contact may have scientific implications as well. Extraterrestrial civilizations may, as Harrison (2002) speculates, cause a profound technological and societal impact of a magnitude much greater than the Industrial Revolution of the late-eighteenth to nineteenth centuries, and extraterrestrial life in general may give us knowledge of extraterrestrial biochemistry. However, extraterrestrial civilizations, if they know terrestrial biology and its weaknesses and are malevolent, may conduct biological warfare by means of pathogens and invasive species completely unknown to the Earth previously.