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The Fermi Paradox is a physical paradox that was brought to light by a simple question posed by the physicist Enrico Fermi when speculating about the existence of technologically advanced civilizations within the observable universe, and exactly how common they would be.
The age of the universe and the vast number of stars in our galaxy alone suggest that extraterrestrial life should not be rare — a notion later supported by many estimates based on the Drake equation. However, Fermi is said (perhaps apocryphally) to have asked, "Where are they?" If there are a multitude of advanced extraterrestrial civilizations in our galaxy (the Milky Way) then why haven't we seen any evidence, such as probes, spacecraft or radio transmissions? The paradox can be stated as follows:
The belief that the universe contains many technologically advanced civilizations, combined with our lack of observational evidence to support that view, is inconsistent. Either this assumption is incorrect (and technologically advanced intelligent life is much rarer than we believe), our current observations are incomplete (and we simply have not detected them yet), or our search methodologies are flawed (we are not searching for the correct indicators).
Those who believe that the lack of such overt evidence is a conclusive argument for the non-existence of technologically advanced extraterrestrial civilization within communication distance of earth refer to this lack of evidence as the Fermi principle.
There have been attempts to resolve the Fermi Paradox by locating evidence of technologically advanced civilizations, or to respond to it by explaining how extraterrestrial civilizations could exist and yet remain undetected by us.
Theorizing about extraterrestrial life: The basis of the paradox
A great deal of effort has gone into developing scientific theories and possible models of extraterrestrial life. Since we have very little empirical evidence — and that which we do have is not yet exhaustive or comprehensive — we are in a position much like ancient Greek philosophers trying to form a cosmology a priori without direct physical evidence. Still, given what we know about physical cosmology, astronomy, biology and ecology we attempt to come to as reasonable a view as we can.
The questions around the idea of extraterrestrial life break down into several parts: does intelligent extraterrestrial life exist; and if it exists how common is it, how may we find it, and how may we communicate with it?
The various answers to these questions are the basis out of which the Fermi paradox arose.
Does it exist?
Pro: The argument by scale
Some speculate that if life is possible at all in the universe — and we are an example of it — then given the vast scale of the universe, and the age of the universe, it should not only be possible, but almost certain that there are large numbers of extraterrestrial civilizations somewhere in the Universe. This view is based on the mediocrity principle, which states that Earth is not particularly unique in the universe. It is one of trillions of worlds which are all subject to the same laws, effects, and chances. Even if intelligent life occurs once for every few billion of these "ordinary" planets and takes billions of years, they argue, there are potentially trillions of planets (or more) and the universe is billions of years old as well. The vast universal scales of time and space make even infinitesimal probabilities of any one planet producing intelligent life an almost certainty when large numbers of planets are considered together.
This is the argument and assumption that lay at the root of Fermi's question, and the classic response of "Where are they then?" the spark that has fueled the debate. Given the lack of evidence to the contrary, we cannot yet know for certain that we are not alone — at least in our part of the Milky Way galaxy.
It cannot be denied that intelligent life is possible within our universe — at least at this stage of its development — since we exist.
Con: The 'Rare Earth' hypothesis
The rare Earth hypothesis is a repudiation of mediocrity principle, and claims that Earth is an unusual world — maybe even unique — within the universe. While some believe that this is true for philosophical or religious reasons, most arguments based on the rare earth hypothesis are based on a statistical evaluation of Earth's position in our solar system and in our galaxy.
Most proponents of the rare Earth hypothesis, argue that multicellular life may be exceedingly rare in the universe because Earth-like planets are most likely very rare. They argue that many improbable coincidences converged to make complex life on Earth possible, such as,
- The solar system orbits the galactic center between the spiral arms, in an almost perfectly circular orbit, at an orbital velocity which matches the speed of the shock front formed in the intergalactic medium by the movements of the spiral arms. This orbit — which has lasted for the last 30 galactic orbits, almost the entire time that "higher life forms" have existed on Earth — shields the solar system from the high levels of radiation (which is thought to interfere with the development of life) within the spiral arms themselves, caused by numerous novae.
- The unlikely possession of such a relatively large Moon has stabilized the precession of the Earth's axis to a large degree — resulting in a relative uniformity of climate, which makes the development of "higher life forms" easier. The Moon has also likely protected Earth from numerous asteroidal collisions.
- The unlikely possession of such a relatively large Moon, and the internal tidal stresses it creates within the Earth, may have caused heating of the core, strengthening and prolonging the life of the "dynamos" that generate Earth's magnetic field. The possession of a strong magnetic field has helped prevent atmospheric damage from solar wind and shielded life on Earth from gene-damaging high-energy solar radiation.
- The placement of Jupiter in our solar system acts as a gravitational "broom", sweeping up debris in the inner solar system, and reducing the frequency of impact events on the Earth. Such an advantageous placement of a "protector planet" is probably uncommon.
Critics of the Rare Earth hypothesis admit that the probability of the specific conditions on Earth being widely replicated is low. However they point out that complex life may not require exclusively Earth-like conditions in order to evolve (see Evolving the Alien: The Science of Extraterrestrial Life and Alternative biochemistry). There may be other, more probable and wide-spread, conditions which will allow the development of other types of intelligent life.
Pro: The anthropic principle
Various formulations of the anthropic principle have been applied to speculation about the probability of the existence of alien civilizations.
The Anthropic Principle notes that the universe seems uniquely suited to the development of human intelligence, i.e. that any variation in any one of a myriad of universal constants would make the development of intelligent life more difficult. Thus, human intelligence has a "privileged" position in the universe.
Various formulations of the principle disagree on whether this principle is descriptive (if a condition must exist in the universe for human life to arise, then the universe must already meet that condition, as we are here), or teleological (the universe has to be this way, or it was designed to be this way, for the express purpose of creating human intelligence).
Ironically, the Anthropic Principle can be — and has been — used by both opponents and proponents of the idea of the existence of extraterrestrial intelligent life.
Opponents of the idea point out that the universe seems ideally suited for human life. Other, alien, forms of life would not have the same unique advantages as humans, and therefore the probability that they exist is low. The conditions required for human life are rare (see the Rare Earth hypothesis), and the likelihood of other forms of life are low.
Proponents of the idea state that the universe is ideally suited for intelligent, not just human life, and as such, we can expect to see many forms of intelligent life in the universe.
There are those on both sides of the debate that deny that the Anthropic Principle is a meaningful argument at all.
Con: The lack of extraterrestrial colonization evidence
Adherents to the Fermi principle argue that from what we know about life's ability to overcome scarcity and colonize new habitats on our own planet, we can reasonably assume that life elsewhere will follow similar principles. Thus, they reason that any advanced civilization will almost certainly try to seek out new resources and colonize first their solar system, and then surrounding solar systems. Several writers have tried to estimate how fast an alien civilization might spread through the galaxy. There have been estimates of anywhere from 5 million to 50 million years to colonize the entire galaxy; a relatively small amount of time on a geological scale, let alone a cosmological one. The very fact that we see no evidence on Earth, or anywhere else in the solar system of attempted alien colonization is presented as an argument for the rarity of intelligent life in our galaxy.
Even if colonization is impractical or undesirable to an alien civilization, large scale exploration of the galaxy is still possible with minor investment in energy and resources. An alien civilization might dispatch a small fleet of self-replicating Von Neumann probes to explore the galaxy (or a malevolent civilization might dispatch a fleet of Berserker probes (after Fred Saberhagen's Berserker novels). Given the age of the universe, if intelligent life is common, it seems likely that there would be at least one civilization which would have dispatched such a machine. Given that it is estimated that such machines could spread through the entire galaxy in as little as half a million years, it seems likely that we should have been visited by such a craft in the past, or see evidence of them today.
Proponents of the existence of extraterrestrial life counter that it is quite possible that we have been visited by such a machine, or there is evidence out there that such machines exist; we simply haven't seen it yet. Arthur C. Clarke's story "The Sentinel" and David Brin's story "Lungfish" explore such possible scenarios. Another possibility is that we have only recently become detectable to a Bracewell probe (through our radio transmissions), should any have been launched by an alien civilization, and perhaps too little time has elapsed since then for contact to have been established.
How common is it?: the Drake equation
In an attempt to find a systematic means to estimate the number of extraterrestrial civilizations that we might reasonably come in contact with, Dr. Frank Drake formulated The Drake equation. While it was formulated after the objections raised by Enrico Fermi, Drake's equation has become a common and respected means of estimating the frequency of occurrence of interstellar civilizations.
The values assigned to the various "factors" of the equation are only estimates or educated guesses, however the range of "reasonable" estimates seems to imply that contact with extraterrestrial life might not be impossible. Several scientists have published different estimated values for the Drake equation's factors, with quite varied results. The estimates published by Dr. Carl Sagan, for example, imply that intelligent life should be common in the Universe, and thus easy to detect. Other sets of estimates have placed the probability of the existence of other civilizations in our galaxy close to zero.
Critics of the Drake equation claim that since we cannot yet determine the variables of the equation with any real confidence, we cannot determine the number of extraterrestrial civilizations based on it. Instead, we must rely on empirical data, which we are only now beginning to collect and analyze in a significant manner; only with further observation can we hope to derive meaningful values for the Drake equation factors.
Trying to resolve the paradox empirically: What we look for, and how we look
One obvious way to resolve the Fermi paradox would be to find conclusive evidence of extraterrestrial intelligence. Various efforts to find such evidence have been made since 1960, and several are ongoing. There has also been a lot of conjecture and theorizing about the best ways to conduct such searches.
The limits of searching
It must be remembered that as we do not have interstellar travel capability, such searches are being carried out at great distances and rely on careful analysis of very subtle evidence. We are therefore limited to detecting civilizations which alter their environment in a detectable way, or produce effects that are detectable at a distance — such as radio emissions. Non-technological civilizations are very unlikely to be detectable by us in the near future.
The dangers of anthropomorphism
One of the difficulties with the search for extraterrestrial intelligence is avoiding an overly anthropomorphic viewpoint. Our first inclination as humans is to look for evidence of the type of activities that humans have performed, or likely would perform if we had more advanced technology. In fact, we cannot know with certainty how intelligent aliens might think or behave. Thus aliens may be broadcasting evidence of their existence that we are not looking for, or they may not do things that we would expect any technological civilization to do and thus not broadcast the types of evidence we expect. We can speculate on possible differences between ourselves and alien species, and try to take these into consideration, but all such efforts may be flawed.
What we might look for
Since the development of radio technology, we have been broadcasting signals into space, both accidentally and deliberately. To a nearby observer — or one with sufficiently sensitive equipment — our solar system would appear to have unusually intense radio wave emissions for an otherwise unremarkable main sequence star. The increased intensity is caused by broadcasters using this part of the electromagnetic spectrum for television and other telecommunication. Our solar system would appear unusual to an alien civilization — a star system emitting anomalous radio energy with no apparent natural cause — and would likely draw the attention of any alien civilization that detected it.
Keeping in mind the warning above about anthropomorphism, it would seem that radio technology is a simple and natural technology for any technological species to develop. Because it creates effects that can theoretically be detected over interstellar distances, a search for alien radio emissions — whether the "accidental" byproducts of a radio capable civilization, or deliberate radio broadcasts (like CETI's Arecibo message) — may yield evidence of extraterrestrial civilizations. Careful analysis of radio emissions from space, searching for signals that cannot be attributed to natural processes, may lead to detection of such alien civilizations.
Such a search is not as easy as might be thought. It's a popular notion that broadband, omnidirectional transmissions such as radio and television shows could be detected over a distance of light years. However, it has been estimated  that with a radio telescope as sensitive as the Arecibo Observatory, Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light years (the closest star to our sun is Proxima Centauri at 4.3 light years).
The detectability of signals is vastly improved if the signal energy is focused in either a narrow range of frequencies (Narrowband transmissions), and/or concentrated directionally at a specific part of the sky. Such signals can be detected at ranges of hundreds to tens of thousands of light-years distance . This increase in range is offset by a requirement for more finely tuned receivers (listening to the specific bandwidth the signal is in), and/or the receiver existing in the direction that a more narrowly focused beam is sent.
If we expect to detect alien civilizations through their radio emissions, we either need to create much more sensitive instruments, hope that the broadband radio emissions of alien radio technology are much stronger than our own, hope that one of our SETI programs are listening to the correct frequencies from the right regions of space, or hope that aliens are sending focused transmissions such as the Arecibo message in our general direction.
Direct planetary observation
Relatively recent developments in astronomical instruments, and
methods of analyzing astronomical data, have led to the detection of planets outside our solar system.
While this is a new field in astronomy, it is hoped that we may
eventually be able to find planets which are likely to be able to
support some form of life, or perhaps even find direct observational
evidence for the existence of life such as the absorption spectrum of chlorophyll
in light filtered through a planet's atmosphere. Such location of
potential or actual "life bearing" planets would help narrow the search
for intelligent life, and perhaps even find direct observational evidence of an alien technological civilization (see right).
Probes, colonies, and other artifacts
As already noted, if we assume that technologically advanced extraterrestrial life exists (or has existed) in our galaxy, then given the age of the universe, and the relative rapidity at which dispersion of intelligent life can occur — even at sub-light speeds — it may be that some day we will find evidence of alien colonization attempts. While it is clear that there are no obvious alien colonies nearby, perhaps we should try to find evidence of such colonization behavior, both within our solar system and abroad.
Additionally, we might look for evidence of "unbeinged" exploration in the form of probes and information gathering devices. Some theoretical exploration techniques such as the Von Neumann probe could exhaustively explore a galaxy the size of the Milky Way in as little as half a million years, with relatively little investment in materials and energy compared to the results. If even a single civilization in our galaxy attempt this, such probes would spread throughout the entire galaxy. We might eventually find evidence of such probes in our solar system — perhaps in the asteroids where raw materials would be plentiful and easily accessed.
Another possibility for contact with an alien probe — one that would be trying to find us — is an alien Bracewell probe . Such a device would be an autonomous space probe whose purpose is to seek out and communicate with alien civilizations (as opposed to Von Neumann probes, which are usually described as purely exploratory). These were proposed as an alternative to carrying a slow speed-of-light dialog between vastly distant neighbors. Rather than contending with the long delays a radio dialog would suffer, a probe housing an artificial intelligence would seek out an alien civilization to carry on a close range dialog with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilization at light speed, but an information-gathering dialog could be conducted in real time. There have been attempts to scan for such probes lying dormant within our solar system by scientists Robert Freitas and Francisco Valdes .
Advanced stellar scale artifacts
Dr. Freeman Dyson observed that every developing human civilization constantly increases its energy consumption. Theoretically, a civilization of sufficient age would need all the energy produced by its sun. The Dyson Sphere was the thought experiment solution that he derived. A Dyson sphere is a shell or cloud of objects (there are several variants of the Dyson sphere) enclosing a star to harness as much of the radiant energy of that star as possible. Were such a feat of astroengineering accomplished by an advanced civilization, it would consume energy from the star and reradiate a portion into space as waste energy. A star surrounded by such an object (or objects) would thus emit a distinctive altered spectrum; it would be at least partly a black body spectrum without the strong emission lines of a natural stellar atmosphere. Dyson himself speculated that advanced alien civilizations might be detected by examining the spectra of stars, searching for such an altered spectrum.
However, it may be that such constructs are more difficult to detect than originally thought. Dyson spheres may have different emission spectra depending on the desired internal environment. For example, life based on high-temperature reactions may require high internal temperatures for their Dyson sphere that would result in "waste radiation" in the visible spectrum, not the infrared. Additionally, variants on the Dyson sphere have been proposed that might be almost undetectable: the multiple concentric spheres of a Matrioshka Brain — each one radiating less energy per area than the next smallest one, with the outermost sphere radiating at close to the background radiation — would be difficult to observe from any great distance.
It is also possible that civilizations may find alternate solutions to their energy demands, using technology that is beyond our current theoretical understanding, or engineering capability to realize. Such a civilization would not need to construct a Dyson sphere.
Other stellar-scale artifacts, that might be created by civilizations high on the Kardashev scale (see Ringworlds, Alderson disks, Shkadov thrusters, and Stellar engines) might also be directly observable at interstellar distances.
How we've been looking
Radio and signal emissions: SETI
Main article: SETI.
Assumptions behind SETI searches
Given the sheer size of the radio search needed to look for signals from extraterrestrial civilizations — searching an entire galaxy is a big job — and the limited amount of resources committed to SETI, most SETI programs have tried to narrow their search by making some assumptions about alien life.
- SETI assumes that most alien life will occur around planets that orbit main sequence stars — and concentrates on Sun like stars in particular .
- SETI assumes that extraterrestrial intelligent life will most likely be based on carbon, and will most likely be found on water bearing worlds.
- Many SETI searches assume that extraterrestrial civilizations will be broadcasting a deliberate signal (like our own Arecibo message), in order to be found. Project SENTINAL went so far as to assume that a signal might be beamed directly and deliberately at our own Sun.
- SETI assumes that such signals will be a sine wave based carrier wave with no complex modulation, and using a narrow band transmission.
Essentially, SETI is only searching for simple deliberate signals, from carbon/water based life-forms, living on Earth-like planets, orbiting Sun-like stars.
SETI programs are doing what they can given the resources allocated for such a large problem, but these assumptions might very well be incorrect. It may be that there are civilizations in space that do not match SETI's profile, and which we will not detect until the form of our search changes.
A brief history of SETI searches
Radio and observational data have for several decades been collected and analyzed by such projects as Project Ozma, the Search for Extraterrestrial Intelligence (SETI), and the various projects searching for extrasolar planets. So far the SETI data show no known main sequence stars with unusually bright radio emissions.
In 1960, Cornell University astronomer Frank Drake (creator of the Drake equation) 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.
Project SERENDIP is an ongoing SETI program, which takes advantage of ongoing "mainstream" radio telescope observations as a "piggyback" program. Rather than having it's own observation program, SERENDIP analyzes deep space radio telescope data that it obtains from other astronomers and examines it for extraterrestrial signals. The program has gone through several upgrades in techniques and equipment and is now in its 4th incarnation as SERENDIP IV, where data from the Arecibo Observatory is analyzed through the SETI@home program.
Project Sentinel, META and BETA
Running 1983 to 1985, Project Sentinel was the first SETI high-resolution "all sky" scan. Over a period of 200 days, Sentinel scanned the northern hemisphere of the sky examining a region of the electromagnetic spectrum within 2 kHz of the 21 cm neutral hydrogen band. It had several technical limitations: narrow scanning bandwidth, slow response time to re-examine "candidate signals", and it was only capable of detecting signals deliberately targeted at our Sun. Sentinel was superseded by the META and BETA programs in 1985 and 1995, respectively.
MOP and Project Phoenix
In 1992, the U.S. government funded an operational SETI program in the form of the NASA "Microwave Observing Program (MOP)". This program was intended to be two-fold: a "Targeted Search" of 800 specific nearby stars, and a less-rigorous "Sky Survey" to scan the sky. The program was ridiculed in Congress, and was canceled after a year.
An independently funded program called Project Phoenix was begun to pick up the work started by MOP. Project Phoenix undertook only the targeted search, looking at 800 stars within 200 light years. The search was performed using the Arecibo Observatory and has been called the most sensitive SETI search to date. It analyzed radio frequency bands narrow as 1 Hz in the range between 1,000 and 3,000 MHz. On completion of the search, project leader Peter Backus remarked "[W]e live in a quiet neighborhood".
ATA and the ongoing search
SETIs search programs are ongoing, and expanding. As well as the "piggyback program" of SERENDIP IV, SETI is planning to use the Allen Telescope Array — currently under construction — for its most comprehensive deep space survey yet: a survey of 1,000,000 stars within 1,000 light years of Earth, for medium-strength signals, plus a survey of the 4×1010 stars of the inner Galactic Plane from 1.42 to 1.72 GHz for very powerful transmitters. Observations began in late 2005, even though the telescope is still undergoing construction and expansion. The ATA holds the current best hope for future artificial extraterrestrial radio source detection.
Direct planetary observation: the exoplanet hunters
Detection and classification of exoplanets did not come about as part of the search for extraterrestrial life. Rather it has come out of mainstream astronomy as astronomical instruments and methods of data analysis have improved to the point where it is now possible to isolate the effects of planetary bodies, and to infer their existence. Exoplanet detection and cataloging is a very new sub-discipline of astronomy, with the first published paper claiming to have discovered an exoplanet released in 1989.
Exoplanets have not yet been directly observed, only their effects have been noted. At present we can only estimate the size and orbit of exoplanets, and, combined with the stellar classification of their sun, we can estimate the planets' probable range of environments.
From the perspective of the search for extraterrestrial life, the means of detecting exoplanets is not likely to be useful yet; the types and sizes of planets being detected are not those which have high probability of being able to support life. As of 2005 only a handful of possible terrestrial type planets have been detected. So far, only two such planets have been located in orbit of a main sequence stars: Gliese 876 d and OGLE-2005-BLG-390Lb. Neither seem likely to be able to support life as we know it. It is hoped that refinement of instruments and techniques of data analysis will push the envelope of exoplanet detection to make more information available, and to increase the probability of finding more Earth-like worlds.
Alien constructs: Alien artifacts and the Dyson sphere survey
We have only been exploring our own solar system for a handful of decades, and we have only explored a minuscule percentage of it. While we have not uncovered any evidence that our solar system has ever been visited by alien colonists, or probes, we cannot yet rule out the existence of such evidence. Detailed exploration of areas of the solar system where resources would be plentiful and easily accessed — such as the asteroids, the Kuiper belt, the Oort cloud and the various planetary ring systems — may yet uncover evidence of alien exploration. Unfortunately, these regions where evidence of self-replicating alien probes is more likely to be found are also massive. Finding such evidence, if it exists, may be very difficult.
There have been preliminary efforts to do just this, however. The SETA (Search for Extraterrestrial Artifacts) and SETV (Search for Extraterrestrial Visitation) projects have attempted to locate such evidence within our own solar system — although it must be admitted that many of the projects that fall under this umbrella are considered "fringe" science by many astronomers. There have also been attempts to signal, attract, or activate Bracewell probes in our local vicinity.
It may be that should we find alien artifacts, even here on Earth, we may not recognize them as such. The products of an alien mind and an advanced alien technology might not be perceptible to us, or recognizable as artificial constructs. Exploratory devices in the form of bio engineered life forms created through synthetic biology would presumably disintegrate after their "demise" leaving us no evidence; an alien information gathering system based on molecular nanotechnology could theoretically be swarming all around us at this very moment, completely undetected by us; and Clarke's third law tells us that an alien civilization well in advance of our own might have means of investigation that we cannot even conceive of yet.
On the other end of the spectrum, there have been some preliminary attempts to find evidence of the existence of Dyson spheres or other large Type-II or Type-III Kardashev scale artifacts that would alter the spectra of their core stars. Fermilab has an ongoing program to find Dyson spheres, but such searches are as of yet preliminary and incomplete.
What we've found
So far, no unambiguous evidence of extraterrestrial civilizations, or even extraterrestrial life, has been found.
The various radio telescope SETI search programs have not discovered any unambiguous extraterrestrial signals, although there have been several candidate signals. On August 15, 1977 the "Wow! signal" was picked up by The Big Ear radio telescope. However it lasted for only 72 seconds, and has not been repeated. In 2003, Radio source SHGb02+14a was isolated by SETI@home analysis, although it has largely been discounted by further study. In neither case can the candidate signal be said to be unambiguously from an extraterrestrial intelligence.
So far, all extrasolar planets that have been detected appear to be harsh environments for advanced life-forms. However, this should not be taken as an argument against the existence of hospitable planets, and thus against the existence of complex extraterrestrial life. It must be remembered that current means of detecting exoplanets work best with very massive planets on the order of Jupiter or larger. Only a few terrestrial planets have yet been detected.
So far, our searches for evidence of technologically advanced extraterrestrial civilization have not resulted in incontestable positive evidence.
Trying to resolve the paradox theoretically: Explaining the silence
Despite the belief by many that the existence of an alien technological civilization is not impossible, as of 2005 there has been no unambiguous evidence of intelligent extraterrestrial life. While many believe this is a vindication of the idea that we are alone in our galaxy (if not the universe) many theoreticians have proposed alternate explanations as to why the skies appear to be silent.
They do not exist...
The simplest explanation is that we are alone in the galaxy. Several theories along these lines have been proposed, explaining why intelligent life might be either very rare, or very short lived.
...and they never did.
Those who believe that extraterrestrial intelligent life does not exist in our galaxy argue that the conditions needed for life — or at least complex life — to evolve are rare or even unique to Earth (see the Rare Earth hypothesis above). While some have pointed out that complex life may evolve through other mechanisms than those found specifically here on Earth, the fact that in the extremely long history of life on the Earth only one species has developed a civilization to the point of being capable of space flight and radio technology seems to lead more credence to the idea of technologically advanced civilization being a rare commodity in the universe.
While the lack of evidence for extraterrestrial intelligence is not conclusive proof of their non-existence, those that believe that we are alone have yet to be disproven.
...because an inhospitable universe destroys complex intelligent life.
Another possibility is that life can and does arise elsewhere, but events such as ice ages, impact events, or other catastrophic planetary events prevent complex life forms from evolving. Even if conditions for the development of life are not unique to Earth, it may be that on most worlds such events routinely and periodically destroy such life. Even if a "benign local environment" might exist on some world long enough for intelligent life to finally arise despite the odds, such life might also be exterminated by cosmological events (such as supernovae, or gamma ray bursts) suddenly sterilizing previously hospitable regions of space. 
...because it is the nature of intelligent life to destroy itself.
Technological civilizations may usually or invariably destroy themselves (via nuclear war, biological warfare, nanotechnological catastrophe, or in a Malthusian catastrophe after destroying their planet's ecosphere) before or shortly after developing radio or space flight technology. This general theme is explored in The Mote in God's Eye by Larry Niven and Jerry Pournelle, which has as its central premise a civilization that overtaxes its resource base and cyclically self-destructs, but which tries to preserve its culture from one cycle to the next.
It would be anthropocentric to suggest that humanity is immune from such a fate. Therefore it is possible that we ourselves will not exist long enough to encounter alien life. Indeed, there are probabilistic arguments which suggest that our end may occur sooner rather than later. See Doomsday argument.
Such argument might be extended to intelligent life elsewhere. Intelligent life on Earth evolved as a result of the competition for scarce resources. The evolutionary psychology that developed during this struggle has left its mark on our characters, and left human beings subject to involuntary, instinctual drives to consume resources and to breed. It can be argued that this is the very aspect of our nature that lead us to develop a technological society — that our technology is a result of our quest to access more resources (and utilize them effectively) in order that we can continue to breed. If this is true, then it seems likely that either intelligent life on other planets has evolved subject to similar constraints — and they have developed a technology — or they are not subject to such constraints and do not have the drive to develop a technological civilization. In the former case their long term viability — and ours — may be in doubt. In the latter case, they may be difficult or impossible to detect. Either way, it seems possible that the evolutionary character of life would make contact between intelligence less likely than originally thought.
...because it is the nature of intelligent life to destroy others.
Science fiction authors have proposed another possible explanation — that someone, or something, is destroying intelligent life in the universe as fast as it is created. This theme can be found in novels such as Frederik Pohl's Heechee novels, Fred Saberhagen's Berserker novels, Alastair Reynolds's Revelation Space novels, Greg Bear's novel The Forge of God, Ian Douglas's series The Heritage Trilogy, K.A. Applegate's novel The Ellimist Chronicles and Jack McDevitt's novel The Engines of God. This explanation is also featured in Marvel Comics with the being Galactus.
If several intelligent species arise in a galaxy it is possible that some may view other civilizations as a threat, or as competition. It is possible that they may pursue a policy of violent extermination of other civilizations. Nor is this an unrealistic goal. The concept of self replicating spacecraft need not be limited to exploration or communication, but can be applied to aggression (see Berserker probe). Even if such a civilization were to fall, or go extinct, such machines could outlive their creators, destroying civilizations far into the future.
Fortunately, there are good arguments for such an approach not being used by any civilization in our galaxy for several billion years (see Berserker probe). However, this does not rule out all other aggressive acts and methods by an aggressive civilization. It may be that intelligent life tends to suppress other intelligent life, and as such, becomes a rare commodity in a galaxy.
...because God created humans alone.
Although not generally considered a testable scientific explanation, this theory is one of the lines of thought contributing the Rare Earth Hypothesis. Several schools of thought within the Judeo-Cristian and Islamic religions maintain that that man is uniquely special in the universe, and thus could be viewed as the only physical creatures in the universe with intelligence (many religions do include non-physical created intelligences, for example angels, jinn and demons).
They do exist, but...
It may be that technological extraterrestrial civilizations may exist, but that we do not or can not communicate with them because of technical constraints, or because their nature is simply too alien for perception of them as intelligent life, or for meaningful communication. Perhaps our belief that we can communicate with an alien civilization is unrealistic anthropormorphization of alien life.
... we cannot communicate for the technical reason that ...
... we are too far apart in space to communicate.
It may be that technologically capable alien civilizations exist, but are rare enough such that there is a high probablility that they are simply too far apart for meaningful two-way communication. If two civilizations are separated by several thousand light years, it is very possible that one, or the other, or both cultures may become extinct before meaningful dialog can be established. We may be able to detect their existence, but we may find it impossible to communicate with them. This problem might be ameliorated somewhat if contact/communication is made through a Bracewell probe. In this case at least one partner in the exchange is guaranteed to obtain meaningful information.
... we are too far apart in time to communicate.
If we look at the length of time that intelligent life has existed on Earth — or is likely to exist — the "window of opportunity" for detection or contact might be quite small. Intelligent civilizations may arise, and fall, periodically thoughout our galaxy, but this may be such a rare event that the odds of two or more such civilizations existing at the same time may be low. There may have been intelligent civilizations in our galaxy before us, and there may be intelligent civilizations after our race is extinct, but it is possible that we are the only intelligent civilization in existence now.
... it is too expensive to spread physically throughout the galaxy.
Many assumptions on the ability of an alien culture to colonize other stars, let alone come near the solar system, are based on the idea that interstellar travel is technologically feasible. While our current understanding of physics rules out the possibility of faster than light travel, we believe that there are no major theoretical barriers to the construction of "slow" interstellar ships (see Project Daedalus, Project Orion, and Project Longshot). This idea underlies the concept of the Von Neumann probe and the Bracewell probe as evidence of extraterrestrial intelligence, or even as means of communication. It is also important to the idea of alien colonization attempts and the possible evidence that such attempts may furnish us with.
It is possible, however, that we lack sufficient theoretical knowledge to gauge the feasibility and/or costs of such ventures. Perhaps there are theoretical barriers to such efforts that we do not yet understand. Perhaps the cost of materials and energy for such ventures are so high as to make it unlikely that any civilization could afford to attempt it.
In such a case, it would be unlikely that any physical evidence of extraterrestrial life would ever be found.
If we conceive of a hypothetical scenerio where physical evidence is ruled out, and — as in the above possible technical limits — there are vast separating distances in time and space, communication with, or detection of, extraterrestrial civilization would be very unlikely, even if such civilizations exist.
... we have not been searching long enough.
Some commentators (such as British science fiction author Stephen Baxter) have pointed out that humanity's ability to detect and comprehend intelligent extraterrestrial life has existed for only a very brief period — by the early 21st century, perhaps only a century at best — and that Homo sapiens itself is a recent species, given the apparent size and age of the universe.
According to this view, humanity has simply not been around sufficiently long to encounter alien life. For example, one million years ago — a relatively brief period in cosmological terms — there would have been no humans for alien emissaries to meet, as modern humans only appeared about 200,000 years ago. For each step back further in time, there would arguably have been increasingly fewer indications to such emissaries that intelligent life as we know it would develop on Earth. In a large and already ancient universe, a space-faring alien species may well have had many other more promising worlds to visit and revisit.
Even if alien emissaries visited in more recent times, they may have been misinterpreted by early human cultures as supernatural entities.
This hypothesis depends upon all visiting civilizations ultimately stagnating or dying out, rather than expanding. However, this cannot be ruled out, since the whole period of modern human existence to date (about 200,000 years) is a very brief period on a cosmological scale, a position which changes little even if our species survives for many more hundreds of thousand of years. Even if intelligent life undergoes a continuous cycle of birth, extinction and rebirth across the universe, civilizations may simply be too far apart in either time or space to actually meet.
... most people have yet to see them, even though they are here on Earth, because...
There is a growing "fringe" belief (or at least it is considered "fringe" by the majority of the scientific community) that believes that intelligent alien life forms not only exist, but are already present here on earth, but we do not detect them either because they do not wish it, or we refuse to observe them.
... they are cloaking themselves from us.
Some believe that it is not unreasonable that a life form intelligent enough to travel to our planet would also be sufficiently intelligent to exist here undetected. In this view, the aliens have arrived and are observing us, but are debating when or whether to establish contact. Such observation could be conducted in a number of ways that would be very difficult to detect, for example via molecular nanotechnology on Earth, or passive monitoring from elsewhere.
... we refuse to see the evidence.
Many UFO researchers and watchers argue that society as a whole is unfairly biased against claims of alien abduction, sightings, and encounters, and as a result may not be fully receptive to claims of proof that aliens are visiting our planet. Others use complex conspiracy theories to allege that evidence of alien visits is being concealed from the public by political elites who seek to hide the true extent of contact between aliens and humans. Scenarios such as these have been depicted in popular culture for decades, with recent favorites being The X-files television series, and the eponymous Men in Black, named for the hypothetical government agents who suppress knowledge of alien contact.
... civilizations only broadcast detectable radio signals for a brief period of time...
... because of evolving technology.
It may be that alien civilizations might not be detectable though their radio emissions after all. As seen earlier, use of radio technology is probably very difficult to detect even at the peak of its use — unless it is used for deliberate high-energy messages or beacons. However, radio itself may be a technology that civilizations outgrow. The "fiber optic objection" notes that the use of broadcast technologies like radio for the transmission of information are fundamentally wasteful of energy: broadcasts are radiated in all directions evenly, and a large amount of power is needed for a transmitter to send messages any significant distance. Human technology is currently moving away from broadcast for long-distance communication and replacing it with wires, optical fibers, and focused electromagnetic technologies like aimed narrow-beam radio, microwave, or laser transmission. Most recent technologies that employ broadcasting, such as mobile phones and Wi-Fi networks, use very short-range transmitters to communicate with fixed stations that are themselves connected by wires or narrow beams. It is argued that this trend may make Earth itself even more difficult to detect (remember that the plausible "range of detection" of our current telecommunication broadcasts is only 0.3 light years) within a few decades. It seems plausible that many civilizations would only be detectable for a short period of time between the discovery of radio and the switch to more efficient technologies, even if we use extremely sensitive detectors.
... because of depleted energy resources.
The problem of peak oil has raised another possibility for short-lived radio technology within a civilization. It has been pointed out that our civilization has been capable of interstellar radio communication for only a few decades. It has been argued that we are running out of fossil fuels, and it may only be a few more decades before energy becomes too expensive, and the necessary electronics and computers too difficult to manufacture, for us to continue the search. If the same conditions regarding energy supplies hold true for other civilizations, then it may be that a given civilization would only be able to transmit for a very short time. Therefore, unless two civilizations happen to be near each other and develop the ability to communicate at the same time — which is statistically unlikely — it would be virtually impossible for anyone to talk to anyone else.
Critics of this idea point out that an energy consuming civilization is not dependant solely on fossil fuels. Alternate energy sources exist — such as solar power which has the potential to generate more energy than current energy consumption. For depletion of fossil fuels to end the "technological phase" of a civilization some form of technological regression would have to consistently occur, preventing the exploitation of renewable energy sources.
... they choose not to communicate, or are too alien.
It is possible that the belief that alien races would communicate with us is a fallacy, and that alien civilizations may not wish to communicate, for reasons that we can only speculate about, even if they have the technical ability. Possible reasons that have been proposed include,
- They may not wish to communicate for ethical reasons, like the desire to encourage our independent development, or perhaps Earth is under quarantine for some reason, possibly because of mankind's warlike tendencies. (see below).
- They may wish to avoid detection and possible destruction at the hands of other civilizations, or to avoid calling malicious attention to us (see: Berserker probe and above section).
- Their psychologies may simply be too different to communicate with, and realizing this, they do not make the attempt, or the very concept of communication with other species is one which they cannot even conceive. (see: They Are Made Of Meat)
- An intelligence could experience the universe in a manner that would make their perception of us (or communication with us) difficult, if not impossible. For example, if an alien life form had a metabolic rate far slower than our own (such as uttering one word every 12 hours or so, having a life span of millions of years), they might not consider us a life form, or even observe us at all for that matter. We would also have problems perceiving or communicating with them. See also technological singularity below.
Whatever the reason, it is possible that our enthusiasm and desire to communicate with other species may not be shared by alien civilizations.
... Earth is purposely isolated (The zoo hypothesis).
A particular reason that alien civilizations may choose not to communicate with us in the so-called Zoo hypothesis: the idea that Earth is being monitored by advanced civilizations for study, or is being preserved in isolation for ethical reasons. This idea is similar to the Prime Directive of the "United Federation of Planets" in the fictional Star Trek television series. This possibility has caused some to speculate that perhaps humanity needs to pass a certain ethical, technological or social boundary before we will be allowed to make contact with existing advanced alien civilizations.
This idea is at least possible if there is a single alien civilization within contact range, or there is a homogenous culture or law amongst alien civilizations which dictates that Earth be thus shielded from contact; this is the concept behind the "Prime Directive" that makes it plausible within its fictional universe. If there is a plurality of cultures, then this theory starts to break down under the uniformity of motive flaw: all it takes is single culture or civilization which decides to act contrary to such a law for it to break down, and the probability of such a violation increases with the number of civilizations which do not subscribe to this law.
... they have experienced a technological singularity.
Another possibility is that technological civilizations invariably experience a technological singularity. This would possibly preclude communication for several reasons. Possibilities include,
- The beings of the civilization would have altered in such a drastic way as to make them alien enough that communication is no longer possible. I.e. no meaningful communication might be possible between a human and the alien equivalent of a transhuman intelligence any more than we can have meaningful communication with — or attempt to talk to — ants.
- Such beings may divest themselves of physical form, create massive artificial virtual environments (see Matrioshka Brain), transfer themselves into these environments through mind transfer, and exist totally within virtual worlds, ignoring the external physical universe (this theme is explored by Greg Bear in his novel Blood Music, and in Charles Stross's Accelerando cycle. It is also mentioned briefly in Greg Bear's Anvil of Stars).
- The new form of the intelligence of a post-singularity civilization would simply require more information exchange than is possible with theoretical means of interstellar communication in order to have meaningful communication between intelligences. Because of this, they do not try.
Whatever the reasons, it seems unlikely that pre-singularity and post-singularity civilizations could have meaningful communication. If most civilizations experience a technological singularity soon after developing a technological civilization, then the "window of time" with which to communicate with a pre-singularity civilization would be brief, and the chances of us communicating with them within that time frame, low.
It may be that most older alien civilizations are post-singularity civilizations, with which we will not be able to communicate until and unless we experience our own singularity.
However, the possibility, probability, and the effects of a technological singularity have not even been resolved for human civilization yet. It is impossible to judge with any certainty the likelihood of alien civilizations experiencing a singularity.
... but we do not understand them, even though they are communicating, because...
Another series of views, some of which tend to be disregarded by contemporary science, consider that alien entities have been communicating with humans throughout history, but for any number of reasons we are unable to scientifically detect these attempts; or that the accounts of communication that have been reported are generally dismissed by scientists for sociological reasons.
... we are not listening properly.
As noted above, there are some assumptions that underly the SETI search programs. It may be that any number of these may be incorrect, and are causing us to miss signals that are present.
For example, the radio searches to date would completely miss highly compressed data streams (which would be almost indistinguishable from "white noise" to anyone who did not understand the compression algorithm). They might also use frequencies that we have decided are unlikely to carry signals, or use modulation strategies we have not thought to look for yet. They may even use the expected "simple" broadcast techniques, but are broadcasting from non-main sequence stars which are searched with lower priority. Even if only a few civilizations attempted to communicate in such an "obtuse" manner, it would reduce the odds of us locating an alien civilization. If there are only a few civilizations in our galaxy to begin with, our "narrow mindedness" may cause us to miss their existence completely.
... we misunderstand their attempts, or we dismiss the evidence.
Another proposed possibility is that alien intelligences are attempting to communicate using methods and technologies that are outside our experience or even our speculation. It is possible that such "signals" are reaching us, but we do not perceive them, or perceive them in a distorted manner.
As an example: Some have proposed that if the human brain utilizes quantum mechanical processes in its operation (as theorized by Roger Penrose, Stuart Hameroff, and others) then it may be open to receiving some form of nonlocal "psychic" communication — perhaps using quantum entanglement. Some have proposed that at least some accounts of mystics, shamans, schizophrenics, and channelers may be such "garbled" communications, transmitted by non-human intelligences in this manner.
Although classical communication in the context of information theory is not possible using quantum nonlocal correlations, supporters of this view believe that it may explain the 'garbled', associative, and inspirational nature of the 'messages' recorded in the world's religious and anthropological history.
Terence McKenna has proposed that the psychoactive drug Dimethyltryptamine (DMT) is an alien technology, "seeded" here on Earth by non-human intelligence, as part of a "biological communication strategy", in order to alter the perceptive processes of the human mind so that it may receive messages being transmitted to us.
While this may seem to be "fringe science" to some, it is an example of a theoretical means of communication that would appear very alien to our way of thinking, and would most likely be (some say is being) misinterpreted by us. Apart from this example it is possible that there are other signalling systems that would be likewise outside our current assumptions about inter-species communication, and would most likely be missed if used as a means of attempted communication by an alien civilization.
- Arecibo Observatory
- Doomsday argument
- Drake equation
- Fermi problem
- Von Neumann probe
- Zoo hypothesis
- Planetary habitability
- The Fermi Paradox: An Approach Based on Percolation Theory by Geoffrey A. Landis
- Space.com: Our Galaxy Should Be Teeming With Civilizations, But Where Are They? by Seth Shostak
- The Possibilities of FTL: Or Fermi's Paradox Reconsidered by F.E. Freiheit IV
- Fermi's Paradox (i.e. Where are They?) by James Schombert
- Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension by John Smart
- The Great Filter — Are We Almost Past It? by Robin Hanson
- Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox, which argues that our observations are incomplete, and There Is No Fermi Paradox, arguing that the paradox is based on a logical flaw, both by Robert Freitas
- Beyond Kardaschev: Possible Answer to Fermi's Paradox by Paul Hughes
- SETI and the Cosmic Quarantine Hypothesis by Steven Soter.
- The Millennial Project: Colonizing the Galaxy in 8 Easy Steps by Marshall T. Savage (Empyrean Publishing; Denver; 1992) pages 341–354 ISBN 0-9633914-8-8
- Scientific American — June 2000: "Where Are They? Maybe we are alone in the galaxy after all" by Ian Crawford
- If the Universe Is Teeming with Aliens... Where Is Everybody? by Stephen Webb (Copernicus Books; 2002) ISBN 0-387-95501-1
- The Berserker novels by Fred Saberhagen
- Childhood's End by Arthur C. Clarke
- The Engines of God by Jack McDevitt
- The Forge of God, Anvil of Stars, and Blood Music by Greg Bear
- The Heechee novels by Frederik Pohl
- The Manifold Trilogy: Manifold: Time, Manifold: Space, and Manifold: Origin by Stephen Baxter
- The Revelation Space novels by Alastair Reynolds
- Accelerando by Charles Stross
- Marvel's Ultimate Galactus trlogy by Warren Ellis uses the concept of Galactus to explain the paradox and it is mentioned in great detail during the series.