Fermi paradox

A graphical representation of the Arecibo message, Humanity's first attempt to communicate its existence to alien civilizations

The Fermi Paradox is a physical paradox in which high estimates of the probability of the existence of extraterrestrial life are contrasted with a lack of evidence.

The age of the universe and its vast number of stars seem to suggest that extraterrestrial life should be common. Considering this with colleagues over lunch in 1950, the physicist Enrico Fermi is said to have asked: "Where are they?" If there are a multitude of advanced extraterrestrial civilizations in the Milky Way galaxy then why have we not seen any evidence, such as probes, spacecraft or radio transmissions? The simple question "Where are they?" (alternatively, "Where is everybody?") is possibly apocryphal but Fermi is widely credited with simplifying and clarifying the problem of the probability of extraterrestrial life.

There have been attempts to resolve the Fermi Paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without our knowledge. Counterarguments suggest that intelligent extraterrestrial life does not exist or at least occurs so rarely that we will never make contact with it. The belief that the lack of evidence conclusively demonstrates the non-existence of extraterrestrial civilizations is known as the Fermi principle.

A great deal of effort has gone into developing scientific theories and possible models of extraterrestrial life and the Fermi paradox has become a theoretical reference point in much of this work. The problem has spawned numerous scholarly works addressing it directly, while various questions that relate to it have been addressed in fields as diverse as astronomy, biology, ecology and philosophy. The emerging field of astrobiology has brought an interdisciplinary approach to the Fermi paradox and the question of extraterrestrial life.

Basis of the paradox: argument by scale and a lack of evidence

The Fermi paradox is a conflict between an argument of scale and probability, and a lack of evidence. A more complete definition could be stated thus:

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 the initial 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 and we are not searching for the correct indicators.

The first aspect of the paradox, "the argument by scale", is a function of the raw numbers involved: there are an estimated 250 billion stars in the Milky Way and seventy sextillion (7 x 10²²) in the universe. Even if intelligent life occurs on only a miniscule percentage of planets, there should still be a great number of civilizations extant in our galaxy alone. This argument also assumes the mediocrity principle, which states that Earth is not special, but merely a typical planet, subject to the same laws, effects, and likely outcomes as any other world. Some estimates using the Drake equation (see below) support this argument, although the assumptions behind those calculations have themselves been challenged.

The second cornerstone of the Fermi paradox is a rejoinder to the argument by scale: given intelligent life's ability to overcome scarcity and colonize new habitats, we can reasonably assume that any advanced civilization will seek out new resources and colonize first their solar system, and then surrounding solar systems. As there is no evidence on Earth or anywhere else of attempted alien colonization after 13 billion years of the universe's history, we must assume that intelligent life is extremely rare and possibly unique to Earth.

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. 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. The means of exploration and theoretical probes involved are discussed extensively below.

Supporting concepts

Drake Equation

In an attempt to find a systematic means to evaluate the numerous probabilities involved, Dr. Frank Drake formulated The Drake equation in 1960, a decade after the objections raised by Enrico Fermi.

The Drake equation has been used by both both optimists and pessimists, with varying results. Dr. Carl Sagan, for example, suggested as many 1 million communicating civilizations in the Milky Way in 1966 . Another published estimate from Frank Tipler in 1982 placed the value at just one—e.g., human beings are the only extant intelligent life .

Critics of the Drake equation claim that since the variables cannot yet be determined with any real confidence, estimating the number of extraterrestrial civilizations based on it is methodologically flawed, an idea which the wide diverengence in estimates seems to support. Focusing on empirical data—which scientists are only now beginning to collect and analyze in a significant manner—rather than theory is the only sound approach. Only with further observation can meaningful values for the Drake equation factors be assigned.

Rare Earth Hypothesis

Further information: Rare Earth hypothesis

Another important argument that relates to the Fermi paradox is the Rare Earth hypothesis which suggests that Earth is not typical, but unusual, and perhaps even unique. This is a rejection of the mediocrity principle and supports the Fermi principle—that a lack of evidence equals a lack of extraterrestrial life. While a unique Earth has had historical support on philosophical or religious grounds, the Rare Earth Hypothesis deploys quantifiable and statistical arguments in support of the theory that multicellular life is exceedingly rare in the universe because Earth-like planets are themselves exceedingly rare. Supporters argue that many improbable coincidences that have converged to make complex life on Earth possible (see Rare Earth hypothesis for detailed arguments).

Critics of the Rare Earth hypothesis suggest that the argument is essentially circular: intelligent Earth-life appears rare and therefore intelligent life is rare. Rather than a true hypothesis, the Rare Earth idea may simply be a description of how life arose here . While the probability of the specific conditions on Earth being widely replicated may be low, 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 for further information).

The anthropic principle

Various formulations of the anthropic principle have also 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 it 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).

The Anthropic Principle can 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 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 also those on both sides of the debate that deny that the Anthropic Principle is a meaningful argument at all.

Trying to resolve the paradox empirically: Detection possibilities and methods

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. As human beings do not have interstellar travel capability, such searches are being carried out at great distances and rely on careful analysis of very subtle evidence. This limits possible discoveries to 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 from Earth in the near future (though microbial life may be deduced in the Solar System).

One difficulty in searching is avoiding an overly anthropomorphic viewpoint. Conjecture on the type of evidence likely to be found often focuses on the types of activities that humans have performed, or likely would perform given more advanced technology. Intelligent aliens might avoid these "expected" activities, or perform totally novel activities we would not think to look for.

Radio emissions

File:Arecibo.arp.750pix.jpg

The Arecibo Observatory: a radio telescope which has played a key role in attempts to resolve the Fermi paradox

See also: SETI, Project Ozma, Project Phoenix, SERENDIP, and Allen Telescope Array.

Radio technology and the ability to construct a radio telescope are presumed to be a natural advance for technological species theoretically creating effects that might be detected over interstellar distances. Sensitive observers of our solar system, for example, would note unusually intense radio waves for a G2 star due to Earth's television and telecommunication broadcasts. In the absence of an apparent natural cause, alien observers might infer the existence of terrestrial civilization.

Therefore, the careful searching of radio emissions from space for non-natural signals may lead to the detection of alien civilizations. Such signals could be either "accidental" byproducts of a civilization, or deliberate attempts to communicate, such as the Communication with Extraterrestrial Intelligence's Arecibo message. A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly gathered into the SETI organization, although other approaches, such as optical SETI also exist.

Several decades of SETI analysis has not revealed any main sequence stars with unusually bright, or meaningfully repetitive radio emissions, although there have been several candidate signals: on August 15, 1977 the "Wow! signal" was picked up by The Big Ear radio telescope. 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. There are numerous technical assumptions underlying SETI that may cause human beings to miss radio emissions with present search techniques; this is discussed below.

Direct planetary observation

A composite picture of Earth at night. Human civilization is detectable from space.

Detection and classification of exoplanets has not come about as part of the search for extraterrestrial life, but out of recent refinements in mainstream astronomical instruments and analysis. While this is a new field in astronomy—the first published paper claiming to have discovered an exoplanet was released in 1989—it is possible that planets which are likely to be able to support life will be found in the near future. Direct observational evidence for the existence of life such as the absorption spectrum of chlorophyll in light filtered through a planet's atmosphere is possible assuming future refinements in detection methods. Locating potential or actual "life bearing" planets would help narrow the search for intelligent life, and perhaps even lead to observational evidence of an alien technological civilization (see right).

Exoplanets are rarely directly observed (the first claim to have done so was in 2005) rather their existence is inferred based on effects caused in their orbiting a star. Currently, the size and orbit of an exoplanet can be deduced. This information, along with the stellar classification of its sun, and educated guesses as to its composition based on its size and comparisons to studied bodies, allows for rough approximations of the planetary environment.

The methods for exoplanet detection are not likely to deduce Earth-like life at present, given that most exoplanets discovered are Jupiter mass or larger. As of 2005 only a handful of possible terrestrial type planets have been detected, and only two of these have been located in orbit of main sequence stars: Gliese 876 d and OGLE-2005-BLG-390Lb. Neither are likely to be able to support life as we know it. Refinements in instruments and analysis will push the envelope of exoplanet detection and increase the probability of finding more Earth-like worlds.

Alien constructs

Probes, colonies, and other artifacts

Further information: Von Neumann probe, Bracewell probe, and Search for Extraterrestrial Artifacts (SETA)

As already noted, assuming 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 — evidence of alien colonization attempts might plausibly be discovered. Additionally, evidence of "unbeinged" exploration in the form of probes and information gathering devices may await discovery.

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 relative to the results. If even a single civilization in our galaxy attempted this, such probes could spread throughout the entire galaxy. Evidence of such probes might be found in our solar system — perhaps in the asteroid belt 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.

Since the 1950s direct exploration has been carried out on a small fraction of our solar system and no evidence that it has ever been visited by alien colonists, or probes, has been uncovered. Detailed exploration of areas of the solar system where resources would be plentiful—such as the asteroids, the Kuiper belt, the Oort cloud and the various planetary ring systems—may yet produce evidence of alien exploration. Unfortunately, these regions where evidence of alien probes are more likely to be found are also massive and investigation 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[1] have attempted to locate such evidence within our own solar system. There have also been attempts to signal, attract, or activate Bracewell probes in our local vicinity, including by scientists Robert Freitas and Francisco Valdes. Admittedly, many of the projects that fall under this umbrella are considered "fringe" science by many astronomers and none of the various projects have located any artifacts.

Should alien artifacts be discovered, even here on Earth, they may not be recognizable as such. The products of an alien mind and an advanced alien technology might not be perceptible or recognizable as artificial constructs. Exploratory devices in the form of bio-engineered life forms created through synthetic biology would presumably disintegrate after a point, leaving no evidence; an alien information gathering system based on molecular nanotechnology could theoretically be swarming at any moment, completely undetected. Clarke's third law suggests that an alien civilization well in advance of our own might have means of investigation that is not yet conceivable to human beings.

Advanced stellar scale artifacts

File:Dysonspherediagram.gif

A variant of the speculative Dyson sphere. Such large scale artifacts would drastically alter the spectrum of a star.

Further information: Dyson Sphere, Kardashev scale, Astroengineering, Megastructure, Alderson disk, Matrioshka brain, Ringworld, and Stellar engine

In 1959, Dr. Freeman Dyson observed that every developing human civilization constantly increases its energy consumption, and theoretically, a civilization of sufficient age would require all the energy produced by its sun. The Dyson Sphere was the thought experiment solution that he derived: a shell or cloud of objects enclosing a star to harness as much of the radiant energy of that star as possible. Such a feat of astroengineering would drastically alter the observed spectrum of the sun, changing it at least partly from the normal emission lines of a natural stellar atmosphere, to that of a a black body radiation, probably with a peak in the infrared. Dyson himself speculated that advanced alien civilizations might be detected by examining the spectra of stars, searching for such an altered spectrum.

Since then, several other theoretical stellar-scale megastructures have been proposed (see links above), but the central idea remains that a highly advanced civilization — Type II or greater on the Kardashev scale — could alter its environment enough as to be detectable from interstellar distances.

However, such constructs may be more difficult to detect than originally thought. Dyson spheres might have different emission spectra depending on the desired internal environment; life based on high-temperature reactions may require a high temperature environment, with resulting "waste radiation" in the visible spectrum, not the infrared. Additionally, a variant of the Dyson sphere has been proposed which would be difficult to observe from any great distance. A Matrioshka Brain is a series concentric spheres, each radiating less energy per area than its inner neighbour. The outermost sphere of such a structure could be close to the temperature of the interstellar background radiation, and thus all but invisible.

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.

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 but optical surveys have not located anything. Fermilab has an ongoing program to find Dyson spheres, but such searches are preliminary and incomplete as of yet.

Trying to resolve the paradox theoretically: Explaining the silence

Certain theoreticians accept that the absence of evidence proves the absence of extraterrestrials and attempt to explain why. Others offer possible frameworks in which the silence may be explained without ruling the possibility of such life, including assumptions about extraterrestrial behaviour and technology.

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.

For example, the emergence of intelligence may have been an evolutionary accident. Geoffrey Miller proposes that human intelligence is the result of runaway sexual selection, which takes unpredictable directions.

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 before or shortly after developing radio or space flight technology. Possible means of annilihation include nuclear war, biological warfare or accidental contamination, nanotechnological catastrophe, or a Malthusian catastrophe after the deterioration of a planet's ecosphere. This general theme is explored both in fiction and in mainstream scientific theorizing. Indeed, there are probabilistic arguments which suggest that humanity's end may occur sooner rather than later (see Doomsday argument). In 1966 Sagan and Shklovskii suggested that technological civilizations will tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales [2]. Self-annilihation may also be viewed in terms of thermodynamics: insofar as life is an ordered system that can sustain itself against the tendency to disorder, the "external transmission" or interstellar communicative phase may be the point at which the system becomes unstable and self-destructs [3].

From a Darwinian perspective, self-destruction is a paradoxical outcome of evolutionary success. The evolutionary psychology that developed during the competition for scarce resources over the course of human evolution has left our species subject to aggressive, instinctual drives to consume resources, increase longevity, and to reproduce—in part, the very motives that lead to the development of technological society. It seems likely that intelligent extraterrestrial life would evolve subject to similar conditions and thus face the same possibility of self-destruction. It has been suggested, for instance, that a successful alien species will be a superpredator, as is homo sapiens [4]. In the absence of an aggressive, aquisitive character, technological civilization would be a less likely outcome amongst extraterrestrials, rendering them difficult or impossible to detect.

...because it is the nature of intelligent life to destroy others.

Another possibility is that intelligent species beyond a certain point of technological capability will destroy other intelligence as it appears. The idea that someone, or something, is destroying intelligent life in the universe is well explored in science fiction, for instance. The primary motive would be perceived competition for an aggressive, expansionist species. In 1981, cosmologist Edward Harrison also pointed out that such behaviour would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies would view any other species bent on galactic expansion as a kind of virus .

Violent extermination of other civilizations is not 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 a civilization who created such machines were to fall or go extinct, the probes could outlive their creators, destroying civilizations far into the future.

While it appears plausible intelligent life tends to suppress other intelligent life, the idea can be criticized as continuing to beg the question at the heart of the Fermi Paradox: if intelligence destroys upstart intelligence, why are we still here?

...because God created humans alone.

Although not generally considered a testable scientific explanation, this theory is one of the lines of thought contributing to the Rare Earth Hypothesis. Several schools of thought within the Judeo-Christian and Islamic religions maintain 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 exist, but that we do not or cannot communicate with them because of various constraints: problems of scale or of technology; because their nature is simply too alien for meaningful communication; or because we refuse to admit to evidence of their presence.

...we cannot communicate due to problems of scale

NASA's conception of the Terrestrial Planet Finder. Is it possible alien civilizations are too far away for meaningful communication?

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 (see intelligent life may destroy itself discussion above), the "window of opportunity" for detection or contact might be quite small. Advanced civilizations may periodically arise and fall throughout 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. The term "now" is somewhat complicated by the finite speed of light and the nature of space-time under relativity (see Relativity of simultaneity). Assuming that an extraterrestrial intelligence is not able to travel to our vicinity at faster-than-light speeds, in order to detect an intelligence 1,000 light-years distant, that intelligence will need to have been active 1,000 years ago.

There is a possibility that we may detect archeological evidence of past civilizations through deep space observations — especially if they left behind large artifacts such as Dyson spheres — but this seems less likely than detecting the output of a thriving civilization.

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, it appears 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 present scientific knowledge cannot properly gauge the feasibility and/or costs of such ventures. Perhaps there are theoretical barriers to such efforts that are not yet understood and 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 let alone communicated with.

We have not been searching long enough

"... begging your pardon sir, but it's a big-ass sky." — Billy Bob Thornton as Truman in Armageddon

Some commentators have pointed out that humanity's ability to detect and comprehend intelligent extraterrestrial life has existed for only a very brief period — from 1937 onwards, if the invention of the radio telescope is taken as the dividing line — and that Homo sapiens itself is a recent species, given the apparent size and age of the universe. 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 hundreds of thousands of years more. Thus it remains possible that our species has neither been searching long enough to find other intelligences, nor existed long enough to be found.

For example, one million years ago — a relatively brief period in cosmological terms — there would have been no humans for alien emissaries to meet. For each further step back 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 is more plausible if alien civilizations tend to stagnate or die out, rather than expand. However "the probability of a site never being visited, even [with an] infinite time limit, is a non-zero value." Thus, even if intelligent life expands elsewhere, it remains statistically possible that terrestrial life will go undiscovered.

...we cannot communicate for technical reasons

We are not listening properly

There are some assumptions that underly the SETI search programs that may cause 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). Extraterrestrials 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. "Simple" broadcast techniques might be employed, but sent from from non-main sequence stars which are searched with lower priority; current programs assume that most alien life will be orbiting Sun-like stars.

The greatest problem is the sheer size of the radio search needed to look for signals and the limited amount of resources committed to SETI. SETI estimates, for instance, 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) . A signal is much easier to detect if the signal energy is focused in either a narrow range of frequencies (Narrowband transmissions), and/or directed at a specific part of the sky. Such signals can be detected at ranges of hundreds to tens of thousands of light-years distance . However this means that detectors must be listening to an appropriate range of frequencies, and be in that region of space to which the beam is being sent. Many SETI searches go so far as to assume that extraterrestrial civilizations will be broadcasting a deliberate signal (like the Arecibo message), in order to be found.

Thus to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortuitous circumstances: that the broadband radio emissions of alien radio technology are much stronger than our own; that one of SETI's programs is listening to the correct frequencies from the right regions of space; or that aliens are sending focused transmissions such as the Arecibo message in our general direction.

Civilizations only broadcast detectable radio signals for a brief period of time

It may be that alien civilizations are not detectable though their radio emissions after all, or at least not for timeframes that allow for meaningful communication. There are two possibilities in this regard: civilizations outgrow radio through technological advance or, conversely, resource depletion cuts short the the time in which a species broadcasts.

The first idea, that civilizations advance beyond radio, is based in part on the "fiber optic objection": the use of broadcast technologies like radio (as opposed to fiber optics) for the transmission of information is fundamentally wasteful of energy as 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. 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; when this fact is considered alongside probable advances beyond radio, 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.

The converse problem of resource depletion and a concomitant decline in technological capability is also based on the human example. Our civilization has been capable of interstellar radio communication for only a few decades and we are already running out of fossil fuels and grappling with the problem of peak oil. 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 radio technology may be a short-lived phenomenon. 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 have experienced a technological singularity

Another possibility is that technological civilizations invariably experience a technological singularity, such as the development of strong artificial intelligence or a posthuman character. Theoretical civilizations of this sort may have altered drastically enough to render communication impossible. The intelligences of a post-singularity civilization might require more information exchange than is possible through interstellar communication, for example. Or perhaps any information humanity might provide would appear elementary. Because of this they do not try to communicate, any more than human beings attempt to talk to ants.

Even more extreme forms of post-singularity have been suggested, particularly in fiction: beings that 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.

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.

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.

...they choose not to communicate

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)

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. A particular reason that alien civilizations may choose not to communicate is the so-called Zoo hypothesis: the idea that Earth is being monitored by advanced civilizations for study, or is being preserved in an isolated "zoo or wilderness area"..

The reason may be ethical (encouraging our independent development) or strategic (they wish to avoid detection and possible destruction at the hands of other civilizations). These ideas are 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 most plausible if there is a single alien civilization within contact range, or there is a homogenous culture or law amongst alien civilizations which dictates that the Earth be shielded (the latter is the concept behind the "Prime Directive" in Star Trek). If there is a plurality of alien cultures, however, this theory breaks down under the uniformity of motive flaw: all it takes is single culture or civilization to decide to act contrary to the imperative for it to be abrogated, and the probability of such a violation increases with the number of civilizations .

They are too alien.

Another possibility that is that we have underestimated how much alien life might differ from that on Earth. Their psychologies may simply be too different to communicate with, and realizing this, they do not make the attempt (see: They Are Made Of Meat). It is also possible that the very concept of communication with other species is one which they cannot even conceive. Human mathematics, language, tool use, and other cornerstones of technology and communicative capacity may be parochial to Earth and not shared by other life [5]. See also technological singularity above.

...they are here and we don't know it

There is a widespread belief, generally considered "fringe" by mainstream science, that intelligent alien life forms not only exist, but are already present here on Earth. We do not detect them either because they do not wish it, we are technically unable, or because we refuse to admit to the evidence.

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.

We misunderstand their attempts

Another series of views consider that alien entities have been communicating with humans throughout history, but for any number of reasons we are unable to scientifically detect them. Accounts of communication have perhaps have been reported in, for example, ancient religious texts, but have been dismissed or overlooked.

It is also possible that alien intelligences are attempting to communicate using methods and technologies that are outside our experience or even our speculation. "Signals" are reaching us, but we do not perceive them, or perceive them in a distorted manner, which perhaps accounts for the wide variety of ancient and modern anecdotal reports of angels, demons, and so on.

As an example: 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. It has been proposed that some accounts of mystics, shamans, schizophrenics, and channelers may be such "garbled" communications, transmitted by non-human intelligences in this manner. According to quantum mechanics the transfer of information in the context of information theory is not possible using quantum nonlocal correlations. However, supporters of the idea of this form of communication idea believe that this 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 particular example may appear dubious, 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 misinterpreted. It is possible that there are other signalling systems that would likewise fall outside our current assumptions about inter-species communication, and would most likely be missed if used as a means of attempted communication.

Fictional treatment

• The Berserker novels by Fred Saberhagen
• Childhood's End by Arthur C. Clarke
• The Ellimist Chronicles by K.A. Applegate
• 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 Heritage Trilogy of Ian Douglas
• The Manifold Trilogy: Manifold: Time, Manifold: Space, and Manifold: Origin by Stephen Baxter
• The Revelation Space novels by Alastair Reynolds
• Accelerando by Charles Stross

See also

• SETI
• Arecibo Observatory
• Astrosociobiology
• Doomsday argument
• Drake equation
• Fermi problem
• Von Neumann probe
• Zoo hypothesis
• Planetary habitability

References

• 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
• "Where Are They? Maybe we are alone in the galaxy after all", by Ian Crawford. Scientific American — June 2000.
• If the Universe Is Teeming with Aliens... Where Is Everybody? by Stephen Webb (Copernicus Books; 2002) ISBN 0-387-95501-1

External links

• 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.