Fermi paradox
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The Fermi Paradox is a physical paradox in which estimates of the high probability of the existence of extraterrestrial life are contrasted with a lack of evidence.
The age of the universe and the vast number of stars in the universe suggest that extraterrestrial life should be common, an idea supported by many estimates based on the Drake equation. Considering this, the physicist Enrico Fermi is said to have asked his colleagues over lunch in 1950: "Where are they?"<ref> Template:Cite web</ref> 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 technologically advanced civilizations, or to respond to it by explaining how extraterrestrial civilizations could exist and yet remain undetected by us. That human beings have not been searching long enough, carefully enough, or for the right types of evidence have all been offered as suggestions to explain why the paradox remains unsolved. The belief that the lack of evidence is a conclusive argument for the non-existence of technologically advanced extraterrestrial civilizations is referred to as the Fermi principle. Several theories have been put forward in favour of this more skeptical stance.
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The basis of the paradox
Stated simply, the Fermi paradox is a conflict between an argument of scale and probability, and a lack of evidence: given the vast size and age of the universe, it seems unreasonable that humanity is the only intelligent species in existence, yet there is no evidence that extraterrestrial intelligence exists either through direct contact or distant observation. 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 combined with our lack of observational evidence to support it, is logically inconsistent. 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.
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 physical cosmology, astronomy, biology, ecology and philosophy.
The emerging field of astrobiology has brought an interdisciplinary approach to the question of extraterrestrial life. Critical astrobiological work includes research into the origins of life, geophysical comparisons between the Earth and other bodies, and theoretical and observational analysis of habitability parameters.
Despite the various scientific difficulties involved in astrobiological research, core questions remain simple. Does intelligent extraterrestrial life exist? If it exists, how common is it? How may human beings find it and communicate with it?
Arguments
The argument by scale
As noted, the argument by scale is a fundamental component of the paradox. Since intelligent life is possible in the universe (as proven by the example of human beings) then given its vast size and age there should be large numbers of extraterrestrial civilizations.
This view is a function of the raw numbers involved—an estimated 250 billion stars in the Milky Way and a seventy sextillion (7 x 1022) in the universe<ref>Template:Cite web</ref> — and of 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. Even if intelligent life only occurs on a small percentage of "ordinary" planets, there should still be a great number of civilizations extant in our galaxy alone. Some estimates using the Drake equation (see below) support this argument, although the assumptions behind those calculations have themselves been challenged.
The lack of evidence and the Rare Earth Hypothesis
Template:See Adherents to the Fermi principle—that a lack of evidence equals a lack of extraterrestrial intelligence—have 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<ref>Crawford, I.A., ``Searching for Extraterrestrials: Where are They?, Scientific American, July 2000, 38-43, (2000).</ref>. 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.
Skeptis have also begun to reject the mediocrity principle in favor of the rare Earth hypothesis which states that Earth is not typical, but unusual — perhaps even unique. 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. They argue many improbable coincidences that have converged to make complex life on Earth possible (see Rare Earth hypothesis for detailed arguments)<ref>Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000. ISBN 0387987010.</ref>.
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 <ref>Athena Andreadis. E. T., Call Springer-Verlag! SETI League Publications. 2000.</ref>. 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).
Supporting concepts
The 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. 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 Drake equation has been used by both sides of the debate, using different estimates for its various factors.
The values assigned to the various factors of the equation are only estimates, however the range of "reasonable" estimates seems to imply that contact with extraterrestrial life should be possible. Several scientists have published different estimated values for the Drake equation's factors, with 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 closer to 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. 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.
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: 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. 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.
What we might look for
Radio emissions
Since developing radio, human societies have been broadcasting signals into space both accidentally and deliberately. To a sensitive observer our solar system would appear to emit unusually intense radio waves for a G2 star due to our television and telecommunication broadcasts. As such energy has no apparent natural cause, alien observers might infer the existence of terrestrial civilization from observing this energy.
The ability to construct a radio telescope has been suggested as a definition of alien intelligence and radio technology generally is presumed to be a natural advance for technological species<ref>Template:Cite web</ref>. As radio creates effects that can theoretically be detected over interstellar distances, 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 signals could be either "accidental" byproducts of a civilization, or deliberate attempts to communicate, such as CETI's Arecibo message.
Such a search is not easy. SETI estimates 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, while the closest star to our sun is Proxima Centauri at 4.3 light years<ref>SETI's FAQ, Sec 1.2.3</ref>.
A signal is much easier to detect 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 <ref>SETI's FAQ, Sec 1.6</ref>. 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.
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; that aliens are sending focused transmissions such as the Arecibo message in our general direction.
Direct planetary observation
Image:Earthlights dmsp.jpg
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).
Alien constructs
Probes, colonies, and other artifacts
Template:See 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 attempted 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 <ref>Papagiannis, M. D. "Are We Alone or Could They be in the Asteroid Belt?," Quarterly Journal of the Royal Astronomical Society, 19, 277-281 (1978).</ref>
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<ref>Bracewell, R. N. "Communications from Superior Galactic Communities," Nature, 186, 670-671 (1960). Reprinted in A.G. Cameron (ed.), Interstellar Communication, W. A. Benjamin, Inc., New York, pp. 243-248, 1963.</ref>. There have been attempts to scan for such probes lying dormant within our solar system by scientists Robert Freitas and Francisco Valdes<ref>Freitas Jr., Robert A. and Valdes, Francisco. "The Search for Extraterrestrial Artifacts," Acta Astronautica, 12, No. 12, 1027-1034 (1985).</ref>.
Advanced stellar scale artifacts
Template:See 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<ref>Dyson, Freeman, "Search for Artificial Stellar Sources of Infra-Red Radiation", Science, June 1960.</ref>.
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<ref>Niven, Larry, Analog, "Bigger than Worlds", March 1974.</ref>. 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.
How we have been looking
Radio and signal emissions: SETI
- Further information: SETI, Project Ozma, Project Phoenix, SERENDIP, and Allen Telescope Array
A number of astronomers and observatories have attempted — and are still attempting — to detect evidence of technological alien civilizations, mostly by looking for radio signals, although other approaches also exist (see optical SETI). Most of these search programs (see links above) have been gathered under the umbrella of the SETI organization.
Given the sheer size of the radio search needed to look for signals from extraterrestrial civilizations and the limited amount of resources committed to SETI, most programs have tried to narrow their search by making some assumptions about alien life. Current SETI programs assume that most alien life will be orbiting Sun-like stars: late F to early K spectral class, metal-rich, and with low stellar variation <ref>Margaret C. Turnbull and Jill C. Tarter. "Target selection for SETI: A catalog of nearby habitable stellar systems," The Astrophysical Journal Supplement Series, 145: 181-198, March 2003.</ref>.
Many SETI searches also assume that extraterrestrial civilizations will be broadcasting a deliberate signal (like the Arecibo message), in order to be found. Project SENTINEL went so far as to assume that a signal might be beamed directly and deliberately at our own Sun. It is also usually assumed that such signals will be a sine wave based carrier wave with no complex modulation, transmitted in a narrow band of radio frequencies. Template:Cite-needed
Given the limited resources allocated for such a large problem, SETI's assumptions are understandable, but might very well be incorrect; extraterrestrial civilizations that do not match SETI's profile, if they exist, cannot be detected until the form of the searches change. Although data has been collected and analyzed by SETI over several decades, no main sequence stars with unusually bright, or meaningfully repetitive radio emissions, has yet been detected.
Direct planetary observation: the exoplanet hunters
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. 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. Currently we can only deduce the size and orbit of an exoplanet. This information, along with the stellar classification of its sun, and "educated guesses" as to its composition (based on its size), allows us to make rough estimates of the environments that such a planet might have.
From the perspective of the search for extraterrestrial life, the methods for exoplanet detection are not likely to be useful yet. The types and sizes of planets being detected through these methods are not those likely to be able to support Earth-like life. As of 2005 only a handful of possible terrestrial type planets have been detected, and 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. Hopefully, refinements in instruments and analysis will push the envelope of exoplanet detection, making more information available, and increasing the probability of finding more Earth-like worlds.
Alien constructs: Alien artifacts and the Dyson sphere survey
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 — admittedly, 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.
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.
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 preliminary and incomplete as of yet.
What we have 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 (Earth-like) 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. As our methods of planet-detection improve over time, many more terrestrial planets will probably be discovered.
None of the various SETA and SETV projects have located any artifacts.
The optical surveys for Dyson spheres did not locate anything; neither has the Fermilab search, although the latter is ongoing.
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
Cetain 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. <ref>Template:Cite web</ref>
...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 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 may exist, but that we do not or cannot 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 anthropomorphization 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 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 lightyears distant, that intelligence will need to have been active 1,000 years ago.)
There is a very slight 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, 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 scenario 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.
"... begging your pardon sir, but it's a big-ass sky." — Billy Bob Thornton as Truman in Armageddon
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).
- Human culture is part of a nursery maintained by other intelligent life, which must mature enough to join "society" as a whole. Perhaps, upon reaching a specific enough level of maturity there will be some kind of ceremonial recognition.
- 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 perhaps have been reported in, for example, ancient religious texts, 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 which perhaps accounts for the wide variety of ancient and modern anecdotal reports of angels and demons, for example.
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. According to quantum mechanics however, 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 may seem to be "fringe science" to many, 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.
Fictional treatment
- 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
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.Template:Featured article
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