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{{otheruses4|Enrico Fermi's observation regarding extraterrestrial life|the music album|Fermi Paradox (album)|Fermi's estimation methods|Fermi problem}}
[[File:Arecibo message.svg|thumb|right|140px|A graphical representation of the [[Arecibo message]] - Humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations]]

The '''Fermi paradox''' is the apparent contradiction between high estimates of the probability of the existence of [[Extraterrestrial life|extraterrestrial civilizations]] and the lack of evidence for, or contact with, such civilizations.

The extreme [[age of the universe]] and its vast number of stars suggest that if the Earth is typical, extraterrestrial life should be common.<ref name="sagan">Sagan, Carl. ''Cosmos,'' Ballantine Books 1985</ref>  In an informal discussion in 1950, the [[physicist]] [[Enrico Fermi]] questioned why, if a multitude of advanced extraterrestrial civilizations exist in the [[Milky Way]] [[galaxy]], evidence such as [[alien spacecraft|spacecraft]] or [[Von Neumann probe|probes]] are not seen. A more detailed examination of the implications of the topic began with a paper by [[Michael H. Hart]] in 1975, and it is sometimes referred to as the '''Fermi-Hart paradox'''.<ref>{{cite journal | last = Wesson | first = Paul | year = 1992 | month = June | title = Cosmology, extraterrestrial intelligence, and a resolution of the Fermi-Hart paradox | journal = Royal Astronomical Society, Quarterly Journal | volume = 31 | pages = 161–170 | issn = 0035-8738 | url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1990QJRAS..31..161W&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf | accessdate = 2007-05-06|format=PDF}}</ref>  Another closely related question is the '''Great Silence'''<ref name="Brin">{{cite journal |author= Brin, Glen David |year=1983 |title= The 'Great Silence': The Controversy Concerning Extraterrestial Intelligent Life |url= http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1983QJRAS..24..283B&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf |journal=Quarterly Journal of Royal Astronomical Society |volume=24 |pages=283–309.|format=PDF}}</ref>—even if travel is hard, if life is common, why don't we detect their [[Radio|radio transmissions]]?  

There have been attempts to resolve the Fermi Paradox by locating evidence of extraterrestrial civilizations, along with proposals that such life could exist without human knowledge. Counterarguments suggest that intelligent [[extraterrestrial life]] does not exist or occurs so rarely that humans will never make contact with it.

Starting with Hart, a great deal of effort has gone into developing scientific theories about, 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 ==
The Fermi paradox is a conflict between an argument of [[Scale (spatial)|scale]] and [[probability]] and a lack of [[evidence]]. A more complete [[definition]] could be stated thus: 

{{quotation|The apparent size and age of the universe suggests that many technologically advanced extraterrestrial civilizations ought to exist.<br /> However, this hypothesis seems inconsistent with the lack of observational evidence to support it.}}

The first aspect of the paradox, "the argument by scale", is a function of the raw numbers involved: there are an estimated 250 billion (2.5 x [[1 E9|10<sup>11</sup>]]) stars in the [[Milky Way]] and 70 sextillion (7 x 10<sup>22</sup>) in the [[visible universe]].<ref>{{cite web | last = Craig | first = Andrew  
| year = 2003 | url = http://news.bbc.co.uk/1/hi/sci/tech/3085885.stm | title = "Astronomers count the stars" | work = BBC News | publisher = BBC | dateformat = mdy | accessdate = April 8 2006 }}</ref> Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there should still be a great number of civilizations extant in the Milky Way [[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]] 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 its tendency to colonize new [[Habitat (ecology)|habitats]], it seems likely that any advanced civilization would seek out new resources and colonize first their own [[star system]], and then the surrounding star systems. As there is no conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13.7 billion years of the universe's history, it may be assumed that intelligent life is rare or that our assumptions about the general behavior of intelligent species are flawed.

The Fermi paradox can be asked in two ways. The first is, "Why are no aliens or their artifacts physically here?" If [[interstellar travel]] is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy.<ref>Crawford, I.A., [http://www.cc.nctu.edu.tw/~tseng327/nctu-origin/article/sc200007a.pdf "Where are They? Maybe we are alone in the galaxy after all"], '' Scientific American,'' July 2000, 38-43, (2000).</ref>  This is a relatively small amount of time on a [[geological time|geological scale]], let alone a [[Timeline of the Big Bang|cosmological one]]. Since there are many stars older than the sun, or since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large scale ''exploration'' of the galaxy is still possible; the means of exploration and theoretical probes involved are discussed extensively [[#Alien constructs|below]]. However, no signs of either colonization or exploration have been generally acknowledged.

The argument above may not hold for the universe as a whole, since travel times may well explain the lack of physical presence on Earth of alien inhabitants of far away galaxies. However, the question then becomes "Why do we see no signs of intelligent life?" as a sufficiently advanced civilization<ref>The [[Soviet]] [[astronomer]] [[Nikolai Kardashev]] has stated that an [[alien civilization]] on [[Kardashev scale]] of 3 could send signals up to 10 billion light years.</ref> could potentially be seen over a significant fraction of the [[Observable universe#Size|size of the observable universe]].<ref>Intelligent Life in the Universe by I. S. Shklovskii(Author), Carl Sagan (Author)  p394</ref>  Even if such civilizations are rare, the scale argument indicates they should exist somewhere at some point during the history of the universe, and since they could be detected from far away over a considerable period of time, many more potential sites for their origin are within our view. However, no incontrovertible signs of such civilizations have been detected.

It is currently unclear which version of the paradox is stronger.<ref>Let <math>N(r)</math> be the number of civilizations (per unit volume) that can be seen at a radius <math>r</math>. Let <math>R_g</math> be the radius of the galaxy. So the number of civilizations we see is:
:<math> \int_0^{R_g} N(r) 4 \pi r^2\,dr + \int_{R_g}^\infty N(r) 4 \pi r^2 \,dr</math>
where the first integral are those in the galaxy, and the second those outside. Which integral is bigger depends on how fast N(r) decreases, which is completely unknown. This observation is due to Kardashev.</ref>

== Name ==
In 1950, while working at [[Los Alamos National Laboratory]], the physicist [[Enrico Fermi]] had a casual conversation while walking to lunch with colleagues [[Emil Konopinski]], [[Edward Teller]] and [[Herbert York]].  The men lightly discussed a recent spate of [[Unidentified flying object|UFO]] reports and an [[Alan Dunn (cartoonist)|Alan Dunn]] cartoon<ref>{{cite web |url=http://home.fnal.gov/~carrigan/pillars/New_Yorker_aliens.png |title=Uncaptioned cartoon |author=Alan Dunne |publisher=New Yorker, 20 May 1950 |year=1950 |accessdate=2008-09-29}}</ref> facetiously blaming the disappearance of municipal trashcans on marauding aliens.  They then had a more serious discussion regarding the [[Odds|chances]] of humans observing [[faster-than-light]] travel of some material object within the next ten years, which Teller put at one in a million, but Fermi put closer to one in ten.  The conversation shifted to other subjects, until during lunch Fermi suddenly exclaimed, "Where are they?" (alternatively, ''"Where is everybody?"'')<ref name="autogenerated1">Eric Jones, [http://www.fas.org/sgp/othergov/doe/lanl/la-10311-ms.pdf "Where is everybody?", An account of Fermi's question"], Los Alamos Technical report LA-10311-MS, March, 1985.</ref>  One participant recollects that Fermi then made a series of rapid calculations using estimated figures (Fermi was known for his ability to make good estimates from first principles and minimal data, see [[Fermi problem]].)  According to this account, he then concluded that Earth should have been visited long ago and many times over.<ref name="autogenerated1" /><ref>{{cite web | last = Shostak | first = Seth | date = [[25 October]] [[2001]] | url = http://www.space.com/searchforlife/shostak_paradox_011024.html | title = "Our Galaxy Should Be Teeming With Civilizations, But Where Are They?" | work = Space.com | publisher = [http://www.space.com Space.com] | dateformat = mdy | accessdate = April 8 2006 }}</ref>

== Drake equation ==
{{main|Drake equation}}
While numerous theories and principles attend to the Fermi paradox, the most closely related is the [[Drake equation]]. 

The equation was formulated by Dr. [[Frank Drake]] in 1960, a decade after the objections raised by [[Enrico Fermi]], in an attempt to find a systematic means to evaluate the numerous probabilities involved in alien life. The speculative equation factors: the rate of [[star formation]] in the galaxy; the number of stars with planets and the number that are habitable; the number of those planets which develop life and subsequently intelligent communicating life; and finally the expected lifetimes of such civilizations. The fundamental problem is that the last four terms (fraction of planets with life, odds life becomes intelligent, odds intelligent life becomes communicative, and lifetime of communicating civilizations) are completely unknown. We have only one example, rendering statistical estimates impossible, and even the example we have is subject to a strong [[anthropic bias]].

A deeper objection is that the very form of the Drake equation assumes that civilizations arise and then die out within their original solar systems.  If interstellar colonization is possible, then this assumption is invalid, and the equations of [[population dynamics]] would apply instead.<ref>{{cite book |author= [[Jack Cohen (scientist)|Jack Cohen]] and [[Ian Stewart (mathematician)|Ian Stewart]] |title=[[Evolving the Alien]] |publisher=John Wiley and Sons, Inc., Hoboken, NJ |date=2002}} Chapter 6, ''What does a Martian look like?''.</ref>

The Drake equation has been used by both optimists and pessimists with wildly differing results. Dr. [[Carl Sagan]], using optimistic numbers, suggested as many as one million communicating civilizations in the [[Milky Way]] in 1966, though he later suggested that the number could be far smaller. Skeptics, such as [[Frank J. Tipler|Frank Tipler]], have put in pessimistic numbers and concluded that the average number of civilizations in a galaxy is much less than one.<ref>{{cite book| author=[[John D. Barrow|Barrow J. D.]] and [[Frank J. Tipler|Tipler, F. J.]]| year=1986| title=The Anthropic Cosmological Principle p588| publisher=Oxford Univ. Press| isbn = 0-19-282147-4}}</ref> (Note that, even though there is at least one civilization in our galaxy, the average or "most likely" number of civilizations in our galaxy as described by this equation may still be smaller than one. In other words, the fact that there is at least one civilization in our galaxy does not mean that this was a likely outcome. This is an excellent example of [[anthropic bias]]. No civilization can use itself to estimate the average number of civilizations in a galaxy, since if there was not at least one civilization the question could not arise.)

Frank Drake himself has commented that the Drake Equation is unlikely to settle the Fermi paradox; instead it is just a way of organizing our ignorance on the subject.

== Resolving the paradox empirically ==

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. It is very unlikely that non-technological civilizations will be detectable from Earth in the near future.

One difficulty in searching is avoiding an overly [[anthropocentric]] 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 activities totally novel to humans. 

=== Radio emissions ===
{{see|SETI|Project Ozma|Project Cyclops|Project Phoenix (SETI)|SERENDIP|Allen Telescope Array}}
[[Image:parkes.arp.750pix.jpg|thumb|left|255px|[[Radio telescope]]s are often used by SETI projects]]
Radio technology and the ability to construct a [[radio telescope]] are presumed to be a natural advance for technological species<ref>{{cite web | last = Mullen | first = Leslie  
| year = 2002 | url = http://www.space.com/scienceastronomy/alien_intelligence_021202.html  | title = "Alien Intelligence Depends on Time Needed to Grow Brains"  | work = Astrobiology Magazine | publisher = [http://www.space.com Space.com] | dateformat = mdy | accessdate = April 21 2006 }}</ref> theoretically creating effects that might be detected over interstellar distances.
Sensitive observers of the solar system, for example, would note unusually intense [[radio]] waves for a [[Star#Classification|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" by-products 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 through the [[SETI]] organization, although other approaches, such as 
[[SETI#Optical experiments|optical SETI]] also exist. 

Several decades of SETI analysis have not revealed any [[main sequence star]]s 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.  However, the Big Ear only looks at each point on the sky for 72 seconds, and re-examinations of the same spot have found nothing.  In 2003, [[Radio source SHGb02 plus 14a|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; these are discussed [[#Communication is impossible for technical reasons|below]].

=== Direct planetary observation ===
[[File:Earthlights dmsp.jpg|350px|thumb|A composite picture of Earth at night, created with data from the [[Defense Meteorological Satellite Program]] (DMSP) Operational Linescan System (OLS). Human [[civilization]] is detectable from space.]]

Detection and classification of [[exoplanets]] has come 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 evidence for the existence of life may eventually be observable, such as the detection of biotic signature gases (such as [[methane]] and [[oxygen]]) — or even the industrial [[air pollution]] of a technologically advanced civilization — in an exoplanet's atmosphere by means of [[spectroscopy|spectrographic analysis]].<ref>[http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2085&mode=thread&order=0&thold=0 Habitable Planet Signposts], ''Astrobiology magazine''.</ref> With improvements in our observational capabilities, it may eventually even be possible to detect direct evidence such as that which humanity produces (see right).

However, exoplanets are rarely directly observed (the first claim to have done so was made in 2004<ref>{{cite journal | author=G. Chauvin; A.M. Lagrange; C. Dumas; B. Zuckerman; D. Mouillet; I. Song; J.-L. Beuzit; P. Lowrance | title=A giant planet candidate near a young brown dwarf | journal=Astronomy & Astrophysics | year=2004 | volume=425 | issue= | pages=L29 – L32 | url=http://www.edpsciences.org/articles/aa/abs/2004/38/aagg222/aagg222.html | doi = 10.1051/0004-6361:200400056}}</ref>); rather, their existence is usually inferred from the effects they have on the star(s) they orbit. This means that usually only the mass 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 (usually based on the mass of the planet, and its distance from its sun), allows only for rough approximations of the planetary environment.

Prior to 2009, [[Methods of detecting extrasolar planets|methods for exoplanet detection]] were not likely to detect life-bearing Earth-like worlds.  Methods such as [[gravitational microlensing]] can detect the presence of "small" worlds, potentially even smaller than the Earth, but can only detect such worlds for very brief moments of time, and no follow-up is possible.   Other methods such as [[Methods of detecting extrasolar planets#Radial velocity|radial velocity]], [[astrometry]], and the [[Methods of detecting extrasolar planets#Transit method|transit method]] allow prolonged observations of exoplanet effects, but only work with worlds that are many times the mass of Earth, at least when performed while looking through the atmosphere. These seem unlikely candidates to harbor Earth-like life. However, exoplanet detection and classification is a very active sub-discipline in astronomy, with 241 such planets being detected between 1988 and 2007,<ref name="Encyclopedia">{{cite web |title=Interactive Extra-solar Planets Catalog |work=The Extrasolar Planets Encyclopedia |url=http://exoplanet.eu/ |last=Schneider |first=Jean |date=2007-04-25 |accessdate=2007-04-25}}</ref> and the first possibly [[terrestrial planet]] discovered within a star's [[habitable zone]] being found in 2007.<ref>Udry et al. (2007). "The HARPS search for southern extra-solar planets, XI. An habitable super-Earth (5 M⊕) in a 3-planet system". Astronomy and Astrophysics preprint: preprint.</ref>  New [[Methods of detecting extrasolar planets#Future detection methods|refinements in exoplanet detection methods]], and use of existing methods from space, (such as the [[Kepler Mission]], launched in 2009) are expected to detect and characterize terrestrial-size planets, and determine if they are within the habitable zones of their stars.  Such observational refinements may allow us to better gauge how common potentially habitable worlds are, and therefore allow us a much better idea of how common life in the universe might be; this would have a profound influence over the expectations behind the Fermi Paradox itself.

=== Alien constructs ===
==== Probes, colonies, and other artifacts ====
{{see|Von Neumann probe|Bracewell probe}}
As noted, given the size and age of the universe, and the relative rapidity at which dispersion of intelligent life can occur, evidence of alien colonization attempts might plausibly be discovered. Also, evidence of exploration not containing extraterrestrial life, such as 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 the Milky Way attempted this, such probes could spread throughout the entire galaxy. Evidence of such probes might be found in the solar system—perhaps in the [[asteroid belt]] where raw materials would be plentiful and easily accessed.<ref>{{cite journal |author=Papagiannis, M. D. |title=Are We Alone or Could They be in the Asteroid Belt? |journal=Quarterly Journal of the Royal Astronomical Society |volume=19 |pages=277–281 |year=1978 |url = http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1978QJRAS..19..277P&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf |format=PDF}}</ref>

Another possibility for contact with an alien probe—one that would be trying to find human beings—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|speed-of-light]] dialogue between vastly distant neighbours. Rather than contending with the long delays a radio dialogue would suffer, a probe housing an [[artificial intelligence]] would seek out an alien civilisation to carry on a close range communication with the discovered civilization. The findings of such a probe would still have to be transmitted to the home civilisation at light speed, but an information-gathering dialogue 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> 

Since the 1950s direct exploration has been carried out on a small fraction of the 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 [[asteroid]]s, the [[Kuiper belt]], the [[Oort cloud]] and the various planetary ring systems—may yet produce evidence of alien exploration, though these regions are vast and difficult to investigate. There have been preliminary efforts in this direction in the form of the SETA and SETV projects to search for extraterrestrial artifacts or other evidence of extraterrestrial visitation within the solar system.<ref>[http://www.setv.org/ SETV projects]</ref> There have also been attempts to signal, attract, or activate [[Bracewell probe]]s in Earth's local vicinity, including 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> Many of the projects that fall under this umbrella are considered "fringe" science by 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 be all around us at this very moment, completely undetected. [[Clarke's three laws|Clarke's third law]] suggests that an alien civilization well in advance of humanity's might have means of investigation that are not yet conceivable to human beings.

==== Advanced stellar-scale artifacts ====
{{further| [[Dyson sphere]], [[Kardashev scale]], [[Alderson disk]], [[Matrioshka brain]], [[Stellar engine]]}}

[[File:Dyson Sphere Diagram-en.svg|thumb|right|250px|A variant of the speculative [[Dyson sphere]]. Such large scale artifacts would drastically alter the spectrum of a star.]]

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]] that he derived as a solution: a shell or cloud of objects enclosing a star to harness as much radiant energy as possible. Such a feat of [[astroengineering]] would drastically alter the observed [[spectroscopy|spectrum]] of the sun, changing it at least partly from the normal [[emission line]]s of a natural [[stellar atmosphere]], to that of a [[blackbody|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 (journal)|Science]],'' June 1960.</ref> 

Since then, several other theoretical stellar-scale [[megastructure]]s have been proposed, 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>[[Larry Niven|Niven, Larry]], "Bigger than Worlds", ''[[Astounding (magazine)|Analog]],'' 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 of 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 be all but invisible.

There have been some preliminary attempts to find evidence of the existence of [[Dyson sphere]]s 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,<ref>{{cite web | url = http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm | title = Fermilab Dyson Sphere search program | publisher = [http://www.fnal.gov/ ''Fermi National Accelerator Laboratory''] | dateformat = mdy | accessdate = February 10 2008 }}</ref> but such searches are preliminary and incomplete as yet.

A variant on this topic is galactic scale constructs.  This evidence is even more detrimental to the "life is common" theory.  Direct observation of thousands of galaxies shows no evidence of an artificial construction or modification.  This implies that the rate of evolution of a species which can create galaxy spanning superstructures is less than once per 10,000 galaxies per 13 billion years or 10^-18 per year.

== Explaining the paradox theoretically ==

Certain theoreticians accept that the apparent 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 out the possibility of such life, including assumptions about extraterrestrial behaviour and technology. Each of these hypothesized explanations is essentially an argument for decreasing the value of one or more of the terms in the [[Drake equation]]. The arguments are not, in general, mutually exclusive. For example, it could be that both life is rare, and technical civilizations tend to destroy themselves, or many other combinations of the explanations below.

=== No other civilizations currently exist ===

One explanation is that the human civilization ''is'' 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.  Implications of these hypotheses are examined as [[The Great Filter]].<ref>{{cite web |url=http://hanson.gmu.edu/greatfilter.html |title=The Great Filter — Are We Almost Past It? |author= [[Robin Hanson]] |year=1998}}</ref>

==== No other civilizations have arisen ====
{{see also|Rare Earth hypothesis}}
Those who believe that extraterrestrial intelligent life does not exist argue that the conditions needed for [[life]]—or at least complex life—to evolve are rare or even unique to Earth. This is known as the ''Rare Earth'' hypothesis, which attempts to resolve the Fermi paradox by rejecting the [[mediocrity principle]], and asserting that Earth is not typical, but unusual or even unique. While a unique Earth has historically been assumed on philosophical or religious grounds, the Rare Earth [[Hypothesis]] uses quantifiable and [[statistical]] arguments to argue that multicellular life is exceedingly rare in the universe because Earth-like planets are themselves exceedingly rare and/or many improbable coincidences have converged to make complex [[life on Earth]] possible.<ref>[[Peter Ward (paleontologist)|Peter Ward]] and [[Donald Brownlee]]. ''Rare Earth: Why Complex Life is Uncommon in the Universe.'' Copernicus Books. January 2000. ISBN 0-387-98701-0.</ref>  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 lend 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 (evolutionary psychologist)|Geoffrey Miller]] proposes that human intelligence is the result of runaway [[sexual selection]], which takes unpredictable directions. [[Steven Pinker]], in his book ''[[How the Mind Works]]'', cautions that the idea that evolution of life (once it has reached a certain minimum complexity) is bound to produce intelligent beings, relies on the fallacy of the "ladder of evolution": As [[evolution]] does not strive for a goal but just happens, it uses the [[adaptation]] most useful for a given [[ecological niche]], and the fact that, on Earth, this led to language-capable sentience only once so far may suggest that this adaptation is only rarely a good choice and hence by no means a sure endpoint of the evolution of a [[Tree of life (science)|tree of life]].

Another theory along these lines is that even if the conditions needed for [[life]] might be common in the universe, that the formation of life itself, a complex array of molecules that are capable simultaneously of reproduction, of extraction of base components from the environment, and of obtaining energy in a form that life can use to maintain the reaction (or the initial [[abiogenesis]] on a potential life-bearing planet), might ultimately be very rare.

Additionally, in the nondirectional meandering from initial life to humans, other low-probability happenings may have been the transition from [[Prokaryote|prokaryotic]] cells to [[Eukaryote|eukaryotic]] cells and the transition from single-cellular life to [[Multicellular organism|multicellular life]].

It is also possible that intelligence is common, but industrial civilization is not. For example, the rise of industrialism on Earth was driven by the presence of convenient energy sources such as fossil fuels. If such energy sources are rare or nonexistent elsewhere, then it may be far more difficult for an intelligent race to advance technologically to the point where we could communicate with them. There may also be other unique factors on which our civilization is dependent.

Insofar as the Rare Earth Hypothesis privileges Earth-life and its process of formation, it is a variant of the [[anthropic principle]]. The variant of the anthropic principle states the universe seems uniquely suited towards developing human intelligence. This philosophical stance opposes not only [[mediocrity principle|mediocrity]], but the [[Copernican principle]] more generally, which suggests there is no privileged location in the universe. It is also opposed by increasing evidence that humans are not the only intelligent/language/tool using/making species (or however else you define the concept) on our planet.<ref>Inside Animal Minds, National Geographic, March 2008</ref>

Opponents dismiss both Rare Earth and the anthropic principle as [[Tautology (logic)|tautological]] — if a condition must exist in the [[universe]] for human life to arise, then the universe must already meet that condition, as human life exists — and as an [[argument from ignorance|unimaginative argument]]. According to this analysis, the Rare Earth hypothesis confuses a description of how life on Earth arose with a uniform conclusion of how life ''must'' arise.<ref>Athena Andreadis. [http://www.setileague.org/reviews/rarearth.htm "E. T., Call Springer-Verlag!"] ''[[SETI]] League Publications,'' 2000.</ref> While the probability of the specific conditions on Earth being widely replicated is low, we do not know what complex life may require in order to evolve.

==== It is the nature of intelligent life to destroy itself ====
{{see also|Doomsday argument}}
Technological civilizations may usually or invariably destroy themselves before or shortly after developing radio or space flight technology. Possible means of annihilation include  [[Nuclear warfare|nuclear war]], [[biological warfare]] or accidental contamination, [[grey goo|nanotechnological catastrophe]], ill-advised physics experiments<ref>An [[Relativistic Heavy Ion Collider#Fears among the public|example of fears of civilization destroying physics experiments]].  This particular fear (particle colliders creating black holes, destroying the false vacuum, etc.) is discounted among scientists, since cosmic rays of much higher energy have been striking the Earth and moon for eons ([http://www.nytimes.com/2008/06/21/science/21blackhole.html NYT article], [http://lsag.web.cern.ch/lsag/LSAG-Report.pdf Technical report]).  But the general concern is valid.</ref>, a badly programmed super-intelligence, 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.<ref>{{cite web |title=Existential Risks
Analyzing Human Extinction Scenarios and Related Hazards |author=Nick Bostrom |url=http://www.nickbostrom.com/existential/risks.doc |accessdate=2008-07-29|format=DOC}}
</ref> Indeed, there are probabilistic arguments which suggest that humanity's end may occur sooner rather than later. In 1966 [[Carl Sagan|Sagan]] and [[Iosif Shklovsky|Shklovskii]] suggested that technological civilizations will either tend to destroy themselves within a century of developing interstellar communicative capability or master their self-destructive tendencies and survive for billion-year timescales.<ref>{{cite web | last = Darling | first = David | url = http://www.daviddarling.info/encyclopedia/E/etcivhaz.html | title = "Extraterrestrial intelligence, hazards to" | work = ''[http://www.daviddarling.info/encyclopedia/ETEmain.html The Encyclopedia of Astrobiology, Astronomy, and Spaceflight]'' | publisher = Worlds of David Darling | dateformat = mdy | accessdate = May 11 2006 }}</ref> Self-annihilation 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.<ref>{{cite web | last = Hawking | first = Stephen | url = http://www.hawking.org.uk/lectures/life.html | title = "Life in the Universe" | work = Public Lectures | publisher = University of Cambridge | dateformat = mdy | accessdate = May 11 2006}}</ref>

From a [[Charles Darwin|Darwinian]] perspective, self-destruction would be 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 the species subject to aggressive, instinctual drives. These compel humanity to consume resources, increase longevity, and to reproduce — in part, the very motives that led to the development of technological society. It seems likely that intelligent extraterrestrial life would evolve similarly and thus face the same possibility of self-destruction. And yet, for species self-destruction to provide a good answer to Fermi's Question, it would have to be very nearly universal. That is, this possibility would have a probability of very nearly 1.0. It has been suggested that a successful alien species would be a [[superpredator]], as is ''[[Homo sapiens]]''.<ref>Archer, Michael. "Slime Monsters Will Be Human Too," ''Nature Australia,'' vol. 22, 1989.</ref>

This argument does not require the civilization to entirely self-destruct, only to become once again non-technological.  In other ways it could persist and even thrive according to evolutionary standards, which postulates creating producing offspring as the sole goal of life - not "progress," technology or even intelligence.

==== It is the nature of intelligent life to destroy others ====
{{see also|technological singularity|Von Neumann probe}}
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 has been well explored in [[science fiction]]<ref name="Berserker">See, for example, [[Berserker (Saberhagen)]], [[Heechee|The Heechee Saga]] ([[Frederick Pohl|Pohl]]), [[Revelation Space]] (Reynolds)</ref> and scientific literature.<ref name="Brin"/> A species might undertake such extermination out of expansionist motives, paranoia, or simple aggression. In 1981, cosmologist [[Edward Robert Harrison|Edward Harrison]] argued that such behavior would be an act of prudence: an intelligent species that has overcome its own self-destructive tendencies might view any other species bent on galactic expansion as a kind of virus.<ref>{{cite web | last = Soter | first = Steven  
| year = 2005 | url = http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1745  | title = "SETI and the Cosmic Quarantine Hypothesis" | work = Astrobiology Magazine | publisher = [http://www.space.com Space.com] | dateformat = mdy | accessdate = May 3 2006 }}</ref>

This hypothesis requires at least one civilization to have arisen in the past, and the first civilization would not have faced this problem.<ref> Webb, Stephen. ''If the Universe Is Teeming With Aliens...Where Is Everybody?'', Springer, 2002, pp. 112</ref>  However, it could still be that Earth is alone now.  Like exploration, the extermination of other civilizations might be carried out with self-replicating spacecraft. Under such a scenario,<ref name="Berserker"/> even if a civilization that created such machines were to disappear, the probes could outlive their creators, destroying civilizations far into the future. 

If true, this argument reduces the number of visible civilizations in two ways - by destroying some civilizations, and forcing all others to remain quiet, under fear of discovery. (see [[#They choose not to interact with us|They choose not to interact with us]]) as we have seen no signs of them.

==== Human beings were created alone ====  	 
Religious and philosophical speculation about extraterrestrial intelligent life long predates the modern scientific inquiry into the subject. Some religious thinkers, including the Jewish rationalist commentator Rabbi [[Hasdai Crescas]] (c. 1340–1410/1411)<ref>Or Hashem 4:2</ref> and the Christian philosopher [[Nicholas of Cusa]] (1401–1464), posited the possibility of such extraterrestrial intelligence. On the other hand, at least some strains within the various Western religious traditions suggest the uniqueness of human beings in the divine plan and would counsel against belief in intelligent life on other worlds.<ref>{{cite web| url=http://catholiceducation.org/articles/religion/re0591.html| publisher=crisismagazine.com| title=Christianity and the Search for Extraterrestrial Life| first=Benjamin D.| last=Wiker}}</ref> 
	
Religious reasons for doubting the existence of extraterrestrial intelligent life resemble some forms of the [[Rare Earth Hypothesis]]. The argument here would be a [[teleological]] form of the strong [[anthropic principle]]: the universe was designed for the express purpose of creating human (and only human) intelligence.<ref>See, for instance: Gonzalez, Guillermo and Richards, Jay W. ''[[The Privileged Planet]],'' Regnery, 2004.</ref>

=== They do exist, but we see no evidence ===

It may be that technological extraterrestrial civilizations exist, but that human beings 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 human society refuses to admit to evidence of their presence.  

==== Communication is impossible due to problems of scale ====
{{see also|Relativity of simultaneity}}

'''&nbsp;&nbsp;&nbsp;&nbsp;Intelligent civilizations are too far apart in space or time'''
[[File:Terrestrial Planet Finder PIA04499.jpg|thumb|right|250px|[[NASA]]'s conception of the [[Terrestrial Planet Finder]].]]

It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.<ref> Webb, Stephen. ''If the Universe Is Teeming With Aliens...Where Is Everybody?'', Springer, 2002, pp. 62–71</ref> If two civilizations are separated by several thousand [[light year]]s, it is very possible that one or both cultures may become extinct before meaningful dialogue can be established. Human searches may be able to detect their existence, but communication will remain impossible because of distance. 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 may obtain meaningful information. Alternatively, a civilization may simply broadcast its knowledge, and leave it to the receiver to make what they may of it. This is similar to the transmission of information from ancient civilizations to the present.<ref>
Douglas Vakoch, [http://www.seti.org/site/apps/nl/content2.asp?c=ktJ2J9MMIsE&b=194993&ct=221014 Decoding E.T.: Ancient Tongues Point Way To Learning Alien Languages], SETI Institute, November 15, 2001</ref>

The problem of distance is compounded by the fact that timescales affording a "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, relatively speaking, that the odds of two or more such civilizations existing at the same time are low. There may have been intelligent civilizations in the galaxy before the emergence of intelligence on Earth, and there may be intelligent civilizations after its extinction, but it is possible that human beings are the only intelligent civilization in existence ''now''. The term "now" is somewhat complicated by the finite [[speed of light]] and the nature of [[spacetime]] under relativity. 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. Strictly speaking, only the portions of the universe lying within the past [[light cone]] of Earth need be considered, since any civilizations outside it could not be detected.

There is a possibility that [[Archaeology|archaeological]] evidence of past civilizations may be detected through deep space observations — especially if they left behind large artifacts such as [[Dyson sphere]]s — but this seems less likely than detecting the output of a thriving civilization.{{Fact|date=April 2009}}

A related argument holds that other civilizations exist, and are transmitting and exploring, but their signals and probes simply have not arrived yet.<ref>{{cite journal |author=Newman, W.T. and Sagan, C. |year=1981 |title=Galactic civilizations: Population. dynamics and interstellar diffusion |journal=Icarus |volume=46 |pages=293–327 |doi=10.1016/0019-1035(81)90135-4 }}</ref>  However, critics have noted that this is unlikely, since it requires we are currently at a very special point in time, when the galaxy is transitioning from empty to full.  This particular portion is just a tiny fraction of the life of a galaxy, so the odds we exist at such a moment are low.<ref>Brin, pp. 287 and 298.</ref>

'''&nbsp;&nbsp;&nbsp;&nbsp;It is too expensive to spread physically throughout the galaxy'''
{{see also|Project Daedalus|Project Orion (nuclear propulsion)|Project Longshot}}
Many assumptions about the ability of an alien culture to colonize other stars are based on the idea that interstellar travel is technologically feasible. While the 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. This idea underlies the concept of the [[Von Neumann probe]] and the [[Bracewell probe]] as evidence of extraterrestrial intelligence. 

It is possible, however, that present scientific knowledge cannot properly gauge the feasibility and costs of such interstellar colonization. Theoretical barriers may not yet be understood and the cost of materials and energy for such ventures may be so high as to make it unlikely that any civilization could afford to attempt it. Even if interstellar travel and colonization are possible, they may be difficult, leading to a colonization model based on [[percolation theory]].<ref>[[Geoffrey A. Landis|Landis, Geoffrey]]. [http://www.sff.net/people/Geoffrey.Landis/percolation.htp "The Fermi Paradox: An Approach Based on Percolation Theory"], ''Journal of the British Interplanetary Society,'' London, vol 51, page 163–166, 1998.</ref>   Colonization efforts may not occur as an unstoppable rush, but rather as an uneven tendency to "percolate" outwards, within an eventual slowing and termination of the effort given the enormous costs involved and the fact that colonies will inevitably develop a culture and civilization of their own. [[Colonization]] may thus occur in "clusters," with large areas remaining uncolonized at any one time.

A similar argument holds that interstellar physical travel may be possible, but is much more expensive than interstellar communication. Furthermore, to an advanced civilization, travel itself may be replaced by communication, through [[mind uploading]] and similar technologies.<ref>{{cite journal |author=Scheffer, L.K. |year= 1994 |title=Machine Intelligence, the Cost of Interstellar Travel and Fermi's Paradox |url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1994QJRAS..35..157S&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf
|journal=Quarterly Journal of the Royal Astronomical Society |volume=35 |pages=157|format=PDF}}</ref> Therefore the first civilization may have physically explored or colonized the galaxy, but subsequent civilizations find it cheaper, faster, and easier to travel and get information through contacting existing civilizations rather than physically exploring or traveling themselves. In this scenario, since there is little or no physical travel, and directed communications are hard to see except to the intended receiver, there could be many technical and interacting civilizations with few signs visible across interstellar distances.

'''&nbsp;&nbsp;&nbsp;&nbsp;Human beings have not been searching long enough'''

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 ''[[Homo sapiens]]'' is a geologically recent species. The whole period of modern human existence to date (about [[human evolution|200,000 years]]) is a very brief period on a cosmological scale, while [[radio]] transmissions have only been propagated since 1895. Thus it remains possible that human beings have neither been searching long enough to find other intelligences, nor been in existence long enough to be found.

One million years ago there would have been no humans for any extraterrestrial emissaries to meet. For each further step back in time, there would have been increasingly fewer indications to such emissaries that intelligent life 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. (As proposed by [[Erich von Däniken]])

<!--This quote seems to actually argue ''against'' what is said here - it should be reworked, or better explained if that is indeed the conclusion of the essay-->
This hypothesis is more plausible if alien civilizations tend to stagnate or die out, rather than expand. In addition, "the probability of a site never being visited, even [with an] infinite time limit, is a non-zero value."<ref>Kinouchi, Osame. [http://arxiv.org/abs/cond-mat/0112137 "Persistence solves Fermi Paradox but challenges SETI projects,"] ''Condensed Matter,'' 0112137 v1, December 2001.</ref> Thus, even if intelligent life expands elsewhere, it remains statistically possible that such extraterrestrial life might never discover Earth.

==== Communication is impossible for technical reasons ====

'''&nbsp;&nbsp;&nbsp;&nbsp;Human beings are not listening properly'''

There are some assumptions that underlie the [[SETI]] search programs that may cause searchers 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 scientists have decided are unlikely to carry signals, or do not penetrate our atmosphere, or use [[modulation]] strategies that are not being looked for. The signals might be at a datarate that is too fast for our electronics to handle, or too slow to be recognised as attempts at communication.  "Simple" broadcast techniques might be employed, but sent from non-[[main sequence]] stars which are searched with lower priority; current programs assume that most alien life will be orbiting [[Solar twins|Sun-like stars]].<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> 

The greatest problem is the sheer size of the radio search needed to look for signals, the limited amount of resources committed to SETI, and the sensitivity of modern instruments. 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 year]]s.<ref>[http://setifaq.org/faq.html#1.2.3 ''SETI's FAQ,'' Sec 1.2.3]</ref> Clearly detecting an Earth type civilization at great distances is difficult. 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.<ref>[http://setifaq.org/faq.html#1.6 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. Many SETI searches, starting with the venerable [[Project Cyclops]], 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.

'''&nbsp;&nbsp;&nbsp;&nbsp;Civilizations only broadcast detectable radio signals for a brief period of time'''

It may be that alien civilizations are detectable through their radio emissions for only a short time, reducing the likelihood of spotting them. There are two possibilities in this regard: civilizations outgrow radio through technological advance or, conversely, resource depletion cuts short the time in which a species broadcasts.

The first idea, that civilizations advance beyond [[radio]], is based in part on the "[[Optical fiber|fiber optic]] objection": the use of high power radio with low-to-medium gain (i.e., non-directional) antennas for long-distance [[transmission (communications)|transmission]] is wasteful of spectrum, yet this "waste" is precisely what makes these systems conspicuous at interstellar distances. Humans are moving to directional or guided transmission channels such as electrical cables, optical fibers, narrow-beam [[microwave]] and [[laser]]s, and conventional radio with non-directional antennas is increasingly reserved for low-power, short-range applications such as [[mobile phone|cell phones]] and [[Wi-Fi]] networks. These signals are far less detectable from space. Analog television, developed in the mid-twentieth century, contains strong [[carrier wave|carriers]] to aid reception and demodulation. Carriers are spectral lines that are very easily
detected yet do not convey any information beyond their highly artificial nature. Nearly every SETI project is looking for carriers for just this reason, and UHF TV carriers are currently the most conspicuous and artificial signals from Earth that could be detected at interstellar distances. But advances in technology are replacing analog TV with [[digital television]] which uses spectrum more efficiently
precisely by eliminating or reducing components such as carriers that make them so conspicuous. Using our own experience as an example, we could set the date of radio-visibility for Earth as [[December 12]], [[1901]], when [[Guglielmo Marconi]] sent radio signals from Cornwall, England, to Newfoundland, Canada.<ref>[http://www.bbc.co.uk/cornwall/marconi/marconibg.shtml BBC - Cornwall - Marconi 100<!-- Bot generated title -->]</ref>. Visibility is now ending, or at least becoming orders of magnitude more difficult, as [[Digital television#Analog switch-off|analog TV is being phased out]]. And so, if our experience is typical, a civilization remains radio-visible for approximately a hundred years. So a civilization may have been very visible from 1325 to 1483, but we were just not listening at that time. This is essentially the solution, "Everyone is listening, no one is sending."

More hypothetically, advanced alien civilizations evolve beyond broadcasting at all in the electromagnetic spectrum and communicate by principles of physics we don't yet understand. Some scientists have hypothesized that advanced civilizations may send [[neutrino]] signals.<ref>[http://www.bigear.org/vol1no3/neutrino.htm Cosmic Search Vol. 1 No. 3]</ref> If such signals exist they could be detectable by [[neutrino detector]]s that are now under construction.<ref>[http://arxiv.org/abs/0805.2429 Galactic Neutrino Communication]</ref> If stable [[wormholes]] could be created and used for communications then interstellar broadcasts would become largely redundant. Thus it may be that other civilizations would only be detectable for a relatively short period of time between the discovery of radio and the switch to more efficient technologies.

A different argument is that resource depletion will soon result in a decline in technological capability. Human civilization has been capable of interstellar radio communication for only a few decades and is already rapidly depleting fossil fuels and confronting possible problems such as [[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 societies 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 it would be virtually impossible for any one civilization to "talk" to another.

Critics of the resource depletion argument point out that an energy-consuming civilization is not dependent solely on [[fossil fuel]]s. Alternate energy sources exist, such as [[solar energy|solar power]] which is renewable and has enormous potential relative to technical barriers.<ref>[http://www.solarenergy.com/info_history.html History of Solar Energy], www.solarenergy.com</ref> For depletion of fossil fuels to end the "technological phase" of a civilization, some form of [[end of civilization|technological regression]] would have to invariably occur, preventing the exploitation of [[renewable energy]] sources.

'''&nbsp;&nbsp;&nbsp;&nbsp;They tend to experience a technological singularity'''
<!-- I think this should be a subsection of the previous one, which is about civilizations going silent. A singularity is merely a ''mechanism'' by which civilizations would only briefly use mechanisms wasteful enough to be detected at interstellar distances (or interested enough in interstellar space to deliberately attempt to communicate). -->
{{See also|Sentience Quotient|Matrioshka brain}}
Another possibility is that technological civilizations invariably experience a [[technological singularity]] and attain a [[Posthuman (Human evolution)|posthuman]] (or post''alien'') 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, and thus 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, transfer themselves into these environments through [[mind transfer]], and exist totally within virtual worlds, ignoring the external physical universe. Surprisingly early treatments, such as [[Lewis Padgett]]'s short story ''[[Mimsy were the Borogoves]]'' (1943), suggest a migration of advanced beings out of the presently known physical universe into a different and presumably more agreeable alternative one.

One version of this perspective, which makes predictions for future SETI findings of transcension "fossils" and includes a variation of the Zoo hypothesis below, has been proposed by singularity scholar [[John Smart (futurist)|John Smart]].<ref>Smart, John, [http://www.accelerating.org/articles/answeringfermiparadox.html "Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension"], ''Journal of Evolution and Technology'', June 2002.</ref>

==== They choose not to interact with us ====

'''&nbsp;&nbsp;&nbsp;&nbsp;Earth is purposely isolated (The zoo hypothesis)'''
{{main|Zoo hypothesis}}

It is possible that the belief that alien races would communicate with the human species is merely an assumption, and that alien civilizations may not wish to communicate, 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 alien civilizations avoid contact with Earth so as not to interfere with our development, or to preserve an isolated "zoo or wilderness area".<ref>John A. Ball. "The Zoo Hypothesis," ''Icarus,'' vol 19, issue 3, pp 347–349, July 1973.</ref> Another possible reason has been discussed above under ''technological singularity'' – they may be too absorbed in pursuits of their own making.

Many other reasons that an alien race might avoid contact have been proposed. Aliens might only choose to allow contact once the human race has passed certain ethical, political, or technological standards, e.g., ending poverty/war or being able to master interstellar travel. They may not want to interfere with our natural independent progress<ref>For an example from popular culture, see the Prime Directive of [[Star Trek]]</ref>, or the Earth may have been set as an explicit experiment that contact would ruin<ref>Laura Knight-Jadczyk (2006), "The High Strangeness"</ref>.

These ideas are most plausible if there is a single alien civilization within contact range, or there is a homogeneous culture or law amongst alien civilizations which dictates that the Earth be shielded. If there is a plurality of alien cultures, however, this theory may break down under the [[uniformity of motive]] flaw: all it takes is a single culture or civilization to decide to act contrary to the imperative within our range of detection for it to be abrogated, and the probability of such a violation increases with the number of civilizations.<ref>Crawford, July 2000.</ref>  This idea, and many others, become more plausible if we estimate that our galaxy has only a relatively small number of civilizations, or that all civilizations tend to evolve similar cultural values in regard to contact, or that all civilizations follow the lead of some particularly distinguished civilization (a [[hegemony]]).{{fact|date=May 2009}}

A related idea is that the perceived universe is a [[simulated reality]]. The ''planetarium hypothesis''<ref>Baxter, Stephen, 2001, [http://www.bis-spaceflight.com/sitesia.aspx/page/358/id/890/l/en The Planetarium Hypothesis: A Resolution of the Fermi Paradox], Journal of the British Interplanetary Society, Vol. 54, no. 5/6, pp. 210-216.</ref> holds that beings may have simulated a universe for us that appears to be empty of other life, by design. The ''simulation argument''<ref>Nick Bostrom, 2003, Are You Living In a Computer Simulation? Philosophical Quarterly, 2003, Vol. 53, No. 211, pp. 243-255. See also [http://www.simulation-argument.com/ this web site about the simulation argument].</ref> by Bostrom holds that although such a simulation may contain other life, such life cannot be much in advance of us since a far more advanced civilization may be correspondingly hard to simulate. 

'''&nbsp;&nbsp;&nbsp;&nbsp;It is dangerous to communicate'''

An alien civilization might feel it is too dangerous to communicate, either for us or for them.  After all, when very different civilizations have met on Earth, the results have often been disastrous for one side or the other, and the same may well apply to interstellar contact<ref><{{cite web |url=http://www.setileague.org/iaaseti/brin.pdf |title=A CONTRARIAN PERSPECTIVE ON ALTRUISM: THE DANGERS OF FIRST CONTACT |author=David Brin |accessdate=2008-12-10}}</ref>.  Even contact at a safe distance could lead to infection by computer code, or even ideas themselves ( see [[meme]] ).  Perhaps prudent civilizations actively hide not only from us but from everyone, for they fear [[#It is the nature of intelligent life to destroy others|it is the nature of intelligent life to destroy others]].

Perhaps the Fermi paradox itself - or the alien equivalent of it - is the ultimate reason for any civilization to avoid contact with other civilizations, even if no other obstacles existed. From any one civilization's point of view, it would be unlikely for them to be the first ones to make first contact and therefore likely for them to face the same possibly fatal problems that supposedly prevented the earlier civilizations from contacting them. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so. 

'''&nbsp;&nbsp;&nbsp;&nbsp;They are too alien'''
{{See also|technological singularity}}
Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Alien psychologies may simply be too different to communicate with human beings - or even to understand the concept of communication - and they are unable or unwilling to make the attempt. Human [[mathematics]], [[language]], [[tool]] use, and other concepts and communicative capacity may be parochial to Earth and not shared by other life.<ref>Schombert, James. [http://zebu.uoregon.edu/~js/cosmo/lectures/lec28.html "Fermi's paradox (i.e. Where are they?)"] ''Lectures'', University of Oregon.</ref>

'''&nbsp;&nbsp;&nbsp;&nbsp;They are non-technological'''

It is not clear that a civilization of intelligent beings must be technological. If an alien species does not develop technology, because it is difficult in its environment, because it chooses not to, or for any other reason, it will be very hard for human beings to detect.<ref name="Tarter-NYAS">J. Tarter, (2001)[http://www.annalsnyas.org/cgi/content/abstract/950/1/269 What is SETI], Annals of the New York Academy of Sciences '''950''' , pp. 269–275 (2001)</ref> Intelligence alone, as opposed to life, is not necessarily visible across interstellar distances. While there are various remote sensing techniques which could perhaps detect life-bearing planets, none of them has any ability to distinguish intelligent but non-technical life from non-intelligent life. Not even any theoretical methods for doing so have been proposed, short of an actual physical visit by an astronaut or probe. This is sometimes referred to as the "algae vs. alumnae" problem.<ref name="Tarter-NYAS"/>

==== They are here unobserved ====

It may be that intelligent alien life forms not only exist, but are already present here on Earth. They are not detected because they do not wish it, human beings are technically unable to, or because societies refuse to admit to the evidence.<ref>Webb, pp. 27–60</ref>
 
It is possible that a life form technologically advanced enough to travel to Earth might also be sufficiently advanced to exist here undetected. In this view, the aliens have arrived on Earth, or in our solar system, and are observing the planet, while concealing their presence. Observation could conceivably be conducted in a number of ways that would be very difficult to detect. For example, a complex system of microscopic monitoring devices constructed via [[molecular nanotechnology]] could be deployed on Earth and remain undetected, or sophisticated instruments could conduct passive monitoring from elsewhere.

Many [[UFO]] researchers and watchers argue that society as a whole is unfairly biased against claims of [[alien abduction]], [[List of major UFO sightings|sightings]], and [[Roswell UFO incident|encounters]], and as a result may not be fully receptive to claims of proof that aliens are visiting our planet.<ref>Swords, Michael D. "The Extraterrestrial Hypothesis and Science", pp. 368-373 in [[Jerome Clark]]'s ''The UFO Encyclopedia: The Phenomenon from the Beginning''. 2nd edition. 1998. Omingraphics. ISBN 0-7808-0097-4</ref> Others use complex [[conspiracy theory|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, as for instance the movie [[Men in Black]].

This theory was (jokingly) suggested to Fermi himself by his fellow physicist, [[Leó Szilárd]], who suggested that "they are already among us - but they call themselves [[Hungarian people|Hungarians]]", a humorous reference to the peculiar [[Hungarian language]], unrelated to most other languages spoken in Europe.

== See also ==
*[[Fermi problem]]
*[[Anthropic principle]]
*[[Interstellar travel]]

== References ==
{{reflist|2}}

== Further reading ==
*  Ben Zuckerman and Michael H. Hart, Extraterrestrials: Where Are They? ISBN 0-521-44803-4 [http://www.amazon.com/dp/0521448034 Amazon]
* {{cite book | last = Savage | first = Marshall T. | title = The Millennial Project: Colonizing the Galaxy in 8 Easy Steps | publisher = Empyrean Publishing | location = Denver | year = 1992 | isbn = 0-9633914-8-8}}
* {{cite book | last = Webb | first = Stephen | title = If the Universe Is Teeming with Aliens... Where Is Everybody? | publisher = Copernicus Books | year = 2002 | isbn = 0-387-95501-1}}
* {{cite book | last = Michaud | first = Michael | title = Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials | publisher = Copernicus Books | year = 2006 | isbn = 978-0387-28598-6}}
* {{cite book | last = Gold | first = Thomas | title = The Deep Hot Biosphere | publisher = Springer | year = 1998 | isbn = 978-0387952536 }}
* ''Evolving the Alien: The Science of Extraterrestrial Life''

== External links ==
{{Spoken Wikipedia-3|2008-05-29|Fermi_paradox_1.ogg|Fermi_paradox_2.ogg|Fermi_paradox_3.ogg}}
* [http://www.terrybisson.com/page6/page6.html They're Made Out Of Meat]
* [http://www.guardian.co.uk/science/story/0,,1993006,00.html So much space, so little time: why aliens haven't found us yet] by Ian Sample,''The Guardian'' [[January 18]], [[2007]]
* [http://www-personal.engin.umich.edu/~fritx/Ftlessay/essay.html The Possibilities of FTL: Or Fermi's Paradox Reconsidered] by F.E. Freiheit IV 
* [http://members.verizon.net/~vze3fs8i/astro/litu.pdf Life in the Universe], by [[Eric Schulman]], ''[[Mercury (magazine)|Mercury]]'' Magazine (May/June to November/December 2000)
* [http://www.accelerating.org/articles/answeringfermiparadox.html Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension] by [[John Smart (futurist)|John Smart]]
* ''[http://www.rfreitas.com/Astro/ResolvingFermi1983.htm Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox]'', which argues that our observations are incomplete, and ''[http://www.rfreitas.com/Astro/ThereIsNoFermiParadox1985.htm There Is No Fermi Paradox]'', arguing that the paradox is based on a logical flaw, both by [[Robert Freitas]]
* [http://astranaut.org/library/exotic-civilizations-beyond-ka.php Exotic Civilizations: Possible Answer to Fermi's Paradox] by Paul Hughes
* [http://www.astrobio.net/news/article242.html Fermi Paradox debate] ''Astrobiology Magazine'' July 2002. Michael Meyer, Frank Drake, Christopher McKay, Donald Brownlee, & David Grinspoon.
* [http://www.fermisparadox.com Introduction and Drake equations for the Fermi paradox]
* [http://www.sentientdevelopments.com/2007/08/fermi-paradox-back-with-vengeance.html The Fermi Paradox: Back With a Vengeance] by [[George Dvorsky]].
* [http://michaelgr.com/2008/05/09/virtual-reality-could-explain-the-fermi-paradox/ Virtual Reality Could Explain the Fermi Paradox] by Michael Graham Richard
* [http://www.manxman.com/spb/fermi/index.html Songs about Fermi's Paradox]

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[[Category:Physical paradoxes]]
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