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| {{About|the absence of evidence for extraterrestrial intelligence|the type of estimation problem|Fermi problem}}
| | I'm a 33 years old and work at the high school (Modern Languages and Classics).<br>In my free time I learn Hindi. I have been twicethere and look forward to go there sometime in the future. I like to read, preferably on my ipad. I like to watch Arrested Development and Doctor Who as well as docus about anything technological. I love Weightlifting.<br><br>my webpage; [http://happynewyeartotalboxofficecollection.in/happy-new-year-box-office-prediction-daywise/ Happy new year 6th day collection] |
| [[File:Arecibo message.svg|thumb|upright|A graphical representation of the [[Arecibo message]] – Humanity's first attempt to use radio waves to actively communicate its existence to alien civilizations]]
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| The '''Fermi paradox''' (or '''Fermi's paradox''') is the apparent contradiction between high estimates of the [[probability]] of the existence of [[extraterrestrial life|extraterrestrial civilization]] and [[human]]ity's lack of contact with, or [[evidence]] for, such civilizations.<ref>[http://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html The Washington Post]</ref> The basic points of the argument, made by [[physicists]] [[Enrico Fermi]] and [[Michael H. Hart]], are:
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| * The [[Sun]] is a young star. There are billions of [[stars]] in the [[galaxy]] that are [[1,000,000,000|billions]] of years older;
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| * some of these stars probably have Earth-like planets<ref>{{cite web |title=Alien Earths Have Been Around for a While |url=http://news.sciencemag.org/sciencenow/2012/06/scienceshot-alien-earths-have.html}}</ref> which, if the [[Earth]] is typical, may develop intelligent life;
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| * presumably, some of these civilizations will develop interstellar travel, a technology [[100 Year Starship|Earth is investigating even now]];
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| * at any practical pace of [[interstellar travel]], the [[galaxy]] can be completely [[colonized]] in a few tens of millions of years.
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| According to this line of thinking, the Earth should already have been colonized, or at least visited. But no convincing evidence of this exists. Furthermore, no confirmed [[#Empirical resolution attempts|signs of intelligence]] elsewhere have been spotted, either in our galaxy or in the more than 80 billion other galaxies of the [[observable universe]]. Hence Fermi's question, "Where is everybody?"<ref name="OSTI-19850301">{{cite web |last=Jones |first=E. M. |title="Where is everybody?" An account of Fermi's question" |url=http://www.osti.gov/cgi-bin/rd_accomplishments/display_biblio.cgi?id=ACC0055&numPages=20&fp=N |date=March 1, 1985 |publisher=[[Los Alamos National Laboratory]] (LANL), [[United States Department of Energy]] |accessdate=January 12, 2013 }}</ref>
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| {{TOC limit|limit=3}}
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| ==Overview==
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| The [[age of the universe]] and its vast number of stars suggest that unless the Earth is very atypical, extraterrestrial life should be common.<ref name="sagan">[[Carl Sagan|Sagan, Carl]] ''Cosmos,'' Random House 2002 ISBN 0-375-50832-5</ref> In an informal discussion in 1950, the [[physicist]] [[Enrico Fermi]] questioned why, if a multitude of advanced extraterrestrial civilizations exists in the [[Milky Way]] [[galaxy]], evidence such as [[alien spacecraft|spacecraft]] or [[Von Neumann probe|probes]] is not seen. A more detailed examination of the implications of the topic began with a paper by [[Michael H. Hart]] in 1975,<ref>{{cite journal
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| | title = Explanation for the Absence of Extraterrestrials on Earth
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| | last = Hart
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| | first = Michael H.
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| | journal = [[Quarterly Journal of the Royal Astronomical Society]]
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| | volume = 16 | pages = 128–135
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| | year = 1975
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| | bibcode=1975QJRAS..16..128H
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| | url=http://adsabs.harvard.edu/full/1975QJRAS..16..128H
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| }}</ref>
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| and it is sometimes referred to as the '''Fermi–Hart paradox'''.<ref>{{cite journal
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| | last = Wesson
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| | first = Paul
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| | year = 1990
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| |title = Cosmology, extraterrestrial intelligence, and a resolution of the Fermi-Hart paradox
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| | journal = [[Quarterly Journal of the Royal Astronomical Society]]
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| | volume = 31
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| | pages = 161–170
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| | bibcode = 1990QJRAS..31..161W
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| }}</ref> Other common names for the same phenomenon are ''Fermi's question'' ("Where are they?"), the ''Fermi Problem'', the ''Great Silence'',<ref name="Brin">{{cite journal
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| | author = Brin, Glen David
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| | author-link = David Brin
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| | year = 1983
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| | title = The 'Great Silence': The Controversy Concerning Extraterrestrial Intelligent Life
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| | bibcode = 1983QJRAS..24..283B
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| | journal = [[Quarterly Journal of the Royal Astronomical Society]]
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| | volume = 24
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| | pages = 283–309
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| }}</ref><ref name=Annis>{{cite arXiv
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| |eprint=astro-ph/9901322
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| |author1=James Annis
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| |title=An Astrophysical Explanation for the Great Silence
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| |class=astro-ph
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| |year=1999
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| }}</ref><ref name=Filter>{{cite web
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| | url = http://hanson.gmu.edu/greatfilter.html
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| | title = The Great Filter – Are We Almost Past It?
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| | author = [[Robin Hanson|Hanson, Robin]]
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| |year=1998
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| }}</ref><ref name=Hope>{{Cite journal
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| | last = Bostrom
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| | first = Nick
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| | author-link = Nick Bostrom
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| | title = In Great Silence there is Great Hope
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| | year = 2007
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| | url = http://www.nickbostrom.com/papers/fermi.pdf
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| | accessdate = 2010-09-06
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| }}</ref><ref name="Cirkovic2009"/> and ''silentium universi''<ref name=Cirkovic2009>{{cite journal
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| |author1=Milan M. Ćirković
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| |title=Fermi's Paradox – The Last Challenge for Copernicanism?
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| |year=2009
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| |doi=10.2298/SAJ0978001C
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| |journal=Serbian Astronomical Journal
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| |issue=178
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| |pages=1–20
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| |volume=178
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| |arxiv=0907.3432
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| |bibcode = 2009SerAJ.178....1C }}</ref><ref name=Lem>{{cite book
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| | title = His Master's Voice
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| | last = Lem
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| | first = Stanisław
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| | author-link = Stanisław Lem
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| | year = 1983
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| | publisher = Harvest Books
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| | isbn = 0-15-640300-5
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| | url = http://books.google.com/?id=I5gYWtcfMioC&lpg=PA73&dq=%22silentium%20universi%22%20lem&pg=PA73}}</ref> (Latin for "silence of the universe").
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| 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 and/or briefly that humans will never make contact with it.
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| Starting with Hart, a great deal of effort has gone into developing scientific [[hypotheses]] 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 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.
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| ==Basis==
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| 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:
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| {{quotation|The apparent size and age of the universe suggest that many technologically advanced extraterrestrial civilizations ought to exist.<br /> However, this hypothesis seems inconsistent with the lack of observational evidence to support it.}}
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| The first aspect of the paradox, "the argument by scale", is a function of the raw numbers involved: there are an estimated 200–400 billion<ref>{{cite web|url=http://www.nasa.gov/worldbook/galaxy_worldbook.html |title=NASA – Galaxy |publisher=Nasa.gov |date=November 29, 2007|accessdate = August 19, 2010}}{{dead link|date=May 2012}}</ref> (2–4 ×[[orders of magnitude|10<sup>11</sup>]]) stars in the Milky Way and 70 sextillion (7×10<sup>22</sup>) in the [[visible universe]].<ref>{{cite news|url = http://news.bbc.co.uk/1/hi/sci/tech/3085885.stm|title = Astronomers count the stars|work = BBC News|publisher = BBC|accessdate = April 8, 2010|author=Craig, Andrew |date=July 22, 2003}}</ref> Even if intelligent life occurs on only a minuscule percentage of planets around these stars, there might 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.
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| 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 at least some civilizations would be technologically advanced, seek out new resources in space and then colonize first their own [[star system]] and subsequently the surrounding star systems. Since there is no conclusive or certifiable evidence on Earth or elsewhere in the known universe of other intelligent life after 13.8 billion years of the universe's history, we have the conflict requiring a resolution. Some examples of possible resolutions are that intelligent life is rarer than we think, that our assumptions about the general development or behavior of intelligent species are flawed, or, more radically, that our current scientific understanding of the nature of the universe or reality itself is seriously incomplete.
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| 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 name=cr>Crawford, I.A., [http://www.scientificamerican.com/article.cfm?id=where-are-they "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.
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| 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?" since a sufficiently advanced civilization<ref group="Note">The [[Soviet Union|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 observable over a significant fraction of the [[Observable universe#Size|size of the observable universe]].<ref>{{harvnb|Shklovskii|Sagan|1966|p=364}}</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 range of our observation. However, no incontrovertible signs of such civilizations have been detected.
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| It is unclear whether the paradox is stronger for our galaxy or for the universe as a whole.<ref group="Note">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:
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| :<math> \int_0^{R_g} N(r) 4 \pi r^2\,dr + \int_{R_g}^\infty N(r) 4 \pi r^2 \,dr,</math>
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| 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>
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| ==Name==
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| In 1950, while working at [[Los Alamos National Laboratory]], Fermi had a casual conversation while walking to lunch with colleagues [[Emil Konopinski]], [[Edward Teller]] and [[Herbert York]].<ref>{{cite web|last = Shostak|first = Seth|date = October 25, 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]| accessdate = April 8, 2006 }}</ref> The men 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=Dunne, Alan|publisher=New Yorker, 20 May 1950 |year=1950 |accessdate = August 19, 2010}}</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 by some material object within the next ten years. Teller thinks Fermi directed the question at him, asking "Edward, what do you think? How probable is it that within the next ten years we shall have clear evidence of a material object moving faster than light?" Teller answered one in a million. Teller remembers Fermi said, "This is much too low. The probability is more like ten percent" [the well known figure for a Fermi miracle]. Konopinski did not remember the exact numbers "except that they changed rapidly as Edward and Fermi bounced arguments off each other."<ref name="Eric-Jones">Jones, Eric [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>
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| The conversation shifted to other subjects, until during lunch Fermi suddenly exclaimed, "Where are they?" (alternatively, ''"Where is everybody?"''). Teller remembers, "The result of his question was general laughter because of the strange fact that in spite of Fermi’s question coming from the clear blue, everybody around the table seemed to understand at once that he was talking about extraterrestrial life."<ref name="Eric-Jones"/>
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| Edward Teller further remembers, "I do not believe that much came of this conversation, except perhaps a statement that the distances to the next location of living beings may be very great and that, indeed, as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center." Herbert York 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]].) York writes that Enrico Fermi "followed up with a series of calculations on the probability of earthlike planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over." If so, Fermi anticipated and pre-dated many of the elements that went into the Drake equation.<ref name="Eric-Jones"/>
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| ==Drake equation==
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| {{Main|Drake equation}}
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| The theories and principles in the [[Drake equation]] are closely related to the Fermi paradox. The equation was formulated by [[Frank Drake]] in 1961, a decade after the objections raised by Enrico Fermi, in an attempt to find a systematic means to evaluate the numerous probabilities involved in the existence of alien life. The speculative equation factors in: the rate of [[star formation]] in the galaxy; the fraction of stars with planets and the number per star that are [[Planetary habitability|habitable]]; the fraction of those planets which develop life, the fraction of intelligent life, and the further fraction of detectable technological intelligent life; and finally the length of time such civilizations are detectable. The fundamental problem is that the last four terms (fraction of planets with life, odds life becomes intelligent, odds intelligent life becomes detectable, and detectable lifetime of 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]].
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| A deeper objection is that the very form of the Drake equation assumes that civilizations arise and then die out within their original star 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)|Cohen, Jack]] and [[Ian Stewart (mathematician)|Stewart, Ian]] |title=[[Evolving the Alien]] |publisher=John Wiley and Sons, Inc., Hoboken, NJ |year=2002|isbn=0-09-187927-2}} Chapter 6, ''What does a Martian look like?''.</ref>
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| The Drake equation has been used by both optimists and pessimists with wildly differing results. [[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 actual number could be far smaller. [[Frank J. Tipler|Frank Tipler]] and [[John D Barrow]] used pessimistic numbers and concluded that the average number of civilizations in a galaxy is much less than one.<ref>{{BarrowTipler1986|page=588}}</ref><ref group="Note">Note that, even though there is at least one civilization in our galaxy (namely our own), 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 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. The Drake equation computes only the long-term average number of civilizations; even if the average number of civilizations per galaxy is less than one, there could be more than one in any given galaxy at any given time.</ref> 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.<ref>Gowdy, Robert H., VCU Department of Physics [http://www.courses.vcu.edu/PHY-rhg/astron/html/mod/019/s5.html SETI: Search for ExtraTerrestrial Intelligence. The Interstellar Distance Problem], 2008</ref>
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| ==Empirical resolution attempts==
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| One obvious way to resolve the Fermi paradox would be to find conclusive evidence of extraterrestrial intelligence. Efforts to find such evidence have been made since 1960, and several are ongoing.<ref>See, for example, the [[SETI Institute]], [http://seti.harvard.edu/seti/ The Harvard SETI Home Page], or [http://seti.berkeley.edu/ The Search for Extra Terrestrial Intelligence at Berkeley]</ref> As human beings do not possess [[interstellar travel]] capability, such searches are being remotely carried out at great distances and rely on analysis of very subtle evidence. This limits possible discoveries to civilizations which alter their environment in a detectable way, or produce effects that are observable at a distance, such as radio emissions. It is very unlikely that non-technological civilizations will be detectable from Earth in the near future.
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| 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 dissimilar to those of humans.
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| ===Mainstream astronomy and SETI===
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| There are two ways that astronomy might find evidence of an extraterrestrial civilization. One is that conventional astronomers, studying stars, planets, and galaxies, might serendipitously observe some phenomenon that cannot be explained without positing an intelligent civilization as the source. This has been suspected several times. [[Pulsar]]s, when first discovered, were called [[Pulsar#History|LGMs (Little Green Men)]], because of the precise repetition of their pulses (they rival the best atomic clocks). Likewise [[Seyfert galaxy|Seyfert galaxies]] were suspected to be ''industrial accidents''<ref>{{cite book |title=Sins of Our Fathers |author=Schmidt, Stanley |year=1976|place=Berkeley}}</ref> because their enormous and directed energy output had no initial explanation. Eventually, natural explanations not involving intelligent life have been found for all such observations to date,<ref>Pulsars are now attributed to neutron stars, and Seyfert galaxies to an end-on view of the accretion onto the black holes.</ref> but the possibility of discovery remains.<ref>{{cite web |title=NASA/CP2007-214567: Workshop Report on the Future of Intelligence In The Cosmos |url=http://event.arc.nasa.gov/main/home/reports/CP2007-214567_Langhoff.pdf |publisher=NASA}}</ref> Proposed examples include asteroid mining that would change the appearance of debris disks around stars<ref>{{cite journal |title=Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence |author= Duncan Forgan, Martin Elvis |journal=International Journal of Astrobiology |date=28 March 2011 |arxiv=1103.5369 |bibcode = 2011IJAsB..10..307F |doi = 10.1017/S1473550411000127 |volume=10 |issue=4 |page=307 }}</ref> or large-scale use of solar power changing the light curve of planets measured near eclipse.<ref>{{cite conference |url=http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5207.pdf |title=Large Scale Use of Solar Power May be Visible Across Interstellar Distances |author=L. K. Scheffer |conference=Astrobiology Science Conference 2010 |year=2010}}</ref>
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| The other way astronomy might settle the Fermi paradox is through a search specifically dedicated to finding evidence of life.
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| ===Radio emissions===
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| {{Further|SETI|Project Ozma|Project Cyclops|Project Phoenix (SETI)|SERENDIP|Allen Telescope Array}}
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| [[File:parkes.arp.750pix.jpg|thumb|left|[[Radio telescope]]s are often used by SETI projects]]
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| 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
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| | 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]| accessdate = April 21, 2006 }}</ref> theoretically creating effects that might be detected over interstellar distances.
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| Sensitive observers of the [[Solar System]], for example, would note unusually intense [[radio wave]]s 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 a terrestrial civilization. It should be noted however that even much more sensitive radio telescopes than those currently available on Earth would not be able to detect non-directional radio signals even at a fraction of a light year, so it is questionable whether any such signals could be detected by an extraterrestrial civilization.<ref>SETI@home FAQ, [http://setifaq.org/faq.html#1.2.3 How far away could we detect radio transmissions?]</ref>
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| 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
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| [[optical SETI]], also exist.
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| 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 looked at each point on the sky for 72 seconds, and re-examinations of the same spot have found nothing. In 2003, [[Radio source SHGb02+14a]] was isolated by [[SETI@home]] analysis, although it has largely been discounted by further study. There are numerous technical assumptions underlying SETI that may cause human beings to miss radio emissions with present search techniques; these are discussed [[#Communication is impossible for technical reasons|below]].
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| ===Direct planetary observation===
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| [[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). Large-scale artificial lighting as produced by the human [[civilization]] is detectable from space.<ref>[http://www.centauri-dreams.org/?p=20482 The Light of Alien Cities<!-- Bot generated title -->]</ref>]]
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| Detection and classification of [[Extrasolar planet|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 able to support life will be found in the near future.
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| 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>{{cite web|author=Matsos, Helen |url=http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2085&mode=thread&order=0&thold=0 |title=Habitable Planet Signposts|publisher=Astrobiology magazine|date=June 16, 2009|accessdate = August 19, 2010}}</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).
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| However, exoplanets are rarely directly observed (the first claim to have done so was made in 2004<ref>{{cite journal|author=Chauvin, G. ''et al.''|title=A giant planet candidate near a young brown dwarf|journal=Astronomy & Astrophysics|year=2004|volume=425|issue=2|pages=L29–L32|doi = 10.1051/0004-6361:200400056|bibcode=2004A&A...425L..29C|arxiv = astro-ph/0409323|first2=A.-M.|first3=C.|first4=B.|first5=D.|first6=I.|first7=J.-L.|first8=P. }}</ref>); rather, their existence is usually inferred from the effects they have on the stars 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.
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| 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 [[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 424 such planets being detected between 1988 and 2010,<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=January 20, 2010 |accessdate = August 19, 2010}}</ref> and the first possibly [[terrestrial planet]] discovered within a star's [[habitable zone]] being found in 2007.<ref>{{cite journal|last1=Udry|first1=S.|last2=Bonfils|first2=X.|last3=Delfosse|first3=X.|last4=Forveille|first4=T.|last5=Mayor|first5=M.|last6=Perrier|first6=C.|last7=Bouchy|first7=F.|last8=Lovis|first8=C.|last9=Pepe|first9=F.|title=The HARPS search for southern extra-solar planets|doi=10.1051/0004-6361:20077612|year=2007|pages=L43|issue=3|volume=469|journal=Astronomy and Astrophysics|url=http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf|bibcode=2007A&A...469L..43U|arxiv = 0704.3841|last10=Queloz|first10=D.|last11=Bertaux|first11=J.-L. }}</ref> New [[Methods of detecting extrasolar planets#Other possible 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. Using methods like the Drake equation with this data would therefore allow 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.
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| ===Alien constructs===
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| ====Probes, colonies, and other artifacts====
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| {{Further|Von Neumann probe|Bracewell probe}}
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| As noted, given the size and age of the universe, and the relative rapidity at which dispersion of intelligent life can in principle occur, evidence of alien colonization attempts might plausibly be discovered. Evidence of exploration not containing extraterrestrial life, such as probes and information gathering devices, may also await discovery.
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| Some theoretical exploration techniques such as the [[Von Neumann probe]] (a self-replicating device) could exhaustively explore a galaxy the size of the [[Milky Way]] in as little as half a million years, with comparatively 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 all Alone, or could They be in the Asteroid Belt? |journal=[[Quarterly Journal of the Royal Astronomical Society]] |volume=19 |pages=277–281 |year=1978 |bibcode = 1978QJRAS..19..277P}}</ref>
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| 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]] 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 civilization 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 civilization at light speed, but an information-gathering dialogue could be conducted in real time.<ref>{{cite journal|last1=Bracewell|first1=R. N.|title=Communications from Superior Galactic Communities|journal=Nature|volume=186|issue=4726|page=670|year=1960|doi=10.1038/186670a0|bibcode = 1960Natur.186..670B }}</ref>
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| 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 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>{{cite web|url=http://www.setv.org/ |title=SETV projects |publisher=Setv.org |date= February 21, 2009 |accessdate=August 19, 2010}}</ref> There have also been attempts to signal, attract, or activate Bracewell probes in Earth's local vicinity, including by scientists [[Robert Freitas]] and Francisco Valdes.<ref>{{cite journal|last1=Freitas|first1=Robert A, Jr|last2=Valdes|first2=F|doi=10.1016/0094-5765(85)90031-1|title=The search for extraterrestrial artifacts (SETA)|url=http://www.setv.org/online_mss/seta83.html|year=1985|page=1027|issue=12|volume=12|journal=Acta Astronautica|accessdate = August 19, 2010}}</ref> Many of the projects that fall under this umbrella are considered [[fringe science|"fringe" science]] by astronomers and none of the projects has located any artifacts.
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| 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. The same might be true of civilizations that actively hide their investigations from us, for possible reasons described further in this article. Also, [[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.
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| ====Advanced stellar-scale artifacts====
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| {{further2| [[Dyson sphere]], [[Kardashev scale]], [[Alderson disk]], [[Matrioshka brain]], [[Stellar engine]]}}
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| [[File:Dyson Sphere Diagram-en.svg|thumb|300px|A variant of the speculative [[Dyson sphere]]. Such large scale artifacts would drastically alter the spectrum of a star.]]
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| In 1959, [[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 star. 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 star involved, 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 and searching for such an altered spectrum.<ref>{{cite journal|journal=[[Science (journal)|Science]]|year= 1960|url=http://www.islandone.org/LEOBiblio/SETI1.HTM|title=Search for Artificial Stellar Sources of Infra-Red Radiation|author= [[Freeman Dyson|Dyson, Freeman J.]]|pages= 1667–1668|volume =131|doi= 10.1126/science.131.3414.1667|pmid=17780673|issue=3414|bibcode = 1960Sci...131.1667D }}</ref>
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| 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 so as to be detectable from interstellar distances.
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| 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.
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| 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.<ref>{{cite web |url=http://www.planetary.org/news/2004/0323_Can_a_Stars_Glow_Reveal_an_Advanced.html |title=Can a Star's Glow Reveal an Advanced Civilization? |author=Amir Alexander |date=23 March 2004 |publisher=Planetary Society}}</ref><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'']| accessdate = February 10, 2008 }}</ref> These surveys have not located anything yet, though they are still incomplete. Similarly, direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.
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| ==Explaining the paradox hypothetically==
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| Certain theoreticians accept that the apparent absence of evidence implies 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 both that life is rare and that technical civilizations are short lived, or many other combinations of the explanations below.<ref>{{cite journal |title=W (h) ither the Drake equation?
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| |author=Burchell, M.J.
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| |journal=International Journal of Astrobiology
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| |volume=5
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| |issue=3
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| |pages=243–250
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| |year=2006
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| |doi = 10.1017/S1473550406003107
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| |bibcode = 2006IJAsB...5..243B }}</ref>
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| ===Few, if any, other civilizations currently exist===
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| One explanation is that the human civilization ''is'' alone (or very nearly so) 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 name="Filter" />
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| ====No other civilizations have arisen====
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| {{See also|Rare Earth hypothesis}}
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| 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 name="rare-earth">{{cite book |title=Rare Earth: Why Complex Life is Uncommon in the Universe |first1=Peter D. |last1=Ward |authorlink1=Peter Ward (paleontologist) |first2=Donald |last2=Brownlee |authorlink2=Donald Brownlee |date=January 14, 2000 |page=368 |publisher=Springer |edition=1st |isbn=978-0-387-98701-9}}</ref> It is possible that complex life may evolve through other mechanisms than those found specifically here on Earth,<ref name="rare-earth"/> but the fact that in the 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 lends more credence to the idea of technologically advanced civilizations being rare in the universe.
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| 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": Since [[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 technological intelligence only once so far may suggest that this outcome of natural selection is rare and hence by no means a certain development of the evolution of a [[tree of life (science)|tree of life]].
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| Another hypothesis estimates that [[evolution]] to the stage of intelligent life required about ten billion years, rather than five billion years normally assumed.<ref>{{cite arXiv |author=Alexei A. Sharov, Richard Gordon |title=Life Before Earth |year=1970 |eprint=1304.3381 |class=physics.gen-ph }}</ref> This implies that the first five billion years of our own evolution occurred in other locations, for instance planets that existed before the Earth. According to this hypothesis, intelligent life is rare because evolution has only now reached the stage where intelligent life has a reasonable probability of occurring. (See [[Rare Earth hypothesis#Enough time elapsed since the big bang for evolution to occur|Enough time elapsed since the big bang for evolution to occur]].)
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| Another hypothesis along these lines is that, even if conditions needed for [[life]] are common in the universe, the initial [[abiogenesis]] on a potentially life-bearing planet – the formation of life itself – the existence of a complex array of molecules that are simultaneously capable of reproduction, extraction of base components from the environment, and obtaining energy in a form that can be used to maintain the reaction, might ultimately be very rare.
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| 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 (with separate nucleus, organelles, specialization, and a cytoskeleton allowing the cell to take on different shapes) and the transition from single-cellular life to [[multicellular organism|multicellular life]], which was recorded in the [[Cambrian Explosion]] of 530 million years ago when significant numbers of organisms evolved hard body parts, although multicellular life perhaps first started to evolve a couple of hundred million years before that. Single celled life emerged [[Evolutionary history of life|c. 3.5 billion years ago]], and for most of Earth's history and for reasons not fully understood there have only been single-celled creatures. And even fundamental conditions such as the chemical composition of the nursery nebula from which a planetary system forms could have unusual or detrimental consequences for the emergence and survival of life.
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| And there are many other potential branching points. For example, perhaps the transition from ocean creatures to land-dwelling creatures crucially depends on an unusually large moon and significant [[tide]]s. Many astronomers refer to our Earth-Moon pairing as a double planet. This ratio between parent and satellite is rare in our planetary system. There is no observational data on the numbers of "double planets" in other planetary systems. The obliquity of Earth's axis of rotation is 23.5° with {{unicode|±}}1.5° variation over 41,000 years. Mars has an obliquity of 25° with {{unicode|±}}10° variation over 100,000 years, and calculations indicate over longer time periods, Mars' obliquity changes chaotically, perhaps ranging from 0° to 60° over the last 10 million years. With no Moon to act as a stabilizing influence, Earth's obliquity might also wander chaotically.<ref>{{harvnb|Webb|2002|pp=188–89}}</ref>
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| 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 alien race to advance technologically to the point where humans could communicate with them. There may also be other unique factors on which our civilization is dependent. Or, on a water world, where the intelligent creatures are something like dolphins, it may be difficult to build fire and forge metals.
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| Another possibility is that Earth is the first planet in the Milky Way on which industrial civilization has arisen.<ref>{{harvnb|Webb|2002|p=153}}</ref> However, critics note that, according to current understanding, many Earth-like planets were created many billions of years prior to Earth, so this explanation requires repudiation of the [[mediocrity principle]].<ref>{{harvnb|Webb|2002|pp=153–56}}</ref>
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| Insofar as the Rare Earth Hypothesis privileges life on Earth 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 the [[mediocrity principle]], but also the wider [[Copernican principle]], which suggests there is no privileged location in the universe.
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| 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 incredulity|argument from incredulity or lack of imagination]]. 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>Andreadis, Athena [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.<ref>{{cite journal |doi=10.1098/rstb.1999.0532 |title=Life: past, present and future
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| |author=Nealson, K.H. and Conrad, P.G.
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| |journal=Philosophical Transactions of the Royal Society B
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| |volume=354
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| |issue=1392
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| |pages=1923–39
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| |year=1999
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| |publisher=The Royal Society |pmid=10670014
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| |pmc=1692713 }}</ref><ref>{{cite journal |title=How Rare Is Complex Life in the Milky Way?
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| |author=Bounama, C. and von Bloh, W. and Franck, S.
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| |journal=Astrobiology
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| |volume=7
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| |issue=5
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| |pages=745–756
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| |year=2007 |pmid=17963474
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| |publisher=Mary Ann Liebert, Inc.
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| |doi=10.1089/ast.2006.0084
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| |bibcode=2007AsBio...7..745B}}</ref>
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| ====It is the nature of intelligent life to destroy itself====
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| This is the argument that 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, [[climate change]], [[grey goo|nanotechnological catastrophe]], ill-advised physics experiments,<ref group="Note">An [[Relativistic Heavy Ion Collider#Critics of high energy experiments|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]).</ref> a badly programmed super-intelligence, or a [[Malthusian catastrophe]] after the deterioration of a planet's [[Earth science#Earth's spheres|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=Bostrom, Nick|url=http://www.nickbostrom.com/existential/risks.html |accessdate=October 4, 2009}}
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| </ref> Indeed, there are probabilistic arguments which suggest that human extinction may occur sooner rather than later. In 1966 Sagan and [[Iosif Shklovsky|Shklovskii]] speculated 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|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|archiveurl = http://web.archive.org/web/20060421051343/http://www.hawking.org.uk/lectures/life.html|archivedate = April 21, 2006|title = Life in the Universe|work = Public Lectures|publisher = University of Cambridge|accessdate = May 11, 2006}}</ref>
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| From a [[Charles Darwin|Darwinian]] perspective, self-destruction would be an ironic 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, extend 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 in a similar fashion and thus face the same possibility of self-destruction. And yet, to provide a good answer to Fermi's Question, self-destruction by technological species (or ''any'' sociological explanation) would have to be a near universal occurrence. Otherwise, the few civilizations to which it does not apply would colonize the galaxy.
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| 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 postulate producing offspring as the sole goal of life—not "progress", be it in terms of technology or even intelligence.<ref>{{cite book |title=The Selfish Gene
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| |author=Dawkins, Richard
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| |year=2006
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| |publisher=Oxford University Press, USA}}</ref>
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| ====It is the nature of intelligent life to destroy others====
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| {{See also|Technological singularity|Von Neumann probe}}
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| Another possibility is that an intelligent species beyond a certain point of technological capability will destroy other intelligence as it appears, as is exemplified by the theorised extermination of Neanderthals by early humans. The idea that something, or someone, is destroying intelligent life in the universe has been well explored in [[science fiction]]<ref name="Berserker" group="Note">See, for example, [[Berserker (Saberhagen)]], [[Heechee|The Heechee Saga]] ([[Frederik 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
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| | 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]|accessdate = May 3, 2006 }}</ref> It has also been suggested that a successful alien species would be a [[superpredator]], as is ''[[Homo sapiens]]''.<ref>{{cite journal|author=Archer, Michael|title=Slime Monsters Will Be Human Too|journal=Aust. Nat. Hist|volume=22|pages=546–547|year=1989}}</ref>
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| This hypothesis requires at least one civilization to have arisen in the past, and the first civilization would not have faced this problem.<ref>{{harvnb|Webb|2002|p=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" group="Note"/> even if a civilization that created such machines were to disappear, the probes could outlive their creators, destroying civilizations far into the future.
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| If true, this argument reduces the number of visible civilizations in two ways—by destroying some civilizations, and forcing others to remain quiet, under fear of discovery (see [[#They choose not to interact with us|They choose not to interact with us]]) so we would see no signs of them. This may also make it impossible for life to evolve in regions of the universe close to a developed civilization, assuring that any new civilizations will start off far away from preexisting ones.
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| ====Life is periodically destroyed by naturally occurring events====
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| On Earth, there have been numerous major [[extinction events]] that destroyed the majority of complex species alive at the time. The extinction of the [[dinosaur]]s is the best known example. These are believed to be caused by events such as impact from a large meteorite, massive volcanic eruptions, or astronomical events such as [[gamma ray burst]]s.<ref>{{cite journal |doi=10.1017/S1473550404001910 |title=Did a gamma-ray burst initiate the late Ordovician mass extinction? |author=Melott, A.L. and Lieberman, BS and Laird, CM and Martin, LD and Medvedev, MV and Thomas, BC and Cannizzo, JK and Gehrels, N. and Jackman, CH |journal=International Journal of Astrobiology |volume=3 |issue=1 |pages=55–61 |year=2004 |publisher=Cambridge Univ Press |url = http://acdb-ext.gsfc.nasa.gov/People/Jackman/Melott_2004.pdf|arxiv = astro-ph/0309415 |bibcode = 2004IJAsB...3...55M }}</ref> It may be the case that such extinction events are common throughout the universe and periodically destroy intelligent life (or at least destroy their civilizations) before the species is able to develop the technology to communicate with other species.<ref>{{cite book |title=Global catastrophic risks |author=Nick Bostrom, Milan M. Ćirković}} Section 12.5 – The Fermi Paradox and Mass Extinctions.</ref>
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| ====Human beings were created alone====
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| Religious and philosophical speculation about extraterrestrial intelligent {{not a typo|life long}} predates modern scientific inquiry into the subject. Greek philosophers [[Leucippus]], [[Democritus]], and [[Epicurus]] (5th and 4th centuries BC) suggested that there may be [[Cosmic pluralism|other inhabited worlds]]. Some religious thinkers, including the Jewish philosopher Rabbi [[Hasdai Crescas]] (c. 1340–1410/1411) and the Christian philosopher [[Nicholas of Cusa]] (1401–1464), also put forward their views of the possibility of such extraterrestrial intelligence.
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| On the other hand, philosophers such as [[Aristotle]] and religious thinkers such as [[Thomas Aquinas]] claim that human beings are unique in the divine plan and 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|year=2002| first=Benjamin D.| last=Wiker}}</ref> Aristotle believed the element of the heavens was [[Fire (classical element)|Fire]], as opposed to [[Earth (classical element)|Earth]], and so the heavens could not support life.<ref>{{harvnb|Crowe|2008|p=9}}</ref> Thomas Aquinas additionally believed the uniqueness of [[God in Christianity|God]] implied the uniqueness of Earth, and also notes the [[Christian biblical canons|Bible]] refers to the world in the singular.<ref>{{harvnb|Crowe|2008|p=19}}</ref>
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| 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>{{cite book|url=http://books.google.com/books?id=KFdu4CyQ1k0C&pg=PA306|page=306|author=Gonzalez, Guillermo and Richards, Jay W. |title=The Privileged Planet|year=2004|isbn=0-89526-065-4|publisher=Regnery Publishing}}</ref> This argument presupposes that a prior advanced intelligence existed in order to create human life, which might pose the question whether that intelligence was the only one to exist before it created us, but the perspective is a philosophical and abstract one.
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| ====Inflation hypothesis and the Youngness Argument====
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| Cosmologist [[Alan Guth]] proposed a multi-verse solution to the Fermi paradox. In this scenario, using the synchronous gauge probability distribution, young universes exceedingly outnumber older ones (by a factor of e<sup>10<sup>37</sup></sup> for every second of age). Therefore, averaged over all universes, universes with civilizations will almost always have just one, the first to develop. However, Guth notes "Perhaps this argument explains why SETI has not found any signals from alien civilizations, but I find it more plausible that it is merely a symptom that the synchronous gauge probability distribution is not the right one."<ref>{{cite journal|url = http://arxiv.org/PS_cache/hep-th/pdf/0702/0702178v1.pdf|journal = Journal of Physics A: Mathematical and Theoretical|title = Eternal Inflation and its Implications|volume = 40|issue = 25|page = 6811 | first = Alan|year = 2007 | last = Guth|doi=10.1088/1751-8113/40/25/S25|arxiv = hep-th/0702178 |bibcode = 2007JPhA...40.6811G }}</ref>
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| ===They do exist, but we see no evidence===
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| It may be that technological extraterrestrial civilizations exist, but that human beings cannot communicate with them because of constraints: problems of scale or of technology; because they do not wish to communicate or their nature is simply too alien for meaningful communication, or perhaps even be recognized as technology. Think of the immense differences between a blue whale and a common ant, both from the same planet.
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| ====Communication is improbable due to problems of scale====
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| {{See also|Relativity of simultaneity}}
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| =====Intelligent civilizations are too far apart in space or time=====
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| [[File:Terrestrial Planet Finder PIA04499.jpg|thumb|[[NASA]]'s conception of the [[Terrestrial Planet Finder]]]]
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| 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>{{harvnb|Webb|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>{{cite web|author=Vakoch, Douglas |title=Decoding E.T.: Ancient Tongues Point Way To Learning Alien Languages|publisher=SETI Institute|date= November 15, 2001|accessdate = August 19, 2010|url=http://www.space.com/searchforlife/seti_decode_011115.html}}</ref> and humanity has undertaken similar activities like the [[Arecibo message]], which could transfer information about Earth's intelligent species, even if it never yields a response (or does not yield a response in time for humanity to receive it). It is also possible 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.
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| 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. Another issue is the possibly very small length of time (even in historical timescales) that a civilization might be "loudly" broadcasting material that could be reasonably detected (see below).
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| 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 |issue=3 |pages=293–327 |doi=10.1016/0019-1035(81)90135-4 |bibcode=1981Icar...46..293N}}</ref> However, critics have noted that this is unlikely, since it requires that humanity's advancement has occurred at a very special point in time, while the Milky Way is in transition from empty to full. This is a tiny fraction of the life of a galaxy under ordinary assumptions and calculations resulting from them, so the likelihood that we're in the midst of this transition is considered low in the paradox.<ref>{{cite journal |author= Brin, Glen David |year=1983 |title= The 'Great Silence': The Controversy Concerning Extraterrestrial Intelligent Life |bibcode= 1983QJRAS..24..283B|journal=[[Quarterly Journal of the Royal Astronomical Society]]|volume=24 |pages=287, 298}}</ref> Work on the theory of [[neocatastrophism]], wherein galactic and even super-galactic dynamics are seen as possibly frequently injurious to extant biospheres in a way that is roughly analogous to the way [[Geology|geological]] and [[Climate|climatological]] [[catastrophy|catastrophes]] have occasionally set back [[Biology|biological]] developments on [[Earth]], might be given as a partial, if not full, resolution to the paradox, as advanced species might well be fragile to major events at a pace that would argue against a short transition.
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| =====It is too expensive to spread physically throughout the galaxy=====
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| {{See also|Project Daedalus|Project Orion (nuclear propulsion)|Project Longshot}}
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| 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, even though the engineering required is considerably beyond present capabilities. This idea underlies the concept of the [[Von Neumann probe]] and the [[Bracewell probe]] as evidence of extraterrestrial intelligence.
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| 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>{{cite journal|authorlink=Geoffrey A. Landis|author=Landis, Geoffrey|url=http://www.sff.net/people/Geoffrey.Landis/percolation.htp |title=The Fermi Paradox: An Approach Based on Percolation Theory|journal=Journal of the British Interplanetary Society|volume= 51|pages= 163–166|year= 1998|bibcode = 1998JBIS...51..163L }}</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.{{or|date=December 2013}}
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| 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 |bibcode=1994QJRAS..35..157S|journal=[[Quarterly Journal of the Royal Astronomical Society]] |volume=35 |page=157}}</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.
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| Another economic argument is that although advanced virtual civilizations — possibly en route developmentally to a [[Matrioshka Brain]] — could engage in travel to other star systems, they choose not to. This is not due to a lack of curiosity, but more through a set of energy-information economic choices, whereby in an information market predicated on available stellar energy and planetary matter for building more computing capacity, the most successful virtual intelligences have to remain central to the star. Energy and proximity (and therefore wireless communication bandwidth and speed) are much greater closer to the matter and energy sources of the star, and larger planets, and so to be successful requires focus on their home planetary system. In this scenario, economic incentives to travel out of a star system are inhibited.<ref>{{cite book |author=[[Charles Stross]] |title=[[Accelerando (novel)|Accelerando]] |publisher=Ace |year=2006 |isbn=978-0441014156}}</ref>
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| =====Human beings have not been searching long enough=====
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| 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.
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| 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 [[ancient astronauts|interpreted by early human cultures as supernatural entities]].
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| <!--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-->
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| 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>{{cite arXiv|author=Kinouchi, Osame|eprint=cond-mat/0112137 |title=Persistence solves Fermi Paradox but challenges SETI projects|date=December 2001 |pages=12137|class=cond-mat.dis-nn }}</ref> Thus, even if intelligent life expands elsewhere, it remains statistically possible that such extraterrestrial life might never discover Earth.
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| ====Communication is improbable for technical reasons====
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| =====Humans are not listening properly=====
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| 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 [[Data compression|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 (e.g., [[gamma ray]]s), or use [[modulation]] strategies that are not being looked for. The signals might be at a data rate 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>{{cite journal|last1=Turnbull|first1=Margaret C.|last2=Tarter|first2=Jill C.|doi=10.1086/345779|title=Target Selection for SETI. I. A Catalog of Nearby Habitable Stellar Systems|url=http://www.projectrho.com/HabCat.pdf|year=2003|page=181|issue=1|volume=145|journal=The Astrophysical Journal Supplement Series|accessdate=August 19, 2010|bibcode=2003ApJS..145..181T|arxiv = astro-ph/0210675 }}</ref>
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| The greatest problem is the sheer size of the radio search needed to look for signals (effectively spanning the entire visible universe), 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>{{cite web|url=http://setifaq.org/faq.html#1.2.3 |title=''SETI's FAQ,'' Sec 1.2.3 |publisher=Setifaq.org |date=November 2003 |accessdate=July 3, 2010}}</ref> Clearly detecting an Earth type civilization at great distances is difficult. A signal is much easier to detect if the signal energy is limited to 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>{{cite web|url=http://setifaq.org/faq.html#1.6 |title=SETI's FAQ, Sec 1.6 |publisher=Setifaq.org |date=November 2003 |accessdate=July 3, 2010}}</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.
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| Thus to detect alien civilizations through their radio emissions, Earth observers either need more sensitive instruments or must hope for fortunate circumstances: that the broadband radio emissions of alien radio technology are much stronger than our own (e.g., [[gamma-ray burst]]s); 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.
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| =====Aliens aren't monitoring Earth because Earth is not superhabitable=====
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| The Milky Way contains billions of worlds that may support life. Listening for radio signals from all planets is too complex and requires too many separate telescopes and monitoring devices, so aliens focus on superhabitable planets. They ignore the Earth, which is only marginally habitable.<ref>{{cite web|last=Heller|first=Rene|title=Superhabitable Worlds|url=http://arxiv.org/abs/1401.2392|work=Astrobiology|accessdate=28 January 2014}}</ref>
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| =====Civilizations broadcast detectable radio signals only for a brief period of time=====
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| 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.
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| 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-20th 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 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 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>{{cite web|url=http://www.bbc.co.uk/cornwall/marconi/marconibg.shtml |author=Anonymous|title=BBC – Cornwall – Marconi 100 |publisher=Bbc.co.uk |date=December 2001|accessdate=July 3, 2010}}</ref> Visibility is now ending, or at least becoming orders of magnitude more difficult, as [[Digital television transition|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."
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| 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>{{cite web|url=http://www.bigear.org/vol1no3/neutrino.htm |title=Cosmic Search Vol. 1 No. 3 |publisher=Bigear.org |date=September 21, 2004 |accessdate=July 3, 2010}}</ref> If such signals exist they could be detectable by [[neutrino detector]]s that are now under construction.<ref>{{cite journal|last1=Learned|first1=J|last2=Pakvasa|first2=S|last3=Zee|first3=A|title=Galactic neutrino communication|arxiv=0805.2429|journal=Physics Letters B|volume=671|issue=1|page=15|year=2009 |doi=10.1016/j.physletb.2008.11.057|bibcode = 2009PhLB..671...15L }}</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.
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| One counter to this argument is that although broadcast communication may become difficult to detect, other uses for radio such as radar and power transmission cannot be replaced by low power technologies or fiber optics. These will potentially remain visible even after broadcast emission are replaced by less observable technology.<ref>{{cite journal |title=Solar Power Satellites as Interstellar Beacons |author=Stephenson, D. G |bibcode=1984QJRAS..25...80S |journal=[[Quarterly Journal of the Royal Astronomical Society]]|volume=25 |issue=1 |page=80 |year=1984 |publisher=Royal Astronomical Society }}</ref>
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| 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 us to continue the search. If the same conditions regarding energy supplies hold true for other civilizations, then radio technology may be a short-lived phenomenon. Unless two civilizations happen to be near each other and develop the ability to communicate at the same time it would be virtually impossible for any one civilization to "talk" to another.
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| Critics of the resource depletion argument point out that alternate energy sources exist, such as [[solar energy|solar power]], which are renewable and have enormous potential relative to technical barriers.<ref>{{cite web|url=http://www.solarenergy.com/info_history.html |title= History of Solar Energy|publisher= Technology Review: Solar Power|date=July 1995 |author=Smith, Charles |archiveurl = http://web.archive.org/web/20080225114548/http://www.solarenergy.com/info_history.html |archivedate = 2008-02-25}}</ref> For depletion of fossil fuels to end the "technological phase" of a civilization, some form of technological regression would have to invariably occur, preventing the exploitation of [[renewable energy]] sources.
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| =====They tend to experience a technological singularity=====
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| <!-- 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). -->
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| {{See also|Sentience Quotient|Matrioshka brain}}
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| Another possibility is that technological civilizations invariably experience a [[technological singularity]] and attain a [[posthuman]] (or more properly, post-biological) character. Hypothetical civilizations of this sort may have advanced 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 though we do ascribe a form of intelligence to them. For example, a [[Superintelligence|superintelligent]] civilization might consist of an advanced megastructure such as a [[Matrioshka brain]] or a [[black hole]] and communicate using [[neutrinos]] or by [[gamma-ray bursts]] at bandwidths that exceed our receiving capabilities.
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| 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 uploading]], 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.
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| 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://accelerating.org/articles/transcensionhypothesis.html "The Transcension Hypothesis: Sufficiently Advanced Civilizations May Invariably Leave Our Universe, and Implications for METI and SETI"], ''Acta Astronautica'', 78:55-68, 2012.</ref>
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| =====They are too busy online=====
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| It may be that intelligent alien life forms cause their own "increasing disinterest" in the outside world.<ref>{{harvnb|Webb|2002|p=86}}</ref> Perhaps any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, and that the rate of appeal of these social contrivances is destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization.
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| =====They are too alien=====
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| Another possibility is that human theoreticians have underestimated how much alien life might differ from that on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Perhaps human [[mathematics]] is parochial to Earth and not shared by other life,<ref>Schombert, James. [http://abyss.uoregon.edu/~js/cosmo/lectures/lec28.html "Fermi's paradox (i.e. Where are they?)"] ''Cosmology Lectures'', University of Oregon.</ref> though others argue this can only apply to abstract math since the math associated with physics must be similar (in results, if not in methods.)<ref>{{cite journal |title=Mathematics on a distant planet
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| |author=Hamming, RW
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| |journal=The American mathematical monthly
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| |volume=105
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| |issue=7
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| |pages=640–650
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| |year=1998
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| |publisher=
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| |jstor=2589247 |doi=10.2307/2589247 }}</ref>
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| Physiology might also cause a communication barrier. In ''[[Contact (novel)|Contact]]'', Carl Sagan briefly speculated that an alien species might have a thought process orders of magnitude slower (or faster) than humans. Such a species could conceivably speak so slowly that it requires years to say even a simple phrase like "Hello". A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.
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| =====They are non-technological=====
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| It may be that at least some civilizations of intelligent beings are not technological, perhaps because it is difficult in their environment, or because they choose not to, or for other reasons yet unknown. Such civilizations would be very hard for humans to detect.<ref name="Tarter-NYAS">{{cite journal|last1=Tarter|first1=Jill|title=What is SETI?a|journal=Annals of the New York Academy of Sciences|volume=950|issue=1|pages=269–75|year=2006|pmid=11797755|doi=10.1111/j.1749-6632.2001.tb02144.x|bibcode = 2001NYASA.950..269T }}</ref> While there are remote sensing techniques which could perhaps detect life-bearing planets without relying on the signs of technology,<ref>{{cite journal
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| |title=Detecting Life-bearing Extrasolar Planets with Space Telescopes
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| |bibcode=2008ApJ...684.1404B
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| |author1=Steven V. W. Beckwith
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| |doi=10.1086/590466
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| |issue=2,
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| |journal=The Astrophysical Journal
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| |volume=684
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| |page=1404
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| |year=2008
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| |publisher=IOP Publishing|arxiv = 0710.1444 }} [http://arxiv.org/abs/0710.1444 Arxiv preprint].</ref><ref>{{cite journal
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| |title={Detection of circular polarization in light scattered from photosynthetic microbes
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| |author=Sparks, W.B. and Hough, J. and Germer, T.A. and Chen, F. and DasSarma, S. and DasSarma, P. and Robb, F.T. and Manset, N. and Kolokolova, L. and Reid, N. and others},
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| |journal=Proceedings of the National Academy of Sciences
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| |volume=106
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| |number=19
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| |pages=7816
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| |year=2009
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| |publisher=National Acad Sciences
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| |url=http://www.pnas.org/content/early/2009/04/28/0810215106.full.pdf
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| |doi=10.1016/j.jqsrt.2009.02.028}}
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| </ref> none of them has any ability to tell if any detected life is intelligent. 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"/>
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| =====The evidence is being suppressed=====
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| It is theoretically possible that SETI groups are not reporting positive detections, or governments have been blocking extraterrestrial signals or suppressing publication of detections. This response might be attributed to National Security and Trade Interests from the potential use of advanced extraterrestrial technology or weapons. It has been suggested that the detection of an extraterrestrial radio signal or technology could well be the most highly classified military information that exists.<ref>{{cite journal |author=A. Tough |title=A critical examination of factors that might encourage secrecy |journal=Acta Astronautica |issue=21, |year=1990 |pages=97–102 |doi=10.1016/0094-5765(90)90134-7 |volume=21}}</ref> Claims that this has already happened are common in the popular press,<ref>{{cite web |url=http://www.theregister.co.uk/2006/07/31/signals_seti/ |title=SETI urged to fess up over alien signals |publisher=The Guardian |author=Ashley Vance | date=31 July 2006}}</ref><ref>{{cite web |url=http://www.huffingtonpost.com/2011/12/06/new-round-of-ufo-white-house-fight_n_1125873.html |title=UFO Hunters Keep Pressing White House For Answers Through 'We The People' Petitions |publisher=Huffington Post |date=6 Dec 2011}}</ref> but the scientists involved report the opposite experience – the press becomes informed and interested in a potential detection even before a signal can be confirmed.<ref>{{cite book |author=G. Seth Shostak |title=Confessions of an Alien Hunter: A Scientist's Search for Extraterrestrial Intelligence |publisher=National Geographic |isbn=978-1-4262-0392-3 |year=2009}} Page 17.</ref> Another issue is the diverse number of organisations and governments involved in science activities that might chance upon detections, of which SETI forms only a small part. Numerous [[conspiracy theories]] have been proposed, including the possibility of extraterrestrial life at [[Area 51]].
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| ====They choose not to interact with us====
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| In these scenarios, alien civilizations exist that are technically capable of contacting Earth, but explicitly choose not to do so.
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| This is the official position of the Earth today; we listen ([[SETI]]), but except for a few small efforts, do not [[Active SETI|explicitly transmit]].
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| Of course if all, or even most, civilizations act the same way, the galaxy could be full of civilizations eager for contact, but everyone is listening and no-one is transmitting. This is the so-called ''SETI Paradox''.<ref>{{cite arXiv |eprint=physics/0611283 |title=The SETI paradox |author=Alexander Zaitsen |class=physics.gen-ph |year=2006}}</ref>
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| =====They don't agree among themselves=====
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| Official policy within SETI community<ref>{{cite web |url=http://www.seti.org/post-detection.html |title=Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence }}</ref> is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place." However, given the possible impact of any reply<ref>{{cite doi|10.1016/S0265-9646(03)00019-5}}</ref> it may be very difficult to obtain any consensus on "Who speaks for Earth?" and "What should we say?" Other civilizations might suffer from this same lack of consensus, and therefore send no messages at all.
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| =====Earth is purposely not contacted (The zoo hypothesis)=====
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| [[File:Matrix sphere.jpg|thumb|Schematic representation of a planetarium simulating the universe to humans. The "[[reality|real]]" universe is outside the black sphere, the simulated one projected on/filtered through it.]]
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| {{Main|Zoo hypothesis}}
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| The zoo hypothesis states that superintelligent extraterrestrial life exists and does not contact life on Earth to allow for its natural evolution and development.<ref>{{cite journal|last1=Ball|first1=J|title=The zoo hypothesis|journal=Icarus|volume=19|issue=3|page=347|year=1973|doi=10.1016/0019-1035(73)90111-5|bibcode=1973Icar...19..347B}}</ref>
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| These ideas are perhaps most plausible if there is a relatively universal cultural or legal policy among a plurality of extraterrestrial civilizations necessitating isolation with respect to alien life. In a Universe without a hegemonic power, random civilizations with independent principles would, in all likelihood, make contact. This makes a crowded Universe with clearly defined rules seem more plausible.
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| 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 name=cr/> However, perhaps a sufficiently technologically and socially advanced civilization would be capable of enforcing rules.
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| T. W. Hair<ref>Hair, Thomas W. (2011). "Temporal Dispersion of the Emergence of Intelligence: An Inter-arrival Time Analysis", ''International Journal of Astrobiology'' 10(2): 131–135 (2011), {{doi|10.1017/S1473550411000024}} [http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8143571]</ref> has done [[Monte Carlo method|Monte Carlo]] analysis of the inter-arrival times between civilizations in the galaxy based on common astrobiological assumptions that suggest that since the initial civilization would have such a commanding lead over the later arrivals, it may have established what we call ''zoo hypothesis'' as a galactic/universal norm and the resultant "paradox" by a cultural [[founder effect]] with or without the continued activity of the founder.
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| Compare with the [[Prime Directive]] from ''[[Star Trek]]''.
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| =====Earth is purposely isolated (planetarium hypothesis)=====
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| {{Main|Planetarium hypothesis}}
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| A related idea is that, beyond a certain distance, the perceived universe is a [[simulated reality]]. The planetarium hypothesis<ref>{{cite journal|author=Baxter, Stephen|year=2001|title=The Planetarium Hypothesis: A Resolution of the Fermi Paradox|journal=Journal of the British Interplanetary Society|volume= 54|issue=5/6|pages=210–216|bibcode = 2001JBIS...54..210B }}</ref> holds that beings may have created this simulation so that the universe appears to be empty of other life.
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| =====It is dangerous to communicate=====
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| 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 [[Spanish conquest of the Inca Empire|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=Brin, David|accessdate=August 19, 2010}}</ref> Even contact at a safe distance could lead to infection by computer code<ref>{{cite journal|title=Do potential SETI signals need to be decontaminated? |author=Carrigan, Richard A.|journal=Acta Astronautica |volume=58 |issue=2 |pages=112–117|year=2006 |doi=10.1016/j.actaastro.2005.05.004 |bibcode=2006AcAau..58..112C }}</ref> or even ideas themselves<ref>{{cite journal|last=Marsden|first=P.|title=Memetics and social contagion: Two sides of the same coin|journal=Journal of Memetics-Evolutionary Models of Information Transmission|year=1998|volume=2|issue=2|pages=171–185|url=http://cfpm.org/jom-emit/1998/vol2/marsden_p.html}}</ref> (see [[meme]]).
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| Perhaps prudent civilizations actively hide not only from us but from everyone, out of [[#It is the nature of intelligent life to destroy others|fear of other civilizations]].<ref>{{cite arXiv |title=A Solution to the Fermi Paradox: The Solar System, Part of a Galactic Hypercivilization? |author=Beatriz Gato-Rivera |year=1970 |eprint=physics/0512062 |class=physics.pop-ph}}</ref>
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| =====The Fermi paradox itself is what prevents communication=====
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| 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. Therefore it is likely that previous civilizations faced fatal problems with first contact. So perhaps every civilization keeps quiet because of the possibility that there is a real reason for others to do so.<ref name="Brin"/>
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| ====They are here unobserved====
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| 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>{{harvnb|Webb|2002|pp=27–60}}</ref> Several variations of this idea have been proposed:
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| [[Carl Sagan]] and [[Iosif Shklovsky]]<ref>{{harvnb|Shklovskii|Sagan|1966}}</ref> argued for serious consideration of "[[Intelligent Life in the Universe (book)|paleocontact]]" with extraterrestrials in the early historical era, and for examination of myths and religious lore for evidence of such contact. Sagan and Shklovsky noted that many or most religions were founded by men who claimed contact with supernatural entities who bestowed wisdom, guidance and technology, citing the fish-god [[Oannes]] as a particularly salient example. This can be a case of [[any sufficiently advanced technology is indistinguishable from magic]], [[Clarke's three laws|Clarke's third law]].
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| 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 while concealing themselves with stealth technologies that need not be much more advanced than current terrestrial ones.
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| [[UFO]] researchers note that the Fermi Paradox arose within the context of a wave of UFO reports, yet Fermi, Teller, York and Konopinski apparently dismissed the possibility that flying saucers might be extraterrestrial – despite contemporary US Air Force investigations that judged a small portion of UFO reports as inexplicable by contemporary technology. Mainstream scientific publications have occasionally addressed the possibility of extraterrestrial contact,<ref>
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| {{cite journal
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| |last=Steel |first=D.
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| |year=1995
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| |title=SETA and 1991 VG
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| |journal=[[The Observatory (journal)|The Observatory]]
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| |volume=115 |pages=78–83
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| |url=http://www.setv.org/online_mss/1991vg.pdf|accessdate=August 19, 2010|bibcode = 1995Obs...115...78S }}</ref> but the scientific community in general has given little serious attention to claims of [[unidentified flying object]]s or to the [[extraterrestrial hypothesis]]. Given that UFO investigators argue compelling evidence supports the reality of UFOs as [[Anomalistics|anomalies]], but that extant UFO evidence does not support an extraterrestrial origin, it is suggested that closer examination of UFO data may support or falsify the Fermi paradox and/or the [[extraterrestrial hypothesis]] of UFO origins: "Any refusal of interest [by mainstream scientists] in investigating the UFO phenomenon, using an ETI [extraterrestrial intelligence] concept as one working hypothesis, should surely be astonishing."<ref>[[Michael D. Swords|Swords, Michael D.]] (1989) "Science and the Extraterrestrial Hypothesis", Journal of UFO Studies, New Series 1, 1989, 67–102, p. 88; reprinted in 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 however argue that the lack of solid photographic evidence, despite billions of cell-phone, dashboard, and surveillance cameras, indicates that pursuing UFO reports is unlikely to be fruitful.<ref>{{cite web |url=http://www.nytimes.com/2010/07/04/weekinreview/04schwartz.html |title=Out of This World, Out of Our Minds |publisher=New York Times |date=3 July 2010 |author=John Schwartz}}</ref>
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| This extraterrestrial hypothesis was jokingly suggested in response to Fermi's paradox by his fellow physicist, [[Leó Szilárd]], who suggested to Fermi that extraterrestrials "are already among us—but they call themselves [[Hungarian people|Hungarians]]",<ref name=w28>{{harvnb|Webb|2002|p=28}}</ref><ref>{{cite book |last=Crick |first=Francis |authorlink=Francis Crick |year=1981 |title=Life Itself |publisher=Simon and Schuster |isbn=0-671-25562-2}}</ref> a humorous reference to the peculiar [[Hungarian language]], [[Uralic|unrelated]] to [[Indo-European languages|most other languages]] [[Languages of Europe|spoken in Europe]].<ref name=w28/><ref group="Note">See, for example, George Marx's 1995 lecture, "[http://epa.oszk.hu/00300/00342/00113/marx1.html Conflicts and Creativity – The Hungarian Lesson]", which was based on his 1994 book, ''The Voice of the Martians'', Roland Eötvös Physical Society ISBN 963-05-7427-6, and his article {{cite journal|last1=Marx|first1=George|title=The myth of the martians and the golden age of Hungarian science|journal=Science & Education|volume=5|issue=3|page=225|year=1996|doi=10.1007/BF00414313|bibcode=1996Sc&Ed...5..225M}}</ref>
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| ==In science fiction and other media==
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| Many, perhaps most, of the serious explanations for the Fermi Paradox have appeared in science fiction, along with many that are not so serious. Examples include: [[Fermi and Frost]], [[Manifold: Space]] and the other books in this series, the novel ''Existence'' by [[David Brin]], [[The Fermi Paradox Is Our Business Model]], and many others.
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| Less commonly the Fermi Paradox appears in other media. Examples are the album [[Fermi Paradox (album)|Fermi Paradox]], the movie ''The Fermi Paradox,''<ref>{{cite web |title=The Fermi Paradox |url=http://www.imdb.com/title/tt2256865/ |publisher=IMDB}}</ref> and noted YouTuber Hank Green's song: [http://www.youtube.com/watch?v=zqzMAnPKa_s&list=PL30E188D6D5930BF4&index=64 The Fermi Paradox: A Song].
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| ==See also==
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| {{div col|cols=2}}
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| * [[Anthropic principle]]
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| * [[Drake equation]]
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| * [[Extraterrestrial hypothesis]]
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| * [[Fermi problem]]
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| * [[Interstellar travel]]
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| * [[Rare earth hypothesis]]
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| * [[Zoo hypothesis]]
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| {{div col end}}
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| ==Notes==
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| {{reflist|group=Note}}
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| ==References==
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| {{Reflist|colwidth=30em}}
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| ==Bibliography==
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| * {{cite book |title=The Extraterrestrial Life Debate, Antiquity to 1915 |last=Crowe |first=Michael J. |publisher=University of Notre Dame Press |year=2008 |isbn=978-0-268-02368-3 |ref=harv}}
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| * {{cite book |last=Shklovskii |first=Iosif |author-link=Iosif Shklovsky |last2=Sagan |first2=Carl |author2-link=Carl Sagan |title=Intelligent Life in the Universe |location=San Francisco |isbn=1-892803-02-X |publisher=Holden–Day |year=1966 |ref=harv}}
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| * {{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 |url=http://books.google.co.uk/books?id=-vZ0BVSHix4C |ref=harv}}
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| ==Further reading==
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| * [[Benjamin Zuckerman|Ben Zuckerman]] and Michael H. Hart, Extraterrestrials: Where Are They? ISBN 0-521-44803-4 [http://www.amazon.com/dp/0521448034 Amazon]
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| * {{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}}
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| * {{cite book|last = Michaud|first = Michael|title = Contact with Alien Civilizations: Our Hopes and Fears about Encountering Extraterrestrials|publisher = Copernicus Books|year = 2006|isbn = 978-0-387-28598-6}}
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| * {{cite book|last = Gold|first = Thomas|title = The Deep Hot Biosphere|publisher = Springer|year = 1998|isbn = 978-0-387-95253-6 }}
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| ==External links==
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| {{Spoken Wikipedia-3|2008-05-29|Fermi_paradox_1.ogg|Fermi_paradox_2.ogg|Fermi_paradox_3.ogg}}
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| * {{wiktionary-inline|Fermi paradox}}
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| * [http://www.smbc-comics.com/index.php?db=comics&id=2004 SMBC]
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| * [http://www.cplire.ru/html/ra&sr/irm/index.html Interstellar Radio Messages]
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| * [http://www.enthea.org/writing/exotic-civilizations-a-possible-answer-to-fermis-paradox/ Exotic Civilizations: Possible Answer to Fermi's Paradox] by Paul Hughes
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| * [http://www.fermisparadox.com/ Introduction and Drake equations for the Fermi paradox]
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| * [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
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| * [http://www-personal.engin.umich.edu/~fritx/Ftlessay/essay.html The Possibilities of FTL: Or Fermi's Paradox Reconsidered] by F.E. Freiheit IV
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| * [http://zebu.uoregon.edu/~js/cosmo/lectures/lec28.html Fermi's Paradox (i.e. Where are They?)] by [http://zebu.uoregon.edu/~js/ James Schombert]
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| * [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)
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| * [http://www.accelerating.org/articles/answeringfermiparadox.html Answering the Fermi Paradox: Exploring the Mechanisms of Universal Transcension] by [[John Smart (futurist)|John Smart]]
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| * ''[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]]
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| * [http://www.astrobio.net/news/article242.html Fermi Paradox debate] ''Astrobiology Magazine'' July 2002. Michael Meyer, Frank Drake, Christopher McKay, Donald Brownlee, & David Grinspoon.
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| * [http://www.sentientdevelopments.com/2007/08/fermi-paradox-back-with-vengeance.html The Fermi Paradox: Back With a Vengeance] by [[George Dvorsky]]
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| * [http://michaelgr.com/2008/05/09/virtual-reality-could-explain-the-fermi-paradox/ Virtual Reality Could Explain the Fermi Paradox] by Michael Graham Richard
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| * [http://www.aleph.se/andart/archives/2010/10/finding_fermi.html A list of possible answers to the Fermi paradox] (with references to published works discussing some of these options) by [[Anders Sandberg]]
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| * [http://www.setileague.org/press/silence.htm Translation] of the documentary "Overcome the Great Silence!" provided by [[Aleksandr Leonidovich Zaitsev]]
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