Editing Fermi paradox (section)

== Empirical evidence ==
{{Main|Search for extraterrestrial intelligence|Technosignature}}

There are two parts of the Fermi paradox that rely on empirical evidence—that there are many potentially [[Planetary habitability|habitable planets]], and that humans see no evidence of life. The first point, that many suitable planets exist, was an assumption in Fermi's time, but is since supported by the discovery that [[exoplanet]]s are common. Existing models predict billions of habitable worlds in the Milky Way.<ref>{{cite journal |title=On The History and Future of Cosmic Planet Formation |journal=MNRAS |date=December 1, 2015 |volume=454 |issue=2 |pages=1811–1817 |doi=10.1093/mnras/stv1817|arxiv = 1508.01202 |bibcode = 2015MNRAS.454.1811B |last1=Behroozi |first1=Peter |last2=Peeples |first2=Molly S. |doi-access=free |s2cid=35542825 }}</ref>

The second part of the paradox, that humans see no evidence of extraterrestrial life, is also an active field of scientific research. This includes both efforts to find any indication of life,<ref>{{cite journal |journal=Astrophys Space Sci |date=2013 |volume=348 |issue=1 |pages=1–10 |doi=10.1007/s10509-013-1536-9 |title=Final frontiers: the hunt for life elsewhere in the Universe |author=Sohan Jheeta|bibcode = 2013Ap&SS.348....1J |s2cid=122750031 }}</ref> and efforts specifically directed to finding intelligent life. These searches have been made since 1960, and several are ongoing.<ref group=note>See, for example, the [[SETI Institute]], [http://seti.harvard.edu/seti/ The Harvard SETI Home Page] {{Webarchive|url=https://web.archive.org/web/20100816170426/http://seti.harvard.edu/seti/ |date=August 16, 2010 }}, or [http://seti.berkeley.edu/ The Search for Extra Terrestrial Intelligence at Berkeley] {{Webarchive|url=https://web.archive.org/web/20190406232538/http://seti.berkeley.edu/ |date=April 6, 2019 }}</ref>

Although astronomers do not usually search for extraterrestrials, they have observed phenomena that they could not immediately explain without positing an intelligent civilization as the source. For example, [[pulsar]]s, when [[PSR B1919+21|first discovered]] in 1967, were called [[little green men]] (LGM) because of the precise repetition of their pulses.<ref>{{cite journal |title=Discovery of pulsars: a graduate student's story |author=Wade, Nicholas |journal=Science |volume=189 |issue=4200 |pages=358–364 |year=1975 |url=https://www.science.org/doi/abs/10.1126/science.189.4200.358 |bibcode=1975Sci...189..358W |doi=10.1126/science.189.4200.358 |pmid=17840812 |access-date=July 21, 2015 |archive-url=https://web.archive.org/web/20150924142852/http://www.sciencemag.org/content/189/4200/358.short |archive-date=September 24, 2015 |url-status=live }}</ref> In all cases, explanations with no need for intelligent life have been found for such observations,<ref group=note>Pulsars are 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 |url-status=dead |archive-url=https://web.archive.org/web/20140811194232/http://event.arc.nasa.gov/main/home/reports/CP2007-214567_Langhoff.pdf |archive-date=August 11, 2014 }}</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=March 28, 2011 |arxiv=1103.5369 |bibcode = 2011IJAsB..10..307F |doi = 10.1017/S1473550411000127 |volume=10 |issue=4 |pages=307–313 |last2= Elvis |s2cid= 119111392 }}</ref> or spectral lines from [[nuclear waste]] disposal in stars.<ref>{{cite journal |author1=Whitmire, Daniel P. |author2=David P. Wright. |title=Nuclear waste spectrum as evidence of technological extraterrestrial civilizations |journal=Icarus |volume=42 |issue=1 |date=1980 |pages=149–156 |doi=10.1016/0019-1035(80)90253-5 |bibcode=1980Icar...42..149W}}</ref>

=== Electromagnetic emissions ===
{{Further|Project Phoenix (SETI)|SERENDIP|Allen Telescope Array}}
[[File:parkes.arp.750pix.jpg|thumb|left|[[Radio telescope]]s are often used by SETI projects.]]
Radio technology and the ability to construct a [[radio telescope]] are presumed to be a natural advance for technological species,<ref>{{cite web
|last=Mullen
|first=Leslie
|date=2002
|url=http://www.space.com/scienceastronomy/alien_intelligence_021202.html
|archive-url=https://web.archive.org/web/20030212141854/http://www.space.com/scienceastronomy/alien_intelligence_021202.html
|url-status=dead
|archive-date=February 12, 2003
|title=Alien Intelligence Depends on Time Needed to Grow Brains
|work=Astrobiology Magazine
|publisher=Space.com
|access-date=April 21, 2006
}}</ref> theoretically creating effects that might be detected over interstellar distances. The careful searching for non-natural radio emissions from space may lead to the detection of alien civilizations. Sensitive alien 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. Such signals could be either "accidental" by-products of a civilization, or deliberate attempts to communicate, such as the [[Arecibo message]]. It is unclear whether "leakage", as opposed to a deliberate beacon, could be detected by an extraterrestrial civilization. The most sensitive radio telescopes on Earth, {{as of|2019|lc=y}}, would not be able to detect non-directional radio signals (such as [[broadband]]) even at a fraction of a [[light-year]] away,<ref>{{cite journal | title=The benefits and harm of transmitting into space | last1=Haqq-Misra | first1=Jacob | last2=Busch | first2=Michael W. | last3=Som | first3=Sanjoy M. | last4=Baum | first4=Seth D. | display-authors=1 | journal=Space Policy | volume=29 | issue=1 | pages=40–48 | date=February 2013 | doi=10.1016/j.spacepol.2012.11.006 | arxiv=1207.5540 | bibcode=2013SpPol..29...40H }} See table 1.</ref> but other civilizations could hypothetically have much better equipment.<ref>{{cite journal |last=Scheffer | first=L. |year=2004 |title=Aliens can watch 'I Love Lucy' |journal=Contact in Context |volume=2 |issue=1 | url=https://lscheffer.com/tv.pdf | access-date=2024-02-02}}</ref><ref>{{cite web |title=Radio Leakage: Is anybody listening? |first=Brian |last=von Konsky |date=October 23, 2000 |url=https://www.researchgate.net/publication/242237960}}</ref>

A number of astronomers and observatories have attempted and are attempting to detect such evidence, mostly through SETI organizations such as the [[SETI Institute]] and [[Breakthrough Listen]]. Several decades of SETI analysis have not revealed any unusually bright or meaningfully repetitive radio emissions.<ref>{{cite arXiv |author=Participants, NASA |year=2018 |title=NASA and the Search for Technosignatures: A Report from the NASA Technosignatures Workshop |eprint=1812.08681|class=astro-ph.IM }}</ref>

=== Direct planetary observation ===
[[File:Earthlights dmsp 1994–1995.jpg|thumb|upright=1.35|A composite picture of Earth at night, created using data from the [[Defense Meteorological Satellite Program]] (DMSP) Operational Linescan System (OLS). Large-scale artificial lighting produced by human civilization is detectable from space.]]
Exoplanet detection and classification is a very active sub-discipline in astronomy; the first candidate [[terrestrial planet]] discovered within a star's [[habitable zone]] was found in 2007.<ref>{{Cite journal |doi=10.1051/0004-6361:20077612 |arxiv=0704.3841 |title=The HARPS search for southern extra-solar planets XI. Super-Earths (5 and 8&nbsp;{{Earth mass}}) in a 3-planet system |journal=Astronomy & Astrophysics |volume=469 |issue=3 |pages=L43–L47 |last1=Udry |first1=Stéphane |last2=Bonfils |first2=Xavier |last3=Delfosse |first3=Xavier |last4=Forveille |first4=Thierry |last5=Mayor |first5=Michel |last6=Perrier |first6=Christian |last7=Bouchy |first7=François |last8=Lovis |first8=Christophe |last9=Pepe |first9=Francesco |last10=Queloz |first10=Didier |last11=Bertaux |first11=Jean-Loup |year=2007 |bibcode=2007A&A...469L..43U |s2cid=119144195 |url=http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf |url-status=dead |archive-url=https://web.archive.org/web/20101008120426/http://exoplanet.eu/papers/udry_terre_HARPS-1.pdf |archive-date=October 8, 2010 }}</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 space telescope|Kepler]] and [[Transiting Exoplanet Survey Satellite|TESS]] missions) are starting to detect and characterize Earth-size planets, to determine whether they are within the habitable zones of their stars. Such observational refinements may allow for a better estimation of how common these potentially habitable worlds are.<ref>From {{cite web |url=http://kepler.nasa.gov/Mission/QuickGuide/ |title=Kepler: About the Mission |publisher=NASA |date=March 31, 2015 |access-date=March 30, 2016 |archive-url=https://web.archive.org/web/20120420015542/http://kepler.nasa.gov/Mission/QuickGuide/ |archive-date=April 20, 2012 |url-status=dead }} "The Kepler Mission, NASA Discovery mission #10, is specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets."</ref>

=== Conjectures about interstellar probes ===
{{Further|Hart–Tipler conjecture|Von Neumann probe|Bracewell probe}}
The [[Hart–Tipler conjecture]] is a form of [[contraposition]] which states that because no interstellar probes have been detected, there likely is no other intelligent life in the universe, as such life should be expected to eventually create and launch such probes.<ref name="Gray2015">{{cite journal |last1=Gray |first1=Robert H. |title=The Fermi Paradox Is Neither Fermi's Nor a Paradox |journal=Astrobiology |date=March 2015 |volume=15 |issue=3 |pages=195–199 |doi=10.1089/ast.2014.1247|pmid=25719510 |arxiv=1605.09187 |bibcode=2015AsBio..15..195G }}</ref><ref name="AliensSpaceTime2020">{{cite book|last1=Dick |first1=Steven J. |title=Space, Time, and Aliens: Collected Works on Cosmos and Culture |chapter=Bringing Culture to Cosmos: Cultural Evolution, the Postbiological Universe, and SETI |date=2020 |pages=171–190 |doi=10.1007/978-3-030-41614-0_12 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-030-41614-0_12 |access-date=18 October 2022 |publisher=Springer International Publishing |isbn=978-3-030-41613-3 |s2cid=219414685 |language=en}}</ref> Self-replicating probes could exhaustively explore a galaxy the size of the Milky Way in as little as a million years.<ref name="Hart"/> If even a single civilization in the Milky Way attempted this, such probes could spread throughout the entire galaxy. Another speculation for contact with an alien probe—one that would be trying to find human beings—is an alien [[Bracewell probe]]. Such a hypothetical 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 neighbors. 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| pages=670–671| date=1960| doi=10.1038/186670a0| bibcode = 1960Natur.186..670B | s2cid=4222557}}</ref>

Direct exploration of the Solar System has yielded no evidence indicating a visit by aliens or their probes. Detailed exploration of areas of the Solar System where resources would be plentiful may yet produce evidence of alien exploration,<ref name="AsteroidBelt">{{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 |date=1978 |bibcode = 1978QJRAS..19..277P }}</ref><ref>{{cite news |title=Extraterrestrial Intelligence in the Solar System: Resolving the Fermi Paradox |author=Robert A. Freitas Jr. |journal=[[Journal of the British Interplanetary Society]] |volume=36 |date=November 1983 |pages=496–500 |url=http://www.rfreitas.com/Astro/ResolvingFermi1983.htm |bibcode=1983JBIS...36..496F |access-date=November 12, 2004 |archive-url=https://web.archive.org/web/20041208080419/http://www.rfreitas.com/Astro/ResolvingFermi1983.htm |archive-date=December 8, 2004 |url-status=live }}</ref> though the entirety of the Solar System is relatively vast and difficult to investigate. Attempts to signal, attract, or activate hypothetical Bracewell probes in Earth's vicinity have not succeeded.<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)| date=1985 |pages=1027–1034| issue=12| volume=12| journal=Acta Astronautica| bibcode = 1985AcAau..12.1027F | citeseerx=10.1.1.118.4668}}</ref>

=== Searches for stellar-scale artifacts ===
{{further|Dyson sphere|Stellar engine|Kardashev scale}}
[[File:Dyson Sphere Diagram-en.svg|thumb|upright=1.2|A variant of the speculative [[Dyson sphere]]. Such large-scale artifacts would drastically alter the spectrum of a star.]]
In 1959, [[Freeman Dyson]] observed that every developing human civilization constantly increases its energy consumption, and he conjectured that a civilization might try to harness a large part of the energy produced by a star. He proposed a hypothetical "Dyson sphere" as a means: a shell or cloud of objects enclosing a star to absorb and utilize 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 lines]] of a natural [[stellar atmosphere]] to those of [[black-body radiation]], probably with a peak in the [[infrared]]. Dyson 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]]| date=1960| url=http://www.islandone.org/LEOBiblio/SETI1.HTM| title=Search for Artificial Stellar Sources of Infra-Red Radiation| author=Dyson, Freeman J.| pages=1667–1668| volume=131| doi=10.1126/science.131.3414.1667| pmid=17780673| issue=3414| bibcode=1960Sci...131.1667D| s2cid=3195432| author-link=Freeman Dyson| access-date=August 19, 2010| archive-url=https://web.archive.org/web/20190714215002/http://www.islandone.org/LEOBiblio/SETI1.HTM| archive-date=July 14, 2019| url-status=live}}</ref><ref name="G^I">{{Cite journal| arxiv=1408.1133| last1= Wright| first1= J. T.| title= The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. I. Background and Justification| journal= The Astrophysical Journal| volume= 792| issue= 1| pages= 26| last2= Mullan| first2= B.| last3= Sigurðsson| first3= S.|last4= Povich| first4= M. S.| date= 2014| doi= 10.1088/0004-637X/792/1/26| bibcode= 2014ApJ...792...26W| s2cid= 119221206}}</ref><ref name="G^II">{{Cite journal| arxiv=1408.1134| last1= Wright| first1= J. T.| title= The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result| journal= The Astrophysical Journal| volume= 792| issue= 1| pages= 27| last2= Griffith| first2= R.| last3= Sigurðsson| first3= S.| last4= Povich| first4= M. S.| last5= Mullan| first5= B.| date= 2014| doi= 10.1088/0004-637X/792/1/27| bibcode= 2014ApJ...792...27W| s2cid= 16322536}}</ref>

There have been attempts to find evidence of Dyson spheres that would alter the spectra of their core stars.<ref>{{cite web|url = http://home.fnal.gov/~carrigan/infrared_astronomy/Fermilab_search.htm|title = Fermilab Dyson Sphere search program|publisher = Fermi National Accelerator Laboratory|access-date = February 10, 2008|archive-url = https://web.archive.org/web/20060306222359/http://home.fnal.gov/~carrigan/Infrared_Astronomy/Fermilab_search.htm|archive-date = March 6, 2006|url-status = dead}}</ref> Direct observation of thousands of galaxies has shown no explicit evidence of artificial construction or modifications.<ref name="G^I" /><ref name="G^II" /><ref name="G^III">{{Cite journal|first1=J. T. |last1=Wright |first2=B |last2=Mullan |first3=S |last3=Sigurdsson |first4=M. S |last4=Povich |arxiv=1504.03418 |title=The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. III. The Reddest Extended Sources in WISE |journal=The Astrophysical Journal Supplement Series |volume=217 |issue=2 |pages=25 |date=2014|doi=10.1088/0067-0049/217/2/25 |bibcode=2015ApJS..217...25G |s2cid=118463557 }}</ref><ref>{{cite news |title=Alien Supercivilizations Absent from 100,000 Nearby Galaxies |url=http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies/ |work=Scientific American |date=April 17, 2015 |access-date=June 29, 2015 |archive-url=https://web.archive.org/web/20150622003330/http://www.scientificamerican.com/article/alien-supercivilizations-absent-from-100-000-nearby-galaxies/ |archive-date=June 22, 2015 |url-status=live }}</ref> In October 2015, there was speculation that a dimming of light from star [[KIC 8462852]], observed by the [[Kepler space telescope]], could have been a result of Dyson sphere construction.<ref>{{Cite journal|arxiv=1510.04606| last1= Wright| first1= Jason T.|title= The Ĝ Search for Extraterrestrial Civilizations with Large Energy Supplies. IV. The Signatures and Information Content of Transiting Megastructures| journal= The Astrophysical Journal| volume= 816| issue= 1| pages= 17| last2=Cartier| first2= Kimberly M. S.| last3= Zhao| first3= Ming| last4= Jontof-Hutter| first4= Daniel| last5= Ford| first5= Eric B.| year= 2015| doi= 10.3847/0004-637X/816/1/17|bibcode = 2016ApJ...816...17W | s2cid= 119282226| doi-access= free}}</ref><ref name="ATL-20151013">{{cite web| last1=Andersen| first1=Ross| title=The Most Mysterious Star in Our Galaxy| url=https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/| date=October 13, 2015| work=[[The Atlantic]]| access-date=October 13, 2015| archive-url=https://web.archive.org/web/20170720013427/https://www.theatlantic.com/science/archive/2015/10/the-most-interesting-star-in-our-galaxy/410023/| archive-date=July 20, 2017| url-status=live}}</ref> However, in 2018, further observations determined that the amount of dimming varied by the frequency of the light, pointing to dust, rather than an opaque object such as a Dyson sphere, as the cause of the dimming.<ref>{{Cite journal |title=The First Post-Kepler Brightness Dips of KIC 8462852 |journal=The Astrophysical Journal |volume=853 |issue=1 |at=L8 |arxiv=1801.00732 |first1=Tabetha S. |last1=Boyajian |display-authors=etal |doi=10.3847/2041-8213/aaa405 |year=2018 |bibcode=2018ApJ...853L...8B|s2cid=215751718 |doi-access=free }}</ref><ref>{{cite news |last1=Overbye |first1=Dennis |title=Magnetic Secrets of Mysterious Radio Bursts in a Faraway Galaxy |url=https://www.nytimes.com/2018/01/10/science/neutron-star-fast-radio-bursts.html |access-date=April 2, 2019 |work=The New York Times |date=January 10, 2018 |archive-url=https://web.archive.org/web/20180111001837/https://www.nytimes.com/2018/01/10/science/neutron-star-fast-radio-bursts.html |archive-date=January 11, 2018 |url-status=live }}</ref>