Editing Fermi paradox (section)

=== Discovery of extraterrestrial life is too difficult ===
==== Humans have not listened properly ====
There are some assumptions that underlie the [[SETI]] programs that may cause searchers to miss signals that exist. Extraterrestrials might, for example, transmit signals that have a very high or low data rate, or employ unconventional (in human terms) [[Frequency|frequencies]], which would make them hard to distinguish from background noise. Signals might be sent from non-[[main sequence]] star systems that humans search with lower priority; our 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| date=2003| pages=181–198| issue=1| volume=145| journal=The Astrophysical Journal Supplement Series| access-date=August 19, 2010| bibcode=2003ApJS..145..181T| arxiv=astro-ph/0210675| s2cid=14734094| archive-url=https://web.archive.org/web/20100614030445/http://www.projectrho.com/HabCat.pdf| archive-date=June 14, 2010| url-status=live}}</ref>

The greatest challenge is the sheer size of the radio search needed to look for signals (effectively spanning the entire observable 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-years, less than 1/10 the distance to the nearest star. A signal is much easier to detect if it consists of a deliberate, powerful transmission directed at Earth. Such signals could be detected at ranges of hundreds to tens of thousands of light-years distance.<ref>{{cite journal |journal=Icarus |volume=26 |issue=4 |date=December 1975 |pages=462–466 |title=The Arecibo message of November, 1974 |author=The Staff at the National Astronomy and Ionosphere Center |bibcode = 1975Icar...26..462. |doi = 10.1016/0019-1035(75)90116-5 }} "A radio telescope in M13 operating at the transmission frequency, and pointed toward the Sun at the time the message arrives at the receiving site will observe a flux density from the message which will exceed the flux density of the Sun itself by a factor of roughly 10<sup>7</sup>. Indeed, at that unique time, the Sun will appear to the receptors to be by far the brightest star of the Milky Way."</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 assume that extraterrestrial civilizations will be broadcasting a deliberate signal, like the Arecibo message, in order to be found.

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

==== Humans have not listened for long enough ====
Humanity's ability to detect 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 is a very brief period on a cosmological scale, and radio transmissions have only been propagated since 1895. Thus, it remains possible that human beings have neither existed long enough nor made themselves sufficiently detectable to be found by extraterrestrial intelligence.<ref>{{cite journal |author1=Seth D. Baum |author2=Jacob D. Haqq-Misra |author3=Shawn D. Domagal-Goldman |title=Would contact with extraterrestrials benefit or harm humanity? A scenario analysis |journal=Acta Astronautica |volume=68 |issue=11 |date=2011 |pages=2114–2129 |arxiv=1104.4462 |bibcode=2011AcAau..68.2114B |doi=10.1016/j.actaastro.2010.10.012 |url=http://sethbaum.com/ac/2011_ET-Scenarios.pdf |citeseerx=10.1.1.592.1341 |s2cid=16889489 |access-date=August 1, 2018 |archive-url=https://web.archive.org/web/20180721225331/http://sethbaum.com/ac/2011_ET-Scenarios.pdf |archive-date=July 21, 2018 |url-status=live }} "If ETI search for us just as we search for them, i.e. by scanning the sky at radio and optical wavelengths [...] the radiation that has been unintentionally leaking and intentionally transmitted from Earth may have already alerted any nearby ETI to our presence and may eventually alert more distant ETI. Once ETI become alerted to our presence, it will take at least as many years for us to realize that they know."</ref>

==== Intelligent life may be too far away ====
[[File:Terrestrial Planet Finder PIA04499.jpg|thumb|[[NASA]]'s conception of the [[Terrestrial Planet Finder]]]]
It may be that non-colonizing technologically capable alien civilizations exist, but that they are simply too far apart for meaningful two-way communication.<ref name=webb02/>{{rp|62–71}} Sebastian von Hoerner estimated the average duration of civilization at 6,500 years and the average distance between civilizations in the Milky Way at 1,000 light years.<ref name="Hoerner"/> If two civilizations are separated by several thousand light-years, it is 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. It has been suggested that this problem might be ameliorated somewhat if contact and 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 |access-date=August 19, 2010 |url=http://www.space.com/searchforlife/seti_decode_011115.html |url-status=dead |archive-url=https://web.archive.org/web/20090523030456/https://www.space.com/searchforlife/seti_decode_011115.html |archive-date=May 23, 2009 }}</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 possible that observational signatures of self-destroyed civilizations could be detected, depending on the destruction scenario and the timing of human observation relative to it.<ref>{{cite journal |author1=Adam Stevens |author2=Duncan Forgan |author3=Jack O'Malley James |date=2015 |title=Observational Signatures of Self–Destructive Civilisations |journal=International Journal of Astrobiology |volume=15 |issue=4 |pages=333–344 |doi=10.1017/S1473550415000397 |arxiv=1507.08530 |s2cid=118428874 }}</ref>

A related speculation by Sagan and Newman suggests that if other civilizations exist, and are transmitting and exploring, their signals and probes simply have not arrived yet, i.e. that Humans are a relatively early civilization.<ref>{{cite journal |author1=Newman, W.T. |author2=Sagan, C. |date=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|hdl=2060/19790011801 |hdl-access=free }}</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 lifespan of a galaxy under ordinary assumptions, so the likelihood that humanity is in the midst of this transition is considered low in the paradox.<ref name="David Brin, 1983, pages 287 and 298">"The Great Silence: the Controversy . . " (15-page paper), ''Quart. Journ. Royal Astronomical Soc.,'' David Brin, 1983, [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html page 287, sixth paragraph, "Equilibrium is another concept which weaves through the new SETI debate ... "] {{Webarchive|url=https://web.archive.org/web/20190411223152/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000287.000.html |date=April 11, 2019 }}, as well as [http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html page 298, third paragraph, "Newman & Sagan ('''4''') have suggested that population pressure is not ... "] {{Webarchive|url=https://web.archive.org/web/20190411223717/http://adsbit.harvard.edu//full/1983QJRAS..24..283B/0000298.000.html |date=April 11, 2019 }}.</ref> In 2021, Hanson et al. reconsidered this likelihood and concluded it is indeed plausible when assuming that many civilizations are "grabby", i.e. displace other civilizations. Under this assumption there is a [[Anthropic principle|selection effect]] of the sort that provided we exist and are not (yet) destroyed by grabby aliens, we are very unlikely to observe aliens. Specifically, grabby aliens imply a typical civilizational expansion rate at nearly the speed of light because otherwise many other civilizations would be visible. The transition time between detection of an alien [[technosignature]] and extinction would be vanishingly short in cosmological timeframes, making it likely we are before that time period.<ref name="Hanson21"/>

Some SETI skeptics may also believe that humanity is at a very special point of time—specifically, a transitional period from no space-faring societies to one space-faring society, namely that of human beings.<ref name="David Brin, 1983, pages 287 and 298"/>

==== Intelligent life may exist hidden from view ====
Planetary scientist Alan Stern put forward the idea that there could be a number of worlds with subsurface oceans (such as Jupiter's [[Europa (moon)|Europa]] or Saturn's [[Enceladus]]). The surface would provide a large degree of protection from such things as cometary impacts and nearby supernovae, as well as creating a situation in which a much broader range of orbits are acceptable. Life, and potentially intelligence and civilization, could evolve. Stern states, "If they have technology, and let's say they're broadcasting, or they have city lights or whatever—we can't see it in any part of the spectrum, except maybe very-low-frequency [radio]."<ref>{{Cite web |url=https://www.space.com/38577-fermi-paradox-alien-life-buried-oceans.html |title=Where Are All the Intelligent Aliens? Maybe They're Trapped in Buried Oceans |website=Space.com |first=Mike |last=Wall |date=October 26, 2017}}</ref><ref>[https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract An Answer to Fermi’s Paradox In the Prevalence of Ocean Worlds] {{Webarchive|url=https://web.archive.org/web/20191221201926/https://ui.adsabs.harvard.edu/abs/2017DPS....4920203S/abstract |date=December 21, 2019 }}, S. Alan Stern, American Astronomical Society, Division for Planetary Sciences Meeting Abstracts #49, October 2017. "... We suggest another—namely that the great majority of worlds with biology and civilizations are interior water ocean worlds (WOWs)..."</ref>

==== Advanced civilizations may limit their search for life to technological signatures ====
If life is abundant in the universe but the cost of space travel is high, an advanced civilization may choose to focus its search not on signs of life in general, but on those of other advanced civilizations, and specifically on [[radio]] signals. Since humanity has [[History of radio|only recently]] began to use radio communication, its signals may have yet to arrive to other inhabited planets, and if they have, [[Space probe|probes]] from those planets may have yet to arrive on Earth.<ref>{{Cite journal |last=Wandel |first=Amri |date=2022-12-01 |title=The Fermi Paradox Revisited: Technosignatures and the Contact Era |journal=The Astrophysical Journal |volume=941 |issue=2 |pages=184 |doi=10.3847/1538-4357/ac9e00 |arxiv=2211.16505 |bibcode=2022ApJ...941..184W |s2cid=254096277 |issn=0004-637X|doi-access=free }}</ref>