Editing Kardashev scale (section)

== Possible scenarios ==
According to Kardashev, the most important parameters to define the existence of a civilization are three: the presence of very powerful energy sources, the use of non-standard technologies, and the transmission of significant amounts of information of various kinds through space.<ref name="Kardashev-1971">{{Cite book |last=Kardashev |first=Nikolai S. |url=https://archive.org/stream/nasa_techdoc_19710005278/19710005278_djvu.txt |title=Extraterrestrial Civilizations: Problems of Interstellar Communications |publisher=Coronet Books |year=1971 |isbn=978-0706510348 |editor-last=Kaplan |editor-first=S. A. |pages=12–57 |chapter=The Astrophysical Aspect of the Search for Signals From Extraterrestrial Civilizations}}</ref>

=== Energy sources ===
Kardashev's classification is based on the hypothesis that an advanced civilization uses significant energy, which implies that it must be de facto detectable over long distances, as summarized by Zoltan Galántai.<ref name="Galantai-2006" /> For Kardashev, the limit of a civilization's energy consumption is originally located in the region of the [[electromagnetic spectrum]] from 10<sup>6</sup> to 10<sup>8</sup> Hz, which allows two observations related to [[thermodynamics]]. First, all the energy consumed is inevitably converted into heat. Second, this energy can only be dissipated in the form of radiation scattered in space. These two findings are the pillars of Kardashev's theory that cosmic objects with strong radiation could be artificial sources.<ref name="Kardashev-1971" /> He also considered the possibility of detecting an artificial source by emphasizing the [[spectral line]] of [[hydrogen]] in its use for [[nuclear fusion]].<ref>{{Cite journal |last=Kardashev |first=Nikolai S. |date=1960 |title=The Possibility of Detection of Allowed Lines of Atomic Hydrogen in the Radiofrequency Spectrum |journal=Soviet Astromical Journal |volume=3 |pages=813–820}}</ref>

Dutil and Dumas consider several physical limits to continuous energy production, such as [[photosynthesis]] (about 10 [[Terawatt|TW]]), climate (about 127 TW), and [[Solar irradiance|solar flux]] (174,000 TW). The only inexhaustible source of energy that can sustain a civilization for over several billion years, is [[deuterium]] (used in [[nuclear fusion]]).<ref name="Dutil-2007" /> The sustainability of a civilization must therefore involve "strict control of the exploitation of available resources"; this difficulty in exceeding energy limits may explain the fact that the vast majority of civilizations fail to engage in a space colonization project.<ref name="Dutil-2007" />

Astrophysicist Makoto Inoue and economist Hiromitsu Yokoo have explored the possibility that a Type&nbsp;III civilization could extract energy from a [[supermassive black hole]] (SMBH). The captured energy could meet the extraordinary needs of a civilization that requires about 4 × 10<sup>44</sup> erg/s.<ref name="Inoue-2011" /> The energy would be captured in the form of radiation emitted by the matter rushing into the star, by means of collectors located within the [[accretion disk]]. These collectors are similar to Dyson spheres. The overflow, as well as the waste of the civilization, would be redirected towards the black hole. A fraction of this energy, directed as a [[Maser|high-powered beam]], could be useful for space travel. A galactic club of civilizations could transmit the energy through networks within the galaxy. Within the various central power stations that make up the network, power transmission is periodically switched between transmitter and receiver, according to the galactic rotation. To be efficient, this network should be located at the center of the galaxy.<ref name="Inoue-2011" />

=== The technology ===
{{Further|Stellar engineering|Megastructure}}This parameter is one of the most undetectable in the Universe due to the fact that solid matter structures are at low temperatures and emit weak radiation. Their luminosity, which is difficult to observe, also makes it impossible to observe them with telescopes. Likewise, we cannot detect them by their [[Gravity|gravitational effects]].<ref name="Kardashev-1971" /> On the other hand their existence can be detected by analyzing the [[wavelength]]s between 8 and 13 microns, corresponding to surface temperatures of 300 K. A hypothetical [[Dyson sphere]] could thus be detected,<ref name="Carrigan Jr.-20102"/> provided that the observation is made from space. Locally, the significant dip in luminosity that would result from a giant Dyson sphere (or "Fermi bubble") would allow the detection of a Type&nbsp;III civilization.<ref name="Carrigan Jr.-20102"/>

A [[megastructure]] like a Dyson sphere could be the result of a technology based on [[Self-replicating machine|self-replicating probes]], as those imagined by [[John von Neumann|von Neumann]]. A Type&nbsp;III civilization would indeed have the means to disperse a significant number of these spheres throughout the galaxy, which would have the effect of attenuating the light emitted by the galaxy.<ref name="Calissendorff-2013" /> Kaku also considers this to be the most efficient method of colonizing space. For example, a galaxy 100,000 light years in diameter would be explored in half a million years.<ref name="Kaku-2007" /> [[Paul Davies]] has suggested that a civilization could colonize the galaxy by scattering miniature probes, no larger than the palm of a hand, using [[nanotechnology]]. This thesis is realistic, he explains, because it is obvious that the technology is becoming increasingly miniaturized and proportionally less expensive.<ref name="Kaku-2007" />

Type&nbsp;II megastructures would be easier to detect. This would be the case of a [[Dyson sphere]] used as a "[[stellar engine]]",<ref name="Badescu">{{Cite web |last1=Badescu |first1=Viorel |last2=Cathcart |first2=Richard B. |title=Space travel with solar power and a dyson sphere |url=http://www.astronomytoday.com/exploration/solartravel.html |url-status=live |archive-url=https://web.archive.org/web/20230415102053/http://www.astronomytoday.com/exploration/solartravel.html |archive-date=2023-04-15 |access-date=2023-08-22 |website=www.astronomytoday.com}}</ref> as well as the contribution of heavy elements.<ref name="Carrigan Jr.-20102"/> Similarly, "Shkadov thrusters", which would produce a lateral thrust of 4.4 [[parsec]]s on their star by reflecting solar radiation through a structure made of mirrors, would be observable objects. This device would break the symmetry of solar radiation and counteract [[Gravity|gravitational forces]], allowing a Type&nbsp;II civilization to move its home solar system through space.<ref>{{Cite journal |last=Shkadov |first=L. M. |date=1987 |title=Possibility of controlling solar system motion in the Galaxy |url=https://ui.adsabs.harvard.edu/abs/1987brig.iafcR....S/abstract |journal=38th Congress of the International Astronautical Federation |bibcode=1987brig.iafcR....S |archive-date=2023-08-22 |access-date=2023-08-22 |archive-url=https://web.archive.org/web/20230822180535/https://ui.adsabs.harvard.edu/abs/1987brig.iafcR....S/abstract |url-status=live }}</ref><ref name="Badescu" /> Drake and Shklovski have also considered the possibility of "seeding" a star (Stellar salting) by artificially adding extremely rare elements such as [[technetium]] or [[promethium]]. Such an intervention in a star's composition would be detectable.<ref name="Carrigan Jr.-20102"/>

It is still possible that humanity could discover traces of lost Type&nbsp;I, II, or III civilizations. The search for material traces of such civilizations (e.g. Dyson spheres or stellar engines), an "interesting alternative" to the conventional SETI program, lays the foundation for a "[[Xenoarchaeology|cosmic archaeology]]" according to Richard A. Carrigan. Efforts to detect intelligence markers in the atmospheres of [[exoplanet]]s (such as [[freon]], [[oxygen]], or even [[ozone]], residues of biotic activity according to [[James Lovelock]]'s research)<ref name="Carrigan Jr.-20102"/> are one of the most promising avenues. A civilization watching its star die (as a [[red giant]], for example) could have tried to prolong its existence through megastructures that should be detectable.<ref name="Carrigan Jr.-20102"/> The possible traces could be nuclear remnants, to be sought within the [[Stellar classification|spectral types]] going from [[A-type main-sequence star|A5]] to [[Main sequence|F2]] according to Whitmire and Wright.<ref>{{Cite journal |last1=Whitmire |first1=Daniel P. |last2=Wright |first2=David P. |date=1980-04-01 |title=Nuclear waste spectrum as evidence of technological extraterrestrial civilizations |url=https://dx.doi.org/10.1016/0019-1035%2880%2990253-5 |journal=Icarus |volume=42 |issue=1 |pages=149–156 |doi=10.1016/0019-1035(80)90253-5 |bibcode=1980Icar...42..149W |issn=0019-1035}}</ref> It could also be a change in the [[Natural abundance|isotopic ratio]], due to a stellar engine, or an unusual spectral modulation in the composition of the star.<ref name="Carrigan Jr.-20102"/>

=== The interstellar transmission of information ===
{{Further|Interstellar communication}}
[[File:SETI@home (version 8.19).png|alt=Color illustration. Geometric shapes in various colors are drawn on a black background.|thumb|A screenshot of the [[SETI@home]] computer program]]
According to Kardashev, the transmissions of an extraterrestrial civilization (what [[SERENDIP]] is looking for) can be divided into two types. On the one hand, there can be an exchange of information between highly developed civilizations or civilizations at similar stages of evolution. On the other hand, the transmission of information can be aimed at raising the level of other less developed civilizations. If supercivilizations do exist, the transmissions of the first type must remain inaccessible to our observation because they must be unidirectional and not be directed toward the [[Solar System]]. Conversely, those of the second type must be easily detectable by our listening devices.<ref name="Kardashev-1971" /> 

A signal of artificial origin should contain more than 10 and less than 100 [[bit]]s. The latter would be of two types: transient and stable. Several criteria allow us to distinguish a signal of artificial origin from others. First, the optimal region of the spectrum to host artificial signals is the one where the temperature of the [[cosmic microwave background]] is the lowest.<ref name="Kardashev-1971" /> Second, artificial sources must have a minimum angular size. The presence of suspicious data in other regions of the spectrum (such as [[circular polarization]], radio and optical frequencies,<ref>{{Cite book |last1=Shvartsman |first1=V. F. |last2=Beskin |first2=G. M. |last3=Mitronova |first3=S. N. |last4=Neizvestny |first4=S. I. |last5=Plakhotnichenko |first5=V. L. |last6=Pustil'nik |first6=L. A. |date=1993 |editor-last=Shostak |editor-first=G. S. |chapter=Results of the MANIA Experiment: an Optical Search for Extraterrestrial Intelligence |chapter-url=https://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ASPC...47..381S&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf |title=Third Decennial US-USSR Conference on SETI |series=ASP Conference Series |volume=47 |pages=381–392|bibcode=1993ASPC...47..381S }}</ref> or [[Astrophysical X-ray source|X-ray emissions]]) can confirm that it is an intelligent transmission. Two sources among those studied have parameters close to those expected: 1934-63 and [[3C 273|3C 273B]].<ref name="Kardashev-1971" />

For L. M. Gindilis, there are two criteria for a signal to be called artificial: one related to the artificial nature of the source and the other related to a particular radiation, intentionally designed to ensure communication and facilitate detection.<ref name="Gindilis-1971">{{Cite book |last=Gindilis |first=L. M. |title=Extraterrestrial Civilizations – Problems of Interstellar Communication |year=1971 |pages=68–131 |chapter=The Possibility of Radio Communication With Extraterrestrial Civilization}}</ref> Only Type&nbsp;II or III civilizations can communicate using [[Isotropy|isotropic]] transmissions that allow omnidirectional reception. In a 1&nbsp;MHz band (which requires about 10<sup>24</sup> watts), detection of signals from a Type&nbsp;II civilization is possible up to 1,000 [[light-year]]s away, while signals from a Type&nbsp;III civilization are detectable virtually throughout the [[Observable universe|observable Universe]].<ref name="Gindilis-1971" /> However, building an omnidirectional transmitter powerful enough to transmit over a range of 1,000 light years would take several million years. According to V.S. Troitsky, the energy required and the limitations in its production would be two obstacles to completing this project in a reasonable time.<ref>{{Cite journal |last=Troitskij |first=V. S. |date=1989 |title=Development of Extraterrestrial Intelligence and Physical Laws |url=https://www.sciencedirect.com/science/article/abs/pii/0094576589900799 |journal=Acta Astronautica |volume=19 |issue=11 |pages=875–887 |doi=10.1016/0094-5765(89)90079-9 |bibcode=1989AcAau..19..875T |archive-date=2023-08-29 |access-date=2023-08-23 |archive-url=https://web.archive.org/web/20230829130330/https://www.sciencedirect.com/science/article/abs/pii/0094576589900799 |url-status=live }}</ref>

For Zoltan Galántai, we would not be able to distinguish between an intelligent extraterrestrial signal and a signal of natural origin. Therefore, he does not believe that Type&nbsp;II, III or even IV civilizations can be detected. Even if humanity reaches Type&nbsp;IV, it will not be able to detect another supercivilization of a similar level, and we will consider their changes in the universe to be the result of natural causes. Thus, there may be many Type&nbsp;IV civilizations in the universe, but none of them will be able to detect the others. Moreover, the dimensions of the universe make these supercivilizations like islands far from the others, which Dyson defines as a "[[Lewis Carroll|Carroll]] Universe".<ref name="Galántai-2003" />

For Alexander L. Zaitsev, the radio transmission of interstellar messages (IRM) is the most likely method used by civilizations. Planetary [[radio telescope]]s and those installed on [[asteroid]]s would make it possible to listen to the many messages that could be sent to us.<ref>{{Cite journal |last=Zaitsev |first=Alexander |date=2008 |title=Sending and Searching for Interstellar Messages |journal=Acta Astronautica |volume=63 |issue=5–6 |pages=614–617 |doi=10.1016/j.actaastro.2008.05.014|arxiv=0711.2368 |bibcode=2008AcAau..63..614Z |s2cid=55638129 }}</ref> In 2007, the [[Search for extraterrestrial intelligence|SETI program]] analyzed the only television frequencies sent by a Type&nbsp;0 civilization, notes [[Michio Kaku]]. Therefore, our galaxy may have communications from Type&nbsp;II and III civilizations, but our listening devices can only detect Type&nbsp;0 messages.<ref name="Kaku-2007" />