Drake equation: Difference between revisions
→Current estimates of the parameters: rewrite part of fi, mayr was a famous biologist, delete irrelevant link |
m ISBN978 → ISBN 978 |
||
Line 1: | Line 1: | ||
{{Short description|Estimate of extraterrestrial civilizations}} |
|||
{{About|Frank Drake's equation|the [[Tub Ring]] music album|Drake Equation (album)}} |
|||
{{About|Frank Drake's equation|other uses|Drake equation (disambiguation)}} |
|||
{{Use dmy dates|date=July 2019}} |
|||
[[File:Dr. Frank Drake.jpg|thumb|upright=0.9|[[Frank Drake]]]] |
|||
The '''Drake equation''' is a [[probability theory|probabilistic argument]] used to estimate the number of active, communicative [[extraterrestrial life|extraterrestrial civilizations]] in the [[Milky Way]] [[Galaxy]].<ref name="Drake 1961">[[Physics Today]] 14 (4), 40–46 (1961). {{cite web |url=https://pubs.aip.org/physicstoday/article/14/4/40/422237/Project-Ozma |title=Project Ozma |last=Drake |first=F. D. |date=April 1961 |website=pubs.aip.org |publisher=American Institute of Physics |access-date=27 April 2023 |quote=The question of the existence of intelligent life elsewhere in space has long fascinated people, but, until recently, has been properly left to the science‐fiction writers.}}</ref><ref name="Burchell"/><ref>{{cite journal |last1=Glade |first1=N. |last2=Ballet |first2=P. |last3=Bastien |first3=O. |date=2012 |title=A stochastic process approach of the drake equation parameters |journal=[[International Journal of Astrobiology]] |volume=11 |issue=2 |pages=103–108 |arxiv=1112.1506 |bibcode=2012IJAsB..11..103G |doi=10.1017/S1473550411000413|s2cid=119250730 }}</ref> |
|||
The '''Drake equation''' (sometimes called the '''Green Bank equation''' or the '''Green Bank Formula''') is an equation used to estimate the number of detectable [[extraterrestrial]] civilizations in the [[Milky Way]] [[galaxy]]. It is used in the fields of [[exobiology]] and the [[SETI|search for extraterrestrial intelligence]] (SETI). The equation was devised by [[Frank Drake]] in 1961. |
|||
The equation was formulated in 1961 by [[Frank Drake]], not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the [[search for extraterrestrial intelligence]] (SETI).<ref name="December 2002">{{cite web |date=December 2002 |title=Chapter 3 – Philosophy: "Solving the Drake Equation |url=http://www.setileague.org/askdr/drake.htm |work=Ask Dr. SETI |publisher=SETI League |access-date=2013-04-10}}</ref><ref>{{cite web |last=Drake |first=N. |author-link=Nadia Drake |date=30 June 2014 |title=How my Dad's Equation Sparked the Search for Extraterrestrial Intelligence |url=http://news.nationalgeographic.com/news/2014/06/140630-drake-equation-50-years-later-aliens-science/ |archive-url=https://web.archive.org/web/20140705053200/http://news.nationalgeographic.com/news/2014/06/140630-drake-equation-50-years-later-aliens-science/ |url-status=dead |archive-date=5 July 2014 |work=[[National Geographic (magazine)|National Geographic]] |access-date=2 October 2016}}</ref> The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.<ref name="December 2002"/> It is more properly thought of as an approximation than as a serious attempt to determine a precise number. |
|||
==History== |
|||
In 1960, Frank Drake conducted the first search for radio signals from extraterrestrial civilizations at the National Radio Astronomy Observatory in [[Green Bank, West Virginia|Green Bank]], [[West Virginia]]. Soon thereafter, the National Academy of Sciences asked Drake to convene a meeting on detecting extraterrestrial intelligence. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting: |
|||
{{Quotation|As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.|Frank Drake<ref>{{cite web |url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 |title=The Drake Equation Revisited: Part I}}</ref>}} |
|||
This meeting established SETI as a scientific discipline. The meeting's dozen participants — astronomers, physicists, biologists, social scientists, and industry leaders — became known as the "Order of the Dolphin". The Green Bank meeting has been commemorated by a [http://www.setileague.org/photos/miscpix/drakeqn.jpg plaque] at the site. |
|||
The Drake equation is closely related to the [[Fermi paradox]] in that Drake suggested that a large number of extraterrestrial civilizations would form, but that the lack of evidence of such civilizations (the Fermi paradox) suggests that technological civilizations tend to disappear rather quickly. This theory often stimulates an interest in identifying and publicizing ways in which humanity could destroy itself, and then counters with hopes of avoiding such destruction and eventually becoming a space-faring species. A similar argument is [[The Great Filter]],<ref>{{cite web |url=http://hanson.gmu.edu/greatfilter.html |title=The Great Filter — Are We Almost Past It? |author= [[Robin Hanson]] |year=1998}}</ref> which notes that since there are no observed extraterrestrial civilizations, despite the vast number of stars, then some step in the process must be acting as a filter to reduce the final value. According to this view, either it is very hard for intelligent life to arise, or the lifetime of such civilizations must be relatively short. |
|||
Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions. |
|||
[[Carl Sagan]], a great proponent of [[SETI]], quoted the formula often and as a result the formula is sometimes mislabeled as "The Sagan Equation." |
|||
== |
==Equation== |
||
The Drake equation |
The Drake equation is:<ref name="Drake 1961" /> |
||
<math display="block">N = R_* \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot L</math> |
|||
where |
where |
||
* {{math|''N''}} = the number of [[civilization]]s in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past [[light cone]]); |
|||
and |
and |
||
* {{math|''R''<sub>∗</sub>}} = the average rate of [[star formation]] in [[Milky Way|our Galaxy]]. |
|||
* {{math|''f''<sub>p</sub>}} = the fraction of those stars that have [[planet]]s. |
|||
* {{math|''n''<sub>e</sub>}} = the average number of planets that can potentially support [[life]] per star that has planets. |
|||
* {{math|''f''<sub>l</sub>}} = the fraction of planets that could support life that actually develop life at some point. |
|||
* {{math|''f''<sub>i</sub>}} = the fraction of planets with life that go on to develop [[intelligence|intelligent]] life (civilizations). |
|||
* {{math|''f''<sub>c</sub>}} = the fraction of civilizations that develop a technology that releases [[Technosignature|detectable signs of their existence into space]]. |
|||
* {{math|''L''}} = the length of time for which such civilizations release detectable signals into space.<ref name="NOVA"> |
|||
{{cite web |
|||
|last=Aguirre |first=L. |
|||
|date=1 July 2008 |
|||
|title=The Drake Equation |
|||
|url=https://www.pbs.org/wgbh/nova/origins/drake.html |
|||
|work=[[Nova ScienceNow]] |
|||
|publisher=[[PBS]] |
|||
|access-date=2010-03-07 |
|||
}}</ref><ref name="SETI-Drake-Equation">{{cite web | url=http://www.seti.org/drakeequation | title=What do we need to know about to discover life in space? | publisher=[[SETI Institute]] | access-date=2013-04-16}}</ref> |
|||
This form of the equation first appeared in Drake's 1965 paper.<ref>{{Cite book |last=Drake |first=Frank D. |url=https://ui.adsabs.harvard.edu/abs/1965cae..book..323D |title=The Radio Search for Intelligent Extraterrestrial Life |date=1965-01-01|bibcode=1965cae..book..323D }}</ref><ref>{{Cite web |last=jtw13 |date=2019-07-31 |title=Freeman Dyson's First Law of SETI Investigations |url=https://sites.psu.edu/astrowright/2019/07/31/freeman-dysons-first-law-of-seti-investigations/ |access-date=2024-08-02 |website=AstroWright |language=en-US}}</ref> |
|||
===Alternative expression=== |
|||
The number of stars in the galaxy now, ''N''<sup>*</sup>, is related to the star formation rate ''R''<sup>*</sup> by |
|||
: <math> N^{\ast} = \int_0^{T_g} R^{\ast}(t) dt , \,\!</math> |
|||
where ''T''<sub>''g''</sub> = the age of the galaxy. Assuming for simplicity that ''R''<sup>*</sup> is constant, then <math>N^{\ast} = R^{\ast} \times T_g</math> and the Drake equation can be rewritten into an alternate form phrased in terms of the more easily observable value, ''N''<sup>*</sup>.<ref>Michael Seeds, ''Horizons: Exploring the Universe'', Brooks/Cole Publishing Co., 10th edition, ISBN 978-0-495-11358-4</ref> |
|||
==History== |
|||
:<math>N = N^{\ast} \times f_p \times n_e \times f_{\ell} \times f_i \times f_c \times L / T_g \,\!</math> |
|||
In September 1959, physicists [[Giuseppe Cocconi]] and [[Philip Morrison]] published an article in the journal ''[[Nature (journal)|Nature]]'' with the provocative title "Searching for Interstellar Communications".<ref name="Cocconi 1959"> |
|||
{{cite journal |
|||
|last1=Cocconi |first1=G. |
|||
|last2=Morisson |first2=P. |
|||
|year=1959 |
|||
|title=Searching for Interstellar Communications |
|||
|url=http://www.iaragroup.org/_OLD/seti/pdf_IARA/cocconi.pdf |archive-url=https://web.archive.org/web/20110728025232/http://www.iaragroup.org/_OLD/seti/pdf_IARA/cocconi.pdf |archive-date=2011-07-28 |url-status=live |
|||
|journal=[[Nature (journal)|Nature]] |
|||
|volume=184 |issue=4690 |pages=844–846 |
|||
|access-date=2013-04-10 |
|||
|bibcode=1959Natur.184..844C |
|||
|doi=10.1038/184844a0 |
|||
|s2cid=4220318 |
|||
}}</ref><ref name="history">{{cite web |
|||
|last1=Schilling |
|||
|first1=G. |
|||
|last2=MacRobert |
|||
|first2=A. M. |
|||
|year=2013 |
|||
|title=The Chance of Finding Aliens |
|||
|url=https://skyandtelescope.org/astronomy-news/the-chance-of-finding-aliens/ |
|||
|work=[[Sky & Telescope]] |
|||
|access-date=2013-04-10 |
|||
|archive-url=https://web.archive.org/web/20130214073858/http://www.skyandtelescope.com/resources/seti/3304541.html |
|||
|archive-date=14 February 2013 |
|||
|url-status=dead |
|||
}}</ref> Cocconi and Morrison argued that [[radio telescope]]s had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a [[wavelength]] of 21 cm (1,420.4 [[megahertz|MHz]]). This is the wavelength of radio emission by neutral [[hydrogen]], the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the [[radio spectrum]]. |
|||
Two months later, Harvard University astronomy professor [[Harlow Shapley]] speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."<ref name="SydneyMorningHerald"> |
|||
===R factor=== |
|||
{{cite news |
|||
One can question why the number of civilizations should be proportional to the star formation rate, though this makes technical sense. (The product of all the terms except ''L'' tells how many new communicating civilizations are born each year. Then you multiply by the lifetime to get the expected number. For example, if an average of 0.01 new civilizations are born each year, and they each last 500 years on the average, then on the average 5 will exist at any time.) The original Drake Equation can be extended to a more realistic model, where the equation uses not the number of stars that are forming now, but those that were forming several billion years ago. The alternate formulation, in terms of the number of stars in the galaxy, is easier to explain and understand, but implicitly assumes the star formation rate is constant over the life of the galaxy. |
|||
|last=newspaper |first=staff |
|||
|date=8 November 1959 |
|||
|title=Life On Other Planets? |
|||
|url=https://news.google.com/newspapers?nid=1301&dat=19591108&id=s39WAAAAIBAJ&pg=2212,2112869&hl=en |
|||
|work=[[Sydney Morning Herald]] |
|||
|access-date=2015-10-02 |
|||
}}</ref> |
|||
Seven months after Cocconi and Morrison published their article, Drake began [[search for extraterrestrial intelligence|searching for extraterrestrial intelligence]] in an experiment called [[Project Ozma]]. It was the first systematic search for signals from communicative extraterrestrial civilizations. Using the {{convert|85|foot|abbr=on}} dish of the [[Green Bank Observatory|National Radio Astronomy Observatory, Green Bank]] in [[Green Bank, West Virginia]], Drake monitored two nearby Sun-like stars: [[Epsilon Eridani]] and [[Tau Ceti]], slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960.<ref name="history"/> The project was well designed, inexpensive, and simple by today's standards. It detected no signals. |
|||
==Elaborations of the Drake Equation== |
|||
As many observers have pointed out, the Drake equation is a very simple model that does not include potentially relevant parameters. [[David Brin]] states: |
|||
<blockquote> |
|||
<nowiki>[</nowiki>The Drake Equation<nowiki>]</nowiki> merely speaks of the number of sites at which ETIs ''spontaneously arise''. It says nothing directly about the contact cross-section between an ETIS and contemporary human society.<ref name="GS">{{cite web |url=http://adsabs.harvard.edu/abs/1983QJRAS..24..283B |title=The Great Silence - the Controversy Concerning Extraterrestrial Intelligent Life, G. D. Brin, 1983, Quarterly Journal of the Royal Astronomical Society, volume 24, pages 283-309}}</ref> |
|||
</blockquote> |
|||
Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed. These include the number of times a civilization might re-appear on the same planet, the number of nearby stars that might be colonized and form sites of their own, and other factors. |
|||
Soon thereafter, Drake hosted the first search for extraterrestrial intelligence conference on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.<ref name="Astrobiology Magazine">{{cite web | url=http://www.astrobio.net/alien-life/the-drake-equation-revisited-part-i/ | title=The Drake Equation Revisited: Part I | work=[[Astrobiology Magazine]] | date=29 September 2003 | access-date=20 May 2017 |archive-url=https://web.archive.org/web/20210225062139/http://www.astrobio.net/alien-life/the-drake-equation-revisited-part-i/ |archive-date=2021-02-25 |url-status=usurped}}</ref> |
|||
===Colonization=== |
|||
Brin has proposed generalizing the Drake Equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity ''v'', and establishes additional sites that survive for a lifetime ''L<nowiki>'</nowiki>''. The result is a more complex set of 3 equations.<ref name="GS"/> |
|||
{{blockquote|As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.|Frank Drake}} |
|||
===Reappearance number=== |
|||
The Drake equation may furthermore be multiplied by ''how many times'' an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if ''n''<sub>''r''</sub> is the average number of times a new civilization ''re''appears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be (1+''n''<sub>''r''</sub>), which is the actual ''reappearance factor'' added to the equation. |
|||
The ten attendees were conference organizer J. Peter Pearman, Frank Drake, [[Philip Morrison]], businessman and radio amateur Dana Atchley, chemist [[Melvin Calvin]], astronomer [[Su-Shu Huang]], neuroscientist [[John C. Lilly]], inventor [[Barney Oliver]], astronomer [[Carl Sagan]], and radio-astronomer [[Otto Struve]].<ref name="Wende"> |
|||
The factor depends on what generally is the cause of [[civilization extinction]]. If it is generally by temporary uninhabitability, for example a [[nuclear winter]], then ''n''<sub>''r''</sub> may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as [[stellar evolution]], then ''n''<sub>''r''</sub> may be almost zero. |
|||
{{cite news |
|||
|last=Zaun |first=H. |
|||
|date=1 November 2011 |
|||
|title=Es war wie eine 180-Grad-Wende von diesem peinlichen Geheimnis! |
|||
|trans-title=It was like a 180 degree turn from this embarrassing secret |
|||
|url=http://www.heise.de/tp/artikel/35/35756/1.html |
|||
|work=[[Telepolis]] |
|||
|language=de |
|||
|access-date=2013-08-13 |
|||
}}</ref> These participants called themselves "The Order of the Dolphin" (because of Lilly's work on [[dolphin communication]]), and commemorated their first meeting with a plaque at the observatory hall.<ref> |
|||
{{cite web |
|||
|title=Drake Equation Plaque |
|||
|url=http://www.setileague.org/photos/miscpix/drakeqn.jpg |
|||
|access-date=2013-08-13 |
|||
}}</ref><ref>{{cite encyclopedia |title=Green Bank conference (1961) |encyclopedia=[[The Encyclopedia of Science]] |url=https://www.daviddarling.info/encyclopedia/G/GreenBank.html#Green_Bank_SETI_conference |access-date=13 August 2013 |last=Darling |first=D. J. |archive-url=https://web.archive.org/web/20240221024436/https://www.daviddarling.info/encyclopedia/G/GreenBank.html#Green_Bank_SETI_conference |archive-date=21 Feb 2024 |url-status=dead |df=dmy-all}}</ref> |
|||
==Usefulness== |
|||
In the case of total life extinction, a similar factor may be applicable for ''f''<sub>ℓ</sub>, that is, ''how many times'' life may appear on a planet where it has appeared once. |
|||
[[File:C G-K - DSC 0421.jpg|thumb|upright=0.9|The [[Allen Telescope Array]] for SETI]] |
|||
The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life.<ref name="Burchell">{{cite journal |author=Burchell |first=M. J. |date=2006 |title=W(h)ither the Drake equation? |journal=International Journal of Astrobiology |volume=5 |issue=3 |pages=243–250 |bibcode=2006IJAsB...5..243B |doi=10.1017/S1473550406003107 |s2cid=121060763}}</ref><ref name="NOVA"/><ref> |
|||
{{cite web |
|||
|last=Jones |first=D. S. |
|||
|date=26 September 2001 |
|||
|title=Beyond the Drake Equation |
|||
|url=http://frombob.to/drake.html |
|||
|access-date=2013-04-17 |
|||
}}</ref> The last three parameters, {{math|''f''<sub>i</sub>}}, {{math|''f''<sub>c</sub>}}, and {{mvar|L}}, are not known and are very difficult to estimate, with values ranging over many orders of magnitude (see {{Section link|2=Criticism|nopage=y}}). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere,<ref name="Burchell"/><ref name="December 2002"/> and gives the question of life elsewhere a basis for [[Scientific method|scientific analysis]]. The equation has helped draw attention to some particular scientific problems related to life in the universe, for example [[abiogenesis]], the development of [[multi-cellular life]], and the development of [[intelligence]] itself.<ref>{{cite web |
|||
|year=2010 |
|||
|title=The Search For Life : The Drake Equation 2010 – Part 1 |
|||
|url=https://www.youtube.com/watch?list=PL56DCB81E2F59166A&v=U3UyAoYkhTo&feature=player_embedded |
|||
|publisher=[[BBC Four]] |
|||
|access-date=2013-04-17 |
|||
}}</ref> |
|||
Within the limits of existing human technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. After about 50 years, the Drake equation is still of seminal importance because it is a 'road map' of what we need to learn in order to solve this fundamental existential question.<ref name="Burchell"/> It also formed the backbone of [[astrobiology]] as a science; although speculation is entertained to give context, astrobiology concerns itself primarily with [[hypotheses]] that fit firmly into existing [[Theory#Science|scientific theories]]. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved significantly since the early 1960s. SETI efforts since 1961 have conclusively ruled out widespread alien emissions near the 21 cm wavelength of the [[Hydrogen frequencies|hydrogen frequency]].<ref>[http://www.astronomynow.com/news/n1004/SETI/ SETI: A celebration of the first 50 years]. Keith Cooper. ''Astronomy Now''. 2000</ref> |
|||
===METI factor=== |
|||
[[Aleksandr Leonidovich Zaitsev|Alexander Zaitsev]] said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans, although being in a communicative phase, are not a communicative civilization; we do not practice such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the [http://www.cplire.ru/html/ra&sr/irm/Drake_equation.html METI factor] (Messaging to Extra-Terrestrial Intelligence) to the classical Drake Equation. The factor is defined as "The fraction of communicative civilizations with clear and non-paranoid planetary consciousness", or alternatively expressed, the fraction of communicative civilizations that actually engage in deliberate interstellar transmission. |
|||
==Estimates== |
|||
==Historical estimates of the parameters== |
|||
Considerable disagreement on the values of most of these parameters exists, but the values used by Drake and his colleagues in 1961 were: |
|||
* ''R''* = 10/year (10 stars formed per year, on the average over the life of the galaxy) |
|||
* ''f''<sub>p</sub> = 0.5 (half of all stars formed will have planets) |
|||
* ''n''<sub>e</sub> = 2 (stars with planets will have 2 planets capable of developing life) |
|||
* ''f''<sub>l</sub> = 1 (100% of these planets will develop life) |
|||
* ''f''<sub>i</sub> = 0.01 (1% of which will be intelligent life) |
|||
* ''f''<sub>c</sub> = 0.01 (1% of which will be able to communicate) |
|||
* ''L'' = 10,000 years (which will last 10,000 years) |
|||
===Original estimates=== |
|||
Drake's values give ''N ''= 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10. |
|||
There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:<ref name="Drake 1961" /><ref> |
|||
{{cite book |
|||
| last1 = Drake | first1 = F. |
|||
| last2 = Sobel |first2 = D. |
|||
| year = 1992 |
|||
| title = Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence |
|||
| pages = 55–62 |
|||
| publisher = [[Delta (publisher)|Delta]] |
|||
| isbn = 0-385-31122-2 |
|||
}}</ref><ref> |
|||
{{cite journal |
|||
|last1=Glade |first1=N. |
|||
|last2=Ballet |first2=P. |
|||
|last3=Bastien |first3=O. |
|||
|year=2012 |
|||
|title=A stochastic process approach of the drake equation parameters |
|||
|journal=[[International Journal of Astrobiology]] |
|||
|volume=11 |issue=2 |pages=103–108 |
|||
|arxiv=1112.1506 |
|||
|bibcode=2012IJAsB..11..103G |
|||
|doi=10.1017/S1473550411000413 |
|||
|s2cid=119250730 |
|||
}} Note: This reference has a table of 1961 values, claimed to be taken from Drake & Sobel, but these differ from the book.</ref> |
|||
* {{math|''R''<sub>∗</sub>}} = 1 yr<sup>−1</sup> (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative) |
|||
* {{math|''f''<sub>p</sub>}} = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets) |
|||
* {{math|''n''<sub>e</sub>}} = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life) |
|||
* {{math|''f''<sub>l</sub>}} = 1 (100% of these planets will develop life) |
|||
* {{math|''f''<sub>i</sub>}} = 1 (100% of which will develop intelligent life) |
|||
* {{math|''f''<sub>c</sub>}} = 0.1 to 0.2 (10–20% of which will be able to communicate) |
|||
* {{math|''L''}} = somewhere between 1000 and 100,000,000 years |
|||
Inserting the above minimum numbers into the equation gives a minimum N of 20 (see: [[#Range of results|Range of results]]). Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that {{math|''N'' ≈ ''L''}}, and there were probably between 1000 and 100,000,000 planets with civilizations in the [[Milky Way]] Galaxy. |
|||
The value of ''R''* is determined from considerable astronomical data, and is the least disputed term of the equation; ''f''<sub>p</sub> is less certain, but is still much firmer than the values following. The value of ''n''<sub>e</sub> is based on our own solar system, and assumes that two planets had the possibility of having life. This not only has problems with [[anthropic bias]], but also is inconsistent with a ''f''<sub>l</sub> of one unless we do find life on Mars. Also, the discovery of numerous [[gas giant]]s in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the creation of their stellar systems. In addition, most stars in our galaxy are [[red dwarf]]s, which flare violently, mostly in [[X-ray]]s—a property not conducive to life as we know it (simulations also suggest that these bursts erode planetary [[Earth's atmosphere|atmosphere]]s). The possibility of life on [[natural satellite|moons]] of gas giants (such as [[Jupiter]]'s moon [[Europa (moon)|Europa]], or [[Saturn]]'s moon [[Titan (moon)|Titan]]) adds further uncertainty to this figure. |
|||
===Current estimates=== |
|||
Geological evidence from the Earth suggests that ''f''<sub>l</sub> may be very high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that [[abiogenesis]] may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains [[anthropic bias]], as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). Also countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. In addition, from a classical [[hypothesis testing]] standpoint, there are zero [[degrees of freedom (statistics)|degrees of freedom]], permitting no valid estimates to be made. If life were to be found on Mars that developed independently from life on Earth it would imply a value for ''f''<sub>l</sub> close to one. While this would improve the degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent. |
|||
This section discusses and attempts to list the best current estimates for the parameters of the Drake equation. |
|||
<!-- Please state the rationale behind the estimate and a citation to their source. --> |
|||
====Rate of star creation in this Galaxy, {{math|''R''<sub>∗</sub>}}==== |
|||
Similar arguments of bias can be made regarding ''f''<sub>i</sub> and ''f''<sub>c</sub> by considering the Earth as a model: intelligence with the capacity of extraterrestrial communication occurs only in one species in the 4 billion year history of life on Earth. If generalized, this means only relatively old planets may have intelligent life capable of extraterrestrial communication. Again this model has a large [[anthropic bias]] and there are still zero degrees of freedom. Note that the capacity and willingness to participate in extraterrestrial communication has come relatively "quickly", with the Earth having only an estimated 100,000 year history of intelligent human life, and less than a century of technological ability. |
|||
Calculations in 2010, from [[NASA]] and the [[European Space Agency]] indicate that the rate of star formation in this Galaxy is about {{solar mass|0.68–1.45|link=yes}} of material per year.<ref name=Robitaille>{{cite journal |author1=Robitaille, Thomas P. |author2=Barbara A. Whitney |title=The present-day star formation rate of the Milky Way determined from Spitzer-detected young stellar objects |journal=The Astrophysical Journal Letters |volume=710 |issue=1 |year=2010 |pages=L11 |arxiv=1001.3672 |bibcode=2010ApJ...710L..11R |doi=10.1088/2041-8205/710/1/L11|s2cid=118703635 }}</ref><ref name="The Drake Equation"> |
|||
{{cite book |
|||
|last=Wanjek |first=C. |
|||
|year=2015 |
|||
|title=The Drake Equation |
|||
|url=https://books.google.com/books?id=jcnSCQAAQBAJ&q=Robitaille+and+Whitney+came+up+with+a+figure+for+R*+between+0.68+and+1.45&pg=PA45 |
|||
|publisher=[[Cambridge University Press]] |
|||
|access-date=2016-09-09 |
|||
|isbn=9781107073654 |
|||
}}</ref> To get the number of stars per year, we divide this by the [[initial mass function]] (IMF) for stars, where the average new star's mass is about {{solar mass|0.5}}.<ref>{{cite journal |last1=Kennicutt |first1=Robert C. |last2=Evans |first2=Neal J. |title=Star Formation in the Milky Way and Nearby Galaxies |journal=Annual Review of Astronomy and Astrophysics |date=22 September 2012 |volume=50 |issue=1 |pages=531–608 |arxiv=1204.3552 |bibcode=2012ARA&A..50..531K |doi=10.1146/annurev-astro-081811-125610|s2cid=118667387 }}</ref> This gives a star formation rate of about 1.5–3 stars per year. |
|||
{{anchor|eta-earth|}} |
|||
''f''<sub>i</sub>, ''f''<sub>c</sub> and ''L'', like ''f''<sub>l</sub>, are also guesses. Estimates of ''f''<sub>i</sub> have been affected by discoveries that the solar system's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for hundreds of millions of years (evading radiation from [[nova]]e). Also, Earth's large moon may aid the evolution of life by stabilizing the planet's axis of rotation. In addition, while it appears that life developed soon after the formation of Earth, the [[Cambrian explosion]], in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the [[Snowball Earth]] or research into the [[extinction events]] have raised the possibility that life on Earth is relatively fragile. Again, the controversy over life on Mars is relevant since a discovery that life did form on Mars but ceased to exist would affect estimates of these terms. |
|||
====Fraction of those stars that have planets, {{math|''f''<sub>p</sub>}}==== |
|||
The astronomer [[Carl Sagan]] speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of [[nuclear warfare]]. |
|||
Analysis of [[Gravitational microlensing|microlensing]] surveys, in 2012, has found that {{math|''f''<sub>p</sub>}} may approach 1—that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star.<ref name="bbc.co.uk"> |
|||
{{cite news |
|||
|last=Palmer |first=J. |
|||
|date=11 January 2012 |
|||
|title=Exoplanets are around every star, study suggests |
|||
|url=https://www.bbc.co.uk/news/science-environment-16515944 |
|||
|publisher=[[BBC]] |
|||
|access-date=2012-01-12 |
|||
}}</ref><ref name="Nature-20120111"> |
|||
{{cite journal |
|||
|last=Cassan |first=A. |
|||
|display-authors=etal |
|||
|date=11 January 2012 |
|||
|title=One or more bound planets per Milky Way star from microlensing observations |
|||
|journal=[[Nature (journal)|Nature]] |
|||
|volume=481 |issue=7380 |pages=167–169 |
|||
|arxiv=1202.0903 |
|||
|bibcode=2012Natur.481..167C |
|||
|doi=10.1038/nature10684 |
|||
|pmid=22237108 |
|||
|s2cid=2614136 |
|||
}}</ref> |
|||
====Average number of planets that might support life per star that has planets, {{math|''n''<sub>e</sub>}}==== |
|||
By plugging in apparently "plausible" values for each of the parameters above, the resultant value of ''N'' can be made greater than 1. This has provided considerable motivation for the [[SETI]] movement. However, we have no evidence for extraterrestrial civilizations. This conflict is often called the [[Fermi paradox]], after [[Enrico Fermi]] who first asked about our lack of observation of extraterrestrials, and motivates advocates of SETI to continually expand the volume of space in which another civilization could be observed. |
|||
In November 2013, astronomers reported, based on [[Kepler space telescope]] data, that there could be as many as 40 billion [[Terrestrial planet|Earth-sized]] [[extrasolar planets|planets]] orbiting in the [[habitable zone]]s of [[sun-like|sun-like stars]] and [[red dwarf stars]] within the [[Milky Way Galaxy]].<ref name="NYT-20131104">{{cite news |last=Overbye |first=Dennis |title=Far-Off Planets Like the Earth Dot the Galaxy |url=https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |archive-date=2022-01-01 |url-access=limited |date=4 November 2013 |work=[[The New York Times]] |access-date=5 November 2013 }}{{cbignore}}</ref><ref name="PNAS-20131031">{{cite journal |last1=Petigura |first1=Eric A. |last2=Howard |first2=Andrew W. |last3=Marcy |first3=Geoffrey W. |title=Prevalence of Earth-size planets orbiting Sun-like stars |date=31 October 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |doi=10.1073/pnas.1319909110 |arxiv = 1311.6806 |bibcode = 2013PNAS..11019273P |volume=110 |issue=48 |pages=19273–19278 |pmid=24191033 |pmc=3845182|doi-access=free }}</ref> 11 billion of these estimated planets may be orbiting sun-like stars.<ref name="LATimes-20131104">{{cite news |last=Khan |first=Amina |title=Milky Way may host billions of Earth-size planets |url=http://www.latimes.com/science/la-sci-earth-like-planets-20131105,0,2673237.story |date=4 November 2013 |work=[[Los Angeles Times]] |access-date=5 November 2013 }}</ref> Since there are about 100 billion stars in the galaxy, this implies {{math|''f''<sub>p</sub> · ''n''<sub>e</sub>}} is roughly 0.4. The nearest planet in the habitable zone is [[Proxima Centauri b]], which is as close as about 4.2 light-years away. |
|||
The consensus at the Green Bank meeting was that {{math|''n''<sub>e</sub>}} had a minimum value between 3 and 5. Dutch science journalist [[Govert Schilling]] has opined that this is optimistic.<ref name=schilling2011 /> Even if planets are in the [[habitable zone]], the number of planets with the right proportion of elements is difficult to estimate.<ref name="Trimble">{{cite journal |
|||
Some computations of the Drake equation, given different assumptions: |
|||
|last=Trimble |first=V. |
|||
|year=1997 |
|||
|title=Origin of the biologically important elements |
|||
|journal=[[Origins of Life and Evolution of the Biosphere]] |
|||
|volume=27 |issue=1–3 |pages=3–21 |
|||
|doi=10.1023/A:1006561811750 |
|||
|pmid=9150565 |
|||
|bibcode=1997OLEB...27....3T |
|||
|s2cid=7612499 |
|||
}}</ref> Brad Gibson, Yeshe Fenner, and Charley Lineweaver determined that about 10% of [[star system]]s in the Milky Way Galaxy are hospitable to life, by having heavy elements, being far from [[supernova]]e and being stable for a sufficient time.<ref> |
|||
{{cite journal |
|||
|last1=Lineweaver |first1=C. H. |
|||
|last2=Fenner |first2=Y. |
|||
|last3=Gibson |first3=B. K. |
|||
|year=2004 |
|||
|title=The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way |
|||
|journal=[[Science (journal)|Science]] |
|||
|volume=303 |issue=5654 |pages= 59–62 |
|||
|arxiv=astro-ph/0401024 |
|||
|bibcode=2004Sci...303...59L |
|||
|doi=10.1126/science.1092322 |
|||
|pmid=14704421 |
|||
|s2cid=18140737 |
|||
}}</ref> |
|||
The discovery of numerous [[gas giant]]s in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the formation of their stellar systems. So-called [[hot Jupiter]]s may migrate from distant orbits to near orbits, in the process disrupting the orbits of habitable planets. |
|||
Current estimates (see below): |
|||
On the other hand, the variety of [[star system]]s that might have habitable zones is not just limited to solar-type stars and Earth-sized planets. It is now estimated that even tidally locked planets close to [[red dwarf]] stars [[habitability of red dwarf systems|might have habitable zones]],<ref> |
|||
:''R''* = 7/year, ''f''<sub>p</sub> = 0.5, ''n''<sub>e</sub> = 2, ''f''<sub>l</sub> = 0.33, ''f''<sub>i</sub> = 0.01, ''f''<sub>c</sub> = 0.01, and ''L'' = 10,000 years |
|||
{{cite journal |
|||
:''N'' = 7 × 0.5 × 2 × 0.33 × 0.01 × 0.01 × 10,000 = 2.31 (so two communicative civilizations exist in our galaxy at any given time, on average, plus two hundred more that are not trying to communicate). |
|||
|last1=Dressing |first1=C. D. |
|||
|last2=Charbonneau |first2=D. |
|||
|year=2013 |
|||
|title=The Occurrence Rate of Small Planets around Small Stars |
|||
|journal=[[The Astrophysical Journal]] |
|||
|volume=767 |issue= 1|page=95 |
|||
|arxiv=1302.1647 |
|||
|bibcode=2013ApJ...767...95D |
|||
|doi=10.1088/0004-637X/767/1/95 |
|||
|s2cid=29441006 |
|||
}}</ref> although the flaring behavior of these stars might speak against this.<ref>{{cite web|title=Red Dwarf Stars Could Leave Habitable Earth-Like Planets Vulnerable to Radiation|url=http://scitechdaily.com/red-dwarf-stars-could-leave-habitable-earth-like-planets-vulnerable-to-radiation/|website=SciTech Daily|access-date=22 September 2015|date=2 July 2013}}</ref> The possibility of life on [[natural satellite|moons]] of gas giants (such as [[Jupiter]]'s moon [[Europa (moon)|Europa]], or [[Saturn]]'s moons [[Titan (moon)|Titan]] and [[Enceladus]]) adds further uncertainty to this figure.<ref>{{cite journal |last1=Heller |first1=René |last2=Barnes |first2=Rory |title=Constraints on the Habitability of Extrasolar Moons |journal=Proceedings of the International Astronomical Union |date=29 April 2014 |volume=8 |issue=S293 |pages=159–164 |arxiv=1210.5172 |bibcode=2014IAUS..293..159H |doi=10.1017/S1743921313012738|s2cid=92988047 }}</ref> |
|||
The authors of the [[rare Earth hypothesis]] propose a number of additional constraints on habitability for planets, including being in galactic zones with suitably low radiation, high star metallicity, and low enough density to avoid excessive asteroid bombardment. They also propose that it is necessary to have a planetary system with large gas giants which provide bombardment protection without a [[hot Jupiter]]; and a planet with [[plate tectonic]]s, a large moon that creates tidal pools, and moderate [[axial tilt]] to generate seasonal variation.<ref name="RareEarth">{{cite book |last1=Ward |first1=Peter D. |last2=Brownlee |first2=Donald |title=Rare Earth: Why Complex Life is Uncommon in the Universe |publisher=Copernicus Books (Springer Verlag) |date=2000 |isbn=0-387-98701-0 }}</ref> |
|||
But a pessimist might equally well believe that suitable planets are rare, life seldom becomes intelligent, and intelligent civilizations do not last very long: |
|||
====Fraction of the above that actually go on to develop life, {{math|''f''<sub>l</sub>}}==== |
|||
:''R''* = 10/year, ''f''<sub>p</sub> = 0.5, ''n''<sub>e</sub> = 0.01, ''f''<sub>l</sub> = 0.13, ''f''<sub>i</sub> = 0.001, ''f''<sub>c</sub> = 0.01, and ''L'' = 1000 years |
|||
Geological evidence from the Earth suggests that {{math|''f''<sub>l</sub>}} may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that [[abiogenesis]] may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains [[anthropic bias]], as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical [[hypothesis testing]] standpoint, without assuming that the underlying distribution of {{math|''f''<sub>l</sub>}} is the same for all planets in the Milky Way, there are zero [[degrees of freedom (statistics)|degrees of freedom]], permitting no valid estimates to be made. If life (or evidence of past life) were to be found on [[life on Mars|Mars]], [[Europa (moon)|Europa]], [[Enceladus]] or [[Titan (moon)|Titan]] that developed independently from life on Earth it would imply a value for {{math|''f''<sub>l</sub>}} close to 1. While this would raise the number of degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent. |
|||
:''N'' = 10 × 0.5 × 0.01 × 0.13 × 0.001 × 0.01 × 1000 = 0.000065 (we are almost surely alone in our galaxy). |
|||
Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for [[bacteria]] that are unrelated to other life on Earth, but none have been found yet.<ref> |
|||
Alternatively, making some more optimistic assumptions, assuming that planets are common, life always arises when planets are favorable, 10% of civilizations become willing and able to communicate, and then spread through their local star systems for 100,000 years (a very short period in geologic time): |
|||
{{cite journal |
|||
|last=Davies |first=P. |
|||
|year=2007 |
|||
|title=Are Aliens Among Us? |
|||
|journal=[[Scientific American]] |
|||
|volume=297 |issue=6 |pages=62–69 |
|||
|doi=10.1038/scientificamerican1207-62 |
|||
|bibcode = 2007SciAm.297f..62D }}</ref> It is also possible that life arose more than once, but that other branches were out-competed, or died in mass extinctions, or were lost in other ways. Biochemists [[Francis Crick]] and [[Leslie Orgel]] laid special emphasis on this uncertainty: "At the moment we have no means at all of knowing" whether we are "likely to be alone in the galaxy (Universe)" or whether "the galaxy may be pullulating with life of many different forms."<ref> |
|||
{{cite journal |
|||
|last1=Crick |first1=F. H. C. |
|||
|last2=Orgel |first2=L. E. |
|||
|year=1973 |
|||
|title=Directed Panspermia |
|||
|url=http://profiles.nlm.nih.gov/ps/access/SCBCCP.pdf |archive-url=https://web.archive.org/web/20111029060655/http://profiles.nlm.nih.gov/ps/access/SCBCCP.pdf |archive-date=2011-10-29 |url-status=live |
|||
|journal=[[Icarus (journal)|Icarus]] |
|||
|volume=19 |issue=3 |pages=341–346 |
|||
|bibcode=1973Icar...19..341C |
|||
|doi=10.1016/0019-1035(73)90110-3 |
|||
}}</ref> As an alternative to abiogenesis on Earth, they proposed the hypothesis of [[directed panspermia]], which states that Earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship". |
|||
In 2020, a paper by scholars at the [[University of Nottingham]] proposed an "Astrobiological Copernican" principle, based on the [[Principle of Mediocrity]], and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, {{math|''f''<sub>l</sub>}}, {{math|''f''<sub>i</sub>}}, and {{math|''f''<sub>c</sub>}} are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).<ref>{{cite journal |last1=Westby |first1=Tom |last2=Conselice |first2=Christopher J. |title=The Astrobiological Copernican Weak and Strong Limits for Intelligent Life |journal=The Astrophysical Journal |date=15 June 2020 |volume=896 |issue=1 |pages=58 |doi=10.3847/1538-4357/ab8225|arxiv=2004.03968 |bibcode=2020ApJ...896...58W |s2cid=215415788 |doi-access=free }}</ref><ref>{{cite news |last1=Davis |first1=Nicola |title=Scientists say most likely number of contactable alien civilisations is 36 |url=https://www.theguardian.com/science/2020/jun/15/scientists-say-most-likely-number-of-contactable-alien-civilisations-is-36 |access-date=19 June 2020 |work=The Guardian |date=15 June 2020}}</ref> |
|||
:''R''* = 20/year, ''f''<sub>p</sub> = 0.5, ''n''<sub>e</sub> = 2, ''f''<sub>l</sub> = 1, ''f''<sub>i</sub> = 0.1, ''f''<sub>c</sub> = 0.1, and ''L'' = 100,000 years |
|||
:''N'' = 20 × 0.5 × 2 × 1 × 0.1 × 0.1 × 100,000 = 20,000 (there's quite a few civilizations, although the closest one would still be about 1500 light years away). |
|||
====Fraction of the above that develops intelligent life, {{math|''f''<sub>i</sub>}}==== |
|||
==Current estimates of the parameters== |
|||
This value remains particularly controversial. Those who favor a low value, such as the biologist [[Ernst Mayr]], point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for {{math|''f''<sub>i</sub>}}.<ref name="Ernst Mayr on SETI"> |
|||
This section attempts to list best current estimates for the parameters of the Drake equation. |
|||
{{cite web |
|||
<!-- Please list new estimates for these values here, giving the rationale behind the estimate and a citation to their source. --> |
|||
|title = Ernst Mayr on SETI |
|||
|url = http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html |
|||
|publisher = [[The Planetary Society]] |
|||
|url-status = dead |
|||
|archive-url = https://web.archive.org/web/20101206171624/http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html |
|||
|archive-date = 6 December 2010 |
|||
|df = dmy-all |
|||
}}</ref> Likewise, the Rare Earth hypothesis, notwithstanding their low value for {{math|''n''<sub>e</sub>}} above, also think a low value for {{math|''f''<sub>i</sub>}} dominates the analysis.<ref>Rare Earth, p. xviii.: "We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake or Sagan envisioned. However, ''complex'' life—animals and higher plants—is likely to be far more rare than commonly assumed."</ref> Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable,<ref name="acampbell.ukfsn.org"> |
|||
{{cite web |
|||
|last = Campbell |
|||
|first = A. |
|||
|date = 13 March 2005 |
|||
|title = Review of ''Life's Solution'' by Simon Conway Morris |
|||
|url = http://www.acampbell.ukfsn.org/bookreviews/r/morris.html |
|||
|url-status = dead |
|||
|archive-url = https://web.archive.org/web/20110716063324/http://www.acampbell.ukfsn.org/bookreviews/r/morris.html |
|||
|archive-date = 16 July 2011 |
|||
|df = dmy-all |
|||
}}</ref><ref> |
|||
{{cite book |
|||
|last=Bonner |first=J. T. |
|||
|year=1988 |
|||
|title=The evolution of complexity by means of natural selection |
|||
|url=https://archive.org/details/evolutionofcompl0000bonn |url-access=registration |publisher=[[Princeton University Press]] |
|||
|isbn=0-691-08494-7 |
|||
}}</ref> implying an {{math|''f''<sub>i</sub>}} approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See [[#Criticism|Criticism]]). |
|||
In addition, while it appears that life developed soon after the formation of Earth, the [[Cambrian explosion]], in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the [[snowball Earth]] or research into [[extinction events]] have raised the possibility that life on Earth is relatively fragile. Research on any past [[life on Mars]] is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of {{math|''f''<sub>l</sub>}} but would indicate that in half the known cases, intelligent life did not develop. |
|||
''R*'' = ''the rate of star creation in our galaxy'' |
|||
Estimates of {{math|''f''<sub>i</sub>}} have been affected by discoveries that the Solar System's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for tens of millions of years (evading radiation from [[nova]]e). Also, Earth's large moon may aid the evolution of life by [[Rare Earth hypothesis#A large moon|stabilizing the planet's axis of rotation]]. |
|||
:Estimated by Drake as 10/year. Latest calculations from [[NASA]] and the [[European Space Agency]] indicate that the current rate of star formation in our galaxy is about 7 per year.<ref>{{cite web |url=http://www.nasa.gov/centers/goddard/news/topstory/2006/milkyway_seven.html |title=Milky Way Churns Out Seven New Stars Per Year, Scientists Say |publisher=Goddard Space Flight Center, NASA |accessdate=2008-05-08}}</ref> |
|||
There has been quantitative work to begin to define <math>f_\mathrm{l} \cdot f_\mathrm{i}</math>. One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.<ref name="Kipping2020"> |
|||
{{anchor|eta-earth|}}''f''<sub>p</sub> = ''the fraction of those stars that have planets'' |
|||
{{cite journal |
|||
|last1=Kipping |
|||
|first1=David |
|||
|date=18 May 2020 |
|||
|title=An objective Bayesian analysis of life's early start and our late arrival |
|||
|journal=[[Proceedings of the National Academy of Sciences]] |
|||
|volume=117 |
|||
|issue=22 |
|||
|pages=11995–12003 |
|||
|doi=10.1073/pnas.1921655117|pmid=32424083 |
|||
|pmc=7275750 |
|||
|arxiv=2005.09008 |
|||
|bibcode=2020PNAS..11711995K |
|||
|doi-access=free |
|||
}}</ref><ref name="ColumbiaPR"> |
|||
{{cite web |
|||
|author1=Columbia University |
|||
|title=New study estimates the odds of life and intelligence emerging beyond our planet |
|||
|url=https://phys.org/news/2020-05-odds-life-intelligence-emerging-planet.html |
|||
|website=Phys.org |
|||
|access-date=23 May 2020}} |
|||
</ref> |
|||
Planetary scientist [[Pascal Lee]] of the [[SETI Institute]] proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since ''[[Homo erectus]]'' evolved, compared to 4.6 billion years since Earth formed).<ref>{{Cite web|last=Lee|first=Pascal|title=N~1: Alone in the Milky Way, Mt Tam|website=[[YouTube]]|date=24 October 2020 |url=https://www.youtube.com/watch?v=cuJDkIUuDBg| archive-url=https://ghostarchive.org/varchive/youtube/20211211/cuJDkIUuDBg| archive-date=2021-12-11|url-status=live}}{{cbignore}}</ref><ref>{{Cite web |last=Lee |first=Pascal |title=N~1: Alone in the Milky Way – Kalamazoo Astronomical Society |url=https://www.youtube.com/watch?v=wj5nmgoQr50 |url-status=live |archive-url=https://web.archive.org/web/20210315085249/https://www.youtube.com/watch?v=wj5nmgoQr50 |archive-date=2021-03-15 |website=[[YouTube]]|date=6 March 2021 }}</ref> |
|||
:Estimated by Drake as 0.5. It is now known from modern planet searches that at least 40% of [[sun]]-like stars have planets,<ref>{{cite news |url=http://news.bbc.co.uk/1/hi/sci/tech/8314581.stm |title=Scientists announce planet bounty |publisher=BBC |accessdate=2009-10-19 | date=2009-10-19}}</ref> and the true proportion may be much higher, since only planets considerably larger than Earth can be detected with current technology.<ref name="marcyprogth05">{{cite journal | author=Marcy, G.; Butler, R.; Fischer, D.; et al. | title=Observed Properties of Exoplanets: Masses, Orbits and Metallicities | journal=Progress of Theoretical Physics Supplement | year=2005 | volume=158 | issue= | pages=24 – 42 | url=http://ptp.ipap.jp/link?PTPS/158/24 | doi = 10.1086/172208}}</ref> Infra-red surveys of dust discs around young stars imply that 20-60% of sun-like stars may form terrestrial planets.<ref>{{cite web |url=http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20080217.html |title=Many, Perhaps Most, Nearby Sun-Like Stars May Form Rocky Planets}}</ref> Microlensing surveys, sensitive to planets further from their star, see planets in about 1/3 of systems examined–a lower limit since not all planets are seen.<ref>{{cite journal |title=Extrasolar planets: More giants in focus |author=John Chambers|journal=Nature |issue=467 |pages=405–406 |date=23 September 2010 |doi=10.1038/467405a}}</ref> A mid-2010 estimate by Dimitar Sasselov, of the Kepler planet-hunting team estimates the number of terrestrial planets in the Milky Way to be as much as 100 million.<ref>{{cite web |url=http://cosmiclog.msnbc.msn.com/_news/2010/07/26/4756559-millions-of-earths-talk-causes-a-stir |title=Millions of Earths Talk Causes a Stir}}</ref> |
|||
====Fraction of the above revealing their existence via signal release into space, {{math|''f''<sub>c</sub>}}==== |
|||
''n''<sub>e</sub> = ''the average number of planets (satellites may perhaps sometimes be just as good candidates) that can potentially support life per star that has planets'' |
|||
For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are [[Active SETI|some efforts]] covering only a tiny fraction of the stars that might look for human presence. (See [[Arecibo message]], for example). There is [[Fermi paradox#They choose not to interact with us|considerable speculation]] why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans.<ref> |
|||
{{cite journal |
|||
|last1=Forgan |first1=D. |
|||
|last2=Elvis |first2=M. |
|||
|year=2011 |
|||
|title=Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence |
|||
|journal=[[International Journal of Astrobiology]] |
|||
|volume=10 |issue=4 |pages=307–313 |
|||
|arxiv=1103.5369 |
|||
|bibcode=2011IJAsB..10..307F |
|||
|doi=10.1017/S1473550411000127 |
|||
|s2cid=119111392 |
|||
}}</ref> By this standard, the Earth is a communicating civilization. |
|||
Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.<ref>{{cite journal |title=The Search for Extraterrestrial Intelligence (SETI) |journal=Annual Review of Astronomy and Astrophysics |first=Jill C. |last=Tarter |author-link=Jill Tarter |volume=39 |pages=511–548 |date=September 2001 |doi=10.1146/annurev.astro.39.1.511 |bibcode=2001ARA&A..39..511T|s2cid=261531924 }}</ref> |
|||
:Estimated by Drake as 2. The combination of this with ''f''<sub>l</sub>=1 implies that two planets per star develop life, which not only assumes that our solar system is typical but that there is (or was) life on Mars. Marcy et al.<ref name="marcyprogth05" /> note that most of the observed planets have very eccentric orbits, or orbit very close to the sun where the temperature is too high for earth-like life. However, several planetary systems that look more solar-system-like are known, such as [[HD 70642]], [[HD 154345]], [[Gliese 849]] or [[Gliese 581]]. There may well be other, as yet unseen, earth-sized planets in the habitable zones of these stars. Also, the variety of solar systems that might have habitable zones is not just limited to solar-type stars and earth-sized planets; it is now believed that even tidally locked planets close to [[Habitability of red dwarf systems|red dwarfs might have habitable zones]], and some of the large planets detected so far could potentially support life |
|||
====Lifetime of such a civilization wherein it communicates its signals into space, {{math|''L''}}==== |
|||
:In early 2008, two different research groups concluded that [[Gliese 581 d]] may possibly be habitable.<ref>{{cite journal |doi=10.1051/0004-6361:20077939 |title=The habitability of super-Earths in Gliese 581 |author=W. von Bloh, C.Bounama, M. Cuntz, and S. Franck. |journal=Astronomy & Astrophysics |year=2007 |volume=476 |page=1365}}</ref><ref>{{cite journal |title=Habitable planets around the star Gliese 581? |author=F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. |
|||
[[Michael Shermer]] estimated {{math|''L''}} as 420 years, based on the duration of sixty historical Earthly civilizations.<ref name="Why ET Hasn’t Called"> |
|||
|journal=Astronomy & Astrophysics |year=2007 |volume=476 |pages=1373 |doi=10.1051/0004-6361:20078091 }}</ref> Since about 200 planetary systems are known, this very roughly estimates <math> n_e > 0.005</math>. In 2010, researchers announced the discovery of [[Gliese 581 g]], a 3.1 Earth-mass planet in near the middle of the habitable zone of [[Gliese 581]], and a strong candidate for being the first known Earth-like habitable planet.<ref>{{cite web |url=http://arxiv.org/abs/1009.5733 |title=The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581 |author=Steven S. Vogt, R. Paul Butler, Eugenio J. Rivera, Nader Haghighipour, Gregory W. Henry, Michael H. Williamson}}</ref> Given the closeness of the planet's star, and the number of stars examined to the level of detail needed to find such planets, they estimate ε<sub>Earth</sub>, or the fraction of stars with Earth-like planets, as 10-20%. |
|||
{{cite journal |
|||
|last=Shermer |first=M. |
|||
|date=August 2002 |
|||
|title=Why ET Hasn't Called |
|||
|url=http://www.michaelshermer.com/2002/08/why-et-hasnt-called/ |
|||
|journal=[[Scientific American]] |
|||
|volume=287 |
|||
|issue=2 |
|||
|page=21 |
|||
|bibcode=2002SciAm.287b..33S |
|||
|doi=10.1038/scientificamerican0802-33 |
|||
}}</ref> Using 28 civilizations more recent than the Roman Empire, he calculates a figure of 304 years for "modern" civilizations. It could also be argued from Michael Shermer's results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it is doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including ''reappearance number'', this lack of specificity in defining single civilizations does not matter for the result, since such a civilization turnover could be described as an increase in the ''reappearance number'' rather than increase in {{math|''L''}}, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid. |
|||
[[David Grinspoon]] has argued that once a civilization has developed enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for {{math|''L''}} potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed.<ref name="David Grinspoon 2004"> |
|||
:Using different criteria, Lineweaver has also determined that about 10% of star systems in the Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable for a sufficient time.<ref>{{cite web |url=http://www.newscientist.com/article/dn4525-one-tenth-of-stars-may-support-life.html |title=One tenth of stars may support life |publisher=New Scientist |date=2004-01-01 |accessdate=2010-05-12}}</ref> |
|||
{{cite book |
|||
|last=Grinspoon |first=D. |
|||
|year=2004 |
|||
|title=Lonely Planets |
|||
}}</ref> He proposes that the last factor {{math|''L''}} be replaced with {{math|''f''<sub>IC</sub> · ''T''}}, where {{math|''f''<sub>IC</sub>}} is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and {{math|''T''}} representing the length of time during which this process has been going on. This has the advantage that {{math|''T''}} would be a relatively easy-to-discover number, as it would simply be some fraction of the age of the universe. |
|||
It has also been hypothesized that once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.<ref name="GoldsmithOwen"> |
|||
:NASA's [[Kepler mission]] was launched on March 6, 2009. Unlike previous searches, it is sensitive to planets as small as Earth, and with orbital periods as long as a year. If successful, Kepler should provide a much better estimate of the number of planets per star that are found in the habitable zone. |
|||
{{Cite book |
|||
|last1=Goldsmith |first1=D. |
|||
|last2=Owen |first2=T. |
|||
|year=1992 |
|||
|title=The Search for Life in the Universe |
|||
|edition=2nd |page=415 |
|||
|publisher=[[Addison-Wesley]] |
|||
|isbn=1-891389-16-5 |
|||
}}</ref> |
|||
The astronomer [[Carl Sagan]] speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of [[nuclear warfare]]. Paleobiologist [[Olev Vinn]] suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.<ref name=vinn2024>{{cite journal|last=Vinn|first=O.|date=2024|title=Potential incompatibility of inherited behavior patterns with civilization: Implications for Fermi paradox|journal=Science Progress|volume=107|issue=3|pages=1–6|doi=10.1177/00368504241272491|pmid= 39105260|s2cid= |doi-access=free|pmc=11307330}}</ref> |
|||
:Even if planets are in the habitable zone, however, the number of planets with the right proportion of elements may be difficult to estimate.<ref name="Trimble">{{cite journal | author=Trimble, V. | title=Origin of the biologically important elements.. | journal=Orig Life Evol Biosph. | year=1997 | volume=27 | issue=1–3 | pages=3–21 | pmid=9150565 | doi=10.1023/A:1006561811750}}</ref> Also, the [[Rare Earth hypothesis]], which posits that conditions for intelligent life are quite rare, has advanced a set of arguments based on the Drake equation that the number of planets or satellites that could support life is small, and quite possibly limited to Earth alone; in this case, the estimate of ''n''<sub>e</sub> would be infinitesimal. |
|||
An intelligent civilization might not be organic, as some have suggested that [[artificial general intelligence]] may replace humanity.<ref>{{cite news |author=Sulleyman |first=Aatif |date=2 November 2017 |title=Stephen Hawking warns artificial intelligence 'may replace humans altogether' |work=independent.co.uk |url=https://www.independent.co.uk/life-style/gadgets-and-tech/news/stephen-hawking-artificial-intelligence-fears-ai-will-replace-humans-virus-life-a8034341.html}}</ref> |
|||
''f''<sub>l</sub> = ''the fraction of the above that actually go on to develop life'' |
|||
===Range of results=== |
|||
:Estimated by Drake as 1. |
|||
As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions,<ref>"The value of {{mvar|N}} remains highly uncertain. Even if we had a perfect knowledge of the first two terms in the equation, there are still five remaining terms, each of which could be uncertain by factors of 1,000." from {{cite journal |title=The search for extraterrestrial intelligence |
|||
|author=Wilson, TL |
|||
|journal=Nature |
|||
|volume=409 |
|||
|issue=6823 |
|||
|pages=1110–1114 |
|||
|year=2001 |
|||
|publisher=Nature Publishing Group |bibcode = 2001Natur.409.1110W |doi = 10.1038/35059235 |pmid=11234025 |
|||
|s2cid=205014501 |
|||
}}, or more informally, "The Drake Equation can have any value from "billions and billions" to zero", Michael Crichton, as quoted in {{cite book |author=Douglas A. Vakoch |title=The Drake Equation: Estimating the prevalence of extraterrestrial life through the ages |publisher=Cambridge University Press |date=2015 |isbn=978-1-10-707365-4|display-authors=etal}}, p. 13</ref> as the values used in portions of the Drake equation are not well established.<ref name="schilling2011">{{cite web |author=Schilling |first=Govert |date=November 2011 |title=The Chance of Finding Aliens: Reevaluating the Drake Equation |url=http://www.astro-tom.com/technical_data/alien_life.htm |work=astro-tom.com}}</ref><ref name=renamed_from_2011_on_20160211035411>{{cite web|url=https://www.e-education.psu.edu/astro801/content/l12_p5.html|title=The Drake Equation|work=psu.edu}}</ref><ref>{{cite web|url=http://www.space.com/22648-drake-equation-alien-life-seager.html|title=The Drake Equation Revisited: Interview with Planet Hunter Sara Seager|author=Devin Powell, Astrobiology Magazine|work=Space.com|date=4 September 2013}}</ref><ref name=schilling2009>{{cite news|url=https://skyandtelescope.org/astronomy-news/the-chance-of-finding-aliens/|title=The Chance of Finding Aliens|author1=Govert Schilling|author2=Alan M. MacRobert|date=3 June 2009|work=Sky & Telescope}}</ref> In particular, the result can be {{math|''N'' ≪ 1}}, meaning we are likely alone in the galaxy, or {{math|''N'' ≫ 1}}, implying there are many civilizations we might contact. One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.<ref name="Dean">{{better source needed|date=August 2015}} |
|||
{{cite web |
|||
|last = Dean |
|||
|first = T. |
|||
|date = 10 August 2009 |
|||
|title = A review of the Drake Equation |
|||
|url = http://www.cosmosmagazine.com/features/are-we-alone-a-review-drake-equation/ |
|||
|work = [[Cosmos Magazine]] |
|||
|access-date = 16 April 2013 |
|||
|url-status = dead |
|||
|archive-url = https://web.archive.org/web/20130603043832/http://www.cosmosmagazine.com/features/are-we-alone-a-review-drake-equation/ |
|||
|archive-date = 3 June 2013 |
|||
|df = dmy-all |
|||
}}</ref> |
|||
As an example of a low estimate, combining NASA's star formation rates, the [[rare Earth hypothesis]] value of {{math|''f''<sub>p</sub> · ''n''<sub>e</sub> · ''f''<sub>l</sub> {{=}} 10<sup>−5</sup>}},<ref>Rare Earth, page 270: "When we take into account factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001% of all stars might have planets with habitats similar to Earth. [...] If microbial life forms readily, then millions to hundreds of millions of planets in the galaxy have the ''potential'' for developing advanced life. (We expect that a much higher number will have microbial life.)"</ref> Mayr's view on intelligence arising, Drake's view of communication, and Shermer's estimate of lifetime: |
|||
:In 2002, Charles H. Lineweaver and Tamara M. Davis (at the [[University of New South Wales]] and the Australian Centre for Astrobiology) estimated ''f''<sub>l</sub> as > 0.13 on planets that have existed for at least one billion years using a statistical argument based on the length of time life took to evolve on Earth.<ref>{{cite journal | author=Lineweaver, C. H. & Davis, T. M. | title=Does the rapid appearance of life on Earth suggest that life is common in the universe? | journal=Astrobiology | year=2002 | volume=2 | issue=3 | pages=293–304 | pmid=12530239 | doi = 10.1089/153110702762027871}}</ref> |
|||
:{{math|''R''<sub>∗</sub> {{=}} 1.5–3 yr<sup>−1</sup>}},<ref name=Robitaille/> {{math|''f''<sub>p</sub> · ''n''<sub>e</sub> · ''f''<sub>l</sub> {{=}} 10<sup>−5</sup>}},<ref name="RareEarth"/> {{math|''f''<sub>i</sub> {{=}} 10<sup>−9</sup>}},<ref name="Ernst Mayr on SETI"/> {{math|''f''<sub>c</sub> {{=}} 0.2}}<sup>[Drake, above]</sup>, and {{math|''L'' {{=}} 304}} years<ref name="Why ET Hasn’t Called"/> |
|||
gives: |
|||
:{{math|''N'' {{=}} 1.5 × 10<sup>−5</sup> × 10<sup>−9</sup> × 0.2 × 304 {{=}} 9.1 × 10<sup>−13</sup>}} |
|||
i.e., suggesting that we are probably alone in this galaxy, and possibly in the [[observable universe]]. |
|||
On the other hand, with larger values for each of the parameters above, values of {{math|''N''}} can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters: |
|||
''f''<sub>i</sub> = ''the fraction of the above that actually go on to develop ''intelligent'' life'' |
|||
:{{math|''R''<sub>∗</sub> {{=}} 1.5–3 yr<sup>−1</sup>}},<ref name=Robitaille/> {{math|''f''<sub>p</sub> {{=}} 1}},<ref name="bbc.co.uk"/> {{math|''n''<sub>e</sub> {{=}} 0.2}},<ref name="W. von Bloh, C.Bounama, M. Cuntz, and S. Franck. 2007 1365"> |
|||
{{cite journal |
|||
|last1=von Bloh |first1=W. |
|||
|last2=Bounama |first2=C. |
|||
|last3=Cuntz |first3=M. |
|||
|last4=Franck |first4=S. |
|||
|year=2007 |
|||
|title=The habitability of super-Earths in Gliese 581 |
|||
|journal=[[Astronomy & Astrophysics]] |
|||
|volume=476 |issue=3 |pages=1365–1371 |
|||
|arxiv=0705.3758 |
|||
|bibcode=2007A&A...476.1365V |
|||
|doi=10.1051/0004-6361:20077939 |
|||
|s2cid=14475537 |
|||
}}</ref><ref name="F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. 2007 1373"> |
|||
{{cite journal |doi=10.1051/0004-6361:20078091 |bibcode=2007A&A...476.1373S |title=Habitable planets around the star Gl 581? |journal=[[Astronomy and Astrophysics]] |volume=476 |issue=3 |year=2007 |pages=1373–1387 |last1=Selsis |first1=Franck |last2=Kasting |first2=James F. |last3=Levrard |first3=Benjamin |last4=Paillet |first4=Jimmy |last5=Ribas |first5=Ignasi |last6=Delfosse |first6=Xavier |arxiv=0710.5294 |s2cid=11492499 |ref=Selsis |url=https://hal.archives-ouvertes.fr/hal-00182743 }}</ref> {{math|''f''<sub>l</sub> {{=}} 0.13}},<ref name="Lineweaver, C. H. & Davis, T. M. 2002 293–304"> |
|||
{{cite journal |
|||
|last1=Lineweaver |first1=C. H. |
|||
|last2=Davis |first2=T. M. |
|||
|year=2002 |
|||
|title=Does the rapid appearance of life on Earth suggest that life is common in the universe? |
|||
|journal=[[Astrobiology (journal)|Astrobiology]] |
|||
|volume=2 |issue=3 |pages=293–304 |
|||
|arxiv=astro-ph/0205014 |
|||
|bibcode=2002AsBio...2..293L |
|||
|doi=10.1089/153110702762027871 |
|||
|pmid=12530239 |
|||
|s2cid=431699 |
|||
}}</ref> {{math|''f''<sub>i</sub> {{=}} 1}},<ref name="acampbell.ukfsn.org"/> {{math|''f''<sub>c</sub> {{=}} 0.2}}<sup>[Drake, above]</sup>, and {{math|''L'' {{=}} 10<sup>9</sup>}} years<ref name="David Grinspoon 2004"/> |
|||
Use of these parameters gives: |
|||
:{{math|''N'' {{=}} 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 10<sup>9</sup> {{=}} 15,600,000}} |
|||
[[Monte Carlo method|Monte Carlo]] simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.<ref> |
|||
:Estimated by Drake as 0.01 based on little or no evidence. This value remains particularly controversial. The famous biologist [[Ernst Mayr]] pointed out that of the billions of species that have existed on Earth, only one has become intelligent<ref>{{cite web |url=http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html |title=Ernst Mayr on SETI}}</ref> and infer a tiny value for ''f''<sub>i</sub>.Skeptics point out that the large spread of values in this term and others make all estimates unreliable. (See [[#Criticism|criticism]]). |
|||
{{cite journal |
|||
|last1=Forgan |first1=D. |
|||
|year=2009 |
|||
|title=A numerical testbed for hypotheses of extraterrestrial life and intelligence |
|||
|journal=[[International Journal of Astrobiology]] |
|||
|volume=8 |issue=2 |pages=121–131 |
|||
|arxiv=0810.2222 |
|||
|bibcode=2009IJAsB...8..121F |
|||
|doi=10.1017/S1473550408004321 |
|||
|s2cid=17469638 |
|||
}}</ref> |
|||
===Possible former technological civilizations=== |
|||
''f''<sub>c</sub> = ''the fraction of the above that are willing and able to communicate'' |
|||
In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the ''only'' technological species that has ''ever'' arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ''ever'' develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than {{val|2.5|e=-24}}. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than {{val|1.7|e=-11}} (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.<ref>{{cite news |url= http://phys.org/news/2016-04-limits-uniqueness.html |title= Are we alone? Setting some limits to our uniqueness |date= 28 April 2016 |publisher= phys.org }}</ref><ref>{{cite episode |title= Are We Alone? Galactic Civilization Challenge |series= PBS Space Time |date= 5 October 2016 |network= PBS Digital Studios }}</ref><ref>{{cite news |author=Frank |first=Adam |date=10 June 2016 |title=Yes, There Have Been Aliens |work=The New York Times |url=https://www.nytimes.com/2016/06/12/opinion/sunday/yes-there-have-been-aliens.html}}</ref><ref>{{cite journal |author=Frank |first1=Adam |last2=Sullivan III |first2=W. T. |date=22 April 2016 |title=A New Empirical Constraint on the Prevalence of Technological Species in the Universe |journal=Astrobiology |publication-date=13 May 2016 |volume=16 |issue=5 |pages=359–362 |arxiv=1510.08837 |bibcode=2016AsBio..16..359F |doi=10.1089/ast.2015.1418 |pmid=27105054}}</ref><ref>''Bioverse: How the Cellular World Contains the Secrets to Life's Biggest Questions'' William B Miller Jr. {{ISBN|9781633887992}} p50</ref> |
|||
==Modifications== |
|||
:Estimated by Drake as 0.01. There is [[Fermi paradox#They choose not to interact with us|considerable speculation why a civilization might exist but choose not to communicate]], but there is no hard data. |
|||
As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters,<ref> |
|||
{{cite journal |
|||
|last1 = Hetesi |
|||
|first1 = Z. |
|||
|last2 = Regaly |
|||
|first2 = Z. |
|||
|year = 2006 |
|||
|title = A new interpretation of Drake-equation |
|||
|url = http://astro.elte.hu/~hetesizs/Hetesi%20Zsolt%20cikkei/new%20interpretation%20fo%20drake%20eq.pdf |
|||
|archive-url = https://web.archive.org/web/20090205123935/http://astro.elte.hu/~hetesizs/Hetesi%20Zsolt%20cikkei/new%20interpretation%20fo%20drake%20eq.pdf |
|||
|url-status = dead |
|||
|archive-date = 2009-02-05 |
|||
|journal = [[Journal of the British Interplanetary Society]] |
|||
|volume = 59 |
|||
|pages = 11–14 |
|||
|bibcode = 2006JBIS...59...11H |
|||
}}</ref> and many changes and modifications to the equation have been proposed. One line of modification, for example, attempts to account for the uncertainty inherent in many of the terms.<ref> |
|||
{{cite journal |
|||
|last=Maccone |first=C. |
|||
|year=2010 |
|||
|title=The Statistical Drake Equation |
|||
|journal=[[Acta Astronautica]] |
|||
|volume=67 |issue=11–12 |pages=1366–1383 |
|||
|bibcode= 2010AcAau..67.1366M |
|||
|doi=10.1016/j.actaastro.2010.05.003 |
|||
|s2cid=121239391 |
|||
}}</ref> |
|||
Combining the estimates of the original six factors by major researchers via a Monte Carlo procedure leads to a best value for the non-longevity factors of 0.85 1/years.<ref>{{Cite journal |last=Golden |first=Leslie M. |date=2021-08-01 |title=A joint mind consideration of the Drake equation in the search for extraterrestrial intelligence |url=https://www.sciencedirect.com/science/article/pii/S0094576521001338 |journal=Acta Astronautica |volume=185 |pages=333–336 |doi=10.1016/j.actaastro.2021.03.020 |bibcode=2021AcAau.185..333G |s2cid=233663920 |issn=0094-5765}}</ref> This result differs insignificantly from the estimate of unity given both by Drake and the Cyclops report. |
|||
Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, [[David Brin]] states: "The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society".<ref name="GS"> |
|||
''L'' = ''the expected lifetime of such a civilization for the period that it can communicate across interstellar space'' |
|||
{{cite journal |
|||
|last=Brin |first=G. D. |
|||
|year=1983 |
|||
|title=The Great Silence – The Controversy Concerning Extraterrestrial Intelligent Life |
|||
|journal=[[Quarterly Journal of the Royal Astronomical Society]] |
|||
|volume=24 |issue=3 |pages=283–309 |
|||
|bibcode=1983QJRAS..24..283B |
|||
}}</ref> Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed. |
|||
;Colonization : It has been proposed to generalize the Drake equation to include additional effects of alien civilizations colonizing other [[star system]]s. Each original site expands with an expansion velocity {{mvar|v}}, and establishes additional sites that survive for a lifetime {{mvar|L}}. The result is a more complex set of 3 equations.<ref name="GS"/> |
|||
:Estimated by Drake as 10,000 years. |
|||
;Reappearance factor : The Drake equation may furthermore be multiplied by ''how many times'' an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next [[Sociocultural evolution|civilization to evolve]]. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if {{math|''n''<sub>r</sub>}} is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be {{math|1 + ''n''<sub>r</sub>}}, which is the actual ''reappearance factor'' added to the equation. |
|||
:In an article in ''[[Scientific American]]'', [[Michael Shermer]] estimated ''L'' as 420 years, based on compiling the durations of sixty historical civilizations.<ref>{{cite web |url=http://www.michaelshermer.com/2002/08/why-et-hasnt-called/ |title=Why ET Hasn’t Called|publisher=Scientific American |date=August 2002}}</ref> Using twenty-eight civilizations more recent than the Roman Empire he calculates a figure of 304 years for "modern" civilizations. It could also be argued from [[Michael Shermer]]'s results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it's doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including ''reappearance number'', this lack of specificity in defining single civilizations doesn't matter for the end result, since such a civilization turnover could be described as an increase in the ''reappearance number'' rather than increase in ''L'', stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid. |
|||
:The factor depends on what generally is the cause of [[civilization extinction]]. If it is generally by temporary uninhabitability, for example a [[nuclear winter]], then {{math|''n''<sub>r</sub>}} may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as [[stellar evolution]], then {{math|''n''<sub>r</sub>}} may be almost zero. In the case of total life extinction, a similar factor may be applicable for {{math|''f''<sub>l</sub>}}, that is, ''how many times'' life may appear on a planet where it has appeared once. |
|||
:[[David Grinspoon]] has argued that once a civilization has developed it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for ''L'' potentially billions of years. If this is the case, then the galaxy has been steadily accumulating advanced civilizations since it formed.<ref>[[Lonely Planets]], David Grinspoon (2004)</ref> |
|||
;METI factor : [[Aleksandr Leonidovich Zaitsev|Alexander Zaitsev]] said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans, although being in a communicative phase, are not a communicative civilization; we do not practise such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (messaging to extraterrestrial intelligence) to the classical Drake equation.<ref> |
|||
Values based on the above estimates, |
|||
{{cite web |
|||
:''R''* = 7/year, ''f''<sub>p</sub> = 0.5, ''n''<sub>e</sub> = 2, ''f''<sub>l</sub> = 0.33, ''f''<sub>i</sub> = 0.01, ''f''<sub>c</sub> = 0.01, and ''L'' = 10000 years |
|||
|last=Zaitsev |first=A. |
|||
result in |
|||
|date=May 2005 |
|||
:''N'' = 7 × 0.5 × 2 × 0.33 × 0.01 × 0.01 × 10000 = 2.31 |
|||
|title=The Drake Equation: Adding a METI Factor |
|||
|url=http://www.cplire.ru/html/ra&sr/irm/Drake_equation.html |
|||
|publisher=[[SETI League]] |
|||
|access-date=2013-04-20 |
|||
}}</ref> He defined the factor as "the fraction of communicative civilizations with clear and non-paranoid planetary consciousness", or alternatively expressed, the fraction of communicative civilizations that actually engage in deliberate interstellar transmission. |
|||
:The METI factor is somewhat misleading since active, purposeful transmission of messages by a civilization is not required for them to receive a broadcast sent by another that is seeking first contact. It is merely required they have capable and compatible receiver systems operational; however, this is a variable humans cannot accurately estimate. |
|||
James Kasting, in his book "How To Find A Habitable Planet", gives the equation as <math>N=N_g \times f_p \times n_e \times f_l \times f_i \times f_c \times f_L</math>, where the first term on the right hand side of the equation is the number of stars in the galaxy. He estimates the first three terms at 4 * 10 to the ninth power. He then uses Carl Sagan's figures for the next three terms, disclaiming responsibility, and arrives at approximately 10 to the seventh power as an estimate, not considering the final term, <math>f_L</math>, which is the fraction of a planet's lifetime during which it supports a technical civilization. He notes that this is the most uncertain factor in the equation.<ref>Kastings, James. "How To Find A Habitable Planet" Princeton University Press, Princeton, 2010.</ref> |
|||
;Biogenic gases : Astronomer [[Sara Seager]] proposed a revised equation that focuses on the search for planets with biosignature gases.<ref name="NYT-20161207">{{cite news |last=Jones |first=Chris |title='The World Sees Me as the One Who Will Find Another Earth' – The star-crossed life of Sara Seager, an astrophysicist obsessed with discovering distant planets. |url=https://www.nytimes.com/2016/12/07/magazine/the-world-sees-me-as-the-one-who-will-find-another-earth.html |date=7 December 2016 |work=[[The New York Times]] |access-date=8 December 2016 }}</ref> These gases are produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.<ref name="equation">{{Cite web |author1=Devin Powell |date=2013-09-04 |title=The Drake Equation Revisited: Interview with Planet Hunter Sara Seager |url=https://www.space.com/22648-drake-equation-alien-life-seager.html |access-date=2023-10-06 |website=Space.com |language=en}}</ref> |
|||
:The Seager equation looks like this:<ref name=equation />{{refn|group=lower-alpha|The rendering of the equation here is slightly modified for clarity of presentation from the rendering in the cited source.<ref name=equation />}} |
|||
::<math display=block>N = N_* \cdot F_\mathrm{Q} \cdot F_\mathrm{HZ} \cdot F_\mathrm{O} \cdot F_\mathrm{L} \cdot F_\mathrm{S}</math> |
|||
:where: |
|||
::{{math|''N''}} = the number of planets with detectable signs of life |
|||
::{{math|''N''<sub>∗</sub>}} = the number of stars observed |
|||
::{{math|''F''<sub>Q</sub>}} = the fraction of stars that are quiet |
|||
::{{math|''F''<sub>HZ</sub>}} = the fraction of stars with rocky planets in the habitable zone |
|||
::{{math|''F''<sub>O</sub>}} = the fraction of those planets that can be observed |
|||
::{{math|''F''<sub>L</sub>}} = the fraction that have life |
|||
::{{math|''F''<sub>S</sub>}} = the fraction on which life produces a detectable signature gas |
|||
:Seager stresses, "We're not throwing out the Drake Equation, which is really a different topic," explaining, "Since Drake came up with the equation, we have discovered thousands of exoplanets. We as a community have had our views revolutionized as to what could possibly be out there. And now we have a real question on our hands, one that's not related to intelligent life: Can we detect any signs of life in any way in the very near future?"<ref>{{cite web|url=http://io9.com/what-a-brand-new-equation-reveals-about-our-odds-of-fin-531575395|title=A New Equation Reveals Our Exact Odds of Finding Alien Life|date=21 June 2013 |publisher=[[io9]]}}</ref> |
|||
;Carl Sagan's version of the Drake equation:American astronomer [[Carl Sagan]] made some modifications<ref>{{cite web |title=The Drake Equation |url=https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Supplemental_Modules_(Astronomy_and_Cosmology)/Astronomy/Life_beyond_the_Earth/The_Drake_Equation#:~:text=Another%20version%20of%20the%20Drake,graced%20by%20a%20technological%20civilization. |website=phys.libretexts.org | date=13 August 2014 |access-date=4 February 2024}}</ref> in the Drake equation and presented it in the 1980 program ''[[Cosmos: A Personal Voyage]]''.<ref>{{cite web |title=Carl Sagan - Cosmos - Drake Equation | website=[[YouTube]] | date=24 March 2009 |url=https://www.youtube.com/watch?v=MlikCebQSlY}}</ref> The modified equation is shown below |
|||
<math display="block">N = N_\mathrm{*} \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot f_\mathrm{L}</math><ref>{{cite web |title=Carl Sagan - Cosmos - Drake Equation | website=[[YouTube]] | date=24 March 2009 |url=https://www.youtube.com/watch?v=MlikCebQSlY |access-date=4 February 2024}}</ref> |
|||
where |
|||
* {{math|''N''}} = the number of [[civilization]]s in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past [[light cone]]); |
|||
and |
|||
* {{math|''N''<sub>∗</sub>}} = Number of [[stars]] in the [[Milky Way Galaxy]] |
|||
* {{math|''f''<sub>p</sub>}} = the fraction of those stars that have [[planet]]s. |
|||
* {{math|''n''<sub>e</sub>}} = the average number of planets that can potentially support [[life]] per star that has planets. |
|||
* {{math|''f''<sub>l</sub>}} = the fraction of planets that could support life that actually develop life at some point. |
|||
* {{math|''f''<sub>i</sub>}} = the fraction of planets with life that go on to develop [[intelligence|intelligent]] life (civilizations). |
|||
* {{math|''f''<sub>c</sub>}} = the fraction of civilizations that develop a technology that releases [[Technosignature|detectable signs of their existence into space]]. |
|||
* {{math|''f''<sub>L</sub>}} = fraction of a planetary lifetime graced by a technological civilization |
|||
==Criticism== |
==Criticism== |
||
Criticism of the Drake equation is varied. Firstly, many of the terms in the equation are largely or entirely based on conjecture.<ref>{{Cite web |last=Hartsfield |first=Tom |date=2015-03-11 |title=Why the Drake Equation Is Useless {{!}} RealClearScience |url=https://www.realclearscience.com/blog/2015/03/why_the_drake_equation_is_useless.html |access-date=2024-04-29 |website=www.realclearscience.com |language=en}}</ref><ref>{{Cite web |title=The Drake Equation: Could It Be Wrong? |url=https://www.seti.org/drake-equation-could-it-be-wrong |access-date=2024-04-29 |website=SETI Institute |language=en}}</ref> Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.<ref> |
|||
Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. Thus the equation cannot be used to draw firm conclusions of any kind. As T.J. Nelson states:<ref>{{cite web |url=http://brneurosci.org/reviews/scienceofgod.html |title=Review: The Science of God |author = T.J. Watson }}</ref> |
|||
{{cite web |
|||
<blockquote> The Drake equation consists of a large number of probabilities multiplied together. Since each factor is guaranteed to be somewhere between 0 and 1, the result is also guaranteed to be a reasonable-looking number between 0 and 1. Unfortunately, all the probabilities are completely unknown, making the result worse than useless.</blockquote> |
|||
|last=Dvorsky |first=G. |
|||
Likewise, in a 2003 lecture at [[Caltech]], [[Michael Crichton]], a science fiction author, stated:<ref>{{cite web |url=http://www.michaelcrichton.com/speech-alienscauseglobalwarming.html |author=Michael Crichton |title=Aliens cause Global Warming}}</ref> |
|||
|date=31 May 2007 |
|||
<blockquote>The problem, of course, is that none of the terms can |
|||
|title=The Drake Equation is obsolete |
|||
be known, and most cannot even be estimated. The only way to work the |
|||
|url=http://www.sentientdevelopments.com/2007/05/drake-equation-is-obsolete.html |
|||
equation is to fill in with guesses. [...] As a result, the Drake equation can have any value from "billions and |
|||
|work=Sentient Developments |
|||
billions" to zero. An expression that can mean anything means nothing. |
|||
|access-date=2013-08-21 |
|||
Speaking precisely, the Drake equation is literally meaningless...</blockquote> |
|||
}}</ref><ref>{{Cite web|url=https://www.space.com/42739-stop-using-the-drake-equation.html|title=Alien Hunters, Stop Using the Drake Equation|last=Sutter|first=Paul|date=2018-12-27|website=Space.com|language=en|access-date=2019-02-18}}</ref> |
|||
Others point out that the equation was formulated before our understanding of the universe had matured. Astrophysicist Ethan Siegel, said: |
|||
{{blockquote|The Drake equation, when it was put forth, made an assumption about the Universe that we now know is untrue: It assumed that the Universe was eternal and static in time. As we learned only a few years after Frank Drake first proposed his equation, the Universe doesn’t exist in a steady state, where it’s unchanging in time, but rather has evolved from a hot, dense, energetic, and rapidly expanding state: a hot Big Bang that occurred over a finite duration in our cosmic past.<ref>{{Cite web |date=2024-04-23 |title=The unsurprising non-detection of intelligent aliens |url=https://bigthink.com/starts-with-a-bang/unsurprising-non-detection-intelligent-aliens/ |access-date=2024-04-29 |website=Big Think |language=en-US}}</ref>}} |
|||
One reply to such criticisms<ref> |
|||
{{Cite journal |
|||
|last=Tarter |first=Jill C. |
|||
|date=May–June 2006 |
|||
|title=The Cosmic Haystack Is Large |
|||
|url=http://www.csicop.org/si/show/cosmic_haystack_is_large/ |
|||
|journal=[[Skeptical Inquirer]] |
|||
|volume=30 |issue=3 |
|||
|access-date=2013-08-21 |
|||
}}</ref> is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.<ref> |
|||
{{cite web |
|||
|last=Alexander |first=A. |
|||
|title=The Search for Extraterrestrial Intelligence: A Short History – Part 7: The Birth of the Drake Equation |
|||
|url=http://www.planetary.org/html/UPDATES/seti/history/History07.htm |
|||
|publisher=[[The Planetary Society]] |
|||
|archive-url=https://web.archive.org/web/20050306072552/http://www.planetary.org/html/UPDATES/seti/history/History07.htm |
|||
|archive-date=2005-03-06 |
|||
}}</ref> |
|||
===Fermi paradox=== |
|||
{{main|Fermi paradox}} |
|||
A civilization lasting for tens of millions of years could be able to spread throughout the galaxy, even at the slow speeds foreseeable with present-day technology. However, no confirmed signs of civilizations or intelligent life elsewhere have been found, either in this Galaxy or in the [[observable universe]] of 2 [[trillion (short scale)|trillion]] galaxies.<ref name="Conselice">{{cite journal|title=The Evolution of Galaxy Number Density at {{math|''z'' < 8}} and its Implications|author=Christopher J. Conselice|journal=The Astrophysical Journal|volume=830|issue=2|year=2016|arxiv=1607.03909|bibcode= 2016ApJ...830...83C|doi=10.3847/0004-637X/830/2/83|display-authors=etal|pages=83|s2cid=17424588 |doi-access=free }}</ref><ref name="NYT-20161017">{{cite news |last=Fountain |first=Henry |title=Two Trillion Galaxies, at the Very Least |url=https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2016/10/18/science/two-trillion-galaxies-at-the-very-least.html |archive-date=2022-01-01 |url-access=limited |date=17 October 2016 |work=[[The New York Times]] |access-date=17 October 2016 }}{{cbignore}}</ref> According to this line of thinking, the tendency to fill (or at least explore) all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".<ref name="OSTI-19850301"> |
|||
{{cite report |
|||
|last=Jones |first=E. M. |
|||
|date=1 March 1985 |
|||
|title="Where is everybody?" An account of Fermi's question |
|||
|url=http://www.osti.gov/accomplishments/documents/fullText/ACC0055.pdf |archive-url=https://web.archive.org/web/20071012123516/http://www.osti.gov/accomplishments/documents/fullText/ACC0055.pdf |archive-date=2007-10-12 |url-status=live |
|||
|publisher=[[Los Alamos National Laboratory]] |
|||
|access-date=2013-08-21 |
|||
|bibcode=1985STIN...8530988J |
|||
|osti=5746675 |
|||
|osti-access=free |
|||
|doi=10.2172/5746675 |
|||
|doi-access=free |
|||
}}</ref><ref> |
|||
{{Cite news |
|||
|last=Krauthammer |first=C. |
|||
|date=29 December 2011 |
|||
|title=Are we alone in the Universe? |
|||
|url=https://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html |
|||
|newspaper=[[The Washington Post]] |
|||
|access-date=2013-08-21 |
|||
}}</ref> |
|||
A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations.<ref> |
|||
{{cite book |
|||
|last=Webb |first=S. |
|||
|year=2015 |
|||
|title=If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life |
|||
|url=https://books.google.com/books?id=QWKyrQEACAAJ&q=Universe-Teeming-Aliens-WHERE-EVERYBODY |
|||
|publisher=Springer International Publishing |
|||
|isbn=978-3319132358 |
|||
}}</ref> In terms of the Drake Equation, the explanations can be divided into three classes: |
|||
*Few intelligent civilizations ever arise. This is an argument that at least one of the first few terms, {{math|''R''<sub>∗</sub> · ''f''<sub>p</sub> · ''n''<sub>e</sub> · ''f''<sub>l</sub> · ''f''<sub>i</sub>}}, has a low value. The most common suspect is {{math|''f''<sub>i</sub>}}, but explanations such as the rare Earth hypothesis argue that {{math|''n''<sub>e</sub>}} is the small term. |
|||
*Intelligent civilizations exist, but we see no evidence, meaning {{math|''f''<sub>c</sub>}} is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, communication is dangerous, and many others. |
|||
*The lifetime of intelligent, communicative civilizations is short, meaning the value of {{mvar|L}} is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to be destroyed by natural events, and others. |
|||
These lines of reasoning lead to the [[Great Filter]] hypothesis,<ref> |
|||
{{cite web |last=Hanson |first=R. |date=15 September 1998 |title=The Great Filter – Are We Almost Past It? |url=http://hanson.gmu.edu/greatfilter.html |access-date=2013-08-21}}</ref> which states that since there are no observed extraterrestrial civilizations despite the vast number of stars, at least one step in the process must be acting as a filter to reduce the final value. According to this view, either it is very difficult for intelligent life to arise, or the lifetime of technologically advanced civilizations, or the period of time they reveal their existence must be relatively short. |
|||
An analysis by [[Anders Sandberg]], [[Eric Drexler]] and [[Toby Ord]] suggests "a substantial ''ex ante'' (predicted) probability of there being no other intelligent life in our observable universe".<ref>{{cite arXiv|last1=Sandberg|first1=Anders|last2=Drexler|first2=Eric|last3=Ord|first3=Toby|date=2018-06-06|title=Dissolving the Fermi Paradox|class=physics.pop-ph|eprint=1806.02404}}</ref> |
|||
==In fiction and popular culture== |
|||
[[File:Europa Clipper commemorative plate.jpg|thumb|Commemorative plate on [[Europa Clipper]]]] |
|||
The equation was cited by [[Gene Roddenberry]] as supporting the multiplicity of inhabited planets shown on ''[[Star Trek]]'', the television series he created. However, Roddenberry did not have the equation with him, and he was forced to "invent" it for his original proposal.<ref>''The Making of Star Trek'' by Stephen E. Whitfield and Gene Roddenberry, New York: Ballantine Books, 1968</ref> The invented equation created by Roddenberry is: |
|||
Another objection is that the very form of the Drake equation assumes that civilizations arise and then die out within their original solar systems. If interstellar colonization is possible, then this assumption is invalid, and the equations of [[population dynamics]] would apply instead.<ref>{{cite book |author= [[Jack Cohen (scientist)|Jack Cohen]] and [[Ian Stewart (mathematician)|Ian Stewart]] |title=[[Evolving the Alien]] |publisher=John Wiley and Sons, Inc., Hoboken, NJ |year=2002}} Chapter 6, ''What does a Martian look like?''</ref> |
|||
<math display="block">Ff^2 (MgE)-C^1 Ri^1 \cdot M=L/So </math> |
|||
One reply to such criticisms<ref>Jill Tarter, ''The Cosmic Haystack Is Large'', Skeptical Inquirer magazine, May 2006.</ref> is that even though the Drake equation currently involves speculation about unmeasured parameters, it was not meant to be science, but intended as a way to stimulate dialog on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.<ref>{{cite web|url=http://www.planetary.org/explore/topics/seti/seti_history_07.html|author=Amir Alexander|title=The Search for Extraterrestrial Intelligence: A Short History - Part 7: The Birth of the Drake Equation}}</ref> |
|||
Regarding Roddenberry's fictional version of the equation, Drake himself commented that a number raised to the first power is just the number itself.<ref>{{cite book |title=[[The Star Trek Encyclopedia]] |author=[[Michael Okuda|Okuda, Mike]] and Denise Okuda, with Debbie Mirek |publisher=Pocket Books |page=122 |isbn=0-671-53609-5 |year=1999}}</ref> |
|||
Another reply to such criticisms is that the Drake Equation is a [[Fermi problem]] which involves the multiplication of several estimated factors, and such calculations (e.g. the number of piano tuners in Chicago) will ''probably'' be more accurate than might be first supposed (assuming that there is no consistent bias in the estimated factors). This is because if there is no consistent bias, then there will probably (with a binomial distribution) be some factors that are estimated too high and other factors that are estimated too low, and such errors will partially cancel each other out. |
|||
A commemorative plate on NASA's [[Europa Clipper]] mission, planned for launch in October 2024, features a poem by the U.S. Poet Laureate [[Ada Limón]], waveforms of the word 'water' in 103 languages, a schematic of the [[water hole (radio)|water hole]], the Drake equation, and a portrait of planetary scientist [[Ron Greeley]] on it.<ref>{{cite web |title=NASA Unveils Design for Message Heading to Jupiter's Moon Europa |url=https://www.jpl.nasa.gov/news/nasa-unveils-design-for-message-heading-to-jupiters-moon-europa |website=NASA Jet Propulsion Laboratory (JPL) |access-date=11 March 2024}} {{PD-notice}}</ref> |
|||
==In fiction== |
|||
The Drake equation and the [[Fermi paradox]] have been discussed many times in [[science fiction]], including both serious takes in stories such as [[Frederik Pohl]]'s [[Hugo Award for Best Short Story|Hugo award]]-winning "Fermi and Frost", which cites the paradox as evidence for the short lifetime of technical civilizations — that is, the possibility that once a civilization develops the power to destroy itself (perhaps by [[nuclear winter]]), it does, and humorous commentary in stories such as [[Terry Bisson]]'s classic short story "They're Made Out of Meat."<ref>{{cite web|url=http://baetzler.de/humor/meat_beings.html |title=They're made out of Meat, by Hugo and Nebula Winner Terry Bisson |publisher=Baetzler.de |date= |accessdate=2010-03-07}}</ref> |
|||
* The equation was cited by [[Gene Roddenberry]] as supporting the multiplicity of inhabited planets shown in ''[[Star Trek]],'' the television show he created. However, Roddenberry didn't have the equation with him, and he was forced to "invent" it for his original proposal.<ref>''The Making of STAR TREK'' by Stephen E. Whitfield and Gene Roddenberry, Ballantine Books, N. Y., 1968</ref> The invented equation created by Roddenberry is: |
|||
::<math>Ff^2 (MgE)-C^1 Ri^1 ~ \times ~ M=L/So.\ </math> |
|||
The track ''Abiogenesis'' on the [[Carbon Based Lifeforms]] album [[World of Sleepers]] features the Drake equation in a spoken voice-over. |
|||
:Drake has gently pointed out, however, that a number raised to the first power is merely the number itself. A poster with both versions of the equation was seen in the ''[[Star Trek: Voyager]]'' episode "[[Future's End]]." |
|||
* The formula is also cited in [[Michael Crichton]]'s ''[[Sphere (novel)|Sphere]]''. |
|||
* In the Evolution-based game ''[[Spore (2008 video game)|Spore]]'', after eventually coming into contact with living beings on other planets, a picture is shown, along with the comment, "Drake's Equation was right...a living alien race!" |
|||
* [[George Alec Effinger]]'s short story "One" uses an expedition confident in the Drake Equation as a backdrop to explore the psychological implications of a lonely humanity. |
|||
* [[Alastair Reynolds]]' [[Revelation Space]] trilogy and short stories focus very much on the Drake Equation and the Fermi Paradox, using genocidal self-replicating machines as a [[Great Filter|great filter]]. |
|||
* [[Stephen Baxter|Stephen Baxter's]] [[Manifold Trilogy]] explores the Drake Equation and the Fermi Paradox in three distinct perspectives. |
|||
* [[Ian R. MacLeod]]'s 2001 novella "New Light On The Drake Equation" concerns a man who is obsessed by the Drake Equation. |
|||
* The [[Ultimate Marvel]] [[comic book]] mini-series ''[[Ultimate Secret]]'' has [[Ultimate Fantastic Four|Reed Richards]] examining the Drake Equation and considering the Fermi Paradox. He claims that when Drake plugged in his numbers, he came up with 10,000 alien races that would have a civilization advanced enough to contact Earth, but only one that Richards knew of had done it (actually 10 others already had, and two more were about to, but Richards did not know that at the time). In discussion with [[Ultimate Fantastic Four|Sue Storm]], it is revealed that he believes that advanced civilizations destroy themselves. In the story it turns out that they are also destroyed by [[Gah Lak Tus#Ultimate Gah Lak Tus|Gah Lak Tus]]. |
|||
* Eleanor Ann Arroway paraphrases the Drake equation several times in the film ''[[Contact (film)|Contact]]'', using the magnitude of ''N''<sup>*</sup> and its implications on the output value to justify the SETI program. However, in one scene of the 1997 motion picture based upon the [[Carl Sagan]] novel, she misstates her Drake Equation result by several billion. |
|||
* This equation was mentioned by Howard and detailed by Sheldon in the 20th episode of the second season of the television series ''[[The Big Bang Theory]]''. Howard goes on to modify the terms in the equation to project the likelihood of a member of the group [[hooking up]] with a member of the opposite sex. He uses the "Wallowitz Coefficient" which equals neediness times dress size squared to achieve this. |
|||
==See also== |
==See also== |
||
* {{annotated link|Astrobiology}} |
|||
* [[Big Bounce]] |
|||
* {{annotated link|Goldilocks principle}} |
|||
* [[Black swan theory]] |
|||
* {{annotated link|Kardashev scale}} |
|||
* [[Doomsday argument]] |
|||
* {{annotated link|Planetary habitability}} |
|||
* [[Final anthropic principle]] |
|||
* {{annotated link|Ufology}} |
|||
* [[Goldilocks Principle]] |
|||
* {{annotated link|Lincoln index}} |
|||
* [[Great Filter]] |
|||
* ''[[The Search for Life: The Drake Equation]]'', BBC documentary |
|||
* [[Inverse gambler's fallacy]] |
|||
* [[Kardashev scale]] |
|||
* [[Metaphysical naturalism]] |
|||
* [[Neocatastrophism]] |
|||
* [[Nick Bostrom]] |
|||
* [[Principle of mediocrity]] |
|||
* [[Rare Earth hypothesis]] |
|||
* [[Selection bias]] |
|||
* [[Zoo hypothesis]] |
|||
== |
==Notes== |
||
{{ |
{{notelist}} |
||
==References== |
==References== |
||
{{reflist|38em}} |
|||
* {{cite book | first = Frank | last = Drake | coauthors = Dava Sobel | title = Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence | publisher = Delacorte Pr. | location = New York | year = 1992 | isbn = 0-385-30532-X | unused_data = ISBN status = May be invalid - please double check}} |
|||
* {{cite book | first = Robert T. | last = Rood | coauthors = James S. Trefil | title = Are We Alone? The Possibility of Extraterrestrial Civilizations | publisher = Scribner | location = New York | year = 1981 | isbn = 0-684-16826-X | unused_data = ISBN status = May be invalid - please double check}} |
|||
== Further reading == |
|||
* {{cite journal | first = Charles H. | last = Lineweaver | coauthors = Tamara M. Davis | title = Does the Rapid Appearance of Life on Earth Suggest that Life is Common in the Universe? | id = {{ArXiv|archive=astro-ph|id=0205014}} | date = 2 May 2002}} |
|||
* {{cite |
* {{cite book | first = Oliver | last = Morton | editor=Graham Formelo |chapter=A Mirror in the Sky | title = It Must Be Beautiful | publisher = Granta Books | year = 2002 | isbn = 1-86207-555-7}} |
||
* {{cite book | first = |
* {{cite book | first = Robert T. | last = Rood |author2=James S. Trefil | title = Are We Alone? The Possibility of Extraterrestrial Civilizations | publisher = Scribner | location = New York | year = 1981 | isbn = 0684178427}} |
||
* {{Cite book |editor1-link=Douglas Vakoch |editor1-first=Douglas A. |editor1-last=Vakoch |editor2-first=Matthew F. |editor2-last=Dowd |year=2015 |title=The Drake Equation: Estimating the Prevalence of Extraterrestrial Life Through the Ages |url=https://books.google.com/books?id=jcnSCQAAQBAJ |location=Cambridge, UK |publisher=Cambridge University Press |isbn=978-1-10-707365-4}} |
|||
* {{cite book |
|||
| first = Oliver | last = Morton |
|||
|editor=Graham Formelo |
|||
| title = It Must Be Beautiful | publisher = Granta Books | year = 2002 | isbn = 0-86207-555-7 |
|||
| unused_data = ISBN status = May be invalid - please double check}} Chapter ''A Mirror in the Sky'' is dedicated to Drake equation |
|||
==External links== |
==External links== |
||
{{Wiktionary|Drake equation}} |
|||
*<span class="plainlinks">[http://www.cosmosmagazine.com/features/online/3384/qa-with-frank-drake "Only a matter of time, says Frank Drake"]</span> A Q&A with Frank Drake about his famous equation and the meaning of SETI, from an interview in February 2010, leading up to the 50th birthday of SETI. |
|||
* [http://spacegeek.org/calc/ Interactive Drake Equation Calculator] |
|||
* {{cite web | url = http://wired.com/wired/archive/12.12/life.html | title = The E.T. Equation, Recalculated | author = [[Frank Drake]] | month = December | year =2004 | publisher = [[Wired magazine|Wired]]}} |
|||
* [https://astrosociety.org/file_download/inline/58ee6041-5f61-4f88-8b15-d2d3d22ab83d Frank Drake's 2010 article on "The Origin of the Drake Equation"] |
|||
* [http://spacegeek.org/canvas2/ Drake Calculator] Calculates the Drake equation with each parameter a variable that can be changed and is explained. |
|||
* [https://web.archive.org/web/20100411202244/http://www.cosmosmagazine.com/features/online/3384/qa-with-frank-drake "Only a matter of time, says Frank Drake"]. A Q&A with Frank Drake in February 2010 |
|||
* http://www.skypub.com/news/special/9812seti_aliens.html |
|||
* {{cite magazine |url=https://www.wired.com/wired/archive/12.12/life.html |title=The E.T. Equation, Recalculated |author=Drake |date=December 2004 |magazine=[[Wired (magazine)|Wired]] |first=Frank |author-link=Frank Drake}} |
|||
* [http://frombob.to/drake.html Beyond the Drake Equation] |
|||
* [https://www.pbs.org/wgbh/nova/origins/drake.html Macromedia Flash page allowing the user to modify Drake's values] from [[Public Broadcasting Service|PBS]]'s ''[[Nova (American TV series)|Nova]]'' |
|||
* [http://space.com/scienceastronomy/astronomy/jupiter_typical_020128.html January 2002 space.com article about estimated prevalence of extrasolar planets] |
|||
* [http://www.astronomycast.com/solar-system/episode-23-the-drake-equation/ "The Drake Equation"], ''[[Astronomy Cast]]'' episode #23; includes full transcript |
|||
* [http://www.arxiv.org/abs/astro-ph/0205014 Preprint by Lineweaver and Davis estimating f<sub>l</sub> as > 0.13] |
|||
* [http://www.area52online.com/sections/simulations/drake/guessinggame.htm Animated simulation of the Drake equation]. ({{Webarchive|url=https://web.archive.org/web/20151208133744/http://www.area52online.com/sections/simulations/drake/guessinggame.htm |date=8 December 2015 }}) |
|||
* [http://www.astrobio.net/news/article517.html July 2003 discovery of a planetary system similar to our solar system] |
|||
* [http://www.bbc.co.uk/programmes/p009rtr0 "The Alien Equation"], BBC Radio program ''Discovery'' (22 September 2010) |
|||
* [http://www.pbs.org/wgbh/nova/origins/drake.html Macromedia Flash page allowing the user to modify Drake's values] from [[PBS]] [[Nova (TV series)|Nova]] |
|||
* [https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1473550413000207 "Reflections on the Equation"] (PDF), by Frank Drake, 2013 |
|||
*[http://www.astronomycast.com/solar-system/episode-23-the-drake-equation/ The Drake Equation] [[Astronomy Cast]] episode #23, includes full transcript. |
|||
*[http://www.dbskeptic.com/2009/04/19/the-drake-equation/ The Drake Equation] A critical examination of the Drake Equation |
|||
*[http://www.area52online.com/sections/simulations/simulations.htm The Drake Equation] Animated simulation of the Drake Equation |
|||
{{Extraterrestrial life}} |
|||
{{Astrobiology}} |
|||
{{Extraterrestrial life}} |
|||
{{Interstellar messages}} |
{{Interstellar messages}} |
||
{{Molecules detected in outer space}} |
|||
{{Portal bar|Stars|Spaceflight|Solar System|Science}} |
|||
[[Category:1961 introductions]] |
|||
{{DEFAULTSORT:Drake Equation}} |
|||
[[Category: |
[[Category:Astrobiology]] |
||
[[Category:Astronomical controversies]] |
|||
[[Category:Astronomical hypotheses]] |
|||
[[Category:Eponymous equations of physics]] |
|||
[[Category:Fermi paradox]] |
|||
[[Category:Interstellar messages]] |
[[Category:Interstellar messages]] |
||
[[Category: |
[[Category:Search for extraterrestrial intelligence]] |
||
<!-- Interwikis --> |
|||
[[bn:ড্রেকের সূত্র]] |
|||
[[bs:Drakeova jednačina]] |
|||
[[bg:Уравнение на Дрейк]] |
|||
[[ca:Equació de Drake]] |
|||
[[cs:Drakeova rovnice]] |
|||
[[da:Drakes ligning]] |
|||
[[de:Drake-Gleichung]] |
|||
[[es:Ecuación de Drake]] |
|||
[[eo:Ekvacio de Drake]] |
|||
[[eu:Drakeren ekuazioa]] |
|||
[[fr:Équation de Drake]] |
|||
[[ko:드레이크 방정식]] |
|||
[[hr:Drakeova jednadžba]] |
|||
[[it:Equazione di Drake]] |
|||
[[he:נוסחת דרייק]] |
|||
[[lt:Dreiko lygtis]] |
|||
[[hu:Drake-formula]] |
|||
[[ml:ഡ്രേക്ക് സമവാക്യം]] |
|||
[[nl:Vergelijking van Drake]] |
|||
[[ja:ドレイクの方程式]] |
|||
[[no:Drakes ligning]] |
|||
[[nn:Drake-likninga]] |
|||
[[pl:Równanie Drake'a]] |
|||
[[pt:Equação de Drake]] |
|||
[[ro:Ecuația lui Drake]] |
|||
[[ru:Уравнение Дрейка]] |
|||
[[sq:Ekuacioni Drejk]] |
|||
[[sk:Drakeova rovnica]] |
|||
[[sl:Drakova enačba]] |
|||
[[sr:Дрејкова једначина]] |
|||
[[fi:Draken kaava]] |
|||
[[sv:Drakes ekvation]] |
|||
[[th:สมการของเดรก]] |
|||
[[tr:Drake denklemi]] |
|||
[[uk:Рівняння Дрейка]] |
|||
[[zh:德雷克公式]] |
Latest revision as of 12:27, 10 December 2024
The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.[1][2][3]
The equation was formulated in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI).[4][5] The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.[4] It is more properly thought of as an approximation than as a serious attempt to determine a precise number.
Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.
Equation
[edit]The Drake equation is:[1]
where
- N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
and
- R∗ = the average rate of star formation in our Galaxy.
- fp = the fraction of those stars that have planets.
- ne = the average number of planets that can potentially support life per star that has planets.
- fl = the fraction of planets that could support life that actually develop life at some point.
- fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- L = the length of time for which such civilizations release detectable signals into space.[6][7]
This form of the equation first appeared in Drake's 1965 paper.[8][9]
History
[edit]In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications".[10][11] Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.
Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."[12]
Seven months after Cocconi and Morrison published their article, Drake began searching for extraterrestrial intelligence in an experiment called Project Ozma. It was the first systematic search for signals from communicative extraterrestrial civilizations. Using the 85 ft (26 m) dish of the National Radio Astronomy Observatory, Green Bank in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti, slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960.[11] The project was well designed, inexpensive, and simple by today's standards. It detected no signals.
Soon thereafter, Drake hosted the first search for extraterrestrial intelligence conference on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.[13]
As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This was aimed at the radio search, and not to search for primordial or primitive life forms.
— Frank Drake
The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio-astronomer Otto Struve.[14] These participants called themselves "The Order of the Dolphin" (because of Lilly's work on dolphin communication), and commemorated their first meeting with a plaque at the observatory hall.[15][16]
Usefulness
[edit]The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life.[2][6][17] The last three parameters, fi, fc, and L, are not known and are very difficult to estimate, with values ranging over many orders of magnitude (see § Criticism). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere,[2][4] and gives the question of life elsewhere a basis for scientific analysis. The equation has helped draw attention to some particular scientific problems related to life in the universe, for example abiogenesis, the development of multi-cellular life, and the development of intelligence itself.[18]
Within the limits of existing human technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. After about 50 years, the Drake equation is still of seminal importance because it is a 'road map' of what we need to learn in order to solve this fundamental existential question.[2] It also formed the backbone of astrobiology as a science; although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved significantly since the early 1960s. SETI efforts since 1961 have conclusively ruled out widespread alien emissions near the 21 cm wavelength of the hydrogen frequency.[19]
Estimates
[edit]Original estimates
[edit]There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:[1][20][21]
- R∗ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
- fp = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
- ne = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)
- fl = 1 (100% of these planets will develop life)
- fi = 1 (100% of which will develop intelligent life)
- fc = 0.1 to 0.2 (10–20% of which will be able to communicate)
- L = somewhere between 1000 and 100,000,000 years
Inserting the above minimum numbers into the equation gives a minimum N of 20 (see: Range of results). Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that N ≈ L, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy.
Current estimates
[edit]This section discusses and attempts to list the best current estimates for the parameters of the Drake equation.
Rate of star creation in this Galaxy, R∗
[edit]Calculations in 2010, from NASA and the European Space Agency indicate that the rate of star formation in this Galaxy is about 0.68–1.45 M☉ of material per year.[22][23] To get the number of stars per year, we divide this by the initial mass function (IMF) for stars, where the average new star's mass is about 0.5 M☉.[24] This gives a star formation rate of about 1.5–3 stars per year.
Fraction of those stars that have planets, fp
[edit]Analysis of microlensing surveys, in 2012, has found that fp may approach 1—that is, stars are orbited by planets as a rule, rather than the exception; and that there are one or more bound planets per Milky Way star.[25][26]
Average number of planets that might support life per star that has planets, ne
[edit]In November 2013, astronomers reported, based on Kepler space telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy.[27][28] 11 billion of these estimated planets may be orbiting sun-like stars.[29] Since there are about 100 billion stars in the galaxy, this implies fp · ne is roughly 0.4. The nearest planet in the habitable zone is Proxima Centauri b, which is as close as about 4.2 light-years away.
The consensus at the Green Bank meeting was that ne had a minimum value between 3 and 5. Dutch science journalist Govert Schilling has opined that this is optimistic.[30] Even if planets are in the habitable zone, the number of planets with the right proportion of elements is difficult to estimate.[31] Brad Gibson, Yeshe Fenner, and Charley Lineweaver determined that about 10% of star systems in the Milky Way Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable for a sufficient time.[32]
The discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the formation of their stellar systems. So-called hot Jupiters may migrate from distant orbits to near orbits, in the process disrupting the orbits of habitable planets.
On the other hand, the variety of star systems that might have habitable zones is not just limited to solar-type stars and Earth-sized planets. It is now estimated that even tidally locked planets close to red dwarf stars might have habitable zones,[33] although the flaring behavior of these stars might speak against this.[34] The possibility of life on moons of gas giants (such as Jupiter's moon Europa, or Saturn's moons Titan and Enceladus) adds further uncertainty to this figure.[35]
The authors of the rare Earth hypothesis propose a number of additional constraints on habitability for planets, including being in galactic zones with suitably low radiation, high star metallicity, and low enough density to avoid excessive asteroid bombardment. They also propose that it is necessary to have a planetary system with large gas giants which provide bombardment protection without a hot Jupiter; and a planet with plate tectonics, a large moon that creates tidal pools, and moderate axial tilt to generate seasonal variation.[36]
Fraction of the above that actually go on to develop life, fl
[edit]Geological evidence from the Earth suggests that fl may be high; life on Earth appears to have begun around the same time as favorable conditions arose, suggesting that abiogenesis may be relatively common once conditions are right. However, this evidence only looks at the Earth (a single model planet), and contains anthropic bias, as the planet of study was not chosen randomly, but by the living organisms that already inhabit it (ourselves). From a classical hypothesis testing standpoint, without assuming that the underlying distribution of fl is the same for all planets in the Milky Way, there are zero degrees of freedom, permitting no valid estimates to be made. If life (or evidence of past life) were to be found on Mars, Europa, Enceladus or Titan that developed independently from life on Earth it would imply a value for fl close to 1. While this would raise the number of degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.
Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for bacteria that are unrelated to other life on Earth, but none have been found yet.[37] It is also possible that life arose more than once, but that other branches were out-competed, or died in mass extinctions, or were lost in other ways. Biochemists Francis Crick and Leslie Orgel laid special emphasis on this uncertainty: "At the moment we have no means at all of knowing" whether we are "likely to be alone in the galaxy (Universe)" or whether "the galaxy may be pullulating with life of many different forms."[38] As an alternative to abiogenesis on Earth, they proposed the hypothesis of directed panspermia, which states that Earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship".
In 2020, a paper by scholars at the University of Nottingham proposed an "Astrobiological Copernican" principle, based on the Principle of Mediocrity, and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, fl, fi, and fc are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).[39][40]
Fraction of the above that develops intelligent life, fi
[edit]This value remains particularly controversial. Those who favor a low value, such as the biologist Ernst Mayr, point out that of the billions of species that have existed on Earth, only one has become intelligent and from this, infer a tiny value for fi.[41] Likewise, the Rare Earth hypothesis, notwithstanding their low value for ne above, also think a low value for fi dominates the analysis.[42] Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable,[43][44] implying an fi approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See Criticism).
In addition, while it appears that life developed soon after the formation of Earth, the Cambrian explosion, in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the snowball Earth or research into extinction events have raised the possibility that life on Earth is relatively fragile. Research on any past life on Mars is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of fl but would indicate that in half the known cases, intelligent life did not develop.
Estimates of fi have been affected by discoveries that the Solar System's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for tens of millions of years (evading radiation from novae). Also, Earth's large moon may aid the evolution of life by stabilizing the planet's axis of rotation.
There has been quantitative work to begin to define . One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.[45][46]
Planetary scientist Pascal Lee of the SETI Institute proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since Homo erectus evolved, compared to 4.6 billion years since Earth formed).[47][48]
Fraction of the above revealing their existence via signal release into space, fc
[edit]For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for human presence. (See Arecibo message, for example). There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans.[49] By this standard, the Earth is a communicating civilization.
Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.[50]
Lifetime of such a civilization wherein it communicates its signals into space, L
[edit]Michael Shermer estimated L as 420 years, based on the duration of sixty historical Earthly civilizations.[51] Using 28 civilizations more recent than the Roman Empire, he calculates a figure of 304 years for "modern" civilizations. It could also be argued from Michael Shermer's results that the fall of most of these civilizations was followed by later civilizations that carried on the technologies, so it is doubtful that they are separate civilizations in the context of the Drake equation. In the expanded version, including reappearance number, this lack of specificity in defining single civilizations does not matter for the result, since such a civilization turnover could be described as an increase in the reappearance number rather than increase in L, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid.
David Grinspoon has argued that once a civilization has developed enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for L potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed.[52] He proposes that the last factor L be replaced with fIC · T, where fIC is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and T representing the length of time during which this process has been going on. This has the advantage that T would be a relatively easy-to-discover number, as it would simply be some fraction of the age of the universe.
It has also been hypothesized that once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.[53]
The astronomer Carl Sagan speculated that all of the terms, except for the lifetime of a civilization, are relatively high and the determining factor in whether there are large or small numbers of civilizations in the universe is the civilization lifetime, or in other words, the ability of technological civilizations to avoid self-destruction. In Sagan's case, the Drake equation was a strong motivating factor for his interest in environmental issues and his efforts to warn against the dangers of nuclear warfare. Paleobiologist Olev Vinn suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.[54]
An intelligent civilization might not be organic, as some have suggested that artificial general intelligence may replace humanity.[55]
Range of results
[edit]As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions,[56] as the values used in portions of the Drake equation are not well established.[30][57][58][59] In particular, the result can be N ≪ 1, meaning we are likely alone in the galaxy, or N ≫ 1, implying there are many civilizations we might contact. One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.[60]
As an example of a low estimate, combining NASA's star formation rates, the rare Earth hypothesis value of fp · ne · fl = 10−5,[61] Mayr's view on intelligence arising, Drake's view of communication, and Shermer's estimate of lifetime:
- R∗ = 1.5–3 yr−1,[22] fp · ne · fl = 10−5,[36] fi = 10−9,[41] fc = 0.2[Drake, above], and L = 304 years[51]
gives:
- N = 1.5 × 10−5 × 10−9 × 0.2 × 304 = 9.1 × 10−13
i.e., suggesting that we are probably alone in this galaxy, and possibly in the observable universe.
On the other hand, with larger values for each of the parameters above, values of N can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters:
- R∗ = 1.5–3 yr−1,[22] fp = 1,[25] ne = 0.2,[62][63] fl = 0.13,[64] fi = 1,[43] fc = 0.2[Drake, above], and L = 109 years[52]
Use of these parameters gives:
- N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 15,600,000
Monte Carlo simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.[65]
Possible former technological civilizations
[edit]In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the only technological species that has ever arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ever develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than 2.5×10−24. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than 1.7×10−11 (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.[66][67][68][69][70]
Modifications
[edit]As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters,[71] and many changes and modifications to the equation have been proposed. One line of modification, for example, attempts to account for the uncertainty inherent in many of the terms.[72] Combining the estimates of the original six factors by major researchers via a Monte Carlo procedure leads to a best value for the non-longevity factors of 0.85 1/years.[73] This result differs insignificantly from the estimate of unity given both by Drake and the Cyclops report.
Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, David Brin states: "The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society".[74] Because it is the contact cross-section that is of interest to the SETI community, many additional factors and modifications of the Drake equation have been proposed.
- Colonization
- It has been proposed to generalize the Drake equation to include additional effects of alien civilizations colonizing other star systems. Each original site expands with an expansion velocity v, and establishes additional sites that survive for a lifetime L. The result is a more complex set of 3 equations.[74]
- Reappearance factor
- The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if nr is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be 1 + nr, which is the actual reappearance factor added to the equation.
- The factor depends on what generally is the cause of civilization extinction. If it is generally by temporary uninhabitability, for example a nuclear winter, then nr may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as stellar evolution, then nr may be almost zero. In the case of total life extinction, a similar factor may be applicable for fl, that is, how many times life may appear on a planet where it has appeared once.
- METI factor
- Alexander Zaitsev said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans, although being in a communicative phase, are not a communicative civilization; we do not practise such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (messaging to extraterrestrial intelligence) to the classical Drake equation.[75] He defined the factor as "the fraction of communicative civilizations with clear and non-paranoid planetary consciousness", or alternatively expressed, the fraction of communicative civilizations that actually engage in deliberate interstellar transmission.
- The METI factor is somewhat misleading since active, purposeful transmission of messages by a civilization is not required for them to receive a broadcast sent by another that is seeking first contact. It is merely required they have capable and compatible receiver systems operational; however, this is a variable humans cannot accurately estimate.
- Biogenic gases
- Astronomer Sara Seager proposed a revised equation that focuses on the search for planets with biosignature gases.[76] These gases are produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.[77]
- The Seager equation looks like this:[77][a]
- where:
- N = the number of planets with detectable signs of life
- N∗ = the number of stars observed
- FQ = the fraction of stars that are quiet
- FHZ = the fraction of stars with rocky planets in the habitable zone
- FO = the fraction of those planets that can be observed
- FL = the fraction that have life
- FS = the fraction on which life produces a detectable signature gas
- Seager stresses, "We're not throwing out the Drake Equation, which is really a different topic," explaining, "Since Drake came up with the equation, we have discovered thousands of exoplanets. We as a community have had our views revolutionized as to what could possibly be out there. And now we have a real question on our hands, one that's not related to intelligent life: Can we detect any signs of life in any way in the very near future?"[78]
- Carl Sagan's version of the Drake equation
- American astronomer Carl Sagan made some modifications[79] in the Drake equation and presented it in the 1980 program Cosmos: A Personal Voyage.[80] The modified equation is shown below
[81] where
- N = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);
and
- N∗ = Number of stars in the Milky Way Galaxy
- fp = the fraction of those stars that have planets.
- ne = the average number of planets that can potentially support life per star that has planets.
- fl = the fraction of planets that could support life that actually develop life at some point.
- fi = the fraction of planets with life that go on to develop intelligent life (civilizations).
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
- fL = fraction of a planetary lifetime graced by a technological civilization
Criticism
[edit]Criticism of the Drake equation is varied. Firstly, many of the terms in the equation are largely or entirely based on conjecture.[82][83] Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.[84][85]
Others point out that the equation was formulated before our understanding of the universe had matured. Astrophysicist Ethan Siegel, said:
The Drake equation, when it was put forth, made an assumption about the Universe that we now know is untrue: It assumed that the Universe was eternal and static in time. As we learned only a few years after Frank Drake first proposed his equation, the Universe doesn’t exist in a steady state, where it’s unchanging in time, but rather has evolved from a hot, dense, energetic, and rapidly expanding state: a hot Big Bang that occurred over a finite duration in our cosmic past.[86]
One reply to such criticisms[87] is that even though the Drake equation currently involves speculation about unmeasured parameters, it was intended as a way to stimulate dialogue on these topics. Then the focus becomes how to proceed experimentally. Indeed, Drake originally formulated the equation merely as an agenda for discussion at the Green Bank conference.[88]
Fermi paradox
[edit]A civilization lasting for tens of millions of years could be able to spread throughout the galaxy, even at the slow speeds foreseeable with present-day technology. However, no confirmed signs of civilizations or intelligent life elsewhere have been found, either in this Galaxy or in the observable universe of 2 trillion galaxies.[89][90] According to this line of thinking, the tendency to fill (or at least explore) all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".[91][92]
A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations.[93] In terms of the Drake Equation, the explanations can be divided into three classes:
- Few intelligent civilizations ever arise. This is an argument that at least one of the first few terms, R∗ · fp · ne · fl · fi, has a low value. The most common suspect is fi, but explanations such as the rare Earth hypothesis argue that ne is the small term.
- Intelligent civilizations exist, but we see no evidence, meaning fc is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, communication is dangerous, and many others.
- The lifetime of intelligent, communicative civilizations is short, meaning the value of L is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to be destroyed by natural events, and others.
These lines of reasoning lead to the Great Filter hypothesis,[94] which states that since there are no observed extraterrestrial civilizations despite the vast number of stars, at least one step in the process must be acting as a filter to reduce the final value. According to this view, either it is very difficult for intelligent life to arise, or the lifetime of technologically advanced civilizations, or the period of time they reveal their existence must be relatively short.
An analysis by Anders Sandberg, Eric Drexler and Toby Ord suggests "a substantial ex ante (predicted) probability of there being no other intelligent life in our observable universe".[95]
In fiction and popular culture
[edit]The equation was cited by Gene Roddenberry as supporting the multiplicity of inhabited planets shown on Star Trek, the television series he created. However, Roddenberry did not have the equation with him, and he was forced to "invent" it for his original proposal.[96] The invented equation created by Roddenberry is:
Regarding Roddenberry's fictional version of the equation, Drake himself commented that a number raised to the first power is just the number itself.[97]
A commemorative plate on NASA's Europa Clipper mission, planned for launch in October 2024, features a poem by the U.S. Poet Laureate Ada Limón, waveforms of the word 'water' in 103 languages, a schematic of the water hole, the Drake equation, and a portrait of planetary scientist Ron Greeley on it.[98]
The track Abiogenesis on the Carbon Based Lifeforms album World of Sleepers features the Drake equation in a spoken voice-over.
See also
[edit]- Astrobiology – Science concerned with life in the universe
- Goldilocks principle – Analogy for optimal conditions
- Kardashev scale – Measure of a civilization's evolution
- Planetary habitability – Known extent to which a planet is suitable for life
- Ufology – Study of UFOs
- Lincoln index – Statistical measure
- The Search for Life: The Drake Equation, BBC documentary
Notes
[edit]References
[edit]- ^ a b c Physics Today 14 (4), 40–46 (1961). Drake, F. D. (April 1961). "Project Ozma". pubs.aip.org. American Institute of Physics. Retrieved 27 April 2023.
The question of the existence of intelligent life elsewhere in space has long fascinated people, but, until recently, has been properly left to the science‐fiction writers.
- ^ a b c d Burchell, M. J. (2006). "W(h)ither the Drake equation?". International Journal of Astrobiology. 5 (3): 243–250. Bibcode:2006IJAsB...5..243B. doi:10.1017/S1473550406003107. S2CID 121060763.
- ^ Glade, N.; Ballet, P.; Bastien, O. (2012). "A stochastic process approach of the drake equation parameters". International Journal of Astrobiology. 11 (2): 103–108. arXiv:1112.1506. Bibcode:2012IJAsB..11..103G. doi:10.1017/S1473550411000413. S2CID 119250730.
- ^ a b c "Chapter 3 – Philosophy: "Solving the Drake Equation". Ask Dr. SETI. SETI League. December 2002. Retrieved 10 April 2013.
- ^ Drake, N. (30 June 2014). "How my Dad's Equation Sparked the Search for Extraterrestrial Intelligence". National Geographic. Archived from the original on 5 July 2014. Retrieved 2 October 2016.
- ^ a b Aguirre, L. (1 July 2008). "The Drake Equation". Nova ScienceNow. PBS. Retrieved 7 March 2010.
- ^ "What do we need to know about to discover life in space?". SETI Institute. Retrieved 16 April 2013.
- ^ Drake, Frank D. (1 January 1965). The Radio Search for Intelligent Extraterrestrial Life. Bibcode:1965cae..book..323D.
- ^ jtw13 (31 July 2019). "Freeman Dyson's First Law of SETI Investigations". AstroWright. Retrieved 2 August 2024.
{{cite web}}
: CS1 maint: numeric names: authors list (link) - ^ Cocconi, G.; Morisson, P. (1959). "Searching for Interstellar Communications" (PDF). Nature. 184 (4690): 844–846. Bibcode:1959Natur.184..844C. doi:10.1038/184844a0. S2CID 4220318. Archived (PDF) from the original on 28 July 2011. Retrieved 10 April 2013.
- ^ a b Schilling, G.; MacRobert, A. M. (2013). "The Chance of Finding Aliens". Sky & Telescope. Archived from the original on 14 February 2013. Retrieved 10 April 2013.
- ^ newspaper, staff (8 November 1959). "Life On Other Planets?". Sydney Morning Herald. Retrieved 2 October 2015.
- ^ "The Drake Equation Revisited: Part I". Astrobiology Magazine. 29 September 2003. Archived from the original on 25 February 2021. Retrieved 20 May 2017.
{{cite web}}
: CS1 maint: unfit URL (link) - ^ Zaun, H. (1 November 2011). "Es war wie eine 180-Grad-Wende von diesem peinlichen Geheimnis!" [It was like a 180 degree turn from this embarrassing secret]. Telepolis (in German). Retrieved 13 August 2013.
- ^ "Drake Equation Plaque". Retrieved 13 August 2013.
- ^ Darling, D. J. "Green Bank conference (1961)". The Encyclopedia of Science. Archived from the original on 21 February 2024. Retrieved 13 August 2013.
- ^ Jones, D. S. (26 September 2001). "Beyond the Drake Equation". Retrieved 17 April 2013.
- ^ "The Search For Life : The Drake Equation 2010 – Part 1". BBC Four. 2010. Retrieved 17 April 2013.
- ^ SETI: A celebration of the first 50 years. Keith Cooper. Astronomy Now. 2000
- ^ Drake, F.; Sobel, D. (1992). Is Anyone Out There? The Scientific Search for Extraterrestrial Intelligence. Delta. pp. 55–62. ISBN 0-385-31122-2.
- ^ Glade, N.; Ballet, P.; Bastien, O. (2012). "A stochastic process approach of the drake equation parameters". International Journal of Astrobiology. 11 (2): 103–108. arXiv:1112.1506. Bibcode:2012IJAsB..11..103G. doi:10.1017/S1473550411000413. S2CID 119250730. Note: This reference has a table of 1961 values, claimed to be taken from Drake & Sobel, but these differ from the book.
- ^ a b c Robitaille, Thomas P.; Barbara A. Whitney (2010). "The present-day star formation rate of the Milky Way determined from Spitzer-detected young stellar objects". The Astrophysical Journal Letters. 710 (1): L11. arXiv:1001.3672. Bibcode:2010ApJ...710L..11R. doi:10.1088/2041-8205/710/1/L11. S2CID 118703635.
- ^ Wanjek, C. (2015). The Drake Equation. Cambridge University Press. ISBN 9781107073654. Retrieved 9 September 2016.
- ^ Kennicutt, Robert C.; Evans, Neal J. (22 September 2012). "Star Formation in the Milky Way and Nearby Galaxies". Annual Review of Astronomy and Astrophysics. 50 (1): 531–608. arXiv:1204.3552. Bibcode:2012ARA&A..50..531K. doi:10.1146/annurev-astro-081811-125610. S2CID 118667387.
- ^ a b Palmer, J. (11 January 2012). "Exoplanets are around every star, study suggests". BBC. Retrieved 12 January 2012.
- ^ Cassan, A.; et al. (11 January 2012). "One or more bound planets per Milky Way star from microlensing observations". Nature. 481 (7380): 167–169. arXiv:1202.0903. Bibcode:2012Natur.481..167C. doi:10.1038/nature10684. PMID 22237108. S2CID 2614136.
- ^ Overbye, Dennis (4 November 2013). "Far-Off Planets Like the Earth Dot the Galaxy". The New York Times. Archived from the original on 1 January 2022. Retrieved 5 November 2013.
- ^ Petigura, Eric A.; Howard, Andrew W.; Marcy, Geoffrey W. (31 October 2013). "Prevalence of Earth-size planets orbiting Sun-like stars". Proceedings of the National Academy of Sciences of the United States of America. 110 (48): 19273–19278. arXiv:1311.6806. Bibcode:2013PNAS..11019273P. doi:10.1073/pnas.1319909110. PMC 3845182. PMID 24191033.
- ^ Khan, Amina (4 November 2013). "Milky Way may host billions of Earth-size planets". Los Angeles Times. Retrieved 5 November 2013.
- ^ a b Schilling, Govert (November 2011). "The Chance of Finding Aliens: Reevaluating the Drake Equation". astro-tom.com.
- ^ Trimble, V. (1997). "Origin of the biologically important elements". Origins of Life and Evolution of the Biosphere. 27 (1–3): 3–21. Bibcode:1997OLEB...27....3T. doi:10.1023/A:1006561811750. PMID 9150565. S2CID 7612499.
- ^ Lineweaver, C. H.; Fenner, Y.; Gibson, B. K. (2004). "The Galactic Habitable Zone and the Age Distribution of Complex Life in the Milky Way". Science. 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421. S2CID 18140737.
- ^ Dressing, C. D.; Charbonneau, D. (2013). "The Occurrence Rate of Small Planets around Small Stars". The Astrophysical Journal. 767 (1): 95. arXiv:1302.1647. Bibcode:2013ApJ...767...95D. doi:10.1088/0004-637X/767/1/95. S2CID 29441006.
- ^ "Red Dwarf Stars Could Leave Habitable Earth-Like Planets Vulnerable to Radiation". SciTech Daily. 2 July 2013. Retrieved 22 September 2015.
- ^ Heller, René; Barnes, Rory (29 April 2014). "Constraints on the Habitability of Extrasolar Moons". Proceedings of the International Astronomical Union. 8 (S293): 159–164. arXiv:1210.5172. Bibcode:2014IAUS..293..159H. doi:10.1017/S1743921313012738. S2CID 92988047.
- ^ a b Ward, Peter D.; Brownlee, Donald (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books (Springer Verlag). ISBN 0-387-98701-0.
- ^ Davies, P. (2007). "Are Aliens Among Us?". Scientific American. 297 (6): 62–69. Bibcode:2007SciAm.297f..62D. doi:10.1038/scientificamerican1207-62.
- ^ Crick, F. H. C.; Orgel, L. E. (1973). "Directed Panspermia" (PDF). Icarus. 19 (3): 341–346. Bibcode:1973Icar...19..341C. doi:10.1016/0019-1035(73)90110-3. Archived (PDF) from the original on 29 October 2011.
- ^ Westby, Tom; Conselice, Christopher J. (15 June 2020). "The Astrobiological Copernican Weak and Strong Limits for Intelligent Life". The Astrophysical Journal. 896 (1): 58. arXiv:2004.03968. Bibcode:2020ApJ...896...58W. doi:10.3847/1538-4357/ab8225. S2CID 215415788.
- ^ Davis, Nicola (15 June 2020). "Scientists say most likely number of contactable alien civilisations is 36". The Guardian. Retrieved 19 June 2020.
- ^ a b "Ernst Mayr on SETI". The Planetary Society. Archived from the original on 6 December 2010.
- ^ Rare Earth, p. xviii.: "We believe that life in the form of microbes or their equivalents is very common in the universe, perhaps more common than even Drake or Sagan envisioned. However, complex life—animals and higher plants—is likely to be far more rare than commonly assumed."
- ^ a b Campbell, A. (13 March 2005). "Review of Life's Solution by Simon Conway Morris". Archived from the original on 16 July 2011.
- ^ Bonner, J. T. (1988). The evolution of complexity by means of natural selection. Princeton University Press. ISBN 0-691-08494-7.
- ^ Kipping, David (18 May 2020). "An objective Bayesian analysis of life's early start and our late arrival". Proceedings of the National Academy of Sciences. 117 (22): 11995–12003. arXiv:2005.09008. Bibcode:2020PNAS..11711995K. doi:10.1073/pnas.1921655117. PMC 7275750. PMID 32424083.
- ^ Columbia University. "New study estimates the odds of life and intelligence emerging beyond our planet". Phys.org. Retrieved 23 May 2020.
- ^ Lee, Pascal (24 October 2020). "N~1: Alone in the Milky Way, Mt Tam". YouTube. Archived from the original on 11 December 2021.
- ^ Lee, Pascal (6 March 2021). "N~1: Alone in the Milky Way – Kalamazoo Astronomical Society". YouTube. Archived from the original on 15 March 2021.
- ^ Forgan, D.; Elvis, M. (2011). "Extrasolar Asteroid Mining as Forensic Evidence for Extraterrestrial Intelligence". International Journal of Astrobiology. 10 (4): 307–313. arXiv:1103.5369. Bibcode:2011IJAsB..10..307F. doi:10.1017/S1473550411000127. S2CID 119111392.
- ^ Tarter, Jill C. (September 2001). "The Search for Extraterrestrial Intelligence (SETI)". Annual Review of Astronomy and Astrophysics. 39: 511–548. Bibcode:2001ARA&A..39..511T. doi:10.1146/annurev.astro.39.1.511. S2CID 261531924.
- ^ a b Shermer, M. (August 2002). "Why ET Hasn't Called". Scientific American. 287 (2): 21. Bibcode:2002SciAm.287b..33S. doi:10.1038/scientificamerican0802-33.
- ^ a b Grinspoon, D. (2004). Lonely Planets.
- ^ Goldsmith, D.; Owen, T. (1992). The Search for Life in the Universe (2nd ed.). Addison-Wesley. p. 415. ISBN 1-891389-16-5.
- ^ Vinn, O. (2024). "Potential incompatibility of inherited behavior patterns with civilization: Implications for Fermi paradox". Science Progress. 107 (3): 1–6. doi:10.1177/00368504241272491. PMC 11307330. PMID 39105260.
- ^ Sulleyman, Aatif (2 November 2017). "Stephen Hawking warns artificial intelligence 'may replace humans altogether'". independent.co.uk.
- ^ "The value of N remains highly uncertain. Even if we had a perfect knowledge of the first two terms in the equation, there are still five remaining terms, each of which could be uncertain by factors of 1,000." from Wilson, TL (2001). "The search for extraterrestrial intelligence". Nature. 409 (6823). Nature Publishing Group: 1110–1114. Bibcode:2001Natur.409.1110W. doi:10.1038/35059235. PMID 11234025. S2CID 205014501., or more informally, "The Drake Equation can have any value from "billions and billions" to zero", Michael Crichton, as quoted in Douglas A. Vakoch; et al. (2015). The Drake Equation: Estimating the prevalence of extraterrestrial life through the ages. Cambridge University Press. ISBN 978-1-10-707365-4., p. 13
- ^ "The Drake Equation". psu.edu.
- ^ Devin Powell, Astrobiology Magazine (4 September 2013). "The Drake Equation Revisited: Interview with Planet Hunter Sara Seager". Space.com.
- ^ Govert Schilling; Alan M. MacRobert (3 June 2009). "The Chance of Finding Aliens". Sky & Telescope.
- ^ [better source needed] Dean, T. (10 August 2009). "A review of the Drake Equation". Cosmos Magazine. Archived from the original on 3 June 2013. Retrieved 16 April 2013.
- ^ Rare Earth, page 270: "When we take into account factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001% of all stars might have planets with habitats similar to Earth. [...] If microbial life forms readily, then millions to hundreds of millions of planets in the galaxy have the potential for developing advanced life. (We expect that a much higher number will have microbial life.)"
- ^ von Bloh, W.; Bounama, C.; Cuntz, M.; Franck, S. (2007). "The habitability of super-Earths in Gliese 581". Astronomy & Astrophysics. 476 (3): 1365–1371. arXiv:0705.3758. Bibcode:2007A&A...476.1365V. doi:10.1051/0004-6361:20077939. S2CID 14475537.
- ^ Selsis, Franck; Kasting, James F.; Levrard, Benjamin; Paillet, Jimmy; Ribas, Ignasi; Delfosse, Xavier (2007). "Habitable planets around the star Gl 581?". Astronomy and Astrophysics. 476 (3): 1373–1387. arXiv:0710.5294. Bibcode:2007A&A...476.1373S. doi:10.1051/0004-6361:20078091. S2CID 11492499.
- ^ Lineweaver, C. H.; Davis, T. M. (2002). "Does the rapid appearance of life on Earth suggest that life is common in the universe?". Astrobiology. 2 (3): 293–304. arXiv:astro-ph/0205014. Bibcode:2002AsBio...2..293L. doi:10.1089/153110702762027871. PMID 12530239. S2CID 431699.
- ^ Forgan, D. (2009). "A numerical testbed for hypotheses of extraterrestrial life and intelligence". International Journal of Astrobiology. 8 (2): 121–131. arXiv:0810.2222. Bibcode:2009IJAsB...8..121F. doi:10.1017/S1473550408004321. S2CID 17469638.
- ^ "Are we alone? Setting some limits to our uniqueness". phys.org. 28 April 2016.
- ^ "Are We Alone? Galactic Civilization Challenge". PBS Space Time. 5 October 2016. PBS Digital Studios.
- ^ Frank, Adam (10 June 2016). "Yes, There Have Been Aliens". The New York Times.
- ^ Frank, Adam; Sullivan III, W. T. (22 April 2016). "A New Empirical Constraint on the Prevalence of Technological Species in the Universe". Astrobiology. 16 (5) (published 13 May 2016): 359–362. arXiv:1510.08837. Bibcode:2016AsBio..16..359F. doi:10.1089/ast.2015.1418. PMID 27105054.
- ^ Bioverse: How the Cellular World Contains the Secrets to Life's Biggest Questions William B Miller Jr. ISBN 9781633887992 p50
- ^ Hetesi, Z.; Regaly, Z. (2006). "A new interpretation of Drake-equation" (PDF). Journal of the British Interplanetary Society. 59: 11–14. Bibcode:2006JBIS...59...11H. Archived from the original (PDF) on 5 February 2009.
- ^ Maccone, C. (2010). "The Statistical Drake Equation". Acta Astronautica. 67 (11–12): 1366–1383. Bibcode:2010AcAau..67.1366M. doi:10.1016/j.actaastro.2010.05.003. S2CID 121239391.
- ^ Golden, Leslie M. (1 August 2021). "A joint mind consideration of the Drake equation in the search for extraterrestrial intelligence". Acta Astronautica. 185: 333–336. Bibcode:2021AcAau.185..333G. doi:10.1016/j.actaastro.2021.03.020. ISSN 0094-5765. S2CID 233663920.
- ^ a b Brin, G. D. (1983). "The Great Silence – The Controversy Concerning Extraterrestrial Intelligent Life". Quarterly Journal of the Royal Astronomical Society. 24 (3): 283–309. Bibcode:1983QJRAS..24..283B.
- ^ Zaitsev, A. (May 2005). "The Drake Equation: Adding a METI Factor". SETI League. Retrieved 20 April 2013.
- ^ Jones, Chris (7 December 2016). "'The World Sees Me as the One Who Will Find Another Earth' – The star-crossed life of Sara Seager, an astrophysicist obsessed with discovering distant planets". The New York Times. Retrieved 8 December 2016.
- ^ a b c Devin Powell (4 September 2013). "The Drake Equation Revisited: Interview with Planet Hunter Sara Seager". Space.com. Retrieved 6 October 2023.
- ^ "A New Equation Reveals Our Exact Odds of Finding Alien Life". io9. 21 June 2013.
- ^ "The Drake Equation". phys.libretexts.org. 13 August 2014. Retrieved 4 February 2024.
- ^ "Carl Sagan - Cosmos - Drake Equation". YouTube. 24 March 2009.
- ^ "Carl Sagan - Cosmos - Drake Equation". YouTube. 24 March 2009. Retrieved 4 February 2024.
- ^ Hartsfield, Tom (11 March 2015). "Why the Drake Equation Is Useless | RealClearScience". www.realclearscience.com. Retrieved 29 April 2024.
- ^ "The Drake Equation: Could It Be Wrong?". SETI Institute. Retrieved 29 April 2024.
- ^ Dvorsky, G. (31 May 2007). "The Drake Equation is obsolete". Sentient Developments. Retrieved 21 August 2013.
- ^ Sutter, Paul (27 December 2018). "Alien Hunters, Stop Using the Drake Equation". Space.com. Retrieved 18 February 2019.
- ^ "The unsurprising non-detection of intelligent aliens". Big Think. 23 April 2024. Retrieved 29 April 2024.
- ^ Tarter, Jill C. (May–June 2006). "The Cosmic Haystack Is Large". Skeptical Inquirer. 30 (3). Retrieved 21 August 2013.
- ^ Alexander, A. "The Search for Extraterrestrial Intelligence: A Short History – Part 7: The Birth of the Drake Equation". The Planetary Society. Archived from the original on 6 March 2005.
- ^ Christopher J. Conselice; et al. (2016). "The Evolution of Galaxy Number Density at z < 8 and its Implications". The Astrophysical Journal. 830 (2): 83. arXiv:1607.03909. Bibcode:2016ApJ...830...83C. doi:10.3847/0004-637X/830/2/83. S2CID 17424588.
- ^ Fountain, Henry (17 October 2016). "Two Trillion Galaxies, at the Very Least". The New York Times. Archived from the original on 1 January 2022. Retrieved 17 October 2016.
- ^ Jones, E. M. (1 March 1985). "Where is everybody?" An account of Fermi's question (PDF) (Report). Los Alamos National Laboratory. Bibcode:1985STIN...8530988J. doi:10.2172/5746675. OSTI 5746675. Archived (PDF) from the original on 12 October 2007. Retrieved 21 August 2013.
- ^ Krauthammer, C. (29 December 2011). "Are we alone in the Universe?". The Washington Post. Retrieved 21 August 2013.
- ^ Webb, S. (2015). If the Universe Is Teeming with Aliens ... WHERE IS EVERYBODY?: Seventy-Five Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life. Springer International Publishing. ISBN 978-3319132358.
- ^ Hanson, R. (15 September 1998). "The Great Filter – Are We Almost Past It?". Retrieved 21 August 2013.
- ^ Sandberg, Anders; Drexler, Eric; Ord, Toby (6 June 2018). "Dissolving the Fermi Paradox". arXiv:1806.02404 [physics.pop-ph].
- ^ The Making of Star Trek by Stephen E. Whitfield and Gene Roddenberry, New York: Ballantine Books, 1968
- ^ Okuda, Mike and Denise Okuda, with Debbie Mirek (1999). The Star Trek Encyclopedia. Pocket Books. p. 122. ISBN 0-671-53609-5.
{{cite book}}
: CS1 maint: multiple names: authors list (link) - ^ "NASA Unveils Design for Message Heading to Jupiter's Moon Europa". NASA Jet Propulsion Laboratory (JPL). Retrieved 11 March 2024. This article incorporates text from this source, which is in the public domain.
Further reading
[edit]- Morton, Oliver (2002). "A Mirror in the Sky". In Graham Formelo (ed.). It Must Be Beautiful. Granta Books. ISBN 1-86207-555-7.
- Rood, Robert T.; James S. Trefil (1981). Are We Alone? The Possibility of Extraterrestrial Civilizations. New York: Scribner. ISBN 0684178427.
- Vakoch, Douglas A.; Dowd, Matthew F., eds. (2015). The Drake Equation: Estimating the Prevalence of Extraterrestrial Life Through the Ages. Cambridge, UK: Cambridge University Press. ISBN 978-1-10-707365-4.
External links
[edit]- Interactive Drake Equation Calculator
- Frank Drake's 2010 article on "The Origin of the Drake Equation"
- "Only a matter of time, says Frank Drake". A Q&A with Frank Drake in February 2010
- Drake, Frank (December 2004). "The E.T. Equation, Recalculated". Wired.
- Macromedia Flash page allowing the user to modify Drake's values from PBS's Nova
- "The Drake Equation", Astronomy Cast episode #23; includes full transcript
- Animated simulation of the Drake equation. (Archived 8 December 2015 at the Wayback Machine)
- "The Alien Equation", BBC Radio program Discovery (22 September 2010)
- "Reflections on the Equation" (PDF), by Frank Drake, 2013