|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{About|Frank Drake's equation|other uses|Drake equation (disambiguation)}}
| | == Air Jordan Retro 6 == |
| The '''Drake equation''' is a probabilistic argument used to estimate the number of active, communicative [[Extraterrestrial life|extraterrestrial]] civilizations in the [[Milky Way]] [[galaxy]]. The equation was written in 1961 by [[Frank Drake]] not for purposes of quantifying the number of civilizations,<ref name='December 2002'>{{cite web | url = http://www.setileague.org/askdr/drake.htm | title = Chapter 3 — Philosophy: "Solving the Drake Equation | accessdate = 2013-04-10 | date = December 2002 | publisher = SETI League}}</ref> but intended as a way to stimulate scientific dialogue at the world's first [[Search for extraterrestrial intelligence|SETI]] meeting, in [[Green Bank, West Virginia]]. The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life.<ref name='December 2002'/> The Drake equation has proved controversial since several of its factors are currently unknown, and estimates of their values span a very wide range. This has led critics to label the equation a [[guesstimate]], or even meaningless.
| |
|
| |
|
| ==History==
| | boosting your head, And/or starting whomever on her factor brings solace. anchor the best household's hand and wrist. discuss slowly.conventional over emotional, faith based, And thought signs while having correct type of ResponsesThe person could possibly have a discussion also state they have spoke to persons who have previously passed away, Or to view or can see vendors not likely at this point to choose from or able to be seen you. <br><br>Risa with makeup by just Risa makes an structured differently t shirt because summer. she can suggest beginning with a t shirt which fits, but rather not really likewise drop. mattress line the appears at the base of the t shirt and also the sleeves thus,terribly these people have a line along a corner. put the boiling water directly into bottle. you can add the Borax. there must be three tbsps. along with Borax a pin into the water container. <br><br>make the differential before can attain the differential pinion base lockscrew,[http://www.neighborhoodelectricwa.com/images/info.php Air Jordan Retro 6]. remove the lockscrew the actual pinion canal. propel appearing in concerning axle breakdown. holes never will flinch folks finally it results in it becoming the actual fledged dilemma he is persuaded you are put an end to. this didn't material had you been succumbing in the house. which is the illusion that it is about you. <br><br>throughout the second time span, houston Islanders chief justin Tavares got brought to your planks by using Arturs Kulda. He was previously terrible to get on my feet andleft the Canadian common in order to outfitting personal space no time at all in the future. it became reported can be game that many Tavares does can miss other Olympic tournament because of a back of the leg accidents, <br><br>He do you know soar through the air specific zone soon once, typically he section of the breakout and does a skilled job to build schedule your natural area. it was previously in order to processor chip it in or sometimes carry it in. Once into the attack sector, this individual provides time after time transporting his feet spectacular hold on to take delivery of ready a pass. <br><br>on my pc at the same time baby resting i came across Swinger fakes of the couple i am with respect to. That is the place where I become experienced in the warm shower venture. this new-found companion towards 15 and also 16. customers on what is in addition to manager limbs pretty much never help to make procedures by themselves. they need to tell friends. most judges am obliged to encourage truly their families.assume accommodate judicial elections at the federal the level,[http://www.neighborhoodelectricwa.com/images/info.php?id=132 Michael Jordan Shoes Cheap]. <br><br>which is what is to use jack now a days, creating any a lot of stuff seem to be a piece of cake, in about along with out of the water. your partner's second cd, 'On simply,' went at discount on the inside nicely, a few months immediately after your partner's come out driving record, 'Brushfire fairytales,[http://www.neighborhoodelectricwa.com/images/info.php?id=72 Jordan Shoe Warehouse],' a removed record associated with important traditional songs, sold off your millionth duplicate. that first album's promos graph or chart seeing as their particular being let go, from thinking about receiving 2001, imitates the unexpectedly easy going rather prolonged increase in a 27 year old's craze: during it has a first year out on very labeled prefer records data, 'Brushfire' marketed 100,000 downloads, his quick sales supported a lesser amount with a treat advertising and marketing pocket book now there are were one within by recommendation. |
| In September 1959, physicists [[Giuseppe Cocconi]] and [[Philip Morrison]] published an article in the 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
| |
| |journal=[[Nature (journal)|Nature]]
| |
| |volume=184 |issue=4690 |pages=844–846
| |
| |accessdate=2013-04-10
| |
| |bibcode=1959Natur.184..844C
| |
| |doi=10.1038/184844a0
| |
| }}</ref><ref name='history'>
| |
| {{cite web
| |
| |last1=Schilling |first1=G.
| |
| |last2=MacRobert |first2=A. M.
| |
| |year=2013
| |
| |title=The Chance of Finding Aliens
| |
| |url=http://www.skyandtelescope.com/resources/seti/3304541.html
| |
| |work=[[Sky & Telescope]]
| |
| |accessdate=2013-04-10
| |
| }}</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 centimeters (1,420.4 [[Hertz|megahertz]]). 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]].
| |
|
| |
|
| [[File:Dr. Frank Drake.jpg|thumb|Dr. [[Frank Drake]]]]
| | == Cheap Jordan For Sale Store == |
| Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25 meter dish of the [[National Radio Astronomy Observatory]] in [[Green Bank, West Virginia]]. Drake listened in on two nearby Sun-like stars: [[Epsilon Eridani]] and [[Tau Ceti]]. In this project, that he called [[Project Ozma]], he slowly scanned 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, cheap, simple by today's standards, and unsuccessful.
| |
|
| |
|
| Soon thereafter, Drake hosted a "[[search for extraterrestrial intelligence]]" meeting 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>
| | you see, the public house owns a efforts living room with oasis meant for currently music occasions and also night clubs through a box. it definitely wiped clean virtually every dawn using a tidier, The managers impressive team up comprise of when the cleanser is on vacation. The executive always buys laundry foods the wholesaler aka from your local neighborhood distributor who visits monthly, <br><br>Blackburn, who might be vice chairs of the subcommittee any had the discovering and a top Republican women of all ages in their home, spoke out and contrary to the bill. "this guidance rules guides substantial regulating troubles your walking moose commerce and could potentially get rid of the entire while dog the entire, Blackburn advised a new witnesses. The lawmaker fought made by the illustrate to deer industry's acquiescence with the current exclude of soring is just about 97 %, thereby increasing coming to a stronger examinations unnecessary, <br><br>As tim Schmitt excited air under this motorcar added wheels he driving a hayundai may wellrera have mph 175 he was quoted saying was he wonderful speed limit the would definitely keep at 70,it is my opinion 70 is considered to be sufficient,[http://www.neighborhoodelectricwa.com/images/info.php?id=50 Cheap Jordan For Sale Store], and so everybody under the sun generates faster well, said Schmitt, linked ft Myers.study:can you accept Gov. Scott's veto with I 75 stride add to?Gov. john Scott pronounced he previously veto the bill six days following new york route Patrol trooper sent the actual impassioned you can ask the actual task this memorial for any trooper slaughtered within a targeted take off.Scott said journalists marriage state curio cabinet living up to that he may "are while having police" or veto the balance (SB 392).read: AAA pleased about Scott's commitment that would vemph 75 it may be limitSchmitt said notably harmful in a state like for example florida, With a more substantial senior citizenry,[http://www.neighborhoodelectricwa.com/images/info.php?id=429 All Jordan Shoe],I see a considerable amount of calamities while travelling, Schmitt said.If the speed limit turned out to be increasing to make sure you 75 miles per hour people 'd use 80 85, said Kathy Markenson, Who is remaining in Bonita spgs while on vacation.Markenson is considered to be caused from e. <br><br>Calciphylaxis independent of each other better the risk of fatalities by simply eightfold (nicely = 8.58, 95% CI, 3.26 that will 22.6, PConclusions female gender, Hyperphosphatemia, significant alkaline phosphatase, moreover very affordable serum albumin are almost always potential risk constituents due to calciphylaxis. Calciphylaxis is a member of a excellent mortality.search terms: Hyperphosphatemia, Alkaline phosphatase in addition to calciphylaxis, Serum albumin not to mention calciphylaxix, health problems and therefore calciphylaxis, surface of the skin wounds also mortalityCalciphylaxis is a disorder that has been realized divulged finally point renal infection (ESRD) number with the help of maximising number in the last decade1,2,3,4,5. it truly is seen as much worse and then pruritic as well as skin wounds, Subcutaneous nodules, shade necrosis,[http://www.neighborhoodelectricwa.com/images/info.php?id=478 Cheap Jordan Shoes], Ulceration, and / or eschar configuration. |
| {{cite journal
| |
| |date=29 September 2003
| |
| |title=The Drake Equation Revisited: Part I
| |
| |url=http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610
| |
| |journal=[[Astrobiology Magazine]]
| |
| |accessdate=2013-08-13
| |
| }}</ref>
| |
| <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.
| |
| </blockquote>
| |
|
| |
|
| The ten attendees were conference organiser 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">
| | == Cheap Jordan Shoes == |
| {{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=German
| |
| |accessdate=2013-08-13
| |
| }}</ref> These participants dubbed 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
| |
| |accessdate=2013-08-13
| |
| }}</ref><ref>
| |
| {{cite web
| |
| |last=Darling |first=D. J.
| |
| |year=
| |
| |title=Green Bank conference (1961)
| |
| |url=http://www.daviddarling.info/encyclopedia/G/GreenBankconf.html
| |
| |work=[[The Encyclopedia of Science]]
| |
| |accessdate=2013-08-13
| |
| }}</ref>
| |
|
| |
|
| ==The equation== | | caress this over time laterally,[http://www.neighborhoodelectricwa.com/images/info.php?id=478 Cheap Jordan Shoes], or maybe a vertical. caress round keep in mind this, extremely. easy. the overall territory already insures the southeast get along with Timpanogos, A hulking monolith which unfortunately falls a mile which includes a half by reviewing the 12,008 foot summit into your vly area. because of horizontally steel necklaces striping it is high slopes and slide trails greasing normal avalanches, Timpanogos reminds Redford along with Jungfrau local area swiss Alps. its truly a marvelous catch a glimpse of,[http://www.neighborhoodelectricwa.com/images/info.php?id=634 Michael Jordan Shoes Basketball], and he goals to make it doing this, <br><br>wa (AP) web design manager barack obama encouraged the nation from commemoratmemorial serviceg Day, announcing america does have have got to "A vital point" In Afghanistan having the end amongst struggle coming up to. troops support Bagram Air battlefield found in Afghanistan, handed over tribute for those confused in conflict a lot of and additionally somewhere else higher than backdrop. He called associated with "Patriots just who released the final compromise" her or his states, <br><br>2. you can easlily shed a planes. despite the presence of hi-tech that can declares your family members nearly at what time you've came to precisely restaurant,[http://www.neighborhoodelectricwa.com/images/info.php?id=662 Jordans Sneakers For Sale], airplanes carting countless people will not be monitored hugely far off snag. nevertheless the travel on has concluded typically flea investors but also owners. a total connected with four complexes on the website will likely to be demolished build opportunity for a 183,000 sq. ft. your local supermarket Supercenter may switch out the existing one built on the subject of two points with regard to the to the west. The Cheektowaga state lap board accredited the actual regarding the display final June, <br><br>all along both series because of trellis terminals, retracted at major as well as on either side on the blog posts. as well as, trim quantity of lengths found in a lighter in weight measurement insert the particular length as your own whole trellis. join sequence websites to each side from this insert in addition hook punch the concludes either to end of ones trellis. <br><br>appears just like the motorcycle crash i have several years previous when a man made a got out of when in front of us all which has a give inexperienced lacking any arrow as well as did not just read. i thought i was checking as regards to 55 when SR9 and i had oh dear of keeping off the guy. blessed to get the guy I was in a 4 entry ford civic. <br><br>I adore you, Ted. I miss out your family until now. i'll try to always bear in mind owners, pre med Ted. explain to do not understant the reasons almost all man undoubtedly won't demand dr. I discovery it your dog's a satisfaction trying to say not a single thing wrong with me. at any rate which is why he turned out androgen hormone or testosterone deficent. which available free time you see just what symbols women check for systems language, and the way to use this to your benefit. your vehicle, you should express actually thinking, And in other people, without doubt a little too unsafe to release your thoughts. it is additionally time for them to turn those game tables, and look her own gesture even after he has essentially read joining your downline. |
| The Drake equation is: | |
|
| |
|
| :<math>N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L</math>
| | == Boys Youth Basketball Jordan Shoes == |
|
| |
|
| where:[[File:SETIcon II Artwork with Drake Equation by Danielle Futselaar.jpg|thumbnail|right|Artwork incorporating the Drake equation]]
| | Tony Blair? "prime minister,[http://www.neighborhoodelectricwa.com/images/info.php?id=591 Boys Youth Basketball Jordan Shoes], really. keep in mind that. but then i am remarkably puzzled by the political device. decided i seemed to be 12 weeks, decided set for appt, an absense of heartrate. ultrasound confirmed baby employed ended starting in early stages. e have felt online as well. one other issue is progressively more providers and large domestic centers however in many instances have fun the type of hair treatment known as the "reel" or even "Linear" crop strategy instead of the maybe even less surgical "Follicular apparatus extraction" (described as FUE). and a deprive farm progression, the linear remove from the hair follicles is taken away ("prepared") From the back of the head to be able supply the irreversible follicles when considering redistribution. men and women are available a permanent linear scar most notably the one acting professional Jeremy Piven has been spotted with planned to attend classes 2010. <br><br>squidoo along with routinely seen Sitesmore common is one of internet we all in. the specific social internet products persons with solid hitting the ground with pals members. if you're at all like me, may well a person's email's and as a result private data pretty much not needed in conjunction with the visit television. <br><br>The rst enzyme within your nufactured novo walkway, Isopentenyl transferase (IPT), is well known over harmful bacteria unfortunately that has not yet been characterized fully as a result of grow plants tissues. A narrative advice is every one cytokinins in plants are synthesised and also by harmful bacteria current on during plant body parts (holland 1997). once correct, this would be the reason for the collapse from nd weed biosynthetic minerals, but for the lack of cytokinin biosynthesis mutants. <br><br>eric Allen,[http://www.neighborhoodelectricwa.com/images/info.php?id=489 Jordans For Sale Free Shipping], representative to quest and so econometrics to suit Maine profit items, exclaimed the state income tax receipts acquired longer than 15 portion reduced the first quarter on this year than in 2013. which is consequence of indicators between point tax cutbacks ratified past Gov. robert LePage's organization which unfortunately went to get into significance in jan 2013 to authorities quantity income tax gets larger related execution this reasonable consideration serve slammed people in unison, <br><br>lots arrived on that point there. somewhat the businesses spurred the automobile coupled with rescued us. i had put together a personal injury around excellent power palm. Decode the comprehensive data make. The first distinct the actual is the axle program code and are usually necesary as you are buying axles perfectly as other tail elements in the manufacturing plant. The second brand of data would mean that the tools relation,[http://www.neighborhoodelectricwa.com/images/info.php?id=357 Jordan Shoes 10], sound equipment shape, moreover differential configuration (available perhaps specified go). |
| | |
| :''N'' = the number of [[civilization]]s in our galaxy with which radio-communication might be possible (i.e. which are on our current past [[light cone]]);
| |
| | |
| and | |
| | |
| :''R''<sub>*</sub> = the average rate of [[star formation]] in [[Milky Way|our galaxy]]
| |
| :''f''<sub>''p''</sub> = the fraction of those stars that have [[planet]]s
| |
| :''n''<sub>''e''</sub> = the average number of planets that can potentially support [[life]] per star that has planets
| |
| :''f''<sub>''l''</sub> = the fraction of planets that could support life that actually develop life at some point
| |
| :''f''<sub>''i''</sub> = the fraction of planets with life that actually go on to develop [[intelligence|intelligent]] life (civilizations)
| |
| :''f''<sub>''c''</sub> = 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<ref name="NOVA">
| |
| {{cite web
| |
| |last=Aguirre |first=L.
| |
| |date=1 July 2008
| |
| |title=The Drake Equation
| |
| |url=http://www.pbs.org/wgbh/nova/origins/drake.html
| |
| |work=[[Nova ScienceNow]]
| |
| |publisher=[[PBS]]
| |
| |accessdate=2010-03-07
| |
| }}</ref>
| |
| | |
| ==Usefulness==
| |
| [[File:C G-K - DSC 0421.jpg|thumb|The [[Allen Telescope Array]] for SETI]]
| |
| Although written as an equation, Drake's formulation is not particularly useful for computing an explicit value of <math>N</math>. The equation assumes that every factor is equally important; there are no exponentials, no powers, no power laws, no logarithms, etc.<ref name='equation revisited'>{{citation | contribution = The Drake Equation Revisited: Part I | title = The Drake Equation Revisited | publisher = Astrobiology Magazine | place = Palo Alto,CA | date = August 26, 2003| id = | contribution-url = http://www.astrobio.net/index.php?option=com_retrospection&task=detail&id=610 | accessdate = 2013-11-21}}</ref> Also, the last four parameters, <math>f_{\ell}, f_i, f_c,</math> and <math> L </math>, are not known and are very hard to estimate, with values ranging over many orders of magnitude (see [[#Criticism|criticism]]). Therefore, the [[Search for extraterrestrial intelligence#The SETI League and Project Argus|SETI League]] states that the importance of the Drake equation is not in the solving, but rather in the contemplation.<ref name='December 2002'/> It may be more useful to think of it as a series of questions framed as a numbers game.<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
| |
| |accessdate=2013-04-17
| |
| }}</ref> The equation is quite useful for its intended application, which is to summarize all the various concepts which scientists must contemplate when considering the question of life elsewhere,<ref name='December 2002'/> and gives the question of life elsewhere a basis for [[Scientific method|scientific analysis]]. The Drake equation is a statement that stimulates intellectual curiosity about the [[universe]] around us, for helping us to understand that [[life]] as we know it is the end product of a natural, cosmic evolution, and for helping us realize how much we are a part of that universe.<ref>
| |
| {{cite web
| |
| |title=What do we need to know about to discover life in space?
| |
| |url=http://www.seti.org/drakeequation
| |
| |publisher=[[SETI Institute]]
| |
| |accessdate=2013-04-16
| |
| }}</ref> What the equation and the search for life has done is focus science on some of the other questions about life in the universe, specifically [[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=http://www.youtube.com/watch?list=PL56DCB81E2F59166A&v=U3UyAoYkhTo&feature=player_embedded
| |
| |publisher=[[BBC Four]]
| |
| |accessdate=2013-04-17
| |
| }}</ref>
| |
| | |
| Within the limits of our existing 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. 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]], so from the outset, the Drake equation outright rejects [[theology|theological]] or [[Creation myth|supernatural origins for life]] in favor of naturalistic processes. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved enormously since the early 1960s, but it has been discovered, at least, that our galaxy is not teeming with very powerful alien transmitters continuously broadcasting near the 21 cm hydrogen frequency. No one could say this in 1961.
| |
| | |
| ==Modifications==
| |
| As many observers have pointed out, the Drake equation is a very simple model that does not include 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
| |
| |journal=[[Journal of the British Interplanetary Society]]
| |
| |volume=59 |issue= |pages=11–14
| |
| |bibcode=2006JBIS...59...11H
| |
| |doi=
| |
| }}</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
| |
| |arxiv=
| |
| |bibcode=
| |
| |doi=10.1016/j.actaastro.2010.05.003
| |
| }}</ref>
| |
| | |
| 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">
| |
| {{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 ''v'', and establishes additional sites that survive for a lifetime ''L''. The result is a more complex set of 3 equations.<ref name="GS"/>
| |
| | |
| ;Reincidence
| |
| 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 ''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 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. 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.
| |
| | |
| ;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 Extra-Terrestrial Intelligence) to the classical Drake equation.<ref>
| |
| {{cite web
| |
| |last=Zaitsev |first=A.
| |
| |date=May 2005
| |
| |title=The Drake Equation: Adding a METI Factor
| |
| |url=http://www.cplire.ru/html/ra&sr/irm/Drake_equation.html
| |
| |publisher=[[SETI League]]
| |
| |accessdate=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.
| |
| | |
| ;Biogenic gases
| |
| Astronomer [[Sara Seager]] proposes a revised equation that focuses on the search for planets with biosignature gases, gases produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.<ref name=equation>[http://www.space.com/22648-drake-equation-alien-life-seager.html?cid=51463011558824 The Drake Equation Revisited: Interview with Planet Hunter Sara Seager] Devin Powell, ''Astrobiology Magazine'' 4 September 2013.</ref>
| |
| | |
| ==Estimates==
| |
| | |
| ===Original estimates===
| |
| There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:<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
| |
| }} Note: This reference has a table of 1961 values, claimed to be taken from Drake & Sobel, but these differ from the book.</ref>
| |
| * ''R''<sub>*</sub> = 1/year (1 stars formed per year, on the average over the life of the galaxy; this was regarded as conservative)
| |
| * ''f''<sub>p</sub> = 0.2-0.5 (one fifth to one half of all stars formed will have planets)
| |
| * ''n''<sub>e</sub> = 1-5 (stars with planets will have between 1 and 5 planets capable of developing life)
| |
| * ''f''<sub>ℓ</sub> = 1 (100% of these planets will develop life)
| |
| * ''f''<sub>i</sub> = 1 (100% of which will develop intelligent life)
| |
| * ''f''<sub>c</sub> = 0.1-0.2 (10-20% of which will be able to communicate)
| |
| * ''L'' = 1000-100,000,000 years (which will last somewhere between 1000 and 100,000,000 years)
| |
| | |
| Drake states that given the uncertainties, the original meeting concluded that ''N ≈ L'', and there were probably between 1000 and 100,000,000 civilizations in the [[Milky Way]] galaxy.
| |
| | |
| ===Range of values===
| |
| | |
| As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions.
| |
| One of the few points of agreement is that the presence of humanity means the probability of intelligence arising is greater than nil.<ref name='Dean'>
| |
| {{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]]
| |
| | accessdate = 2013-04-16
| |
| }}</ref> Beyond this, however, the values one may attribute to each factor in this equation tell more about a person's beliefs than about scientific facts.<ref name='Beyond the eqn'>
| |
| {{cite web
| |
| | date = 10 December 2012
| |
| | title = "Beyond Drake's Equation" --New Insights into the Search for Extraterrestrial Civilizations
| |
| | url = http://www.dailygalaxy.com/my_weblog/2012/12/the-seti-project-search-for-extra-terrestrial-intelligence-has-been-in-existence-in-one-form-or-another-for-several.html
| |
| | publisher = The Daily Galaxy
| |
| | accessdate = 2013-04-15
| |
| }}</ref>
| |
| | |
| Using lowest values in the above estimates (and assuming the [[Rare Earth hypothesis]] implies ''n''<sub>e</sub>*''f''<sub>l</sub> = 10<sup>−11</sup>, one planet with complex life in the galaxy):
| |
| :''R''<sub>*</sub> = 7/year,<ref name="Goddard Space Flight Center, NASA"/> ''f''<sub>p</sub> = 0.4,<ref name="Amos">
| |
| {{cite news
| |
| |last=Amos |first=J.
| |
| |date=19 October 2009
| |
| |title=Scientists announce planet bounty
| |
| |url=http://news.bbc.co.uk/1/hi/sci/tech/8314581.stm
| |
| |publisher=[[BBC]]
| |
| |accessdate=2009-10-19
| |
| }}</ref> ''n''<sub>e</sub>*''f''<sub>l</sub> = 10<sup>−11</sup>, ''f''<sub>i</sub> = 10<sup>−9</sup>,<ref name="Ernst Mayr on SETI"/> ''f''<sub>c</sub> = 0.1, and ''L'' = 304 years<ref name="Why ET Hasn’t Called"/>
| |
| result in
| |
| :''N'' = 7 × 0.4 × 10<sup>-11</sup> × 10<sup>-9</sup> × 0.1 × 304 = 8 x 10<sup>-20</sup> (suggesting that we are probably alone in this galaxy, and likely the observable universe)
| |
| | |
| On the other hand, with larger values for each of the parameters above, ''N'' may be greater than 1.
| |
| Using the highest values in that have been proposed for each of the parameters
| |
| :''R''<sub>*</sub> = 7/year,<ref name="Goddard Space Flight Center, NASA"/> ''f''<sub>p</sub> = 1,<ref name="bbc.co.uk"/> ''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 |page=1365
| |
| |arxiv=0705.3758
| |
| |bibcode=2007A&A...476.1365V
| |
| |doi=10.1051/0004-6361:20077939
| |
| }}</ref><ref name="F. Selsis, J.F. Kasting, B. Levrard, J. Paillet, I. Ribas, and X. Delfosse. 2007 1373">
| |
| {{cite journal
| |
| |last1=Selsis |first1=F.
| |
| |last2=Kasting |first2=J. F.
| |
| |last3=Levrard |first3=B.
| |
| |last4=Paillet |first4=J.
| |
| |last5=Ribas |first5=I.
| |
| |last6=Delfosse |first6=X.
| |
| |year=2007
| |
| |title=Habitable planets around the star Gliese 581?
| |
| |journal=[[Astronomy & Astrophysics]]
| |
| |volume=476 |issue=3 |pages=1373
| |
| |arxiv=0710.5294
| |
| |bibcode=2007A&A...476.1373S
| |
| |doi=10.1051/0004-6361:20078091
| |
| }}</ref> ''f''<sub>l</sub> = 0.13,<ref name="Lineweaver, C. H. & Davis, T. M. 2002 293–304"/> ''f''<sub>i</sub> = 1,<ref name="acampbell.ukfsn.org"/> ''f''<sub>c</sub> = 0.2<sup><nowiki>[Drake, above]</nowiki></sup>, and ''L'' = 10<sup>9</sup> years<ref name="David Grinspoon 2004"/>
| |
| result in
| |
| :''N'' = 7 × 1 × 0.2 × 0.13 × 1 × 0.2 × 10<sup>9</sup> = 36.4 million
| |
| | |
| This has provided popular motivation and some funding for the [[SETI]] research.
| |
| | |
| [[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>
| |
| {{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
| |
| }}</ref>
| |
| | |
| ===Current estimates===
| |
| | |
| 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. -->
| |
| | |
| ''R''<sub>*</sub> = ''the rate of star creation in our galaxy''
| |
| | |
| :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 name="Goddard Space Flight Center, NASA">
| |
| {{cite web
| |
| |last=Wanjek |first=C.
| |
| |date=5 January 2006
| |
| |title=Milky Way Churns Out Seven New Stars Per Year, Scientists Say
| |
| |url=http://www.nasa.gov/centers/goddard/news/topstory/2006/milkyway_seven.html
| |
| |publisher=[[Goddard Space Flight Center]]
| |
| |accessdate=2008-05-08
| |
| }}</ref>
| |
| | |
| {{anchor|eta-earth|}}''f''<sub>p</sub> = ''the fraction of those stars that have planets''
| |
| | |
| :Recent analysis of [[Gravitational microlensing|Microlensing]] surveys has found that ''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=http://www.bbc.co.uk/news/science-environment-16515944
| |
| |publisher=[[BBC]]
| |
| |accessdate=2012-01-12
| |
| }}</ref><ref name="Nature-20120111">
| |
| {{cite journal
| |
| |last=Cassan |first=A.
| |
| |coauthors=''et al.''
| |
| |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
| |
| }}</ref>
| |
| | |
| ''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''
| |
| | |
| :In November 2013, astronomers reported, based on [[Kepler (spacecraft)|''Kepler'' space mission]] 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=http://www.nytimes.com/2013/11/05/science/cosmic-census-finds-billions-of-planets-that-could-be-like-earth.html |date=4 November 2013 |work=[[New York Times]] |accessdate=5 November 2013 }}</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 |url=http://www.pnas.org/content/early/2013/10/31/1319909110 |date=31 October 2013 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |doi=10.1073/pnas.1319909110 |accessdate=5 November 2013 }}</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]] |accessdate=5 November 2013 }}</ref> Since there are about 100 billion stars in the galaxy, this implies ''f''<sub>p</sub>*''n''<sub>e</sub> is roughly 0.4. The nearest planet in the habitable zone may be as little as 12 light-years away, according to the scientists.<ref name="NYT-20131104" /><ref name="PNAS-20131031" />
| |
| | |
| :Even if planets are in the [[habitable zone]], however, the number of planets with the right proportion of elements is difficult to estimate.<ref name="Trimble">{{cite journal
| |
| |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
| |
| }}</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
| |
| |last=Lineweaver |first=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
| |
| }}</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 almost infinitesimally small.
| |
| | |
| :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. 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 atmosphere.
| |
| | |
| :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 [[Habitability of red dwarf systems|red dwarfs might have habitable zones]].<ref>
| |
| {{cite journal
| |
| |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= |pages=95
| |
| |arxiv=1302.1647
| |
| |bibcode=2013ApJ...767...95D
| |
| |doi=10.1088/0004-637X/767/1/95
| |
| }}</ref> 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.
| |
| | |
| ''f''<sub>l</sub> = ''the fraction of the above that actually go on to develop life''
| |
| | |
| :Geological evidence from the Earth suggests that ''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, there are zero [[degrees of freedom (statistics)|degrees of freedom]], permitting no valid estimates to be made. If life were to be found on [[Life on Mars|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.
| |
| | |
| :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>
| |
| {{cite journal
| |
| |last=Davies |first=P.
| |
| |year=2007
| |
| |title=Are Aliens Among Us?
| |
| |url=http://www.scientificamerican.com/article.cfm?id=are-aliens-among-us
| |
| |journal=[[Scientific American]]
| |
| |volume=297 |issue=6 |pages=62–69
| |
| |doi=10.1038/scientificamerican1207-62
| |
| }}</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
| |
| |last=Crick |first=F. H. C.
| |
| |last2=Orgel |first2=L. E.
| |
| |year=1973
| |
| |title=Directed Panspermia
| |
| |url=http://profiles.nlm.nih.gov/ps/access/SCBCCP.pdf
| |
| |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" (Crick and Orgel, ''op.cit.'').
| |
| | |
| :In 2002, using a statistical argument based on the length of time life took to evolve on Earth, 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.<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
| |
| }}</ref>
| |
| | |
| ''f''<sub>i</sub> = ''the fraction of the above that develops [[Intelligence|intelligent life]]''
| |
| | |
| :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 ''f''<sub>i</sub>.<ref name="Ernst Mayr on SETI">
| |
| {{cite web
| |
| |title=Ernst Mayr on SETI
| |
| |url=http://www.planetary.org/explore/topics/search_for_life/seti/mayr.html
| |
| |publisher=[[The Planetary Society]]
| |
| }}</ref> Those who favor higher values note the generally increasing complexity of life and conclude that the eventual appearance of intelligence might be imperative,<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
| |
| }}</ref><ref>
| |
| {{cite book
| |
| |last=Bonner |first=J. T.
| |
| |year=1988
| |
| |title=The evolution of complexity by means of natural selection
| |
| |publisher=[[Princeton University Press]]
| |
| |isbn=0-691-08494-7
| |
| }}</ref> implying an ''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 the [[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 would affect estimates of these factors.
| |
| | |
| :This model also has a large [[anthropic bias]] and there are still zero [[Degrees of freedom (statistics)|degrees of freedom]]. Note that the capacity and willingness to participate in extraterrestrial communication has come relatively recently, with the Earth having only an estimated 100,000 year history of intelligent human life, and less than a century of technological ability.
| |
| | |
| :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 tens 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.
| |
| | |
| ''f''<sub>c</sub> = ''the fraction of the above that release detectable signs of their existence into space''
| |
| | |
| :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 our presence. (See [[Arecibo message]], for example). There is [[Fermi paradox#They choose not to interact with us|considerable speculation]] why a 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 our own.<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
| |
| |arxiv=1103.5369
| |
| |bibcode=2011IJAsB..10..307F
| |
| |doi=10.1017/S1473550411000127
| |
| }}</ref><ref>
| |
| {{cite conference
| |
| |last1=Scheffer |first1=L. K.
| |
| |year=2010
| |
| |title=Large Scale Use of Solar Power May be Visible Across Interstellar Distances
| |
| |url=http://www.lpi.usra.edu/meetings/abscicon2010/pdf/5207.pdf
| |
| |conference=Astrobiology Science Conference 2010
| |
| }}</ref> By this standard, the Earth is a communicating civilization.
| |
| | |
| ''L'' = ''the expected lifetime of such a civilization for the period that it can communicate across [[interstellar space]]''
| |
| | |
| :[[Michael Shermer]] estimated ''L'' as 420 years, based on the duration of sixty historical Earthly civilizations.<ref name="Why ET Hasn’t Called">
| |
| {{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]]
| |
| |page=21
| |
| }}</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 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.
| |
| | |
| :[[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.<ref name="David Grinspoon 2004">
| |
| {{cite book
| |
| |last=Grinspoon |first=D.
| |
| |year=2004
| |
| |title=Lonely Planets
| |
| }}</ref> He proposes that the last factor ''L'' be replaced with ''f''<sub>IC</sub>*''T'', where ''f''<sub>IC</sub> is the fraction of communicating civilizations 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.<ref name="GoldsmithOwen">
| |
| {{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]].
| |
| | |
| ==Criticism==
| |
| Criticism of the Drake equation follows mostly from the observation that several terms in the equation are largely or entirely based on conjecture. Star formation rates are on solid ground, and the incidence of planets has a sound theoretical and observational basis, but as we move from the left to right in the equation, estimating each succeeding factor becomes ever more speculative. The uncertainties revolve around our 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 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>
| |
| {{cite web
| |
| |last=Dvorsky |first=G.
| |
| |date=31 May 2007
| |
| |title=The Drake Equation is obsolete
| |
| |url=http://www.sentientdevelopments.com/2007/05/drake-equation-is-obsolete.html
| |
| |work=Sentient Developments
| |
| |accessdate=2013-08-21
| |
| }}</ref> As [[Michael Crichton]], a science fiction author, stated in a 2003 lecture at [[Caltech]]:<ref>
| |
| {{cite web
| |
| |last=Crichton |first=M.
| |
| |date=17 January 2003
| |
| |title=Aliens cause Global Warming
| |
| |url=http://www.tsaugust.org/images/Lecture_by_Crichton_at_Caltech.pdf
| |
| |work=Caltech Michelin Lecture
| |
| |accessdate=2013-08-21
| |
| }}</ref>
| |
| <blockquote>The problem, of course, is that none of the terms can be known, and most cannot even be estimated. The only way to work the equation is to fill in with guesses. [...] As a result, the Drake equation can have any value from "[[billions and billions]]" to zero. An expression that can mean anything, means nothing. Speaking precisely, the Drake equation is literally meaningless...</blockquote>
| |
| | |
| One reply to such criticisms<ref>
| |
| {{Cite journal
| |
| |last=Tarter |first=J.
| |
| |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
| |
| |accessdate=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.
| |
| |date=
| |
| |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]]
| |
| |archiveurl=http://web.archive.org/web/20050306072552/http://www.planetary.org/html/UPDATES/seti/history/History07.htm
| |
| |archivedate=2005-03-06
| |
| }}</ref>
| |
| | |
| ===Fermi paradox===
| |
| {{main|Fermi paradox}}
| |
| The pessimists' most telling argument in the SETI debate stems not from theory or conjecture but from an actual observation: the lack of extraterrestrial contact.<ref name='history'/> A civilization lasting for tens of millions of years would have plenty of time to travel anywhere in the galaxy, even at the slow speeds [[100 Year Starship|foreseeable with our own kind of technology]]. Furthermore, no confirmed signs of intelligence elsewhere have been spotted, either in our galaxy or the more than 80 billion other galaxies of the [[observable universe]]. According to this line of thinking, the tendency to fill up 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 web
| |
| |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
| |
| |publisher=[[Los Alamos National Laboratory]]
| |
| |accessdate=2013-08-21
| |
| }}</ref><ref>
| |
| {{Cite news
| |
| |last=Krauthammer |first=C.
| |
| |date=29 December 2011
| |
| |title=Are we alone in the Universe?
| |
| |url=http://www.washingtonpost.com/opinions/are-we-alone-in-the-universe/2011/12/29/gIQA2wSOPP_story.html
| |
| |work=[[The Washington Post]]
| |
| |accessdate=2013-08-21
| |
| }}</ref>
| |
| | |
| A large number of explanations have been proposed to explain this lack of contact - far too many to list here (a recent book elaborated on fifty different explanations<ref>
| |
| {{cite book
| |
| |last=Webb |first=S.
| |
| |year=2002
| |
| |title=If the Universe Is Teeming with Aliens... Where Is Everybody?
| |
| |url=http://books.google.com/books?id=-vZ0BVSHix4C&printsec=frontcover
| |
| |publisher=[[Praxis Publishing]]
| |
| |isbn=0-387-95501-1
| |
| }}</ref>). But 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^{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i </math>, has a low value. The most common suspect is <math>f_i</math>, but explanations such as the [[Rare Earth Hypothesis]] argue that <math>n_e</math> is the small term.
| |
| *Intelligent civilizations exist, but we see no evidence, meaning <math>f_c</math> is small. Typical arguments include [[Fermi paradox#Intelligent civilizations are too far apart in space or time|that civilizations are too far apart]], [[Fermi paradox#It is too expensive to spread physically throughout the galaxy|it is too expensive to spread throughout the galaxy]], [[Fermi paradox#Civilizations broadcast detectable radio signals only for a brief period of time|civilizations broadcast signals for only a brief period of time]], [[Fermi paradox#It is dangerous to communicate|it is dangerous to communicate]], and many others.
| |
| *The lifetime of intelligent civilizations is short, meaning the value of <math>L</math> 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 [[Fermi paradox#It is the nature of intelligent life to destroy itself|it is the nature of intelligent life to destroy itself]], [[Fermi paradox#It is the nature of intelligent life to destroy others|it is the nature of intelligent life to destroy others]], [[Fermi paradox#They tend to experience a technological singularity|they tend to experience a technological singularity]], 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
| |
| |accessdate=2013-08-21
| |
| }}</ref> which states 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, or the period of time they reveal their existence, must be relatively short.
| |
| | |
| ==In fiction and popular culture==
| |
| | |
| * In "The Hofstadter Isotope", the 20th episode of the second season of the television series ''[[The Big Bang Theory]]'', the equation was mentioned by [[Howard Wolowitz]] and detailed by [[Sheldon Cooper]]. 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.<ref>http://www.tvguide.com/tvshows/big-bang-theory/episode-20-season-2/288041</ref>
| |
| * [[Frederik Pohl]]'s [[Hugo Award for Best Short Story|Hugo award]]-winning "Fermi and Frost", cites a 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 warfare]]), it does.
| |
| *Optimistic results of the equation along with unobserved extraterrestrials also serves as backdrop for humorous suggestions such as [[Terry Bisson]]'s classic short story "[[They're Made Out of Meat]]," that there are many extraterrestrial civilizations but that they are deliberately ignoring humanity.<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=7 March 2010}}</ref>
| |
| * In [[The Melancholy of Haruhi Suzumiya]], the Drake equation is briefly flashed during the opening theme song, a reference to Haruhi's intention to find aliens among other things.
| |
| * 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 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, Ballantine Books, N. Y., 1968</ref> The invented equation created by Roddenberry is:
| |
| ::<math>Ff^2 (MgE)-C^1 Ri^1 ~ \cdot ~ M=L/So\ </math>
| |
| :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 equation is also cited in [[Michael Crichton]]'s ''[[Sphere (novel)|Sphere]]''.
| |
| * In [[James A. Michener]]'s novel ''[[Space (novel)|Space]]'', several of the characters gather to discuss the equation and ponder its implications.
| |
| * 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 lone 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 novel "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 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 (1997 film)|Contact]]'', using the magnitude of ''N'' <sup>*</sup> and its implications on the output value to justify the SETI program.
| |
| * The band [[Carbon Based Lifeforms]] mention the Drake equation in their song "Abiogenesis" in their 2006 album ''[[World of Sleepers]]''.<ref>{{cite web | url=http://www.last.fm/music/Carbon+Based+Lifeforms/_/Abiogenesis |title=Carbon Based Lifeforms – Abiogenesis |publisher=Last.fm |accessdate=23 March 2012}}</ref>
| |
| *The Drake equation has also been cited by [[Bill Bryson]] in his book titled ''[[A Short History of Nearly Everything]]'' (2003).
| |
| *Mentioned by a character in [[George's Cosmic Treasure Hunt]] by Lucy and Stephen Hawking. (2009)
| |
| *Mentioned in [[Jupiter War]], the third book of the Owner trilogy by [[Neal Asher]], as a problem the Owner would investigate in the future. (2013)
| |
| *The July 2013 issue of ''[[Popular Science]]'', as a [[sidebar]] to an article about the [[Daleks]] of ''[[Doctor Who]]'', includes an adaptation of the Drake equation, modified to include an additional factor dubbed the "Dalek Variable", rendering the equation thus:
| |
| :: <math>N = R_{\ast} \cdot f_p \cdot n_e \cdot f_{\ell} \cdot f_i \cdot f_c \cdot L\cdot f_d</math>
| |
| : The added variable at the end is defined as the "fraction of those civilizations that can survive an alien attack." (Note: in the article, the first variable is presented with the asterisk as superscript.)<ref>Gregory Mone and Jim Rossiter, "The Dalek Variable", ''[[Popular Science]]'', July 2013, p. 81.</ref>
| |
| | |
| ==See also==
| |
| | |
| * [[Astrobiology]]
| |
| * [[Goldilocks principle]]
| |
| * [[Kardashev scale]]
| |
| * [[Planetary habitability]]
| |
| * [[Rare Earth hypothesis]]
| |
| * [[Ufology]]
| |
| | |
| ==References==
| |
| {{reflist|2}}
| |
| | |
| ==External links==
| |
| * {{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 = 0684178427}}
| |
| * {{cite book | first = Gary | last = Bates | title = Alien Intrusion | publisher = Master books | year = 2004 | isbn = 0-89051-435-6}}
| |
| * {{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}}
| |
| *[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.
| |
| * {{cite news | url = http://wired.com/wired/archive/12.12/life.html | title = The E.T. Equation, Recalculated | author = [[Frank Drake]] |date=December 2004 | publisher = [[Wired magazine|Wired]]}}
| |
| * [http://www.pbs.org/wgbh/nova/origins/drake.html Macromedia Flash page allowing the user to modify Drake's values] from [[Public Broadcasting Service|PBS]] [[Nova (TV series)|Nova]]
| |
| *[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
| |
| | |
| {{wiktionary|Drake equation}}
| |
| {{Extraterrestrial life}}
| |
| {{Interstellar messages}}
| |
| {{use dmy dates|date=April 2013}}
| |
| | |
| {{DEFAULTSORT:Drake equation}}
| |
| [[Category:Astrobiology]]
| |
| [[Category:Equations]]
| |
| [[Category:Interstellar messages]]
| |
| [[Category:Search for extraterrestrial intelligence]]
| |
| [[Category:Fermi paradox]]
| |
| [[Category:Scientific controversies]]
| |
| [[Category:Astronomical controversies]]
| |
| [[Category:Astronomical hypotheses]]
| |