8/22/2019 Drake Equation - Wikipedia, The Free Encyclopedia http://slidepdf.com/reader/full/drake-equation-wikipedia-the-free-encyclopedia 1/21 8/2/13 Drake equation - Wikipedia, the free encyclopedia en.wikipedia.org/wiki/Drake_equation Drake equation From Wikipedia, the free encyclopedia The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. The equation wa written in 1961 by Frank Drake not for purposes of quantifying the number of civilizations, [1] but intended as a way to stimulate scientific dialogue at the world's fir SETI meeting, in Green Bank, West Virginia. The equation summarizes the main factor which scientists must contemplate when considering the question of other radio- communicative life. [1] The Drake equation has proved controversial since several of it 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. Contents 1 History 2 The equation 3 Usefulness 4 Modifications 5 Estimates 5.1 Original estimates 5.2 Range of values 5.3 Current estimates 6 Criticism 6.1 Fermi paradox 7 In fiction and popular culture 8 See also 9 References 10 External links History
Transcript
8/22/2019 Drake Equation - Wikipedia, The Free Encyclopedia
8/2/13 Drake equation - Wikipedia, the free encyclopedia
en.wikipedia.org/wiki/Drake_equation
Drake equationFrom Wikipedia, the free encyclopedia
The Drake equation is a probabilistic argument used to estimate the number of active,
communicative extraterrestrial civilizations in the Milky Way galaxy. The equation wa
written in 1961 by Frank Drake not for purposes of quantifying the number of civilizations,[1] but intended as a way to stimulate scientific dialogue at the world's fir
SETI meeting, in Green Bank, West Virginia. The equation summarizes the main factor
which scientists must contemplate when considering the question of other radio-
communicative life.[1] The Drake equation has proved controversial since several of it
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.
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Dr. Frank Drake
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an
article in the journal Nature with the provocative title "Searching for Interstellar
Communications."[2][3] Cocconi and Morrison argued that radio telescopes had becom
sensitive enough to pick up transmissions that might be broadcast into space by
civilizations orbiting other stars. Such messages, they suggested, might be transmitted a
a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio
emission by neutral hydrogen, the most common element in the universe, and theyreasoned that other intelligences might see this as a logical landmark in the radio
spectrum.
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 twonearby 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.[3] 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.[4]
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 anumber, 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 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,
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As many observers have pointed out, the Drake equation is a very simple model that
does not include potentially relevant parameters,[13] 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.[14]
Others note that the Drake equation ignores many factors that might be relevant to the
odds of contacting other civilizations. For example, David Brin states: "The Drakeequation 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".[15] 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.[15]
Reincidence
The Drake equation may furthermore be multiplied by how many times an intelligentcivilization 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 f ℓ, that is, how many times life may appear on a
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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 practice 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) tothe classical Drake equation.[16] 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 engag
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 andcompatible receiver systems operational; however, this is a variable humans cannot
accurately estimate.
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:[17][18]
R* = 1/year (1 stars formed per year, on the average over the life of the galaxy;
this was regarded as conservative)
f p = 0.2-0.5 (one fifth to one half of all stars formed will have planets)
ne = 1-5 (stars with planets will have between 1 and 5 planets capable of
developing life)
f ℓ = 1 (100% of these planets will develop life)
f i = 1 (100% of which will develop intelligent life)
f c = 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
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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.[1
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.[20]
Using lowest values in the above estimates (and assuming the Rare Earth hypothesis
implies ne* f l = 10−11, one planet with complex life in the galaxy):
R* = 7/year,[21] f p = 0.4,[22] ne* f l = 10-11, f i = 10-9,[23] f c = 0.1, and L = 304
years[24]
result in
N = 7 × 0.4 × 10-11 × 10-9 × 0.1 × 304 = 8 x 10-20 (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* = 7/year,[21] f p = 1,[25] ne = 0.2,[26][27] f l = 0.13,[28] f i = 1,[29] f c = 0.2[Drake,
above], and L = 109 years[30]
result in
N = 7 × 1 × 0.2 × 0.13 × 1 × 0.2 × 109 = 36.4 million
This has provided popular motivation and some funding for the SETI research.
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This section discusses and attempts to list the best current estimates for the parameters
of the Drake equation.
* = 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.[21]
p = the fraction of those stars that have planets
Recent analysis of Microlensing surveys has found that f p 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 [31][25]
ne
= the average number of planets (satellites may perhaps sometimes be just as
good candidates) that can potentially support life per star that has planets
Marcy et al.[32] note that as of 2005, most observed planets have very eccentric
orbits, or orbit very close to their star where the temperature is too high for Earth
like life. However, several planetary systems that look more Solar System-like ar
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 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 dwarfs might have habitable zones.[33]
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.[34]
NASA's Kepler mission was launched in March 2009. Unlike previous searches,
it has the sensitivity to detect planets as small as Earth, and with orbital periods along as a year. Since it looks at a large sample, about 150,000 stars, the ongoing
(as of 2013) mission should eventually provide a fairly reliable estimate of the
number of planets per star that are found in the habitable zone. Estimates from
partial data suggest that 34% ± 14% of FGK stars are predicted to have at least
one terrestrial, habitable-zone planet,[35] and that at least 5.4% of all stars may
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Even if planets are in the habitable zone, however, the number of planets with the
right proportion of elements is difficult to estimate.[37] 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 ne would be infinitesimally small.
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. In addition, most stars in our galaxy are red dwarfs, which flare violently
mostly in X-rays, a property not conducive to life as we know it. Simulations also
suggest that these bursts erode planetary atmosphere. The possibility of life on
moons of gas giants (such as Jupiter's moon Europa, or Saturn's moon Titan) adds
further uncertainty to this figure.
l = the fraction of the above that actually go on to develop life
Geological evidence from the Earth suggests that f l 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, permitting no valid estimateto be made. If life were to be found on Mars that developed independently from
life on Earth it would imply a value for f l 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 commonorigin. If abiogenesis were more common it would be speculated to have occurre
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.[38
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 i
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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.
c = 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 some efforts covering only a tiny
fraction of the stars that might look for our presence. (See Arecibo message, for
example). There is considerable speculation why a extraterrestrial civilization
might exist but choose not to communicate. However, deliberate communication i
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.[41][42] By this standard, the Earth is a communicating civilization.
L = the expected lifetime of such a civilization for the period that it can communicat
across interstellar space
Michael Shermer estimated L as 420 years, based on the duration of sixty
historical Earthly civilizations.[24] 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 reappearanc
number rather than increase in L, stating that a civilization reappears in the form o
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.[30] He proposes that the last factor L be
replaced with f IC*T , where f IC is the fraction of communicating civilizations
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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 theexperiences of the other.[43]
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 ar
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 succeedingfactor 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 i
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.[44] As Michael Crichton, a science fiction author, stated in a
2003 lecture at Caltech:[45]
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...
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One reply to such criticisms,[46] 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.[47]
Fermi paradox
Main article: 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.[3] 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 foreseeable with our own kind of
technology. Furthermore, no confirmed signs of intelligence elsewhere have beenspotted, either in our galaxy or the more than 80 billion other galaxies of the observabl
universe. According to this line of thinking, the tendency to fill up all available territor
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?".[48][49]
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[50]). Butin 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, , has a low value. The most common suspect is
, but explanations such as the Rare Earth Hypothesis argue that is the small
term.
Intelligent civilizations exist, but we see no evidence, meaning is small. Typica
arguments include that civilizations are too far apart, it is too expensive to spreadthroughout the galaxy, civilizations broadcast signals for only a brief period of
time, it is dangerous to communicate, and many others.
The lifetime of intelligent civilizations is short, meaning the value of 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
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nature of intelligent life to destroy others, they tend to experience a technologicalsingularity, and others.
These lines of reasoning lead to the Great Filter hypothesis,[51] 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 lifetimeof such civilizations must be relatively short.
In fiction and popular culture
Frederik Pohl's 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 civilization
but that they are deliberately ignoring humanity.[52]
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.[53] The invented equation created by Roddenberry is:
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 inthe Star Trek: Voyager episode "Future's End."
The equation is also cited in Michael Crichton's Sphere.
In James A. Michener's novel Space, several of the characters gather to discuss th
equation and ponder its implications.
In the evolution-based 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!"
for-extra-terrestrial-intelligence-has-been-in-existence-in-one-form-or-another-for-several.html). The Daily Galaxy. December 10, 2012. Retrieved 2013-04-15.
21. ^ a b c "Milky Way Churns Out Seven New Stars Per Year, Scientists Say"
33. ^ Dressing, Courtney D and Charbonneau, David (2013). "The Occurrence Rate of Smal
Planets around Small Stars" (http://arxiv.org/pdf/1302.1647.pdf). The Astrophysical
Journal .
34. ^ "The Galactic Habitable Zone and the Age Distribution of Complex Life in the MilkyWay" (http://www.sciencemag.org/content/303/5654/59.full). Science. 2 January 2004.
Retrieved 12 May 2010.
35. ^ Traub, W.A. (2012). "Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler"
(http://arxiv.org/pdf/1109.4682.pdf). The Astrophysical Journal (IOP Publishing) 745: 20
universe/2011/12/29/gIQA2wSOPP_story.html). The Washington Post .
50. ^ Webb, Stephen (2002). If the Universe Is Teeming with Aliens... Where Is Everybody(http://books.google.com/books?id=-vZ0BVSHix4C&printsec=frontcover). Copernicus
Books. ISBN 0-387-95501-1.
51. ^ Robin Hanson (1998). "The Great Filter — Are We Almost Past It?"
(http://hanson.gmu.edu/greatfilter.html).
52. ^ "They're made out of Meat, by Hugo and Nebula Winner Terry Bisson"
(http://baetzler.de/humor/meat_beings.html). Baetzler.de. Retrieved 7 March 2010.
53. ^ The Making of Star Trek by Stephen E. Whitfield and Gene Roddenberry, Ballantine
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