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Drake equation

The Drake equation is a probabilistic argument used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy.

The equation was formulated in 1961 by Frank Drake, not for purposes of quantifying the number of civilizations, but as a way to stimulate scientific dialogue at the first scientific meeting on the search for extraterrestrial intelligence (SETI). The equation summarizes the main concepts which scientists must contemplate when considering the question of other radio-communicative life. It is more properly thought of as an approximation than as a serious attempt to determine a precise number.

Criticism related to the Drake equation focuses not on the equation itself, but on the fact that the estimated values for several of its factors are highly conjectural, the combined multiplicative effect being that the uncertainty associated with any derived value is so large that the equation cannot be used to draw firm conclusions.

Equation
The Drake equation is:

$$N = R_* \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot L$$

where

 * $N$ = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);

and

 * $R_{∗}$ = the average rate of star formation in our Galaxy.
 * $f_{p}$ = the fraction of those stars that have planets.
 * $n_{e}$ = the average number of planets that can potentially support life per star that has planets.
 * $f_{l}$ = the fraction of planets that could support life that actually develop life at some point.
 * $f_{i}$ = the fraction of planets with life that go on to develop intelligent life (civilizations).
 * $f_{c}$ = 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.

History
In September 1959, physicists Giuseppe Cocconi and Philip Morrison published an article in the journal Nature with the provocative title "Searching for Interstellar Communications". Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 cm (1,420.4 MHz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe, and they reasoned that other intelligences might see this as a logical landmark in the radio spectrum.

Two months later, Harvard University astronomy professor Harlow Shapley speculated on the number of inhabited planets in the universe, saying "The universe has 10 million, million, million suns (10 followed by 18 zeros) similar to our own. One in a million has planets around it. Only one in a million million has the right combination of chemicals, temperature, water, days and nights to support planetary life as we know it. This calculation arrives at the estimated figure of 100 million worlds where life has been forged by evolution."

Seven months after Cocconi and Morrison published their article, Drake began searching for extraterrestrial intelligence in an experiment called Project Ozma. It was the first systematic search for signals from communicative extraterrestrial civilizations. Using the 85 foot dish of the National Radio Astronomy Observatory, Green Bank in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti, slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960. The project was well designed, inexpensive, and simple by today's standards. It detected no signals.

Soon thereafter, Drake hosted the first search for extraterrestrial intelligence conference on detecting their radio signals. The meeting was held at the Green Bank facility in 1961. The equation that bears Drake's name arose out of his preparations for the meeting.

"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."

The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio-astronomer Otto Struve. These participants called themselves "The Order of the Dolphin" (because of Lilly's work on dolphin communication), and commemorated their first meeting with a plaque at the observatory hall.

Usefulness
The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life. The last three parameters, $f_{i}$, $f_{c}$, and $L$, are not known and are very difficult to estimate, with values ranging over many orders of magnitude (see ). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere, and gives the question of life elsewhere a basis for scientific analysis. The equation has helped draw attention to some particular scientific problems related to life in the universe, for example abiogenesis, the development of multi-cellular life, and the development of intelligence itself.

Within the limits of existing human technology, any practical search for distant intelligent life must necessarily be a search for some manifestation of a distant technology. After about 50 years, the Drake equation is still of seminal importance because it is a 'road map' of what we need to learn in order to solve this fundamental existential question. It also formed the backbone of astrobiology as a science; although speculation is entertained to give context, astrobiology concerns itself primarily with hypotheses that fit firmly into existing scientific theories. Some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved significantly since the early 1960s. SETI efforts since 1961 have conclusively ruled out widespread alien emissions near the 21 cm wavelength of the hydrogen frequency.

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:
 * $R_{∗}$ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)
 * $f_{p}$ = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)
 * $n_{e}$ = 1 to 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 to 0.2 (10–20% of which will be able to communicate)
 * $L$ = somewhere between 1000 and 100,000,000 years

Inserting the above minimum numbers into the equation gives a minimum N of 20 (see: Range of results). Inserting the maximum numbers gives a maximum of 50,000,000. Drake states that given the uncertainties, the original meeting concluded that $N ≈ L$, and there were probably between 1000 and 100,000,000 planets with civilizations in the Milky Way Galaxy.

Current estimates
This section discusses and attempts to list the best current estimates for the parameters of the Drake equation.

Rate of star creation in this Galaxy, $R_{∗}$
Calculations in 2010, from NASA and the European Space Agency indicate that the rate of star formation in this Galaxy is about of material per year. To get the number of stars per year, we divide this by the initial mass function (IMF) for stars, where the average new star's mass is about. This gives a star formation rate of about 1.5–3 stars per year.

Fraction of those stars that have planets, $f_{p}$
Analysis of microlensing surveys, in 2012, 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.

Average number of planets that might support life per star that has planets, $n_{e}$
In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 11 billion of these estimated planets may be orbiting sun-like stars. Since there are about 100 billion stars in the galaxy, this implies $f_{p} · n_{e}$ is roughly 0.4. The nearest planet in the habitable zone is Proxima Centauri b, which is as close as about 4.2 light-years away.

The consensus at the Green Bank meeting was that $n_{e}$ had a minimum value between 3 and 5. Dutch science journalist Govert Schilling has opined that this is optimistic. Even if planets are in the habitable zone, the number of planets with the right proportion of elements is difficult to estimate. 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.

The discovery of numerous gas giants in close orbit with their stars has introduced doubt that life-supporting planets commonly survive the formation of their stellar systems. So-called hot Jupiters may migrate from distant orbits to near orbits, in the process disrupting the orbits of habitable planets.

On the other hand, the variety of star systems that might have habitable zones is not just limited to solar-type stars and Earth-sized planets. It is now estimated that even tidally locked planets close to red dwarf stars might have habitable zones, although the flaring behavior of these stars might speak against this. The possibility of life on moons of gas giants (such as Jupiter's moon Europa, or Saturn's moons Titan and Enceladus) adds further uncertainty to this figure.

The authors of the rare Earth hypothesis propose a number of additional constraints on habitability for planets, including being in galactic zones with suitably low radiation, high star metallicity, and low enough density to avoid excessive asteroid bombardment. They also propose that it is necessary to have a planetary system with large gas giants which provide bombardment protection without a hot Jupiter; and a planet with plate tectonics, a large moon that creates tidal pools, and moderate axial tilt to generate seasonal variation.

Fraction of the above that actually go on to develop life, $f_{l}$
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, without assuming that the underlying distribution of $f_{l}$ is the same for all planets in the Milky Way, there are zero degrees of freedom, permitting no valid estimates to be made. If life (or evidence of past life) were to be found on Mars, Europa, Enceladus or Titan that developed independently from life on Earth it would imply a value for $f_{l}$ close to 1. While this would raise the number of degrees of freedom from zero to one, there would remain a great deal of uncertainty on any estimate due to the small sample size, and the chance they are not really independent.

Countering this argument is that there is no evidence for abiogenesis occurring more than once on the Earth—that is, all terrestrial life stems from a common origin. If abiogenesis were more common it would be speculated to have occurred more than once on the Earth. Scientists have searched for this by looking for bacteria that are unrelated to other life on Earth, but none have been found yet. 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." As an alternative to abiogenesis on Earth, they proposed the hypothesis of directed panspermia, which states that Earth life began with "microorganisms sent here deliberately by a technological society on another planet, by means of a special long-range unmanned spaceship".

In 2020, a paper by scholars at the University of Nottingham proposed an "Astrobiological Copernican" principle, based on the Principle of Mediocrity, and speculated that "intelligent life would form on other [Earth-like] planets like it has on Earth, so within a few billion years life would automatically form as a natural part of evolution". In the authors' framework, $f_{l}$, $f_{i}$, and $f_{c}$ are all set to a probability of 1 (certainty). Their resultant calculation concludes there are more than thirty current technological civilizations in the galaxy (disregarding error bars).

Fraction of the above that develops intelligent life, $f_{i}$
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 $f_{i}$. Likewise, the Rare Earth hypothesis, notwithstanding their low value for $n_{e}$ above, also think a low value for $f_{i}$ dominates the analysis. Those who favor higher values note the generally increasing complexity of life over time, concluding that the appearance of intelligence is almost inevitable, implying an $f_{i}$ approaching 1. Skeptics point out that the large spread of values in this factor and others make all estimates unreliable. (See Criticism).

In addition, while it appears that life developed soon after the formation of Earth, the Cambrian explosion, in which a large variety of multicellular life forms came into being, occurred a considerable amount of time after the formation of Earth, which suggests the possibility that special conditions were necessary. Some scenarios such as the snowball Earth or research into extinction events have raised the possibility that life on Earth is relatively fragile. Research on any past life on Mars is relevant since a discovery that life did form on Mars but ceased to exist might raise the estimate of $f_{l}$ but would indicate that in half the known cases, intelligent life did not develop.

Estimates of $f_{i}$ have been affected by discoveries that the Solar System's orbit is circular in the galaxy, at such a distance that it remains out of the spiral arms for tens of millions of years (evading radiation from novae). Also, Earth's large moon may aid the evolution of life by stabilizing the planet's axis of rotation.

There has been quantitative work to begin to define $$f_\mathrm{l} \cdot f_\mathrm{i}$$. One example is a Bayesian analysis published in 2020. In the conclusion, the author cautions that this study applies to Earth's conditions. In Bayesian terms, the study favors the formation of intelligence on a planet with identical conditions to Earth but does not do so with high confidence.

Planetary scientist Pascal Lee of the SETI Institute proposes that this fraction is very low (0.0002). He based this estimate on how long it took Earth to develop intelligent life (1 million years since Homo erectus evolved, compared to 4.6 billion years since Earth formed).

Fraction of the above revealing their existence via signal release into space, $f_{c}$
For deliberate communication, the one example we have (the Earth) does not do much explicit communication, though there are some efforts covering only a tiny fraction of the stars that might look for human presence. (See Arecibo message, for example). There is considerable speculation why an extraterrestrial civilization might exist but choose not to communicate. However, deliberate communication is not required, and calculations indicate that current or near-future Earth-level technology might well be detectable to civilizations not too much more advanced than present day humans. By this standard, the Earth is a communicating civilization.

Another question is what percentage of civilizations in the galaxy are close enough for us to detect, assuming that they send out signals. For example, existing Earth radio telescopes could only detect Earth radio transmissions from roughly a light year away.

Lifetime of such a civilization wherein it communicates its signals into space, $L$
Michael Shermer estimated $L$ as 420 years, based on the duration of sixty historical Earthly civilizations. 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. He proposes that the last factor $L$ be replaced with $f_{IC} · T$, where $f_{IC}$ is the fraction of communicating civilizations that become "immortal" (in the sense that they simply do not die out), and $T$ representing the length of time during which this process has been going on. This has the advantage that $T$ would be a relatively easy-to-discover number, as it would simply be some fraction of the age of the universe.

It has also been hypothesized that once a civilization has learned of a more advanced one, its longevity could increase because it can learn from the experiences of the other.

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.

An intelligent civilization might not be organic, as some have suggested that artificial general intelligence may replace humanity.

Range of results
As many skeptics have pointed out, the Drake equation can give a very wide range of values, depending on the assumptions, as the values used in portions of the Drake equation are not well established. In particular, the result can be $N ≪ 1$, meaning we are likely alone in the galaxy, or $N ≫ 1$, implying there are many civilizations we might contact. One of the few points of wide agreement is that the presence of humanity implies a probability of intelligence arising of greater than zero.

As an example of a low estimate, combining NASA's star formation rates, the rare Earth hypothesis value of $f_{p} · n_{e} · f_{l} = 10^{−5}$, Mayr's view on intelligence arising, Drake's view of communication, and Shermer's estimate of lifetime:
 * $R_{∗} = 1.5–3 yr^{−1}$, $f_{p} · n_{e} · f_{l} = 10^{−5}$, $f_{i} = 10^{−9}$, $f_{c} = 0.2$[Drake, above], and $L = 304$ years

gives:

i.e., suggesting that we are probably alone in this galaxy, and possibly in the observable universe.

On the other hand, with larger values for each of the parameters above, values of $N = 1.5 × 10^{−5} × 10^{−9} × 0.2 × 304 = 9.1 × 10^{−13}$ can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters: Use of these parameters gives:
 * $N$, $R_{∗} = 1.5–3 yr^{−1}$, $f_{p} = 1$, $n_{e} = 0.2$, $f_{l} = 0.13$, $f_{i} = 1$[Drake, above], and $f_{c} = 0.2$ years

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.

Possible former technological civilizations
In 2016, Adam Frank and Woodruff Sullivan modified the Drake equation to determine just how unlikely the event of a technological species arising on a given habitable planet must be, to give the result that Earth hosts the only technological species that has ever arisen, for two cases: (a) this Galaxy, and (b) the universe as a whole. By asking this different question, one removes the lifetime and simultaneous communication uncertainties. Since the numbers of habitable planets per star can today be reasonably estimated, the only remaining unknown in the Drake equation is the probability that a habitable planet ever develops a technological species over its lifetime. For Earth to have the only technological species that has ever occurred in the universe, they calculate the probability of any given habitable planet ever developing a technological species must be less than $N$. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than $2.5$ (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.

Modifications
As many observers have pointed out, the Drake equation is a very simple model that omits potentially relevant parameters, 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. Combining the estimates of the original six factors by major researchers via a Monte Carlo procedure leads to a best value for the non-longevity factors of 0.85 1/years. This result differs insignificantly from the estimate of unity given both by Drake and the Cyclops report.

Others note that the Drake equation ignores many concepts that might be relevant to the odds of contacting other civilizations. For example, David Brin states: "The Drake equation merely speaks of the number of sites at which ETIs spontaneously arise. The equation says nothing directly about the contact cross-section between an ETIS and contemporary human society". 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 $1.7$, and establishes additional sites that survive for a lifetime $v$. The result is a more complex set of 3 equations.

 * Reappearance factor : The Drake equation may furthermore be multiplied by how many times an intelligent civilization may occur on planets where it has happened once. Even if an intelligent civilization reaches the end of its lifetime after, for example, 10,000 years, life may still prevail on the planet for billions of years, permitting the next civilization to evolve. Thus, several civilizations may come and go during the lifespan of one and the same planet. Thus, if $L = 10^{9}$ 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 $N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 10^{9} = 15,600,000$, 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 $n_{r}$ may be relatively high. On the other hand, if it is generally by permanent uninhabitability, such as stellar evolution, then $1 + n_{r}$ may be almost zero. In the case of total life extinction, a similar factor may be applicable for $n_{r}$, that is, how many times life may appear on a planet where it has appeared once.

 * METI factor : Alexander Zaitsev said that to be in a communicative phase and emit dedicated messages are not the same. For example, humans, although being in a communicative phase, are not a communicative civilization; we do not practise such activities as the purposeful and regular transmission of interstellar messages. For this reason, he suggested introducing the METI factor (messaging to extraterrestrial intelligence) to the classical Drake equation. He defined the factor as "the fraction of communicative civilizations with clear and non-paranoid planetary consciousness", or alternatively expressed, the fraction of communicative civilizations that actually engage in deliberate interstellar transmission.

 * The METI factor is somewhat misleading since active, purposeful transmission of messages by a civilization is not required for them to receive a broadcast sent by another that is seeking first contact. It is merely required they have capable and compatible receiver systems operational; however, this is a variable humans cannot accurately estimate.

 * Biogenic gases : Astronomer Sara Seager proposed a revised equation that focuses on the search for planets with biosignature gases. These gases are produced by living organisms that can accumulate in a planet atmosphere to levels that can be detected with remote space telescopes.

 * The Seager equation looks like this:
 * $$N = N_* \cdot F_\mathrm{Q} \cdot F_\mathrm{HZ} \cdot F_\mathrm{O} \cdot F_\mathrm{L} \cdot F_\mathrm{S}$$
 * where:
 * $n_{r}$ = the number of planets with detectable signs of life
 * $f_{l}$ = the number of stars observed
 * $N$ = the fraction of stars that are quiet
 * $N_{∗}$ = the fraction of stars with rocky planets in the habitable zone
 * $F_{Q}$ = the fraction of those planets that can be observed
 * $F_{HZ}$ = the fraction that have life
 * $F_{O}$ = the fraction on which life produces a detectable signature gas

 * Seager stresses, "We're not throwing out the Drake Equation, which is really a different topic," explaining, "Since Drake came up with the equation, we have discovered thousands of exoplanets. We as a community have had our views revolutionized as to what could possibly be out there. And now we have a real question on our hands, one that's not related to intelligent life: Can we detect any signs of life in any way in the very near future?"

 * Carl Sagan version of Drake Equation:American astronomer Carl Sagan made some modifications in drake equation and presented it in the program  Cosmos: A Personal Voyage. The modified equation is shown below

$$N = N_\mathrm{*} \cdot f_\mathrm{p} \cdot n_\mathrm{e} \cdot f_\mathrm{l} \cdot f_\mathrm{i} \cdot f_\mathrm{c} \cdot f_\mathrm{L}$$ where

 * $F_{L}$ = the number of civilizations in the Milky Way galaxy with which communication might be possible (i.e. which are on the current past light cone);

and

 * $F_{S}$ = Number of stars in the Milky Way Galaxy
 * $N$ = the fraction of those stars that have planets.
 * $N_{∗}$ = the average number of planets that can potentially support life per star that has planets.
 * $f_{p}$ = the fraction of planets that could support life that actually develop life at some point.
 * $n_{e}$ = the fraction of planets with life that go on to develop intelligent life (civilizations).
 * $f_{l}$ = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space.
 * $f_{i}$ = fraction of a planetary lifetime graced by a technological civilization

Criticism
Criticism of the Drake equation is varied. Firstly, many of the terms in the equation are largely or entirely based on conjecture. Star formation rates are well-known, and the incidence of planets has a sound theoretical and observational basis, but the other terms in the equation become very speculative. The uncertainties revolve around the present day understanding of the evolution of life, intelligence, and civilization, not physics. No statistical estimates are possible for some of the parameters, where only one example is known. The net result is that the equation cannot be used to draw firm conclusions of any kind, and the resulting margin of error is huge, far beyond what some consider acceptable or meaningful.

Others point out that the equation was formulated before our understanding of the universe had matured. Astrophysicist Ethan Siegel, said: "The Drake equation, when it was put forth, made an assumption about the Universe that we now know is untrue: It assumed that the Universe was eternal and static in time. As we learned only a few years after Frank Drake first proposed his equation, the Universe doesn’t exist in a steady state, where it’s unchanging in time, but rather has evolved from a hot, dense, energetic, and rapidly expanding state: a hot Big Bang that occurred over a finite duration in our cosmic past."

One reply to such criticisms 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.

Fermi paradox
A civilization lasting for tens of millions of years could be able to spread throughout the galaxy, even at the slow speeds foreseeable with present day technology. However, no confirmed signs of civilizations or intelligent life elsewhere have been found, either in this Galaxy or in the observable universe of 2 trillion galaxies. According to this line of thinking, the tendency to fill (or at least explore) all available territory seems to be a universal trait of living things, so the Earth should have already been colonized, or at least visited, but no evidence of this exists. Hence Fermi's question "Where is everybody?".

A large number of explanations have been proposed to explain this lack of contact; a book published in 2015 elaborated on 75 different explanations. 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, $f_{c}$, has a low value. The most common suspect is $f_{L}$, but explanations such as the rare Earth hypothesis argue that $z < 8$ is the small term.
 * Intelligent civilizations exist, but we see no evidence, meaning $R_{∗} · f_{p} · n_{e} · f_{l} · f_{i}$ is small. Typical arguments include that civilizations are too far apart, it is too expensive to spread throughout the galaxy, civilizations broadcast signals for only a brief period of time, communication is dangerous, and many others.
 * The lifetime of intelligent, communicative civilizations is short, meaning the value of $L$ is small. Drake suggested that a large number of extraterrestrial civilizations would form, and he further speculated that the lack of evidence of such civilizations may be because technological civilizations tend to disappear rather quickly. Typical explanations include it is the nature of intelligent life to destroy itself, it is the nature of intelligent life to destroy others, they tend to be destroyed by natural events, and others.

These lines of reasoning lead to the Great Filter hypothesis, which states that since there are no observed extraterrestrial civilizations despite the vast number of stars, at least one step in the process must be acting as a filter to reduce the final value. According to this view, either it is very difficult for intelligent life to arise, or the lifetime of technologically advanced civilizations, or the period of time they reveal their existence must be relatively short.

An analysis by Anders Sandberg, Eric Drexler and Toby Ord suggests "a substantial ex ante probability of there being no other intelligent life in our observable universe".

In fiction and popular culture

The equation was cited by Gene Roddenberry as supporting the multiplicity of inhabited planets shown on Star Trek, the television series he created. However, Roddenberry did not have the equation with him, and he was forced to "invent" it for his original proposal. The invented equation created by Roddenberry is:

$$Ff^2 (MgE)-C^1 Ri^1 \cdot M=L/So $$

Regarding Roddenberry's fictional version of the equation, Drake himself commented that a number raised to the first power is just the number itself.

Commemorative plate on NASA's Europa Clipper mission, planned for launch in 2024, features a poem by the U.S. Poet Laureate Ada Limón, waveforms of the word 'water' in 103 languages, the Drake Equation, and a portrait of planetary scientist Ron Greeley on it.

The track Abiogenesis on the Carbon Based Lifeforms album World of Sleepers features the Drake equation in a spoken voice-over.

title: Drake equation


text:
Talk:Drake equation/Archive 1

Drake vs. Koran - Finding the Other Six Earths in the Universe As Mentioned in Koran
Koran says that God has created seven Earths, like the seven Firmaments He has created, one we live on & the rest of the six are unknown to us as to where they are. As per the Prophetic Hadith each Earth is inhabited by a human civilization & each civilization is accountable to God for their deeds as we are. As per the Hadith each has its Adam (ie. the first of a progeny on a planet who was created by God's Hand) & its Mohammad (ie., the last of God's Messenger to each Adam's progeny to guide them to His Paradise in the Hereafter) so in fact God created seven Adams & sent seven Mohammads & all the progenies of seven Earths will be together on the Plain of Accounting on the Day of Resurrection. God has revealed to us in Koran that: [{سورة الطلاق}*Q:65('The Divorce' or 'AtTalaq'):12) in which God says: (({It is}Alllah who created seven Firmaments & from the Earth likewise...,God's command descends between them...))((الله الذي خلق سبع سموات و من الأرض مثلهن... {يتنزل الأمر بينهن ...} لتعلموا أن الله على كلّ شئ قدير) & God says that He is telling us all this in Koran: (...that we may know that verily God is capable on {doing} everything) & (و أن الله قد أحاط بكل شئ علما) (ie. And that verily God encompasses {or knows} everything) ie. Seven Earths, on one we live, while the rest of the six, though unknown to humanity as to where they are, are known to God as to where they are, & as earlier stated as per Prophetic Hadith each Earth is populated with humankind & each has its Adam & its Mohammad & as earlier stated so in fact God created seven Adams & seven Mohammads & as earlier stated all the progenies of seven Earths will be together on the Plain of Accounting on the Day of Resurrection, for the successful to enter God's Paradise for Eternity.Ilaila (talk) 08:56, 23 January 2010 (UTC)

Question
I'm trying to figure out what this means, and unfortunately my understanding of stats is weak. Can someone break this down for me and others?

When at the end, the current estimates plugged into the Drake equation give an N of 2.31 -- that's, as I understand it, saying that odds are there are 2.31 possibly-communicating civilizations in the Milky Way. Now:

Does that include us? In other words, does it mean that besides us there are probably 1.31 others? Or is the sample size so big that we haven't scratched the surface, and we're talking about 2.31 *more*?

I'm not taking the Drake equation as infallible, I'm just trying to understand what it means. -- Please help! 71.101.53.18 (talk)
 * N is an estimate of EXTRA-terrestrial (ie. alien) civilizations. But the result of 2.31 is far outweighed by the uncertainty factors in the parameters -- something which the article should mention more clearly. Even the order of magnitude of N is unknown. See Fuzziness Ropata (talk) 00:35, 25 May 2009 (UTC)
 * Thanks! -- that makes sense. I agree, that for a moderately-educated layman like me, conveying the range would definitely help understanding (if it's 2.31 +/- 1, that's a different animal than 2.31 +/- 20 or 3,000!) -- isn't there something about not reporting more digits than your sig figs allow for? (71.101.53.18 (talk) 03:41, 25 May 2009 (UTC))

Typo? Close to novea
At discussing fl (the fraction of the above which actually go on to develop life) it says: "Lineweaver has also determined that about 10% of star systems in the Galaxy are hospitable to life, by having heavy elements, being far from supernovae and being stable themselves for sufficient time."

I gues that should be "by being close to supernovae"?

I'm not an expert in astronomy, but what it says now does not sound logical to me. Or somebody explain me the mechanism. ;-)

For the rest: impressive article!

Arjan202 (talk) 11:06, 26 June 2008 (UTC)

Rare Earth refutation
This really begs to be contrasted with Ward and Brownlee's Rare Earth hypothesis, which many believes lays waste to the Drake Equation. Any thoughts?

 * Surely the Rare Earth hypothesis does not contradict the Drake eqn, but merely says that some of the parameters (notably fi) are a lot lower than originally thought. 80.3.112.87 22:56, 6 May 2007 (UTC)

 * Yes, it's sillly to say that the "Rare Earth" hypothesis "refutes" the Drake equation. What it does it does is to make a set of proposals for what the terms in the Drake equation are, and carry the calculation through to say the answer is that the expected number of other civilizations is <<1.  The Rare Earth hypothesis doesn't even make sense without the Drake equation; it takes the Drake equation as a starting point.

Geoffrey.landis 15:56, 30 May 2007 (UTC)

Drake overly optimistic?
What about the possibility that even if an alien civilization did exist, from what they could gather of our culture they wouldn't want to communicate with us? Was Drake being overly optimistic about us?

Why do you say that the currently observed value of N = 0? What are we, chopped liver? (Or are you saying that we are not able to communicate interstellarly?)

Does "fairly conservative" f_l, f_i, f_c and L has any meaning ? --Taw

 * f_l, f_i and L are the really contentious ones, I think. We can make a fair stab at guesstimating R* and f_p now, given recent advances in astronomy. -- The Anome

-

Existence of human race now acknowledged. -- The Anome

Claim that f_l = 1 is weird. In "planets which can potentially support life" in Solar System they usually list Venus, Earth and Mars. So f_l here is 1/3. Unless they say that only Earth is capable of supporting life. In that case n_e is way too high.

 * This whole thing is rather fuzzy and ill-defined, IMO. For example, if I were listing "planets which can potentially support life" for the solar system, Venus would emphatically _not_ be included. On the other hand, Europa, Callisto, and maybe Ganymede would be on there; these three moons are thought to have liquid water oceans under their crusts (Ganymede is the iffiest, Europa is almost certain). Uranus and Neptune may also have liquid water layers, though even if they do conditions there would be somewhat extreme for life as we know it and may not be suitable. Basically, IMO, we simply don't know enough about the universe yet to be proposing actual numbers for some of these variables yet. -BD

We certainly don't know enough to produce good numbers, but even guesses within a few orders of magnitude are better than nothing. It doesn't take much accuracy to make certain theories more or less likely than others, or to present interesting problems. In particular, the estimates given as "conservative" are sufficient to generate the Fermi paradox; if our guesses are two low, it's even more remarkable; if our guesses are way too high, Fremi becomes expected but then the low numbers themselves are hard to explain. --LDC

 * If there are a couple of orders of magnitude of flexibility to a couple of the terms in an equation like this, then I don't see how it can be reasonably used to suggest _anything_ with enough certainty to base a serious discussion on. I see the Drake equation as more of a suggestion of what we should be trying to find out in the future than as a useful analysis of life in the galaxy at this point, and wanted to make sure the article didn't give the wrong impression (ie, that "scientists say there should be aliens everywhere" or that "scientists say we're alone in the universe").

So what exactly is the Fermi paradox? The estimates for the parameters given in the article yield N=0.01, which doesn't seem to be excessively high. --AxelBoldt

See my examples now in the article -- The Anome

Being new here, I won't take the privledge of removing this particular statement, but I want to alert you that in all of my studies of genetics and molecular biology, I have never encountered the suggestion that outside mutagens (such as UV, as stated in this article) are necessary to produce the genetic variation that alows organisms to adapt to changing environments. I have only encoutered this idea in science fiction books from the 1950s or so (Asimov and Heinlein), and I think that this was just a bunch of speculation from biolgists of the time who had no idea of the mechanism of how mutations occur.

We now know that mutations occur by changes in the DNA sequence. We know that DNA must constantly be copied, and that any copying device has some error rate. We know that there are multiple DNA polymerases (copying enzymes) each with its own characteristic error rate. The error rate is itself an evolvable trait, even as it affects evolution. If it is necessary to have mutations for long term survival of a lineage (which I believe it is), then any lineage that ceases to have mutations will die out a la natural selection, and only the ones that have mutations will persist. Thus, even this trait (mutation rate) will acquire a sort of optimum, just as other traits do, and it will be adapted based upon the lifestyle of the organism and the environment that it lives in.

adam

 * This may be based on a younger hypothesis that Sol's UV was the driving force behind the start of life, but I think that's just as discredited now. In any case, it's not cited, I added a "citation needed". Darekun 02:28, 18 October 2007 (UTC)

Too bad it probably won't be on the actuarial exam. It has a nice mnemonic to it, just think "pelican".

F_l=1 is quite optimistic, since it means every planet that can develop life does. -Padde 00:38, 15 April 2006 (UTC)

 * That seems to be based on the observation that as soon as Earth could develop earthlike life, it did; cases like Venus and Mars seem to be covered by the low n_e. Darekun 02:28, 18 October 2007 (UTC)

Removed paragraph
Removed this paragraph: It's a very common misconception often spread by the popular media that the evidence supports the value of N equal to 1 - there's one human civilization in the galaxy. But the galaxy is just an arbitrary probed area - we could as well probe each two galaxies, each 100 galaxies, or each one thousandth of a galaxy - and the conclusion should be the same number of such civilizations per galaxy. According to this fallacious thinking, the evidence would have supported N equal to one half, one hundredth, and one thousand as well as exactly 1.

 * 1) On the contrary, the popular media gives the impression that N is equal to hundreds, if not thousands of civilizations in our galaxy (Star Trek, Babylon 5, Star Wars, many sci fi movies and shows like the X-Files, stories of UFO and government conspiracies to cover up the "truth"... etc...).
 * 2) The rest doesn't make sense due to the fact that we know of at least one civilization. Anything beyond that is speculation based on the equation here (some parameters we do know better than others). And if there is only one civilization between two galaxies (a lonely thought) that would give an N = 0.5 for each of those galaxies. There is nothing fallacious about that - it is just probability. I suggest a read of anthropic principle. --mav

This probably has changes some of the numbers, but not sure how. http://news.bbc.co.uk/1/hi/sci/tech/3588721.stm

Edward 12:23, 2 Apr 2004 (UTC)

Is there any source discussing what Drake meant by an estimate of 10 years as the value of L? Does such a pessimistic view reflect the fact that he was living during the Cold War, or did he think that in general a civilization that develops interplanetary communications will wipe itself out within 10 years? I mean, it took us humans decades to go from understanding light and using radio, to detonating a nuclear weapon.
 * Yes, I think it's at least generally assumed that he was thinking of nuclear annihilation. BTW we actually detonated a nuclear weapon before the development of our first radio system that had any reasonable chance of communicating with an interstellar civilisation (Trinity test in 1945, Jodrell Bank opened in 1947). Securiger 12:12, 4 Nov 2004 (UTC)

Fuzziness
I do not think that our article adequately conveys the "fuzziness". As BD and someone else remarked above, the basic problem, is that for several of the parameters we really have no idea, or at least the range of plausible estimates varies by several orders of magnitude. And when you multiply variations of orders of magnitude several times, you end up with a huge range. This is occasionally hinted at in the article, but is never really made clear. Rather than listing several people's estimates and results, would it not be more useful to multiply out the ranges of estimates, to show the range of the results? (This is easier if we work in orders of magnitude - OoM being logarithmic we could then add instead of multiply - but that might lose some readers.) An example to explain what I mean:

All of these parameters were taken from either our own article, or one of the ones in our extlinks. But I make no claim they are comprehensive, just an example. Securiger 12:12, 4 Nov 2004 (UTC)

(Is this table a comment on the section 'Current estimates of the parameters' in the main article? One problem with that section is no 'error bars' are quoted for the factors, there must be someone who has estimated the uncertainty in all the factors? Seems Sagan did this several times back in the early days of the Drake Equation.) —Preceding unsigned comment added by Albert.a.jackson (talk • contribs) 14:39, 10 December 2007 (UTC)

Effects of personal bias
As an aside, having discussed this with quite a few people over the years, and read a few learned monographs on it, I have come to the conclusion that most people make up their minds beforehand - perhaps either wanting to believe that human life is especially unique, or wanting to believe that alien visitation is just a few years away - and then choose parameter ranges that satisfy those desires. The Drake equation tends to add excessive dignity to this process of biased, wild guessing. Really for at least four of those parameters, the only intellectually honest estimate is "we have no idea", which means the final answer is "we have no idea". Not very satisfactory, but true nonetheless. Securiger 12:12, 4 Nov 2004 (UTC)

(I totally agree with the statement above, but I think Drake would too, the equation was really only a 'talking point' and he has stated such himself. Some of the factors have almost infinite standard deviations. ) —Preceding unsigned comment added by Aajacksoniv (talk • contribs) 01:00, 12 December 2007 (UTC)

Anthropics and stuff
Another point whilst I'm rambling on; our statement:
 * What evidence is currently visible to humanity suggests that fl is very high; life on Earth appears to have begun almost immediately after conditions arrived in which it was possible, suggesting that abiogenesis is relatively "easy" once conditions are right.

suffers from the Self-Sampling Assumption. We can in fact deduce nothing about fl from our own presence on earth, since our own presence somewhere is a given, or else we would not be making the observation in the first place. For example, suppose that the true value of fl is so extremely low that there probably exists only one planet in the Universe with life on it. Evidently then, we the observers are on that planet, and we know of one civilization (us) and no others, because there are no others. Now consider a parallel universe where fl is billions of times higher, but, for some reason, no contact has been established. Any given group of observers knows of one civilisation (themselves) and no others, for reasons unknown to them. The observations are identical in both cases; by observing the existence of your own civilisation, you can deduce nothing at all about fl. To cut a long story short, in estimating any of these parameters, you can't count yourself. (The exception is L; since it doesn't affect our ability to observe ourselves, we can use ourselves as an estimator for L. I think.)

 * Actually, not an exception. The L estimate will never stop increasing, because if it does, no one is left to make that observation.

The best that we can do is observe that there is another planet nearby where maybe life could have appeared, but apparently didn't, so fl is certainly less than 1. Note incidentally, that this is already implicitly taken care of in the definition of N, which is for extraterrestrial civilisations only; N does not include our own civ. Securiger 12:12, 4 Nov 2004 (UTC)

 * Another way of looking at the question is to ask how many degrees of freedom we have in estimating each parameter. For many of them, we have zero degrees of freedom, since we only have the one sample (ourselves).  Thus, no valid estimate can be made, just various wild-ass guesses.Dogface 20:04, 28 July 2006 (UTC)

What is N << 1 ?
What is this terminology used in the article? N << 1

How does it differ from N < 1 ?

 * N < 1 means N is less than 1 (possibly by an infinitesimal amount); N << 1 means N is much less than 1. Exactly how much "much less" means is a bit vague and usually determined by context. In some contexts, it means "so much less that within the limits of accuracy of our calculation, we can ignore it". In other contexts it can be taken to mean an order of magnitude less. Securiger 18:27, 15 Nov 2004 (UTC)

What you are saying is it should be

N approx zero, an infinitesimal. BernardZ 05:47, 6 March 2007 (UTC)

2?
for N not >>1, there should be some discussion of the conditional probability of the presence of a second civilization, given the presence of one (i.e. we know we are here, but we are interested if there is anyone else). dab (ᛏ) 14:52, 13 Jan 2005 (UTC)

 * I imagine that if N not >> 1 then it is independent of the number of civilizations observed. Just because our star system came up "heads", it wouldn't affect the chances of the other star systems. The only way I can conceive of it not being independent is the chance for intergalactic war between civilizations, and I don't remember us engaging in any genocidal space campaigns =P Intangir 08:32, 13 November 2005 (UTC)

However it probably would be argued to being a Poissonic distribution. Low probabilities and discrete values.

If we assume that the average number we can expect = X Then the probability is PR(N) =(exp(-x) * X^N)/N!

therefore the probability of no ETI life is Pr(0)=exp(-X)

therefore the probability of one ETI life Pr(1)=exp(-X)*X = X*Pr(0)

So we can say that Pr(N>0) = 1-Pr(0) Pr(N>1) = 1-Pr(0)-X*Pr(0)

You could say using the Anthropic principal that the Pr(N>0) could be anything say an infinitesimal as there would be no discussion on this question if N=0.

So what are the odds that some else is there. The odds that there is someone else is

Pr(X>1)/Pr(X>0)= 1- X/(1/Pr(0)-1) = 1 - X /(Exp(X)-1)

Of course I am simplifying it a lot. Another issue is as the universe gets older, I would expect the X to go up over time. BernardZ 23:57, 20 February 2007 (UTC)

Michael Crichton on the Drake equation???
Removed the following from the article:

Michael Crichton denounced the Drake Equation as pseudoscience in the Caltech Michelin Lecture entitled "Aliens Cause Global Warming" (meant to be a serious lecture with a catchy title), 17 January 2003:

 * Cast your minds back to 1960. John F. Kennedy is president, commercial jet airplanes are just appearing, the biggest university mainframes have 12K of memory. And in Green Bank, West Virginia at the new National Radio Astronomy Observatory, a young astrophysicist named Frank Drake runs a two week project called Ozma, to search for extraterrestrial signals. A signal is received, to great excitement. It turns out to be false, but the excitement remains. In 1960, Drake organizes the first SETI conference, and came up with the now-famous Drake equation:

 * N=N*fp ne fl fi fc fL

 * Where N* is the number of stars in the Milky Way galaxy; fp is the fraction with planets; ne is the number of planets per star capable of supporting life; fl is the fraction of planets where life evolves; fi is the fraction where intelligent life evolves; and fc is the fraction that communicates; and fL is the fraction of the planet's life during which the communicating civilizations live.

 * This serious-looking equation gave SETI an serious footing as a legitimate intellectual inquiry. 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. And guesses-just so we're clear-are merely expressions of prejudice. Nor can there be "informed guesses." If you need to state how many planets with life choose to communicate, there is simply no way to make an informed guess. It's simply prejudice.

 * 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, and has nothing to do with science. I take the hard view that science involves the creation of testable hypotheses. The Drake equation cannot be tested and therefore SETI is not science. SETI is unquestionably a religion. Faith is defined as the firm belief in something for which there is no proof. The belief that the Koran is the word of God is a matter of faith. The belief that God created the universe in seven days is a matter of faith. The belief that there are other life forms in the universe is a matter of faith. There is not a single shred of evidence for any other life forms, and in forty years of searching, none has been discovered. There is absolutely no evidentiary reason to maintain this belief. SETI is a religion.

 * One way to chart the cooling of enthusiasm is to review popular works on the subject. In 1964, at the height of SETI enthusiasm, Walter Sullivan of the NY Times wrote an exciting book about life in the universe entitled WE ARE NOT ALONE. By 1995, when Paul Davis wrote a book on the same subject, he titled it ARE WE ALONE? ( Since 1981, there have in fact been four books titled ARE WE ALONE.) More recently we have seen the rise of the so-called "Rare Earth" theory which suggests that we may, in fact, be all alone. Again, there is no evidence either way.

 * Back in the sixties, SETI had its critics, although not among astrophysicists and astronomers. The biologists and paleontologists were harshest. George Gaylord Simpson of Harvard sneered that SETI was a "study without a subject," and it remains so to the present day.

 * But scientists in general have been indulgent toward SETI, viewing it either with bemused tolerance, or with indifference. After all, what's the big deal? It's kind of fun. If people want to look, let them. Only a curmudgeon would speak harshly of SETI. It wasn't worth the bother.

 * And of course it is true that untestable theories may have heuristic value. Of course extraterrestrials are a good way to teach science to kids. But that does not relieve us of the obligation to see the Drake equation clearly for what it is-pure speculation in quasi-scientific trappings.

 * The fact that the Drake equation was not greeted with screams of outrage-similar to the screams of outrage that greet each Creationist new claim, for example-meant that now there was a crack in the door, a loosening of the definition of what constituted legitimate scientific procedure. And soon enough, pernicious garbage began to squeeze through the cracks.

Comment: Crichton is a film-maker and a thriller writer, do his comments really add to this article? In any case, he fails to understand the concept of falsifiability. The Drake equation is not the issue, the value of N is. The Drake equation is just a back-of-envelope calculation made about the Fermi paradox. Falsifiable hypotheses can be made by setting bounds on the value of N, and making observations that can falsify these hypotheses. For example, just just one otherwise-unexplainable signal (prime numbered pulses, for example) recieved by SETI@home would do the job of setting a lowerbound of N=2. Or the sighting of some Dyson spheres, or other examples of engineering capable of being seen from lightyears away. Alternatively, you could go round the galaxy, enumerating civilizations, a task which whilst lengthy (a few hundred thousand years) is entirely physically possible, given von Neumann machines and a lot of civilizational patience. -- The Anome 14:13, May 21, 2005 (UTC)

 * I agree, Crichton is way off the mark here: he appears to think that DE is same thing as e.g. the law of gravity. DE is used for estimation purposes, it is not a physical law. If we knew the real value of N, we wouldn't need DE, would we? Moreover, when he says that DE is not scientific, he is again off the mark, because it exactly follows the scientific approach to cracking hard problems, which is dividing them into smaller subproblems, separating well-defined ones from ill-defined ones along the way. GregorB 17:35, May 30, 2005 (UTC)

 * Wow - just lost a lot of respect for ol' Crichton. I'm not so concerned about the Drake equation - I agree with the substance of what Crichton says - it is a little silly, but as GregorB says, it wasn't meant as a law, just an attempt at an estimate.  However, that attack on SETI is out of the ballpark ridiculous.  His claim that "SETI is a religion" just doesn't make any sense.  If SETI was a group of people who dogmatically believed in the existence of extraterrestrials, then they would be a religion.  However last I checked, they were searching for extraterrestrial life, not assuming its existence.  I see no distinction in that respect between SETI and particle physicists.  No one would reasonably claim that particle physics is a religion because it involves searching for new particles that aren't known to exist!  Religious groups don't spent millions on equipment to search for evidence of their beliefs - they just believe them.  How did he miss that?  Sorry, this doesn't have much to do with the article, but oh well.  --Bmk 04:47, 7 July 2006 (UTC)

A new number for Ne could be 4 being that in our star system there are 2 planets and 2 moons which might develope life. They are Earth, Mars, and the moons Europa and Titan which may be developing life. Theoretically counting only ours as a primary source because we don't know how many planets are actually out there that are capable of creating life let alone sustaining it. Concluding the fact that the majority of the stars in the Milky Way are red dwarfs (which cannot have a planet around it because it would erode away any atmosphere on a planet). We took most of Drake's original numbers and added a few of our own and got N=8 so we think there are only 8 planets out there like ours that could communicate with us... comments please!!! need help with this one...!!!!

WP:NPOV states that various views must be represented fairly, proportionately and without bias. Crichton is not alone in his perception. I encountered this equation in a philosophy class in which it was labelled a pseudo-science. Shawnc 07:09, 17 December 2006 (UTC)

Nitpick: lower bound on L
I changed the following from the article:
 * A lower bound on L can be estimated from the lifetime of our current civilization....

I don't think that's estimating a lower bound, because our civilization might have a longer lifetime than the average (expected) lifetime. Maybe "pessimistic value of L" would be more accurate? Wmahan. 05:53, 2005 Jun 15 (UTC)
 * I think it would make a decent lower bound in that we are not dead yet and do not expect to be dead next year. Our technology is futher expected to outlive us (at least in detectble terms).  But throwing the value of L away I think the deravation of the equation with respect to R* and L needs a bit more fleshing out for the less geek of readers.  The stated goal of counting civlazations which we should be able to detect would seem at first anyway to call for a count of stars of the proper type at the proper distance.  The rest of the equation is pretty simple as its all fractions of fractions of fractions, but I think the years aspect of the equationneeds further explination in the article. Dalf | Talk 06:43, 14 October 2005 (UTC)

 * Yes, but if we use Earth's civilization to estimate L, that's not much of a sample space. The paragraph currently in the article: "The value of L can be estimated from the lifetime of our current civilization from the advent of radio astronomy in 1938 (dated from Grote Reber's parabolic dish radio telescope) to the current date. In 2007, this gives an L of 69 years." is absurd, and should read "at least 69 years" or "69 years and counting". Moreover, the number 69 has no use in an example of the equation, any more than producing the equation with all other variable values estimated based on this one-sample (our solar system) empirical method, which would yield (using the equally absurd 69 for L): fp = 1.0, ne = 1.0 (minimum), fl = 1.0, fi = 1.0, fc = 1.0 and thus: N = 6 × 1.0 × 1.0 × 1.0 × 1.0 × 1.0 × 69 = 414.  I suggest either removing the paragraph concluding that using 69 is logical for L (and the 0.0000001 conclusion for N), or, add my above example of the equation, showing that using our solar system and present history (the extremely pessimistic value 69 for L), the number for N is at least 414.   Anybody's thoughts?  Gekritzl 22:11, 23 June 2007 (UTC)

Different version of the equation
It seems to me that the equation with "R* is the rate of star formation in our galaxy" gives an estimate of the number of civilizations that come on line each tear.

The astronomy book "Horizons: Exploring the Universe", by Michael Seeds gives a different version of the equation:

 * N = N* x f_p x n_LZ x f_L x f_i x F_S (6 terms instead of 7)

where

 * N = number of communicative civilizations in the galaxy

 * N* = # of stars in the galaxy - NOT the number of new stars/year

 * f_p = fraction of stars with planets

 * n_LZ = # planets of star in life zone

 * f_i = fraction that evolve intelligence

 * f_L = fraction of planets suitable for life

 * F_S = fraction of star's life that has a technoligal civilization

Needless to say, with the number of stars in the galaxy in there instead of the number of new stars/year, the result is much larger. He gives the pessimistic result 0.00002 and the optimistic 10,000,000.

Also, another point - f_i is normally limited to <= 1, i.e. at most one intelligent per planet. But there could be more than one per planet. Earth could evolve other intelligent species later. Bubba73 04:36, 22 Jun 2005 (UTC)

P.S. On second thoght, I guss the two versions are pretty much equivalent. One thing, near the end of the article for the current estimates, in the body it lists f_i as 0.01 but 1E-7 is used in the calculations.

Also, it should be noted that the number of intelligent civilizations is extimated to be considerably higher than the number that we can communicate with. Bubba73 04:54, 22 Jun 2005 (UTC)

 * I agree with the point that it's useful to also post the verion of the equation using N* instead of R* as the initial variable in the equation. In fact, I already made this addition; I'll go back and add your reference.

I'll quibble with your point that "with the number of stars in the galaxy in there instead of the number of new stars/year the result is much larger". It's not larger, because making this change means you have to divide the lifetime of the civilization by the lifetime of the galaxy, so the end result comes out the same. Geoffrey.landis 19:18, 30 May 2007 (UTC)

What is N, really?
Should N be the number of communicative civilizations out there (ie right now)?

As currently phrased in the article, the equation instead gives the rate at which new stars are forming which will someday house communicative civilizations times the average length of the civilizations. Of course, these civilizations take time to blossom, that the equation does not take into account the age of the galaxy should be a clear indicator that N is not the number of communicative extraterrestrial civilizations in our galaxy right now (this follows from the observation that a relatively young galaxy with the same rate of star formation would be treated identically to our own). It seems the our version of the equation somehow misrepresents the equation. Either what we are representing N as meaning should be changed, or some of what the terms represent need to be changed, though it would seem to be the former, judging from the above post. Of course we aren't the first to make this mistake, it seems to be everywhere so...

...maybe I'm wrong. Maybe there is some analysis (which is missing from the article) which shows that the age of the galaxy is somehow already taken into account. Or maybe the Drake Equation relies on the assumption of a relatively 'steady-state'(ie all terms in the equation have been the same for billions of years) galaxy, much older than the average time it takes for civilizations to evolve (i think that only in such a case would the rate*length = current number of civilizations).

...or maybe this is a valid criticism of the equation which others somewhere have pointed out. (ie the Fermi Paradox solution which posits that we are among the earliest civilizations).

Anyone know which it is? Intangir 06:17, 13 November 2005 (UTC)

I think the Current Estimates Section Should be removed
The current estimates section should be removed or at least edited to indicate that it's mostly baloney. To the reader, it seems to indicate that current evidence suggests those values are more or less correct while in fact they just certain people's opinions of what the values ought to be. In science, there are not supposed to be experts and the opinion of one person should be given no more weight than the opinion of any other. As the links at the bottom of the document and discussion in this section show, there is wide spread disagreement over the values.

Also, I have taken several astro classes in which the Drake Equation is discussed and never heard claimed values that remotely approach for resemble those $$f_i$$. The claim neither gives a citation for the figure or explains why radiation dosage must be as low as Earth. Indeed, much of the experimentation regarding the formation of life seems to indicate that higher doses of certain sorts of radiation may actually encourage the formation of life.

Many astronomers, Frank Drake included, seem to believe that we really don't have any good ideas of any of the values other than R*. With future space missions such as Kepler and TPF we may get a better idea for $$f_p$$ and $$n_e$$. Everything else is guesswork, not science, and it shouldn't be presented like it is.

 * How about ranges for values instead? Are there papers that say things like "well, the laws of physics say it can't be less than X, and if it were more than Y then the stars at night would outshine the moon"?  Otherwise, I think consensus figures are a good idea, as long as they have an adequate disclaimer.  --noösfractal 17:32, 24 August 2005 (UTC)

 * Ranges are even worse unless we are going to put what the actual ranges are given the unknowns (in which case the ranges will be so large 0-1 for example as to only confuse the matter). I think a discussion of what sort of considerations are made when thinking about each number is worthwhile.  I think doing an example where the numbers used are called out as being guesses (or perhaps two examples one that generates a really high value and oen a really low).  Using talking about the numbers Drake used originall is also ok since that is historically accurate and not claiming any values for the number but simply stating the ones that Drake used.  Dalf | Talk 06:47, 14 October 2005 (UTC)

I agree with the original poster, this section is absolute bollocks. If anything, current estimations place N above 10,000.

The estimates give no rational for their values. Shouldn't they be based on hard data and give the expected error or confidence interval or something like that. For instance with the discovery of Gliese_581_c and the information in Extrasolar_planet we now have earth, mars, venus, and Gliese 581c as the only known possibly habitable planets. (venus and mars are a stretch, and Gliese 581c has too little data to make a call on but it is a possible life supporting planet.) out of 229 extrasolar planets + 8 solar planets = an experimental ratio of 4 / 237 habitable planets per known planet. Similarly the current experimental value for the ratio of stars containing habitable planets is 2(sun and Gliese) / the number of stars. We also have an experimental value for the number of stars that contain planets(given current detection methods). I took some stats classes, but am more than a bit rusty, and of course I suspect that our current sampling method is no where near random but a trained statistician could probably come up with some numbers for the experimental means, standard deviations, errors, and the current confidence level of our testing method. I'm sure the errors would be huge(given such small sample sizes and non random sampling methods) but it would be nice to get some real statistics behind these estimations. Would anyone be willing to write down some math that would give estimates of the values for which we have some data? Anytime the experimental data gets updated the math can be rerun to give a more current estimate. --Dave1g 07:05, 28 April 2007 (UTC)

Possibility of other life
Has the possibility that non-intelligent ET life-forms exist been documented? That would be M = N * (1 / L) * (1 / fc) * (1 / fi). We know that N >= 1, so M must be very high. I guess a flaw with this is observer bias, i.e. N could be less than 1. Neurodivergent 19:27, 11 January 2006 (UTC)

Value of L
Doesn't the value of L (67 years) assume we'll go extinct tomorrow? L should be based on the life expectancy of the species, assuming we'll keep using this technology. The doomsday argument might be helpful, although flawed for various reasons. Neurodivergent 19:41, 11 January 2006 (UTC)

I have many problems with this equation.
 * 1) "fc is the fraction of the above which are willing and able to communicate". These should be broken into seperate parts. The ablity to communicate and the willingness to do so are not congruent.
 * 2) "L is the expected lifetime of such a civilization". Since it's been pointed out that technology and knowledge go with a civilization this should be changed to "expected evolutionary lifetime of a sentient species"(eg: The Roman Empire is long gone but much of their technology, culture, and knowledge is alive and well in almost all current cultures)
 * 3) It doesnt take into account many of the smaller events here on Earth that may have led to the formation of life. Such as, the Earth's magnetic core. As well as, though probably less important, the moon's affects and the rate of rotation.

"(Note, however, that in the year 2001 a value of 50 for L can be used with exactly the same degree of confidence that Drake had in using 10 in the year 1961.)"

Is this supposed to mean 10,000 or 50,000? It doesn't make any sense if not... 82.32.65.149 10:06, 10 May 2006 (UTC)

The article is about a historical equation created by one person, right? Is there a place to argue about the details of what makes up the equation? ADyuaa 23:17, 8 January 2007 (UTC)

I don't understand why the value of L is the number of years that a civilization is capable of radio communication. Wouldn't it make far more sense that L would be the proportion of the length of the life of a star that a civilization is in radio communication mode. Thus if the average life of a star is 10 billion years and the average life of radio communication civiliations is 10,000 years (making us a very, very young civilization), the odds of any given star at a given time (assuming 100% develop radio civilizations) being a radio transmitter is 1 in 1,000,000 (10,000 divided by 10 billion). Given that the other factors in Drake are less then one and that there only 600 million stars with 5,000 light years and 10,000 looks highly optomistic as the length of a civilization and the fact that SETI has not found anybody is not very surprising. TedA 07:09, 18 August 2007 (UTC)

 * The units don't work out for that - given the other terms, it needs a time unit to produce a number of civilizations. However, it now says "L is the length of time such civilizations release detectable signals into space.", which works. - Darekun 03:29, 18 October 2007 (UTC)

I haven't seen anything about the end of L being due to using something better than radio as we know it now. (For example, a message using the orbital angular momentum of single photons would be a single photon - we couldn't even detect a pulse of fifty copies.) Is there nothing encyclopedia-worthy about the idea yet? - Darekun 03:29, 18 October 2007 (UTC)

What about zero?
Question: In mathematics, when any number of variables is multiplied together,if one of the variables equals Zero, the result is Zero. So, doesn't plugging a zero into this equation also make the answer zero? Joeylawn 05:33, 12 March 2006 (UTC)

 * No worries there: none of them is zero, or we wouldn't be here.  -Padde 00:37, 15 April 2006 (UTC)

Please revert f_p
Someone changed the value of f_p to 10.5 from 0.5. That's impossible, f_p is the fraction of stars that have planets and so must be less than or equal to 1.

Notation
fi = fc = 0.01 is poor notation. It suggests that fi and fc are intrinsically equal for some reason. I have fixed this. Padde 00:36, 15 April 2006 (UTC)

Fermi Paradox
If anyone is interested Fermi Paradox is on peer review. I'd like to get some feedback to get it back to FA status. Marskell 10:32, 28 April 2006 (UTC)

Literature
If I remember right, the Drake Equation is mentioned in Sphere by Michael Crichton. I don't have a copy of it to hand, so I can't check it though :'( --H2g2bob 00:43, 26 May 2006 (UTC)

Gary Bates a reliable reference?
The following reference is currently in the article: Can someone verify that this is a reliable reference? A quick look at the amazon (UK) reviews makes me doubt it. It was added by User:68.85.27.47 (talk • contribs) back on the 25 february 2006. Mike Peel 21:05, 6 August 2006 (UTC)

Roddenberry's fudge
I think putting this fake equation in the opening section is confusing to a casual reader - in an article named for an equation, the first thing on the page that looks like a mathematical construct should be the subject of the article!

I'd suggest trimming that section out and placing it in a 'trivia' or 'culture' section, below the main content. Radagast 04:04, 4 December 2006 (UTC)

Equation vs. Variables
Why is the Drake equation considered bad science? From what I can see, it's just a multiplication of probabilities which seem quite reasonable. People who argue with it seem to be mostly quibbling over the estimates of the variables. What's the problem with the equation itself? Maybe we currently have no way of knowing most variables, but we come closer to figuring them (at least some of them) out daily, and it becomes a good guideline of other factors to estimate (scientifically) to determine N. ADyuaa 23:10, 8 January 2007 (UTC)

R*
Shouldn't R*, obviously, be the total number of stars in the galaxy?

 * No, it's correct as Drake wrote it. I added to the main section the other version of the formula, which calculates the same number in terms of N*, the number of stars in the galaxy.  The two equations are identical if you define the average rate of star formation R*, to be equal to the number of stars in the galaxy divided by the age of the galaxy.  Geoffrey.landis 19:31, 30 May 2007 (UTC)

 * I was wondering about this too. If this were the definition (R* = dN*/dt), where dt is the age (change in time) of the galaxy, shouldn't R* be defined as the average rate (or joint rate) of both formation and destruction of stars? This definition assumes that the number of stars in the galaxy N* refers to all stars that have been formed and destroyed in the history of the galaxy. Daniel.kho 11:49, 7 September 2007 (UTC)

Move Crichton to SETI?
I added some discussion of the section with Michael Crichton's critique of SETI, but it occurs to me that this might fit better if it is moved to the SETI article, since it's really a comment about SETI, not about the Drake equation. Should it be moved to the SETI article? Geoffrey.landis 19:58, 30 May 2007 (UTC)

Assumptions about intelligent life's form
This entire equation seems to have (at least) one really glaring fallacy. It requires making a gigantic assumption about the prerequisites for an environment to evolve intelligent life. I mean the very basis of evolution is that life adapts to the environment, not the other way around.

What's to say life has to evolve in conditions even remotely similar to earth? Or even on a planet for that matter? Intelligence could eventually evolve in some complex system that isn't even anything close to what we think of as biological, and it hardly requires ANYTHING other than a complex systems for evolution to take place on a large scale. There are surely plenty of non-planetary systems (not to mention planetary one's discounted here like gas giants) that would have enough complexity to allow evolution to potentially take place.

I tried to scan through the entirety of the talk page and didn't notice anything addressing this. I don't have anything to reference for this, but I wouldn't mind seeing it addressed in the article as well.

Jbrownos (talk) 21:20, 6 December 2007 (UTC)
 * This is one of the most important criticisms of the Drake equation, and is noted in the article. It doesn't make the equation wrong, however.  It simply feeds into fp, ne, and fl - if you can have life in exotic environments, you just change the value of the relevant fractions and the number of possible sites.  The talk page isn't usually used for stuff like this, being about the article, not a forum. Michaelbusch (talk) 21:24, 6 December 2007 (UTC)

Criticism
Does this count as criticism? http://xkcd.com/384/ There's a wikipedia article about this webcomic, so I guess it is relevant: http://en.wikipedia.org/wiki/Xkcd --217.86.19.19 (talk) 11:03, 15 February 2008 (UTC)
 * I believe the criticism section is for constructive, meaningful critics, not just somebody who called the Drake equation "bullshit." The reference under the in fiction header seems appropriate. ~ Booya Bazooka 16:05, 15 February 2008 (UTC)
 * I'm not sure that was the point of the comic, but then I'm not Randall Munroe. But if we're going to call it criticism, then surely the alt-text is more relevant than the comic itself. --Iustinus (talk) 22:20, 15 February 2008 (UTC)
 * Or better yet, the artist's blog entry on the subject. Jruderman (talk) 05:02, 21 February 2008 (UTC)

 * There are better sources for criticism of the Drake equation than Munroe. That should be obvious.  The comic is pretty funny though...I suppose there's no way to include it (even if Munroe would change the licensing), due to NPOV issues.  --C S (talk) 21:47, 20 February 2009 (UTC)

Disabled?
Just curious, why was this article locked out to new and unregistered edits? --Fcsuper (talk) 03:18, 28 February 2008 (UTC)

 * Looks like vandalism stemming from the xkcd webcomic. Kjl (talk) 18:53, 28 February 2008 (UTC)

L should be proportional
Why is L measured in years? Doesn't that make the answer meaningless? The equation is supposed to tell us how many alien species are out there right now, but nowhere is "now" plugged into the equation. L should be "the proportion of the age of the universe that a civilization will be in a signal-emitting phase". This would then give the chance - at any random point in time - that there will be another civilization out there to listen to. In effect, dividing the result by the age of the universe (in years, if L is in years) cancels out the years unit introduced by measuring L in years. Obviously this makes the final number really really small, but that isn't my problem. 14:05, 5 March 2008 (UTC)

Hi! The lifetime has unit 'years' since the star formation rate is in 'stars/year'. The basic idea is that the average number around at any time is (how many form per unit time) * lifetime. So if 0.01 civilizations form every year, and they last 500 years, then there should be about 5 at any time. The unit 'years' is arbitrary and does not change the number at all. If you change to 'stars formed per second' (or fortnight, or any other unit), as long as you measure the lifetime in the same units, you get the same answer. LouScheffer (talk) 16:04, 5 March 2008 (UTC)
 * Ok, the unit cancels out, I see. Thanks. 79.69.241.8 (talk) 01:55, 21 March 2008 (UTC)

Correct Figure?
Drake states here that "We used to think N was about 10,000. Now I think it could be a great deal larger." Which would make the entire historical and current estimates part of this page wrong - it says N=2 or 10... This wiki page gives N as ~10,000. I think someone with more knowledge should have a look at corecting that and trimming the estimates section a bit.MrFire (talk) 01:19, 11 March 2008 (UTC)

Europa & Jupiter
Given that theoreticaly the gravitational effects of Jupiter on Europa could cause core heating leading to liquid oceans on a volcanic ocean floor. Also theoreticly enabling the development of life... The Drake equation requiring R underestimates the possibilities. Indeed any body orbiting another with suficient mass could potentialy support life. Any body in or out of a solar system of relative size could carry a moon that harbours life irrelivent of what we would consider light.. --92.234.248.209 (talk) 04:12, 24 May 2008 (UTC)JBJ UK

A further level should be added to the Drake Equation to take this possibility into account. The potential of a rogue or orbital gas giant or body providing a surragate to a orbital body is not one that can be ignored. A solar system may not be needed!

--92.234.248.209 (talk) 04:16, 24 May 2008 (UTC)JBJ

Why is the unit of L years?
Years are an arbitrary number created by humans to coincide with our planet's patterns. Someone could argue that the unit of measurement should be seconds or minutes and if we used those numbers that would radically alter the equation. Why did Drake pick years in his equation? Maybe I missed something in the article, but it seems like a problem.24.22.28.146 (talk) 03:43, 23 August 2008 (UTC)

 * No, the units make no difference, since the lifetime of the civilization is also measured in years. So if the units were zlotls, which is 1/10 of a year, then the star formation rate would be 10 times lower, but the lifetime would be 10 times longer, and the anser the same.  This holds for any unit of time you might choose.   LouScheffer (talk) 12:58, 23 August 2008 (UTC)

Probabilities
It appears that given the right conditions organic compounds are created by natural processes. Life (including bacteria, viruses and similar) can evolve to make use of almost any environment on Earth (and survived on the camera that stayed on the moon), however hostile. While the process by which organic compounds "become life" is presently unknown, having happened once, is likely to occur again - and, likewise more complicated life forms, and sentient beings albeit with increasingly lower frequencies.

To what extent can the "necessary criteria for life to emerge" be calculated - the orbited star's (or moon's planet's) lifespan and energy etc output (short-lived stars emitting much energy are unlikely to have life bearing planets) the relevant temperature ranges/minimum quantities of water and carbon etc? These should be easier to calculate than the probabilities (as "sufficient time has already elapsed"). Jackiespeel (talk) 17:21, 14 October 2008 (UTC)

Contact
"Eleanor Ann Arroway ... in one scene of the 1997 motion picture based upon the Carl Sagan novel, she misstates her Drake Equation result by several billion." If I recall correctly, then she actually multiplies the Drake equation by the number of galaxies in the visible universe, so I don't think she was quite that far off in her estimate. Mike Peel (talk) 21:49, 24 December 2008 (UTC)

Disputed neutrality: "SETI is unquestionably a religion"
This is the opinion of a popular science fiction writer. A quote like this from a science fiction writer is hardly a fact. Opinions have no business being stated as fact in wikipedia articles (See NPOV). I recommend rephrasing this, citing it as Michael Crichton's opinion, or removing it from the article. 199.111.176.245 (talk) —Preceding undated comment was added at 01:26, 13 December 2008 (UTC).

 * The section in question Criticisms was in flagrant violation of many Wikipedia policies. There certainly may be a “Criticisms” section in Wikipedia articles—even widely accepted theories will have mainstream, respectable detractors and skeptics. But writing about dissenting viewpoints does not somehow create an "equal opportunities" scheme for fringe viewpoints. Over ten million Americans believe the Apollo Moon landings were faked. Notwithstanding the fact that this is a widely shared view, reliable sources state that the landings were real. Accordingly, Wikipedia strictly adheres to this view in our Apollo 11 article. Widespread, particularly famous fringe theories may be discussed and these are sometimes given their own articles (such as Apollo Moon Landing hoax conspiracy theories), but even then, Wikipedia does not depart from a mainstream view as to what is fact and what is discredited opinion. This entire issue is, at this very moment, being discussed here on Jimbo’s talk page as a result of POV-pushing by a certain editor on Cold fusion. The cited criticism by Michael Crichton, who is a science fiction author, is a fringe view that is not shared by most reliable sources. Wikipedia is a mainstream encyclopedia that represents scientific points of views. The fact that Wikipedia always endeavors to maintain a neutral point of view does not mean that equal standing must be afforded to minority viewpoints—even if those views are those of an author who has sold many books—when those views are wildly contrary to the vast preponderance of reliable sources. Clearly, most reliable sources state that SETI is unquestionably science. The offending section has been deleted.  Greg L (talk) 03:48, 30 December 2008 (UTC)

 * In addition to the source of the quote being a science fiction writer and not a scientist, I would add that Crichton in particular does not have great credibility compared to other science fiction writers like Arthur C. Clarke, Isaac Asimov, etc. If any of those guys were to have said SETI was a religion, you could reasonably argue that's worth including...but Crichton?  Give me a break.  --C S (talk) 21:42, 20 February 2009 (UTC)

1960
The 1960 date seems to be based on faulty information from Michael Crichton that someone added to this article. Does anyone have a source for this date besides Crichton? Drake himself said he worked on it in the 1950s, and as everyone is aware, it wasn't "published" until November, 1961. Viriditas (talk) 10:03, 2 March 2009 (UTC)

"Sagan" equation
I admit that I know nothing about this topic, but is it fair to say it is "mislabeled" the Sagan equation? Many mathematical concepts could have diverse names. Maybe "it is also sometimes called" might be better. —Preceding unsigned comment added by 131.182.172.229 (talk) 20:55, 30 April 2009 (UTC)

A question...
Who was it that said the chance of any intelligent life forms, much less a civilation, forming or being created, is 1 in a googol, or another really big number? Was that Drake?70.136.142.54 (talk) 03:36, 11 July 2009 (UTC)

I think the answer to the question "How many intelligent species exist in our Galaxy?" is probably zero. At least it seems that way from here. XAlpha (talk) 18:11, 3 August 2009 (UTC)

Reasoning behind my edits
Just a quick run down of why these changes were made:

Removed:

"One reply to such criticism is that experiments by SETI scientists do not attempt to address the Drake equation for the existence of extraterrestrial civilizations anywhere in the universe, but are focused on specific, testable hypotheses (i.e., "do extraterrestrial civilizations communicating in the radio spectrum exist near sun-like stars within 50 light years of the Earth?")."

Because:

Besides having no verifiable reference, this is not a rebuttal of the criticism lodged against the Drake equation, and in fact has nothing to do with the Drake equation whatsoever. This may be SETI defending it's own internal policies but nothing in this unverified statement claims to disagree with criticism of the Drake equation. In other words, it isn't a "reply to such criticism".

Moved:

"A deeper objection ..." up to join the other criticism

Because:

This could have been misinterpreted as being part of the rebuttal to criticism where it was placed.

Reappearance number
a history channel show recently reported that chromium stainless steel was invented in the Sung dynasty; and that the technology was lost, but re-appeared a thousand years later (Germany, 1945). this fact, and others, suggests that nr has estimable bounds. does anyone have references? -- 99.233.186.4 (talk) 20:16, 8 November 2009 (UTC)

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title: Talk:Drake equation/Archive 1