Cultural relativism

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https://en.wikipedia.org/wiki/Cultural_relativism

Cultural relativism is the view that moral values must be understood in their own cultural context and not judged according to the standards of a different culture. It asserts the equal validity of all points of view and the relative nature of truth, which is determined by an individual or their culture.

The concept was established by anthropologist Franz Boas, who first articulated the idea in 1887: "civilization is not something absolute, but ... is relative, and ... our ideas and conceptions are true only so far as our civilization goes". However, Boas did not use the phrase "cultural relativism". The concept was spread by Boas' students, such as Robert Lowie.

The first use of the term recorded in the Oxford English Dictionary was by philosopher and social theorist Alain Locke in 1924 to describe Lowie's "extreme cultural relativism", found in the latter's 1917 book Culture and Ethnology.

The term became common among anthropologists after Boas' death in 1942, to express their synthesis of a number of ideas he had developed. Boas believed that the sweep of cultures, to be found in connection with any subspecies, is so vast and pervasive that there cannot be a relationship between culture and race. Cultural relativism involves specific epistemological and methodological claims. Whether or not these claims necessitate a specific ethical stance is a matter of debate. Cultural relativism became popularized after World War II in reaction to historical events such as "Nazism, and to colonialism, ethnocentrism and racism more generally."

In antiquity

Herodotus (Histories 3.38) observes on the relativity of mores (νόμοι):

If anyone, no matter who, were given the opportunity of choosing from amongst all the nations in the world the set of beliefs which he thought best, he would inevitably—after careful considerations of their relative merits—choose that of his own country. Everyone without exception believes his own native customs, and the religion he was brought up in, to be the best; and that being so, it is unlikely that anyone but a madman would mock at such things. There is abundant evidence that this is the universal feeling about the ancient customs of one's country.

— translated by Aubrey de Selincourt

He mentions an anecdote of Darius the Great who illustrated the principle by inquiring about the funeral customs of the Greeks and the Callatiae, peoples from the extreme western and eastern fringes of his empire, respectively. They practiced cremation and funerary cannibalism, respectively, and were each dismayed and abhorred at the proposition of the other tribes' practices.

The works of the Pyrrhonist philosopher Sextus Empiricus detail ancient Greek arguments for cultural relativism as part of the tenth of the Ten Modes of Aenesidemus.

As a methodological and heuristic device

According to George E. Marcus and Michael M. J. Fischer:

20th century social and cultural anthropology has promised its still largely Western readership enlightenment on two fronts. The one has been the salvaging of distinct cultural forms of life from a process of apparent global Westernization. With both its romantic appeal and its scientific intentions, anthropology has stood for the refusal to accept this conventional perception of homogenization toward a dominant Western model.

Cultural relativism was, in part, a response to Western ethnocentrism. Ethnocentrism may take obvious forms, in which one consciously believes that one's people's arts are the most beautiful, values the most virtuous, and beliefs the most truthful. Franz Boas, originally trained in physics and geography, and heavily influenced by the thought of Kant, Herder, and von Humboldt, argued that one's culture may mediate and thus limit one's perceptions in less obvious ways. Boas understood "culture" to include not only certain tastes in food, art, and music, or beliefs about religion; he assumed a much broader notion of culture, defined as:

[T]he totality of the mental and physical reactions and activities that characterize the behavior of the individuals composing a social group collectively and individually in relation to their natural environment, to other groups, to members of the group itself, and of each individual to himself.

This view of culture confronts anthropologists with two problems: first, how to escape the unconscious bonds of one's own culture, which inevitably bias our perceptions of and reactions to the world, and second, how to make sense of an unfamiliar culture. The principle of cultural relativism thus forced anthropologists to develop innovative methods and heuristic strategies.

As a methodological tool

Between World War I and II, cultural relativism was the central tool for American anthropologists in this rejection of Western claims to universality and salvage of non-Western cultures. It functioned to transform Boas' epistemology into methodological lessons.

This is most obvious in the case of language. Although language is commonly thought of as a means of communication, Boas called attention especially to the idea that it is also a means of categorizing experiences, hypothesizing that the existence of different languages suggests that people categorize, and thus experience, language differently (this view was more fully developed in the hypothesis of Linguistic relativity).

Thus, although all people perceive visible radiation the same way, in terms of a continuum of color, people who speak different languages slice up this continuum into discrete colors in different ways. Some languages have no word that corresponds to the English word green. When people who speak such languages are shown a green chip, some identify it using their word for blue, others identify it using their word for yellow. Thus, Boas's student Melville Herskovits summed up the principle of cultural relativism thus: "Judgements are based on experience, and experience is interpreted by each individual in terms of his own enculturation."

Boas pointed out that scientists grow up and work in a particular culture, and are thus necessarily ethnocentric. He provided an example of this in his 1889 article "On Alternating Sounds". A number of linguists at Boas's time had observed that speakers of some Native-American languages pronounced the same word with different sounds indiscriminately. They thought that this meant that the languages were unorganized and lacked strict rules for pronunciation, and they took it as evidence that the languages were more primitive than their own. Boas, however, noted that the variant pronunciations were not an effect of lack of organization of sound patterns, but an effect of the fact that these languages organized sounds differently from English. The languages grouped sounds that were considered distinct in English into a single sound, but also having contrasts that did not exist in English. He then argued the case that Native Americans had been pronouncing the word in question the same way, consistently, and the variation was only perceived by someone whose own language distinguishes those two sounds. Boas's student, the linguist Edward Sapir, later noted also that English speakers pronounce sounds differently even when they think they are pronouncing the same sound. For example, few English speakers realize that the sounds written with the letter ⟨t⟩ in the words tick and stick are phonetically different, the first being generally aspirated and the other unaspirated; a speaker of a language where this contrast is meaningful would instantly perceive them as different sounds and tend not to see them as different realizations of a single phoneme.

Boas's students did not draw only on his engagement with German philosophy. They also engaged the work of contemporary philosophers and scientists, such as Karl Pearson, Ernst Mach, Henri Poincaré, William James, and John Dewey in an attempt to move, in the words of Boas's student Robert Lowie, from "a naively metaphysical to an epistemological stage" as a basis for revising the methods and theories of anthropology.

Boas and his students realized that, if they were to conduct scientific research in other cultures, they would need to employ methods that would help them escape the limits of their own ethnocentrism. One such method is that of ethnography: basically, they advocated living with people of another culture for an extended period of time, so that they could learn the local language and be enculturated, at least partially, into that culture.

In this context, cultural relativism is an attitude that is of fundamental methodological importance, because it calls attention to the importance of the local context in understanding the meaning of particular human beliefs and activities. Thus, in 1948 Virginia Heyer wrote: "Cultural relativity, to phrase it in starkest abstraction, states the relativity of the part to the whole. The part gains its cultural significance by its place in the whole, and cannot retain its integrity in a different situation."

As a heuristic tool

Another method was ethnology: to compare and contrast as wide a range of cultures as possible, in a systematic and even-handed manner. In the late nineteenth century, this study occurred primarily through the display of material artifacts in museums. Curators typically assumed that similar causes produce similar effects; therefore, in order to understand the causes of human action, they grouped similar artifacts together—regardless of provenance. Their aim was to classify artifacts, like biological organisms, according to families, genera, and species. Thus organized museum displays would illustrate the evolution of civilization from its crudest to its most refined forms.

In an article in the journal Science, Boas argued that this approach to cultural evolution ignored one of Charles Darwin's main contributions to evolutionary theory:

It is only since the development of the evolutional theory that it became clear that the object of study is the individual, not abstractions from the individual under observation. We have to study each ethnological specimen individually in its history and in its medium.... By regarding a single implement outside of its surroundings, outside of other inventions of the people to whom it belongs, and outside of other phenomena affecting that people and its productions, we cannot understand its meanings.... Our objection...is, that classification is not explanation.

Boas argued that although similar causes produce similar effects, different causes may also produce similar effects. Consequently, similar artifacts found in distinct and distant places may be the products of distinct causes. Against the popular method of drawing analogies in order to reach generalizations, Boas argued in favor of an inductive method. Based on his critique of contemporary museum displays, Boas concluded:

It is my opinion that the main object of ethnological collections should be the dissemination of the fact that civilization is not something absolute, but that it is relative, and that our ideas and conceptions are true only so far as our civilization goes.

Boas's student Alfred Kroeber described the rise of the relativist perspective thus:

Now while some of the interest in (so called solial culture science) anthropology in its earlier stages was in the exotic and the out-of-the-way, yet even this antiquarian motivation ultimately contributed to a broader result. Anthropologists became aware of the diversity of culture. They began to see the tremendous range of its variations. From that, they commenced to envisage it as a totality, as no historian of one period or of a single people was likely to do, nor any analyst of his own type of civilization alone. They became aware of culture as a "universe", or vast field in which we of today and our own civilization occupy only one place of many. The result was a widening of a fundamental point of view, a departure from unconscious ethnocentricity toward relativity. This shift from naive self-centeredness in one's own time and spot to a broader view based on objective comparison is somewhat like the change from the original geocentric assumption of astronomy to the Copernican interpretation of the solar system and the subsequent still greater widening to a universe of galaxies.

This conception of culture, and principle of cultural relativism, were for Kroeber and his colleagues the fundamental contribution of anthropology, and what distinguished anthropology from similar disciplines such as sociology and psychology.

Ruth Benedict, another of Boas's students, also argued that an appreciation of the importance of culture and the problem of ethnocentrism demands that the scientist adopt cultural relativism as a method. Her book, Patterns of Culture, did much to popularize the term in the United States. In it, she explained that:

The study of custom can be profitable only after certain preliminary propositions have been violently opposed. In the first place any scientific study requires that there be no preferential weighting of one or another items in the series it selects for its consideration. In all the less controversial fields like the study of cacti or termites or the nature of nebulae, the necessary method of study is to group the relevant material and to take note of all possible variant forms and conditions. In this way we have learned all that we know of the laws of astronomy, or of the habits of the social insects, let us say. It is only in the study of man himself that the major social sciences have substituted the study of one local variation, that of Western civilization.

Benedict was adamant that she was not romanticizing so-called primitive societies; she was emphasizing that any understanding of the totality of humanity must be based on as wide and varied a sample of individual cultures as possible. Moreover, it is only by appreciating a culture that is profoundly different from our own, that we can realize the extent to which our own beliefs and activities are culture-bound, rather than natural or universal. In this context, cultural relativism is a heuristic device of fundamental importance because it calls attention to the importance of variation in any sample that is used to derive generalizations about humanity.

As a critical device

Marcus and Fischer's attention to anthropology's refusal to accept Western culture's claims to universality implies that cultural relativism is a tool not only in cultural understanding, but in cultural critique. This points to the second front on which they believe anthropology offers people enlightenment:

The other promise of anthropology, one less fully distinguished and attended to than the first, has been to serve as a form of cultural critique for ourselves. In using portraits of other cultural patterns to reflect self-critically on our own ways, anthropology disrupts common sense and makes us reexamine our taken-for-granted assumptions.

The critical function of cultural relativism is widely understood; philosopher John Cook observed that "It is aimed at getting people to admit that although it may seem to them that their moral principles are self-evidently true, and hence seem to be grounds for passing judgement on other peoples, in fact, the self-evidence of these principles is a kind of illusion." Cook recognizes the middle ground in between moral relativism and moral absolutism that cultural relativism straddles, remarking that the ensuing battlegrounds that arise tend to be in the domain of claims of self-evidence made on behalf of a people.

The critical function was indeed one of the ends to which Benedict hoped her own work would meet. The most famous use of cultural relativism as a means of cultural critique is Margaret Mead's research of adolescent female sexuality in Samoa. By contrasting the ease and freedom enjoyed by Samoan teenagers, Mead called into question claims that the stress and rebelliousness that characterize American adolescence is natural and inevitable.

As Marcus and Fischer point out, however, this use of relativism can be sustained only if there is ethnographic research in the United States comparable to the research conducted in Samoa. Although every decade has witnessed anthropologists conducting research in the United States, the very principles of relativism have led most anthropologists to conduct research in foreign countries.

Comparison to moral relativism

According to Marcus and Fischer, when the principle of cultural relativism was popularized after World War II, it came to be understood "more as a doctrine, or position, The principle of cultural relativity does not mean that because the members of some savage tribe are allowed to behave in a certain way that this fact gives intellectual warrant for such behavior in all groups. Cultural relativity means, on the contrary, that the appropriateness of any positive or negative custom must be evaluated with regard to how this habit fits with other group habits. While breeding a healthy scepticism as to the eternity of any value prized by a particular people, anthropology does not as a matter of theory deny the existence of moral absolutes. Rather, the use of the comparative method provides a scientific means of discovering such absolutes. If all surviving societies have found it necessary to impose some of the same restrictions upon the behavior of their members, this makes a strong argument that these aspects of the moral code are indispensable.

Although Kluckhohn was using language that was popular at the time (e.g. "savage tribe") but which is now considered antiquated and coarse by most anthropologists, his point was that although moral standards are rooted in one's culture, anthropological research reveals that the fact that people have moral standards is a universal. He was especially interested in deriving specific moral standards that are universal, although few if any anthropologists think that he was successful.

There is an ambiguity in Kluckhohn's formulation that would haunt anthropologists in the years to come. It makes it clear that one's moral standards make sense in terms of one's culture. He waffles, however, on whether the moral standards of one society could be applied to another. Four years later American anthropologists had to confront this issue head-on.

Vertical and horizontal relativism

It was James Lawrence Wray-Miller who provided an additional clarification tool, or caveat, of the theoretical underpinnings of cultural relativism by dividing it into two binary, analytical continuums: vertical and horizontal cultural relativism. Ultimately, these two analytical continuums share the same basic conclusion: that human morality and ethics are not static but fluid and vary across cultures depending on the time period and current condition of any particular culture.

Vertical relativism describes that cultures, throughout history (vertical—i.e., passage through past and future), are products of the prevailing societal norms and conditions of their respective historical periods. Therefore, any moral or ethical judgments, made during the present, regarding past cultures' belief systems or societal practices must be firmly grounded and informed by these norms and conditions to be intellectually useful. Vertical relativism also accounts for the possibility that cultural values and norms will necessarily change as influencing norms and conditions change in the future.

Horizontal relativism describes that cultures in the present (horizontal in time—i.e., the present period of the culture) are products of the prevailing norms and conditions developed as a result of their unique geographies, histories, and environmental influences. Therefore, moral or ethical judgments, made during the present, regarding a current culture's belief system or societal practices must account for these unique differences to be intellectually useful.

Statement on human rights

The transformation of cultural relativism as a heuristic tool into the doctrine of moral relativism occurred in the context of the work of the Commission of Human Rights of the United Nations in preparing the Universal Declaration of Human Rights (1948).

Melville J. Herskovits prepared a draft "Statement on Human Rights" which Executive Board of the American Anthropological Association revised, submitted to the Commission on Human Rights, and then published. The statement begins with a fairly straightforward explanation of the relevance of cultural relativism:

The problem is thus to formulate a statement of human rights that will do more than phrase respect for the individual as individual. It must also take into full account the individual as a member of a social group of which he is part, whose sanctioned modes of life shape his behavior, and with whose fate his own is thus inextricably bound.

The bulk of this statement emphasizes concern that the Declaration of Human Rights was being prepared primarily by people from Western societies, and would express values that, far from being universal, are really Western:

Today the problem is complicated by the fact that the Declaration must be of world-wide applicability. It must embrace and recognize the validity of many different ways of life. It will not be convincing to the Indonesian, the African, the Chinese, if it lies on the same plane as like documents of an earlier period. The rights of Man in the Twentieth Century cannot be circumscribed by the standards of any single culture, or be dictated by the aspirations of any single people. Such a document will lead to frustration, not realization of the personalities of vast numbers of human beings.

Although this statement could be read as making a procedural point (that the Commission must involve people of diverse cultures, especially cultures that had been or are still under European colonial or imperial domination), the document ended by making two substantive claims:

1. Even where political systems exist that deny citizens the right of participation in their government, or seek to conquer weaker peoples, underlying cultural values may be called on to bring the peoples of such states to a realization of the consequences of the acts of their governments, and thus enforce a brake upon discrimination and conquest.
2. Worldwide standards of freedom and justice, based on the principle that man is free only when he lives as his society defines freedom, that his rights are those he recognizes as a member of his society, must be basic.

These claims provoked an immediate response by a number of anthropologists. Julian Steward (who, as a student of Alfred Kroeber and Robert Lowie, and as a professor at Columbia University, was situated firmly in the Boasian lineage) suggested that the first claim "may have been a loophole to exclude Germany from the advocated tolerance", but that it revealed the fundamental flaw in moral relativism:

"Either we tolerate everything, and keep hands off, or we fight intolerance and conquest—political and economic as well as military—in all their forms." Similarly, he questioned whether the second principle means that anthropologists "approve the social caste system of India, the racial caste system of the United States, or many other varieties of social discrimination in the world."

Steward and others argued that any attempt to apply the principle of cultural relativism to moral problems would only end in contradiction: either a principle that seems to stand for tolerance ends up being used to excuse intolerance, or the principle of tolerance is revealed to be utterly intolerant of any society that seems to lack the (arguably, Western) value of tolerance. They concluded that anthropologists must stick to science, and engage in debates over values only as individuals.

Governmental usage

Several countries have used cultural relativism as a justification for limiting the rights in the Universal Declaration of Human Rights, despite the World Conference on Human Rights rejecting it as a refutation of human rights violations.

A 2011 study by international legal expert Roger Lloret Blackburn, examining the Universal Periodic Reviews, distinguishes several different groups of nations:

• One group consists of nations where the current regime has been installed by revolution, and that deny the need for political plurality: China, Vietnam, Myanmar, Cuba, and Iran.
• Another group are certain Islamic nations that adhere to sharia and certain traditional practices: Yemen, Iran, Saudi Arabia, Pakistan.
• A third possible group is nations that give special rights to specific groups: Malaysia, Mexico, Indonesia, and Colombia.

Criticism

The debate over the "Statement on Human Rights", then, was not merely over the validity of cultural relativism, or the question of what makes a right universal. It forced anthropologists to confront the question of whether anthropological research is relevant to non-anthropologists. Although Steward and Barnett seemed to be suggesting that anthropology as such should restrict itself to purely academic affairs, people within and without the academy have continued to debate the ways non-anthropologists have used this principle in public policy concerning ethnic minorities or in international relations.

Political scientist Alison Dundes Renteln has argued that most debates over moral relativism misunderstand the importance of cultural relativism. Most philosophers understand the Benedictine–Herskovitz formulation of cultural relativism to mean:

[W]hat is right or good for one individual or society is not right or good for another, even if the situations are similar, meaning not merely that what is thought right or good by one is not thought right or good by another...but that what is really right or good in one case is not so in another.

Although this formulation clearly echoes the kinds of example anthropologists used in elaborating cultural relativism, Renteln believes that it misses the spirit of the principle. Accordingly, she supports a different formulation: "there are or can be no value judgements that are true, that is, objectively justifiable, independent of specific cultures."

Renteln faults philosophers for disregarding the heuristic and critical functions of cultural relativism. Her main argument is that in order to understand the principle of cultural relativism, one must recognize the extent to which it is based on enculturation: "the idea that people unconsciously acquire the categories and standards of their culture." This observation, which echoes the arguments about culture that originally led Boas to develop the principle, suggests that the use of cultural relativism in debates of rights and morals is not substantive but procedural. That is, it does not require a relativist to sacrifice his or her values. But it does require anyone engaged in a consideration of rights and morals to reflect on how their own enculturation has shaped their views:

There is no reason why the relativist should be paralyzed, as critics have often asserted. But a relativist will acknowledge that the criticism is based on his own ethnocentric standards and realizes also that the condemnation may be a form of cultural imperialism.

Renteln thus bridges the gap between the anthropologist as scientist (whom Steward and Barnett felt had nothing to offer debates on rights and morality) and as private individual (who has every right to make value judgements). The individual keeps this right, but the scientist requires that the individual acknowledge that these judgements are neither self-evident universals, nor entirely personal (and idiosyncratic), but rather took form in relation to the individual's own culture.

Post-colonial politics

Boas and his students understood anthropology to be a historical, or human science, in that it involves subjects (anthropologists) studying other subjects (humans and their activities), rather than subjects studying objects (such as rocks or stars). Under such conditions, it is fairly obvious that scientific research may have political consequences, and the Boasians saw no conflict between their scientific attempts to understand other cultures, and the political implications of critiquing their own culture. For anthropologists working in this tradition, the doctrine of cultural relativism as a basis for moral relativism was anathema. For politicians, moralists, and many social scientists (but few anthropologists) who saw science and human interests as necessarily independent or even opposed, however, the earlier Boasian principle of cultural relativism was anathema. Thus, cultural relativism came under attack, but from opposing sides and for opposing reasons.

Political critique

On the one hand, many anthropologists began to criticize the way moral relativism, in the guise of cultural relativism, is used to mask the effects of Western colonialism and imperialism. Thus, Stanley Diamond argued that when the term "cultural relativism" entered popular culture, popular culture co-opted anthropology in a way that voided the principle of any critical function:

Relativism is the bad faith of the conqueror, who has become secure enough to become a tourist. Cultural relativism is a purely intellectual attitude; it does not inhibit the anthropologist from participating as a professional in his own milieu; on the contrary, it rationalizes that milieu. Relativism is self-critical only in the abstract. Nor does it lead to engagement. It only converts the anthropologist into a shadowy figure, prone to newsworthy and shallow pronouncements about the cosmic condition of the human race. It has the effect of mystifying the profession, so that the very term anthropologist ("student of man") commands the attention of an increasingly "popular" audience in search of novelty. But the search for self-knowledge, which Montaigne was the first to link to the annihilation of prejudice, is reduced to the experience of culture shock, a phrase used by both anthropologists and the State Department to account for the disorientation that usually follows an encounter with an alien way of life. But culture shock is a condition one recovers from; it is not experienced as an authentic redefinition of the personality but as a testing of its tolerance ... The tendency of relativism, which it never quite achieves, is to detach the anthropologist from all particular cultures. Nor does it provide him with a moral center, only a job.

George Stocking summarized this view with the observation that "Cultural relativism, which had buttressed the attack against racialism, [can] be perceived as a sort of neo-racialism justifying the backward techno-economic status of once colonized peoples."

Defence by Clifford Geertz

By the 1980s many anthropologists had absorbed the Boasian critique of moral relativism, and were ready to reevaluate the origins and uses of cultural relativism. In a distinguished lecture before the American Anthropological Association in 1984, Clifford Geertz claimed that the critics of cultural relativism did not really understand, and were not really responding to, the ideas of Benedict, Herskovits, Kroeber and Kluckhohn. Consequently, the various critics and proponents of cultural relativism were talking past one another. What these different positions have in common, Geertz argued, is that they are all responding to the same thing: knowledge about other ways of life.

The supposed conflict between Benedict's and Herskovits's call for tolerance and the untolerant passion with which they called for it turns out not to be the simple contradiction so many amateur logicians have held it to be, but the expression of a perception, caused by thinking a lot about Zunis and Dahomys, that the world being so full of a number of things, rushing to judgement is more than a mistake, it is a crime. Similarly, Kroeber's and Kluckholn's verities – Kroeber's were mostly about messy creatural matters like delirium and menstruation, Kluckholn's were mostly about messy social ones like lying and killing within the in-group, turn out not to be just the arbitrary personal obsessions they so much look like, but the expression of a much vaster concern, caused by thinking a lot about anthrōpos in general, that if something isn't anchored everywhere nothing can be anchored anywhere. Theory here – if that is what these earnest advices about how we must look at things if we are to be accounted as decent should be called – is more an exchange of warnings than an analytical debate. We are being offered a choice of worries. What the relativists – so-called – want us to worry about is provincialism – the danger that our perceptions will be dulled, our intellects constricted, and our sympathies narrowed by the overlearned and overvalued acceptances of our own society. What the anti-relativists – self-declared – want us to worry about, and worry about and worry about, as though our very souls depended on it, is a kind of spiritual entropy, a heat death of the mind, in which everything is as significant, and thus as insignificant, as everything else: anything goes, to each his own, you pays your money and you takes your choice, I know what I like, not in the couth, tout comprendre, c'est tout pardonner.

Geertz concludes this discussion by commenting, "As I have already suggested, I myself find provincialism altogether the more real concern so far as what actually goes on in the world." Geertz' defense of cultural relativism as a concern which should motivate various inquiries, rather than as an explanation or solution, echoed a comment Alfred Kroeber made in reply to earlier critics of cultural relativism, in 1949:

Obviously, relativism poses certain problems when from trying merely to understand the world we pass on to taking action in the world: and right decisions are not always easy to find. However, it is also obvious that authoritarians who know the complete answers beforehand will necessarily be intolerant of relativism: they should be, if there is only one truth and that is theirs. I admit that hatred of the intolerant for relativism does not suffice to make relativism true. But most of us are human enough for our belief in relativism to be somewhat reinforced just by that fact. At any rate, it would seem that the world has come far enough so that it is only by starting from relativism and its tolerations that we may hope to work out a new set of absolute values and standards, if such are attainable at all or prove to be desirable.

Drake equation

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Drake_equation

Frank Drake

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:

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

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.

This form of the equation first appeared in Drake's 1965 paper.

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 ft (26 m) dish of the National Radio Astronomy Observatory, Green Bank in Green Bank, West Virginia, Drake monitored two nearby Sun-like stars: Epsilon Eridani and Tau Ceti, slowly scanning frequencies close to the 21 cm wavelength for six hours per day from April to July 1960. 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.

— Frank Drake

The ten attendees were conference organizer J. Peter Pearman, Frank Drake, Philip Morrison, businessman and radio amateur Dana Atchley, chemist Melvin Calvin, astronomer Su-Shu Huang, neuroscientist John C. Lilly, inventor Barney Oliver, astronomer Carl Sagan, and radio-astronomer Otto Struve. 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 Allen Telescope Array for SETI

The Drake equation results in a summary of the factors affecting the likelihood that we might detect radio-communication from intelligent extraterrestrial life. 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 § Criticism). Therefore, the usefulness of the Drake equation is not in the solving, but rather in the contemplation of all the various concepts which scientists must incorporate when considering the question of life elsewhere, 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.

Estimates

Original estimates

There is considerable disagreement on the values of these parameters, but the 'educated guesses' used by Drake and his colleagues in 1961 were:

• 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 0.68–1.45 M_☉ 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 0.5 M_☉. 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 telescope data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of sun-like stars and red dwarf stars within the Milky Way Galaxy. 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 ${\displaystyle f_{\mathrm{l}}\cdot f_{\mathrm{i}}}$. 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 result, since such a civilization turnover could be described as an increase in the reappearance number rather than increase in L, stating that a civilization reappears in the form of the succeeding cultures. Furthermore, since none could communicate over interstellar space, the method of comparing with historical civilizations could be regarded as invalid.

David Grinspoon has argued that once a civilization has developed enough, it might overcome all threats to its survival. It will then last for an indefinite period of time, making the value for L potentially billions of years. If this is the case, then he proposes that the Milky Way Galaxy may have been steadily accumulating advanced civilizations since it formed.^[52] He proposes that the last factor L be replaced with 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. Paleobiologist Olev Vinn suggests that the lifetime of most technological civilizations is brief due to inherited behavior patterns present in all intelligent organisms. These behaviors, incompatible with civilized conditions, inevitably lead to self-destruction soon after the emergence of advanced technologies.

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:

N = 1.5 × 10⁻⁵ × 10⁻⁹ × 0.2 × 304 = 9.1 × 10⁻¹³

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

On the other hand, with larger values for each of the parameters above, values of N can be derived that are greater than 1. The following higher values that have been proposed for each of the parameters:

R_∗ = 1.5–3 yr⁻¹, f_p = 1, n_e = 0.2, f_l = 0.13, f_i = 1, f_c = 0.2, and L = 10⁹ years

Use of these parameters gives:

N = 3 × 1 × 0.2 × 0.13 × 1 × 0.2 × 10⁹ = 15,600,000

Monte Carlo simulations of estimates of the Drake equation factors based on a stellar and planetary model of the Milky Way have resulted in the number of civilizations varying by a factor of 100.

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 2.5×10⁻²⁴. Similarly, for Earth to have been the only case of hosting a technological species over the history of this Galaxy, the odds of a habitable zone planet ever hosting a technological species must be less than 1.7×10⁻¹¹ (about 1 in 60 billion). The figure for the universe implies that it is extremely unlikely that Earth hosts the only technological species that has ever occurred. On the other hand, for this Galaxy one must think that fewer than 1 in 60 billion habitable planets develop a technological species for there not to have been at least a second case of such a species over the past history of this Galaxy.

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 v, and establishes additional sites that survive for a lifetime L. 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 n_r is the average number of times a new civilization reappears on the same planet where a previous civilization once has appeared and ended, then the total number of civilizations on such a planet would be 1 + n_r, 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 n_r may be almost zero. In the case of total life extinction, a similar factor may be applicable for f_l, that is, how many times life may appear on a 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:

$${\displaystyle N=N_{\ast }\cdot F_{\mathrm{Q}}\cdot F_{\mathrm{H}\mathrm{Z}}\cdot F_{\mathrm{O}}\cdot F_{\mathrm{L}}\cdot F_{\mathrm{S}}}$$

where:

N = the number of planets with detectable signs of life

N_∗ = the number of stars observed

F_Q = the fraction of stars that are quiet

F_HZ = the fraction of stars with rocky planets in the habitable zone

F_O = the fraction of those planets that can be observed

F_L = the fraction that have life

F_S = 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's version of the Drake equation

American astronomer Carl Sagan made some modifications in the Drake equation and presented it in the 1980 program Cosmos: A Personal Voyage. The modified equation is shown below

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

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

and

• N_∗ = Number of stars in the Milky Way Galaxy
• 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.
• f_L = 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

Main article: 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, R_∗ · f_p · n_e · f_l · f_i, has a low value. The most common suspect is f_i, but explanations such as the rare Earth hypothesis argue that n_e is the small term.
• Intelligent civilizations exist, but we see no evidence, meaning f_c 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 (predicted) probability of there being no other intelligent life in our observable universe".

In fiction and popular culture

Commemorative plate on Europa Clipper

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

$${\displaystyle F{f}^2(MgE)-C^{1}R{i}^1\cdot M=L/So}$$

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

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

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