Photoluminescence

From Wikipedia, the free encyclopedia

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

Fluorescent solutions under UV light. Absorbed photons are rapidly re-emitted under longer electromagnetic wavelengths.

Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photons that excite electrons to a higher energy level in an atom), hence the prefix photo-. Following excitation, various relaxation processes typically occur in which other photons are re-radiated. Time periods between absorption and emission may vary: ranging from short femtosecond-regime for emission involving free-carrier plasma in inorganic semiconductors or metals up to milliseconds for phosphoresence processes in molecular systems; and under special circumstances delay of emission may even span to minutes or hours.

Observation of photoluminescence at a certain energy can be viewed as an indication that an electron transitioned between states separated by this transition energy. While this is generally true in atoms and similar systems, correlations and other more complex phenomena also act as sources for photoluminescence in many-body systems such as semiconductors or metals. A theoretical approach to handle this is given by the semiconductor luminescence equations.

Forms

Schematic for the excitation-relaxation processes of photoluminescence

Photoluminescence processes can be classified by various parameters such as the energy of the exciting photon with respect to the emission. Resonant excitation describes a situation in which photons of a particular wavelength are absorbed and equivalent photons are very rapidly re-emitted. This is often referred to as resonance fluorescence. For materials in solution or in the gas phase, this process involves electrons but no significant internal energy transitions involving molecular features of the chemical substance between absorption and emission. In crystalline inorganic semiconductors where an electronic band structure is formed, secondary emission can be more complicated as events may contain both coherent contributions such as resonant Rayleigh scattering where a fixed phase relation with the driving light field is maintained (i.e. energetically elastic processes where no losses are involved), and incoherent contributions (or inelastic modes where some energy channels into an auxiliary loss mode),

The latter originate, e.g., from the radiative recombination of excitons, Coulomb-bound electron-hole pair states in solids. Resonance fluorescence may also show significant quantum optical correlations.

More processes may occur when a substance undergoes internal energy transitions before re-emitting the energy from the absorption event. Electrons change energy states by either resonantly gaining energy from absorption of a photon or losing energy by emitting photons. In chemistry-related disciplines, one often distinguishes between fluorescence and phosphorescence. The former is typically a fast process, yet some amount of the original energy is dissipated so that re-emitted light photons will have lower energy than did the absorbed excitation photons. The re-emitted photon in this case is said to be red shifted, referring to the reduced energy it carries following this loss (as the Jablonski diagram shows). For phosphorescence, electrons which absorbed photons, undergo intersystem crossing where they enter into a state with altered spin multiplicity (see term symbol), usually a triplet state. Once the excited electron is transferred into this triplet state, electron transition (relaxation) back to the lower singlet state energies is quantum mechanically forbidden, meaning that it happens much more slowly than other transitions. The result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours. This is the basis for "glow in the dark" substances.

Photoluminescence is an important technique for measuring the purity and crystalline quality of semiconductors such as GaN and InP and for quantification of the amount of disorder present in a system.

Time-resolved photoluminescence (TRPL) is a method where the sample is excited with a light pulse and then the decay in photoluminescence with respect to time is measured. This technique is useful for measuring the minority carrier lifetime of III-V semiconductors like gallium arsenide (GaAs).

Photoluminescence properties of direct-gap semiconductors

In a typical PL experiment, a semiconductor is excited with a light-source that provides photons with an energy larger than the bandgap energy. The incoming light excites a polarization that can be described with the semiconductor Bloch equations. Once the photons are absorbed, electrons and holes are formed with finite momenta ${\displaystyle \mathbf{k}}$ in the conduction and valence bands, respectively. The excitations then undergo energy and momentum relaxation towards the band-gap minimum. Typical mechanisms are Coulomb scattering and the interaction with phonons. Finally, the electrons recombine with holes under emission of photons.

Ideal, defect-free semiconductors are many-body systems where the interactions of charge-carriers and lattice vibrations have to be considered in addition to the light-matter coupling. In general, the PL properties are also extremely sensitive to internal electric fields and to the dielectric environment (such as in photonic crystals) which impose further degrees of complexity. A precise microscopic description is provided by the semiconductor luminescence equations.

Ideal quantum-well structures

An ideal, defect-free semiconductor quantum well structure is a useful model system to illustrate the fundamental processes in typical PL experiments. The discussion is based on results published in Klingshirn (2012) and Balkan (1998).

The fictive model structure for this discussion has two confined quantized electronic and two hole subbands, e₁, e₂ and h₁, h₂, respectively. The linear absorption spectrum of such a structure shows the exciton resonances of the first (e1h1) and the second quantum well subbands (e₂, h₂), as well as the absorption from the corresponding continuum states and from the barrier.

Photoexcitation

In general, three different excitation conditions are distinguished: resonant, quasi-resonant, and non-resonant. For the resonant excitation, the central energy of the laser corresponds to the lowest exciton resonance of the quantum well. No, or only a negligible amount of the excess, energy is injected to the carrier system. For these conditions, coherent processes contribute significantly to the spontaneous emission. The decay of polarization creates excitons directly. The detection of PL is challenging for resonant excitation as it is difficult to discriminate contributions from the excitation, i.e., stray-light and diffuse scattering from surface roughness. Thus, speckle and resonant Rayleigh-scattering are always superimposed to the incoherent emission.

In case of the non-resonant excitation, the structure is excited with some excess energy. This is the typical situation used in most PL experiments as the excitation energy can be discriminated using a spectrometer or an optical filter. One has to distinguish between quasi-resonant excitation and barrier excitation.

For quasi-resonant conditions, the energy of the excitation is tuned above the ground state but still below the barrier absorption edge, for example, into the continuum of the first subband. The polarization decay for these conditions is much faster than for resonant excitation and coherent contributions to the quantum well emission are negligible. The initial temperature of the carrier system is significantly higher than the lattice temperature due to the surplus energy of the injected carriers. Finally, only the electron-hole plasma is initially created. It is then followed by the formation of excitons.

In case of barrier excitation, the initial carrier distribution in the quantum well strongly depends on the carrier scattering between barrier and the well.

Relaxation

Initially, the laser light induces coherent polarization in the sample, i.e., the transitions between electron and hole states oscillate with the laser frequency and a fixed phase. The polarization dephases typically on a sub-100 fs time-scale in case of nonresonant excitation due to ultra-fast Coulomb- and phonon-scattering.

The dephasing of the polarization leads to creation of populations of electrons and holes in the conduction and the valence bands, respectively. The lifetime of the carrier populations is rather long, limited by radiative and non-radiative recombination such as Auger recombination. During this lifetime a fraction of electrons and holes may form excitons, this topic is still controversially discussed in the literature. The formation rate depends on the experimental conditions such as lattice temperature, excitation density, as well as on the general material parameters, e.g., the strength of the Coulomb-interaction or the exciton binding energy.

The characteristic time-scales are in the range of hundreds of picoseconds in GaAs; they appear to be much shorter in wide-gap semiconductors.

Directly after the excitation with short (femtosecond) pulses and the quasi-instantaneous decay of the polarization, the carrier distribution is mainly determined by the spectral width of the excitation, e.g., a laser pulse. The distribution is thus highly non-thermal and resembles a Gaussian distribution, centered at a finite momentum. In the first hundreds of femtoseconds, the carriers are scattered by phonons, or at elevated carrier densities via Coulomb-interaction. The carrier system successively relaxes to the Fermi–Dirac distribution typically within the first picosecond. Finally, the carrier system cools down under the emission of phonons. This can take up to several nanoseconds, depending on the material system, the lattice temperature, and the excitation conditions such as the surplus energy.

Initially, the carrier temperature decreases fast via emission of optical phonons. This is quite efficient due to the comparatively large energy associated with optical phonons, (36meV or 420K in GaAs) and their rather flat dispersion, allowing for a wide range of scattering processes under conservation of energy and momentum. Once the carrier temperature decreases below the value corresponding to the optical phonon energy, acoustic phonons dominate the relaxation. Here, cooling is less efficient due their dispersion and small energies and the temperature decreases much slower beyond the first tens of picoseconds. At elevated excitation densities, the carrier cooling is further inhibited by the so-called hot-phonon effect. The relaxation of a large number of hot carriers leads to a high generation rate of optical phonons which exceeds the decay rate into acoustic phonons. This creates a non-equilibrium "over-population" of optical phonons and thus causes their increased reabsorption by the charge-carriers significantly suppressing any cooling. Thus, a system cools slower, the higher the carrier density is.

Radiative recombination

The emission directly after the excitation is spectrally very broad, yet still centered in the vicinity of the strongest exciton resonance. As the carrier distribution relaxes and cools, the width of the PL peak decreases and the emission energy shifts to match the ground state of the exciton (such as an electron) for ideal samples without disorder. The PL spectrum approaches its quasi-steady-state shape defined by the distribution of electrons and holes. Increasing the excitation density will change the emission spectra. They are dominated by the excitonic ground state for low densities. Additional peaks from higher subband transitions appear as the carrier density or lattice temperature are increased as these states get more and more populated. Also, the width of the main PL peak increases significantly with rising excitation due to excitation-induced dephasing and the emission peak experiences a small shift in energy due to the Coulomb-renormalization and phase-filling.

In general, both exciton populations and plasma, uncorrelated electrons and holes, can act as sources for photoluminescence as described in the semiconductor-luminescence equations. Both yield very similar spectral features which are difficult to distinguish; their emission dynamics, however, vary significantly. The decay of excitons yields a single-exponential decay function since the probability of their radiative recombination does not depend on the carrier density. The probability of spontaneous emission for uncorrelated electrons and holes, is approximately proportional to the product of electron and hole populations eventually leading to a non-single-exponential decay described by a hyperbolic function.

Effects of disorder

Real material systems always incorporate disorder. Examples are structural defects in the lattice or disorder due to variations of the chemical composition. Their treatment is extremely challenging for microscopic theories due to the lack of detailed knowledge about perturbations of the ideal structure. Thus, the influence of the extrinsic effects on the PL is usually addressed phenomenologically. In experiments, disorder can lead to localization of carriers and hence drastically increase the photoluminescence life times as localized carriers cannot as easily find nonradiative recombination centers as can free ones.

Researchers from the King Abdullah University of Science and Technology (KAUST) have studied the photoinduced entropy (i.e. thermodynamic disorder) of InGaN/GaN p-i-n double-heterostructure and AlGaN nanowires using temperature-dependent photoluminescence. They defined the photoinduced entropy as a thermodynamic quantity that represents the unavailability of a system's energy for conversion into useful work due to carrier recombination and photon emission. They have also related the change in entropy generation to the change in photocarrier dynamics in the nanowire active regions using results from time-resolved photoluminescence study. They hypothesized that the amount of generated disorder in the InGaN layers eventually increases as the temperature approaches room temperature because of the thermal activation of surface states, while an insignificant increase was observed in AlGaN nanowires, indicating lower degrees of disorder-induced uncertainty in the wider bandgap semiconductor. To study the photoinduced entropy, the scientists have developed a mathematical model that considers the net energy exchange resulting from photoexcitation and photoluminescence.

Photoluminescence from metals

Unlike photoluminescence from semiconductors, photoluminescence from metals is a much weaker effect. In particular, its quantum yield is order of magnitude lower. Nevertheless, metal photoluminecence is of fundamental importance (e.g., for our understanding of electron structure and population dynamics in metals) as well as practical importance (e.g., for thermometry, see next Section).

Photoluminescent materials for temperature detection

In phosphor thermometry, the temperature dependence of the photoluminescence process is exploited to measure temperature. Somewhat similarly, anti-Stokes photoluminescence from metals can be used for thermometry.

Experimental methods

Photoluminescence spectroscopy is a widely used technique for characterisation of the optical and electronic properties of semiconductors and molecules. The technique itself is fast, contactless, and nondestructive. Therefore, it can be used to study the optoelectronic properties of materials of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. For example, photoluminescence measurements of solar cell absorbers can predict the maximum voltage the material could produce. In chemistry, the method is more often referred to as fluorescence spectroscopy, but the instrumentation is the same. The relaxation processes can be studied using time-resolved fluorescence spectroscopy to find the decay lifetime of the photoluminescence. These techniques can be combined with microscopy, to map the intensity (confocal microscopy) or the lifetime (fluorescence-lifetime imaging microscopy) of the photoluminescence across a sample (e.g. a semiconducting wafer, or a biological sample that has been marked with fluorescent molecules). Modulated photoluminescence is a specific method for measuring the complex frequency response of the photoluminescence signal to a sinusoidal excitation, allowing for the direct extraction of minority carrier lifetime without the need for intensity calibrations. It has been used to study the influence of interface defects on the recombination of excess carriers in crystalline silicon wafers with different passivation schemes.

Ecocriticism

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

Ecocriticism is the study of literature and ecology from an interdisciplinary point of view, where literature scholars analyze texts that illustrate environmental concerns and examine the various ways literature treats the subject of nature.^[1] It was first originated by Joseph Meeker as an idea called "literary ecology" in his The Comedy of Survival: Studies in Literary Ecology (1972).

The term 'ecocriticism' was coined in 1978 by William Rueckert in his essay "Literature and Ecology: An Experiment in Ecocriticism". It takes an interdisciplinary point of view by analyzing the works of authors, researchers and poets in the context of environmental issues and nature. Some ecocritics brainstorm possible solutions for the correction of the contemporary environmental situation, though not all ecocritics agree on the purpose, methodology, or scope of ecocriticism.

In the United States, ecocriticism is often associated with the Association for the Study of Literature and Environment (ASLE), which hosts a biennial conference for scholars who deal with environmental matters in literature and the environmental humanities in general. ASLE publishes a journal—Interdisciplinary Studies in Literature and Environment (ISLE)—in which current international scholarship can be found.

Ecocriticism is an intentionally broad approach that is known by a number of other designations, including "green (cultural) studies", "ecopoetics", and "environmental literary criticism", and is often informed by other fields such as ecology, sustainable design, biopolitics, environmental history, environmentalism, and social ecology, among others.

Definition

In comparison with other 'political' forms of criticism, there has been relatively little dispute about the moral and philosophical aims of ecocriticism, although its scope has broadened from nature writing, romantic poetry, and canonical literature to take in film, television, theatre, animal stories, architectures, scientific narratives and an extraordinary range of literary texts. At the same time, ecocriticism has borrowed methodologies and theoretically informed approaches liberally from other fields of literary, social and scientific study.

Cheryll Glotfelty's working definition in The Ecocriticism Reader is that "ecocriticism is the study of the relationship between literature and the physical environment", and one of the implicit goals of the approach is to recoup professional dignity for what Glotfelty calls the "undervalued genre of nature writing". Lawrence Buell defines "'ecocriticism' ... as [a] study of the relationship between literature and the environment conducted in a spirit of commitment to environmentalist praxis".

Simon Estok noted in 2001 that "ecocriticism has distinguished itself, debates notwithstanding, firstly by the ethical stand it takes, its commitment to the natural world as an important thing rather than simply as an object of thematic study, and, secondly, by its commitment to making connections".

More recently, in an article that extends ecocriticism to Shakespearean studies, Estok argues that ecocriticism is more than "simply the study of Nature or natural things in literature; rather, it is any theory that is committed to effecting change by analyzing the function–thematic, artistic, social, historical, ideological, theoretical, or otherwise–of the natural environment, or aspects of it, represented in documents (literary or other) that contribute to material practices in material worlds". This echoes the functional approach of the cultural ecology branch of ecocriticism, which analyzes the analogies between ecosystems and imaginative texts and posits that such texts potentially have an ecological (regenerative, revitalizing) function in the cultural system.

As Michael P. Cohen has observed, "if you want to be an ecocritic, be prepared to explain what you do and be criticized, if not satirized." Certainly, Cohen adds his voice to such critique, noting that one of the problems of ecocriticism has been what he calls its "praise-song school" of criticism. All ecocritics share an environmentalist motivation of some sort, but whereas the majority are 'nature endorsing', some are 'nature sceptical'. In part this entails a shared sense of the ways in which 'nature' has been used to legitimize gender, sexual and racial norms (so homosexuality has been seen as 'unnatural', for example), but it also involves scepticism about the uses to which 'ecological' language is put in ecocriticism; it can also involve a critique of the ways cultural norms of nature and the environment contribute to environmental degradation. Greg Garrard has dubbed 'pastoral ecology' the notion that nature undisturbed is balanced and harmonious, while Dana Phillips has criticised the literary quality and scientific accuracy of nature writing in The Truth of Ecology. Similarly, there has been a call to recognize the place of the environmental justice movement in redefining ecocritical discourse.

In response to the question of what ecocriticism is or should be, Camilo Gomides has offered an operational definition that is both broad and discriminating: "The field of enquiry that analyzes and promotes works of art which raise moral questions about human interactions with nature, while also motivating audiences to live within a limit that will be binding over generations". He tests it for a film adaptation about Amazonian deforestation. Implementing the Gomides definition, Joseph Henry Vogel makes the case that ecocriticism constitutes an "economic school of thought" as it engages audiences to debate issues of resource allocation that have no technical solution. Ashton Nichols has recently argued that the historical dangers of a romantic version of nature now need to be replaced by "urbanatural roosting", a view that sees urban life and the natural world as closely linked and argues for humans to live more lightly on the planet, the way virtually all other species do.

The interdisciplinary nature of Ecocriticism and Islam as well as their mutual interest in nature led to the coinage of Islamecocriticism in 2021. Islemecocriticism is fully introduced by ISLE: Interdisciplinary Studies in Literature and Environment in "Islamecocriticism: Green Islam Introduced to Ecocriticism." The article is shortly followed by a thorough representation of Material Islamecocriticism in "Matter Really Matters: A Poetic Material Islamecocritical Reading of Inanimateness Animism" which appeared in Kritika Kultura.

In literary studies

Ecocritics investigate such things as the underlying ecological values, what, precisely, is meant by the word nature, and whether the examination of "place" should be a distinctive category, much like class, gender or race. Ecocritics examine human perception of wilderness, and how it has changed throughout history and whether or not current environmental issues are accurately represented or even mentioned in popular culture and modern literature. Not only do ecocritics determine the actual meaning of nature writing texts, but they use those texts for analyzing the practices of society in relationship to nature. They also critique visions that are human-centered and man/male centered. Scholars in ecocriticism engage in questions regarding anthropocentrism, and the "mainstream assumption that the natural world be seen primarily as a resource for human beings" as well as critical approaches to changing ideas in "the material and cultural bases of modern society." Recently, "empirical ecocritics" have begun empirically evaluating the influence of ecofiction on its readers and the prevalence of environmental narratives in popular media. Other disciplines, such as history, economics, philosophy, ethics, and psychology, are also considered by ecocritics to be possible contributors to ecocriticism.

While William Rueckert may have been the first person to use the term ecocriticism (Barry 240) in his 1978 essay entitled Literature and Ecology: An Experiment in Ecocriticism, ecocriticism as a movement owes much to Rachel Carson's 1962 environmental exposé Silent Spring. Drawing from this critical moment, Rueckert's intent was to focus on "the application of ecology and ecological concepts to the study of literature".

Ecologically minded individuals and scholars have been publishing progressive works of ecotheory and criticism since the explosion of environmentalism in the late 1960s and 1970s. However, because there was no organized movement to study the ecological/environmental side of literature, these important works were scattered and categorized under a litany of different subject headings: pastoralism, human ecology, regionalism, American Studies etc. British marxist critic Raymond Williams, for example, wrote a seminal critique of pastoral literature in 1973, The Country and the City.

Another early ecocritical text, Joseph Meeker's The Comedy of Survival (1974), proposed a version of an argument that was later to dominate ecocriticism and environmental philosophy; that environmental crisis is caused primarily by a cultural tradition in the West of separation of culture from nature, and elevation of the former to moral predominance. Such anthropocentrism is identified in the tragic conception of a hero whose moral struggles are more important than mere biological survival, whereas the science of animal ethology, Meeker asserts, shows that a "comic mode" of muddling through and "making love not war" has superior ecological value. In the later, "second wave" ecocriticism, Meeker's adoption of an ecophilosophical position with apparent scientific sanction as a measure of literary value tended to prevail over Williams's ideological and historical critique of the shifts in a literary genre's representation of nature.

As Glotfelty noted in The Ecocriticism Reader, "One indication of the disunity of the early efforts is that these critics rarely cited one another's work; they didn't know that it existed...Each was a single voice howling in the wilderness." Nevertheless, ecocriticism—unlike feminist and Marxist criticisms—failed to crystallize into a coherent movement in the late 1970s, and indeed only did so in the US in the 1990s.

In the mid-1980s, scholars began to work collectively to establish ecocriticism as a genre, primarily through the work of the Western Literature Association in which the revaluation of nature writing as a non-fictional literary genre could function. During the late-1980s poet Jack Collom was awarded a 2nd National Endowment for the Arts grant, for his ground-breaking work in this emerging genre. Collom taught an influential Eco-Lit course at Naropa University in Boulder, Colorado, for nearly two decades. In 1990, at the University of Nevada, Reno, Glotfelty became the first person to hold an academic position as a professor of Literature and the Environment, and UNR, with the aid of the now-retired Glotfelty and the remaining professor Michael P. Branch, has retained the position it established at that time as the intellectual home of ecocriticism even as ASLE has burgeoned into an organization with thousands of members in the US alone. From the late 1990s, new branches of ASLE and affiliated organizations were started in the UK, Japan, Korea, Australia and New Zealand (ASLEC-ANZ), India (OSLE-India), Southeast Asia (ASLE-ASEAN), Taiwan, Canada and continental Europe. The emergence of ecocriticism in British literary criticism is usually dated to the publication in 1991 of Romantic Ecology: Wordsworth and the Environmental Tradition by Jonathan Bate.

Ecocriticism’s second wave emerged in the 2000s through a more complex understanding of the overall history of global environmentalism and environmental justice. According to Lawrence Buell, former Harvard professor and proponent of ecocriticism, the second wave of ecocriticism aligns with public health environmentalism, with ethics and politics that are sociocentric rather than ecocentric. The second wave not only considers rural landscapes or wilderness, but also landscapes of urban and industrial transformation. It is inspired by writers such as Charles Dickens, who wrote about Victorian-era public health concerns, and the American novelist Upton Sinclair, as well as by global activists, such as Ken Saro-Wiwa, who was executed for his protests against ecological devastation in Nigeria, and Michiko Ishimure, who wrote about Minamata disease and the effects of mercury poisoning. The second wave of ecocriticism distinguishes itself from the first wave by prioritizing the exploration of issues such as environmental resource distribution, environmental justice, minority and socioeconomic impacts related to environmental circumstances. A representative of second-wave ecocriticism is the 2002 Environmental Justice Reader: Politics, Poetics, and Pedagogy.

Recent eco-critical scholarship has seen issues concerning eco-anxiety, veganism, and activism take precedence. Scholars are now starting to take into consideration the impact of the Covid-19 pandemic on environmental activism and the climate crisis more broadly. In their 2022 Special Issue for Green Letters entitled 'A New Poetics of Space', the co-editors, Lucy Jeffery and Vicky Angelaki, remark: 'We hope that through the analysis of the act of walking in the creative arts we can understand our role in shaping the environmental state in which we find ourselves. Throughout this volume we ask: is there a way yet to describe how our attitude towards our being in the environment has shifted since the Covid-19 pandemic?' This scholarly volume is evidence of a current trend in eco-critical scholarship to explore the impact of the twenty-first century's technological developments, societal shifts, environmental challenges, and political situation through the perspective of creative works that are concerned with the poetics of space, health, and the environment (both urban and rural).

Collective unconscious

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

In psychology, the collective unconsciousness (German: kollektives Unbewusstes) is a term coined by Carl Jung, which is the belief that the unconscious mind comprises the instincts of Jungian archetypes—innate symbols understood from birth in all humans. Jung considered the collective unconscious to underpin and surround the unconscious mind, distinguishing it from the personal unconscious of Freudian psychoanalysis. He believed that the concept of the collective unconscious helps to explain why similar themes occur in mythologies around the world. He argued that the collective unconscious had a profound influence on the lives of individuals, who lived out its symbols and clothed them in meaning through their experiences. The psychotherapeutic practice of analytical psychology revolves around examining the patient's relationship to the collective unconscious.

Psychiatrist and Jungian analyst Lionel Corbett argues that the contemporary terms "autonomous psyche" or "objective psyche" are more commonly used in the practice of depth psychology rather than the traditional term of the "collective unconscious". Critics of the collective unconscious concept have called it unscientific and fatalistic, or otherwise very difficult to test scientifically (due to the mystical aspect of the collective unconscious). Proponents suggest that it is borne out by findings of psychology, neuroscience, and anthropology.

Basic explanation

The term "collective unconscious" first appeared in Jung's 1916 essay, "The Structure of the Unconscious". This essay distinguishes between the "personal", Freudian unconscious, filled with sexual fantasies and repressed images, and the "collective" unconscious encompassing the soul of humanity at large.

In "The Significance of Constitution and Heredity in Psychology" (November 1929), Jung wrote:

And the essential thing, psychologically, is that in dreams, fantasies, and other exceptional states of mind the most far-fetched mythological motifs and symbols can appear autochthonously at any time, often, apparently, as the result of particular influences, traditions, and excitations working on the individual, but more often without any sign of them. These "primordial images" or "archetypes," as I have called them, belong to the basic stock of the unconscious psyche and cannot be explained as personal acquisitions. Together they make up that psychic stratum which has been called the collective unconscious.
The existence of the collective unconscious means that individual consciousness is anything but a tabula rasa and is not immune to predetermining influences. On the contrary, it is in the highest degree influenced by inherited presuppositions, quite apart from the unavoidable influences exerted upon it by the environment. The collective unconscious comprises in itself the psychic life of our ancestors right back to the earliest beginnings. It is the matrix of all conscious psychic occurrences, and hence it exerts an influence that compromises the freedom of consciousness in the highest degree, since it is continually striving to lead all conscious processes back into the old paths.

On October 19, 1936, Jung delivered a lecture "The Concept of the Collective Unconscious" to the Abernethian Society at St. Bartholomew's Hospital in London. He said:

My thesis then, is as follows: in addition to our immediate consciousness, which is of a thoroughly personal nature and which we believe to be the only empirical psyche (even if we tack on the personal unconscious as an appendix), there exists a second psychic system of a collective, universal, and impersonal nature which is identical in all individuals. This collective unconscious does not develop individually but is inherited. It consists of pre-existent forms, the archetypes, which can only become conscious secondarily and which give definite form to certain psychic contents.

Jung linked the collective unconscious to "what Freud called 'archaic remnants' – mental forms whose presence cannot be explained by anything in the individual's own life and which seem to be aboriginal, innate, and inherited shapes of the human mind". He credited Freud for developing his "primal horde" theory in Totem and Taboo and continued further with the idea of an archaic ancestor maintaining its influence in the minds of present-day humans. Every human being, he wrote, "however high his conscious development, is still an archaic man at the deeper levels of his psyche."

As modern humans go through their process of individuation, moving out of the collective unconscious into mature selves, they establish a persona—which can be understood simply as that small portion of the collective psyche which they embody, perform, and identify with.

The collective unconscious exerts overwhelming influence on the minds of individuals. These effects of course vary widely, however, since they involve virtually every emotion and situation. At times, the collective unconscious can terrify, but it can also heal.

Archetypes

Main article: Archetypal psychology

In an early definition of the term, Jung writes: "Archetypes are typical modes of apprehension, and wherever we meet with uniform and regularly recurring modes of apprehension we are dealing with an archetype, no matter whether its mythological character is recognized or not." He traces the term back to Philo, Irenaeus, and the Corpus Hermeticum, which associate archetypes with divinity and the creation of the world, and notes the close relationship of Platonic ideas.

These archetypes dwell in a world beyond the chronology of a human lifespan, developing on an evolutionary timescale. Regarding the animus and anima, the male principle within the woman and the female principle within the man, Jung writes:

They evidently live and function in the deeper layers of the unconscious, especially in that phylogenetic substratum which I have called the collective unconscious. This localization explains a good deal of their strangeness: they bring into our ephemeral consciousness an unknown psychic life belonging to a remote past. It is the mind of our unknown ancestors, their way of thinking and feeling, their way of experiencing life and the world, gods, and men. The existence of these archaic strata is presumably the source of man's belief in reincarnations and in memories of "previous experiences". Just as the human body is a museum, so to speak, of its phylogenetic history, so too is the psyche.

Jung also described archetypes as imprints of momentous or frequently recurring situations in the lengthy human past.

A complete list of archetypes cannot be made, nor can differences between archetypes be absolutely delineated. For example, the Eagle is a common archetype that may have a multiplicity of interpretations. It could mean the soul leaving the mortal body and connecting with the heavenly spheres, or it may mean that someone is sexually impotent, in that they have had their spiritual ego body engaged. In spite of this difficulty, Jungian analyst June Singer suggests a partial list of well-studied archetypes, listed in pairs of opposites:

Ego	Shadow
Sacred Progenitor	Tyrannical Progenitor
Old Wise Man	Trickster
Animus	Anima
Meaning	Absurdity
Centrality	Diffusion
Order	Chaos
Opposition	Conjunction
Time	Eternity
Sacred	Profane
Light	Darkness
Transformation	Fixity

Jung made reference to contents of this category of the unconscious psyche as being similar to Levy-Bruhl's use of "collective representations", Hubert and Mauss's "categories of the imagination", and Adolf Bastian's "primordial thoughts". He also called archetypes "dominants" because of their profound influence on mental life.

Instincts

Jung's exposition of the collective unconscious builds on the classic issue in psychology and biology regarding nature versus nurture. If we accept that nature, or heredity, has some influence on the individual psyche, we must examine the question of how this influence takes hold in the real world.

On exactly one night in its entire lifetime, the yucca moth discovers pollen in the opened flowers of the yucca plant, forms some into a pellet, and then transports this pellet, with one of its eggs, to the pistil of another yucca plant. This activity cannot be "learned"; it makes more sense to describe the yucca moth as experiencing intuition about how to act. Archetypes and instincts coexist in the collective unconscious as interdependent opposites, Jung would later clarify. Whereas for most animals intuitive understandings completely intertwine with instinct, in humans the archetypes have become a separate register of mental phenomena.

Humans experience five main types of instinct, wrote Jung: hunger, sexuality, activity, reflection, and creativity. These instincts, listed in order of increasing abstraction, elicit and constrain human behavior, but also leave room for freedom in their implementation and especially in their interplay. Even a simple hungry feeling can lead to many different responses, including metaphorical sublimation. These instincts could be compared to the "drives" discussed in psychoanalysis and other domains of psychology. Several readers of Jung have observed that in his treatment of the collective unconscious, Jung suggests an unusual mixture of primordial, "lower" forces, and spiritual, "higher" forces.

Exploration

Moře (Sea), Eduard Tomek (cs), 1971

Jung believed that proof of the existence of a collective unconscious, and insight into its nature, could be gleaned primarily from dreams and from active imagination, a waking exploration of fantasy.

Jung considered that 'the shadow' and the anima and animus differ from the other archetypes in the fact that their content is more directly related to the individual's personal situation'. These archetypes, a special focus of Jung's work, become autonomous personalities within an individual psyche. Jung encouraged direct conscious dialogue of the patients with these personalities within. While the shadow usually personifies the personal unconscious, the anima or the Wise Old Man can act as representatives of the collective unconscious.

Jung suggested that parapsychology, alchemy, and occult religious ideas could contribute understanding of the collective unconscious. Based on his interpretation of synchronicity and extra-sensory perception, Jung argued that psychic activity transcended the brain. In alchemy, Jung found that plain water, or seawater, corresponded to his concept of the collective unconscious.

In humans, the psyche mediates between the primal force of the collective unconscious and the experience of consciousness or dream. Therefore, symbols may require interpretation before they can be understood as archetypes. Jung writes:

We have only to disregard the dependence of dream language on environment and substitute "eagle" for "aeroplane," "dragon" for "automobile" or "train," "snake-bite" for "injection," and so forth, in order to arrive at the more universal and more fundamental language of mythology. This give us access to the primordial images that underlie all thinking and have a considerable influence even on our scientific ideas.

A single archetype can manifest in many different ways. Regarding the Mother archetype, Jung suggests that not only can it apply to mothers, grandmothers, stepmothers, mothers-in-law, and mothers in mythology, but to various concepts, places, objects, and animals:

Other symbols of the mother in a figurative sense appear in things representing the goal of our longing for redemption, such as Paradise, the Kingdom of God, the Heavenly Jerusalem. Many things arousing devotion or feelings of awe, as for instance the Church, university, city or country, heaven, earth, the woods, the sea or any still waters, matter even, the underworld and the moon, can be mother-symbols. The archetype is often associated with things and places standing for fertility and fruitfulness: the cornucopia, a ploughed field, a garden. It can be attached to a rock, a cave, a tree, a spring, a deep well, or to various vessels such as the baptismal font, or to vessel-shaped flowers like the rose or the lotus. Because of the protection it implies, the magic circle or mandala can be a form of mother archetype. Hollow objects such as ovens or cooking vessels are associated with the mother archetype, and, of course, the uterus, yoni, and anything of a like shape. Added to this list there are many animals, such as the cow, hare, and helpful animals in general.

Care must be taken, however, to determine the meaning of a symbol through further investigation; one cannot simply decode a dream by assuming these meanings are constant. Archetypal explanations work best when an already-known mythological narrative can clearly help to explain the confusing experience of an individual.

Evidence

In his clinical psychiatry practice, Jung identified mythological elements which seemed to recur in the minds of his patients—above and beyond the usual complexes which could be explained in terms of their personal lives. The most obvious patterns applied to the patient's parents: "Nobody knows better than the psychotherapist that the mythologizing of the parents is often pursued far into adulthood and is given up only with the greatest resistance."

Jung cited recurring themes as evidence of the existence of psychic elements shared among all humans. For example: "The snake-motif was certainly not an individual acquisition of the dreamer, for snake-dreams are very common even among city-dwellers who have probably never seen a real snake." Still better evidence, he felt, came when patients described complex images and narratives with obscure mythological parallels. Jung's leading example of this phenomenon was a paranoid-schizophrenic patient who could see the sun's dangling phallus, whose motion caused wind to blow on earth. Jung found a direct analogue of this idea in the "Mithras Liturgy", from the Greek Magical Papyri of Ancient Egypt—only just translated into German—which also discussed a phallic tube, hanging from the sun, and causing wind to blow on earth. He concluded that the patient's vision and the ancient Liturgy arose from the same source in the collective unconscious.

Going beyond the individual mind, Jung believed that "the whole of mythology could be taken as a sort of projection of the collective unconscious". Therefore, psychologists could learn about the collective unconscious by studying religions and spiritual practices of all cultures, as well as belief systems like astrology.

Criticism of Jung's evidence

Popperian critic Ray Scott Percival disputes some of Jung's examples and argues that his strongest claims are not falsifiable. Percival takes special issue with Jung's claim that major scientific discoveries emanate from the collective unconscious and not from unpredictable or innovative work done by scientists. Percival charges Jung with excessive determinism and writes: "He could not countenance the possibility that people sometimes create ideas that cannot be predicted, even in principle." Regarding the claim that all humans exhibit certain patterns of mind, Percival argues that these common patterns could be explained by common environments (i.e. by shared nurture, not nature). Because all people have families, encounter plants and animals, and experience night and day, it should come as no surprise that they develop basic mental structures around these phenomena.

This latter example has been the subject of contentious debate, and Jung critic Richard Noll has argued against its authenticity.

Ethology and biology

Animals all have some innate psychological concepts which guide their mental development. The concept of imprinting in ethology is one well-studied example, dealing most famously with the Mother constructs of newborn animals. The many predetermined scripts for animal behavior are called innate releasing mechanisms.

Proponents of the collective unconscious theory in neuroscience suggest that mental commonalities in humans originate especially from the subcortical area of the brain: specifically, the thalamus and limbic system. These centrally located structures link the brain to the rest of the nervous system and are said to control vital processes including emotions and long-term memory .

Archetype research

A more common experimental approach investigates the unique effects of archetypal images. An influential study of this type, by Rosen, Smith, Huston, & Gonzalez in 1991, found that people could better remember symbols paired with words representing their archetypal meaning. Using data from the Archive for Research in Archetypal Symbolism and a jury of evaluators, Rosen et al. developed an "Archetypal Symbol Inventory" listing symbols and one-word connotations. Many of these connotations were obscure to laypeople. For example, a picture of a diamond represented "self"; a square represented "Earth". They found that even when subjects did not consciously associate the word with the symbol, they were better able to remember the pairing of the symbol with its chosen word. Brown & Hannigan replicated this result in 2013, and expanded the study slightly to include tests in English and in Spanish of people who spoke both languages.

Maloney (1999) asked people questions about their feelings to variations on images featuring the same archetype: some positive, some negative, and some non-anthropomorphic. He found that although the images did not elicit significantly different responses to questions about whether they were "interesting" or "pleasant", but did provoke highly significant differences in response to the statement: "If I were to keep this image with me forever, I would be". Maloney suggested that this question led the respondents to process the archetypal images on a deeper level, which strongly reflected their positive or negative valence.

Ultimately, although Jung referred to the collective unconscious as an empirical concept, based on evidence, its elusive nature does create a barrier to traditional experimental research. June Singer writes:

But the collective unconscious lies beyond the conceptual limitations of individual human consciousness, and thus cannot possibly be encompassed by them. We cannot, therefore, make controlled experiments to prove the existence of the collective unconscious, for the psyche of man, holistically conceived, cannot be brought under laboratory conditions without doing violence to its nature. ... In this respect, psychology may be compared to astronomy, the phenomena of which also cannot be enclosed within a controlled setting. The heavenly bodies must be observed where they exist in the natural universe, under their own conditions, rather than under conditions we might propose to set for them.

Application to psychotherapy

Psychotherapy based on analytical psychology would seek to analyze the relationship between a person's individual consciousness and the deeper common structures which underlie them. Personal experiences both activate archetypes in the mind and give them meaning and substance for individual. At the same time, archetypes covertly organize human experience and memory, their powerful effects becoming apparent only indirectly and in retrospect. Understanding the power of the collective unconscious can help an individual to navigate through life.

In the interpretation of analytical psychologist Mary Williams, a patient who understands the impact of the archetype can help to dissociate the underlying symbol from the real person who embodies the symbol for the patient. In this way, the patient no longer uncritically transfers their feelings about the archetype onto people in everyday life, and as a result, can develop healthier and more personal relationships.

Practitioners of analytic psychotherapy, Jung cautioned, could become so fascinated with manifestations of the collective unconscious that they facilitated their appearance at the expense of their patient's well-being. Individuals with schizophrenia, it is said, fully identify with the collective unconscious, lacking a functioning ego to help them deal with actual difficulties of life.

Application to politics and society

Elements from the collective unconscious can manifest among groups of people, who by definition all share a connection to these elements. Groups of people can become especially receptive to specific symbols due to the historical situation they find themselves in. The common importance of the collective unconscious makes people ripe for political manipulation, especially in the era of mass politics. Jung compared mass movements to mass psychoses, comparable to demonic possession in which people uncritically channel unconscious symbolism through the social dynamic of the mob and the leader.

Although civilization leads people to disavow their links with the mythological world of uncivilized societies, Jung argued that aspects of the primitive unconscious would nevertheless reassert themselves in the form of superstitions, everyday practices, and unquestioned traditions such as the Christmas tree.

Based on empirical inquiry, Jung felt that all humans, regardless of racial and geographic differences, share the same collective pool of instincts and images, though these manifest differently due to the moulding influence of culture. However, above and in addition to the primordial collective unconscious, people within a certain culture may share additional bodies of primal collective ideas.

Jung called the UFO phenomenon a "living myth", a legend in the process of consolidation. Belief in a messianic encounter with UFOs demonstrated the point, Jung argued, that even if a rationalistic modern ideology repressed the images of the collective unconscious, its fundamental aspects would inevitably resurface. The circular shape of the flying saucer confirms its symbolic connection to repressed but psychically necessary ideas of divinity.

The universal applicability of archetypes has not escaped the attention of marketing specialists, who observe that branding can resonate with consumers through appeal to archetypes of the collective unconscious.

Distinction from related concepts

Jung contrasted the collective unconscious with the personal unconscious, the unique aspects of an individual study which Jung says constitute the focus of Sigmund Freud and Alfred Adler. Psychotherapy patients, it seemed to Jung, often described fantasies and dreams which repeated elements from ancient mythology. These elements appeared even in patients who were probably not exposed to the original story. For example, mythology offers many examples of the "dual mother" narrative, according to which a child has a biological mother and a divine mother. Therefore, argues Jung, Freudian psychoanalysis would neglect important sources for unconscious ideas, in the case of a patient with neurosis around a dual-mother image.

This divergence over the nature of the unconscious has been cited as a key aspect of Jung's famous split from Sigmund Freud and his school of psychoanalysis. Some commentators have rejected Jung's characterization of Freud, observing that in texts such as Totem and Taboo (1913) Freud directly addresses the interface between the unconscious and society at large. Jung himself said that Freud had discovered a collective archetype, the Oedipus complex, but that it "was the first archetype Freud discovered, the first and only one".

Probably none of my empirical concepts has been met with so much misunderstanding as the idea of the collective unconscious.

Jung, October 19, 1936

Jung also distinguished the collective unconscious and collective consciousness, between which lay "an almost unbridgeable gulf over which the subject finds himself suspended". According to Jung, collective consciousness (meaning something along the lines of consensus reality) offered only generalizations, simplistic ideas, and the fashionable ideologies of the age. This tension between collective unconscious and collective consciousness corresponds roughly to the "everlasting cosmic tug of war between good and evil" and has worsened in the time of the mass man.

Organized religion, exemplified by the Catholic Church, lies more with the collective consciousness; but, through its all-encompassing dogma it channels and molds the images which inevitably pass from the collective unconscious into the minds of people. (Conversely, religious critics including Martin Buber accused Jung of wrongly placing psychology above transcendental factors in explaining human experience.)

Minimal and maximal interpretations

In a minimalist interpretation of what would then appear as "Jung's much misunderstood idea of the collective unconscious", his idea was "simply that certain structures and predispositions of the unconscious are common to all of us ... [on] an inherited, species-specific, genetic basis". Thus "one could as easily speak of the 'collective arm' – meaning the basic pattern of bones and muscles which all human arms share in common."

Others point out however that "there does seem to be a basic ambiguity in Jung's various descriptions of the Collective Unconscious. Sometimes he seems to regard the predisposition to experience certain images as understandable in terms of some genetic model" – as with the collective arm. However, Jung was "also at pains to stress the numinous quality of these experiences, and there can be no doubt that he was attracted to the idea that the archetypes afford evidence of some communion with some divine or world mind', and perhaps 'his popularity as a thinker derives precisely from this" – the maximal interpretation.

Marie-Louise von Franz accepted that "it is naturally very tempting to identify the hypothesis of the collective unconscious historically and regressively with the ancient idea of an all-extensive world-soul." New Age writer Sherry Healy goes further, claiming that Jung himself "dared to suggest that the human mind could link to ideas and motivations called the collective unconscious ... a body of unconscious energy that lives forever." This is the idea of monopsychism.

RNA-based evolution

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/RNA-based_evolution 

RNA-based evolution is a theory that posits that RNA is not merely an intermediate between the Watson and Crick model of the DNA molecule and proteins, but rather a far more dynamic and independent role-player in determining phenotype. By regulating the transcription in DNA sequences, the stability of RNA, and the capability of messenger RNA to be translated, RNA processing events allow for a diverse array of proteins to be synthesized from a single gene. Since RNA processing is heritable, it is subject to natural selection suggested by Darwin and contributes to the evolution and diversity of most eukaryotic organisms.

Role of RNA in conventional evolution

In accordance with the central dogma of molecular biology, RNA passes information between the DNA of a genome and the proteins expressed within an organism. Therefore, from an evolutionary standpoint, a mutation within the DNA bases results in an alteration of the RNA transcripts, which in turn leads to a direct difference in phenotype. RNA is also believed to have been the genetic material of the first life on Earth. The role of RNA in the origin of life is best supported by the ease of forming RNA from basic chemical building blocks (such as amino acids, sugars, and hydroxyl acids) that were likely present 4 billion years ago. Molecules of RNA have also been shown to effectively self-replicate, catalyze basic reactions, and store heritable information. As life progressed and evolved over time only DNA, which is much more chemically stable than RNA, could support large genomes and eventually took over the role as the major carrier of genetic information.

Single-Stranded RNA can fold into complex structures

Single-stranded RNA molecules can single handedly fold into complex structures. The molecules fold into secondary and tertiary structures by intramolecular base pairing. There is a fine dynamic of disorder and order that facilitate an efficient structure formation. RNA strands form complementary base pairs. These complementary strands of RNA base pair with another strand, which results in a three-dimensional shape from the paired strands folding in on itself. The formation of the secondary structure results from base pairing by hydrogen bonds between the strands, while tertiary structure results from folding of the RNA. The three-dimensional structure consists of grooves and helices. The formation of these complex structure gives reason to suspect that early life could have formed by RNA.

Variability of RNA processing

Research within the past decade has shown that strands of RNA are not merely transcribed from regions of DNA and translated into proteins. Rather RNA has retained some of its former independence from DNA and is subject to a network of processing events that alter the protein expression from that bounded by just the genomic DNA. Processing of RNA influences protein expression by managing the transcription of DNA sequences, the stability of RNA, and the translation of messenger RNA.

Alternative splicing

Splicing is the process by which non-coding regions of RNA are removed. The number and combination of splicing events varies greatly based on differences in transcript sequence and environmental factors. Variation in phenotype caused by alternative splicing is best seen in the sex determination of D. melanogaster. The Tra gene, determinant of sex, in male flies becomes truncated as splicing events fail to remove a stop codon that controls the length of the RNA molecule. In others the stop signal is retained within the final RNA molecule and a functional Tra protein is produced resulting in the female phenotype. Thus, alternative RNA splicing events allow differential phenotypes, regardless of the identity of the coding DNA sequence.

RNA stability

Phenotype may also be determined by the number of RNA molecules, as more RNA transcripts lead to a greater expression of protein. Short tails of repetitive nucleic acids are often added to the ends of RNA molecules in order to prevent degradation, effectively increasing the number of RNA strands able to be translated into protein. During mammalian liver regeneration RNA molecules of growth factors increase in number due to the addition of signaling tails. With more transcripts present the growth factors are produced at a higher rate, aiding the rebuilding process of the organ.

RNA silencing

Silencing of RNA occurs when double stranded RNA molecules are processed by a series of enzymatic reactions, resulting in RNA fragments that degrade complementary RNA sequences. By degrading transcripts, a lower amount of protein products are translated and the phenotype is altered by yet another RNA processing event.

RNA and Protein

In Earth's early developmental history RNA was the primary substance of life. RNA served as a blueprint for genetic material and was the catalyst to multiply said blueprint. Currently RNA acts by forming proteins. protein enzymes carry out catalytic reactions. RNAs are critical in gene expression and that gene expression depends on mRNA, rRNA, and tRNA. There is a relationship between protein and RNAs. This relationship could suggest that there is a mutual transfer of energy or information. In vitro RNA selection experiments have produced RNA that bind tightly to amino acids. It has been shown that the amino acids recognized by the RNA nucleotide sequences had a disproportionately high frequency of codons for said amino acids. There is a possibility that the direct association of amino acids containing specific RNA sequences yielded a limited genetic code.

Evolutionary mechanism

Most RNA processing events work in concert with one another and produce networks of regulating processes that allow a greater variety of proteins to be expressed than those strictly directed by the genome. These RNA processing events can also be passed on from generation to generation via reverse transcription into the genome. Over time, RNA networks that produce the fittest phenotypes will be more likely to be maintained in a population, contributing to evolution. Studies have shown that RNA processing events have especially been critical with the fast phenotypic evolution of vertebrates—large jumps in phenotype explained by changes in RNA processing events. Human genome searches have also revealed RNA processing events that have provided significant “sequence space for more variability”. On the whole, RNA processing expands the possible phenotypes of a given genotype and contributes to the evolution and diversity of life.

RNA virus evolution

RNA virus evolution appears to be facilitated by a high mutation rate caused by the lack of a proofreading mechanism during viral genome replication. In addition to mutation, RNA virus evolution is also facilitated by genetic recombination. Genetic recombination can occur when at least two RNA viral genomes are present in the same host cell and has been studies in numerous RNA viruses. RNA recombination appears to be a major driving force in viral evolution among Picornaviridae ((+)ssRNA) (e.g. poliovirus). In the Retroviridae ((+)ssRNA)(e.g. HIV), damage in the RNA genome appears to be avoided during reverse transcription by strand switching, a form of genetic recombination. Recombination also occurs in the Coronaviridae ((+)ssRNA) (e.g. SARS). Recombination in RNA viruses appears to be an adaptation for coping with genome damage. Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans.