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Atomic physics | See also | See also
Particle physics
Isomeric shift
Atomism
Ionisation
Quantum Mechanics
Electron Correlation
Quantum Chemistry
Bound State |
Atomic physics | Bibliography | Bibliography
Sommerfeld, A. (1923) Atomic structure and spectral lines. (translated from German "Atombau und Spektrallinien" 1921), Dutton Publisher.
Smirnov, B.E. (2003) Physics of Atoms and Ions, Springer. .
Szász, L. (1992) The Electronic Structure of Atoms, John Willey & Sons. .
Bethe, H.A. & Salpeter E.E. (1957) Quantum Mechanics of One- and Two Electron Atoms. Springer.
Born, M. (1937) Atomic Physics. Blackie & Son Limited.
Cox, P.A. (1996) Introduction to Quantum Theory and Atomic Spectra. Oxford University Press. ISBN 0-19-855916
|
Atomic physics | References | References |
Atomic physics | External links | External links
MIT-Harvard Center for Ultracold Atoms
Stanford QFARM Initiative for Quantum Science & Enginneering
Joint Quantum Institute at University of Maryland and NIST
Atomic Physics on the Internet
JILA (Atomic Physics)
ORNL Physics Division
Category:Atomic, molecular, and optical physics |
Atomic physics | Table of Content | Short description, Isolated atoms, Electronic configuration, Bohr Model of the Atom, History and developments, Significant atomic physicists, See also, Bibliography, References, External links |
American Sign Language | Short description | American Sign Language (ASL) is a natural languageAbout American Sign Language , Deaf Research Library, Karen Nakamura that serves as the predominant sign language of Deaf communities in the United States and most of Anglophone Canada. ASL is a complete and organized visual language that is expressed by employing both manual and nonmanual features. Besides North America, dialects of ASL and ASL-based creoles are used in many countries around the world, including much of West Africa and parts of Southeast Asia. ASL is also widely learned as a second language, serving as a lingua franca. ASL is most closely related to French Sign Language (LSF). It has been proposed that ASL is a creole language of LSF, although ASL shows features atypical of creole languages, such as agglutinative morphology.
ASL originated in the early 19th century in the American School for the Deaf (ASD) in Hartford, Connecticut, from a situation of language contact. Since then, ASL use has been propagated widely by schools for the deaf and deaf community organizations. Despite its wide use, no accurate count of ASL users has been taken. Reliable estimates for American ASL users range from 250,000 to 500,000 persons, including a number of children of deaf adults (CODA) and other hearing individuals.
Signs in ASL have a number of phonemic components, such as movement of the face, the torso, and the hands. ASL is not a form of pantomime, although iconicity plays a larger role in ASL than in spoken languages. English loan words are often borrowed through fingerspelling, although ASL grammar is unrelated to that of English. ASL has verbal agreement and aspectual marking and has a productive system of forming agglutinative classifiers. Many linguists believe ASL to be a subject–verb–object language. However, there are several other proposals to account for ASL word order. |
American Sign Language | Classification | Classification
thumb|Travis Dougherty explains and demonstrates the ASL alphabet. Voice-over interpretation by Gilbert G. Lensbower.
ASL emerged as a language in the American School for the Deaf (ASD), founded by Thomas Gallaudet in 1817, which brought together Old French Sign Language, various village sign languages, and home sign systems. ASL was created in that situation by language contact. ASL is influenced by its forerunners, yet linguistically distinct.
The influence of French Sign Language (LSF) on ASL is readily apparent; for example, it has been found that about 58% of signs in modern ASL are cognate to Old French Sign Language signs. However, that is far less than the standard 80% measure used to determine whether related languages are actually dialects. That suggests nascent ASL was highly affected by the other signing systems brought by the ASD students although the school's original director, Laurent Clerc, taught in LSF. In fact, Clerc reported that he often learned the students' signs rather than conveying LSF:
It has been proposed that ASL is a creole in which LSF is the superstrate language and the native village sign languages are substrate languages. However, more recent research has shown that modern ASL does not share many of the structural features that characterize creole languages. ASL may have begun as a creole and then undergone structural change over time, but it is also possible that it was never a creole-type language. There are modality-specific reasons that signed languages tend towards agglutination, such as the ability to simultaneously convey information via the face, head, torso, and other body parts. That might override creole characteristics such as the tendency towards isolating morphology. Additionally, Clerc and Thomas Hopkins Gallaudet may have used an artificially constructed form of manually coded language in instruction rather than true LSF.
Although the United States, the United Kingdom, and Australia share English as a common oral and written language, ASL is not mutually intelligible with either British Sign Language (BSL) or Auslan. All three languages show degrees of borrowing from English, but that alone is not sufficient for cross-language comprehension. It has been found that a relatively high percentage (37–44%) of ASL signs have similar translations in Auslan, which for oral languages would suggest that they belong to the same language family. However, that does not seem justified historically for ASL and Auslan, and it is likely that the resemblance is caused by the higher degree of iconicity in sign languages in general as well as contact with English.
American Sign Language is growing in popularity in many states. Many high school and university students desire to take it as a foreign language, but until recently, it was usually not considered a creditable foreign language elective. ASL users, however, have a very distinct culture, and they interact very differently when they talk. Their facial expressions and hand movements reflect what they are communicating. They also have their own sentence structure, which sets the language apart.
American Sign Language is now being accepted by many colleges as a language eligible for foreign language course credit; many states are making it mandatory to accept it as such. In some states however, this is only true with regard to high school coursework. |
American Sign Language | History | History
thumb|upright|alt=signing man sitting in the foreground, with a speaker standing at a podium in the background|A sign language interpreter at a presentation
Prior to the birth of ASL, sign language had been used by various communities in the United States. In the United States, as elsewhere in the world, hearing families with deaf children have historically employed ad hoc home sign, which often reaches much higher levels of sophistication than gestures used by hearing people in spoken conversation. As early as 1541 at first contact by Francisco Vásquez de Coronado, there were reports that the Indigenous peoples of the Great Plains widely spoke a sign language to communicate across vast national and linguistic lines.Ceil Lucas, 1995, The Sociolinguistics of the Deaf Community
In the 19th century, a "triangle" of village sign languages developed in New England: one in Martha's Vineyard, Massachusetts; one in Henniker, New Hampshire, and one in Sandy River Valley, Maine. Martha's Vineyard Sign Language (MVSL), which was particularly important for the history of ASL, was used mainly in Chilmark, Massachusetts. Due to intermarriage in the original community of English settlers of the 1690s, and the recessive nature of genetic deafness, Chilmark had a high 4% rate of genetic deafness. MVSL was used even by hearing residents whenever a deaf person was present, and also in some situations where spoken language would be ineffective or inappropriate, such as during church sermons or between boats at sea.
ASL is thought to have originated in the American School for the Deaf (ASD), founded in Hartford, Connecticut, in 1817. Originally known as The American Asylum, At Hartford, For The Education And Instruction Of The Deaf And Dumb, the school was founded by the Yale graduate and divinity student Thomas Hopkins Gallaudet. Gallaudet, inspired by his success in demonstrating the learning abilities of a young deaf girl Alice Cogswell, traveled to Europe in order to learn deaf pedagogy from European institutions. Ultimately, Gallaudet chose to adopt the methods of the French Institut National de Jeunes Sourds de Paris, and convinced Laurent Clerc, an assistant to the school's founder Charles-Michel de l'Épée, to accompany him back to the United States. Upon his return, Gallaudet founded the ASD on April 15, 1817.
The largest group of students during the first seven decades of the school were from Martha's Vineyard, and they brought MVSL with them. There were also 44 students from around Henniker, New Hampshire, and 27 from the Sandy River valley in Maine, each of which had their own village sign language. Other students brought knowledge of their own home signs. Laurent Clerc, the first teacher at ASD, taught using French Sign Language (LSF), which itself had developed in the Parisian school for the deaf established in 1755. From that situation of language contact, a new language emerged, now known as ASL.
thumb|alt=man standing on a stage in the foreground addressing a seated crowd|American Sign Language Convention of March 2008 in Austin, Texas
More schools for the deaf were founded after ASD, and knowledge of ASL spread to those schools. In addition, the rise of Deaf community organizations bolstered the continued use of ASL. Societies such as the National Association of the Deaf and the National Fraternal Society of the Deaf held national conventions that attracted signers from across the country. All of that contributed to ASL's wide use over a large geographical area, atypical of a sign language.
While oralism, an approach to educating deaf students focusing on oral language, had previously been used in American schools, the Milan Congress made it dominant and effectively banned the use of sign languages at schools in the United States and Europe. However, the efforts of Deaf advocates and educators, more lenient enforcement of the Congress's mandate, and the use of ASL in religious education and proselytism ensured greater use and documentation compared to European sign languages, albeit more influenced by fingerspelled loanwords and borrowed idioms from English as students were societally pressured to achieve fluency in spoken language. Nevertheless, oralism remained the predominant method of deaf education up to the 1950s. Linguists did not consider sign language to be true "language" but as something inferior. Recognition of the legitimacy of ASL was achieved by William Stokoe, a linguist who arrived at Gallaudet University in 1955 when that was still the dominant assumption. Aided by the Civil Rights Movement of the 1960s, Stokoe argued for manualism, the use of sign language in deaf education.Stokoe, William C. 1960. Sign Language Structure: An Outline of the Visual Communication Systems of the American Deaf , Studies in linguistics: Occasional papers (No. 8). Buffalo: Dept. of Anthropology and Linguistics, University of Buffalo. Stokoe noted that sign language shares the important features that oral languages have as a means of communication, and even devised a transcription system for ASL. In doing so, Stokoe revolutionized both deaf education and linguistics. In the 1960s, ASL was sometimes referred to as "Ameslan", but that term is now considered obsolete. |
American Sign Language | Population | Population
Counting the number of ASL signers is difficult because ASL users have never been counted by the American census. The ultimate source for current estimates of the number of ASL users in the United States is a report for the National Census of the Deaf Population (NCDP) by Schein and Delk (1974). Based on a 1972 survey of the NCDP, Schein and Delk provided estimates consistent with a signing population between 250,000 and 500,000. The survey did not distinguish between ASL and other forms of signing; in fact, the name "ASL" was not yet in widespread use.
Incorrect figures are sometimes cited for the population of ASL users in the United States based on misunderstandings of known statistics. Demographics of the deaf population have been confused with those of ASL use since adults who become deaf late in life rarely use ASL in the home. That accounts for currently-cited estimations that are greater than 500,000; such mistaken estimations can reach as high as 15,000,000. A 100,000-person lower bound has been cited for ASL users; the source of that figure is unclear, but it may be an estimate of prelingual deafness, which is correlated with but not equivalent to signing.
ASL is sometimes incorrectly cited as the third- or fourth-most-spoken language in the United States. Those figures misquote Schein and Delk (1974), who actually concluded that ASL speakers constituted the third-largest population "requiring an interpreter in court". Although that would make ASL the third-most used language among monolinguals other than English, it does not imply that it is the fourth-most-spoken language in the United States since speakers of other languages may also speak English. |
American Sign Language | Geographic distribution | Geographic distribution
ASL is used throughout Anglo-America. That contrasts with Europe, where a variety of sign languages are used within the same continent. The unique situation of ASL seems to have been caused by the proliferation of ASL through schools influenced by the American School for the Deaf, wherein ASL originated, and the rise of community organizations for the Deaf.
Throughout West Africa, ASL-based sign languages are signed by educated Deaf adults. Such languages, imported by boarding schools, are often considered by associations to be the official sign languages of their countries and are named accordingly, such as Nigerian Sign Language and Ghanaian Sign Language. Such signing systems are found in Benin, Burkina Faso, Ivory Coast, Ghana, Liberia, Mauritania, Mali, Nigeria, and Togo. Due to lack of data, it is still an open question how similar those sign languages are to the variety of ASL used in America.
In addition to the aforementioned West African countries, ASL is reported to be used as a first language in Barbados, Bolivia, CambodiaBenoit Duchateau-Arminjon, 2013, Healing Cambodia One Child at a Time, p. 180. (alongside Cambodian Sign Language), the Central African Republic, Chad, China (Hong Kong), the Democratic Republic of the Congo, Gabon, Jamaica, Kenya, Madagascar, the Philippines, Singapore, and Zimbabwe. ASL is also used as a lingua franca throughout the deaf world, widely learned as a second language. |
American Sign Language | Regional variation | Regional variation |
American Sign Language | Sign production | Sign production
Sign production can often vary according to location. Signers from the South tend to sign with more flow and ease. Native signers from New York have been reported as signing comparatively quicker and sharper. Sign production of native Californian signers has also been reported as being fast. Research on that phenomenon often concludes that the fast-paced production for signers from the coasts could be due to the fast-paced nature of living in large metropolitan areas. That conclusion also supports how the ease with which Southerners sign could be caused by the easygoing environment of the South in comparison to that of the coasts.
Sign production can also vary depending on age and native language. For example, sign production of letters may vary in older signers. Slight differences in finger spelling production can be a signal of age. Additionally, signers who learned American Sign Language as a second language vary in production. For Deaf signers who learned a different sign language before learning American Sign Language, qualities of their native language may show in their ASL production. Some examples of that varied production include fingerspelling towards the body, instead of away from it, and signing certain movement from bottom to top, instead of top to bottom. Hearing people who learn American Sign Language also have noticeable differences in signing production. The most notable production difference of hearing people learning American Sign Language is their rhythm and arm posture. |
American Sign Language | Sign variants | Sign variants
Most popularly, there are variants of the signs for English words such as "birthday", "pizza", "Halloween", "early", and "soon", just a sample of the most commonly recognized signs with variants based on regional change. The sign for "school" is commonly varied between black and white signers; the variants used by black signers are sometimes called Black American Sign Language. Social variation is also found between citation forms and forms used by Deaf gay men for words such as "pain" and "protest". |
American Sign Language | History and implications | History and implications
The prevalence of residential Deaf schools can account for much of the regional variance of signs and sign productions across the United States. Deaf schools often serve students of the state in which the school resides. That limited access to signers from other regions, combined with the residential quality of Deaf Schools promoted specific use of certain sign variants. Native signers did not have much access to signers from other regions during the beginning years of their education. It is hypothesized that because of that seclusion, certain variants of a sign prevailed over others due to the choice of variant used by the student of the school/signers in the community.
However, American Sign Language does not appear to be vastly varied in comparison to other signed languages. That is because when Deaf education was beginning in the United States, many educators flocked to the American School for the Deaf in Hartford, Connecticut, whose central location for the first generation of educators in Deaf education to learn American Sign Language allows ASL to be more standardized than its variant. |
American Sign Language | Varieties | Varieties
Varieties of ASL are found throughout the world. There is little difficulty in comprehension among the varieties of the United States and Canada.
Mutual intelligibility among those ASL varieties is high, and the variation is primarily lexical. For example, there are three different words for English about in Canadian ASL; the standard way, and two regional variations (Atlantic and Ontario). Variation may also be phonological, meaning that the same sign may be signed in a different way depending on the region. For example, an extremely common type of variation is between the handshapes /1/, /L/, and /5/ in signs with one handshape.
There is also a distinct variety of ASL used by the Black Deaf community. Black ASL evolved as a result of racially segregated schools in some states, which included the residential schools for the deaf. Black ASL differs from standard ASL in vocabulary, phonology, and some grammatical structure. While African American English (AAE) is generally viewed as more innovating than standard English, Black ASL is more conservative than standard ASL, preserving older forms of many signs. Black sign language speakers use more two-handed signs than in mainstream ASL, are less likely to show assimilatory lowering of signs produced on the forehead (e.g. KNOW) and use a wider signing space. Modern Black ASL borrows a number of idioms from AAE; for instance, the AAE idiom "I feel you" is calqued into Black ASL.
ASL is used internationally as a lingua franca, and a number of closely related sign languages derived from ASL are used in many different countries. Even so, there have been varying degrees of divergence from standard ASL in those imported ASL varieties. Bolivian Sign Language is reported to be a dialect of ASL, no more divergent than other acknowledged dialects. On the other hand, it is also known that some imported ASL varieties have diverged to the extent of being separate languages. For example, Malaysian Sign Language, which has ASL origins, is no longer mutually comprehensible with ASL and must be considered its own language. For some imported ASL varieties, such as those used in West Africa, it is still an open question how similar they are to American ASL.
When communicating with hearing English speakers, ASL-speakers often use what is commonly called Pidgin Signed English (PSE) or 'contact signing', a blend of English structure with ASL vocabulary. Various types of PSE exist, ranging from highly English-influenced PSE (practically relexified English) to PSE which is quite close to ASL lexically and grammatically, but may alter some subtle features of ASL grammar. Fingerspelling may be used more often in PSE than it is normally used in ASL. There have been some constructed sign languages, known as Manually Coded English (MCE), which match English grammar exactly and simply replace spoken words with signs; those systems are not considered to be varieties of ASL.
Tactile ASL (TASL) is a variety of ASL used throughout the United States by and with the deaf-blind. It is particularly common among those with Usher's syndrome. It results in deafness from birth followed by loss of vision later in life; consequently, those with Usher's syndrome often grow up in the Deaf community using ASL, and later transition to TASL. TASL differs from ASL in that signs are produced by touching the palms, and there are some grammatical differences from standard ASL in order to compensate for the lack of nonmanual signing.
ASL changes over time and from generation to generation. The sign for telephone has changed as the shape of phones and the manner of holding them have changed. The development of telephones with screens has also changed ASL, encouraging the use of signs that can be seen on small screens. |
American Sign Language | Stigma | Stigma
In 2013, the White House published a response to a petition that gained over 37,000 signatures to officially recognize American Sign Language as a community language and a language of instruction in schools. The response is titled "there shouldn't be any stigma about American Sign Language" and addressed that ASL is a vital language for the Deaf and hard of hearing. Stigmas associated with sign languages and the use of sign for educating children often lead to the absence of sign during periods in children's lives when they can access languages most effectively. Scholars such as Beth S. Benedict advocate not only for bilingualism (using ASL and English training) but also for early childhood intervention for children who are deaf. York University psychologist Ellen Bialystok has also campaigned for bilingualism, arguing that those who are bilingual acquire cognitive skills that may help to prevent dementia later in life.
Most children born to deaf parents are hearing. Known as CODAs ("Children of Deaf Adults"), they are often more culturally Deaf than deaf children, most of whom are born to hearing parents. Unlike many deaf children, CODAs acquire ASL as well as Deaf cultural values and behaviors from birth. Such bilingual hearing children may be mistakenly labeled as being "slow learners" or as having "language difficulties" because of preferential attitudes towards spoken language. |
American Sign Language | Writing systems | Writing systems
thumb|alt=text written in Stokoe notation|The ASL phrase "American Sign Language", written in Stokoe notation
Although there is no well-established writing system for ASL, written sign language dates back almost two centuries. The first systematic writing system for a sign language seems to be that of Roch-Ambroise Auguste Bébian, developed in 1825. However, written sign language remained marginal among the public. In the 1960s, linguist William Stokoe created Stokoe notation specifically for ASL. It is alphabetic, with a letter or diacritic for every phonemic (distinctive) hand shape, orientation, motion, and position, though it lacks any representation of facial expression, and is better suited for individual words than for extended passages of text.Armstrong, David F., and Michael A. Karchmer. "William C. Stokoe and the Study of Signed Languages." Sign Language Studies 9.4 (2009): 389-397. Academic Search Premier. Web. 7 June 2012. Stokoe used that system for his 1965 A Dictionary of American Sign Language on Linguistic Principles.Stokoe, William C.; Dorothy C. Casterline; Carl G. Croneberg. 1965. A dictionary of American sign languages on linguistic principles. Washington, D.C.: Gallaudet College Press
thumb|alt=text written in Sutton SignWriting|The ASL phrase "American Sign Language", written in SignWriting
SignWriting, proposed in 1974 by Valerie Sutton, is the first writing system to gain use among the public and the first writing system for sign languages to be included in the Unicode Standard. SignWriting consists of more than 5000 distinct iconic graphs/glyphs. Currently, it is in use in many schools for the Deaf, particularly in Brazil, and has been used in International Sign forums with speakers and researchers in more than 40 countries, including Brazil, Ethiopia, France, Germany, Italy, Portugal, Saudi Arabia, Slovenia, Tunisia, and the United States. Sutton SignWriting has both a printed and an electronically produced form so that persons can use the system anywhere that oral languages are written (personal letters, newspapers, and media, academic research). The systematic examination of the International Sign Writing Alphabet (ISWA) as an equivalent usage structure to the International Phonetic Alphabet for spoken languages has been proposed.Charles Butler, Center for Sutton Movement Writing, 2014 According to some researchers, SignWriting is not a phonemic orthography and does not have a one-to-one map from phonological forms to written forms. That assertion has been disputed, and the process for each country to look at the ISWA and create a phonemic/morphemic assignment of features of each sign language was proposed by researchers Msc. Roberto Cesar Reis da Costa and Madson Barreto in a thesis forum on June 23, 2014. The SignWriting community has an open project on Wikimedia Labs to support the various Wikimedia projects on Wikimedia Incubator and elsewhere involving SignWriting. The ASL Wikipedia request was marked as eligible in 2008 and the test ASL Wikipedia has 50 articles written in ASL using SignWriting.
The most widely used transcription system among academics is HamNoSys, developed at the University of Hamburg. Based on Stokoe Notation, HamNoSys was expanded to about 200 graphs in order to allow transcription of any sign language. Phonological features are usually indicated with single symbols, though the group of features that make up a handshape is indicated collectively with a symbol.
Several additional candidates for written ASL have appeared over the years, including SignFont, ASL-phabet, and Si5s.
thumb|center|upright=2.05|Comparison of ASL writing systems: Sutton SignWriting, Si5s, Stokoe notation, SignFont, and ASLphabet
For English-speaking audiences, ASL is often glossed using English words. Such glosses are typically all-capitalized and are arranged in ASL order. For example, the ASL sentence DOG NOW CHASE>IX=3 CAT, meaning "the dog is chasing the cat", uses NOW to mark ASL progressive aspect and shows ASL verbal inflection for the third person (>IX=3). However, glossing is not used to write the language for speakers of ASL. |
American Sign Language | Phonology | Phonology
Each sign in ASL is composed of a number of distinctive components, generally referred to as parameters. A sign may use one hand or both. All signs can be described using the five parameters involved in signed languages, which are handshape, movement, palm orientation, location and nonmanual markers. Just as phonemes of sound distinguish meaning in spoken languages, those parameters are the phonemes that distinguish meaning in signed languages like ASL. Changing any one of them may change the meaning of a sign, as illustrated by the ASL signs THINK and DISAPPOINTED:
+ THINK Handshape Closed fist with index finger extended Orientation Facing signer's body Location Tip of finger in contact with forehead Movement Unidirectional single contacting movement+ DISAPPOINTED Handshape (as for THINK) Orientation (as for THINK) Location Tip of finger in contact with chin Movement (as for THINK)
There are also meaningful nonmanual signals in ASL, which may include movement of the eyebrows, the cheeks, the nose, the head, the torso, and the eyes.
William Stokoe proposed that such components are analogous to the phonemes of spoken languages. There has also been a proposal that they are analogous to classes like place and manner of articulation. As in spoken languages, those phonological units can be split into distinctive features. For instance, the handshapes /2/ and /3/ are distinguished by the presence or absence of the feature [± closed thumb], as illustrated to the right. ASL has processes of allophony and phonotactic restrictions. There is ongoing research into whether ASL has an analog of syllables in spoken language. |
American Sign Language | Grammar | Grammar
thumb|alt=two men and a woman signing|Two men and a woman signing |
American Sign Language | Morphology | Morphology
ASL has a rich system of verbal inflection, which involves both grammatical aspect: how the action of verbs flows in time—and agreement marking. Aspect can be marked by changing the manner of movement of the verb; for example, continuous aspect is marked by incorporating rhythmic, circular movement, while punctual aspect is achieved by modifying the sign so that it has a stationary hand position. Verbs may agree with both the subject and the object, and are marked for number and reciprocity. Reciprocity is indicated by using two one-handed signs; for example, the sign SHOOT, made with an L-shaped handshape with inward movement of the thumb, inflects to SHOOT[reciprocal], articulated by having two L-shaped hands "shooting" at each other.
ASL has a productive system of classifiers, which are used to classify objects and their movement in space. For example, a rabbit running downhill would use a classifier consisting of a bent V classifier handshape with a downhill-directed path; if the rabbit is hopping, the path is executed with a bouncy manner. In general, classifiers are composed of a "classifier handshape" bound to a "movement root". The classifier handshape represents the object as a whole, incorporating such attributes as surface, depth, and shape, and is usually very iconic. The movement root consists of a path, a direction and a manner.
In linguistics, there are two primary ways of changing the form of a word: derivation and inflection. Derivation involves creating new words by adding something to an existing word, while inflection involves changing the form of a word to convey grammatical information without altering its fundamental meaning or category.
For example, adding the suffix "-ship" to the noun "friend" creates the new word "friendship", which has a different meaning than the original word. Inflection, on the other hand, involves modifying a word's form to indicate grammatical features such as tense, number, gender, person, case, and degree of comparison.
In American Sign Language (ASL), inflection is conveyed through facial expressions, body movements, and other non-manual markers. For instance, to indicate past tense in ASL, one might sign the present tense of a verb (such as "walk"), and then add a facial expression and head tilt to signify that the action occurred in the past (i.e., "walked").
According to the book Linguistics of American Sign Language, ASL signs have two main components: hold segments and movement segments. Hold segments consist of hand-shape, location, orientation, and non-manual features, while movement segments possess similar features.
Morphology is the study of how languages form words by using smaller units to construct larger units. The smallest meaningful unit in a language is known as a "morpheme", with some morphemes able to stand alone as independent units (free morphemes), while others must occur with other morphemes (bound morphemes).
For example, the plural "-s" and third person "-s" in English are bound morphemes. In ASL, the 3 handshape in signs like THREE-WEEKS and THREE-MONTHS are also bound morphemes.
Affixes, which are morphemes added to words to create new words or modify their meanings, are part of the derivational process. For example, in English, prefixes like "re-" and suffixes like "-able" are affixes. In ASL, affixation can be used to modify the sign for CHAIR to indicate different types of chairs. The inflectional process, on the other hand, adds grammatical information to existing units.
By studying morphemes and how they can be combined or modified, linguists gain insight into the underlying structure of language and the creative ways in which it can be used to express meaning. Understanding morphology is essential to understanding how languages are built and how new signs or words can be formed.
Furthermore, understanding morphology has practical applications in language learning and teaching. For example, teaching students the basic morphological structures of a language can help them to better understand the language's grammar and syntax, and can also aid in their acquisition of new vocabulary.
In summary, morphology is an essential component of language and provides valuable insights into the structure and function of languages. By understanding the morphological processes involved in language formation, we can gain a deeper understanding of how languages work and how they can be effectively taught and learned. |
American Sign Language | Fingerspelling | Fingerspelling
thumb|upright=1.1|right|alt=chart of letters in the American manual alphabet, with Latin script equivalents|The American manual alphabet and numbers
American Sign Language possesses a set of 26 signs known as the American manual alphabet, which can be used to spell out words from the English language. It is rather a representation of the English alphabet, and not a unique alphabet of ASL, although commonly labeled as the "ASL alphabet". It is borrowed from French Sign Language (LSF), as much of ASL is derived from LSF. Such signs make use of the 19 handshapes of ASL. For example, the signs for 'p' and 'k' use the same handshape but different orientations. A common misconception is that ASL consists only of fingerspelling; although such a method (Rochester Method) has been used, it is not ASL.
Fingerspelling is a form of borrowing, a linguistic process wherein words from one language are incorporated into another. In ASL, fingerspelling is used for proper nouns and for technical terms with no native ASL equivalent. There are also some other loan words which are fingerspelled, either very short English words or abbreviations of longer English words, e.g. O-N from English 'on', and A-P-T from English 'apartment'. Fingerspelling may also be used to emphasize a word that would normally be signed otherwise. |
American Sign Language | Syntax | Syntax
ASL is a subject–verb–object (SVO) language, but various phenomena affect that basic word order. Basic SVO sentences are signed without any pauses:
However, other word orders may also occur since ASL allows the topic of a sentence to be moved to sentence-initial position, a phenomenon known as topicalization. In object–subject–verb (OSV) sentences, the object is topicalized, marked by a forward head-tilt and a pause:
Besides, word orders can be obtained through the phenomenon of subject copy in which the subject is repeated at the end of the sentence, accompanied by head nodding for clarification or emphasis:
ASL also allows null subject sentences whose subject is implied, rather than stated explicitly. Subjects can be copied even in a null subject sentence, and the subject is then omitted from its original position, yielding a verb–object–subject (VOS) construction:
Topicalization, accompanied with a null subject and a subject copy, can produce yet another word order, object–verb–subject (OVS).
Those properties of ASL allow it a variety of word orders, leading many to question which is the true, underlying, "basic" order. There are several other proposals that attempt to account for the flexibility of word order in ASL. One proposal is that languages like ASL are best described with a topic–comment structure whose words are ordered by their importance in the sentence, rather than by their syntactic properties. Another hypothesis is that ASL exhibits free word order, in which syntax is not encoded in word order but can be encoded by other means such as head nods, eyebrow movement, and body position. |
American Sign Language | Iconicity | Iconicity
Common misconceptions are that signs are iconically self-explanatory, that they are a transparent imitation of what they mean, or even that they are pantomime. In fact, many signs bear no resemblance to their referent because they were originally arbitrary symbols, or their iconicity has been obscured over time. Even so, in ASL iconicity plays a significant role; a high percentage of signs resemble their referents in some way. That may be because the medium of sign, three-dimensional space, naturally allows more iconicity than oral language.
In the era of the influential linguist Ferdinand de Saussure, it was assumed that the mapping between form and meaning in language must be completely arbitrary. Although onomatopoeia is a clear exception, since words like "choo-choo" bear clear resemblance to the sounds that they mimic, the Saussurean approach was to treat them as marginal exceptions. ASL, with its significant inventory of iconic signs, directly challenges that theory.
Research on acquisition of pronouns in ASL has shown that children do not always take advantage of the iconic properties of signs when they interpret their meaning. It has been found that when children acquire the pronoun "you", the iconicity of the point (at the child) is often confused, being treated more like a name. That is a similar finding to research in oral languages on pronoun acquisition. It has also been found that iconicity of signs does not affect immediate memory and recall; less iconic signs are remembered just as well as highly-iconic signs. |
American Sign Language | See also | See also
American Sign Language grammar
American Sign Language literature
Baby sign language
Bimodal bilingualism
Great ape language, of which ASL has been one attempted mode
Inspirisles
Legal recognition of sign languages
Pointing
Sign name
ASL interpreting
Billingual-Bicultural Education |
American Sign Language | Notes | Notes |
American Sign Language | References | References |
American Sign Language | Bibliography | Bibliography
|
American Sign Language | External links | External links
Accessible American Sign Language vocabulary site
American Sign Language discussion forum
One-stop resource American Sign Language and video dictionary
National Institute of Deafness ASL section
National Association of the Deaf ASL information
American Sign Language
The American Sign Language Linguistics Research Project
Video Dictionary of ASL
American Sign Language Dictionary
Category:American Sign Language family
Category:Articles containing video clips
Category:Deaf culture in the United States
Category:French Sign Language family
Category:Fusional languages
Category:Languages of Barbados
Category:Languages of Belize
Category:Languages of Botswana
Category:Languages of Burundi
Category:Sign languages of Canada
Category:Languages of Grenada
Category:Languages of Guyana
Category:Languages of Haiti
Category:Languages of Saint Kitts and Nevis
Category:Languages of Saint Vincent and the Grenadines
Category:Languages of the United States Virgin Islands
Category:Sign languages of the United States
Category:Languages of Zimbabwe
Category:Subject–verb–object languages
Category:1817 introductions
Category:Languages attested from the 19th century
Category:Languages of Canada
Category:Languages of the United States |
American Sign Language | Table of Content | Short description, Classification, History, Population, Geographic distribution, Regional variation, Sign production, Sign variants, History and implications, Varieties, Stigma, Writing systems, Phonology, Grammar, Morphology, Fingerspelling, Syntax, Iconicity, See also, Notes, References, Bibliography, External links |
Applet | short description | In computing, an applet is any small application that performs one specific task that runs within the scope of a dedicated widget engine or a larger program, often as a plug-in."AskOxford: applet", Oxford Dictionaries. Accessed on July 21, 2009 The term is frequently used to refer to a Java applet, a program written in the Java programming language that is designed to be placed on a web page. Applets are typical examples of transient and auxiliary applications that do not monopolize the user's attention. Applets are not full-featured application programs, and are intended to be easily accessible. |
Applet | History | History
The word applet was first used in 1990 in PC Magazine. However, the concept of an applet, or more broadly a small interpreted program downloaded and executed by the user, dates at least to RFC 5 (1969) by Jeff Rulifson, which described the Decode-Encode Language, which was designed to allow remote use of the oN-Line System over ARPANET, by downloading small programs to enhance the interaction. This has been specifically credited as a forerunner of Java's downloadable programs in RFC 2555. |
Applet | Applet as an extension of other software | Applet as an extension of other software
In some cases, an applet does not run independently. These applets must run either in a container provided by a host program, through a plugin, or a variety of other applications including mobile devices that support the applet programming model. |
Applet | Web-based applets | Web-based applets
Applets were used to provide interactive features to web applications that historically could not be provided by HTML alone. They could capture mouse input and also had controls like buttons or check boxes. In response to the user action, an applet could change the provided graphic content. This made applets well suited for demonstration, visualization, and teaching. There were online applet collections for studying various subjects, from physics to heart physiology. Applets were also used to create online game collections that allowed players to compete against live opponents in real-time.
An applet could also be a text area only, providing, for instance, a cross-platform command-line interface to some remote system. If needed, an applet could leave the dedicated area and run as a separate window. However, applets had very little control over web page content outside the applet dedicated area, so they were less useful for improving the site appearance in general (while applets like news tickers or WYSIWYG editors are also known). Applets could also play media in formats that are not natively supported by the browser.
HTML pages could embed parameters that were passed to the applet. Hence, the same applet could appear differently depending on the parameters that were passed.
Examples of Web-based applets include:
QuickTime movies
Flash movies
Windows Media Player applets, used to display embedded video files in Internet Explorer (and other browsers that supported the plugin)
3D modeling display applets, used to rotate and zoom a model
Browser games that were applet-based, though some developed into fully functional applications that required installation. |
Applet | Applet Vs. Subroutine | Applet Vs. Subroutine
A larger application distinguishes its applets through several features:
Applets execute only on the "client" platform environment of a system, as contrasted from "Servlet". As such, an applet provides functionality or performance beyond the default capabilities of its container (the browser).
The container restricts applets' capabilities.
Applets are written in a language different from the scripting or HTML language that invokes it. The applet is written in a compiled language, whereas the scripting language of the container is an interpreted language, hence the greater performance or functionality of the applet. Unlike a subroutine, a complete web component can be implemented as an applet. |
Applet | Java applets | Java applets
A Java applet is a Java program that is launched from HTML and run in a web browser. It takes code from server and run in a web browser. It can provide web applications with interactive features that cannot be provided by HTML. Since Java's bytecode is platform-independent, Java applets can be executed by browsers running under many platforms, including Windows, Unix, macOS, and Linux. When a Java technology-enabled web browser processes a page that contains an applet, the applet's code is transferred to the client's system and executed by the browser's Java virtual machine. An HTML page references an applet either via the deprecated tag or via its replacement, the tag."HTML applet tag", W3Schools. Access on July 21, 2009 |
Applet | Security | Security
Recent developments in the coding of applications, including mobile and embedded systems, have led to the awareness of the security of applets. |
Applet | Open platform applets | Open platform applets
Applets in an open platform environment should provide secure interactions between different applications. A compositional approach can be used to provide security for open platform applets. Advanced compositional verification methods have been developed for secure applet interactions. |
Applet | Java applets | Java applets
A Java applet contains different security models: unsigned Java applet security, signed Java applet security, and self-signed Java applet security. |
Applet | Web-based applets | Web-based applets
In an applet-enabled web browser, many methods can be used to provide applet security for malicious applets. A malicious applet can infect a computer system in many ways, including denial of service, invasion of privacy, and annoyance. A typical solution for malicious applets is to make the web browser to monitor applets' activities. This will result in a web browser that will enable the manual or automatic stopping of malicious applets. |
Applet | See also | See also
Application posture
Bookmarklet
Java applet
Widget engine
Abstract Window Toolkit |
Applet | References | References |
Applet | External links | External links
Category:Technology neologisms
Category:Component-based software engineering
Category:Java (programming language) libraries |
Applet | Table of Content | short description, History, Applet as an extension of other software, Web-based applets, Applet Vs. Subroutine, Java applets, Security, Open platform applets, Java applets, Web-based applets, See also, References, External links |
Alternate history | Short description | thumb|right|260px|A painting by Jakub Różalski depicts an alternate history of the 1920s in Iron Harvest, in which rural peasants must contend with giant mechanical walking tanks.
Alternate history (also referred to as alternative history, allohistory, althist, or simply A.H.) is a subgenre of speculative fiction in which one or more historical events have occurred but are resolved differently than in actual history.Brave New Words: The Oxford Dictionary of Science Fiction (Oxford University Press, 2007) notes the preferred usage is "Alternate History", which was coined in 1954; "Alternative History" was first used in 1977, pp. 4–5. As conjecture based upon historical fact, alternate history stories propose What if? scenarios about crucial events in human history, and present outcomes very different from the historical record. Some alternate histories are considered a subgenre of science fiction, or historical fiction.
Since the 1950s, as a subgenre of science fiction, some alternative history stories have featured the tropes of time travel between histories, the psychic awareness of the existence of an alternative universe by the inhabitants of a given universe, and time travel that divides history into various timestreams. |
Alternate history | Definition | Definition
Often described as a subgenre of science fiction, alternative history is a genre of fiction wherein the author speculates upon how the course of history might have been altered if a particular historical event had an outcome different from the real life outcome. An alternate history requires three conditions: (i) A point of divergence from the historical record, before the time in which the author is writing; (ii) A change that would alter known history; and (iii) An examination of the ramifications of that alteration to history. Occasionally, some types of genre fiction are misidentified as alternative history, specifically science fiction stories set in a time that was the future for the writer, but now is the past for the reader, such as the novels 2001: A Space Odyssey (1968) by Arthur C. Clarke, 1984 (1949) by George Orwell and the movie 2012 (2009) because the authors did not alter the real history of the past when they wrote the stories.
Similar to the genre of alternative history, there is also the genre of secret history - which can be either fictional or non-fictional - which documents events that might have occurred in history, but which had no effect upon the recorded historical outcome. Alternative history also is thematically related to, but distinct from, counterfactual history, which is a form of historiography that explores historical events in an extrapolated timeline in which key historical events either did not occur or had an outcome different from the historical record, in order to understand what did happen."It [alternative history] is, at the very root, the idea of conjecturing on what did not happen, or what might have happened, in order to understand what did happen."Martin Bunzl, "Counterfactual History: A User's Guide", American Historical Review (2004) 109 No. 3, pp. 845–858 in JSTOR |
Alternate history | History of literature | History of literature |
Alternate history | Antiquity and medieval | Antiquity and medieval
thumb|Title page of the first Spanish-language translation of Joanot Martorell's Tirant lo Blanch (originally in Catalan)
The earliest example of alternate (or counterfactual) history is found in Livy's Ab Urbe Condita Libri (book IX, sections 17–19). Livy contemplated an alternative 4th century BC in which Alexander the Great had survived to attack Europe as he had planned; asking, "What would have been the results for Rome if she had been engaged in a war with Alexander?" Livy concluded that the Romans would likely have defeated Alexander. An even earlier possibility is Herodotus's Histories, which contains speculative material.
Another example of counterfactual history was posited by cardinal and Doctor of the Church Peter Damian in the 11th century. In his famous work De Divina Omnipotentia, a long letter in which he discusses God's omnipotence, he treats questions related to the limits of divine power, including the question of whether God can change the past, for example, bringing about that Rome was never founded:I see I must respond finally to what many people, on the basis of your holiness's [own] judgment, raise as an objection on the topic of this dispute. For they say: If, as you assert, God is omnipotent in all things, can he manage this, that things that have been made were not made? He can certainly destroy all things that have been made, so that they do not exist now. But it cannot be seen how he can bring it about that things that have been made were not made. To be sure, it can come about that from now on and hereafter Rome does not exist; for it can be destroyed. But no opinion can grasp how it can come about that it was not founded long ago...One early work of fiction detailing an alternate history is Joanot Martorell's 1490 epic romance Tirant lo Blanch, which was written when the fall of Constantinople to the Turks was still a recent and traumatic memory for Christian Europe. It tells the story of the knight Tirant the White from Brittany who travels to the embattled remnants of the Byzantine Empire. He becomes a Megaduke and commander of its armies and manages to fight off the invading Ottoman armies of . He saves the city from Islamic conquest, and even chases the Turks deeper into lands they had previously conquered. |
Alternate history | 19th century | 19th century
One of the earliest works of alternate history published in large quantities for the reception of a large audience may be Louis Geoffroy's Histoire de la Monarchie universelle : Napoléon et la conquête du monde (1812–1832) (History of the Universal Monarchy: Napoleon and the Conquest of the World) (1836), which imagines Napoleon's First French Empire emerging victorious in the French invasion of Russia in 1812 and in an invasion of England in 1814, later unifying the world under Bonaparte's rule.
thumb|220x220px|The Glorious Appearing of Jesus to the Nephites by William Armitage
The Book of Mormon (published 1830) is described as an "alternative history" by Richard Lyman Bushman, a biographer of Joseph Smith. Smith claimed to have translated the document from golden plates, which told the story of a Jewish group who migrated from Israel to the Americas and inhabited the region from about 600 B.C. to 400 A.D., becoming the ancestors of Native Americans. In the 2005 biography Joseph Smith: Rough Stone Rolling, Bushman wrote that the Book of Mormon "turned American history upside down [and] works on the premise that a history—a book—can reconstitute a nation. It assumes that by giving a nation an alternative history, alternative values can be made to grow."Richard Lyman Bushman. Knopf, ISBN 1-4000-4270-4, p. 104
In the English language, the first known complete alternate history may be Nathaniel Hawthorne's short story "P.'s Correspondence", published in 1845. It recounts the tale of a man who is considered "a madman" due to his perceptions of a different 1845, a reality in which long-dead famous people, such as the poets Robert Burns, Lord Byron, Percy Bysshe Shelley, and John Keats, the actor Edmund Kean, the British politician George Canning, and Napoleon Bonaparte, are still alive.
The first novel-length alternate history in English would seem to be Castello Holford's Aristopia (1895). While not as nationalistic as Geoffroy's Napoléon et la conquête du monde, 1812–1823, Aristopia is another attempt to portray a Utopian society. In Aristopia, the earliest settlers in Virginia discover a reef made of solid gold and are able to build a Utopian society in North America. |
Alternate history | Early 20th century and the era of the pulps | Early 20th century and the era of the pulps
In 1905, H. G. Wells published A Modern Utopia. As explicitly noted in the book itself, Wells's main aim in writing it was to set out his social and political ideas, the plot serving mainly as a vehicle to expound them. This book introduced the idea of a person being transported from a point in our familiar world to the precise geographical equivalent point in an alternate world in which history had gone differently. The protagonists undergo various adventures in the alternate world, and then are finally transported back to our world, again to the precise geographical equivalent point. Since then, that has become a staple of the alternate history genre.
A number of alternate history stories and novels appeared in the late 19th and early 20th centuries (see, for example, Joseph Edgar Chamberlin's The Ifs of History [1907] and Charles Petrie's If: A Jacobite Fantasy [1926]). In 1931, British historian Sir John Squire collected a series of essays from some of the leading historians of the period for his anthology If It Had Happened Otherwise. In that work, scholars from major universities, as well as important non-academic authors, turned their attention to such questions as "If the Moors in Spain Had Won" and "If Louis XVI Had Had an Atom of Firmness". The essays range from serious scholarly efforts to Hendrik Willem van Loon's fanciful and satiric portrayal of an independent 20th-century New Amsterdam, a Dutch city-state on the island of Manhattan. Among the authors included were Hilaire Belloc, André Maurois, and Winston Churchill.
One of the entries in Squire's volume was Churchill's "If Lee Had Not Won the Battle of Gettysburg", written from the viewpoint of a historian in a world in which the Confederacy had won the American Civil War. The entry considers what would have happened if the North had been victorious (in other words, a character from an alternate world imagines a world more like the real one we live in, although it is not identical in every detail). Speculative work that narrates from the point of view of an alternate history is variously known as "recursive alternate history", a "double-blind what-if", or an "alternate-alternate history". Churchill's essay was one of the influences behind Ward Moore's alternate history novel Bring the Jubilee in which General Robert E. Lee won the Battle of Gettysburg and paved the way for the eventual victory of the Confederacy in the American Civil War (named the "War of Southron Independence" in this timeline). The protagonist, the autodidact Hodgins Backmaker, travels back to the aforementioned battle and inadvertently changes history, which results in the emergence of our own timeline and the consequent victory of the Union instead.
The American humorist author James Thurber parodied alternate history stories about the American Civil War in his 1930 story "If Grant Had Been Drinking at Appomattox", which he accompanied with this very brief introduction: "Scribner's magazine is publishing a series of three articles: 'If Booth Had Missed Lincoln', 'If Lee Had Won the Battle of Gettysburg', and 'If Napoleon Had Escaped to America'. This is the fourth".
Another example of alternate history from this period (and arguably Herbert Millingchamp Vaughan's The Dial of Ahaz (1917) posits a multiverse filled with alternate versions of planet Earth. the first that explicitly posited cross-time travel from one universe to another as anything more than a visionary experience) is H.G. Wells' Men Like Gods (1923) in which the London-based journalist Mr. Barnstable, along with two cars and their passengers, is mysteriously teleported into "another world", which the "Earthlings" call Utopia. Being far more advanced than Earth, Utopia is some 3000 years ahead of humanity in its development. Wells describes a multiverse of alternative worlds, complete with the paratime travel machines that would later become popular with American pulp writers. However, since his hero experiences only a single alternate world, the story is not very different from conventional alternate history.
In the 1930s, alternate history moved into a new arena. The December 1933 issue of Astounding published Nat Schachner's "Ancestral Voices", which was quickly followed by Murray Leinster's "Sidewise in Time" (1934). While earlier alternate histories examined reasonably-straightforward divergences, Leinster attempted something completely different. In his "World gone mad", pieces of Earth traded places with their analogs from different timelines. The story follows Professor Minott and his students from a fictitious Robinson College as they wander through analogues of worlds that followed a different history. "Sidewise in Time" has been described as "the point at which the alternate history narrative first enters science fiction as a plot device" and is the story for which the Sidewise Award for Alternate History is named.
thumb |right | 300px|The world in 1964 in the novel Fatherland in which the Nazis won World War II
A somewhat similar approach was taken by Robert A. Heinlein in his 1941 novelette Elsewhen in which a professor trains his mind to move his body across timelines. He then hypnotizes his students so that they can explore more of them. Eventually, each settles into the reality that is most suitable for him or her. Some of the worlds they visit are mundane, some are very odd, and others follow science fiction or fantasy conventions.
World War II produced alternate history for propaganda: both British and American authors wrote works depicting Nazi invasions of their respective countries as cautionary tales. |
Alternate history | Time travel to create historical divergences | Time travel to create historical divergences
The period around World War II also saw the publication of the time travel novel Lest Darkness Fall by L. Sprague de Camp in which an American academic travels to Italy at the time of the Byzantine invasion of the Ostrogoths. De Camp's time traveler, Martin Padway, is depicted as making permanent historical changes and implicitly forming a new time branch, thereby making the work an alternate history.
In William Tenn's short story Brooklyn Project (1948), a tyrannical US Government brushes aside the warnings of scientists about the dangers of time travel and goes on with a planned experiment - with the result that minor changes to the prehistoric past cause Humanity to never have existed, its place taken by tentacled underwater intelligent creatures - who also have a tyrannical government which also insists on experimenting with time-travel.
In Ray Bradbury's classic short story "A Sound of Thunder" (1952) a group of hunters travel to the Late Cretaceous to hunt dinosaurs whose death would not be considered consequential as they are about to die a natural death within two minutes of the encounter. To minimize risking changes history they are told to stay on a levitating antigravity path that touches nothing. However one of the hunters stumbles off the path, inadvertently crushing a butterfly. When the group returns they find that history became significantly harsher and a fascist is now President.
Time travel as the cause of a point of divergence (POD), which can denote either the bifurcation of a historical timeline or a simple replacement of the future that existed before the time-travelling event, has continued to be a popular theme. In Ward Moore's Bring the Jubilee (1953), the protagonist lives in an alternate history in which the Confederacy has won the American Civil War. He travels backward through time and brings about a Union victory at the Battle of Gettysburg.
When a story's assumptions about the nature of time travel lead to the complete replacement of the visited time's future, rather than just the creation of an additional time line, the device of a "time patrol" is often used where guardians move through time to preserve the "correct" history.
A more recent example is Making History by Stephen Fry in which a time machine is used to alter history so that Adolf Hitler was never born. That ironically results in a more competent leader of Nazi Germany and results in the country's ascendancy and longevity in the altered timeline. |
Alternate history | Quantum theory of many worlds | Quantum theory of many worlds
While many justifications for alternate histories involve a multiverse, the "many world" theory would naturally involve many worlds, in fact a continually exploding array of universes. In quantum theory, new worlds would proliferate with every quantum event, and even if the writer uses human decisions, every decision that could be made differently would result in a different timeline. A writer's fictional multiverse may, in fact, preclude some decisions as humanly impossible, as when, in Night Watch, Terry Pratchett depicts a character informing Vimes that while anything that can happen, has happened, nevertheless there is no history whatsoever in which Vimes has ever murdered his wife. When the writer explicitly maintains that all possible decisions are made in all possible ways, one possible conclusion is that the characters were neither brave, nor clever, nor skilled, but simply lucky enough to happen on the universe in which they did not choose the cowardly route, take the stupid action, fumble the crucial activity, etc.; few writers focus on this idea, although it has been explored in stories such as Larry Niven's story All the Myriad Ways, where the reality of all possible universes leads to an epidemic of suicide and crime because people conclude their choices have no moral import.
In any case, even if it is true that every possible outcome occurs in some world, it can still be argued that traits such as bravery and intelligence might still affect the relative frequency of worlds in which better or worse outcomes occurred (even if the total number of worlds with each type of outcome is infinite, it is still possible to assign a different measure to different infinite sets). The physicist David Deutsch, a strong advocate of the many-worlds interpretation of quantum mechanics, has argued along these lines, saying that "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives. When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen." This view is perhaps somewhat too abstract to be explored directly in science fiction stories, but a few writers have tried, such as Greg Egan in his short story The Infinite Assassin, where an agent is trying to contain reality-scrambling "whirlpools" that form around users of a certain drug, and the agent is constantly trying to maximize the consistency of behavior among his alternate selves, attempting to compensate for events and thoughts he experiences, he guesses are of low measure relative to those experienced by most of his other selves.
Many writers—perhaps the majority—avoid the discussion entirely. In one novel of this type, H. Beam Piper's Lord Kalvan of Otherwhen, a Pennsylvania State Police officer, who knows how to make gunpowder, is transported from our world to an alternate universe where the recipe for gunpowder is a tightly held secret and saves a country that is about to be conquered by its neighbors. The paratime patrol members are warned against going into the timelines immediately surrounding it, where the country will be overrun, but the book never depicts the slaughter of the innocent thus entailed, remaining solely in the timeline where the country is saved.
The cross-time theme was further developed in the 1960s by Keith Laumer in the first three volumes of his Imperium sequence, which would be completed in Zone Yellow (1990). Piper's politically more sophisticated variant was adopted and adapted by Michael Kurland and Jack Chalker in the 1980s; Chalker's G.O.D. Inc trilogy (1987–89), featuring paratime detectives Sam and Brandy Horowitz, marks the first attempt at merging the paratime thriller with the police procedural. Kurland's Perchance (1988), the first volume of the never-completed "Chronicles of Elsewhen", presents a multiverse of secretive cross-time societies that utilize a variety of means for cross-time travel, ranging from high-tech capsules to mutant powers. Crosstime Traffic is a 6-book series written by Harry Turtledove aimed at teenagers featuring a variant of H. Beam Piper's paratime trading empire. While the home timeline appears to be the same in each of the books there is no overlap in characters or repetition of the alternative worlds. |
Alternate history | Rival paratime worlds | Rival paratime worlds
The concept of a cross-time version of a world war, involving rival paratime empires, was developed in Fritz Leiber's Change War series, starting with the Hugo Award winning The Big Time (1958); followed by Richard C. Meredith's Timeliner trilogy in the 1970s, Michael McCollum's A Greater Infinity (1982) and John Barnes' Timeline Wars trilogy in the 1990s.
Such "paratime" stories may include speculation that the laws of nature can vary from one universe to the next, providing a science fictional explanation—or veneer—for what is normally fantasy. Aaron Allston's Doc Sidhe and Sidhe Devil take place between our world, the "grim world" and an alternate "fair world" where the Sidhe retreated to. Although technology is clearly present in both worlds, and the "fair world" parallels our history, about fifty years out of step, there is functional magic in the fair world. Even with such explanation, the more explicitly the alternate world resembles a normal fantasy world, the more likely the story is to be labelled fantasy, as in Poul Anderson's "House Rule" and "Loser's Night". In both science fiction and fantasy, whether a given parallel universe is an alternate history may not be clear. The writer might allude to a POD only to explain the existence and make no use of the concept, or may present the universe without explanation of its existence. |
Alternate history | Major writers explore alternate histories | Major writers explore alternate histories
Isaac Asimov's short story "What If—" (1952) is about a couple who can explore alternate realities by means of a television-like device. This idea can also be found in Asimov's novel The End of Eternity (1955), in which the "Eternals" can change the realities of the world, without people being aware of it. Poul Anderson's Time Patrol stories feature conflicts between forces intent on changing history and the Patrol who work to preserve it. One story, Delenda Est, describes a world in which Carthage triumphed over the Roman Republic. The Big Time, by Fritz Leiber, describes a Change War ranging across all of history.
Keith Laumer's Worlds of the Imperium is one of the earliest alternate history novels; it was published by Fantastic Stories of the Imagination in 1961, in magazine form, and reprinted by Ace Books in 1962 as one half of an Ace Double. Besides our world, Laumer describes a world ruled by an Imperial aristocracy formed by the merger of European empires, in which the American Revolution never happened, and a third world in post-war chaos ruled by the protagonist's doppelganger.
thumb|A map of the United States as depicted in The Man in the High Castle TV series, based on Philip K. Dick's The Man in the High Castle
Philip K. Dick's novel, The Man in the High Castle (1962), is an alternate history in which Nazi Germany and Imperial Japan won World War II. This book contains an example of "alternate-alternate" history, in that one of its characters authored a book depicting a reality in which the Allies won the war, itself divergent from real-world history in several aspects. The several characters live within a divided United States, in which the Empire of Japan takes the Pacific states, governing them as a puppet, Nazi Germany takes the East Coast of the United States and parts of the Midwest, with the remnants of the old United States' government as the Neutral Zone, a buffer state between the two superpowers. The book has inspired an Amazon series of the same name.
Vladimir Nabokov's novel, Ada or Ardor: A Family Chronicle (1969), is a story of incest that takes place within an alternate North America settled in part by Czarist Russia and that borrows from Dick's idea of "alternate-alternate" history (the world of Nabokov's hero is wracked by rumors of a "counter-earth" that apparently is ours). Some critics believe that the references to a counter-earth suggest that the world portrayed in Ada is a delusion in the mind of the hero (another favorite theme of Dick's novels). Strikingly, the characters in Ada seem to acknowledge their own world as the copy or negative version, calling it "Anti-Terra", while its mythical twin is the real "Terra". Like history, science has followed a divergent path on Anti-Terra: it boasts all the same technology as our world, but all based on water instead of electricity; e.g., when a character in Ada makes a long-distance call, all the toilets in the house flush at once to provide hydraulic power.
Guido Morselli described the defeat of Italy (and subsequently France) in World War I in his novel, Past Conditional (1975; ), wherein the static Alpine front line which divided Italy from Austria during that war collapses when the Germans and the Austrians forsake trench warfare and adopt blitzkrieg twenty years in advance.
Kingsley Amis set his novel, The Alteration (1976), in the 20th century, but major events in the Reformation did not take place, and Protestantism is limited to the breakaway Republic of New England. Martin Luther was reconciled to the Roman Catholic Church and later became Pope Germanian I.
In Nick Hancock and Chris England's 1997 book What Didn't Happen Next: An Alternative History of Football it is suggested that, had Gordon Banks been fit to play in the 1970 FIFA World Cup quarter-final, there would have been no Thatcherism and the post-war consensus would have continued indefinitely.
Kim Stanley Robinson's novel, The Years of Rice and Salt (2002), starts at the point of divergence with Timur turning his army away from Europe, and the Black Death has killed 99% of Europe's population, instead of only a third. Robinson explores world history from that point in AD 1405 (807 AH) to about AD 2045 (1467 AH). Rather than following the great man theory of history, focusing on leaders, wars, and major events, Robinson writes more about social history, similar to the Annales School of history theory and Marxist historiography, focusing on the lives of ordinary people living in their time and place.
Philip Roth's novel, The Plot Against America (2004), looks at an America where Franklin D. Roosevelt is defeated in 1940 in his bid for a third term as President of the United States, and Charles Lindbergh is elected, leading to a US that features increasing fascism and anti-Semitism.
Michael Chabon, occasionally an author of speculative fiction, contributed to the genre with his novel The Yiddish Policemen's Union (2007), which explores a world in which the State of Israel was destroyed in its infancy and many of the world's Jews instead live in a small strip of Alaska set aside by the US government for Jewish settlement. The story follows a Jewish detective solving a murder case in the Yiddish-speaking semi-autonomous city state of Sitka. Stylistically, Chabon borrows heavily from the noir and detective fiction genres, while exploring social issues related to Jewish history and culture. Apart from the alternate history of the Jews and Israel, Chabon also plays with other common tropes of alternate history fiction; in the book, Germany actually loses the war even harder than they did in reality, getting hit with a nuclear bomb instead of just simply losing a ground war (subverting the common "what if Germany won WWII?" trope). |
Alternate history | Contemporary alternate history in popular literature | Contemporary alternate history in popular literature
thumb|The world of 1942, as depicted at the start of S. M. Stirling's The Domination series
thumb|World War I from Harry Turtledove's Southern Victory ("Timeline 191") series
The late 1980s and the 1990s saw a boom in popular-fiction versions of alternate history, fueled by the emergence of the prolific alternate history author Harry Turtledove, as well as the development of the steampunk genre and two series of anthologies—the What Might Have Been series edited by Gregory Benford and the Alternate ... series edited by Mike Resnick. This period also saw alternate history works by S. M. Stirling, Kim Stanley Robinson, Harry Harrison, Howard Waldrop, Peter Tieryas, and others.
In 1986, a sixteen-part epic comic book series called Captain Confederacy began examining a world where the Confederate States of America won the American Civil War. In the series, the Captain and others heroes are staged government propaganda events featuring the feats of these superheroes.
Since the late 1990s, Harry Turtledove has been the most prolific practitioner of alternate history and has been given the title "Master of Alternate History" by some. His books include those of Timeline 191 (a.k.a. Southern Victory, also known as TL-191), in which, while the Confederate States of America won the American Civil War, the Union and Imperial Germany defeat the Entente Powers in the two "Great War"s of the 1910s and 1940s (with a Nazi-esque Confederate government attempting to exterminate its black population), and the Worldwar series, in which aliens invaded Earth during World War II. Other stories by Turtledove include A Different Flesh, in which the Americas were not populated from Asia during the last ice age; In the Presence of Mine Enemies, in which the Nazis won World War II; and Ruled Britannia, in which the Spanish Armada succeeded in conquering England in the Elizabethan era, with William Shakespeare being given the task of writing the play that will motivate the Britons to rise up against their Spanish conquerors. He also co-authored a book with actor Richard Dreyfuss, The Two Georges, in which the United Kingdom retained the American colonies, with George Washington and King George III making peace. He did a two-volume series in which the Japanese not only bombed Pearl Harbor but also invaded and occupied the Hawaiian Islands.
Perhaps the most incessantly explored theme in popular alternate history focuses on the aftermath of an Axis victory in World War II. In some versions, the Nazis and/or Axis Powers win; or in others, they conquer most of the world but a "Fortress America" exists under siege; while in others, there is a Nazi/Japanese Cold War comparable to the US/Soviet equivalent in 'our' timeline. Fatherland (1992), by Robert Harris, is set in Europe following the Nazi victory. The novel Dominion by C.J. Sansom (2012) is similar in concept but is set in England, with Churchill the leader of an anti-German Resistance and other historic persons in various fictional roles. In the Mecha Samurai Empire series (2016), Peter Tieryas focuses on the Asian-American side of the alternate history, exploring an America ruled by the Japanese Empire while integrating elements of Asian pop culture like mechas and videogames.
Several writers have posited points of departure for such a world but then have injected time splitters from the future. For instance James P. Hogan's The Proteus Operation. Norman Spinrad wrote The Iron Dream in 1972, which is intended to be a science fiction novel written by Adolf Hitler after fleeing from Europe to North America in the 1920s.
In Jo Walton's "Small Change" series, the United Kingdom made peace with Hitler before the involvement of the United States in World War II, and slowly collapses due to severe economic depression. Former House Speaker Newt Gingrich and William R. Forstchen have written a novel, 1945, in which the US defeated Japan but not Germany in World War II, resulting in a Cold War with Germany rather than the Soviet Union. Gingrich and Forstchen neglected to write the promised sequel; instead, they wrote a trilogy about the American Civil War, starting with Gettysburg: A Novel of the Civil War, in which the Confederates win a victory at the Battle of Gettysburg - however, after Lincoln responds by bringing Grant and his forces to the eastern theater, the Army of Northern Virginia is soon trapped and destroyed in Maryland, and the war ends within weeks.
While World War II has been a common point of divergence in alternate history literature, several works have been based on other points of divergence. For example, Martin Cruz Smith, in his first novel, posited an independent American Indian nation following the defeat of Custer in The Indians Won (1970). Beginning with The Probability Broach in 1980, L. Neil Smith wrote several novels that postulated the disintegration of the US Federal Government after Albert Gallatin joins the Whiskey Rebellion in 1794 and eventually leads to the creation of a libertarian utopia. In the 2022 novel Poutine and Gin by Steve Rhinelander, the point of divergence is the Battle of the Plains of Abraham of the French and Indian War. That novel is a mystery set in 1940 of that time line.
A recent time traveling splitter variant involves entire communities being shifted elsewhere to become the unwitting creators of new time branches. These communities are transported from the present (or the near-future) to the past or to another timeline via a natural disaster, the action of technologically advanced aliens, or a human experiment gone wrong. S. M. Stirling wrote the Island in the Sea of Time trilogy, in which Nantucket Island and all its modern inhabitants are transported to Bronze Age times to become the world's first superpower. In Eric Flint's 1632 series, a small town in West Virginia is transported to 17th century central Europe and drastically changes the course of the Thirty Years' War, which was then underway. John Birmingham's Axis of Time trilogy deals with the culture shock when a United Nations naval task force from 2021 finds itself back in 1942 helping the Allies against the Empire of Japan and the Germans (and doing almost as much harm as good in spite of its advanced weapons). The series also explores the cultural impacts of people with 2021 ideals interacting with 1940s culture. Similarly, Robert Charles Wilson's Mysterium depicts a failed US government experiment which transports a small American town into an alternative version of the US run by Gnostics, who are engaged in a bitter war with the "Spanish" in Mexico (the chief scientist at the laboratory where the experiment occurred is described as a Gnostic, and references to Christian Gnosticism appear repeatedly in the book).
Although not dealing in physical time travel, in his alt-history novel Marx Returns, Jason Barker introduces anachronisms into the life and times of Karl Marx, such as when his wife Jenny sings a verse from the Sex Pistols's song "Anarchy in the U.K.", or in the games of chess she plays with the Marxes' housekeeper Helene Demuth, which on one occasion involves a Caro–Kann Defence. In her review of the novel, Nina Power writes of "Jenny's 'utopian' desire for an end to time", an attitude which, according to Power, is inspired by her husband's co-authored book The German Ideology. However, in keeping with the novel's anachronisms, the latter was not published until 1932. By contrast, the novel's timeline ends in 1871.
In the 2022 novel Hydrogen Wars: Atomic Sunrise by R.M. Christianson, a small change in post-war Japanese history leads to the election of General Douglas MacArthur as President of the United States. This minor change ultimately leads to all-out atomic war between the major Cold War powers.
Through crowdfunding on Kickstarter, Alan Jenkins and Gan Golan produced a graphic novel series called 1/6 depicting a dystopian alternate reality in which the January 6 United States Capitol attack was successful. What follows is the burning down of the Capitol building and the hanging of Vice President Mike Pence. Under Donald Trump's second term as president, a solid gold statue of him is erected and armed thugs patrol the streets of Washington DC suppressing civilian resistance with brutal violence under the banner of the Confederate flag. |
Alternate history | In fantasy genre | In fantasy genre
thumb|The Angevin Empire in 1172, before the point of divergence of Randall Garrett's Lord Darcy series
Many works of straight fantasy and science fantasy take place in historical settings, though with the addition of, for example, magic or mythological beasts. Some present a secret history in which the modern day world no longer believes that these elements ever existed. Many ambiguous alternate/secret histories are set in Renaissance or pre-Renaissance times, and may explicitly include a "retreat" from the world, which would explain the current absence of such phenomena. Other stories make plan a divergence of some kind.
In Poul Anderson's Three Hearts and Three Lions in which the Matter of France is history and the fairy folk are real and powerful. The same author's A Midsummer Tempest occurs in a world in which the plays of William Shakespeare (called here "the Great Historian"), presented the literal truth in every instance. The novel itself takes place in the era of Oliver Cromwell and Charles I. Here, the English Civil War had a different outcome, and the Industrial Revolution has occurred early.
Randall Garrett's "Lord Darcy" series presents a point of divergence: a monk systemizes magic rather than science, so the use of foxglove to treat heart disease is regarded as superstition. Another point of divergence occurs in 1199, when Richard the Lionheart survives the Siege of Chaluz and returns to England and makes the Angevin Empire so strong that it survives into the 20th century.
Jonathan Strange & Mr Norrell by Susanna Clarke takes place in an England where a separate Kingdom ruled by the Raven King and founded on magic existed in Northumbria for over 300 years. In Patricia Wrede's Regency fantasies, Great Britain has a Royal Society of Wizards.
The Tales of Alvin Maker series by Orson Scott Card (a parallel to the life of Joseph Smith, founder of the Latter Day Saint movement) takes place in an alternate America, beginning in the early 19th century. Prior to that time, a POD occurred: England, under the rule of Oliver Cromwell, had banished "makers", or anyone else demonstrating "knacks" (an ability to perform seemingly supernatural feats) to the North American continent. Thus the early American colonists embraced these gifts as perfectly ordinary, and counted on them as a part of their daily lives. The political division of the continent is considerably altered, with two large English colonies bookending a smaller "American" nation, one aligned with England, and the other governed by exiled Cavaliers. Actual historical figures are seen in a much different light: Ben Franklin is revered as the continent's finest "maker", George Washington was executed after being captured, and "Tom" Jefferson is the first president of "Appalachia", the result of a compromise between the Continentals and the British Crown.
On the other hand, when the "Old Ones" (fairies) still manifest themselves in England in Keith Roberts's Pavane, which takes place in a technologically backward world after a Spanish assassination of Elizabeth I allowed the Spanish Armada to conquer England, the possibility that the fairies were real but retreated from modern advances makes the POD possible: the fairies really were present all along, in a secret history.
Again, in the English Renaissance fantasy Armor of Light by Melissa Scott and Lisa A. Barnett, the magic used in the book, by Dr. John Dee and others, actually was practiced in the Renaissance; positing a secret history of effective magic makes this an alternate history with a point of departure. Sir Philip Sidney survives the Battle of Zutphen in 1586, and shortly thereafter saving the life of Christopher Marlowe.
When the magical version of our world's history is set in contemporary times, the distinction becomes clear between alternate history on the one hand and contemporary fantasy, using in effect a form of secret history (as when Josepha Sherman's Son of Darkness has an elf living in New York City, in disguise) on the other. In works such as Robert A. Heinlein's Magic, Incorporated where a construction company can use magic to rig up stands at a sporting event and Poul Anderson's Operation Chaos and its sequel Operation Luna, where djinns are serious weapons of war—with atomic bombs—the use of magic throughout the United States and other modern countries makes it clear that this is not secret history—although references in Operation Chaos to degaussing the effects of cold iron make it possible that it is the result of a POD. The sequel clarifies this as the result of a collaboration of Einstein and Planck in 1901, resulting in the theory of "rhea tics". Henry Moseley applies this theory to "degauss the effects of cold iron and release the goetic forces." This results in the suppression of ferromagnetism and the re-emergence of magic and magical creatures.
Alternate history shades off into other fantasy subgenres when the use of actual, though altered, history and geography decreases, although a culture may still be clearly the original source; Barry Hughart's Bridge of Birds and its sequels take place in a fantasy world, albeit one clearly based on China, and with allusions to actual Chinese history, such as the Empress Wu. Richard Garfinkle's Celestial Matters incorporates ancient Chinese physics and Greek Aristotelian physics, using them as if factual.
Alternate history has long been a staple of Japanese speculative fiction with such authors as Futaro Yamada and Ryō Hanmura writing novels set in recognizable historical settings with added supernatural or science fiction elements. Ryō Hanmura's 1973 Musubi no Yama Hiroku which recreated 400 years of Japan's history from the perspective of a secret magical family with psychic abilities. The novel has since come to be recognized as a masterpiece of Japanese speculative fiction. Twelve years later, author Hiroshi Aramata wrote the groundbreaking Teito Monogatari which reimagined the history of Tokyo across the 20th century in a world heavily influenced by the supernatural.
Disney's Pirates of the Caribbean series takes place in an alternate history. The filmmakers of The Curse of the Black Pearl made no secret about taking liberties with the time period in which their story takes place. Producer Jerry Bruckheimer explained that the film is a fantasy, but did want to be true to the overall feel of the era, paying particular attention to the years between 1720 and 1750 "in an effort to find an approximation." Director Gore Verbinski asserted that it takes place "roughly at the tail end of the Golden Age of Piracy, when the Morgans lived. Maybe the late 1720s." The crew went to great lengths to maintain authenticity, such as Jack Sparrow's sword being an original that dates from the 1750s.Pirates of the Caribbean presskit , accessed December 9, 2006 Ann C. Crispin knew about the Pirates universe being an alternate history writing the prequel novel The Price of Freedom,A. C. Crispin interview – The Price of Freedom – Fast Forward: Contemporary Science Fiction – YouTube with Disney's instructions for Crispin being to "stick to historical fact, unless it conflicts with established Pirates of the Caribbean continuity." Crispin made a faithful effort to do this, having done plenty of research, with Under the Black Flag by David Cordingly being one of the four pirate-related books she found herself using the most consistently.Crispin, A. C. (2011). Pirates of the Caribbean: The Price of Freedom According to production designer John Myhre, the filmmakers of the fourth film, On Stranger Tides, picked the date of 1750, or in the range of the mid-1700s.Pirates of the Caribbean: On Stranger Tides Set Visit! - ComingSoon.net - Part 1 - ArchivedPirates of the Caribbean: On Stranger Tides Set Visit! - ComingSoon.net - Part 2 - Archived The film also featured Blackbeard, based on the historical figure and an element retained from the novel On Stranger Tides by Tim Powers. The history prior to On Stranger Tides is also slightly different from real-world history, with Blackbeard's death at Ocracoke Inlet in 1718 was considered a legend in the film, with Jack Sparrow saying he was beheaded, and that his headless body swam three times around his ship before climbing back on board. The fifth film, Dead Men Tell No Tales, also took place in the 1750s, with an early draft taking place sometime the Seven Years' War.PIRATES OF THE CARIBBEAN: DEAD MEN TELL NO TALES by Terry Rossio - Wordplayer.com |
Alternate history | Television | Television
1983 is set on a world where the Iron Curtain never fell and the Cold War continues until the present (2003).
An Englishman's Castle tells the story of the writer of a soap opera in a 1970s England which lost World War II. England is run by a collaborator government which strains to maintain a normal appearance of British life. Slowly, however, the writer begins to uncover the truth.
In the Community episode "Remedial Chaos Theory," each of the six members of the study group rolls a die to decide who has to go downstairs to accept a pizza delivery for the group, creating 6 different alternative worlds. Characters from the worst universe, "darkest timeline," would later appear in the "prime universe".
Confederate was a planned HBO series set on a world where the south won the US Civil War. Social media backlash during pre-production led to the series being cancelled with no episodes produced.
Counterpart tells of a United Nations agency that is responsible for monitoring passage between alternative worlds. Two of the worlds, Alpha and Prime, are locked in a cold war.
The Court-Martial of George Armstrong Custer is a 1977 telemovie where George Custer survives the Battle of Little Bighorn and faces a court martial hearing over his incompetence.
C.S.A.: The Confederate States of America presents itself as a British TV documentary uncovering some of the dark secrets of the Confederacy on a world where the south won the US Civil War.
Dark Skies tells that much of history having been shaped since the 1940s by a government conspiracy with aliens. One race of aliens can take over humans, while those immune to the alien's control fight back.
Doctor Who'''s main character has visited two alternative worlds in the TV show and several in its spin off media. The Third Doctor visits a world with a fascist Great Britain on the brink of destruction in Inferno, while the Tenth Doctor visits a Britain that has a President and blimps are a common form of transportation beset by Cybermen in Rise of the Cybermen / The Age of Steel. The Seventh Doctor faces a threat from an alternative world in Battlefield, where magic is real and the alternative version of The Doctor is hinted to be that reality's Merlin.
thumb|196x196px|The U.S. flag in the Fallout franchise Fallout shows a 1950s retro-future world that suffers a global nuclear war on the Amazon streaming service.Fatherland is a TV movie set in a 1960 alternative world where US President Joseph Kennedy and Adolf Hitler have agreed to meet to discuss an end to their country's Cold War 15 years after the Axis victory in World War II. However, an American reporter has discovered proof of the long denied Final Solution threatens the meeting.
The anime Fena: Pirate Princess featured an alternate 18th century.For All Mankind depicts an alternate timeline in which the Soviet crewed lunar program successfully lands on the Moon before the US Apollo program, resulting in a continued and intensified Space Race.Fringe has the father of one of the main characters cross into another reality to steal that world's version of his son after his son dies. The second world has a slightly different history, with a few different states in the United States, such as only one Carolina and Upper Michigan as a state. In addition, the 9/11 attack didn't take down the Twin Towers but the White House. Also, several major DC Comics events are different, such as Superman not Supergirl dying during Crisis on Infinite Earths. The incursion to steal the son has many negative effects on that world, and while the realities start out as antagonist, they eventually work together to repair the damage.The Man in the High Castle, an adaptation of the novel of the same name, showed a world where the Axis Powers won World War II.Motherland: Fort Salem explores a female-dominated world in which witchcraft is real. Its world diverged from our timeline when the Salem witch trials are resolved by an agreement between witches and ungifted humans.Noughts + Crosses is a British TV show set on a world where a powerful West African empire colonizes Europe 700 years before the start of the series.Parallels was a planned TV show whose pilot was later released as a Netflix movie. The plot concerns a building which can shift realities every 36 hours and those who use the building to travel to other realities.The Plot Against America is an HBO miniseries where Charles Lindbergh wins the 1940 US presidential election as an anti-war candidate who moves the country toward fascism.
Primal features Spear and Fang from the Prehistoric encountering with Ancient Egypt and Vikings era, and in episode The Primal Theory where Charles Darwin is alive in 1890 instead of 1882.
The TV show Sliders explores different possible alternate realities by having the protagonist "slide" into different parallel dimensions of the same planet Earth.The Great Martian War 1913-1917An alternate history documentary where giant martians with machines invaded the Earth during WW1, causing huge technological upgrades and the entente and central powers fighting alongside each other.SS-GB shows a world where the Axis Powers quickly win World War II, killing Churchill and installing a puppet government. However, British resistance fights back.
In the various Star Trek TV shows and spin off media a Mirror Universe has been encountered where Earth has an empire that subjugates other planets. Doppelgängers of the main cast of many the TV shows appear in that reality.
The Watchmen series is set on a world where costumed heroes were initially welcomed but later outlawed. It is set 34 years after the events of the comic book on which the series shares a name.
The Marvel Cinematic Universe series, Loki (2021 & 2023), on Disney+, shows an agency which prevents alterations to the timeline. Alternate versions of Loki from various universes appear.
The Marvel Cinematic Universe series, What If...? (2021–2024), on Disney+, shows alternate universes that depict alternate events from the MCU films.
Online
Fans of alternate history have made use of the internet from a very early point to showcase their own works and provide useful tools for those fans searching for anything alternate history, first in mailing lists and usenet groups, later in web databases and forums. The "Usenet Alternate History List" was first posted on 11 April 1991, to the Usenet newsgroup rec.arts.sf-lovers. In May 1995, the dedicated newsgroup soc.history.what-if was created for showcasing and discussing alternate histories. Its prominence declined with the general migration from unmoderated usenet to moderated web forums, most prominently AlternateHistory.com, the self-described "largest gathering of alternate history fans on the internet" with over 10,000 active members.
In addition to these discussion forums, in 1997 Uchronia: The Alternate History List was created as an online repository, now containing over 2,900 alternate history novels, stories, essays, and other printed materials in several different languages. Uchronia was selected as the Sci Fi Channel's "Sci Fi Site of the Week" twice.
Uchronia
In Spanish, French, German, Portuguese, Italian, Catalan, and Galician, the words , , and are native versions of alternate history, from which comes the English loanword uchronia. The English term uchronia is a neologism that is sometimes used in its original meaning as a straightforward synonym for alternate history.de Sa, Alexandre F. (2012). From modern utopias to contemporary uchronia. Existential Utopia: New Perspectives on Utopian Thought.Loyer, Emmanuelle (2019). Uchronia. Booksandideas.net.Schmid, Helga (2020). Uchronia: Designing Time. Germany: Walter de Gruyter GmbH. p. 26 However, it may also now refer to other concepts, namely an umbrella genre of fiction that encompasses alternate history, parallel universes in fiction, and fiction based in futuristic or non-temporal settings.Worth, Aaron (2018). Uchronia. Victorian Literature and Culture, 46(3-4), 928-930.Craveiro, Joanna (2016). A live/living museum of small, forgotten and unwanted memories: performing narratives, testimonies and archives of the Portuguese Dictatorship and Revolution (Doctoral dissertation, University of Roehampton), p. 46.Schmid, 2020, p. 11, 28.
See also
20th century in science fiction
Alien space bats
Alternate ending
Alternative future
American Civil War alternate histories
Dieselpunk
Dystopian
Fictional universe
Future history
The Garden of Forking Paths Historical revisionism
Hypothetical Axis victory in World War II
Invasion literature
Jonbar hinge
List of alternate history fiction
Possible worlds
Pulp novels
Ruritanian romance
References
Further reading
Chapman, Edgar L., and Carl B. Yoke (eds.). Classic and Iconoclastic Alternate History Science Fiction. Mellen, 2003.
Collins, William Joseph. Paths Not Taken: The Development, Structure, and Aesthetics of the Alternative History. University of California, Davis 1990.
Darius, Julian. "58 Varieties: Watchmen and Revisionism". In Minutes to Midnight: Twelve Essays on Watchmen. Sequart Research & Literacy Organization, 2010. Focuses on Watchmen as alternate history.
Cowley, Robert, ed., What If? Military Historians Imagine What Might Have Been. Pan Books, 1999.
Gevers, Nicholas. Mirrors of the Past: Versions of History in Science Fiction and Fantasy. University of Cape Town, 1997
Hellekson, Karen. The Alternate History: Refiguring Historical Time. Kent State University Press, 2001
Keen, Antony G. "Alternate Histories of the Roman Empire in Stephen Baxter, Robert Silverberg and Sophia McDougall". Foundation: The International Review of Science Fiction 102, Spring 2008.
McKnight, Edgar Vernon Jr. Alternative History: The Development of a Literary Genre. University of North Carolina at Chapel Hill, 1994.
Morgan, Glyn, and C. Palmer-Patel (eds.). Sideways in Time: Critical Essays on Alternate History Fiction. Liverpool University Press, 2019.
Nedelkovh, Aleksandar B. British and American Science Fiction Novel 1950–1980 with the Theme of Alternative History (an Axiological Approach). 1994 , 1999 .
Rosenfeld, Gavriel David. The World Hitler Never Made: Alternate History and the Memory of Nazism. 2005
Rosenfeld, Gavriel David. "Why Do We Ask 'What If?' Reflections on the Function of Alternate History." History and Theory 41, Theme Issue 41: Unconventional History (December 2002), 90–103. .
Schneider-Mayerson, Matthew. "What Almost Was: The Politics of the Contemporary Alternate History Novel". American Studies 30, 3–4 (Summer 2009), 63–83.
Singles, Kathleen. Alternate History: Playing with Contingency and Necessity''. De Gruyter, 2013. |
Alternate history | External links | External links
Alternate History on TV Tropes
Category:Alternate history
Category:Science fiction genres
Category:Speculative fiction |
Alternate history | Table of Content | Short description, Definition, History of literature, Antiquity and medieval, 19th century, Early 20th century and the era of the pulps, Time travel to create historical divergences, Quantum theory of many worlds, Rival paratime worlds, Major writers explore alternate histories, Contemporary alternate history in popular literature, In fantasy genre, Television, External links |
Atomic orbital | Short description | right|thumb|upright=2|The shapes of the first five atomic orbitals are 1s, 2s, 2px, 2py, and 2pz. The two colors show the phase or sign of the wave function in each region. Each picture is domain coloring of a function which depends on the coordinates of one electron. To see the elongated shape of functions that show probability density more directly, see pictures of d-orbitals below.
In quantum mechanics, an atomic orbital () is a function describing the location and wave-like behavior of an electron in an atom. This function describes an electron's charge distribution around the atom's nucleus, and can be used to calculate the probability of finding an electron in a specific region around the nucleus.
Each orbital in an atom is characterized by a set of values of three quantum numbers , , and , which respectively correspond to electron's energy, its orbital angular momentum, and its orbital angular momentum projected along a chosen axis (magnetic quantum number). The orbitals with a well-defined magnetic quantum number are generally complex-valued. Real-valued orbitals can be formed as linear combinations of and orbitals, and are often labeled using associated harmonic polynomials (e.g., xy, ) which describe their angular structure.
An orbital can be occupied by a maximum of two electrons, each with its own projection of spin . The simple names s orbital, p orbital, d orbital, and f orbital refer to orbitals with angular momentum quantum number and respectively. These names, together with their n values, are used to describe electron configurations of atoms. They are derived from description by early spectroscopists of certain series of alkali metal spectroscopic lines as sharp, principal, diffuse, and fundamental. Orbitals for continue alphabetically (g, h, i, k, ...), omitting j because some languages do not distinguish between letters "i" and "j".
Atomic orbitals are basic building blocks of the atomic orbital model (or electron cloud or wave mechanics model), a modern framework for visualizing submicroscopic behavior of electrons in matter. In this model, the electron cloud of an atom may be seen as being built up (in approximation) in an electron configuration that is a product of simpler hydrogen-like atomic orbitals. The repeating periodicity of blocks of 2, 6, 10, and 14 elements within sections of periodic table arises naturally from total number of electrons that occupy a complete set of s, p, d, and f orbitals, respectively, though for higher values of quantum number , particularly when the atom bears a positive charge, energies of certain sub-shells become very similar and so, the order in which they are said to be populated by electrons (e.g., Cr = [Ar]4s13d5 and Cr2+ = [Ar]3d4) can be rationalized only somewhat arbitrarily.
thumb|upright=1.5|Cross-sections of atomic orbitals of the electron in a hydrogen atom at different energy levels. The probability of finding the electron is given by the color, as shown in the key at upper right. |
Atomic orbital | Electron properties | Electron properties
With the development of quantum mechanics and experimental findings (such as the two slit diffraction of electrons), it was found that the electrons orbiting a nucleus could not be fully described as particles, but needed to be explained by wave–particle duality. In this sense, electrons have the following properties:
Wave-like properties:
Electrons do not orbit a nucleus in the manner of a planet orbiting a star, but instead exist as standing waves. Thus the lowest possible energy an electron can take is similar to the fundamental frequency of a wave on a string. Higher energy states are similar to harmonics of that fundamental frequency.
The electrons are never in a single point location, though the probability of interacting with the electron at a single point can be found from the electron's wave function. The electron's charge acts like it is smeared out in space in a continuous distribution, proportional at any point to the squared magnitude of the electron's wave function.
Particle-like properties:
The number of electrons orbiting a nucleus can be only an integer.
Electrons jump between orbitals like particles. For example, if one photon strikes the electrons, only one electron changes state as a result.
Electrons retain particle-like properties such as: each wave state has the same electric charge as its electron particle. Each wave state has a single discrete spin (spin up or spin down) depending on its superposition.
Thus, electrons cannot be described simply as solid particles. An analogy might be that of a large and often oddly shaped "atmosphere" (the electron), distributed around a relatively tiny planet (the nucleus). Atomic orbitals exactly describe the shape of this "atmosphere" only when one electron is present. When more electrons are added, the additional electrons tend to more evenly fill in a volume of space around the nucleus so that the resulting collection ("electron cloud") tends toward a generally spherical zone of probability describing the electron's location, because of the uncertainty principle.
One should remember that these orbital 'states', as described here, are merely eigenstates of an electron in its orbit. An actual electron exists in a superposition of states, which is like a weighted average, but with complex number weights. So, for instance, an electron could be in a pure eigenstate (2, 1, 0), or a mixed state (2, 1, 0) + (2, 1, 1), or even the mixed state (2, 1, 0) + (2, 1, 1). For each eigenstate, a property has an eigenvalue. So, for the three states just mentioned, the value of is 2, and the value of is 1. For the second and third states, the value for is a superposition of 0 and 1. As a superposition of states, it is ambiguous—either exactly 0 or exactly 1—not an intermediate or average value like the fraction . A superposition of eigenstates (2, 1, 1) and (3, 2, 1) would have an ambiguous and , but would definitely be 1. Eigenstates make it easier to deal with the math. You can choose a different basis of eigenstates by superimposing eigenstates from any other basis (see Real orbitals below). |
Atomic orbital | Formal quantum mechanical definition | Formal quantum mechanical definition
Atomic orbitals may be defined more precisely in formal quantum mechanical language. They are approximate solutions to the Schrödinger equation for the electrons bound to the atom by the electric field of the atom's nucleus. Specifically, in quantum mechanics, the state of an atom, i.e., an eigenstate of the atomic Hamiltonian, is approximated by an expansion (see configuration interaction expansion and basis set) into linear combinations of anti-symmetrized products (Slater determinants) of one-electron functions. The spatial components of these one-electron functions are called atomic orbitals. (When one considers also their spin component, one speaks of atomic spin orbitals.) A state is actually a function of the coordinates of all the electrons, so that their motion is correlated, but this is often approximated by this independent-particle model of products of single electron wave functions.Roger Penrose, The Road to Reality (The London dispersion force, for example, depends on the correlations of the motion of the electrons.)
In atomic physics, the atomic spectral lines correspond to transitions (quantum leaps) between quantum states of an atom. These states are labeled by a set of quantum numbers summarized in the term symbol and usually associated with particular electron configurations, i.e., by occupation schemes of atomic orbitals (for example, 1s2 2s2 2p6 for the ground state of neon-term symbol: 1S0).
This notation means that the corresponding Slater determinants have a clear higher weight in the configuration interaction expansion. The atomic orbital concept is therefore a key concept for visualizing the excitation process associated with a given transition. For example, one can say for a given transition that it corresponds to the excitation of an electron from an occupied orbital to a given unoccupied orbital. Nevertheless, one has to keep in mind that electrons are fermions ruled by the Pauli exclusion principle and cannot be distinguished from each other. Moreover, it sometimes happens that the configuration interaction expansion converges very slowly and that one cannot speak about simple one-determinant wave function at all. This is the case when electron correlation is large.
Fundamentally, an atomic orbital is a one-electron wave function, even though many electrons are not in one-electron atoms, and so the one-electron view is an approximation. When thinking about orbitals, we are often given an orbital visualization heavily influenced by the Hartree–Fock approximation, which is one way to reduce the complexities of molecular orbital theory. |
Atomic orbital | Types of orbital | Types of orbital
thumb|upright=1.5|3D views of some hydrogen-like atomic orbitals showing probability density and phase (g orbitals and higher not shown)
Atomic orbitals can be the hydrogen-like "orbitals" which are exact solutions to the Schrödinger equation for a hydrogen-like "atom" (i.e., atom with one electron). Alternatively, atomic orbitals refer to functions that depend on the coordinates of one electron (i.e., orbitals) but are used as starting points for approximating wave functions that depend on the simultaneous coordinates of all the electrons in an atom or molecule. The coordinate systems chosen for orbitals are usually spherical coordinates in atoms and Cartesian in polyatomic molecules. The advantage of spherical coordinates here is that an orbital wave function is a product of three factors each dependent on a single coordinate: . The angular factors of atomic orbitals generate s, p, d, etc. functions as real combinations of spherical harmonics (where and are quantum numbers). There are typically three mathematical forms for the radial functions which can be chosen as a starting point for the calculation of the properties of atoms and molecules with many electrons:
The hydrogen-like orbitals are derived from the exact solutions of the Schrödinger equation for one electron and a nucleus, for a hydrogen-like atom. The part of the function that depends on distance r from the nucleus has radial nodes and decays as .
The Slater-type orbital (STO) is a form without radial nodes but decays from the nucleus as does a hydrogen-like orbital.
The form of the Gaussian type orbital (Gaussians) has no radial nodes and decays as .
Although hydrogen-like orbitals are still used as pedagogical tools, the advent of computers has made STOs preferable for atoms and diatomic molecules since combinations of STOs can replace the nodes in hydrogen-like orbitals. Gaussians are typically used in molecules with three or more atoms. Although not as accurate by themselves as STOs, combinations of many Gaussians can attain the accuracy of hydrogen-like orbitals. |
Atomic orbital | History | History
The term orbital was introduced by Robert S. Mulliken in 1932 as short for one-electron orbital wave function. Niels Bohr explained around 1913 that electrons might revolve around a compact nucleus with definite angular momentum. Bohr's model was an improvement on the 1911 explanations of Ernest Rutherford, that of the electron moving around a nucleus. Japanese physicist Hantaro Nagaoka published an orbit-based hypothesis for electron behavior as early as 1904. These theories were each built upon new observations starting with simple understanding and becoming more correct and complex. Explaining the behavior of these electron "orbits" was one of the driving forces behind the development of quantum mechanics. |
Atomic orbital | Early models | Early models
With J. J. Thomson's discovery of the electron in 1897, it became clear that atoms were not the smallest building blocks of nature, but were rather composite particles. The newly discovered structure within atoms tempted many to imagine how the atom's constituent parts might interact with each other. Thomson theorized that multiple electrons revolve in orbit-like rings within a positively charged jelly-like substance, and between the electron's discovery and 1909, this "plum pudding model" was the most widely accepted explanation of atomic structure.
Shortly after Thomson's discovery, Hantaro Nagaoka predicted a different model for electronic structure. Unlike the plum pudding model, the positive charge in Nagaoka's "Saturnian Model" was concentrated into a central core, pulling the electrons into circular orbits reminiscent of Saturn's rings. Few people took notice of Nagaoka's work at the time, and Nagaoka himself recognized a fundamental defect in the theory even at its conception, namely that a classical charged object cannot sustain orbital motion because it is accelerating and therefore loses energy due to electromagnetic radiation. Nevertheless, the Saturnian model turned out to have more in common with modern theory than any of its contemporaries. |
Atomic orbital | Bohr atom | Bohr atom
In 1909, Ernest Rutherford discovered that the bulk of the atomic mass was tightly condensed into a nucleus, which was also found to be positively charged. It became clear from his analysis in 1911 that the plum pudding model could not explain atomic structure. In 1913, Rutherford's post-doctoral student, Niels Bohr, proposed a new model of the atom, wherein electrons orbited the nucleus with classical periods, but were permitted to have only discrete values of angular momentum, quantized in units ħ. This constraint automatically allowed only certain electron energies. The Bohr model of the atom fixed the problem of energy loss from radiation from a ground state (by declaring that there was no state below this), and more importantly explained the origin of spectral lines.
thumb|The Rutherford–Bohr model of the hydrogen atom
After Bohr's use of Einstein's explanation of the photoelectric effect to relate energy levels in atoms with the wavelength of emitted light, the connection between the structure of electrons in atoms and the emission and absorption spectra of atoms became an increasingly useful tool in the understanding of electrons in atoms. The most prominent feature of emission and absorption spectra (known experimentally since the middle of the 19th century), was that these atomic spectra contained discrete lines. The significance of the Bohr model was that it related the lines in emission and absorption spectra to the energy differences between the orbits that electrons could take around an atom. This was, however, not achieved by Bohr through giving the electrons some kind of wave-like properties, since the idea that electrons could behave as matter waves was not suggested until eleven years later. Still, the Bohr model's use of quantized angular momenta and therefore quantized energy levels was a significant step toward the understanding of electrons in atoms, and also a significant step towards the development of quantum mechanics in suggesting that quantized restraints must account for all discontinuous energy levels and spectra in atoms.
With de Broglie's suggestion of the existence of electron matter waves in 1924, and for a short time before the full 1926 Schrödinger equation treatment of hydrogen-like atoms, a Bohr electron "wavelength" could be seen to be a function of its momentum; so a Bohr orbiting electron was seen to orbit in a circle at a multiple of its half-wavelength. The Bohr model for a short time could be seen as a classical model with an additional constraint provided by the 'wavelength' argument. However, this period was immediately superseded by the full three-dimensional wave mechanics of 1926. In our current understanding of physics, the Bohr model is called a semi-classical model because of its quantization of angular momentum, not primarily because of its relationship with electron wavelength, which appeared in hindsight a dozen years after the Bohr model was proposed.
The Bohr model was able to explain the emission and absorption spectra of hydrogen. The energies of electrons in the n = 1, 2, 3, etc. states in the Bohr model match those of current physics. However, this did not explain similarities between different atoms, as expressed by the periodic table, such as the fact that helium (two electrons), neon (10 electrons), and argon (18 electrons) exhibit similar chemical inertness. Modern quantum mechanics explains this in terms of electron shells and subshells which can each hold a number of electrons determined by the Pauli exclusion principle. Thus the n = 1 state can hold one or two electrons, while the n = 2 state can hold up to eight electrons in 2s and 2p subshells. In helium, all n = 1 states are fully occupied; the same is true for n = 1 and n = 2 in neon. In argon, the 3s and 3p subshells are similarly fully occupied by eight electrons; quantum mechanics also allows a 3d subshell but this is at higher energy than the 3s and 3p in argon (contrary to the situation for hydrogen) and remains empty. |
Atomic orbital | Modern conceptions and connections to the Heisenberg uncertainty principle | Modern conceptions and connections to the Heisenberg uncertainty principle
Immediately after Heisenberg discovered his uncertainty principle, Bohr noted that the existence of any sort of wave packet implies uncertainty in the wave frequency and wavelength, since a spread of frequencies is needed to create the packet itself. In quantum mechanics, where all particle momenta are associated with waves, it is the formation of such a wave packet which localizes the wave, and thus the particle, in space. In states where a quantum mechanical particle is bound, it must be localized as a wave packet, and the existence of the packet and its minimum size implies a spread and minimal value in particle wavelength, and thus also momentum and energy. In quantum mechanics, as a particle is localized to a smaller region in space, the associated compressed wave packet requires a larger and larger range of momenta, and thus larger kinetic energy. Thus the binding energy to contain or trap a particle in a smaller region of space increases without bound as the region of space grows smaller. Particles cannot be restricted to a geometric point in space, since this would require infinite particle momentum.
In chemistry, Erwin Schrödinger, Linus Pauling, Mulliken and others noted that the consequence of Heisenberg's relation was that the electron, as a wave packet, could not be considered to have an exact location in its orbital. Max Born suggested that the electron's position needed to be described by a probability distribution which was connected with finding the electron at some point in the wave-function which described its associated wave packet. The new quantum mechanics did not give exact results, but only the probabilities for the occurrence of a variety of possible such results. Heisenberg held that the path of a moving particle has no meaning if we cannot observe it, as we cannot with electrons in an atom.
In the quantum picture of Heisenberg, Schrödinger and others, the Bohr atom number n for each orbital became known as an n-sphere in a three-dimensional atom and was pictured as the most probable energy of the probability cloud of the electron's wave packet which surrounded the atom. |
Atomic orbital | Orbital names | Orbital names |
Atomic orbital | Orbital notation and subshells | Orbital notation and subshells
Orbitals have been given names, which are usually given in the form:
where X is the energy level corresponding to the principal quantum number ; type is a lower-case letter denoting the shape or subshell of the orbital, corresponding to the angular momentum quantum number .
For example, the orbital 1s (pronounced as the individual numbers and letters: "'one' 'ess'") is the lowest energy level () and has an angular quantum number of , denoted as s. Orbitals with are denoted as p, d and f respectively.
The set of orbitals for a given n and is called a subshell, denoted
.
The superscript y shows the number of electrons in the subshell. For example, the notation 2p4 indicates that the 2p subshell of an atom contains 4 electrons. This subshell has 3 orbitals, each with n = 2 and = 1. |
Atomic orbital | X-ray notation | X-ray notation
There is also another, less common system still used in X-ray science known as X-ray notation, which is a continuation of the notations used before orbital theory was well understood. In this system, the principal quantum number is given a letter associated with it. For , the letters associated with those numbers are K, L, M, N, O, ... respectively. |
Atomic orbital | Hydrogen-like orbitals | Hydrogen-like orbitals
The simplest atomic orbitals are those that are calculated for systems with a single electron, such as the hydrogen atom. An atom of any other element ionized down to a single electron (He+, Li2+, etc.) is very similar to hydrogen, and the orbitals take the same form. In the Schrödinger equation for this system of one negative and one positive particle, the atomic orbitals are the eigenstates of the Hamiltonian operator for the energy. They can be obtained analytically, meaning that the resulting orbitals are products of a polynomial series, and exponential and trigonometric functions. (see hydrogen atom).
For atoms with two or more electrons, the governing equations can be solved only with the use of methods of iterative approximation. Orbitals of multi-electron atoms are qualitatively similar to those of hydrogen, and in the simplest models, they are taken to have the same form. For more rigorous and precise analysis, numerical approximations must be used.
A given (hydrogen-like) atomic orbital is identified by unique values of three quantum numbers: , , and . The rules restricting the values of the quantum numbers, and their energies (see below), explain the electron configuration of the atoms and the periodic table.
The stationary states (quantum states) of a hydrogen-like atom are its atomic orbitals. However, in general, an electron's behavior is not fully described by a single orbital. Electron states are best represented by time-depending "mixtures" (linear combinations) of multiple orbitals. See Linear combination of atomic orbitals molecular orbital method.
The quantum number first appeared in the Bohr model where it determines the radius of each circular electron orbit. In modern quantum mechanics however, determines the mean distance of the electron from the nucleus; all electrons with the same value of n lie at the same average distance. For this reason, orbitals with the same value of n are said to comprise a "shell". Orbitals with the same value of n and also the same value of are even more closely related, and are said to comprise a "subshell". |
Atomic orbital | Quantum numbers | Quantum numbers
Because of the quantum mechanical nature of the electrons around a nucleus, atomic orbitals can be uniquely defined by a set of integers known as quantum numbers. These quantum numbers occur only in certain combinations of values, and their physical interpretation changes depending on whether real or complex versions of the atomic orbitals are employed. |
Atomic orbital | Complex orbitals | Complex orbitals
thumb|450px|alt=Electronic levels|Energetic levels and sublevels of polyelectronic atoms
In physics, the most common orbital descriptions are based on the solutions to the hydrogen atom, where orbitals are given by the product between a radial function and a pure spherical harmonic. The quantum numbers, together with the rules governing their possible values, are as follows:
The principal quantum number describes the energy of the electron and is always a positive integer. In fact, it can be any positive integer, but for reasons discussed below, large numbers are seldom encountered. Each atom has, in general, many orbitals associated with each value of n; these orbitals together are sometimes called electron shells.
The azimuthal quantum number describes the orbital angular momentum of each electron and is a non-negative integer. Within a shell where is some integer , ranges across all (integer) values satisfying the relation . For instance, the shell has only orbitals with , and the shell has only orbitals with , and . The set of orbitals associated with a particular value of are sometimes collectively called a subshell.
The magnetic quantum number, , describes the projection of the orbital angular momentum along a chosen axis. It determines the magnitude of the current circulating around that axis and the orbital contribution to the magnetic moment of an electron via the Ampèrian loop model. Within a subshell , obtains the integer values in the range .
The above results may be summarized in the following table. Each cell represents a subshell, and lists the values of available in that subshell. Empty cells represent subshells that do not exist.
... ... 0 −1, 0, 1 ... 0 −1, 0, 1 −2, −1, 0, 1, 2 ... 0 −1, 0, 1 −2, −1, 0, 1, 2 −3, −2, −1, 0, 1, 2, 3 ... 0 −1, 0, 1 −2, −1, 0, 1, 2 −3, −2, −1, 0, 1, 2, 3 −4, −3, −2, −1, 0, 1, 2, 3, 4 ... ... ... ... ... ... ... ...
Subshells are usually identified by their - and -values. is represented by its numerical value, but is represented by a letter as follows: 0 is represented by 's', 1 by 'p', 2 by 'd', 3 by 'f', and 4 by 'g'. For instance, one may speak of the subshell with and as a '2s subshell'.
Each electron also has angular momentum in the form of quantum mechanical spin given by spin s = . Its projection along a specified axis is given by the spin magnetic quantum number, ms, which can be + or −. These values are also called "spin up" or "spin down" respectively.
The Pauli exclusion principle states that no two electrons in an atom can have the same values of all four quantum numbers. If there are two electrons in an orbital with given values for three quantum numbers, (, , ), these two electrons must differ in their spin projection ms.
The above conventions imply a preferred axis (for example, the z direction in Cartesian coordinates), and they also imply a preferred direction along this preferred axis. Otherwise there would be no sense in distinguishing from . As such, the model is most useful when applied to physical systems that share these symmetries. The Stern–Gerlach experimentwhere an atom is exposed to a magnetic fieldprovides one such example. |
Atomic orbital | Real orbitals | Real orbitals
thumb|220px|Animation of continuously varying superpositions between the and the orbitals. This animation does not use the Condon–Shortley phase convention.
Instead of the complex orbitals described above, it is common, especially in the chemistry literature, to use real atomic orbitals. These real orbitals arise from simple linear combinations of complex orbitals. Using the Condon–Shortley phase convention, real orbitals are related to complex orbitals in the same way that the real spherical harmonics are related to complex spherical harmonics. Letting denote a complex orbital with quantum numbers , , and , the real orbitals may be defined by
If , with the radial part of the orbital, this definition is equivalent to where is the real spherical harmonic related to either the real or imaginary part of the complex spherical harmonic .
Real spherical harmonics are physically relevant when an atom is embedded in a crystalline solid, in which case there are multiple preferred symmetry axes but no single preferred direction. Real atomic orbitals are also more frequently encountered in introductory chemistry textbooks and shown in common orbital visualizations. In real hydrogen-like orbitals, quantum numbers and have the same interpretation and significance as their complex counterparts, but is no longer a good quantum number (but its absolute value is).
Some real orbitals are given specific names beyond the simple designation. Orbitals with quantum number are called orbitals. With this one can already assign names to complex orbitals such as ; the first symbol is the quantum number, the second character is the symbol for that particular quantum number and the subscript is the quantum number.
As an example of how the full orbital names are generated for real orbitals, one may calculate . From the table of spherical harmonics, with . Then
Likewise . As a more complicated example:
In all these cases we generate a Cartesian label for the orbital by examining, and abbreviating, the polynomial in appearing in the numerator. We ignore any terms in the polynomial except for the term with the highest exponent in .
We then use the abbreviated polynomial as a subscript label for the atomic state, using the same nomenclature as above to indicate the and quantum numbers.
The expression above all use the Condon–Shortley phase convention which is favored by quantum physicists. Other conventions exist for the phase of the spherical harmonics. Under these different conventions the and orbitals may appear, for example, as the sum and difference of and , contrary to what is shown above.
Below is a list of these Cartesian polynomial names for the atomic orbitals. There does not seem to be reference in the literature as to how to abbreviate the long Cartesian spherical harmonic polynomials for so there does not seem be consensus on the naming of orbitals or higher according to this nomenclature.
|
Atomic orbital | Shapes of orbitals | Shapes of orbitals
thumb|Transparent cloud view of a computed 6s hydrogen orbital. The s orbitals, though spherically symmetric, have radially placed wave-nodes for . Only s orbitals invariably have a center anti-node; the other types never do.
Simple pictures showing orbital shapes are intended to describe the angular forms of regions in space where the electrons occupying the orbital are likely to be found. The diagrams cannot show the entire region where an electron can be found, since according to quantum mechanics there is a non-zero probability of finding the electron (almost) anywhere in space. Instead the diagrams are approximate representations of boundary or contour surfaces where the probability density has a constant value, chosen so that there is a certain probability (for example 90%) of finding the electron within the contour. Although as the square of an absolute value is everywhere non-negative, the sign of the wave function is often indicated in each subregion of the orbital picture.
Sometimes the function is graphed to show its phases, rather than which shows probability density but has no phase (which is lost when taking absolute value, since is a complex number). orbital graphs tend to have less spherical, thinner lobes than graphs, but have the same number of lobes in the same places, and otherwise are recognizable. This article, to show wave function phase, shows mostly graphs.
The lobes can be seen as standing wave interference patterns between the two counter-rotating, ring-resonant traveling wave and modes; the projection of the orbital onto the xy plane has a resonant wavelength around the circumference. Although rarely shown, the traveling wave solutions can be seen as rotating banded tori; the bands represent phase information. For each there are two standing wave solutions and . If , the orbital is vertical, counter rotating information is unknown, and the orbital is z-axis symmetric. If there are no counter rotating modes. There are only radial modes and the shape is spherically symmetric.
Nodal planes and nodal spheres are surfaces on which the probability density vanishes. The number of nodal surfaces is controlled by the quantum numbers and . An orbital with azimuthal quantum number has radial nodal planes passing through the origin. For example, the s orbitals () are spherically symmetric and have no nodal planes, whereas the p orbitals () have a single nodal plane between the lobes. The number of nodal spheres equals , consistent with the restriction on the quantum numbers. The principal quantum number controls the total number of nodal surfaces which is . Loosely speaking, is energy, is analogous to eccentricity, and is orientation.
In general, determines size and energy of the orbital for a given nucleus; as increases, the size of the orbital increases. The higher nuclear charge of heavier elements causes their orbitals to contract by comparison to lighter ones, so that the size of the atom remains very roughly constant, even as the number of electrons increases.
thumb|Experimentally imaged 1s and 2p core-electron orbitals of Sr, including the effects of atomic thermal vibration and excitation broadening, retrieved from energy dispersive x-ray spectroscopy (EDX) in scanning transmission electron microscopy (STEM).
Also in general terms, determines an orbital's shape, and its orientation. However, since some orbitals are described by equations in complex numbers, the shape sometimes depends on also. Together, the whole set of orbitals for a given and fill space as symmetrically as possible, though with increasingly complex sets of lobes and nodes.
The single s orbitals () are shaped like spheres. For it is roughly a solid ball (densest at center and fades outward exponentially), but for , each single s orbital is made of spherically symmetric surfaces which are nested shells (i.e., the "wave-structure" is radial, following a sinusoidal radial component as well). See illustration of a cross-section of these nested shells, at right. The s orbitals for all numbers are the only orbitals with an anti-node (a region of high wave function density) at the center of the nucleus. All other orbitals (p, d, f, etc.) have angular momentum, and thus avoid the nucleus (having a wave node at the nucleus). Recently, there has been an effort to experimentally image the 1s and 2p orbitals in a SrTiO3 crystal using scanning transmission electron microscopy with energy dispersive x-ray spectroscopy. Because the imaging was conducted using an electron beam, Coulombic beam-orbital interaction that is often termed as the impact parameter effect is included in the outcome (see the figure at right).
The shapes of p, d and f orbitals are described verbally here and shown graphically in the Orbitals table below. The three p orbitals for have the form of two ellipsoids with a point of tangency at the nucleus (the two-lobed shape is sometimes referred to as a "dumbbell"—there are two lobes pointing in opposite directions from each other). The three p orbitals in each shell are oriented at right angles to each other, as determined by their respective linear combination of values of . The overall result is a lobe pointing along each direction of the primary axes.
Four of the five d orbitals for look similar, each with four pear-shaped lobes, each lobe tangent at right angles to two others, and the centers of all four lying in one plane. Three of these planes are the xy-, xz-, and yz-planes—the lobes are between the pairs of primary axes—and the fourth has the center along the x and y axes themselves. The fifth and final d orbital consists of three regions of high probability density: a torus in between two pear-shaped regions placed symmetrically on its z axis. The overall total of 18 directional lobes point in every primary axis direction and between every pair.
There are seven f orbitals, each with shapes more complex than those of the d orbitals.
Additionally, as is the case with the s orbitals, individual p, d, f and g orbitals with values higher than the lowest possible value, exhibit an additional radial node structure which is reminiscent of harmonic waves of the same type, as compared with the lowest (or fundamental) mode of the wave. As with s orbitals, this phenomenon provides p, d, f, and g orbitals at the next higher possible value of (for example, 3p orbitals vs. the fundamental 2p), an additional node in each lobe. Still higher values of further increase the number of radial nodes, for each type of orbital.
The shapes of atomic orbitals in one-electron atom are related to 3-dimensional spherical harmonics. These shapes are not unique, and any linear combination is valid, like a transformation to cubic harmonics, in fact it is possible to generate sets where all the d's are the same shape, just like the and are the same shape.
thumb|The 1s, 2s, and 2p orbitals of a sodium atom
Although individual orbitals are most often shown independent of each other, the orbitals coexist around the nucleus at the same time. Also, in 1927, Albrecht Unsöld proved that if one sums the electron density of all orbitals of a particular azimuthal quantum number of the same shell (e.g., all three 2p orbitals, or all five 3d orbitals) where each orbital is occupied by an electron or each is occupied by an electron pair, then all angular dependence disappears; that is, the resulting total density of all the atomic orbitals in that subshell (those with the same ) is spherical. This is known as Unsöld's theorem. |
Atomic orbital | Orbitals table | Orbitals table
This table shows the real hydrogen-like wave functions for all atomic orbitals up to 7s, and therefore covers the occupied orbitals in the ground state of all elements in the periodic table up to radium and some beyond. "ψ" graphs are shown with − and + wave function phases shown in two different colors (arbitrarily red and blue). The orbital is the same as the orbital, but the and are formed by taking linear combinations of the and orbitals (which is why they are listed under the label). Also, the and are not the same shape as the , since they are pure spherical harmonics.
s ()p ()d ()f () s pz px py dz2 dxz dyz dxy dx2−y2 fz3 fxz2 fyz2 fxyz fz(x2−y2) fx(x2−3y2) fy(3x2−y2) 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px 50px . . . . . . . . . . . . . . . . . . . . . 50px 50px 50px 50px . . . ‡ . . . ‡ . . . ‡ . . . ‡ . . . ‡ . . . * . . . * . . . * . . . * . . . * . . . * . . . * 50px . . . † . . . † . . . † . . . * . . . * . . . * . . . * . . . * . . . * . . . * . . . * . . . * . . . * . . . * . . . *
* No elements with 6f, 7d or 7f electrons have been discovered yet.
† Elements with 7p electrons have been discovered, but their electronic configurations are only predicted – save the exceptional Lr, which fills 7p1 instead of 6d1.
‡ For the elements whose highest occupied orbital is a 6d orbital, only some electronic configurations have been confirmed. (Mt, Ds, Rg and Cn are still missing).
These are the real-valued orbitals commonly used in chemistry. Only the orbitals where are eigenstates of the orbital angular momentum operator, . The columns with are combinations of two eigenstates. See comparison in the following picture: thumb|Atomic orbitals spdf m-eigenstates and superpositions |
Atomic orbital | Qualitative understanding of shapes | Qualitative understanding of shapes
The shapes of atomic orbitals can be qualitatively understood by considering the analogous case of standing waves on a circular drum. To see the analogy, the mean vibrational displacement of each bit of drum membrane from the equilibrium point over many cycles (a measure of average drum membrane velocity and momentum at that point) must be considered relative to that point's distance from the center of the drum head. If this displacement is taken as being analogous to the probability of finding an electron at a given distance from the nucleus, then it will be seen that the many modes of the vibrating disk form patterns that trace the various shapes of atomic orbitals. The basic reason for this correspondence lies in the fact that the distribution of kinetic energy and momentum in a matter-wave is predictive of where the particle associated with the wave will be. That is, the probability of finding an electron at a given place is also a function of the electron's average momentum at that point, since high electron momentum at a given position tends to "localize" the electron in that position, via the properties of electron wave-packets (see the Heisenberg uncertainty principle for details of the mechanism).
This relationship means that certain key features can be observed in both drum membrane modes and atomic orbitals. For example, in all of the modes analogous to s orbitals (the top row in the animated illustration below), it can be seen that the very center of the drum membrane vibrates most strongly, corresponding to the antinode in all s orbitals in an atom. This antinode means the electron is most likely to be at the physical position of the nucleus (which it passes straight through without scattering or striking it), since it is moving (on average) most rapidly at that point, giving it maximal momentum.
A mental "planetary orbit" picture closest to the behavior of electrons in s orbitals, all of which have no angular momentum, might perhaps be that of a Keplerian orbit with the orbital eccentricity of 1 but a finite major axis, not physically possible (because particles were to collide), but can be imagined as a limit of orbits with equal major axes but increasing eccentricity.
Below, a number of drum membrane vibration modes and the respective wave functions of the hydrogen atom are shown. A correspondence can be considered where the wave functions of a vibrating drum head are for a two-coordinate system and the wave functions for a vibrating sphere are three-coordinate .
None of the other sets of modes in a drum membrane have a central antinode, and in all of them the center of the drum does not move. These correspond to a node at the nucleus for all non-s orbitals in an atom. These orbitals all have some angular momentum, and in the planetary model, they correspond to particles in orbit with eccentricity less than 1.0, so that they do not pass straight through the center of the primary body, but keep somewhat away from it.
In addition, the drum modes analogous to p and d modes in an atom show spatial irregularity along the different radial directions from the center of the drum, whereas all of the modes analogous to s modes are perfectly symmetrical in radial direction. The non-radial-symmetry properties of non-s orbitals are necessary to localize a particle with angular momentum and a wave nature in an orbital where it must tend to stay away from the central attraction force, since any particle localized at the point of central attraction could have no angular momentum. For these modes, waves in the drum head tend to avoid the central point. Such features again emphasize that the shapes of atomic orbitals are a direct consequence of the wave nature of electrons. |
Atomic orbital | Orbital energy | Orbital energy
In atoms with one electron (hydrogen-like atom), the energy of an orbital (and, consequently, any electron in the orbital) is determined mainly by . The orbital has the lowest possible energy in the atom. Each successively higher value of has a higher energy, but the difference decreases as increases. For high , the energy becomes so high that the electron can easily escape the atom. In single electron atoms, all levels with different within a given are degenerate in the Schrödinger approximation, and have the same energy. This approximation is broken slightly in the solution to the Dirac equation (where energy depends on and another quantum number ), and by the effect of the magnetic field of the nucleus and quantum electrodynamics effects. The latter induce tiny binding energy differences especially for s electrons that go nearer the nucleus, since these feel a very slightly different nuclear charge, even in one-electron atoms; see Lamb shift.
In atoms with multiple electrons, the energy of an electron depends not only on its orbital, but also on its interactions with other electrons. These interactions depend on the detail of its spatial probability distribution, and so the energy levels of orbitals depend not only on but also on . Higher values of are associated with higher values of energy; for instance, the 2p state is higher than the 2s state. When , the increase in energy of the orbital becomes so large as to push the energy of orbital above the energy of the s orbital in the next higher shell; when the energy is pushed into the shell two steps higher. The filling of the 3d orbitals does not occur until the 4s orbitals have been filled.
The increase in energy for subshells of increasing angular momentum in larger atoms is due to electron–electron interaction effects, and it is specifically related to the ability of low angular momentum electrons to penetrate more effectively toward the nucleus, where they are subject to less screening from the charge of intervening electrons. Thus, in atoms with higher atomic number, the of electrons becomes more and more of a determining factor in their energy, and the principal quantum numbers of electrons becomes less and less important in their energy placement.
The energy sequence of the first 35 subshells (e.g., 1s, 2p, 3d, etc.) is given in the following table. Each cell represents a subshell with and given by its row and column indices, respectively. The number in the cell is the subshell's position in the sequence. For a linear listing of the subshells in terms of increasing energies in multielectron atoms, see the section below.
s p d f g h 1 1 2 2 3 3 4 5 7 4 6 8 10 13 5 9 11 14 17 21 612 15 18 22 26 31 716 19 23 27 32 37 820 24 28 33 38 44 925 29 34 39 45 51 1030 35 40 46 52 59
Note: empty cells indicate non-existent sublevels, while numbers in italics indicate sublevels that could (potentially) exist, but which do not hold electrons in any element currently known. |
Atomic orbital | Electron placement and the periodic table | Electron placement and the periodic table
right|thumb|upright=1.6|Electron atomic and molecular orbitals. The chart of orbitals (left) is arranged by increasing energy (see Madelung rule). Atomic orbits are functions of three variables (two angles, and the distance from the nucleus). These images are faithful to the angular component of the orbital, but not entirely representative of the orbital as a whole.
thumb|Atomic orbitals and periodic table construction
Several rules govern the placement of electrons in orbitals (electron configuration). The first dictates that no two electrons in an atom may have the same set of values of quantum numbers (this is the Pauli exclusion principle). These quantum numbers include the three that define orbitals, as well as the spin magnetic quantum number . Thus, two electrons may occupy a single orbital, so long as they have different values of . Because takes one of only two values ( or −), at most two electrons can occupy each orbital.
Additionally, an electron always tends to fall to the lowest possible energy state. It is possible for it to occupy any orbital so long as it does not violate the Pauli exclusion principle, but if lower-energy orbitals are available, this condition is unstable. The electron will eventually lose energy (by releasing a photon) and drop into the lower orbital. Thus, electrons fill orbitals in the order specified by the energy sequence given above.
This behavior is responsible for the structure of the periodic table. The table may be divided into several rows (called 'periods'), numbered starting with 1 at the top. The presently known elements occupy seven periods. If a certain period has number i, it consists of elements whose outermost electrons fall in the ith shell. Niels Bohr was the first to propose (1923) that the periodicity in the properties of the elements might be explained by the periodic filling of the electron energy levels, resulting in the electronic structure of the atom.
The periodic table may also be divided into several numbered rectangular 'blocks'. The elements belonging to a given block have this common feature: their highest-energy electrons all belong to the same -state (but the associated with that -state depends upon the period). For instance, the leftmost two columns constitute the 's-block'. The outermost electrons of Li and Be respectively belong to the 2s subshell, and those of Na and Mg to the 3s subshell.
The following is the order for filling the "subshell" orbitals, which also gives the order of the "blocks" in the periodic table:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p
The "periodic" nature of the filling of orbitals, as well as emergence of the s, p, d, and f "blocks", is more obvious if this order of filling is given in matrix form, with increasing principal quantum numbers starting the new rows ("periods") in the matrix. Then, each subshell (composed of the first two quantum numbers) is repeated as many times as required for each pair of electrons it may contain. The result is a compressed periodic table, with each entry representing two successive elements:
1s 2s 2p2p2p3s 3p3p3p4s 3d3d3d3d3d4p4p4p5s 4d4d4d4d4d5p5p5p6s4f4f4f4f4f4f4f5d5d5d5d5d6p6p6p7s5f5f5f5f5f5f5f6d6d6d6d6d7p7p7p
Although this is the general order of orbital filling according to the Madelung rule, there are exceptions, and the actual electronic energies of each element are also dependent upon additional details of the atoms (see ).
The number of electrons in an electrically neutral atom increases with the atomic number. The electrons in the outermost shell, or valence electrons, tend to be responsible for an element's chemical behavior. Elements that contain the same number of valence electrons can be grouped together and display similar chemical properties. |
Atomic orbital | Relativistic effects | Relativistic effects
For elements with high atomic number , the effects of relativity become more pronounced, and especially so for s electrons, which move at relativistic velocities as they penetrate the screening electrons near the core of high- atoms. This relativistic increase in momentum for high speed electrons causes a corresponding decrease in wavelength and contraction of 6s orbitals relative to 5d orbitals (by comparison to corresponding s and d electrons in lighter elements in the same column of the periodic table); this results in 6s valence electrons becoming lowered in energy.
Examples of significant physical outcomes of this effect include the lowered melting temperature of mercury (which results from 6s electrons not being available for metal bonding) and the golden color of gold and caesium.
In the Bohr model, an electron has a velocity given by , where is the atomic number, is the fine-structure constant, and is the speed of light. In non-relativistic quantum mechanics, therefore, any atom with an atomic number greater than 137 would require its 1s electrons to be traveling faster than the speed of light. Even in the Dirac equation, which accounts for relativistic effects, the wave function of the electron for atoms with is oscillatory and unbounded. The significance of element 137, also known as untriseptium, was first pointed out by the physicist Richard Feynman. Element 137 is sometimes informally called feynmanium (symbol Fy). However, Feynman's approximation fails to predict the exact critical value of due to the non-point-charge nature of the nucleus and very small orbital radius of inner electrons, resulting in a potential seen by inner electrons which is effectively less than . The critical value, which makes the atom unstable with regard to high-field breakdown of the vacuum and production of electron-positron pairs, does not occur until is about 173. These conditions are not seen except transiently in collisions of very heavy nuclei such as lead or uranium in accelerators, where such electron-positron production from these effects has been claimed to be observed.
There are no nodes in relativistic orbital densities, although individual components of the wave function will have nodes. |
Atomic orbital | pp hybridization (conjectured) | pp hybridization (conjectured)
In late period 8 elements, a hybrid of 8p3/2 and 9p1/2 is expected to exist, where "3/2" and "1/2" refer to the total angular momentum quantum number. This "pp" hybrid may be responsible for the p-block of the period due to properties similar to p subshells in ordinary valence shells. Energy levels of 8p3/2 and 9p1/2 come close due to relativistic spin–orbit effects; the 9s subshell should also participate, as these elements are expected to be analogous to the respective 5p elements indium through xenon. |
Atomic orbital | Transitions between orbitals | Transitions between orbitals
Bound quantum states have discrete energy levels. When applied to atomic orbitals, this means that the energy differences between states are also discrete. A transition between these states (i.e., an electron absorbing or emitting a photon) can thus happen only if the photon has an energy corresponding with the exact energy difference between said states.
Consider two states of the hydrogen atom:
State , , and
State , , and
By quantum theory, state 1 has a fixed energy of , and state 2 has a fixed energy of . Now, what would happen if an electron in state 1 were to move to state 2? For this to happen, the electron would need to gain an energy of exactly . If the electron receives energy that is less than or greater than this value, it cannot jump from state 1 to state 2. Now, suppose we irradiate the atom with a broad-spectrum of light. Photons that reach the atom that have an energy of exactly will be absorbed by the electron in state 1, and that electron will jump to state 2. However, photons that are greater or lower in energy cannot be absorbed by the electron, because the electron can jump only to one of the orbitals, it cannot jump to a state between orbitals. The result is that only photons of a specific frequency will be absorbed by the atom. This creates a line in the spectrum, known as an absorption line, which corresponds to the energy difference between states 1 and 2.
The atomic orbital model thus predicts line spectra, which are observed experimentally. This is one of the main validations of the atomic orbital model.
The atomic orbital model is nevertheless an approximation to the full quantum theory, which only recognizes many electron states. The predictions of line spectra are qualitatively useful but are not quantitatively accurate for atoms and ions other than those containing only one electron. |
Atomic orbital | See also | See also
Atomic electron configuration table
Condensed matter physics
Electron configuration
Energy level
Hund's rules
Molecular orbital
Orbital overlap
Quantum chemistry
Quantum chemistry computer programs
Solid-state physics
Wave function collapse
Wiswesser's rule |
Atomic orbital | References | References
|
Atomic orbital | External links | External links
3D representation of hydrogenic orbitals
The Orbitron, a visualization of all common and uncommon atomic orbitals, from 1s to 7g
Grand table Still images of many orbitals
Category:Atomic physics
Category:Chemical bonding
Category:Electron states
Category:Quantum chemistry
Category:Articles containing video clips |
Atomic orbital | Table of Content | Short description, Electron properties, Formal quantum mechanical definition, Types of orbital, History, Early models, Bohr atom, Modern conceptions and connections to the Heisenberg uncertainty principle, Orbital names, Orbital notation and subshells, X-ray notation, Hydrogen-like orbitals, Quantum numbers, Complex orbitals, Real orbitals, Shapes of orbitals, Orbitals table, Qualitative understanding of shapes, Orbital energy, Electron placement and the periodic table, Relativistic effects, pp hybridization (conjectured), Transitions between orbitals, See also, References, External links |
Amino acid | short description |
class=skin-invert-image|thumb|upright=1.15|Structure of a typical L-alpha-amino acid in the "neutral" form
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 appear in the genetic code of life.
Amino acids can be classified according to the locations of the core structural functional groups (alpha- (α-), beta- (β-), gamma- (γ-) amino acids, etc.); other categories relate to polarity, ionization, and side-chain group type (aliphatic, acyclic, aromatic, polar, etc.). In the form of proteins, amino-acid residues form the second-largest component (water being the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. It is thought that they played a key role in enabling life on Earth and its emergence.
Amino acids are formally named by the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature in terms of the fictitious "neutral" structure shown in the illustration. For example, the systematic name of alanine is 2-aminopropanoic acid, based on the formula . The Commission justified this approach as follows:
The systematic names and formulas given refer to hypothetical forms in which amino groups are unprotonated and carboxyl groups are undissociated. This convention is useful to avoid various nomenclatural problems but should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules. |
Amino acid | History | History
The first few amino acids were discovered in the early 1800s. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound from asparagus that was subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820. The last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who also determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth.
The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it.Menten, P. Dictionnaire de chimie: Une approche étymologique et historique. De Boeck, Bruxelles. link . The first use of the term "amino acid" in the English language dates from 1898, while the German term, , was used earlier. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide". |
Amino acid | General structure | General structure
thumb|upright=2.75|The 21 proteinogenic α-amino acids found in eukaryotes, grouped according to their side chains' pKa values and charges carried at physiological pH (7.4)
2-, alpha-, or α-amino acids. have the generic formula in most cases, where R is an organic substituent known as a "side chain".
Of the many hundreds of described amino acids, 22 are proteinogenic ("protein-building"). It is these 22 compounds that combine to give a vast array of peptides and proteins assembled by ribosomes. Non-proteinogenic or modified amino acids may arise from post-translational modification or during nonribosomal peptide synthesis. |
Amino acid | Chirality | Chirality
The carbon atom next to the carboxyl group is called the α–carbon. In proteinogenic amino acids, it bears the amine and the R group or side chain specific to each amino acid, as well as a hydrogen atom. With the exception of glycine, for which the side chain is also a hydrogen atom, the α–carbon is stereogenic. All chiral proteogenic amino acids have the L configuration. They are "left-handed" enantiomers, which refers to the stereoisomers of the alpha carbon.
A few D-amino acids ("right-handed") have been found in nature, e.g., in bacterial envelopes, as a neuromodulator (D-serine), and in some antibiotics. Rarely, D-amino acid residues are found in proteins, and are converted from the L-amino acid as a post-translational modification. |
Amino acid | Side chains | Side chains |
Amino acid | Polar charged side chains | Polar charged side chains
Five amino acids possess a charge at neutral pH. Often these side chains appear at the surfaces on proteins to enable their solubility in water, and side chains with opposite charges form important electrostatic contacts called salt bridges that maintain structures within a single protein or between interfacing proteins. Many proteins bind metal into their structures specifically, and these interactions are commonly mediated by charged side chains such as aspartate, glutamate and histidine. Under certain conditions, each ion-forming group can be charged, forming double salts.
The two negatively charged amino acids at neutral pH are aspartate (Asp, D) and glutamate (Glu, E). The anionic carboxylate groups behave as Brønsted bases in most circumstances. Enzymes in very low pH environments, like the aspartic protease pepsin in mammalian stomachs, may have catalytic aspartate or glutamate residues that act as Brønsted acids.
class=skin-invert-image|thumb|upright=2.05 |Functional groups found in histidine (left), lysine (middle) and arginine (right)
There are three amino acids with side chains that are cations at neutral pH: arginine (Arg, R), lysine (Lys, K) and histidine (His, H). Arginine has a charged guanidino group and lysine a charged alkyl amino group, and are fully protonated at pH 7. Histidine's imidazole group has a pKa of 6.0, and is only around 10% protonated at neutral pH. Because histidine is easily found in its basic and conjugate acid forms it often participates in catalytic proton transfers in enzyme reactions. |
Amino acid | Polar uncharged side chains | Polar uncharged side chains
The polar, uncharged amino acids serine (Ser, S), threonine (Thr, T), asparagine (Asn, N) and glutamine (Gln, Q) readily form hydrogen bonds with water and other amino acids. They do not ionize in normal conditions, a prominent exception being the catalytic serine in serine proteases. This is an example of severe perturbation, and is not characteristic of serine residues in general. Threonine has two chiral centers, not only the L (2S) chiral center at the α-carbon shared by all amino acids apart from achiral glycine, but also (3R) at the β-carbon. The full stereochemical specification is (2S,3R)-L-threonine. |
Amino acid | Hydrophobic side chains | Hydrophobic side chains
Nonpolar amino acid interactions are the primary driving force behind the processes that fold proteins into their functional three dimensional structures. None of these amino acids' side chains ionize easily, and therefore do not have pKas, with the exception of tyrosine (Tyr, Y). The hydroxyl of tyrosine can deprotonate at high pH forming the negatively charged phenolate. Because of this one could place tyrosine into the polar, uncharged amino acid category, but its very low solubility in water matches the characteristics of hydrophobic amino acids well. |
Amino acid | Special case side chains | Special case side chains
Several side chains are not described well by the charged, polar and hydrophobic categories. Glycine (Gly, G) could be considered a polar amino acid since its small size means that its solubility is largely determined by the amino and carboxylate groups. However, the lack of any side chain provides glycine with a unique flexibility among amino acids with large ramifications to protein folding. Cysteine (Cys, C) can also form hydrogen bonds readily, which would place it in the polar amino acid category, though it can often be found in protein structures forming covalent bonds, called disulphide bonds, with other cysteines. These bonds influence the folding and stability of proteins, and are essential in the formation of antibodies. Proline (Pro, P) has an alkyl side chain and could be considered hydrophobic, but because the side chain joins back onto the alpha amino group it becomes particularly inflexible when incorporated into proteins. Similar to glycine this influences protein structure in a way unique among amino acids. Selenocysteine (Sec, U) is a rare amino acid not directly encoded by DNA, but is incorporated into proteins via the ribosome. Selenocysteine has a lower redox potential compared to the similar cysteine, and participates in several unique enzymatic reactions. Pyrrolysine (Pyl, O) is another amino acid not encoded in DNA, but synthesized into protein by ribosomes. It is found in archaeal species where it participates in the catalytic activity of several methyltransferases. |
Amino acid | β- and γ-amino acids | β- and γ-amino acids
Amino acids with the structure , such as β-alanine, a component of carnosine and a few other peptides, are β-amino acids. Ones with the structure are γ-amino acids, and so on, where X and Y are two substituents (one of which is normally H). |
Amino acid | Zwitterions | Zwitterions
class=skin-invert-image|thumb|upright=1.5|Ionization and Brønsted character of N-terminal amino, C-terminal carboxylate, and side chains of amino acid residues
The common natural forms of amino acids have a zwitterionic structure, with ( in the case of proline) and functional groups attached to the same C atom, and are thus α-amino acids, and are the only ones found in proteins during translation in the ribosome.
In aqueous solution at pH close to neutrality, amino acids exist as zwitterions, i.e. as dipolar ions with both and in charged states, so the overall structure is . At physiological pH the so-called "neutral forms" are not present to any measurable degree. Although the two charges in the zwitterion structure add up to zero it is misleading to call a species with a net charge of zero "uncharged".
In strongly acidic conditions (pH below 3), the carboxylate group becomes protonated and the structure becomes an ammonio carboxylic acid, . This is relevant for enzymes like pepsin that are active in acidic environments such as the mammalian stomach and lysosomes, but does not significantly apply to intracellular enzymes. In highly basic conditions (pH greater than 10, not normally seen in physiological conditions), the ammonio group is deprotonated to give .
Although various definitions of acids and bases are used in chemistry, the only one that is useful for chemistry in aqueous solution is that of Brønsted: an acid is a species that can donate a proton to another species, and a base is one that can accept a proton. This criterion is used to label the groups in the above illustration. The carboxylate side chains of aspartate and glutamate residues are the principal Brønsted bases in proteins. Likewise, lysine, tyrosine and cysteine will typically act as a Brønsted acid. Histidine under these conditions can act both as a Brønsted acid and a base. |
Amino acid | Isoelectric point | Isoelectric point
class=skin-invert-image|thumb|right|upright=1.5|Composite of titration curves of twenty proteinogenic amino acids grouped by side chain category
For amino acids with uncharged side-chains the zwitterion predominates at pH values between the two pKa values, but coexists in equilibrium with small amounts of net negative and net positive ions. At the midpoint between the two pKa values, the trace amount of net negative and trace of net positive ions balance, so that average net charge of all forms present is zero. This pH is known as the isoelectric point pI, so pI = (pKa1 + pKa2).
For amino acids with charged side chains, the pKa of the side chain is involved. Thus for aspartate or glutamate with negative side chains, the terminal amino group is essentially entirely in the charged form , but this positive charge needs to be balanced by the state with just one C-terminal carboxylate group is negatively charged. This occurs halfway between the two carboxylate pKa values: pI = (pKa1 + pKa(R)), where pKa(R) is the side chain pKa.
Similar considerations apply to other amino acids with ionizable side-chains, including not only glutamate (similar to aspartate), but also cysteine, histidine, lysine, tyrosine and arginine with positive side chains.
Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point. |
Amino acid | Physicochemical properties | Physicochemical properties
The 20 canonical amino acids can be classified according to their properties. Important factors are charge, hydrophilicity or hydrophobicity, size, and functional groups. These properties influence protein structure and protein–protein interactions. The water-soluble proteins tend to have their hydrophobic residues (Leu, Ile, Val, Phe, and Trp) buried in the middle of the protein, whereas hydrophilic side chains are exposed to the aqueous solvent. (In biochemistry, a residue refers to a specific monomer within the polymeric chain of a polysaccharide, protein or nucleic acid.) The integral membrane proteins tend to have outer rings of exposed hydrophobic amino acids that anchor them in the lipid bilayer. Some peripheral membrane proteins have a patch of hydrophobic amino acids on their surface that sticks to the membrane. In a similar fashion, proteins that have to bind to positively charged molecules have surfaces rich in negatively charged amino acids such as glutamate and aspartate, while proteins binding to negatively charged molecules have surfaces rich in positively charged amino acids like lysine and arginine. For example, lysine and arginine are present in large amounts in the low-complexity regions of nucleic-acid binding proteins. There are various hydrophobicity scales of amino acid residues.
Some amino acids have special properties. Cysteine can form covalent disulfide bonds to other cysteine residues. Proline forms a cycle to the polypeptide backbone, and glycine is more flexible than other amino acids.
Glycine and proline are strongly present within low complexity regions of both eukaryotic and prokaryotic proteins, whereas the opposite is the case with cysteine, phenylalanine, tryptophan, methionine, valine, leucine, isoleucine, which are highly reactive, or complex, or hydrophobic.
Many proteins undergo a range of posttranslational modifications, whereby additional chemical groups are attached to the amino acid residue side chains sometimes producing lipoproteins (that are hydrophobic), or glycoproteins (that are hydrophilic) allowing the protein to attach temporarily to a membrane. For example, a signaling protein can attach and then detach from a cell membrane, because it contains cysteine residues that can have the fatty acid palmitic acid added to them and subsequently removed. |
Amino acid | Table of standard amino acid abbreviations and properties | Table of standard amino acid abbreviations and properties
Although one-letter symbols are included in the table, IUPAC–IUBMB recommend that "Use of the one-letter symbols should be restricted to the comparison of long sequences".
The one-letter notation was chosen by IUPAC-IUB based on the following rules:
Initial letters are used where there is no ambiguity: C cysteine, H histidine, I isoleucine, M methionine, S serine, V valine,
Where arbitrary assignment is needed, the structurally simpler amino acids are given precedence: A Alanine, G glycine, L leucine, P proline, T threonine,
F PHenylalanine and R aRginine are assigned by being phonetically suggestive,
W tryptophan is assigned based on the double ring being visually suggestive to the bulky letter W,
K lysine and Y tyrosine are assigned as alphabetically nearest to their initials L and T (note that U was avoided for its similarity with V, while X was reserved for undetermined or atypical amino acids); for tyrosine the mnemonic tYrosine was also proposed,
D aspartate was assigned arbitrarily, with the proposed mnemonic asparDic acid; E glutamate was assigned in alphabetical sequence being larger by merely one methylene –CH2– group,
N asparagine was assigned arbitrarily, with the proposed mnemonic asparagiNe; Q glutamine was assigned in alphabetical sequence of those still available (note again that O was avoided due to similarity with D), with the proposed mnemonic Qlutamine.
Amino acid 3- and 1-letter symbols Side chain Hydropathy index Molar absorptivity Molecular mass Abundance in proteins (%) Standard genetic coding,IUPAC notation 3 1 Class Chemical polarity Net chargeat pH 7.4 Wavelength,λmax (nm) Coefficient ε(mM−1·cm−1) Alanine Ala A Aliphatic Nonpolar Neutral 1.8 89.094 8.76 GCN Arginine Arg R Fixed cation Basic polar Positive −4.5 174.203 5.78 MGR, CGY Asparagine Asn N Amide Polar Neutral −3.5 132.119 3.93 AAY Aspartate Asp D Anion Brønsted base Negative −3.5 133.104 5.49 GAY Cysteine Cys C Thiol Brønsted acid Neutral 2.5 250 0.3 121.154 1.38 UGY Glutamine Gln Q Amide Polar Neutral −3.5 146.146 3.9 CAR Glutamate Glu E Anion Brønsted base Negative −3.5 147.131 6.32 GAR Glycine Gly G Aliphatic Nonpolar Neutral −0.4 75.067 7.03 GGN Histidine His H Cationic Brønsted acid and base Positive, 10%Neutral, 90% −3.2 211 5.9 155.156 2.26 CAY Isoleucine Ile I Aliphatic Nonpolar Neutral 4.5 131.175 5.49 AUH Leucine Leu L Aliphatic Nonpolar Neutral 3.8 131.175 9.68 YUR, CUY Lysine Lys K Cation Brønsted acid Positive −3.9 146.189 5.19 AAR Methionine Met M Thioether Nonpolar Neutral 1.9 149.208 2.32 AUG Phenylalanine Phe F Aromatic Nonpolar Neutral 2.8 257, 206, 188 0.2, 9.3, 60.0 165.192 3.87 UUY Proline Pro P Cyclic Nonpolar Neutral −1.6 115.132 5.02 CCN Serine Ser S Hydroxylic Polar Neutral −0.8 105.093 7.14 UCN, AGY Threonine Thr T Hydroxylic Polar Neutral −0.7 119.119 5.53 ACN Tryptophan Trp W Aromatic Nonpolar Neutral −0.9 280, 219 5.6, 47.0 204.228 1.25 UGG Tyrosine Tyr Y Aromatic Brønsted acid Neutral −1.3 274, 222, 193 1.4, 8.0, 48.0 181.191 2.91 UAY Valine Val V Aliphatic Nonpolar Neutral 4.2 117.148 6.73 GUN
Two additional amino acids are in some species coded for by codons that are usually interpreted as stop codons:
21st and 22nd amino acids 3-letter 1-letter Molecular mass Selenocysteine Sec U 168.064 Pyrrolysine Pyl O 255.313
In addition to the specific amino acid codes, placeholders are used in cases where chemical or crystallographic analysis of a peptide or protein cannot conclusively determine the identity of a residue. They are also used to summarize conserved protein sequence motifs. The use of single letters to indicate sets of similar residues is similar to the use of abbreviation codes for degenerate bases.
Ambiguous amino acids 3-letter 1-letter Amino acids included Codons included Any / unknown Xaa X All NNN Asparagine or aspartate Asx B D, N RAY Glutamine or glutamate Glx Z E, Q SAR Leucine or isoleucine Xle J I, L YTR, ATH, CTY Hydrophobic Φ V, I, L, F, W, Y, M NTN, TAY, TGG Aromatic Ω F, W, Y, H YWY, TTY, TGG Aliphatic (non-aromatic) Ψ V, I, L, M VTN, TTR Small π P, G, A, S BCN, RGY, GGR Hydrophilic ζ S, T, H, N, Q, E, D, K, R VAN, WCN, CGN, AGY Positively-charged + K, R, H ARR, CRY, CGR Negatively-charged − D, E GAN
Unk is sometimes used instead of Xaa, but is less standard.
Ter or * (from termination) is used in notation for mutations in proteins when a stop codon occurs. It corresponds to no amino acid at all.
In addition, many nonstandard amino acids have a specific code. For example, several peptide drugs, such as Bortezomib and MG132, are artificially synthesized and retain their protecting groups, which have specific codes. Bortezomib is Pyz–Phe–boroLeu, and MG132 is Z–Leu–Leu–Leu–al. To aid in the analysis of protein structure, photo-reactive amino acid analogs are available. These include photoleucine (pLeu) and photomethionine (pMet). |
Amino acid | Occurrence and functions in biochemistry | Occurrence and functions in biochemistry |
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