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Sunday, July 10, 2022

Language attrition

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Language_attrition

Language attrition is the process of losing a native or first language. This process is generally caused by both isolation from speakers of the first language ("L1") and the acquisition and use of a second language ("L2"), which interferes with the correct production and comprehension of the first. Such interference from a second language is probably experienced to some extent by all bilinguals, but is most evident among speakers for whom a language other than their first has started to play an important, if not dominant, role in everyday life; these speakers are more likely to experience language attrition. It is common among immigrants that travel to countries where languages foreign to them are used.

There are several factors which affect the process. Frequent exposure and use of a particular language is often assumed adequate to maintain the native language system intact. However, research has often failed to confirm this prediction. A positive attitude towards the potentially attriting language or its speech community and motivation to retain the language are other factors which may reduce attrition. These factors are too difficult to confirm by research. However, a person's age can well predict the likelihood of attrition; children are demonstrably more likely to lose their first language than adults.

These factors are similar to those that affect second-language acquisition and the two processes are sometimes compared. However, the overall impact of these factors is far less than that for second language acquisition.

Language attrition results in a decrease of language proficiency. The current consensus is that it manifests itself first and most noticeably in speakers' vocabulary (in their lexical access and their mental lexicon), while grammatical and especially phonological representations appear more stable among speakers who emigrated after puberty.

Study

The term first language attrition (FLA) refers to the gradual decline in native language proficiency. As speakers use their L2 frequently and become proficient (or even dominant) in it, some aspects of the L1 can deteriorate or become subject to L2 influence.

The study of language attrition became a subfield of linguistics with a 1980 conference at the University of Pennsylvania called "Loss of Language Skills". The aim of the conference was to discuss areas of second language attrition and to discuss ideas for possible future research. The conference revealed that attrition is a wide topic, with numerous factors and taking many forms. Decades later, the field of first language attrition gained new momentum with two conferences held in Amsterdam in 2002 and 2005, as well as a series of graduate workshops and panels at international conferences, such as the International Symposium on Bilingualism (2007, 2009), the annual conferences of the European Second Language Association, and the AILA World Congress (2008). The outcomes of some of these meetings were later published in edited volumes.

To study the process of language attrition, researchers initially looked at neighboring areas of linguistics to identify which parts of the L1 system attrite first; lacking years of direct experimental data, linguists studied language contact, creolization, L2 acquisition, and aphasia, and applied their findings to language acquisition.

One issue that is faced when researching attrition is distinguishing between normal L2 influence on the L1 and actual attrition of the L1. Since all bilinguals experience some degree of cross linguistic influence, where the L2 interferes with the retrieval of the speaker's L1, it is difficult to determine if delays and/or mistakes in the L1 are due to attrition or caused by CLI. Also, simultaneous bilinguals may not have a language that is indistinguishable from that of a native speaker or a language where their knowledge of it is less extensive than a native speaker's; therefore testing for attrition is difficult.

Manifestations

Lexical attrition

The first linguistic system to be affected by first language attrition is the lexicon. The lexical-semantic relationship usually starts to deteriorate first and most quickly, driven by Cross Linguistic Interference (CLI) from the speaker's L2, and it is believed to be exacerbated by continued exposure to, and frequent use of, the L2. Evidence for such interlanguage effects can be seen in a study by Pavlenko (2003, 2004) which shows that there was some semantic extension from the L2, which was English, into the L1 Russian speakers' lexicons. In order to test for lexical attrition, researchers used tests such as picture naming tasks, where they place a picture of an item in front of the participant and ask them to name it, or by measuring lexical diversity in the speaker's spontaneous speech (speech that is unprompted and improvised). In both cases, attriters performed worse than non-attriters. One hypothesis suggests that when a speaker tries to access a lexical item from their L1 they are also competing with the translation equivalents of their L2 and that there is either a problem with activating the L1 due to infrequent use or with the inhibition of the competing L2.

Grammatical attrition

Grammatical attrition can be defined as "the disintegration of the structure of a first language (L1) in contact situations with a second language (L2)". In a study of bilingual Swedes raised outside of Sweden, who in their late twenties returned to their home country for schooling, there was noted to be attrition of their L1. The participants demonstrated a complete retention of the underlying syntactic structure of their L1. Notably, they exhibited the V2, verb second, word order present in most Germanic languages, except English. This rule requires the tense-marked verb of a main clause to occur in the second position of the sentence, even if that means it comes before the subject (e.g. there is an adverb at the beginning of the sentence). These speakers' ability to form sentences with V2 word order was compared against L2 learners who often overproduce the rigid SVO word order rather than applying the V2 rule. Although the study did not show evidence for attrition of syntax of the person's L1, there was evidence for attrition in the expatriates' morphology, especially in terms of agreement. They found that the bilinguals would choose to use the unmarked morphemes in place of the marked one when having to differentiate between gender and plurality; also they tend to overgeneralize where certain morphemes can be used. For example, they may use the suffix /-a/, which is used to express an indefinite plural, and overextend this morpheme to also represent the indefinite singular. There is little evidence to support the view that there is a complete restructuring of the language systems. That is, even under language attrition the syntax is largely unaffected and any variability observed is thought to be due to interference from another language, rather than attrition.

L1 attriters, like L2 learners, may use language differently from native speakers. In particular, they can have variability on certain rules which native speakers apply deterministically. In the context of attrition, however, there is strong evidence that this optionality is not indicative of any underlying representational deficits: the same individuals do not appear to encounter recurring problems with the same kinds of grammatical phenomena in different speech situations or on different tasks. This suggests that problems of L1 attriters are due to momentary conflicts between the two linguistic systems and not indicative of a structural change to underlying linguistic knowledge (that is, to an emerging representational deficit of any kind). This assumption is in line with a range of investigations of L1 attrition which argue that this process may affect interface phenomena (e.g. the distribution of overt and null subjects in pro-drop languages) but will not touch the narrow syntax.

Phonological attrition

Phonological attrition is a form of language loss that affects the speaker's ability to produce their native language with their native accent. A study of five native speakers of American English who moved to Brazil and learned Portuguese as their L2 demonstrates the possibility that one could lose one's L1 accent in place of an accent that is directly influenced by the L2. It is thought that phonological loss can occur to those who are closer to native-like fluency in the L2, especially in terms of phonological production, and for those who have immersed themselves and built a connection to the culture of the country for the L2. A sociolinguistic approach to this phenomenon is that the acquisition of a native-like L2 accent and the subsequent loss of one's native accent is influenced by the societal norms of the country and the speakers' attempt to adapt in order to feel a part of the culture they are trying to assimilate into. This type of attrition is not to be confused with contact-induced change since that would mean speech production changes due to an increased use of another language and not due to the less frequent use of the L1.

Studies and hypotheses

Lambert and Moore attempted to define numerous hypotheses regarding the nature of language loss, crossed with various aspects of language. They envisioned a test to be given to American State Department employees that would include four linguistic categories (syntax, morphology, lexicon, and phonology) and three skill areas (reading, listening, and speaking). A translation component would feature on a sub-section of each skill area tested. The test was to include linguistic features that are the most difficult, according to teachers, for students to master. Such a test may confound testing what was not acquired with what was lost. Lambert, in personal communication with Köpke and Schmid, described the results as 'not substantial enough to help much in the development of the new field of language skill attrition'.

The use of translation tests to study language loss is inappropriate for a number of reasons: it is questionable what such tests measure; too much variation; the difference between attriters and bilinguals is complex; activating two languages at once may cause interference. Yoshitomi attempted to define a model of language attrition that was related to neurological and psychological aspects of language learning and unlearning. She discussed four possible hypotheses and five key aspects related to acquisition and attrition. The hypotheses are:

  • 1. Reverse order: last learned, first forgotten. Studies by Russell and Hayashi both looked at the Japanese negation system and both found that attrition was the reverse order of acquisition. Yoshitomi and others, including Yukawa, argue that attrition can occur so rapidly, it is impossible to determine the order of loss.
  • 2. Inverse relation: better learned, better retained. Language items that are acquired first also happen to be those that are most reinforced. As a result, hypotheses 1 and 2 capture the main linguistic characteristics of language attrition
  • 3. Critical period: at or around age 9. As a child grows, he becomes less able to master native-like abilities. Furthermore, various linguistic features (for example phonology or syntax) may have different stages or age limits for mastering. Hyltenstam & Abrahamsson argue that after childhood, in general, it becomes more and more difficult to acquire "native-like-ness", but that there is no cut-off point in particular. Furthermore, they discuss a number of cases where a native-like L2 was acquired during adulthood.
  • 4. Affect: motivation and attitude.

According to Yoshitomi, the five key aspects related to attrition are neuroplasticity, consolidation, permastore/savings, decreased accessibility, and receptive versus productive abilities.

The regression hypothesis

The regression hypothesis, first formulated by Roman Jakobson in 1941 and originally formulated on the phonology of only Slavic languages, goes back to the beginnings of psychology and psychoanalysis. It states that which was learned first will be retained last, both in 'normal' processes of forgetting and in pathological conditions such as aphasia or dementia. As a template for language attrition, the regression hypothesis has long seemed an attractive paradigm. However, regression is not in itself a theoretical or explanatory framework. Both order of acquisition and order of attrition need to be put into the larger context of linguistic theory in order to gain explanatory adequacy.

Keijzer (2007) conducted a study on the attrition of Dutch in Anglophone Canada. She finds some evidence that later-learned rules, such as diminutive and plural formation, indeed erode before earlier learned grammatical rules. However, there is also considerable interaction between the first and second language and so a straightforward 'regression pattern' cannot be observed. Also, parallels in noun and verb phrase morphology could be present because of the nature of the tests or because of avoidance by the participants. In a follow up 2010 article, Keijzer suggests that the regression hypothesis may be more applicable to morphology than to syntax.

Citing the studies on the regression hypothesis that have been done, Yukawa says that the results have been contradictory. It is possible that attrition is a case-by-case situation depending on a number of variables (age, proficiency,& literacy, the similarities between the L1 and L2, and whether the L1 or the L2 is attriting). The threshold hypothesis states that there may be a level of proficiency that once attained, enables the attriting language to remain stable.

Factors

Age effect

Children are more susceptible to (first) language attrition than adults. Research shows an age effect around the ages of 8 through 13. Before this time period, a first language can attrite under certain circumstances, the most prominent being a sudden decline in exposure to the first language. Various case studies show that children who emigrate before puberty and have little to no exposure to their first language end up losing the first language. In 2009, a study compared two groups of Swedish-speaking groups: native Swedish speakers and Korean international adoptees who were at risk of losing their Korean. Of the Korean adoptees, those who were adopted the earliest essentially lost their Korean and those adopted later still retained some of it, although it was primarily their comprehension of Korean that was spared. A 2007 study looked at Korean adoptees in France and found that they performed on par with native French speakers in French proficiency and Korean.

Attrition of a first language does not guarantee an advantage in learning a second language. Attriters are outperformed by native speakers of the second language in proficiency. A 2009 study tested the Swedish proficiency of Swedish speakers who had attrited knowledge of Spanish. These participants did show almost but not quite native-like proficiency when compared to native Swedish speakers, and they did not show an advantage when compared with bilingual Swedish-Spanish speakers.

On the other hand, L1 attrition may also occur if the overall effort to maintain the first language is insufficient while exposed to a dominant L2 environment. Another recent investigation, focusing on the development of language in late bilinguals (i.e. adults past puberty), has claimed that maintenance of the mother tongue in an L1 environment requires little to no maintenance for individuals, whereas those in the L2 environment have an additive requirement for the maintenance of the L1 and the development of the L2 (Opitz, 2013).

There have been cases in which adults have undergone first language attrition. A 2011 study tested adult monolingual English speakers, adult monolingual Russian speakers and adult bilingual English-Russian speakers on naming various liquid containers (cup, glass, mug, etc.) in both English and Russian. The results showed that the bilinguals had attrited Russian vocabulary because they did not label these liquid containers the same way as the monolingual Russian speakers. When grouped according to Age of Acquisition (AoA) of English, the bilinguals showed an effect of AoA (or perhaps the length of exposure to the L2) in that bilinguals with earlier AoA (mean AoA 3.4 years) exhibited much stronger attrition than bilinguals with later AoA (mean AoA 22.8 years). That is, the individuals with earlier AoA were the more different from monolingual Russian speakers in their labeling and categorization of drinking vessels, than the people with later AoA. However, even the late AoA bilinguals exhibited some degree of attrition in that they labeled the drinking vessels differently from native monolingual Russian-speaking adults.

Critical period hypothesis

Given that exposure to an L2 at a younger age typically leads to stronger attrition of the L1 than L2 exposure at later ages, there may be a relationship between language attrition and the critical period hypothesis. The critical period for language claims that there is an optimal time period for humans to acquire language, and after this time language acquisition is more difficult (though not impossible). Language attrition also seems to have a time period; before around age 12, a first language is most susceptible to attrition if there is reduced exposure to that language. Research shows that the complete attrition of a language would occur before the critical period ends.

All available evidence on the age effect for L1 attrition, therefore, indicates that the development of susceptibility displays a curved, not a linear, function. This suggests that in native language learning there is indeed a Critical Period effect, and that full development of native language capacities necessitates exposure to L1 input for the entire duration of this CP.

L2 attrition

In Hansen & Reetz-Kurashige (1999), Hansen cites her own research on L2-Hindi and Urdu attrition in young children. As young pre-school children in India and Pakistan, the subjects of her study were often judged to be native speakers of Hindi or Urdu; their mother was far less proficient. On return visits to their home country, the United States, both children appeared to lose all their L2 while the mother noticed no decline in her own L2 abilities. Twenty years later, those same young children as adults comprehend not a word from recordings of their own animated conversations in Hindi-Urdu; the mother still understands much of them.

Yamamoto (2001) found a link between age and bilinguality. In fact, a number of factors are at play in bilingual families. In her study, bicultural families that maintained only one language, the minority language, in the household, were able to raise bilingual, bicultural children without fail. Families that adopted the one parent – one language policy were able to raise bilingual children at first but when the children joined the dominant language school system, there was a 50% chance that children would lose their minority language abilities. In families that had more than one child, the older child was most likely to retain two languages, if it was at all possible. Younger siblings in families with more than two other brothers and sisters had little chance of maintaining or ever becoming bilingual.

Age of arrival

There are few principled and systematic investigations of FLA specifically investigating the impact of AoA. However, converging evidence suggests an age effect on FLA which is much stronger and more clearly delineated than the effects that have been found in SLA research. Two studies that consider prepuberty and postpuberty migrants (Ammerlaan, 1996, AoA 0–29 yrs; Pelc, 2001, AoA 8–32 years) find that AoA is one of the most important predictors of ultimate proficiency, and a number of studies that investigate the impact of age among postpuberty migrants fail to find any effect at all (Köpke, 1999, AoA 14–36 yrs; Schmid, 2002, AoA 12–29 yrs; Schmid, 2007, AoA 17–51 yrs). A range of studies conducted by Montrul on Spanish heritage speakers in the US as well as Spanish-English bilinguals with varying levels of AoA also suggests that the L1 system of early bilinguals may be similar to that of L2 speakers, while later learners pattern with monolinguals in their L1 (e.g. Montrul, 2008; Montrul, 2009). These findings therefore indicate strongly that early (prepuberty) and late (postpuberty) exposure to an L2 environment have a different impact on possible fossilization and/or deterioration of the linguistic system.

Frequency of use

Frequency of use has been shown to be an important factor in language attrition. Decline in use of a given language leads to gradual loss of that language.

In the face of much evidence to the contrary, one study is often cited to suggest that frequency of use does not correlate strongly with language attrition. Their methodology, however, can be called into question, especially concerning the small sample size and the reliance on self reported data. The researchers themselves state that their findings may be inaccurate. The overall evidence suggests that frequency of use is a strong indicator of language attrition.

Motivation

Motivation could be defined as the willingness and desire to learn a second language, or, in the case of attrition, the incentive to maintain a language. Motivation can be split into four categories, but it is often simply split into two distinct forms: the instrumental and the integrative. Instrumental motivation, in the case of attrition, is the desire to maintain a language in order to complete a specific goal, i.e. maintaining a language to maintain a job. Integrative motivation, however, is motivation that comes from a desire to fit in or maintain one's cultural ties. These inferences can be drawn, as strategies for knowledge maintenance will, by definition, precisely oppose actions that lead to forgetting.

There are differences in attrition related to motivation depending on the type at hand. Instrumental motivation is often less potent than integrative motivation, but, given sufficient incentives, it can be equally as powerful. A 1972 study by Gardner and Lambert emphasized the importance of integrative motivation in particular in regards to factors relating to language acquisition, and, by extension, language attrition.

Impact crater

From Wikipedia, the free encyclopedia

Crater Engelier on Saturn's moon Iapetus Fresh crater on Mars showing a ray system of ejecta
Impact crater Tycho on the Moon
The Barringer Crater (Meteor Crater) east of Flagstaff, Arizona
Impact craters in the Solar System:
  • Top-left: 500-kilometre-wide (310 mi) crater Engelier on Saturn's moon Iapetus
  • Top-right: Recently formed (between July 2010 and May 2012) impact crater on Mars showing a pristine ray system of ejecta
  • Bottom-left: 50,000-year-old Meteor Crater east of Flagstaff, Arizona, U.S. on Earth
  • Bottom-right: The prominent crater Tycho in the southern highlands of the Moon

An impact crater is a depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Lunar impact craters range from microscopic craters on lunar rocks returned by the Apollo Program and small, simple, bowl-shaped depressions in the lunar regolith to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Venus, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics over time. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more commonly used. In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth.

The cratering records of very old surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been considerably lower, but it is appreciable nonetheless; Earth experiences from one to three impacts large enough to produce a 20-kilometre-diameter (12 mi) crater about once every million years on average. This indicates that there should be far more relatively young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. Formed in a collision 80 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate. Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System.

Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified. These range in diameter from a few tens of meters up to about 300 km (190 mi), and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation was witnessed in 1947) to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters. They are also selectively found in the stable interior regions of continents. Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, and the subduction of the ocean floor into Earth's interior by processes of plate tectonics.

Impact craters are not to be confused with landforms that may appear similar, including calderas, sinkholes, glacial cirques, ring dikes, salt domes, and others.

History

Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned, Meteor Crater, was of cosmic origin. Yet most geologists at the time assumed it formed as the result of a volcanic steam eruption.

Eugene Shoemaker, pioneer impact crater researcher, here at a crystallographic microscope used to examine meteorites

In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States. He concluded they had been created by some great explosive event, but believed that this force was probably volcanic in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts.

Grove Karl Gilbert suggested in 1893 that the Moon's craters were formed by large asteroid impacts. Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960, Gene Shoemaker revived the idea. According to David H. Levy, Gene "saw the craters on the Moon as logical impact sites that were formed not gradually, in eons, but explosively, in seconds." For his Ph.D. degree at Princeton (1960), under the guidance of Harry Hammond Hess, Shoemaker studied the impact dynamics of Barringer Meteor Crater. Shoemaker noted Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at the Nevada Test Site, notably Jangle U in 1951 and Teapot Ess in 1955. In 1960, Edward C. T. Chao and Shoemaker identified coesite (a form of silicon dioxide) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within suevite at Nördlinger Ries, proving its impact origin.

Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Astrophysical Observatory in Victoria, British Columbia, Canada and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American Apollo Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon. Because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.

Crater formation

Impact cratering involves high velocity collisions between solid objects, typically much greater than the speed of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the slowing effects of travel through the atmosphere, the lowest impact velocity with an object from space is equal to the gravitational escape velocity of about 11 km/s. The fastest impacts occur at about 72 km/s in the "worst case" scenario in which an object in a retrograde near-parabolic orbit hits Earth. The median impact velocity on Earth is about 20 km/s.

However, the slowing effects of travel through the atmosphere rapidly decelerate any potential impactor, especially in the lowest 12 kilometres where 90% of the earth's atmospheric mass lies. Meteorites of up to 7,000 kg lose all their cosmic velocity due to atmospheric drag at a certain altitude (retardation point), and start to accelerate again due to Earth's gravity until the body reaches its terminal velocity of 0.09 to 0.16 km/s. The larger the meteoroid (i.e. asteroids and comets) the more of its initial cosmic velocity it preserves. While an object of 9,000 kg maintains about 6% of its original velocity, one of 900,000 kg already preserves about 70%. Extremely large bodies (about 100,000 tonnes) are not slowed by the atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs.

Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater. Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.

This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of Hyperion, may produce internal compression without ejecta, punching a hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon.

It is convenient to divide the impact process conceptually into three distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. In practice, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse is already underway in others.

Contact and compression

Nested Craters on Mars, 40.104° N, 125.005° E. These nested craters are probably caused by changes in the strength of the target material. This usually happens when a weaker material overlies a stronger material.

In the absence of atmosphere, the impact process begins when the impactor first touches the target surface. This contact accelerates the target and decelerates the impactor. Because the impactor is moving so rapidly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate across the impactor. As a result, the impactor is compressed, its density rises, and the pressure within it increases dramatically. Peak pressures in large impacts exceed 1 TPa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions.

In physical terms, a shock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses the impactor, and it accelerates and compresses the target. Stress levels within the shock wave far exceed the strength of solid materials; consequently, both the impactor and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher-density phases by shock waves; for example, the common mineral quartz can be transformed into the higher-pressure forms coesite and stishovite. Many other shock-related changes take place within both impactor and target as the shock wave passes through, and some of these changes can be used as diagnostic tools to determine whether particular geological features were produced by impact cratering.

As the shock wave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the shock wave raises the temperature of the material. In all but the smallest impacts this increase in temperature is sufficient to melt the impactor, and in larger impacts to vaporize most of it and to melt large volumes of the target. As well as being heated, the target near the impact is accelerated by the shock wave, and it continues moving away from the impact behind the decaying shock wave.

Excavation

Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for a large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic. During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing a paraboloid (bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity.

Herschel Crater on Saturn's moon Mimas

The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater do not include material excavated from the full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth. As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials, a significant crater volume may also be formed by the permanent compaction of the pore space. Such compaction craters may be important on many asteroids, comets and small moons.

In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor. Some of this impact melt rock may be ejected, but most of it remains within the transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so. As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts, the expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space.

Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed escape velocity and leave the impacted planet or moon entirely. The majority of the fastest material is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic.

Small volumes of un-melted and relatively un-shocked material may be spalled at very high relative velocities from the surface of the target and from the rear of the impactor. Spalling provides a potential mechanism whereby material may be ejected into inter-planetary space largely undamaged, and whereby small volumes of the impactor may be preserved undamaged even in large impacts. Small volumes of high-speed material may also be generated early in the impact by jetting. This occurs when two surfaces converge rapidly and obliquely at a small angle, and high-temperature highly shocked material is expelled from the convergence zone with velocities that may be several times larger than the impact velocity.

Modification and collapse

Weathering may change the aspect of a crater drastically. This mound on Mars' north pole may be the result of an impact crater that was buried by sediment and subsequently re-exposed by erosion.

In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down the crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse breccia, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.

Multi-ringed impact basin Valhalla on Jupiter's moon Callisto

Above a certain threshold size, which varies with planetary gravity, the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a complex crater. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of elastic rebound, which is a process in which a material with elastic strength attempts to return to its original geometry; rather the collapse is a process in which a material with little or no strength attempts to return to a state of gravitational equilibrium.

Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls. At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an impact basin rather than an impact crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: small complex craters with a central topographic peak are called central peak craters, for example Tycho; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called peak-ring craters, for example Schrödinger; and the largest craters contain multiple concentric topographic rings, and are called multi-ringed basins, for example Orientale. On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at the largest sizes may contain many concentric rings. Valhalla on Callisto is an example of this type.

Identifying impact craters

Impact structure of craters: simple and complex craters
 
Wells Creek crater in Tennessee, United States: a close-up of shatter cones developed in fine grained dolomite
 
Decorah crater: aerial electromagnetic resistivity map (USGS)
 
Meteor Crater in the U.S. state of Arizona, was the world's first confirmed impact crater.
 
Shoemaker Crater in Western Australia was renamed in memory of Gene Shoemaker.

Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics.

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.

Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:

  • A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".
  • Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.
  • High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: scientific literature has reported that some "shock" features, such as small shatter cones, which are often associated only with impact events, have been found also in terrestrial volcanic ejecta.
  • Microscopic pressure deformations of minerals. These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or stishovite and coesite, varieties of shocked quartz.
  • Buried craters, such as the Decorah crater, can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.

Economic importance of impacts

On Earth impact craters have resulted in useful minerals. Some of the ores produced from impact related effects on Earth include ores of iron, uranium, gold, copper, and nickel. It is estimated that the value of materials mined from impact structures is five billion dollars/year just for North America. The eventual usefulness of impact craters depends on several factors especially the nature of the materials that were impacted and when the materials were affected. In some cases the deposits were already in place and the impact brought them to the surface. These are called “progenetic economic deposits.” Others were created during the actual impact. The great energy involved caused melting. Useful minerals formed as a result of this energy are classified as “syngenetic deposits.” The third type, called “epigenetic deposits,” is caused by the creation of a basin from the impact. Many of the minerals that our modern lives depend on are associated with impacts in the past. The Vredeford Dome in the center of the Witwatersrand Basin is the largest goldfield in the world which has supplied about 40% of all the gold ever mined in an impact structure (though the gold did not come from the bolide). The asteroid that struck the region was 9.7 km (6 mi) wide. The Sudbury Basin was caused by an impacting body over 9.7 km (6 mi) in diameter. This basin is famous for its deposits of nickel, copper, and Platinum Group Elements. An impact was involved in making the Carswell structure in Saskatchewan, Canada; it contains uranium deposits. Hydrocarbons are common around impact structures. Fifty percent of impact structures in North America in hydrocarbon-bearing sedimentary basins contain oil/gas fields.

Martian craters

Because of the many missions studying Mars since the 1960s, there is good coverage of its surface which contains large numbers of craters. Many of the craters on Mars differ from those on the Moon and other moons since Mars contains ice under the ground, especially in the higher latitudes. Some of the types of craters that have special shapes due to impact into ice-rich ground are pedestal craters, rampart craters, expanded craters, and LARLE craters.

Lists of craters

Impact craters on Earth

World map in equirectangular projection of the craters on the Earth Impact Database as of November 2017 (in the SVG file, hover over a crater to show its details)
 

On Earth, the recognition of impact craters is a branch of geology, and is related to planetary geology in the study of other worlds. Out of many proposed craters, relatively few are confirmed. The following twenty are a sample of articles of confirmed and well-documented impact sites.

See the Earth Impact Database, a website concerned with 190 (as of July 2019) scientifically-confirmed impact craters on Earth.

Some extraterrestrial craters

Balanchine crater in Caloris Basin, photographed by MESSENGER, 2011

Largest named craters in the Solar System

Tirawa crater straddling the terminator on Rhea, lower right.
 
  1. North Polar Basin/Borealis Basin (disputed) – Mars – Diameter: 10,600 km
  2. South Pole-Aitken basin – Moon – Diameter: 2,500 km
  3. Hellas Basin – Mars – Diameter: 2,100 km
  4. Caloris Basin – Mercury – Diameter: 1,550 km
  5. Imbrium Basin – Moon – Diameter: 1,100 km
  6. Isidis Planitia – Mars – Diameter: 1,100 km
  7. Mare Tranquilitatis – Moon – Diameter: 870 km
  8. Argyre Planitia – Mars – Diameter: 800 km
  9. Rembrandt – Mercury – Diameter: 715 km
  10. Serenitatis Basin – Moon – Diameter: 700 km
  11. Mare Nubium – Moon – Diameter: 700 km
  12. Beethoven – Mercury – Diameter: 625 km
  13. Valhalla – Callisto – Diameter: 600 km, with rings to 4,000 km diameter
  14. Hertzsprung – Moon – Diameter: 590 km
  15. Turgis – Iapetus – Diameter: 580 km
  16. Apollo – Moon – Diameter: 540 km
  17. Engelier – Iapetus – Diameter: 504 km
  18. Mamaldi – Rhea – Diameter: 480 km
  19. Huygens – Mars – Diameter: 470 km
  20. Schiaparelli – Mars – Diameter: 470 km
  21. Rheasilvia – 4 Vesta – Diameter: 460 km
  22. Gerin – Iapetus – Diameter: 445 km
  23. Odysseus – Tethys – Diameter: 445 km
  24. Korolev – Moon – Diameter: 430 km
  25. Falsaron – Iapetus – Diameter: 424 km
  26. Dostoevskij – Mercury – Diameter: 400 km
  27. Menrva – Titan – Diameter: 392 km
  28. Tolstoj – Mercury – Diameter: 390 km
  29. Goethe – Mercury – Diameter: 380 km
  30. Malprimis – Iapetus – Diameter: 377 km
  31. Tirawa – Rhea – Diameter: 360 km
  32. Orientale Basin – Moon – Diameter: 350 km, with rings to 930 km diameter
  33. Evander – Dione – Diameter: 350 km
  34. Epigeus – Ganymede – Diameter: 343 km
  35. Gertrude – Titania – Diameter: 326 km
  36. Telemus – Tethys – Diameter: 320 km
  37. Asgard – Callisto – Diameter: 300 km, with rings to 1,400 km diameter
  38. Vredefort impact structure – Earth – Diameter: 300 km
  39. Kerwan – Ceres – Diameter: 284 km
  40. Powehiwehi – Rhea – Diameter: 271 km

There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars. Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus.

Worlds in Collision

From Wikipedia, the free encyclopedia

Worlds in Collision
Wic-cover.jpg
First UK edition (publ. Gollancz)
AuthorImmanuel Velikovsky
CountryUnited States
LanguageEnglish
SubjectMythology
Published3 April 1950

Worlds in Collision is a book by Immanuel Velikovsky published in 1950. The book postulates that around the 15th century BC, the planet Venus was ejected from Jupiter as a comet or comet-like object and passed near Earth (an actual collision is not mentioned). The object allegedly changed Earth's orbit and axis, causing innumerable catastrophes that are mentioned in early mythologies and religions from around the world. The book has been heavily criticized as a work of pseudoscience and catastrophism, and many of its claims are completely rejected by the established scientific community as they are not supported by any available evidence.

Publication

Worlds in Collision was first published on April 3, 1950, by Macmillan Publishers. Macmillan's interest in publishing it was encouraged by the knowledge that Velikovsky had obtained a promise from Gordon Atwater, Director of the Hayden Planetarium, for a sky show based on the book when it was published. The book, Velikovsky's most criticized and controversial, was an instant New York Times bestseller, topping the charts for eleven weeks while being in the top ten for twenty-seven straight weeks. Despite this popularity, overwhelming rejection of its thesis by the scientific community forced Macmillan to stop publishing it and to transfer the book to Doubleday within two months.

Core ideas

In the book's preface, Velikovsky summarizes his arguments:

Worlds in Collision is a book of wars in the celestial sphere that took place in historical times. In these wars the planet Earth participated too. [...] The historical-cosmological story of this book is based in the evidence of historical texts of many people around the globe, on classical literature, on epics of the northern races, on sacred books of the peoples of the Orient and Occident, on traditions and folklore of primitive peoples, on old astronomical inscriptions and charts, on archaeological finds, and also on geological and paleontological material.

The book proposes that around the 15th century BCE, Venus was ejected from Jupiter as a comet or comet-like object and subsequently passed near Earth, though an actual collision with the Earth is not mentioned. In doing so it changed Earth's orbit and axial inclination, causing innumerable catastrophes which were identified in early mythologies and religious traditions from human civilizations around the world. Fifty-two years later, it again made a close approach, stopping the Earth's rotation for a while and causing more catastrophes. Then, in the 8th and 7th centuries BCE, Mars (itself displaced by Venus) made close approaches to the Earth; this incident caused a new round of disturbances and disasters. After that, the current "celestial order" was established. The courses of the planets stabilized over the centuries and Venus gradually became a "normal" planet.

These events led to several key statements:

  1. Venus must be still very hot as young planets radiate heat.
  2. Venus must be rich in petroleum and hydrocarbon gases.
  3. Venus has an abnormal orbit in consequence of the unusual disasters stemming from its planetary origins.

Velikovsky suggested some additional ideas that he said derived from these claims, including:

  1. Jupiter emits radio noises.
  2. The magnetosphere of the Earth reaches at least up to the Moon.
  3. The Sun has an electric potential of approximately 1019 volts.
  4. The rotation of the Earth can be affected by electromagnetic fields.

Velikovsky arrived at these proposals using a methodology which would today be called comparative mythology – he looked for concordances in the myths and written histories of unconnected cultures across the world, following a literal reading of their accounts of the exploits of planetary deities. He argues on the basis of ancient cosmological myths from places as disparate as India and China, Greece and Rome, Assyria and Sumer. For example, ancient Greek mythology asserts that the goddess Athena sprang from the head of Zeus. Velikovsky identifies Zeus (whose Roman counterpart was the god Jupiter) with the planet Jupiter and Athena (the Roman Minerva) with the planet Venus. This myth, along with others from ancient Egypt, Israel, Mexico, etc. are used to support the claim that "Venus was expelled as a comet and then changed to a planet after contact with a number of members of our solar system" (Velikovsky 1972:182).

Critical reaction

Contemporary reactions

The plausibility of the theory was summarily rejected by the physics community, as the cosmic chain of events proposed by Velikovsky contradicts basic laws of physics. Velikovsky's ideas had been known to astronomers for years before the publication of the book, partially by his writing to astronomer Harlow Shapley of Harvard, partially through his 1946 pamphlet Cosmos Without Gravitation, and partially by a preview of his work in an article in the August 11, 1946, edition of the New York Herald Tribune. An article about the upcoming book was published by Harper's Magazine in January 1950, which was followed by additional articles in Newsweek (Bauer 1984:3–4) and Reader's Digest in March 1950.

Shapley, along with others such as astronomer Cecilia Payne-Gaposchkin (also at Harvard), instigated a campaign against the book before its publication. Initially, they were highly critical of a publisher as reputable as Macmillan publishing such a pseudoscientific book, even as a trade book. Their disapproval was re-invigorated when Macmillan included Worlds in Collision among other trade books of possible interest to professors listed under the category "Science" in the back of a textbook catalog mailed to college professors. Within two months of the book's initial release, the publishing of the book was transferred to Doubleday, which has no textbook division.

The fundamental criticism against the book from the astronomy community was that its celestial mechanics were irreconcilable with Newtonian mechanics, requiring planetary orbits which could not be made to conform to the laws of conservation of energy and conservation of angular momentum (Bauer 1984:70). Velikovsky conceded that the behavior of the planets in his theories is not consistent with Newton's laws of motion and universal gravitation. He proposed that electromagnetic forces could be the cause of the movements of the planets, although such forces between astronomical bodies are known to be essentially zero.

Velikovsky tried to protect himself from criticism of his proposed celestial mechanics by removing the original Appendix on the subject from Worlds in Collision, hoping that the merit of his ideas would be evaluated on the basis of his comparative mythology and use of literary sources alone. This strategy did not protect him: the Appendix was an expanded version of the Cosmos Without Gravitation monograph, which he had already distributed to Shapley and others in the late 1940s — and they had regarded the physics within it as egregiously in error.

Carl Sagan

In his 1979 science book Broca's Brain: Reflections on the Romance of Science, astronomer Carl Sagan wrote that the high surface temperature of Venus was known prior to Velikovsky, and that Velikovsky misunderstood the mechanism for this heat. Velikovsky believed that Venus was heated by its close encounter with the Earth and Mars. He also did not understand the greenhouse effect caused by Venus' atmosphere, which had earlier been elucidated by astronomer Rupert Wildt. Ultimately, Venus is hot due to its proximity to the Sun; it does not emit more heat than it receives from the Sun, and any heat produced by its celestial movements would have long since dissipated. Sagan concludes: "(1) the temperature in question was never specified [by Velikovsky]; (2) the mechanism proposed for providing this temperature is grossly inadequate; (3) the surface of the planet does not cool off with time as advertised; and (4) the idea of a high surface temperature on Venus was published in the dominant astronomical journal of its time and with an essentially correct argument ten years before the publication of Worlds in Collision" (p. 118).

Sagan also noted that "Velikovsky's idea that the clouds of Venus are composed of hydrocarbons or carbohydrates is neither original nor correct." Sagan notes that the presence of hydrocarbon gases (such as petroleum gases) on Venus was earlier suggested, and abandoned, again by Rupert Wildt, whose work is not credited by Velikovsky. Also, the 1962 Mariner 2 probe was erroneously reported in the popular press to have discovered hydrocarbons on Venus. These errors were subsequently corrected, and Sagan later concluded that "[n]either Mariner 2 nor any subsequent investigation of the Venus atmosphere has found evidence for hydrocarbons or carbohydrates" (p. 113).

Regarding Jupiter's radio emissions, Sagan noted that "all objects give off radio waves if they are at temperatures above absolute zero. The essential characteristics of the Jovian radio emission — that it is nonthermal, polarized, intermittent radiation, connected with the vast belts of charged particles which surround Jupiter, trapped by its strong magnetic field — are nowhere predicted by Velikovsky. Further, his 'prediction' is clearly not linked in its essentials to the fundamental Velikovskian theses. Merely guessing something right does not necessarily demonstrate prior knowledge or a correct theory." Sagan concluded that "there is not one case where [Velikovsky's] ideas are simultaneously original and consistent with simple physical theory and observation."

He also noted that it was Athena and not Venus who was born from the head of Zeus – two utterly different goddesses. Athena was never identified with a planet.

Later reactions

Tim Callahan, religion editor of Skeptic, presses the case further in claiming that the composition of the atmosphere of Venus is a complete disproof of Worlds in Collision. "...Velikovsky's hypothesis stands or falls on Venus having a reducing atmosphere made up mainly of hydrocarbons. In fact, the atmosphere of Venus is made up mainly of carbon dioxide—carbon in its oxidized form—along with clouds of sulfuric acid. Therefore, it couldn't have carried such an atmosphere with it out of Jupiter and it couldn't be the source of hydrocarbons to react with oxygen in our atmosphere to produce carbohydrates. Velikovsky's hypothesis is falsified by the carbon dioxide atmosphere of Venus."

Astronomer Philip Plait has pointed out that Velikovsky's hypothesis is also falsified by the presence of the Moon with its nearly circular orbit, for which the length of the month has not changed sensibly in the more than 2,000 years the Jews have used their lunar calendar. "If Venus were to get so close to the Earth that it could actually exchange atmospheric contents [i.e., closer than 1,000 kilometers (620 mi) from the surface of the Earth]," as Velikovsky claimed, ". . . the Moon would have literally been flung into interplanetary space. At the very least its orbit would have been profoundly changed, made tremendously elliptical... Had Venus done any of the things Velikovsky claimed, the Moon's orbit would have changed."

Controversy

By 1974, the controversy surrounding Velikovsky's work had reached the point where the American Association for the Advancement of Science felt obliged to address the situation, as they had previously done in relation to UFOs, and devoted a scientific meeting to Velikovsky. The meeting featured, among others, Velikovsky himself and Carl Sagan. Sagan gave a critique of Velikovsky's ideas and attacked most of the assumptions made in Worlds in Collision. His criticism is published in Scientists Confront Velikovsky (Ithaca, New York, 1977), edited by Donald Goldsmith, and presented in a revised and corrected version in his book Broca's Brain: Reflections on the Romance of Science and is much longer than that given in the talk. Sagan further critiqued Velikovsky's ideas in his PBS television series Cosmos. In Cosmos, Sagan also criticizes the scientific community for their attitude toward Velikovsky, stating that while science is a process in which all ideas are subject to a process of extensive scrutiny before any idea can be accepted as fact, the attempt by some scientists to suppress outright Velikovsky's ideas was "the worst aspect of the Velikovsky affair."

In November 1974, at the Biennial Meeting of the Philosophy of Science Association held at the University of Notre Dame, Michael W. Friedlander, professor of physics at Washington University in St. Louis, confronted Velikovsky in the symposium "Velikovsky and the Politics of Science" with examples of his "substandard scholarship" involving the "distortion of the published scientific literature in quotations that he used to support his theses". For example, contrary to Velikovsky, R.A. Lyttleton did not write "the terrestrial planets, Venus included, must [emphasis added] have originated from the giant planets…" Rather, Lyttleton wrote "…it is even possible…" As Friedlander recounts, "When I gave each example, [Velikovsky's] response was 'Where did I write that?'; when I showed a photo copy of the quoted pages, he simply switched to a different topic."

A thorough examination of the original material cited in Velikovsky's publications, and a severe criticism of its use, was published by Bob Forrest. Earlier in 1974, James Fitton published a brief critique of Velikovsky's interpretation of myth, drawing on the section "The World Ages" and the later interpretation of the Trojan War, that was ignored by Velikovsky and his defenders whose indictment began: "In at least three important ways Velikovsky's use of mythology is unsound. The first of these is his proclivity to treat all myths as having independent value; the second is the tendency to treat only such material as is consistent with his thesis; and the third is his very unsystematic method." A short analysis of the position of arguments in the late 20th century was given by Velikovsky's ex-associate C. Leroy Ellenberger, a former senior editor of Kronos (a journal to promote Velikovsky's ideas) (Bauer 1995:11), in his essay. Almost ten years later, Ellenberger criticized some Velikovskian and neo-Velikovskian qua "Saturnist" ideas in an invited essay.

The storm of controversy that was created by Velikovsky's works, especially Worlds in Collision, may have helped revive the Catastrophist movements in the last half of the 20th century; it is also held by some working in the field that progress has actually been retarded by the negative aspects of the so-called Velikovsky Affair. The assessment of Velikovsky's work by tree-ring expert Mike Baillie is instructive: "But fundamentally, Velikovsky did not understand anything about comets … As if to comfort his readers, at one point he says that no planet at present has a course which poses a danger to this planet: '…only a few asteroids—mere rocks, a few kilometres in diameter—have orbits which cross the path of the earth.' … He did not know about the hazard posed by relatively small objects, and, just in case there is any doubt about his mistake, he repeats the notion by noting that a possibility exists of some future collision between planets, 'not a mere encounter between a planet and an asteroid'. This failure to recognize the power of comets and asteroids means that it is reasonable to go back to Velikovsky and delete all the physically impossible text about Venus and Mars passing close to the earth."

More recently, the absence of supporting material in ice core studies (such as the Greenland Dye-3 and Vostok cores), bristlecone pine tree ring data, Swedish clay varves, and many hundreds of cores taken from ocean and lake sediments from all over the world has ruled out any basis for the proposition of a global catastrophe of the proposed dimension within the Late Holocene age. Also, the fossils, geological deposits, and landforms in Earth in Upheaval, which Velikovsky regards as corroborating the hypothesis presented in Worlds in Collision have been, since their publication, explained in terms of mundane non-catastrophic geologic processes. So far, the only piece of the geologic evidence which has shown to have a catastrophic origin is a "raised beach" containing coral-bearing conglomerates found at an elevation of 1,200 feet above sea level within the Hawaiian Islands. The sediments, which were misidentified as a "raise beach", are now attributed to megatsunamis generated by massive landslides created by the periodic collapse of the sides of the islands. In addition, these conglomerates, as many of the items cited as evidence for his ideas in Earth in Upheaval, are far too old to be used as valid evidence supporting the hypothesis presented in Worlds in Collision.

Rydberg atom

From Wikipedia, the free encyclopedia https://en.wikipedia.org/wiki/Rydberg_atom Figure 1: Electron orbi...