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Rolling hairpin replication ( RHR ) is a unidirectional, strand displacement form of DNA replication used by parvoviruses, a group of viruses that constitute the family Parvoviridae . Parvoviruses have linear, single-stranded DNA (ssDNA) genomes in which the coding portion of the genome is flanked by telomeres at each end that form hairpin loops . During RHR, these hairpin loops repeatedly unfold and refold to change the direction of DNA replication so that replication progresses in a continuous manner back and forth across the genome. RHR is initiated and terminated by an endonuclease encoded by parvoviruses that is variously called NS1 or Rep, and RHR is similar to rolling circle replication , which is used by ssDNA viruses that have circular genomes.
Before RHR begins, a host cell DNA polymerase converts the genome to a duplex form in which the coding portion is double-stranded and connected to the terminal hairpins. From there, messenger RNA (mRNA) that encodes the viral initiator protein is transcribed and translated to synthesize the protein. The initiator protein commences RHR by binding to and nicking the genome in a region adjacent to a hairpin called the origin and establishing a replication fork with its helicase activity. Nicking leads to the hairpin unfolding into a linear, extended form. The telomere is then replicated and both strands of the telomere refold back in on themselves to their original turn-around forms. This repositions the replication fork to switch templates to the other strand and move in the opposite direction. Upon reaching the other end, the same process of unfolding, replication, and refolding occurs.
Parvoviruses vary in whether both hairpins are the same or different. Homotelomeric parvoviruses such as adeno-associated viruses (AAV), i.e. those that have identical or similar telomeres, have both ends replicated by terminal resolution, the previously described process. Heterotelomeric parvoviruses such as minute virus of mice (MVM), i.e. those that have different telomeres, have one end replicated by terminal resolution and the other by an asymmetric process called junction resolution. During asymmetric junction resolution, the duplex extended form of the telomere reorganizes into a cruciform-shaped junction , and the correct orientation of the telomere is replicated off the lower arm of the cruciform. As a result of RHR, a replicative molecule that contains numerous copies of the genomes is synthesized. The initiator protein periodically excises progeny ssDNA genomes from this replicative concatemer.
Parvoviruses are a family of DNA viruses that have single-stranded DNA (ssDNA) genomes enclosed in rugged, icosahedral protein capsids 18–26 nanometers (nm) in diameter. [ 1 ] Unlike most other ssDNA viruses, which have circular genomes that form a loop, parvoviruses have linear genomes with short terminal sequences at each end of the genome. These termini are capable of being formed into structures called hairpins or hairpin loops and consist of short, imperfect palindromes. [ 2 ] [ 3 ] Varying from virus to virus, the coding region of the genome is 4–6 kilobases (kb) in length, and the termini are 116–550 nucleotides (nt) in length each. The hairpin sequences provide most of the cis -acting information needed for DNA replication and packaging. [ 1 ] [ 4 ]
Parvovirus genomes may be either positive-sense or negative-sense . Some species, such as adeno-associated viruses (AAV) like AAV2, package a roughly equal number of positive-sense and negative-sense strands into virions, others, such as minute virus of mice (MVM), show preference toward packaging negative-sense strands, and others have varying proportions. [ 4 ] Because of this disparity, the 5′-end (usually pronounced "five prime end") of the strand that encodes the non-structural proteins is called the "left end", and the 3′-end (usually pronounced "three prime end") is called the "right end". [ 3 ] In reference to the negative-sense strand, the 3′-end is the left side and the 5′-end is the right side. [ 4 ] [ 5 ]
Parvoviruses replicate their genomes through a process called rolling hairpin replication (RHR), which is a unidirectional, strand displacement form of DNA replication. Before replication, the coding portion of the ssDNA genome is converted to a double-strand DNA (dsDNA) form, which is then cleaved by a viral protein to initiate replication. Sequential unfolding and refolding of the hairpin termini acts to reverse the direction of synthesis, which allows replication to go back and forth along the genome to synthesize a continuous duplex replicative form (RF) DNA intermediate. Progeny ssDNA genomes are then excised from the RF intermediate. [ 4 ] [ 6 ] While the general aspects of RHR are conserved across genera and species, the exact details likely vary. [ 7 ]
Parvovirus genomes have distinct starting points of replication that contain palindromic DNA sequences. These sequences are able to alternate between inter- and intrastrand basepairing throughout replication, and they serve as self-priming telomeres at each end of the genome. [ 2 ] They also contain two key sites necessary for replication used by the initiator protein: a binding site and a cleavage site. [ 8 ] Telomere sequences have significant complexity and diversity, suggesting that they perform additional functions for many species. [ 1 ] [ 9 ] In MVM, for example, the left-end hairpin contains binding sites for transcription factors that modulate gene expression from an adjacent promoter . For AAV, the hairpins can bind to MRE11/Rad50/NBS1 (MRN) complexes and Ku70/80 heterodimers, which are involved in sensing and repairing DNA. [ 5 ] In general, however, they have the same basic structure: imperfect palindromes in which a fully or primarily basepaired region terminates into an axial symmetry. These palindromes can fold into a variety of structures such as a Y-shaped structure and a cruciform-shaped structure. During replication, the termini act as hinges in which the imperfectly basepaired or partial cruciform regions surrounding the axis provide a favorable environment for unfolding and refolding of the hairpin. [ 2 ] [ 3 ] [ 4 ]
Some parvoviruses, such as AAV2, are homotelomeric, meaning the two palindromic telomeres are similar or identical and form part of larger (inverted) terminal repeat ((I)TR) sequences. Replication at each terminal ending is therefore similar. Other parvoviruses, such as MVM, are heterotelomeric, meaning they have two physically different telomeres. As a result, heterotelomeric parvoviruses tend to have a more complex replication process since the two telomeres have different replication processes. [ 2 ] [ 3 ] [ 4 ] In general, homotelomeric parvoviruses replicate both ends via a process called terminal resolution, whereas heterotelomeric parvoviruses replicate one end by terminal resolution and the other end by an asymmetric process called junction resolution. [ 4 ] [ 5 ] [ 6 ] [ 10 ] Whether a genus is hetero- or homotelomeric, along with other genomic characteristics, is shown in the following table. [ 4 ]
The entire process of rolling hairpin replication, which has distinct, sequential stages, can be summarized as follows: [ 4 ] [ 5 ] [ 7 ]
Upon cell entry, a tether about 24 nucleotides in length that attaches the viral protein NS1, essential in replication, to the virion is cleaved off the virion to be reattached later. [ 3 ] After cell entry, virions accumulate in the cell nucleus while the genome is still contained within the capsid. These capsids may be reconfigured to an open or transitioned state during entry. The exact mechanism by which the genome leaves the capsid is unclear. [ 9 ] For AAV, it has been suggested that nuclear factors disassemble the capsid, whereas for MVM, it appears as if the genome is ejected in a 3′-to-5′ direction from an opening in the capsid called a portal. [ 5 ]
Parvoviruses lack genes capable of inducing resting cells to enter their DNA synthesis phase (S-phase). Additionally, naked ssDNA is likely to be unstable, perceived as foreign by the host cell, or improperly replicated by host DNA repair . For these reasons, the genome must either be converted rapidly to its less obstructive, more stable duplex form or retained within the capsid until it is uncoated during S-phase. Typically, the latter occurs and virion remains silent in the nucleus until the host cell enters S-phase by itself. During this waiting period, virions may make use of certain strategies to evade host defense mechanisms to protect their hairpins and DNA to reach S-phase, [ 9 ] though it is unclear how this occurs. [ 4 ] Since the genome is packaged as ssDNA, creation of a complementary strand is necessary before gene expression . [ 5 ] [ 9 ]
DNA polymerases are only able to synthesize DNA in a 5′ to 3′ direction, and they require a basepair primer to begin synthesis. Parvoviruses address these limitations by using their termini as primers for complementary strand synthesis. [ 9 ] A 3′ hydroxyl end of the left-hand (3′) terminus pairs with an internal base to prime initial DNA synthesis, resulting in the conversion of the ssDNA genome to its first duplex form. [ 1 ] [ 7 ] This is a monomeric double-stranded DNA molecule in which the two strands are covalently cross-linked to each other at the left-end by a single copy of the viral telomere. Synthesis of the duplex form precedes NS1 expression so that when the replication fork during initial complementary strand synthesis reaches the right (5′) end, it does not displace and copy the right-end hairpin. This allows the 3′-end of the new DNA strand to be covalently ligated to the 5′-end of the right hairpin by a host ligase, thereby creating the duplex molecule. During this step, the tether sequence that was present before viral entry into the cell is resynthesized. [ 6 ]
Once an infected cell enters S-phase, parvovirus genomes are converted to their duplex form by host replication machinery, and mRNA that encodes non-structural (NS) proteins is transcribed starting from a viral promoter (P4 for MVM). [ 4 ] [ 5 ] [ 9 ] One of these NS proteins is usually called NS1 but also Rep1 or Rep68/78 for the genus Dependoparvovirus , which AAV belongs to. [ 4 ] NS1 is a site-specific DNA binding protein that acts as the replication initiator protein [ 9 ] via nickase activity. [ 15 ] It also mediates excision of both ends of the genome from duplex RF intermediates via a transesterification reaction that introduces a nick into specific duplex origin sequences. [ 4 ] Key components of NS1 include an HUH endonuclease domain toward the N-terminus of the protein and a superfamily 3 (SF3) helicase toward the C-terminus , [ 16 ] as well as ATPase activity. [ 1 ] It binds to ssDNA, RNA, and site-specifically on duplex DNA at reiterations of the tetranucleotide sequence 5′-ACCA-3′ 1–3 . [ 1 ] [ 9 ] These sequences are present in the viral replication origin sites and repeated at multiple sites throughout the genome in more or less degenerative forms. [ 15 ]
NS1 nicks the covalently-closed right-end telomere via a transesterification reaction that liberates a basepaired 3′ nucleotide as a free hydroxyl (-OH). [ 4 ] This reaction is assisted by a host DNA-binding protein from the high mobility group 1/2 (HMG1/2) family and is made in the replication origin, OriR , which was created by sequences in and immediately adjacent to the right hairpin. The left-end telomere of MVM, a heterotelomeric parvovirus, contains sequences that can give rise to replication origins in higher-order duplex intermediates, but these sequences are inactive in the hairpin terminus of the monomeric molecule, so NS1 always initiates replication at the right end. [ 6 ]
The 3′-OH that is freed by nicking acts as a primer for the DNA polymerase to start complementary strand synthesis [ 8 ] while NS1 remains covalently attached to the 5′-end via a tyrosine residue. [ 1 ] Consequently, a copy of NS1 remains attached to the 5′-end of all RF and progeny DNA throughout replication, packaging, and virion release. [ 4 ] [ 6 ] NS1 is only able to bind to this specific site by assembling into homodimers or higher order multimers, which happens naturally with the addition of adenosine triphosphate (ATP) that is likely mediated by NS1's helicase domain. In vivo studies have shown that NS1 can form into a variety of oligomeric states, but it most likely assembles into hexamers to fulfill the functions of both the endonuclease domain and helicase domain. [ 15 ]
Starting from the location at the nick, it is thought that NS1 organizes a replication fork and acts as the replicative 3′-to-5′ helicase. Near its C-terminus, NS1 contains an acidic transcriptional activation domain. This domain acts to upregulate transcription starting from a viral promoter (P38 for MVM) when NS1 is bound to a series of 5′-ACCA-3′ motifs, called the tar sequence, positioned upstream (toward the 5′-end) of the promoter unit, and via interaction with NS1 and various transcription factors. [ 15 ] NS1 also recruits the cellular replication protein A (RPA) complex, which is essential for establishing the new replication fork and for binding and stabilizing displaced single strands. [ 6 ]
While NS1 is the only non-structural protein essential for all parvoviruses, some have other individual proteins that are essential for replication. For MVM, NS2 appears to reprogram the host cell for efficient DNA amplification, single-strand progeny synthesis, capsid assembly, and virion export, though it seems to lack direct involvement in these processes. NS2 initially accumulates up to three times more quickly than NS1 in the early S-phase but is turned-over rapidly by a proteasome-mediated pathway. As the infectious cycle progresses, NS2 becomes less common as P38-driven transcription becomes more prominent. [ 15 ] Another example is the nuclear phosphoprotein NP1 of bocaviruses, which, if not synthesized, results in non-viable progeny genomes. [ 5 ]
As viral NS proteins accumulate, they commandeer host cell replication apparati, terminating host cell DNA synthesis and causing viral DNA amplification to begin. Interference with host DNA replication may be due to direct effects on host replication proteins that are not essential for viral replication, by extensive nicking of host DNA, or by the restructuring of the nucleus during viral infection. Early in infection, parvoviruses establish replication foci in the nucleus that are termed autonomous parvovirus-associated replication (APAR) bodies. NS1 co-localizes with replicating viral DNA in these structures with other cellular proteins necessary for viral DNA synthesis, [ 15 ] while other complexes not required for replication are sequestered from APAR bodies. The exact manner by which proteins are included or excluded from APAR bodies is unclear and appears to vary from species to species and between cell types. [ 5 ] As infection progresses, APAR microdomains begin to coalesce with other, formerly distinct, nuclear bodies to form progressively larger nuclear inclusions where viral replication and virion assembly occur. After S-phase begins, the host cell is forced to synthesize viral DNA and cannot leave S-phase. [ 17 ]
The right-end hairpin of MVM contains 248 nucleotides [ 10 ] organized into a cruciform shape. [ 1 ] This region is almost perfectly basepaired, with just three unpaired bases at the axis and a mismatched region positioned 20 nucleotides from the axis. A three nucleotide insertion, AGA or TCT, on one strand separates opposing pairs of NS1 binding sites, creating a 36 basepair-length palindrome that can assume an alternate cruciform configuration. This configuration is expected to destabilize the duplex, which facilitates its ability to function as a hinge. The mismatch of the unpaired bases, rather than the three-nucleotide sequence itself, may help to promote instability of duplex DNA. [ 10 ]
Fully-duplex linear forms of the right-end hairpin sequence also function as NS1-dependent origins. For many parvoviral telomeres, however, only an initiator binding site next to the nick site is required for the origin function so that the minimal sequences required for nicking are less than 40 basepairs in length. For MVM, the minimal right-end origin is around 125 basepairs in length and includes most of the hairpin sequence because at least three recognition elements are involved: the nick site 5′-CTWWTCA-3′ (element 1), positioned seven nucleotides upstream from a duplex NS1-binding site (element 2) that is oriented to have the attached NS1 complex extending over the nick site, and a second NS1-binding site (element 3), which is adjacent to the hairpin axis. [ 10 ]
The second binding site is over 100 basepairs away from the nick site but is required for NS1-mediated cleavage. [ 10 ] In vivo , there is slight variation in the position of the nick, plus or minus one nucleotide, with one position preferred. During nicking, this site is likely exposed as a single strand and is potentially stabilized as a minimal stem-loop by the tetranucleotide inverted repeats to the sides of the site. Optimal forms of the NS1-binding site contain at least three tandem copies of the 5′-ACCA-3′ sequence. Modest alterations to these motifs only have a small effect on affinity, which suggests that each tetranucleotide motif is recognized by different molecules in the NS1 complex. The NS1-binding site that positions NS1 over the nick site in the right-end origin is a high affinity site. [ 18 ]
With ATP, NS1 binds asymmetrically over the aforementioned sequence, protecting a region 41 basepairs in length from digestion. This footprint extends just five nucleotides beyond the 3′-end of the ACCA repeat but 22 nucleotides beyond the 5′-end so that the footprint ends 15 nucleotides beyond the nick site, placing NS1 in position to nick the origin. Nicking only occurs if the second, distant NS1-binding site is also present in the origin and the entire complex is activated by addition of HMG1. [ 18 ]
In the absence of NS1, HMG1 binds the hairpin sequence independently, causing it to bend, without protecting any region from digestion. HMG1 can also directly bind to NS1 and mediates interactions between NS1 molecules bound to their recognition elements in the origin, so it is essential for formation of the cleavage complex. The ability of the axis region to reconfigure into a cruciform does not appear to be important in this process. Cleavage is dependent on the correct spacing of the elements of the origin, so additions and deletions can be lethal, whereas substitutions can be tolerated. Addition of HMG1 appears to only slightly adjust the sequences protected by NS1, but the conformation of the intervening DNA changes, folding into a double helical loop that extends about 30 basepairs through a guanine -rich element in the hairpin stem. Between this element and the nick site there are five thymidine residues included in the loop, and the site has a region to its side containing many alternating adenine and thymine residues, which likely increases flexibility. The creation of the loop likely allows the terminus to assume a specific 3-dimensional structure required to activate the nickase since origins that fail to reconfigure into a double-helical loop once HMG1 is added are not nicked. [ 18 ]
Following nicking, a replication fork is established at the newly exposed 3′ nucleotide that proceeds to unfold and copy the right-end hairpin through a series of melting and reannealing reactions. [ 9 ] [ 18 ] This process begins once NS1 nicks the inboard end of the original hairpin. The terminal sequence is then copied in the opposite direction, which produces an inverted copy of the original sequence. [ 9 ] The end result is a duplex extended-form terminus that contains two copies of the terminal sequence. [ 18 ] While NS1 is required for this, it is unclear if unfolding is mediated by its helicase activity in front of the fork or by destabilization of the duplex following DNA binding at one of its 5′-(ACCA) n -3′ recognition sites. [ 6 ] This process is usually called terminal resolution but also hairpin transfer or hairpin resolution. [ 6 ] [ 9 ] Terminal resolution occurs with each round of replication, so progeny genomes contain an equal number of each terminal orientation. The two orientations are termed "flip" and "flop", [ 5 ] [ 6 ] and may be represented as R and r, or B and b, for the flip and flop of the right-end telomere and L and l, or A and a, for the flip and flop of the left-end telomere. [ 7 ] [ 19 ] Since parvoviral terminal palindromes are imperfect, it is easy to identify which orientation is which. [ 1 ]
The extended-form duplex telomeres generated during terminal resolution are melted, mediated by NS1 with ATP hydrolysis , causing individual strands to fold back on themselves to create hairpin "rabbit ear" structures that have the flip and flop of the termini. This requires the NS1 helicase activity as well as its site-specific binding activity, the latter of which enables NS1 to bind to symmetrical copies of NS1-binding sites that surround the axis of the extended-form terminus. [ 10 ] [ 20 ]
Rabbit ear formation allows the 3′ nucleotide of the newly synthesized DNA strand to pair with an internal base, which repositions the replication fork in a strand-switching maneuver that primes synthesis of additional linear sequences. [ 10 ] Switching from DNA synthesis to rabbit-ear formation at the end of terminal resolution may require different types of NS1 complexes. Alternatively, the NS1 complex may remain intact during this switch, being ready to start stand displacement synthesis following refolding into rabbit ears. [ 20 ] After the replication fork is repositioned, replication continues toward the left end, using the newly synthesized DNA strand as a template. [ 7 ]
At the left end of the genome, NS1 is probably required to unfold the hairpin. NS1 appears to be directly involved in melting-out and reconfiguring the resulting extended-form left-end duplexes into rabbit ear structures, though this reaction seems to be less efficient than at the right-end terminus. Dimeric and tetrameric concatemers of the genome are generated successively for MVM. In these concatemers, alternating unit-length genomes are fused through a palindromic junction in left-end to left-end and right-end to right-end orientations. [ 1 ] [ 10 ] In total, RHR results in coding sequences of the genome being copied twice as often as the termini. [ 1 ] [ 7 ] [ 10 ] Both linear and hairpin configurations of the right-end telomere support initiation of RHR, so resolution of duplex right-end to right-end junctions can occur symmetrically on the basepaired duplex sequence or after this complex is melted and reconfigured into two hairpins. It is unclear which of these two reactions is more common since both appear to produce identical results. [ 20 ]
For AAV, each telomere is 125 bases in length and capable folding into a T-shaped hairpin. AAV contains a Rep gene that encodes for four Rep proteins, two of which, Rep68 and Rep78, act as replication initiator proteins and fulfill the same functions, such the nickase and helicase activities, as NS1. They recognize and bind to a (GAGC) 3 sequence in the stem region of the terminus and nick a site 20 bases away termed trs . The same process of terminal resolution as MVM is done for AAV, but at both ends. The other two Rep proteins, Rep52 and Rep40, are not involved in DNA replication but are implicated in synthesis of progeny. AAV replication is dependent on a helper virus that is either an adenovirus or a herpesvirus that coinfects the cell. In the absence of coinfection, the AAV genome is integrated into the host cell's DNA until coinfection occurs. [ 1 ]
A general rule is that parvoviruses with identical termini, i.e. homotelomeric parvoviruses such as AAV and B19, replicate both ends by terminal resolution, generating equal numbers of flips and flops of each telomere. [ 1 ] [ 4 ] [ 6 ] Parvoviruses that have different termini, i.e. heterotelomeric parvoviruses like MVM, replicate one end by terminal resolution and the other end by asymmetric junction resolution, which conserves a single-sequence orientation and requires different structural arrangements and cofactors to activate NS1's nickase. [ 4 ] [ 10 ] AAV DNA intermediates containing covalently linked sense and antisense strands yield genomic concatemers under denaturing conditions, indicating that AAV replication also synthesizes duplex concatemers that require some form of junction resolution. [ 10 ]
In negative-sense MVM genomes, the left-end hairpin is 121 nucleotides in length and exists in a single flip sequence orientation. This telomere is Y-shaped and contains small internal palindromes that fold into the "ears" of the Y, a duplex stem region 43 nucleotides in length that is interrupted by an asymmetric thymidine residue, and a mismatched "bubble" sequence in which the 5′-GAA-3′ sequence on the inboard arm is opposite of 5′-GA-3′ in the outboard strand. [ 1 ] [ 20 ] Sequences in this hairpin are involved in both replication and regulation of transcription. The elements involved in these two functions separate the two arms of the hairpin. [ 20 ]
The left-end telomere of MVM, and likely of all heterotelomeric parvoviruses, cannot function as a replication origin in its hairpin configuration. Instead, a single origin on the lower strand is created when the hairpin is unfolded, extended, and copied to form a duplex basepaired sequence that spans adjacent genomes in the dimer RF. Within this structure, the sequence from the outboard arm that surrounds a GA/TC [ 1 ] dinucleotide serves as an origin, OriL TC . The equivalent GAA/TTC sequence on the inboard arm that contains the bubble trinucleotide, called OriL GAA , does not serve as an origin. The inboard arm and hairpin configuration of the terminus instead appear to function as upstream control elements for the viral transcriptional promoter P4. Additionally, the ability to segregate one arm from nicking appears essential for replication. [ 20 ]
The minimal linear left-end origin is about 50 basepairs long and extends from two 5′-ACGT-3′ motifs, spaced five nucleotides apart at one end, to a position seven basepairs beyond the nick site. The bubble's GA sequence itself is relatively unimportant, but the space that it occupies is necessary for the origin to function. [ 1 ] [ 20 ] Within the origin, there are three recognition sequences: an NS1-binding site that orients the NS1 complex over the nick site 5′-CTWWTCA-3′, which is located 17 nucleotides downstream (toward the 3′-end), and the two ACGT motifs. These motifs bind a heterodimeric cellular factor called either parvovirus initiation factor (PIF) or glucocorticoid modulating element-binding protein (GMEB). [ 21 ]
PIF is a site-specific DNA-binding heterodimeric complex that contains two subunits, p96 and p79, and functions as a transcription modulator in the host cell. It binds DNA via a KDWK fold and recognizes two ACGT half-sites. The spacing between these sites can vary significantly for PIF, from one to nine nucleotides, with an optimal spacing of six. PIF stabilizes the binding of NS1 on the active form of the left-end origin, OriL TC , but not on the inactive form, OriL GAA , because the two complexes are able to establish contact over the bubble binucleotide. The left-end hairpin of all other species in the Protoparvovirus genus, [ note 6 ] of which MVM belongs, have bubble asymmetries and PIF-binding sites, though with slight variation in spacing. This suggests that they all share a similar origin segregation mechanism. [ 21 ]
Due to the location of the active origin OriL TC in the dimer junction, synthesis of new copies of the left-end hairpin in the correct, i.e.flip, orientation is not straightforward since a replication fork moving from this site through the linear bridge structure should synthesize new DNA in the flop orientation. Instead, the left-hand MVM dimer junction is resolved asymmetrically in a process that creates a cruciform intermediate. This maneuver accomplishes two things: it allows synthesis of the new DNA in the correct sequence orientation, and it creates a structure that can be resolved by NS1. This "heterocruciform" model of synthesis suggests that resolution is driven by the NS1 helicase activity and depends on the inherent instability of the duplex palindrome, a property that allows it to switch between its linear and cruciform configurations. [ 21 ]
NS1 initially introduces a single-strand nick in OriL TC in the B ("right") arm of the junction and becomes covalently attached to the DNA on the 5′ side of the nick, exposing a basepaired 3′ nucleotide. Two outcomes can then occur, depending on the speed with which a replication fork is assembled. If assembly is rapid, then while the junction is in its linear configuration, "read-through" synthesis copies the upper strand, which regenerates the duplex junction and displaces a positive-sense strand that feeds back into the replicative pool. This promotes MVM DNA amplification but does not lead to synthesis of new terminal sequences in the correct orientation or to junction resolution. [ 22 ]
To create a resolvable structure, the initial nicking must be followed by melting and rearrangement of the dimer junction into a cruciform. This is driven by the 3′-to-5′ helicase activity of the 5′-linked NS1 complex. Once this cruciform extends to include sequences beyond the nick site, the exposed primer at the nick site in OriL TC undergoes template switching by annealing with its complement in the lower arm of the cruciform. If a fork assembles after this point, then the subsequent synthesis unfolds and copies the lower cruciform arm. This creates a heterocruciform intermediate that contains the newly synthesized telomere in the flip sequence orientation that is attached to the lower strand of the B arm. [ 22 ] This modified junction is called MJ2. [ 23 ]
The lower arm of MJ2 is an extended-form duplex palindrome that is essentially identical to those generated during terminal resolution. Once MJ2 is synthesized, the lower arm becomes susceptible to rabbit-ear formation. This repositions the 3′ nucleotide of the newly synthesized copy of the lower arm so that it pairs with inboard sequences on the junction's B arm to prime strand displacement synthesis. If a replication fork is created at this 3′ nucleotide, then the lower strand of the B arm is copied, creating an intermediate junction called MJ1 and progressively displacing the upper strand. This leads to the release of the newly synthesized B turn-around (B-ta) sequence. The residual cruciform, called δJ, is partially single-stranded at the upper part of the B arm and contains the intact upper strand of the junction paired to the lower strand of the A ("left") arm, with an intact copy of the left-end hairpin, ending in a 5′ NS1 complex. Since δJ carries the NS1 helicase, it is presumed to periodically alter configuration. [ 22 ] [ 23 ]
The next step is less certain but can be inferred based on what is known about the process thus far. The NS1 helicase is expected to create a dynamic structure in which the nick site in δJ in the normally inactive A side is temporarily but repeatedly exposed in a single-stranded form during duplex-to-hairpin rearrangements, which allows NS1 to engage the nick site in the origin OriL GAA without the help of a cofactor. The nick would leave NS1 covalently attached to the positive-sense "B" strand of δJ and lead to the release of this strand. Nicking also leaves open a basepaired 3′ nucleotide on the "A" strand of δJ to prime DNA synthesis. If a replication fork is established here, then the A strand is unfolded and copied to create its duplex extended form. [ 23 ]
When MVM genomes replicate in vivo , the aforementioned nick may not occur because both ends of the dimer replicative form contain an efficient number of right-end hairpin origins. Therefore, replication forks may progress back toward the dimer junction from the genome's right end, copying the top strand of the B arm before the final resolution nick. This bypasses dimer bridge resolution and recycles the top strand into a replicating duplex dimer pool. In a closely related virus, LuIII, the single-strand nick releases a positive-sense strand with its left-end hairpin in the flop orientation. Unlike MVM, LuIII packages strands of both sense with equal frequency. In the negative-sense strands, the left-end hairpins are all in the flip orientation, while in the positive-sense strands, there are an equal number of flip and flop orientations. Compared to MVM, LuIII contains a two-base insertion immediately 3′ of the nick site in the right origin, which impairs its efficiency. Because of this, the reduced efficiency of replication fork assembly in the genome's right end may favor single-strand nicking by giving it more time to occur. [ 23 ]
Individual progeny genomes are excised from genomic replicative concatemers starting by introducing breaks in replication origins, usually by the replication initiator protein. This results in the establishment of new replication forks that replicate the telomeres in a combination of terminal resolution and junction resolution and displaces individual ssDNA genomes from the replicative molecule. [ 7 ] [ 20 ] At the end of this process, the telomeres are folded back inwards to form hairpins on excised genomes. The extended-form termini created during excision resemble the extended-form molecules prior to terminal resolution, so they can be melted out and refolded into rabbit ears for additional rounds of replication. [ 1 ] Within an infected cell, numerous replicative concatemers are therefore able to arise. [ 7 ]
Displacement of progeny ssDNA genomes either occurs: predominantly or exclusively during active DNA replication, or when cells are assembling viral particles. Displacement of single strands may therefore be associated with packaging viral DNA into capsids. Earlier research suggested that the preassembled viral particle may sequester the genome in a 5′-to-3′ direction as it is displaced from the fork, but more recent research suggests that packaging is performed in a 3′-to-5′ direction driven by the NS1 helicase using newly synthesized single strands. [ 24 ]
It is not clear if these single strands are released into the nucleoplasm so that packaging complexes are physically separate from replication complexes or if the replication intermediates serve as both replication and packaging substrates. In the latter case, newly displaced progeny genomes would be kept in the replication complex via interactions between their 5′-linked NS1 molecules and NS1 or capsid proteins that are physically associated with replicating DNA. [ 24 ] Genomes are inserted into the capsid via an entrance called a portal situated at one of the icosahedral 5-fold axes of the capsid, [ 4 ] which is possibly opposite of the opening from which genomes are expelled early in the replication cycle. [ 5 ]
Strand selection for encapsidation likely does not involve specific packaging signals but may be predictable by the Kinetic Hairpin Transfer (KHT) mathematical model, which explains the distribution of the strands and terminal conformations of packaged genomes in terms of the efficiency with which each terminus type can undergo reactions that allow it to be copied and reformed. In other words, the KHT model postulates that the relative efficiency with which two genomic termini are resolved and replicated determines the distribution of amplified replication intermediates created during infection and ultimately the efficiency with which ssDNAs of characteristic polarity and terminal orientations are excised, which will then be packaged with equal efficiency. [ 4 ] [ 24 ]
Preferential excision of particular genomes is only apparent during packaging. Therefore, among parvoviruses that package strands of one sense, replication appears to be biphasic. At early times, both sense strands are excised. This is followed by a switch in the replication mode that allows for exclusive synthesis of a single sense for packaging. A modified form of the KHT model, called the preferential strand displacement model, proposes that the aforementioned switch in replication is caused by the onset of packaging because the substrate for packaging is probably a newly displaced DNA molecule. [ 24 ] For heterotelomeric parvoviruses, imbalance of origin firing leads to preferential displacement of negative sense strands from the right-end origin. The relative frequency of sense strands in packaged virions can therefore be used to infer the type of resolution mechanism used during excision. [ 5 ]
Shortly after the start of S-phase, translation of viral mRNA leads to the accumulation of capsid proteins in the nucleus. These proteins form into oligomers that are assembled into intact empty capsids. After encapsidation, complete virions may be exported from the nucleus to the exterior of the cell before disintegration of the nucleus. Disruption of the host cell environment may also occur later on in infection. This results in cell lysis via necrosis or apoptosis , which releases virions to the outside of the cell. [ 4 ] [ 17 ]
Many small replicons that have circular genomes such as circular ssDNA viruses and circular plasmids replicate via rolling circle replication (RCR), which is a unidirectional, strand displacement form of DNA replication similar to RHR. In RCR, successive rounds of replication, which proceeds in a loop around the genome, are initiated and terminated by site-specific single-strand nicks made by a replicon-encoded endonuclease, variously called the nickase, relaxase, mobilization protein (mob), transesterase, or replication protein (Rep). The replication initiator protein of parvoviruses is genetically related to these other endonucleases. [ 17 ]
RCR initiator proteins contain three motifs considered to be important for replication. Two of these are retained within parvovirus initiator proteins: an HUHUUU cluster, which is presumed to bind to a Mg 2+ ion required for nicking, and a YxxxK motif that contains the active-site tyrosine residue that attacks the phosphodiester bond of target DNA. In contrast to RCR initiator proteins, which can join together DNA strands, RHR initiator proteins have only vestigial traces of being able to perform ligation. [ 17 ]
RCR begins when the initiator protein nicks a DNA strand at a specific sequence in the replication origin region. This is done through a transesterification reaction that forms a 5′-phosphate bond that connects the DNA to the active-site tyrosine and frees the 3′-end hydroxyl (3′-OH) adjacent to the nick site. The 3′-end is then used as a primer for the host DNA polymerase to begin replication while the initiator protein remains attached to the 5′-end of the "original" strand. After one loop of replication around the circular genome, the initiator protein returns to the nick site, i.e. the original initiator complex, while still attached to the parent strand and attacks the regenerated duplex nick site, or a nearby second site in some cases, by means of a topoisomerase -like nicking-joining reaction. [ 17 ]
During the aforementioned reaction, the initiator protein cleaves a new nick site and is transferred across the analogous phosphodiester bond. It thereby becomes attached to the new 5′-end while ligating the 5′-end of the first strand to which it was originally attached to the 3′-end of the same strand. This second mechanism varies depending on the replicon. Some replicons such as the virus ΦX174 contain a second active tyrosine residue in the initiator protein. Others use the analogous active-site tyrosine in a second initiator protein that is present as part of a multimeric nickase complex. [ 17 ]
This second nicking reaction may occur after one loop or successive loops may occur in which a concatemer containing multiple copies of the genome is created. The result of this nick is that displaced genomes become detached from the replicative molecule. These copies of the genome are ligated and may either be encapsidated into progeny capsids, provided they are monomeric, or converted to a covalently-closed double-stranded form by a host DNA polymerase for further replication. While RHR generally involves replication of both sense strands in a continuous process, RCR has complementary strand synthesis and genomic strand synthesis occur separately. [ 7 ]
The strategies used in RHR to engage the nick site are also present in RCR. Most RCR origins are in the form of duplex DNA that has to be melted before nicking. RCR initiators accomplish this by binding to specific DNA-binding sequences in the origin next to the initiation site. [ 17 ] The latter site is then melted in a process that consumes ATP and which is assisted by the ability of the separated strands to reconfigure into stem-loop structures. In these structures, the nick site is presented on an exposed loop. Like RHR initiator proteins, many RCR initiator proteins contain helicase activity, which allows them to melt the DNA prior to nicking and serve as the 3′-to-5′ helicase in the replication fork. [ 19 ] | https://en.wikipedia.org/wiki/Terminal_resolution |
Terminal restriction fragment length polymorphism ( TRFLP or sometimes T-RFLP ) is a molecular biology technique for profiling of microbial communities based on the position of a restriction site closest to a labelled end of an amplified gene. The method is based on digesting a mixture of PCR amplified variants of a single gene using one or more restriction enzymes and detecting the size of each of the individual resulting terminal fragments using a DNA sequencer . The result is a graph image where the x-axis represents the sizes of the fragment and the y-axis represents their fluorescence intensity.
TRFLP is one of several molecular methods aimed to generate a fingerprint of an unknown microbial community. Other similar methods include DGGE, TGGE , ARISA , ARDRA , PLFA , etc. These relatively high throughput methods were developed in order to reduce the cost and effort in analyzing microbial communities using a clone library . The method was first described by Avaniss-Aghajani et al in 1994 [ 1 ] and later by Liu in 1997 [ 2 ] which employed the amplification of the 16S rDNA target gene from the DNA of several isolated bacteria as well as environmental samples. Since then the method has been applied for the use of other marker genes such as the functional marker gene pmoA to analyze methanotrophic communities.
Like most other community analysis methods, TRFLP is also based on PCR amplification of a target gene. In the case of TRFLP, the amplification is performed with one or both the primers having their 5’ end labeled with a fluorescent molecule. In case both primers are labeled, different fluorescent dyes are required. While several common fluorescent dyes can be used for the purpose of tagging such as 6-carboxyfluorescein (6-FAM), ROX, carboxytetramethylrhodamine (TAMRA, a rhodamine -based dye), and hexachlorofluorescein (HEX), the most widely used dye is 6-FAM. The mixture of amplicons is then subjected to a restriction reaction, normally using a four-cutter restriction enzyme . Following the restriction reaction, the mixture of fragments is separated using either capillary or polyacrylamide electrophoresis in a DNA sequencer and the sizes of the different terminal fragments are determined by the fluorescence detector. Because the excised mixture of amplicons is analyzed in a sequencer, only the terminal fragments (i.e. the labeled end or ends of the amplicon) are read while all other fragments are ignored. Thus, T-RFLP is different from ARDRA and RFLP in which all restriction fragments are visualized. In addition to these steps the TRFLP protocol often includes a cleanup of the PCR products prior to the restriction and in case a capillary electrophoresis is used a desalting stage is also performed prior to running the sample.
The result of a T-RFLP profiling is a graph called electropherogram which is an intensity plot representation of an electrophoresis experiment (gel or capillary). In an electropherogram the X-axis marks the sizes of the fragments while the Y-axis marks the fluorescence intensity of each fragment. Thus, what appears on an electrophoresis gel as a band appears as a peak on the electropherogram whose integral is its total fluorescence. In a T–RFLP profile each peak assumingly corresponds to one genetic variant in the original sample while its height or area corresponds to its relative abundance in the specific community. Both assumptions listed above, however, are not always met. Often, several different bacteria in a population might give a single peak on the electropherogram due to the presence of a restriction site for the particular restriction enzyme used in the experiment at the same position. To overcome this problem and to increase the resolving power of this technique a single sample can be digested in parallel by several enzymes (often three) resulting in three T-RFLP profiles per sample each resolving some variants while missing others. Another modification which is sometimes used is to fluorescently label the reverse primer as well using a different dye, again resulting in two parallel profiles per sample each resolving a different number of variants.
In addition to convergence of two distinct genetic variants into a single peak artifacts might also appear, mainly in the form of false peaks. False peaks are generally of two types: background “noises” and “pseudo” TRFs. [ 3 ] Background (noise) peaks are peaks resulting from the sensitivity of the detector in use. These peaks are often small in their intensity and usually form a problem in case the total intensity of the profile is low (i.e. low concentration of DNA). Because these peaks result from background noise they are normally irreproducible in replicate profiles, thus the problem can be tackled by producing a consensus profile from several replicates or by eliminating peaks below a certain threshold. Several other computational techniques were also introduced in order to deal with this problem. [ 4 ] Pseudo TRFs, on the other hand, are reproducible peaks and are linear to the amount of DNA loaded. These peaks are thought to be the result of ssDNA annealing on to itself and creating double stranded random restriction sites which are later recognized by the restriction enzyme resulting in a terminal fragment which does not represent any genuine genetic variant. It has been suggested that applying a DNA exonuclease such as the Mung bean exonuclease prior to the digestion stage might eliminate such artifact.
The data resulting from the electropherogram is normally interpreted in one of the following ways.
In pattern comparison the general shapes of electropherograms of different samples are compared for changes such as presence-absence of peaks between treatments, their relative size, etc.
If a clone library is constructed in parallel to the T-RFLP analysis then the clones can be used to assess and interpret the T-RFLP profile. In this method the TRF of each clone is determined either directly (i.e. performing T-RFLP analysis on each single clone) or by in silico analysis of that clone’s sequence. By comparing the T-RFLP profile to a clone library it is possible to validate each of the peaks as genuine as well as to assess the relative abundance of each variant in the library.
Several computer applications attempt to relate the peaks in an electropherogram to specific bacteria in a database. Normally this type of analysis is done by simultaneously resolving several profiles of a single sample obtained with different restriction enzymes. The software then resolves the profile by attempting to maximize the matches between the peaks in the profiles and the entries in the database so that the number of peaks left without a matching sequence is minimal. The software withdraws from the database only those sequences which have their TRFs in all analyzed profiles.
A recently growing way to analyze T-RFLP profiles is use multivariate statistical methods to interpret the T-RFLP data. [ 5 ] Usually the methods applied are those commonly used in ecology and especially in the study of biodiversity. Among them ordinations and cluster analysis are the most widely used.
In order to perform multivariate statistical analysis on T-RFLP data, the data must first be converted to table known as a “sample by species table“ which depicts the different samples (T-RFLP profiles) versus the species (T-RFS) with the height or area of the peaks as values.
As T-RFLP is a fingerprinting technique its advantages and drawbacks are often discussed in comparison with other similar techniques, mostly DGGE.
The major advantage of T-RFLP is the use of an automated sequencer which gives highly reproducible results for repeated samples. Although the genetic profiles are not completely reproducible and several minor peaks which appear are irreproducible the overall shape of the electropherogram and the ratios of the major peaks are considered reproducible. The use of an automated sequencer which outputs the results in a digital numerical format also enables an easy way to store the data and compare different samples and experiments. The numerical format of the data can and has been used for relative (though not absolute) quantification and statistical analysis. Although sequence data cannot be definitively inferred directly from the T-RFLP profile, ‘’in-silico’’ assignment of the peaks to existing sequences is possible to a certain extent.
Because T-RFLP relies on DNA extraction methods and PCR, the biases inherent to both will affect the results of the analysis. [ 6 ] [ 7 ] Also, the fact that only the terminal fragments are being read means that any two distinct sequences which share a terminal restriction site will result in one peak only on the electropherogram and will be indistinguishable. Indeed, when T-RFLP is applied on a complex microbial community the result is often a compression of the total diversity to normally 20-50 distinct peaks only representing each an unknown number of distinct sequences. Although this phenomenon makes the T-RFLP results easier to handle, it naturally introduces biases and oversimplification of the real diversity. Attempts to minimize (but not overcome) this problem are often done by applying several restriction enzymes and/ or labeling both primers with a different fluorescent dye. The inability to retrieve sequences from T-RFLP often leads to the need to construct and analyze one or more clone libraries in parallel to the T-RFLP analysis which adds to the effort and complicates analysis. The possible appearance of false (pseudo) T-RFs, as discussed above, is yet another drawback. To handle this researchers often only consider peaks which can be affiliated to sequences in a clone library. | https://en.wikipedia.org/wiki/Terminal_restriction_fragment_length_polymorphism |
Terminal velocity is the maximum speed attainable by an object as it falls through a fluid ( air is the most common example). It is reached when the sum of the drag force ( F d ) and the buoyancy is equal to the downward force of gravity ( F G ) acting on the object. Since the net force on the object is zero, the object has zero acceleration . [ 1 ] [ 2 ] For objects falling through air at normal pressure, the buoyant force is usually dismissed and not taken into account, as its effects are negligible.
As the speed of an object increases, so does the drag force acting on it, which also depends on the substance it is passing through (for example air or water). At some speed, the drag or force of resistance will be equal to the gravitational pull on the object. At this point the object stops accelerating and continues falling at a constant speed called the terminal velocity (also called settling velocity ).
An object moving downward faster than the terminal velocity (for example because it was thrown downwards, it fell from a thinner part of the atmosphere, or it changed shape) will slow down until it reaches the terminal velocity. Drag depends on the projected area , here represented by the object's cross-section or silhouette in a horizontal plane.
An object with a large projected area relative to its mass, such as a parachute, has a lower terminal velocity than one with a small projected area relative to its mass, such as a dart. In general, for the same shape and material, the terminal velocity of an object increases with size. This is because the downward force (weight) is proportional to the cube of the linear dimension, but the air resistance is approximately proportional to the cross-section area which increases only as the square of the linear dimension.
For very small objects such as dust and mist, the terminal velocity is easily overcome by convection currents which can prevent them from reaching the ground at all, and hence they can stay suspended in the air for indefinite periods. Air pollution and fog are examples.
Based on air resistance, for example, the terminal speed of a skydiver in a belly-to-earth (i.e., face down) free fall position is about 55 m/s (180 ft/s). [ 3 ] This speed is the asymptotic limiting value of the speed, and the forces acting on the body balance each other more and more closely as the terminal speed is approached. In this example, a speed of 50.0% of terminal speed is reached after only about 3 seconds, while it takes 8 seconds to reach 90%, 15 seconds to reach 99%, and so on.
Higher speeds can be attained if the skydiver pulls in their limbs (see also freeflying ). [ 3 ] In this case, the terminal speed increases to about 90 m/s (300 ft/s), [ citation needed ] which is almost the terminal speed of the peregrine falcon diving down on its prey. [ 4 ] The same terminal speed is reached for a typical .30-06 bullet dropping downwards—when it is returning to the ground having been fired upwards or dropped from a tower—according to a 1920 U.S. Army Ordnance study. [ 5 ]
Competition speed skydivers fly in a head-down position and can reach speeds of 150 m/s (490 ft/s). [ citation needed ] The current record is held by Felix Baumgartner who jumped from an altitude of 38,887 m (127,582 ft) and reached 380 m/s (1,200 ft/s), though he achieved this speed at high altitude where the density of the air is much lower than at the Earth's surface, producing a correspondingly lower drag force. [ 6 ]
The biologist J. B. S. Haldane wrote,
To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal's length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force. [ 7 ]
For terminal velocity in falling through air , where viscosity is negligible compared to the drag force, and without considering buoyancy effects, terminal velocity is given by V t = 2 m g ρ A C d {\displaystyle V_{t}={\sqrt {\frac {2mg}{\rho AC_{d}}}}} where
In reality, an object approaches its terminal speed asymptotically .
Buoyancy effects, due to the upward force on the object by the surrounding fluid, can be taken into account using Archimedes' principle : the mass m {\displaystyle m} has to be reduced by the displaced fluid mass ρ V {\displaystyle \rho V} , with V {\displaystyle V} the volume of the object. So instead of m {\displaystyle m} use the reduced mass m r = m − ρ V {\displaystyle m_{r}=m-\rho V} in this and subsequent formulas.
The terminal speed of an object changes due to the properties of the fluid, the mass of the object and its projected cross-sectional surface area .
Air density increases with decreasing altitude, at about 1% per 80 metres (260 ft) (see barometric formula ). For objects falling through the atmosphere, for every 160 metres (520 ft) of fall, the terminal speed decreases 1%. After reaching the local terminal velocity, while continuing the fall, speed decreases to change with the local terminal speed.
Using mathematical terms, defining down to be positive, the net force acting on an object falling near the surface of Earth is (according to the drag equation ):
F net = m a = m g − 1 2 ρ v 2 A C d , {\displaystyle F_{\text{net}}=ma=mg-{\frac {1}{2}}\rho v^{2}AC_{d},}
with v ( t ) the velocity of the object as a function of time t .
At equilibrium , the net force is zero ( F net = 0) [ 9 ] and the velocity becomes the terminal velocity lim t →∞ v ( t ) = V t :
m g − 1 2 ρ V t 2 A C d = 0. {\displaystyle mg-{1 \over 2}\rho V_{t}^{2}AC_{d}=0.}
Solving for V t yields:
The drag equation is—assuming ρ , g and C d to be constants: m a = m d v d t = m g − 1 2 ρ v 2 A C d . {\displaystyle ma=m{\frac {\mathrm {d} v}{\mathrm {d} t}}=mg-{\frac {1}{2}}\rho v^{2}AC_{d}.}
Although this is a Riccati equation that can be solved by reduction to a second-order linear differential equation, it is easier to separate variables .
A more practical form of this equation can be obtained by making the substitution α 2 = ρAC d / 2 mg .
Dividing both sides by m gives d v d t = g ( 1 − α 2 v 2 ) . {\displaystyle {\frac {\mathrm {d} v}{\mathrm {d} t}}=g\left(1-\alpha ^{2}v^{2}\right).}
The equation can be re-arranged into d t = d v g ( 1 − α 2 v 2 ) . {\displaystyle \mathrm {d} t={\frac {\mathrm {d} v}{g(1-\alpha ^{2}v^{2})}}.}
Taking the integral of both sides yields ∫ 0 t d t ′ = 1 g ∫ 0 v d v ′ 1 − α 2 v ′ 2 . {\displaystyle \int _{0}^{t}{\mathrm {d} t'}={1 \over g}\int _{0}^{v}{\frac {\mathrm {d} v'}{1-\alpha ^{2}v^{\prime 2}}}.}
After integration, this becomes t − 0 = 1 g [ ln ( 1 + α v ′ ) 2 α − ln ( 1 − α v ′ ) 2 α + C ] v ′ = 0 v ′ = v = 1 g [ ln 1 + α v ′ 1 − α v ′ 2 α + C ] v ′ = 0 v ′ = v {\displaystyle t-0={1 \over g}\left[{\ln(1+\alpha v') \over 2\alpha }-{\frac {\ln(1-\alpha v')}{2\alpha }}+C\right]_{v'=0}^{v'=v}={1 \over g}\left[{\ln {\frac {1+\alpha v'}{1-\alpha v'}} \over 2\alpha }+C\right]_{v'=0}^{v'=v}}
or in a simpler form t = 1 2 α g ln 1 + α v 1 − α v = a r t a n h ( α v ) α g , {\displaystyle t={1 \over 2\alpha g}\ln {\frac {1+\alpha v}{1-\alpha v}}={\frac {\mathrm {artanh} (\alpha v)}{\alpha g}},} with artanh the inverse hyperbolic tangent function.
Alternatively, 1 α tanh ( α g t ) = v , {\displaystyle {\frac {1}{\alpha }}\tanh(\alpha gt)=v,} with tanh the hyperbolic tangent function. Assuming that g is positive (which it was defined to be), and substituting α back in, the speed v becomes v = 2 m g ρ A C d tanh ( t g ρ A C d 2 m ) . {\displaystyle v={\sqrt {\frac {2mg}{\rho AC_{d}}}}\tanh \left(t{\sqrt {\frac {g\rho AC_{d}}{2m}}}\right).}
Using the formula for terminal velocity V t = 2 m g ρ A C d {\displaystyle V_{t}={\sqrt {\frac {2mg}{\rho AC_{d}}}}} the equation can be rewritten as v = V t tanh ( t g V t ) . {\displaystyle v=V_{t}\tanh \left(t{\frac {g}{V_{t}}}\right).}
As time tends to infinity ( t → ∞), the hyperbolic tangent tends to 1, resulting in the terminal speed V t = lim t → ∞ v ( t ) = 2 m g ρ A C d . {\displaystyle V_{t}=\lim _{t\to \infty }v(t)={\sqrt {\frac {2mg}{\rho AC_{d}}}}.}
For very slow motion of the fluid, the inertia forces of the fluid are negligible (assumption of massless fluid) in comparison to other forces. Such flows are called creeping or Stokes flows and the condition to be satisfied for the flows to be creeping flows is the Reynolds number , R e ≪ 1 {\displaystyle Re\ll 1} . The equation of motion for creeping flow (simplified Navier–Stokes equation ) is given by:
∇ p = μ ∇ 2 v {\displaystyle {\mathbf {\nabla } }p=\mu \nabla ^{2}{\mathbf {v} }}
where:
The analytical solution for the creeping flow around a sphere was first given by Stokes in 1851. [ 10 ] From Stokes' solution, the drag force acting on the sphere of diameter d {\displaystyle d} can be obtained as
where the Reynolds number, R e = ρ d μ V {\displaystyle Re={\frac {\rho d}{\mu }}V} . The expression for the drag force given by equation ( 6 ) is called Stokes' law .
When the value of C d {\displaystyle C_{d}} is substituted in the equation ( 5 ), we obtain the expression for terminal speed of a spherical object moving under creeping flow conditions: [ 11 ]
V t = g d 2 18 μ ( ρ s − ρ ) , {\displaystyle V_{t}={\frac {gd^{2}}{18\mu }}\left(\rho _{s}-\rho \right),} where ρ s {\displaystyle \rho _{s}} is the density of the object.
The creeping flow results can be applied in order to study the settling of sediments near the ocean bottom and the fall of moisture drops in the atmosphere. The principle is also applied in the falling sphere viscometer , an experimental device used to measure the viscosity of highly viscous fluids, for example oil, paraffin, tar etc.
When the buoyancy effects are taken into account, an object falling through a fluid under its own weight can reach a terminal velocity (settling velocity) if the net force acting on the object becomes zero. When the terminal velocity is reached the weight of the object is exactly balanced by the upward buoyancy force and drag force. That is
where
If the falling object is spherical in shape, the expression for the three forces are given below:
where
Substitution of equations ( 2 – 4 ) in equation ( 1 ) and solving for terminal velocity, V t {\displaystyle V_{t}} to yield the following expression
In equation ( 1 ), it is assumed that the object is denser than the fluid. If not, the sign of the drag force should be made negative since the object will be moving upwards, against gravity. Examples are bubbles formed at the bottom of a champagne glass and helium balloons. The terminal velocity in such cases will have a negative value, corresponding to the rate of rising up. | https://en.wikipedia.org/wiki/Terminal_velocity |
The terminal web is a filamentous structure found at the apical surface of epithelial cells that possess microvilli . It is composed primarily of actin filaments stabilized by spectrin , which also anchors the terminal web to the apical cell membrane. The presence of myosin II and tropomyosin helps to explain the contractile ability of the terminal web. When contracted, the terminal web causes a decrease in diameter of the apex of the cell, causing the microvilli, which are anchored into the terminal web through their stiff actin fibers, to spread apart. This spreading apart of the microvilli aids cells in absorption. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
This anatomy article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terminal_web |
In molecular biology , a termination factor is a protein that mediates the termination of RNA transcription by recognizing a transcription terminator and causing the release of the newly made mRNA . This is part of the process that regulates the transcription of RNA to preserve gene expression integrity and are present in both eukaryotes and prokaryotes , although the process in bacteria is more widely understood. [ 1 ] The most extensively studied and detailed transcriptional termination factor is the Rho (ρ) protein of E. coli . [ 2 ]
Prokaryotes use one type of RNA polymerase, transcribing mRNAs that code for more than one type of protein. Transcription, translation and mRNA degradation all happen simultaneously. Transcription termination is essential to define boundaries in transcriptional units, a function necessary to maintain the integrity of the strands and provide quality control. Termination in E. coli may be Rho dependent, utilizing Rho factor, or Rho independent, also known as intrinsic termination . Although most operons in DNA are Rho independent, Rho dependent termination is also essential to maintain correct transcription. [ 1 ]
ρ factor The Rho protein is an RNA translocase that recognizes a cytosine -rich region of the elongating mRNA, but the exact features of the recognized sequences and how the cleaving takes place remain unknown. Rho forms a ring-shaped hexamer and advances along the mRNA, hydrolyzing ATP toward RNA polymerase (5' to 3' with respect to the mRNA). [ 3 ] [ 4 ] When the Rho protein reaches the RNA polymerase complex, transcription is terminated by dissociation of the RNA polymerase from the DNA . The structure and activity of the Rho protein is similar to that of the F 1 subunit of ATP synthase , supporting the theory that the two share an evolutionary link. [ 4 ]
Rho factor is widely present in different bacterial sequences and is responsible for the genetic polarity in E. coli. It works as a sensor of translational status, inhibiting non-productive transcriptions, [ 5 ] suppressing antisense transcriptions and resolving conflicts that happen between transcription and replication. [ 6 ] The process of termination by Rho factor is regulated by attenuation and antitermination mechanisms, competing with elongation factors for overlapping utilization sites ( ruts and nut s), and depends on how fast Rho can move during the transcription to catch up with the RNA polymerase and activate the termination process. [ 7 ]
Inhibition of Rho dependent termination by bicyclomycin is used to treat bacterial infections. The use of this mechanism along with other classes of antibiotics is being studied as a way to address antibiotic resistance, by suppressing the protective factors in RNA transcription while working in synergy with other inhibitors of gene expression such as tetracycline or rifampicin . [ 8 ]
The process of transcriptional termination is less understood in eukaryotes, which have extensive post-transcriptional RNA processing, and each of the three types of eukaryotic RNA polymerase have a different termination system.
In RNA polymerase I , Transcription termination factor, RNA polymerase I binds downstream of the pre-rRNA coding regions, causing the dissociation of the RNA polymerase from the template and the release of the new RNA strand.
In RNA polymerase II , the termination occurs via a polyadenylation/cleaving complex. The 3' tail on the ending of the strand is bound at the polyadenylation site, but the strand will continue to code. The newly synthesised ribonucleotides are removed one at a time by the cleavage factors CSTF and CPSF , in a process that is still not fully understood. The remainder of the strand is disengaged by a 5′-exonuclease when the transcription is finished.
RNA polymerase III terminates after a series of uracil polymerization residues in the transcribed mRNA. [ 1 ] Unlike in bacteria and in polymerase I, the termination RNA hairpin needs to be upstream to allow for correct cleaving. [ 9 ] | https://en.wikipedia.org/wiki/Termination_factor |
In computer science , termination analysis is program analysis which attempts to determine whether the evaluation of a given program halts for each input. This means to determine whether the input program computes a total function.
It is closely related to the halting problem , which is to determine whether a given program halts for a given input and which is undecidable . The termination analysis is even more difficult than the halting problem: the termination analysis in the model of Turing machines as the model of programs implementing computable functions would have the goal of deciding whether a given Turing machine is a total Turing machine , and this problem is at level Π 2 0 {\displaystyle \Pi _{2}^{0}} of the arithmetical hierarchy and thus is strictly more difficult than the halting problem.
Now as the question whether a computable function is total is not semi-decidable , [ 1 ] each sound termination analyzer (i.e. an affirmative answer is never given for a non-terminating program) is incomplete , i.e. must fail in determining termination for infinitely many terminating programs, either by running forever or halting with an indefinite answer.
A termination proof is a type of mathematical proof that plays a critical role in formal verification because total correctness of an algorithm depends on termination.
A simple, general method for constructing termination proofs involves associating a measure with each step of an algorithm. The measure is taken from the domain of a well-founded relation , such as from the ordinal numbers . If the measure "decreases" according to the relation along every possible step of the algorithm, it must terminate, because there are no infinite descending chains with respect to a well-founded relation.
Some types of termination analysis can automatically generate or imply the existence of a termination proof.
An example of a programming language construct which may or may not terminate is a loop , as they can be run repeatedly. Loops implemented using a counter variable as typically found in data processing algorithms will usually terminate, demonstrated by the pseudocode example below:
If the value of SIZE_OF_DATA is non-negative, fixed and finite, the loop will eventually terminate, assuming process_data terminates too.
Some loops can be shown to always terminate or never terminate through human inspection. For example, the following loop will, in theory, never stop. However, it may halt when executed on a physical machine due to arithmetic overflow : either leading to an exception or causing the counter to wrap to a negative value and enabling the loop condition to be fulfilled.
In termination analysis one may also try to determine the termination behaviour of some program depending on some unknown input. The following example illustrates this problem.
Here the loop condition is defined using some value UNKNOWN, where the value of UNKNOWN is not known (e.g. defined by the user's input when the program is executed). Here the termination analysis must take into account all possible values of UNKNOWN and find out that in the possible case of UNKNOWN = 0 (as in the original example) the termination cannot be shown.
There is, however, no general procedure for determining whether an expression involving looping instructions will halt, even when humans are tasked with the inspection. The theoretical reason for this is the undecidability of the halting problem: there cannot exist some algorithm which determines whether any given program stops after finitely many computation steps.
In practice one fails to show termination (or non-termination) because every algorithm works with a finite set of methods being able to extract relevant information out of a given program. A method might look at how variables change with respect to some loop condition (possibly showing termination for that loop), other methods might try to transform the program's calculation to some mathematical construct and work on that, possibly getting information about the termination behaviour out of some properties of this mathematical model. But because each method is only able to "see" some specific reasons for (non)termination, even through combination of such methods one cannot cover all possible reasons for (non)termination. [ citation needed ]
Recursive functions and loops are equivalent in expression; any expression involving loops can be written using recursion, and vice versa. Thus the termination of recursive expressions is also undecidable in general. Most recursive expressions found in common usage (i.e. not pathological ) can be shown to terminate through various means, usually depending on the definition of the expression itself. As an example, the function argument in the recursive expression for the factorial function below will always decrease by 1; by the well-ordering property of natural numbers , the argument will eventually reach 1 and the recursion will terminate.
Termination check is very important in dependently typed programming language and theorem proving systems like Coq and Agda . These systems use Curry-Howard isomorphism between programs and proofs. Proofs over inductively defined data types were traditionally described using induction principles. However, it was found later that describing a program via a recursively defined function with pattern matching is a more natural way of proving than using induction principles directly. Unfortunately, allowing non-terminating definitions leads to logical inconsistency in type theories [ citation needed ] , which is why Agda and Coq have termination checkers built-in.
One of the approaches to termination checking in dependently typed programming languages are sized types. The main idea is to annotate the types over which we can recurse with size annotations and allow recursive calls only on smaller arguments. Sized types are implemented in Agda as a syntactic extension.
There are several research teams that work on new methods that can show (non)termination. Many researchers include these methods into programs [ 2 ] that try to analyze the termination behavior automatically (so without human interaction). An ongoing aspect of research is to allow the existing methods to be used to analyze termination behavior of programs written in "real world" programming languages. For declarative languages like Haskell , Mercury and Prolog , many results exist [ 3 ] [ 4 ] [ 5 ] (mainly because of the strong mathematical background of these languages). The research community also works on new methods to analyze termination behavior of programs written in imperative languages like C and Java.
Research papers on automated program termination analysis include:
System descriptions of automated termination analysis tools include: | https://en.wikipedia.org/wiki/Termination_proof |
In genetics , a transcription terminator is a section of nucleic acid sequence that marks the end of a gene or operon in genomic DNA during transcription . This sequence mediates transcriptional termination by providing signals in the newly synthesized transcript RNA that trigger processes which release the transcript RNA from the transcriptional complex . These processes include the direct interaction of the mRNA secondary structure with the complex and/or the indirect activities of recruited termination factors . Release of the transcriptional complex frees RNA polymerase and related transcriptional machinery to begin transcription of new mRNAs.
Two classes of transcription terminators, Rho-dependent and Rho-independent, have been identified throughout prokaryotic genomes. These widely distributed sequences are responsible for triggering the end of transcription upon normal completion of gene or operon transcription, mediating early termination of transcripts as a means of regulation such as that observed in transcriptional attenuation , and to ensure the termination of runaway transcriptional complexes that manage to escape earlier terminators by chance, which prevents unnecessary energy expenditure for the cell.
Rho-dependent transcription terminators require a large protein called a Rho factor which exhibits RNA helicase activity to disrupt the mRNA-DNA-RNA polymerase transcriptional complex. Rho-dependent terminators are found in bacteria and phages . The Rho-dependent terminator occurs downstream of translational stop codons and consists of an unstructured, cytosine-rich sequence on the mRNA known as a Rho utilization site ( rut ), [ 1 ] and a downstream transcription stop point ( tsp ). The rut serves as a mRNA loading site and as an activator for Rho; activation enables Rho to efficiently hydrolyze ATP and translocate down the mRNA while it maintains contact with the rut site. Rho is able to catch up with the RNA polymerase because it is being stalled at the downstream tsp sites. Multiple different sequences can function as a tsp site. [ 2 ] Contact between Rho and the RNA polymerase complex stimulates dissociation of the transcriptional complex through a mechanism involving allosteric effects of Rho on RNA polymerase. [ 3 ] [ 4 ]
Intrinsic transcription terminators or Rho-independent terminators require the formation of a self-annealing hairpin structure on the elongating transcript, which results in the disruption of the mRNA-DNA-RNA polymerase ternary complex . The terminator sequence in DNA contains a 20 basepair GC-rich region of dyad symmetry followed by a short poly-A tract or "A stretch" which is transcribed to form the terminating hairpin and a 7–9 nucleotide "U tract" respectively. The mechanism of termination is hypothesized to occur through a combination of direct promotion of dissociation through allosteric effects of hairpin binding interactions with the RNA polymerase and "competitive kinetics". The hairpin formation causes RNA polymerase stalling and destabilization, leading to a greater likelihood that dissociation of the complex will occur at that location due to increased time spent paused at that site and reduced stability of the complex. [ 5 ] [ 6 ] Additionally, the elongation protein factor NusA interacts with the RNA polymerase and the hairpin structure to stimulate transcriptional termination. [ 7 ]
In eukaryotic transcription of mRNAs, terminator signals are recognized by protein factors that are associated with the RNA polymerase II and which trigger the termination process. The genome encodes one or more polyadenylation signals . Once the signals are transcribed into the mRNA, the proteins cleavage and polyadenylation specificity factor (CPSF) and cleavage stimulation factor (CstF) transfer from the carboxyl terminal domain of RNA polymerase II to the poly-A signal. These two factors then recruit other proteins to the site to cleave the transcript, freeing the mRNA from the transcription complex, and add a string of about 200 A-repeats to the 3' end of the mRNA in a process known as polyadenylation . During these processing steps, the RNA polymerase continues to transcribe for several hundred to a few thousand bases and eventually dissociates from the DNA and downstream transcript through an unclear mechanism; there are two basic models for this event known as the torpedo and allosteric models. [ 8 ] [ 9 ]
After the mRNA is completed and cleaved off at the poly-A signal sequence, the left-over (residual) RNA strand remains bound to the DNA template and the RNA polymerase II unit, continuing to be transcribed. After this cleavage, a so-called exonuclease binds to the residual RNA strand and removes the freshly transcribed nucleotides one at a time (also called 'degrading' the RNA), moving towards the bound RNA polymerase II. This exonuclease is XRN2 (5'-3' Exoribonuclease 2) in humans. This model proposes that XRN2 proceeds to degrade the uncapped residual RNA from 5' to 3' until it reaches the RNA pol II unit. This causes the exonuclease to 'push off' the RNA pol II unit as it moves past it, terminating the transcription while also cleaning up the residual RNA strand.
Similar to Rho-dependent termination, XRN2 triggers the dissociation of RNA polymerase II by either pushing the polymerase off of the DNA template or pulling the template out of the RNA polymerase. [ 10 ] The mechanism by which this happens remains unclear, however, and has been challenged not to be the sole cause of the dissociation. [ 11 ]
In order to protect the transcribed mRNA from degradation by the exonuclease, a 5' cap is added to the strand. This is a modified guanine added to the front of mRNA, which prevents the exonuclease from binding and degrading the RNA strand. A 3' poly(A) tail is added to the end of a mRNA strand for protection from other exonucleases as well.
The allosteric model suggests that termination occurs due to the structural change of the RNA polymerase unit after binding to or losing some of its associated proteins, making it detach from the DNA strand after the signal. [ 9 ] This would occur after the RNA pol II unit has transcribed the poly-A signal sequence, which acts as a terminator signal.
RNA polymerase is normally capable of transcribing DNA into single-stranded mRNA efficiently. However, upon transcribing over the poly-A signals on the DNA template, a conformational shift is induced in the RNA polymerase from the proposed loss of associated proteins from its carboxyl terminal domain . This change of conformation reduces RNA polymerase's processivity making the enzyme more prone to dissociating from its DNA-RNA substrate. In this case, termination is not completed by degradation of mRNA but instead is mediated by limiting the elongation efficiency of RNA polymerase and thus increasing the likelihood that the polymerase will dissociate and end its current cycle of transcription. [ 8 ]
The several RNA polymerases in eukaryotes each have their own means of termination. Pol I is stopped by TTF1 (yeast Nsi1), which recognizes a downstream DNA sequence; the endonuclease is XRN2 (yeast Rat1). Pol III is able to terminate transcription on a stretch of As on the template strand. [ 12 ]
Finally, Pol II also have poly(A)-independent modes of termination, which is required when it transcribes snRNA and snoRNA genes. In yeast, the protein Nrd1 is responsible, [ 9 ] along with Nab3 (multiple human homologs including HNRNPC and RALY ) [ 13 ] and Sen1 , collectively making up the "NNS" pathway. [ 14 ]
Some human mechanism, possibly PCF11 , seems to cause premature termination when pol II transcribes HIV genes. [ 15 ] | https://en.wikipedia.org/wiki/Terminator_(genetics) |
The Terminologia Histologica ( TH ) is the controlled vocabulary for use in cytology and histology . [ 1 ] [ 2 ] In April 2011, Terminologia Histologica was published online [ 3 ] by the Federative International Programme on Anatomical Terminologies (FIPAT), the successor of FCAT.
It was intended to replace Nomina Histologica . The Nomina Histologica was introduced in 1977, with the fourth edition of Nomina Anatomica . [ 4 ]
It was developed by the Federative International Committee on Anatomical Terminology . | https://en.wikipedia.org/wiki/Terminologia_Histologica |
Terminology for the Description of Dynamics ( TEDDY ) aims to provide an ontology for dynamical behaviours, observable dynamical phenomena, and control elements of bio-models and biological systems in Systems Biology and Synthetic Biology . [ 1 ] [ 2 ]
This science article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terminology_for_the_Description_of_Dynamics |
Alternative medicine is a term often used to describe medical practices where are untested or untestable . Complementary medicine (CM), complementary and alternative medicine (CAM), integrated medicine or integrative medicine (IM), functional medicine, and holistic medicine are among many rebrandings of the same phenomenon.
The terms alternative medicine , complementary medicine , integrative medicine, holistic medicine , natural medicine , unorthodox medicine , fringe medicine , unconventional medicine , and new age medicine are used interchangeably as having the same meaning and are almost synonymous in most contexts. [ 1 ] [ 2 ] [ 3 ] [ 4 ]
There is concern that a lack of stabilized terminology for these practices may give the appearance of effectiveness. [ 5 ] Loose terminology may also be used to suggest that a dichotomy exists when it does not, e.g., the use of the expressions "Western medicine" and "Eastern medicine" to suggest that the difference is a cultural difference between the Asiatic east and the European west, rather than that the difference is between evidence-based medicine and other forms of treatment. [ 6 ]
Some scholars adopt this "Western" and "Eastern" language. For example, in a study done on musculoskeletal pain acupuncture treatment, researchers use the term “Western Acupuncture”, which is defined as the acupuncture practices that are evidence based. [ 7 ] This term also removes the cultural connotations that are used in acupuncture such as "qi", or "primordial energy". [ 8 ]
Complementary medicine ( CM ) or integrative medicine ( IM ) is when alternative medicine is used together with mainstream medical treatment, in a belief that it improves the effect of treatments. [ n 1 ] [ 10 ] [ 11 ] [ 12 ] [ 13 ] For example, acupuncture (piercing the body with needles to influence the flow of a supernatural energy) might be believed to increase the effectiveness or "complement" science-based medicine when used at the same time. [ 14 ] [ 15 ] [ 16 ] Instead, significant drug interactions caused by alternative therapies may make treatments less effective, notably in cancer therapy . [ 17 ] [ 18 ] Integrative medicine has been described as an attempt to bring pseudoscience into academic science-based medicine . [ 19 ] Due to its many names, the field has been criticized by writer Rose Shapiro for what she describes as intense rebranding of what are essentially the same practices. [ 1 ]
CAM is an abbreviation of the phrase complementary and alternative medicine . [ 20 ] [ 21 ] The 2019 World Health Organization (WHO) Global Report on Traditional and Complementary Medicine states that the terms complementary and alternative medicine "refer to a broad set of health care practices that are not part of that country's own traditional or conventional medicine and are not fully integrated into the dominant health care system. They are used interchangeably with traditional medicine in some countries." [ 22 ]
Traditional medicine comprises medical aspects of traditional knowledge that developed over generations within the folk beliefs of various societies, including indigenous peoples , before the era of modern medicine. The 2019 WHO study defines traditional medicine as "the sum total of the knowledge, skill and practices based on the theories, beliefs and experiences indigenous to different cultures, whether explicable or not, used in the maintenance of health as well as in the prevention, diagnosis, improvement or treatment of physical and mental illness." [ 22 ]
Holistic medicine is another rebranding of alternative medicine. In this case, the words balance and holism are often used alongside complementary or integrative , claiming to take into account a "whole" person, in contrast to the supposed reductionism of medicine. The “whole” person idea is referring to the ‘analysis of physical, nutritional, environmental, emotional, spiritual and lifestyle elements’ as defined by the American Holistic Health Association. [ 23 ] When specific ailments are diagnosed, or health is being analyzed, things like mental health are factors that are also taken into consideration alongside physical health. According to a random national survey in America of US osteopathic physicians, less than a third of the respondents agreed with the fact that holism is a concept in osteopathic medicine that is distinct from allopathic medicine. [ 24 ] This study highlights that osteopathic physicians view that practitioners in the allopathic field also practice holism in their respective practices.
Functional medicine is a marketing term for alternative medicine created by Jeffrey Bland, who founded The Institute for Functional Medicine (IFM). [ 25 ] [ 26 ] [ 27 ]
Alternative medicine is defined loosely as a set of products, practices, and theories that are believed or perceived by their users to have the healing effects of medicine , [ n 2 ] [ n 3 ] but whose effectiveness has not been established using scientific methods , [ n 2 ] [ n 4 ] [ 6 ] [ 30 ] [ 31 ] [ 32 ] or whose theory and practice is not part of biomedicine , [ n 3 ] [ n 5 ] [ n 6 ] [ n 7 ] or whose theories or practices are directly contradicted by scientific evidence or scientific principles used in biomedicine. [ 6 ] [ 30 ] [ 36 ] "Biomedicine" or "medicine" is that part of medical science that applies principles of biology , physiology , molecular biology , biophysics , and other natural sciences to clinical practice , using scientific methods to establish the effectiveness of that practice. Unlike medicine, [ n 5 ] an alternative product or practice does not originate from using scientific methods, but may instead be based on hearsay , religion , tradition, superstition , belief in supernatural energies, pseudoscience , errors in reasoning , propaganda , fraud , or other unscientific sources. [ n 4 ] [ 6 ] [ 10 ] [ 30 ] [ 36 ]
Terminology has shifted over time, reflecting the preferred branding of practitioners. [ 37 ] For example, the United States National Institutes of Health department studying alternative medicine, currently named the National Center for Complementary and Integrative Health (NCCIH), was established as the Office of Alternative Medicine (OAM) and was renamed the National Center for Complementary and Alternative Medicine (NCCAM) before obtaining its current name. Therapies are often framed as "natural" or "holistic", in apparent opposition to conventional medicine which is "artificial" and "narrow in scope", statements which are intentionally misleading. [ 38 ] [ 39 ]
Prominent members of the science [ 40 ] [ 41 ] and biomedical science community [ 29 ] say that it is not meaningful to define an alternative medicine that is separate from a conventional medicine, because the expressions "conventional medicine", "alternative medicine", "complementary medicine", "integrative medicine", and "holistic medicine" do not refer to any medicine at all. [ 29 ] [ 40 ] [ 41 ] [ 42 ] Others say that alternative medicine cannot be precisely defined because of the diversity of theories and practices it includes, and because the boundaries between alternative and conventional medicine overlap, are porous, and change. [ 33 ] [ 43 ] The systems and practices it refers to are diffuse, and its boundaries poorly defined. [ 44 ] [ 45 ] Healthcare practices categorized as alternative may differ in their historical origin, theoretical basis, diagnostic technique , therapeutic practice and in their relationship to the medical mainstream. [ 46 ] Some alternative therapies, such as traditional Chinese medicine (TCM) and Ayurveda , have antique origins in East or South Asia and are entirely alternative medical systems; [ 46 ] : 13 IOM Report 2005 , p. 18 .</ref> others, such as homeopathy and chiropractic, have origins in Europe or the United States and emerged in the eighteenth and nineteenth centuries. [ 47 ] Some, such as osteopathy and chiropractic, employ manipulative physical methods of treatment; others, such as meditation and prayer , are based on mind-body interventions . [ 48 ] Under a definition of alternative medicine as "non-mainstream", treatments considered alternative in one location may be considered conventional in another. [ 49 ]
Critics say the expression is deceptive because it implies there is an effective alternative to science-based medicine, and that complementary is deceptive because it implies that the treatment increases the effectiveness of (complements) science-based medicine, while alternative medicines that have been tested nearly always have no measurable positive effect compared to a placebo . [ 6 ] [ 19 ] [ 50 ] [ 51 ] It has been said that "there is really no such thing as alternative medicine, just medicine that works and medicine that doesn't", [ 41 ] and that the very idea of "alternative" treatments is paradoxical because any treatment proven to work is by definition "medicine." [ 52 ]
Some definitions seek to specify alternative medicine in terms of its social and political marginality to mainstream healthcare. [ 53 ] This can refer to the lack of support that alternative therapies receive from medical scientists regarding access to research funding , sympathetic coverage in the medical press , or inclusion in the standard medical curriculum . [ 53 ] In 1993, the British Medical Association (BMA) stated that it [ n 8 ] referred to "...those forms of treatment which are not widely used by the conventional healthcare professions, and the skills of which are not taught as part of the undergraduate curriculum of conventional medical and paramedical healthcare courses" . [ 54 ] In a US context, a definition coined in 1993 by the Harvard-based physician David M. Eisenberg [ 55 ] [ 56 ] described alternative medicine as "interventions neither taught widely in medical schools nor generally available in US hospitals" . [ 57 ] In a definition published in 2000 by the World Health Organization (WHO), CAM was defined as a broad set of health care practices that are not part of that country's own tradition and are not integrated into the dominant health care system. [ 58 ] [ 59 ] A widely used [ 60 ] descriptive definition devised by the US NCCIH calls it "a group of diverse medical and health care systems, practices, and products that are not generally considered part of conventional medicine" . [ 61 ] However, these descriptive definitions are inadequate in the present-day when some conventional doctors offer alternative medical treatments and introductory courses or modules can be offered as part of standard undergraduate medical training; [ 62 ] alternative medicine is taught in more than half of US medical schools and US health insurers are increasingly willing to provide reimbursement for alternative therapies. [ 63 ] In 1999, 7.7% of US hospitals reported using some form of alternative therapy; this proportion had risen to 37.7% by 2008. [ 64 ] A 15-year systematic review published in 2022 on the global acceptance and use of CAM among medical specialists found the overall acceptance of CAM at 52% and the overall use at 45%. [ 65 ]
An expert panel at a conference hosted in 1995 by the US Office for Alternative Medicine (OAM), [ 66 ] [ n 9 ] devised a theoretical definition [ 66 ] of alternative medicine as "a broad domain of healing resources ... other than those intrinsic to the politically dominant health system of a particular society or culture in a given historical period". [ 68 ] This definition has been widely adopted, [ 66 ] and has been cited by the UK Department of Health, [ 69 ] attributed as the definition used by the Cochrane Collaboration , [ 70 ] and, with some modification, [ dubious – discuss ] was preferred in the 2005 consensus report of the US Institute of Medicine . [ n 3 ] This definition, an expansion of Eisenberg's 1993 formulation, is silent regarding questions of the medical effectiveness of alternative therapies. [ 71 ] Its proponents hold that it thus avoids relativism about differing forms of medical knowledge and, while it is an essentially political definition, this should not imply that the dominance of mainstream medicine is solely due to political forces. [ 71 ] According to this definition, alternative and mainstream medicine can only be differentiated with reference to what is "intrinsic to the politically dominant health system of a particular society of culture". [ 72 ] However, there is neither a reliable method to distinguish between cultures and subcultures , nor to attribute them as dominant or subordinate, nor any accepted criteria to determine the dominance of a cultural entity. [ 72 ] If the culture of a politically dominant healthcare system is held to be equivalent to the perspectives of those charged with the medical management of leading healthcare institutions and programs, the definition fails to recognize the potential for division either within such an elite or between a healthcare elite and the wider population. [ 72 ]
Evidence-based definitions distinguish alternative medicine based on its provision of therapies that are unproven, unvalidated, or ineffective and support of theories with no recognized scientific basis. [ 73 ] These definitions characterize practices as constituting alternative medicine when, used independently or in place of evidence-based medicine , they are put forward as having the healing effects of medicine, but are not based on evidence gathered with the scientific method . [ 10 ] [ 29 ] [ 14 ] [ 15 ] [ 61 ] [ 74 ] Exemplifying this perspective, a 1998 editorial co-authored by Marcia Angell , a former editor of The New England Journal of Medicine , argued that:
It is time for the scientific community to stop giving alternative medicine a free ride. There cannot be two kinds of medicine – conventional and alternative. There is only medicine that has been adequately tested and medicine that has not, medicine that works and medicine that may or may not work. Once a treatment has been tested rigorously, it no longer matters whether it was considered alternative at the outset. If it is found to be reasonably safe and effective, it will be accepted. But assertions, speculation, and testimonials do not substitute for evidence. Alternative treatments should be subjected to scientific testing no less rigorous than that required for conventional treatments. [ 29 ]
This line of division has been subject to criticism on the grounds that not all forms of standard medical practice have adequately demonstrated evidence of benefit, [ n 5 ] [ 75 ] [ 76 ] and that most conventional therapies, if proven to be ineffective, would not later be classified as alternative. [ 66 ] Another definition is that alternative medicine refers to a diverse range of related and unrelated products, practices, and theories ranging from biologically plausible practices and products and practices with some evidence, to practices and theories that are directly contradicted by basic science or clear evidence, and products that have been conclusively proven to be ineffective or even toxic and harmful. [ n 3 ] [ 44 ] [ 77 ] [ 6 ]
Proponents of an evidence-base for medicine [ n 10 ] [ 79 ] [ 80 ] [ 81 ] [ 82 ] such as the Cochrane Collaboration take a position that all systematic reviews of treatments, whether "mainstream" or "alternative", ought to be held to the current standards of scientific method. [ 83 ] In 2011, the Cochrane Collaboration proposed that indicators of a therapy's level of acceptance include government licensing of practitioners, coverage by health insurance , statements of approval by government agencies, and recommendation as part of a practice guideline; and that if something is currently a standard, accepted therapy, then it is not likely to be widely considered as alternative. [ 66 ]
The public information website maintained by the National Health and Medical Research Council (NHMRC) of the Commonwealth of Australia uses the acronym "CAM" for a wide range of health care practices, therapies, procedures and devices not within the domain of conventional medicine. In the Australian context this is stated to include acupuncture; aromatherapy; chiropractic; homeopathy; massage; meditation and relaxation therapies; naturopathy; osteopathy; reflexology, traditional Chinese medicine; and the use of vitamin supplements. [ 84 ]
The Danish National Board of Health's Council for Alternative Medicine (Sundhedsstyrelsens Råd for Alternativ Behandling (SRAB)), an independent institution under the National Board of Health (Danish: Sundhedsstyrelsen ), uses the term "alternative medicine" for:
Allopathic medicine or allopathy is a pejorative term used by proponents of alternative medicine to refer to modern scientific systems of medicine , [ 86 ] such as the use of pharmacologically active agents or physical interventions to treat or suppress symptoms or pathophysiologic processes of diseases or conditions. [ 87 ] [ 88 ] The expression was coined in 1810 by the creator of homeopathy , Samuel Hahnemann (1755–1843). [ 89 ] Among homeopaths and other alternative medicine advocates, the expression "allopathic medicine" is still used to refer to "the broad category of medical practice that is sometimes called Western medicine, biomedicine , evidence-based medicine , or modern medicine." [ 90 ]
Use of the term remains common among homeopaths and has spread to other alternative medicine practices. The meaning implied by the label has never been accepted by conventional medicine and is still considered pejorative by some. [ 91 ] William Jarvis, an expert on alternative medicine and public health, [ 92 ] states that "although many modern therapies can be construed to conform to an allopathic rationale (e.g., using a laxative to relieve constipation), standard medicine has never paid allegiance to an allopathic principle" and that the label "allopath" was "considered highly derisive by regular medicine." [ 93 ]
Many modern science-based medical treatments (antibiotics, vaccines, and chemotherapeutics, for example) do not fit Samuel Hahnemann's definition of allopathy, as they seek to prevent illness, or remove the cause of an illness by acting on the cause of disease. [ 94 ] [ 95 ] | https://en.wikipedia.org/wiki/Terminology_of_alternative_medicine |
A terminology server is a piece of software providing a range of terminology -related software services through an applications programming interface to its client applications .
Typical terminology services might include:
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terminology_server |
Termitophiles are macro-organisms adapted to live in association with termites or their nests. They include vertebrates, invertebrates and fungi and can either be obligate termitophiles (those that cannot live without the termites) or non-obligate termitophiles (those that can live independently and make use of the termite nests facultatively or opportunistically). Termitophiles may spend a just a part or the whole of their lifecycle inside a termite nest. The term termitariophily has been suggested as a term to describe the situation where a foreign organism merely uses the termite nest. [ 1 ]
Termites live in colonies and construct nests whose environments are controlled. The temperature, humidity, and other conditions inside the nests may be more favourable than the outdoor environment for the termitophiles while potentially also making use of the food resources within the nest, including the fungi grown by the colony or the eggs or larvae being reared.
Termitophilous insects avoid the defenses of the termite colony through one or more of a number of adaptations including having a rounded and smooth body, having bristles (often yellow) on their body surface, masking their odor to avoid detection, exuding chemicals from their body that the termites find pleasing, or by appearing like inanimate objects or mimicking termites. [ 2 ]
A number of species of staphylinid beetles are known to be termitophiles. Cretotrichopsenius burmiticus has been described from 99 million year old Burmese amber and shows termitophilous adaptations. [ 3 ] Some like Trichopsenius frosti and Xenistusa hexagonalis are known to follow the trail pheromones of their termite host Reticulitermes virginicus . [ 4 ] Trichopsenius frosti also has a cuticular hydrocarbon profile closely matching that of its host. [ 5 ] Staphylinid termitophiles mostly in the subfamily Aleocharinae curl their abdomen over their body. [ 6 ] The abdomen may also show enlargement of physogastry and in a few species there are protruding appendages that mimic the body structure of a termite. The Australian species Austrospirachtha mimetes and Austrospirachtha carrijoi have abdomen resembling termites. [ 7 ] Similar adaptations are seen in the South American Thyreoxenus alakazam and the African Coatonachthodes ovambolandicus . [ 8 ] [ 9 ]
A subfamily of scarab beetles, the Termitotroginae, are small, blind, and with reduced antennae. The genus Termitotrox (includes Aphodiocopris ) is known from the fungus combs of termites in India and Africa. They are thought to be obligate termitophiles. [ 10 ]
Some flies in the family Phoridae are termitophilous and grow as larvae within the termite nests. Some species have larvae that feed on the fungus comb while others are termite endoparasites or predators. [ 11 ]
Termite nest specific fungi include the Basidiobolus , Antennopsis , and some species of Xylaria . Several species of Termitomyces are grown intentionally as food by termites within their comb. [ 12 ] [ 13 ] | https://en.wikipedia.org/wiki/Termitophile |
Terms of orientation , terms of location , or spatial words are common linguistic descriptors used to indicate the spatial positioning of objects in three-dimensional space , including notions of top , bottom , front , back , left side , and right side as used in everyday language and interactions. Assigning these to objects then allows things to be described in relation to the object, above , below , in front of , behind , beside , and so forth.
Linguist Eve V. Clark notes that "many objects in the world around us have an inherent orientation that we usually take for granted". [ 1 ] One of the first learning tasks that children are presented with is learning the difference between the top and bottom of things, and the front and back of things. [ 1 ] Children tend to first learn to understand the concept of things having a top, as demonstrated by the tendency to initially identify the uppermost surface of a set of shelves as the place to add a new object, ignoring lower shelves. [ 1 ] The orientation assigned to an object can differ depending on the vantage point and intent of the observer:
The reference frame [for an object] can be established in different ways. One way is to use the intrinsic orientation of the reference object. In this case, the regions that are above, below, in front of, behind, to the left of, and to the right of the reference objects are the regions which are adjacent to the top, bottom, front, back, left side, and right side, respectively. If the intrinsic orientation of the reference object is used to establish the reference frame, I am referring to the intrinsic use of the corresponding prepositions. Thus, in intrinsic use, two arguments are needed for a locative description: the primary object and the reference object. [ 2 ]
For objects having a clearly discernible top and bottom, these aspects are determined by gravity , with the surface of the object tending to be closer to gravitational pull being the bottom, and the surface of the object tending to be farther from gravitational pull being the top. [ 3 ] However, these distinction "do not distinguish between intrinsic tops and bottoms and absolute or environmental tops and bottoms", such as when an object has fallen over. For example, "a candle has an intrinsic top and bottom, because its canonical position is upright with certain defining features at each end. Even when a candle has fallen over, we can still talk about its top and bottom". [ 3 ] Where an object does not inherently have such characteristics , the assignment of a top, bottom, and the like, is temporary and contingent. "If the reference object doesn't have an intrinsic orientation, or its intrinsic orientation isn't used for establishing the frame of reference, factors of the situational context determine the reference frame". [ 2 ] [ 4 ]
Terms describing the orientation of objects extend to the positional relationships of those objects relative to other objects, such as above , below , in front of , behind , and beside . The Cambridge Dictionary notes that "we usually use above, but not over, when there is no contact between the things referred to. Over or on top of have a more general meaning, and can be used when one thing touches or covers another". [ 5 ] These universal terms are then easily translated to metaphorical concepts, such as being "on top of things", or of happiness being at the "top" of emotional states and sadness being at the "bottom". [ 6 ]
Various specialized language is used in specific fields to identify, for example, anatomical terms of location (such as "anterior and posterior", "dorsal and ventral", or "proximal and distal") or geometric terms of location (such as "circumferential", "tangential", "parallel", and "orthogonal"). In everyday life , however, people generally use simpler and more universal terms. [ 1 ] [ 2 ] Orientational terms are often relative to the viewer, such that a person facing multiple objects from one vantage point may see one object as being on the right side of another, while a person facing those objects from a different vantage point may see the relationship differently. For some uses, where it is necessary to avoid confusion from differences in viewpoint, separate terminology is used to describe the sides of things. For example, proper right and proper left are conceptual terms used to unambiguously convey relative direction when describing an image or other object. The "proper right" hand of a figure is the hand that would be regarded by that figure as its right hand. [ 7 ]
In stagecraft, blocking is used to designate portions of the stage in an absolute sense, with stage right, stage left, upstage, and downstage being used to refer to the same direction relative to the stage, irrespective of the position of the viewer. [ 8 ] Similarly, port and starboard are nautical terms for watercraft , aircraft and spacecraft , referring respectively to the left and right sides of the vessel, when aboard and facing the bow (front). Port and starboard unambiguously refer to the left and right side of the vessel, not the observer. That is, the port side of the vessel always refers to the same portion of the vessel's structure, and does not depend on which way the observer is facing . The port side is the side to the left of an observer aboard the vessel and facing the bow , towards the direction the vehicle is heading when underway . The starboard side is thus to the right of such an observer. [ 9 ]
The existence of anatomical terms of location is a reflection of the tendency of living things, more than naturally occurring nonliving objects, to have an orientation, described by the concept of body relative direction . Aristotle reasoned that concepts of "front" and "back" were only relevant to animals with the ability to perceive these relative positions. An analysis of Aristotle's writings on the subject summed it up as follows:
The reason why plants do not have their own front and back, Aristotle says, is that they lack sense-perception. Animals... have the dimension of depth, defined by the opposites front and back. Front and back are grounded in sense-perception, and since to be an animal is to have sense-perception, all animals have a front and a back. [ 10 ]
Notably, Aristotle's assertion that "all animals have a front and a back" is not entirely accurate, with some uncharacteristic sea animals such as jellyfish , sea urchins , and starfish lacking these characteristics. With respect to jellyfish, Aristotle denied that they were animals at all. [ 11 ] With respect to the small set of spherical animals, such as sea urchins, it has been noted that "since all radii are alike, a spherical animal can be divided into two similar pieces by a cut in any direction through the center. There is no front or rear, no top or bottom, no right or left sides, no ends—at least no permanent ones". [ 12 ] It is further observed that an animal lacking a distinct front and back has "a disadvantage in directed locomotion", meaning that this form is "most typical of free-floating organisms that do not move under their own power". [ 12 ]
A ping pong ball , like the orange one pictured below, is a uniform sphere , and is therefore a typical example of an object that has no set top , bottom , front , back , or sides ; [ 13 ] it only has these characteristics in a contingent and temporary sense relative to the viewer. The ball, seen from above in the picture, could be described as having its "top" facing the viewer and its "bottom" obscured, or could be described as having its "top" and "bottom" as the uppermost and lowermost points visible to the viewer relative to the screen on which the object is being viewed.
In the images, both the cones of the Korean fir and the man-made traffic cone have a clearly discernible top and bottom , but are not clearly differentiated along other dimensions. A person viewing either kind of cone would be likely to provisionally identify the surface of the cone facing them as "the front", and would further identify an object between themselves and the cone as being "in front of" the fir cone. The mountain has a clearly discernible top and bottom , but the assignment of a front and back would be arbitrary.
In the third image below, a cat and a mouse are sitting on top of a dog , which is lying on its side; all three animals have a clearly discernible top and bottom , and a clearly discernible front , back , and sides . Although the cat and mouse are "on top of the dog", they are not sitting on the part that would be considered the top of the dog. Rather, they are sitting on the side of the dog, which serves as a provisional "top" surface, even though it continues to be understood to be the side of the animal. | https://en.wikipedia.org/wiki/Terms_of_orientation |
A ternary complex is a protein complex containing three different molecules that are bound together. In structural biology , ternary complex can also be used to describe a crystal containing a protein with two small molecules bound, such as a cofactor and a substrate ; or a complex formed between two proteins and a single substrate. [ 1 ] In Immunology , ternary complex can refer to the MHC–peptide–T-cell-receptor complex formed when T cells recognize epitopes of an antigen.
Another important example is the ternary complex formed during eukaryotic translation, in which ternary complex composed of eIF2 + GTP + Met-tRNA i Met is formed. [ 2 ] A ternary complex can be a complex formed between two substrate molecules and an enzyme. This is seen in multi-substrate enzyme-catalyzed reactions where two substrates and two products can be formed. The ternary complex is an intermediate species in this type of enzyme-catalyzed reaction. An example for a ternary complex is seen in the random-order mechanism or the compulsory-order mechanism of enzyme catalysis for multiple substrates. [ 3 ]
The term ternary complex can also refer to a polymer formed by electrostatic interactions. [ 4 ]
Trevor Palmer (Enzymes, 2nd edition)
This protein -related article is a stub . You can help Wikipedia by expanding it .
This biochemistry article is a stub . You can help Wikipedia by expanding it .
This molecular biology article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Ternary_complex |
In inorganic chemistry and materials chemistry , a ternary compound or ternary phase is a chemical compound containing three different elements.
While some ternary compounds are molecular, e.g. chloroform ( HCCl 3 ), more typically ternary phases refer to extended solids. The perovskites are a famous example. [ 1 ]
Binary phases , with only two elements, have lower degrees of complexity than ternary phases. With four elements, quaternary phases are more complex.
The number of isomers of a ternary compound provide a distinction between inorganic and organic chemistry: "In inorganic chemistry one or, at most, only a few compounds composed of any two or three elements were known, whereas in organic chemistry the situation was very different." [ 2 ]
An example is sodium phosphate , Na 3 PO 4 . The sodium ion has a charge of 1+ and the phosphate ion has a charge of 3–. Therefore, three sodium ions are needed to balance the charge of one phosphate ion. Another example of a ternary compound is calcium carbonate , CaCO 3 . In naming and writing the formulae for ternary compounds, rules are similar to binary compounds.
According to Rustum Roy and Olaf Müller, [ 3 ] "the chemistry of the entire mineral world informs us that chemical complexity can easily be accommodated within structural simplicity." The example of zircon is cited, where various metal atoms are replaced in the same crystal structure . "The structural entity ... remains ternary in character and is able to accommodate an enormous range of chemical elements." The great variety of ternary compounds is therefore reduced to relatively few structures: "By dealing with approximately ten ternary structural groupings we can cover the most important structures of science and technology specific to the non-metallics world. It is a remarkable instance of nature's simplexity ." [ 3 ] : 3, 4
Letting A and B represent cations and X an anion, these ternary groupings are organized by stoichiometric types A 2 BX 4 , ABX 4 , and ABX 3 .
A ternary compound of type A 2 BX 4 may be in the class of olivine , the spinel group , or phenakite . Examples include K 2 NiF 4 , β- K 2 SO 4 , and CaFe 2 O 4 .
One of type ABX 4 may be of the class of zircon , scheelite , barite or an ordered silicon dioxide derivative .
In the ABX 3 class of ternary compounds, there are the structures of perovskite (structure) , calcium carbonate , pyroxenes , corundum and hexagonal ABX 2 types. [ 3 ] : figure 1, page 3
Other ternary compounds are described as crystals of types ABX 2 , A 2 B 2 X 7 , ABX 5 , A 2 BX 6 , and A 3 BX 5 .
A particular class of ternary compounds are the ternary semiconductors , particularly within the III-V semiconductor family. In this type of semiconductor, the ternary can be considered to be an alloy of the two binary endpoints. Varying the composition between the endpoints allows both the lattice constant and the energy bandgap to be adjusted to produce the properties desired, for example, in emitting light (for example, as a LED ) or absorbing light (as a photodetector or a photovoltaic cell ). An example would be the semiconductor indium gallium arsenide ( In x Ga 1− x As ), a material with band gap dependent on In/Ga ratio.
Important examples of ternary semiconductors can also be found in other semiconductor families, such as the II-VI family ( e.g. , Mercury cadmium telluride , Hg 1− x Cd x Te ), or the I-II-VI2 family, with examples such as CuInSe 2 .
In organic chemistry , the carbohydrates and carboxylic acids are ternary compounds with carbon, oxygen, and hydrogen. Other organic ternary compounds replace oxygen with another atom to form functional groups .
The multiplicity of ternary compounds based on {C, H, O} has been noted. For example, C 9 H 10 O 3 {\displaystyle {\ce {C9 H10 O3}}} corresponds to more than 60 ternary compounds. [ 4 ] [ 2 ] | https://en.wikipedia.org/wiki/Ternary_compound |
In mathematics , a ternary equivalence relation is a kind of ternary relation analogous to a binary equivalence relation . A ternary equivalence relation is symmetric, reflexive, and transitive, where those terms are meant in the sense defined below. The classic example is the relation of collinearity among three points in Euclidean space . In an abstract set, a ternary equivalence relation determines a collection of equivalence classes or pencils that form a linear space in the sense of incidence geometry . In the same way, a binary equivalence relation on a set determines a partition .
A ternary equivalence relation on a set X is a relation E ⊂ X 3 , written [ a , b , c ] , that satisfies the following axioms: | https://en.wikipedia.org/wiki/Ternary_equivalence_relation |
Ternary fission is a comparatively rare (0.2 to 0.4% of events) type of nuclear fission in which three charged products are produced rather than two. As in other nuclear fission processes, other uncharged particles such as multiple neutrons and gamma rays are produced in ternary fission.
Ternary fission may happen during neutron-induced fission or in spontaneous fission (the type of radioactive decay). About 25% more ternary fission happens in spontaneous fission compared to the same fission system formed after thermal neutron capture, [ 1 ] illustrating that these processes remain physically slightly different, even after the absorption of the neutron, possibly because of the extra energy present in the nuclear reaction system of thermal neutron-induced fission.
Quaternary fission, at 1 per 10 million fissions, is also known (see below).
The most common nuclear fission process is "binary fission." It produces two charged asymmetrical fission products with maximally probable charged product at 95±15 and 135±15 u atomic mass. However, in this conventional fission of large nuclei, the binary process happens merely because it is the most energetically probable.
In anywhere from 2 to 4 fissions per 1000 in a nuclear reactor, the alternative ternary fission process produces three positively charged fragments (plus neutrons, which are not charged and not counted in this reckoning). The smallest of the charged products may range from so small a charge and mass as a single proton (Z=1), up to as large a fragment as the nucleus of argon (Z=18).
Although particles as large as argon nuclei may be produced as the smaller (third) charged product in the usual ternary fission, the most common small fragments from ternary fission are helium-4 nuclei, which make up about 90% of the small fragment products. This high incidence is related to the stability (high binding energy) of the alpha particle , which makes more energy available to the reaction. The second-most common particles produced in ternary fission are Tritons (the nuclei of tritium ), which make up 7% of the total small fragments, and the third-most are helium-6 nuclei (which decay in about 0.8 seconds to lithium-6). Protons and larger nuclei are in the small fraction (< 2%) which make up the remainder of the small charged products. The two larger charged particles from ternary fission, particularly when alphas are produced, are quite similar in size distribution to those produced in binary fission.
The energy of the third much-smaller product usually ranges between 10 and 20 MeV. In keeping with their origin, alpha particles produced by ternary fission typically have mean energies of about ~ 16 MeV (energies this great are never seen in alpha decay). Since these typically have significantly more energy than the ~ 5 MeV alpha particles from alpha decay , they are accordingly called " long-range alphas " (referring to their longer range in air or other media).
The other two larger fragments carry away, in their kinetic energies, the remainder of the fission kinetic energy (typically totalling ~ 170 MeV in heavy element fission) that does not appear as the 10 to 20 MeV kinetic energy carried away by the third smaller product. Thus, the larger fragments in ternary fission are each less energetic, by a typical 5 to 10 MeV, than they are seen to be in binary fission.
Although the ternary fission process is less common than the binary process, it still produces significant helium-4 and tritium gas buildup in the fuel rods of modern nuclear reactors. [ 2 ] This phenomenon was initially detected in 1957, within the environs of the Savannah River National Laboratory . [ 3 ]
A very rare type of ternary fission process is sometimes called "true ternary fission." It produces three nearly equal-sized charged fragments (Z ~ 30) but only happens in about 1 in 100 million fission events. In this type of fission, the product nuclei split the fission energy in three nearly equal parts and have kinetic energies of ~ 60 MeV. True ternary fission has so far only been observed in nuclei bombarded by heavy, high energy ions. [ 4 ]
Another rare fission process, occurring in about 1 in 10 million fissions, is Quaternary fission. It is analogous to ternary fission, save that four charged products are seen. Typically two of these are light particles, with the most common mode of Quaternary fission apparently being two large particles and two alpha particles (rather than one alpha, the most common mode of ternary fission). [ 5 ] | https://en.wikipedia.org/wiki/Ternary_fission |
A ternary / ˈ t ɜːr n ər i / numeral system (also called base 3 or trinary [ 1 ] ) has three as its base . Analogous to a bit , a ternary digit is a trit ( tri nary dig it ). One trit is equivalent to log 2 3 (about 1.58496) bits of information .
Although ternary most often refers to a system in which the three digits are all non–negative numbers; specifically 0 , 1 , and 2 , the adjective also lends its name to the balanced ternary system; comprising the digits −1 , 0 and +1, used in comparison logic and ternary computers .
Representations of integer numbers in ternary do not get uncomfortably lengthy as quickly as in binary . For example, decimal 365 (10) or senary 1 405 (6) corresponds to binary 1 0110 1101 (2) (nine bits ) and to ternary 111 112 (3) (six digits). However, they are still far less compact than the corresponding representations in bases such as decimal – see below for a compact way to codify ternary using nonary (base 9) and septemvigesimal (base 27).
As for rational numbers , ternary offers a convenient way to represent 1 / 3 as same as senary (as opposed to its cumbersome representation as an infinite string of recurring digits in decimal); but a major drawback is that, in turn, ternary does not offer a finite representation for 1 / 2 (nor for 1 / 4 , 1 / 8 , etc.), because 2 is not a prime factor of the base; as with base two, one-tenth (decimal 1 / 10 , senary 1 / 14 ) is not representable exactly (that would need e.g. decimal); nor is one-sixth (senary 1 / 10 , decimal 1 / 6 ).
The value of a binary number with n bits that are all 1 is 2 n − 1 .
Similarly, for a number N ( b , d ) with base b and d digits, all of which are the maximal digit value b − 1 , we can write:
Then
For a three-digit ternary number, N (3, 3) = 3 3 − 1 = 26 = 2 × 3 2 + 2 × 3 1 + 2 × 3 0 = 18 + 6 + 2 .
Nonary / ˈ n ɒ n ər i / (base 9, each digit is two ternary digits) or septemvigesimal (base 27, each digit is three ternary digits) can be used for compact representation of ternary, similar to how octal and hexadecimal systems are used in place of binary .
In certain analog logic, the state of the circuit is often expressed ternary. This is most commonly seen in CMOS circuits, and also in transistor–transistor logic with totem-pole output . The output is said to either be low ( grounded ), high, or open ( high- Z ). In this configuration the output of the circuit is actually not connected to any voltage reference at all. Where the signal is usually grounded to a certain reference, or at a certain voltage level, the state is said to be high impedance because it is open and serves its own reference. Thus, the actual voltage level is sometimes unpredictable.
A rare "ternary point" in common use is for defensive statistics in American baseball (usually just for pitchers ), to denote fractional parts of an inning. Since the team on offense is allowed three outs , each out is considered one third of a defensive inning and is denoted as .1 . For example, if a player pitched all of the 4th, 5th and 6th innings, plus achieving 2 outs in the 7th inning, his innings pitched column for that game would be listed as 3.2 , the equivalent of 3 + 2 ⁄ 3 (which is sometimes used as an alternative by some record keepers). In this usage, only the fractional part of the number is written in ternary form. [ 2 ] [ 3 ]
Ternary numbers can be used to convey self-similar structures like the Sierpinski triangle or the Cantor set conveniently. Additionally, it turns out that the ternary representation is useful for defining the Cantor set and related point sets, because of the way the Cantor set is constructed. The Cantor set consists of the points from 0 to 1 that have a ternary expression that does not contain any instance of the digit 1. [ 4 ] [ 5 ] Any terminating expansion in the ternary system is equivalent to the expression that is identical up to the term preceding the last non-zero term followed by the term one less than the last non-zero term of the first expression, followed by an infinite tail of twos. For example: 0.1020 is equivalent to 0.1012222... because the expansions are the same until the "two" of the first expression, the two was decremented in the second expansion, and trailing zeros were replaced with trailing twos in the second expression.
Ternary is the integer base with the lowest radix economy , followed closely by binary and quaternary . This is due to its proximity to the mathematical constant e . It has been used for some computing systems because of this efficiency. It is also used to represent three-option trees , such as phone menu systems, which allow a simple path to any branch.
A form of redundant binary representation called a binary signed-digit number system, a form of signed-digit representation , is sometimes used in low-level software and hardware to accomplish fast addition of integers because it can eliminate carries . [ 6 ]
Simulation of ternary computers using binary computers, or interfacing between ternary and binary computers, can involve use of binary-coded ternary (BCT) numbers, with two or three bits used to encode each trit. [ 7 ] [ 8 ] BCT encoding is analogous to binary-coded decimal (BCD) encoding. If the trit values 0, 1 and 2 are encoded 00, 01 and 10, conversion in either direction between binary-coded ternary and binary can be done in logarithmic time . [ 9 ] A library of C code supporting BCT arithmetic is available. [ 10 ]
Some ternary computers such as the Setun defined a tryte to be six trits [ 11 ] or approximately 9.5 bits (holding more information than the de facto binary byte ). [ 12 ] | https://en.wikipedia.org/wiki/Ternary_numeral_system |
In mathematics , a ternary relation or triadic relation is a finitary relation in which the number of places in the relation is three. Ternary relations may also be referred to as 3-adic , 3-ary , 3-dimensional , or 3-place .
Just as a binary relation is formally defined as a set of pairs , i.e. a subset of the Cartesian product A × B of some sets A and B , so a ternary relation is a set of triples, forming a subset of the Cartesian product A × B × C of three sets A , B and C .
An example of a ternary relation in elementary geometry can be given on triples of points, where a triple is in the relation if the three points are collinear . Another geometric example can be obtained by considering triples consisting of two points and a line, where a triple is in the ternary relation if the two points determine (are incident with) the line.
A function f : A × B → C in two variables, mapping two values from sets A and B , respectively, to a value in C associates to every pair ( a , b ) in A × B an element f ( a , b ) in C . Therefore, its graph consists of pairs of the form (( a , b ), f ( a , b )) . Such pairs in which the first element is itself a pair are often identified with triples. This makes the graph of f a ternary relation between A , B and C , consisting of all triples ( a , b , f ( a , b )) , satisfying a in A , b in B , and f ( a , b ) in C .
Given any set A whose elements are arranged on a circle, one can define a ternary relation R on A , i.e. a subset of A 3 = A × A × A , by stipulating that R ( a , b , c ) holds if and only if the elements a , b and c are pairwise different and when going from a to c in a clockwise direction one passes through b . For example, if A = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 } represents the hours on a clock face , then R (8, 12, 4) holds and R (12, 8, 4) does not hold.
The ordinary congruence of arithmetics
which holds for three integers a , b , and m if and only if m divides a − b , formally may be considered as a ternary relation. However, usually, this instead is considered as a family of binary relations between the a and the b , indexed by the modulus m . For each fixed m , indeed this binary relation has some natural properties, like being an equivalence relation ; while the combined ternary relation in general is not studied as one relation.
A typing relation Γ ⊢ e : σ indicates that e is a term of type σ in context Γ, and is thus a ternary relation between contexts, terms and types.
Given homogeneous relations A , B , and C on a set, a ternary relation ( A , B , C ) can be defined using composition of relations AB and inclusion AB ⊆ C . Within the calculus of relations each relation A has a converse relation A T and a complement relation A . Using these involutions , Augustus De Morgan and Ernst Schröder showed that ( A , B , C ) is equivalent to ( C , B T , A ) and also equivalent to ( A T , C , B ) . The mutual equivalences of these forms, constructed from the ternary relation ( A , B , C ), are called the Schröder rules . [ 1 ] | https://en.wikipedia.org/wiki/Ternary_relation |
" Ternatin " is a term used for two unrelated categories of biochemical compounds: | https://en.wikipedia.org/wiki/Ternatin |
Terpene synthases include:
These synthases' structures may include: | https://en.wikipedia.org/wiki/Terpene_synthase |
Terra Incognita: The Perils and Promise of Stem Cell Research , also known as Terra Incognita: Mapping Stem Cell Research , is a documentary film released by Kartemquin Films in 2007. The film follows Dr. Jack Kessler of Northwestern University in his search for a cure for spinal cord injuries using embryonic stem cells . When Kessler was invited to head up the Neurology Department at Northwestern, his focus was on using stem cells to help cure diabetes. However, soon after his move to Chicago, his daughter Allison – then age 15, was injured in a skiing accident and paralyzed from the waist down. In the moments following the accident, Dr. Kessler made the decision to change the focus of his research to begin looking for a cure for spinal cord injuries using embryonic stem cells.
Kessler's story brings the stem cell debate to the public for discussion. The film follows the constantly evolving interplay between the promise of new discoveries, the controversy of modern science and the resilience and courage of people living every day with devastating disease and injury. [ 1 ]
The film was directed by Maria Finitzo ( 5 Girls ), and was broadcast on PBS' award-winning series Independent Lens in 2008. Terra Incognita won a 2008 Peabody Award recognizing the film's uncompromising look at stem-cell research. [ 2 ] The film also won Best Documentary Feature at the 2009 Kos International Health Film Festival in Greece. [ 3 ] | https://en.wikipedia.org/wiki/Terra_Incognita:_The_Perils_and_Promise_of_Stem_Cell_Research |
In chemistry , the terrace ledge kink ( TLK ) model , which is also referred to as the terrace step kink ( TSK ) model , describes the thermodynamics of crystal surface formation and transformation, as well as the energetics of surface defect formation. It is based upon the idea that the energy of an atom’s position on a crystal surface is determined by its bonding to neighboring atoms and that transitions simply involve the counting of broken and formed bonds. The TLK model can be applied to surface science topics such as crystal growth , surface diffusion , roughening, and vaporization .
The TLK model is credited as having originated from papers published in the 1920s by the German chemist Walther Kossel [ 1 ] and the Bulgarian chemist Ivan Stranski [ 2 ]
Depending on the position of an atom on a surface, it can be referred to by one of several names. Figure 1 illustrates the names for the atomic positions and point defects on a surface for a simple cubic lattice .
Figure 2 shows a scanning tunneling microscopy topographic image of a step edge that shows many of the features in Figure 1 . Figure 3 shows a crystal surface with steps, kinks, adatoms, and vacancies in a closely packed crystalline material, [ 3 ] which resembles the surface featured in Figure 2.
Although intuitively evident, it has only recently been explicitly recognized that the attachment of crystal building units to kink positions plays a pivotal role in perpetuating the crystal's symmetry. At a kink position, the attaching unit does not form all its potential bonds; rather, it forms only half the bonds in each given direction. These bonds are grouped in such a way in order to create a concave structure, which naturally accommodates the incoming building unit. This unique arrangement not only minimizes the system's free energy but also aligns the new unit with the symmetry of the underlying lattice. Consequently, kink positions serve as the primary sites where the crystal's structural order is reproduced and propagated, enabling the transition from microscopic nucleation to a macroscopic, ordered crystal form. This subtle yet fundamental mechanism distinguishes kink-mediated growth from other aggregation processes and underscores its critical role in maintaining the uniformity and symmetry of growing crystals. [ 4 ]
The energy required to remove an atom from the surface depends on the number of bonds to other surface atoms which must be broken. For a simple cubic lattice in this model, each atom is treated as a cube and bonding occurs at each face, giving a coordination number of 6 nearest neighbors. Second-nearest neighbors in this cubic model are those that share an edge and third-nearest neighbors are those that share corners. The number of neighbors, second-nearest neighbors, and third-nearest neighbors for each of the different atom positions are given in Table 1 . [ 5 ]
Most crystals, however, are not arranged in a simple cubic lattice. The same ideas apply for other types of lattices where the coordination number is not six, but these are not as easy to visualize and work with in theory, so the remainder of the discussion will focus on simple cubic lattices. Table 2 indicates the number of neighboring atoms for a bulk atom in some other crystal lattices. [ 5 ]
The kink site is of special importance when evaluating the thermodynamics of a variety of phenomena. This site is also referred to as the “half-crystal position” and energies are evaluated relative to this position for processes such as adsorption , surface diffusion, and sublimation. [ 6 ] The term “half-crystal” comes from the fact that the kink site has half the number of neighboring atoms as an atom in the crystal bulk, regardless of the type of crystal lattice. [ 5 ]
For example, the formation energy for an adatom —ignoring any crystal relaxation—is calculated by subtracting the energy of an adatom from the energy of the kink atom.
This can be understood as the breaking of all of the kink atom’s bonds to remove the atom from the surface and then reforming the adatom interactions. This is equivalent to a kink atom diffusing away from the rest of the step to become a step adatom and then diffusing away from the adjacent step onto the terrace to become an adatom. In the case where all interactions are ignored except for those with nearest neighbors, the formation energy for an adatom would be the following, where ϕ {\displaystyle \phi } is the bond energy in the crystal is given by Equation 2 .
This can be extended to a variety of situations, such as the formation of an adatom-surface vacancy pair on a terrace, which would involve the removal of a surface atom from the crystal and placing it as an adatom on the terrace. This is described by Equation 3 .
The energy of sublimation would simply be the energy required to remove an atom from the kink site. This can be envisioned as the surface being disassembled one terrace at a time by removing atoms from the edge of each step, which is the kink position. It has been demonstrated that the application of an external electric field will induce the formation of additional kinks in a surface, which then leads to a faster rate of evaporation from the surface. [ 7 ]
The number of adatoms present on a surface is temperature dependent. The relationship between the surface adatom concentration and the temperature at equilibrium is described by equation 4, where n 0 is the total number of surface sites per unit area:
This can be extended to find the equilibrium concentration of other types of surface point defects as well. To do so, the energy of the defect in question is simply substituted into the above equation in the place of the energy of adatom formation. | https://en.wikipedia.org/wiki/Terrace_ledge_kink_model |
Terraform is an infrastructure-as-code software tool created by HashiCorp . Users define and provide data center infrastructure using a declarative configuration language known as HashiCorp Configuration Language (HCL), or optionally JSON . [ 3 ]
Terraform manages external resources (such as public cloud infrastructure, private cloud infrastructure, network appliances, software as a service , and platform as a service ) with "providers". HashiCorp maintains an extensive list of official providers, and can also integrate with community-developed providers. [ 4 ] Users can interact with Terraform providers by declaring resources [ 5 ] or by calling data sources. [ 6 ] Rather than using imperative commands to provision resources, Terraform uses declarative configuration to describe the desired final state. Once a user invokes Terraform on a given resource, Terraform will perform CRUD actions on the user's behalf to accomplish the desired state. [ 7 ] The infrastructure as code can be written as modules, promoting reusability and maintainability. [ 8 ]
Terraform supports a number of cloud infrastructure providers such as Amazon Web Services , Cloudflare , [ 9 ] Microsoft Azure , IBM Cloud , Serverspace, Selectel [ 10 ] Google Cloud Platform , [ 11 ] DigitalOcean , [ 12 ] Oracle Cloud Infrastructure , Yandex.Cloud , [ 13 ] VMware vSphere , and OpenStack . [ 14 ] [ 15 ] [ 16 ] [ 17 ] [ 18 ]
HashiCorp launched the Terraform Module Registry in 2017. [ 19 ] In 2019, the paid version Terraform Enterprise was introduced. [ 20 ]
Terraform was previously free software available under version 2.0 of the Mozilla Public License (MPL). On August 10, 2023, HashiCorp announced that all products produced by the company would be relicensed under the Business Source License (BUSL), with HashiCorp prohibiting commercial use of the community edition by those who offer "competitive services". [ 21 ]
OpenTofu was created as a fork resulting from the HashiCorp changing the Terraform license to a BSL . [ 22 ] | https://en.wikipedia.org/wiki/Terraform_(software) |
Terraforming or terraformation ("Earth-shaping") is the hypothetical process of deliberately modifying the atmosphere , temperature , surface topography or ecology of a planet , moon , or other body to be similar to the environment of Earth to make it habitable for humans to live on.
The concept of terraforming developed from both science fiction and actual science . Carl Sagan , an astronomer , proposed the planetary engineering of Venus in 1961, which is considered one of the first accounts of the concept. [ 1 ] The term was coined by Jack Williamson in a science-fiction short story (" Collision Orbit ") published in 1942 in Astounding Science Fiction . [ 2 ]
Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. While Venus and the Moon have been studied in relation to the subject, Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods for the terraforming of Mars may be within humanity's technological capabilities, but according to Martin Beech, the economic attitude of preferring short-term profits over long-term investments will not support a terraforming project. [ 3 ]
The long timescales and practicality of terraforming are also the subject of debate. As the subject has gained traction, research has expanded to other possibilities including biological terraforming, para-terraforming, and modifying humans to better suit the environments of planets and moons . Despite this, questions still remain in areas relating to the ethics , logistics , economics , politics , and methodology of altering the environment of an extraterrestrial world, presenting issues to the implementation of the concept.
The astronomer Carl Sagan proposed the planetary engineering of Venus in an article published in the journal Science in 1961. [ 1 ] Sagan imagined seeding the atmosphere of Venus with algae , which would convert water, nitrogen and carbon dioxide into organic compounds . As this process removed carbon dioxide from the atmosphere, the greenhouse effect would be reduced until surface temperatures dropped to "comfortable" levels. The resulting plant matter, Sagan proposed, would be pyrolyzed by the high surface temperatures of Venus, and thus be sequestered in the form of "graphite or some involatile form of carbon" on the planet's surface. [ 4 ] However, later discoveries about the conditions on Venus made this particular approach impossible. One problem is that the clouds of Venus are composed of a highly concentrated sulfuric acid solution. Even if atmospheric algae could thrive in the hostile environment of Venus's upper atmosphere, an even more insurmountable problem is that its atmosphere is simply far too thick: the high atmospheric pressure would result in a "atmosphere of nearly pure molecular oxygen" [ 4 ] at high pressure. This volatile combination could not be sustained through time. Any carbon that had been reduced by photosynthesis would be quickly oxidized in this atmosphere through combustion, "short-circuiting" the terraforming process. [ 4 ]
Sagan also visualized making Mars habitable for human life in an article published in the journal Icarus , "Planetary Engineering on Mars" (1973). [ 5 ] Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term "planetary ecosynthesis" instead. [ 6 ] The study concluded that it was possible for Mars to support life and be made into a habitable planet . The first conference session on terraforming, then referred to as "Planetary Modeling", was organized that same year.
In March 1979, NASA engineer and author James Oberg organized the First Terraforming Colloquium, a special session at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his book New Earths (1981). [ 7 ] Not until 1982 was the word terraforming used in the title of a published journal article. Planetologist Christopher McKay wrote "Terraforming Mars", a paper for the Journal of the British Interplanetary Society . [ 8 ] The paper discussed the prospects of a self-regulating Martian biosphere, and the word "terraforming" has since become the preferred term. [ citation needed ]
In 1984, James Lovelock and Michael Allaby published The Greening of Mars . [ 9 ] Lovelock's book was one of the first to describe a novel method of warming Mars, where chlorofluorocarbons (CFCs) are added to the atmosphere to produce a strong greenhouse effect.
Motivated by Lovelock's book, biophysicist Robert Haynes worked behind the scenes [ citation needed ] to promote terraforming, and contributed the neologism Ecopoiesis , [ 10 ] forming the word from the Greek οἶκος , oikos , "house", [ 11 ] and ποίησις , poiesis , "production". [ 12 ] Ecopoiesis refers to the origin of an ecosystem . In the context of space exploration, Haynes describes ecopoiesis as the "fabrication of a sustainable ecosystem on a currently lifeless, sterile planet". Fogg defines ecopoiesis as a type of planetary engineering and is one of the first stages of terraformation. This primary stage of ecosystem creation is usually restricted to the initial seeding of microbial life. [ 13 ] A 2019 opinion piece by Lopez, Peixoto and Rosado has reintroduced microbiology as a necessary component of any possible colonization strategy based on the principles of microbial symbiosis and their beneficial ecosystem services . [ 14 ] As conditions approach that of Earth, plant life could be brought in, and this will accelerate the production of oxygen, theoretically making the planet eventually able to support animal life.
In 1985, Martyn Fogg started publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1992. In his book Terraforming: Engineering Planetary Environments (1995), Fogg proposed the following definitions for different aspects related to terraforming: [ 13 ]
Fogg also devised definitions for candidate planets of varying degrees of human compatibility: [ 15 ]
Fogg suggests that Mars was a biologically compatible planet in its youth, but is not now in any of these three categories, because it can only be terraformed with greater difficulty. [ 16 ]
Planetary habitability, broadly defined as the capacity for an astronomical body to sustain life, requires that various geophysical , geochemical , and astrophysical criteria must be met before the surface of such a body is considered habitable. Modifying a planetary surface such that it is able to sustain life, particularly for humans, is generally the end-goal of the hypothetical process of terraforming. Of particular interest in the context of terraforming is the set of factors that have sustained complex, multicellular animals in addition to simpler organisms on Earth. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology .
Classifications of the criteria of habitability can be varied, but it is generally agreed upon that the presence of water, non-extreme temperatures, and an energy source put broad constraints on habitability. [ 18 ] Other requirements for habitability have been defined as the presence of raw materials, a solvent, and clement conditions, [ 19 ] or elemental requirements (such as carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur), and reasonable physiochemical conditions. [ 20 ] When applied to organisms present on the earth, including humans, these constraints can substantially narrow.
In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules , and energy sources to sustain metabolism ." [ 21 ]
The general temperature range for all life on Earth is -20 °C to 122 °C, [ 18 ] set primarily by the ability of water (possibly saline, or under high pressure in the ocean bottom) to be available in liquid form. This may constitute a bounding range for the development of life on other planets, in the context of terraforming. For Earth, the temperature is set by the equilibrium of incident solar radiation absorbed and outgoing infrared radiation, including the effect of greenhouse gasses in modifying the planetary equilibrium temperature ; terraforming concepts may include modifying temperature by methods including solar reflectors to increase or decrease the amount of solar illumination, and hence modify temperature.
All known life requires water; [ 19 ] thus the capacity for planetary body to sustain water is a critical aspect of habitability. The habitable zone of a solar system is generally defined as the region in which stable surface liquid water may be present on a planetary body. [ 19 ] [ 22 ] The boundaries of the habitable zone were originally defined by water loss by photolysis and hydrogen escape, setting a limit on how close a planet may be to its orbited star, and the prevalence of CO 2 clouds that would increase albedo , setting an outer boundary on stable liquid water. [ 22 ] These constraints are applicable in particular to Earth-like planets, and would not as easily apply to moons like Europa and Enceladus with ice-covered oceans, where the energy source to keep the water liquid is from tidal heating , rather than solar energy.
On the most fundamental level, the only absolute requirement of life may be thermodynamic disequilibrium , or the presence of Gibbs free energy . [ 19 ] It has been argued that habitability can be conceived of as a balance between life's demand for energy and the capacity for the environment to provide such energy. [ 19 ] For humans, energy comes in the form of sugars, fats, and proteins provided by consuming plants and animals, necessitating in turn that a habitable planet for humans can sustain such organisms. [ 23 ]
Much of Earth's biomass (~60%) relies on photosynthesis for an energy source, while a further ~40% is chemotropic . [ 18 ] For the development of life on other planetary bodies, chemical energy may have been critical, [ 18 ] while for sustaining life on another planetary body in the Solar System , sufficiently high solar energy may also be necessary for phototrophic organisms.
On Earth, life generally requires six elements in high abundance: carbon , hydrogen , nitrogen , oxygen , phosphorus , and sulfur . [ 20 ] These elements are considered "essential" for all known life and plentiful within biological systems. [ 24 ] Additional elements crucial to life include the cations Mg 2+ , Ca 2+ , K + and Na + and the anion Cl − . [ 24 ] Many of these elements may undergo biologically facilitated oxidation or reduction to produce usable metabolic energy. [ 24 ]
Terraforming a planet would involve making it fit the habitability requirements listed in the previous section. For example, a planet may be too cold for liquid water to exist on its surface. Its temperature could be raised by adding greenhouse gases to the atmosphere, [ 25 ] using orbiting mirrors to reflect more sunlight onto the planet, [ 26 ] or lowering the albedo of the planet. [ 5 ] Conversely, a planet too hot for liquid water could be cooled down by removing greenhouse gases (if these are present), placing a sunshade in the L 1 point to reduce sunlight reaching the planet, or increasing the albedo. [ 27 ] Atmospheric pressure is another issue: various celestial bodies including Mars, Mercury and most moons have lower pressure than Earth. At pressures below the triple point of water (611.7 Pa), water cannot be liquid at any temperature. Human survival requires a still-higher pressure of at least 6.3 kPa, the Armstrong limit ; below this pressure, exposed body fluids boil at body temperature. Furthermore, a thick atmosphere protects the surface from cosmic rays . [ 28 ] A thin atmosphere could be thickened using gases produced locally (e.g. the Moon could be given an atmosphere of oxygen by reducing lunar rock [ 29 ] ) or gases could be imported from elsewhere.
Once conditions become more suitable for life of the introduced species , the importation of microbial life could begin. [ 13 ] As conditions approach that of Earth, plant life could also be brought in. This would accelerate the production of oxygen, which theoretically would make the planet eventually able to support animal life.
In many respects, Mars is the most Earth-like planet in the Solar System. [ 30 ] [ 31 ] It is thought that Mars once had a more Earth-like environment early in its history, with a thicker atmosphere and abundant water that was lost over the course of hundreds of millions of years. [ 32 ]
The exact mechanism of this loss is still unclear, though three mechanisms, in particular, seem likely: First, whenever surface water is present, carbon dioxide ( CO 2 ) reacts with rocks to form carbonates , thus drawing atmosphere off and binding it to the planetary surface. On Earth, this process is counteracted when plate tectonics works to cause volcanic eruptions that vent carbon dioxide back to the atmosphere. On Mars, the lack of such tectonic activity worked to prevent the recycling of gases locked up in sediments. [ 33 ]
Second, the lack of a magnetosphere around Mars may have allowed the solar wind to gradually erode the atmosphere. [ 33 ] [ 34 ] Convection within the core of Mars, which is made mostly of iron , [ 35 ] originally generated a magnetic field . However the dynamo ceased to function long ago, [ 36 ] and the magnetic field of Mars has largely disappeared, probably due to "loss of core heat, solidification of most of the core, and/or changes in the mantle convection regime." [ 37 ] Results from the NASA MAVEN mission show that the atmosphere is removed primarily due to coronal mass ejection events, where outbursts of high-velocity protons from the Sun impact the atmosphere. Mars does still retain a limited magnetosphere that covers approximately 40% of its surface. Rather than uniformly covering and protecting the atmosphere from solar wind, however, the magnetic field takes the form of a collection of smaller, umbrella-shaped fields, mainly clustered together around the planet's southern hemisphere. [ 38 ]
Finally, between approximately 4.1 and 3.8 billion years ago, asteroid impacts during the Late Heavy Bombardment caused significant changes to the surface environment of objects in the Solar System. The low gravity of Mars suggests that these impacts could have ejected much of the Martian atmosphere into deep space. [ 39 ]
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. [ 40 ] A thicker atmosphere of greenhouse gases such as carbon dioxide would trap incoming solar radiation . Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other. [ 41 ] Carbon dioxide alone would not suffice to sustain a temperature above the freezing point of water, so a mixture of specialized greenhouse molecules might be manufactured. [ 42 ]
Terraforming Venus requires two major changes: removing most of the planet's dense 9 MPa (1,300 psi; 89 atm) carbon dioxide atmosphere, and reducing the planet's 450 °C (842 °F) surface temperature. [ 43 ] [ 27 ] These goals are closely interrelated because Venus's extreme temperature may result from the greenhouse effect caused by its dense atmosphere.
Venus's atmosphere currently contains little oxygen, so an additional step would be to inject breathable O 2 into the atmosphere. An early proposal for such a process comes from Carl Sagan , who suggested the injection of floating, photosynthetic bacteria into the Venusian atmosphere to reduce CO 2 to organic form , and increase the atmospheric concentration of O 2 in the atmosphere. [ 1 ] This concept, however, was based in a flawed 1960s understanding of Venus's atmosphere as much lower pressure; in reality, the Venusian atmospheric pressure (9300 kPa) is far higher than early estimates. Sagan's idea is therefore untenable, as he later conceded. [ 44 ]
An additional step noted by Martin Beech includes the injection of water and/or hydrogen into the planetary atmosphere; [ 3 ] this step follows after sequestering CO 2 and reducing the mass of the atmosphere. In order to combine hydrogen with O 2 produced by other means, an estimated 4×10 19 kg of hydrogen is necessary; this may need to be mined from another source, such as Uranus or Neptune. [ 3 ]
Although the gravity on Earth's Moon is too low to hold an atmosphere for geological spans of time, if given one, it would retain it for spans of time that are long compared to human lifespans. [ 45 ] [ 29 ] Landis [ 29 ] and others [ 46 ] [ 47 ] have thus proposed that it could be feasible to terraform the Moon, although not all agree with that proposal. [ 48 ] Landis estimates that a 6.89 kPa atmosphere of pure oxygen on the Moon would require on the order of two hundred trillion tons of oxygen, and suggests it could be produced by reducing the oxygen from an amount of lunar rock equivalent to a cube about fifty kilometers on an edge. Alternatively, he suggests that the water content of "fifty to a hundred comets" the size of Halley's Comet would do the job, "assuming that the water doesn't splash away when the comets hit the moon." [ 29 ] Likewise, Benford calculates that terraforming the moon would require "about 100 comets the size of Halley's." [ 46 ]
Mercury would be difficult to terraform. Beech [ 49 ] states "There seems little prospect of terraforming Mercury such that any animals or plants might exist there," and suggests that its primary use in a terraforming project would be as a mining source for minerals. Nevertheless, terraforming has been considered. [ 50 ] Mercury's magnetic field is only 1.1% that of Earth's, and, being closer to the Sun, any atmosphere would be stripped rapidly unless it can be protected from the solar wind. It is conjectured that Mercury's magnetic field should be much stronger, up to 30% of Earth's, if it weren't being suppressed by certain solar wind feedback effects. [ 51 ] If some means of shielding Mercury from solar wind by placing an artificial magnetic shield at Mercury-Sun L 1 (similar to the proposal for Mars), then Mercury's magnetic field could possibly grow in intensity to a point where Mercury's magnetic field could be self-sustaining provided the field wasn't made to "stall" by another solar event. [ citation needed ]
Despite being much smaller than Mars, Mercury has an escape velocity only slightly less than that of Mars due to its higher density and could, if a magnetosphere prevents atmospheric stripping, hold a nitrogen / oxygen atmosphere for millions of years.
To provide one atmosphere of pressure, roughly 1.1×10 18 kilograms of gas would be required; [ 50 ] or a somewhat lower amount if lower pressure is acceptable. Water could be delivered from the outer Solar System . Once this water has been delivered, it would split the water into its constituent oxygen and hydrogen molecules, possibly using a photo-catalytic dust, with the hydrogen rapidly being lost to space. At an oxygen pressure of 20-30 kPa, the atmosphere would be breathable and nitrogen may be added as required to allow for plant growth in the presence of nitrates .
Temperature management would be required, due to the equilibrium average temperature of ~159°C. However, millions of square kilometers at the poles have an average temperature of 0-50°C ( i.e., an area the size of Mexico at each pole with habitable temperatures). The total habitable area could be even larger if the planetary albedo were increased from 0.12 to ~0.6, potentially increasing the habitable area. Roy proposes that the temperature could be further managed by decreasing the solar flux at Mercury to near the terrestrial value by solar sails reflecting sunlight. He calculates that 16 to 17 million sails, each with an area of one square kilometer would be needed. [ 50 ]
It has been recently proposed [ when? ] that due to the effects of climate change , an interventionist program might be designed to return Earth to pre-industrial climate parameters. In order to achieve this, multiple approaches have been proposed, such as the management of solar radiation , the sequestration of carbon dioxide, and the design and release of climate altering genetically engineered organisms. [ 52 ] [ 53 ] These are typically referred to as geoengineering or climate engineering , rather than terraforming.
Other possible candidates for terraforming (possibly only partial or paraterraforming) include large moons of Jupiter or Saturn ( Europa , Ganymede , Callisto , Enceladus , Titan ), and the dwarf planet Ceres .
The moons are covered in ice, so heating them would make some of this ice sublimate into an atmosphere of water vapour, ammonia and other gases. [ 54 ] [ 55 ] For Jupiter's moons, the intense radiation around Jupiter would cause radiolysis of water vapour, splitting it into hydrogen and oxygen. [ 54 ] The former would be rapidly lost to space, leaving behind the oxygen (this already occurs on the moons to a minor extent, giving them thin atmospheres of oxygen). [ 54 ] For Saturn's moons, the water vapour could be split by using orbital mirrors to focus sunlight, causing photolysis . [ 55 ] The ammonia could be converted to nitrogen by introducing bacteria such as Nitrosomonas , Pseudomonas and Clostridium , resulting in an Earth-like nitrogen-oxygen atmosphere. [ 54 ] [ 55 ] This atmosphere would protect the surface from Jupiter's radiation, [ 28 ] but it would also be possible to clear said radiation using orbiting tethers [ 56 ] or radio waves. [ 57 ]
Challenges to terraforming the moons include their high amounts of ice and their low gravity. [ 54 ] [ 55 ] If all of the ice were fully melted, it would result in deep moon-spanning oceans, meaning any settlements would have to be floating (unless some of the ice was allowed to remain, to serve as land). [ 54 ] [ 55 ] Low gravity would cause atmospheric escape over time and may cause problems for human health . However, atmospheric escape would take place over spans of time that are long compared to human lifespans, as with the Moon. [ 29 ]
One proposal for terraforming Ceres would involve heating it (using orbital mirrors, detonating thermonuclear devices or colliding small asteroids with Ceres), creating an atmosphere and deep ocean. [ 58 ] However, this appears to be based on a misconception that Ceres' surface is icy in a similar way to the gas giant moons. In reality, Ceres' surface is "a layer of mixed ice, silicates and light strong phases best matched by hydrated salts and clathrates". [ 59 ] It is unclear what the result of heating this up would be.
Many proposals for planetary engineering involve the use of genetically engineered bacteria. [ 60 ] [ 61 ]
As synthetic biology matures over the coming decades it may become possible to build designer organisms from scratch that directly manufacture desired products efficiently. [ 62 ] Lisa Nip, Ph.D. candidate at the MIT Media Lab 's Molecular Machines group, said that by synthetic biology, scientists could genetically engineer humans, plants and bacteria to create Earth-like conditions on another planet. [ 63 ] [ 64 ]
Gary King, microbiologist at Louisiana State University studying the most extreme organisms on Earth, notes that "synthetic biology has given us a remarkable toolkit that can be used to manufacture new kinds of organisms specially suited for the systems we want to plan for" and outlines the prospects for terraforming, saying "we'll want to investigate our chosen microbes, find the genes that code for the survival and terraforming properties that we want (like radiation and drought resistance ), and then use that knowledge to genetically engineer specifically Martian-designed microbes". He sees the project's biggest bottleneck in the ability to genetically tweak and tailor the right microbes, estimating that this could take "a decade or more" to be solved. He also notes that it would be best to develop "not a single kind of microbe but a suite of several that work together". [ 65 ]
DARPA is researching the use of photosynthesizing plants, bacteria, and algae grown directly on the Mars surface that could warm up and thicken its atmosphere. In 2015 the agency and some of its research partners created a software called DTA GView , in which genomes of several organisms can be pulled up on the program to immediately show a list of known genes and where they are located in the genome. According to Alicia Jackson, deputy director of DARPA's Biological Technologies Office , they have developed a "technological toolkit to transform not just hostile places here on Earth, but to go into space not just to visit, but to stay". [ 66 ] [ 67 ] [ 68 ] [ 69 ]
Also known as the "world house" concept, para-terraforming involves the construction of a habitable enclosure on a planet that encompasses most of the planet's usable area. [ 70 ] The enclosure would consist of a transparent roof held one or more kilometers above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. The world house concept is similar to the concept of a domed habitat , but one which covers all (or most) of the planet.
Potential targets for paraterraforming include Mercury, the Moon, Ceres and the gas giant moons. [ 71 ]
It has also been suggested that instead of or in addition to terraforming a hostile environment humans might adapt to these places by the use of genetic engineering , biotechnology and cybernetic enhancements . [ 72 ] [ 73 ] [ 74 ] [ 75 ] [ 76 ] This is known as pantropy .
There is a philosophical debate within biology and ecology as to whether terraforming other worlds is an ethical endeavor. From the point of view of a cosmocentric ethic , this involves balancing the need for the preservation of human life against the intrinsic value of existing planetary ecologies. [ 77 ] Lucianne Walkowicz has even called terraforming a "planetary-scale strip mining operation". [ 78 ]
On the pro-terraforming side of the argument, there are those like Robert Zubrin , Martyn J. Fogg , Richard L. S. Taylor, and the late Carl Sagan who believe that it is humanity's moral obligation to make other worlds suitable for human life , as a continuation of the history of life-transforming the environments around it on Earth. [ 79 ] [ 80 ] They also point out that Earth would eventually be destroyed if nature takes its course , so that humanity faces a very long-term choice between terraforming other worlds or allowing all terrestrial life to become extinct . Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life.
The opposing argument posits that terraforming would be an unethical interference in nature , and that given humanity's past treatment of Earth, other planets may be better off without human interference. [ citation needed ] Still others strike a middle ground, such as Christopher McKay , who argues that terraforming is ethically sound only once it is completely certain that an alien planet does not harbor life of its own; but that if it does, it should not try be reshaped to fit humans' own use, but rather to engineer its environment to artificially nurture the alien life and help it thrive and co-evolve, or even co-exist with humans. [ 81 ] Even this would be seen as a type of terraforming to the strictest of ecocentrists, who would say that all life has the right, in its home biosphere, to evolve without outside interference.
The initial cost of such projects as planetary terraforming would be massive, and the infrastructure of such an enterprise would have to be built from scratch. Such technology has not yet been developed, let alone financially feasible at the moment. John Hickman has pointed out that almost none of the current schemes for terraforming incorporate economic strategies , and most of their models and expectations seem highly optimistic. [ 82 ]
Terraforming is a common concept in science fiction , ranging from television , movies and novels to video games . [ 83 ]
A related concept from science fiction is xenoforming – a process in which aliens change the Earth or other planets to suit their own needs, already suggested in the classic The War of the Worlds (1898) of H.G. Wells . [ 84 ] | https://en.wikipedia.org/wiki/Terraforming |
Terraforming is well represented in contemporary literature, usually in the form of science fiction , as well as in popular culture . [ 1 ] [ 2 ] While many stories involving interstellar travel feature planets already suited to habitation by humans and supporting their own indigenous life, some authors prefer to address the unlikeliness of such a concept by instead detailing the means by which humans have converted inhospitable worlds to ones capable of supporting life through artificial means.
Author Jack Williamson is credited with inventing and popularizing the term "terraform". In July 1942, under the pseudonym Will Stewart, Williamson published a science fiction novella entitled " Collision Orbit " in Astounding Science-Fiction magazine. The series was later published as two novels, Seetee Shock (1949) and Seetee Ship (1951). [ 3 ] American geographer Richard Cathcart successfully lobbied for formal recognition of the verb "to terraform", and it was first included in the fourth edition of the Shorter Oxford English Dictionary in 1993. [ 4 ]
The concept of terraforming in popular culture predates Williamson's work; for example, the idea of turning the Moon into a habitable environment with atmosphere was already present in La Journée d'un Parisien au XXI e siècle ("A Day of a Parisian in the 21st Century", 1910) by Octave Béliard [ fr ] . [ 5 ] In fact, perhaps predating the concept of terraforming, is that of xenoforming – a process in which aliens change the Earth to suit their own needs, already suggested in the classic The War of the Worlds (1898) of H.G. Wells . [ 6 ]
Deformable terrain, as used in e.g. Perimeter and Red Faction , is occasionally called terraforming but is not a form of planetary engineering . [ citation needed ] | https://en.wikipedia.org/wiki/Terraforming_in_popular_culture |
The terraforming of Venus or the terraformation of Venus is the hypothetical process of engineering the global environment of the planet Venus in order to make it suitable for human habitation. [ 1 ] [ 2 ] [ 3 ] Adjustments to the existing environment of Venus to support human life would require at least three major changes to the planet's atmosphere: [ 3 ]
These three changes are closely interrelated because Venus's extreme temperature is due to the high pressure of its dense atmosphere and the greenhouse effect .
The most simple proposal is to "veil" the planet from the sun, thus dropping the temperature low enough to condense or solidify carbon dioxide which would then need to be removed or stored in some way.
Poul Anderson , a successful science fiction writer, had proposed the idea in his 1954 novelette "The Big Rain", a story belonging to his Psychotechnic League future history .
The first known suggestion to terraform Venus in a scholarly context was by the astronomer Carl Sagan in 1961. [ 5 ]
Prior to the early 1960s, the atmosphere of Venus was believed by many astronomers to have an Earth-like temperature. When Venus was understood to have a thick carbon dioxide atmosphere with a consequence of a very large greenhouse effect , [ 6 ] some scientists began to contemplate the idea of altering the atmosphere to make the surface more Earth-like. This hypothetical prospect, known as terraforming , was first proposed by Carl Sagan in 1961, as a final section of his classic article in the journal Science discussing the atmosphere and greenhouse effect of Venus. [ 5 ] Sagan proposed injecting photosynthetic bacteria into the Venus atmosphere, which would convert the carbon dioxide into reduced carbon in organic form, thus reducing the carbon dioxide from the atmosphere.
The knowledge of Venus's atmosphere was still inexact in 1961, when Sagan made his original proposal. Thirty-three years after his original proposal, in his 1994 book Pale Blue Dot , Sagan conceded his original proposal for terraforming would not work because the atmosphere of Venus is far denser than was known in 1961: [ 7 ]
"Here's the fatal flaw: In 1961, I thought the atmospheric pressure at the surface of Venus was a few bars ... We now know it to be 90 bars, so if the scheme worked, the result would be a surface buried in hundreds of meters of fine graphite, and an atmosphere made of 65 bars of almost pure molecular oxygen. Whether we would first implode under the atmospheric pressure or spontaneously burst into flames in all that oxygen is open to question. However, long before so much oxygen could build up, the graphite would spontaneously burn back into CO 2 , short-circuiting the process."
Following Sagan's paper, there was little scientific discussion of the concept until a resurgence of interest in the 1980s. [ 8 ] [ 9 ] [ 10 ]
A number of approaches to terraforming are reviewed by Martyn J. Fogg (1995) [ 2 ] [ 11 ] and by Geoffrey A. Landis (2011). [ 3 ]
The main problem with Venus today, from a terraformation standpoint, is the very thick carbon dioxide atmosphere. The ground level pressure of Venus is 9.2 MPa (91 atm; 1,330 psi). This also, through the greenhouse effect, causes the temperature on the surface to be several hundred degrees too hot for any significant organisms. Therefore, all approaches to the terraforming of Venus include somehow removing almost all the carbon dioxide in the atmosphere.
The method proposed in 1961 by Carl Sagan involves the use of genetically engineered algae to fix carbon into organic compounds . [ 5 ] Although this method is still proposed [ 10 ] in discussions of Venus terraforming, later discoveries showed that biological means alone would not be successful. [ 12 ]
Difficulties include the fact that the production of organic molecules from carbon dioxide requires hydrogen, which is very rare on Venus. [ 13 ] Because Venus lacks a protective magnetosphere , the upper atmosphere is exposed to direct erosion by the solar wind and has lost most of its original hydrogen to space. And, as Sagan noted, any carbon that was bound up in organic molecules would quickly be converted to carbon dioxide again by the hot surface environment. Venus would not begin to cool down until after most of the carbon dioxide had already been removed.
Although it is generally conceded that Venus could not be terraformed by introduction of photosynthetic biota alone, use of photosynthetic organisms to produce oxygen in the atmosphere continues to be a component of other proposed methods of terraforming. [ citation needed ]
On Earth nearly all carbon is sequestered in the form of carbonate minerals or in different stages of the carbon cycle , while very little is present in the atmosphere in the form of carbon dioxide. On Venus, the situation is the opposite. Much of the carbon is present in the atmosphere, while comparatively little is sequestered in the lithosphere . [ 14 ] Many approaches to terraforming therefore focus on getting rid of carbon dioxide by chemical reactions trapping and stabilising it in the form of carbonate minerals.
Modelling by astrobiologists Mark Bullock and David Grinspoon [ 14 ] of Venus's atmospheric evolution suggests that the equilibrium between the current 92-bar atmosphere and existing surface minerals, particularly calcium and magnesium oxides, is quite unstable, and that the latter could serve as a sink of carbon dioxide and sulfur dioxide through conversion to carbonates. If these surface minerals were fully converted and saturated, then the atmospheric pressure would decline and the planet would cool somewhat. One of the possible end states modelled by Bullock and Grinspoon was an atmosphere of 43 bars (42 atm; 620 psi) and a surface temperature of 400 K (127 °C; 260 °F). To convert the rest of the carbon dioxide in the atmosphere, a larger portion of the crust would have to be artificially exposed to the atmosphere to allow more extensive carbonate conversion. In 1989, Alexander G. Smith proposed that Venus could be terraformed by lithosphere overturn, allowing crust to be converted into carbonates. [ 15 ] Landis 2011 calculated that it would require the involvement of the entire surface crust down to a depth of over 1 km to produce enough rock surface area to convert enough of the atmosphere. [ 3 ]
Natural formation of carbonate rock from minerals and carbon dioxide is a very slow process. Recent research into sequestering carbon dioxide into carbonate minerals in the context of mitigating global warming on Earth however points out that this process can be considerably accelerated (from hundreds or thousands of years to just 75 days) through the use of catalysts such as polystyrene microspheres . [ 16 ] It could therefore be theorised that similar technologies might also be used in the context of terraformation on Venus. It can also be noted that the chemical reaction that converts minerals and carbon dioxide into carbonates is exothermic , in essence producing more energy than is consumed by the reaction. This opens up the possibility of creating self-reinforcing conversion processes with potential for exponential growth of the conversion rate until most of the atmospheric carbon dioxide can be converted.
Bombardment of Venus with refined magnesium and calcium from off-world could also sequester carbon dioxide in the form of calcium and magnesium carbonates . About 8 × 10 20 kg of calcium or 5 × 10 20 kg of magnesium would be required to convert all the carbon dioxide in the atmosphere, which would entail a great deal of mining and mineral refining (perhaps on Mercury which is notably mineral rich). [ 17 ] 8 × 10 20 kg is a few times the mass of the asteroid 4 Vesta (more than 500 kilometres (310 mi) in diameter).
Research projects in Iceland and the US state of Washington have shown that potentially large amounts of carbon dioxide could be removed from the atmosphere by high-pressure injection into subsurface porous basalt formations, where carbon dioxide is rapidly transformed into solid inert minerals. [ 18 ] [ 19 ]
Other studies [ 20 ] predict that one cubic meter of porous basalt has the potential to sequester 47 kilograms of injected carbon dioxide. According to these estimates a volume of about 9.86 × 10 9 km 3 of basalt rock would be needed to sequester all the carbon dioxide in the Venusian atmosphere. This is equal to the entire crust of Venus down to a depth of about 21.4 kilometers. Another study [ 21 ] concluded that under optimal conditions, on average, 1 cubic meter of basalt rock can sequester 260 kg of carbon dioxide. Venus's crust appears to be 70 kilometres (43 mi) thick and the planet is dominated by volcanic features. The surface is about 90% basalt , and about 65% consists of a mosaic of volcanic lava plains. [ 22 ] There should therefore be ample volumes of basalt rock strata on the planet with very promising potential for carbon dioxide sequestration .
Research has also demonstrated that under the high temperature and high pressure conditions in the mantle , silicon dioxide , the most abundant mineral in the mantle (on Earth and probably also on Venus) can form carbonates that are stable under these conditions. This opens up the possibility of carbon dioxide sequestration in the mantle. [ 23 ]
According to Birch, [ 24 ] bombarding Venus with hydrogen and reacting it with carbon dioxide could produce elemental carbon ( graphite ) and water by the Bosch reaction . It would take about 4 × 10 19 kg of hydrogen to convert the whole Venusian atmosphere, [ citation needed ] and such a large amount of hydrogen could be obtained from the gas giants or their moons' ice. Another possible source of hydrogen could be somehow extracting it from possible reservoirs in the interior of the planet itself. According to some researchers, the Earth's mantle and/or core might hold large quantities of hydrogen left there since the original formation of Earth from the nebular cloud . [ 25 ] [ 26 ] Since the original formation and inner structure of Earth and Venus are generally believed to be somewhat similar, the same might be true for Venus.
Iron aerosol in the atmosphere will also be required for the reaction to work, and iron can come from Mercury , asteroids, or the Moon . (Loss of hydrogen due to the solar wind is unlikely to be significant on the timescale of terraforming.) Due to the planet's relatively flat surface, this water would cover about 80% of the surface, compared to 70% for Earth, even though it would amount to only roughly 10% of the water found on Earth. [ citation needed ]
The remaining atmosphere, at around 3 bars (about three times that of Earth), would mainly be composed of nitrogen, some of which will dissolve into the new oceans of water, reducing atmospheric pressure in accordance with Henry's law . To further reduce the pressure even more, nitrogen could also be fixated into nitrates .
Futurist Isaac Arthur has suggested using the hypothesized processes of starlifting and stellasing to create a particle beam of ionized hydrogen from the sun, tentatively dubbed a "hydro-cannon". This device could be used both to thin the dense atmosphere of Venus, but also to introduce hydrogen to react with carbon dioxide to create water, thereby further lowering the atmospheric pressure. [ 27 ]
The thinning of the Venusian atmosphere could be attempted by a variety of methods, possibly in combination. Directly lifting atmospheric gas from Venus into space would probably prove difficult. Venus has sufficiently high escape velocity to make blasting it away with asteroid impacts impractical. Pollack and Sagan calculated in 1994 [ 28 ] that an impactor of 700 km diameter striking Venus at greater than 20 km/s, would eject all the atmosphere above the horizon as seen from the point of impact, but because this is less than a thousandth of the total atmosphere and there would be diminishing returns as the atmosphere's density decreases, a very great number of such giant impactors would be required. Landis calculated [ 3 ] that to lower the pressure from 92 bar to 1 bar would require a minimum of 2,000 impacts, even if the efficiency of atmosphere removal was perfect. Smaller objects would not work, either, because more would be required. The violence of the bombardment could well result in significant outgassing that would replace removed atmosphere. Most of the ejected atmosphere would go into solar orbit near Venus, and, without further intervention, could be captured by the Venerian gravitational field and become part of the atmosphere once again.
Another variant method involving bombardment would be to perturb a massive Kuiper belt object to put its orbit onto a collision path with Venus. If the object, made of mostly ices, had enough velocity to penetrate just a few kilometers past the Venusian surface, the resulting forces from the vaporization of ice from the impactor and the impact itself could stir the lithosphere and mantle thus ejecting a proportional amount of matter (as magma and gas) from Venus. A by-product of this method would be either a new moon for Venus or a new impactor-body of debris that would fall back to the surface at a later time.
Removal of atmospheric gas in a more controlled manner could also prove difficult. Venus's extremely slow rotation means that space elevators would be very difficult to construct because the planet's geostationary orbit lies an impractical distance above the surface, and the very thick atmosphere to be removed makes mass drivers useless for removing payloads from the planet's surface. Possible workarounds include placing mass drivers on high-altitude balloons or balloon-supported towers extending above the bulk of the atmosphere, using space fountains , or rotovators .
In addition, if the density of the atmosphere (and corresponding greenhouse effect) were dramatically reduced, the surface temperature (now effectively constant) would probably vary widely between day side and night side. Another side effect to atmospheric-density reduction could be the creation of zones of dramatic weather activity or storms at the terminator because large volumes of atmosphere would undergo rapid heating or cooling.
Venus receives about twice the sunlight that Earth does, which is thought to have contributed to its runaway greenhouse effect . One means of terraforming Venus could involve reducing the insolation at Venus's surface to prevent the planet from heating up again.
Solar shades could be used to reduce the total insolation received by Venus, cooling the planet somewhat. [ 29 ] A shade placed in the Sun–Venus L 1 Lagrangian point also would serve to block the solar wind , removing the radiation exposure problem on Venus.
A suitably large solar shade would be four times the diameter of Venus itself if at the L 1 point. This would necessitate construction in space. There would also be the difficulty of balancing a thin-film shade perpendicular to the Sun's rays at the Sun–Venus Lagrange point with the incoming radiation pressure , which would tend to turn the shade into a huge solar sail . If the shade were simply left at the L 1 point, the pressure would add force to the sunward side and the shade would accelerate and drift out of orbit. The shade could instead be positioned nearer to the Sun, using the solar pressure to balance the gravitational forces, in practice becoming a statite .
Other modifications to the L 1 solar shade design have also been suggested to solve the solar-sail problem. One suggested method is to use polar-orbiting , solar-synchronous mirrors that reflect light toward the back of the sunshade, from the non-sunward side of Venus. Photon pressure would push the support mirrors to an angle of 30 degrees away from the sunward side. [ 2 ]
Paul Birch proposed [ 24 ] a slatted system of mirrors near the L 1 point between Venus and the Sun. The shade's panels would not be perpendicular to the Sun's rays, but instead at an angle of 30 degrees, such that the reflected light would strike the next panel, negating the photon pressure. Each successive row of panels would be +/- 1 degree off the 30-degree deflection angle, causing the reflected light to be skewed 4 degrees from striking Venus.
Solar shades could also serve as solar power generators. Space-based solar shade techniques, and thin-film solar sails in general, are only in an early stage of development. The vast sizes require a quantity of material that is many orders of magnitude greater than any human-made object that has ever been brought into space or constructed in space.
Venus could also be cooled by placing reflectors in the atmosphere. Reflective balloons floating in the upper atmosphere could create shade. The number and/or size of the balloons would necessarily be great. Geoffrey A. Landis has suggested [ 30 ] that if enough floating cities were built, they could form a solar shield around the planet, and could simultaneously be used to process the atmosphere into a more desirable form, thus combining the solar shield theory and the atmospheric processing theory with a scalable technology that would immediately provide living space in the Venusian atmosphere. If made from carbon nanotubes or graphene (a sheet-like carbon allotrope ), then the major structural materials can be produced using carbon dioxide gathered in situ from the atmosphere. [ citation needed ] The recently synthesised amorphous carbonia might prove a useful structural material if it can be quenched to Standard Temperature and Pressure (STP) conditions, perhaps in a mixture with regular silica glass. According to Birch's analysis, such colonies and materials would provide an immediate economic return from colonizing Venus, funding further terraforming efforts. [ citation needed ]
Increasing the planet's albedo by deploying light-colored or reflective material on the surface (or at any level below the cloud tops) would not be useful, because the Venerian surface is already completely enshrouded by clouds, and almost no sunlight reaches the surface. Thus, it would be unlikely to be able to reflect more light than Venus's already-reflective clouds, with Bond albedo of 0.77. [ 31 ]
Birch proposed that solar shades could be used to not merely cool the planet but to also reduce atmospheric pressure as well, by the process of freezing of the carbon dioxide. [ 24 ] This requires Venus's temperature to be reduced, first to the liquefaction point, requiring a temperature less than 304.128(15) K [ 32 ] ( 30.978(15) °C or 87.761(27) °F ) and partial pressures of CO 2 to bring the atmospheric pressure down to 73.773(30) bar [ 32 ] ( carbon dioxide 's critical point ); and from there reducing the temperature below 216.592(3) K [ 32 ] ( −56.558(3) °C or −69.8044(54) °F ) (carbon dioxide's triple point ). Below that temperature, freezing of atmospheric carbon dioxide into dry ice will cause it to deposit onto the surface. He then proposed that the frozen CO 2 could be buried and maintained in that condition by pressure, or even shipped off-world (perhaps to provide greenhouse gas needed for terraforming of Mars or the moons of Jupiter ). After this process was complete, the shades could be removed or solettas added, allowing the planet to partially warm again to temperatures comfortable for Earth life. A source of hydrogen or water would still be needed, and some of the remaining 3.5 bar of atmospheric nitrogen would need to be fixed into the soil. Birch suggests disrupting an icy moon of Saturn, for example Hyperion , and bombarding Venus with its fragments.
Paul Birch suggests that, in addition to cooling the planet with a sunshade in L1, "heat pipes" could be built on the planet to accelerate the cooling. The proposed mechanism would transport heat from the surface to colder regions higher up in the atmosphere, similar to a solar updraft tower , thereby facilitating radiation of excess heat out into space. [ 24 ] A newly proposed variation of this technology is the atmospheric vortex engine , where instead of physical chimney pipes, the atmospheric updraft is achieved through the creation of a vortex, similar to a stationary tornado. In addition to this method being less material intensive and potentially more cost effective, this process also produces a net surplus of energy, which could be utilised to power venusian colonies or other aspects of the terraforming effort, while simultaneously contributing to speeding up the cooling of the planet. Another method to cool down the planet could be with the use of radiative cooling [ 33 ] This technology could utilise the fact that in certain wavelengths, thermal radiation from the lower atmosphere of Venus can "escape" to space through partially transparent atmospheric "windows" – spectral gaps between strong CO 2 and H 2 O absorption bands in the near infrared range 0.8–2.4 μm (31–94 μin). The outgoing thermal radiation is wavelength dependent and varies from the very surface at 1 μm (39 μin) to approximately 35 km (22 mi) at 2.3 μm (91 μin). [ 34 ] Nanophotonics and construction of metamaterials opens up new possibilities to tailor the emittance spectrum of a surface via properly designing periodic nano/micro-structures. [ 35 ] [ 36 ] Recently there has been proposals of a device named a "emissive energy harvester" that can transfer heat to space through radiative cooling and convert part of the heat flow into surplus energy, [ 37 ] opening up possibilities of a self-replicating system that could exponentially cool the planet.
Since Venus has only a fraction of the water of Earth (less than half the Earth's water content in the atmosphere, and none on the surface), [ 38 ] water would have to be introduced either by the aforementioned method of introduction of hydrogen, or from some other interplanetary or extraplanetary source.
Paul Birch suggests the possibility of colliding Venus with one of the ice moons from the outer solar system, [ 24 ] thereby bringing in all the water needed for terraformation in one go. This could be achieved through gravity assisted capture of Saturn's moons Enceladus and Hyperion or the Uranian moon Miranda . Simply changing the velocity of these moons enough to move them from their current orbit and enable gravity-assisted transport to Venus would require large amounts of energy. However, through complex gravity-assisted chain reactions the propulsion requirements could be reduced by several orders of magnitude. As Birch puts it, "[t]heoretically one could flick a pebble into the asteroid belt and end up dumping Mars into the Sun." [ 24 ]
Studies have shown that substantial amounts of water (in the form of hydrogen) might be present in the mantle of terrestrial planets. [ 39 ] It has therefore been speculated [ 40 ] that it would be technically possible to extract this water from the mantle to the surface even if no feasible method to accomplish this exists currently.
Venus rotates once every 243 Earth days—by far the slowest rotation period of any known object in the Solar System. A Venusian sidereal day thus lasts more than a Venusian year (243 versus 224.7 Earth days). However, the length of a solar day on Venus is significantly shorter than the sidereal day ; to an observer on the surface of Venus, the time from one sunrise to the next would be 116.75 days. Therefore, the slow Venerian rotation rate would result in extremely long days and nights, similar to the day-night cycles in the polar regions of earth—shorter, but global. The exact period of a solar day is very important for terraforming since 117 days of daytime would be the equivalent of a summer in the more temperate regions of Alaska whereas 58 days of daytime would result in a very short growing season found in the high arctic. It could mean the difference between permafrost and perpetual ice or green lush boreal forests. The slow rotation might also account for the lack of a significant magnetic field.
It has until recently been assumed that the rotation rate or day-night cycle of Venus would have to be increased for successful terraformation to be achieved. More recent research has shown, however, that the current slow rotation rate of Venus is not at all detrimental to the planet's capability to support an Earth-like climate. Rather, the slow rotation rate would, given an Earth-like atmosphere, enable the formation of thick cloud layers on the side of the planet facing the sun. This in turn would raise planetary albedo and act to cool the global temperature to Earth-like levels, despite the greater proximity to the Sun. According to calculations, maximum temperatures would be just around 35 °C (95 °F), given an Earth-like atmosphere. [ 41 ] [ 42 ] Speeding up the rotation rate would therefore be both impractical and detrimental to the terraforming effort. A terraformed Venus with the current slow rotation would result in a global climate with "day" and "night" periods each roughly 2 months (58 days) long, resembling the seasons at higher latitudes on Earth. The "day" would resemble a short summer with a warm, humid climate, a heavy overcast sky and ample rainfall. The "night" would resemble a short, very dark winter with quite cold temperature and snowfall. There would be periods with more temperate climate and clear weather at sunrise and sunset resembling a "spring" and "autumn". [ 41 ]
The problem of very dark conditions during the roughly two-month long "night" period could be solved through the use of a space mirror in a 24-hour orbit (the same distance as a geostationary orbit on Earth) similar to the Znamya (satellite) project experiments. Extrapolating the numbers from those experiments and applying them to Venerian conditions would mean that a space mirror just under 1700 meters in diameter could illuminate the entire nightside of the planet with the luminosity of 10-20 full moons and create an artificial 24-hour light cycle. An even bigger mirror could potentially create even stronger illumination conditions. Further extrapolation suggests that to achieve illumination levels of about 400 lux (similar to normal office lighting or a sunrise on a clear day on earth) a circular mirror about 55 kilometers across would be needed.
Paul Birch suggested keeping the entire planet protected from sunlight by a permanent system of slated shades in L1 , and the surface illuminated by a rotating soletta mirror in a polar orbit , which would produce a 24-hour light cycle. [ 24 ]
If increasing the rotation speed of the planet would be desired (despite the above-mentioned potentially positive climatic effects of the current rotational speed), it would require energy of a magnitude many orders greater than the construction of orbiting solar mirrors, or even than the removal of the Venerian atmosphere. Birch calculates that increasing the rotation of Venus to an Earth-like solar cycle would require about 1.6 × 10 29 Joules [ 43 ] (50 billion petawatt-hours).
Scientific research suggests that close flybys of asteroids or cometary bodies larger than 100 kilometres (60 mi) across could be used to move a planet in its orbit, or increase the speed of rotation. [ 44 ] The energy required to do this is large. In his book on terraforming, one of the concepts Fogg discusses is to increase the spin of Venus using three quadrillion objects circulating between Venus and the Sun every 2 hours, each traveling at 10% of the speed of light. [ 2 ]
G. David Nordley has suggested, in fiction, [ 45 ] that Venus might be spun up to a day length of 30 Earth days by exporting the atmosphere of Venus into space via mass drivers . A proposal by Birch involves the use of dynamic compression members to transfer energy and momentum via high-velocity mass streams to a band around the equator of Venus. He calculated that a sufficiently high-velocity mass stream, at about 10% of the speed of light, could give Venus a day of 24 hours in 30 years. [ 43 ]
Protecting the new atmosphere from the solar wind, to avoid the loss of hydrogen, would require an artificial magnetosphere. Venus presently lacks an intrinsic magnetic field, therefore creating an artificial planetary magnetic field is needed to form a magnetosphere via its interaction with the solar wind. According to two NIFS Japanese scientists, it is feasible to do that with current technology by building a system of refrigerated latitudinal superconducting rings, each carrying a sufficient amount of direct current . In the same report, it is claimed that the economic impact of the system can be minimized by using it also as a planetary energy transfer and storage system (SMES). [ 46 ]
Another study proposes the possibility of deployment of a magnetic dipole shield at the L1 Lagrange point , thereby creating an artificial magnetosphere that would protect the whole planet from solar wind and radiation. [ 47 ] | https://en.wikipedia.org/wiki/Terraforming_of_Venus |
The terrainability of a machine or robot is defined as its ability to negotiate terrain irregularities. [ 1 ]
Terrainability is a term coined in the research community and related to locomotion in the field of mobile robotics . Its various definitions generically describe the ability of the robot to handle various terrains in terms of their ground support, obstacle sizes and spacing, passive/dynamic stability, etc. [ 2 ]
This robotics-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terrainability |
TerreStar Corporation ( TSTR ), formerly Motient Corp. ( MNCP - 2000–2007) and American Mobile Satellite Corp. (AMSC - 1988–2000), was the controlling shareholder of TerreStar Networks Inc. , TerreStar National Services, Inc. and TerreStar Global Ltd. , and a shareholder of SkyTerra Communications.
TerreStar Networks was a Reston, Virginia -based company that operated integrated satellite and terrestrial telecommunications systems. The company declared bankruptcy in 2010 and is now owned by Dish Network . [ 1 ]
XM Satellite Radio was a spin-off of American Mobile Satellite Corp.
Arianespace launched TerreStar-1 on the morning of July 1, 2009. With a launch mass of 6,910 kg, it has been deemed "the largest commercial telecommunications satellite ever launched." It was built by Space Systems/Loral and was launched from the Guiana Space Center with an Ariane 5 rocket in French Guiana . TerreStar-1 with the plan of providing mobile voice, messaging and data communications services to North America. [ 2 ]
Terrestar system is based on GMR standard.
On January 14, 2010, TerreStar announced that the Federal Communications Commission had approved the company's deployment of a terrestrial wireless network using the same S-band frequencies used by TerreStar-1. [ 3 ]
In October 2010, TerreStar filed for a "prepackaged" bankruptcy, led by its largest secured creditor, EchoStar. Together the secured creditors exchanged $940 million of debt for about 97 percent of the company. The plan, along with $75 million of debtor-in-possession financing , was approved in November 2010. [ 4 ] In December 2010, an NPO called " A Human Right " mounted an effort to buy the satellite for use over Africa, with basic free internet service for those who don't have it, and internet access for nations that have cut off international internet connections. [ 5 ] [ 6 ]
After successfully bidding $1.375 billion for the acquisition of the TerreStar-1 satellite in a bankruptcy-court auction, [ 7 ] Dish Network on August 22, 2011, asked the Federal Communications Commission to let the company utilize the wireless spectrum of TerreStar to offer its own wireless broadband service. [ 8 ]
This article about a telecommunications corporation or company in the United States is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/TerreStar_Corporation |
The Terrestrial Planet Finder ( TPF ) was a proposed project by NASA to construct a system of space telescopes for detecting extrasolar terrestrial planets . TPF was postponed several times and finally cancelled in 2011. [ 1 ] [ 2 ] There were two telescope systems under consideration, the TPF-I , which had several small telescopes, and TPF-C , which used one large telescope.
In May 2002, NASA chose two TPF mission architecture concepts for further study and technology development. Each would use a different means to achieve the same goal—to block the light from a parent star in order to see its much smaller, dimmer planets. The technological challenge of imaging planets near their much brighter star has been likened to finding a firefly near the beam of a distant searchlight . Additional goals of the mission would include the characterization of the surfaces and atmospheres of newfound planets, and looking for the chemical signatures of life.
The two planned architectures were:
NASA and Jet Propulsion Laboratory (JPL) were to issue calls for proposals seeking input on the development and demonstration of technologies to implement the two architectures, and on scientific research relevant to planet finding. Launch of TPF-C had been anticipated to occur around 2014, and TPF-I possibly by 2020.
According to NASA's 2007 budget documentation, released on 6 February 2006, [ 3 ] the project was deferred indefinitely. [ 4 ]
In June 2006, a House of Representatives subcommittee voted to provide funding for the TPF along with the long-sought mission to Europa , a moon of Jupiter that might harbor extraterrestrial life . [ 5 ] Congressional spending limits under House Resolution 20 passed on 31 January 2007, by the United States House of Representatives and 14 February by the U.S. Senate postponed the program indefinitely. Actual funding has not materialized, and TPF remains a concept. [ 6 ] In June 2011, the TPF (and SIM ) programs were reported as "cancelled". [ 1 ] | https://en.wikipedia.org/wiki/Terrestrial_Planet_Finder |
Terrestrial Time ( TT ) is a modern astronomical time standard defined by the International Astronomical Union , primarily for time-measurements of astronomical observations made from the surface of Earth. [ 1 ] For example, the Astronomical Almanac uses TT for its tables of positions ( ephemerides ) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), [ 2 ] which succeeded ephemeris time (ET) . TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth .
The unit of TT is the SI second , the definition of which is based currently on the caesium atomic clock , [ 3 ] but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it.
TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT is indirectly the basis of UTC, via International Atomic Time (TAI). Because of the historical difference between TAI and ET when TT was introduced, TT is 32.184 s ahead of TAI.
A definition of a terrestrial time standard was adopted by the International Astronomical Union (IAU) in 1976 at its XVI General Assembly and later named Terrestrial Dynamical Time (TDT). It was the counterpart to Barycentric Dynamical Time (TDB), which was a time standard for Solar system ephemerides , to be based on a dynamical time scale . Both of these time standards turned out to be imperfectly defined. Doubts were also expressed about the meaning of 'dynamical' in the name TDT.
In 1991, in Recommendation IV of the XXI General Assembly, the IAU redefined TDT, also renaming it "Terrestrial Time". TT was formally defined in terms of Geocentric Coordinate Time (TCG), defined by the IAU on the same occasion. TT was defined to be a linear scaling of TCG, such that the unit of TT is the "SI second on the geoid ", [ 4 ] i.e. the rate approximately matched the rate of proper time on the Earth's surface at mean sea level. Thus the exact ratio between TT time and TCG time was 1 − L G {\displaystyle 1-L_{\mathrm {G} }} , where L G = U G / c 2 {\displaystyle L_{\mathrm {G} }=U_{\mathrm {G} }/c^{2}} was a constant and U G {\displaystyle U_{\mathrm {G} }} was the gravitational potential at the geoid surface, a value measured by physical geodesy . In 1991 the best available estimate of L G {\displaystyle L_{\mathrm {G} }} was 6.969 291 × 10 −10 .
In 2000, the IAU very slightly altered the definition of TT by adopting an exact value, L G = 6.969 290 134 × 10 −10 . [ 5 ]
TT differs from Geocentric Coordinate Time (TCG) by a constant rate. Formally it is defined by the equation
T T = ( 1 − L G ) × T C G + E , {\displaystyle \mathrm {TT} ={\bigl (}1-L_{\mathrm {G} }{\bigr )}\times \mathrm {TCG} +E,}
where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, L G {\displaystyle L_{\mathrm {G} }} is the constant difference in the rates of the two time scales, and E {\displaystyle E} is a constant to resolve the epochs (see below). L G {\displaystyle L_{\mathrm {G} }} is defined as exactly 6.969 290 134 × 10 −10 . Due to the term 1 − L G {\displaystyle 1-L_{\mathrm {G} }} the rate of TT is very slightly slower than that of TCG.
The equation linking TT and TCG more commonly has the form given by the IAU,
T T = T C G − L G × ( J D T C G − 2443144.5003725 ) × 86400 , {\displaystyle \mathrm {TT} =\mathrm {TCG} -L_{\mathrm {G} }\times {\bigl (}\mathrm {JD_{TCG}} -2443144.5003725{\bigr )}\times 86400,}
where J D T C G {\displaystyle \mathrm {JD_{TCG}} } is the TCG time expressed as a Julian date (JD) . The Julian Date is a linear transformation of the raw count of seconds represented by the variable TCG, so this form of the equation is not simplified . The use of a Julian Date specifies the epoch fully. The above equation is often given with the Julian Date 2443 144.5 for the epoch, but that is inexact (though inappreciably so, because of the small size of the multiplier L G {\displaystyle L_{\mathrm {G} }} ). The value 2443 144.500 3725 is exactly in accord with the definition.
Time coordinates on the TT and TCG scales are specified conventionally using traditional means of specifying days, inherited from non-uniform time standards based on the rotation of Earth. Specifically, both Julian Dates and the Gregorian calendar are used. For continuity with their predecessor Ephemeris Time (ET), TT and TCG were set to match ET at around Julian Date 2443 144.5 (1977-01-01T00Z). More precisely, it was defined that TT instant 1977-01-01T00:00:32.184 and TCG instant 1977-01-01T00:00:32.184 exactly correspond to the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000. This is also the instant at which TAI introduced corrections for gravitational time dilation .
TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation
J D T T = E J D + ( J D T C G − E J D ) × ( 1 − L G ) , {\displaystyle \mathrm {JD_{TT}} =E_{\mathrm {JD} }+{\bigl (}\mathrm {JD_{TCG}} -E_{\mathrm {JD} }{\bigr )}\times {\bigl (}1-L_{\mathrm {G} }{\bigr )},}
where E J D {\displaystyle E_{\mathrm {JD} }} is 2443 144.500 3725 exactly.
TT is a theoretical ideal, not dependent on a particular realization. For practical use, physical clocks must be measured and their readings processed to estimate TT. A simple offset calculation is sufficient for most applications, but in demanding applications, detailed modeling of relativistic physics and measurement uncertainties may be needed. [ 6 ]
The main realization of TT is supplied by TAI. The BIPM TAI service, performed since 1958, estimates TT using measurements from an ensemble of atomic clocks spread over the surface and low orbital space of Earth. TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings shown by that particular group of atomic clocks at the time. Estimates of TAI are also provided in real time by the institutions that operate the participating clocks. Because of the historical difference between TAI and ET when TT was introduced, the TAI realization of TT is defined thus: [ 7 ]
T T ( T A I ) = T A I + 32.184 s . {\displaystyle \mathrm {TT(TAI)=TAI+32.184~s} .}
The offset 32.184 s arises from history. The atomic time scale A1 (a predecessor of TAI) was set equal to UT2 at its conventional starting date of 1 January 1958, [ 8 ] when Δ T (ET − UT) was about 32 seconds. The offset 32.184 seconds was the 1976 estimate of the difference between Ephemeris Time (ET) and TAI, "to provide continuity with the current values and practice in the use of Ephemeris Time". [ 9 ]
TAI is never revised once published and TT(TAI) has small errors relative to TT(BIPM), [ 6 ] on the order of 10-50 microseconds. [ 10 ]
The GPS time scale has a nominal difference from atomic time (TAI − GPS time = +19 seconds) , [ 11 ] so that TT ≈ GPS time + 51.184 seconds . This realization introduces up to a microsecond of additional error, as the GPS signal is not precisely synchronized with TAI, but GPS receiving devices are widely available. [ 12 ]
Approximately annually since 1992, the International Bureau of Weights and Measures ( BIPM ) has produced better realizations of TT based on reanalysis of historical TAI data. BIPM's realizations of TT are named in the form "TT(BIPM08)", with the digits indicating the year of publication. They are published in the form of a table of differences from TT(TAI), along with an extrapolation equation that may be used for dates later than the table. The latest as of July 2024 [update] is TT(BIPM23). [ 13 ]
Researchers from the International Pulsar Timing Array collaboration have created a realization TT(IPTA16) of TT based on observations of an ensemble of pulsars up to 2012. This new pulsar time scale is an independent means of computing TT. The researchers observed that their scale was within 0.5 microseconds of TT(BIPM17), with significantly lower errors since 2003. The data used was insufficient to analyze long-term stability, and contained several anomalies, but as more data is collected and analyzed, this realization may eventually be useful to identify defects in TAI and TT(BIPM). [ 14 ]
TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET). It was designed for continuity with ET, [ 15 ] and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations ). The JPL ephemeris time argument T eph is within a few milliseconds of TT.
TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as Δ T = TT − UT1. Δ T was measured at +67.6439 seconds (TT ahead of UT1) at 0 h UTC on 1 January 2015; [ 16 ] and by retrospective calculation, Δ T was close to zero about the year 1900. Δ T is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future. In fine detail, Δ T is somewhat unpredictable, with 10-year extrapolations diverging by 2-3 seconds from the actual value. [ 17 ]
Observers in different locations, that are in relative motion or at different altitudes, can disagree about the rates of each other's clocks, owing to effects described by the theory of relativity . As a result, TT (even as a theoretical ideal) does not match the proper time of all observers.
In relativistic terms, TT is described as the proper time of a clock located on the geoid (essentially mean sea level ). [ 18 ] However, [ 19 ] TT is now actually defined as a coordinate time scale . [ 20 ] The redefinition did not quantitatively change TT, but rather made the existing definition more precise. In effect it defined the geoid (mean sea level) in terms of a particular level of gravitational time dilation relative to a notional observer located at infinitely high altitude.
The present definition of TT is a linear scaling of Geocentric Coordinate Time (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. TCG is used to date mainly for theoretical purposes in astronomy. From the point of view of an observer on Earth's surface the second of TCG passes in slightly less than the observer's SI second. The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster. | https://en.wikipedia.org/wiki/Terrestrial_Time |
Terrestrial analogue sites (also called " space analogues ") are places on Earth with assumed past or present geological, environmental or biological conditions of a celestial body such as the Moon or Mars . Analogue sites are used in the frame of space exploration to either study geological or biological processes observed on other planets, or to prepare astronauts for surface extra-vehicular activity .
Analogue sites are places on Earth with assumed, past or present, geological, environmental or biological conditions of a celestial body. Analogue site studies are necessary because they help to understand geological processes (on Earth) which can be extrapolated to other Solar System bodies in order to interpret and validate the data received from orbiters or planetary rovers . Analogue sites are also important for optimizing scientific and technological needs and exploration strategies in robotic or crewed missions to the Moon or Mars. [ 2 ] The definition of space analogues is therefore rather vast, reaching from places on Earth that exhibit geologic or atmospheric characteristics which are close to those observed on other celestial bodies, to sites that are used for space mission simulations to test sampling or drilling equipment, space suits , or the performance of astronauts in reduced gravity.
Some sites are therefore suited to test instruments for exobiological research or to train sampling procedures for field explorations. Other sites offer an extreme environment that can be used by astronauts to prepare for the difficulties in future space missions.
An important notion in the evaluation of analogue sites is that of "fidelity", which describes the resemblance of the analogue to its extraterrestrial correspondent. Fidelity is used in comparative planetary science to express the analogy of a terrestrial site to a target extraterrestrial surface. This classification is possible based on various criteria such as geomorphology , geochemistry , exobiology or exploration conditions.
Geomorphology is the scientific study of landforms and the processes that shape them. In terms of analogue sites, scientists search for locations on Earth that exhibit similar landforms such as can be found on exploration targets like the Moon , Mars or even asteroids and comets . The idea is to confront astronauts, robots or scientific equipment with sites that resemble in their geologic appearance those extraterrestrial surfaces. Examples are volcanic sites which resemble lunar terrain ( regolith ), polar locations and glaciers that can be compared to the poles of Mars or of Jupiter moon Europa , or terrestrial lava tubes which can also be found on the Moon or Mars.
Geochemistry is the science that uses the principles of chemistry to explain the mechanisms behind major geological systems. The aspect of geochemistry is of importance for analogue sites when locations offer the possibility to test analysis instruments for future space missions (crewed or robotic). Geochemical fidelity is also of importance for the development and test of equipment used for in-situ resource utilization . Examples for such analogue sites are terrestrial volcanoes that offer rocks similar to those found on the Moon or hematite concretions which can be found in Earth deserts and also on Mars (so-called "Blueberries").
Exobiology or astrobiology is the study of the origin and evolution of extraterrestrial life . In terrestrial analogues efforts are put on the identification of so-called extremophile organisms, which are life forms that live and survive in extreme conditions such as can be found on other planets or moons. The objective of this research is to understand how such organisms survive and how they can be identified (or their remnants).
Examples of exobiology analogue sites are the Rio Tinto in Spain , which hosts bacteria that can survive high temperatures and harsh chemical conditions, or black smokers in the deep sea that host colonies of life forms in high-pressure and high-temperature conditions. The cold dry hyperarid core of the Atacama desert is one of the closest analogues for Martian surface conditions and is often used for testing rovers and life detection equipment that one day may be sent to Mars. [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] Other extreme environments, such as the polar regions, high-altitude mountainous areas, or remote islands are also used in studies to better understanding of life under such conditions. Scientists can test at such analogue sites sampling equipment designed to search and identify lifeforms.
Another criterion to search for analogue sites are locations where the exploration conditions of future astronauts can be simulated. Future explorers of the Moon or Mars will have to handle various conditions, such as reduced gravity , radiation , work in pressurized space suits and extreme temperatures. Preparing astronauts for these conditions calls for training on sites that exhibit some of those conditions. The operations that can be simulated reach from living in isolation, to extra-vehicular activity (EVA) in reduced gravity to the construction of habitats. Examples for analogue sites that offer such exploration conditions are research stations at the poles or underwater EVA training as it is done at NEEMO by NASA , at the Marseilles subsea analogue by COMEX , or by using parabolic flights to simulate lower gravity for shorter durations. [ 8 ] Underwater analogue sites allow for the training of astronauts in neutral buoyancy conditions (such as is done in test pools at NASA, ESA or Star City in Russia) while operating on a natural terrain. Potential targets for such training are missions to the Moon and Mars, to test sampling, drilling and field explorations in 1/6th or 1/3rd of Earth's gravity, or asteroids, and to test anchoring systems in microgravity.
The notion of space analogues is not new. NASA has used such sites for a long time to train its astronauts for space missions. The following data are taken from the official website of NASA. [ 9 ]
The first analog mission was undertaken in 1997 in Arizona . Since then, NASA leads annual missions there to evaluate and test EVAs and outpost systems and operations. This site was chosen to test the materials in a desolate environment with rugged terrain, dust storms, extreme temperatures...
In the same year, the Haughton-Mars Project (HMP) was started on Devon Island in the Arctic. Since then, 14 missions have been conducted there to test technology and operations in a remote, extreme environment and conduct science research on the Mars-like terrain.
In 2001, NASA conducted the mission named NEEMO near Florida , 62 feet (19 m) underwater, that was supposed to be a simulation for six aquanauts living in a confined space. It was also the way to test the exploration equipment in an extreme and isolated environment. Since 2001, 14 missions have been undertaken there in a multi-organizational environment.
Since 2004, two-week missions are conducted every summer in Pavilion Lake in Canada . This analogue site allows astronauts to train in searching for evidence of life in an extreme environment with reduced-gravity conditions. This is an international and multi-organizational project conducted underwater.
The last analogue site used by NASA is at Mauna Kea on the Big Island of Hawaii, after which an analogue base under the name of HI-SEAS was founded on Mauna Loa . In total, six NASA missions have started in this base between 2013 and 2018, until the sixth HI-SEAS mission was halted due to a medical emergency. This project was led to test technologies for sustaining human exploration on desolate planetary surfaces like the Moon or Mars and explore social wellbeing and crew dynamics on long-duration missions. HI-SEAS is currently under management of the International Moonbase Alliance, founded by Henk Rogers .
Keen interest for space analogues has emerged through the student community, the 2017 NASA Ames Grand Prize Winning entry Anastasi [ dead link ] , explores the possibility of an underwater settlement as a preliminary to space settlement infrastructure.
The history of the use of terrestrial analogues shows the importance attached to using analogue sites in validating spatial technologies and scientific instruments. But analogue sites also have other uses:
Space analogues can help to train personnel in using technologies and instruments, and in knowing how to behave with a spacesuit. Thus two types of analogue sites exist: underwater sites and surface sites.
Space analogues may have potential similarities to environments for exobiology. In some places on Earth the conditions allow only certain types of organisms - extremophile organisms - to live.
Following table lists currently used space analogues on Earth.
Mars 12,000 sq-ft (1115 sq-m) outdoor Mars yard sculpted per mission; 3200 sq-ft (300 sq-m) indoor Mars yard and terrain park with fine-grained basalt and varied terrain beds and features for pressure suit, rover, and drone tests and exploration; synthetic lava tube being constructed fall of 2023.
Basalt bed is geologically accurate.
Additional sedimentary and metamorphic terrain provide varied topology and navigation challenges.
Mars Basalt dust was built from crushed volcanic rock (northern Arizona) with minimal original organic material for the Biosphere 2 Landscape Evolution Observatory (LEO) experiment.
Confinement . Confinement in isolated space habitat. Programmed, delayed communication.
Habitat and LSS. Installed space habitat simulation for days to months. Hermetically sealed, pressurized. On-site Mission Control Center will include (early '24) officer desks, data and communications monitoring.
(several locations)
Underground facility.
Soils, rocks. Mars Soils, sand, rocks.
Deep space analog research in isolation (regolith simulant)
Soils, rocks, volcanoclastics . Mars Sand, soils, fresh volcanic rocks. | https://en.wikipedia.org/wiki/Terrestrial_analogue_site |
Terrestrial animals are animals that live predominantly or entirely on land (e.g. cats , chickens , ants , most spiders ), as compared with aquatic animals , which live predominantly or entirely in the water (e.g. fish , lobsters , octopuses ), and semiaquatic animals, which rely on both aquatic and terrestrial habitats (e.g. platypus , most amphibians ). Some groups of insects are terrestrial , such as ants , butterflies , earwigs , cockroaches , grasshoppers and many others, while other groups are partially aquatic, such as mosquitoes and dragonflies , which pass their larval stages in water.
Alternatively, terrestrial is used to describe animals that live on the ground, as opposed to arboreal animals that live in trees.
The term "terrestrial" is typically applied to species that live primarily on or in the ground, in contrast to arboreal species, who live primarily in trees, even though the latter are actually a specialized subgroup of the terrestrial fauna.
There are other less common terms that apply to specific subgroups of terrestrial animals:
Terrestrial invasion is one of the most important events in the history of life . [ 1 ] [ 2 ] [ 3 ] Terrestrial lineages evolved in several animal phyla , among which arthropods, vertebrates and mollusks are representatives of more successful groups of terrestrial animals.
Terrestrial animals do not form a unified clade ; rather, they are a polyphyletic group that share only the fact that they live on land. The transition from an aquatic to terrestrial life by various groups of animals has occurred independently and successfully many times. [ 3 ] Most terrestrial lineages originated under a mild or tropical climate during the Paleozoic and Mesozoic , whereas few animals became fully terrestrial during the Cenozoic .
If internal parasites are excluded, eleven phyla include free living species in terrestrial environments. These can be grouped as follows:
Three phyla contain species that have adapted totally to dry terrestrial environments, and which have no aquatic phase in their life cycles:
Four phyla include species that depend on more or less moist habitats:
Species in four more phyla, as well as some smaller species of arthropods and annelids, are microscopic animals that require a film of water to live in, and are therefore considered semi-terrestrial: [ 4 ]
Labeling an animal species "terrestrial" or "aquatic" is often obscure and becomes a matter of judgment. Many animals considered terrestrial have a life-cycle that is partly dependent on being in water. Penguins , seals , and walruses sleep on land and feed in the ocean, yet they are all considered terrestrial. Many insects, e.g. mosquitos , and all terrestrial crabs , as well as other clades, have an aquatic life cycle stage: their eggs need to be laid in and to hatch in water; after hatching, there is an early aquatic form, either a nymph or larva .
There are crab species that are completely aquatic, crab species that are amphibious, and crab species that are terrestrial. Fiddler crabs are called "semi-terrestrial" since they make burrows in the muddy substrate, to which they retreat during high tides. When the tide is out, fiddler crabs search the beach for food. The same is true in the mollusca . Many hundreds of gastropod genera and species live in intermediate situations, such as for example, Truncatella . Some gastropods with gills live on land, and others with a lung live in the water.
As well as the purely terrestrial and the purely aquatic animals, there are many borderline species. There are no universally accepted criteria for deciding how to label these species, thus some assignments are disputed.
Fossil evidence has shown that sea creatures, likely arthropods, first began to make forays onto land around 530 million years ago, in the Early Cambrian . There is little reason to believe, however, that animals first began living permanently on land around that time. A more likely hypothesis is that these early arthropods' motivation for venturing onto dry land was to mate (as modern horseshoe crabs do) or to lay eggs out of the reach of predators. [ 5 ]
Three groups of arthropods had independently adapted to land by the end of the Cambrian: myriapods , hexapods and arachnids . [ 6 ] By the late Ordovician , they may have fully terrestrialized. There are other groups of arthropods, all from malacostracan crustaceans, which independently became terrestrial at a later date: woodlice , sandhoppers , and terrestrial crabs . Additionally, the sister panarthropodan groups Onychophora (velvet worms) are also terrestrial, while the Eutardigrada are also adapted for land to some degree; both groups probably becoming so during the Early Devonian . [ 7 ] Among arthropods, many microscopic crustacean groups like copepods and amphipods and seed shrimp can go dormant when dry and live in transient bodies of water. [ citation needed ]
By approximately 375 million years ago [ 3 ] the bony fish best adapted to life in shallow coastal/swampy waters (such as Tiktaalik roseae ). Thanks to relatively strong, muscular limbs (which were likely weight-bearing, thus making them a preferable alternative to traditional fins in extremely shallow water), [ 8 ] and lungs which existed in conjunction with gills, Tiktaalik and animals like it were able to establish a strong foothold on land by the end of the Devonian period. In the Carboniferous , tetrapods (losing their gills) became fully terrestrialized, allowing their expansion into most terrestrial niches, though later on some will return to being aquatic and conquer the air also.
Gastropod mollusks are one of the most successful animals that have diversified in the fully terrestrial habitat. [ 9 ] They have evolved terrestrial taxa in more than nine lineages. [ 9 ] They are commonly referred to as land snails and slugs .
Terrestrial invasion of gastropod mollusks has occurred in Neritopsina , Cyclophoroidea , Littorinoidea , Rissooidea , Ellobioidea , Onchidioidea , Veronicelloidea , Succineoidea , and Stylommatophora , and in particular, each of Neritopsina, Rissooidea and Ellobioidea has likely achieved land invasion more than once. [ 9 ]
Most terrestrialization events have occurred during the Paleozoic or Mesozoic . [ 9 ] Gastropods are especially unique due to several fully terrestrial and epifaunal lineages that evolved during the Cenozoic . [ 9 ] Some members of rissooidean families Truncatellidae , Assimineidae , and Pomatiopsidae are considered to have colonized to land during the Cenozoic. [ 9 ] Most truncatellid and assimineid snails amphibiously live in intertidal and supratidal zones from brackish water to pelagic areas. [ 9 ] Terrestrial lineages likely evolved from such ancestors. [ 9 ] The rissooidean gastropod family Pomatiopsidae is one of the few groups that have evolved fully terrestrial taxa during the late Cenozoic in the Japanese Archipelago only. [ 9 ] Shifts from aquatic to terrestrial life occurred at least twice within two Japanese endemic lineages in Japanese Pomatiopsidae and it started in the Late Miocene . [ 9 ]
About one-third of gastropod species are terrestrial. [ 10 ] In terrestrial habitats they are subjected to daily and seasonal variation in temperature and water availability. [ 10 ] Their success in colonizing different habitats is due to physiological, behavioral, and morphological adaptations to water availability, as well as ionic and thermal balance. [ 10 ] They are adapted to most of the habitats on Earth. [ 10 ] The shell of a snail is constructed of calcium carbonate , but even in acidic soils one can find various species of shell-less slugs. [ 10 ] Land-snails, such as Xerocrassa seetzeni and Sphincterochila boissieri , also live in deserts, where they must contend with heat and aridity. [ 10 ] Terrestrial gastropods are primarily herbivores and only a few groups are carnivorous. [ 11 ] Carnivorous gastropods usually feed on other gastropod species or on weak individuals of the same species; some feed on insect larvae or earthworms. [ 11 ]
Semi-terrestrial animals are macroscopic animals that rely on very moist environments to thrive, they may be considered a transitional point between true terrestrial animals and aquatic animals. Among vertebrates, amphibians have this characteristic relying on a moist environment and breathing through their moist skin while reproducing in water.
Many other animal groups solely have terrestrial animals that live like this: land planarians , land ribbon worms , roundworms (nematodes), and land annelids (clitellates) who are very primitive and breathe through skin .
Clitellates or terrestrial annelids demonstrate many unique terrestrial adaptations especially in their methods of reproduction, they tend towards being simpler than their marine relatives, the bristleworms , lacking many of the complex appendages the latter have.
Velvet worms are prone to desiccation not due to breathing through their skin but due to their spiracles being inefficient at protecting from desiccation, like clitellates they demonstrate extensive terrestrial adaptations and differences from their marine relatives including live birth.
Many animals live in terrestrial environments by thriving in transient often microscopic bodies of water and moisture, these include rotifers and gastrotrichs which lay resilient eggs capable of surviving years in dry environments, and some of which can go dormant themselves. Nematodes are usually microscopic with this lifestyle. Although eutardigrades only have lifespans of a few months, they famously can enter suspended animation during dry or hostile conditions and survive for decades, which allows them to be ubiquitous in terrestrial environments despite needing water to grow and reproduce. Many microscopic crustacean groups like copepods and amphipods and seed shrimps are known to go dormant when dry and live in transient bodies of water too. [ 4 ]
This article incorporates CC-BY-2.0 text from the reference [ 9 ] and CC-BY-2.5 text from the reference [ 10 ] and CC-BY-3.0 text from the reference [ 11 ] | https://en.wikipedia.org/wiki/Terrestrial_animal |
Terrestrial ecosystems are ecosystems that are found on land. Examples include tundra , taiga , temperate deciduous forest , tropical rain forest , grassland , deserts . [ 1 ]
Terrestrial ecosystems differ from aquatic ecosystems by the predominant presence of soil rather than water at the surface and by the extension of plants above this soil/water surface in terrestrial ecosystems. There is a wide range of water availability among terrestrial ecosystems (including water scarcity in some cases), whereas water is seldom a limiting factor to organisms in aquatic ecosystems. Because water buffers temperature fluctuations, terrestrial ecosystems usually experience greater diurnal and seasonal temperature fluctuations than do aquatic ecosystems in similar climates. [ 2 ]
Terrestrial ecosystems are of particular importance especially in meeting Sustainable Development Goal 15 that targets the conservation-restoration and sustainable use of terrestrial ecosystems. [ 3 ]
Organisms in terrestrial ecosystems have adaptations that allow them to obtain water when the entire body is no longer bathed in that fluid, means of transporting the water from limited sites of acquisition to the rest of the body, and means of preventing the evaporation of water from body surfaces. They also have traits that provide body support in the atmosphere, a much less buoyant medium than water, and other traits that render them capable of withstanding the extremes of temperature, wind, and humidity that characterize terrestrial ecosystems. Finally, the organisms in terrestrial ecosystems have evolved many methods of transporting gametes in environments where fluid flow is much less effective as a transport medium. [ 4 ] This is terrestrial ecosystems.
Common Types of Terrestrial Plants
Four main groupings for terrestrial plants are bryophytes, pteridophytes, gymnosperms, and angiosperms, have been existing for many years and have allowed diversity into our ecosystems .
Terrestrial ecosystems occupy 55,660,000 mi 2 (144,150,000 km 2 ), or 28.26% of Earth's surface. [ 5 ] Major plant taxa in terrestrial ecosystems are members of the division Magnoliophyta (flowering plants), of which there are about 275,000 species, and the division Pinophyta (conifers), of which there are about 500 species. Members of the division Bryophyta (mosses and liverworts), of which there are about 24,000 species, are also important in some terrestrial ecosystems. Major animal taxa in terrestrial ecosystems include the classes Insecta (insects) with about 900,000 species, Aves (birds) with 8,500 species, and Mammalia (mammals) with approximately 4,100 species. [ 6 ] | https://en.wikipedia.org/wiki/Terrestrial_ecosystem |
Terrestrial locomotion has evolved as animals adapted from aquatic to terrestrial environments. Locomotion on land raises different problems than that in water, with reduced friction being replaced by the increased effects of gravity .
As viewed from evolutionary taxonomy , there are three basic forms of animal locomotion in the terrestrial environment:
Some terrains and terrestrial surfaces permit or demand alternative locomotive styles. A sliding component to locomotion becomes possible on slippery surfaces (such as ice and snow ), where locomotion is aided by potential energy , or on loose surfaces (such as sand or scree ), where friction is low but purchase (traction) is difficult. Humans, especially, have adapted to sliding over terrestrial snowpack and terrestrial ice by means of ice skates , snow skis , and toboggans .
Aquatic animals adapted to polar climates , such as ice seals and penguins also take advantage of the slipperiness of ice and snow as part of their locomotion repertoire. Beavers are known to take advantage of a mud slick known as a "beaver slide" over a short distance when passing from land into a lake or pond. Human locomotion in mud is improved through the use of cleats . Some snakes use an unusual method of movement known as sidewinding on sand or loose soil. Animals caught in terrestrial mudflows are subject to involuntary locomotion; this may be beneficial to the distribution of species with limited locomotive range under their own power. There is less opportunity for passive locomotion on land than by sea or air, though parasitism ( hitchhiking ) is available toward this end, as in all other habitats .
Many species of monkeys and apes use a form of arboreal locomotion known as brachiation , with forelimbs as the prime mover. Some elements of the gymnastic sport of uneven bars resemble brachiation, but most adult humans do not have the upper body strength required to sustain brachiation. Many other species of arboreal animal with tails will incorporate their tails into the locomotion repertoire, if only as a minor component of their suspensory behaviors .
Locomotion on irregular, steep surfaces require agility and dynamic balance known as sure-footedness . Mountain goats are famed for navigating vertiginous mountainsides where the least misstep could lead to a fatal fall .
Many species of animals must sometimes locomote while safely conveying their young. Most often this task is performed by adult females. Some species are specially adapted to conveying their young without occupying their limbs, such as marsupials with their special pouch. In other species, the young are carried on the mother's back, and the offspring have instinctual clinging behaviours. Many species incorporate specialized transportation behaviours as a component of their locomotion repertoire, such as the dung beetle when rolling a ball of dung, which combines both rolling and limb-based elements.
The remainder of this article focuses on the anatomical and physiological distinctions involving terrestrial locomotion from the taxonomic perspective.
Movement on appendages is the most common form of terrestrial locomotion, it is the basic form of locomotion of two major groups with many terrestrial members, the vertebrates and the arthropods . Important aspects of legged locomotion are posture (the way the body is supported by the legs), the number of legs, and the functional structure of the leg and foot . There are also many gaits , ways of moving the legs to locomote, such as walking , running , or jumping .
Appendages can be used for movement in a lot of ways: the posture, the way the body is supported by the legs, is an important aspect. There are three main ways [ 1 ] in which vertebrates support themselves with their legs – sprawling, semi-erect, and fully erect. Some animals may use different postures in different circumstances, depending on the posture's mechanical advantages. There is no detectable difference in energetic cost between stances.
The "sprawling" posture is the most primitive, and is the original limb posture from which the others evolved. The upper limbs are typically held horizontally, while the lower limbs are vertical, though upper limb angle may be substantially increased in large animals. The body may drag along the ground, as in salamanders, or may be substantially elevated, as in monitor lizards . This posture is typically associated with trotting gaits , and the body flexes from side-to-side during movement to increase step length. All limbed reptiles , excluding birds , and salamanders use this posture, as does the platypus and several species of frogs that walk. Unusual examples can be found among amphibious fish , such as the mudskipper , which drag themselves across land on their sturdy fins. Among the invertebrates , most arthropods – which includes the most diverse group of animals, the insects – have a stance best described as sprawling. There is also anecdotal evidence that some octopus species (such as the genus Pinnoctopus ) can also drag themselves across land a short distance by hauling their body along by their tentacles (for example to pursue prey between rockpools) [ 2 ] – there may be video evidence of this. [ 3 ] The semi-erect posture is more accurately interpreted as an extremely elevated sprawling posture. This mode of locomotion is typically found in large lizards such as monitor lizards and tegus .
Mammals and birds typically have a fully erect posture, though each evolved it independently. In these groups the legs are placed beneath the body. This is often linked with the evolution of endothermy , as it avoids Carrier's constraint and thus allows prolonged periods of activity. [ 4 ] The fully erect stance is not necessarily the "most-evolved" stance; evidence suggests that crocodilians evolved a semi-erect stance in their forelimbs from ancestors with fully erect stance as a result of adapting to a mostly aquatic lifestyle, [ 5 ] though their hindlimbs are still held fully erect. For example, the mesozoic prehistoric crocodilian Erpetosuchus is believed to have had a fully erect stance and been terrestrial. [ 6 ]
The number of locomotory appendages varies much between animals, and sometimes the same animal may use different numbers of its legs in different circumstances. The best contender for unipedal movement is the springtail , which while normally hexapedal , hurls itself away from danger using its furcula , a tail -like forked rod that can be rapidly unfurled from the underside of its body.
A number of species move and stand on two legs, that is, they are bipedal . The group that is exclusively bipedal is the birds , which have either an alternating or a hopping gait. There are also a number of bipedal mammals . Most of these move by hopping – including the macropods such as kangaroos and various jumping rodents . Only a few mammals such as humans and the ground pangolin commonly show an alternating bipedal gait. In humans, alternating bipedalism is characterized by a bobbing motion, which is due to the utilization of gravity when falling forward. This form of bipedalism has demonstrated significant energy savings. Cockroaches and some lizards may also run on their two hind legs.
With the exception of the birds, terrestrial vertebrate groups with legs are mostly quadrupedal – the mammals, reptiles , and the amphibians usually move on four legs. There are many quadrupedal gaits.
The most diverse group of animals on earth, the insects , are included in a larger taxon known as hexapods , most of which are hexapedal, walking and standing on six legs. Exceptions among the insects include praying mantises and water scorpions , which are quadrupeds with their front two legs modified for grasping, some butterflies such as the Lycaenidae (blues and hairstreaks) which use only four legs, and some kinds of insect larvae that may have no legs (e.g., maggots ), or additional prolegs (e.g., caterpillars ).
Spiders and many of their relatives move on eight legs – they are octopedal . However, some creatures move on many more legs. Terrestrial crustaceans may have a fair number – woodlice having fourteen legs. Also, as previously mentioned, some insect larvae such as caterpillars and sawfly larvae have up to five (caterpillars) or nine (sawflies) additional fleshy prolegs in addition to the six legs normal for insects.
Some species of invertebrate have even more legs, the unusual velvet worm having stubby legs under the length of its body, with around several dozen pairs of legs. Centipedes have one pair of legs per body segment, with typically around 50 legs, but some species have over 200. The terrestrial animals with the most legs are the millipedes . They have two pairs of legs per body segment, with common species having between 80 and 400 legs overall – with the rare species Illacme plenipes having up to 750 legs.
Animals with many legs typically move them in metachronal rhythm , which gives the appearance of waves of motion travelling forward or backward along their rows of legs. Millipedes, caterpillars, and some small centipedes move with the leg waves travelling forward as they walk, while larger centipedes move with the leg waves travelling backward.
The legs of tetrapods , the main group of terrestrial vertebrates (which also includes amphibious fish ), have internal bones, with externally attached muscles for movement, and the basic form has three key joints : the shoulder joint, the knee joint, and the ankle joint, at which the foot is attached. Within this form there is much variation in structure and shape. An alternative form of vertebrate 'leg' to the tetrapod leg is the fins found on amphibious fish . Also a few tetrapods , such as the macropods , have adapted their tails as additional locomotory appendages.
The fundamental form of the vertebrate foot has five digits, however some animals have fused digits, giving them less, and some early fishapods had more; Acanthostega had eight toes. Only ichthyosaurs evolved more than 5 digits within tetrapods, while their transition from land to water again (limb terminations were becoming flippers). Feet have evolved many forms depending on the animal's needs. One key variation is where on the foot the animal's weight is placed. Some vertebrates: amphibians, reptiles, and some mammals such as humans , bears , and rodents, are plantigrade. This means the weight of the body is placed on the heel of the foot, giving it strength and stability. Most mammals, such as cats and dogs , are digitigrade , walking on their toes, giving them what many people mistake as a “backward knee”, which is really their ankle. The extension of the joint helps store momentum and acts as a spring, allowing digitigrade creatures more speed. Digitigrade mammals are also often adept at quiet movement. Birds are also digitigrade. [ 7 ] Hooved mammals are known as ungulates , walking on the fused tips of their fingers and toes. This can vary from odd-toed ungulates, such as horses, rhinos, and a few wild African ungulates, to even-toed ungulates, such as pigs, cows, deer, and goats.
Mammals whose limbs have adapted to grab objects have what are called prehensile limbs. This term can be attributed to front limbs as well as tails for animals such as monkeys and some rodents. All animals that have prehensile front limbs are plantigrade, even if their ankle joint looks extended (squirrels are a good example).
Among terrestrial invertebrates there are a number of leg forms. The arthropod legs are jointed and supported by hard external armor, with the muscles attached to the internal surface of this exoskeleton . The other group of legged terrestrial invertebrates, the velvet worms , have soft stumpy legs supported by a hydrostatic skeleton . The prolegs that some caterpillars have in addition to their six more-standard arthropod legs have a similar form to those of velvet worms, and suggest a distant shared ancestry.
Animals show a vast range of gaits , the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds , there are three main categories: walking gaits, running gaits, and leaping gaits . In one system (relating to horses), [ 8 ] there are 60 discrete patterns: 37 walking gaits, 14 running gaits, and 9 leaping gaits .
Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. In an informal sense, running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension . Technically, however, moments of suspension occur in both running gaits (such as trot) and leaping gaits (such as canter and gallop). Gaits involving one or more moments of suspension can be found in many animals, and compared to walking they are faster but more energetically costly forms of locomotion.
Animals will use different gaits for different speeds, terrain, and situations. For example, horses show four natural gaits, the slowest horse gait is the walk , then there are three faster gaits which, from slowest to fastest, are the trot , the canter , and the gallop . Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits are bipedal walking and running , but they employ many other gaits occasionally, including a four-legged crawl in tight spaces.
In walking, and for many animals running, the motion of legs on either side of the body alternates, i.e. is out of phase. Other animals, such as a horse when galloping, or an inchworm , alternate between their front and back legs.
In saltation (hopping) all legs move together, instead of alternating. As a main means of locomotion, this is usually found in bipeds, or semi-bipeds. Among the mammals saltation is commonly used among kangaroos and their relatives, jerboas , springhares , kangaroo rats , hopping mice , gerbils , and sportive lemurs . Certain tendons in the hind legs of kangaroos are very elastic , allowing kangaroos to effectively bounce along conserving energy from hop to hop, making saltation a very energy efficient way to move around in their nutrient poor environment. Saltation is also used by many small birds, frogs , fleas , crickets , grasshoppers , and water fleas (a small planktonic crustacean ).
Most animals move in the direction of their head. However, there are some exceptions. Crabs move sideways, and naked mole rats , which live in tight tunnels and can move backward or forward with equal facility. Crayfish can move backward much faster than they can move forward.
Gait analysis is the study of gait in humans and other animals. This may involve videoing subjects with markers on particular anatomical landmarks and measuring the forces of their footfall using floor transducers ( strain gauges ). Skin electrodes may also be used to measure muscle activity.
There are a number of terrestrial and amphibious limbless vertebrates and invertebrates. These animals, due to lack of appendages, use their bodies to generate propulsive force. These movements are sometimes referred to as "slithering" or "crawling", although neither are formally used in the scientific literature and the latter term is also used for some animals moving on all four limbs. All limbless animals come from cold-blooded groups; there are no endothermic limbless animals, i.e. there are no limbless birds or mammals.
Where the foot is important to the legged mammal, for limbless animals the underside of the body is important. Some animals such as snakes or legless lizards move on their smooth dry underside. Other animals have various features that aid movement. Molluscs such as slugs and snails move on a layer of mucus that is secreted from their underside, reducing friction and protecting from injury when moving over sharp objects. Earthworms have small bristles ( setae ) that hook into the substrate and help them move. Some animals, such as leeches , have suction cups on either end of the body allowing two anchor movement .
Some limbless animals, such as leeches, have suction cups on either end of their body, which allow them to move by anchoring the rear end and then moving forward the front end, which is then anchored and then the back end is pulled in, and so on. This is known as two-anchor movement . A legged animal, the inchworm , also moves like this, clasping with appendages at either end of its body.
Limbless animals can also move using pedal locomotory waves , rippling the underside of the body. This is the main method used by molluscs such as slugs and snails, and also large flatworms, some other worms, and even earless seals . The waves may move in the opposite direction to motion, known as retrograde waves , or in the same direction as motion, known as direct waves. Earthworms move by retrograde waves alternatively swelling and contracting down the length of their body, the swollen sections being held in place using setae . Aquatic molluscs such as limpets , which are sometimes out of the water, tend to move using retrograde waves. However, terrestrial molluscs such as slugs and snails tend to use direct waves. Lugworms and seals also use direct waves.
Most snakes move using lateral undulation where a lateral wave travels down the snake's body in the opposite direction to the snake's motion and pushes the snake off irregularities in the ground. This mode of locomotion requires these irregularities to function. Another form of locomotion, rectilinear locomotion , is used at times by some snakes, especially large ones such as pythons and boa . Here, large scales on the underside of the body known as scutes are used to push backwards and downwards. This is effective on a flat surface and is used for slow, silent movement, such as when stalking prey. Snakes use concertina locomotion for moving slowly in tunnels, here the snake alternates in bracing parts of its body on it surrounds. Finally the caenophidian snakes use the fast and unusual method of movement known as sidewinding on sand or loose soil. The snake cycles through throwing the front part of its body in the direction of motion and bringing the back part of its body into line crosswise.
Although animals have never evolved wheels for locomotion, [ 9 ] [ 10 ] a small number of animals will move at times by rolling their whole body. Rolling animals can be divided into those that roll under the force of gravity or wind and those that roll using their own power.
The web-toed salamander , a 10-centimetre (3.9 in) salamander, lives on steep hills in the Sierra Nevada mountains. When disturbed or startled it coils itself up into a ball, often causing it to roll downhill. [ 11 ] [ 12 ]
The pebble toad ( Oreophrynella nigra ) lives atop tepui in the Guiana highlands of South America . When threatened, often by tarantulas , it rolls into ball, and typically being on an incline, rolls away under gravity like a loose pebble. [ 13 ]
Namib wheeling spiders ( Carparachne spp. ), found in the Namib desert, will actively roll down sand dunes. This action can be used to successfully escape predators such as the Pompilidae tarantula wasps , which lay their eggs in a paralyzed spider for their larvae to feed on when they hatch. The spiders flip their body sideways and then cartwheel over their bent legs. The rotation is fast, the golden wheel spider ( Carparachne aureoflava ) moving up to 20 revolutions per second, moving the spider at 1 metre per second (3.3 ft/s). [ 14 ]
Coastal tiger beetle larvae when threatened can flick themselves into the air and curl their bodies to form a wheels, which the wind blows, often uphill, as far as 25 m (80 ft) and as fast as 11 km/h (3 m/s; 7 mph). They also may have some ability to steer themselves in this state. [ 15 ]
Pangolins , a type of mammal covered in thick scales, roll into a tight ball when threatened. Pangolins have been reported to roll away from danger, by both gravity and self-powered methods. A pangolin in hill country in Sumatra , to flee from a researcher, ran to the edge of a slope and curled into a ball to roll down the slope, crashing through the vegetation, and covering an estimated 30 metres (100 ft) or more in 10 seconds. [ 16 ]
Caterpillars of the mother-of-pearl moth, Pleuroptya ruralis , when attacked, will touch their heads to their tails and roll backwards, up to 5 revolutions at about 40 centimetres per second (16 in/s), which is about 40 times its normal speed. [ 12 ]
Nannosquilla decemspinosa , a species of long-bodied, short-legged mantis shrimp , lives in shallow sandy areas along the Pacific coast of Central and South America. When stranded by a low tide the 3 cm (1.2 in) stomatopod lies on its back and performs backwards somersaults over and over. The animal moves up to 2 metres (6.5 ft) at a time by rolling 20–40 times, with speeds of around 72 revolutions per minute. That is 1.5 body lengths per second (3.5 cm/s or 1.4 in/s). Researchers estimate that the stomatopod acts as a true wheel around 40% of the time during this series of rolls. The remaining 60% of the time it has to "jumpstart" a roll by using its body to thrust itself upwards and forwards. [ 12 ] [ 17 ]
Pangolins have also been reported to roll away from danger by self-powered methods. Witnessed by a lion researcher [ 18 ] in the Serengeti in Africa, a group of lions surrounded a pangolin, but could not get purchase on it when it rolled into a ball, and so the lions sat around it waiting and dozing. Surrounded by lions, it would unroll itself slightly and give itself a push to roll some distance, until by doing this multiple times it could get far enough away from the lions to be safe. Moving like this would allow a pangolin to cover distance while still remaining in a protective armoured ball.
Moroccan flic-flac spiders , if provoked or threatened, can escape by doubling their normal walking speed using forward or backward flips similar to acrobatic flic-flac movements. [ 19 ]
The fastest terrestrial animal is the cheetah , which can attain maximal sprint speeds of approximately 104 km/h (64 mph). [ 20 ] [ 21 ] The fastest running lizard is the black iguana , which has been recorded moving at speed of up to 34.9 km/h (21.7 mph). [ citation needed ] | https://en.wikipedia.org/wiki/Terrestrial_locomotion |
A terrestrial planet , tellurian planet , telluric planet , or rocky planet , is a planet that is composed primarily of silicate , rocks or metals . Within the Solar System , the terrestrial planets accepted by the IAU are the inner planets closest to the Sun : Mercury , Venus , Earth and Mars . Among astronomers who use the geophysical definition of a planet , two or three planetary-mass satellites – Earth's Moon , Io , and sometimes Europa – may also be considered terrestrial planets. The large rocky asteroids Pallas and Vesta are sometimes included as well, albeit rarely. [ 1 ] [ 2 ] [ 3 ] The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth ( Terra and Tellus ), as these planets are, in terms of structure, Earth-like . Terrestrial planets are generally studied by geologists , astronomers , and geophysicists .
Terrestrial planets have a solid planetary surface , making them substantially different from larger gaseous planets , which are composed mostly of some combination of hydrogen , helium , and water existing in various physical states .
All terrestrial planets in the Solar System have the same basic structure, such as a central metallic core (mostly iron ) with a surrounding silicate mantle .
The large rocky asteroid 4 Vesta has a similar structure; possibly so does the smaller one 21 Lutetia . [ 4 ] Another rocky asteroid 2 Pallas is about the same size as Vesta, but is significantly less dense; it appears to have never differentiated a core and a mantle. The Earth's Moon and Jupiter's moon Io have similar structures to terrestrial planets, but Earth's Moon has a much smaller iron core. Another Jovian moon Europa has a similar density but has a significant ice layer on the surface: for this reason, it is sometimes considered an icy planet instead.
Terrestrial planets can have surface structures such as canyons , craters , mountains , volcanoes , and others, depending on the presence at any time of an erosive liquid or tectonic activity or both.
Terrestrial planets have secondary atmospheres , generated by volcanic out-gassing or from comet impact debris. This contrasts with the outer , giant planets , whose atmospheres are primary; primary atmospheres were captured directly from the original solar nebula . [ 5 ]
The Solar System has four terrestrial planets under the dynamical definition: Mercury , Venus , Earth and Mars . The Earth's Moon as well as Jupiter's moons Io and Europa would also count geophysically, as well as perhaps the large protoplanet-asteroids Pallas and Vesta (though those are borderline cases). Among these bodies, only the Earth has an active surface hydrosphere . Europa is believed to have an active hydrosphere under its ice layer.
During the formation of the Solar System, there were many terrestrial planetesimals and proto-planets , but most merged with or were ejected by the four terrestrial planets, leaving only Pallas and Vesta to survive more or less intact. These two were likely both dwarf planets in the past, but have been battered out of equilibrium shapes by impacts. Some other protoplanets began to accrete and differentiate but suffered catastrophic collisions that left only a metallic or rocky core, like 16 Psyche [ 4 ] or 8 Flora respectively. [ 6 ] Many S-type [ 6 ] and M-type asteroids may be such fragments. [ 7 ]
The other round bodies from the asteroid belt outward are geophysically icy planets . They are similar to terrestrial planets in that they have a solid surface, but are composed of ice and rock rather than of rock and metal. These include the dwarf planets, such as Ceres , Pluto and Eris , which are found today only in the regions beyond the formation snow line where water ice was stable under direct sunlight in the early Solar System. It also includes the other round moons, which are ice-rock (e.g. Ganymede , Callisto , Titan , and Triton ) or even almost pure (at least 99%) ice ( Tethys and Iapetus ). Some of these bodies are known to have subsurface hydrospheres (Ganymede, Callisto, Enceladus , and Titan), like Europa, and it is also possible for some others (e.g. Ceres, Mimas , Dione , Miranda , Ariel , Triton, and Pluto). [ 8 ] [ 9 ] Titan even has surface bodies of liquid, albeit liquid methane rather than water. Jupiter's Ganymede, though icy, does have a metallic core like the Moon, Io, Europa, and the terrestrial planets.
The name Terran world has been suggested to define all solid worlds (bodies assuming a rounded shape), without regard to their composition. It would thus include both terrestrial and icy planets. [ 10 ]
The uncompressed density of a terrestrial planet is the average density its materials would have at zero pressure . A greater uncompressed density indicates a greater metal content. Uncompressed density differs from the true average density (also often called "bulk" density) because compression within planet cores increases their density; the average density depends on planet size, temperature distribution, and material stiffness as well as composition.
Calculations to estimate uncompressed density inherently require a model of the planet's structure. Where there have been landers or multiple orbiting spacecraft, these models are constrained by seismological data and also moment of inertia data derived from the spacecraft's orbits. Where such data is not available, uncertainties are inevitably higher. [ 11 ]
The uncompressed densities of the rounded terrestrial bodies directly orbiting the Sun trend towards lower values as the distance from the Sun increases, consistent with the temperature gradient that would have existed within the primordial solar nebula. The Galilean satellites show a similar trend going outwards from Jupiter; however, no such trend is observable for the icy satellites of Saturn or Uranus. [ 12 ] The icy worlds typically have densities less than 2 g·cm −3 . Eris is significantly denser ( 2.43 ± 0.05 g·cm −3 ), and may be mostly rocky with some surface ice, like Europa. [ 2 ] It is unknown whether extrasolar terrestrial planets in general will follow such a trend.
The data in the tables below are mostly taken from a list of gravitationally rounded objects of the Solar System and planetary-mass moon . All distances from the Sun are averages.
Most of the planets discovered outside the Solar System are giant planets, because they are more easily detectable. [ 14 ] [ 15 ] [ 16 ] But since 2005, hundreds of potentially terrestrial extrasolar planets have also been found, with several being confirmed as terrestrial. Most of these are super-Earths , i.e. planets with masses between Earth's and Neptune's; super-Earths may be gas planets or terrestrial, depending on their mass and other parameters.
During the early 1990s, the first extrasolar planets were discovered orbiting the pulsar PSR B1257+12 , with masses of 0.02, 4.3, and 3.9 times that of Earth, by pulsar timing .
When 51 Pegasi b , the first planet found around a star still undergoing fusion , was discovered, many astronomers assumed it to be a gigantic terrestrial, [ citation needed ] because it was assumed no gas giant could exist as close to its star (0.052 AU) as 51 Pegasi b did. It was later found to be a gas giant.
In 2005, the first planets orbiting a main-sequence star and which showed signs of being terrestrial planets were found: Gliese 876 d and OGLE-2005-BLG-390Lb . Gliese 876 d orbits the red dwarf Gliese 876 , 15 light years from Earth, and has a mass seven to nine times that of Earth and an orbital period of just two Earth days. OGLE-2005-BLG-390Lb has about 5.5 times the mass of Earth and orbits a star about 21,000 light-years away in the constellation Scorpius.
From 2007 to 2010, three (possibly four) potential terrestrial planets were found orbiting within the Gliese 581 planetary system . The smallest, Gliese 581e , is only about 1.9 Earth masses, [ 17 ] but orbits very close to the star. [ 18 ] Two others, Gliese 581c and the disputed Gliese 581d , are more-massive super-Earths orbiting in or close to the habitable zone of the star, so they could potentially be habitable, with Earth-like temperatures.
Another possibly terrestrial planet, HD 85512 b , was discovered in 2011; it has at least 3.6 times the mass of Earth. [ 19 ] The radius and composition of all these planets are unknown.
The first confirmed terrestrial exoplanet , Kepler-10b , was found in 2011 by the Kepler space telescope , specifically designed to discover Earth-size planets around other stars using the transit method. [ 20 ]
In the same year, the Kepler space telescope mission team released a list of 1235 extrasolar planet candidates , including six that are "Earth-size" or "super-Earth-size" (i.e. they have a radius less than twice that of the Earth) [ 21 ] and in the habitable zone of their star. [ 22 ] Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range (see image).
In 2016, statistical modeling of the relationship between a planet's mass and radius using a broken power law appeared to suggest that the transition point between rocky, terrestrial worlds and mini-Neptunes without a defined surface was in fact very close to Earth and Venus's, suggesting that rocky worlds much larger than our own are in fact quite rare. [ 10 ] This resulted in some advocating for the retirement of the term "super-earth" as being scientifically misleading. [ 23 ] Since 2016 the catalog of known exoplanets has increased significantly, and there have been several published refinements of the mass-radius model. As of 2024, the expected transition point between rocky and intermediate-mass planets sits at roughly 4.4 earth masses, and roughly 1.6 earth radii. [ 24 ]
In September 2020, astronomers using microlensing techniques reported the detection , for the first time, of an Earth-mass rogue planet (named OGLE-2016-BLG-1928 ) unbounded by any star, and free-floating in the Milky Way galaxy . [ 25 ] [ 26 ] [ 27 ]
The following exoplanets have a density of at least 5 g/cm 3 and a mass below Neptune's and are thus very likely terrestrial:
Kepler-10b , Kepler-20b , Kepler-36b , Kepler-48d , Kepler 68c , Kepler-78b , Kepler-89b , Kepler-93b , Kepler-97b , Kepler-99b , Kepler-100b , Kepler-101c , Kepler-102b , Kepler-102d , Kepler-113b , Kepler-131b , Kepler-131c , Kepler-138c , Kepler-406b , Kepler-406c , Kepler-409b .
In 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth- and super-Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way . [ 28 ] [ 29 ] [ 30 ] Eleven billion of these estimated planets may be orbiting Sun-like stars. [ 31 ] The nearest such planet may be 12 light-years away, according to the scientists. [ 28 ] [ 29 ] However, this does not give estimates for the number of extrasolar terrestrial planets, because there are planets as small as Earth that have been shown to be gas planets (see Kepler-138d ). [ 32 ]
Estimates show that about 80% of potentially habitable worlds are covered by land, and about 20% are ocean planets. Planets with rations more like those of Earth, which was 30% land and 70% ocean, only make up 1% of these worlds. [ 33 ]
Several possible classifications for solid planets have been proposed. [ 34 ]
Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow ( → ) may be read as "within" or "part of". | https://en.wikipedia.org/wiki/Terrestrial_planet |
Terreulactone A is a meroterpenoid isolate of Aspergillus with anti- acetylcholinesterase activity. [ 2 ] | https://en.wikipedia.org/wiki/Terreulactone_A |
A terricolous lichen is a lichen that grows on the soil as a substrate . [ 1 ] Examples include some members of the genus Peltigera . [ 2 ]
This article about lichens or lichenology is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terricolous_lichen |
TERSE is an IBM archive file format that supports lossless compression . A TERSE file may contain a sequential data set, a partitioned data set (PDS), partitioned data set extended (PDSE), or a large format dataset (DSNTYPE=LARGE). Any record format (RECFM) is allowed as long as the record length is less than 32 K (64 K for RECFM=VBS). Records may contain printer control characters. [ 1 ]
Terse files are compressed using a modification of Ziv, Lempel compression algorithm developed by Victor S. Miller and Mark Wegman at the Thomas J. Watson Research Center in Yorktown Heights, New York . [ 2 ] [ 3 ]
The Terse algorithm was proprietary to IBM; however, IBM has released an open source Java decompressor under the Apache 2 license. [ 4 ] The compression/decompression program (called terse and unterse )—AMATERSE or TRSMAIN—is available from IBM for z/OS ; the z/VM equivalents are the TERSE and DETERSE commands, for sequential datasets only. Versions for PC DOS , OS/2 , AIX , Windows (2000, XP, 2003), Linux , and Mac OS/X are available online. [ 5 ]
The following JCL can be used to invoke AMATERSE on z/OS (TRSMAIN uses INFILE and OUTFILE instead of SYSUT1 and SYSUT2): [ 6 ] [ 7 ]
Terse can be used as a general-purpose compression/decompression tool. IBM also distributes downloadable Program temporary fixs (PTFs) as tersed datasets. Terse is also used by IBM customers to package diagnostic information such as z/OS dumps and traces, for transmission to IBM.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Terse |
tert -Butanesulfinamide (also known as 2-methyl-2-propanesulfinamide or Ellman's sulfinamide ) is an organosulfur compound and a member of the class of sulfinamides . Both enantiomeric forms are commercially available and are used in asymmetric synthesis as chiral auxiliaries , often as chiral ammonia equivalents for the synthesis of amines . [ 1 ] [ 2 ] [ 3 ] tert -Butanesulfinamide and the associated synthetic methodology was introduced in 1997 by Jonathan A. Ellman et al . [ 4 ]
Enantiopure tert -butanesulfinamide can be prepared by enantioselective oxidation of inexpensive di- tert -butyl disulfide to the thiosulfinate followed by disulfide bond cleavage by lithium amide . In the original scope the chiral ligand used together with vanadyl acetylacetonate was prepared by condensing an optically pure chiral aminoindanol with 3,5-di- tert -butyl salicylaldehyde .
Condensation with ketones and aldehydes yields the corresponding N - tert -butanesulfinyl aldimines and ketimines . These intermediates are more resistant to hydrolysis than other imines but more reactive towards nucleophiles . A nucleophile adds diastereoselectively over the imine group in an electrophilic addition with the tert -butanesulfinyl group acting as a chiral auxiliary. This tert -butanesulfinyl group is also a protecting group . On addition of hydrochloric acid the tert -butanesulfinyl group is removed, forming the chiral primary ammonium salt or amine (from aldehyde precursor) or the chiral secondary amine (ketone precursor).
Typical nucleophiles are Grignard reagents , organozinc compounds , organolithium compounds , and enolates .
Chiral sulfinimines as intermediates for the asymmetric synthesis of amines have also been developed by Franklin A. Davis . [ 5 ]
tert -Butanesulfinamide has been used as an auxiliary in an asymmetric synthesis of cetirizine (more potent than the racemic mixture of the drug) starting from p -chlorobenzaldehyde and phenylmagnesium bromide . [ 6 ] | https://en.wikipedia.org/wiki/Tert-Butanesulfinamide |
tert -Butyl hydroperoxide (tBuOOH) is the organic compound with the formula (CH 3 ) 3 COOH. It is one of the most widely used hydroperoxides in a variety of oxidation processes, like the Halcon process . [ 3 ] It is normally supplied as a 69–70% aqueous solution. Compared to hydrogen peroxide and organic peracids, tert -butyl hydroperoxide is less reactive and more soluble in organic solvents. Overall, it is renowned for the convenient handling properties of its solutions. Its solutions in organic solvents are highly stable.
Industrially, tert -butyl hydroperoxide is used to prepare propylene oxide. In the Halcon process, molybdenum-based catalysts are used for this reaction:
The byproduct t-butanol can be dehydrated to isobutene and converted to MTBE .
On a much smaller scale, tert -butyl hydroperoxide is used to produce some fine chemicals by the Sharpless epoxidation . [ 4 ]
Many synthetic routes are available, [ 5 ] e.g. by the auto-oxidation of isobutane . [ 3 ]
tert -butyl hydroperoxide is potentially dangerous, but explosions are rare. [ 3 ]
A solution of tert -butyl hydroperoxide and water with a concentration of greater than 90% is forbidden to be shipped according to US Department of Transportation Hazardous Materials Table 49 CFR 172.101.
In some sources it also has an NFPA 704 rating of 4 for health, 4 for flammability, 4 for reactivity and is a potent oxidant , [ 6 ] however other sources claim lower ratings of 3-2-2 or 1-4-4. [ 7 ] [ 8 ] | https://en.wikipedia.org/wiki/Tert-Butyl_hydroperoxide |
tert -Butyldimethylsilyl chloride is an organosilicon compound with the formula (Me 3 C)Me 2 SiCl (Me = CH 3 ). It is commonly abbreviated as TBSCl or TBDMSCl. It is a chlorosilane containing two methyl groups and a tert -butyl group. As such it is more bulky that trimethylsilyl chloride . It is a colorless or white solid that is soluble in many organic solvents but reacts with water and alcohols. The compound is used to protect alcohols in organic synthesis . [ 1 ]
tert -Butyldimethylsilyl chloride reacts with alcohols in the presence of base to give tert -butyldimethyl silyl ethers : [ 2 ] [ 3 ]
These silyl ethers hydrolyze much more slowly than the trimethylsilyl ethers.
It also can silylate terminal alkynes. [ 4 ]
The triflate derivative (Me 3 C)Me 2 SiOTf is used similarly but is more reactive. [ 5 ] [ 6 ] | https://en.wikipedia.org/wiki/Tert-Butyldimethylsilyl_chloride |
sec -Butyllithium
tert -Butyllithium is a chemical compound with the formula (CH 3 ) 3 CLi. As an organolithium compound , it has applications in organic synthesis since it is a strong base , capable of deprotonating many carbon molecules, including benzene . tert -Butyllithium is available commercially as solutions in hydrocarbons (such as pentane ); it is not usually prepared in the laboratory.
tert -Butyllithium is produced commercially by treating tert-butyl chloride with lithium . Its synthesis was first reported by R. B. Woodward in 1941. [ 1 ]
Like other organolithium compounds, tert -butyllithium is a cluster compound . Whereas n -butyllithium exists both as a hexamer and a tetramer, tert -butyllithium exists exclusively as a tetramer with a cubane structure . Bonding in organolithium clusters involves sigma delocalization and significant Li−Li bonding. [ 2 ] Despite its complicated structure, tert -butyllithium is usually depicted in equations as a monomer.
The lithium–carbon bond in tert -butyllithium is highly polarized, having about 40 percent ionic character . The molecule reacts like a carbanion , as is represented by these two resonance structures : [ 3 ]
tert -Butyllithium is renowned for deprotonation of carbon acids (C-H bonds). One example is the double deprotonation of allyl alcohol . [ 4 ] Other examples are the deprotonation of vinyl ethers . [ 5 ] [ 6 ] [ 7 ]
In combination with n -butyllithiium, tert -butylllithium monolithiates ferrocene . [ 8 ] tert -Butyllithium deprotonates dichloromethane : [ 9 ]
Similar to n -butyllithium, tert -butyllithium can be used for lithium–halogen exchange reactions. [ 10 ] [ 11 ]
To minimize degradation by solvents, reactions involving tert -butyllithium are often conducted at very low temperatures in special solvents, such as the Trapp solvent mixture.
More so than other alkyllithium compounds, tert -butyllithium reacts with ethers . [ 2 ] In diethyl ether , the half-life of tert -butyllithium is about 60 minutes at 0 °C. It is even more reactive toward tetrahydrofuran (THF); the half-life in THF solutions is about 40 minutes at −20 °C. [ 12 ] In dimethoxyethane , the half-life is about 11 minutes at −70 °C [ 13 ]
In this example, the reaction of tert -butyllithium with (THF) is shown:
tert -butyllithium is a pyrophoric substance, meaning that it spontaneously ignites on exposure to air. Air-free techniques are important so as to prevent this compound from reacting violently with oxygen and moisture:
The solvents used in common commercial preparations are themselves flammable. While it is possible to work with this compound using cannula transfer , traces of tert -butyllithium at the tip of the needle or cannula may ignite and clog the cannula with lithium salts. While some researchers take this "pilot light" effect as a sign that the product is "fresh" and has not degraded due to time or improper storage/handling, others prefer to enclose the needle tip or cannula in a short glass tube, which is flushed with an inert gas and sealed at each end with septa. [ 14 ] Serious laboratory accidents involving tert -butyllithium have occurred. For example, in 2008 a staff research assistant, Sheharbano Sangji , in the lab of Patrick Harran [ 15 ] at the University of California, Los Angeles , died after being severely burned by a fire ignited by tert -butyllithium. [ 16 ] [ 17 ] [ 18 ]
Large-scale reactions may lead to runaway reactions, fires, and explosions when tert -butyllithium is mixed with ethers such as diethyl ether, and tetrahydrofuran. The use of hydrocarbon solvents may be preferred. | https://en.wikipedia.org/wiki/Tert-Butyllithium |
Tertiary is a term used in organic chemistry to classify various types of compounds (e. g. alcohols, alkyl halides, amines) or reactive intermediates (e. g. alkyl radicals, carbocations).
Tertiary central atoms compared with primary , secondary and quaternary central atoms. | https://en.wikipedia.org/wiki/Tertiary_(chemistry) |
A tertiary carbon atom is a carbon atom bound to three other carbon atoms. [ 1 ] For this reason, tertiary carbon atoms are found only in hydrocarbons containing at least four carbon atoms. They are called saturated hydrocarbons because they only contain carbon-carbon single bonds. [ 2 ] Tertiary carbons have a hybridization of sp3. Tertiary carbon atoms can occur, for example, in branched alkanes , but not in linear alkanes . [ 3 ]
The R is the functional group attached to a tertiary carbon. If the functional group was an OH group, this compound would be commonly called tert- butanol or t- butanol. When a functional group is attached to a tertiary carbon, the prefix - tert (- t ) is used in the common name for the compound. [ 4 ] An example of this is shown in the figure.
Tertiary carbons form the most stable carbocations due to a combination of factors. The three alkyl groups on the tertiary carbon contribute to a strong inductive effect . This is because each alkyl group will share its electron density with the central carbocation to stabilize it. Additionally, the surrounding sp3 hybridized carbons can stabilize the carbocation through hyperconjugation . [ 5 ] This occurs when adjacent sp3 orbitals have a weak overlap with the vacant p orbital; since there are 3 surrounding carbons with sp3 hybridization , there are more opportunities for overlap, which contributes to increasing carbocation stability.
A tertiary carbocation will maximize the rate of reaction for an SN1 reaction by producing a stable carbocation. This happens because the rate determining step of a SN1 reaction is the formation of the carbocation. The rate of the reaction is therefore reliant on the stability of the carbocation because it means that the transition state has a lower energy level which makes the activation energy lower. [ 6 ] Tertiary carbons are similarly preferred in E1 for the same reasons as it has a carbocation intermediate. E1 and E2 reactions follow Zaitsev's rule which states that the most substituted product in an elimination reactions is going to be the major product because it will be favored for its stability. This leads to tertiary carbons being preferred for their stability in elimination reactions. [ 7 ] In general, SN2 reactions do not occur with tertiary carbons because of the steric hindrance produced by the substituted groups. However, recent research has shown there are exceptions to this rule; for the first time, a bimolecular nucleophilic substitution, aka SN2 reaction , can happen to a tertiary carbon. [ 8 ] | https://en.wikipedia.org/wiki/Tertiary_carbon |
In mathematics , a tertiary ideal is a two-sided ideal in a perhaps noncommutative ring that cannot be expressed as a nontrivial intersection of a right fractional ideal with another ideal. Tertiary ideals generalize primary ideals to the case of noncommutative rings . Although primary decompositions do not exist in general for ideals in noncommutative rings, tertiary decompositions do, at least if the ring is Noetherian .
Every primary ideal is tertiary. Tertiary ideals and primary ideals coincide for commutative rings. To any (two-sided) ideal, a tertiary ideal can be associated called the tertiary radical, defined as
Then t ( I ) always contains I .
If R is a (not necessarily commutative) Noetherian ring and I a right ideal in R , then I has a unique irredundant decomposition into tertiary ideals
This algebra -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tertiary_ideal |
In software engineering , a tertiary review is a systematic review of systematic reviews. [ 1 ] It is also referred to as a tertiary study in the software engineering literature. However, Umbrella review is the term more commonly used in medicine.
Kitchenham et al. [ 1 ] suggest that methodologically there is no difference between a systematic review and a tertiary review. However, as the software engineering community has started performing tertiary reviews new concerns unique to tertiary reviews have surfaced. These include the challenge of quality assessment of systematic reviews, [ 2 ] search validation [ 3 ] and the additional risk of double counting. [ 4 ] | https://en.wikipedia.org/wiki/Tertiary_review |
Etherloop is a hybrid technology combining aspects of Ethernet with other technologies to achieve a result not possible with either technology alone.
EtherLoop was originally developed in the 1990s to allow high-speed data communication access to residential customers over standard twisted-pair telephone lines, also known as plain old telephone service or POTS. The technology development effort was begun at Northern Telecom in order to allow telephone companies to compete with the high-speed local data access then beginning to be offered by cable TV providers. [ 1 ] : 5
Etherloop is also a communications architecture with much broader applications. Technically, the initial EtherLoop adopted the protocol concepts of an Ethernet short-distance physical network with digital subscriber line (DSL) technology to facilitate the combination of voice and data transmission on legacy physical infrastructure of standard phone lines over distances of several kilometers. The project goal was to overcome the limitations of ADSL and HDSL while maintaining high-quality and high-speed data transmission. By combining features of Ethernet and DSL, and using digital signal processors (DSP) to enable the "maximum possible bandwidth out of any twisted pair copper pipe," EtherLoop became an architecture able to address a much wider variety of data networking requirements than the original 1990s-2000s application of data over POTS lines. [ 1 ] : 5, 28
Other technologies termed "etherloop" have been developed, including use for automotive intra-vehicle communication in the 2020s, where a gigabit Ethernet physical network has been used with a proprietary time-sliced , network protocol for near real-time , redundant control and feedback of motor vehicle subsystems. [ 2 ] [ 3 ]
EtherLoop was initially developed by Elastic Networks in the 1990s, to allow high-speed data communication access to residential customers over standard twisted-pair telephone lines. The technology development effort had been started by Jack Terry of Northern Telecom in order to allow telephone companies to compete with the high-speed local data access then beginning to be offered by cable TV providers. [ 1 ] : 5
In 1999, EtherLoop technology could, under the right conditions, facilitate speeds of up to 6 megabits per second over a distance of up to 6.4 km (21,000 feet). [ 1 ]
The telco EtherLoop design adopted the basic concepts of digital subscriber line (DSL) communications technology plus Ethernet local area network technology to facilitate the combination of voice and data transmission on legacy physical infrastructure of standard twisted-pair telephone lines, or plain old telephone service (POTS). [ 1 ] : 5
Prior DSL implementations— Asymmetric DSL (ADSL) and High-bit-rate DSL (HDSL)—had technical issues that limited adoption in telephone networks. Sending high-speed data requires substantial power to drive the signal levels across copper lines. More signal delivered results in crosstalk with other copper lines in the typical 25 or 50 tightly-bundled pairs used in telephone wiring.
For DSL services to reach their theoretical performance maximums, a near-ideal subscriber loop is required. In the real world, however, most subscriber loops are far from ideal. The wire may change gauge [ranging from 22 gauge to 26 gauge in POTS services]. This causes distortions and interference in a passing signal. It is also possible to have bridge taps on the loop, where a wire is attached to the main loop, but not connected to anything at the far end. Unconnected bridge taps cause reflections in the signal – some of the incoming signal will bounce backwards, and this reflection will interfere with the original signal. [ 1 ] : 7
The continuous power level required to operate DSL in the telco environment also increased the heat that needed to be dissipated over traditional phone service and increased the cost of the components. [ 1 ] : 7–10
Telco EtherLoop overcame some of the limitations while maintaining high-quality and high-speed data transmission by combining features of Ethernet and DSL, and using digital signal processors (DSP) to enable the "maximum possible bandwidth out of any twisted pair copper pipe," EtherLoop became an architecture able to address a much wider variety of data networking requirements than the original 1990s-2000s application of data over POTS lines. [ 1 ] : 5, 28 The initial EtherLoop implementation in 1999 used a half-duplex /bi-directional communication approach—but in only a single direction at a time, not simultaneously—plus burst packet delivery to mitigate several of the serious side effects of the legacy high-speed DSL offerings of the late 1990s. As such, EtherLoop transmission is less susceptible to interference caused by poor line quality, bridge taps , etc. in telephone company applications. [ 1 ] : 8–12
Later applications of EtherLoop in automotive systems overcame a different set of problems with EtherLoop-design solutions, as described in the Applications section below. | https://en.wikipedia.org/wiki/Tesla_EtherLoop |
Tessaleno Campos Devezas (born 4 December 1946 in Rio de Janeiro ) is a Brazilian -born Portuguese physicist , systems theorist , and materials scientist . He is best known for his contributions to the long waves theory in socioeconomic development, technological evolution, energy systems as well as world system analysis. [ 2 ]
In March 2002, Devezas was honored with the " Elsevier Best Paper Prize 2001" [ citation needed ] for his paper proposing a model explaining the mechanism underlying the long economic waves ( Kondratieff waves ) and, in 2006, another of his papers (about the growth dynamics of the Internet) received an Honor Mention from Elsevier. [ citation needed ] In 2004, he was awarded with the Silver Kondratieff Medal [ 3 ] by the International N. D. Kondratieff Foundation and the Russian Academy of Natural Sciences (RAEN) for his written contributions for the understanding of the Kondratieff waves , and, in 2005, he was honored as Honorary Member of the International Kondratieff Foundation. [ citation needed ]
The following list does not include this author's publications on materials science and engineering. | https://en.wikipedia.org/wiki/Tessaleno_Devezas |
Tessellated roof is a frame and a self-supporting structural system in architecture . A simple ridged roof may inside be a tessellated system. The interlinking shapes are replicated across the moulded surface using curvilinear coordinates , a specific technique with rigid interlinking beams, having characteristics similar to woven fabric . A tessellated roof is one of the most flexible framed systems to design. The measurements and precision are complex and commonly part of a computer-aided design process of production. It is used in a honeycomb geometry form, in the biomes of the Eden Project .
It can be fabricated to fit a wide range of situations. The size of the repeated geometric shape used can be customised, with a multitude of the same shape throughout the structure. An even and equal load is shared by the interlocking structural integrity of the frame as a whole. The use of a tessellated roof for public areas is an increasingly implemented architectural feature of modern public buildings, covering walkways and over retail centers . A transparent roof being for shelter from the weather, has an advantage during daylight with electricity for artificial lighting in solid roof buildings being a financial cost.
A modern tessellated roof for roofing public areas is a variation of a greenhouse or glass roof in different shapes and sized. The roof can be held aloft with columns , that may have branches to support and connect to the roof latticework, which stabilise the roof to create a strong structure. The material of the roof in-between or covering the tessellated frame may be a light composite , toughened glass or insulated glazing . There are roofed boulevards with columns that can form a colonnade. Some tessellated roof shapes connect to the ground in place of conventional rain gutters , for example the FieraMilano , or it can be supported entirely by the surrounding buildings. A tessellated roof can convert previous outdoor space into a dry public area; some examples of this method are Galleria Vittorio Emanuele II and many other shopping complexes or the Queen Elizabeth II Great Court at the British Museum in London by Norman Foster. | https://en.wikipedia.org/wiki/Tessellated_roof |
In computer science , the test-and-set instruction is an instruction used to write (set) 1 to a memory location and return its old value as a single atomic (i.e., non- interruptible ) operation. The caller can then "test" the result to see if the state was changed by the call. If multiple processes may access the same memory location, and if a process is currently performing a test-and-set, no other process may begin another test-and-set until the first process's test-and-set is finished. A central processing unit (CPU) may use a test-and-set instruction offered by another electronic component, such as dual-port RAM ; a CPU itself may also offer a test-and-set instruction.
A lock can be built using an atomic test-and-set [ 1 ] instruction as follows:
This code assumes that the memory location was initialized to 0 at some point prior to the first test-and-set. The calling process obtains the lock if the old value was 0, otherwise the while-loop spins waiting to acquire the lock. This is called a spinlock . At any point, the holder of the lock can simply set the memory location back to 0 to release the lock for acquisition by another--this does not require any special handling as the holder "owns" this memory location. " Test and test-and-set " is another example.
Maurice Herlihy (1991) proved that test-and-set (1-bit comparand) has a finite consensus number and can solve the wait-free consensus problem for at-most two concurrent processes. [ 2 ] In contrast, compare-and-swap (32-bit comparand) offers a more general solution to this problem, and in some implementations wider compare-and-swap (64- or 128-bit comparand) is also available for extended utility.
DPRAM test-and-set instructions can work in many ways. Here are two variations, both of which describe a DPRAM which provides exactly 2 ports, allowing 2 separate electronic components (such as 2 CPUs) access to every memory location on the DPRAM.
When CPU 1 issues a test-and-set instruction, the DPRAM first makes an "internal note" of this by storing the address of the memory location in a special place. If at this point, CPU 2 happens to issue a test-and-set instruction for the same memory location, the DPRAM first checks its "internal note", recognizes the situation, and issues a BUSY interrupt, which tells CPU 2 that it must wait and retry. This is an implementation of a busy waiting or spinlock using the interrupt mechanism. Since all this happens at hardware speeds, CPU 2's wait to get out of the spin-lock is very short.
Whether or not CPU 2 was trying to access the memory location, the DPRAM performs the test given by CPU 1. If the test succeeds, the DPRAM sets the memory location to the value given by CPU 1. Then the DPRAM wipes out its "internal note" that CPU 1 was writing there. At this point, CPU 2 could issue a test-and-set, which would succeed.
CPU 1 issues a test-and-set instruction to write to "memory location A". The DPRAM does not immediately store the value in memory location A, but instead simultaneously moves the current value to a special register, while setting the contents of memory location A to a special "flag value". If at this point, CPU 2 issues a test-and-set to memory location A, the DPRAM detects the special flag value, and as in Variation 1, issues a BUSY interrupt.
Whether or not CPU 2 was trying to access the memory location, the DPRAM now performs CPU 1's test. If the test succeeds, the DPRAM sets memory location A to the value specified by CPU 1. If the test fails, the DPRAM copies the value back from the special register to memory location A. Either operation wipes out the special flag value. If CPU 2 now issues a test-and-set, it will succeed.
Some instruction sets have an atomic test-and-set machine language instruction. Examples include x86 [ 3 ] and IBM System/360 and its successors (including z/Architecture ). [ 4 ] Those that do not can still implement an atomic test-and-set using a read-modify-write or compare-and-swap instruction.
The test and set instruction, when used with boolean values, uses logic like that shown in the following function, except that the function must execute atomically . That is, no other process must be able to interrupt the function mid-execution, thereby seeing a state that only exists while the function executes. That requires hardware support; it cannot be implemented as shown. Nevertheless, the code shown helps to explain the behaviour of test-and-set. NOTE: In this example, 'lock' is assumed to be passed by reference (or by name) but the assignment to 'initial' creates a new value (not just copying a reference).
Not only is the code shown not atomic, in the sense of the test-and-set instruction, it also differs from the descriptions of DPRAM hardware test-and-set above. Here, the value being set and the test are fixed and invariant, and the value is updated regardless of the outcome of the test, whereas for the DPRAM test-and-set, the memory is set only when the test succeeds, and the value to set and the test condition are specified by the CPU. Here, the value to set can only be 1, but if 0 and 1 are considered the only valid values for the memory location, and "value is nonzero" is the only allowed test, then this equates to the case described for DPRAM hardware (or, more specifically, the DPRAM case reduces to this under these constraints). From that viewpoint, this can, correctly, be called "test-and-set" in the full, conventional sense of that term. The essential point to note is the general intent and principle of test-and-set: a value is both tested and set in one atomic operation such that no other program thread or process can change the target memory location after it is tested but before it is set. (This is because the location must only be set if it currently has a certain value, not if it had that value sometime earlier.)
In the C programming language , the implementation would be like:
The code also shows that there are really two operations: an atomic read-modify-write and a test. Only the read-modify-write needs to be atomic. (This is true because delaying the value comparison by any amount of time will not change the result of the test once the value to test has been obtained. Once the code writes the initial value, the result of the test has been established, even if it has not been computed yet — e.g., by the == operator.)
One way to implement mutual exclusion is by using a test-and-set based lock [ 5 ] [ 6 ] as follows:
The lock variable is a shared variable i.e. it can be accessed by all processors/threads. Note the volatile keyword. In absence of volatile, the compiler and/or the CPU(s) may optimize access to lock and/or use cached values, thus rendering the above code erroneous. Conversely, and unfortunately, the presence of volatile does not guarantee that reads and writes are committed to memory. Some compilers issue memory barriers to ensure that operations are committed to memory, but since the semantics of volatile in C/C++ is quite vague, not all compilers will do that. Consult your compiler's documentation to determine if it does.
This spin lock function can be called by multiple processes, but it is guaranteed that only one process will be in the critical section at a time. The rest of the processes will keep spinning until they get the lock. It is possible that a process is never granted the lock. In such a case it will loop endlessly. This is a drawback of a spin lock implementation as it doesn't ensure fairness. These issues are further elaborated in the performance section .
Here tsl is an atomic instruction and flag is the lock variable. The process doesn't return unless it acquires the lock.
The four major evaluation metrics for locks in general are uncontended lock-acquisition latency, bus traffic, fairness, and storage. [ 7 ]
Test-and-set scores low on two of them, namely, high bus traffic and unfairness.
When processor P1 has obtained a lock and processor P2 is also waiting for the lock, P2 will keep incurring bus transactions in attempts to acquire the lock. When a processor has obtained a lock, all other processors which also wish to obtain the same lock keep trying to obtain the lock by initiating bus transactions repeatedly until they get hold of the lock. This increases the bus traffic requirement of test-and-set significantly. This slows down all other traffic from cache and coherence misses. It slows down the overall section, since the traffic is saturated by failed lock acquisition attempts. Test-and-test-and-set is an improvement over TSL since it does not initiate lock acquisition requests continuously.
When we consider fairness, we consider if a processor gets a fair chance of acquiring the lock when it is set free. In an extreme situation the processor might starve i.e. it might not be able to acquire the lock for an extended period of time even though it has become free during that time.
Storage overhead for TSL is next to nothing since only one lock is required. Uncontended latency is also low since only one atomic instruction and branch are needed. | https://en.wikipedia.org/wiki/Test-and-set |
In biology, a test is the hard shell of some spherical marine animals and protists , notably sea urchins and microorganisms such as testate foraminiferans , radiolarians , and testate amoebae . The term is also applied to the covering of scale insects . The related Latin term testa is used for the hard seed coat of plant seeds.
The anatomical term "test" derives from the Latin testa (which means a rounded bowl, amphora or bottle).
The test is a skeletal structure, made of hard material such as calcium carbonate , silica , chitin or composite materials . [ 1 ] As such, it allows the protection of the internal organs and the attachment of soft flesh.
The structure is notable for its ambulacra , alternating in wide and narrow patterns. Small serrations, bumps, ridges or thorns are frequently found along the outer cortex. [ 1 ] Magnesium calcites in the structures share three common features: lack of uniformity in Mg distribution, calcite minerals that maintain crystallographic orientations, and formation of Mg-calcites that are thermodynamically unstable. [ 2 ]
The test of sea urchins is made of calcium carbonate, strengthened by a framework of calcite monocrystals, in a characteristic "stereomic" structure. These two ingredients provide sea urchins with a great solidity and a moderate weight, as well as the capacity to regenerate the mesh from the cuticle. According to a 2012 study, [ 3 ] the skeletal structures of sea urchins consist of 92% of "bricks" of calcite monocrystals (conferring solidity and hardness) and 8% of a "mortar" of amorphous lime (allowing flexibility and lightness). This lime is constituted itself of 99.9% of calcium carbonate, with 0.1% structural proteins, which make sea urchins animals with an extremely mineralized skeleton (which also explains their excellent conservation as fossils). [ 3 ] . The endoskeletal matrix is formed by spicules of calcite and extracellular matrix proteins that form concentric folded layers. [ 4 ]
The test of foraminifera , a group of single-celled organisms, is extremely evolutionarily diverse. Many different methods of constructing the test are present, from lacking a test in Reticulomyxa , proteinaceous tests in the " allogromiids ", agglomerated tests made from foreign particles in many groups including textulariids , silica tests in silicoloculinids , and aragonite or calcite tests in many forms including miliolids and rotaliids . It can be of many types, including proteinaceous, agglutinated (exogenous agglomerate), porcelain-like (smooth calcite) or hyalin (lens). Foraminifera with multi-chambered tests are referred to as multilocular and develop by building new chambers in their test. These are arranged according to a geometry particular to each species: they can be rectilinear, curved, rolled up or cyclic, uniserial or multiserial. These organizational types can also be mixed, or even more complex. Miliolids have a particular arrangement of chambers known as "milioline". The surface of the test can be smooth or textured and may be perforated with small holes. [ 5 ]
In ascidians , the sheath is sometimes called test as well and is composed largely of a particular type of cellulose historically termed "tunicine". From 1845 (when this was discovered by Schmidt ) until 1958 (when cellulose fibres were found in mammalian connective tissue), ascidians were believed to be the only animals that synthesized cellulose. [ 6 ]
Tests are valuable tools in the fossil record used as proxies for reconstructing environmental conditions. Urchins appeared in the Phanerozoic and are globally distributed, and the skeletal nature of their tests allowed for consistent conservation in the fossil record. [ 7 ] The rapid growth and incorporation of isotopes including oxygen, magnesium, calcium and carbon allow scientists to evaluate the relative conditions of the oceans throughout Earth's history. [ 7 ]
The effects of ocean acidification and sea temperature change can be detrimental to test formation and function due to their incorporation of calcium and carbonate. Increase in pCO2 has decreased structural integrity resulting in skeletal failure. [ 1 ] Alteration and decreased test robustness results in lower growth rates and smaller adult diameters of urchin tests. [ 1 ] Other studies indicate that there some species able to adapt to long term exposure to higher acidity, due to evidence of enhanced growth after prolonged proximity to a hydrothermal vent [ 8 ] or seasonal hypercapnia events. [ 9 ]
On a strictly scientific point of view, the term "test" should be restricted to the hard shell protecting sea urchins and foraminiferans. For sessile echinoderms (like crinoids , but also many fossile groups such as cistoids or blastoids ), the correct word is " theca ". For diatoms , the term in use is " frustule ", and for radiolarians it should be " capsule ". The more common word " shell " is used for mollusks , arthropods and turtles (even if the latter ones belong to the order " Testudines "). | https://en.wikipedia.org/wiki/Test_(biology) |
The Test Methods Regulation is a Regulation (European Union) No. 440/2008 of May 30, 2008. It, and its subsequent amendments, define tests, testing of chemicals for the REACH Regulation . They are based on the OECD Guidelines for the Testing of Chemicals .
This article about the European Union is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Test_Methods_Regulation |
The Test Template Framework ( TTF ) is a model-based testing (MBT) framework proposed by Phil Stocks and David Carrington ( Stocks & Carrington 1996 ) for the purpose of software testing . Although the TTF was meant to be notation-independent, the original presentation was made using the Z formal notation . It is one of the few MBT frameworks approaching unit testing .
The TTF is a specific proposal of model-based testing (MBT). It considers models to be Z specifications . Each operation within the specification is analyzed to derive or generate abstract test cases . This analysis consists of the following steps:
One of the main advantages of the TTF is that all of these concepts are expressed in the same notation of the specification, i.e. the Z notation . Hence, the engineer has to know only one notation to perform the analysis down to the generation of abstract test cases .
In this section the main concepts defined by the TTF are described.
Let O p {\displaystyle Op} be a Z operation. Let x 1 … x n {\displaystyle x_{1}\dots x_{n}} be all the input and (non-primed) state variables referenced in O p {\displaystyle Op} , and T 1 … T n {\displaystyle T_{1}\dots T_{n}} their corresponding types. The Input Space (IS) of O p {\displaystyle Op} , written O p i s {\displaystyle Op_{is}} , is the Z schema box defined by [ x 1 : T 1 … x n : T n ] {\displaystyle [x_{1}:T_{1}\dots x_{n}:T_{n}]} .
Let O p {\displaystyle Op} be a Z operation. Let pre O p {\displaystyle {\text{pre }}Op} be the precondition of O p {\displaystyle Op} . The Valid Input Space (VIS) of O p {\displaystyle Op} , written O p v i s {\displaystyle Op_{vis}} , is the Z schema box defined by [ O p i s | pre O p ] {\displaystyle [Op_{is}|{\text{pre }}Op]} .
Let O p {\displaystyle Op} be a Z operation and let P {\displaystyle P} be any a conjunction of atomic predicates depending on one or more of the variables defined in O p v i s {\displaystyle Op_{vis}} . Then, the Z schema box [ O p v i s | P ] {\displaystyle [Op_{vis}|P]} is a test class of O p {\displaystyle Op} . Note that this schema is equivalent to [ O p i s | pre O p ∧ P ] {\displaystyle [Op_{is}|{\text{pre }}Op\land P]} . This observation can be generalized by saying that if O p c {\displaystyle Op_{c}} is a test class of O p {\displaystyle Op} , then the Z schema box defined by [ O p c | P ] {\displaystyle [Op_{c}|P]} is also a test class of O p {\displaystyle Op} . According to this definition the VIS is also a test class.
If O p c {\displaystyle Op_{c}} is a test class of O p {\displaystyle Op} , then the predicate P {\displaystyle P} in O p c ′ == [ O p c | P ] {\displaystyle Op_{c'}==[Op_{c}|P]} is said to be the characteristic predicate of O p c ′ {\displaystyle Op_{c'}} or O p c ′ {\displaystyle Op_{c'}} is characterized by P {\displaystyle P} .
Test classes are also called test objectives ( Utting & Legeard 2007 ), test templates ( Stocks & Carrington 1996 ), test specifications and test conditions.
In the context of the TTF a testing tactic [ 1 ] is a means to partition any test class of any operation. However, some of the testing tactics used in practice actually do not always generate a partition of some test classes.
Some testing tactics originally proposed for the TTF are the following:
Some other testing tactics that may also be used are the following:
The application of a testing tactic to the VIS generates some test classes. If some of these test classes are further partitioned by applying one or more testing tactics, a new set of test classes is obtained. This process can continue by applying testing tactics to the test classes generated so far. Evidently, the result of this process can be drawn as a tree with the VIS as the root node, the test classes generated by the first testing tactic as its children, and so on. Furthermore, Stocks and Carrington ( Stocks & Carrington 1996 ) propose to use the Z notation to build the tree, as follows.
V I S == [ I S | P ] T C L T 1 1 == [ V I S | P T 1 1 ] … T C L T 1 n == [ V I S | P T 1 n ] T C L T 2 1 == [ T C L T 1 i | P T 2 1 ] … T C L T 2 m == [ T C L T 1 i | P T 2 m ] … T C L T 3 1 == [ T C L T 2 j | P T 3 1 ] … T C L T 3 k == [ T C L T 2 j | P T 3 k ] … … … {\displaystyle {\begin{aligned}VIS&==[IS|P]\\TCL_{T_{1}}^{1}&==[VIS|P_{T_{1}}^{1}]\\&\dots \\TCL_{T_{1}}^{n}&==[VIS|P_{T_{1}}^{n}]\\TCL_{T_{2}}^{1}&==[TCL_{T_{1}}^{i}|P_{T_{2}}^{1}]\\&\dots \\TCL_{T_{2}}^{m}&==[TCL_{T_{1}}^{i}|P_{T_{2}}^{m}]\\&\dots \\TCL_{T_{3}}^{1}&==[TCL_{T_{2}}^{j}|P_{T_{3}}^{1}]\\&\dots \\TCL_{T_{3}}^{k}&==[TCL_{T_{2}}^{j}|P_{T_{3}}^{k}]\\&\dots \\&\dots \\&\dots \end{aligned}}}
Testing tactics in the TTF tend to produce unsatisfiable test classes. These test classes must be pruned from the testing tree because they represent impossible combinations of input values, i.e. no abstract test case can be derived out of them.
An abstract test case is an element belonging to a test class . The TTF indicates that abstract test cases should be derived only from the leaves of the testing tree . Abstract test cases can also be written as Z schema boxes. Let O p {\displaystyle Op} be some operation, let O p v i s {\displaystyle Op_{vis}} be the VIS of O p {\displaystyle Op} , let x 1 : T 1 … x n : T n {\displaystyle x_{1}:T_{1}\dots x_{n}:T_{n}} be all the variables declared in O p v i s {\displaystyle Op_{vis}} , let O p c {\displaystyle Op_{c}} be a (leaf) test class of the testing tree associated to O p {\displaystyle Op} , let P 1 … P m {\displaystyle P_{1}\dots P_{m}} be the characteristic predicates of each test class from O p c {\displaystyle Op_{c}} up to O p v i s {\displaystyle Op_{vis}} (by following the edges from child to parent ), and let v 1 : T 1 … v n : T n {\displaystyle v_{1}:T_{1}\dots v_{n}:T_{n}} be n {\displaystyle n} constant values satisfying P 1 ∧ ⋯ ∧ P m {\displaystyle P_{1}\land \dots \land P_{m}} . Then, an abstract test case of O p c {\displaystyle Op_{c}} is the Z schema box defined by [ O p c | x 1 = v 1 ∧ ⋯ ∧ x n = v n ] {\displaystyle [Op_{c}|x_{1}=v_{1}\land \dots \land x_{n}=v_{n}]} . | https://en.wikipedia.org/wiki/Test_Template_Framework |
Test and evaluation master plan ( TEMP ) is a critical aspect of project management involving complex systems that must satisfy specification requirements. The TEMP is used to support programmatic events called milestone decisions that separate the individual phases of a project. For military systems, the level of funding determines the Acquisition Category and the organization responsible for the milestone decision. [ 1 ] [ 2 ]
A traceability matrix is generally used to link items within the TEMP to items within specifications .
The Test and Evaluation Master Plan documents the overall structure and objectives of the Test & Evaluation for a program. [ 3 ] It covers activities over a program’s life-cycle and identifies evaluation criteria for the testers. [ 4 ]
The test and evaluation master plan consists of individual tests. Each test contains the following.
The test scenario establishes test conditions. This is typically associated with a specific mission profile. For military systems, this would be a combat scenario, and it may involve Live Fire Test and Evaluation (LFT&E). For commercial systems, this would involve a specific kind of situation involving the use of the item being developed.
For example, cold weather operation may require operation to be evaluated at temperatures below − 40 o {\displaystyle -40^{o}} C using an environmental chamber. Evaluation of operation with a vehicle interface may require compatibility evaluation with a vibration test . Evaluation of an Internet store would require the system to take the user through a product purchase while the system is loaded with other traffic.
The test scenario identifies the following.
Data collection identifies information that must be collected during the test. This involves preliminary setup before the test begins. This may involve preparation for any of the following.
Systems that incorporate a computer typically require the ability to extract and record specific kinds of data from the system while it is operating normally.
Electronic data collection may be started and stopped as one of the actions described in the test scenario.
When data access is restricted, so the transfer of data between organizations may require a Data Collection Plan . This can occur with classified military systems .
Data is analyzed after testing is complete. This analysis is called performance evaluation.
Measures of effectiveness are specific metrics that are used to measure results in the overall mission and execution of assigned tasks.
These may have flexible performance limits associated with the outcome of a specific event. For example, the first round fired from a gun aimed using a radar would not impact a specific location, but the position can be measured using the radar, so the system should be able to deliver a round within a specific radius after several rounds have been fired. The number of rounds required to land one inside the limit is the MOE. The radius would be a Measure Of Performance (MOP).
Measures of performance are specific metrics that have a pass or fail limit that must be satisfied. These are generally identified with the words shall or must in the specification.
One type of MOP is the distance that a vehicle with a specific load must travel at a specific speed before running out of fuel.
Another type of MOP is the distance that a radar can detect a 1 square meter reflector.
Measures of suitability evaluate the ability to be supported in its intended operational environment.
As an example, this may be an evaluation of the Mean Time Between Failure (MTBF) that is evaluated during other testing. A system with excessive failures may satisfy all other requirements and not be suitable for use. A gun that jams when dirty is not suitable for military use.
These requirements are associated with ilities.
The results of the TEMP evaluation are used for multiple purposes.
Mission planning involves translation of MOEs, MOPs, and MOSs into the following.
ROC and POE are specific expectations evaluated by the TEMP that are used to determine how to deploy assets to satisfy a specific mission requirement. Diagnostic testing ensures these expectations are satisfied for the duration of the mission.
The term 'test and evaluation master plan' as a distinct overall guide to the Test and Evaluation functions in a development has also been used by the Australian Department of Defence. [ 5 ] Others do use the term, or similar terms such as 'Master Test and Evaluation Plan'. | https://en.wikipedia.org/wiki/Test_and_evaluation_master_plan |
In computer architecture , the test-and-set CPU instruction (or instruction sequence) is designed to implement mutual exclusion in multiprocessor environments. Although a correct lock can be implemented with test-and-set, the test and test-and-set optimization lowers resource contention caused by bus locking, especially cache coherency protocol overhead on contended locks.
Given a lock:
the entry protocol is:
and the exit protocol is:
The difference to the simple test-and-set protocol is the additional spin-loop (the test in test and test-and-set ) at the start of the entry protocol, which utilizes ordinary load instructions. The load in this loop executes with less overhead compared to an atomic operation (resp. a load-exclusive instruction). E.g., on a system utilizing the MESI cache coherency protocol, the cache line being loaded is moved to the Shared state, whereas a test-and-set instruction or a load-exclusive instruction moves it into the Exclusive state.
This is particularly advantageous if multiple processors are contending for the same lock: whereas an atomic instruction or load-exclusive instruction requires a coherency-protocol transaction to give that processor exclusive access to the cache line (causing that line to ping-pong between the involved processors), ordinary loads on a line in Shared state require no protocol transactions at all: processors spinning in the inner loop operate purely locally.
Cache-coherency protocol transactions are used only in the outer loop, after the initial check has ascertained that they have a reasonable likelihood of success.
If the programming language used supports short-circuit evaluation , the entry protocol could be implemented as:
Although this optimization is useful in system programming , test-and-set is to be avoided in high-level concurrent programming : spinning in applications deprives the operating system scheduler
the knowledge of who is blocking on what. Consequently, the scheduler will have to guess on how to allocate CPU time among the threads -- typically just allowing the threads to use up their timing quota. Threads will end up spinning unproductively, waiting for threads that are not scheduled.
By using operating-system provided lock objects, such as mutexes, the OS can schedule exactly the unblocked threads. | https://en.wikipedia.org/wiki/Test_and_test-and-set |
In computer software testing, a test assertion is an expression which encapsulates some testable logic specified about a target under test. The expression is formally presented as an assertion , along with some form of identifier , to help testers and engineers ensure that tests of the target relate properly and clearly to the corresponding specified statements about the target.
Usually the logic for each test assertion is limited to one single aspect specified. A test assertion may include prerequisites which must be true for the test assertion to be valid.
This computing article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Test_assertion |
Test compression is a technique used to reduce the time and cost of testing integrated circuits . The first ICs were tested with test vectors created by hand. It proved very difficult to get good coverage of potential faults, so Design for testability (DFT) based on scan and automatic test pattern generation (ATPG) were developed to explicitly test each gate and path in a design. These techniques were very successful at creating high-quality vectors for manufacturing test, with excellent test coverage. However, as chips got bigger and more complex the ratio of logic to be tested per pin increased dramatically, and the volume of scan test data started causing a significant increase in test time, and required tester memory. This raised the cost of testing.
Test compression was developed to help address this problem. When an ATPG tool generates a test for a fault, or a set of faults, only a small percentage of scan cells need to take specific values. The rest of the scan chain is don't care , and are usually filled with random values. Loading and unloading these vectors is not a very efficient use of tester time. Test compression takes advantage of the small number of significant values to reduce test data and test time. In general, the idea is to modify the design to increase the number of internal scan chains, each of shorter length. These chains are then driven by an on-chip decompressor, usually designed to allow continuous flow decompression where the internal scan chains are loaded as the data is delivered to the decompressor. Many different decompression methods can be used. [ 1 ] One common choice is a linear finite state machine, where the compressed stimuli are computed by solving linear equations corresponding to internal scan cells with specified positions in partially specified test patterns. Experimental results show that for industrial circuits with test vectors and responses with very low fill rates, ranging from 3% to 0.2%, the test compression based on this method often results in compression ratios of 30 to 500 times. [ 2 ]
With a large number of test chains, not all the outputs can be sent to the output pins. Therefore, a test response compactor is also required, which must be inserted between the internal scan chain outputs and the tester scan channel outputs. The compactor must be synchronized with the data decompressor, and must be capable of handling unknown (X) states. (Even if the input is fully specified by the decompressor, these can result from false and multi-cycle paths, for example.) Another design criteria for the test result compressor is that it should give good diagnostic capabilities, not just a yes/no answer. | https://en.wikipedia.org/wiki/Test_compression |
Test data are sets of inputs or information used to verify the correctness, performance, and reliability of software systems. Test data encompass various types, such as positive and negative scenarios, edge cases, and realistic user scenarios, and aims to exercise different aspects of the software to uncover bugs and validate its behavior. Test data is also used in regression testing to verify that new code changes or enhancements do not introduce unintended side effects or break existing functionalities. [ 1 ]
Test data may be used to verify that a given set of inputs to a function produces an expected result. Alternatively, data can be used to challenge the program's ability to handle unusual, extreme, exceptional, or unexpected inputs. [ 2 ]
Test data can be produced in a focused or systematic manner, as is typically the case in domain testing, or through less focused approaches, such as high-volume randomized automated tests. [ 3 ] Test data can be generated by the tester or by a program or function that assists the tester. It can be recorded for reuse or used only once. Test data may be created manually, using data generation tools (often based on randomness), [ 4 ] or retrieved from an existing production environment. The data set may consist of synthetic (fake) data, but ideally, it should include representative (real) data. [ 5 ]
Due to privacy regulations such as GDPR, PCI, and the HIPAA , the use of privacy-sensitive personal data for testing is restricted. [ 6 ] However, anonymized (and preferably subsetted [ clarification needed ] ) production data may be used as representative data for testing and development. [ 7 ] Programmers may also choose to generate synthetic data as an alternative to using real or anonymized data. While synthetic data can offer significant advantages, such as enhanced privacy and flexibility, it also comes with limitations. For instance, generating synthetic data that accurately reflects real-world complexity can be challenging. There is also a risk of synthetic data not fully capturing the nuances of real data, potentially leading to gaps in test coverage. [ 8 ]
Domain testing is a set of techniques focusing on test data. This includes identifying critical inputs, values at the boundaries between equivalence classes, and combinations of inputs that drive the system toward specific outputs. Domain testing helps ensure that various scenarios are effectively tested, including edge cases and unusual conditions. [ 9 ] | https://en.wikipedia.org/wiki/Test_data |
Test environment management ( TEM ) is a function in a software delivery process which aids the software testing cycle by providing a validated, stable and usable test environment to execute the test scenarios or replicate bugs.
As with a scientific experiment, in Testing repeatability and control of variables is essential. In testing a key component of this control is to manage the environment in which testing is taking place. This environment specifically includes the underlying hardware and software which supports the actual software under test. This encompasses items such as servers, operating systems , communications tools, databases , cloud ecosystems browsers.
In early testing stages only limited formal management of environments is required, if any. For example programmers may typically perform their testing within standardised IDEs which provide control by default. However at later stages, test execution will tend to work across multiple technologies and development streams, and typically involving multiple (teams of) testers. In these circumstances individual testers cannot reasonably be expected to exercise control over the technical landscape. This is where the need for some formal Test Environment Management function arises.
The activities under the TEM function include:
Many teams use spreadsheets instead of using specific tools for the first two areas if the data is less. However, if the data is more, it is recommended to use specialized tools for it. | https://en.wikipedia.org/wiki/Test_environment_management |
A test probe is a physical device used to connect electronic test equipment to a device under test (DUT). Test probes range from very simple, robust devices to complex probes that are sophisticated, expensive, and fragile. Specific types include test prods , oscilloscope probes and current probes . A test probe is often supplied as a test lead , which includes the probe, cable and terminating connector.
Voltage probes are used to measure voltages present on the DUT. To achieve high accuracy, the test instrument and its probe must not significantly affect the voltage being measured. This is accomplished by ensuring that the combination of instrument and probe exhibit a sufficiently high impedance that will not load the DUT. For AC measurements, the reactive component of impedance may be more important than the resistive.
A typical voltmeter probe consists of a single wire test lead that has on one end a connector that fits the voltmeter and on the other end a rigid, tubular plastic section that comprises both a handle and probe body. The handle allows a person to hold and guide the probe without influencing the measurement (by becoming part of the electric circuit) or being exposed to dangerous voltages that might cause electric shock . Within the probe body, the wire is connected to a rigid, pointed metal tip that contacts the DUT. Some probes allow an alligator clip to be attached to the tip, thus enabling the probe to be attached to the DUT so that it need not be held in place.
Test leads are usually made with finely stranded wire to keep them flexible, of wire gauges sufficient to conduct a few amperes of electric current . The insulation is chosen to be both flexible and have a breakdown voltage higher than the voltmeter's maximum input voltage. The many fine strands and the thick insulation make the wire thicker than ordinary hookup wire.
Two probes are used together to measure voltage, current, and two-terminal components such as resistors and capacitors. When making DC measurements it is necessary to know which probe is positive and which is negative, so by convention the probes are colored red for positive and black for negative. Depending upon the accuracy required, they can be used with signal frequencies ranging from DC to a few kilohertz .
When sensitive measurements must be made (e.g., very low voltages, or very low or very high resistances) shields, guards, and techniques such as four-terminal Kelvin sensing (using separate leads to carry the measuring current and to sense the voltage) are used.
Tweezer probes are a pair of simple probes fixed to a tweezer mechanism, operated with one hand, for measuring voltages or other electronic circuit parameters between closely spaced pins.
Spring probes (a.k.a. " pogo pins ") are spring-loaded pins used in electrical test fixtures to contact test points, component leads, and other conductive features of the DUT (Device Under Test). These probes are usually press-fit into probe sockets, to allow their easy replacement on test fixtures which may remain in service for decades, testing many thousands of DUTs in automatic test equipment .
Oscilloscopes display the instantaneous waveform of varying electrical quantities, unlike other instruments which give numerical values of relatively stable quantities.
Scope probes fall into two main categories: passive and active.
Passive scope probes contain no active electronic parts, such as transistors , so they require no external power.
Because of the high frequencies often involved, oscilloscopes do not normally use simple wires ("flying leads") to connect to the DUT. Flying leads are likely to pick up interference, so they are not suitable for low-level signals. Furthermore, the inductance of flying leads make them unsuitable for high frequency signals. Instead, a specific scope probe is used, which uses a coaxial cable to transmit the signal from the tip of the probe to the oscilloscope. This cable has two main benefits: it protects the signal from external electromagnetic interference, improving accuracy for low-level signals; and it has a lower inductance than flying leads, making the probe more accurate for high-frequency signals.
Although coaxial cable has lower inductance than flying leads, it has higher capacitance: a typical 50 ohm cable has about 90 pF per meter. Consequently, a one-meter high-impedance direct (1×) coaxial probe may load the circuit with a capacitance of about 110 pF and a resistance of 1 megohm.
Oscilloscope probes are characterised by their frequency limit, where the amplitude response has fallen by 3 dB, and/or by their rise time t r {\displaystyle t_{r}} . These are related as (in round figures)
Thus a 50 MHz probe has a rise time of 7 ns. The response of the combination of an oscilloscope and a probe is given by
For example, a 50 MHz probe feeding a 50 MHz scope will give a 35 MHz system. It is therefore advantageous to use a probe with a higher frequency limit to minimize the effect on the overall system response.
To minimize loading, attenuator probes (e.g., 10× probes) are used. A typical probe uses a 9 megohm series resistor shunted by a low-value capacitor to make an RC compensated divider with the cable capacitance and scope input. The RC time constants are adjusted to match. For example, the 9 megohm series resistor is shunted by a 12.2 pF capacitor for a time constant of 110 microseconds. The cable capacitance of 90 pF in parallel with the scope input of 20 pF (total capacitance 110 pF) and 1 megohm also gives a time constant of 110 microseconds. In practice, there will be an adjustment so the operator can precisely match the low frequency time constant (called compensating the probe). Matching the time constants makes the attenuation independent of frequency. At low frequencies (where the resistance of R is much less than the reactance of C ), the circuit looks like a resistive divider; at higher frequencies (resistance much greater than reactance), the circuit looks like a capacitive divider. [ 1 ]
The result is a frequency compensated probe for modest frequencies that presents a load of about 10 megohms shunted by 12 pF. Although such a probe is an improvement, it does not work when the time scale shrinks to several cable transit times (transit time is typically 5 ns). In that time frame, the cable looks like its characteristic impedance, and there will be reflections from the transmission line mismatch at the scope input and the probe that causes ringing. [ 2 ] The modern scope probe uses lossy low capacitance transmission lines and sophisticated frequency shaping networks to make the 10× probe perform well at several hundred megahertz. Consequently, there are other adjustments for completing the compensation. [ 3 ] [ 4 ] [ 5 ]
A directly connected test probe (so called 1× probe) puts the unwanted lead capacitance across the circuit under test. For a typical coaxial cable , loading is of the order of 100pF per meter (the length of a typical test lead).
Attenuator probes minimize capacitive loading with an attenuator, but reduce the magnitude of the signal delivered to the instrument. A 10× attenuator will reduce the capacitive load by a factor of about 10. The attenuator must have an accurate ratio over the whole range of frequencies of interest; the input impedance of the instrument becomes part of the attenuator. A DC attenuator with resistive divider is supplemented with capacitors, so that the frequency response is predictable over the range of interest. [ 6 ]
The RC time constant matching method works as long as the transit time of the shielded cable is much less than the time scale of interest. That means that the shielded cable can be viewed as a lumped capacitor rather than an inductor. Transit time on a 1-meter cable is about 5 ns. Consequently, these probes will work to a few megahertz, but after that transmission line effects cause trouble.
At high frequencies, the probe impedance will be low. [ 7 ]
The most common design inserts a 9 megohm resistor in series with the probe tip. The signal is then transmitted from the probe head to the oscilloscope over a special lossy coaxial cable that is designed to minimize capacitance and ringing . The invention of this cable has been traced [ 8 ] to John Kobbe, an engineer working for Tektronix . The resistor serves to minimize the loading that the cable capacitance would impose on the DUT. In series with the normal 1 megohm input impedance of the oscilloscope, the 9 megohm resistor creates a 10× voltage divider so such probes are normally known as either low cap(acitance) probes or 10× probes, often printed with the letter X or x instead of the multiplication sign, and usually spoken of as "a times-ten probe".
Because the oscilloscope input has some parasitic capacitance in parallel with the 1 megohm resistance, the 9 megohm resistor must also be bypassed by a capacitor to prevent it from forming a severe RC low-pass filter with the 'scope's parasitic capacitance. The amount of bypass capacitance must be carefully matched with the input capacitance of the oscilloscope so that the capacitors also form a 10× voltage divider. In this way, the probe provides a uniform 10× attenuation from DC (with the attenuation provided by the resistors) to very high AC frequencies (with the attenuation provided by the capacitors).
In the past, the bypass capacitor in the probe head was adjustable (to achieve this 10× attenuation). More modern probe designs use a laser-trimmed thick-film electronic circuit in the head that combines the 9 megohm resistor with a fixed-value bypass capacitor; they then place a small adjustable capacitor in parallel with the oscilloscope's input capacitance. Either way, the probe must be adjusted so that it provides uniform attenuation at all frequencies. This is referred to as compensating the probe . Compensation is usually accomplished by probing a 1 kHz square wave and adjusting the compensating capacitor until the oscilloscope displays the most square waveshape. Most oscilloscopes have a 1 kHz calibration source on their front panels since probe compensation must be done every time a 10:1 probe is attached to an oscilloscope input. Newer, faster probes have more complex compensation arrangements and may occasionally require further adjustments.
100× passive probes are also available, as are some designs specialized for use at very high voltages (up to 25 kV).
Passive probes usually connect to the oscilloscope using a BNC connector . Most 10× probes are equivalent to a load of about 10-15 pF and 10 megohms on the DUT, while 100× probes typically present a 100 megohm load and a smaller capacitance, and therefore load the circuit less.
Z 0 probes are a specialized type of low-capacitance passive probe used in low-impedance , very-high-frequency circuits. They are similar in design to 10× passive probes but at much lower impedance levels. The probe cables usually have a characteristic impedance of 50 ohms and connect to oscilloscopes with a matched 50 ohm (rather than a 1 megohm) input impedance). High-impedance scope probes are designed for the conventional 1 megohm oscilloscope, but the 1 megohm input impedance is only at low frequency; the input impedance is not a constant 1 megohm across the probe's bandwidth but rather decreases with frequency. For example, a Tektronix P6139A input impedance starts falling above 10 kHz and is about 100 ohms at 100 MHz. [ 9 ] A different probe technique is needed for high frequency signals.
A high frequency oscilloscope presents a matched load (usually 50 ohms) at its input, which minimizes reflections at the scope. Probing with a matching 50-ohm transmission line would offer high frequency performance, but it would unduly load most circuits. An attenuator (resistive divider) can be used to minimize loading. At the tip, these probes use a 450 ohm (for 10× attenuation) or 950 ohm (for 20× attenuation) series resistor. [ 10 ] [ 11 ] Tektronix sells a 10× divider probe with a 9 GHz bandwidth with a 450 ohm series resistor. [ 12 ] [ failed verification ] These probes are also called resistive divider probes, since a 50 ohm transmission line presents a purely resistive load.
The Z 0 name refers to the characteristic impedance of the oscilloscope and cable. The matched impedances provide better high-frequency performance than an unmatched passive probe can achieve, but at the expense of the low 500-ohm load offered by the probe tip to the DUT. Parasitic capacitance at the probe tip is very low so, for very high-frequency signals, the Z 0 probe can offer lower loading than any hi-Z probe and even many active probes. [ 13 ]
In principle this type of probe can be used at any frequency, but at DC and lower frequencies circuits often have high impedances that would be unacceptably loaded by the probe's low 500 or 1000 ohm probe impedance. Parasitic impedances limit very-high-frequency circuits to operating at low impedance, so the probe impedance is less of a problem.
Active scope probes use a high-impedance high-frequency amplifier mounted in the probe head, and a screened lead. The purpose of the amplifier is not gain, but isolation (buffering) between the circuit under test and the oscilloscope and cable, loading the circuit with only a low capacitance and high DC resistance, and matching the oscilloscope input. Active probes are commonly seen by the circuit under test as a capacitance of 1 picofarad or less in parallel with 1 megohm resistance. Probes are connected to the oscilloscope with a cable matching the characteristic impedance of the oscilloscope input. Tube based active probes were used before the advent of high-frequency solid-state electronics , using a small vacuum tube as cathode follower amplifier.
Active probes have several disadvantages which have kept them from replacing passive probes for all applications:
Many active probes allow the user to introduce an offset voltage to allow measurement of voltages with excessive DC level. The total dynamic range is still limited, but the user may be able to adjust its centerpoint so that voltages in the range of, for example, zero to five volts may be measured rather than -2.5 to +2.5.
Because of their inherent low voltage rating, there is little need to provide high-voltage insulation for operator safety. This allows the heads of active probes to be extremely small, making them very convenient for use with modern high-density electronic circuits.
Passive probes and a modest active probe design is discussed in an application note by Williams. [ 14 ]
The Tektronix P6201 is an early DC to 900 MHz active FET probe. [ 15 ]
At extreme high frequencies a modern digital scope requires that the user solder a preamp to the DUT to get 50GS/s, 20 GHz performance. [ 16 ]
Differential probes are optimized for acquiring differential signals . To maximize the common-mode rejection ratio (CMRR), differential probes must provide two signal paths that are as nearly identical as possible, matched in overall attenuation, frequency response, and time delay.
In the past, this was done by designing passive probes with two signal paths, requiring a differential amplifier stage at or near the oscilloscope. (A very few early probes fitted the differential amplifier into a rather-bulky probe head using vacuum tubes.) With advances in solid-state electronics, it has become practical to put the differential amplifier directly within the probe head, greatly easing the requirements on the rest of the signal path (since it now becomes single-ended rather than differential and the need to match parameters on the signal path is removed). A modern differential probe usually has two metal extensions which can be adjusted by the operator to simultaneously touch the appropriate two points on the DUT. Very high CMRRs are thereby made possible.
All scope probes contain some facility for grounding (earthing) the probe to the circuit's reference voltage. This is usually accomplished by connecting a very short pigtail wire from the probe head to ground. Inductance in the ground wire can lead to distortion in the observed signal, so this wire is kept as short as possible. Some probes use a small ground foot instead of any wire, allowing the ground link to be as short as 10 mm.
Most probes allow a variety of "tips" to be installed. A pointed tip is the most common, but a seizer probe or "test hook" with a hooked tip that can secure to the test point, is also commonly used. Tips that have a small plastic insulating foot with indentations into it can make it easier to probe very-fine-pitch integrated circuits ; the indentations mate with the pitch of the IC leads, stabilizing the probe against the shaking of the user's hand and thereby help to maintain contact on the desired pin. Various styles of feet accommodate various pitches of the IC leads. Different types of tips can also be used for probes for other instruments.
Some probes contain a push button. Pressing the button will either disconnect the signal (and send a ground signal to the 'scope) or cause the 'scope to identify the trace in some other way. This feature is very useful when simultaneously using more than one probe as it lets the user correlate probes and traces on the 'scope screen.
Some probe designs have additional pins surrounding the BNC or use a more complex connector than a BNC. These extra connections allow the probe to inform the oscilloscope of its attenuation factor (10×, 100×, other). The oscilloscope can then adjust its user displays to automatically take into account the attenuation and other factors caused by the probe. These extra pins can also be used to supply power to active probes.
Some ×10 probes have a "×1/×10" switch. The "×1" position bypasses the attenuator and compensating network, and can be used when working with very small signals that would be below the scope's sensitivity limit if attenuated by ×10.
Because of their standardized design, passive probes (including Z 0 probes) from any manufacturer can usually be used with any oscilloscope (although specialized features such as the automatic readout adjustment may not work). Passive probes with voltage dividers may not be compatible with a particular scope. The compensation adjustment capacitor only allows for compensation over a small range of oscilloscope input capacitance values. The probe compensation range must be compatible with the oscilloscope input capacitance.
On the other hand, active probes are almost always vendor-specific due to their power requirements, offset voltage controls, etc. Probe manufacturers sometimes offer external amplifiers or plug-in AC power adapters that allow their probes to be used with any oscilloscope.
A high voltage probe allows an ordinary voltmeter to measure voltages that would otherwise be too high to measure or even destructive. It does this by reducing the input voltage to a safe, measurable level with a precision voltage divider circuit within the probe body.
Probes intended for up to 100 kV typically employ a resistor voltage divider, with an input resistance of hundreds or thousands of megohms to minimize circuit loading. High linearity and accuracy is achieved by using resistors with extremely low voltage coefficients, in matched sets that maintain a consistent, precise divider ratio across the probe's operating temperature. Voltmeters have input resistance that effectively alters the probe's divider ratio, and parasitic capacitance that combines with the probe's resistance to form an RC circuit ; these can easily reduce DC and AC accuracy, respectively, if left uncompensated. To mitigate these effects, voltage divider probes usually include additional components that improve frequency response and allow them to be calibrated for different meter loads.
Even higher voltages can be measured with capacitor divider probes, though the larger physical size and other mechanical features (e.g., corona rings ) of these devices often preclude their use as handheld probes.
A current probe generates a voltage proportional to a current in the circuit being measured; as the proportionality constant is known, instruments that respond to voltage can be calibrated to indicate current. Current probes can be used both by measuring instruments and oscilloscopes.
The classic current probe is a low valued resistor (a "sampling resistor" or "current shunt") inserted in the current's path. The current is determined by measuring the voltage drop across the resistor and using Ohm's law . ( Wedlock & Roberge 1969 , p. 152.) The sampling resistance needs to be small enough not to affect circuit operation significantly, but large enough to provide a good reading. The method is valid for both AC and DC measurements. A disadvantage of this method is the need to break the circuit to introduce the shunt. Another problem is measuring the voltage across the shunt when common-mode voltages are present; a differential voltage measurement is needed.
Alternating currents are relatively easy to measure as transformers can be used.
A current transformer is commonly used to measure alternating currents. The current to be measured is forced through the primary winding (often a single turn) and the current through the secondary winding is found by measuring the voltage across a current-sense resistor (or " burden resistor "). The secondary winding has a burden resistor to set the current scale. The properties of a transformer offer many advantages. The current transformer rejects common mode voltages, so an accurate single-ended voltage measurement can be made on a grounded secondary. The effective series resistance R s {\displaystyle R_{s}} of the primary winding is set by the burden resistor on the secondary winding R {\displaystyle R} and the transformer turns ratio N {\displaystyle N} , where: R s = R / N 2 {\displaystyle R_{s}=R/N^{2}} .
The core of some current transformers is split and hinged; it is opened and clipped around the wire to be sensed, then closed, making it unnecessary to free one end of the conductor and thread it through the core.
Another clip-on design is the Rogowski coil . It is a magnetically balanced coil that measures current by electronically evaluating the line integral around a current.
High-frequency, small-signal, passive current probes typically have a frequency range of several kilohertz to over 100 MHz. The Tektronix P6022 has a range from 935 Hz to 200 MHz. ( Tektronix 1983 , p. 435)
Transformers cannot be used to probe direct currents (DC).
Some DC probe designs use the nonlinear properties of a magnetic material to measure DC.
Other current probes use Hall effect sensors to measure the magnetic field around a wire produced by an electric current through the wire without the need to interrupt the circuit to fit the probe. They are available for both voltmeters and oscilloscopes. Most current probes are self-contained, drawing power from a battery or the instrument, but a few require the use of an external amplifier unit. (See also: Clamp meter )
More advanced current probes combine a Hall effect sensor with a current transformer. The Hall effect sensor measures the DC and low frequency components of the signal and the current transformer measures the high frequency components. These signals are combined in the amplifier circuit to yield a wide band signal extending from DC to over 50 MHz. ( Wedlock & Roberge 1969 , p. 154) The Tektronix A6302 current probe and AM503 amplifier combination is an example of such a system. ( Tektronix 1983 , p. 375) ( Tektronix 1998 , p. 571)
Near-field probes allow the measurement of an electromagnetic field . They are commonly used to measure electrical noise and other undesirable electromagnetic radiation from the DUT, although they can also be used to spy on the workings of the DUT without introducing much loading into the circuitry.
They are commonly connected to spectrum analyzers .
Temperature probes are used to make contact measurements of surface temperatures. They employ a temperature sensor such as a thermistor , thermocouple , or RTD , to produce a voltage that varies with temperature. In the case of thermistor and RTD probes, the sensor must be electrically stimulated to produce a voltage, whereas thermocouple probes do not require stimulation because a thermocouple will independently produce an output voltage.
Voltmeters can sometimes be used to measure temperature probes, but this task is usually delegated to specialized instruments that will stimulate the probe's sensor (if necessary), measure the probe's output voltage, and convert the voltage to temperature units.
To measure or display the modulating waveform of a modulated high-frequency signal—for example, an amplitude-modulated radio signal—a probe fitted with a simple diode demodulator can be used. The probe will output the modulating waveform without the high-frequency carrier .
A logic probe is used for observing digital signals . | https://en.wikipedia.org/wiki/Test_probe |
In computer science and engineering , a test vector is a set of inputs provided to a system in order to test that system. In software development , test vectors are a methodology of software testing and software verification and validation .
In computer science and engineering, a system acts as a computable function . An example of a specific function could be y = f ( x ) {\displaystyle y=f(x)} where y {\displaystyle y} is the output of the system and x {\displaystyle x} is the input; however, most systems' inputs are not one-dimensional. When the inputs are multi-dimensional, we could say that the system takes the form y = f ( x 1 , x 2 , . . . ) {\displaystyle y=f(x_{1},x_{2},...)} ; however, we can generalize this equation to a general form Y = C ( X ) {\displaystyle Y=C(X)} where Y {\displaystyle Y} is the result of the system's execution, C {\displaystyle C} belongs to the set of computable functions , and X {\displaystyle X} is an input vector. While testing the system, various test vectors must be used to examine the system's behavior with differing inputs.
For example, consider a login page with two input fields: a username field and a password field. In that case, the login system can be described as:
y = L ( u , p ) {\displaystyle y=L(u,p)}
with y ∈ { t r u e , f a l s e } {\displaystyle y\in \{true,false\}} and u , p ∈ { S t r i n g } {\displaystyle u,p\in \{String\}} , with t r u e {\displaystyle true} designating login successful, and f a l s e {\displaystyle false} designating login failure, respectively.
Making things more generic, we can suggest that the function L {\displaystyle L} takes input as a 2-dimensional vector and outputs a one-dimensional vector ( scalar ).
This can be written in the following way:-
Y = L ( X ) {\displaystyle Y=L(X)}
with X = [ x 1 , x 2 ] = [ u , p ] ; Y = [ y 1 ] {\displaystyle X=[x_{1},x_{2}]=[u,p]\;;\;Y=[y_{1}]}
In this case, X {\displaystyle X} is called the input vector, and Y {\displaystyle Y} is called the output vector.
In order to test the login page, it is necessary to pass some sample input vectors { X 1 , X 2 , X 3 , . . . } {\displaystyle \{X_{1},X_{2},X_{3},...\}} . In this context X i {\displaystyle X_{i}} is called a test vector.
Alternatively, the concatenation of X {\displaystyle X} and Y {\displaystyle Y} , e.g., [ x 1 , x 2 , y 1 ] {\displaystyle [x_{1},x_{2},y_{1}]} , can be called a test vector. | https://en.wikipedia.org/wiki/Test_vector |
A testbed aircraft is an aeroplane, helicopter or other kind of aircraft intended for flight research or testing the aircraft concepts or on-board equipment. These could be specially designed or modified from serial production aircraft. [ 1 ] [ 2 ]
For example, in development of new aircraft engines , these are fitted to a testbed aircraft for flight testing, before certification. New instruments wiring and equipment, a fuel system and piping, structural alterations to the wings, and other adjustments are needed for this adaptation. [ 3 ] [ 4 ]
The Folland Fo.108 (nicknamed the "Folland Frightful") was a dedicated engine testbed aircraft in service from 1940. The aircraft had a mid-fuselage cabin for test instrumentation and observers. Twelve were built and provided to British aero-engine companies. A large number of aircraft-testbeds have been produced and tested since 1941 in the USSR and Russia by the Gromov Flight Research Institute . [ 2 ] [ 5 ]
AlliedSignal , [ 6 ] Honeywell Aerospace , [ 7 ] Pratt & Whitney , [ 8 ] and other aerospace companies used Boeing jetliners as flying testbed aircraft. [ 9 ]
This aircraft-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Testbed_aircraft |
In software engineering , tester-driven development , or bug-driven development , is an anti-pattern where the requirements are determined by bug reports or test results rather than, for example, the value or cost of a feature . The concept is generally invoked facetiously, and comes with the implication that high volumes of computer code are written with little regard for unit testing by the programmers.
The term itself is a tongue-in-cheek reference to test-driven development , a widely used methodology in agile software practices . In test-driven development tests are used to drive the implementation towards fulfilling the requirements. Tester-driven development instead shortcuts the process by removing the determination of requirements and letting the testers (or the QA team ) drive what they think the software should be through the testing (or QA) process. [ 1 ]
Projects that are developed using this anti-pattern often suffer from being extremely late. Another common problem is poor code quality .
Common causes for projects ending up being run this way are often:
Things get worse when the testers realize that they don't know what the requirements are and therefore don't know how to test any particular code changes. The onus then falls on the developers of individual changes to write their own test cases and they are happy to do so because their own tests normally pass and their performance measurements improve. Project leaders are also delighted by the rapid reduction in the number of open change requests.
This software-engineering -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tester-driven_development |
Testican is a type of proteoglycan . Testican-1 is a highly conserved, multidomain proteoglycan that is most prominently expressed in the thalamus , and is upregulated in activated astroglial cells of the cerebrum. Several functions of this gene product have now been demonstrated in vitro including membrane-type matrix metalloproteinase inhibition, cathepsin L inhibition, and low-affinity calcium binding. The purified gene product has been shown to inhibit cell attachment and neurite extensions in culture. Functions of testican in vivo have yet to be demonstrated in knockout mice or other models. Testican has been shown to carry substantial amounts of chondroitin sulfate as well as other oligosaccharides , but the biological significance of these embellishments is not yet known.
In humans there are three testicans:
Testican-1 plays a role in lapatinib resistance, which is a drug used to treat HER2-positive gastric cancer. [ 1 ] When testican-1 levels are artificially reduced, sensitivity towards lapatinib was once again increased. [ 1 ] This shows the potential for future use in combating drug resistance.
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Testican |
Testicular Immunology is the study of the immune system within the testis . It includes an investigation of the effects of infection , inflammation and immune factors on testicular function. Two unique characteristics of testicular immunology are evident: (1) the testis is described as an immunologically privileged site , where suppression of immune responses occurs; and, (2) some factors which normally lead to inflammation are present at high levels in the testis, where they regulate the development of sperm instead of promoting inflammation.
Immune cells of the human testis are not as well characterized as those from rodents, due to the rarity of normal human testes available for experiment. The majority of experiments have studied the rat testis due to its convenience: it is of relatively large size and is easily extracted from experimental animals.
Macrophages are directly involved in the fight against invading micro-organisms as well as being antigen-presenting cells which activate lymphocytes . Early studies demonstrated the presence of macrophages in the rat testis [ 5 ] Testicular macrophages are the largest population of immune cells in the rodent testis. [ 6 ] [ 7 ] Macrophages have also been found in the testes of humans, [ 8 ] guinea pigs, hamsters, [ 9 ] boars, [ 10 ] horses [ 11 ] and bulls. [ 12 ] They originate from blood monocytes which move into the testis then mature into macrophages. In the rat, testicular macrophages have been described as either “resident” or “newly arrived” from the blood supply. [ 13 ] [ 14 ] It is likely that most of the adult population of testicular macrophages in adult rats are a result of very rapid proliferation of early precursors that entered the testis during postnatal maturation [ 15 ]
Testicular macrophages can respond to infectious stimuli and become activated (undergo changes enabling the killing of the invading micro-organism), but do so to a lesser extent than other types of macrophages. [ 16 ] [ 17 ] An example is production of the inflammatory cytokines TNFα and IL-1 β by activated rat testicular macrophages: these macrophages produce significantly less TNFα and IL-1β than activated rat peritoneal macrophages. [ 17 ] [ 18 ] Aside from responding to infectious stimuli, testicular macrophages are also involved in maintaining normal testis function. They have been shown to secrete 25-hydroxycholesterol, a sterol that can be converted to testosterone by Leydig cells. [ 19 ] Their presence is necessary for the normal development and function of the Leydig cells , [ 20 ] [ 21 ] [ 22 ] which are the testosterone-producing cells of the testis.
B-lymphocytes take part in the adaptive immune response and produce antibodies. These cells are not normally found in the testis, even during inflammatory conditions. The lack of B-lymphocytes in the testis is significant, since these are the antibody-producing cells of the immune system. Since anti-sperm antibodies can cause infertility, it is important that antibody-producing B-lymphocytes are kept separated from the testis.
T-lymphocytes (T-cells) are white blood cells which take part in cell-mediated immunity. They are often found within tissues where they can be activated by antigen-presenting cells upon infection. They are present in rat [ 23 ] [ 24 ] and human testes, [ 25 ] where they constitute approximately 10 to 20% of the immune cells present, as well as mouse [ 26 ] and ram [ 24 ] testes. Both cytotoxic T-cells and Helper T cells are found in the testes of rats. [ 27 ] Also present in the testes of rats and humans are natural killer cells [ 1 ] [ 27 ] and Natural killer T cells have been found in rats and mice.
Mast cells are regulators of immune responses, particularly those against parasites . They are also involved in the development of autoimmune diseases and allergies . Mast cells have been found in relatively low numbers in the testes of humans, rats, mice, dogs, cats, bulls, boars and deer. [ 28 ] [ 29 ] In the mammalian testis mast cells regulate testosterone production. [ 29 ] There are two lines of evidence that restriction of mast cell activation in the testis could be beneficial during treatment of inflammatory conditions; (1) In experimental models of testicular inflammation, mast cells were present in 10-fold greater numbers and showed signs of activation, [ 30 ] and (2) Treatment with drugs which stabilize mast cell activation has proved beneficial in treating some types of male infertility. [ 31 ] [ 32 ] [ 33 ]
Eosinophils directly fight parasitic infections and are involved in allergic reactions. They have been found in relatively low numbers in the rat, mouse, dog, cat, bull and deer testes. [ 28 ] Almost nothing is known about their significance or function in the testis.
Dendritic cells initiate adaptive immune responses . Relatively small amounts of dendritic cells have been found in the testes of humans, [ 34 ] rats [ 35 ] and mice. [ 36 ] [ 37 ] The functional role of dendritic cells in the testis is not well understood, although they have been shown to be involved in autoimmune orchitis during animal experiments. [ 28 ] [ 35 ] When autoimmune orchitis is induced in rats, the dendritic cell population of the testis greatly increases. [ 35 ] This is likely to contribute to testicular inflammation, considering the well-established role of dendritic cells in other types of autoimmune inflammation. [ 38 ]
Neutrophils are white blood cells which are present in the blood but not normally in tissues. They move out from the blood into tissues and organs upon infection or damage. They directly fight invading pathogens such as bacteria. Neutrophils are not found in the rodent testis under normal conditions but can enter from the blood supply upon infection or inflammatory stimulus. This has been demonstrated in the rat after injection with bacterial cell wall components to produce an immune reaction. [ 39 ] Neutrophils also enter the rat testis after treatment with hormones that increase the permeability of blood vessels. [ 40 ] In humans, neutrophils have been found in the testis when associated with some tumors. [ 41 ] In rat experiments, testicular torsion leads to neutrophil entry into the testis. [ 42 ] Neutrophil activity in the testis is an inflammatory response which needs to be tightly regulated by the body, since inflammation-induced damage to the testis can lead to infertility. [ 43 ] [ 44 ] It is assumed that the role of the immunosuppressive environment of the testis is to protect developing sperm from inflammation.
Sperm are immunogenic - that is they will cause an autoimmune reaction if transplanted from the testis into a different part of the body. This has been demonstrated in experiments using rats by Landsteiner (1899) and Metchinikoff (1900), [ 1 ] [ 29 ] mice [ 45 ] and guinea pigs. [ 46 ] The likely reason for this is that sperm first mature at puberty, after immune tolerance is established, therefore the body recognizes them as foreign and mounts an immune reaction against them. Therefore, mechanisms for their protection must exist in this organ to prevent any autoimmune reaction. The blood-testis barrier is likely to contribute to the survival of sperm. However, it is believed in the field of testicular immunology that the blood-testis barrier cannot account for all immune suppression in the testis, due to (1) its incompleteness at a region called the rete testis [ 29 ] and (2) the presence of immunogenic molecules outside the blood-testis barrier, on the surface of spermatogonia . [ 1 ] [ 29 ] Another mechanism which is likely to protect sperm is the suppression of immune responses in the testis. [ 17 ] [ 47 ] Both the suppression of immune responses and the increased survival of grafts in the testis have led to its recognition as an immunologically privileged site . Other immunologically privileged sites include the eye, brain and uterus. [ 48 ]
The two main features of immune privilege in the rat testis are;
It is also predicted that the high level of inflammatory cytokines in the testis contributes to immune privilege. [ 29 ]
The existence of immune privilege in the testes of rodents is well accepted, due to many experiments demonstrating prolonged, and sometimes indefinite, survival of tissue transplanted into the testis, [ 49 ] [ 50 ] or testicular tissue transplanted elsewhere. [ 51 ] [ 52 ] Evidence includes the tolerance of testicular grafts in mice and rats, as well as the increased survival of transplants of pancreatic insulin-producing cells in rats, when cells from the testes ( Sertoli cells ) are added to the transplanted material. [ 53 ] Complete spermatogenesis , forming functional pig or goat sperm, can be established by the grafting of pig or goat testicular tissue onto the backs of mice - however, immunodeficient mice needed to be used. [ 51 ]
The presence of immune-privilege in the human testis is controversial and insufficient evidence exists to either confirm or rule out this phenomenon.
Sperm are protected from autoimmune attack, which when it occurs in humans leads to infertility. [ 54 ] Local injury of seminiferous tubules caused by fine-needle biopsies in humans does not cause testicular inflammation ( orchitis ). [ 55 ] Furthermore, human testis cells tolerate early HIV infection with little response. [ 56 ]
In transplant experiments, primate testes fail to support grafts of monkey thyroid tissue. [ 57 ] Human testis tissue transplanted into the mouse elicited an immune response and was rejected, however, this immune response was not as extensive as that against other types of grafted tissue. [ 58 ]
How the testicular environment suppresses the immune response is only partially understood. Recent experiments have uncovered a number of biological processes that most likely contribute to immune privilege in the testes of rodents:
Since protection of developing sperm is so important to the survival of a species, it would not be surprising if more than one mechanism were in use.
Curiously, the testis contains factors such as cytokines , which are usually only produced upon infections and tissue damage. The cytokines interleukin-1α ( IL-1 α), IL-6 and Activin A are found in the testis, often at high levels. [ 62 ] [ 63 ] [ 64 ] [ 65 ] [ 66 ] In other tissues, these cytokine would promote inflammation, but here they control testis function. They regulate the development of sperm by controlling their cell division and survival. [ 67 ] [ 68 ] [ 69 ] [ 70 ] [ 71 ]
Other immune factors found in the testis include the enzyme inducible nitric oxide synthase (iNOS), and its product nitric oxide (NO), [ 72 ] [ 73 ] [ 74 ] transforming growth factor beta (TGFβ), [ 75 ] the enzyme cyclooxygenase -2 (COX-2) and its product prostaglandin E2 , [ 76 ] and many others. Further research is required to define the functional roles of these immune factors in the testis.
Mumps is a viral disease which causes swelling of the salivary glands and testes. The mumps virus lives in the upper respiratory tract and spreads through direct contact with saliva. [ 77 ] Prior to widespread vaccination programs, it was a common childhood disease. Mumps is generally not serious in children, but in adults, where sperm have matured in the testis, it can cause more severe complications, such as infertility.
Gonorrhea is a sexually transmitted disease caused by the bacteria Niesseria gonorrhea which can lead to testicular pain and swelling. Gonorrhea also infects the female reproductive system around the cervix and uterus, and can grow in the mouth, throat, eyes and anus. [ 78 ] It can be effectively treated with antibiotics, however, if untreated, gonorrhea can cause infertility in men. Chlamydia is caused by the sexually transmitted bacteria Chlamydia trachomatis which infects the genitals. It more commonly affects women, and if untreated, can lead to pelvic inflammatory disease and infertility. [ 79 ] Serious symptoms in men are rare, but include swollen testicles and an unusual discharge from the penis. It is effectively treated with antibiotics.
Antisperm antibodies (ASA) have been considered as infertility cause in around 10–30% of infertile couples. [ 80 ] ASA production are directed against surface antigens on sperm, which can interfere with sperm motility and transport through the female reproductive tract, inhibiting capacitation and acrosome reaction , impaired fertilization , influence on the implantation process, and impaired growth and development of the embryo . Risk factors for the formation of antisperm antibodies in men include the breakdown of the blood‑testis barrier, trauma and surgery, orchitis, varicocele , infections, prostatitis , testicular cancer , failure of immunosuppression and unprotected receptive anal or oral sex with men. [ 80 ] [ 81 ]
Testicular torsion is a condition of physical twisting of the testis which results in cutting off the blood supply. It leads to damage that, if not treated within a few hours, causes the death of testicular tissue, and requires removal of the testis to prevent gangrene , and therefore can cause infertility. [ 82 ]
Orchitis is a condition of testicular pain involving swelling, inflammation and possibly infection. Orchitis can be caused by an autoimmune reaction (autoimmune orchitis) leading to a reduction in fertility. Autoimmune orchitis is rare in humans, compared to anti-sperm antibodies. [ 1 ] To study orchitis in the testis, autoimmune orchitis has been induced in the rodent testis. The disease starts with the appearance of testicular antibodies, then movement of macrophages and lymphocytes from the blood stream into the testis, breaking of the physical interactions between the developing sperm and Sertoli cells , entry of neutrophils or eosinophils , and finally death of the developing sperm, leading to infertility. [ 83 ] [ 84 ] [ 85 ]
Experiments in rats have examined, in fine detail, the course of testicular events during a bacterial infection. In the short term (3 hours) multiple inflammatory factors are produced and released by testicular macrophages . Examples are prostaglandin E2 , [ 86 ] [ 76 ] inducible nitric oxide synthase (iNOS), [ 39 ] [ 87 ] TNFα [ 88 ] and IL-1β, although at lower levels than other tissues. [ 86 ] [ 47 ] Non-immune cells of the testis such as Sertoli cells and Leydig cells also able to respond to bacteria. [ 63 ] [ 89 ] During a bacterial infection, testosterone levels and the amount of testicular interstitial fluid are reduced. [ 39 ] Neutrophils enter the testis about 12 hours after infection. [ 39 ] Importantly, there is damage to the developing sperm, which start to die under severe infections. [ 39 ] [ 90 ] Despite all the data on the effects of bacteria on normal testis parameters, there is little experimental data regarding its effect on rodent fertility.
Testicular inflammation can be a symptom of the following diseases: Coxsackie A virus, [ 91 ] [ 92 ] varicella (chicken pox) [ 91 ] [ 93 ] human immunodeficiency virus ( HIV ), [ 94 ] dengue fever , [ 95 ] Epstein Barr virus -associated infectious mononucleosis, [ 91 ] [ 96 ] syphilis , [ 97 ] leprosy , [ 98 ] tuberculosis . [ 99 ] | https://en.wikipedia.org/wiki/Testicular_immunology |
Transportable pressure vessels for high-pressure gases are routinely inspected and tested as part of the manufacturing process. They are generally marked as evidence of passing the tests, either individually or as part of a batch (some tests are destructive), and certified as meeting the standard of manufacture by the authorised testing agency, making them legal for import and sale. [ citation needed ] When a cylinder is manufactured, its specification, including manufacturer , working pressure , test pressure , date of manufacture , capacity and weight are stamped on the cylinder. [ 1 ]
Most countries require diving cylinders to be checked on a regular basis. This usually consists of an internal visual inspection and a hydrostatic test. The inspection and testing requirements for scuba cylinders may be very different from the requirements for other compressed gas containers due to the more corrosive environment in which they are used. [ 2 ] After a cylinder passes the test, the test date, (or the test expiry date in some countries such as Germany ), is punched into the shoulder of the cylinder for easy verification at fill time. [ note 1 ] The international standard for the stamp format is ISO 13769, Gas cylinders - Stamp marking . [ 1 ]
A hydrostatic test involves pressurising the cylinder to its test pressure (usually 5/3 or 3/2 of the working pressure) and measuring its volume before and after the test. A permanent increase in volume above the tolerated level means the cylinder fails the test and must be permanently removed from service. [ 3 ] : sect. 5.7.3
An inspection may include external and internal inspection for damage, corrosion, and correct colour and markings. The failure criteria vary according to the published standards of the relevant authority, but may include inspection for bulges, overheating, dents, gouges, electrical arc scars, pitting, line corrosion, general corrosion, cracks, thread damage, defacing of permanent markings, and colour coding. [ 3 ] : sect. 5.7.3 [ 2 ]
Gas filling operators may be required to check the cylinder markings and perform an external visual inspection before filling the cylinder and may refuse to fill non-standard or out-of-test cylinders. [ note 2 ]
Standard seamless high pressure aluminium cylinders do not have a limited life. They may be used until they fail test or inspection. Three cylinders from each batch are pulsation hydrostatically tested for 10,000 cycles from 0 to test pressure at 12 cycles per minute. An alternative test approved by the US Department of Transportation is 100,000 cycles from 0 to working pressure. For the batch to pass, there must be no leaks or failures. Bursting pressure when new is about 2.5 times working pressure. [ 4 ]
A cylinder is due to be inspected and tested at the first time it is to be filled after the expiry of the interval as specified by the United Nations Recommendations on the Transport of Dangerous Goods, Model Regulations , or as specified by national or international standards applicable in the region of use. [ 5 ] [ 6 ] An external visual pre-fill inspection should be done before filling a cylinder.
If a cylinder passes the listed procedures, but the condition remains doubtful, further tests can be applied to ensure that the cylinder is fit for use. Cylinders that fail the tests or inspection and cannot be fixed should be rendered unserviceable after notifying the owner of the reason for failure. [ 11 ] [ 12 ]
Before starting work the cylinder must be identified from the labelling and permanent stamp markings , and the ownership and contents verified. [ 13 ] [ 14 ]
Before internal inspection the valve must be removed after depressurising and verifying that the valve is open. Cylinders containing breathing gases do not need special precautions for discharge except that high oxygen fraction gases should not be released in an enclosed space because of the fire hazard. If the valve is blocked or stuck closed it may be necessary to release the pressure by removing the burst disc or drilling into the valve body below the valve seat. These operations require care to avoid injury. [ 15 ] [ 16 ]
Before inspection the cylinder must be clean and free of loose coatings, corrosion products and other materials which may obscure the surface. Foreign materials may be removed by brushing, controlled shot-blasting, water-jet cleaning chemical cleaning or other non-destructive methods. The method used must not remove a significant amount of cylinder material, [ 17 ] and steel cylinders may not be heated above 300 °C. [ 18 ] Aluminium cylinders are even more restricted in the temperatures permitted, which are specified by the manufacturer. [ 19 ]
The cylinder is inspected for dents, cracks, gouges, cuts, bulges, laminations and excessive wear, heat damage, torch or electric arc burns, and corrosion damage. The cylinder is also checked for illegible, incorrect or unauthorised permanent stamp markings, and unauthorised additions or modifications. If the cylinder exceeds the rejection criteria for these items it is unsuitable for further service and will be made permanently unserviceable. [ 20 ] [ 21 ]
Typical rejection criteria include: [ 2 ]
(details to be added)
Unless the cylinder walls are examined by ultrasonic methods , the interior must be visually inspected using sufficient illumination to identify any damage and defects, particularly corrosion. If the inner surface is not clearly visible it should be cleaned by approved method which does not remove a significant amount of wall material. Methods allowed include shot-blasting , water jet cleaning , flailing , steam or hot water jet, rumbling and chemical cleaning . The cylinder must be internally inspected after cleaning. [ 22 ] [ 23 ]
Typical rejection criteria include: [ 2 ]
(details to be added)
When there is uncertainty whether a defect found during visual inspection meets the rejection criteria, additional tests may be applied, such as ultrasonic measurement of pitting wall thickness, or weight checks to establish total weight lost to corrosion. [ 24 ] Hardness tests on aluminium cylinders are done on the cylindrical body and must avoid making deep impressions. [ 25 ]
While the valve is off, the threads of cylinder and valve must be checked to identify the thread type and condition. The threads of cylinder and valve must be of matching thread specification, clean and full form, undamaged and free of cracks, burrs and other imperfections. Tap marks [ note 3 ] are acceptable and should not be confused with cracks. Other neck surfaces will also be examined to be sure they are free from cracks. In some cases threads may be re-tapped, but if the threads are altered they must be checked with the appropriate thread gauges. [ 26 ] [ 27 ]
The aluminium alloys used for diving cylinders are 6061 and 6351. 6351 alloy is subject to sustained load cracking and cylinders manufactured of this alloy should be periodically eddy current tested according to national legislation and manufacturer's recommendations. [ 28 ] [ 29 ] 6351 alloy has been superseded for new manufacture, but many old cylinders are still in service, and are still legal and considered safe if they pass the periodic hydrostatic, visual and eddy current tests required by regulation and as specified by the manufacturer. The number of cylinders that have failed catastrophically is in the order of 50 out of some 50 million manufactured. A larger number have failed the eddy current test and visual inspection of neck threads, or have leaked and been removed from service without harm to anyone. [ 30 ]
Ultrasonic inspection may be substituted for the pressure test, which is usually a hydrostatic test and may be either a proof test or a volumetric expansion test, depending on the cylinder design specification. Test pressure is specified in the stamp markings of the cylinder. The results of a correctly performed pressure test are final. [ 31 ] [ 32 ]
Valves that are to be reused must be inspected and maintained to ensure they remain fit for service. [ 33 ] [ 34 ]
The recommended practice for valve inspection and maintenance includes inspection, and where applicable correction of threads, cleaning of components, replacement of excessively worn and damaged parts, packing and safety devices, lubrication as applicable with approved lubricants for the gas service, checks for correct operation and sealing at intended operating pressure. Checks may be done with the valve fitted to the cylinder after inspection and testing, or before the valve is fitted. [ 35 ] [ 36 ]
Gauging of threads may be mandatory to ensure the integrity of parallel threads. If the gauge exceeds the maximum gauge limit for taper threads, re-tapping may be considered at the discretion of the competent person. [ 37 ]
The interior of the cylinder must be thoroughly dried immediately after cleaning or hydrostatic testing, and the interior inspected to ensure that there is no trace of free water or other contaminants. [ 38 ] [ 39 ]
If the cylinder is repainted or plastic coated, the temperature must not exceed 300 °C for steel cylinders, [ 40 ] or the temperature specified by the manufacturer for aluminium cylinders. [ 19 ]
Before fitting the valve the thread type must be checked to ensure that a valve with matching thread specification is fitted. [ 41 ] Fitting of valves should follow the procedures specified in ISO 13341 Transportable gas cylinders - Fitting of valves to gas cylinders. [ 42 ]
After the tests have been satisfactorily completed, a cylinder passing the test will be marked accordingly. Stamp marking will include the registered mark of the inspection facility and the date of testing (month and year). [ 43 ] [ 44 ]
Records of a periodic inspection and test are made by the test station and kept available for inspection. These include: [ 45 ] [ 46 ]
Identification of the cylinder:
Records of the tests and inspections:
If a cylinder fails inspection or testing and cannot be recovered, the owner must be notified before making the empty cylinder unserviceable by crushing, burning a hole in the shoulder, irregular cutting of the neck or cylinder or bursting using a safe method. If the owner does not give permission they become legally responsible for any consequences. [ 47 ]
Before filling a cylinder the filling operator may be required by regulations, code of practice, or operations manual, to inspect the cylinder and valve for any obvious external defects or damage, and to reject for filling any cylinder that does not comply with the standards. They may also be required to record cylinder details in the filling log. [ 2 ]
In South Africa test stations are accredited by the South African National Accreditation System (SANAS) under the approval of the Department of Employment and Labour . [ 48 ]
"Gas cylinders - Seamless steel gas cylinders - Periodic inspection and testing" (PDF) . ISO 6406:2005(E) . Geneva: International Organization for Standardization. 2005 . Retrieved 4 August 2016 .
"Gas cylinders - Seamless aluminium-alloy gas cylinders - Periodic inspection and testing" . ISO 10461:2005(E) . Geneva: International Organization for Standardization. 2005 . Retrieved 5 August 2016 . | https://en.wikipedia.org/wiki/Testing_and_inspection_of_diving_cylinders |
Cosmetic testing on animals is a type of animal testing used to test the safety and hypoallergenic properties of cosmetic products for use by humans.
Since this type of animal testing is often harmful to the animal subjects, it is opposed by animal rights activists and others. Cosmetic animal testing is banned in many parts of the world, including Colombia , the European Union , the United Kingdom , India , [ 1 ] [ 2 ] and Norway . [ 3 ]
Cosmetics that have been produced without any testing on animals are sometimes known as " cruelty-free cosmetics ". [ 4 ] Some popular cruelty-free beauty brands include: E.L.F., Charlotte Tilbury, Farsali, Fenty Beauty, Fenty Skin, Glow Recipe and others. The website "Cruelty-Free Kitty" was created to assess which brands are cruelty-free. [ 5 ] Furthermore, some brands have participated in animal testing in the past, however, if they currently do not test on animals, these cosmetics are considered "cruelty-free". [ 6 ]
Using animal testing in the development of cosmetics may involve testing either a finished product or the individual ingredients of a finished product on animals, often rabbits , as well as mice , rats , monkeys , dogs , guinea pigs and other animals. Cosmetics can be defined as products applied to the body to enhance the body's appearance or to cleanse the body. This includes all hair products, makeup, and skin products. [ 7 ]
The United States Food and Drug Administration (FDA) continues to endorse animal testing methods. [ 8 ]
Re-using existing test data obtained from previous animal testing is generally not considered to be cosmetic testing on animals; however, the acceptability of this to opponents of testing is inversely proportional to how recent the data is. [ citation needed ]
Methods of testing cosmetics on animals include various tests that are categorized differently based on which areas the cosmetics will be used for. One new ingredient in any cosmetic product used in these tests could lead to the deaths of at least 1,400 animals. [ 9 ]
Dermal penetration: Rats are mostly used in this method that analyzes chemical movement, through the penetration of the chemical into the bloodstream. Dermal penetration is a method that creates a better understanding of skin absorption. [ 8 ]
Skin sensitization: This is a method that tests for allergic reactions to different chemicals. In some tests, a chemical adjuvant is injected to boost the immune system, which was typically performed on guinea pigs. In some tests, no chemical adjuvant is injected with the test chemical, or the chemical is applied on a shaved patch of skin. The reaction is then recorded by the appearance of the skin afterward. [ 8 ]
Acute toxicity: This test is used to determine the danger of exposure to a chemical by mouth, skin, or inhalation. It shows the various dangerous effects of a substance that result from a short period of exposure. Large amounts of rats and mice are injected in Lethal Dose 50 (LD50) tests that continue until half of the test subjects die. Other tests can use a smaller number of animals but can cause convulsions, loss of motor function, and seizures. The animals are often then killed afterward to gather information about the internal effects of the chemicals. [ 8 ]
Draize test : This is a method of testing that may cause irritation or corrosion to the skin or eye on animals, dermal sensitization, airway sensitization, endocrine disruption, and LD 50 (which refers to the lethal dose which kills 50% of the treated animals). [ 8 ]
Skin corrosivity or irritation: This method of the test assesses the potential of a substance causing irreversible damage to the skin. It is typically performed on rabbits and involves putting chemicals on a shaved patch of skin. This determines the level of damage to the skin including itching, inflammation, swelling, etc. [ 8 ]
A variety of alternatives exists to of animal testing. Cosmetics manufacturers who do not test on animals may use in vitro screens to test for endpoints that can determine the potential risk to humans with very high sensitivity and specificity. Companies such as CeeTox in the USA, acquired by Cyprotex, specialize in such testing and organizations like the Center for Alternatives to Animal Testing (CAAT), PETA and many other organizations advocate the use of in vitro and other non-animal tests in the development of consumer products.
Using safe ingredients from a list of 5,000 that have already been tested in conjunction with modern methods of cosmetics testing, the need for tests using animals is negated. [ 10 ]
EpiSkin, EpiDerm, SkinEthic and BioDEpi are lab-made reconstructed artificial human skin models that are non-animal alternative testing platforms with histological similarity with native skin tissues. Artificial skin can imitate the actual human skin, on which cosmetic products can be tested. For example, using UV light on EpiSkin can cause it to resemble older skin and adding melanocytes will turn the skin a darker color. This helped create a spectrum of different skin colors that are then used to compare the results of sunblock on a different variety of people. [ 11 ] To address potential issues with other parts of the human body, research companies such as NOTOX have developed a synthetic model of the human liver, which is the main organ to detox the body, to test harmful ingredients and chemicals to see if the liver can detox those elements. [ 12 ]
Lab-grown tissues are now being used to test chemicals in makeup products. MatTek is one of the companies that do this. It sells small amounts of skin cells to companies to test their products on them. Some of these companies are those that make laundry detergent, makeup, toilet bowl cleaner, anti-aging creams, and tanning lotion. Without these tissues, companies would be testing their products on living animals. Lab-grown tissues are a great alternative to testing harmful products on animals. [ 13 ] One lab was able to grow 11 different types of tissues in a petri dish. The downfall was that the tissues were not fully functional on their own, in fact, many of these tissues only resembled tiny parts of an actual-sized human organ, most of which were too small to transplant into humans. This technology could potentially be great, but it was a major downfall, 'Ministomachs that took about nine weeks to cultivate in a petri dish formed "oval-shaped, hollow structures". [ 14 ]
Research companies can also use body parts and organs taken from animals slaughtered for the meat industry to perform tests such as the Bovine Corneal Opacity and Permeability Test and Isolated Chicken Eye Test. [ 15 ]
Many companies have not made the switch to cruelty-free yet for many reasons, one of them being the time it takes for lab-grown tissues to be useable. Animals, on the other hand, can mature quickly. Rats, for example, have a much quicker growth rate "From birth to adult, rats take about three weeks to mature and begin fending for themselves. The rodents reach sexual maturity in about five weeks and begin mating soon after to produce the next generation to start the rat life cycle over again". [ citation needed ] On top of the extremely short time it takes a rat to mature, they can provide us with a complete set of organ systems, not just a paper-thin sheet of cells. Rats can also reproduce, and they do so at a very fast pace "In general, rats produce about seven offspring per litter and can reach up to 14 at times. Typical gestation periods last only a few weeks, allowing each female rat to produce around five litters a year". [ citation needed ]
The first known tests on animals were done as early as 300 BC. "Writings of ancient civilizations all document the use of animal testing. These civilizations, led by men like Aristotle and Erasistratus, used live animals to test various medical procedures". [ 16 ] This testing was important because it led to new discoveries such as how blood circulated and the fact that living beings needed air to survive. The idea of taking an animal and comparing it to how human beings survived was a completely new idea. It would not have existed (at least not as quickly as it did) without our ancestors studying animals and how their bodies worked.
"Proving the germ theory of disease was the crowning achievement of the French scientist Louis Pasteur. He was not the first to propose that diseases were caused by microscopic organisms, but the view was controversial in the 19th century and opposed the accepted theory of 'spontaneous generation'". [ 17 ] The idea of germs and other microscopic organisms was an entirely new idea and would not have come to be without the use of animals. In 1665, scientists Robert Hooke and Antoni van Leeuwenhoek discovered and studied how germs worked. They published a book about their discovery, which was not accepted by very many people, including the science community, at first. After some time, scientists were able to give animals diseases from microbes and realized that microbes really did exist. From there, they were able to use animals to understand how the disease worked, and the effects it could potentially have on the human body.
All of this has led up to something a bit more recent, the use of animals to test beauty products. This has become a very controversial topic in recent years. There are various people who are extremely against the use of animals for this purpose, and for a good reason. "Typically, animal tests for cosmetics include skin and eye irritation tests where chemicals are rubbed onto the shaved skin or dripped into the eyes of rabbits; repeated oral force-feeding studies lasting weeks or months to look for signs of general illness or specific health hazards, such as cancer or birth defects; and even widely condemned "lethal dose" tests, in which animals are forced to swallow massive amounts of a test chemical to determine the dose that causes death". [ 18 ] This kind of testing can be vital in finding important information about products, but can be harmful to the animals it is tested on.
In 1937, a mistake was made that ended up changing the pharmaceutical industry drastically. A company created a medicine ( elixir sulfanilamide ) "to treat streptococcal infections", and without any scientific research the medicine was out on shelves. [ 19 ] This medicine turned out to be extremely poisonous to people, leading to large poisoning outbreaks followed by over 100 deaths. [ 19 ] This epidemic led to a law being passed in 1938, called the U.S. Federal Food, Drug, and Cosmetic Act , enforcing more rigorous guidelines on cosmetic products. [ 19 ] After this law was passed, companies looked to animals to test their products, in turn, creating the first encounters of cosmetic animal testing.
There is a strategy used in animal testing laboratories titled the 'Three R's:' Reduction, refinement, and replacement' (Doke, "Alternatives to Animal Testing: A Review").
Due to the strong public backlash against cosmetic testing on animals, most cosmetic manufacturers say their products are not tested on animals. However, they are still required by trading standards and consumer protection laws in most countries to show their products are not toxic and not dangerous to public health. They also need to show that the ingredients are not dangerous in large quantities, such as when in transport or in the manufacturing plant. In some countries, it is possible to meet these requirements without any further tests on animals. Other countries, may require animal testing to meet legal requirements. The United States and Japan are frequently criticized for their insistence on stringent safety measures, which often require animal testing.
Some retailers distinguish themselves in the marketplace by their stance on animal testing.
Although Japanese law does not require non-medicated cosmetics to be tested on animals, it does not prohibit it either, leaving the decision to individual companies. [ 26 ] Animal testing is required when the product contains newly-developed tar colours
, ultraviolet ray protective ingredients or preservatives, and when the amount of any ingredient regulated in terms of how much can be added is increased. [ 27 ]
Japanese brands such as Shiseido and Mandom have ended much, but not all, of their animal testing. However, most other leading cosmetics companies in Japan still test on animals. [ 26 ] [ 28 ] [ 29 ]
São Paulo in Brazil banned cosmetic animal testing in 2014. [ 30 ]
In June 2023, the Government of Canada banned the testing of cosmetics on animals, and the sale of cosmetics tested on animals. Amendments to the Food and Drugs Act to end cosmetic animal testing through Bill C-47, the Budget Implementation Act, 2023, No. 1, went into effect on December 22, 2023. [ 31 ] [ 32 ]
In June 2020, the Senate of the Republic of Colombia approved a resolution banning the commercialization and testing of cosmetics on animals. [ 33 ] In August 2020, presidential assent was granted to the resolution, thus effectively banning the testing of cosmetics on animals in Colombia. [ 34 ]
The European Union (EU) followed suit, after it agreed to phase in a near-total ban on the sale of animal-tested cosmetics throughout the EU from 2009, and to ban cosmetics-related animal testing. [ 35 ] Animal testing is regulated in EC Regulation 1223/2009 on cosmetics . Imported cosmetics ingredients tested on animals were phased out for EU consumer markets in 2013 by the ban, [ 35 ] but can still be sold to outside of the EU. [ 36 ] Norway banned cosmetics animal testing at the same time as the EU. [ 37 ] In May 2018, the European Parliament voted for the EU and its Member States to work towards a UN convention against the use of animal testing for cosmetics. [ 38 ]
The four EFTA countries that are not in the EU, i.e. Norway, Liechtenstein, Switzerland, and Iceland, also banned cosmetic testing. [ 39 ]
In 2017, Guatemala banned cosmetic animal testing. [ 40 ]
In early 2014, India announced a ban on testing cosmetics on animals in the country, thereby becoming the second country in Asia to do so. [ 41 ] Later India banned import of cosmetics tested on animals in November 2014. [ 42 ]
Israel banned "the import and marketing of cosmetics, toiletries, or detergents that were tested on animals" in 2013. [ 43 ]
In 2015, New Zealand also banned animal testing. [ 44 ] However, the ban on testing cosmetics on animals was unlikely to lead to products being stripped from shelves in New Zealand, as around 90 percent of cosmetic products sold in New Zealand were made overseas. [ 45 ]
In 2015, Taiwan launched a bill proposing a ban on cosmetic testing on animals. [ 46 ] It passed in 2016 and went into effect in 2019. [ 47 ] [ 48 ] Shortly before the ban went into effect on 9 November 2019, however, it was noted that most Taiwan cosmetic companies already did not experiment with animals. [ 47 ]
Turkey "banned any animal testing for cosmetic products that have already been introduced to the market." [ 49 ]
Animal testing on cosmetics or their ingredients was banned in the UK in 1998. [ 50 ]
The Association of Southeast Asian Nations (ASEAN) is potentially "making strides toward ending cosmetics testing on animals." [ 3 ]
In Australia, the End Cruel Cosmetics Bill was introduced to Parliament in March 2014, which would ban local testing, which generally does not happen there, and importation of cosmetics tested on animals. [ 51 ] In 2016 a bill was passed to ban the sale of cosmetics tested on animals, which came into effect in July 2017. [ 52 ]
In March 2014, the Humane Cosmetics Act was introduced to the U.S. Congress . It would ban cosmetic testing on animals and eventually would ban the sale of cosmetics tested on animals. [ 3 ] The bill did not advance.
Similar bills have been introduced and passed at the state level, and testing cosmetics on animals has been banned in ten US states as of 2023: California , Nevada , Illinois , Hawaii , Maryland , Maine , New Jersey , Virginia , Louisiana , and New York . [ 53 ]
On 19 March 2020, the Mexican Senate unanimously passed legislation banning testing cosmetics on animals. [ 54 ] The proposed ban now awaits approval from the lower house of the Mexican Congress, the Mexican Chamber of Deputies. [ 55 ]
South Korea is also potentially "making strides toward ending cosmetics testing on animals." [ 3 ]
China passed a law on 30 June 2014 to eliminate the requirement for animal testing of cosmetics. Though domestically-produced ordinary cosmetic goods do not require testing, animal testing is still mandated by law for Chinese-made "cosmeceuticals" (cosmetic goods which make a functional claim) which are available for sale in China. Cosmetics intended solely for export are exempt from the animal testing requirement. [ 56 ] As of March 2019, post-market testing (i.e. tests on cosmetics after they hit the market) for finished imported and domestically produced cosmetic products will no longer require animal testing. [ 57 ] Chinese law was further amended in April 2020, fully dropping all remaining mandatory animal testing requirements for all cosmetics - both locally produced and imported, instead creating a regulatory 'preference' for non-animal based testing methods in the safety certification of cosmetic products. [ 58 ] [ 59 ]
In 2013, the Russian Ministry of Health stated "Toxicological testing is performed by means of testing for skin allergic reaction or test on mucous tissue /eye area (with use of lab animals) or by use of alternative general toxicology methods (IN VITRO). In this manner the technical regulations include measures which provide an alternative to animal testing". [ 60 ] | https://en.wikipedia.org/wiki/Testing_cosmetics_on_animals |
Welding of advanced thermoplastic composites is a beneficial method of joining these materials compared to mechanical fastening and adhesive bonding . Mechanical fastening requires intense labor, and creates stress concentrations, while adhesive bonding requires extensive surface preparation, and long curing cycles. Welding these materials is a cost-effective method of joining concerning preparation and execution, and these materials retain their properties upon cooling, so no post processing is necessary. These materials are widely used in the aerospace industry to reduce weight of a part while keeping strength. [ 1 ]
For many industries there are codes and standards that need to be followed when being implemented into service. The quality of the welds made on these materials are important in ensuring people receive safe products. [ 2 ] There are not codes made specifically for the welding of advanced thermoplastic composite welds, so the codes for adhesive bonding of plastics and metals [ 1 ] [ 3 ] are slightly altered, and used in order to properly test these materials. Even though the joining method is different these materials have mechanical requirements they need to meet.
There are several mechanical properties that need to be tested to ensure the quality of welds. The testing methods talked about in this article will be referenced from the ASTM adhesive bonding standards. The properties needed to be tested are shear strength, fracture toughness, and fatigue properties. Optical microscopy is also often done to look for weld defects.
According to ASTM D1002 The specimens tested will be configured as lap joints. They will need to be sectioned in a way that they can fit in the grips used for the tensile testing . The length of the overlap for the lap joint is determined by the thickness of the material, the yield point of the metal, and the value that is 50% of the estimated average shear strength in an adhesive bond, but for the purpose of this article it will be specified for a welded joint. The code also specifies the required capabilities of the machine used to test the shear strength. The breaking load of the specimens must fall between 15 and 85 percent of the full scale capabilities of the apparatus. For thermoplastic composites these machines need to be able to maintain a loading rate of 80–100 kg/cm 2 . The jaws of the machine must align with test specimen so as soon as the test gets started the long axis of the test specimen will be aligned with the direction of the applied tension. The machines grip on the test specimen must be 63 mm (2.5 in). The code specifies what needs to be recorded from the test such as material used, material thickness and other necessary sample measurements, and material properties. ASTM also governs on precision of testing, and avoiding bias in the results. [ 4 ]
ASTM D3166 specifies fatigue testing methods for metal to metal adhesive joints. It references ASTM D1002 for creating test specimens. The testing machine must be capable of applying a sinusoidal cyclic axial load. The cycle rate and type of equipment can influence the results of the tests being run. 1800 cycles/minute are recommended unless otherwise specified. Tests are generally run at ambient temperature and humidity which is specified at 50% relative humidity ±4%, and 23 °C ±1.1 °C. At least 5 S-N curves need to be generated for a welded joint to give a usable range of cyclical loads on the material. The loads need to be varied from a minimal value that is ideally above 2000 cycles, to a load with 10% of the materials max strength. [ non-primary source needed ]
Impact testing is done to test the fracture toughness of the joints welded. ASTM D5041 is used for reference when doing impact tests on advanced thermoplastic composites. Impact tests can get data for figuring out how much energy is needed to break the material, and it can also shed light on modes of failure for a certain joint. The testing machine needs to be something that moves at a constant speed before impact, it needs to be able to give a force readout on impact, generally a wedge is used as the impact tool, along other requirements specified by the code. The code calls for the tests to be done at ambient lab conditions, but depending on the application of the material this may change. Standard speed of testing for impacts is 127 mm/min, where the standard chart speed is 250 mm/min. [ non-primary source needed ]
Optical microscopy is a necessary testing method in order to observe the quality of the weld joint. There are defects that can occur during welded that can weaken the joint or cause stress concentrations. [ 1 ] [ 3 ] Voids can occur during processes such as induction, ultrasonic, and resistance welding, so visual inspection is important to ensuring a quality joint, while developing weld procedures, and for using parts for service. Inspection can be done with the naked eye, an optical microscope, and more high-powered devices such as a scanning electron microscope (SEM). [ 1 ] Taking a cross section of the welded joint will allow the joint to be inspected for defects.
Many of the nondestructive testing (NDT) methods available for testing of thermoplastic composite base materials can be used for welds in thermoplastic composites as well. In some cases, modifications are necessary. [ 5 ] International standards like EN 13100-1, 13100-2, 13100-3 & 13100-4 govern inspection of the base materials. [ non-primary source needed ] While these standards were not necessarily developed specifically for the welds in said materials, the physical principles are often still applicable. [ citation needed ]
The methods include:
Visual Inspection (VT) is typically the first option for any attempt at NDT, being the least expensive, as it requires the least specialized training and usually few if any special tools. Defects on the surfaces of thermoplastic composite welds can be detected visually if they are of sufficient size. Weld defects such as misalignment, porosity, lack of fusion and degradation of the matrix and/or fibers may be visually apparent. Subsurface defects may not be visible, unless the composite matrix was nearly transparent and the embedded fibers did not obscure them. [ 6 ]
Ultrasonic testing (UT) can offer detailed NDT information for welded thermoplastic composites. [ 7 ] Tests can be done with shear wave or transverse waves, though the composite materials often attenuate the signals significantly and care must be taken to account for this. Contact methods using either manual or automated transducers coupled to the part being inspected or non-contact methods using water immersion or a bubbler (i.e. a continuous stream of water through which the ultrasound passes) can be effective if designed and calibrated properly. Amplitude of reflection data may be used to generate B-scan [ 8 ] or C-scan images, which can show the materials being welded at various, discrete depths or cross sections, a capability not available with traditional radiographic methods. Ultrasound can detect delaminations, lack of fusion, porosity, voids, inclusions and other defects mostly regardless of their orientation. Deterring factors include that the method is time consuming and the data are open to some interpretation, requiring skilled technician to perform and interpret the test.
Radiographic testing (RT) can be performed in several ways. Typically low energies are required [ 9 ] for testing of composites in order to see any detail, which restricts the radiation sources to be used to x-ray types rather than gamma sources like Ir-192 or Cobalt-60 , which tend to have higher energy levels. Data may be recorded either on film or digitally, using specially developed screens for detecting and saving an image than can be manipulated later with the proper software and hardware. Because radiographic testing relies on differences in material density to provide an image, resolution of fibers like carbon from the thermoplastic matrix is not always very high, since the density of the plastic does not differ much from that of the carbon or glass filaments. For digital imaging, the lack of contrast may be partially addressed after the radiographic images are taken, using digital imaging software. Radiography can detect porosity, voids and possibly differences in fiber density or orientation in the composite matrix due to the welding process. Lack of fusion may not be visible by RT unless it is perpendicular to the direction of the source of radiation. Computed Tomography (CT) , a subset of radiographic testing, is proving useful for the inspection of thermoplastic composite welds. CT involves the computerized building of a 3-D image using X-rays taken from numerous, incremental angles. It is particularly useful for the determination of fiber orientation in welds of glass reinforced composites. [ 10 ]
Thermography [ 11 ] involves testing the part for discontinuities that can be seen by an infrared camera when the part is heated or cooled. It offers a significant improvement on some of the more traditional NDT methods in that it can be used on large areas of, for example, airplane parts or storage tanks.
Eddy Current testing (ET) has been found to be useful for characterizing the nature of fibers and their orientation in certain composite materials, particularly those with conductive reinforcing fibers. It would not be useful for composites reinforced with glass or aramid fibers, for example, as no currents can be induced in these insulating materials. Much higher magnetic field frequencies are used to generate the eddy currents used for testing plastic composites than are typically used for metals. [ 12 ] Though delaminations in the material were either undetectable or nearly so, more recent research has found that by induction heating the part in addition to exciting an alternating magnetic field, some delaminations could also be detected in CFRP .
Laser Shearography involves accurately measuring perturbations in the surfaces of a (usually thin) part under load or strain with the aid of lasers scanning across the surface being evaluated. [ 13 ] Voids, pores, delaminations and other defects in composite welds can be detected by this method.
Acoustic Emission testing provides qualitative information on the presence and potential growth of defects such as cracks and delaminations in welded composite materials. [ 14 ] Typically this method is used to help narrow down the locations(s) of defects in large structures before using a more precise NDT method such as radiography or ultrasonic testing to help localize and characterize the nature of the defect. | https://en.wikipedia.org/wiki/Testing_of_advanced_thermoplastic_composite_welds |
Testosterone 17beta-dehydrogenase (NADP + ) ( EC 1.1.1.64 , 17-ketoreductase , NADP-dependent testosterone-17beta-oxidoreductase , testosterone 17beta-dehydrogenase (NADP) ) is an enzyme with systematic name 17beta-hydroxysteroid:NADP + 17-oxidoreductase . [ 1 ] [ 2 ] [ 3 ] This enzyme catalyses the following chemical reaction
Also oxidizes 3-hydroxyhexobarbital to 3-oxohexobarbital.
This EC 1.1.1 enzyme -related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Testosterone_17b-dehydrogenase_(NADP+) |
Testosterone glucuronide is an endogenous , naturally occurring steroid and minor urinary metabolite of testosterone . [ 1 ]
This article about a steroid is a stub . You can help Wikipedia by expanding it .
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Testosterone_glucuronide |
Testosterone sulfate is an endogenous , naturally occurring steroid and minor urinary metabolite of testosterone . [ 1 ]
This article about a steroid is a stub . You can help Wikipedia by expanding it .
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Testosterone_sulfate |
In human biology , the testosterone–cortisol ratio describes the ratio between testosterone , the primary male sex hormone and an anabolic steroid , and cortisol , another steroid hormone , in the human body. [ 1 ]
The ratio is often used as a biomarker of physiological stress in athletes during training, during athletic performance, and during recovery, and has been explored as a predictor of performance. [ 1 ] [ 2 ] [ 3 ] At least among weight-lifters , the ratio tracks linearly with increases in training volume over the first year of training but the relationship breaks down after that. [ 1 ] A lower ratio in weight-lifters just prior to performance appears to predict better performance. [ 1 ]
The ratio has been studied as a possible biomarker for criminal aggression, but as of 2009 its usefulness was uncertain. [ 4 ] | https://en.wikipedia.org/wiki/Testosterone–cortisol_ratio |
"Testure" is a song by Canadian electro-industrial band Skinny Puppy , taken from its 1988 album VIVIsectVI and released as a single in 1989. "Testure" was the group's first and last song to chart on Billboards's Dance Club Songs , and it was accompanied with a controversial music video.
Three primary versions of "Testure" exist, two of which appear on the single. The album version of "Testure" is a five-minute track with smooth electronics, fretless bass , and a profusion of samples from Martin Rosen 's 1982 film The Plague Dogs . [ 2 ] Its lyrics contain both the title of its host album, VIVIsectVI (1988), and the album's main themes: animal rights and testing . [ 3 ] [ 4 ] See Magazine considered "Testure" to be VIVIsectVI' s "centerpiece". [ 5 ]
"Testure (S.F. Mix)" shortens the song to four minutes and introduces a greater emphasis on samples from The Plague Dogs . The fretless bass, played by Dale Plevin, is also featured more prominently than on the album version. [ 6 ] This mix went on to appear on the band's 1999 compilation album, The Singles Collect , and was used in the song's music video. "Testure (12" Mix)" acts as an extended version of the song, clocking in at eight and a half minutes. This version, remixed and re-edited by Skinny Puppy's own cEvin Key and Dave Ogilvie , [ 6 ] also begins with a protracted series of samples from The Plague Dogs . [ 2 ] The song's main synthesizer riff does not begin until nearly two minutes in, and Nivek Ogre 's vocals are not introduced until the three minute mark. This extended mix appeared on 1990's Twelve Inch Anthology compilation.
"Testure's" title is most likely a play on the words "test" and "torture" intended to equate live animal experimentation to torture.
Prompted by airplay and club attention, [ 7 ] "Testure" was released as a single in 1989, a year after the release of VIVIsectVI . [ 8 ] It charted at place nineteen and spent five weeks on Billboards's Dance Club Songs , [ 9 ] [ 10 ] making it Skinny Puppy's most successful single. [ 5 ] Two variations of the release's track listing exist: one with the song "The Second Opinion", and one with "Cage". All versions feature the S.F. and extended mixes of "Testure", and all versions feature "Serpents", a B-side unique to this single. [ 11 ] [ 8 ]
"The Second Opinion", which also appeared appended to the end of VIVIsectVI's CD release, [ 12 ] includes the line "that machine has got to be destroyed" from Stuart Gordon's 1986 adaptation of H. P. Lovecraft's From Beyond . [ 2 ] The song itself is built around a repeating drum machine loop interspersed with modulated and distorted vocal samples. "The Second Opinion" began as a live jam titled "Snub" and was later refined and mixed in studio. [ 13 ] "Cage", which originally appeared on Skinny Puppy's 1987 single " Chainsaw ", concludes with the line "It's just a little blood... it'll wash out" from William Lustig 's 1980 horror film, Maniac . [ 2 ] "Serpents" is percussion-focused song that blends programmed industrial beats with tribal drums.
The single's artwork was designed by Steven R. Gilmore , and the back features a large syringe provided by his friend from the University of British Columbia , who also supplied the X-ray images used on VIVIsectVI's artwork. [ 14 ]
"Testure's" music video, directed by Ogre and produced by Gary Blair Smith, [ 8 ] begins with a definition of the word vivisection . What follows is the story of a dog-abusing man who, in turn, becomes a test subject operated on and caged by surgeons. [ 15 ] Interspersed with the narrative sections are shots of actual animal testing footage from the 1981 documentary The Animals Film and the 1984 PETA film Unnecessary Fuss . [ 8 ] According to Ogre and Key, the video was pulled from airplay following an internal poll by Citytv , an associate of Canada's MuchMusic . [ 16 ] The poll came out nearly split, but, regardless, the video was ultimately banned by "the powers that be". [ 16 ] The video, despite depicting vivisection in "vivid detail", was broadcast on Horizon , the Soviet Union 's primary satellite channel, as a critique on materialism. [ 5 ]
All credits adapted from liner notes [ 6 ]
Skinny Puppy
Additional personnel | https://en.wikipedia.org/wiki/Testure |
The TetTag mouse is a bi- transgenic mutant used in neuroscience research that expresses a persistent marker (e.g. beta-galactosidase ) under control of the immediate early gene fos . This mouse strain allows the stable labeling of activated neurons in mice in a defined time window of several hours. [ 2 ] [ 3 ]
Two independently generated transgenic strains were crossed to produce the TetTag strain. In the first transgenic construct, the tetracycline-controlled transactivator (tTA) protein and a two hour half-life Green Fluorescent Protein (shEGFP) are expressed under the direction of the fos minimal promoter . The second transgenic construct expresses a nuclear-localizing beta-galactosidase gene and the tetracycline regulated transactivator (tTA) under the control of the TetO tetracycline-responsive regulatory element. [ 2 ]
The TetTag mouse allows researchers to label activated neurons during a learning experiment (e.g. fear conditioning , [ 4 ] water maze training [ 5 ] ). When the memory is retrieved days later, the acutely activated neurons can be labeled by immunohistochemistry against immediate early genes . These experiments test whether the same neurons are responsible for storing and retrieving a memory, a key question to understand the cellular mechanism of memory. [ citation needed ]
The removal of doxycycline from the chow opens the window for activity-dependent labeling, but it takes several hours for the drug to be cleared from the brain. Thus, the labeling window is not very precise in time. Also, it is not entirely clear which activity patterns lead to Fos activation in neurons. [ 6 ] | https://en.wikipedia.org/wiki/TetTag |
Tetanolysin is a toxin produced by Clostridium tetani bacteria. Its function is unknown, but it is believed to contribute to the pathogenesis of tetanus . The other C. tetani toxin, tetanospasmin , is more definitively linked to tetanus. It is sensitive to oxygen.
Tetanolysin belongs to a family of protein toxins known as thiol-activated cytolysins , which bind to cholesterol . [ 1 ] It is related to streptolysin O and the θ-toxin of Clostridium perfringens . [ 2 ] Cytolysins form pores in the cytoplasmic membrane that allows for the passage of ions and other molecules into the cell. The molecular weight of tetanolysin is around 55,000 daltons . [ 3 ]
This biochemistry article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetanolysin |
A tether is a form of cell surface protrusion, separated from the cytoskeleton after the application of low pulling forces to the cell surface membrane . They are thin, viscoelastic tubes which can be observed in vivo due to shear flow caused by molecular bonds between blood cells and vessel walls, for example. [ 1 ]
In biochemistry, a tether is a molecule that carries one or two carbon intermediates from one active site to another. They are commonly used in lipid synthesis , gluconeogenesis , and the conversion of pyruvate into Acetyl CoA via PDH complex. Common tethers are lipoate -lysine residue complex associated with dihydrolipoyl transacetylase, which is used for carrying hydroxyethyl from hydroxyethyl TPP. This compound forms Acetyl- CoA, a convergent molecule in metabolic pathways. [ citation needed ]
Another tether is biotin-lysine residue complex associated with pyruvate carboxylase, an enzyme which plays an important role in gluconeogenesis. It is involved in the production of oxaloacetate from pyruvate. [ citation needed ]
One of the biological tethers used in the synthesis of fats is a β- mercaptoethylamine-pantothenate complex associated with an acyl carrier protein. [ citation needed ] | https://en.wikipedia.org/wiki/Tether_(cell_biology) |
Tethered particle motion ( TPM ) is a biophysical method that is used for studying various polymers such as DNA and their interaction with other entities such as proteins .
The method allows observers to measure various physical properties on the substances, as well as to measure the properties of biochemical interactions with other substances such as proteins and enzymes.
TPM is a single molecule experiment method.
TPM was first introduced by Schafer, Gelles, Sheetz and Landick in 1991. [ 1 ] In their research, they attached RNA polymerase to the surface, and gold beads were attached to one end of the DNA molecules. In the beginning, the RNA polymerase "captures" the DNA near the gold bead. During the transcription , the DNA "slides" on the RNA polymerase so the distance between the RNA polymerase and the gold bead (the tether length)is increased. Using an optical microscope the area that the bead moves in was detected. The transcription rate was extracted from data. Since then, a lot of TPM experiments have been done, and the method was improved in many ways such as bead types, biochemistry techniques, imaging (faster cameras, different microscopy methods etc.) data analysis and combination with other single-molecule techniques (e.g. optical or magnetical tweezers).
One end of a polymer is attached to a small bead (tens to hundreds of nanometer), while the other end is attached to a surface.
Both the polymer and the bead stay in an aqueous environment, so the bead moves in Brownian motion . Because of the tether, the motion is restricted. Using an optical microscope and CCD camera , one can track the bead position in a time series. Although the bead is usually smaller than the diffraction limit , so the image is a spot which is larger than the bead itself ( point spread function ), the center of the spot represents the projection on the X-Y plane of the end of the polymer ( end-to-end vector ). Analyzing the distribution of the bead position can tell us a lot of information about the polymer.
In order that the motion would be polymer dominated, and not bead dominated, one should notice that the excursion number, N R , [ 2 ] will be less than 1:
where r {\displaystyle r} is the bead radius, L {\displaystyle L} is the contour length of the polymer and l p {\displaystyle l_{p}} is the persistence length (50 nm in physiological conditions) of the polymer. (It is possible to work also when N R > 1 {\displaystyle N_{R}>1} , but it should be treated carefully.)
Metallic beads (usually gold) scatter light with high intensity, so one can use very small beads (~40 nm diameter), and still have a good picture. From the other hand, metallic beads are not the appropriate tool for optical tweezers experiments.
Polystyrene beads scatter light weaker than metallic (in order to get the same intensity as getting from 40 nm gold bead, the polystyrene bead should be ~125 nm! [ 3 ] ), but it has the advantage that it can be combined with optical tweezers experiments.
The major advantage of fluorospheres is that the excitation wavelength and the emission wavelength are not the same, so dichroic filter can be used to give a cleaner signal. The disadvantage of the fluorospheres is photobleaching .
All of the bead types and diameters (with the biochemistry marker, look at the tether assembly section) are manufactured by commercial companies, and can purchased easily.
A chip can be made of two coverslips. One of them should be drilled to make two hole, allowing the reagents to be injected into the flowcell. The slides should be cleaned to remove dirt. A bath sonicator is a good tool for that, 15 minutes in Isopropanol should do the trick. Next, the a channel should be made. One way of doing so is to cut parafilm in the center, leaving a frame of parafilm that would be used as a spacer between the slides. The slides should be assembled one on the other with the cut parafilm between them. The final step is to heat the chip so that the parafilm will melt and glue the slides together.
First, the chip has to be passivated so that the polymer won't stick to the glass, there are plenty of blocking reagents available (BSA, alpha-casein, etc.) and one should find what works best for the specific situation
Next, the surface should be coated with an antibody or other reactive molecule (such as anti- digoxigenin ) that will bind to an antigen ( digoxigenin ) at one end of the polymer. After an incubation of about 45min, the excess antibody has to be washed away.
After washing the excess antibody, the polymer should be injected into the chip and incubated for about the same time. The polymer had been modified before at the ends. One end has a biotin tail and the other has a digoxigenin tail. After incubation, unbound polymer has to be washed out from the cell. Then, anti- biotin coated beads should be injected to the flowcell and incubate for about 30-45min. Excess beads should be washed out.
As mentioned above, the image doesn't show the bead itself but a larger spot according to its PSF ( Point spread function ). In addition, the pixel size on the camera may reduce the resolution of the measure.
In order to extract the exact bead's position (that corresponds to the end-to-end vector), the center of the spot should be found as accurate as possible. It can be done with good resolution using two different techniques, both based on spot characteristics. The light intensity in the focal plane distributed as airy disk , and has circular symmetry.
A 2-dimensional Gaussian function is a good approximation for airy disk . By fitting this function to the spot one can find the parameters x 0 {\displaystyle x_{0}} and y 0 {\displaystyle y_{0}} that are the coordinates of the center of the spot, and of the end-to-end vector.
The second technique is to find the center of intensity, [ 4 ] using the definition of center of mass :
where R → c m {\displaystyle {\vec {R}}_{cm}} is the center of mass coordinate, I t o t {\displaystyle \textstyle I_{tot}} is the total intensity of the spot, and I k {\displaystyle \textstyle I_{k}} and r → k {\displaystyle \textstyle {\vec {r}}_{k}} are the intensity and coordinate of the k -th pixel. Because of the circular symmetry, the coordinate of the center of intensity is the coordinate of the center of the bead. Both techniques give us the coordinate of the end-to-end vector in a resolution better than pixel size.
Usually, the whole system drifts during the measuring. There are several methods to correct the drift, generally these can divided into 3 groups:
The Brownian motion frequency is much larger than the drift frequency, so one can use high-pass filter in order to remove the drift. Similar effect can achieve by smoothing the data, and subtraction of the smoothed from the data (see figure).
If few beads are shown in the frame, because every bead moving randomly, averaging over the position of them for every frame should give us the drift (it should subtracted from the data for having clean data).
If an immobilized bead is shown in the frame, we can take its position as a reference, and correct the data by the immobilized bead's position. (Another advantage of looking at immobilized bead, is the fact that the motion of it can tell us about the accuracy of the measure.) Of course one can use more than one method.
It is common to fit random walk statistics to the end-to-end vector of the polymer. [ 5 ] For 1-dimensional we'll get the Normal distribution , and for 2-dimensional the Rayleigh distribution :
where L {\displaystyle L} is the contour length and l p {\displaystyle l_{p}} is the persistence length. After collecting the data of time series, one should fitting the histogram of the data to the distribution function (one or two dimensional). If the contour length of the polymer is known, the only fitting parameter is the persistence length.
Due to entropic force , the polymer acts like Hookian spring. According to Boltzmann distribution , the distribution is proportional to exponent of the ratio between the elastic energy and the thermal energy :
where k {\displaystyle k} is the spring constant , K B {\displaystyle K_{B}} is Boltzmann constant and T {\displaystyle T} is the temperature. By taking the logarithm of the distribution P ( x ) {\displaystyle P(x)} and fitting it to a parabola shape, one can get the spring constant of the polymer: [ 6 ]
where α {\displaystyle \alpha } is the coefficient of x 2 {\displaystyle x^{2}} from the parabola fit.
Advantages include a simple setup, cost, the fact that observations are made in the polymer's natural environment (no external forces are used), it is suitable for various microscopy methods (e.g. TIRFM , dark field , differential interference contrast microscopy , etc.), it can be combined and manipulated using other methods, and there are a high variety of applications. Disadvantages include low spatial resolution (~30 nm) and that it fits to in vitro experiments only. [ citation needed ] | https://en.wikipedia.org/wiki/Tethered_particle_motion |
Tetra-amido macrocyclic ligands ( TAMLs ) constitute a class of macrocyclic ligands . When complexed to metals, TAMLs are proposed as environmentally friendly catalysts. Although never commercialized, iron-TAML complexes catalyze the degradation of pesticides, effluent streams from paper mills , dibenzothiophenes from diesel fuels , and anthrax spores. [ 1 ]
This chemical reaction article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetra-amido_macrocyclic_ligand |
Tetra- n -butylammonium fluoride , commonly abbreviated to TBAF and n -Bu 4 NF , is a quaternary ammonium salt with the chemical formula (CH 3 CH 2 CH 2 CH 2 ) 4 N + F − . It is commercially available as the white solid trihydrate and as a solution in tetrahydrofuran . TBAF is used as a source of fluoride ion in organic solvents. [ 1 ]
TBAF can be prepared by passing hydrofluoric acid through an ion-exchange resin , followed by tetrabutylammonium bromide . Upon evaporation of the water, TBAF can be collected as an oil in quantitative yield. [ 1 ]
Preparing anhydrous samples is of interest as the basicity of fluoride increases by more than 20 p K units on passing from aqueous to aprotic solvent . [ citation needed ] However, heating samples of the hydrated material to 77 °C under vacuum causes decomposition to the hydrogen difluoride salt. [ 2 ] Similarly, samples dried at 40 °C under high vacuum still contain 10-30 mol% of water and some 10% of difluoride. [ 3 ] Instead, anhydrous TBAF has been prepared by the reaction of hexafluorobenzene and tetrabutylammonium cyanide. Solutions of the salt in acetonitrile and dimethyl sulfoxide are stable. [ 4 ]
Because the fluoride ion is such a strong hydrogen bond acceptor, its salts tend to be hydrated and of limited solubility in organic solvents. As a fluoride ion source, TBAF solves this problem, although the nature of the fluoride is uncertain because TBAF samples are almost always hydrated, resulting in the formation of bifluoride (HF 2 − ) hydroxide (OH − ) as well as fluoride. Many applications tolerate heterogeneous or ill-defined fluoride sources.
As a fluoride source in organic solvents, TBAF is used to remove silyl ether protecting groups . It is also used as a phase transfer catalyst and as a mild base . As a deprotecting agent, TBAF in DMSO will convert O-silylated enolates into carbonyls. With C-Si bonds, TBAF gives carbanions that can be trapped with electrophiles or undergo protonolysis. [ 1 ] [ 5 ]
TBAF is also used, when in an organic solvent, as an activator compound to allow super glue to stick to low surface energy polymers such as polyethylene and polypropylene. [ 6 ] | https://en.wikipedia.org/wiki/Tetra-n-butylammonium_fluoride |
Tetra- tert -butylethylene is a hypothetical organic compound , a hydrocarbon with formula C 18 H 36 , or ((H 3 C−) 3 C−) 2 C=C(−C(−CH 3 ) 3 ) 2 . As the name indicates, its molecular structure can be viewed as an ethylene molecule H 2 C=CH 2 with the four hydrogens replaced by tert -butyl −C(−CH 3 ) 3 groups.
As of 2006, this compound had not yet been synthesized, in spite of many efforts. It is of interest in chemical research as an alkene whose double bond is strained but protected by steric hindrance . Theoretical studies indicate that the molecule should be stable, with a strain energy of about 93 kcal/mol (390 kJ/mol). [ 2 ]
This article about theoretical chemistry is a stub . You can help Wikipedia by expanding it .
This article about a hydrocarbon is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetra-tert-butylethylene |
Tetra- tert -butylmethane is a hypothetical organic compound with formula C 17 H 36 , consisting of four tert -butyl groups bonded to a central carbon atom. It would be an alkane , specifically the most compact branched isomer of heptadecane .
Some calculations suggest this compound cannot exist due to the steric hindrance among the closely packed tert -butyl groups, which would make it one of the smallest, if not the smallest itself, saturated and acyclic hydrocarbon that cannot exist. [ 1 ]
Other calculations suggest that the molecule would be stable, with the C–C bonds to the central ("methane") carbon having a length of 166.1 pm — longer than the typical C−C bond in order to reduce steric effects, but still shorter than those found in some other real molecules. [ 2 ]
This article about theoretical chemistry is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetra-tert-butylmethane |
In organic chemistry , tetraaminoethylene is a hypothetical , organic compound with formula C 2 N 4 H 8 or (H 2 N) 2 C=C(NH 2 ) 2 . Like all polyamines that are geminal , this compound has never been synthesised and is believed to be extremely unstable. [ 2 ]
However, there are many stable compounds that can be viewed as derivatives of tetraaminoethylene, with various organic functional groups substituted for some or all hydrogen atoms. These compounds, which have the general formula (R 2 N) 2 C=C(NR 2 ) 2 , are collectively called tetraaminoethylenes .
Tetraaminoethylenes are important in organic chemistry as dimers of diaminocarbenes , a type of stable carbene with the general formula (R 2 N) 2 C : . | https://en.wikipedia.org/wiki/Tetraaminoethylene |
Tetraazidomethane , C(N 3 ) 4 , is a colorless, highly explosive liquid. Its chemical structure consists of a carbon atom covalently bonded to four azide functional groups .
It was first prepared by Klaus Banert in 2006 by reaction of trichloroacetonitrile with sodium azide . [ 1 ]
As with other polyazides , tetraazidomethane has interest as a high-energy-density material with potential uses in explosives, propellants, or fireworks. [ 2 ] Silicon tetraazide is also a known compound.
Banert has reported that tetraazidomethane participates in a number of reactions including hydrolysis , cycloaddition reactions with alkenes and alkynes , and reaction with phosphines to form phosphazenes . [ 1 ] | https://en.wikipedia.org/wiki/Tetraazidomethane |
Tetrabutylammonium is a quaternary ammonium cation with the formula [N(C 4 H 9 ) 4 ] + , also denoted [NBu 4 ] + (where Bu = butyl group ). It is used in the research laboratory to prepare lipophilic salts of inorganic anions. Relative to tetraethylammonium derivatives, tetrabutylammonium salts are more lipophilic but crystallize less readily.
Some tetrabutylammonium salts of simple anions include:
Some tetrabutylammonium salts of more complex examples include: | https://en.wikipedia.org/wiki/Tetrabutylammonium |
Tetrabutylammonium tribromide , abbreviated to TBATB , is a pale orange solid with the formula [N(C 4 H 9 ) 4 ]Br 3 . It is a salt of the lipophilic tetrabutylammonium cation and the linear tribromide anion. [ 3 ] [ 4 ] The salt is sometimes used as a reagent used in organic synthesis as a conveniently weighable, solid source of bromine .
The compound is prepared by treatment of solid tetra- n -butylammonium bromide with bromine vapor: [ 5 ]
Instead of bromine, tetra- n -butylammonium bromide can also be reacted with vanadium pentoxide and aqueous hydrogen peroxide , or alternatively with ceric ammonium nitrate . [ 1 ]
This article about an organic compound is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetrabutylammonium_tribromide |
This page provides supplementary chemical data on tetrachloroethylene .
The handling of this chemical may incur notable safety precautions. It is highly recommended that you seek the Material Safety Datasheet ( MSDS ) for this chemical from a reliable source such as SIRI , and follow its directions. MSDS is available from Fisher Scientific .
Table data obtained from CRC Handbook of Chemistry and Physics 47th ed. Note that "(s)" annotation indicates equilibrium temperature of vapor pressure of solid. Otherwise indication is equilibrium temperature of vapor of liquid.
See also | https://en.wikipedia.org/wiki/Tetrachloroethylene_(data_page) |
The tetractys ( Greek : τετρακτύς ), or tetrad , [ 1 ] or the tetractys of the decad [ 2 ] is a triangular figure consisting of ten points arranged in four rows: one, two, three, and four points in each row, which is the geometrical representation of the fourth triangular number . As a mystical symbol, it was very important to the secret worship of Pythagoreanism . There were four seasons, and the number was also associated with planetary motions and music. [ 3 ]
A prayer of the Pythagoreans shows the importance of the Tetractys (sometimes called the "Mystic Tetrad"), as the prayer was addressed to it.
Bless us, divine number, thou who generated gods and men! O holy, holy Tetractys, thou that containest the root and source of the eternally flowing creation! For the divine number begins with the profound, pure unity until it comes to the holy four; then it begets the mother of all, the all-comprising, all-bounding, the first-born, the never-swerving, the never-tiring holy ten, the keyholder of all. [ 5 ]
The Pythagorean oath also mentioned the Tetractys:
It is said [ 6 ] [ 7 ] [ 8 ] that the Pythagorean musical system was based on the Tetractys as the rows can be read as the ratios of 4:3 (perfect fourth), 3:2 ( perfect fifth ), 2:1 (octave), forming the basic intervals of the Pythagorean scales. That is, Pythagorean scales are generated from combining pure fourths (in a 4:3 relation), pure fifths (in a 3:2 relation), and the simple ratios of the unison 1:1 and the octave 2:1 . Note that the diapason , 2:1 (octave), and the diapason plus diapente, 3:1 (compound fifth or perfect twelfth), are consonant intervals according to the tetractys of the decad, but that the diapason plus diatessaron, 8:3 (compound fourth or perfect eleventh), is not. [ 9 ] [ 10 ]
The Tetractys [also known as the decad] is an equilateral triangle formed from the sequence of the first ten numbers aligned in four rows. It is both a mathematical idea and a metaphysical symbol that embraces within itself—in seedlike form—the principles of the natural world, the harmony of the cosmos, the ascent to the divine, and the mysteries of the divine realm. So revered was this ancient symbol that it inspired ancient philosophers to swear by the name of the one who brought this gift to humanity.
In the work by anthropologist Raphael Patai entitled The Hebrew Goddess , the author argues that the tetractys and its mysteries influenced the early Kabbalah . [ 11 ] A Hebrew tetractys has the letters of the Tetragrammaton inscribed on the ten positions of the tetractys, from right to left. It has been argued that the Kabbalistic Tree of Life , with its ten spheres of emanation, is in some way connected to the tetractys, but its form is not that of a triangle. The occultist Dion Fortune writes:
The relationship between geometrical shapes and the first four Sephirot is analogous to the geometrical correlations in Tetraktys, shown above under #Pythagorean symbol , and unveils the relevance of the Tree of Life with the Tetraktys.
The tetractys occurs (generally coincidentally) in the following:
In English-language poetry, a tetractys is a syllable-counting form with five lines. The first line has one syllable, the second has two syllables, the third line has three syllables, the fourth line has four syllables, and the fifth line has ten syllables. [ 13 ] A sample tetractys would look like this:
The tetractys was created by Ray Stebbing , who said the following about his newly created form: | https://en.wikipedia.org/wiki/Tetractys |
Tetracyanoethylene ( TCNE ) is organic compound with the formula C 2 (CN) 4 . It is a colorless solid, although samples are often off-white. It is an important member of the cyanocarbons .
TCNE is prepared by brominating malononitrile in the presence of potassium bromide to give the KBr-complex, and dehalogenating with copper . [ 1 ]
Oxidation of TCNE with hydrogen peroxide gives the corresponding epoxide , which has unusual properties. [ 2 ]
In the presence of base , TCNE reacts with malononitrile to give salts of pentacyanopropenide: [ 3 ]
TCNE is an electron acceptor . Cyano groups have low energy π* orbitals , and the presence of four such groups, with their π systems (conjugated) to the central C=C double bond , gives rise to an electrophilic alkene . TCNE is reduced at −0.27 V vs ferrocene/ ferrocenium : [ 4 ]
Because of its ability to accept an electron, TCNE has been used to prepare numerous charge-transfer salts [ 5 ] and magnetic molecular materials.
The central C=C distance in TCNE is 135 pm . [ 6 ] Upon reduction, this bond elongates to 141–145 pm, depending on the counterion. [ 7 ]
TCNE hydrolyzes in moist air to give hydrogen cyanide and should be handled accordingly. [ 1 ] | https://en.wikipedia.org/wiki/Tetracyanoethylene |
Tetracyclics are cyclic chemical compounds that contain four fused rings of atoms, for example, Tröger's base .
Some tricyclic compounds having three fused and one tethered ring (connected to main nucleus by a single bond) can also classified as tetracyclic, for example, ciclazindol . [ 1 ]
Tetracyclic compounds have various pharmaceutical uses, such as:
This organic chemistry article is a stub . You can help Wikipedia by expanding it .
Steroids | https://en.wikipedia.org/wiki/Tetracyclic |
Tetracycline , sold under various brand names, is an antibiotic in the tetracyclines family of medications, used to treat a number of infections , [ 3 ] including acne , cholera , brucellosis , plague , malaria , and syphilis . [ 3 ] It is available in oral and topical formulations. [ 4 ] [ 5 ]
Common side effects include vomiting, diarrhea , rash, and loss of appetite. [ 3 ] Other side effects include poor tooth development if used by children less than eight years of age, kidney problems , and sunburning easily. [ 3 ] Use during pregnancy may harm the baby. [ 3 ] It works by inhibiting protein synthesis in bacteria. [ 3 ]
Tetracycline was patented in 1953 [ 6 ] and was approved for prescription use in 1954. [ 7 ] [ 8 ] It is on the World Health Organization's List of Essential Medicines . [ 9 ] Tetracycline is available as a generic medication . [ 3 ] Tetracycline was originally made from bacteria of the genus Streptomyces . [ 3 ]
Tetracyclines have a broad spectrum of antibiotic action. Originally, they possessed some level of bacteriostatic activity against almost all medically relevant aerobic and anaerobic bacterial genera, both Gram-positive and Gram-negative , with a few exceptions, such as Pseudomonas aeruginosa and Proteus spp. , which display intrinsic resistance. However, acquired (as opposed to inherent) resistance has proliferated in many pathogenic organisms and greatly eroded the formerly vast versatility of this group of antibiotics. Resistance amongst Staphylococcus spp. , Streptococcus spp. , Neisseria gonorrhoeae , anaerobes, members of the Enterobacteriaceae , and several other previously sensitive organisms is now quite common. Tetracyclines remain especially useful in the management of infections by certain obligately intracellular bacterial pathogens such as Chlamydia , Mycoplasma , and Rickettsia . They are also of value in spirochaetal infections, such as syphilis , and Lyme disease . Certain rare or exotic infections, including anthrax , plague , and brucellosis , are also susceptible to tetracyclines. Tetracycline tablets were used in the plague outbreak in India in 1994. [ 10 ] Tetracycline is first-line therapy for Rocky Mountain spotted fever ( Rickettsia ), Lyme disease ( B. burgdorferi ), Q fever ( Coxiella ), psittacosis , Mycoplasma pneumoniae , and nasal carriage of meningococci . [ citation needed ]
It is also one of a group of antibiotics which together may be used to treat peptic ulcers caused by bacterial infections. The mechanism of action for the antibacterial effect of tetracyclines relies on disrupting protein translation in bacteria, thereby damaging the ability of microbes to grow and repair; however, protein translation is also disrupted in eukaryotic mitochondria leading to effects that may confound experimental results. [ 11 ] [ 12 ]
The following list presents MIC susceptibility data for some medically significant microorganisms:
The tetracyclines also have activity against certain eukaryotic parasites, including those responsible for diseases such as dysentery caused by an amoeba , malaria (a plasmodium ), and balantidiasis (a ciliate ). [ citation needed ]
Since tetracycline is absorbed into bone, it is used as a marker of bone growth for biopsies in humans. Tetracycline labeling is used to determine the amount of bone growth within a certain period of time, usually a period around 21 days. Tetracycline is incorporated into mineralizing bone and can be detected by its fluorescence . [ 14 ] In "double tetracycline labeling", a second dose is given 11–14 days after the first dose, and the amount of bone formed during that interval can be calculated by measuring the distance between the two fluorescent labels. [ 15 ]
Tetracycline is also used as a biomarker in wildlife to detect consumption of medicine- or vaccine -containing baits. [ 16 ]
Use of tetracycline antibiotics can: [ 17 ]
Caution should be exercised in long-term use when breastfeeding. Short-term use is safe; bioavailability in milk is low to nil. [ 23 ] According to the U.S. Food and Drug Administration (FDA), cases of Stevens–Johnson syndrome , toxic epidermal necrolysis , and erythema multiforme associated with doxycycline use have been reported, but a causative role has not been established. [ 24 ]
Tetracycline inhibits protein synthesis by blocking the attachment of charged tRNA at the P site peptide chain. Tetracycline blocks the A-site so that a hydrogen bond is not formed between the amino acids. Tetracycline binds to the 30S and 50S subunit of microbial ribosomes. [ 3 ] Thus, it prevents the formation of a peptide chain. [ 25 ] The action is usually not inhibitory and irreversible even with the withdrawal of the drug. Mammalian cells are not vulnerable to the effect of Tetracycline as these cells contain no 30S ribosomal subunits so do not accumulate the drug. [ 26 ] This accounts for the relatively small off-site effect of tetracycline on human cells. [ 27 ]
Bacteria usually acquire resistance to tetracycline from horizontal transfer of a gene that either encodes an efflux pump or a ribosomal protection protein. Efflux pumps actively eject tetracycline from the cell, preventing the build up of an inhibitory concentration of tetracycline in the cytoplasm . [ 28 ] Ribosomal protection proteins interact with the ribosome and dislodge tetracycline from the ribosome, allowing for translation to continue. [ 29 ]
The tetracyclines, a large family of antibiotics, were discovered by Benjamin Minge Duggar in 1948 as natural products, and first prescribed in 1948. [ 30 ] Benjamin Duggar, working under Yellapragada Subbarow at Lederle Laboratories , discovered the first tetracycline antibiotic, chlortetracycline (Aureomycin), in 1945. [ 31 ] The structure of Aureomycin was elucidated in 1952 and published in 1954 by the Pfizer-Woodward group. [ 32 ] After the discovery of the structure, researchers at Pfizer began chemically modifying aureomycin by treating it with hydrogen in the presence of a palladized carbon catalyst . This chemical reaction replaced a chlorine moiety with a hydrogen, creating a compound named tetracycline via hydrogenolysis . [ 33 ] Tetracycline displayed higher potency, better solubility, and more favorable pharmacology than the other antibiotics in its class, leading to its FDA approval in 1954. The new compound was one of the first commercially successful semi-synthetic antibiotics that was used, and laid the foundation for the development of Sancycline, Minocycline , and later the Glycylcyclines . [ 7 ]
Tetracycline has a high affinity for calcium and is incorporated into bones during the active mineralization of hydroxyapatite . When incorporated into bones, tetracycline can be identified using ultraviolet light. [ 34 ]
There is evidence that early inhabitants of Northeastern Africa consumed tetracycline antibiotics. Nubian mummies from between 350 and 550 A.D. were found to exhibit patterns of fluorescence identical with that of modern tetracycline labelled bone. [ 35 ]
It is conjectured that the beer brewed by the Nubians was the source of the tetracycline found in these bones. [ 36 ]
According to data from EvaluatePharma and published in the Boston Globe , in the USA the price of tetracycline rose from $0.06 per 250- mg pill in 2013 to $4.06 a pill in 2015. [ 37 ] The Globe described the "big price hikes of some generic drugs" as a "relatively new phenomenon" which has left most pharmacists "grappling" with large upswings" in the "costs of generics, with 'overnight' price changes sometimes exceeding 1,000%." [ 37 ]
It is marketed under the brand names Sumycin, Tetracyn, and Panmycin, among others. Actisite is a thread-like fiber formulation used in dental applications. [ 38 ]
It is also used to produce several semisynthetic derivatives, which together are known as the tetracycline antibiotics . The term "tetracycline" is also used to denote the four-ring system of this compound; "tetracyclines" are related substances that contain the same four-ring system. [ citation needed ]
Due to the drug's association with fighting infections, it serves as the main "commodity" in the science fiction series Aftermath , with the search for tetracycline becoming a major preoccupation in later episodes. [ 39 ]
Tetracycline is also represented in Bohemia Interactive 's survival sandbox, DayZ . In the game, players may find the antibiotic to treat the common cold, influenza, cholera and infected wounds, but does not portray any side effects associated with tetracycline. [ citation needed ]
In genetic engineering , tetracycline is used in transcriptional activation . It has been used as an engineered "control switch" in chronic myelogenous leukemia models in mice. Engineers were able to develop a retrovirus that induced a particular type of leukemia in mice, and could then "switch" the cancer on and off through tetracycline administration. This could be used to grow the cancer in mice and then halt it at a particular stage to allow for further experimentation or study. [ 40 ]
A technique being developed for the control of the mosquito species Aedes aegypti (the infection vector for yellow fever , dengue fever , Zika fever , and several other diseases) uses a strain that is genetically modified to require tetracycline to develop beyond the larval stage. Modified males raised in a laboratory develop normally as they are supplied with this chemical and can be released into the wild. Their subsequent offspring inherit this trait, but find no tetracycline in their environments, so never develop into adults. [ 41 ] | https://en.wikipedia.org/wiki/Tetracycline |
In geometry , a tetrad is a set of four simply connected disjoint planar regions in the plane, each pair sharing a finite portion of common boundary. It was named by Michael R. W. Buckley in 1975 in the Journal of Recreational Mathematics . A further question was proposed that became a puzzle, whether the 4 regions could be congruent, with or without holes , other enclosed regions. [ 1 ]
The solutions with four congruent tiles include some with five sides. [ 2 ] However, their placement surrounds an uncovered hole in the plane. Among solutions without holes, the ones with the fewest possible sides are given by a hexagon identified by Scott Kim as a student at Stanford University. [ 1 ] It is not known whether five-sided solutions without holes are possible. [ 2 ]
Kim's solution has 16 vertices, while some of the pentagon solutions have as few as 11 vertices. It is not known whether fewer vertices are possible. [ 2 ]
Gardner offered a number of polyform ( polyomino , polyiamond , and polyhex ) solutions, with no holes. [ 1 ]
This geometry-related article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetrad_(geometry_puzzle) |
The tetrad formalism is an approach to general relativity that generalizes the choice of basis for the tangent bundle from a coordinate basis to the less restrictive choice of a local basis, i.e. a locally defined set of four [ a ] linearly independent vector fields called a tetrad or vierbein . [ 1 ] It is a special case of the more general idea of a vielbein formalism , which is set in (pseudo-) Riemannian geometry . This article as currently written makes frequent mention of general relativity; however, almost everything it says is equally applicable to (pseudo-) Riemannian manifolds in general, and even to spin manifolds . Most statements hold simply by substituting arbitrary n {\displaystyle n} for n = 4 {\displaystyle n=4} . In German, " vier " translates to "four", " viel " to "many", and " bein " to "leg".
The general idea is to write the metric tensor as the product of two vielbeins , one on the left, and one on the right. The effect of the vielbeins is to change the coordinate system used on the tangent manifold to one that is simpler or more suitable for calculations. It is frequently the case that the vielbein coordinate system is orthonormal, as that is generally the easiest to use. Most tensors become simple or even trivial in this coordinate system; thus the complexity of most expressions is revealed to be an artifact of the choice of coordinates, rather than a innate property or physical effect [ citation needed ] . That is, as a formalism, it does not alter predictions; it is rather a calculational technique.
The advantage of the tetrad formalism over the standard coordinate-based approach to general relativity lies in the ability to choose the tetrad basis to reflect important physical aspects of the spacetime. The abstract index notation denotes tensors as if they were represented by their coefficients with respect to a fixed local tetrad. Compared to a completely coordinate free notation , which is often conceptually clearer, it allows an easy and computationally explicit way to denote contractions.
The significance of the tetradic formalism appear in the Einstein–Cartan formulation of general relativity. The tetradic formalism of the theory is more fundamental than its metric formulation as one can not convert between the tetradic and metric formulations of the fermionic actions despite this being possible for bosonic actions [ citation needed ] . This is effectively because Weyl spinors can be very naturally defined on a Riemannian manifold [ 2 ] [ citation needed ] and their natural setting leads to the spin connection . Those spinors take form in the vielbein coordinate system, and not in the manifold coordinate system.
The privileged tetradic formalism also appears in the deconstruction of higher dimensional Kaluza–Klein gravity theories [ 3 ] and massive gravity theories, in which the extra-dimension(s) is/are replaced by series of N lattice sites such that the higher dimensional metric is replaced by a set of interacting metrics that depend only on the 4D components. [ 4 ] Vielbeins commonly appear in other general settings in physics and mathematics. Vielbeins can be understood as solder forms .
The tetrad formulation is a special case of a more general formulation, known as the vielbein or n -bein formulation, with n =4. Vielbien is spelt with an "l", not an "r": in German, "viel" means "many", not to be confused with "vier", meaning "four".
In the vielbein formalism, [ 5 ] an open cover of the spacetime manifold M {\displaystyle M} and a local basis for each of those open sets is chosen: a set of n {\displaystyle n} independent vector fields
for a = 1 , … , n {\displaystyle a=1,\ldots ,n} that together span the n {\displaystyle n} -dimensional tangent bundle at each point in the set. Dually, a vielbein (or tetrad in 4 dimensions) determines (and is determined by) a dual co-vielbein (co-tetrad) — a set of n {\displaystyle n} independent 1-forms .
such that
where δ b a {\displaystyle \delta _{b}^{a}} is the Kronecker delta . A vielbein is usually specified by its coefficients e μ a {\displaystyle e^{\mu }{}_{a}} with respect to a coordinate basis, despite the choice of a set of (local) coordinates x μ {\displaystyle x^{\mu }} being unnecessary for the specification of a tetrad. Each covector is a solder form .
From the point of view of the differential geometry of fiber bundles , the n vector fields { e a } a = 1 … n {\displaystyle \{e_{a}\}_{a=1\dots n}} define a section of the frame bundle i.e. a parallelization of U ⊂ M {\displaystyle U\subset M} which is equivalent to an isomorphism T U ≅ U × R n {\displaystyle TU\cong U\times {\mathbb {R} ^{n}}} . Since not every manifold is parallelizable, a vielbein can generally only be chosen locally ( i.e. only on a coordinate chart U {\displaystyle U} and not all of M {\displaystyle M} .)
All tensors of the theory can be expressed in the vector and covector basis, by expressing them as linear combinations of members of the (co)vielbein. For example, the spacetime metric tensor can be transformed from a coordinate basis to the tetrad basis .
Popular tetrad bases in general relativity include orthonormal tetrads and null tetrads. Null tetrads are composed of four null vectors , so are used frequently in problems dealing with radiation, and are the basis of the Newman–Penrose formalism and the GHP formalism .
The standard formalism of differential geometry (and general relativity) consists of using the coordinate tetrad in the tetrad formalism. The coordinate tetrad is the canonical set of vectors associated with the coordinate chart . The coordinate tetrad is commonly denoted { ∂ μ } {\displaystyle \{\partial _{\mu }\}} whereas the dual cotetrad is denoted { d x μ } {\displaystyle \{dx^{\mu }\}} . These tangent vectors are usually defined as directional derivative operators: given a chart φ = ( φ 1 , … , φ n ) {\displaystyle {\varphi =(\varphi ^{1},\ldots ,\varphi ^{n})}} which maps a subset of the manifold into coordinate space R n {\displaystyle \mathbb {R} ^{n}} , and any scalar field f {\displaystyle f} , the coordinate vectors are such that:
The definition of the cotetrad uses the usual abuse of notation d x μ = d φ μ {\displaystyle dx^{\mu }=d\varphi ^{\mu }} to define covectors (1-forms) on M {\displaystyle M} . The involvement of the coordinate tetrad is not usually made explicit in the standard formalism. In the tetrad formalism, instead of writing tensor equations out fully (including tetrad elements and tensor products ⊗ {\displaystyle \otimes } as above) only components of the tensors are mentioned. For example, the metric is written as " g a b {\displaystyle g_{ab}} ". When the tetrad is unspecified this becomes a matter of specifying the type of the tensor called abstract index notation . It allows to easily specify contraction between tensors by repeating indices as in the Einstein summation convention.
Changing tetrads is a routine operation in the standard formalism, as it is involved in every coordinate transformation (i.e., changing from one coordinate tetrad basis to another). Switching between multiple coordinate charts is necessary because, except in trivial cases, it is not possible for a single coordinate chart to cover the entire manifold. Changing to and between general tetrads is much similar and equally necessary (except for parallelizable manifolds ). Any tensor can locally be written in terms of this coordinate tetrad or a general (co)tetrad.
For example, the metric tensor g {\displaystyle \mathbf {g} } can be expressed as:
(Here we use the Einstein summation convention ). Likewise, the metric can be expressed with respect to an arbitrary (co)tetrad as
Here, we use choice of alphabet ( Latin and Greek ) for the index variables to distinguish the applicable basis.
We can translate from a general co-tetrad to the coordinate co-tetrad by expanding the covector e a = e a μ d x μ {\displaystyle e^{a}=e^{a}{}_{\mu }dx^{\mu }} . We then get
from which it follows that g μ ν = g a b e a μ e b ν {\displaystyle g_{\mu \nu }=g_{ab}e^{a}{}_{\mu }e^{b}{}_{\nu }} . Likewise expanding d x μ = e μ a e a {\displaystyle dx^{\mu }=e^{\mu }{}_{a}e^{a}} with respect to the general tetrad, we get
which shows that g a b = g μ ν e μ a e ν b {\displaystyle g_{ab}=g_{\mu \nu }e^{\mu }{}_{a}e^{\nu }{}_{b}} .
The manipulation with tetrad coefficients shows that abstract index formulas can, in principle, be obtained from tensor formulas with respect to a coordinate tetrad by "replacing greek by latin indices". However care must be taken that a coordinate tetrad formula defines a genuine tensor when differentiation is involved. Since the coordinate vector fields have vanishing Lie bracket (i.e. commute: ∂ μ ∂ ν = ∂ ν ∂ μ {\displaystyle \partial _{\mu }\partial _{\nu }=\partial _{\nu }\partial _{\mu }} ), naive substitutions of formulas that correctly compute tensor coefficients with respect to a coordinate tetrad may not correctly define a tensor with respect to a general tetrad because the Lie bracket is non-vanishing: [ e a , e b ] ≠ 0 {\displaystyle [e_{a},e_{b}]\neq 0} . Thus, it is sometimes said that tetrad coordinates provide a non-holonomic basis .
For example, the Riemann curvature tensor is defined for general vector fields X , Y {\displaystyle X,Y} by
In a coordinate tetrad this gives tensor coefficients
The naive "Greek to Latin" substitution of the latter expression
is incorrect because for fixed c and d , ( ∇ c ∇ d − ∇ d ∇ c ) {\displaystyle \left(\nabla _{c}\nabla _{d}-\nabla _{d}\nabla _{c}\right)} is, in general, a first order differential operator rather than a zeroth order operator which defines a tensor coefficient. Substituting a general tetrad basis in the abstract formula we find the proper definition of the curvature in abstract index notation, however:
where [ e a , e b ] = f a b c e c {\displaystyle [e_{a},e_{b}]=f_{ab}{}^{c}e_{c}} . Note that the expression ( ∇ c ∇ d − ∇ d ∇ c − f c d e ∇ e ) {\displaystyle \left(\nabla _{c}\nabla _{d}-\nabla _{d}\nabla _{c}-f_{cd}{}^{e}\nabla _{e}\right)} is indeed a zeroth order operator, hence (the ( c d )-component of) a tensor. Since it agrees with the coordinate expression for the curvature when specialised to a coordinate tetrad it is clear, even without using the abstract definition of the curvature, that it defines the same tensor as the coordinate basis expression.
Given a vector (or covector) in the tangent (or cotangent) manifold, the exponential map describes the corresponding geodesic of that tangent vector. Writing X ∈ T M {\displaystyle X\in TM} , the parallel transport of a differential corresponds to
The above can be readily verified simply by taking X {\displaystyle X} to be a matrix.
For the special case of a Lie algebra , the X {\displaystyle X} can be taken to be an element of the algebra, the exponential is the exponential map of a Lie group , and group elements correspond to the geodesics of the tangent vector. Choosing a basis e i {\displaystyle e_{i}} for the Lie algebra and writing X = X i e i {\displaystyle X=X^{i}e_{i}} for some functions X i , {\displaystyle X^{i},} the commutators can be explicitly written out. One readily computes that
for [ e i , e j ] = f i j k e k {\displaystyle [e_{i},e_{j}]={f_{ij}}^{k}e_{k}} the structure constants of the Lie algebra. The series can be written more compactly as
with the infinite series
Here, M {\displaystyle M} is a matrix whose matrix elements are M j k = X i f i j k {\displaystyle {M_{j}}^{k}=X^{i}{f_{ij}}^{k}} . The matrix W {\displaystyle W} is then the vielbein; it expresses the differential d X j {\displaystyle dX^{j}} in terms of the "flat coordinates" (orthonormal, at that) e i {\displaystyle e_{i}} .
Given some map N → G {\displaystyle N\to G} from some manifold N {\displaystyle N} to some Lie group G {\displaystyle G} , the metric tensor on the manifold N {\displaystyle N} becomes the pullback of the metric tensor B m n {\displaystyle B_{mn}} on the Lie group G {\displaystyle G} :
The metric tensor B m n {\displaystyle B_{mn}} on the Lie group is the Cartan metric, aka the Killing form . Note that, as a matrix, the second W is the transpose. For N {\displaystyle N} a (pseudo-) Riemannian manifold , the metric is a (pseudo-) Riemannian metric . The above generalizes to the case of symmetric spaces . [ 6 ] These vielbeins are used to perform calculations in sigma models , of which the supergravity theories are a special case. [ 7 ] | https://en.wikipedia.org/wiki/Tetrad_formalism |
Tetraethylammonium trichloride (also known as Mioskowski reagent ) [ 2 ] is a chemical compound with the formula [NEt 4 ][Cl 3 ] consisting of a tetraethylammonium cation and a trichloride as anion. The trichloride is also known as trichlorine monoanion representing one of the simplest polychlorine anions. [ 3 ] Tetraethylammonium trichloride is used as reagent for chlorinations and oxidation reactions.
At room temperature, tetraethylammonium trichloride is a yellow solid which is soluble in polar organic solvents (e.g., methylene chloride or acetonitrile ). As it is a strong oxidant and chlorinating agent it is reacting with most organic solvents. [ 1 ] The trichloride can be considered as an symmetric anion as found in [N n Pr 4 ][Cl 3 ], which is formed by a 3c-4e bond. [ 4 ]
Commonly, tetraethylammonium trichloride is prepared by the reaction of tetraethylammonium chloride and elemental chlorine in methylene chloride at room temperature . After evaporation of the solvent, tetraethylammonium trichloride is obtained as a yellow solid.
[NEt 4 ]Cl + Cl 2 → [NEt 4 ][Cl 3 ]
Recently, an alternative preparation of tetraethylammonium trichloride has been described using tetraethylammonium chloride and potassium peroxymonosulfate as oxidant. [ 2 ]
In general, tetraethylammonium trichloride has a similar reactivity compared to elemental chlorine and other trichlorides, e.g., triethylmethylammonium trichloride . As tetraethylammonium trichloride is a solid and can be dissolved in methylene chloride or acetonitrile, it is used as an easier to handle alternative to elemental chlorine, in particular for the synthesis of intermediates in natural product synthesis. [ 5 ] [ 6 ] [ 7 ] Tetraethylammonium trichloride reacts with alkenes to the corresponding vicinal 1,2-dichlorinated alkanes and similarly with alkynes to the corresponding trans -dichlorinated alkenes. Electron rich arenes are chlorinated in para -position. While aldehydes are dichlorinated in alpha -position, ketones react to the monochlorinated alpha -chloroketones. In presence of 1,4-diazabicyclo[2.2.2]octane tetraethylammonium trichloride is a useful oxidant for the oxidation of primary alcohols to the corresponding aldehydes and of secondary alcohols to the corresponding ketones. For compounds bearing both a primary and a secondary alcohol, selective oxidation of the secondary alcohol is observed. Acetals undergo C-H chlorination of the tertiary C-H bond providing the corresponding chlorinated acetals. [ 1 ] | https://en.wikipedia.org/wiki/Tetraethylammonium_trichloride |
In 4-dimensional geometry , the tetrahedral bipyramid is the direct sum of a tetrahedron and a segment, {3,3} + { }. Each face of a central tetrahedron is attached with two tetrahedra, creating 8 tetrahedral cells, 16 triangular faces, 14 edges, and 6 vertices. [ 1 ] A tetrahedral bipyramid can be seen as two tetrahedral pyramids augmented together at their base.
It is the dual of a tetrahedral prism , , so it can also be given a Coxeter-Dynkin diagram , , and both have Coxeter notation symmetry [2,3,3], order 48.
Being convex with all regular cells (tetrahedra) means that it is a Blind polytope .
This bipyramid exists as the cells of the dual of the uniform rectified 5-simplex , and rectified 5-cube or the dual of any uniform 5-polytope with a tetrahedral prism vertex figure . And, as well, it exists as the cells of the dual to the rectified 24-cell honeycomb .
This 4-polytope article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetrahedral_bipyramid |
In 4-dimensional geometry , the tetrahedral cupola is a polychoron bounded by one tetrahedron , a parallel cuboctahedron , connected by 10 triangular prisms , and 4 triangular pyramids . [ 1 ]
The tetrahedral cupola can be sliced off from a runcinated 5-cell , on a hyperplane parallel to a tetrahedral cell. The cuboctahedron base passes through the center of the runcinated 5-cell, so the Tetrahedral cupola contains half of the tetrahedron and triangular prism cells of the runcinated 5-cell. The cupola can be seen in A 2 and A 3 Coxeter plane orthogonal projection of the runcinated 5-cell:
This 4-polytope article is a stub . You can help Wikipedia by expanding it . | https://en.wikipedia.org/wiki/Tetrahedral_cupola |
In a tetrahedral molecular geometry , a central atom is located at the center with four substituents that are located at the corners of a tetrahedron . The bond angles are arccos (− 1 / 3 ) = 109.4712206...° ≈ 109.5° when all four substituents are the same, as in methane ( CH 4 ) [ 1 ] [ 2 ] as well as its heavier analogues . Methane and other perfectly symmetrical tetrahedral molecules belong to point group T d , but most tetrahedral molecules have lower symmetry . Tetrahedral molecules can be chiral .
The bond angle for a symmetric tetrahedral molecule such as CH 4 may be calculated using the dot product of two vectors . As shown in the diagram at left, the molecule can be inscribed in a cube with the tetravalent atom (e.g. carbon ) at the cube centre which is the origin of coordinates, O. The four monovalent atoms (e.g. hydrogens) are at four corners of the cube (A, B, C, D) chosen so that no two atoms are at adjacent corners linked by only one cube edge.
If the edge length of the cube is chosen as 2 units, then the two bonds OA and OB correspond to the vectors a = (1, –1, 1) and b = (1, 1, –1) , and the bond angle θ is the angle between these two vectors. This angle may be calculated from the dot product of the two vectors, defined as a ⋅ b = ‖ a ‖ ‖ b ‖ cos θ where ‖ a ‖ denotes the length of vector a . As shown in the diagram, the dot product here is –1 and the length of each vector is √ 3 , so that cos θ = – 1 / 3 and the tetrahedral bond angle θ = arccos (– 1 / 3 ) ≃ 109.47° .
An alternative proof using trigonometry is shown in the diagram at right.
Aside from virtually all saturated organic compounds, most compounds of Si, Ge, and Sn are tetrahedral. Often tetrahedral molecules feature multiple bonding to the outer ligands, as in xenon tetroxide (XeO 4 ), the perchlorate ion ( ClO − 4 ), the sulfate ion ( SO 2− 4 ), the phosphate ion ( PO 3− 4 ). Thiazyl trifluoride ( SNF 3 ) is tetrahedral, featuring a sulfur-to-nitrogen triple bond. [ 3 ]
Other molecules have a tetrahedral arrangement of electron pairs around a central atom; for example ammonia ( NH 3 ) with the nitrogen atom surrounded by three hydrogens and one lone pair . However the usual classification considers only the bonded atoms and not the lone pair, so that ammonia is actually considered as pyramidal . The H–N–H angles are 107°, contracted from 109.5°. This difference is attributed to the influence of the lone pair which exerts a greater repulsive influence than a bonded atom. [ citation needed ]
Again the geometry is widespread, particularly so for complexes where the metal has d 0 or d 10 configuration. Illustrative examples include tetrakis(triphenylphosphine)palladium(0) ( Pd[P(C 6 H 5 ) 3 ] 4 ), nickel carbonyl ( Ni(CO) 4 ), and titanium tetrachloride ( TiCl 4 ). Many complexes with incompletely filled d-shells are often tetrahedral, e.g. the tetrahalides of iron(II), cobalt(II), and nickel(II).
In the gas phase, a single water molecule has an oxygen atom surrounded by two hydrogens and two lone pairs, and the H 2 O geometry is simply described as bent without considering the nonbonding lone pairs. [ citation needed ]
However, in liquid water or in ice, the lone pairs form hydrogen bonds with neighboring water molecules. The most common arrangement of hydrogen atoms around an oxygen is tetrahedral with two hydrogen atoms covalently bonded to oxygen and two attached by hydrogen bonds. Since the hydrogen bonds vary in length many of these water molecules are not symmetrical and form transient irregular tetrahedra between their four associated hydrogen atoms. [ 4 ]
Many compounds and complexes adopt bitetrahedral structures. In this motif, the two tetrahedra share a common edge. The inorganic polymer silicon disulfide features an infinite chain of edge-shared tetrahedra. In a completely saturated hydrocarbon system, bitetrahedral molecule C 8 H 6 has been proposed as a candidate for the molecule with the shortest possible carbon-carbon single bond . [ 5 ]
Inversion of tetrahedra occurs widely in organic and main group chemistry. The Walden inversion illustrates the stereochemical consequences of inversion at carbon. Nitrogen inversion in ammonia also entails transient formation of planar NH 3 .
Geometrical constraints in a molecule can cause a severe distortion of idealized tetrahedral geometry. In compounds featuring "inverted" tetrahedral geometry at a carbon atom, all four groups attached to this carbon are on one side of a plane. [ 6 ] The carbon atom lies at or near the apex of a square pyramid with the other four groups at the corners. [ 7 ] [ 8 ]
The simplest examples of organic molecules displaying inverted tetrahedral geometry are the smallest propellanes , such as [1.1.1]propellane ; or more generally the paddlanes , [ 9 ] and pyramidane ([3.3.3.3]fenestrane). [ 7 ] [ 8 ] Such molecules are typically strained , resulting in increased reactivity.
A tetrahedron can also be distorted by increasing the angle between two of the bonds. In the extreme case, flattening results. For carbon this phenomenon can be observed in a class of compounds called the fenestranes . [ citation needed ]
A few molecules have a tetrahedral geometry with no central atom. An inorganic example is tetraphosphorus ( P 4 ) which has four phosphorus atoms at the vertices of a tetrahedron and each bonded to the other three. An organic example is tetrahedrane ( C 4 H 4 ) with four carbon atoms each bonded to one hydrogen and the other three carbons. In this case the theoretical C−C−C bond angle is just 60° (in practice the angle will be larger due to bent bonds ), representing a large degree of strain. [ citation needed ] | https://en.wikipedia.org/wiki/Tetrahedral_molecular_geometry |
Tetrahedrane is a hypothetical platonic hydrocarbon with chemical formula C 4 H 4 and a tetrahedral structure. The molecule would be subject to considerable angle strain and has not been synthesized as of 2023 [update] . However, a number of derivatives have been prepared. In a more general sense, the term tetrahedranes is used to describe a class of molecules and ions with related structure, e.g. white phosphorus .
Tetrahedrane ( C 4 H 4 ) is one of the possible platonic hydrocarbons and has the IUPAC name tricyclo[1.1.0.0 2,4 ]butane. [ citation needed ]
Unsubstituted tetrahedrane remains elusive, although predicted kinetically stable. One strategy that has been explored (but thus far failed) is reaction of propene with atomic carbon . [ 1 ]
Contrariwise, several organic compounds with the tetrahedrane core are known. All have multiply bulky substituents, tert -butyl ( t -Bu) or larger. Günther Maier has proposed the corset effect , in which the bulky substituents stabilize the core because decomposition would force the substituents closer together. [ 2 ] Locking a tetrahedrane molecule inside a fullerene has only been attempted in silico . [ 3 ]
All known syntheses have relied on rearrangement from another unstable molecule. In Maier's original synthesis, photochemical cheletropic decarbonylation converts a cyclopentadienone to the tetrahedrane. [ 2 ] In a later synthesis, irradiation directly converted a cyclobutadiene to tetrahedrane. [ 4 ] And more recently, single-electron oxidation can induce a radical chain isomerization with the same effect. [ 5 ]
Tetrahedrane with small substituents would have a variety of interesting properties. Due to its bond strain and stoichiometry, tetranitrotetrahedrane has potential as a high-performance energetic material (explosive). [ 6 ]
Calculations suggest that tetrahedrane's molecular strain reduces if slightly-flexible diyne spacers separate the vertices. [ 7 ]
In 1978, Günther Maier first prepared tetra- tert -butyl-tetrahedrane, [ 2 ] with a deceptively short and simple synthesis that required "astonishing persistence and experimental skill". [ 8 ] "The relatively straightforward scheme shown [...] conceals both the limited availability of the starting material and the enormous amount of work required in establishing the proper conditions for each step." [ 9 ] In Maier's own account, it took several years of careful observation and optimization to develop the correct conditions for the reactions. For instance, the synthesis of tetrakis( t- butyl)cyclopentadienone from the tris( t -butyl)bromocyclopentadienone (itself synthesized with much difficulty) required over 50 attempts before working conditions could be found. [ 10 ]
Maier began with cycloaddition of an alkyne to t -Bu substituted maleic anhydride . [ 11 ] Rearrangement and decarboxylation gave a corset-stabilized cyclopentadienone . To add the fourth t -Bu group, Maier brominated the only labile hydrogen to give an electrophile that coupled directly to tert-butyllithium . Photochemical cheletropic decarbonylation then gave the target.
Heating tetra- tert -butyltetrahedrane gives tetra- tert -butyl cyclobutadiene . The reversibility of this rearrangement proved key to developing a more scalable synthesis. In the last step, photolysis of a cyclopropenyl-substituted diazomethane affords the desired product through a tetrakis( tert -butyl)cyclobutadiene intermediate: [ 4 ] [ 12 ]
Tetrakis(trimethylsilyl)tetrahedrane can be prepared by treatment of the cyclobutadiene precursor with tris(pentafluorophenyl)borane [ 5 ] and is far more stable than the tert -butyl analogue. The silicon–carbon bond is longer than a carbon–carbon bond, and therefore the corset effect is reduced. [ 13 ] Whereas the tert -butyl tetrahedrane melts at 135 °C concomitant with rearrangement to the cyclobutadiene, tetrakis(trimethylsilyl)tetrahedrane, which melts at 202 °C, is stable up to 300 °C, at which point it cracks to bis(trimethylsilyl)acetylene.
The tetrahedrane skeleton is made up of banana bonds , and hence the carbon atoms are high in s-orbital character. From NMR , sp- hybridization can be deduced, normally reserved for triple bonds . As a consequence the bond lengths are unusually short with 152 picometers .
Reaction with methyllithium with tetrakis(trimethylsilyl)tetrahedrane yields tetrahedranyllithium. [ 14 ] The lithium compound can then couple to electrophiles , even relatively small ones. [ 15 ] [ 16 ] [ 17 ]
A bis(tetrahedrane) has also been reported. [ 18 ] The connecting bond is even shorter with 143.6 pm. An ordinary carbon–carbon bond has a length of 154 pm.
The tetrahedrane motif occurs broadly in chemistry. White phosphorus (P 4 ) and yellow arsenic (As 4 ) naturally form tetrahedrane-like clusters. There are a wide variety of synthetic pnictogen-substituted tetrahedranes , and metallatetrahedranes with a single metal (or phosphorus atom) capping a cyclopropyl trianion also exist. [ 20 ] Several metal carbonyl clusters are referred to as tetrahedranes, e.g. tetrarhodium dodecacarbonyl .
Silicon also can be induced to form a tetrahedral core, [ 21 ] but heavier adamantogens tend to form cubane -like clusters. [ citation needed ]
In tetrasilatetrahedrane features a core of four silicon atoms. The standard silicon–silicon bond is much longer (235 pm) and the cage is again enveloped by a total of 16 trimethylsilyl groups, which confer stability. The silatetrahedrane can be reduced with potassium graphite to the tetrasilatetrahedranide potassium derivative. In this compound one of the silicon atoms of the cage has lost a silyl substituent and carries a negative charge. The potassium cation can be sequestered by a crown ether , and in the resulting complex potassium and the silyl anion are separated by a distance of 885 pm. One of the Si − –Si bonds is now 272 pm and the tetravalent silicon atom of that bond has an inverted tetrahedral geometry . Furthermore, the four cage silicon atoms are equivalent on the NMR timescale due to migrations of the silyl substituents over the cage. [ 21 ]
The dimerization reaction observed for the carbon tetrahedrane compound is also attempted for a tetrasilatetrahedrane. [ 22 ] In this tetrahedrane the cage is protected by four so-called supersilyl groups in which a silicon atom has 3 tert -butyl substituents. The dimer does not materialize but a reaction with iodine in benzene followed by reaction with the tri- tert -butylsilaanion results in the formation of an eight-membered silicon cluster compound which can be described as a Si 2 dumbbell (length 229 pm and with inversion of tetrahedral geometry) sandwiched between two almost-parallel Si 3 rings. | https://en.wikipedia.org/wiki/Tetrahedrane |
The Tetrahedron Computer Methodology was a short lived journal that was published by Pergamon Press [ 1 ] (now Elsevier ) to experiment with electronic submission of articles in the ChemText format, [ 2 ] and the sharing source code to enable reproducibility. [ 3 ] [ 4 ] [ 5 ] It was the first chemical journal to be published electronically, [ 6 ] with issues distributed in print and on floppy disks . [ 7 ] It is likely it was also the first journal to accept submissions in a non-paper format (on floppy disks). [ 1 ] The journal ceased publication owing to technical and non-technical reasons, and may have lacked sufficient institutional support. [ 8 ] The last issue appeared in 1992 but was dated 1990. [ 9 ] | https://en.wikipedia.org/wiki/Tetrahedron_Computer_Methodology |
In geometry , tetrahedron packing is the problem of arranging identical regular tetrahedra throughout three-dimensional space so as to fill the maximum possible fraction of space.
Currently, the best lower bound achieved on the optimal packing fraction of regular tetrahedra is 85.63%. [ 1 ] Tetrahedra do not tile space, [ 2 ] and an upper bound below 100% (namely, 1 − (2.6...)·10 −25 ) has been reported. [ 3 ]
Aristotle claimed that tetrahedra could fill space completely. [ 4 ] [ 5 ]
In 2006, Conway and Torquato showed that a packing fraction about 72% can be obtained by constructing a non-Bravais lattice packing of tetrahedra (with multiple particles with generally different orientations per repeating unit), and thus they showed that the best tetrahedron packing cannot be a lattice packing (with one particle per repeating unit such that each particle has a common orientation). [ 6 ] These packing constructions almost doubled the optimal Bravais-lattice-packing fraction 36.73% obtained by Hoylman. [ 7 ] In 2007 and 2010, Chaikin and coworkers experimentally showed that tetrahedron-like dice can randomly pack in a finite container up to a packing fraction between 75% and 76%. [ 8 ] In 2008, Chen was the first to propose a packing of hard, regular tetrahedra that packed more densely than spheres, demonstrating numerically a packing fraction of 77.86%. [ 9 ] [ 10 ] A further improvement was made in 2009 by Torquato and Jiao, who compressed Chen's structure using a computer algorithm to a packing fraction of 78.2021%. [ 11 ]
In mid-2009 Haji-Akbari et al. showed, using MC simulations of initially random systems that at packing densities >50% an equilibrium fluid of hard tetrahedra spontaneously transforms to a dodecagonal quasicrystal , which can be compressed to 83.24%. They also reported a glassy, disordered packing at densities exceeding 78%. For a periodic approximant to a quasicrystal with an 82-tetrahedron unit cell, they obtained a packing density as high as 85.03%. [ 12 ]
In late 2009, a new, much simpler family of packings with a packing fraction of 85.47% was discovered by Kallus, Elser, and Gravel. [ 13 ] These packings were also the basis of a slightly improved packing obtained by Torquato and Jiao at the end of 2009 with a packing fraction of 85.55%, [ 14 ] and by Chen, Engel, and Glotzer in early 2010 with a packing fraction of 85.63%. [ 1 ] The Chen, Engel and Glotzer result currently stands as the densest known packing of hard, regular tetrahedra. Surprisingly, the square-triangle tiling [ 12 ] packs denser than this double lattice of triangular bipyramids when tetrahedra are slightly rounded (the Minkowski sum of a tetrahedron and a sphere), making the 82-tetrahedron crystal the largest unit cell for a densest packing of identical particles to date. [ 15 ]
Because the earliest lower bound known for packings of tetrahedra was less than that of spheres , it was suggested that the regular tetrahedra might be a counterexample to Ulam's conjecture that the optimal density for packing congruent spheres is smaller than that for any other convex body. However, the more recent results have shown that this is not the case. | https://en.wikipedia.org/wiki/Tetrahedron_packing |
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