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Sun protective clothing	History	The original UPF rating system was enhanced in the United States by the American Society for Testing and Materials (ASTM) Committee D13.65, at the behest of the Federal Trade Commission (FTC) and the Consumer Product Safety Commission, to qualify and standardize the emerging sun protective clothing and textile industry. When the Food and Drug Administration (FDA) discontinued regulating sun-protective clothing, the Solar Protective Factory (whose CEO chaired the ASTM Committee) took the lead in developing the UPF testing protocols and labeling standards that are presently used in the United States.In 1992, the FDA reviewed clothing that was being marketed with claims of sun protection (SPF, % UV blockage, or skin cancer prevention). Only one brand of sun protective clothing, Solumbra, was cleared under medical device regulations. The FDA initially regulated sun protective clothing as a medical device, but later transferred oversight for general sun protective clothing to the FTC. The UPF rating system may eventually be adopted by interested apparel/textile/fabric manufacturers as a "value added" program for consumer safety and awareness. Before UPF standards were in place (which directly measure a fabric's ability to block UV radiation), clothing was previously rated using SPF standards (which measure how long a person's skin takes to redden).
Sun protective clothing	Fabric	Factors that affect the level of sun protection provided by a fabric, in approximate order of importance, include weave, color, weight, stretch, and wetness. The less open or more dense the fabric (weave, weight, stretch), the better the protection. Getting a fabric wet reduces the protection as much as half, except for silk and viscose which can get more protective when wet. Polyester contains a benzene ring that absorbs UV light. In addition, UV absorbers may be added at various points in the manufacturing process to enhance protection levels. In 2003, chemical company BASF embedded nanoparticles of titanium dioxide into a nylon fabric, which can be used for sun protective clothing that maintains its UV protection when wet.There is some indication that washing fabrics in detergents containing fabric brighteners, which absorb UV radiation, might increase their protective capability. Studies at the University of Alberta also indicate that darker-colored fabrics offer more protection than lighter-colored fabrics.While there is some correlation between the percentages of visible light and UV that pass through the same fabric, it is not a strong relationship. With new-technology textiles designed for the sole purpose of UV blocking, it is not always possible to judge the UV protection level simply by holding up the fabric and examining how much visible light passes through.Provide more protection: specially manufactured fabrics cotton viscose fabrics black or dark blue denim jeans wool garments satin-finished silk of any weight tightly woven Bamboo/Lycra fabric polyacrylonitrile 100% polyester shiny polyester blends tightly woven fabrics REPREVE fabric unbleached cotton (most cotton sold is bleached) bamboo/cotton blendProvide less protection: polyester crepe bleached cotton viscose knits undyed/white jeans worn/old fabric
Sun protective clothing	UPF rating	A relatively new rating designation for sun protective textiles and clothing is UPF (ultraviolet protection factor), which represents the ratio of sunburn-causing UV measured without and with the protection of the fabric. For example, a fabric rated UPF 30 means that, if 30 units of UV fall on the fabric, only 1 unit will pass through to the skin. A UPF 30 fabric that blocks 29 out of 30 units of UV is therefore blocking 96.7%. Unlike SPF (sun protection factor) measurements that traditionally use human sunburn testing, UPF is measured using a laboratory instrument (spectrophotometer or spectroradiometer) and an artificial light source, and then applying a sunburn weighting curve (erythemal action spectrum) across the relevant UV wavelengths. Theoretically, human SPF testing and instrument UPF testing both generate comparable measurements of a product's ability to protect against sunburn.
Sun protective clothing	UPF rating	Below is the ASTM Standard for Sun Protective Clothing and Swimwear: According to testing by Consumer Reports, UPF 30+ is typical for protective fabrics, while UPF 20 is typical for standard summer fabrics.
Sun protective clothing	UPF testing protocol	Developed in 1998 by Committee RA106, the testing standard for sun protective fabrics in the United States is the American Association of Textile Chemists and Colorists (AATCC) Test Method 183. This method is based on the original guidelines established in Australia in 1994.
Sun protective clothing	UPF testing protocol	AATCC 183 should be used in conjunction with other related standards including ASTM D 6544 and ASTM D 6603. ASTM D 6544 specifies simulating the life cycle of a fabric so that a UPF test can be done near the end of the fabric's life, when it typically provides the least UV protection. ASTM D 6603 is a consumer format recommended for visible hangtag and care labeling of sun protective clothing and textiles. A manufacturer may publish a test result to a maximum of UPF 50+.
Sun protective clothing	UPF testing protocol	Sun protective clothing and textile/fabric manufacturers are currently a self-regulating industry in North America, prescribed by the AATCC and ASTM methods of testing.
Psychoticism	Psychoticism	Psychoticism is one of the three traits used by the psychologist Hans Eysenck in his P–E–N model (psychoticism, extraversion and neuroticism) model of personality.
Psychoticism	Nature	Psychoticism is conceptually similar to the constraint factor in Tellegen's three-factor model of personality. Psychoticism may be divided into narrower traits such as impulsivity and sensation-seeking. These may in turn be further subdivided into even more specific traits. For example, impulsivity may be divided into narrow impulsivity (unthinking responsivity), risk taking, non-planning, and liveliness. Sensation seeking has also been analysed into a number of separate facets.
Psychoticism	Nature	Eysenck argued that there might be a correlation between psychoticism and creativity.
Psychoticism	Critics	Critics of the trait have suggested that the trait is too heterogeneous to be taken as a single trait. Costa and McCrae believe that agreeableness and conscientiousness (both of which represent low levels of psychoticism) need to be distinguished in personality models. It has also been suggested that "psychoticism" may be a misnomer and that "psychopathy" or "Impulsive Unsocialized Sensation Seeking" would be better labels.
Psychoticism	Biological bases	Psychoticism is believed to be associated with levels of dopamine. Other biological correlates of psychoticism include low conditionability and low levels of monoamine oxidase; beta-hydroxylase, cortisol, norepinephrine in cerebrospinal fluid also appear relevant to psychoticism level. Eysenck's theoretical basis for the model was the theory of Einheitspsychosen (unitary psychosis) of the nineteenth-century German psychiatrist Heinrich Neumann.
Psychoticism	More information	Eysenck, H.J. & Eysenck, S.B.G. (1976). Psychoticism as a Dimension of Personality. London: Hodder and Stoughton
Pistachio ice cream	Pistachio ice cream	Pistachio ice cream or pistachio nut ice cream is an ice cream flavor made with pistachio nuts or flavoring. It is often distinctively green in color. Pistachio is also a flavor of sorbet and gelato. Pistachio ice cream is a layer in spumoni.
Pistachio ice cream	Pistachio ice cream	At the Bakdash in Damascus, Syria, a pounded ice cream covered with pistachio called Booza is produced. It has an elastic texture made of mastic and sahlab and is famous around the Arab World. Tripoli's Al Mina district is known for its Arabic ice cream including "ashta" with pistachios.It is widely produced including by Brigham's Ice Cream, Ben & Jerry's, Graeter's and major brands. According to a poll among adults in the US 23 percent of respondents stated that they enjoyed pistachio ice cream while only 4 percent of respondents stated that pistachio ice cream was their favorite flavor. [1]
Play-by-post role-playing game	Play-by-post role-playing game	A play-by-post role-playing game (or sim) is an online text-based role-playing game in which players interact with each other and a predefined environment via text. It is a subset of the online role-playing community which caters to both gamers and creative writers. Play-by-post games may be based on other role-playing games, non-game fiction including books, television and movies, or original settings. This activity is closely related to both interactive fiction and collaborative writing. Compared to other roleplaying game formats, this type tends to have the loosest rulesets.
Play-by-post role-playing game	History	Play-by-post roleplaying has its origins on the large computer networks and bulletin board systems of major universities in the United States in the 1980s. It drew heavily upon the traditions of fanzines and off-line role-playing games. The introduction of IRC enabled users to engage in real-time chat-based role-playing and resulted in the establishment of open communities.Development of forum hosting software and browser-based chat services such as AOL and Yahoo Chat increased the availability of these mediums to the public and improved accessibility to the general public.
Play-by-post role-playing game	Rules	Unlike other forms of online role-playing games such as MUDs or MMORPGs, the events in play-by-post games are rarely handled by software and instead rely on participants or moderators to make decisions or improvise. Players create their own characters and descriptions of events and their surroundings during play. Results of combat, which may include Player versus player encounters, may be determined by chance through dice rolls or software designed to provide a random result. The results of random chance may need to be provided to the players in order to avoid disputes that may be a result of cheating or favoritism. Alternatively a forum may be diceless and rely on cooperation among players to agree on outcomes of events and thus forgo the use of randomisers.
Play-by-post role-playing game	Rules	In the latter case, combat and other measures are handled by requiring players to avoid detailing the results of their actions, and thus leave an opening for a response by other involved players. Consider the following possible post from a character named Bob attacking Joe: This post makes the assumption that Joe takes no further action to avoid the attack from Bob and that he will drop as a result. These types of actions are often called "autohits" as they "automatically hit" without allowing for a response by the affected character, and there may be rules against such actions (commonly referred to as the 'no power playing' rule). Alternatively, Bob may be required to write something like the following: This allows Joe to respond to the action without contradicting the post.
Play-by-post role-playing game	Rules	Depending on the rules established on the forum, role-playing and story can be pushed forward through moderation by a gamemaster, specific rules (often existing role-playing game systems), or by mutual agreement between players.
Play-by-post role-playing game	Rules	Some games allow members of any writing proficiency to join, while others may require members to provide a sample of writing for review before allowing participation. In addition, a minimum word-count for each post may be required in order to encourage more detailed writing. Forums that cater to all levels of role-playing may have specific sections for various difficulty levels.
Play-by-post role-playing game	Characters	In general, each player plays and develops his or her own character. Characters may be original creations of the player, or may be based on a character taken from canon if the setting and rules provide this option. Each community may have its own rules regarding the process of character creation and either allow characters to be liberally created and used with minimal review, or require characters to undergo a review process in which administrators examine the character application and decide whether to approve or reject the application.
Play-by-post role-playing game	Characters	In many cases, characters are regarded as belonging to the players who created them, and others are not allowed to make drastic changes to them without the creator's prior consent (referred to as god-modding). In addition to standard characters, games may also incorporate non-player characters (NPCs). Some NPCs have recurring roles, while others appear only briefly to aid in the writing of a scene. The use and control of NPCs varies widely among role-playing games.
Play-by-post role-playing game	Setting	Games vary in the degree to which the setting is established; some go as far as to include a virtual "world" to roleplay in, while others allow players to improvise the setting as they progress. Settings may be derived from novels, TV shows or movies (often resulting in collaborative fan-fiction) or may be original and unique to the game.
Play-by-post role-playing game	Style	Play-by-post games are frequently written in the third person perspective due to the fact that multiple players must share each scene, each with his or her character as the focus of attention. Common online game terms such as OOC (Out of character) or OOG (Out of Game) are used to differentiate in-character from personal posting.
Play-by-post role-playing game	Style	The opening message or post of each scene typically lays down the scenario and describes a scene, or continues from a previously started scene. Threads then become an ongoing story in which players periodically advance the plot by reading the latest reply and then responding with what their character does and how the environment changes in response. These replies are often open-ended so that other players can continue.
Play-by-post role-playing game	Mediums	Message-board role-playing Internet forums (aka Play-By-Message-Board or PBMB, Forum Role-Playing or simply Forum-Games) are the most common medium for Play-by-Post gaming. Forums may provide features such as online dice rolling, maps, character profiling and game history. Using a forum (as opposed to a live-chat interface) allows players to re-read what they have previously written at a later date, and to read posts made by players in other threads. Many online services provide free game hosting specifically for gamemasters, or provide general forum services that can be used for role-playing purposes (such as Proboards or Invisionfree).
Play-by-post role-playing game	Mediums	As an asynchronous collaborative editing tool, forums lack safeguards to prevent two writers from posting simultaneously and contradicting each other. House Rules may require players to take turns sequentially in order to avoid such conflicts, or players may require posts to be edited or deleted to rectify the situation which may result in dispute and intervention from a moderator if one is available. To avoid this, many boards offer guides and tips on roleplay etiquette. Many message boards are listed in roleplay directories, such as TopRPsites to make them easy to find.
Play-by-post role-playing game	Mediums	Twitter and Tumblr are also very popular mediums for Roleplaying.
Play-by-post role-playing game	Mediums	Play-by-post role-playing Play-by-post role-playing is generally devoted to advancing a single overarching storyline that all board members participate in, rather than many different non-related stories proceeding in separate threads [the latter being known as "multi-genre"]. They vary in organization, but the primary formation includes a full set of rules governing role-playing, out-of-character conduct, combat between players, threads detailing a set storyline (often contributed to by plot-advancing, staff-organized events, or player role-plays), character approval forums, and a full staff with admin(s) and moderators. Larger boards set in a single setting are often organized by cutting up the setting into separate forums, each based on locations within the setting.
Play-by-post role-playing game	Mediums	Many message board based games, such as NationStates, establish a hierarchy of moderators to manage plot, pacing and continuity. To keep story threads organized the message board is often organized into forums based on geographical location within the game setting. Other message boards, however, may choose to sort their board on genre.
Play-by-post role-playing game	Mediums	Play-by-email Play-by-email (PBeM) games are played as other play-by-mail games, using email as the postal medium. Players email their actions to the gamemaster or to each other using a mailing list. Play-by-email games are often slow, since the players must wait for each post before replying, but have the advantage that replies may be tailored to the players, allowing the gamemaster to keep information secret from the other players.
Play-by-post role-playing game	Mediums	This should not be confused with simming style of post or email games. Sims are more collaborative storytelling, where each player tells a portion of the story, usually utilizing other characters in the area as they wish in order to complete their portion of the story. PBeM games more closely resemble table-top role playing games where players react to gamemaster presented scenarios, and characters actions are controlled by individuals.
Play-by-post role-playing game	Mediums	Play-by-chat Online Chat Rooms may be used in a similar fashion as forums for role-playing purposes. Unlike forums, posts are displayed to the screen in real-time and thus may increase the pace at which responses are written. Play-by chat games require users to be present for the duration of a scene which may last several hours. The game may be supplemented by external character profiles or may rely on users to provide information about their character upon request or upon entering a room. Discord has become a major medium for play-by-chat games due to its rapidly-increasing popularity and its user-friendly administrative features that allow users to create private chat rooms (known on the site as "servers") in very little time. Discord servers listing roleplay servers have also become popular and many have thousands of members.
Play-by-post role-playing game	Mediums	Real-time interaction between characters in chat rooms are similar to those encountered in MUDs but lack automated features of MUDs such as combat resolution and item descriptions. Players in chat rooms are required to describe objects and events through manually written text.
Play-by-post role-playing game	Mediums	Play-by-internet Play-by-internet (PBI) refers to fully automated games which take place using server-based software. Play-by-internet games differ from other play-by-post games in that, for most computerized multiplayer games, the players have to be online at the same time, and players can make their moves independently of any other players in the game. The turn-time is usually fixed. A server updates the game after the turn-time has elapsed evaluating all the player's moves sent to the server. The turn-time duration can be hours, days, weeks or even months.
Play-by-post role-playing game	Mediums	Play-by-wiki A play-by-wiki game is played using wiki software instead of a forum. Because players' previous posts are editable and the gamemaster takes responsibility as the overall editor of the story, plot holes can be avoided and writing skills may not be as important for each writer. Wiki space provides not only a means of communication, but also a permanent archive and a designated off-topic discussion area for each page. Players can edit posts freely because records are automatically maintained and changes can be easily undone. Sites such as Wetpaint are commonly used for this.
Play-by-post role-playing game	Mediums	Role-playing blog The role-playing blog (RPB) is a game which is played out online using posts within a blog or weblog. Unlike message board role-playing, a role-playing blog is generally restricted to one gaming group, and the blog contains static files such as maps, archives, and character sheets specific for that group. RPBs often incorporate mixed elements of message board role-playing, play-by-chat, as well as play-by-email styles, allowing players to mix and match the style of play that they prefer. Popular blog sites used to host these games are Tumblr and LiveJournal.The style of role-playing on Tumblr often comes in the format of a 'main blog', the headquarters of the game, and multiple character blogs from which each player posts. The main blog often advertises for players through the tags of Tumblr. The main blog is where applicants to the game apply by filling out an application on the main blog, and where other such administration of the game occurs. The Roleplay game creator is often referred to as the "Admin" short for "Administrator" and players may be required to run major plots and game changes by the Admin before proceeding, making the Admin function in a way that a traditional GM might have. Players role-play by reblogging each other's posts and adding paragraphs of interaction from their own character to the end, each of which are called 'paras.' Often seen are text posts with dialogue and an accompanying .gif image expressing the character mood or intended expression of emotion. Though, recently .jpg icons have risen in popularity in some games.
Play-by-post role-playing game	Mediums	The style of role-playing on Livejournal style RPB's is maintained through "community" blogs that connect "character blogs or journals." Character Blogs/Journals are generally written in first-person character driven context. These character journals are then open to all players of the community to interact on a first person style of writing. Interaction on the "Community" blog is done mostly in third person storybook fashion. RPB's on a "livejournal platform" are frequently run by an individual referred to as MOD (moderator). MOD's are in charge of creating the community/game setting, character limitations, rules, style of play, frequency of play and general worldly game views. MOD's are also in charge of creating worldly events for game play response for individually plotted characters.
Play-by-post role-playing game	Mediums	Role-playing Google documents Somewhat similar to blogs and wikis, Google's documents can have permissions set to allow users to access and modify a document online. This allows multiple users to edit the document at the same time, meaning that others can modify the story online. There is also a revision history that can be split allows commenting on particular words or phrases, or even a general comment, as well as a chat bar for that particular document. Since this form of role-playing is relatively new, it's not a common way of role-playing, and it has drawbacks in the content being editable by anyone with permissions.
Play-by-post role-playing game	Player-player combat	In diceless games where randomisers are not used to determine the outcome of combat, the onus is on players to come to an agreement. Disputes may arise from players in competitive engagements if neither player is able to come to a compromise that is acceptable to both. Players may write their characters in a way that makes them overly powerful or invulnerable, a practice referred to as "power-gaming", "god-modding", or "superhero syndrome". In such cases, a moderator may be required to review the conflict and make a ruling as to what should be accepted as the final result. In some rulesets, the winner of a contest may be a foregone conclusion agreed upon out-of-character and the battle itself a ceremonial description of each character's prowess. In other systems, the only rule may be that the first character to yield or surrender is the loser of the combat. No matter the case, some sort of player consent to wins and losses is required in this type of role-playing game.
Play-by-post role-playing game	Player-player combat	For this reason, text-based role-playing games tend to be focused slightly more on story and player-character interactions, negotiations, and relationships rather than combat. Combat-focused games tend to flow more smoothly in more rules based role-playing platforms and venues such as MUDs, D&D-style games, or video games, where character building or playing skill rather than consent determines the winner of a combat, and play-by-post games usually focus on exploration, negotiation, or romance storylines where characters aren't usually directly opposing one another.
Hand to hand acrobatics	Hand to hand acrobatics	Hand to hand acrobatics is a type of performance in which an acrobatic base and flyer balance on top of each other in either a gymnastic or acrobatic medium. It combines strength, agility, flexibility, and balance. For it to be considered hand to hand acrobatics, the top performer (flyer) must be making physical contact only with the base's hands, with the flyer's hands keeping them balanced. Positions the top can perform in this style of acrobatics are straddles, handstands, pikes, press to handstand, one arm handstands, planches, flags, and many others. Hand to hand acrobatics can also include dynamic catches and throws that either begin with a throw from a hand to hand position or end in a catch in the hand to hand position.
Hand to hand acrobatics	Hand to hand acrobatics	Hand to hand acrobatics has been used in a variety of contexts throughout history. These include circus acrobatics, acrobatic gymnastics, muscle beaches, acroyoga, and strongman competitions.
Hand to hand acrobatics	Circus acrobatics	In circus shows such as Le Rêve or Cirque du Soleil: Worlds Away, hand to hand acrobatics have featured as an important part of the show. Often the circus will recruit gymnasts from sports such as acrobatic gymnastics because hand to hand acrobatics is such a big part of that sport.
Hand to hand acrobatics	Circus acrobatics	Hand to hand acrobatics also appears within other disciplines in the circus arts. Aerialists, trapeze flyers, and contortionists often use hand to hand acrobatics. There are times when aerialists and contortionists perform alone, but when performing in pairs or groups, simple hand to hand acrobatics is often incorporated. For example, this may involve one person hanging from a hoop or curtain while holding another person in the air below them. In the case of trapeze artists, many of the throws and catches are hand to hand grabs, and handstands are often performed between throws to not only show strength but to give the flyer a short amount of time to collect themselves before being thrown across the stage again.
Hand to hand acrobatics	Acrobatic gymnastics	According to Chrissy Antoniades, an acrobat who represented Team USA and is now working at The House of Dancing Water in Macau, China, acrobatic gymnastics is not a well-known sport. The routines are performed on the same spring floor gymnasts use for floor exercise competitions. Partner balances, tosses, catches, dance, and tumbling elements are all choreographed and performed to music. Working in groups or pairs, hand to hand acrobatics is especially stressed in the sport. While you are allowed to perform skills on other body parts, it is very common to see skills performed in the hand to hand fashion. This is true especially in the case of pairs—two athletes working together in a routine.
Hand to hand acrobatics	Acrobatic gymnastics	The first use of acrobatics as a specific sport was in the Soviet Union in the 1930s, and the first acrobatic world championships were in 1974. At this time many acrobats were not performing the same types of routines one would see today. Instead, for groups of four boys, they would have only one balance skill where all four of them are stacked on top of each other, and hand to hand acrobatic moves of the competitors choice were performed.
Hand to hand acrobatics	Acroyoga	Acroyoga is a recent practice stemming from acrobatic gymnastics. It features hand to hand acrobatics in a similar way, but routines are not performed as in the sport. Jason Nemer, the creator of acroyoga, made it a combination of hand to hand acrobatics, yoga, fitness, and healing within balance. While hand to hand acrobatics is a small part in acroyoga, it is more common to do many different types of balancing positions.
Hand to hand acrobatics	College campuses	A small group of college students perform hand to hand acrobatics in their own acrobatic competitions. Currently, only girls are allowed to compete in the sport. The sport resembles cheerleading more than it does acrobatics. Teams that compete come from Baylor University, West Liberty University, East Texas Baptist University, Quinnipiac University, University of Oregon, Hawaii Pacific University, Converse College, and Arizona Christian University.
Hand to hand acrobatics	Strongman	In strongman culture, handstands are often an impressive feat due to the massive size of these men. While their form is far different from those in gymnastics, their hand to hand acrobatics resemble acts closer to what one would find in places such as muscle beach. California is a huge breeding ground for those types of athletes. Instructions from strongmen on how to perform handstands vary from what one would find in gymnastics, but they fall under the same category.
Sea salt	Sea salt	Sea salt is salt that is produced by the evaporation of seawater and saltwater. It is used as a seasoning in foods, cooking, cosmetics and for preserving food. It is also called bay salt, solar salt, or simply salt. Like mined rock salt, production of sea salt has been dated to prehistoric times.
Sea salt	Composition	Commercially available sea salts on the market today vary widely in their chemical composition. Although the principal component is sodium chloride, the remaining portion can range from less than 0.2 to 10% of other salts. These are mostly calcium, potassium, and magnesium salts of chloride and sulfate with substantially lesser amounts of many trace elements found in natural seawater. Though the composition of commercially available salt may vary, the ionic composition of natural saltwater is relatively constant.
Sea salt	Historical production	Sea salt is mentioned in the Vinaya Pitaka, a Buddhist scripture compiled in the mid-5th century BC. The principle of production is evaporation of the water from the sea brine. In warm and dry climates this may be accomplished entirely by using solar energy, but in other climates fuel sources have been used. Modern sea salt production is almost entirely found in Mediterranean and other warm, dry climates.
Sea salt	Historical production	Such places are today called salt works, instead of the older English word saltern. An ancient or medieval saltern was established where there was: Access to a market for the salt A gently shelving coast, protected from exposure to the open sea An inexpensive and easily worked fuel supply, or preferably the sun Another trade, such as pastoral farming or tanning—which benefited from proximity to the saltern (by producing leather, salted meat, etc.) and provided the saltern with a local marketIn this way, salt marsh, pasture (salting), and salt works (saltern) enhanced each other economically. This was the pattern during the Roman and medieval periods around The Wash, in eastern England. There, the tide brought the brine, the extensive saltings provided the pasture, the fens and moors provided the peat fuel, and the sun sometimes shone.
Sea salt	Historical production	The dilute brine of the sea was largely evaporated by the sun. In Roman areas, this was done using ceramic containers known as briquetage. Workers scraped up the concentrated salt and mud slurry and washed it with clean sea water to settle impurities out of the now concentrated brine. They poured the brine into shallow pans (lightly baked from local marine clay) and set them on fist-sized clay pillars over a peat fire for final evaporation. Then they scraped out the dried salt and sold it. In traditional salt production in the Visayas Islands of the Philippines, salt are made from coconut husks, driftwood, or other plant matter soaked in seawater for at least several months. These are burned into ash then seawater is run through the ashes on a filter. The resulting brine is then evaporated in containers. Coconut milk is sometimes added to the brine before evaporation. The practice is endangered due to competition with cheap industrially-produced commercial salt. Only two traditions survive to the present day: asín tibuok and túltul (or dúkdok).In the colonial New World, slaves were brought from Africa to rake salt on various islands in the West Indies, Bahamas and particularly Turks and Caicos Islands.
Sea salt	Historical production	Today, salt labelled "sea salt" in the US might not have actually come from the sea, as long as it meets the FDA's purity requirements. All mined salts were originally sea salts since they originated from a marine source at some point in the distant past, usually from an evaporating shallow sea.
Sea salt	Taste	Some gourmets believe sea salt tastes better and has a better texture than ordinary table salt. In applications that retain sea salt's coarser texture, it can provide a different mouthfeel, and may change flavor due to its different rate of dissolution. The mineral content also affects the taste. The colors and variety of flavors are due to local clays and algae found in the waters the salt is harvested from. For example, some boutique salts from Korea and France are pinkish gray and some from India are black. Black and red salts from Hawaii may even have powdered black lava and baked red clay added in. Some sea salt contains sulfates. It may be difficult to distinguish sea salt from other salts, such as pink Himalayan salt, Maras salt from the ancient Inca hot springs, or rock salt (halite).
Sea salt	Taste	Black lava salt is a marketing term for sea salt harvested from various places around the world that has been blended and colored with activated charcoal. The salt is used as a decorative condiment to be shown at the table.
Sea salt	Health	The nutritional value of sea salt and table salt are about the same as they are both primarily sodium chloride. Table salt is more processed than sea salt to eliminate minerals and usually contains an additive such as silicon dioxide to prevent clumping.Iodine, an element essential for human health, is present only in small amounts in sea salt. Iodised salt is table salt mixed with a minute amount of various salts of the element iodine.
Sea salt	Health	Studies have found some microplastic contamination in sea salt from the US, Europe and China. Sea salt has also been shown to be contaminated by fungi that can cause food spoilage as well as some that may be mycotoxigenic.In traditional Korean cuisine, jugyeom (죽염, 竹鹽), which means "bamboo salt", is prepared by roasting salt at temperatures between 800 and 2000 °C in a bamboo container plugged with mud at both ends. This product absorbs minerals from the bamboo and the mud, and is claimed to increase the anticlastogenic and antimutagenic properties of the fermented soybean paste known in Korea as doenjang. However, these claims are not substantiated by high-quality studies.
Shift rule	Shift rule	The shift rule is a mathematical rule for sequences and series. Here n and N are natural numbers. For sequences, the rule states that if (an) is a sequence, then it converges if and only if (an+N) also converges, and in this case both sequences always converge to the same number.For series, the rule states that the series ∑n=1∞an converges to a number if and only if ∑n=1∞an+N converges.
Schaefer's dichotomy theorem	Schaefer's dichotomy theorem	In computational complexity theory, a branch of computer science, Schaefer's dichotomy theorem states necessary and sufficient conditions under which a finite set S of relations over the Boolean domain yields polynomial-time or NP-complete problems when the relations of S are used to constrain some of the propositional variables. It is called a dichotomy theorem because the complexity of the problem defined by S is either in P or NP-complete as opposed to one of the classes of intermediate complexity that is known to exist (assuming P ≠ NP) by Ladner's theorem. Special cases of Schaefer's dichotomy theorem include the NP-completeness of SAT (the Boolean satisfiability problem) and its two popular variants 1-in-3 SAT and not-all-equal 3SAT (often denoted by NAE-3SAT). In fact, for these two variants of SAT, Schaefer's dichotomy theorem shows that their monotone versions (where negations of variables are not allowed) are also NP-complete.
Schaefer's dichotomy theorem	Original presentation	Schaefer defines a decision problem that he calls the Generalized Satisfiability problem for S (denoted by SAT(S)), where S={R1,…,Rm} is a finite set of relations over propositional variables. An instance of the problem is an S-formula, i.e. a conjunction of constraints of the form Rj(xi1,…,xin) where Rj∈S and the xij are propositional variables. The problem is to determine whether the given formula is satisfiable, in other words if the variables can be assigned values such that they satisfy all the constraints as given by the relations from S.
Schaefer's dichotomy theorem	Original presentation	Schaefer identifies six classes of sets of Boolean relations for which SAT(S) is in P and proves that all other sets of relations generate an NP-complete problem. A finite set of relations S over the Boolean domain defines a polynomial time computable satisfiability problem if any one of the following conditions holds: all relations which are not constantly false are true when all its arguments are true; all relations which are not constantly false are true when all its arguments are false; all relations are equivalent to a conjunction of binary clauses; all relations are equivalent to a conjunction of Horn clauses; all relations are equivalent to a conjunction of dual-Horn clauses; all relations are equivalent to a conjunction of affine formulae. Otherwise, the problem SAT(S) is NP-complete.
Schaefer's dichotomy theorem	Modern presentation	A modern, streamlined presentation of Schaefer's theorem is given in an expository paper by Hubie Chen. In modern terms, the problem SAT(S) is viewed as a constraint satisfaction problem over the Boolean domain. In this area, it is standard to denote the set of relations by Γ and the decision problem defined by Γ as CSP(Γ).
Schaefer's dichotomy theorem	Modern presentation	This modern understanding uses algebra, in particular, universal algebra. For Schaefer's dichotomy theorem, the most important concept in universal algebra is that of a polymorphism. An operation f:Dm→D is a polymorphism of a relation R⊆Dk if, for any choice of m tuples 11 ,…,t1k),…,(tm1,…,tmk) from R, it holds that the tuple obtained from these m tuples by applying f coordinate-wise, i.e. 11 ,…,tm1),…,f(t1k,…,tmk)) , is in R. That is, an operation f is a polymorphism of R if R is closed under f: applying f to any tuples in R yields another tuple inside R. A set of relations Γ is said to have a polymorphism f if every relation in Γ has f as a polymorphism. This definition allows for the algebraic formulation of Schaefer's dichotomy theorem.
Schaefer's dichotomy theorem	Modern presentation	Let Γ be a finite constraint language over the Boolean domain. The problem CSP(Γ) is decidable in polynomial-time if Γ has one of the following six operations as a polymorphism: the constant unary operation 0; the constant unary operation 1; the binary AND operation ∧; the binary OR operation ∨; the ternary majority operation Majority ⁡(x,y,z)=(x∧y)∨(x∧z)∨(y∧z); the ternary minority operation Minority ⁡(x,y,z)=x⊕y⊕z.
Schaefer's dichotomy theorem	Modern presentation	Otherwise, the problem CSP(Γ) is NP-complete. In this formulation, it is easy to check if any of the tractability conditions hold.
Schaefer's dichotomy theorem	Properties of Polymorphisms	Given a set Γ of relations, there is a surprisingly close connection between its polymorphisms and the computational complexity of CSP(Γ). A relation R is called primitive positive definable, or short pp-definable, from a set Γ of relations if R(v1, ... , vk) ⇔ ∃x1 ... xm. C holds for some conjunction C of constraints from Γ and equations over the variables {v1,...,vk, x1,...,xm}. For example, if Γ consists of the ternary relation nae(x,y,z) holding if x,y,z are not all equal, and R(x,y,z) is x∨y∨z, then R can be pp-defined by R(x,y,z) ⇔ ∃a. nae(0,x,a) ∧ nae(y,z,¬a); this reduction has been used to prove that NAE-3SAT is NP-complete. The set of all relations which are pp-definable from Γ is denoted by ≪Γ≫. If Γ' ⊆ ≪Γ≫ for some finite constraint sets Γ and Γ', then CSP(Γ') reduces to CSP(Γ).Given a set Γ of relations, Pol(Γ) denotes the set of polymorphisms of Γ. Conversely, if O is a set of operations, then Inv(O) denotes the set of relations having all operations in O as a polymorphism. Pol and Inv together build a Galois connection.
Schaefer's dichotomy theorem	Properties of Polymorphisms	For any finite set Γ of relations over a finite domain, ≪Γ≫ = Inv(Pol(Γ)) holds, that is, the set of relations pp-definable from Γ can be derived from the polymorphisms of Γ. Moreover, if Pol(Γ) ⊆ Pol(Γ') for two finite relation sets Γ and Γ', then Γ' ⊆ ≪Γ≫ and CSP(Γ') reduces to CSP(Γ). As a consequence, two relation sets having the same polymorphisms lead to the same computational complexity.
Schaefer's dichotomy theorem	Generalizations	The analysis was later fine-tuned: CSP(Γ) is either solvable in co-NLOGTIME, L-complete, NL-complete, ⊕L-complete, P-complete or NP-complete and given Γ, one can decide in polynomial time which of these cases holds.Schaefer's dichotomy theorem was recently generalized to a larger class of relations.
Schaefer's dichotomy theorem	Related work	If the problem is to count the number of solutions, which is denoted by #CSP(Γ), then a similar result by Creignou and Hermann holds. Let Γ be a finite constraint language over the Boolean domain. The problem #CSP(Γ) is computable in polynomial time if Γ has a Mal'tsev operation as a polymorphism. Otherwise, the problem #CSP(Γ) is #P-complete. A Mal'tsev operation m is a ternary operation that satisfies m(x,y,y)=m(y,y,x)=x.
Schaefer's dichotomy theorem	Related work	An example of a Mal'tsev operation is the Minority operation given in the modern, algebraic formulation of Schaefer's dichotomy theorem above. Thus, when Γ has the Minority operation as a polymorphism, it is not only possible to decide CSP(Γ) in polynomial time, but to compute #CSP(Γ) in polynomial time. There are a total of 4 Mal'tsev operations on Boolean variables, determined by the values of m(T,F,T) and m(F,T,F) . An example of less symmetric one is given by m(x,y,z)=(x∧z)∨(¬y∧(x∨z)) . On another domains, such as groups, examples of Mal'tsev operations include x−y+z and xy−1z.
Schaefer's dichotomy theorem	Related work	For larger domains, even for a domain of size three, the existence of a Mal'tsev polymorphism for Γ is no longer a sufficient condition for the tractability of #CSP(Γ). However, the absence of a Mal'tsev polymorphism for Γ still implies the #P-hardness of #CSP(Γ).
Spin-stabilisation	Spin-stabilisation	In aerospace engineering, spin stabilization is a method of stabilizing a satellite or launch vehicle by means of spin, i.e. rotation along the longitudinal axis. The concept originates from conservation of angular momentum as applied to ballistics, where the spin is commonly obtained by means of rifling. For most satellite applications this approach has been superseded by three-axis stabilization.
Spin-stabilisation	Use	Spin-stabilization is used on rockets and spacecraft where attitude control is required without the requirement for on-board 3-axis propulsion or mechanisms, and sensors for attitude control and pointing. On rockets with a solid motor upper stage, spin stabilization is used to keep the motor from drifting off course as they don't have their own thrusters. Usually small rockets are used to spin up the spacecraft and rocket then fire the rocket and send the craft off.
Spin-stabilisation	Use	Rockets and spacecraft that use spin stabilization: The Jupiter-C and Minotaur V launch vehicles used spin-stabilization. The upper stages on both system employ spin-stabilization to stabilize the system during propulsive maneuvers. The Aryabhata satellite used spin-stabilization The Pioneer 4 spacecraft, the second object sent on a lunar flyby in 1959, maintained its attitude using spin-stabilization. The Schiaparelli EDM lander was spun up to 2.5 RPM before being ejected from the ExoMars Trace Gas Orbiter prior to its attempted landing on Mars in October 2016. The Juno was spin-stabilized and arrived at Jupiter orbit in 2016. The launches of Pioneer 10 and Pioneer 11 probes on two Atlas Centaur vehicles in 1972 and 1973 employed Star 37 rocket motors that were spin-stabilized in order to inject the satellites into the high-energy hyperbolic orbits necessary to achieve solar system escape velocity. Additionally, both probes were spin-stabilized during their flights and rotated at approximately 5 rpm.
Spin-stabilisation	Use	In operation as a third stage, the Star 48 rocket booster sits on top of spin table, and before it is separated it is spun up to stabilize it during the separation from the previous stage. The Delta II launch vehicle third stage employed a Star 48 motor and was spin-stabilized and depended on the second stage for proper orientation prior to stage separation, but was sometimes equipped with a nutation control system to maintain proper spin axis. It also included a yo-weight system to induce tumbling in the third stage after payload separation to prevent recontact, or a yo-yo de-spin mechanism to slow the rotation before payload release.Despinning can be achieved by various techniques, including yo-yo de-spin.With advancements in attitude control propulsion systems, guidance systems, and the needs for satellites to point instruments and communications systems precisely, 3-axis attitude control has become much more common than spin-stabilization for systems operating in space.
Ava (company)	Ava (company)	Ava is a medical technology company that developed the Ava bracelet, a wearable device that functions as a fertility tracker.
Ava (company)	History	The company was founded in Zurich, Switzerland by Lea Von Bidder, Pascal Koenig, Philipp Tholen, and Peter Stein. In September 2015, the company took part in TechCrunch’s Startup Battlefield. In November 2015, Ava raised a $2.6 million funding round led by Swisscom and ZKB. The company began shipping the Ava bracelet to customers in July 2016. The company raised nearly $40 million in funding between 2017 and 2018.
Ava (company)	Technology	The device is intended to allow wearers to estimate their fertile window by tracking their menstrual cycle and ovulation based on measurements of their skin temperature, heart rate, perfusion, breathing rate, and heart rate variability. Data collected from the bracelet is displayed on an app, so that the wearer can track their fertility or monitor their health during pregnancy. An independent study of the bracelet's validity found that it provided accurate assessments of sleep duration but that its estimates of other data such as heart rate were inaccurate in comparison to other monitoring methods such as actigraphy.
Lateral thyrohyoid ligament	Lateral thyrohyoid ligament	The lateral thyrohyoid ligament (lateral hyothyroid ligament) is a round elastic cord, which forms the posterior border of the thyrohyoid membrane and passes between the tip of the superior cornu of the thyroid cartilage and the extremity of the greater cornu of the hyoid bone. The internal branch of the superior laryngeal nerve typical lies lateral to this ligament.
Lateral thyrohyoid ligament	Triticeal cartilage	A small cartilaginous nodule (cartilago triticea), sometimes bony, is frequently found in the lateral thyrohyoid ligament.
Tesetaxel	Tesetaxel	Tesetaxel is an orally administered taxane being investigated as a chemotherapy agent for various types of cancer, including breast cancer, gastric cancer, colorectal cancer, and other solid tumors. It differs from other members of the taxane class (e.g. paclitaxel or docetaxel) in that it is administered orally, not intravenously.
Y Centauri	Y Centauri	Y Centauri or Y Cen (HD 127233, HIP 70969) is a semiregular variable star in the constellation of Centaurus.
Y Centauri	Y Centauri	The variability in the star was discovered by Williamina Fleming in 1895 and published in the Third Catalogue of Variable Stars. The photographic magnitude range was given as 7.7 - 8.8, but the variability was described as "somewhat doubtful". It was later given the designation HV 52 in the Harvard Catalogue of Variable Stars. The General Catalogue of Variable Stars lists it as a semiregular variable star with a period of 180 days and a visual magnitude range of 8.0 - 9.1. A study of Hipparcos satellite photometry found a small amplitude range of 0.2 magnitudes at a visual magnitude of 8.53.The distance of the star is poorly known. The revised Hipparcos annual parallax of 3.50 mas gives a distance of 900 light years. A study taking into account the variability of the star found a parallax of 5.57 mas, corresponding to a distance of 585 light years. Both estimates have a margin of error over 20%. The Gaia Data Release 2 parallax lies between these two values and appears more accurate with a margin of error around 5%, but with a large value for astrometric noise. Gaia EDR3 does not list a parallax for this star.Y Centauri is an asymptotic giant branch star 330 times as luminous as the sun. Its spectral type varies between M4 and M7 as it pulsates.The star has been observed to produce 22 GHz water maser emission, although later searches did not find any maser emission.
Philatelic auction	Philatelic auction	A philatelic auction, or stamp auction is a sale of stamps, covers and other philatelic material usually run by stamp dealers or specialist collectibles auctioneers, such as, David Feldman, Christie's and Sotheby's, where prospective purchasers place bids in an attempt to obtain the desired items. The highest bidder for each lot (described item or items) makes the purchase. Auctions are generally divided into mail sales, where bids are accepted by mail, and public sales, where mail bids are combined with live bidding from individuals present at the auction or participating by telephone. Auctions usually allow prospective purchasers to view the items beforehand, either in a catalogue, in the auction house, or both.
Quantum gravity	Quantum gravity	Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics. It deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vicinity of black holes or similar compact astrophysical objects, such as neutron stars as well as in the early stages of the universe moments after the Big BangThree of the four fundamental forces of nature are described within the framework of quantum mechanics and quantum field theory: the electromagnetic interaction, the strong force, and the weak force; this leaves gravity as the only interaction that has not been fully accommodated. The current understanding of gravity is based on Albert Einstein's general theory of relativity, which incorporates his theory of special relativity and deeply modifies the understanding of concepts like time and space. Although General Relativity is highly regarded for its elegance it is not without limitations: the gravitational singularities inside of black holes, the ad. hoc. postulation of Dark Matter, as well as Dark Energy and its relation to the Cosmological Constant are among the current unsolved mysteries regarding gravity; all of which signal the collapse of the general theory of relativity at different scales and highlight the need for a gravitational theory that goes into the quantum realm. At distances close to the Planck length, like those near the center of the black hole, quantum fluctuations of spacetime are expected to play an important role. The breakdown of general relativity at galactic and cosmological scales also points out the necessity for a more robust theory. Finally the discrepancies between the predicted value for the vacuum energy and the observed values (which, depending on the considerations, can be of 60 or 120 orders of magnitude) highlight the necessity for a quantum theory of gravity.
Quantum gravity	Quantum gravity	The field of quantum gravity is actively developing, and theorists are exploring a variety of approaches to the problem of quantum gravity, the most popular being M-theory and loop quantum gravity. All of these approaches aim to describe the quantum behavior of the gravitational field, which does not necessarily include unifying all fundamental interactions into a single mathematical framework. However, many approaches to quantum gravity, such as string theory, try to develop a framework that describes all fundamental forces. Such a theory is often referred to as a theory of everything. Some of the approaches, such as loop quantum gravity, make no such attempt; instead, they make an effort to quantize the gravitational field while it is kept separate from the other forces. Other lesser-known but no less important theories include Causal dynamical triangulation, Noncommutative geometry, and Twistor theory.One of the difficulties of formulating a quantum gravity theory is that direct observation of quantum gravitational effects is thought to only appear at length scales near the Planck scale, around 10−35 meters, a scale far smaller, and hence only accessible with far higher energies, than those currently available in high energy particle accelerators. Therefore, physicists lack experimental data which could distinguish between the competing theories which have been proposed.Thought experiment approaches have been suggested as a testing tool for quantum gravity theories. In the field of quantum gravity there are several open questions - e.g., it is not known how spin of elementary particles sources gravity, and thought experiments could provide a pathway to explore possible resolutions to these questions, even in the absence of lab experiments or physical observations.
Quantum gravity	Quantum gravity	In the early 21st century, new experiment designs and technologies have arisen which suggest that indirect approaches to testing quantum gravity may be feasible over the next few decades. This field of study is called phenomenological quantum gravity.
Quantum gravity	Overview	Much of the difficulty in meshing these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. General relativity models gravity as curvature of spacetime: in the slogan of John Archibald Wheeler, "Spacetime tells matter how to move; matter tells spacetime how to curve." On the other hand, quantum field theory is typically formulated in the flat spacetime used in special relativity. No theory has yet proven successful in describing the general situation where the dynamics of matter, modeled with quantum mechanics, affect the curvature of spacetime. If one attempts to treat gravity as simply another quantum field, the resulting theory is not renormalizable. Even in the simpler case where the curvature of spacetime is fixed a priori, developing quantum field theory becomes more mathematically challenging, and many ideas physicists use in quantum field theory on flat spacetime are no longer applicable.It is widely hoped that a theory of quantum gravity would allow us to understand problems of very high energy and very small dimensions of space, such as the behavior of black holes, and the origin of the universe.
Quantum gravity	Quantum mechanics and general relativity	Graviton The observation that all fundamental forces except gravity have one or more known messenger particles leads researchers to believe that at least one must exist for gravity. This hypothetical particle is known as the graviton. These particles act as a force particle similar to the photon of the electromagnetic interaction. Under mild assumptions, the structure of general relativity requires them to follow the quantum mechanical description of interacting theoretical spin-2 massless particles.
Quantum gravity	Quantum mechanics and general relativity	Many of the accepted notions of a unified theory of physics since the 1970s assume, and to some degree depend upon, the existence of the graviton. The Weinberg–Witten theorem places some constraints on theories in which the graviton is a composite particle. While gravitons are an important theoretical step in a quantum mechanical description of gravity, they are generally believed to be undetectable because they interact too weakly. Nonrenormalizability of gravity General relativity, like electromagnetism, is a classical field theory. One might expect that, as with electromagnetism, the gravitational force should also have a corresponding quantum field theory.
Quantum gravity	Quantum mechanics and general relativity	However, gravity is perturbatively nonrenormalizable. For a quantum field theory to be well defined according to this understanding of the subject, it must be asymptotically free or asymptotically safe. The theory must be characterized by a choice of finitely many parameters, which could, in principle, be set by experiment. For example, in quantum electrodynamics these parameters are the charge and mass of the electron, as measured at a particular energy scale.
Quantum gravity	Quantum mechanics and general relativity	On the other hand, in quantizing gravity there are, in perturbation theory, infinitely many independent parameters (counterterm coefficients) needed to define the theory. For a given choice of those parameters, one could make sense of the theory, but since it is impossible to conduct infinite experiments to fix the values of every parameter, it has been argued that one does not, in perturbation theory, have a meaningful physical theory. At low energies, the logic of the renormalization group tells us that, despite the unknown choices of these infinitely many parameters, quantum gravity will reduce to the usual Einstein theory of general relativity. On the other hand, if we could probe very high energies where quantum effects take over, then every one of the infinitely many unknown parameters would begin to matter, and we could make no predictions at all.It is conceivable that, in the correct theory of quantum gravity, the infinitely many unknown parameters will reduce to a finite number that can then be measured. One possibility is that normal perturbation theory is not a reliable guide to the renormalizability of the theory, and that there really is a UV fixed point for gravity. Since this is a question of non-perturbative quantum field theory, finding a reliable answer is difficult, pursued in the asymptotic safety program. Another possibility is that there are new, undiscovered symmetry principles that constrain the parameters and reduce them to a finite set. This is the route taken by string theory, where all of the excitations of the string essentially manifest themselves as new symmetries.
Quantum gravity	Quantum mechanics and general relativity	Quantum gravity as an effective field theory In an effective field theory, all but the first few of the infinite set of parameters in a nonrenormalizable theory are suppressed by huge energy scales and hence can be neglected when computing low-energy effects. Thus, at least in the low-energy regime, the model is a predictive quantum field theory. Furthermore, many theorists argue that the Standard Model should be regarded as an effective field theory itself, with "nonrenormalizable" interactions suppressed by large energy scales and whose effects have consequently not been observed experimentally.
Quantum gravity	Quantum mechanics and general relativity	Works pioneered by Barvinsky and Vilkovisky suggest as a starting point up to second order in curvature the following action, consisting of local and non-local terms: 16 ln ln ln ⁡(◻μ2)Rμνρσ], where μ is an energy scale. The exact values of the coefficients c1,c2,c3 are unknown, as they depend on the nature of the ultra-violet theory of quantum gravity. ln ⁡(◻/μ2) is an operator with the integral representation ln ⁡(◻μ2)=∫0+∞ds(1μ2+s−1◻+s).
Quantum gravity	Quantum mechanics and general relativity	By treating general relativity as an effective field theory, one can actually make legitimate predictions for quantum gravity, at least for low-energy phenomena. An example is the well-known calculation of the tiny first-order quantum-mechanical correction to the classical Newtonian gravitational potential between two masses. Moreover, one can compute the quantum gravitational corrections to classical thermodynamic properties of black holes, most importantly the entropy. A rigorous derivation of the quantum gravitational corrections to the entropy of Schwarzschild black holes was provided by Calmet and Kuipers. A generalisation for charged (Reissner–Nordström) black holes was subsequently carried out by Campos Delgado.
Quantum gravity	Quantum mechanics and general relativity	Spacetime background dependence A fundamental lesson of general relativity is that there is no fixed spacetime background, as found in Newtonian mechanics and special relativity; the spacetime geometry is dynamic. While simple to grasp in principle, this is a complex idea to understand about general relativity, and its consequences are profound and not fully explored, even at the classical level. To a certain extent, general relativity can be seen to be a relational theory, in which the only physically relevant information is the relationship between different events in space-time.

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