title: Legends of Runeterra

text: "Legends of Runeterra" is a digital collectible card game played one versus one. At the beginning, both players' Nexus has 20 health points; the first to fall to zero loses. Players begin each match with a hand of four cards, which they may trade away for another random card from their deck. Each round, both players draw one card. Cards are played by spending mana; players begin with zero mana, and gain one additional mana crystal per round up to a maximum of ten. A maximum of three unspent mana is stored automatically at the end of a round as spell mana; this can be used in future rounds to cast spells but cannot summon unit cards.


title: Legends of Runeterra

text: One of the game's distinguishing features is its combat pacing. Each round, the "attack token", a symbol which indicates which player may attack and who will defend, alternates from player to player. This is reflected visually on each players' half of the board, with a sword icon representing attack or a shield for defense. Some cards enable players to attack when they do not have the attack token.


title: Legends of Runeterra

text: Each card in the game belongs to a region; in standard play, one deck can use cards from up to two regions. Regions have a distinct style of play and identity. Unlike other trading card games, there are no neutral cards that can be used in every deck. The regions originated in the wider "League of Legends" expanded universe. Upon the game's initial release, there were three types of card: champions, followers, and spells. Champion cards are the playable characters from "League of Legends." These cards are unique within the game because they can level up. Levelling a champion transforms the card—and all copies of it in the player's deck—into a more powerful version of the card. Unit cards, which includes champions and non-champions (followers), have a number representing their attack and health statistic; attack is how much damage a unit deals to either the Nexus or its blocker, while health reflects the maximum damage a card can take before being removed from play.


title: Legends of Runeterra

text: Spell cards have a "speed", denoting when they can be played and in what way the opponent is able to respond, if at all. At launch, there were three speeds: slow, fast, and burst. Slow-speed spells cannot be played during active combat, pass priority over to the opponent, and can be responded to with fast or burst spells; fast spells can be played during combat and do not pass priority; and burst speed spells resolve their effect instantly with no opportunity for opponent response. A fourth speed, Focus, resolves immediately and does not pass over turn priority, but can only be used outside of combat. Unit cards do not have a speed, but end a player's turn within a round.


title: Legends of Runeterra

text: Another card type was added in the Monuments of Power expansion—landmarks. Landmarks are played with regular unit mana and consume a position on the player's board; they cannot block or attack. Some landmarks have a "countdown" mechanic, wherein they cause a set effect after a certain number of rounds.


title: Legends of Runeterra

text: Riot Games employees have considered making a card game since early in the company's history. The company has a significant number of fans of the collectible card game genre. "Legends of Runeterra"'s balancing director, Steve Rubin, pointed to Jeff Jew, the game's executive producer and an early Riot Games employee, and Andrew Yip, as big fans of "". There were several different concepts of the game, but "Legends of Runeterra" was primarily developed over three years beginning in 2017. Riot recruited professional "Magic" competitors as early playtesters; of them, Steve Rubin was invited to return permanently and later moved into the design team. Rubin noted that the announcement of "Artifact" caused the developers to consider rushing the game's release, but ultimately decided to polish the game and aim for wider demographics.


title: Legends of Runeterra

text: A significant challenge in development was determining the mechanics of card acquisition; an early iteration in which players simply unlocked region combinations was poorly received by playtesters, who missed the satisfaction of collecting all cards. Accessibility was a priority for the developers, who sought to provide a familiar experience while not forcing players to buy booster packs, a random bundle of cards otherwise common in the CCG genre. The developers placed a limit on how many cards could be bought in exchange for real money each week. Instead, players are given a number of random cards each week that scales with how frequently they play, and a mechanic called Wild Cards, a way for players to directly craft desired cards. Jeff Jew said that frictionless card collection for players enables the developers to balance more responsively, as players would not be upset that a deck they spent up to building had been weakened.


title: Legends of Runeterra

text: "Legends of Runeterra" was revealed at Riot Games' celebration event of the tenth anniversary of "League of Legends" on October 15, 2019; applications for the closed beta period began following the conclusion of the stream. "Eurogamer" observed the unusual timing of the reveal, given the recent failure of Valve's "Artifact" and the waning audience for Blizzard Entertainment's "Hearthstone". The first closed beta period ended in October 2019. A second provided access to an additional mode called Expeditions from November 14–19, 2019. The open beta, giving access to all players, commenced on January 24, 2020; unlike in the closed beta period, cards and cosmetics purchased in the open beta carried over to the live release of the game.


title: Legends of Runeterra

text: The game was released on April 29, 2020; although the beta period was limited to Windows users, the launch accompanied the game's release on mobile operating systems iOS and Android. During beta, the game had included six regions, with four champion cards per region, and 294 total cards. The official launch also brought a new set to the game, "Rising Tides," introducing 120 new cards and a new region—Bilgewater. Along with new cards, sets contain new game mechanics and further development to existing ones. Every existing region was given an additional champion, with Bilgewater having six.


title: Legends of Runeterra

text: With the game's second set, "Call of the Mountain", Riot Games altered the release schedule, with each set spanning three "expansions". "Call of the Mountain" introduced the region of Mount Targon and was released for PC and mobile devices on August 26, 2020. The region of Shurima became part of the game with the "Empires of the Ascended" set, released on March 3, 2021. The tenth and final region of the game, Bandle City, was released on August 25, 2021, and brought four expansions instead of the usual three. Between the region expansions are Event or Champion Expansions, with Aphelios for the "Call of the Mountain" set, Viego & Akshan for the "Empires of the Ascended" set, and Path of Champions for the "Beyond the Bandlewood", all of which belongs to the Event Set. After the final region, Bandle City, fully released in 2022, Legends of Runeterra started releasing stand-alone expansions, with content for both PvP and PvE aspects of the game. The first stand-alone expansion, Worldwalker, was released on May 25, 2022, introducing Runeterran champions.


title: Legends of Runeterra

text: "Legends of Runeterra" received positive reviews from critics. According to review aggregator Metacritic, the game has a weighted average of 87/100.


title: Legends of Runeterra

text: Many outlets highlighted that the game was both accessible for newcomers to the genre while preserving its depth. "IGN"'s Cam Shea awarded the game a 9/10, noting that it managed to maintain its complexity while also streamlining elements from other collectible card games, such as "Magic: The Gathering". Jason Coles of "NME" wrote that it "may well be the most accessible card game out there".


title: Legends of Runeterra

text: Also of note was the game's generous free-to-play business model, especially in relation to other games in the same genre. Giving the game an 85/100, Steven Messner, writing for "PC Gamer" noted the absence of "booster packs", bundles of cards purchasable with real currency, having been replaced with a generous battle pass system which gives out an abundance of free cards and crafting material every week. Messner also mentioned the ease of achieving the maximum level of the battle pass every week.


title: Legends of Runeterra

text: The game was nominated for Best Mobile Game at The Game Awards 2020. Apple named it the iPad Game of the Year for 2020. It also won the Mobile Game of the Year award at the 24th Annual D.I.C.E. Awards in 2021.


title: Rann of Kutch

text: The Rann of Kutch (alternately spelled as Kuchchh) is a large area of salt marshes that span the border between India and Pakistan. It is located in Gujarat (primarily the Kutch district), India, and in Sindh, Pakistan. It is divided into the Great Rann and Little Rann.


title: Rann of Kutch

text: The Rann of Kutch is located mostly in the Indian state of Gujarat, specifically Kutch district, for which it is named. Some parts extend into the Pakistani province of Sindh. The word "Rann" means "desert".


title: Rann of Kutch

text: The Rann of Kutch covers around 26,000 square kilometres (10,000 square miles). The Great Rann of Kutch is the larger portion of the Rann. It extends east and west, with the Thar Desert to the north and the low hills of Kutch to the south. The Indus River Delta lies to the west in southern Pakistan. The Little Rann of Kutch lies southeast of the Great Rann, and extends southwards to the Gulf of Kutch.


title: Rann of Kutch

text: Many rivers originating in Rajasthan and Gujarat flow into the Rann of Kutch, including the Luni, Bhuki, Bharud, Nara, Kharod, Banas, Saraswati, Rupen, Bambhan, and Machchhu. Kori Creek and Sir Creek, tidal creeks which are part of the Indus River Delta, are located at the western end of the Great Rann.


title: Rann of Kutch

text: The surface is generally flat and very close to sea level, and most of the Rann floods annually during the monsoon season. There are areas of sandy higher ground, known as "bet"s or s, which lie two to three metres above flood level. Trees and shrubs grow on the bets, and they provide refuges for wildlife during the annual floods.


title: Rann of Kutch

text: The climate of the ecoregion is subtropical. Temperatures average 44 °C during the hot summer months, and can reach highs of 50 °C. During the winter the temperature can drop to or below freezing.


title: Rann of Kutch

text: Rainfall is highly seasonal. The Rann of Kutch is dry for most of the year, and rainfall is concentrated in the June to September monsoon season. During the monsoon season, local rainfall and river runoff flood much of the Rann to a depth of 0.5 metres. The waters evaporate during the long dry season, leaving the Rann dry again by the start of the next monsoon season.


title: Rann of Kutch

text: The Rann of Kutch is the only large flooded grasslands zone in the Indomalayan realm. The area has desert on one side and the sea on the other enables various ecosystems, including mangroves and desert vegetation. Its grassland and deserts are home to forms of wildlife that have adapted to its often harsh conditions. These include endemic and endangered animal and plant species.


title: Rann of Kutch

text: The predominant vegetation in the Rann of Kutch is grassland and thorn scrub. Common grass species include "Apluda aristata", "Cenchrus" spp., "Pennisetum" spp., "Cymbopogon" spp., "Eragrostis" spp., and "Elionurus" spp.


title: Rann of Kutch

text: Trees are rare except on the bets which rise above the flood zone. The non-native tree "Prosopis juliflora" has become established on the bets, and its seed pods provide year-round food for the wild asses.


title: Rann of Kutch

text: The Rann of Kutch is home to about 50 species of mammals. They include several large herbivores, including Indian wild ass ("Equus hemionus khur"), chinkara ("Gazella bennettii"), nilgai ("Boselaphus tragocamelus"), and blackbuck ("Antilope cervicapra"), and the large predators wolf ("Canis lupus"), striped hyena ("Hyaena hyaena"), desert wildcat ("Felis lybica"), and caracal ("Felis caracal"). The Indian wild ass once had a wider distribution but is now limited to the Rann of Kutch. The nilgai and blackbuck are threatened species.


title: Rann of Kutch

text: There are over 200 bird species in the Rann of Kutch, including the threatened species lesser florican ("Sypheotides indicus") and houbara bustard ("Chlamydotis undulata"). The seasonal wetlands provide habitat for many water birds, including the demoiselle crane ("Grus virgo") and lesser flamingo ("Phoeniconaias minor").


title: Rann of Kutch

text: The history of the Rann of Kutch began with early neolithic settlements. It was later inhabited by the Indus Valley civilization as well as the Maurya and Gupta empires of India.


title: Rann of Kutch

text: The people of the Indus civilization appear to have settled in the Rann of Kutch around 3500 BCE. The Indus city of Dholavira, the largest Indus site in India, is located in the Rann of Kutch. This city was built on the Tropic of Cancer, possibly indicating that Dholavira's inhabitants were skilled in astronomy.


title: Rann of Kutch

text: Many Indologists such as A. S. Gaur and Mani Murali hold the view that the Rann of Kutch was, rather than the salt marsh that it is today, a navigable archipelago at the time of the Indus Civilization. The Indus Civilization was known to have an extensive maritime trade system, so it has been proposed by Gaur et al. that there were perhaps ports in the Rann of Kutch.


title: Rann of Kutch

text: The Rann of Kutch came under the control of the British Raj, who imposed a ban on salt harvesting. This ban was protested and overturned by Indian activist Mahatma Gandhi. More recently, the residents of the Rann of Kutch began holding the Rann Utsav festival, a three-month long carnival, which marks the peak tourist season.


title: Rann of Kutch

text: A 2017 assessment found that 20,946 km, or 76%, of the ecoregion is in protected areas. They include the Kutch Desert Wildlife Sanctuary (7506.22 km), which was established in 1986 and covers much of the Great Rann, and the Indian Wild Ass Sanctuary (4953.71 km), which was established in 1973 and covers much of the Little Rann. Pakistan's Rann of Kutch Wildlife Sanctuary protects the northern portion of the Great Rann and adjacent Thar Desert.


title: Barong (mythology)

text: Barong is a panther-like creature and character in the Balinese mythology of Bali, Indonesia. He is the king of the spirits, leader of the hosts of good, and enemy of Rangda, the demon queen and mother of all spirit guarders in the mythological traditions of Bali. The battle between Barong and Rangda is featured in the Barong dance to represent the eternal battle between good and evil.


title: Barong (mythology)

text: Barong animal mask dance, together with "sanghyang" dance are considered native Balinese dances, predating Hindu influences. The native Indonesians of Austronesian heritage often have similar mask dances that represent either ancestral or natural spirits; an example is Dayak's "Hudoq" dance or any similar bear worship practice. The term "barong" is thought to have been derived from the local term "bahruang", which today corresponds to the Indonesian word "beruang" which means "bear". It refers to a good spirit, that took the form of an animal as the guardian of forest.


title: Barong (mythology)

text: In Balinese mythology, the good spirit is identified as Banas Pati Raja. Banas Pati Raja is the fourth "brother" or spirit child that accompanies a child throughout their life, which is a similar concept to guardian angels. Banas Pati Raja is the spirit which animates Barong. A protector spirit, he is often represented as a lion. The Barong is often portrayed accompanied by two monkeys.


title: Barong (mythology)

text: Barong is portrayed as a lion with a red head, covered in white thick fur, and wearing gilded jewelry adorned with pieces of mirrors. The shape of lion Barong is somewhat similar to a Pekingese dog. The origins of the Barong are far back in time and quite uncertain. Its origins could be from animist worship, before Hinduism appeared, when villagers still believed in the supernatural protective power of animals.


title: Barong (mythology)

text: Rangda is Barong's opposite. While Barong represents good, Rangda represents evil. Rangda is known as a demon queen, the incarnation of Calon Arang, the legendary witch that wreaked havoc in ancient Java during the reign of Airlangga in the tenth century. It is said that Calon Arang was a widow who had mastered the art of black magic, who often damaged farmer's crops and caused disease to come. She had a girl, named Ratna Manggali, who, though beautiful, could not get a husband because people were afraid of her mother. Because of the difficulties faced by her daughter, Calon Arang was angry and she intended to take revenge by kidnapping a young girl. She brought the girl to a temple to be sacrificed to the goddess Durga. The next day, a great flood engulfed the village and many people died. Disease also appeared.


title: Barong (mythology)

text: King Airlangga, who had heard of this matter, then asked his advisor, Empu Bharada, to deal with this problem. Empu Bharada then sent his disciple, Empu Bahula, to be married to Ratna. Both were married with a huge feast that lasted seven days and seven nights, and the situation returned to normal. Calon Arang had a book that contained magic incantations. One day, this book was found by Empu Bahula, who turned it over to Empu Bharada. As soon as Calon Arang knew that the book had been stolen, she became angry and decided to fight Empu Bharada. Without the help of Durga, Calon Arang was defeated. Since she was defeated, the village was safe from the threat of Calon Arang's black magic.


title: Barong (mythology)

text: Another version holds that Rangda was actually linked to historical queen Mahendradatta or Gunapriyadharmapatni, a Javanese princess sister of Dharmawangsa of East Javanese Ishana dynasty of late Mataram Kingdom period. She was the queen consort of Balinese king Udayana and Airlangga's own mother. Mahendradatta is known for her devotion to the worship of Durga in Bali. The story goes that Mahendradatta, the mother of Airlangga, was condemned by Airlangga's father because she practiced black magic. After she became a widow (Rangda in old Javanese means: 'widow'), hurt and humiliated, she sought revenge upon her ex-husband's court and the whole of his kingdom. She summoned all the evil spirits in the jungle, the and the demons that caused plague and death in the kingdom. Airlangga had to face his own mother to save the kingdom. A fight occurred, but she and her black magic troops were so powerful that Airlangga had to ask for the help of the spirit king, Barong in order to defeat her. Barong came with Airlangga's soldiers, and a fight ensued. Rangda cast a spell that made all of Airlangga's soldiers feel suicidal, pointing their poisoned keris into their own stomachs and chests. In response, Barong cast a spell which caused the soldiers' bodies to be resistant to the sharp keris. At the end, Barong won, and Rangda ran away.


title: Barong (mythology)

text: The lion is the popular one, as it comes from the Gianyar region, where Ubud (the home of tourist's Balinese dance ritual viewing) is located. Within the calon arang, the dance drama in which the Barong appears, the barong responds to Rangda's use of magic to control and kill her to restore balance. In traditional Barong dance performances, he is portrayed in his struggles against Rangda, it is the popular part of Balinese culture. The mythical creature would dance along the street to the calon arang dance. A priest would throw holy water at it.


title: Barong (mythology)

text: The lion barong is one of five traditional Barongs. In Bali each region of the island has its own protective spirit for its forests and lands. Each Barong for each region is modeled after a different animal. They are:


title: Barong (mythology)

text: Lion barong as "banaspati raja" means king of the jungle, the most common Barong, it is the symbol of a good spirit, headdress uses "sekar taji" made of carved leather, sliced with "prada" and added with some small mirror decoration.


title: Barong (mythology)

text: "Bangkal" is an old pig in Bali and is considered a mythical animal that has strength. It is made of velvet fabric. Paraded on Galungan and Kuningan holidays.


title: Barong (mythology)

text: Barong shaped like a tiger that lives in the forest, its fur resembles a tiger fur made of velvet fabric. Tigers are famous mythical animals in stories in Bali, especially "tantri".


title: Barong (mythology)

text: The shape resembles an elephant from India. Barong is paraded around the village on Galungan and Kuningan holidays.


title: Barong (mythology)

text: Barong landung has a different form with barongs in Bali. This barong is not danced by 2 dancers as in general, this barong manifests like two male and female dolls, which are called 'Jero Luh' and 'Jero Gede', the form is similar to Betawi Ondel-ondel.


title: WWE SmackDown vs. Raw 2011

text: WWE SmackDown vs. Raw 2011 (also shortened to WWE SvR 2011 or "SVR 11") is a professional wrestling video game developed by Yuke's and published by THQ for PlayStation 2 (PS2), PlayStation 3 (PS3), PlayStation Portable (PSP), Xbox 360, and Wii systems. It was released on October 26, 2010 in North America, October 28 in Australia, October 29 in Europe, and February 3, 2011 in Japan for the PS3 and Xbox 360 versions. A demo version of the game was released in Japan for Xbox 360 in early February 2011.


title: WWE SmackDown vs. Raw 2011

text: The game is the twelfth overall installment in the series, the successor to "SmackDown vs. Raw 2010", as well as the seventh and last game to be released under the "SmackDown vs. Raw" name; which is based on the WWE professional wrestling promotion and its Raw and SmackDown brands.


title: WWE SmackDown vs. Raw 2011

text: One of the main changes in the game is the incorporation of a new physics system that allows objects to be used more realistically, such as objects involved in the Tables, Ladders, and Chairs match. Tables now break differently depending on the impact placed upon them. Inside and outside the ring, ladders can now be rested onto the ring ropes and be broken in half. For the first time since the original "WWE SmackDown! vs. Raw" game, chairs can be thrown at the player's opponents.


title: WWE SmackDown vs. Raw 2011

text: The Hell in a Cell match was revamped with expanded cell walls, weapons underneath the ring, steel steps near the poles, and the removal of the cell door. In addition, new ways to exit the cell were introduced, such as barreling through the cage wall. This game was also the last to feature Batista and Shawn Michaels as non-legends or unlockables until the release of later installments of the game.


title: WWE SmackDown vs. Raw 2011

text: The game's new "WWE Universe" builds storylines, and integrates cut scenes and rivalries between wrestlers based on the matches that are wrestled. These cut scenes appear randomly before, during, and after the match. The game features more than 100 cutscenes to push storylines, which can revolve around alliances, rivalries and titles, forward. The game reacts to the players actions and depending on the goal of the player's superstar, original or created, puts them in one of, or several, the top ten contenders lists for a specific title. However, one can only get into the contendership list depending on their rivalries, alliances, match types and most importantly, win–loss ratio.


title: WWE SmackDown vs. Raw 2011

text: Road to WrestleMania mode has been modified to include new features such as backstage roaming, mini-quests, and further interaction with other Superstars. Playable storylines involve Rey Mysterio, John Cena, Christian, and Chris Jericho. Along with that is the new storyline in which players choose Kofi Kingston, John Morrison, Dolph Ziggler, R-Truth, or a created superstar in a quest to end The Undertaker's undefeated WrestleMania Streak. Like previous iterations, the game features several storylines starting during the card leading to the Royal Rumble, a time period dubbed the Road to WrestleMania (WrestleMania XXVI).


title: WWE SmackDown vs. Raw 2011

text: The Create A Superstar mode now includes pre-loaded attribute points for created superstars, while in Create A Finisher mode, a new finisher position is the "Corner", in which finishers can be created in corner position (Cannot be used during the Royal Rumble match). Also attributes such as speed and trajectory can now be modified by single increments rather than 25% increments. The number of available created finishers has been expanded to 130 as well. Online mode also returns – each new copy of the game contains an Online Pass allowing one user to play the game online for free. Players who rent or purchase the game used will obtain a trial of Online Pass.


title: WWE SmackDown vs. Raw 2011

text: A new feature called "Match Creator" allows players to create new match types. In addition, Story Designer mode now has new cutscenes, usable sound clips from announcers and wrestler entrance music, support for up to fifteen created superstars, 'branching' story paths depending on which character won the match and the ability to include championship belts in cutscenes and event triggers based on a wrestler reaching a certain state.


title: WWE SmackDown vs. Raw 2011

text: "WWE SmackDown vs. Raw 2011" was first announced along with "WWE All Stars" at E3 in June 2010 by THQ. A playable demo was featured at E3 with Randy Orton, The Undertaker, Chris Jericho, and The Miz as playable characters.


title: WWE SmackDown vs. Raw 2011

text: Special editions of the game featuring The Undertaker, Bret Hart and Randy Orton were released in PAL regions, each offering exclusive in-game content, an art book and DVD relating to each wrestler. Various retailers offered in-game bonuses include a playable Bret Hart, The Undertaker's alternate Ministry of Darkness attire, three alternate outfits for Randy Orton and the WWE Tribute to the Troops arena.


title: WWE SmackDown vs. Raw 2011

text: Three downloadable content packs have been confirmed. The first pack, "Online Axxess" was available at release and adds playable character Chris Masters, Halloween-themed attires for Kelly Kelly and Rey Mysterio and online multiplayer mode. The second pack was released in North America and United Kingdom on December 21, 2010. This pack contains the playable characters British Bulldog, Lex Luger, Layla and Nexus members Wade Barrett, David Otunga, and Justin Gabriel along with alternate attires for Shad Gaspard and Shawn Michaels and the NXT Arena. The third pack includes alternate attires for John Cena, The Undertaker, Sheamus and CM Punk as well as the WCW Nitro Arena. THQ has offered players a package deal called "Fan Axxess" to purchase the last two content packs beforehand.


title: WWE SmackDown vs. Raw 2011

text: On January 25, 2011, Bret Hart was made available as a downloadable character for the Xbox 360 and PlayStation 3 versions. He had previously been excluded from THQ's "Fan Axxess" package.


title: WWE SmackDown vs. Raw 2011

text: The game was met with positive to mixed reception upon release. GameRankings and Metacritic gave it a score of 80% and 80 out of 100 for the PlayStation 2 version; 77.47% and 74 out of 100 for the PlayStation 3 version; 76.84% and 75 out of 100 for the Xbox 360 version; 74% and 72 out of 100 for the Wii version; and 60% and 62 out of 100 for the PSP version.


title: WWE SmackDown vs. Raw 2011

text: Greg Miller of IGN gave the PS3 and Xbox 360 versions of the game an eight out of ten. He praised the game's Universe Mode and the wrestler animations, but criticized the limitations of Road to WrestleMania mode and the inaccurate commentary. In addition, he stated that the grappling system could be "frustrating at times." Miller gave the Wii version of the game a 7.5 out of ten, however. He mentioned the same criticisms from the PS3 and Xbox 360 versions, but also criticized the graphics. GameSpot's Chris Watters rated the PS3 and X360 versions seven out of ten, lauding the game's customization options, while finding fault with the game's collision detection and the lag during online multiplayer games. He also claimed that the series was starting to "show its age."


title: WWE SmackDown vs. Raw 2011

text: GameZone's William Haley criticized the PS3 and Xbox 360 versions for not progressing much beyond their predecessors, claiming that the game targeted the "lowest common denominator." He called the WWE Universe feature, online multiplayer mode and creation tools "compelling", but also wrote that "the foundation upon which they are built is so decrepit that it completely invalidates the meager value this game has to offer."


title: Introduction to general relativity

text: General relativity is a theory of gravitation developed by Albert Einstein between 1907 and 1915. The theory of general relativity says that the observed gravitational effect between masses results from their warping of spacetime.


title: Introduction to general relativity

text: By the beginning of the 20th century, Newton's law of universal gravitation had been accepted for more than two hundred years as a valid description of the gravitational force between masses. In Newton's model, gravity is the result of an attractive force between massive objects. Although even Newton was troubled by the unknown nature of that force, the basic framework was extremely successful at describing motion.


title: Introduction to general relativity

text: Experiments and observations show that Einstein's description of gravitation accounts for several effects that are unexplained by Newton's law, such as minute anomalies in the orbits of Mercury and other planets. General relativity also predicts novel effects of gravity, such as gravitational waves, gravitational lensing and an effect of gravity on time known as gravitational time dilation. Many of these predictions have been confirmed by experiment or observation, most recently gravitational waves.


title: Introduction to general relativity

text: General relativity has developed into an essential tool in modern astrophysics. It provides the foundation for the current understanding of black holes, regions of space where the gravitational effect is strong enough that even light cannot escape. Their strong gravity is thought to be responsible for the intense radiation emitted by certain types of astronomical objects (such as active galactic nuclei or microquasars). General relativity is also part of the framework of the standard Big Bang model of cosmology.


title: Introduction to general relativity

text: Although general relativity is not the only relativistic theory of gravity, it is the simplest such theory that is consistent with the experimental data. Nevertheless, a number of open questions remain, the most fundamental of which is how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity.


title: Introduction to general relativity

text: In September 1905, Albert Einstein published his theory of special relativity, which reconciles Newton's laws of motion with electrodynamics (the interaction between objects with electric charge). Special relativity introduced a new framework for all of physics by proposing new concepts of space and time. Some then-accepted physical theories were inconsistent with that framework; a key example was Newton's theory of gravity, which describes the mutual attraction experienced by bodies due to their mass.


title: Introduction to general relativity

text: Several physicists, including Einstein, searched for a theory that would reconcile Newton's law of gravity and special relativity. Only Einstein's theory proved to be consistent with experiments and observations. To understand the theory's basic ideas, it is instructive to follow Einstein's thinking between 1907 and 1915, from his simple thought experiment involving an observer in free fall to his fully geometric theory of gravity.


title: Introduction to general relativity

text: A person in a free-falling elevator experiences weightlessness; objects either float motionless or drift at constant speed. Since everything in the elevator is falling together, no gravitational effect can be observed. In this way, the experiences of an observer in free fall are indistinguishable from those of an observer in deep space, far from any significant source of gravity. Such observers are the privileged ("inertial") observers Einstein described in his theory of special relativity: observers for whom light travels along straight lines at constant speed.


title: Introduction to general relativity

text: Einstein hypothesized that the similar experiences of weightless observers and inertial observers in special relativity represented a fundamental property of gravity, and he made this the cornerstone of his theory of general relativity, formalized in his equivalence principle. Roughly speaking, the principle states that a person in a free-falling elevator cannot tell that they are in free fall. Every experiment in such a free-falling environment has the same results as it would for an observer at rest or moving uniformly in deep space, far from all sources of gravity.


title: Introduction to general relativity

text: Most effects of gravity vanish in free fall, but effects that seem the same as those of gravity can be "produced" by an accelerated frame of reference. An observer in a closed room cannot tell which of the following is true:


title: Introduction to general relativity

text: Conversely, any effect observed in an accelerated reference frame should also be observed in a gravitational field of corresponding strength. This principle allowed Einstein to predict several novel effects of gravity in 1907, as explained in the next section.


title: Introduction to general relativity

text: An observer in an accelerated reference frame must introduce what physicists call fictitious forces to account for the acceleration experienced by the observer and objects around them. In the example of the driver being pressed into their seat, the force felt by the driver is one example; another is the force one can feel while pulling the arms up and out if attempting to spin around like a top. Einstein's master insight was that the constant, familiar pull of the Earth's gravitational field is fundamentally the same as these fictitious forces. The apparent magnitude of the fictitious forces always appears to be proportional to the mass of any object on which they act – for instance, the driver's seat exerts just enough force to accelerate the driver at the same rate as the car. By analogy, Einstein proposed that an object in a gravitational field should feel a gravitational force proportional to its mass, as embodied in Newton's law of gravitation.


title: Introduction to general relativity

text: In 1907, Einstein was still eight years away from completing the general theory of relativity. Nonetheless, he was able to make a number of novel, testable predictions that were based on his starting point for developing his new theory: the equivalence principle.


title: Introduction to general relativity

text: The first new effect is the gravitational frequency shift of light. Consider two observers aboard an accelerating rocket-ship. Aboard such a ship, there is a natural concept of "up" and "down": the direction in which the ship accelerates is "up", and unattached objects accelerate in the opposite direction, falling "downward". Assume that one of the observers is "higher up" than the other. When the lower observer sends a light signal to the higher observer, the acceleration causes the light to be red-shifted, as may be calculated from special relativity; the second observer will measure a lower frequency for the light than the first. Conversely, light sent from the higher observer to the lower is blue-shifted, that is, shifted towards higher frequencies. Einstein argued that such frequency shifts must also be observed in a gravitational field. This is illustrated in the figure at left, which shows a light wave that is gradually red-shifted as it works its way upwards against the gravitational acceleration. This effect has been confirmed experimentally, as described below.


title: Introduction to general relativity

text: This gravitational frequency shift corresponds to a gravitational time dilation: Since the "higher" observer measures the same light wave to have a lower frequency than the "lower" observer, time must be passing faster for the higher observer. Thus, time runs more slowly for observers who are lower in a gravitational field.


title: Introduction to general relativity

text: It is important to stress that, for each observer, there are no observable changes of the flow of time for events or processes that are at rest in his or her reference frame. Five-minute-eggs as timed by each observer's clock have the same consistency; as one year passes on each clock, each observer ages by that amount; each clock, in short, is in perfect agreement with all processes happening in its immediate vicinity. It is only when the clocks are compared between separate observers that one can notice that time runs more slowly for the lower observer than for the higher. This effect is minute, but it too has been confirmed experimentally in multiple experiments, as described below.


title: Introduction to general relativity

text: In a similar way, Einstein predicted the gravitational deflection of light: in a gravitational field, light is deflected downward. Quantitatively, his results were off by a factor of two; the correct derivation requires a more complete formulation of the theory of general relativity, not just the equivalence principle.


title: Introduction to general relativity

text: The equivalence between gravitational and inertial effects does not constitute a complete theory of gravity. When it comes to explaining gravity near our own location on the Earth's surface, noting that our reference frame is not in free fall, so that fictitious forces are to be expected, provides a suitable explanation. But a freely falling reference frame on one side of the Earth cannot explain why the people on the opposite side of the Earth experience a gravitational pull in the opposite direction.


title: Introduction to general relativity

text: A more basic manifestation of the same effect involves two bodies that are falling side by side towards the Earth. In a reference frame that is in free fall alongside these bodies, they appear to hover weightlessly – but not exactly so. These bodies are not falling in precisely the same direction, but towards a single point in space: namely, the Earth's center of gravity. Consequently, there is a component of each body's motion towards the other (see the figure). In a small environment such as a freely falling lift, this relative acceleration is minuscule, while for skydivers on opposite sides of the Earth, the effect is large. Such differences in force are also responsible for the tides in the Earth's oceans, so the term "tidal effect" is used for this phenomenon.


title: Introduction to general relativity

text: The equivalence between inertia and gravity cannot explain tidal effects – it cannot explain variations in the gravitational field. For that, a theory is needed which describes the way that matter (such as the large mass of the Earth) affects the inertial environment around it.


title: Introduction to general relativity

text: In exploring the equivalence of gravity and acceleration as well as the role of tidal forces, Einstein discovered several analogies with the geometry of surfaces. An example is the transition from an inertial reference frame (in which free particles coast along straight paths at constant speeds) to a rotating reference frame (in which extra terms corresponding to fictitious forces have to be introduced in order to explain particle motion): this is analogous to the transition from a Cartesian coordinate system (in which the coordinate lines are straight lines) to a curved coordinate system (where coordinate lines need not be straight).


title: Introduction to general relativity

text: A deeper analogy relates tidal forces with a property of surfaces called "curvature". For gravitational fields, the absence or presence of tidal forces determines whether or not the influence of gravity can be eliminated by choosing a freely falling reference frame. Similarly, the absence or presence of curvature determines whether or not a surface is equivalent to a plane. In the summer of 1912, inspired by these analogies, Einstein searched for a geometric formulation of gravity.


title: Introduction to general relativity

text: The elementary objects of geometry – points, lines, triangles – are traditionally defined in three-dimensional space or on two-dimensional surfaces. In 1907, Hermann Minkowski, Einstein's former mathematics professor at the Swiss Federal Polytechnic, introduced Minkowski space, a geometric formulation of Einstein's special theory of relativity where the geometry included not only space but also time. The basic entity of this new geometry is four-dimensional spacetime. The orbits of moving bodies are curves in spacetime; the orbits of bodies moving at constant speed without changing direction correspond to straight lines.


title: Introduction to general relativity

text: The geometry of general curved surfaces was developed in the early 19th century by Carl Friedrich Gauss. This geometry had in turn been generalized to higher-dimensional spaces in Riemannian geometry introduced by Bernhard Riemann in the 1850s. With the help of Riemannian geometry, Einstein formulated a geometric description of gravity in which Minkowski's spacetime is replaced by distorted, curved spacetime, just as curved surfaces are a generalization of ordinary plane surfaces. Embedding Diagrams are used to illustrate curved spacetime in educational contexts.


title: Introduction to general relativity

text: After he had realized the validity of this geometric analogy, it took Einstein a further three years to find the missing cornerstone of his theory: the equations describing how matter influences spacetime's curvature. Having formulated what are now known as Einstein's equations (or, more precisely, his field equations of gravity), he presented his new theory of gravity at several sessions of the Prussian Academy of Sciences in late 1915, culminating in his final presentation on November 25, 1915.


title: Introduction to general relativity

text: Paraphrasing John Wheeler, Einstein's geometric theory of gravity can be summarized thus: "spacetime tells matter how to move; matter tells spacetime how to curve". What this means is addressed in the following three sections, which explore the motion of so-called test particles, examine which properties of matter serve as a source for gravity, and, finally, introduce Einstein's equations, which relate these matter properties to the curvature of spacetime.


title: Introduction to general relativity

text: In order to map a body's gravitational influence, it is useful to think about what physicists call probe or test particles: particles that are influenced by gravity, but are so small and light that we can neglect their own gravitational effect. In the absence of gravity and other external forces, a test particle moves along a straight line at a constant speed. In the language of spacetime, this is equivalent to saying that such test particles move along straight world lines in spacetime. In the presence of gravity, spacetime is non-Euclidean, or curved, and in curved spacetime straight world lines may not exist. Instead, test particles move along lines called geodesics, which are "as straight as possible", that is, they follow the shortest path between starting and ending points, taking the curvature into consideration.


title: Introduction to general relativity

text: A simple analogy is the following: In geodesy, the science of measuring Earth's size and shape, a geodesic (from Greek "geo", Earth, and "daiein", to divide) is the shortest route between two points on the Earth's surface. Approximately, such a route is a segment of a great circle, such as a line of longitude or the equator. These paths are certainly not straight, simply because they must follow the curvature of the Earth's surface. But they are as straight as is possible subject to this constraint.


title: Introduction to general relativity

text: The properties of geodesics differ from those of straight lines. For example, on a plane, parallel lines never meet, but this is not so for geodesics on the surface of the Earth: for example, lines of longitude are parallel at the equator, but intersect at the poles. Analogously, the world lines of test particles in free fall are spacetime geodesics, the straightest possible lines in spacetime. But still there are crucial differences between them and the truly straight lines that can be traced out in the gravity-free spacetime of special relativity. In special relativity, parallel geodesics remain parallel. In a gravitational field with tidal effects, this will not, in general, be the case. If, for example, two bodies are initially at rest relative to each other, but are then dropped in the Earth's gravitational field, they will move towards each other as they fall towards the Earth's center.


title: Introduction to general relativity

text: Compared with planets and other astronomical bodies, the objects of everyday life (people, cars, houses, even mountains) have little mass. Where such objects are concerned, the laws governing the behavior of test particles are sufficient to describe what happens. Notably, in order to deflect a test particle from its geodesic path, an external force must be applied. A chair someone is sitting on applies an external upwards force preventing the person from falling freely towards the center of the Earth and thus following a geodesic, which they would otherwise be doing without matter in between them and the center of the Earth. In this way, general relativity explains the daily experience of gravity on the surface of the Earth "not" as the downwards pull of a gravitational force, but as the upwards push of external forces. These forces deflect all bodies resting on the Earth's surface from the geodesics they would otherwise follow. For matter objects whose own gravitational influence cannot be neglected, the laws of motion are somewhat more complicated than for test particles, although it remains true that spacetime tells matter how to move.


title: Introduction to general relativity

text: In Newton's description of gravity, the gravitational force is caused by matter. More precisely, it is caused by a specific property of material objects: their mass. In Einstein's theory and related theories of gravitation, curvature at every point in spacetime is also caused by whatever matter is present. Here, too, mass is a key property in determining the gravitational influence of matter. But in a relativistic theory of gravity, mass cannot be the only source of gravity. Relativity links mass with energy, and energy with momentum.


title: Introduction to general relativity

text: The equivalence between mass and energy, as expressed by the formula "E" = "mc", is the most famous consequence of special relativity. In relativity, mass and energy are two different ways of describing one physical quantity. If a physical system has energy, it also has the corresponding mass, and vice versa. In particular, all properties of a body that are associated with energy, such as its temperature or the binding energy of systems such as nuclei or molecules, contribute to that body's mass, and hence act as sources of gravity.


title: Introduction to general relativity

text: In special relativity, energy is closely connected to momentum. Just as space and time are, in that theory, different aspects of a more comprehensive entity called spacetime, energy and momentum are merely different aspects of a unified, four-dimensional quantity that physicists call four-momentum. In consequence, if energy is a source of gravity, momentum must be a source as well. The same is true for quantities that are directly related to energy and momentum, namely internal pressure and tension. Taken together, in general relativity it is mass, energy, momentum, pressure and tension that serve as sources of gravity: they are how matter tells spacetime how to curve. In the theory's mathematical formulation, all these quantities are but aspects of a more general physical quantity called the energy–momentum tensor.


title: Introduction to general relativity

text: Einstein's equations are the centerpiece of general relativity. They provide a precise formulation of the relationship between spacetime geometry and the properties of matter, using the language of mathematics. More concretely, they are formulated using the concepts of Riemannian geometry, in which the geometric properties of a space (or a spacetime) are described by a quantity called a metric. The metric encodes the information needed to compute the fundamental geometric notions of distance and angle in a curved space (or spacetime).


title: Introduction to general relativity

text: A spherical surface like that of the Earth provides a simple example. The location of any point on the surface can be described by two coordinates: the geographic latitude and longitude. Unlike the Cartesian coordinates of the plane, coordinate differences are not the same as distances on the surface, as shown in the diagram on the right: for someone at the equator, moving 30 degrees of longitude westward (magenta line) corresponds to a distance of roughly , while for someone at a latitude of 55 degrees, moving 30 degrees of longitude westward (blue line) covers a distance of merely . Coordinates therefore do not provide enough information to describe the geometry of a spherical surface, or indeed the geometry of any more complicated space or spacetime. That information is precisely what is encoded in the metric, which is a function defined at each point of the surface (or space, or spacetime) and relates coordinate differences to differences in distance. All other quantities that are of interest in geometry, such as the length of any given curve, or the angle at which two curves meet, can be computed from this metric function.


title: Introduction to general relativity

text: The metric function and its rate of change from point to point can be used to define a geometrical quantity called the Riemann curvature tensor, which describes exactly how the Riemannian manifold, the spacetime in the theory of relativity, is curved at each point. As has already been mentioned, the matter content of the spacetime defines another quantity, the energy–momentum tensor T, and the principle that "spacetime tells matter how to move, and matter tells spacetime how to curve" means that these quantities must be related to each other. Einstein formulated this relation by using the Riemann curvature tensor and the metric to define another geometrical quantity G, now called the Einstein tensor, which describes some aspects of the way spacetime is curved. "Einstein's equation" then states that


title: Introduction to general relativity

text: i.e., up to a constant multiple, the quantity G (which measures curvature) is equated with the quantity T (which measures matter content). Here, "G" is the gravitational constant of Newtonian gravity, and "c" is the speed of light from special relativity.


title: Introduction to general relativity

text: This equation is often referred to in the plural as "Einstein's equations", since the quantities G and T are each determined by several functions of the coordinates of spacetime, and the equations equate each of these component functions. A solution of these equations describes a particular geometry of spacetime; for example, the Schwarzschild solution describes the geometry around a spherical, non-rotating mass such as a star or a black hole, whereas the Kerr solution describes a rotating black hole. Still other solutions can describe a gravitational wave or, in the case of the Friedmann–Lemaître–Robertson–Walker solution, an expanding universe. The simplest solution is the uncurved Minkowski spacetime, the spacetime described by special relativity.


title: Introduction to general relativity

text: No scientific theory is self-evidently true; each is a model that must be checked by experiment. Newton's law of gravity was accepted because it accounted for the motion of planets and moons in the Solar System with considerable accuracy. As the precision of experimental measurements gradually improved, some discrepancies with Newton's predictions were observed, and these were accounted for in the general theory of relativity. Similarly, the predictions of general relativity must also be checked with experiment, and Einstein himself devised three tests now known as the classical tests of the theory:


title: Introduction to general relativity

text: Of these tests, only the perihelion advance of Mercury was known prior to Einstein's final publication of general relativity in 1916. The subsequent experimental confirmation of his other predictions, especially the first measurements of the deflection of light by the sun in 1919, catapulted Einstein to international stardom. These three experiments justified adopting general relativity over Newton's theory and, incidentally, over a number of alternatives to general relativity that had been proposed.


title: Introduction to general relativity

text: Further tests of general relativity include precision measurements of the Shapiro effect or gravitational time delay for light, measured in 2002 by the Cassini space probe. One set of tests focuses on effects predicted by general relativity for the behavior of gyroscopes travelling through space. One of these effects, geodetic precession, has been tested with the Lunar Laser Ranging Experiment (high-precision measurements of the orbit of the Moon). Another, which is related to rotating masses, is called frame-dragging. The geodetic and frame-dragging effects were both tested by the Gravity Probe B satellite experiment launched in 2004, with results confirming relativity to within 0.5% and 15%, respectively, as of December 2008.


title: Introduction to general relativity

text: By cosmic standards, gravity throughout the solar system is weak. Since the differences between the predictions of Einstein's and Newton's theories are most pronounced when gravity is strong, physicists have long been interested in testing various relativistic effects in a setting with comparatively strong gravitational fields. This has become possible thanks to precision observations of binary pulsars. In such a star system, two highly compact neutron stars orbit each other. At least one of them is a pulsar – an astronomical object that emits a tight beam of radiowaves. These beams strike the Earth at very regular intervals, similarly to the way that the rotating beam of a lighthouse means that an observer sees the lighthouse blink, and can be observed as a highly regular series of pulses. General relativity predicts specific deviations from the regularity of these radio pulses. For instance, at times when the radio waves pass close to the other neutron star, they should be deflected by the star's gravitational field. The observed pulse patterns are impressively close to those predicted by general relativity.


title: Introduction to general relativity

text: One particular set of observations is related to eminently useful practical applications, namely to satellite navigation systems such as the Global Positioning System that are used for both precise positioning and timekeeping. Such systems rely on two sets of atomic clocks: clocks aboard satellites orbiting the Earth, and reference clocks stationed on the Earth's surface. General relativity predicts that these two sets of clocks should tick at slightly different rates, due to their different motions (an effect already predicted by special relativity) and their different positions within the Earth's gravitational field. In order to ensure the system's accuracy, either the satellite clocks are slowed down by a relativistic factor, or that same factor is made part of the evaluation algorithm. In turn, tests of the system's accuracy (especially the very thorough measurements that are part of the definition of universal coordinated time) are testament to the validity of the relativistic predictions.


title: Introduction to general relativity

text: A number of other tests have probed the validity of various versions of the equivalence principle; strictly speaking, all measurements of gravitational time dilation are tests of the weak version of that principle, not of general relativity itself. So far, general relativity has passed all observational tests.


title: Introduction to general relativity

text: Models based on general relativity play an important role in astrophysics; the success of these models is further testament to the theory's validity.


title: Introduction to general relativity

text: Since light is deflected in a gravitational field, it is possible for the light of a distant object to reach an observer along two or more paths. For instance, light of a very distant object such as a quasar can pass along one side of a massive galaxy and be deflected slightly so as to reach an observer on Earth, while light passing along the opposite side of that same galaxy is deflected as well, reaching the same observer from a slightly different direction. As a result, that particular observer will see one astronomical object in two different places in the night sky. This kind of focussing is well known when it comes to optical lenses, and hence the corresponding gravitational effect is called gravitational lensing.


title: Introduction to general relativity

text: Observational astronomy uses lensing effects as an important tool to infer properties of the lensing object. Even in cases where that object is not directly visible, the shape of a lensed image provides information about the mass distribution responsible for the light deflection. In particular, gravitational lensing provides one way to measure the distribution of dark matter, which does not give off light and can be observed only by its gravitational effects. One particularly interesting application are large-scale observations, where the lensing masses are spread out over a significant fraction of the observable universe, and can be used to obtain information about the large-scale properties and evolution of our cosmos.


title: Introduction to general relativity

text: Gravitational waves, a direct consequence of Einstein's theory, are distortions of geometry that propagate at the speed of light, and can be thought of as ripples in spacetime. They should not be confused with the gravity waves of fluid dynamics, which are a different concept.


title: Introduction to general relativity

text: In February 2016, the Advanced LIGO team announced that they had directly observed gravitational waves from a black hole merger.


title: Introduction to general relativity

text: Indirectly, the effect of gravitational waves had been detected in observations of specific binary stars. Such pairs of stars orbit each other and, as they do so, gradually lose energy by emitting gravitational waves. For ordinary stars like the Sun, this energy loss would be too small to be detectable, but this energy loss was observed in 1974 in a binary pulsar called PSR1913+16. In such a system, one of the orbiting stars is a pulsar. This has two consequences: a pulsar is an extremely dense object known as a neutron star, for which gravitational wave emission is much stronger than for ordinary stars. Also, a pulsar emits a narrow beam of electromagnetic radiation from its magnetic poles. As the pulsar rotates, its beam sweeps over the Earth, where it is seen as a regular series of radio pulses, just as a ship at sea observes regular flashes of light from the rotating light in a lighthouse. This regular pattern of radio pulses functions as a highly accurate "clock". It can be used to time the double star's orbital period, and it reacts sensitively to distortions of spacetime in its immediate neighborhood.


title: Introduction to general relativity

text: The discoverers of PSR1913+16, Russell Hulse and Joseph Taylor, were awarded the Nobel Prize in Physics in 1993. Since then, several other binary pulsars have been found. The most useful are those in which both stars are pulsars, since they provide accurate tests of general relativity.


title: Introduction to general relativity

text: Currently, a number of land-based gravitational wave detectors are in operation, and a mission to launch a space-based detector, LISA, is currently under development, with a precursor mission (LISA Pathfinder) which was launched in 2015. Gravitational wave observations can be used to obtain information about compact objects such as neutron stars and black holes, and also to probe the state of the early universe fractions of a second after the Big Bang.


title: Introduction to general relativity

text: When mass is concentrated into a sufficiently compact region of space, general relativity predicts the formation of a black hole – a region of space with a gravitational effect so strong that not even light can escape. Certain types of black holes are thought to be the final state in the evolution of massive stars. On the other hand, supermassive black holes with the mass of millions or billions of Suns are assumed to reside in the cores of most galaxies, and they play a key role in current models of how galaxies have formed over the past billions of years.


title: Introduction to general relativity

text: Matter falling onto a compact object is one of the most efficient mechanisms for releasing energy in the form of radiation, and matter falling onto black holes is thought to be responsible for some of the brightest astronomical phenomena imaginable. Notable examples of great interest to astronomers are quasars and other types of active galactic nuclei. Under the right conditions, falling matter accumulating around a black hole can lead to the formation of jets, in which focused beams of matter are flung away into space at speeds near that of light.


title: Introduction to general relativity

text: There are several properties that make black holes the most promising sources of gravitational waves. One reason is that black holes are the most compact objects that can orbit each other as part of a binary system; as a result, the gravitational waves emitted by such a system are especially strong. Another reason follows from what are called black-hole uniqueness theorems: over time, black holes retain only a minimal set of distinguishing features (these theorems have become known as "no-hair" theorems), regardless of the starting geometric shape. For instance, in the long term, the collapse of a hypothetical matter cube will not result in a cube-shaped black hole. Instead, the resulting black hole will be indistinguishable from a black hole formed by the collapse of a spherical mass. In its transition to a spherical shape, the black hole formed by the collapse of a more complicated shape will emit gravitational waves.


title: Introduction to general relativity

text: One of the most important aspects of general relativity is that it can be applied to the universe as a whole. A key point is that, on large scales, our universe appears to be constructed along very simple lines: all current observations suggest that, on average, the structure of the cosmos should be approximately the same, regardless of an observer's location or direction of observation: the universe is approximately homogeneous and isotropic. Such comparatively simple universes can be described by simple solutions of Einstein's equations. The current cosmological models of the universe are obtained by combining these simple solutions to general relativity with theories describing the properties of the universe's matter content, namely thermodynamics, nuclear- and particle physics. According to these models, our present universe emerged from an extremely dense high-temperature state – the Big Bang – roughly 14 billion years ago and has been expanding ever since.


title: Introduction to general relativity

text: Einstein's equations can be generalized by adding a term called the cosmological constant. When this term is present, empty space itself acts as a source of attractive (or, less commonly, repulsive) gravity. Einstein originally introduced this term in his pioneering 1917 paper on cosmology, with a very specific motivation: contemporary cosmological thought held the universe to be static, and the additional term was required for constructing static model universes within the framework of general relativity. When it became apparent that the universe is not static, but expanding, Einstein was quick to discard this additional term. Since the end of the 1990s, however, astronomical evidence indicating an accelerating expansion consistent with a cosmological constant – or, equivalently, with a particular and ubiquitous kind of dark energy – has steadily been accumulating.


title: Introduction to general relativity

text: General relativity is very successful in providing a framework for accurate models which describe an impressive array of physical phenomena. On the other hand, there are many interesting open questions, and in particular, the theory as a whole is almost certainly incomplete.


title: Introduction to general relativity

text: In contrast to all other modern theories of fundamental interactions, general relativity is a classical theory: it does not include the effects of quantum physics. The quest for a quantum version of general relativity addresses one of the most fundamental open questions in physics. While there are promising candidates for such a theory of quantum gravity, notably string theory and loop quantum gravity, there is at present no consistent and complete theory. It has long been hoped that a theory of quantum gravity would also eliminate another problematic feature of general relativity: the presence of spacetime singularities. These singularities are boundaries ("sharp edges") of spacetime at which geometry becomes ill-defined, with the consequence that general relativity itself loses its predictive power. Furthermore, there are so-called singularity theorems which predict that such singularities "must" exist within the universe if the laws of general relativity were to hold without any quantum modifications. The best-known examples are the singularities associated with the model universes that describe black holes and the beginning of the universe.


title: Introduction to general relativity

text: Other attempts to modify general relativity have been made in the context of cosmology. In the modern cosmological models, most energy in the universe is in forms that have never been detected directly, namely dark energy and dark matter. There have been several controversial proposals to remove the need for these enigmatic forms of matter and energy, by modifying the laws governing gravity and the dynamics of cosmic expansion, for example modified Newtonian dynamics.


title: Introduction to general relativity

text: Beyond the challenges of quantum effects and cosmology, research on general relativity is rich with possibilities for further exploration: mathematical relativists explore the nature of singularities and the fundamental properties of Einstein's equations, and ever more comprehensive computer simulations of specific spacetimes (such as those describing merging black holes) are run.


title: Introduction to general relativity

text: "Additional resources, including more advanced material, can be found in General relativity resources."


title: Eurocopter Fennec

text: The Eurocopter (now Airbus Helicopters) AS550 Fennec (now H125M) and AS555 Fennec 2 are lightweight, multipurpose military helicopters manufactured by Eurocopter Group (now Airbus Helicopters). Based on the AS350 Ecureuil and AS355 Ecureuil 2 series, they are named after the fennec fox. The armed versions of the AS550 and AS555 can be fitted with coaxial weapons, rockets, torpedoes and various other munitions.


title: Eurocopter Fennec

text: As a result of the Vienna Treaty, military variants of the Écureuil were marketed under a separate designation; thus from 1990 onwards the type was marketed as the Fennec.


title: Eurocopter Fennec

text: In February 2007, India selected the AS550 C3 Fennec over the Bell 407 helicopter in a deal for 197 helicopters worth US$500 million. This deal was suspended in June 2007 because of concerns of alleged corruption in the bidding process, and cancelled outright on 6 December 2007. The contract is now being re-competed, and the AS550 C3 is among the helicopters competing for the deal. However India chose Kamov Ka-226T over AS550 C3 in the refreshed tender.


title: Eurocopter Fennec

text: On 20 January 2011, Royal Malaysian Navy PASKAL commandos rescued a hijacked Malaysian chemical tanker and its crew from Somali pirates. A Fennec helicopter provided reconnaissance and aerial gunfire to keep the pirate's mother ship at bay while commandos boarded the tanker.


title: The Battle Cats

text: The plot in the game is taught through screens of scrolling text, usually appearing at the beginning and end of each "Chapter".


title: The Battle Cats

text: The story of Empire of Cats (commonly shortened to EoC) consists of an army of "Battle Cats" appearing amidst the world's varying crises. The identity of the creator of these Battle Cats is the fictional deity The Cat God, which can be unlocked as a power up early into the story, and beating chapter two of Cats Of The Cosmos unlocks it as a unit. The cats then proceed to take over the world, to harness the world's power.


title: The Battle Cats

text: The second main story chapter of the game, Into the Future (shortened to ItF)'s story revolves around a time machine the cats obtained in the beginning of Stories of Legend. Using this, they travelled to the future, and find that Aliens have taken over Earth. Battles for the world then commence between the 2 forces.


title: The Battle Cats

text: The current final main chapter of the game, Cats of the Cosmos (often shortened to CotC) begins when interplanetary travel becomes possible. Utilizing this technology, the cats begin to invade outer space, attacking a variety of places in the Cosmos.


title: The Battle Cats

text: The Battle Cats is a tower defense game where the player selects a team of in-game unlockable "cats" with different stat amounts to defeat enemies in order to protect their base (called "Cat Base"). The gameplay involves sending a wide roster of cats out onto a 2D battlefield to defend the Cat Base. The enemies may have in-game traits such as "Red", "Floating", "Black", "Metal", "Alien", "Starred Aliens" (which fall under the Alien category), "Angel", "Zombie", "Aku", "Relic", and "Behemoth". Enemies without a trait are considered "Traitless" enemies.


title: The Battle Cats

text: The Battle Cats contains 3 main story chapters: Empire of Cats (EoC), Into the Future (ItF) and Cats of the Cosmos (CotC). Each main story contains 3 chapters. Additionally, it contains two extra chapters: The Legend Stages and the Catclaw Dojo. The Legend Stages contain multiple stages filled with new enemies and some event stages (including collaborated stages). The Catclaw Dojo contains a stage to test how many enemies you can defeat within a given period of time, with an addition of some collaborated stages of the same premise, except with collaborated enemies.


title: The Battle Cats

text: The Legend Stages are further divided into two categories: Stories of Legend (SoL) and Uncanny Legends (UL), with the latter being unlocked after completing the former. There is also an additional category for scheduled and general event stages, and another category for Collaboration stages. The Stories of Legends take place within the same dimension of the three previous Stories, except it was back in time; before the Legends/Legend Cats were sealed. Uncanny Legends takes place even before Stories of Legend timeline wise, with some of the Legend cats you get from this chapter being indicated to be the ancestors of some Stories of Legend cats.


title: The Battle Cats

text: The cats are the protagonists that the player deploys throughout the game. The cats are all unique in their own sense, design, and ability. Many cats may not look like or be a typical cat (and are sometimes equipped with external objects such as weapons or machines) or are characters earned in collaboration with other games. Almost all units in the game have a distinctive cat face, cat or cat shaped object/being somewhere on their sprite. Cats have level caps, and reaching one of these means one will not be able to upgrade that cat further. The level caps differ from rarity to rarity and unit to unit. All cats can be upgraded to a maximum level of lv50, with the exception of the "Legend cats" obtained from Stories of legends stages and Uncanny Legends stages, which can only be upgraded to lv40 at maximum. Double-ups obtained in Gacha can be used to add additional "plus levels" to a cat unit."Catseyes" can also be used to increase the level cap of some units. The cats are divided into six categories: Basic, Special, Rare, Super Rare, Uber Super Rare (commonly shortened to Uber Rare or just Uber), and Legend Rare (??? Rare before you unlock one). The Battle Cats frequently has collaboration events (colloquialised as collabs) where players can earn cats called collaboration units. The nine basic cats are the first cats available and are the easiest to upgrade. They have a max upgrade level of 20 but can be increased with Cat Capsule duplicates. After upgrading to level 30 with "Plus levels", the player can evolve the cat into what is called its True Form. Later on in the game, a 10th Basic cat is unlocked, titled "Superfeline". It can be unlocked via the "Cat Capsule+" after beating the 3rd story chapter of "Cats of the Cosmos". True Forms for other units are obtained by having the required "Catfruit/behemoth stones" from 11.5 and XP and having them on a certain level. The Special Cats are the second type of cat unlocked. They can be unlocked by drop rewards of certain stages, be bought by cat food (the premium currency of the game), or just by simply collecting the unit by opening the game at certain times. The Rare, Super Rare, Uber Super Rare and ?? Rare (Legend Rare) cats can be unlocked with a rare cat capsule, which requires golden or platinum tickets to operate or to be unlocked in special stages. They are more expensive to upgrade than basic cats and most special cats, but are usually stronger than the basic cats (not to mention "crazed" cats and "manic" cats, the latter being the true forms of the crazed cats), which are buffed versions of the basic cats in the Super Rare Tier, which can be unlocked in special levels in the Legend stages chapter category.


title: The Battle Cats

text: Fighting in battle requires "energy", which slowly regenerates over time. Every one minute, one energy is recovered (the energy recovery rate can be sped up with treasures, up to 1 energy every 30 seconds). The app requires an internet connection in order for energy to update, but if the app has already been opened, the energy amount increases without internet. Max energy can be raised with upgrades and treasures. The player can bring items into the battle that assist such as "Sniper the Cat," a tool that pushes enemies back from the base and deals small damage, and "Cat CPU," a tool that automatically deploys specific cats as soon as money is available, and so on. Some of the stages have special restrictions such as "Special and Rare Cats only."(That mostly applying to "4 crown" stages, formerly known as "4 star stages") The battle takes place on a 2D plane with the player's base on the right, and the enemy base on the left. The player gains a continuous stream of money, which can be spent to either deploy cats or to upgrade the money production speed along with the maximum amount of money you can receive in a game. Deploying cats requires a cooldown, which can be reduced by upgrades and treasures, and "Cat Combos". The battle ends when one of the bases is destroyed. Winning the battle in the main story chapters (1-9) gives the player a chance to get a treasure. Collecting all of them can buff cats' abilities and can also enhance other game mechanics, such as decreased energy recharge time. There are three levels of treasure: Inferior, Normal, and Superior, with an Inferior treasure being the weakest and Superior treasure being the strongest. Players can replay stages to get all the treasures. Players can also use a Treasure Radar powerup to have a 100% chance to get a superior treasure if they complete the battle. The treasure radar can also be used to get a reward in certain stages such as a true form or new unit.


title: The Battle Cats

text: There is a huge roster of enemies all of different stats and types, with more being added with each new update.


title: The Battle Cats

text: Gamatoto is an expedition minigame where the player sends out servant cats to search for different resources such as XP, Cat Food and various boost items (speed-up, rich cat, cat cpu, etc) which can be used in the battle for various purposes, like the Cat jobs, which increases the amount of XP you receive in a stage by 1.5 times. There are 5 levels of Gamatoto helpers that each give a boost to item drop rates, and can be identified by the color of their hats, with white being the lowest and the color of the legend rare crystals being the best (although you do have to obtain 10 golden helpers to get one).


title: The Battle Cats

text: "Cat Capsule," often referred to as Gacha, is an area where the player can spend Cat Tickets, Rare Cat Tickets, Lucky Tickets and Cat Food to obtain Cat Units and Ability Capsules. The rewards earned are then either immediately used, immediately exchanged for XP/NP, saved in the Cat Storage to be used later, or exchanged to get a Rare Ticket.


title: The Battle Cats

text: "The Battle Cats POP!" (often abbreviated to BCP) was released for the Nintendo 3DS worldwide on June 27, 2016. The game featured mechanics similar to "The Battle Cats", but also introduced a 2 player VS mode and made use of the 3DS 3D capabilities.


title: The Battle Cats

text: "The Battle Cats POP!" received average reviews. CJ Andriessen of "Destructoid" gave the game a 7 out of 10 and wrote that "The Battle Cats POP! is a game that's as fun as it is weird. When it's not putting you in time out with the energy meter, it can be addictive like Pokémon. Too much of its free-to-play roots are still present, but even with these elements, it's still a worthwhile time killer." Matt S. of "Digitally Downloaded" gave 3.5 stars out of 5 and noted "what sets Battle Cats apart from the many similar games of similar depth and strategy is that hugely creative edge. This game is the distinctive and memorable example of the genre, because the artists had the sense to do something that makes no sense, but we end up looking forward to each new level just to see the insanity that it brings." However, the stamina system was criticized by reviewers, as Louisa Bhairam with "Nintendo Life" stated "It's a bit of an odd system for a game which requires no micro-transactions, [...] It's a feature which should have been removed entirely for the 3DS version.


title: The Battle Cats

text: The Battle Cats Unite! was released for the Nintendo Switch by Bandai Namco Entertainment in Japan and Southeast Asia on December 9, 2021. The game is a revamped port of The Battle Cats, with a new 2-player co-op mode and four different minigames being exclusive to this version.


title: The Battle Cats

text: "Battle Cats Rangers," an idle/clicker game published by MEMORY Inc and developed with YD Online Corp, was released for Android and iOS devices on April 27, 2017. In "Battle Cats Rangers," players tap the screen as quick as possible to attack incoming enemies and bosses that swarm their team of cats. 2 weeks prior to the release, a pre-registration campaign was announced which encouraged users to sign up for in-game currency. By using coins and cat food, the in-game currency, players can level up their cats to deal more damage and progress through the game. Overtime, players can add more cats to their team and encounter stronger enemies they have to defeat. After 2 years, the game shut down its online servers and the game was no longer playable after March 29, 2019.


title: The Battle Cats

text: 5 years after the release of The Battle Cats, PONOS Corp released "Go! Go! Pogo Cat", a simple infinite platformer similar to Flappy Bird. The player directs a cat riding a pogo stick to gain as much distance as possible, collecting items like gems. For players that have The Battle Cats downloaded, they can complete certain missions in-game for rewards redeemable in The Battle Cats"."


title: The Battle Cats

text: On November 14, 2021, PONOS Corp released "Battle Cats Quest." In this game, players roll a spherical cat on land to knock off enemies from various stages and earn coins. Similarly to "Go! Go! Pogo Cat", players can complete in-game missions for rewards redeemable in The Battle Cats.


title: Sara Waisglass

text: Waisglass made her way into the entertainment industry as a child actress, making her television debut playing 8 year old Jane in "The Jane Show" in 2007. To follow this Waisglass made her big screen debut in the Gilles Bourdos directed psychological thriller "Afterwards" alongside John Malkovich and Evangeline Lilly in 2008. In 2015, Waisglass appeared in the Anton Corbijn directed biographical drama film "Life" amongst a cast that included Robert Pattinson and Sir Ben Kingsley. Waisglass also ventured into modelling at aged 15 for the Youth Apparel brand. In 2015, Waisglass was nominated for "Best Recurring Young Actress 16-21" at the "Young Entertainer Awards 2015" for her main role performance of Frankie Hollingsworth in "."


title: Sara Waisglass

text: In 2017, Waisglass played Mary’s half-sister Robyn in the Molly McGlynn directed film "Mary Goes Round" which featured at the 2017 Toronto International Film Festival. In 2021, Waisglass starred as Maxine, best friend of Ginny and part of the MANG group (Max, Abby, Norah and Ginny) in the Netflix comedy series "Ginny and Georgia".


title: History of the city

text: Towns and cities have a long history, although opinions vary on which ancient settlements are truly cities. The benefits of dense settlement included reduced transport costs, exchange of ideas, sharing of natural resources, large local markets, and in some cases amenities such as running water and sewerage. Possible costs would include higher rate of crime, higher mortality rates, higher cost of living, worse pollution, traffic and high commuting times. Cities grow when the benefits of proximity between people and firms are higher than the cost.


title: History of the city

text: There is not enough evidence to assert what conditions gave rise to the first cities. Some theorists have speculated on what they consider suitable pre-conditions and basic mechanisms that might have been important driving forces.


title: History of the city

text: The conventional view holds that cities first formed after the Neolithic revolution. The Neolithic revolution brought agriculture, which made denser human populations possible, thereby supporting city development. Whether farming immigrants replaced foragers or foragers began farming is not clear. The increased food production per unit of land supported higher population density and more city-like activities. In his book, "Cities and Economic Development", Paul Bairoch takes up this position in his argument that agricultural activity appears necessary before true cities can form.


title: History of the city

text: According to Vere Gordon Childe, for a settlement to qualify as a city, it must have enough surplus of raw materials to support trade and a relatively large population. Bairoch points out that, due to sparse population densities that would have persisted in pre-Neolithic, hunter-gatherer societies, the amount of land that would be required to produce enough food for subsistence and trade for a large population would make it impossible to control the flow of trade. To illustrate this point, Bairoch offers an example: "Western Europe during the pre-Neolithic, [where] the density must have been less than 0.1 person per square kilometre". Using this population density as a base for calculation, and allotting 10% of food towards surplus for trade and assuming that city dwellers do no farming, he calculates that "...to maintain a city with a population of 1,000, and without taking the cost of transport into account, an area of 100,000 square kilometres would have been required. When the cost of transport is taken into account, the figure rises to 200,000 square kilometres ...". Bairoch noted that this is roughly the size of Great Britain. The urban theorist Jane Jacobs suggests that city formation preceded the birth of agriculture, but this view is not widely accepted.


title: History of the city

text: In his book "City Economics", Brendan O'Flaherty asserts "Cities could persist—as they have for thousands of years—only if their advantages offset the disadvantages". O'Flaherty illustrates two similar attracting advantages known as increasing returns to scale and economies of scale, which are concepts usually associated with businesses. Their applications are seen in more basic economic systems as well. Increasing returns to scale occurs when "doubling all inputs more than doubles the output [and] an activity has economies of scale if doubling output less than doubles cost".


title: History of the city

text: Similarly, "Are Cities Dying?", a paper by Harvard economist Edward L. Glaeser, delves into similar reasons for city formation: reduced transport costs for goods, people and ideas. Discussing the benefits of proximity, Glaeser claims that if a city is doubled in size, workers get a ten percent increase in earnings. Glaeser furthers his argument by stating that bigger cities do not pay more for equal productivity than in a smaller city, so it is reasonable to assume that workers become more productive if they move to a city twice the size as they initially worked in. The workers do not benefit much from the ten percent wage increase, because it is recycled back into the higher cost of living in a larger city. They do gain other benefits from living in cities, though.


title: History of the city

text: The first true towns are sometimes considered large settlements where the inhabitants were no longer simply farmers of the surrounding area, but began to take on specialized occupations, and where trade, food storage and power were centralized. In 1950 Gordon Childe attempted to define a historic city with ten general metrics. These are:


title: History of the city

text: This categorisation is descriptive, and it is used as a general touchstone when considering ancient cities, although not all have each of its characteristics.


title: History of the city

text: The more complex human societies, called the first civilizations, emerged around 3000 BCE in the river valleys of Mesopotamia, Minoan Crete, India, China, and Egypt. An increase in food production led to the significant growth in human population and the rise of cities. The peoples of Southwest Asia and Egypt laid the foundations of Western civilization: they developed cities and struggled with the problems of organised states as they moved from individual communities to larger territorial units and eventually to empires. Among these early civilizations, Egypt is exceptional for its apparent lack of big cities.


title: History of the city

text: The growth of the population of ancient civilizations, the formation of ancient empires concentrating political power, and the growth in commerce and manufacturing led to ever greater capital cities and centres of commerce and industry, with Alexandria, Antioch and Seleucia of the Hellenistic civilization, Pataliputra (now Patna) in India, Chang'an (now Xi'an) in China, Carthage, ancient Rome, its eastern successor Constantinople (now Istanbul).


title: History of the city

text: The roster of early urban traditions is notable for its diversity. Excavations at early urban sites show that some cities were sparsely populated political capitals, others were trade centers, and still other cities had a primarily religious focus. Some cities had large dense populations, whereas others carried out urban activities in the realms of politics or religion without having large associated populations. Theories that attempt to explain ancient urbanism by a single factor, such as economic benefit, fail to capture the range of variation documented by archaeologists.


title: History of the city

text: The earliest known city is Çatalhöyük, a settlement of some 10,000 people in southern Anatolia that existed from approximately 7400 BCE to 5200 BCE. Hunting, agriculture and animal domestication all played a role in the society of Çatalhöyük.


title: History of the city

text: Ancient Mesopotamia, the area of the Tigris and Euphrates within modern day Iraq and Syria, was home to numerous cities by the third millennium BCE. These cities formed the basis of the Sumerian and subsequent cultures. Cities such as Jericho, Uruk, Ur, Nineveh, and Babylon, made legendary by the Bible, have been located and excavated, while others such as Damascus and Jerusalem have been continuously populated.


title: History of the city

text: The Phoenician trading empire, flourishing around the turn of the first millennium BC, encompassed numerous cities extending from Tyre, Cydon, and Byblos, across the Mediterranean to Carthage (in modern Tunisia) and Cádiz (in modern Spain). The name of Melqart, an important Phoenician deity, comes from M-L-K and Q-R-T, meaning "king" and "city".


title: History of the city

text: Beginning in the early first millennium, independent city-states in Greece began to flourish, evolving the notion of citizenship, becoming in the process the archetype of the free city, the polis. The agora, meaning "gathering place" or "assembly", was the center of athletic, artistic, spiritual and political life of the polis. These Greek city-states reached great levels of prosperity that resulted in an unprecedented cultural boom, that of classical Greece, expressed in architecture, drama, science, mathematics and philosophy, and nurtured in Athens under a democratic government. The Greek Hippodamus of Miletus () has been dubbed the "Father of City Planning" for his design of Miletus; the Hippodamian, or grid plan, was the basis for subsequent Greek and Roman cities. In the 4th century BCE, Alexander the Great commissioned Dinocrates of Rhodes to lay out his new city of Alexandria, the grandest example of idealized urban planning of the ancient Mediterranean world, where the city's regularity was facilitated by its level site near a mouth of the Nile.


title: History of the city

text: The rise of Rome again shifted the locus of political power, resulting in economic and demographic gain for the city of Rome itself, and a new political regime in the form of the Roman Empire. Rome founded many cities ("coloniae"), characteristically imposing a grid pattern made of north–south "cardines" and east–west "decumani". The intersection of the "cardo maximus" and the "decumanus maximus" marked the origin of the city grid. Following these standard plans, Rome founded hundreds of cities and exerted substantial influence toward urbanizing the Mediterranean. In the process, Rome developed sanitation, public housing, public buildings and the forum. In the late Roman Empire political power was increasingly held by bishops of the Christian Church.


title: History of the city

text: The Indus Valley civilization and ancient China are two other areas with major indigenous urban traditions. Among the early Old World cities, Mohenjo-daro of the Indus Valley Civilization in present-day Pakistan, existing from about 2600 BCE, was one of the largest, with a population of 50,000 or more and a sophisticated sanitation system.


title: History of the city

text: China's planned cities date to the turn of the second millennium BCE. City-states emerging at this time used geomancy to locate and plan cities, orienting their walls to cardinal points. Symbolic cities were constructed as celestial microcosms, with the central point corresponding to the pole star representing harmony and connection between the earthly and other realms. In Chang'an the imperial palace lay to the north, facing south, absorbing the light of the sun, and royalty slept with their heads to the north and their feet to the south. Next came the Imperial City, and then the people's city, divided into eastern and western halves.


title: History of the city

text: Agriculture was practiced in sub-Saharan Africa since the third millennium BC. Because of this, cities could develop as centers of non-agricultural activity, well before the influence of Arab urban culture. One of the oldest sites documented thus far, Jenné-Jeno in what is today Mali, has been dated to the third century BCE. According to Roderick and Susan McIntosh, Jenné-Jeno did not fit into traditional Western conceptions of urbanity as it lacked monumental architecture and a distinctive elite social class, but it should indeed be considered a city based on a functional redefinition of urban development. In particular, Jenné-Jeno featured settlement mounds arranged according to a horizontal, rather than vertical, power hierarchy, and served as a center of specialized production and exhibited functional interdependence with the surrounding hinterland. Archaeological evidence from Jenné-Jeno, specifically the presence of non-West African glass beads dated from the third century BCE to the fourth century CE, indicates that pre-Arabic trade contacts probably existed between Jenné-Jeno and North Africa. Additionally, other early urban centers in sub-Saharan Africa, dated to around 500 CE, include Awdaghust, Kumbi-Saleh the ancient capital of Ghana, and Maranda a center located on a trade rout between Egypt and Gao.


title: History of the city

text: In the ancient Americas, early urban traditions developed in the Andes and Mesoamerica. In the Andes, the first urban centers developed in the Norte Chico civilization (also Caral or Caral-Supe civilization), Chavin and Moche cultures, followed by major cities in the Huari, Chimu and Inca cultures. The Norte Chico civilization included as many as 30 major population centers in what is now the Norte Chico region of north-central coastal Peru. It is the oldest known civilization in the Americas, flourishing between the 30th century BCE and the 18th century BCE. Mesoamerica saw the rise of early urbanism in several cultural regions, including the Preclassic Maya, the Zapotec of Oaxaca, and Teotihuacan in central Mexico. Later cultures such as the Aztec drew on these earlier urban traditions.


title: History of the city

text: Teotihuacan, flourishing from 200 BCE to 750 CE, was the largest American city of the pre-Columbian era, possibly reaching a population of 125,000 in 200 CE. The city's grid plan originated with the "Avenue of the Dead", connecting the Temple of the Feathered Serpent and the Pyramid of the Moon. Beyond its ceremonial center the city featured religious buildings (23 temple complexes) and myriad workshops. Although its religious system was clearly expansive and significant, details of its political and economic functioning remain matters of speculation.


title: History of the city

text: In the remnants of the Roman Empire, cities of late antiquity at first gained independence, but lost their population and their importance, starting in Roman Britain and Germania. The locus of power in the West shifted to Constantinople and to the ascendant Islamic civilization with its major cities Baghdad, Cairo, and Córdoba.


title: History of the city

text: From the 9th through the end of the 12th century, Constantinople, capital of the Byzantine Empire, was the largest and wealthiest city in Europe, with a population approaching 1 million. Following the Byzantine–Ottoman wars and other conflicts, the Ottoman Empire gained control over many cities in the Mediterranean area, including Constantinople in 1453.


title: History of the city

text: During the European Middle Ages, a town was as much a political entity as a collection of houses. City residence brought freedom from customary rural obligations to lord and community: ""Stadtluft macht frei"" ("City air makes you free") was a saying in Germany. In Continental Europe cities with a legislature of their own were not unheard of, the laws for towns as a rule other than for the countryside, the lord of a town often being another than for surrounding land. In the Holy Roman Empire, some cities had no other lord than the emperor. Some planned towns were created, in Britain by King Edward I to colonize Wales and in France, "bastides", fortified cities designed on a regular plan.