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A Stargate is an Einstein–Rosen bridge portal device within the Stargate fictional universe that allows practical, rapid travel between two distant locations. The devices first appear in the 1994 Roland Emmerich film Stargate, and thereafter in the television series Stargate SG-1, Stargate Atlantis, and Stargate Universe. In these productions the Stargate functions as a plot generator, allowing the main characters to visit alien planets without the need for spaceships or any other type of technology. The device allows for near-instantaneous travel across intra- and even intergalactic distances.

Within the Stargate fictional universe, Stargates are large rings composed of a fictional superconductive mineral called "naqahdah". Each Stargate has nine points (chevrons) spaced equally around its circumference which are used to determine the address being dialed. On the inner ring is a set of unique glyphs; on Milky Way and Pegasus gates, all but one of these represent star constellations, with the remaining symbol representing the planet or point of origin, while the meaning of the glyphs on Destiny-style gates is unknown. The number of glyphs is dependent on the network in which the gate belongs; Milky Way gates feature 39 glyphs, while Pegasus and Destiny gates have 36. Six of these symbols plus the point of origin serve to map out a specific location in space to which one can dial. Additional glyphs may also be selected which increase the distance of travel, allowing gates outside the current galaxy to be reached, a process that requires significantly more energy than interstellar dialing. Pairs of Stargates function by generating an artificial stable wormhole between them, allowing one-way travel through. A typical Stargate measures 4.6 m (15 ft) in diameter and weighs 29 metric tonnes (64,000 lb). The Stargates were created millions of years ago by an alien civilization known as the Ancients; their modern history begins when Egyptologist Daniel Jackson deciphers their workings in the Stargate film.

Conceptual origin
The basic notion of the stargate concept is that it has at least two devices (stargates) in two distant positions, such that, when active, the rings of each become similar to a physical, singular gateway or door-frame between the two locations. The concept was developed by the writers of the feature film Stargate, Dean Devlin and Roland Emmerich. Similar devices had been seen in previous fiction, but their complete conception as seen in the film was quite original – though there has been contention as to whether they plagiarized the idea from a previous script submission from a student of Egyptology named Omar Zuhdi who submitted a screenplay to them about ten years before the movie was made. Zuhdi pursued legal action regarding this, and the case was eventually settled out of court.

The idea of a "portal" for travelers has been seen often throughout the history of both fantasy and science fiction, often taking a similar form, a device or magical object shaped as a regular or irregular closed geometric form filled with a water-like, rippling puddle that represents the boundary point between the two locations. The stargate picks up heavily on this conception, emphasizing the "watery puddle" for the sake of an alien mystique.

Some early portal appearances in science fiction include A. E. van Vogt's novella Secret Unattainable (July 1942 Astounding), a radio episode of Space Patrol which aired October 25, 1952 (in which it was called a "cycloplex" or a "hole in space"), and Robert A. Heinlein's Tunnel in the Sky (1955) and its "Ramsbotham jump". In 2001: A Space Odyssey (novel), Arthur C. Clarke uses the term 'Star Gate' for the large sentinel TMA-2, which is a classic stargate portal to another part of the universe.

Much of the inspiration for the functioning of the device is drawn heavily from theoretical astrophysics, particularly that of black holes and wormholes, a staple of science fiction, often used to create "shortcuts" through space. Although these may exist in reality, it is not widely held to be true that any such phenomenon could safely transport a human being, as such wormholes would most likely be created by excessive gravity (e.g., from a black hole) which would destroy any potential traveler. In Stargate however, this is circumvented by transporting a traveller through as an energy signature, and reintegrating them at the other end.

Surrounding plot
The Stargate film begins in 1928, when the titular alien device is first discovered and unearthed at Giza, with a young Catherine Langford watching as her father, the archaeologist who found it, directs its unearthing. Stargate SG-1 has since revealed more of the backstory of the Earth Stargate. The American ship Achilles brought the gate to America in 1939 to prevent it from falling into the hands of the Nazis. The United States Air Force then stored the device in various locations —including Washington, DC (Episode "1969")—before installing it at its location of the film and series. The Stargate was studied in the 1940s as a potential weapon and was later mothballed. As the Stargate film quickly skips to the "present day" (1994), unsuccessful archaeologist Daniel Jackson is giving a lecture about his outlandish theories that the Great Pyramid of Giza was not built by the pharaoh Khufu. After he is laughed away, an aged Catherine Langford meets with him, and recruits his egyptological talent, taking him to a top-secret military base at Creek Mountain, where he is instructed to decipher the unique Egyptian hieroglyphs present on a set of cover-stones. He realizes that the indecipherable glyphs are in fact not words but images of constellations, such that by identifying 6 of them a position in space can be extrapolated. He is then shown the stargate itself, uses his new understanding to identify the 7th symbol (the point of origin allowing a route to be extrapolated), and the gate is opened for the first time.

Because thousands of combinations had been previously tried and had failed, it was believed at the time that only two stargates existed, connecting Earth and the planet Abydos, which was visited in the film. At the beginning of the Stargate SG-1 series, however, a large set of additional valid coordinates were discovered engraved in ruins on Abydos. Because the coordinates pick out stars, and because time leads to stellar drift, other addresses were impossible to dial until Samantha Carter reworked the dialing system on Earth to account for this movement. After this, a massive network of possible connections suddenly became available. Even more addresses were later uncovered by Colonel Jack O'Neill from a repository of Ancient knowledge. In order to allow for dialing back to Earth from other locations (without altering the dialing system), it was later stated that the DHD ("Dial-Home Device") normally attached to each stargate automatically updates for stellar drift; Earth's stargate lacks its DHD, requiring other accommodation.

The alien race encountered in the original movie is later developed in SG-1 as the Goa'uld, the dominant evil power in the Milky Way. The leaders of this race, the System Lords, pose as gods and use the stargates to transport slaves between worlds. This has resulted in a large number of planets throughout the galaxy supporting human life, often in civilizations more primitive than Earth. The majority of these civilizations, descended from former Goa'uld slaves, treat the Stargate as a religious relic, often as a source of long-forgotten fear and evil.

For most of the run of Stargate SG-1, Earth was under constant threat from the Goa'uld, and is no match for their superior technology. In the face of this threat, the US Air Force established a top-secret base, the SGC (Stargate Command), as a frontline defence. Multiple teams are formed and sent on missions through the stargate, their primary objective being exploration, and through it the discovery of intelligence, technology and allies to help in the fight against the Goa'uld. The primary team is called SG-1, and the series follows their adventures.

For a long time it was thought that the Goa'uld were the builders of the Stargate Network, but it was later discovered that they had merely made use of the relics left behind by a different and extinct race, the Ancients. At the climax of SG-1 ' s 6th season, Daniel Jackson discovers that the Earth myth of Atlantis is in fact founded upon the Lost City of the Ancients, and Season 7 is spent trying to locate it. At the beginning of the show Stargate Atlantis, which coincides with the beginning of SG-1 ' s 8th season, the city is found in the Pegasus Galaxy, and 8 chevrons are dialed to send an expedition there on what could be a one-way trip. It is there that they discover a new network of stargates, and are plagued by the nemesis of the Ancients, the Wraith. During the events of The Ark of Truth it is revealed that the pre-ascended Ancient known as Amelius originated the concept of the Stargate and wormhole travel.

In the events of the third television series Stargate Universe, a third generation of stargate is discovered which allegedly predates the model originally discovered in the Milky Way galaxy. This model, discovered as a result of a three-month expedition to unlock the stargate's ninth and final chevron, was first encountered on board the ancient research vessel "Destiny" which has been traversing the universe for several million years unmanned, and is several billion light years away from Earth. It is discovered that the Ancients constructed the vessel to be launched after a number of stargate seed ships were dispersed in the universe in order to follow in their path and stop at each planet which a stargate was deposited. Destiny would then extract any relevant data from the planetary stargate in order to further complete research into an apparent signal embedded in the Cosmic microwave background radiation. This "prototype" or "beta" generation of gates, has a limited range. In addition, when a dialing sequence commences, the entire ring (as opposed to an inner track, like Milky Way-era gates) rotates clockwise and counterclockwise in an alternating pattern until the final chevron is locked and a wormhole is established. Finally, the event horizon of the wormhole also appears a slightly more silver color than later generations. Possibly due to the nature of how these stargates were deposited on hundreds of thousands of planets, no planetary DHD is present. Rather, explorers from Destiny are required to bring an Ancient remote control which can command the gate to dial an address in addition to other functions.

Internal names
The Ancients call the Stargates the "Astria Porta". The Goa'uld and the Jaffa refer to it as the "Chappa'ai" (Cha-Pa-Eye), a term also used by the human inhabitants of many worlds that were under Goa'uld control at one point such as Abydos. The Wraith and some Pegasus human civilizations call Stargates "Portals". Stargates have also been referred to as the "Ring of the Gods", "Great Ring of Abydon", "Circle of Standing Water", "Doorway", "Stone Ring", "Gateway", "Annulus", "Ring of the Ancestors", "Wraith Well", "Circle of Darkness" (particularly by primitive societies that fear or revere the device), and even (in a deliberate self-parody) as "The Old Orifice". In the Stargate movie, the text on the cover stones calls it  sbȝ n sbȝw "gate/portal/door of stars." Numerous academics on Earth mistranslated the Egyptian writings for Stargate as "Doorway to Heaven," or some variant thereof.

Operation
The film Stargate rushed very quickly over how a Stargate actually works and is operated, but the subsequent television shows go into this area in a great amount of detail. In the series SG-1, it is explained that a stargate's destination is not fixed, but is singled out by a process known as "dialing". Once a destination is selected by the traveler, the Stargate generates a wormhole between itself and a complementary device at the destination, by being supplied with a threshold amount of raw energy. Objects in transit between gates are broken down into their individual elemental components, and then into energy as they pass through the event horizon, and then travel through a wormhole before being reconstructed on the other side.

Dialing
Each location in the Stargate universe has its own unique "address", which is a combination of seven or more non-repeating symbols appearing on the dialing stargate. By "dialing" these symbols in the correct order, the traveler selects a destination.

The show is consistent with the mechanics of address-dialing. The process involves associating a unique symbol of the inner ring to each of at least the first seven of the chevrons on the outer circumference. The main "address" is invariably dialed first, and the last symbol is the "point of origin", representing the gate being used, which acts as the final trigger for the completion of the address sequence. As each symbol is dialed, the chevron is said to "engage" or "encode" and usually responds by lighting up or moving. When the final symbol of an address is dialed, that chevron is said to "lock" and the wormhole opens (this terminology is arbitrary and often interchangeable, but preferred by the recurring character Walter Harriman). If the address is incorrect or does not correspond to an existing or otherwise functional stargate, the last chevron will not lock, and all of the chevrons will disengage.

Addresses
The symbols used to compose addresses are actually pictorial representations of star constellations. By identifying six constellations in space, a single point can be interpolated that corresponds to the destination desired. As only a small portion of the possible combinations of Stargate symbols represent valid addresses, dialing the Gate at random is largely futile. In, SG-1 discovers a room on Abydos with a list of invalid Stargate addresses and (luckily) a map that allows the SGC to compensate for thousands of years of stellar drift. In the series, the fictional planet Abydos could be dialed because it is relatively close to Earth, although in the film, Abydos was located in the Kaliam Galaxy. Another list of Stargate addresses is provided by Jack O'Neill in from knowledge downloaded into his mind by a Repository of the Ancients. In, a list of Stargate addresses in the Pegasus galaxy is found in the Atlantis database. The SGC assigns designations to Stargate-accessible planets in the form Pxx-xxx or Mxx-xxx; P standing for Planet and M for Moon. Samantha Carter explains in that the designation "is based on a binary code the computer uses for extrapolation".

The symbols dialed are often referred to as "coordinates", and are written as an ordered string; for example, this is the address used in the show for the planet Abydos:      (corresponding to the constellations of Taurus, Serpens Caput, Capricornus, Monoceros, Sagittarius and Orion). As explained by Dr. Daniel Jackson in the movie, the Stargate requires seven correct symbols to connect to another Stargate. As shown in the picture opposite, the first six symbols act as co-ordinates, creating three intersecting lines, the destination. The Stargate uses the seventh symbol as the point of origin allowing one to plot a straight line course to the destination.

Eight-symbol addresses are introduced in, opening up new plot lines by connecting Stargates to different galaxies. The additional symbol acts as a type of "area code". Such connections, in comparison to seven symbol codes, require substantially more energy to complete a functional wormhole – much more than any standard dialing method can provide. In the first instance, opening an intergalactic wormhole is shown to exceed the total power generation capacity of the SGC at the time. O'Neill fashioned an additional power source using spare parts and the liquid naquadah power core of a staff weapon. A fully charged Zero Point Module (ZPM) can provide enough power for regular travel between galaxies. The 8th chevron is a key element in the Stargate Atlantis series, allowing travel to the Pegasus Galaxy.

Stargate Universe introduces the concept of a nine-symbol address, the purpose of the ninth chevron never having been explored in the previous series. The nine-symbol addresses act as codes to dial specific Stargates, with the only two known nine-symbol addresses used to dial from the Milky Way galaxy to Destiny, a massive Ancient vessel that was part of a project to explore the universe, with the project being abandoned when they started researching into ascension among other things, and from Destiny to Earth. Like eight-symbol addresses, the dialing of this address requires a significant amount of power, such that the scientists on Icarus Base had to tap into the planet's naqahdriah core.

Dial-Home Device


There are a handful of methods used in the shows to dial a Stargate, and the most common is with the use of a Dial-Home Device. Almost always referred to as the "DHD" for short, it is depicted as a pedestal-shaped device with a round inclined control panel on top, consisting of two concentric circles of "keys" and a translucent red (Milky Way) or blue (Pegasus) hemisphere in the center; the keys represent the symbols on the rim of the Stargate. By pressing these keys a traveler builds an address. The central hemisphere serves as an "Enter" key to activate the Stargate once a destination has been dialed. Each DHD only has 38 keys, 19 on each ring. According to Dr. Zelenka, dialing an address leaves a small imprint on the control crystals of the DHD, and about fifty addresses can be recovered from a DHD using the proper equipment. However, this gives no indication of the order in which the addresses were dialed, and no guarantee can be made as to the accuracy of the recovered addresses.

The Atlantis DHD is more similar to the Earth's dialing computer than an actual DHD, and looks more like a set of crystal panels. It can block out certain gate addresses. The Atlantis DHD also has an extra control-crystal allowing the dialing of an eighth chevron during the dialing sequence and is the only DHD in the Pegasus Galaxy capable of dialing Earth. A similar DHD is also used on Puddle Jumpers, where the set of used glyphs corresponds to the galaxy of the Puddle Jumper. The Wraith also travel through Stargates in small spacecraft called darts and have some means of remote-dialing them in a manner similar to Ancient ships.

The show makes it clear that every Stargate originally had its own DHD, located directly in front of the gate and facing it. Over time, however, some DHDs have been damaged or lost. This has been the source of plot-difficulties for the protagonists on several occasions, as it is still possible to travel to a Stargate that lacks a DHD, meaning that dialing home again will be much more difficult, if not impossible. One of the primary functions of the MALP that precedes an SG team is to confirm the presence of a DHD. In the absence of a DHD, a user must select the address by manually rotating the inner ring of the Stargate, and use an external power source, as the ring will not rotate unless it is energised. Pegasus Galaxy Stargates do not have a movable ring, so manually dialing these is impossible. Travelers can also emulate a DHD through a Dialing computer as present at Stargate Command. Remote dialers have been used by several races like the Goa'uld and Asgard in various episodes. As the Stargates in Stargate Universe are a different (less advanced model) the crew of the Destiny are forced to use such a device as no planet visited so far has any variation of DHD present.

Milky Way DHDs have sparked numerous discussions as the design and operation of the device has resulted in contradiction. In the Milky Way, the Dial Home Device contains 38 of the 39 symbols on the Stargate, meaning there is always a missing glyph on each DHD. This missing glyph, however, is not the point of origin for the planet. This means that only certain addresses can be reached from specific positions in the galaxy safely. The only way to intervene and reach all destinations in the Milky Way from a specific stargate, is to manually dial the gate using an external power source. Another method for reaching any stargate is to use a Dialing computer like Stargate Command- however, this could lead to unforeseen side effects such wormholes traveling through stars or solar flares. DHDs are designed to prevent these incidents from taking place by only allowing access to certain stargates from specific locations in the galaxy.

Wormhole
Once an address is dialed, the gate is said to have created a "stable wormhole" between itself and the gate dialed. The creation process is depicted with great consistency, and hence has become one of the defining motifs of Stargate, at times being central in both the SG-1 and Atlantis title sequences. It involves the generation of the "puddle of water" portal which lasts roughly 2 seconds, and is completed by the ejection of an unstable energy vortex resembling a surge of water or quicksilver. The vortex is portrayed as a symbol of the stargate's power, invariably causing characters to become affected by awe. Any matter which comes into contact with the vortex is annihilated on a molecular level, as is dramatically demonstrated by a pair of smoking shoes in the episode "Prisoners". In season 9's "Crusade", the unstable vortex was onomatopoeidiacally referred to by Col. Carter as the "Kawoosh", emulating the sound of the initial vortex. This aspect has been used in some cases to dispose of highly hazardous materials. The vortex is also used on one occasion to dispose of a body in a formal funeral service - the body was placed on a pyre in front of the gate, which was then activated.

The actual portal of a Stargate appears inside the inner ring when an address is correctly dialed. This has the appearance of a vertical puddle of water which represents the "event horizon" in the show. In non-fictional parlance, an event horizon is the perimeter around a black hole or wormhole beyond which the gravitational pull of the singularity would be too strong to overcome. The wavering undulations characteristic of water are supposed to represent the "fluctuations in the event horizon". This puddle may then be entered (usually accompanied by a watery squishing sound), and the traveler will emerge from a similar pool at the destination stargate.

The show makes it clear that transit is strictly one-way; an attempt to travel "backwards" causes the traveler to be destroyed. Although in the first episode the Goa'uld who come through at the beginning of the first episode appear to walk back through the event horizon after taking a hostage, in actuality, they dialed out again using a hand-held device (the whooshing sound is audible in the background). As matter is only transmitted through a stargate once the whole object has passed the event horizon, a person or object could be retrieved from the event horizon before entering completely, as the stargate would automatically reintegrate the traveller.



Passage through a Stargate's wormhole is depicted as a visual effect of shooting through a tunnel in space, although this is just a visual aid as travelers are not conscious during the trip. The average travel time between Stargates is 3.2 seconds. In the movie and early SG-1 episodes, travelers exit from the Stargate "frozen stiff" and at high speed (often being knocked from their feet), feeling as though they have been on a "roller coaster ride". The character Major Charles Kawalsky describes Stargate travel as worse than pulling "out of a simulated bombing run in an F-16 at eight-plus g". In later episodes the experience is no different from stepping through a door, explained as a result of refinements made to the dialing computer at the SGC.

Under normal circumstances, a wormhole can only be maintained for slightly more than 38 minutes. Extending the wormhole duration beyond that requires tremendous amounts of power, such as that provided by a nearby black hole.

While the "kawoosh" effect in the movie was created by filming the actual swirl of water in a glass tube, and looked like a vortex on the back of the Gate, on the TV series, this effect was completely created in CG by the Canadian visual effects company Rainmaker. At the beginning of Season 9, the original movie wormhole sequence was substituted by a new sequence similar to the one already used on Stargate Atlantis, but being blue as it was in the movie and SG-1, whereas in Atlantis it's green. Stargate Universe uses a lighter shade of blue.

Throughout the run of the television franchise, it cost $5,000 to show a person stepping through the event horizon, using visual effects.

Known statements about wormhole physics
As portrayed in the fictional universe of Stargate, wormhole physics is a field of study that describes the functioning of stargates and wormholes. It was pioneered by Samantha Carter. In the episode "Upgrades", possessing super-human abilities Samantha Carter wrote an entire book on the subject, claiming she had been wanting to write it for a long time previously but had never been able to find the time as it was such an incredibly complex subject.
 * Energy to maintain an established wormhole can come from either side, though the energy to form a wormhole can only come from the dialing stargate.
 * Substantial gravitational force can pass through a wormhole from either side. (i.e., the effects of a Black Hole).
 * Outgoing wormholes can be affected by exterior gravitational and electromagnetic forces, causing them to connect to stargates other than their intended targets.
 * When traveling from one gate to another, exterior forces such as solar flares can cause the wormhole to connect to a Stargate in another time period. In this case, the wormhole will then sometimes, under some unknown circumstances, connect to the source stargate. (to source gate: "1969", "2010" and "Time", to another gate: "The Last Man", Stargate: Continuum).
 * Matter can only travel one way but weak energy such as radio communication can travel both ways through a Stargate. This can be explained by the hypothesis that the diameter of the wormhole is very small, possibly microscopic in size. Energy would have no problem passing through on its own but matter would need to be converted into energy and then re-integrated at the other side. Radio communications cannot pass through a wormhole traveling to a nine-chevron address (e.g. between Earth and Destiny, per "Twin Destinies.)
 * Artificial wormholes created by Stargates can only stay open for up to 38 minutes without an extremely powerful energy source powering the dialing gate. Such as "an infinite number of ZPMs or a black hole." (Dr. McKay, "First Strike") After 38 minutes relativistic effects come into play.  No matter can be transmitted through a wormhole after 38 minutes as it is presumed unsafe.
 * The Stargate Atlantis series finale demonstrates the wormhole drive, essentially taking the Stargate concept and applying it to an interstellar vessel. The drive allows Atlantis to move from the edge of the Milky Way galaxy to Earth in moments, similar to a Stargate, but uses a tremendous amount of power. It also requires precise calculation to successfully arrive at the target destination without destroying the ship.

Complexities of function
Both Stargate SG-1 and Stargate Atlantis introduce complicated facets of stargates to contrive more interesting plotlines. Some of these have been developed into integral parts of how Stargates function.

Matter transmission
Stargates only transmit entire objects at a time. Matter from an object that is only partially through the event horizon is stored in a "hyperspatial buffer". Until an object is completely through, it can still be pulled back out of the event horizon. Matter inside a buffer is effectively held in stasis. The buffer is normally wiped clean after the wormhole disconnects, at which time any matter still stored inside ceases to exist. The exception to this is if the wormhole is terminated artificially with an object already en route, in which case it remains stored in the buffer of the receiving Gate until the next wormhole connection. In such a circumstance, the object can be rematerialized by establishing an event horizon without opening a wormhole, done by removing the control crystal of the DHD.

Momentum is conserved through a wormhole, so an object will exit a Stargate at the same speed at which it entered. However, in early episodes (and the film) objects are seen to exit at a faster speed than entry. This is possibly explained by the 'freezing effect' also witnessed – heat energy is converted to kinetic energy, both cooling the object and increasing its velocity. These effects were countered during an upgrade of Stargate Command's dialing computer.

A Stargate is able to discern whether something is "trying" to pass through as opposed to pressure from its ambient environment. Thus, active Stargates in an atmosphere or underwater do not allow gas or water molecules through.

Gate obstruction
A wormhole is prevented from forming if a significant obstruction is present inside the Stargate's ring. Consequently, a simple way to seal a Stargate is to bury it, as was done to Earth's Giza Stargate. Another way of controlling travel through a Stargate is by placing a barrier just above the event horizon, which will prevent incoming matter from being reintegrated (essentially annihilating it). The Iris at the SGC and the Gate shield at Atlantis function in this way. These barriers also prevent the formation of the unstable vortex, although radio communication is still possible. In Stargate Universe, the Stargate at the Icarus base was "modified" to only allow outgoing wormholes, as any incoming wormhole could potentially further destabilize the already unstable Naquadria core of the planet, which the Gate had been tied into.

Power source
Power is always required to establish an outgoing wormhole, and is usually supplied by the DHD, but any Stargate can receive a wormhole whether it has a power supply or not; the dialing gate is the one that supplies power to both. In a few cases, Stargates have been dialed "manually" when more sophisticated means were not available. This was accomplished by providing sufficient raw power to the gate and then rotating the symbol ring by hand to encode each chevron. Power can be fed directly into the Naquadah that composes the gate; power harnessed from lightning strikes has been shown to be sufficient, as has the energy from the core of a naquadah bomb., however the minimum requirement for a stable wormhole appears to be a 12 or 24 volt current (depending on wiring) as shown in the episode "1969" where the team powers the gate using jumper cables hooked up to two military transport trucks. The Stargate that establishes an outgoing wormhole determines how long the wormhole is held open, and can generally close the wormhole at will. Under some conditions, a gate only needs enough power to connect briefly, then the receiving gate can provide enough power to maintain the connection. The same is true if the outgoing gate loses power while transmitting; if the incoming gate has a DHD, it will take over powering the gate until reintegration is complete. The System Lord Anubis possessed an Ancient weapon that would channel energy into a receiving Stargate through an open dialed gate; prolonged use of this weapon would cause the gate's Naquadah to continue to absorb the energy until a significant explosion would take place.

Secondary gates
Some planets are known to possess "secondary" or "backup" Stargates. The second Stargate is normally inactive, with the primary Stargate (defined by the presence of a functioning Dial-Home Device) receiving all incoming wormholes. If a Stargate experiences a power surge while an outgoing wormhole is open, the other end of the wormhole has been observed to "jump" to the next closest gate in the network. (The effect can also be used as a defensive measure, as seen in the episode "Prototype" or to close a connection with the receiving Stargate as demonstrated in the episode "A Matter of Time"). In the case of a planet with two gates, the closest is the inactive secondary gate. This scenario occurred in SG-1's first season, and resulted in a second gate being discovered on Earth, located beneath the ice of Antarctica.

The Antarctic gate was later revealed to have originally been the primary Stargate on Earth, built by the Ancients. The Stargate originally used in the SGC, found in Giza, was brought to Earth by Ra from another planet. Since Stargate addresses correspond to planetary locations and not individual gates, the new gate inherited the same address as the one in Antarctica. Because the Antarctic gate had been abandoned millennia earlier by the Ancients and no longer had a connected DHD, Ra's gate became the primary.

In the episode "Nemesis", SG-1 transports the Stargate from Stargate Command onto a crashing spaceship in order to escape. The gate is replaced at the SGC by the Antarctic gate. The original gate survives the crash, however, and the Russian military takes possession of it to conduct their own off-world travel. Because they were also in possession of a DHD (not found in the original Giza dig but recovered from Germany after WWII), which they activated and deactivated at pre-defined times, they were able to selectively become the primary gate. Using a strict schedule for returning teams, they were able to avoid detection by the US Air Force for some time.

In Redemption, the second Stargate was destroyed by Anubis. In order to continue their Stargate program, the US Air Force took on loan the Giza Stargate from the Russians at a hefty price, and this gate is now the only one left on Earth. This Stargate was eventually traded from the Russians for the Korolev.

In the episode, "Irresistible", it is revealed that Pegasus galaxy Stargates supersede the older, more outdated Stargates, such as the one on Earth. The first time this was encountered at the Midway Station with the Milky Way and the Pegasus gates, which was resolved by use of a programming macro. A similar situation occurred when Earth was unable to dial out from the SGC, due to a Wraith "super-hive" ship in orbit around the moon; with its own Pegasus Stargate, the programming of the newer gates forced it to take precedence over incoming wormholes to the older Stargates, and therefore prevented the Earth gate at the SGC from dialing out.

Durability
Stargates are very durable; the oldest in the Milky Way is probably the Antarctic Gate, 50 million years old yet still perfectly functioning; the power source may have been younger.

The Stargates themselves are extremely resistant to damage or destruction: in one case, a Stargate survived a direct hit from a meteor, while another was still capable of creating a stable wormhole while on a planet near a newly formed black hole. A Stargate has also been seen to continue functioning while entering a star, though it was protected by a portable forcefield for a portion of its journey. In the SG-1 fourth season episode "Chain Reaction", the SGC sent a naquadah-enhanced nuclear bomb to a planet whose surface had trace amounts of naquadah in its topsoil; the explosion destroyed the entire planet, yet the gate still remained open and intact. In the ninth season of Stargate SG-1 the United States develops a naqahdriah-enhanced "Gatebuster" nuclear bomb that is theoretically capable of destroying a Stargate (the "Mark IX"). However, when it was first used it failed to destroy the intended Stargate, as an Ori shield defending the gate was being powered by the weapon attacks attempting to destroy it.

However, a later attempt (in the episode "The Shroud") successfully destroyed an active Stargate in the Pegasus Galaxy (which is connected to the Ori Supergate) by placing a Mark IX warhead behind the gate, thus preventing the energy from being absorbed by the open wormhole.

One of the stargate's more useful features is also its biggest weakness: Its ability to absorb energy from almost any source has been used several times as a means to destroy an active stargate. If enough energy builds up in the gates superconductor, it can cause the naquadah to go critical effectively turning an active gate into a naquadah bomb as seen in the season 6 premiere where Anubis fired a weapon through an active wormhole triggering the detonation of Earth's stargate, which would have rendered the planet uninhabitable had the stargate not been successfully removed to outer space. Additionally several gates were accidentally destroyed in the Stargate Atlantis episode "First Contact" As an unintended side effect of the Attero Device build by the Ancient scientist Janus. While these stargates did not build up enough energy to threaten entire planets, the explosions caused severe local damage.

Susceptibility
The Stargate and its network is susceptible to computer viruses. In the episode "Avenger 2.0" the entire Stargate network is brought down by a virus created by Dr. Felger. Although not intentional, the virus was altered by Ba'al to take the network down. It was previously unknown that each DHD does periodic correlative updates by dialing other gates in close proximity to one another. The updates were designed to compensate for stellar drift to thus maintain the proper coordinates. The Stargate at the SGC uses a human developed "Dialing Computer" rather than a DHD which accounts for the lack of in depth knowledge regarding the DHDs. It is also what saved the network as the Earth Stargate was the only gate unaffected by the virus because it has no DHD.

Stargates are also susceptible to a precise form of subspace interference which causes active Stargates to build up energy until they explode, vaporizing much of the surrounding area for a significant distance. To date the only known source of the interference is the Ancient Attero device, which created it as a side effect to its primary purpose and has since been destroyed.

Other uses
In several episodes of the series, the Stargate Network was used for a purpose other than interplanetary travel. In the plots in which these extra functions feature, they are almost always discovered by a fluke, and were not intended in the design of the Stargates. Two such occurrences regard the Stargate's interaction with time, such as "1969", in which the SG-1 team accidentally travels backward in time to the year 1969, as a result of the matter transmission stream passing through a solar flare. The character Samantha Carter intentionally uses this phenomenon in the episode "2010", where she uses advanced technology to predict a flare and send a message back in time. Time is also a factor in the episode "Window of Opportunity", when a scientist uses a failed time machine built by the Ancients to isolate a region defined by 14 stargates from the rest of the space-time continuum, causing a time loop.

In several episodes, the Stargate, and the cobbled-together dialing program used by the SGC, are nearly the cause of disaster. In the episode "Red Sky", the bypassing of a system error caused the Stargate to introduce atoms of plutonium into the center of a star, causing the star to become unstable. In the episode "Ripple Effect", an unknown device or method was used to connect different realities and was reversed by use of an Asgard beam weapon.

Later in Stargate SG-1 a feature of the Stargate Network, whereby one stargate can be caused to dial multiple other gates simultaneously, is revealed. This allows a blast wave such as that of the Dakara Superweapon to extend almost indefinitely throughout the galaxy, as seen in "Reckoning".

In the episode, a Stargate was used as a weapon capable of destroying an entire solar system. The gate was dialed into a planet in close proximity to a black hole. The gate (protected by a force field) was then sent into a star. The resulting loss of mass caused the sun to go supernova, destroying the solar system and Apophis's fleet, which was orbiting the star at that time.

Features
Stargates are all held to be made of the fictional heavy mineral "Naquadah", a superconducting material which is also extremely durable. Milky Way stargates are held to be 64000 lb in weight.

Milky Way
Milky Way Stargates have 39 inscribed symbols on their inner ring. In the original Stargate film, the inner ring rotates until the dialed symbol is aligned with the appropriate chevron in the sequence, at which point the ring pauses, and the chevron separates and retracts to indicate it is engaged. Dialing then resumes. In the Stargate SG-1 series, the sequence is similar except that all symbols are dialed to the seventh (topmost) chevron, which is the only one that moves, and the chevrons glow orange as they are engaged. (The film prop differs slightly from the series Stargate, in that the topmost chevron is different in design from the others, and none of the chevrons glow.)

With 38 symbols, the Stargate Network in the Milky Way has (theoretically) 1,987,690,320 (38×37×36×35×34×33) possible 7-symbol addresses. However, since only a small fraction of these make up valid destinations, randomly dialing the Stargate is largely futile. If the person dialing does not know the point-of-origin symbol, there are many more possible combinations.

Because the gate on Earth was found without a DHD, the Stargate team on Earth developed the technology to interface with the gate in order to power it and dial it by the use of computers. Unlike a DHD, their version is essentially manual dialing, as the system physically moves the inner ring to lock in the address. Dialing on a DHD automatically locks in the symbols, making the DHD much faster at dialing. However, the SGC's dialing computer has the advantage of controlling access to the Stargate and storing working addresses, whereas a DHD can be used by anyone and has no easy to access list (a limited number of previously accessed gate addresses are stored in the DHD crystals, but the technology and knowledge to access them is not widespread).

Symbols at Giza
As mentioned in the original film, the symbols on the Giza Stargate correspond to constellations as seen from Earth, except for the point of origin symbol. The SG-1 season 7 finale reveals that each symbol has a corresponding syllable in the Ancient language, so that Gate addresses can be spoken aloud (e.g. the Earth symbol is pronounced "at" in Ancient).

†This symbol is supposed to be unique to the planet Earth. In the movie, Dr. Jackson interprets it as representing the Sun over the peak of a pyramid. Other planets are described as having their own unique origin symbols. That said, certain other stargates, including the one on Icarus Base and the one on Klorel's mothership in the Season One finale, included the Earth Origin symbol. In those cases, the Earth symbol replaces another glyph, implying the possibility that Earth is a legitimate coordinate in off-world Stargate dialing, and that certain Stargates may not be able to dial all others due to missing glyphs.

Pegasus galaxy
In the spinoff series Stargate Atlantis, an expedition dials the 8-symbol address from Stargate Command to travel to the Ancients' Lost City of Atlantis, located in the Pegasus galaxy. They find that the Ancients seeded planets throughout the Pegasus galaxy with Stargates, too. Unlike the gray and red Milky Way version, Pegasus Stargates have a brighter silver and blue color scheme. Functionally, the Pegasus Stargates are unique in that their inner ring does not move. Instead, the symbols flash in a circular pattern until reaching the relevant points. This makes manual dialing of Pegasus Stargates impossible. The alternative Stargate design in the Pegasus galaxy is one of many things the producers of the shows employed to differentiate between SG-1 and Atlantis.

Another important difference between the two forms of Stargate is that Pegasus gates supersede Milky Way gates when in close proximity; this may be because Pegasus Stargates are a newer model, having been created after the Ancients left the Milky Way. This is demonstrated in the Stargate Atlantis finale "Enemy at the Gate", where a Wraith Super-Hive brings a Pegasus Gate with them to Earth, thereby intercepting any incoming travelers.

Pegasus Stargates contain 36 symbols as opposed to the Milky Way's 39. Seven symbols are still required to dial an interplanetary address, adhering to the same constraints as a Milky Way gate. With 35 symbols, the Stargate network in the Pegasus galaxy has 1,168,675,200 (35×34×33×32×31×30) possible 7-symbol addresses.

The only Stargate in the Pegasus galaxy capable of reaching Stargates in the Milky Way is the one in Atlantis. This is due to a special "control crystal" in the Atlantis DHD, without which a Pegasus Stargate cannot encode its eighth chevron. The crystal is not restricted to be used only in that particular DHD, as in "Home", the crystal was temporarily removed and installed into the "normal" DHD of M5S-224 by McKay.

Some Pegasus Stargates are placed in orbit around a planet (sometimes called "Spacegates" in the show) rather than on its surface, something that has never been seen in the Milky Way. Ships such as the Ancient puddle jumper and the Wraith dart are designed to fit through Stargates, and have built-in DHDs. Orbital Stargates are kept in place by the three station-keeping thrusters placed equidistantly around the circumference. As seen in the Atlantis season 5 episode "Ghost in the Machine", they will correct for sudden impacts to the gate, such as being hit by a Puddle Jumper.

Symbols at Atlantis
The symbols depicted on the Atlantis gate again correspond to constellations, although in this case the constellations are fictional. The Atlantis glyphs do have names, as seen in a shot of McKay's laptop in the episode "Sateda".

†As with SG-1, this origin symbol is unique to the stargate at Atlantis, and other stargates have their own unique origin symbols as well. Atlantis has since been moved; it is unclear if this has changed the origin symbol, since it is a constellation rather than an abstract as it is on SG-1.

Destiny
In Stargate Universe, a group of civilians and military dials the 9-symbol address           from Icarus Base to escape to the Ancient ship called Destiny. Unlike other addresses, this set of symbols specifically uses the symbol for the final chevron instead of the local point of origin. This version of the Stargate appears to spin the entire ring of the gate, locking the relevant symbols in as they reach the top. Destiny Stargates contain 36 symbols like Pegasus Stargates. The worlds seeded with Stargates ahead of the Destiny use the same model. The Destiny Stargate is possibly the oldest model of Stargate used by the Ancients, and as such is incapable of dialing anywhere in the galaxy as an ordinary Stargate can. Instead, the ship has to move within range of other Stargates in order to dial them.

Similar to Atlantis, the Destiny has a built-in dialing computer which can automatically dial the gate and stores a list of addresses. It locks out certain addresses, presumably because they are either dangerous or inconsequential to the current problem. Two expedition members who went to one of the locked-out addresses could not be contacted upon redialing. The Destiny also has hand-held dialing computers for those that travel through the Gate to make use of, as there do not appear to be on-site DHDs thus far. Gates on planets are each equipped with a platform, which transmits information about planet through subspace to Destiny. Since DHDs are absent, it is presumed the platforms are used to power the Gates.

It is doubtful the glyphs on the Destiny Stargate are related to constellations, as Destiny can dial addresses in numerous galaxies, and each galaxy would have its own unique set of constellations. These glyphs likely incorporate some form of ancient Ancient numeral system (the glyphs are similar to a mix of Morse code and Arabic/Chinese numeral systems on Earth) that calculates the direction and distance for the wormhole to go relative to Destiny's position in space from the destination Stargate.

Other variants

 * Orlin's Stargate In the Stargate SG-1 episode "Ascension", the outcast Ancient, Orlin, builds a miniature Stargate in Samantha Carter's basement. Its stated components included 100 pounds of pure raw titanium, 200 ft of fiber optic cable, seven 100,000 watt industrial strength capacitors, and a toaster. This gate was hooked up to the main power supply of the house and only connected once, to Velona, before burning out.
 * Tollan Stargate In "Pretense", the advanced Tollan civilization is shown to have a new Stargate, built with the assistance of the Nox. Jack O'Neill sarcastically mocks the Tollan gate, saying "Ours is bigger". The Tollan Stargate is destroyed by the Goa'uld in "Between Two Fires".
 * Ori "Supergate" (main article) In the ninth season of Stargate SG-1, the Ori were introduced as the new main enemy for the show. The Ori employ extremely large Stargates to move their fleet of warships from their home galaxy to the Milky Way. Dubbed "Supergates", these devices are composed of 90 individual segments and are powered by a quantum singularity.
 * McKay-Carter Intergalactic Gate Bridge (main article) Introduced in season 3 of Stargate Atlantis, the Gate Bridge is a chain of Stargates placed between the Milky Way and the Pegasus galaxies, allowing movement between Atlantis and Earth without the need for a ZPM or the Daedalus. Halfway along the Bridge is Midway Station, where travelers switch from one galaxy's gate system to the other. The Bridge is hijacked by the Wraith in the episode, and the Midway Station is destroyed as a result of Wraith tampering in the control systems. As a result of the destruction of the Midway Station, the gate bridge has since become inoperative.
 * Asuran satellite weapon (main article) In the Stargate Atlantis season 3 finale, "First Strike", the Asurans send a satellite weapon to attack Atlantis in response to the Apollo ' s bombing of their homeworld. The weapon consists of an eight-chevron Stargate, hyperdrive, shield, and a navigation system. Once it reaches its target, the Stargate activates and the Asurans fire an energy beam through.

Making of the props
Two full Stargate props were originally built for the SG-1 pilot, the second of which was reconstructed from the prop used in the film. They are made of steel and fiberglass, and are 15 ft in diameter. The second prop is less detailed, and is used for exterior scenes; in the pilot it was used solely on the planet Chulak. The primary one is fully automated and capable of rotating and emitting light. This is achieved by the use of a specially designed 15 ft circular gear, which turns the inner ring on a precise pinion drive wheel, using an eight horsepower electric motor. The top seven chevrons emit laser pulses which are read by a sensor fed into a computer responsible for the gate's movement, which is consequently able to start and stop the rotation very quickly. This main prop is kept almost immovably at the permanent set of the SGC, at Bridge Studios, Vancouver.

There are further Stargate props which are no more than two-dimensional or semi-three-dimensional (jar-lid shaped) Stargates, being more lightweight and easier to erect on location. These are always filmed front-on to preserve the illusion. If a shot involves the iris, this is added in post-production, as the mechanics of it opening and closing would be very difficult to build. However, when a Stargate is filmed with just a closed iris (i.e. without it moving), a tangible prop is inserted into place.

The visual effects for Stargate SG-1 and Stargate Atlantis are predominantly produced by Rainmaker Digital Effects, a notable visual-effects studio. However, some effects, including the entire Ori battle sequence in the episode, were done "in-house". The unstable vortex effect, both in the film and the early seasons of the series, on account of being "difficult to achieve" was generated only once and recorded from various angles; this recording was the same used for all gate opening shots early in the series. Rather than being a jet of water, it is actually the image of high-pressure air being blasted into a tank of water. The effect was achieved by mounting a jet airplane engine two feet above a water tank, and using its 180 mi/h windstream to create the sufficient water displacement. In post production, the surrounding water was removed with computer editing, and the image of the air-jet pasted into the center of the opening stargate. This technique was only used for earlier episodes, and the effect was replicated digitally soon after to allow more flexibility in shots.

To cut down on costs, the opening of a Stargate is often just implied rather than shown, by a costless sound-effect followed by distinct lighting effects characteristic of light shining through water (as the event horizon is depicted). The DVD commentary for Stargate SG-1 explains that these effects are produced by reflecting light off large sheets of vibrating Mylar.

The Stargate itself is nearly always filmed against a blue or green backdrop, not only making it easier to paste the vortex imagery onto the scene, but also facilitating the superimposition of the "event horizon ripple effect", which is entirely computer-generated. However, if a shot only involves an open wormhole without anyone stepping through it, the crew may choose to use a "practical puddle," which is simply a back lit screen placed in the gate displaying a video of the wormhole effect. This only works, however, on a darker set, as otherwise the projection will get washed out. On occasion, the Stargate itself is also completely swapped out for a computer generated model, usually in cases where it is being moved, or is depicted in space. Series producer Robert C. Cooper explained that it often costs a lot to erect a Stargate on location, and so in some cases offworld gates are also entirely a visual effect.