Alternate reality

Alternate Realities, or parallel universes, are a concept that stems from Hugh Everett's many-worlds interpretation of quantum mechanics. It is believed there are an infinite number of different realities, some more similar than others. Sometimes there are the same people, places, and objects, but with notable differences. 

Summary
Two major timeline theories have been shown to exist: (1) alternate realities (also known as alternate universes or parallel universes) and (2) alternate timelines. Alternate realities are presented when various versions of SG-1 live in parallel universes, some almost identical; others, so extremely different so as to be unrecognizable. Alternate timelines are presented when it appears that the timeline in which "our" SG-1 exists is modified by a time traveling event.

Multiverse Theory
In a Multiverse, there exist a near-infinite number of universes, or realities, that are each created to reflect a different outcome to a decision. One could use the fork in the road as an analogy, where going in one direction defines the individual's current reality, whereas the roads not taken represent the alternate realities. These realities are progressing through time at the same rate as ours, but different paths are being taken, thus the term parallel universes is also used to describe them. Additional terms for "alternate realities" have included "alternate universes", "quantum universes", and "parallel worlds", and all have basically been used interchangeably, even though there are some differences in their definitions. But in the world of Stargate SG-1, these differences have not been expounded upon, and thus, the two more widely used terms "alternate reality" and "parallel universe" describe the same thing.

Some of these alternate realities have been created through the impact of a time traveling event (see Alternate Timelines), but most were the results of a decision so that all possible outcomes have their own realities. For example, one alternate reality included Daniel Jackson's turning down Catherine Langford's offer to translate the Stargate's coverstone instead of taking up the offer and solving the riddle of the Stargate. From that one event alone, several alternate realities were most likely created, carrying forward the idea that someone else opened the Stargate and went on the first mission, resulting in various realities where Ra was not defeated, and so on.

Entropic Cascade Failure
Traveling between alternate realities comes with some problems, the main one being Entropic Cascade Failure. This phenomenon results when two people representing the same person, such as two Samantha Carters, exist in the same reality at the same time, and is when the visiting person, the one out of his/her own reality, will start to suffer a physical breakdown at the cellular level due to temporal distortion. A traveler who has no living counterpart will not suffer from this breakdown.

Based on this definition, the more "in sync" two realities are, perhaps having only a few differences in their "paths", the less the chance becomes that the traveler will suffer from these fatal effects. Entropic Cascade Failure was observed when the civilian Dr. Samantha Carter who worked at the SGA visited SG-1's reality, but not when several Carters who were in the Air Force and members of the SGC's SG-1 team visited.

Forming the Bridge
In order to travel to an alternate reality, one must form a link, called a "trans-universal bridge" or the more commonly used "inter-universal bridge", between the two realities. Various means of creating the bridge between realities have been discovered by the SGC.

SG-1 was introduced to the existence of alternate realities when they discovered the Quantum Mirror on P3R-233. This device appeared to be constructed of naquadah, the same material used to build the Stargates, and might have been the creation of the Ancients, but its actual origin was never determined. Using the hand-held remote control of the Quantum Mirror, an individual could "dial" an alternate reality, touch the mirror-like device, and be transported into that reality. As long as the link between the two realities was maintained, a traveler could go back and forth, but if the Mirror was turned off, the traveler would have to use a control device in their current reality to find the reality from which they had originally traveled. The method of determining the correct reality was purely by visual confirmation through the Mirror, as neither the Mirror nor its remote control seemed to record the placement of the originating reality or the last reality dialed.

Gen. George Hammond commanded that the Quantum Mirror in his reality be destroyed because he felt that there were some lines that were never meant to be crossed ; but other means of inter-universal travel were later discovered. If one were to direct a controlled energy blast within a wormhole traveling through a black hole, one could actually open bridges among several alternate realities and another, making the wormhole's destination reality a point of confluence. This effect was reversed at the point of confluence with the use of an Asgard energy weapon discharged in the wormhole connected to the original point of origin's corresponding Stargate. Because the Stargate's wormhole was used to create the "inter-universal bridge," travel was only one-way, or uni-directional.

Another means of creating a unidirectional inter-universal bridge was developed by Dr. Rodney McKay and his sister, Jeanne Miller, to draw zero point energy from a parallel universe. A traveler could traverse this uni-directional bridge (from the reality from which the energy was being drawn) if he was beamed across with Asgard beaming technology while wearing an Ancient personal shield. This attempt to draw zero point energy into a containment field created universe-destroying exotic particles, and their research had to be abandoned. Similar research was done in an alternate reality by Maj. Samantha Carter and Doctor Bennett, but that research was destroyed in a power overload when they accidentally tapped into SG-1's reality and drew Lt. Col. Samantha Carter into their reality while she was inside a contained field and out of phase. Maj. Carter and Dr. Bennett were totally annihilated in the explosion that resulted when Carter was drawn in. With the help of an alternate Dr. Rodney McKay, Carter was transported back to her own reality through another inter-universal bridge, but how her specific reality was determined as this bridge's destination was not revealed.

Within Stargate SG-1, several alternate realities and timelines have been glimpsed briefly throughout the series, providing an insight on how several alternate realities may have turned out.

SG1: "There But for the Grace of God"
Daniel Jackson is transported to a different reality via what is later called the Quantum Mirror.

Differences

 * The Stargate program's headquarters is known as the SGA.
 * Earth is currently under attack from the Goa'uld and is slowly being wiped out.
 * Daniel was never a part of the Stargate Program, his alternate self having rudely turned down Catherine Langford's offer to work on the Stargate. He is also highly likely that he is dead as he lived in Egypt, which was hevily bombarded by the Goa'uld ships.
 * Teal'c is also not a part of the Program, and has remained the First Prime of Apophis and is currently leading the assault on Earth. Any chance of him turning was ended when the SGA sent a nuclear explosive to his homeworld Chulak and desimated his people and family.
 * George Hammond is a colonel rather than a general.
 * Jack O'Neill is not a colonel, but rather a general and commanding officer of the SGA.
 * Samantha Carter has not joined the military and is a civilian scientist working for the SGA.
 * Jack O'Neill and Samantha Carter are engaged to be married.
 * Catherine Langford is still working as part of the SGA.

SG1: "Point of View"
An alternate Samantha Carter and Charles Kawalsky come through the Quantum Mirror, fleeing Apophis's takeover of their Earth.

Differences

 * The Stargate program's headquarters is known as the SGA.
 * Charles Kawalsky is still alive and working for the SGA.
 * Samantha Carter did not join the military and works as a civilian scientist for the SGA.
 * Samantha Carter discovered how to open the Stargate rather than Daniel Jackson.
 * Samantha Carter and Jack O'Neill had been married for just over a year.
 * Teal'c is still the First Prime of Apophis and, apparently, is very loyal to his master.
 * The uniforms worn by Apophis' Jaffa are different.
 * Teal'c had a beard, as does Apophis. (This may be a nod to the Star Trek episode "Mirror, Mirror," where Spock's mirror counterpart had a beard.)
 * The symbols on the dialing computer are yellow instead of blue.
 * The people of this reality never met the Asgard, only coming into contact with them when Dr. Carter goes through the Stargate to ask for their help in freeing Earth from Apophis.

Other Realities Glimpsed in this Episode

 * At least two realties where the Quantum Mirror rests in an apparently undisturbed Cheyenne Mountain.
 * At least two realties where the Quantum Mirror rests in a Goa'uld-controlled Cheyenne Mountain.
 * At least two realties where the Quantum Mirror rests in Area 51. One of them is on a Goa'uld-controlled world.
 * One in which the Quantum Mirror rests in an apparently Earth-bound laboratory.
 * One in which the Quantum Mirror rests on the beach of an alien planet.
 * One, apparently identical to our reality, except for the fact Carter is still a Captain.

SG1: "Ripple Effect"
Several different realities are discovered and several teams of SG-1 come through the Stargate. Two realities is mostly seen on screen, where the command center is also called SGC, as in our reality. The "real" SG-1 team is distinguished as wearing "green" SGC uniforms throughout the episode.

Alternate Reality #1

 * Samantha Carter is married and has been on a honeymoon.
 * The Ori successfully planted a second beachhead in our galaxy.
 * During the second Ori incursion, at least two planetary systems, including Dakara, were destroyed.
 * The Prometheus has recently been lost in battle.
 * Selmak/Jacob Carter is still alive.
 * Bratac is the leader of the Free Jaffa.
 * Earth does not have access to a functioning ZPM.
 * This team was distinguished by wearing "Black" SGC uniforms.
 * This team is desperate enough that it caused the problem with all of the alternate realities bleeding over into ours in order to steal the Prometheus and Atlantis' ZPM to protect their Earth with the weapons chair in Antarctica. They fail and are sent home with nothing.

Alternate Reality #2

 * Samantha Carter is currently on maternity leave.
 * Both Janet Fraiser and Martouf are still alive; SG-1 consists of Martouf, Daniel Jackson and Cameron Mitchell. Fraiser is accompanying the mission, but wears an SGC patch.
 * General O'Neill is still Commander of the SGC.
 * Carter and Martouf had a romantic relationship, which led to Martouf joining the SGC and the two living together for a time. The relationship did not work out, however, and Carter is currently involved with someone else.
 * An Ori plague is ravaging Earth, with hundreds of thousands of people dying. As the team prepares to return, General Landry provides Fraiser with the antidote developed in our reality.
 * This team was distinguished by wearing "Camo-Desert" SGC uniforms.

Other Realities Glimpsed in the Episode
At least 17 different SG-1s were stranded at the SGC during this episode. After the arrival of the first, "black" SG-1, General Landry ordered that all further teams be instructed not to proceed through the gate, making exceptions only for those units that were under immediate fire and could not afford to shut down the gate and dial out again.
 * The second alternate SG-1 to arrive, distinguished by wearing "Tiger-Blue Camo" SGC uniforms, arrived from a firefight offworld. Its composition appears identical to the SG-1 of our reality, but its members are much more heavily armed than our universe's counterpart; notably, Teal'c is carrying a staff cannon. This team was still being examined (or examined again) in the SGC's infirmary after several other SG-1's had arrived.
 * Another SG-1 team, this one in green camo uniforms, can be seen in a side room as Teal'c, Daniel, and Landry passed by them. Mitchell and Carter were the only identifiable members of the team.
 * Another SG-1 team is seen exiting an elevator, consisting of Colonel Mitchell and three men who appear to be Jaffa. They also apear to be wearing the bracers from the episode upgrades
 * Another SG-1 team, in hazmat suits passes by as the "real" Teal'c, Daniel, and Landry goes off-screen. This Colonel Mitchell turns to look after Landry, Daniel and Teal'c, then smiles and countinues walking down with his team.
 * Samantha Carter appeared to be a member of nearly every alternate SG-1. In the SGC's science lab, a room full of "Alternate-Carters" can be seen, some isolated and by themselves, others working together to help find a solution on the problem. The "real" Carter along with the "Black Carter" from the first SG-1 team who arrived, converses with Doctor Lee on finding a solution to return the alternate SG-1's back to their realities, just before the "Alternate" Martouf steps in.

Deleted Scenes

 * This episode was originally too long, so many scenes had to be edited and cut out for time. Joseph Mallozzi managed post the script sections of all cut scenes in his producer installment on Gateworld.

ATL: "McKay and Mrs. Miller"
An alternate version of Dr. Rodney McKay (known as "Rod") travels to our universe, seeking to shut down an experiment that might destroy his.

Differences
Since Rod's reality was never viewed directly, nearly everything we know about it is based on his description. No indication was given that Rod was lying, although our universe's Colonel Sheppard did state that he found him "creepy."
 * Both Earth and Atlantis have working Zero Point Modules.
 * Though Sheppard's team appears to have the same composition, the personalities of the team members are somewhat different: in Rod's words, Sheppard is an active member of Atlantis' Mensa chapter and something of a know-it-all that makes the McKay of our reality look humble, and Teyla is hard to talk to (though Ronon, apparently, is about the same). Rod himself is more easygoing and personal than our universe's McKay, and an honorary member of the Athosian council.
 * Jeanne Miller has three children; Rod, their godfather, carries photographs of them in his wallet.
 * The Ancient personal shield found in Atlantis has not yet exhausted its power source (or they found another one.)

SG1: "The Road Not Taken"
In this reality, Anubis' attack on Earth forced the Stargate program into the public eye, and chaos ensued.

Differences

 * Hank Landry is President of the United States; his offices have been moved into Cheyenne Mountain for security purposes.
 * Major General George Hammond is commander of the SGC..
 * Major Evan Lorne is the leader of SG-1.
 * Doctor Daniel Jackson had recently been captured by the Ori forces. (This was at the same time that Daniel Jackson was undercover as a Prior in the main universe, so this may be the same scenario.)
 * Samantha Carter was never promoted to Lieutenant Colonel and served on SG-1 until she was killed while trying to draw zero-point energy from alternate universes.
 * Rodney McKay apparently never went to work for the U.S. Air Force, instead making millions in the dot-com boom. He and Carter were divorced.
 * Teal'c left the SGC several years earlier.
 * Vala Mal Doran was imprisoned at a cell in Area 51.
 * Cameron Mitchell became a national hero after the battle with Anubis' forces over Antarctica, but was cut loose after he refused to support the harsh policies of the American government. He left the military and is currently wheelchair-bound and quite bitter.
 * Though Jack O'Neill is never mentioned, the sequence of events suggest that he may not have ever been removed from the Ancient stasis pod in Antarctica.
 * The Prometheus is still operational, and functions as Air Force One.
 * When the Stargate program became public as a result of Anubis' attack, panic and widespread riots forced the president to declare martial law. The United States' diplomatic relations were severely damaged as well; the Russian Federation took control of the Ancient outpost in Antarctica, though American forces managed to remove the Ancient control chair.
 * Widespread unrest led Congress to suspend the electoral process; the president is now re-elected by plebiscite, in which voters are given a simple yes or no choice as to whether he should remain in office.
 * Earth has managed to defeat an attack by an Ori mothership using the control chair, but its ZPM was nearly depleted in the process. The Ori subsequently sent at least five more ships to Earth, but our universe's Carter was able to use Merlin's phasing device to hide the planet from their attack.
 * The Alpha Site has been abandoned, and Earth has cut ties with off-world allies including the Jaffa.
 * Technology derived from the Stargate program is on display in normal planetary operations as well; American F-302s were used to bomb an alleged terrorist training camp in Ireland, and the U.S. Secret Service employs Goa'uld pain sticks and Zats.

ATL: "The Daedalus Variations"
In one universe, the Earth ship Daedalus was installed with an Alternate reality drive, where it kept jumping to other universes, including our own.

Known universes

 * In one universe, Rodney McKay had developed the Alternate reality drive and fitted it on the Daedalus from that universe. Colonel Sobel was in command of the Daedalus rather than Caldwell, and she and her crew explored other realities.
 * In one universe, Sobel and her crew had to flee the Daedalus after they couldn't figure out a way how to stop the alternate reality drive. They arrived over an abandoned, yet habitable planet, which happens to be M35-117 from our reality.
 * One of the alternate Sheppard's team found the abandoned Daedalus and later found themselves stuck for several weeks. They couldn't find a way out and eventually ran out of supplies. Eventually they starved to death.
 * In the first universe our Sheppard's team visited, Atlantis never made it M35-117. The city was either still on Lantea, or the Atlantis Expedition may not have been formed in the first place.
 * In the second visited reality, their Daedalus was destroyed during a Replicator attack sometime in 2006. Atlantis was on M35-117 and had apparently made enemies with an unknown race of aliens, where one of their flagships attacked Atlantis. The expedition retaliated by sending an F-302 assault against them, and protected the alternate Daedalus. Sheppard in this reality was also a pilot and held the same rank of Lieutenant Colonel.
 * In the third visited reality, the star of M35-117 was turning Red giant and swelled in size roughly 48,000 years before it was supposed to.
 * In the fourth visited reality, a massive object of some sort impacted M35-117, destroying the planet. All that remained was a field of asteroids. This may have occurred thousands, even millions of years ago.

ATL: "Vegas/Enemy at the Gate"
In an alternate reality very similar to ours, John Sheppard had recieved a dishounarable discharge from the military. The Rodney Mckay of this reality apparently met an alternative version of Sheppard in a situation similar to the events of "McKay and Mrs Miller". However this reality is distinct from Rod's reality and it was not our reality's Sheppard that Vegas's Rodney met. When a Hive Ship attempted an attack on Earth, it was quickly destroyed by the powerful Drone weapons of the Antarctic outpost. One of the darts was shot down by a squad of 302s and so ignored by the drones. The pilot survived and pretended to be human to try to send a signal to the wraith in Pegasus to rescue him.

The signal transferred into our reality, informing the ZPM powered Hive Ship of the location of Earth and warning them of the drones protecting it as well as the location of the Control chair at Area 51. This led to the Battle of the ZPM powered Hive Ship which ended when John Sheppard destroyed the Hive Ship above Earth with a nuke planted on the inside.

Also See

 * Alternate timeline
 * Quantum Mirror
 * Matter bridge