Alternate timeline

Alternate timelines are an alteration or change in the current timeline, causing an alternate reality to form in the universe. An alteration in a timeline is usually caused by altering the past through Time travel, which manipulates, shapes, and forms the current present and future. This results in a different outcome, and gives birth to a new alternate reality or universe.

It is possible to restore a timeline to its original state by returning to the past and correcting the damage from there.

SG1: "2010"
Ten years into the future, the Goa'uld have been defeated and the Stargate program is public. Thanks to the Aschen, Earth is safe and humans are being given an anti-aging vaccine. But not all that glitters is gold. This timeline is negated due to the actions of the team from this reality. The Aschen are encountered in the present timeline, but the note sent back makes the team wary and they learn the truth early on.

Differences

 * The Stargate Program has become public knowledge.
 * Stargate Command is a museum; the Stargate is located in the J.R. Reed Space Terminal, a large public complex not unlike an airport or train station.
 * Robert Kinsey is President of the United States.
 * Samantha Carter and Jack O'Neill are both retired from the United States Air Force.
 * Samantha Carter is married to Joseph Faxon, the ambassador to the Aschen.
 * SG-1 is dismantled, with O'Neill estranged from his former teammates due to their difference in opinion on the Aschen's intentions.
 * George S. Hammond has been dead since 2004; it is suspected by Samantha Carter, Dr. Janet Fraiser, Dr. Daniel Jackson, and Teal'c that he may have been murdered by the Aschen in order to preserve the secret behind the anti-aging vaccine.
 * The Aschen were met on their homeworld by SG-1 but have been attempting to take over the Earth through attrition
 * All of SG-1 is now dead. The only surviving member of the group that attempted to change the future is Fraiser as she escaped through the Stargate to Chulak before the attempt began to alert Teal'c.

ATL: "Before I Sleep"
The Atlantis expedition arrives in the Lost City to find its power supply failing, and are trapped undersea as the city floods, this is in fact the original timeline.

Differences

 * All three Zero Point Modules that powered Atlantis had been running in tandem since the Ancients abandoned the city, and were all but depleted when the expedition team arrived.
 * The city's control systems failed to initialize, presumably for lack of power.
 * Colonel Marshall Sumner drowned along with several security teams searching the city before they could be recalled to the control room.
 * Atlantis' failsafe mechanism took the form of emergency bulkheads that sought to close off vital sections of the city; unfortunately, these had the effect of trapping still more members of the expedition with water still rising.
 * Major John Sheppard, Dr. Elizabeth Weir and Dr. Radek Zelenka are the only members of the expedition to escape the city, when Sheppard inadvertently activates a time-travel device inside one of the Puddle Jumpers.
 * The time machine takes them into orbit around Lantea during the siege by the Wraith; the Jumper crashes and Sheppard and Zelenka are killed, but Weir is rescued by the Lanteans and lives out her days on Atlantis, simultaneously setting up events so that a 'fail-safe' is installed that will cause Atlantis to rise to the surface rather than be flooded when power drops to a critical level. These events are not part of an alternate timeline, as they are in fact in the past of "our" timeline.

SG1: "Moebius, Parts 1" and "2"
Two alternate timelines result from this episode. First, Dr. Daniel Jackson discovers that Ra was in possession of a Zero Point Module during his reign on Earth, and Brigadier General Jack O'Neill takes SG-1 back in time in order to retrieve it. Then, SG-1 is stranded in ancient Egypt, where it leads the rebellion and forces Ra to leave Earth, but inadvertently causes Ra to take the Stargate with him. As a result, the Stargate program never came into being.

Differences

 * Daniel Jackson teaches English as a second language.
 * Samantha Carter is not a member of the Air Force, and holds little of the confidence she is very well-known to have.
 * Jack O'Neill is retired from the Air Force and lives on the fishing boat S.S. Homer (named after his favorite show The Simpsons).
 * Teal'c is still First Prime of Apophis, but still has his many doubts as to whether or not his master is a god.
 * George S. Hammond is a Brigadier General and still commanding officer of the Cheyenne Mountain facility.
 * Charles Kawalsky is still alive, though he dies before he can escape Chulak with the rest of the team.
 * Rodney McKay is still working for the Air Force, and is involved with research on the Time Jumper. He is also apparently not allergic to citrus.
 * Robert Kinsey is President of the United States whereas Henry Hayes is.
 * After O'Neill, Carter and Teal'c travel back once more to ancient Egypt, they manage to orchestrate the rebellion in such a way that the Stargate remains on Earth. This changes the "real" reality that exists from these episodes, onward. The timeline after SG-1 went back in time to retrieve the ZPM had these differences from the one that existed prior to these episodes:
 * The rebellion in ancient Egypt happened somewhat earlier; though it's unclear what difference, if any, this made.
 * The "original" fate of Ra's ZPM was unknown, but it's assumed that he took it with him when he left Earth. In the new timeline, it was buried with the camera, preventing any need to go back in time in the first place.
 * Because of the ZPM, Earth was able to send reinforcements to Atlantis. In the original timeline the expedition team presumably would have followed through with its original plan to abandon and destroy the city.
 * There were fish in Brigadier General Jack O'Neill's pond. At the beginning of the episodes, and in the videotape, O'Neill admits that there are no fish in his pond, and in all episodes prior to these, when shown fishing, no one has any success.
 * The fishing scene in "Threads" didn't happen until the end of "Moebius".

SG1: "Unending"
SG-1 is trapped aboard the Odyssey for 50-60 years in a Time dilation field, as the team seeks a means to prevent the ship's destruction by an Ori energy beam weapon. During this period, Dr. Daniel Jackson and Vala Mal Doran became romantically involved, Major General Henry Landry cultivated a garden in his quarters, and Lt. Colonel Samantha Carter learned to play the Cello. Landry died of old age after about twenty years; all of SG-1 save Teal'c are killed when the Ori beam finally struck the ship. Teal'c used the energy to travel back in time and prevent the time dilation field from ever being established.


 * From Teal'c's perspective, all these events remain part of his past; but as far as everyone else in the universe is concerned, they belong to a timeline that has not and will not occur. For his part, Teal'c has declined to discuss any of these events with the rest of the team.

ATL: "The Last Man"
Lt. Colonel John Sheppard travels 48,000 years into the future to find a timeline where Michael Kenmore took over the galaxy and all of his friends but Dr. Rodney McKay and Dr. Jennifer Keller were killed in the war. This timeline was changed when Sheppard traveled back in time, rescued Teyla Emmagan and destroyed Michael's cruiser and army over M2S-445 and was prevented forever when Michael was later killed by Teyla.

Differences

 * Sheppard arrives in the future Atlantis and meets a holographic version of McKay, before he returns to the present, creating the second (current) timeline.
 * Michael Kenmore is presumably still alive and has taken over everything.
 * Teyla Emmagan was killed by Michael after her baby was born and he was taken and used to perfect Michael's Hybrids.
 * Ronon Dex and Todd died fighting against Michael's Hybrids together.
 * The next 304 is named the Phoenix (in the new timeline its renamed the George Hammond in honor of Lt. General George S. Hammond who died of a heart attack.)
 * The Phoenix is destroyed in the Battle of the Phoenix and Colonel Samantha Carter is killed in the destruction.
 * An unknown amount of humans die as a result of the Hoffan drug.
 * Michael destroys the Wraith, exterminates the weak of the galaxy's human populations and turns the strong into more of his Hybrids.
 * After Carter's death, Richard Woolsey orders all support pulled out of helping other worlds and focus on protecting Atlantis leaving at least one ship in orbit at all times. This causes Keller and McKay to quit.
 * Keller and McKay return to Earth on the Daedalus and start a relationship. McKay gets a job with an aerospace firm and Keller starts her own medical practice until a year later she falls fatally ill from complications due to repeated exposure to the Hoffan drug and she dies.
 * McKay spends the next 25 years trying to come up with a way to change the past and succeeds ultimately.
 * Evan Lorne is now a Major General and commander of Stargate Command. The Atlantis expedition is still going on and McKay convinces Lorne to help him get back there and set his plan in motion.
 * McKay implements his plan and then dies at an unknown point in time of presumably natural causes.
 * At some point at least 25 years after McKay left the first time, Atlantis is abandoned for unknown reasons.

Stargate: Continuum
Ba'al travels back to 1939 and sends Achilles, the ship carrying the Stargate to America, to the North Pole and heads out into the galaxy to create his own empire. In the new timeline, Cameron Mitchell doesn't exist- his grandfather was on the ship carrying the Stargate and was killed before he could have children-, Samantha Carter is dead, Charlie O'Neill is still alive, and the Stargates were never found. Carter, Mitchell and Dr. Daniel Jackson end up here due to being in transit in the Stargate at the time of the changes and are thus protected from them. Daniel loses his leg to frostbite and the team are not allowed to travel through the Stargate and set things right and are forced to lead normal lives for a year. The gate in Antarctica is found, but is not figured out how to use. Meanwhile the Russians find and take the Gate from the North Pole. Ba'al launches an attack with Teal'c as his First Prime and Vala Mal Doran (as Qetesh) as his queen. He now commands all of the System Lords and with their combined forces, comes after Earth believing to be unaware of the Goa'uld as he doesn't know SG-1 is there. He attempts to present an air of benevolence to President Henry Hayes by calling him on a satellite phone and setting up a time to meet face to face, but is killed by Qetesh before he can finish. He also has the Gate in Antarctica and the Antarctic outpost destroyed under Qetesh's forcing before his death leaving Earth with no real defenses. Qetesh plans to use the Solar observation outpost to gain power herself but is thwarted by Daniel, Teal'c, Carter, Mitchell and two other Jaffa who get to Praxyon before she does through the Gate that the Russians have and use the time travel device to set things right, but as Teal'c dies, he kills her with a grenade. The timeline is erased when Mitchell, in 1939, sets an ambush and kills Ba'al and his soldiers before they can change time.

SGU: Time

 * In the original timeline a team travels to a Jungle planet and a few members of the team fall ill with an unknown fatal illness. They are unable to dial back to the Destiny due to a very unstable wormhole forming and are trapped on the planet for 36 hours. During that time, a bunch of nocturnal creatures attack and kill Chloe Armstrong, Adam Brody, Lt. Vanessa James, Cpl. Marsden, Sgt. Spencer and badly wound Lt. Matthew Scott by biting him which sends him into a coma. Another member of the team, Dale Volker dies of his illness that night while Eli Wallace records everything with a Kino. In the morning the survivors:Eli, Lt. Tamara Johansen, Dr. Nicholas Rush, Jeremy Franklin and Scott who's still in a coma hide in a cave after still being unable to dial Destiny and MSgt. Ronald Greer kills some of the sleeping creatures with Eli by placing C-4 in their nests and "fire in the hole". Meanwhile, Franklin dies of the illness. That night, the creatures attack again and everyone but Eli, the still comatose Scott and Rush are killed. Rush dials Destiny and heads through the still unstable wormhole in a desperate attempt to get help while Eli returns to the cave and is killed by the creatures. The next morning Scott regains consciousness and finds himself in pain from the wound but mysteriously cured of his illness. With little time left before Destiny leaves, he dials the ship and sends the Kino through the still unstable wormhole, planning to follow. Due to a solar flare that has been disrupting the wormhole, the Kino travels back in time and lands near Rush's dead body, creating the next timeline. Presumably everyone on Destiny dies.
 * Like in the original timeline, the team arrives, but this time finds the Kino from the original timeline and Rush's skeletal remains. The team returns to Destiny immediately and reviews the Kino's recordings. After learning of the illness, the team who went to the planet and everyone who came into contact with them quarantine themselves, but several people fall ill and T.J. learns that the illness is not from the planet, but from a contaminated water supply and their antibiotics won't work. After observing how the original Scott was cured, the crew determines that the creature's venom kills the contagion and realize that a solar flare caused the time travel. With an hour before the flare hits, Colonel Everett Young, Scott, Greer and Spencer travel to the planet to retrieve a live specimen. On the ship, four people: Volker, Franklin, James and Chloe die of the illness. Meanwhile, Greer is unfortunately attacked and killed by the creatures. Young and Scott start firing on the creatures as Spencer attempts to save Greer by taking the creature off. However, the creature kills him as well. Young and Scott still fight, however Young is also killed. Scott is very unhappy to find out that he's the only survivor of the attack. He records a message on a Kino, detailing the illness and what needs to be done before sending it back in time via the wormhole and solar flare. Again everyone on the Destiny likely dies of the illness and Scott is probably killed by the creatures.
 * In the current timeline, the team finds 2 Kinos with their databanks full and acquire the venom in the daylight.

SGU: Twin Destinies

 * In the first timeline, as a result of heavy damage sustained during the Drone attack on Destiny, Eli proposes trying to dial home from inside a star, believing he figured out how to do so with Ginn's help. Colonel Young decides to go for it despite reservations from Rush, but is convinced by Rush to call for ten volunteers to stay along with the two of them to complete Destiny's mission.  The two give a speech to the crowd and among the volunteers are Greer, Volker, Brody, TJ, Chloe, Scott, Camile Wray, Varro and Eli.  The plan works and Earth is succesfully dialed and Telford goes through, but the wormhole becomes unstable.  Rush and Eli manage to correct the problem and the evacuees head through the Stargate, but power starts to overload so Young orders everyone through, but Rush is forced to stay behind to mantain the connection so they can get through.  Rush then connects to Earth, but learns that only Telford, who he's connected to, made it through the Stargate.  Rush is injured in an overload and as the air gets toxic, escapes in the shuttle.
 * The second timeline is the same up to the point where Rush and Young determine how many people they should ask to volunteer. Before they can make a speech, they are called to the bridge where a message from Rush from the first timeline is making contact.  He is allowed to land and tells his story, beleiving that the combination of a solar flare and dialing Earth must've thrown his Destiny back in time 12 hours.  As a result of Future Rush's story, Young calls off dialing Earth and they locate the first Destiny, finding it mostly intact with all the damage seeming to be in the engineering spaces.  Desperatly needing parts and supplies and seeing a chance, the crew dials the second Destiny and raids it for parts and supplies, discovering they have forty minutes until its shields collapse and it falls into the star.  Camile connects to Earth to find out that the first Telford is there.  While trying to get relays for the weapons system, Future Rush is confronted by Telford and in a moment of anger, shoves him into a damaged relay, electrocuting him to death.  Having nothing left to lose, Future Rush heads to the Destiny interface chair to download the knowledge of the Ancients before he dies and convinces present Rush to help him.  Rush and Scott, the last two on the second Destiny, escape at the last second before the ship falls into the star and disintergrates.  Thanks to the parts gotten from the second Destiny, the crew can now repair the damage to the ship and continue the mission.

Note
With the exception of the events in "Moebius, Part 1", "Moebius, Part 2", and "Stargate: Continuum", all these "alternate timelines" are in fact timelines lacking the intervention of a given individual or individuals — SG-1 and Dr. Janet Fraiser in "2010 (episode)," Dr. Elizabeth Weir in "Before I Sleep," Teal'c in "Unending", Lt. Colonel John Sheppard in "The Last Man", Lt. Colonel Cameron Mitchell in "Stargate: Continuum", Lt. Matthew Scott in "Time" and Doctor Nicholas Rush in "Twin Destinies"— all originating from futures that their interventions have negated. (Since "Moebius" actually depicts two altered timelines, one of which is the "final" one, it could fall in this category as well). In effect, then, it might be said that the primary timeline is the "alternate" timeline, as it is the result of repeated historical interventions.