"What is with you people? Time machines are nothin' but trouble. "
Jack O'Neill to Thor[src]

Time travel is a colloquial term for journeying into the past or the future.

There are several known ways to travel through time, including wormholes under special conditions (such as intersecting with a solar flare), special devices (Time machines) and local time manipulations within a Time dilation field. If the past is altered, the following chain of events will change, creating a new timeline. The new timeline is considered as "Main," while timelines with lack of intervention are considered as "Alternate." (SG1: "1969", "Unending", SGA: "Before I Sleep")

Methods of time travel

Wormholes and solar flares

When a wormhole travels close enough to a star while there is a solar flare, its path may be altered and lead through time. Sometimes, objects sent through such a wormhole will reach the destination, but in the other time period, sometimes wormholes will connect with the same gate in an other time.

The first instance of the Tau'ri traveling through time occurred in 1999. This time it happened solely by accident, when SG-1 ended up in 1969. Later, in 2010, SG-1 successfully and intentionally opened a wormhole to the past, altering the future and preventing depopulation of Earth by the Aschen. (SG1: "1969", "2010", "2001")

The computations done normally by the Dial Home Device to compensate for stellar drift had to be accounted for by Captain Samantha Carter in her dialing computer software. These computations also had to account for how close the wormhole formed to the Sun. Carter had done research into other uses of the Stargate technology, including time travel. In her research, she investigated the impact solar flares would have on a wormhole and proposed that these flares, if predicted, could make time travel via the Stargate possible. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. This magnetic energy along with Earth's gravitational field, according to Carter, would bend the wormhole and cause it to return to Earth. Basically, they wouldn't travel through space as much as they would time in the space-time continuum.

The exact time of the occurrence of a solar flare is not predictable because of the amount of time it takes light to travel to Earth from the sun. Major General George S. Hammond used Carter's research to record the times of the solar flares which were observed in the year 1969, so Carter didn't have to predict those. The device Cassandra Fraiser used to send SG-1 back to the past was later revealed to be an Ancient amplifier bracelet, which allows the user to dial to a specific gate. (SG1: "1969", "Small Victories")

Ba'al once managed to travel back in time and altered the past to prevent the Stargate from ever reaching the United States of America on its trip from Giza, Egypt, where it was discovered in 1928. In the alternate timeline, he managed to conquer almost the entire Galaxy before SG-1 traveled back in time to prevent this from happening. (SG1: "Continuum")

The Destiny expedition ran into this phenomenon during the first year of operation, the wormhole between Destiny and the planet the crew was visiting interacted with a solar flare for at least a day. It made the wormhole very unstable and anything that went through it ended up in the past. An alternate Dr. Nicholas Rush and a Kino were sent through it and discovered by another version of the team who viewed the Kino. After a disastrous mission to retrieve some creatures from the planet, Lt. Matthew Scott used another Kino and the solar flare to send a message into the past to change their own timeline. (SGU: "Time")

In an alternate timeline, the combination of a solar flare and dialing Earth from within a star caused a temporal event that threw Destiny itself back in time twelve hours before it was eventually consumed by a star, but allowed the Dr. Nicholas Rush of that timeline to alter the course of events. (SGU: "Twin Destinies")

The crew of the alternate Destiny get thrown back in time again by the combination of another solar flare and the wormhole to Earth. As there is no Destiny for the wormhole to land on, it lands on another planet and later evacuated to one they named Novus 2,000 years in the past where the crew builds a new civilization. (SGU: "Common Descent")

Time machines

One time machine was developed by the Ancients who lived on the planet designated P4X-639. The Ancients had been dying out from a mysterious plague. They took advantage of the geomagnetic disturbance which occurred every fifty or so years to power the device. Unfortunately, the machine only generated a short-term continuous loop. The creators of the machine attempted to fix it, but were unsuccessful. They shut down the machine, recorded their failure on the walls and obelisks surrounding the device, and let the end come. Dr. Daniel Jackson translated the text and determined how to shut down the machine which had been activated by an alien archaeologist named Malakai. Malakai did not have the same translating skills as Daniel and didn't realize that the machine would never work in the way he intended (to go back in time over twelve years to see his dead wife alive once more). The machine was never able to send the traveler back further than ten hours, and as long as the machine was operational, the traveler looped in time for those ten hours. (SG1: "Window of Opportunity")

The second time machine was built by a Lantean Scientist known as Janus. He created the time machine in the form of a small space craft with a time machine built in. This time machine generated a field of some kind around the spacecraft rather than using the wormhole to travel through time.

The Time Jumper discovered by SG-1 on Harold Maybourne's planet could only make jumps of at least a few hundred years or more at a time, so a traveler, for example, couldn't travel to 1918 from 2005.

The technology of the two timeships presented in the Stargate universe are so nearly identical that it is possible that they were both built by the same Ancient, Janus. Janus' machine was used to travel back in time 10,000 years in the Pegasus Galaxy and was destroyed in the past on orders of Moros. Janus, however, was one of the Ancients who returned to Earth 10,000 years ago and he could have built a second machine which was found in Maybourne's planet in the Milky Way Galaxy [Correction: Janus downloads his time research before departing Atlantis in SGA: Before I Sleep, with the stated intention of creating another time ship]. (SG1: "It's Good to Be King", "Moebius, Part 1", SGA: "Before I Sleep")

Time dilation fields

Not quite a time machine, the Asgard's time dilation technology has been used to slow down time, to advance the speed of time, or to reverse time in a localized field, which means that the time is only altered within that field and not throughout the entire universe. For example, the Asgard created a limited-range time dilation field around the planet Hala in order to keep the Replicators isolated until a solution to the problem could be found. They slowed down the progression of time such that one year to the Replicators would be about ten thousand years outside the field. The Replicators managed to turn the device in the other direction and advance their evolution into Human form. They later used the time dilation device to escape the effects of a black hole that resulted when the Asgard purposefully destroyed Hala's sun. The same type of time dilation device was about to be used to send a Mark IX Gatebuster bomb into the Kallana Singularity in order to close multiple inter-universal bridges (between alternate realities) when wormholes passed through it, but a way was discovered to reverse these bridges and close them off individually. (SG1: "Unnatural Selection", "New Order, Part 1", "Ripple Effect")

After the Asgard gave Stargate Command their entire legacy of knowledge and technology, the ability to create a time dilation field (or "bubble") was given to the starship Odyssey. Lt. Colonel Samantha Carter activated this field in order to buy some time to figure a way to save the "Legacy" and to avoid destruction by an Ori warship's energy beam. Approximately fifty years passed inside the field, but a mere 0.86 seconds were all that stood between the ship and the Ori's destructive beam outside the field. During those fifty years, Carter used the Asgard's knowledge base (the Asgard Core) and eventually configured a field in which time was reversed. In order to not repeat the events that caused them to go into the time dilation field in the first place, Teal'c volunteered to isolate himself outside of the time reversal so that he could give Carter the program contained on a crystal to save the ship. (SG1: "Unending")

Time reversal and traveling back in time are not quite the same thing. Those inside the reversal are not aware of the "alternate timeline" they experienced while Teal'c is. Their time in the field is a version of an alternate timeline for their own futures, but not for Teal'c, who has aged those fifty years and remembers the events inside the field. (SG1: "Unending")

Instances of time travel

Object or individual to travel Time of departure Time of arrival Consequences Reference
SG-1 1999 1969 A solar flare accidentally throws SG-1 back in time when traveling through the wormhole. There is no alterations to the future as this instance of time travel alluded to a linear timeline, where everything that happened as a consequence of time travel already happened before SG-1 traveled back in time. With the help of a note from Major General George S. Hammond they try to return to their own time through the Stargate, but are flung into the future by mistake as they leave a few seconds too early. (SG1: "1969")
SG-1 1969 Approximately 60 or so years in the future After leaving 1969 through the wormhole a few seconds too early, SG-1 are thrown into the future where they encounter an elderly Cassandra Fraiser who tells them that their journey is just beginning before using a hand-held device to return them to their own time through the Stargate. (SG1: "1969")
SG-1 60 or so years in the future 1999 SG-1 is able to return to their own time where nothing has changed and they learn from Major General George S. Hammond how he knew when they'd travel back in time. (SG1: "1969")
Colonel Jack O'Neill and Teal'c Localized time loop in 2000 Time loop eventually broken, but excluded Earth and other planets from normal timeline for at least three months. During the time loop, Colonel Jack O'Neill and Teal'c did many things without consequences as the time loop just erased them all. (SG1: "Window of Opportunity")
Piece of paper containing message "UNDER NO CIRCUMSTANCES GO TO P4C-970, COLONEL JACK O'NEILL" 2010 2001 The Tau'ri exercised greater caution in their first contact with the Aschen, thus preventing their plans to slowly depopulate Earth and leading to their possible destruction by the black hole of P3W-451. (SG1: "2001", "2010")
Jade O'Neill, Jack O'Neill and a Reetou Approximately 20 years in the future 2003 Came from a future where the galaxy was devastated by an Aschen bioweapon to change the past. Attempted many times to change things unsuccessfully and had to undo efforts after each failure. With the help of the present day SG-1, they stopped the Aschen plot and changed the future. (SG1: "Relativity")
Jack O'Neill and Jade O'Neill 2003 Approximately 20 years in the future After changing the future, father and daughter decided to return to it even if the time change would result in their erasure from history. (SG1: "Relativity")
Time Jumper with Dr. Elizabeth Weir, Major John Sheppard and Dr. Radek Zelenka aboard 2004 approximately 8,000 BC Using the information gained from Dr. Elizabeth Weir, Janus created a Failsafe mechanism to prevent Atlantis from destruction and the time traveling Weir helped by rotating the city's Zero Point Modules every three thousand years or so. Major John Sheppard and Dr. Radek Zelenka both died in the crash of the Jumper on the ocean floor. (SGA: "Before I Sleep")
Janus and Time Jumper Unknown Unknown Several "prophecies" created by Janus were later discovered and used by Harold Maybourne. (SG1: "It's Good to Be King")
SG-1 and Time Jumper 2005 c. 3,000 B.C. Their rebellion eventually made Ra flee and take Earth's Stargate with him, preventing the Stargate Program from ever being created. (SG1: "Moebius, Part 1")
Time Jumper with SG-1 excluding Dr. Daniel Jackson Unknown c. 3,000 B.C. Successfully stopped Ra from taking the Stargate with him and obtained a Zero Point Module, hiding it until discovery in the future, preventing even the idea of traveling back in time to obtain it. (SG1: "Moebius, Part 2")
Lt. Colonel Cameron Mitchell, Lt. Colonel Samantha Carter, Teal'c 2006 1947 An attack by Priors with energy beams from their staffs results in the Stargate sending the team back to 1947, trapping them in a cavern beneath Cheyenne Mountain created by the unstable vortex. (SG1: "Roswell")
General Jack O'Neill and Vala Mal Doran 2006 1947 Sent back in the Time Jumper by a future version of Samantha Carter and H.G. Wells to rescue SG-1 and the Asgard An. Collision with an Asgard ship causes the Roswell crash and damages Time Jumper. SG-1 and Loki rescued. (SG1: "Roswell")
General Jack O'Neill, SG-1 and Loki 1947 1908 As Vala used a Goa'uld healing device on O'Neill, she thought of the Cubs winning the World Series as Carter's future self had instructed her to do and it resulted in O'Neill inadvertently taking the Time Jumper to 1908 where they emerged inside a Goa'uld Ha'tak, causing the ship to crash to Earth and causing the Tunguska Impact while the rest of the Goa'uld fleet it was a part of fled. In 1908, they work with H. P. Lovecraft to get the parts they need to fix the Jumper, but accidentally cause a fire that results in someone dying that shouldn't have. Vala and Cameron Mitchell are also inadvertently left behind. (SG1: "Roswell")
General Jack O'Neill, Lt. Colonel Samantha Carter, Teal'c and Loki 1908 1947 The team travel back to 1947 on the insistence of Loki who claims he can fix the Time Jumper's Asgard transporter there. However, its a trick so he can escape. In 1947, the team realizes that they are in an alternate timeline that has some differences due to the presence of Vala and Mitchell in 1908 and the death the fire they started caused. After rescuing the Asgard An, they face a Goa'uld invasion of Earth led by Ra and Qetesh with Mitchell as Qetesh's First Prime and Vala as Qestesh's host. They manage to avert the invasion with An's help and get the time travel device needed to make things right. (SG1: "Roswell")
General Jack O'Neill, Lt. Colonel Samantha Carter and Teal'c 1947 1908 The team returns to 1908 where they rescue Mitchell and Vala and heal the man who died in the fire, setting the timeline right. (SG1: "Roswell")
General Jack O'Neill and SG-1 1908 1947 They return to the corrected 1947 where they heal Loki and return him to his ship and rescue An (SG1: "Roswell")
General Jack O'Neill, SG-1 and An 1947 2006 After SG-1 and An's rescue and repairing the timeline, they return to their correct present where An is turned over to the future Carter and H.G. Wells. (SG1: "Roswell")
Lt. Colonel John Sheppard 2008 c. 50,000 Lt. Colonel John Sheppard declared dead, Atlantis expedition failed to stop Michael Kenmore. Dr. Rodney McKay eventually developed a way to bring John back and created a hologram of himself to deliver the instructions to John. In this timeline, the key was Michael having Teyla Emmagan's baby to perfect his Human-Wraith Hybrids. (SGA: "The Last Man")
Lt. Colonel John Sheppard c. 50,800 2008 Using intel obtained from the future, Lt. Colonel John Sheppard manages to rescue Teyla Emmagan and her baby in the Battle of M2S-445, with Michael's cruiser being destroyed and Michael Kenmore is later killed, preventing his rise to power. (SGA: "The Last Man")
Ba'al 2008 1939 Ba'al managed to stop the Achilles and the onboard Stargate from reaching the United States of America, preventing the Tau'ri from becoming a galactic power. Using his knowledge of the future, he became the supreme leader of a Goa'uld empire. (SG1: "Continuum")
Colonel Cameron Mitchell 2009 1929 Having arrived ten years earlier, Colonel Cameron Mitchell managed to ambush and kill Ba'al in 1939, thus preventing the time alteration from ever happening. (SG1: "Continuum")
Three rogue Tok'ra, Doctor Carolyn Lam and SG-1 2008 2492 BC In order to save their race, three rogue Tok'ra kidnap Doctor Lam and steal the Time Jumper to travel into the past and get Egeria to make more Tok'ra. SG-1 follows using Ba'al's time machine and work with their alternate selves to stop Ra from retaking the planet and changing history. (SG1: "Moebius Squared")
SG-1, Doctor Carolyn Lam and a rogue Tok'ra 2492 BC 2008 After capturing the surviving rogue Tok'ra, rescuing Doctor Lam and preventing Ra from retaking Earth and changing history, the group uses the Time Jumper to return to their own time. (SG1: "Moebius Squared")
Dr. Nicholas Rush, Kino 2009 Unknown past Dr. Nicholas Rush didn't survive the trip and left only his skull and a Kino to be found. The Kino was later discovered by an alternate off-world team, who aborted their mission and escaped the encounter with the squigglers. However, a small group had to go to the Jungle planet to capture a squiggler to help cure the Hoth virus. (SGU: "Time")
Kino 2009 Unknown past A Kino with more detailed intel was discovered and used to cure the Hoth virus, and escape the squiggler attacks. (SGU: "Time")
Destiny and everything aboard 2009 2009 A combination of a solar flare and dialing Earth from within a star created a temporal event that threw Destiny back in time twelve hours. Colonel David Telford stepped through the wormhole and returned to Earth, but the wormhole destabilized after that. Destiny's entire crew, save for Dr. Nicholas Rush, were thrown further back in time trying to make it. Rush himself abandoned ship and warned the current Destiny not to make the attempt. The derelict Destiny was scavenged for parts prior to being consumed by the star. The version of Telford aboard Destiny was accidentally killed by the future Rush, and that Rush decided to use the interface chair on the derelict Destiny rather than face accusations of murder, being consumed in the star along with the ship. (SGU: "Twin Destinies")
Alternate Destiny crew 2009 Approximately 2,000 years in the past Instead of being killed in the wormhole, the Destiny crew is thrown back in time 2,000 years and lands on a planet near the star they were attempting to dial Earth from. They abandoned that planet for another they named Novus, not knowing for sure they'd been thrown through time and formed two countries, Tenara and Futura which had separate belief: the Tenarans focused on building a new civilization while the Futurans (led by Dr. Adam Brody) blamed Eli Wallace for their predicament and believed that Dr. Nicholas Rush would return and save them. (SGU: "Common Descent")