Time travel

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

Summary
Stargate SG-1 has presented two major timeline theories over the run of the show: (1) alternate realities, 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 is modified by a time traveling event.

This article presents Alternate Timelines in which SG-1's timeline appears to be changed due to one or more time traveling event.

Methods of Time Travel
According to Albert Einstein, "The distinction between past, present, and future is only a stubbornly persistent illusion." His theories in special relativity, as well as those of the mathematician Hendrick A. Lorentz, propose that time and space are somehow bound together in a four-dimensional space-time continuum. Einstein's general relativity theories propose that velocity and gravity warp space-time in ways that cause time to be relative to the observer measuring it. Through special and general relativity, travel to the future is more easily accomplished than that to the past, but because Stargate SG-1 is science fiction, traveling to either the past or the future is possible.

Physicists have proposed various theories on how time travel could be made possible in our reality, but in the Stargate SG-1 universe, it is already proven as fact.

Wormholes and Solar Flares
The idea of using wormholes as time machines has been explored by Kip Thorne, in his book Black Holes and Time Warps. Various problems exist in maintaining a wormhole long enough to be used to travel through, but since this is the Stargate SG-1 universe and stable wormholes are a given, then traveling through a wormhole in order to travel through time is no problem at all. In the near-instantaneous nature of Stargate travel, SG-1 basically covers a large distance in a relatively short period of time, whereas light would take a while to travel that same distance.

Interference with the formation of the wormhole through the magnetic discharge of solar flares is one way in which Stargate SG-1 proposes that SG-1 could travel through time. In the episodes, 2.21 "1969" and 4.16 "2010", the wormhole is bent by solar flares. Depending on which side of the sun the wormhole is formed, the traveler is either transported to the past or to the future.

The computations done normally by the DHD to compensate for stellar drift had to be accounted for by 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. In the episode, 2.21 "1969", General Hammond used Carter's research to record the times of the solar flares which we observed in the year 1969, so Carter didn't have to predict those. The manner in which Cassandra sent them back to the present when they overshot their return and were flung into the future was not identified in the episode, but it could be assumed that by the time Cassandra was an elderly woman, solar flares were predictable. It could also be assumed that another form of wormhole manipulation was performed by Cassandra's use of advanced technology to get them back to their time.

In Stargate Continuum, the last Ba'al managed to discover an Ancient Solar observation outpost, which allowed him to predict solar flares and thus travel back in time under approximately controlled conditions. Using this, Ba'al was able to change history by preventing the Achilles, the ship that transported the Stargate from Africa to America, from reaching its destination, thus erasing the Stargate Program from existence. Fortunately, when history began to change, Colonel Mitchell, Colonel Carter and Doctor Jackson travelled back to Earth through the Stargate, the wormhole apparently protecting them from changes in the timeline as they arrived in the alternate timeline with their own memories and histories intact (Despite Mitchell being a living grandfather paradox as Ba'al killed his grandfather on the Achilles before he could have children, Carter's other self having died four years ago and Daniel's counterpart still being alive in Egypt). After being forced to live for a year in the alternate timeline, the three of them were able to gain access to the Stargate and travel to the outpost, using the machine to change history back to the way it was by sending Mitchell back to 1929- the closest time to 1939 they could manage-, Mitchell subsequently waiting ten years to get on board the Achilles to kill Ba'al before he could change history.

Time Machines
Technology was the means of time travel in the other time travel episodes of Stargate SG-1. Both the technologies used were created by the Ancients, a race of humans who lived millions of years before humans on Earth. As a matter of fact, in the Stargate SG-1 universe, the Ancients were the ones who seeded the Milky Way Galaxy with our present life forms after a plague swept the galaxy millions of years ago (8.18 "Threads").

One such time machine was developed by the Ancients who lived on the planet designated P4X-639. 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 surrounding the device, and let the end come. Daniel 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. (4.06 "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 small craft is able to fly through the wormhole of a Stargate, and thus, it was nicknamed a "Puddle Jumper" by Major John Sheppard (because the event horizon of a Stargate's wormhole looks like a puddle of water) when he discovered a bay full of them on Atlantis. The term "Jumper" was also used by Carter (8.19 "Moebius Part 1"). This time machine generated a field of some kind around the spacecraft rather than using the wormhole to travel through time.

The timeship discovered by SG-1 on Harry 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 1912 from 2005 (8.13 "It's Good To Be King", 8.19 "Moebius Part 1"). 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, who was introduced in the Stargate Atlantis episode, 1.15 "Before I Sleep". Janus' machine was used to travel back in time 10,000 years in the Pegasus Galaxy and was destroyed in the past in a crash. 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 Harry's planet in the Milky Way Galaxy.

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 Othalla 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. (6.12 "Unnatural Selection", 8.01 "New Order Part 1", 9.13 "Ripple Effect")

After the Asgard gave the SGC their entire legacy of knowledge and technology, the ability to create a time dilation field (or "bubble") was given to the starship Odyssey. 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. (10.20 "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. (10.20 "Unending")

Episodes or films involving time travel or another form of time manipulation

 * "1969"
 * "Window of Opportunity"
 * "2010"
 * "Before I Sleep"
 * "It's Good to Be King"
 * "Moebius, Part 1"
 * "Moebius, Part 2"
 * "Unending"
 * "The Last Man"
 * "Stargate: Continuum"