Time travel

"What is with you people? Time machines are nothin' but trouble."

- Jack O'Neill

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". This terms should not be confused with Alternate reality every of which co-exists at the same moment, and may be traveled between, under certain circumstances, relatively freely.

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 the 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, SG1 successfully and intentionally opened a wormhole to the past, altering the future and preventing depopulation of Earth by Aschens.

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. General 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 used to send SG-1 back to the past was later revealed to be a Tau'ri built Asgard Auto-dialer, which allows the user to dial to a specific gate and time.

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.

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 Doctor 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, Lieutenant Scott used another Kino and the solar flare to send a message into the past to change their own timeline.

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 through the galaxy millions of years ago.

One such 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. 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.

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 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 the Atlantis expedition discovered a bay full of them on Atlantis. The term "Jumper" was also used by Carter. . 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. 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 Maybourne'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 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.

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.

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.