Time travel
Time travel is having the ability to or the act of travel forward or backward in time, thus breaking the seemingly irreversible flow of time. The study of this phenomenon is one of the focuses of temporal mechanics. Alterations to historical events can cause, among other things, alternate timelines and realities.

As late as 2154, time travel was deemed impossible by the Vulcan Science Directorate. ()

The United Federation of Planets had some form of time travel ability since at least 2268. Regulations on time travel were later imposed to prevent alterations from being made to the timeline. (; ; )

In addition to the Federation, the Borg Collective, Bajorans, and the Klingon Empire were known to have some form of time travel capabilities. (; ; ; ; )

History

Overview 
Historical research in 2268 suggested the Federation was capable of controlled time travel and used this ability at the very least for research purposes. ()

As of 2378, at least forty different instances of some form of time travel had been noted in Federation records. Technologically speaking, the Federation's capabilities concerning time travel, for the most part, remained unclear.

In the 25th century, time travel was mostly possible but still risky. ()

As of the 26th century, historians and anthropologists used time-pods to travel through time to observe historical events. ()

In the 29th century, the Federation had the ability to scan time as well as space, and travel back (and presumably forward) in time to attempt to fix issues and anomalies. ()

In the 31st century, time travel was commonplace. Federation temporal agents from this time became involved in a Temporal Cold War, which eventually led to temporal wars. After these wars, time travel was banned and the technology used to travel in time was destroyed. (; )

Events 
Crewman , a temporal agent from the 31st century, showed Jonathan Archer a group of time-traveling anthropologists from 2769 observing the building of the s of Giza. ()

In the 2230s, Section 31 created a special Red Angel time-traveling suit in response to Klingon time-traveling studies. This suit would use a special time crystal that created micro-wormholes to travel through time. A Klingon attack forced its test pilot, Gabrielle Burnham, to use it, bouncing throughout time. Section 31 thought the technology destroyed. ()

Time crystal-related technology was available outside of Federation oversight by late 2256, when Harry Mudd procured a time crystal-powered device that allowed him to rob a Betazoid bank and commandeer the , while repeatedly (and creatively) murdering her captain, Gabriel Lorca. ()

The , after being transported to another universe in late 2256 or early 2257 due to Captain 's machinations, returned to her original universe nine months later due to difficulties navigating the mycelial network. Her crew ended the  shortly thereafter. ()

Captain Christopher Pike was transported via time crystal by his own future self to 2266 to witness the terrible future that would occur if Pike attempted to change his own fate. ()

The  first experienced time travel in 2266, when an emergency cold start of its warp drive slingshot the ship and its crew seventy-one hours into the past. ()

In 2267, after an encounter with a black star, the Enterprise was hurled three hundred years into the past. Eventually, the Enterprises science officer Spock devised a method for returning the Enterprise back to its original period. ()

Later that year, officers Kirk, Spock, and McCoy traveled to Earth's early 20th century and back via the Guardian of Forever. ()

Sometime in 2268, the Enterprise traveled back in time, using the light-speed breakaway factor, to 1968, on a historical research mission. ()

In 2286, Admiral James T. Kirk and his senior staff took a stolen Klingon Bird-of-Prey back in time to the year 1986, to retrieve a pair of humpback whales, a species which was extinct by 2286, to save Earth from destruction by an alien probe. ()

In 2371, Captains Jean-Luc Picard and James T. Kirk used the Nexus as a means of controlled time travel, going to just before the destruction of the Veridian star, which Picard intended to save. (Picard arrived at the Nexus from just after the destruction of said star, whereas Kirk arrived from some seventy-eight years before.) ()

Data, unknowingly at first, traveled back in time to Earth 1893 while investigating an alien species that was later found to be ingesting the neural energy from Humans of that time period. The aliens acquired their sustenance by traveling through a portal back to that time and place from Devidia II, and were able to shield their presence from outsiders by being slightly out of temporal sync with everyone else, disguising their presence further by taking advantage of a local cholera outbreak. Picard, Riker, Crusher, La Forge, and Troi soon followed to retrieve Data and assess the situation. As the Enterprise crewmembers covertly endeavored to stop the aliens and return to their own time, Samuel Clemens was suspicious of them, and was accidentally sent to the 24th century with the Enterprise crewmembers (except Picard) when they used the alien portal to return to their own time. Clemens managed to go back through the portal and retrieve Picard, returning each man to his own time period before the Enterprise destroyed the aliens and their portal. ()

Twice, Q took Picard back in time. The first time was when Picard lay dying in a starbase medical facility during a replacement of a defective artificial heart that he had received as a result of a Nausicaan knife being stabbed through his heart immediately after Picard had graduated from Starfleet Academy on Starbase Earhart. () The other example was to the beginning of life on Earth, and also back and forth in Picard's lifetime. ()

During a transporter accident above Earth caused by chroniton particles being accidentally fused to the hull of the , Benjamin Sisko, Julian Bashir, and Jadzia Dax were sent back in time to the early-21st century, just before the Bell Riots. This caused the Federation and Starfleet to vanish in the present, after the time travelers' interference led to the death of Gabriel Bell before he would have played a crucial role in history. Only the Defiant was left protected by the chroniton particles. Sisko took on the identity of Gabriel Bell to ensure that he was remembered for saving hostages during the events, and ultimately allowing the Federation to come into being over a century later. ()

While on a trip to Earth to take Nog to Starfleet Academy, Quark, Rom, Nog, and Odo were accidentally propelled back to circa 1950, crashing near Roswell, New Mexico, due to a sabotaged warp core on a ship that had been given to them by Quark's cousin . They managed to return to their own time using the reaction between kemocite and the radiation of a nuclear explosion on Earth. These events – which included Quark attempting to alter the timeline by selling goods to the United States of America during the Cold War, and the Ferengi subsequently being interrogated on the suspicion that they were secretly planning an invasion – were kept secret from the public on Earth at the time. ()

Akorem Laan, a poet and seer from Bajor's past (whom history had recorded had mysteriously disappeared) was sent to the future by the Prophets, believing he was the Emissary of the Prophets. Ultimately, Sisko convinced the Prophets to send Laan back into his own period. Laan later went on to finish one of his prophetic poems, which Major Kira remembered as unfinished, after Laan had disappeared. Sisko pointed out that the ending of the poem had appeared in databases, along with new history of his life after his return to Bajor. Although Kira wondered why she could remember the previous timeline if that timeline had been wiped out, Sisko commented that time worked in mysterious ways whenever the Prophets were involved. ()

Quinn took the  back in time to the Big Bang to hide from Q. ()

In 2373, the , commanded by Captain Sisko, traveled back in time to 2268 via the Bajoran Orb of Time, in an attempt by Arne Darvin to kill James T. Kirk and change the timeline. The crew of the Defiant stopped Darvin without any notable damage. Major Kira found a way to use the Orb to get the Defiant and its crew back to their own time. ()

In 2373, around Stardate 50814.2, the Defiant became stuck behind a barrier on a planet in the Gamma Quadrant, where the ship's crew met inhabitants of the planet, who told them they were actually descendants of the Defiant crewmembers themselves and that, in two days, the crew would be sent two hundred years into the past and crash on the planet. With foreknowledge of the crash, the crew surmised that, if they escaped that fate, the timeline would collapse and everything on the planet would cease to exist. A descendant of Jadzia Dax, Yedrin, surmised that it would be possible to save their colony, due to an energy discharge that had hit Major Kira, which had created a subspace doubling effect and, for an instant, created corresponding quantum duplicates of every molecule in her body. By amplifying the doubling effect through modifications to the Defiant, they could make a quantum duplicate of the entire ship, and this second ship would then be the one that would crash on the planet, preserving the original timeline. However, the other Defiant would be able to escape as well. Yedrin, though, had falsified information in his plan, with the intent of making sure that the original (un-duplicated) single Defiant would crash on the planet, thus ensuring that the colony would continue to exist. The Defiant crewmembers ultimately made the decision to sacrifice their ship by flying into the temporal anomaly, in order to save the eight thousand colony members. However, the ship's flight plan was changed, by a version of Odo who lived on the colony, so that it would clear the barrier, the future Odo wanting to save Kira's life. This erased the settlement and all of its inhabitants from the planet. ()

Kira used the Orb of Time to learn about her mother's life on Terok Nor. Before she began her Orb experience, she was warned not to interfere with the timeline by Sisko. Kira nearly assassinated her own mother and Dukat, but changed her mind at the last moment. The Orb then returned her to the present. ()

During a visit to the planet Golana with her family, Molly O'Brien was accidentally sent three hundred years into the past via a time portal created by a long-lost civilization that had once inhabited the planet. She was retrieved by her parents, but was now a teenager reduced to a feral mindset, forcing them to use the portal to send her back. However, their subsequent use of the portal sent the teenage Molly back to shortly after her past self arrived in the past in the first place, allowing her to send her past self back to the present. Shortly after saving the younger Molly, the older Molly ceased to exist, vanishing from existence as the timeline corrected itself. ()

Also in 2373, an encounter with the 29th century timeship Aeon threw Voyager back to 1996, while the Aeon crash-landed in 1967 and was discovered by Henry Starling, who used its future technology to make himself rich. History was corrected when Captain Janeway destroyed the timeship before it could travel to the future and accidentally cause a temporal explosion. Voyager was returned to the correct time by an alternate version of the Aeon from the correct timeline. ()

An alternate future version of Janeway (from 2404) traveled back to 2378 through the use of a chrono deflector, to bring Voyager home from the Delta Quadrant sixteen years earlier than it had arrived in that alternate future. This future Janeway was killed in the attempt but succeeded in returning Voyager home and even delivered a crippling if not fatal blow to the Borg. ()

In 2387, Nero and Spock were sent back in time through a red matter black hole which had prevented a supernova from destroying the galaxy (but not before it had destroyed Romulus in the process). Nero was sent back to the past to 2233, inadvertently creating a new spacetime continuum where he destroyed the . He and his crew then went into hiding for twenty-five years, awaiting Spock's arrival. Spock arrived in Nero's new timeline in 2258. Nero destroyed , while Spock watched from . The crew of the , with the help of , stopped Nero from destroying Earth. ()

In 2401, Q transported Picard, Elnor, Seven of Nine, Agnes Jurati, Cristóbal Rios and Raffaela Musiker into an alternate timeline where Earth was now the Confederation of Earth. () With the help of an alternate Borg Queen, the group used the slingshot effect to travel back in time to 2024 in order to right history. () Q later returned the group, minus Rios and Jurati, to 2401 using his own powers at the cost of Q's life due to his weakened state. The group was returned to just moments before they had left, giving them the opportunity to alter their own future slightly. ()

Temporal Cold War and temporal wars
The events of the Temporal Cold War further complicated the understanding of Federation time travel capabilities.

As of the 28th century, a time-traveling faction, represented by a mysterious individual, could only communicate and exchange information and technologies through time, appearing as non-corporeal entities. By the 29th century, this technology was perfected to allow physical travel.

According to a Temporal Enforcement Agent from the 31st century named "Daniels", all time travelers were bound by the observance of strict procedures set forth in the Temporal Accords. Daniels stated that the Temporal Accords were enacted after time travel was developed by all time-travel-capable species to prevent catastrophic tampering with the timeline.

In 2153, the Sphere-Builders, a group previously thought to possess the ability only to examine alternate timelines, sent Xindi-Reptilians back to 21st-century Detroit to develop a bio-weapon to destroy the Human race. ()

Ultimately, the war ended when the Na'kuhl returned from an alternate 1944 (created by their very actions) and caused the Cold War to become a war with time travelers changing time as they liked. Daniels brought Enterprise and Archer back to 1944 to stop Vosk, leader of the Na'kuhl, from returning to the future. With the help of Silik, a member of another faction of the Temporal Cold War, Archer shut off Vosk's compound's shields, allowing Enterprise to destroy it and thus destroying the temporal conduit and killing Vosk, just as he was about to travel back to the future. Daniels pulled Archer to a place where they watched the time stream, as all of the events that had occurred while the Temporal War had been ongoing were undone and time reset itself to normal. Archer refused to have anything more to do with Daniels' "damn Temporal Cold War" and Daniels agreed, claiming it was finally coming to an end thanks to Archer's actions. ()

After the temporal wars, time travel was banned, and its technology destroyed. ()

In 3189, after  started to die from being separated from her own time and the mirror universe for so long, the crew of the temporarly-displaced  sought help from a being who turned out to be the embodiment of the Guardian of Forever. After testing Georgiou and Michael Burnham, the Guardian of Forever sent Georgiou back to a time when the two universes were still aligned and thus she could survive. ()

Opinions on time travel
Neelix believed that food could be like time travel; in his own words, "You inhale an aroma, take a bite of something and suddenly, bam! You're back at the moment you first tasted it." ()

When Captain Jonathan Archer saw an old clipper ship on an ocean on the Akaali homeworld, he noted that it was almost like "traveling back in time." ()

Methods 
Black hole created by red matter
Chrono deflector
Nexus
Orb of Time
Q powers
Red Angel suit using a time crystal
Slingshot effect (Light-speed breakaway factor)
Temporal causality loop
Temporal conduit
Temporal transporter
Temporal vortex
Time portal
Timeships
Wormholes

Appendices

See also 
Department of Temporal Investigations
Janeway Factor
Temporal anomaly
Temporal Integrity Commission
Temporal Prime Directive

Background information 
A list of episodes that feature time travel.

Apocrypha 
In the  , Captain Spock embarked incognito aboard a Klingon ore transport, refitted with time travel capacity. The ore transport managed to go back to 1867 Earth after an acceleration close to a white dwarf near Starbase 12. The  was modified to follow the ship to 1867, becoming the first instance that a Federation ship utilized intentional and controlled time travel.

In the video game Star Trek: The Next Generation - Future's Past for the Super Nintendo, the computer mentions several temporal near-catastrophes in the 23rd century: the New York incident, the above-mentioned "Ishmael incident", and the "Eyeglass Loop Paradox" referring to what Kirk does in  with his glasses.

The novel trilogy Star Trek: Deep Space Nine - Millennium features the Defiant inadvertently travelling twenty-five years into the future when they slingshot around a newly-formed red wormhole. Upon arriving in the future, Dax explains that time-travel leavers the travellers molecularly out of sync with the local time period in a manner that can only be averted if they either exactly repeat and reverse the process that sent them through time in the first place or if they travel in the opposite direction of their original trip by a considerably greater distance (in this case, it was suggested that the Defiant crew could travel back in time 25 000 years). It is left unclear what would happen if someone tried to go back in time via a different method- the use of the Orb of Time is suggested as one such method that could be used- but it is generally agreed that such an attempt either wouldn't work or prove unpleasantly fatal to the time traveller. 

The Department of Temporal Investigations novels Watching the Clock and Forgotten History go into detail about the principles of time travel, as well as the hows and whys of the various time travel incidents in Star Treks history. The most significant of these demonstrates how Starfleet became aware that history could change – previously, the organization had assumed that changing history just created an alternate timeline that those who changed history happened to get 'stuck' in-; when the DTI created Timeship One with the intention of deliberately traveling three days into the future, a future version of the ship appeared in their time thirty-two hours before it was due to launch, the crew all dead or dying from an accident that had occurred when they overshot. With this discovery postponing the planned launch, the future Timeship One and its crew disappeared once the time of the original launch had passed, confirming that history could be changed.

External links

 
Events