Temporal mechanics

"I hate temporal mechanics..."
 * - Miles O'Brien, echoed by both his past and future selves

Temporal mechanics is the study of the workings of time and its effects on the space-time continuum, and is particularly relevant when dealing with time travel. As late as 2154, time travel was deemed impossible by the Vulcan Science Directorate. 

The United Federation of Planets has had some form of time travel ability since at least 2268. It is possible that at least some of the current and future regulations regarding time travel can trace their origins back to this date. However, capability at this point was most likely inaccurate, making any attempt at time travel experimental and dangerous.

Temporal mechanics is taught at Starfleet Academy. Julian Bashir took the class, where he learned about predestination paradoxes, as did Miles O'Brien. Both Kathryn Janeway and B'Elanna Torres knew the 'finer points' of temporal mechanics. 

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

Rules of Temporal Mechanics

 * Sometimes effect can precede cause. The reaction can be observed before the action that initiated it.

Overview
A historical research in 2268 suggests the Federation was capable of controlled time travel and used this ability at the very least for research purposes. 

As of 2378, at least 40 different instances of some form of time travel had been noted in Federation records. Technologically speaking, the Federation's capabilities concerning time travel have for the most part remained unclear.

In the 25th century, time travel is mostly possible but still risky. 

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

Events
Crewman Daniels, an alleged time traveler from the 31st century, showed Jonathan Archer a group of time-traveling anthropologists from 2769 observing the building of the pyramids of Giza. 

The USS Enterprise (NCC-1701) first experienced time travel in 2266 when an emergency cold start of its warp drive slingshot the ship and crew 71 hours into the past. 

In 2267, after an encounter with a black star, the Enterprise was hurled 300 years into the past. Eventually, the Enterprise's science officer Spock devised a method for returning the Enterprise back to its original period. 

Though other instances of time travel had been noted, this appears to be the first instance of controlled time travel by the Federation. 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 then (2286)-extinct humpback whales 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 78 years before). 

At least once, Q took Captain Jean-Luc Picard back in time to the beginning of life on Earth. 

During a transporter accident above Earth in the USS Defiant (2370) caused by Chroniton particles, Captain Benjamin Sisko, Julian Bashir, and Jadzia Dax were sent back in time to mid 21st century, just before the Bell Riots. This causes the Federation and Startfleet to vanish in the present, after the time travelers inteferrence leads to the death of Gabriel Bell. Only the Defiant is 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 centuries later. 

Quark, Rom, Nog and Odo traveled back to the past, crashing near Roswell, New Mexico. 

Akorem Laan, a poet and seer from Bajor's past (which history had recorded had mysteriously disapeared) 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 Kira remembered as unfinished, after Laan had disappeared. Sisko points out that the ending of the poem had appeared in databases, along with new history of his life after his return to Bajor. Kira wonders why she could remember the previous timeline, if previous timeline had been wiped out. Sisko commented that time works in mysterious ways whenever the Prophets are involved. 

Quinn took USS Voyager back in time to the Big Bang to hide from Q. 

In 2373, the USS Defiant (2370), commanded by Captain Benjamin Sisko traveled back in time to 2268 via the Bajoran Orb of Time, by an attempt by Arne Darvin to kill Captain James T. Kirk and change the timeline. The crew of the Defiant stopped Darvin without any notable damage. 

Around Stardate 50814.2 (of an unknown year), the Defiant became stuck behind a barrier on a planet in the Gamma Quadrant, where they met the inhabitants who told them they were the descendents of the crew of that crashed on the planet 200 years in the past. With foreknowledge of the crash, the crew surmised that if they escaped that fate, the timeline would collapse and everything on the planet cease to exist. The descendent 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.

However, Yedrin 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 crew ultimately made the decision to sacrifice the ship by flying into the temporal anomaly, in order to save the 8000 colony members. However, the ship's flight plan was changed by Odo so that it would clear the barrier. This erased the settlement and all of its inhabitants from the planet. 

Kira uses the Orb of Time to learn about her mother's life on Terok Nor. Before she goes she is warned not to interfere with the timeline by Sisko. Kira nearly kills her own mother and Dukat, but changes her mind at the last moment. The orb then returns her to the present. 

Upon returning to Golana through the time portal 300 years into the past, an adult Molly helped helped the younger Molly return 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 an alternate 1996. History was set right 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 Captain Janeway (from 2404) traveled back to 2378 through the use of a chrono deflector to bring Voyager home from the Delta Quadrant 16 years earlier then 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, where he destroyed the USS Kelvin, then went into hiding for 25 years awaiting Spock's arrival. Spock arrived in Nero's new timeline in 2258. Nero destroyed, while Spock watched from Delta Vega. The crew of the USS Enterprise (alternate reality) with the help of, stopped Nero from destroying Earth. 

Temporal Cold War
The events of the Temporal Cold War further complicate the understanding of Federation time travel capabilities.

As of the 29th century, a time traveling faction represented by a mysterious individual, could only communicate and exchange information and technologies through time, appearing as a non-corporeal entities. By the 31st century, this technology was perfected to allow physical travel.

According to 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.

The act of Daniel's bringing Archer into the future, wipes out the Federation, as Archer was not around to found it. This effectively causes Daniels and Archer to end up in an alternate future, where the Federation never existed, and Earth was wiped out. Returning Archer to his own time restored the future.

By 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 bioweapon 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 go hot with time-travelers changing time as they liked. Daniels brought Enterprise and Archer back to 1944 to stop Vosk, leader of the Nak'uhl, 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 occurred when the Temporal Cold War went hot were undone and time reset itself to normal. Archer refused to have anything to do anymore with Daniels' "damn Temporal Cold War" and Daniels agreed, claiming it was finally coming to an end thanks to Archer's actions. 

Methods

 * Black hole created by red matter
 * Chrono deflector
 * Light-speed breakaway factor (Slingshot effect)
 * Nexus
 * Orb of Time
 * Q powers
 * Temporal causality loop
 * Temporal conduit
 * Temporal transporter
 * Temporal vortex
 * Time portal
 * Timeships
 * Wormholes

Paradoxes

 * Dali paradox
 * Grandfather paradox
 * Pogo paradox
 * Predestination paradox

Examples

 * List of time travel episodes
 * Alternate timelines
 * Temporal Cold War

Apocrypha
In the novel Ishmael, 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 USS Enterprise (NCC-1701) 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 did in Voyage Home with his glasses.

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 Trek's history. The most significant of these demonstrates how Starfleet became aware that history could change- previously they 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 travelling 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 link
Temporale Mechanik Mécanique temporelle Tijdmechanica