Temporal paradox

Time or temporal paradoxes were events arising from means other than the normal flow of cause and effect. Paradoxes vary greatly in terms of origin and severity: at the least-harmful end are fairly benign sorts of paradox, where effect precedes cause, resulting in a confusing-but-tenable event; on the more severe end, are thoroughly impossible paradoxes, such as the descendants of a race travelling back in time to exterminate their predecessors. Perhaps the most destructive form of paradox involves travellers "interfering" with the Web of Time or altering a fixed point in time, an act which could destabilize or destroy the entire space-time continuum.

The Time Lords
The Time Lords enforced Laws of Time to prevent people from meeting themselves (TV: The Three Doctors) from the present to the past. (TV: The Hand of Fear) Except for the the Doctor and, the Time Lords all died in the Last Great Time War. The Ninth Doctor said that when the Time Lords existed, paradoxes were all but impossible and that any paradoxes that occurred were fixed. (TV: Father's Day) The Time Lords used their stewardship of time travel to prevent "unauthorised" time travel (TV: The Time Warrior) which Dastari of the Third Zone, for example, believed had a selfish, political motivation (TV: The Two Doctors)

Other civilisations
Before the Last Great Time War, the Seventh Doctor noted to his companion Ace that even the Daleks would not dare to blatantly change history, presumably for the sake of their own self-preservation. (TV: Remembrance of the Daleks)

Blinovitch Limitation Effect
A natural law, the Blinovitch Limitation Effect ensured that the course of time corrected itself and ran smoothly.


 * See main article.

Reapers
Reapers acted "neutralising" the consequences of time paradoxes by destroying the people and planets involved, as when Rose Tyler changed history. (TV: Father's Day)


 * See main article.

Weeping Angels
One time paradox was deadly to the Weeping Angels - if a paradox was created at a location where Weeping Angels were feeding, the paradox would poison and kill the Angels. (TV: The Angels Take Manhattan) However, the Weeping Angels could also attempt to create a time paradox itself to use as a food source. (PROSE: Touched by an Angel)


 * See main article.

Paradox machine
A temporal paradox could be held in place by a paradox machine such as one created by out of the Doctor's TARDIS (TV: The Sound of Drums/ Last of the Time Lords)


 * See main article.

Faction Paradox
Faction Paradox opposed the Time Lords. They practically worshiped the act of a paradox, not just for the idea, but for the power that a paradox evoked. (PROSE: Alien Bodies onwards)


 * See main article.

The Trickster
The Trickster and his brigade deliberately changed history to cause destruction. (TV: Whatever Happened to Sarah Jane?, Turn Left, The Temptation of Sarah Jane Smith, The Wedding of Sarah Jane Smith, Immortal Sins)


 * See The Trickster and Trickster's Brigade.

Origin of the universe
According to the Fifth Doctor, his universe itself began because of an ontological paradox, since the Big Bang would have been triggered by the explosion of fuel from the future space station Terminus. (TV: Terminus)

Creating one's own past or future
Using foreknowledge to try and prevent the future would result in a temporal negation paradox. (AUDIO: Cobwebs)

Guerilla fighters from an alternate timeline where the Daleks ruled the world came back in time to prevent its creation by assassinating Sir Reginald Styles, but unknowingly created their timeline with their assassination attempt. Eventually the Third Doctor and Jo Grant figured out the truth and convinced Shura not to go through with the plot. Instead he used his dalekanium bomb to destroy a group of attacking Daleks while the delegates for the peace conference were evacuated. This changed the timeline to a better future. (TV: Day of the Daleks)

When the Seventh Doctor's companion Ace visited 1943 with him, she, in Fenric's words, created her own future. She rescued her own mother, Audrey, and her grandmother, Kathleen Dudman. (TV: The Curse of Fenric) Another example was when the Doctor's TARDIS took the Tenth Doctor to find the source of his daughter's signal. The TARDIS arrived early, and the Doctor inadvertently created his daughter, causing the signal. (TV: The Doctor's Daughter)

A powerful example occurred when the amalgamation of beings known as Chiyoko used her godlike powers to go through time setting in motion the events that would lead to her creation: sending a Vorlax Regeneration Drone to Earth in 2011 (COMIC: Do Not Go Gentle Into That Good Night), Eldritch Valdemar to 19th century Paris (COMIC: The Screams of Death) and Axos out of the time loop and back to Earth at some point in its history (COMIC: The Golden Ones), before finally ensuring a war between the human race and the Galatean race. (COMIC: The Child of Time)

While not technically creating her past, Melody Pond was named after her mother, Amy Pond's childhood friend, who was really a future regeneration of her, making her named after herself. She also provided the spark that began her parents' relationship, thereby essentially conceiving herself. Later, when shown that her latest incarnation would use the alias "River Song," she adopted the name for herself. (TV: Let's Kill Hitler)

After being sent back to 1994 by the Weeping Angels, Mark Whitaker, assisted occasionally by the Doctor, Amy and Rory ensured that events that occurred between him and his wife Rebecca happened, in the process causing them to happen. The Angels had sent Mark back in time in hopes he'd create a paradox by saving Rebecca from a car accident that they could feed off of, grow stronger and keep doing it until they'd done it to the entire human race. As the death of Rebecca had caused him to want to save her and led to his actions in the past, saving her would change both the past and the future. Eventually the Doctor convinced Mark not to save his wife and using a trap set up by a time-traveling Rory, the group was able to effectively destroy the Weeping Angels and prevent the paradox they planned. Aside from saving Rebecca from a Weeping Angel and staying with her as she died, Mark did nothing. The Angel that managed to escape the Doctor and Rory's trap got trapped in the CCTV system and its attempt to change its past by sending Mark into the past was a paradox also as it created the situation it tried to avoid. (PROSE: Touched by an Angel)

In an attempt to stop the Eleventh Doctor from reaching Trenzalore and release the Time Lords, the Kovarian Chapter of The Silence decided to travel back into the Doctor's past and kill him to retroactively prevent him from reaching Trenzalore. Their efforts to kill him by blowing up the TARDIS (TV: The Pandorica Opens, The Big Bang) and creating a fixed point in time where River Song "killed" the Doctor only led to the future they were trying to avoid as blowing up the TARDIS created the very Crack the Time Lords were trying to return through and the Doctor never would've made it to Trenzalore without River's help. (TV: The Time of the Doctor)

In some cases, a time traveller had information that came from the act of time travelling, learning it from a future or past version of himself; thus, the information had no real source. An example would be the Doctor's method of saving River Song. Knowing that his future self had given her his sonic screwdriver, the Tenth Doctor was able to preserve her; conversely, his future self gave her sonic screwdriver because he remembered his past self saving her. Neither version truly devised the plan; while the future Doctor remembered the plan, the past Doctor didn't think of it until he deduced what the future Doctor had done (TV: Forest of the Dead) Another example would be how Sally Sparrow received instructions on how to fight the Weeping Angels. Sally created a transcript of the Easter Egg that the Tenth Doctor recorded for her; she then gave this transcript to the past Tenth Doctor, who then read from it to create the Easter Egg, which Sally would later watch. Neither the Doctor nor Sally actually wrote the transcript. (TV: Blink) In another example, a future Doctor left a message for his past self on the planet Heaven (PROSE: Love and War); in yet another, the Fifth Doctor met the Tenth Doctor and saw him cancel out a supernova with a black hole, which meant the Tenth Doctor remembered seeing himself doing it, allowing him to do so. (TV: Time Crash) A prime example of these types of paradoxes occurred when the Tardis appeared inside itself and started sending people who entered the Tardis a short time into the past. The most notable example of a paradox in this instance occurred when the Doctor fixed the problem by waiting for himself to enter the Tardis to tell himself how to solve the problem; to use the Wibbly lever, whereupon the Doctor used the Wibbly lever and entered the Tardis to tell himself this answer. (TV: Time)

Un-creating one's own past or future
When Rose Tyler saved her father, Pete, from his death in 1987, several events occurred at once. Earlier versions of the Ninth Doctor and Rose disappeared and the inside of the Doctor's TARDIS was thrown out of normal space-time, rendering it an empty, police box-shaped shell. There were also anachronisms created. The car which should have killed Pete Tyler kept on disappearing, reappearing and following him to repeat the accident. Reapers appeared to eat up people and landmarks in the vicinity and eventually the world. Pete chose to die, restoring the timeline. (TV: Father's Day)

In a complex example of time "righting itself", Shura, who came from the 22nd century, unintentionally caused World War III by detonating a dalekanium bomb at Auderly House to prevent that very war. The Third Doctor realised the mistake and had the delegates of the World Peace Conference evacuate the house in time to escape so the bomb only killed Daleks and their Ogron slaves. In the new timeline, World War III was averted. (TV: Day of the Daleks)

In another example, the Dalek Emperor of the timeline in which Daleks underwent the Mutant Phase unintentionally allowed the introduction of wasp DNA into the Dalek gene pool which he had travelled back in time to prevent. The Fifth Doctor realised the mistake and convinced the Emperor to destroy the faulty pesticide he had intended to use on the infected Dalek. In the new timeline, the infection was cured and the Mutant Phase never happened. (AUDIO: The Mutant Phase)

Other examples existed of similar corrections of the previous timeline. On other occasions, though, the Fourth Doctor explained that Sutekh, who was even more powerful than the Time Lords, could destroy Sarah Jane Smith's future easily. He took her to an alternative 1980 where this had happened. (TV: Pyramids of Mars) The Ninth Doctor said the same about the Gelth, though by this time the Time Lords had already been destroyed in the Last Great Time War. (TV: The Unquiet Dead)

Some races were only capable of changing the past or future because they existed 'outside' of time, and were therefore not subject to its laws. Examples of this included the Time Lords and the Arboreteans, a race of plant-like aliens who travelled back to their birth at the moment of their deaths and could live their lives over and over, correcting past mistakes. Although they were hunted to extinction by Dr. Kole Paddox in his attempt to copy their 'power' and change his past, they refused to contact past generations to make themselves a warlike people that could have opposed his plans as they did not wish to erase their peaceful society. Although Paddox succeeded in sending himself into his past self, since he was a human who lived 'inside' Time, he couldn't change or influence his life, only watch his past self proceed along the same path as it had before. (PROSE: Festival of Death)

During a visit to Apalapucia, Amy Pond became stuck in a separate timestream in the Two Streams Facility that allowed people to experience the rest of their life in a short amount of time due to the Chen-7 virus. The Eleventh Doctor and Rory Williams worked together to rescue Amy, using the TARDIS to lock onto her timestream and enter it, only to find they arrived 36 years into the future where they meet a much older Amy from an alternate timeline where they'd failed to rescue her. Communicating with her younger self through a mirror, the two Amy's realized that this was because the future Amy refused to help rescue her younger self out of a selfish desire to leave herself. By reminding herself of her love for Rory, young Amy was able to convince her future self to help. Following instructions from the Doctor, Rory was able to bring Amy into her own future and they returned to the TARDIS where the Doctor locked the future Amy out as the TARDIS couldn't sustain the paradox of two Amy's in the same place. Future Amy was killed by the Handbots, but the future was negated by the rescue of her present day self. (TV: The Girl Who Waited)

After being sent to 1938 Manhattan by the Weeping Angels, Rory Williams encountered himself as an old man in the Winter Quay and witnessed himself die. This was caused by the Angels throwing him even further back in time to feed off of his temporal energy. The group realised that if they kept Rory from being sent back in time again and this from happening, it would create a paradox and poison the temporal energy the Angels fed off of, killing them. Rory created this paradox by jumping off the roof of the Quay with his wife Amy Pond. This erased the Quay from existence and killed all but one of the Weeping Angels, which was left very weak. It also restored Amy and Rory to life. However, the Angel sent Rory back to an unknown time, and the Doctor told Amy that creating another paradox wouldn't work, as it would "rip New York apart". (TV: The Angels Take Manhattan)

Meeting one's self
When the Brigadier's "past" (1976) self met his "future" (1983) self, this created a discharge of energy which rendered both of them unconscious and caused amnesia in the 1976 self. The Fifth Doctor cited this as an example of the Blinovitch Limitation Effect. (TV: Mawdryn Undead)

When dug out from beneath Cardiff in 1901, Jack Harkness ordered Alice Guppy to put him in suspended animation, as he knew meeting his younger self working for Torchwood Three at the time had consequences. (TV: Exit Wounds)

When the seventh incarnation of the Doctor met his his earlier self, the "elder" Doctor made sure his younger self would not remember. (PROSE: Cold Fusion) At the same time, on other occasions the Doctor has met himself in a way that influenced the past, creating a past for them both. (TV: Time Crash)

At the Two Streams Facility, Amy Pond was able to meet a version of her future self due to the temporal engines keeping two separate timelines in sync. This didn't interrupt Amy's time differential due to the Amys not coming in physical contact and the eventual negation of the older Amy's timeline. (TV: The Girl Who Waited)

Behind the scenes

 * The outcomes of time paradoxes in the Doctor Who universe have varied according to the needs of the story. The Virgin New Adventures version of canon stated that the Time Lords of Gallifrey's evolved before any other sentient being in the universe and lived in the distant past (relative to the timeline). Therefore, they could visit any number of possible futures though they would not affect their own pasts out of fear for undoing that future. (The New Adventures novel Lungbarrow by Marc Platt changed this when it explained that the First Doctor had visited Gallifrey's Dark Times in his personal past.) Fans continue to debate, discuss and theorise as to the nature of time paradox in the Whoniverse.