Temporal paradox

Temporal paradoxes, or time paradoxes, were events arising from means other than the normal flow of cause and effect. Paradoxes varied greatly in terms of origin and severity: at the least-harmful end were fairly benign sorts of paradox, where effect preceded cause, resulting in a confusing-but-tenable event; (TV: The Time of the Doctor, Time Heist) on the more severe end, however, were thoroughly impossible paradoxes, such as the descendants of a race travelling back in time to exterminate their predecessors. (TV: Last of the Time Lords) Perhaps the most destructive form of paradox involved travellers "interfering" with the Web of Time or altering a fixed point in time, an act which could destabilise or destroy the entire space-time continuum. (TV: Attack of the Cybermen, The Wedding of River Song)

The Time Lords
The Time Lords enforced the 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 Doctor and, (TV: Utopia) the Time Lords all disappeared 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. (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. It occurred when an entity came into contact with its past or future self. (TV: Day of the Daleks, Mawdryn Undead)

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

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)

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)

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 et al.)

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)

Origin of the universe
According to the Fifth Doctor, the 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)

The existence of the Trojan Horse was the result of a temporal paradox, as the First Doctor had heard about it but considered it absurd and impractical, believing that it had been invented by Homer as a dramatic device. However, he was eventually forced to suggest the idea to the Greeks, whose victory was then recounted by Homer, the source for the Doctor's knowledge of the Horse. (TV: The Myth Makers)

Guerrilla 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)

According to one account, the existence of K9 was the result of an temporal paradox, as Professor Frederick Marius got the inspiration to construct K9 Mark I from examining K9 Mark III, who was himself built by the Doctor after meeting K9 Mark I. (PROSE: Tautology)

All TARDISes were equipped with a time safe, which allowed Time Lords to send items or information to their past selves by placing the items in the safe in the future and setting the coordinates to send the items into the past. The Fifth Doctor described it as a permitted temporal paradox that had to be used sparingly, such as when he sent himself the diary of Captain Richard Haliwell of the British Imperial Spacefleet so that the Doctor, Turlough and Kamelion would be able to assist the expedition against the Vrall they encountered on the Moon. (PROSE: Imperial Moon)

While attempting to join the Cult of the Heretic, the Master was told that he would have to kill his own past self to prove his dedication to their cause. To this end, the Master attacked his past self at a point when he was attempting to attack the Time Lord complex on Tersurus, but claimed in hindsight that he had chosen to attack his past self because he knew that the younger Master would survive the attack (although the Cult used this opportunity to use the Masters as pawns in their own plans). (AUDIO: The Two Masters)

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 existence of the Doctor's daughter Jenny forcibly brought the Doctor's TARDIS to her. However, the TARDIS arrived too soon, and shortly after the Tenth Doctor exited the TARDIS, Jenny was created from him, which then made the TARDIS being brought to her and her creation the reason the TARDIS was brought to her to begin with. (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)

A species that the Eighth Doctor met, The Unnoticed, was created by a temporal paradox. The Lebensweltian criminals Darlow, Gimcrack and Svadhisthana became fused together when they were near the Unnoticed. Touching one of them caused an energy discharge since the Unnoticed were their descendants. The criminals were thrown millennia back in time and evolved into the Unnoticed, forgetting their origin. (PROSE: The Book of the Still)

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) Additionally, River never would have been born at all if she hadn't sacrificed herself in the Library (TV: Forest of the Dead) to save the Doctor's life, because if the Doctor had died that day, he never would have met Amy Pond and Rory Williams, so they never would have spent their wedding night on the TARDIS, and they would not have had a child who was part Time Lord.

Another example is when Rose Tyler, as the Bad Wolf entity scattered the words "Bad Wolf" through time and space in order to draw herself to the Game Station in 200,100 where she became the Bad Wolf to destroy the Daleks attacking that time period and save the Ninth Doctor. This is another example of an ontological paradox, in which a meme self-perpetually creates itself. (TV: The Parting of the Ways)

After being sent back to 1994 by the Weeping Angels, Mark Whitaker, assisted occasionally by the Eleventh Doctor, Amy Pond and Rory Williams 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, allowing them to grow stronger and keep doing it to other people until they'd done it to the entire human race, trapping them in a loop where people would shape and alter their pasts from the future. As the death of Rebecca had caused Mark 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-travelling 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 and sending a "psychopath", River Song, to murder 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, known as the bootstrap paradox. (TV: Before the Flood) 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. In that same meeting, the Doctor learned from River that it was possible to open the TARDIS doors by snapping his fingers, a fact River was aware of because she had seen the Eleventh Doctor do it, who in turn only knew it was possible because the Tenth Doctor had been informed by River. (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)

The Twelfth Doctor was called by Madame Karabraxos to rescue the Teller from the Bank of Karabraxos before its destruction. The Doctor set up the heist and erased his own memory of everything after the phone rang to the start of the heist. As a result, he was unaware he set it up. During the heist, he gave Karabraxos his phone number as well as the information that he was a time traveller to offer his help with her greatest regret when she grew old. Thus Madame Karabraxos got the number to call the Doctor from the Doctor himself and called him for help because he told her to after realising that he'd set the whole thing up. (TV: Time Heist)

While visiting NASA in 2017, the Twelfth Doctor and Bill Potts discovered a message reading "God Save The Queen" under Mars' northern polar ice cap. The Doctor, Bill and Nardole travelled back in time to 1881 which was when the TARDIS determined that the message had been written. After the TARDIS mysteriously departed with Nardole, the Doctor and Bill helped settle a conflict between the Ice Warriors led by Queen Iraxxa and British Army soldiers led by Colonel Godsacre. After the Doctor contacted Alpha Centauri for help, Alpha Centauri requested a physical marker to help guide in the rescue fleet that he was sending. Thus Godsacre, the Doctor and Bill created the message that had drawn them to Mars in the first place in order to fulfil Alpha Centauri's request. (TV: Empress of Mars)

After genuinely deciding to change, the incarnation of the Master chose to mortally wound  to ensure that he would regenerate into her and make the choices she had made. Due to their time lines being out of synch, the Master and thus Missy would not recall their meeting on the Mondasian colony ship. Also, during her time on the ship, Missy recalled when a woman had shoved her up against a wall in her previous incarnation and made him promise to always keep a spare dematerialisation circuit on him. She realised that she herself did this and performed the action after learning that the Master had burned out the circuit in his TARDIS, ensuring that she herself had the spare he needed on her at that moment. (TV: The Doctor Falls)

Un-creating one's own past or future
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 disappeared 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 Arboretans, 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. Koel Paddox in his attempt to copy their 'power' and change his past, the Arboreteans 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 his mind 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)

Using the Time Scoop, Borusa was able to remove the first four incarnations of the Doctor from their proper points in the Doctor's timeline and transport them to the Death Zone. Each incarnation of the Doctor was able to continue to exist even with the earlier ones removed, however, the Fifth Doctor demonstrated considerable ill effects from each abduction, claiming to be "whittled away piece by piece", and retreated to the safety of the TARDIS. He also claimed that his timeline had been destabilised and that his existence was threatened by the Fourth Doctor being trapped in a time eddy, even briefly fading from existence before contact with the First Doctor strengthened him. (TV: The Five Doctors)

Making contact with the Cult of the Heretic, the Master agreed to help them in their plan to regenerate the universe if they would permit him to join them in the anomaly cage. The Cult claimed that they would only allow the Master to join them if he killed his past self, but both sides betrayed the other; the Master claimed that he attacked his past self in a manner that he knew his past self would survive, but then the Cult interfered, attacking the older Master and transferring the past Master into his future self, simultaneously trapping the older Master in his past self, telling the younger Master that the other Master was just another Time Lord. The Cult's intention was to allow the young Master to depart in the body of his own future self and then kill the older Master in his past self, with the plan being that the paradox of the younger Master in his own future self after the death of his past body would unmake the universe, allowing the Cult to recreate it. The scale of the paradox was limited as the future-in-past Master was able to escape before the Cult could kill him, but the two Masters travelling in each other's bodies still caused serious damage to reality. The timeline was damaged beyond repair by the time the Seventh Doctor was able to reveal the truth to both Masters, but the three were able to reach the Anomaly Cage before they were destroyed along with the universe, allowing the Doctor to reset reality back to the way it had been before the Cult's actions destroyed reality. The only consequence of these events was that the future Master essentially created his crippled past incarnation. (AUDIO: The Two Masters)

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)

created a paradox with the Toclafane invasion during the Year That Never Was as the Toclafane were humanity from the future. (TV: Last of the Time Lords) The Toclafane slaughtered at least one tenth of the people of Earth. (TV: The Sound of Drums) As the Master turned the Doctor's TARDIS into a paradox machine, the paradox was able to be held in place and not cause problems. When Captain Jack Harkness destroyed the paradox machine, the paradox caused time to rewind to the start of the invasion, sending the Toclafane back where they came from and changing the future. (TV: Last of the Time Lords)

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 realised 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 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) When the Time Lords are involved in the Doctor meeting his past selves, they apparently erase the memories themselves. (TV: The Three Doctors, PROSE: The Empire of Glass)

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 Amy's not coming in physical contact and the eventual negation of the older Amy's timeline. (TV: The Girl Who Waited)

The Eleventh Doctor, in an effort to change Kazran Sardick into a better man, took his twelve-year old self into his future to see himself as an old man and learn what he would become. The two Kazrans were able to touch without any negative effects and seeing his own future caused Kazran to change what he would become after returning to his own time. (TV: A Christmas Carol)

The War Doctor, the Tenth Doctor and the Eleventh Doctor met each other and worked with each other to stop a Zygon plot. They later called upon all of their other incarnations to save Gallifrey and the Time Lords from destruction. (TV: The Day of the Doctor)

When Mark Whitaker- trapped in the past after a Weeping Angel sent him back to 1994 from 2011- was visited by his past self in 2001, the paradox when he grabbed his younger self's wrist triggered a fire that burned down the elder Mark's apartment. Shortly after this, the Eleventh Doctor, Amy Pond and Rory Williams were visited by a future version of Rory who had been sent back from two years in the future by another Weeping Angel; the Doctor protected the two Rorys from causing a similar explosion with the sonic screwdriver and then travelled forward to the time that the future Rory had come from, allowing him to send the younger Rory back and preserve history. (PROSE: Touched by an Angel)

While visiting a Mondasian colony ship, the incarnation of the Master met. (TV: World Enough and Time) Like when the Doctors met due to the Moment, the two Masters' time lines were out of synch and the Master would not retain the memory and thus Missy would not have it as well. At the end of the encounter, Missy created her own future by mortally wounding her past self to force his regeneration into her. In return, the Master killed his future self with his laser screwdriver which prevented her from regenerating and apparently killed Missy permanently. (TV: The Doctor Falls)

An encounter between the Tenth and Thirteenth Doctors in 1969 caused a paradox which rewrote the history of Earth, creating an alternate timeline. (COMIC: A Little Help From my Friends, Alternating Current)

Rewriting established events
Paradoxes could also be created by time travellers rewriting the course of events, even if the events did not directly pertain to their personal chronology. The Eighth Doctor and Charley inadvertently created a paradox by arriving at Edward Grove in 1906, as Charley met Edith Thompson who would later work for Charley’s family in the 1930s and commit suicide after Charley’s disappearance. Her meeting Charley on her travels removed the reason for her suicide, creating a paradox which briefly gave Edward Grove sentience. Grove used the paradox to create a time loop focusing on the downstairs staff and repeatedly murdered them to create traumatic events to sustain itself. When Charley convinced Edith of her self-worth, so she wouldn’t commit suicide at all, the paradox was undone and loop broken. (AUDIO: The Chimes of Midnight)

The Tenth Doctor saving Adelaide Brooke from her fate on Mars, which was a fixed point in time, (TV: The Waters of Mars) and bringing her back to Davies Street created a paradox. As observed by the Doctor’s TARDIS this paradox widened into a time fracture, centred on Davies Street. (PROSE: What the TARDIS thought of “Time Lord Victorious”)

Other references
The Aja'ib contained tales involving paradoxes. (PROSE: Bafflement and Devotion)

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 stated that the Time Lords of Gallifrey 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.

Doctor Who: Legacy
In the story of Doctor Who: Legacy, the Doctors and their companions encounter time paradoxes, created as a result of time traveling Sontarans launching attacks throughout history, which they close to stabilise time, evading Reapers along the way.