Temporal paradox

Time or temporal paradoxes were events in which a travellers "interfered" with the Web of Time.

The Time Lords
The Time Lords had the Laws of Time, which they used to prevent people from meeting themselves (DW: The Three Doctors) or to transport a passenger from the present to the past. (DW: The Hand of Fear) Except for the Ninth Doctor and the Master, the Time Lords all died in the Last Great Time War. The Ninth Doctor mentioned that when the Time Lords still existed, creation of paradoxes was all but impossible and that any paradoxes that did manifest could be fixed. (DW: Father's Day) The Time Lords used their stewardship of time travel to prevent "un-authorised" time travel (DW: The Time Warrior) which Dastari of the Third Zone, for example, believed had a selfish political motivation (DW: 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. (DW: 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
Although their actual nature remains unclear, Reapers might have acted as a way of "neutralising" the consequences of time paradoxes by destroying the people and planets involved, as when Rose Tyler changed history. (DW: Father's Day)


 * See main article.

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


 * See main article.

Faction Paradox
Faction Paradox, (as the name suggests) opposed to the Time Lords, practically worshipped the act of a Paradox, not just for the idea, but for the power that a Paradox evoked. (EDA: Alien Bodies onwards)


 * See main article.

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


 * See The Trickster and Trickster's Brigade.

Creating Your Own Past or Future
When the Seventh Doctor's companion Ace visited 1943 with him, she, in the Doctor's words, created her own future, when she rescued her own mother, Audrey, and her grandmother Kathleen Dudman. (DW: 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 accidentally created his daughter, causing the signal. (DW: The Doctor's Daughter) This is also known as a predestination paradox.

In some cases, a time traveller had information that comes from the act of time travelling, learning it from a future or past version of himself; thus, the information has 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 (DW: Forest of the Dead). Another example would be in the episode (DW: Blink). Sally Sparrow 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. So neither the Doctor, nor Sally actually wrote the transcript. In another example, a future Doctor left a message for his past self on the planet Heaven (NA: Love and War), and in another, the Fifth Doctor met the Tenth Doctor in (DW :Time Crash) and saw him cancel out a supernova with a black hole, which in turn meant the Tenth Doctor remembered seeing himself doing it, allowing him to do so. This is properly called an ontological paradox.

Un-Creating Your Own Past or Future
When Rose Tyler saved her father, Pete from his death in 1987, a number of 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 in order to repeat the accident. Reapers appeared to eat up people and landmarks in the vicinity, and eventually the world. Pete chose to die and restore the timeline. (DW: 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 in order 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 that the bomb only killed Daleks and their Ogron slaves. In the new timeline, World War III never happened. (DW: Day of the Daleks) Other examples exist of similar corrections of the previous timeline. On other occasions, though, the Fourth Doctor explained that the powerful (even more so than the Time Lords) Sutekh could destroy Sarah Jane Smith's future easily, and took her to an alternative 1980 where this had happened. (DW: 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. (DW: The Unquiet Dead)

Meeting Yourself
When the Brigadier's "past" (1976) 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. (DW: Mawdryn Undead)

Though he did not meet himself then (as far as we know), at some time during World War II, at least four versions of Jack Harkness existed on Earth at the same time: the first, a Time Agent, the second the now-immortal ex-Time Agent, the third in cryogenic suspension in Torchwood 3 (TW: Exit Wounds), and the fourth who had fallen through the rift back to 1941 (TW: Captain Jack Harkness.)

When the seventh incarnation of the Doctor met his his earlier self, the "elder" Doctor made sure his younger self would not remember. (MA: 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. (DW: 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 abortion of the older Amy's timeline. (DW: 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 had 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.