Temporal paradox

Temporal Paradoxes are one theoretical element of time travel, not just for the Time Lords but all time active civilizations. It is an event or occurrence that could not have happened had a time traveller appeared to make it happen.

Paradoxes are caused when events of history are radically changed, resulting in all the events flowing from that theory being changed or prevented from happenning. Examples include the (premature) killing of a famous or influential person, or in fact the prevention of the death of such a person. The action results in a domino-like changes of subsequent events - if Lawmaker X is killed, all the laws he helped pass might never be passed, resulting in a different society.

Technically, the only way to un-do a paradox is to cause another, either by travelling back to stop the actions of the time-traveller who caused the paradox before it happens (resulting in a Grandfather paradox; see below) or by attempting to repair the damage to the timeline after the fact.

Examples

Grandfather Paradox

The most commonly known form of a paradox is the Grandfather Paradox. It is defined as an attempt to change an event that, is successful, would render itself moot, since the event would be changed retroactively, therefore, there would be no need to change it. The classic explanation of this concept is as follows (in many variations):

• If I went back in time and killed my grandfather before my father was born, my father would not be born, therefore neither would I, so I would not be able to travel back in time... so my grandfather lives, and my father is born, and thus I am alive and able to travel back in time...

Predestination Paradox

A temporal paradox created through time travel of a subject leading to the subject's passage through time (or a part of said subject's timeline to be formed as a result of time travel).

Ace had a predestination paradox as part of her timeline, when she saved Kathleen Dudman and her baby Audrey (who was Ace's mother) she in turn created her own time line. (DW: The Curse of Fenric) In some ways this particular example is the reverse of the Grandfather Paradox. Another example was when the TARDIS took The Doctor into the future when it detected his daughter's signal. However, the TARDIS arrived early, and The Doctor accidentally created his daughter, causing the signal.

Butterfly Effect

The concept that a single alteration of history, no matter how small, will have reality-changing effects in the present. Theoretically, the farther back in time one goes, the easier the changes are to make.

In practice, the changes to time required to cause a true paradox must be fairly massive, or involve a person of historical significance. If a time traveller goes back and pours water on the first man-made fire, this MAY cause the elimination of that species, but most likely, another caveman will discover fire fairly soon afterwards. The actual name of the man who dicovered fire is rarely known; it is the fact that it is discovered at all that it is important.

The term is a reference to the Ray Bradbury short story A Sound of Thunder, in which a time traveller steps on a prehistoric butterfly in the past, resulting in a causal domino effect that altered the present when the travellers returned.

Prevention of paradoxes

The Doctor has mentioned that when the Time Lords still existed, creation of paradoxes was impossible and that any paradoxes that did manifest could be fixed. This might mean that paradoxes weren't possible or common before the Last Great Time War or they had different effects.

In their absence, there are a number of actions or forces that can prevent or reduce the effects of a paradox:

Paradox Machine

A temporal paradox can be held in place by a paradox machine. One such machine was used to allow the Toclafane, who were Humans from the future, to travel back in time to 2008 where they attacked and killed many of their ancestors. Because they were the future Human-race and they were destroying the Human-race of 2008, they should have cancelled themselves out and prevented their creation, but the paradox machine prevented this from happening. (DW: The Sound of Drums/ Last of the Time Lords)

Blinovitch Limitation Effect

The Blinovitch Limitation Effect prevents time travellers from directly affecting their own timeline, theoretically preventing a number of self-eliminating paradoxes from occurring. However, the Effect is not universal in its efficacy.

When Rose Tyler saved her father, Pete Tyler (DW: Father's Day), the BL Effect shold have prevented it from occurring. There was also the problem that an earlier Doctor and Rose Tyler were watching this change in the timeline.

When the paradox occurred, the earlier versions of the Doctor and Rose disappeared and the inside of the Doctor's TARDIS disappeared, rendering it an empty police box-shaped shell. There also seemed to be unusual occurrences such as a song being played before it was made or the first phone call being repeated. As well, the car which should have killed Pete Tyler, kept on following him in order to repeat the accident.

Reapers

Reapers are a naturally occurring event/effect caused by the creation of a paradox. The participants of the paradox (both those who caused it and those affected) are sealed off in a time-loop, and the Reapers systematically eliminate those in the loop from existence, allowing the remaining timeline to heal itself.

In the aforementioned event, Pete Tyler was able to fix the paradox by getting killed by the car, restoring the original series of events. This caused all the damage that was done by the Reapers and the paradox to repair itself. Though there were minor changes to the timeline, Peter's death was apparently enough. (See earlier reference to the Butterfly Effect.)

Faction Paradox

In some organizations the paradox is a symbol of almost god-like power. The Faction Paradox (as the name suggests) practically worshipped the act of a Paradox, not just for the idea, but the power that a Paradox evokes. (EDA: Alien Bodies, Unnatural History, Interference)