Fixed points in Time were points in time at which events could not change. Time Lords see this in their minds at all times and was described as 'the curse of the Time Lord' by the Doctor.
Definition
Fixed points were events and/or individuals who have such a long-standing impact on the timeline that no one, not even Time Lords, were allowed to interfere with their natural progression. The Doctor, while free to interfere in alien invasions and save planets in most cases, could not interfere/interact with these fixed points.
Notable fixed points
Jack Harkness became a fixed point in time after his rejuvenation by Rose Tyler, which also resulted in him becoming immortal. (DW: The Parting of the Ways, Utopia) Jack later lost his immortality due to the Miracle Day phenomenon but it is unknown if this meant he was no longer a fixed point. (TW: The New World)
The destruction of Pompeii by the Vesuvius volcano was a fixed point in time, caused by the Doctor. As well as its eruption, the deaths of most of the citizens of Pompeii were fixed. The Doctor, however, did save one family. (DW: The Fires of Pompeii)
The death of explorer Adelaide Brooke marks one of the few times the Doctor intentionally interfered with a fixed point, under the rationale that, as the last surviving Time Lord, the Laws of Time were his to command. Ultimately, he was unable to prevent Brooke from committing suicide, thereby allowing the timeline to unfold with only minor changes. (DW: The Waters of Mars)
The Doctor also interfered with a fixed point in time a short while before or after the previous event, by saving the life of Emily Winter, a film actress in 1920s-era Hollywood. He was put on trial by the Shadow Proclamation for this. (IDW: Fugitive)
The death of the Doctor himself, in his eleventh incarnation, was a fixed point in time; the Doctor was killed in Utah, at Lake Silencio, on 22nd April, 2011, at 5:02pm. (DW: Let's Kill Hitler)
Time in flux
The opposite of fixed points was time being in flux. At these points time could change completely.
- It is unknown what happens to fixed points if the future is changed while time is in flux.
Flux points were insignificant events that could be altered with no consequence. Due to this, the Doctor normally meddled at these points in time. (DW: The Christmas Invasion, NSA: I am a Dalek) When the Tenth Doctor first met Martha Jones, he told her that 'Crossing into established events is strictly forbidden ... except for cheap tricks.'. (DW: Smith and Jones)
Fluxing points
- 1969 was in flux, as discovered by the Doctor when it was revealed that WOTAN was supposed to conquer Earth (PDA: The Time Travellers) instead of being defeated by the Doctor. (DW: The War Machines)
- Sarah Jane Smith was shown an alternative timeline by the Doctor, explaining how 1911 was now in flux because of the threat of Sutekh. (DW: Pyramids of Mars)
- As the Doctor explained to Rose Tyler, when she said that he couldn't give dead Human corpses to the Gelth, as she knew for a fact that dead bodies weren't walking around in 1869, time was in flux, changing every second and her "cosy little world can be rewritten like that". (DW: The Unquiet Dead)
- Although the Doctor noted to Rose that Harriet Jones would serve three successive terms as British Prime Minister, he himself would later, in his next incarnation, cause her political downfall, leading to her losing the title of Prime Minister to Harold Saxon. (DW: The Sound of Drums) She would later die the following year at the hands of the Daleks during their invasion of Earth. (DW: The Stolen Earth)
- After they saw Frank Openshaw exterminated by a Dalek, Rose Tyler and the Doctor meddled in history for him so that he met his wife Sandra years earlier than in the original timeline. (NSA: I am a Dalek)
- In 2020, when a drilling operation in Cwmtaff disturbed a Silurian civilisation, the Doctor told Amy Pond, Nasreen Chaudhry and Eldane that this encounter could lead to either a peaceful relationship or a devastating war. The Doctor described these events as opportunities. (DW: Cold Blood)
- When the Dalek Emperor and his flagship survived the end of the Time war in 199,909, the Daleks secretly altered the history of the Fourth Great and Bountiful Human Empire, controlling it via the Jagrafess. Unlike what the Ninth Doctor remembered it to be, the Human race at its height, humanity's development was instead stunted. (DW: The Long Game,Bad Wolf)
- At one point, the Korven altered the history of Earth (which they previously invaded in 2480), causing part of it to be controlled by the corrupt, totalitarian Department. (K9TV: The Eclipse of the Korven)