Alternate timeline

An alternate timeline is a reality that diverges from the "true" timeline due to the actions of time travellers or other temporal phenomena. They are different from a parallel universe in that they do not exist as seperate continuums.

Physics

Alternate timelines can be visited accidentally by contact with a time track (DW: The Space Museum, BFA: The Mutant Phase) or other anomalies (BFA: Colditz) in the Time Vortex. Deliberate travel to these timelines is possible if the traveller possesses something, such as the sigil ring, that allows them to change time streams. (MA: Goth Opera) Whether it is possible or not to travel to a timeline after it has been negated is unknown, though every point in time has its alternative (DW: Pyramids of Mars) and so every alternative may always exist.

They may diverge from points in history where there is temporal fluxuation, but beings with enough power are capable of changing time enough to create them, such as Sutekh and the Chronovores. Because the universe only has a finite amount of mass and energy, the creation of an alternate timeline adversely affects the real timeline, causing chaos throughout the universe; releasing enough energy can destroy the timeline and set history back on its correct course. (NA: Blood Heat) This may explain why the effects of the reality bomb (DW: Journey's End) and Rassilon's Ultimate Sanction (DW: The End of Time) could be observed before either event had even occurred.

Though a timeline may be negated, memories of it could still be retained.

Timelines encountered by the Doctor

Timeline	Negated By	Featured in
The First Doctor and his companions become exhibits in the Moroks' Space Museum	First Doctor, Ian Chesterton, Barbara Wright, Vicki	DW: The Space Museum
Barbara Wright marries Ptolemy I and becomes Queen of Alexandria	alternate Barbara	VD: The Book of Shadows
WOTAN conquers Earth (this was the correct timeline before it was changed by the Doctor)	First Doctor	PDA: The Time Travellers, DW: The War Machines
Nancy Grover uses Grold technology to become a goddess	Third Doctor and Liz Shaw	MA: The Eye of the Giant
The Power Elite take over Earth	Third Doctor	VD: Prisoners of the Sun
Freedom fighters from the 22nd century assassinate Sir Reginald Styles	Third Doctor and Jo Grant	DW: Day of the Daleks
Theodore Cassells's experiments lead to a global war which devastates Earth	Third Doctor	TVA: A Stitch in Time
The Third Doctor dies on Dust and regenerates	Eighth Doctor by destroying Gallifrey (EDA: The Ancestor Cell)	EDA: Interference - Book One, Interference - Book Two
Sutekh is freed and devastates Earth	Fourth Doctor and Sarah Jane Smith	DW: Pyramids of Mars
Daleks undergo the Mutant Phase	Dalek Emperor	BFA: The Mutant Phase
Melanicus creates the Millennium Wars	Fifth Doctor, Sir Justin and Shayde	DWM: The Tides of Time
Humans evolve into Haemovores	Seventh Doctor and Ace	DW: The Curse of Fenric
The Dark Matrix terrorises England	Seventh Doctor and Ace	PDA: Matrix
The Nazis win World War II after a piece of laser technology is left at Colditz Castle	Alternate Eighth Doctor and Elizabeth Klein (inadvertantly)	BFA: Colditz, Klein's Story
Adolf Hitler uses the Timewyrm's power to enable Nazi Germany to conquer Earth	Seventh Doctor and Ace	NA: Timewyrm: Exodus
Chad Boyle kills Dorothy McShane with a brick to the head	Ace	NA: Timewyrm: Revelation
The Process enslaves the Time Scaphe crew	Seventh Doctor	NA: Cat's Cradle: Time's Crucible
Silurian Earth	Destroyed, not negated, by Seventh Doctor	NA: Blood Heat
The Doctor doesn't leave Gallifrey, allowing many alien races to invade Earth	Seventh Doctor, Ace and Bernice Summerfield	DWM: Time & Time Again
The Knights of Velyshaa conquer Gallifrey	Fifth Doctor, Sixth Doctor, Seventh Doctor, the Temperon	BFA: The Sirens of Time
Humans don't evolve and Earth is a Tractis colony known as Paratractis	Eighth Doctor, Sam Jones, Jo Grant	EDA: Genocide
Pete Tyler doesn't die and Reapers begin consuming humanity	Pete Tyler	DW: Father's Day
The Year That Never Was	Tenth Doctor, Martha Jones and Captain Jack Harkness	DW: Last of the Time Lords
Donna's World	alternate Donna Noble, with assistance from Rose Tyler	DW: Turn Left
The Progenitor Daleks repopulate Skaro and exterminate humanity to extinction	Eleventh Doctor and Amy Pond	VG: City of the Daleks
The Doctor's TARDIS explodes, creating a Time Field which erases parts of history	Eleventh Doctor, Amy Pond, Rory Williams (Auton) and River Song (DW: The Big Bang)	DW: Series 5
Amy Pond is stranded on Apalapucia for 36 years	Eleventh Doctor	DW: The Girl Who Waited
River Song's World	Eleventh Doctor, Amy Pond, Rory Williams, River Song, the Teselecta	DW: The Wedding of River Song

Timelines encountered by others

Timeline	Negated By	Featured in
Andrea Yates' World	alternate Andrea Yates	SJA: Whatever Happened to Sarah Jane?
The Trickster's Earth	Eddie and Barbara Smith	SJA: The Temptation of Sarah Jane Smith
Ship erases Sarah Jane Smith, Luke Smith and Clyde Langer	Adam Lloyd	SJA: The Mad Woman in the Attic	Ironside World (Doctor never arrived in Victory of the Daleks and Churchill wins the war with the Daleks	Not Negated but Daleks are destroyed	Doctor Who Annual 2011