How does Doctor Who use time travel to create alternate realities?

Doctor Who is a British science fiction television series that has been running for over 50 years. The show's central premise revolves around the Time Lords, a race of aliens with the ability to manipulate time and space. The Doctor, the main character of the show, is the last of the Time Lords and travels through time and space in a time machine known as the TARDIS.

One of the recurring themes in Doctor Who is the use of time travel to create alternate realities. The Doctor and his companions often find themselves in situations where they must alter the course of history or prevent catastrophic events from occurring. In doing so, they create alternate timelines and realities.

For example, in the episode 'The Wedding of River Song,' the Doctor finds himself in a universe where time has stopped and all of history is happening at once. In order to restore normality, the Doctor must marry River Song, a character who is revealed to be his wife. This marriage creates an alternate timeline in which the Doctor survives his death.

Another example is the episode 'Father's Day,' in which the Doctor and his companion Rose Tyler travel back in time to witness Rose's father's death. When Rose saves her father's life, she creates an alternate reality in which the Earth is overrun by monstrous creatures known as the Reapers. The Doctor must help Rose repair the timeline and prevent the alternate reality from becoming permanent.

Doctor Who's use of time travel to create alternate realities is a key aspect of the show's appeal. It allows for endless storytelling possibilities and enables the show to explore complex themes and ideas.