Single Timeline

The single timeline theory suggests that time is fixed and unchangeable. There is only one timeline, and everything that happens — including time travel — is already part of it. However, there are two major interpretations:

1. Single Timeline With the Grandfather Paradox

In this version, if a person goes back in time and changes something significant (like killing their grandfather), it would create a paradox. They would never be born to commit that action, creating a logical contradiction. This form of the theory suggests that time travel to the past is either impossible or leads to universe-breaking paradoxes.

2. Single Timeline Without the Grandfather Paradox

This version avoids paradoxes by assuming that anything a time traveler does in the past has already happened. If you go back in time, your actions were always part of history — and you cannot change the outcome. You may try to kill your grandfather, but you’ll always fail, or something will intervene. You fulfill history, not rewrite it.

Examples in Media

  • 12 Monkeys: The protagonist’s actions in the past don’t change the future — they cause it.
  • Harry Potter and the Prisoner of Azkaban: Time travel is used to help, but everything was already part of what happened.
  • Tenet: Time is deterministic. Events are set in stone, and characters can only fulfill, not alter, them.

Consequences

In a single timeline, time travel becomes a dangerous tool. If an evil person goes to the past, they might unintentionally cause or reinforce tragic events — but not change them. Likewise, people with good intentions might cause disasters they tried to stop. The past resists change; attempts to alter it may create self-fulfilling loops.

In the version with paradoxes, time travel risks breaking logic entirely. It opens up contradictions that can’t be resolved, which suggests time travel should be either impossible or heavily restricted.

Result

Whether or not paradoxes are allowed, a single timeline means events are locked in. Free will may be limited or an illusion. The worst-case scenario: a destructive event in the past — caused by a time traveler — cannot be prevented, even if we know it’s coming. The more people interfere, the more likely they are to ensure the very future they fear.

In short, time travel in a single timeline is either paradoxical and dangerous — or bound by fate, with no way to truly change history.