Time travel concept used in many sci-fi works, also known as a causal loop, or an ontological paradox. An object or idea travels into the past and becomes its own future travelling self, effectively having no origin or ending.

The term bootstrap paradox was originated by a Robert Heinlein story "By His Boostraps", which plays with a tangle of time travel loops and contains at least one such paradox in the form of a notebook that seems to have written itself. The title itself refers to the phrase "pull yourself up by your bootstraps" that generalizes the idea of starting something from nothing.

For a physical idea of the paradox, imagine a ball. It is hit by another ball and rolls foward into some sort of warp, that sends it into the past and a few feet backwards, where it continues to roll forward to be the ball that hit itself originally, stopping to be the ball that gets hit again. The event of a ball getting hit has no apparent starting point or ending point, and the ball and the rolling momentum both appear to have literally sprung from nothing.

Another example that highlights an idea created from such a paradox was delivered in monologue in the opening of the Doctor Who episode Under the Lake. A fan of music time travels to the past to meet the original composer. To the traveler's shock, the composer never wrote such a work. To keep time stable, the traveler gives a copy of the music to the composer, who claims it as their own and publishes the music to much fame, thus creating a loop of cause and effect. The true bootstrap paradox comes from the resulting question: who actually wrote the music? The loop of personal choices can be broken down into parts, but the origin of the music cannot be, and is thus labeled as a paradox.

Use this tag for questions directly about such situations in any work or media. Do not use tag for more general questions about works which have simply have time loops (there are many different kinds) or questions which are not asking about the paradox itself.

For more information, see the Wikipedia entry on causal loops.

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