The bootstrap paradox, or ontological paradox, is a paradox of time travel that refers to scenarios whereby items or information are passed from the future to the past, which in turn become the same items or information that are subsequently passed from the past to the future - this creates a circularity of cause-effect such that the items or information have no discernible origin. Thus, the paradox raises the ontological questions of where, when and by whom the items were created or the information derived.
After information or an object is sent back in time, it is recovered in the present and becomes the very object or information that was initially brought back in time in the first place. Numerous science fiction stories are based on this paradox, which has also been the subject of serious physics articles.
The term "bootstrap paradox" refers to the expression "pulling yourself up by your bootstraps"; the use of the term for the time-travel paradox was popularized by Robert A. Heinlein's story By His Bootstraps.
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Because of the possibility of influencing the past while time traveling, one way of explaining why history does not change is to posit that these changes already are contained self-consistently in the past timeline. A time traveler attempting to alter the past in this model, intentionally or not, would only be fulfilling his or her role in creating history, not changing it. The Novikov self-consistency principle proposes that contradictory causal loops cannot form, but that consistent ones can.
However, a scenario can occur where items or information are passed from the future to the past, which then become the same items or information that are subsequently passed back. This not only creates a loop, but a situation where these items have no discernible origin. Physical items are even more problematic than pieces of information, since they should ordinarily age and increase in entropy according to the Second law of thermodynamics. But if they age by any nonzero amount at each cycle, they cannot be the same item to be sent back in time, creating a contradiction.
Another problem is the "reverse grandfather paradox", where whatever is sent to the past allows the time travel in the first place (such as saving your past self's life, or sending vital information about the time travel mechanism).
The paradox raises the ontological questions of where, when and by whom the items were created or the information derived. Time loop logic operates on similar principles, sending the solutions to computation problems back in time to be checked for correctness without ever being computed "originally".
Whether or not a scenario described in this paradox would actually be possible, even if time travel itself were possible, is not presently known.
The bootstrap paradox is similar to, but distinct from, the predestination paradox, in which individuals or information travel back in time and ultimately trigger events they already experienced in their own present. In the latter case, information and objects involved have definite origins.
- On her 30th birthday, a woman who wishes to build a time machine is visited by a future version of herself. This future self explains to her that she should not worry about designing the time machine, as she has done it in the future. The woman receives the schematics from her future self and starts building the time machine. Time passes until she finally completes the time machine. She then uses it to travel back in time to her 30th birthday, where she gives the schematics to her past self, closing the loop.
- A professor travels forward in time, and reads in a physics journal about a new equation that was recently derived. He travels back to his own time, and relates it to one of his students who writes it up, and the article is published in the same journal which the professor reads in the future.
- A woman builds a time machine. She goes into the future and steals a valuable gadget. She then returns and reveals the gadget to the world, claiming it as her own. Eventually, a copy of the device ends up being the item the woman originally steals. In other words, the device is a copy of itself and it is not possible to state where the original idea for the device came from.
- A man with a time machine takes the complete plays of Shakespeare, translated into Elizabethan English, and travels back in time to Tudor England. He then gives the plays to the young William Shakespeare before he wrote them, telling him to publish them as his own work. He does, and a copy of the 'original' publication is what is taken back in time. This means that nobody wrote the plays of Shakespeare, as he essentially gave them to himself, thereby closing the loop.
Involving physical items
A woman is locked outside her house because she has lost her keys. While she searches her pockets, a set of keys fall to the ground next to her, which she realises are hers. When she enters the empty house five minutes later, she encounters a time machine which she uses to transport herself and her keys back in time five minutes, allowing her to drop them out of an upstairs window to herself and leave the house, closing the loop.
A further paradox present for any physical item is that the keys should age each time around the loop and eventually wear out. Bringing back a copy of the keys would prevent this "wearing out" issue as would finding the "lost" keys and bringing them back.
A man travels back in time and falls in love with and marries a woman, who he later learns was his own mother, who then gives birth to him. He is therefore his own father (and thus also his father's father, father's father's father and so on), creating a closed loop in his ancestry and giving him no origin for his paternal genetic material.
The bootstrap paradox has been used in fictional stories and films. In the 1980 romance film Somewhere in Time, based on Richard Matheson's 1975 novel Bid Time Return, an elderly woman gives a young man a pocket watch in 1972. He travels back in time to 1912 and gives the pocket watch to her, which she carries with her until 1972 when she meets the young man and gives the watch to him. The concept is named from the Robert Heinlein story "By His Bootstraps", which is considered the "ultimate time travel paradox tale" of its time. Don D'Amassa states that "The greatest difficulty in creating a story of this type is not so much the plotting of the various times loops, but to render them in such a way that the reader can follow the logic."
Other notable examples that feature bootstrap loops as central plot elements include the feature films Bill and Ted's Excellent Adventure, La Jetée, 12 Monkeys, the Terminator franchise, and Interstellar, as well as frequently played upon in episodes of the television show Doctor Who.
- Grandfather paradox
- Newcomb's paradox
- Predestination paradox
- Self-fulfilling prophecy
- Strange loop
- Temporal paradox
- The chicken or the egg
- Time travel in fiction
- Matt Visser (1995). Lorentzian wormholes. "Bootstrap paradoxes A second class of logical paradoxes ..."
- Klosterman, Chuck (2009-10-20). Eating the Dinosaur. Simon and Schuster. p. 60. ISBN 9781439168486. Retrieved 2 February 2013.
- Everett, Allen; Roman, Thomas (2011-12-15). Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space. University of Chicago Press. p. 138. ISBN 9780226224985. Retrieved 3 February 2013.
- D'Ammassa, Don (2005). Encyclopedia Of Science Fiction. Infobase Publishing. p. 67. ISBN 9780816059249. Retrieved 3 February 2013.
- "Stable Time Loop". TVTropes. Retrieved August 23, 2014.