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 travelling, 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 traveller 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 incorrectly thought of as different than the predestination paradox, in which individuals or information travel back in time and ultimately trigger events they already experienced in their own present, but is actually just another form of the same paradox. The difference being that in the latter case, the events are consistent with what happened the first time. In the former, events ultimately go in a new direction.
- On her 40th 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 40th 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
An old woman gives a young man a watch; the young man then goes back in time and hands the watch to a young woman; she later grows into the older woman who hands the watch to him. The watch therefore has no point of origin.
A further paradox present for any physical item is that the watch should age each time around the loop and eventually wear out. Bringing back a copy of the watch would prevent this "wearing out" issue as would it being a "grandfather's axe".
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, 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."
Universal War One, a French comic book series by Denis Bajram, addresses the Novikov self-consistency principle: In 2128, an anti-gravity technology (used to fly spaceships, generate artificial wormholes as weapons of mass destruction, and for subsequently time travel and teleportation) is accidentally sent back in time to 2036 when a wormhole, used to destroy the Earth, collapses onto a ship. The ship is later used on Earth by its pilot Mario Delgado to reverse engineer anti-gravity generators, after his realization that he was and always had been the origin of the technology.
In the second installment of the Ordinary Boy series by William Boniface, "The Return of Meteor Boy?", Ordinary Boy wins a contest to become the recreation of Meteor Boy, the sidekick of the Amazing Indestructible who disappeared 25 years prior to the story's timeline. Then, realizing Meteor Boy had merely been sent 25 years into the future with Dr. Brain-Drain's time-travel device, he went back in time to when Meteor Boy existed, only to realize that he was the only "real" Meteor Boy that had ever existed. Then, later in the book, he dropped a jetpack that had been given to him as a Meteor Boy prop from the present Bee Lady. When he dropped it, the past Bee Lady caught it and saved it for twenty-five years to give it to him in the present. Upon asking her who had actually invented it, Lord Pincushion advised him not to think about it, as it would confuse him.
The game "TimeSplitters: Future Perfect" plays with this in every chapter.
- Towards the game's beginning, Cortez is met by his future self, who gives him a key to open a locked door. Later, Cortez finds a wormhole, and steps through it. he then finds himself in the same position as Future Cortez, and Past Cortez is below him looking around. he then calls out to his past self, and hands himself the key that he still had in his possession.
- Cortez arrives in a room with two computer terminals and a locked door while inside the U-Genics lab. Future Cortez (Cortez 2) appears and tells him that the password to the terminal is "Banana". At the same time, Cortez 3 and Cortez 4 come out of the wormhole in the room where Cortez 2 came from. After finishing hacking the terminal, Cortez 1 jumps through the wormhole and is now Cortez 2, finding Cortez 1 and giving him the password. After finishing again, he then goes back in time and becomes Cortez 3, and Cortez 4 informs him the password is lollipop. He then hacks the second terminal, and goes through the portal to become Cortez 4, who then tells Cortez 3 the password. After finishing this time, Cortez 4 continues on while the other Cortez's jump through the wormhole to do it again as he had done before.
- The first stage of the game is a hail of missiles and attacks from an unknown faction trying to kill Cortez, however Cortez is saved from the shadows by a masked individual he couldn't see. Towards the end of the game, Cortez enters a wormhole wearing a disguise and realizes he is the masked individual that saved his own life, and proceeds to destroy the ships and enemies that were attacking his past self.
- Grandfather paradox
- Newcomb's paradox
- Predestination paradox
- Self-fulfilling prophecy
- Strange loop
- Temporal paradox
- The chicken or the egg
- Time travel in fiction
- Schrödinger's Cat
- 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.
- Bajram, Denis (2009-10-20). UW1: Patriarch. Casterman. ISBN 0785132392. Retrieved 2 March 2006.