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The bootstrap paradox is a paradox of time travel in which information or objects can exist without having been created. After information or an object is sent back in time, it is recovered in the present and becomes the very object/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.
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, no information or matter 'appears out of thin air'.
||This section may contain original research. (August 2012)|
Involving information 
- On his 30th birthday, a man who wishes to build a time machine is visited by a future version of himself. This future self explains to him that he should not worry about designing the time machine, as he has done it in the future. The man receives the schematics from his future self and starts building the time machine. Time passes until he finally completes the time machine. He then uses it to travel back in time to his 30th birthday, where he gives the schematics to his 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 man builds a time machine. He goes into the future and steals a valuable gadget. He then returns and reveals the gadget to the world, claiming it as his own. Eventually, a copy of the device ends up being the item the man 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 young physicist receives an old, disintegrating notebook containing information about future events sent by his future self via a time machine; before the book deteriorates so badly as to be unusable, he copies the information in it into a new notebook. Over the years the predictions of the notebook come true, allowing him to become wealthy enough to fund his own research, which results in the development of a time machine, which he uses to send the now old, tattered, disintegrating notebook back to his former self. The notebook is not a paradox (it has an end and a beginning; the beginning where he receives it and the end where he threw it out after he copied the information), but the information is: It's impossible to state where it came from. The professor has transferred the information that he wrote himself, so there was no original information.
Involving physical items 
- A man is locked outside his house because he's lost his keys. Another man approaches him with the keys. When the man enters the house five minutes later, he encounters a time machine which he uses to transport himself and his keys back in time five minutes, allowing him to give the keys to himself and close 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 handed to him would prevent this "wearing out" issue as would finding the "lost" keys and bringing them back.]
Involving people 
- 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 by extension, also his own grandfather, great-grandfather, great-great grandfather, great-great-great grandfather and so on, making his ancestry infinite, and also giving him no origin for his paternal genetic material.
In fiction 
The bootstrap paradox has been used in fictional stories and films. In the film Somewhere in Time an elderly woman gives a young man a pocket watch. He travels back in time to when she is a young woman and gives the pocket watch to her, which she carries with her until in the future 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'Ammassa 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."
See also 
- 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. pp. 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. pp. 138–. ISBN 9780226224985. Retrieved 3 February 2013.
- D'Ammassa, Don (2005). Encyclopedia Of Science Fiction. Infobase Publishing. pp. 67–. ISBN 9780816059249. Retrieved 3 February 2013.