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.[1]
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 (see below).
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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, no information or matter 'appears out of thin air'.
The term bootstrap paradox comes from Robert A. Heinlein's story "By His Bootstraps", in which the protagonist is asked to go through a time portal by a mysterious stranger, a second stranger tries to stop him, and all three get into a fight which results in the protagonist being pushed through anyway. Ultimately, it is revealed that all three are the same person: the first visitor is his future self and the second an even older future self trying to prevent the loop from occurring. The bootstrap paradox here is in where and how the loop started in the first place. Heinlein's "—All You Zombies—" involves an even more convoluted time loop involving kidnapping, seduction, child abandonment and sex reassignment surgery, resulting in the protagonist creating the circumstances where he becomes his own mother, father, son, daughter, forever-lost lover and kidnapper.
One example is in the American drama, Lost. John Locke is given a compass by Richard Alpert in 2007. He later is sent back in time about 60 years where he gives the compass back to Alpert, telling him to bring him the compass in 2007. The compass is the subject of the paradox here, it goes through this loop infinite times when the island is dislodged from time.
One of the most noted examples of the bootstrap paradox in fiction occurs in the film Somewhere in Time, based on the Richard Matheson novel Bid Time Return. In the film, Christopher Reeve's character is given a pocket watch by an old lady. He then goes back in time and gives the pocket watch to the old lady's younger self, played by Jane Seymour, which prompts her to seek him out years in the future and give him the watch, resulting in the watch having no apparent origin.
The bootstrap paradox occurs several times in the Terminator franchise, perhaps most notably in the creation of the main villain Skynet. In the first film, the Terminator cyborg sent back in time to kill Sarah Connor is destroyed, but its components are salvaged to form the basis of Skynet, the artificially intelligent computer network that will, in the future, send it back in time on its murderous mission. The knowledge of how to create an artificially intelligent machine therefore has no ultimate source.
The current series of Doctor Who features many examples of the bootstrap paradox, most constructed by writer and later showrunner Steven Moffat. For example, in Moffat's 2007 episode "Blink", the Doctor records a message on film in 1969 in the form of half a conversation. The other half is filled in when Sally Sparrow views the film on DVD in 2007, which her friend Lawrence Nightingale transcribes. The full transcript, including the Doctor's portion, is eventually handed to the Doctor in 2008, but before he is sent back to 1969 from his subjective viewpoint, so he can use it in creating the message later. The contents of the conversation form a bootstrap paradox. The Doctor explains Sally's confusion by revealing that most people think of time like "a swift progression of cause to effect", when it's actually, "like a big ball of wibbly-wobbly, timey-wimey... stuff".
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