Temporal paradox

A causal loop, also known as a bootstrap paradox, information loop, information paradox, occurs when any event, such as an action, information, an object, or a person, ultimately causes itself, as a consequence of either retrocausality or time travel. A causal loop appears to violate causality by allowing future events to influence the past and cause themselves. This is sometimes called "bootstrapping", which derives from the idiom "." Backward time travel would allow information, people, or objects whose histories seem to "come from nowhere". Sergey Krasnikov writes that both paradoxes, either information or an object looping through time, are the same; the primary apparent paradox is a physical system evolving into a state in a way that is not governed by its laws. He does not find these paradoxical and attributes problems regarding the validity of time travel to other factors in the interpretation of general relativity.) occurs in the 1958 science fiction short story "—All You Zombies—", by Robert A. Heinlein, wherein the main character, an intersex individual, becomes both their own mother and father; the 2014 film Predestination is based on the story. Allen Everett gives the movie Somewhere in Time as an example involving an object with no origin: an old woman gives a watch to a playwright who later travels back in time and meets the same woman when she was young, and shows her the watch that she will later give to him. Smeenk uses the term "predestination paradox" to refer specifically to situations in which a time traveler goes back in time to try to prevent some event in the past. ==Consistency paradox==

'' in the 1920s. Another variant of the grandfather paradox is the "Hitler paradox" or "Hitler's murder paradox", in which the protagonist travels back in time to murder Adolf Hitler before he can rise to power in Germany, thus preventing World War II and the Holocaust. Rather than necessarily physically preventing time travel, the action removes any reason for the travel, along with any knowledge that the reason ever existed. Physicist John Garrison et al. give a variation of the paradox of an electronic circuit that sends a signal through a time machine to shut itself off, and receives the signal before it sends it. ==Free will and causality==

Newcomb's paradox is a thought experiment showing an apparent contradiction between the expected utility principle and the strategic dominance principle. The thought experiment is often extended to explore causality and free will. Predestination sometimes involves a supernatural power, though it could be the result of other "infallible foreknowledge" mechanisms. By allowing for "perfect predictors", for example if time travel exists as a mechanism for making perfect predictions by allowing true knowledge of the future, then perfect predictions appear to contradict free will because decisions apparently made with free will are already known to the perfect predictor, meaning the choice apparently made with free will was already made. Problems arising from infallibility and influence from the future are explored in the infallible predictor version of Newcomb's paradox. == Proposed resolutions ==

Logical impossibility of time travel Even without knowing whether time travel to the past is physically possible, it is possible to show using modal logic that changing the past results in a logical contradiction. If it is necessarily true that the past happened in a certain way, then it is false and impossible for the past to have occurred in any other way. A time traveler would not be able to change the past from the way it is, but would only act in a way that is already consistent with what necessarily happened. Consideration of the grandfather paradox has led some to the idea that time travel is by its very nature paradoxical and therefore logically impossible. For example, the philosopher Bradley Dowden made this sort of argument in the textbook Logical Reasoning, arguing that the possibility of creating a contradiction rules out time travel to the past entirely. However, some philosophers and scientists believe that time travel into the past need not be logically impossible provided that there is no possibility of changing the past, Reversing entropy If time is not an inherent property of the universe but is instead emergent from the laws of entropy, as some modern theories suggest, then it presents a natural solution to the Grandfather Paradox. In this framework, "time travel" is reinterpreted not as movement along a linear continuum but as a reconfiguration of the local space or the entire universe to match a prior entropic configuration. Because the original chronological sequence—including events like the time traveler's birth—remains preserved in the universe's entropic progression, actions within the reconfigured state cannot alter the causal history that produced the traveler. This avoids paradoxes by treating time as a thermodynamic artifact rather than a mutable dimension. It has been argued that since the traveler arrives in a different history and not their own history, this is not "genuine" time travel. He suggested something along the lines of the block time view, in which time is just another dimension like space, with all events at all times being fixed within this four-dimensional "block". Novikov self-consistency principle A 1992 paper by physicists Andrei Lossev and Igor Novikov labeled items without origin (items that seem to "come from nowhere," as mentioned above) as Jinn, with the singular term Jinnee. This terminology was inspired by the Jinn of the Quran, which are described as leaving no trace when they disappear. Lossev and Novikov allowed the term "Jinn" to cover both objects and information with the reflexive origin; they called the former "Jinn of the first kind", and the latter "Jinn of the second kind". expresses one view as to how backward time travel would be possible without the generation of paradoxes. According to this hypothesis, even though general relativity permits some exact solutions that allow for time travel that contain closed timelike curves that lead back to the same point in spacetime, physics in or near closed timelike curves (time machines) can only be consistent with the universal laws of physics, and thus only self-consistent events can occur. Anything a time traveler does in the past must have been part of history all along, and the time traveler can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency. The authors concluded that time travel need not lead to unresolvable paradoxes, regardless of what type of object was sent to the past. Thorne and two of his students at Caltech, Fernando Echeverria and Gunnar Klinkhammer, went on to find a solution that avoided any inconsistencies, and found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each case. Later analysis by Thorne and Robert Forward showed that for certain initial trajectories of the billiard ball, there could be an infinite number of self-consistent solutions. The lack of constraints on initial conditions only applies to spacetime outside of the chronology-violating region of spacetime; the constraints on the chronology-violating region might prove to be paradoxical, but this is not yet known. Krasnikov similarly finds no inherent fault in causal loops but finds other problems with time travel in general relativity. Interacting many-worlds The many-worlds interpretation of quantum physics poses that all possible quantum events can occur in mutually exclusive histories. A variation of this interpretation, called interacting many-worlds, may involve time travelers arriving in a different universe than the one they came from. It's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel. Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history so that time travelers remain within their world rather than traveling to a different one. David Deutsch has proposed that quantum computation with a negative delay—backward time travel—produces only self-consistent solutions, and the chronology-violating region imposes constraints that are not apparent through classical reasoning. Deutsch's self-consistency condition has been demonstrated as capable of being fulfilled to arbitrary precision by any system subject to the laws of classical statistical mechanics, even if it is not built up by quantum systems. Allen Everett has argued that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time, with different particles emerging in different worlds. == See also ==