cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox, ontological paradox, or bootstrap paradox. The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime. The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses the film Somewhere in Time as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that entropy of the watch will increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a statistical law, so decreasing entropy and non-increasing entropy are not impossible, just improbable. Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history. In 2005, Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent. In fiction Time travel themes in science fiction and the media can be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation. The non-scientific term timeline is often used to refer to all physical events in history, so that where events are changed, the time traveler is described as creating a new timeline. See also Claims of time travel Time travel claims and urban legends Culture Time capsule Fiction Time travel in fiction Time travel in The Lord of the Rings List of time travel works of fiction List of games containing time travel Science Krasnikov tube Retrocausality Ring singularity Temporal paradox Wheeler–Feynman absorber theory Time perception