The Chronoverse

Introduction
Chronoverse Types
Tables for Misjumps

Introduction

Time is the dimension which is asymmetrical with regard to entropy.  Many time travel plots derive their dramatic tension from some variant of the question Will this act change history, and/or will my home timeline survive? - the answer to which depends on the fictional world's temporal structure - and are thus (tacitly) experiments testing the paradox-proofing of hypothetical causal frameworks.

Time travel is the concept of moving forward and backward to different points in time, much as we do through space. It also includes travelling sideways in time between parallel realities or universes. 

Unsolved problems in physics: Is time travel theoretically and practically possible? Will such travel invoke paradoxes, such as often used in fiction? Humans are in fact always travelling in time — in a linear fashion, from the present to the immediate future, inexorably, until death. Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future if these geometries or motions are possible. It has been confirmed that the effects of relativistic and gravitational time dilation can cause a traveller who starts at and returns to a point of origin that remains stationary, to arrive at a time farther in the future in that reference frame than their subjective elapsed time would indicate (a constrained form of time travel into the future).

In physics, the concept of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel. However, theories do exist about the possibility of folding time to hop from one point to another.

In science fiction it is a recurring plot device, used to set a character in a particular time not their own, and explore the character's interaction with the people and technology of that time—as a kind of culture shock. Other ramifications explored are change and reactions to it, such as alternate histories.

Albert Einstein's special theory of relativity (and, by extension, the general theory) very explicitly permits a kind of time dilation that would ordinarily be called time travel. The theory holds that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. The effects of this sort of time dilation is popularly known as the "twin paradox" (despite it not being a paradox). A second, similar type of time travel is permitted by general relativity, where a distant observer sees time passing more slowly for a clock at the bottom of a deep gravity well, and a clock of an object lowered into a deep gravity well and pulled back up will indicate that less time has passed than the distant observer's clock. However, these effects allow "time travel" only toward the future: never backward. This is not typical of the "time travel" featured in science fiction, and there is little doubt surrounding its existence. "Time travel" will hereafter refer to travel with some degree of freedom into the past or future.

Many in the scientific community believe that time travel is highly unlikely. This belief is largely due to Occam's Razor. Any theory which would allow time travel would require that issues of causality be resolved. What happens if you try to go back in time and kill your grandfather?—see grandfather paradox. Also, in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. Indeed, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel—a variant of the Fermi paradox, with time travellers instead of alien visitors. However, assuming that time travel cannot happen is also interesting to physicists because it opens up the question of why and what physical laws exist to prevent time travel from occurring.

The general theory of relativity extends the special theory to cover gravity, describing it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of "field equations," and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first and most famous of these was proposed by Kurt Gödel, but all known current examples require the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown. Most physicists believe that it does, largely because assuming some principle against time travel prevents paradoxical situations from occurring.

Surrounding Dimensions