Time travel
Time travel is the concept of moving between different points in time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 in a manner analogous to moving between different points in space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period (at least not at the normal rate). Any technological device – whether fictional or hypothetical – that would be used to achieve time travel is commonly known as a time machine.

Although time travel has been a common plot device
Plot device
A plot device is an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot....

 in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 since the late 19th century, and the theories of special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 suggest methods for forms of one-way travel into the future via time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

, it is currently unknown whether the laws of physics would allow time travel into the past.