Scientists define time as one of the four dimensions of "space-time," a term used to describe the totality of the universe.
Physicists calculate that subatomic particles travel both forward and backward in time, but humans experience time in only one direction, past to future (the "Arrow of Time").
Video: How to Time Travel
Philosophers debate whether the future is in some way already determined, and whether the past is changeable. Another open question: Would time exist if everything in the universe stood still?
At right, a time machine travels backward through time to a different location in space and time (A) and to the same location but backward in time (B).
One of the consequences of Einstein's theory of relativity is that a traveler moving at high speed (or in the high gravity field near a black hole) will find that time passes more slowly than it does for those who stayed home. The homebody might age considerably (or be long dead) by the time the traveler returns. Examples: "Interstellar" (2014), "Ikarie XB-1" (1963).
"tesseract," a vessel where time is represented as a dimension of space. Cooper (Matthew McConaughey) can move in one direction to see the past and in the other direction to see the future. Cooper cannot interact with the past except via gravitational waves.
"Doctor Who's" (1963-present) TARDIS ("Time And Relative Dimension In Space") moves from one point in space and time to another by traveling through an extra-dimensional vortex. Travelers experience time passing normally within the TARDIS. Outside observers at the departure point see the TARDIS vanish, and those at the destination see the TARDIS appear from nothing.
In the 2006 anime "The Girl Who Leapt Through Time," young Makoto Konno finds a device that allows her to jump through time. Unlike in "Doctor Who," there is no passage of time for the traveler.
Another example of instantaneous time jumping is the DeLorean from "Back to the Future" (1985).
Hermione's Time-Turner from the "Harry Potter" series and H.G. Wells' machine in his novel, "The Time Machine" (1895), both work by moving travelers back and forth in the time dimension while keeping them in one spot in space.
Paul J. Nahin in his book "Time Machines" claims that this would never work, because all scientifically plausible time travel methods involve movement through space as well as time (otherwise, the machine would collide with itself!).
In the 2004 film "Primer," the traveler stays within a box for 1 minute for each minute of time travel. To go back 24 hours, 24 hours must be spent in the box.
The traveler can go back no further than when the time machine was switched on.
On arrival, two versions of the traveler exist: the one who time-traveled and the one who has not yet done so.
In the "Terminator" franchise, a time-displacement machine projects a traveler to a point in the past or future. Return travel to the origin point is not possible. Only living tissue can survive the forces of time travel; inanimate objects would be destroyed.
In Gregory Benford's 1980 novel "Timescape," the protagonist, in a future devastated by disasters, sends messages back to the year 1962 in a desperate bid to prevent the damage. The message is encoded into (hypothetical) faster-than-light tachyon particles, fired at the point in space where the Earth was in 1962.
In 1979's "Superman: The Movie," Christopher Reeve flies faster than light to return to a time before Lois Lane (Margot Kidder) was killed.