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”).
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.
Source SPACE.com: All about our solar system, outer space and exploration.
Source: http://www.space.com/30797-how-time-travel-works-infographic.html
