Time dilation is one of the primary phenomena predicted by Albert Einstein's theory of special relativity published in 1905. The theory states that the measured length of time between two events depends upon the frame of reference of the observer. In particular, time appears to go slower for objects moving with respect to the observer. Hence, time appears to have dilated, or lengthened.

An event is anything that takes place at a specific point in space and at a specific moment in time, like the chiming of a clock. The twelve o'clock chime of a clock occurs at the location of the clock and at the moment when the minute and hour hands of the clock are on the twelve.

The time interval between two events, like the twelve and one o'clock chimes of a clock, depend upon the observer's frame of reference. An observer who is...