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This case is sometimes called special relativistic time dilation.
The faster the relative velocity, the greater the time dilation between one another, with the rate of time reaching zero as one approaches the speed of light (299,792,458 m/s).
The Hafele and Keating experiment involved flying planes around the world with atomic clocks on board.
Upon the trips' completion the clocks were compared to a static, ground based atomic clock.
As a result of the nature of spacetime, a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference.
A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock.
Consider then, a simple clock consisting of two mirrors relative to the resting frame of the clock (diagram at right), the light pulse is seen as tracing out a longer, angled path.This causes massless particles that travel at the speed of light to be unaffected by the passage of time.