Time is relative because it depends on the observer's motion and the strength of the gravitational field they are in. This idea comes from Einstein's theory of relativity, which shows that time does not flow at the same rate for everyone. When an observer moves very fast, close to the speed of light, time slows down for them compared to someone who is stationary. This effect is called time dilation and has been confirmed through experiments with atomic clocks on fast-moving planes.

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