Time dilation is a phenomenon predicted by Albert Einstein's theory of relativity, which states that time can appear to pass at different rates depending on the relative motion between observers and the strength of gravitational fields. Both velocity and mass can contribute to time dilation, but they have distinct effects.
Velocity-induced time dilation (kinematic time dilation): According to the theory of special relativity, as an object approaches the speed of light, time appears to pass more slowly for that object relative to a stationary observer. This effect is known as time dilation. The faster an object moves, the greater the time dilation experienced by observers in relatively slower frames of reference. It is important to note that time dilation due to velocity is relative; it depends on the relative motion between the observer and the moving object.
Gravitational time dilation: According to the theory of general relativity, massive objects, like planets, stars, and black holes, create gravitational fields that cause time to pass more slowly in their vicinity. This gravitational time dilation is a result of the warping of spacetime caused by the presence of mass or energy. The stronger the gravitational field, the greater the time dilation. This effect has been observed and measured in experiments such as the Pound-Rebka experiment and the Hafele-Keating experiment.
In summary, both velocity and mass contribute to time dilation. Velocity-induced time dilation occurs due to the relative motion between observers, while gravitational time dilation occurs in the presence of a gravitational field caused by massive objects. These effects have been confirmed by numerous experiments and observations.