According to the theory of relativity, both acceleration and gravity can cause time dilation. The effects of time dilation arise from the fundamental relationship between space and time known as spacetime.
In general relativity, gravity is described as the curvature of spacetime caused by the presence of mass and energy. Massive objects like planets, stars, or black holes curve spacetime around them. This curvature influences the flow of time in their vicinity, resulting in time dilation. In regions with stronger gravitational fields, time runs slower relative to regions with weaker gravitational fields.
Acceleration, on the other hand, also affects the flow of time. According to special relativity, the theory that describes the behavior of objects moving at constant speeds, the perception of time depends on the relative motion between observers. When an object accelerates, its motion deviates from being constant, and special relativity accounts for this acceleration-induced time dilation.
The phenomenon is known as "gravitational time dilation" for effects caused by gravity and "kinematic time dilation" for effects caused by acceleration. Both types of time dilation have been experimentally observed and verified. For instance, experiments with atomic clocks on fast-moving airplanes or satellites in orbit demonstrate that time dilation occurs due to their velocities, in line with the predictions of special relativity.
In summary, according to the theory of relativity, both gravity and acceleration can cause time dilation. Gravity results from the curvature of spacetime caused by mass and energy, while acceleration affects time due to the principles of special relativity.