Time dilation, a concept from Einstein's theory of special relativity, occurs when the relative motion of two observers causes them to perceive time passing differently. Excluding speed and gravity, there are two main factors that contribute to time dilation:
Relative velocity: When two objects are moving relative to each other at a significant fraction of the speed of light, time dilation occurs. The faster an object moves relative to an observer, the more time dilation is experienced. This phenomenon is known as "kinematic time dilation" and is one of the fundamental principles of special relativity.
Acceleration: While acceleration itself does not directly cause time dilation, it plays a crucial role in the overall context of spacetime. When objects accelerate, they experience changes in their motion and velocity. This, in turn, affects the time dilation experienced by an observer. The Twin Paradox is a classic example of how acceleration and relative motion lead to different aging rates for two observers who reunite after one undergoes accelerated motion.
To summarize, time dilation primarily depends on the relative velocity of objects or observers and is further influenced by acceleration when considering their entire journey in spacetime.