Time dilation is related to both gravity and acceleration. In the theory of relativity, there are two types of time dilation: gravitational time dilation and velocity (or kinematic) time dilation.
Gravitational Time Dilation: According to general relativity, the presence of a massive object, such as a planet or a black hole, causes spacetime to curve. In regions of strong gravity, time runs slower compared to regions with weaker gravity. This means that clocks closer to a massive object tick more slowly relative to clocks farther away. This effect is known as gravitational time dilation. It has been experimentally verified and plays a significant role in phenomena like the gravitational redshift observed in light from distant stars.
Velocity Time Dilation (Kinematic Time Dilation): Special relativity introduces the concept of velocity time dilation, which applies to objects moving at high speeds relative to each other. According to the theory, as an object approaches the speed of light, time appears to run slower for that object relative to a stationary observer. This effect is often referred to as "time dilation" in popular discussions of relativity.
Acceleration also plays a role in time dilation. According to special relativity, an accelerated frame of reference, such as an object undergoing constant acceleration, experiences time dilation as well. This is known as the "twin paradox." In this scenario, if one twin travels at a high speed away from Earth and then decelerates to return, they would have aged less compared to their stationary twin due to the effects of acceleration.
So, both gravity and acceleration contribute to time dilation. However, it's important to note that gravitational time dilation is fundamentally tied to the curvature of spacetime caused by massive objects, while velocity time dilation arises from the constancy of the speed of light and the relative nature of time.