In the theory of special relativity, time dilation occurs when an object or observer is moving relative to another frame of reference at a significant fraction of the speed of light. The concept of time dilation arises from the constancy of the speed of light in all inertial frames of reference.
According to the theory, the passage of time is not absolute but depends on the relative motion between two observers. Specifically, time dilation predicts that time will appear to pass more slowly for a moving object or observer compared to an observer at rest.
To determine which inertial frame of reference experiences time dilation, you need to compare the relative velocities of the two frames. The frame that is moving relative to the other frame at a high velocity will experience time dilation. This means that for observers in that frame, time will appear to pass more slowly compared to observers in the stationary frame.
It's important to note that both frames of reference are equally valid, and the perception of time dilation is relative to the observer's frame. For example, if you are in Frame A and observing Frame B moving relative to you, you would perceive time dilation in Frame B. However, from the perspective of an observer in Frame B, it is Frame A that is moving, and they would perceive time dilation in Frame A.
In general, it is the relative motion between frames of reference that determines which frame experiences time dilation. The frame with the higher velocity relative to the other frame will be the one experiencing time dilation.