An observer will experience time dilation under certain conditions, as predicted by the theory of relativity. Time dilation occurs in two main scenarios: relative motion and gravitational fields.
Relative motion: When two observers are moving relative to each other at significant velocities, they will experience time dilation compared to each other. Specifically, the observer in a relatively faster-moving frame of reference will perceive time as passing more slowly compared to the observer in a relatively slower-moving frame. This effect becomes more pronounced as the velocity approaches the speed of light.
Gravitational fields: Time dilation also occurs in the presence of strong gravitational fields. According to general relativity, the curvature of spacetime caused by mass or energy affects the flow of time. In regions with a stronger gravitational field, time passes more slowly compared to regions with a weaker gravitational field. For example, an observer closer to a massive object like a black hole will experience time passing more slowly than an observer in a region with weaker gravity.
In both cases, time dilation arises from the fundamental interconnection between space and time in the fabric of spacetime. It's important to note that time dilation is a relative effect—it depends on the relative motion or the relative strength of gravitational fields between observers.
To summarize, an observer will experience time dilation when they are in relative motion to another observer or when they are in a region with a different gravitational field strength. These conditions alter the passage of time for the observers involved and can lead to measurable differences in the perceived flow of time between different frames of reference.