The phenomenon you're referring to is known as time dilation, and it is a consequence of Einstein's theory of relativity. According to this theory, the passage of time is not constant and can vary depending on the relative motion between two observers.
When an object or observer is in motion at a significant fraction of the speed of light, time appears to pass more slowly for them compared to a stationary observer. This effect is due to the fundamental relationship between space and time, as described by the theory of special relativity.
Special relativity postulates that the speed of light in a vacuum is constant for all observers, regardless of their relative motion. This means that the observed passage of time is not independent of one's velocity. When an object accelerates to high speeds, its relative motion affects the way time is experienced.
To illustrate this, let's consider an example: Imagine two identical clocks, one stationary on Earth and the other placed in a high-speed spacecraft traveling through space. From the perspective of an observer on Earth, the clock on the spacecraft would appear to be running slower than the stationary clock. Conversely, an observer on the spacecraft would perceive their clock as running normally, while the clock on Earth would appear to be running faster.
This phenomenon occurs because space and time are interconnected in what is known as spacetime. When an object is moving at high speeds, it experiences a distortion in spacetime, which results in the time dilation effect. Essentially, the faster an object moves, the more "stretched" its spacetime becomes, causing time to slow down relative to a stationary observer.
It's important to note that time dilation effects are generally only noticeable at speeds approaching the speed of light, which is approximately 299,792 kilometers per second (or about 186,282 miles per second). For everyday experiences and speeds we encounter in our lives, the effects of time dilation are negligible and not noticeable.