In special relativity, time dilation is a real phenomenon that occurs when an observer moves relative to another observer at a significant fraction of the speed of light. It is not merely a perspectival effect or an illusion but a fundamental consequence of the theory.
According to special relativity, time dilation occurs because the speed of light is constant in all inertial reference frames. When two observers are moving relative to each other, they will measure different time intervals for the same events. From the perspective of an observer in relative motion, time appears to slow down for the moving object or observer compared to their own frame of reference.
This effect has been experimentally verified in numerous ways. For example, high-speed particle accelerators routinely observe the decay rates of unstable particles, which have been found to be slower when they are moving at high velocities. Additionally, experiments using synchronized atomic clocks that are compared after one has been subjected to high speeds and then brought back to rest have consistently shown a time difference between the two clocks.
It is important to note that time dilation is not a subjective perception or a matter of perspective. It is a real physical effect that can be measured and quantified. The slowing down of time for a moving object is a consequence of the relativistic nature of spacetime and the constancy of the speed of light in all inertial frames of reference.