No, time dilation is not an illusion. It is a well-established phenomenon in the theory of relativity and has been experimentally confirmed. Time dilation arises due to the fundamental nature of spacetime and the interplay between time and space.
In special relativity, time dilation occurs due to the relative motion between observers. When two observers are moving at different velocities relative to each other, they will measure different elapsed times. This effect has been verified through numerous experiments, such as high-speed particle accelerators and measurements of fast-moving particles.
Similarly, in general relativity, time dilation arises in the presence of gravitational fields. Clocks closer to massive objects experience a slower passage of time compared to clocks in weaker gravitational fields. This gravitational time dilation has been observed and confirmed through experiments, such as the Pound-Rebka experiment, where the frequency of gamma-ray photons was measured as they moved between different heights in Earth's gravitational field.
The effects of time dilation have significant practical implications and are utilized in various technological applications, including satellite systems like GPS, where precise timekeeping is essential for accurate positioning.
While time dilation may seem counterintuitive compared to our everyday experiences, it is a consequence of the theory of relativity and is supported by a vast body of experimental evidence. Time dilation is not an illusion but a fundamental aspect of the nature of spacetime and our understanding of the physical universe.