In the theory of relativity, proper time is the time interval measured by a clock that is moving with the object being observed. It is the time experienced by an observer who is at rest relative to the event or process being timed—essentially, it’s the time “felt” by the object itself.
Proper time is important because it provides an absolute measure of time between two events occurring at the same location in an object’s own frame of reference, even though time can appear to pass differently for observers in other, relatively moving frames due to time dilation.
For example, if an astronaut travels at near-light speed and then returns to Earth, their wristwatch (which measures proper time) will show less elapsed time than the clocks on Earth. This is not an illusion—less proper time has actually passed for the astronaut due to the effects of special relativity.
Proper time is a key concept in both special and general relativity, ensuring that time measurements are consistent and meaningful within each object’s own frame of reference.