Proper time is the term used in special relativity to describe the time measured by a clock that is moving along with an object or observer. It represents the time experienced by an object or observer in its own frame of reference.
In special relativity, the concept of proper time is important because it is an invariant quantity, meaning it is the same for all observers in all inertial frames of reference. The proper time interval, denoted by Δτ, is determined by the trajectory of an object in spacetime.
When an external observer, who is in a different inertial frame of reference, observes a moving object with a clock, they will measure a different time interval compared to the proper time experienced by the object itself. This difference in time measurement is due to time dilation, an effect predicted by special relativity.
According to the theory of special relativity, when an object is moving relative to an observer, time appears to pass more slowly for the moving object compared to the external observer. This time dilation effect occurs because the relative motion between the observer and the moving object affects the way time is experienced.
In summary, proper time refers to the time measured by a clock that is moving along with an object or observer. It represents the time experienced in the object's own frame of reference. The time measured by an external observer observing the moving object will be different due to time dilation caused by their relative motion.