What Is the Interval Between Two Events?

In the theory of relativity in the study of physics, the interval between two events is called proper time. Proper time is based on a clock that measures both events.

Proper time is measured by a clock that creates the same point of reference, and is used to compare the two events based on their length. For one common example of measuring proper time, consider a spaceship leaving Earth with one person on it and one person left behind, measuring how long it takes. The spaceship goes to a planet five light years away. The person on Earth measures a time that reads five light years, and the person on the spaceship measures a time that is much shorter. The time the person on the spaceship measures is the proper time, because he was present at both the start and finish.