proper time interval clock event
Consider the scenario illustrated in Fig. 4-3A. Observer A holds a light-clock of length 



L


{\displaystyle L}

 as well as an electronic timer with which she measures how long it takes a pulse to make a round trip up and down along the light-clock. Although observer A is traveling rapidly along a train, from her point of view the emission and receipt of the pulse occur at the same place, and she measures the interval using a single clock located at the precise position of these two events. For the interval between these two events, observer A finds 




t

A


=
2
L

/

c
.


{\displaystyle t_{A}=2L/c.}

 A time interval measured using a single clock which is motionless in a particular reference frame is called a proper time interval.
