# Question about proper time

by nwdavis1
Tags: proper, time
By definition, th eproper time is the time between two events in the frame where the two events occur at the same position. So all you have to say is that when you are in the frame where $\Delta x =0$ then $\Delta t = \tau$. Plug that in the spacetime invariant and you get the answer. Note that this definition works only for events for which $(\Delta s)^2 \geq 0$.