PROPER TIME!

The “proper time” interval for a process, which physicists usually call Δτ, is the time interval that would be accurately measured by a standard clock attached to a frame of reference in which the process is at rest.  This is a vital definition, because it is a fact that when an observer in one frame of reference observes a process taking place in a different frame of reference, he finds it takes longer than if it occurred in his own frame!

Seen by Rod, it is Bob's processes that are slow; seen by Bob, it is Rod's processes that are slow. Any process moving relative to you takes longer than the same process at rest in your frame of reference.

Δt = Δτ/(1 - (v/c)²)^1/2

Interactive Time-Dilation Animation.
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