High School Moving clock as seen by an observer at rest

[Herman Trivilino]

We should choose the frame where referred body is at rest and measure decay time in that frame. Thus observed value is "decay time".

More generally, it's called the proper time. It's the time that elapses between two events that occur at the same place. It's equal to the interval between the events, and is a relativistic invariant.

 

[teo del fuego]

your simple act of distinguishing between "see" and "calculate" was immensely helpful and answered a question I had pondered for a long time. Thank you.