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- Since the time like spacetime interval equals proper time then it seems time dilation is frame invariant

Since the time like spacetime interval is equal to proper time for stationary or traveling observers, then it seems time dilation (proper time) seen with traveling clock is necessarily frame invariant. Then the so called time between ticks of both identical clocks, with stationary and traveling observers, are also equal/invariant. If traveler has v = .6c then both observers agree that ∆t' records .8∆t elapsed time. However some references state "time between ticks is not equal.