special relativity
The equation explained
In wordsThe time the carried clock records is the integral, from the start to the end of the trip, of the square root of one minus its speed squared over c squared, over stationary time.Full explanationAdd up, moment by moment, the rate at which the carried clock runs: one when it is at rest, less when it moves. Two legs at 0.6c lasting 10 stationary-system seconds give the travelling clock 8 seconds; a clock left at rest records 10.