Now that we know that time is relative, we can see how a clock appears to two different observers. One observer is at rest with respect to the clock, and the other observer sees the clock moving. The time viewed in the frame in which the clock is at rest is called the proper time for that clock. The word 'proper' does not denote anything superior about this frame; it just happens to be the frame in which the clock is at rest. We can think of proper time as being the time interval between two events at the same place.
A simple clock is shown in Fig. 7.4. In this clock a light beam bounces back and forth between two mirrors, separated by a distance L. We would keep time by counting the light bounces. The time for the light pulse to make a round trip is t0 = 2L/c
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