Herong's Notes on Physics - Version 3.21, by Dr. Herong Yang
Demonstration of Time Dilation - Bob on the Ground
This section continues the thought experiment to demonstrate time dilation with Bob observing the same bouncing light pulse clock installed on a moving train from a stationary frame.
Part 2 - Bob on the Ground: The second part of the thought experiment is to synchronize the bouncing light pulse with a clock stationary on the ground. This part consists of the following:
Based on Bob's observations in his frame, we can derive a formula to express time T' in terms of distance L':
2*L' = c*T' (3) - Bob's observation - Light pulse speed, time and distance relation T' = 2*L'/c (4) - moving variables around - Time on the stationary clock in Bob's frame
Notice that T' also represents the elapsed time observed by Bob between the same two events: event A when the light pulse is leaving the meter, and event B when the same light pulse is returning to the meter reflected back from the mirror.
If we compare Amy's observation and Bob's observation, we will get the following conclusion:
2*L = c*T (1) - Amy's observation 2*L' = c*T' (3) - Bob's observation L' > L (5) - diagonal vs. vertical path - Diagonal path is longer than vertical path 2*L' > 2*L (6) - multiplying (5) by 2 c*T' > c*T (7) - merging (1) and (3) into (6) T' > T (8) - removing c from (7) - Elapsed time on moving clock is less than stationary clock
Congratulations, we have demonstrated the time dilation phenomenon. Bob observed a greater value T' of the elapsed time from his stationary frame between event A and event B.
Since event A and event B represents two consecutive clicks of the bouncing light pulse clock, we can say that Bob's clock is faster than the bouncing light pulse clock. Faster clocks produce greater elapsed time values than slower clocks between two given events.
Or we can say that bouncing light pulse clock in the moving frame is slower.
We can also say that Amy's clock in the moving frame is slower, since her clock is synchronized with the bouncing light pulse clock.
To see how much slower the moving clock is comparing with the stationary clock, continue with the third part of the thought experiment in the next section.
Last update: 2014.
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