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Parable #4: The twin paradox.
Suppose Betty and Barney are twins and Betty of them travels to a star 4 light years away at a speed that would result in a time and length contraction of a factor of two (0.866c)
Betty sees the distance as 2 light years, so the round trip will take 4.62 years. During the trip she calculates Barney’s clock is running at half the rate of hers.
Barney sees the distance as 4 light years so the round trip will take 9.24 years. During the trip he calculates Betty’s clock is running at half the rate of his.
Barney will be 4.62 years older than Betty when she gets back, but where did the extra clock ticks come from? (Think Doppler shift!)
On the way out, Betty sees (with light) ticks from Barney’s clock every 3.231 seconds. On the way back, every 0.232 seconds.