Neddy Bate
Valued Senior Member
Sissy knows that 1 second has elapsed, because the train traveled to the red mark at a velocity of .866c.
Well, Sissy is too far away from the red mark to actually see what is happening there in real time. She can look with a telescope, but what she sees happening there would be on a 0.866 second delay, due to the light rays requiring that much time to reach her eyes. So that is why it makes more sense to just let her have a clock, and when it elapses one second, she calculates that Bro is arriving at the red mark.
Bro knows that being at the red mark at a velocity of .866c means 1 second has elapsed. But he also knows his clock changed rate compared to a standard second. His clock no longer keeps time accurately.
Bro can't discredit his own clock, because he can use that clock to measure the round-trip speed of light on the train, and it measures c as it should. So he trusts his clock, because he knows the speed of light is a constant.
He can verify this by watching the Earth make 1 lap around the Sun, while he rides the train. His clock will not show 1 year has elapsed while riding the train, but he knows that 1 lap of Earth around the Sun has occurred, and that is 1 year. He throws his clock out the window and calls Sissy to get an accurate time check, which she reveals to him that 1 year has elapsed, which is substantiated by the Earth completing 1 lap around the Sun. Lesson learned for Bro, never trust a clock on a train moving at a velocity of .866c!
Nice try, but the train tracks are not on earth. I forgot to tell you that the tracks themselves are in deep space, moving at a speed of 0.866c away from earth. So that means the earth is also time-dilated according to Sissy. By the time she reaches old age (what her body tells her is about 80 years old), the earth has only gone around the sun 40 times. However, her own clock displays 80 years, so she trusts it more than she trusts the earth years!
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