What does she see

# What does she see - c takes her 4/0.6 years or 80 months...

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What does she see? At 0.6 c , she sees the distance to alpha-centauri to be contracted by the familiar to a distance of 3.2 light years, which at 0.6 c will take her a time 5.333 years, or, more conveniently, 64 months. During the outward trip, then, she will see 32 flashes from home, she will see her brother to age by 32 months. Her return trip will also take 64 months, during which time she will see 128 flashes, so over the whole trip she will see 128 + 32 = 160 flashes. That means she will have seen her brother to age by 160 months or 13 years 4 months. What does he see? As he watches for flashes through his telescope, the stay-at-home brother will see his sister to be aging at half his own rate of aging as long as he sees her to be moving away from him, then aging at twice his rate as he sees her coming back. At first glance, this sounds the same as what she sees—but it isn’t! The important question to ask is when does he see her turn around? To him, her outward journey of 4 light years’ distance at a speed of 0.6
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Unformatted text preview: c takes her 4/0.6 years, or 80 months. BUT he doesn’t see her turn around until 4 years later, because of the time light takes to get back to earth from alpha-centauri! In other words, he will actually see her aging at half his rate for 80 + 48 = 128 months, during which time he will see 64 flashes. When he sees his sister turn around, she is already more than half way back! Remember, in his frame the whole trip takes 160 months (8 light years at 0.6 c ) so he will only see her aging at twice his rate during the last 160 - 128 = 32 months, during which period he will see all 64 flashes she sent out on her return trip. Therefore, by counting the flashes of light she transmitted once a month, he will conclude she has aged 128 months on the trip, which by his clock and calendar took 160 months. So when she steps off the spacecraft 32 months younger than her twin brother, neither of them will be surprised!...
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