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[Question] About the radio signal for Twin Paradox

  1. Nov 9, 2013 #1
    There is an example for the radio signal for Twin Paradox from the book "Classical Dynamics of Particle and Systems, Thornton, Marion". Refer "1.jpeg", example 14.5, it said that "However, Frank continues to receive a signal every 3 years for the 8 years it takes the last signal Mary sends when she reaches the planet to travel to Frank". I cannot understand, why there is another 8 years Frank receive signal for every 3 years? Anyone can explain more about this? Thanks.

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  3. Nov 9, 2013 #2


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    Staff: Mentor

    The signal travels at the speed of light. When does the last signal sent by Mary before she reaches the planet get to Frank? She's a lot closer to the planet than to Frank when she sends that signal.
  4. Nov 9, 2013 #3


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    I think a couple of spacetime diagrams would clear everything up for you. Here's one showing the radio signals going from Frank (blue) to Mary (Red):


    As the book says, Mary receives only two signals on her way out but 18 more on her way back (counting the last instantaneous one when she gets back).

    Now here's a spacetime diagram showing the radio signals going from Mary (Red) to Frank (blue):


    As the book says, Frank receives 3 and a third signals during the time that Mary is traveling away (although he has no way of knowing that). He continues to see Mary's signals arriving at the same rate until 8 more years (the length of time the signal sent from Mary at her turnaround takes to get to Frank) and then he starts seeing them come in at three times his own rate for a total of six more in two of his years.

    Does it all make sense now?

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  5. Nov 10, 2013 #4
    Thanks all
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