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Homework Help: Astronaut travels to distant planet, finding age?

  1. Apr 18, 2015 #1
    1. The problem statement, all variables and given/known data
    A 26 year-old biologist makes a trip to study alien life forms in a distant planet 10 light-years away. The round trip including a stay of 1 year in the distance planet takes 21.5 years according to the clock on earth. The biologist’s son is 3 year-old when she left. Assume that the planet is not moving with respect to earth, and the speed is the same for both outbound and inbound part of the journey.

    What is the difference in age between the mother and son after she returns back to earth?

    2. Relevant equations
    T' = T √(1-(v2 /c2))

    3. The attempt at a solution
    A. 7.0 years B. 16.8 years C. 23.0 years D. 30.0 years E. 37.0 years

    The answer is 7 years according to the solution manual. I tried to find the speed of the astronaut, then plugged it in to the relevant equation. I'm not sure how to find the speed of astronaut actually. I think I have to solve for the reference frame of the astronaut T, as T' is the frame of earth.
  2. jcsd
  3. Apr 18, 2015 #2


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    Science Advisor

    Don't over-think it. You know the distance travelled. 10 light years out, 10 light years back. You know the time the journey took. 21.5 years minus the 1 year layover = 20.5 years.
    Last edited: Apr 18, 2015
  4. Apr 18, 2015 #3
    (20ly)/(20.5) = 0.97c
    Using that, I solved for T to get 4.8 years. About 5 years to the astronaut's age is 31, and the son is 24.5, the difference is 7 then. I think this is correct.
  5. Apr 18, 2015 #4


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    2017 Award

    Staff: Mentor

    I get exactly 4.5 years for the time the astronaut gets older while on the spaceship, while the son ages by 20.5 years => astronaut gets 31.5, son gets 24.5 => 7 difference.

    The given options here are very convenient - once you see that the son will age more than the astronaut you are down to two options. The timescale suggests a trip very close to the speed of light, it is possible to see that the second option does not fit even without actual calculations, which just leaves one option.
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