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Homework Help: Age comparison to observer on Earth (time dilation)

  1. Sep 19, 2010 #1
    1. The problem statement, all variables and given/known data

    Alpha Centauri is about 4 light years from the Earth. If you were to travel to Alpha Centauri and back at the speed of the electron that you calculated in question A (0.780c), how much would you age compared to an observer on Earth?

    2. Relevant equations

    There equations were not provided in the actual problem, but so far we've used these 2 (time dilation and Lorentz contraction) equations:

    T' = (T) / sqrt[1 - (v^2/c^2) ]

    L' = L * sqrt[1 - (v^2/c^2) ]

    3. The attempt at a solution

    v = 0.780c

    Both ways:
    L = 8cY

    L = v*T

    T = L / v

    T = (8cY) / (0.780c)

    T = 10.3Y

    T' = ?

    T' = (T) / sqrt[1 - (v^2/c^2) ]

    T' = (T) / sqrt[1 - (v/c)^2 ]

    T' = (T) / sqrt[1 - (0.780c/c)^2 ]

    T' = (T) / sqrt[1 - (0.780)^2 ]

    T' = (T) / sqrt[1 - (0.608) ]

    T' = (T) / sqrt(0.392)

    T' = (T) / sqrt(0.392)

    T' = (T) / (0.626)

    T' = (10.3Y) / (0.626)

    T' = 16.5Y

    From what I recall, T' is the change caused by motion of the moving object, relative to the observer.
    In this case, relative to the person on Earth, the travel time to Alhpa Centauri and back would be 16.5 years.

    There only other time variable left is T, so I assume this applies to the person/partice traveling to Alpha Centauri.

    Am I correct? (I want to avoid using Lorentz contraction to ensure I understand the time dilation equation and use it properly.)

    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 19, 2010 #2


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    Less time passes for the guy in the spaceship. Remember the guy in Heinlein's story who comes home and marries his great-great-grand niece? (Time For The Stars)
    The Earth observer says 10.3 years.
    The traveler says 10.3 * .626
    (ignoring general relativistic effects during the accelerations)
  4. Sep 20, 2010 #3
    OK. Doesn't that mean I did it right so far? Don't I have half of the answer (the time for the observer on Earth)?
  5. Sep 20, 2010 #4


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    You said 16.5 years for the Earth observer; I said 10.3.
    You said 10.3 for the traveler; I said 10.3*.626.
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