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Posted by on Friday, April 6, 2012 at 4:32pm.

Twins who are 19.0 years of age leave the earth and travel to a distant planet 12.0 light-years away. Assume that the planet and earth are at rest with respect to each other. The twins depart at the same time on different spaceships. One twin travels at a speed of 0.854c, and the other twin travels at 0.513c.

(a) According to the theory of special relativity, what is the difference between their ages when they meet again at the earliest possible time?

The answer is 4.3 years, but I'm not sure how they arrived at that...

  • Physics Question - , Friday, April 6, 2012 at 5:06pm

    twin #1:
    speed .854c
    travel time: 12c/.854c = 14.05 yr

    twin #2:
    speed: .513c
    travel time: 12c/.513c = 23.39 yr

    so, twin #1 travels for 14.05 yr and waits 9.34 yr for twin #2 to arrive.

    However, time dilation slowed aging for the twins:

    twin #1: t' = .520t -- sqrt(1-.854^2)
    twin #2: t' = .858t -- sqrt(1-.513^2)

    so,
    twin #1 aged 14.05*.520 = 7.31 yrs in transit
    twin #2 aged 23.29*.858 = 20.07 yrs in transit

    twin #1 age: 7.31+9.34 = 16.65 yrs
    twin #2 age: 20.07 yrs

    difference: 3.4 yrs

    I also did not get 4.3 years, though maybe I'm dyslexic.

    Anyone see where I went astray?

  • Physics Question - , Friday, April 6, 2012 at 5:52pm

    Lo = 12 light years (ly)
    β1 =0.854,
    β2 = 0.513
    Twin1: L1 =Lo•sqrt(1- (β1)^2) = 12• Lo•sqrt(1- (0.854)^2) = 6.24 ly,
    Twin2: L2 =Lo•sqrt(1- (β2)^2) = 12• Lo•sqrt(1- (0.513.)^2) = 10.3 ly.
    The time to reach the planet:
    Twin1: t1 = L1/v1 = 6.24 ly/0.854c = 7.307 years,
    Twin2: t2 = L2/v2 = 10.3 ly/0.513c = 20.08 years.
    The age of each twin when each arrives at the planet
    Twin1: 19 + 7.307 =26.307 years,
    Twin2: 19 + 20.08 =39.08 years.
    Twin1 has to wait for twin 2 to arrive. As seen from Earth
    Δt1 = (12 ly)/0.854 c = 14.05 years,
    Δt2 = (12 ly)/0.513 c = 23.39 years.
    Twin1 must wait another 23.39 – 14.05 = 9.34 years for Twin2 to arrive.
    When Twin2 arrives, twin A has an age of
    24.8 + 9.34 =34.14 years.
    The difference between their ages when they meet is
    39.08 -34.14 = 4.94 years.
    (The answer 4.3 years may be obtained if you take β1 =0.900, β2 = 0.500)

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