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I am not a physicist but a physics enthusiast and while trying to understand time dilation I came up with this thought experiment in order to understand things better based on the answers to my 2 questions below:

  1. a boy is standing still on Earth
  2. a girl is about to leave Earth on a spaceship
  3. the boy starts watching a 2-hour movie
  4. the girl starts watching the same move inside the spaceship, at the same time
  5. as soon as the girl starts watching the movie, she leaves earth onboard her spaceship and accelerates to speeds near the speed of light
  6. the boy remains on earth watching the movie
  7. the girl is inside the spaceship watching the movie while travelling at a speed near the speed of light and while she is watching the movie she uses a camcorder to film the screen that is projecting the movie
  8. back on earth, the boy finishes watching the movie and at that exact point sees the girl landing back on earth
  9. due to time dilation the time experienced by the girl inside the spaceship was more than 2 hours. What it means is that while on the spaceship, the girl watched the movie, the movie ended and the girl had yet to arrive and land back on earth. She had time left to do more stuff inside that spaceship like maybe take a nap or something. And after some time had passed on that spaceship and after she was done watching the movie, only then did she arrive back on earth.
  10. the girl is now back on earth, exits the spaceship sits next to the boy, takes out her camcorder and they both sit and watch the movie she recorded on her spaceship

Question #1: Is point 9 correct?

Question #2: When the boy and the girl sit down on earth to watch the camcorder-movie, how long will that movie last and will the playing-speed of the movie be normal? (please excuse me in advance if this 2nd question in particular is completely ridiculous and laughable)

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  • $\begingroup$ Both of them would agree that the movie lasted 2 hours on their screen assuming they have their own TV that they're taking with them. $\endgroup$
    – Charlie
    Commented Apr 17, 2020 at 20:22
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    $\begingroup$ Two things. The girl comes back younger, not older - or to put it another way, the boy's film finishes sooner than the girl's. Secondly, whichever camcorder movie they watch, the boy's camcorder movie will be longer than the girl's, and that longer movie will have used more tape. There will be no difference in the apparent speed of the camcorder movies - the discrepancy will be accounted for entirely in the fact that the boy will have consumed more tape than the girl whilst recording the whole period. $\endgroup$
    – Steve
    Commented Apr 17, 2020 at 20:23
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    $\begingroup$ If G returns when B finishes their movie, G will have only gotten through a fraction of the movie. $\endgroup$
    – d_b
    Commented Apr 17, 2020 at 20:23
  • $\begingroup$ Steve thanks for your answer. Just to clarify, there is no "boy" camcorder movie. There is only the one recorded by the girl on the spaceship and I would like to know how long will that last while they watch it together back on earth. d_b your answer is the opposite of Steve so now I'm more confused :D $\endgroup$ Commented Apr 17, 2020 at 20:33
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    $\begingroup$ I think Steve and I said the same thing. $\endgroup$
    – d_b
    Commented Apr 18, 2020 at 1:56

1 Answer 1

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You have it backwards. The girl on the spaceship is the one who experiences acceleration. So when she returns to earth, she will have felt less time pass than the boy who stayed on earth. If you travel very very fast, you will return to an earth far in the future. So for her to return right as the boy finishes her movie, she'd experience a very short flight. (And the acceleration required to get up up close to light speed and then deccelerate back to the earth frame would be a horrible, horrible death).

For your second question, the recording would look exactly the way it looked to the girl when she watched the movie. So they wouldn't observe any unusual looking images.

Imagine it were film, instead of digital, just to help make a picture of how this happens. Each frame of film will move through the camera while it is recording at some rate. Say, 50 frames per second. So each second as experienced by the girl and the camera, it would record 50 frames. When the film is back on earth it would be played at a rate of 50 frames per second. Therefore, in one second of playback it would show 50 frames.

But if somehow the boy were watching the girl's screen as she traveled, he would see the movie playing very slowly.

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    $\begingroup$ Thank you @Metropolis. Your last comment about the boy watching the girl's screen while se travelled and seeing it play very slowly was very enlightening, I expected that to be the answer. I'm confused though about how much time she experienced personally while on that spaceship. You're saying her time was less than 2 hours, so she didn't have time to even watch the whole movie on that spaceship?? $\endgroup$ Commented Apr 17, 2020 at 20:39
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    $\begingroup$ Ok I see how I got it completely backwards now :-) $\endgroup$ Commented Apr 17, 2020 at 20:46
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    $\begingroup$ @Metropolis it would play very slowly for most of the journey, but speed up hugely just before the end, stopping just short when they meet. See Doppler analysis diagrams here math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/… $\endgroup$
    – m4r35n357
    Commented Apr 17, 2020 at 21:17
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    $\begingroup$ Thank you. Great point. $\endgroup$
    – Metropolis
    Commented Apr 17, 2020 at 21:28

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