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General Theory of Relativity Question

  1. Dec 3, 2014 #1

    DREAMWALKER

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    Ok, I went through a video to understand it more simply. But there is one point I just don't understand.

    Which is this:
    Screen_Shot_2014_12_04_at_9_20_58_AM.png


    How does Adam think others' clock is SLOW?

    Shouldn't it be FAST?

    Here's my breakdown of the video:

    Adam is in- Space

    Sarah is on - Ground

    Beam of light shone from the spaceship

    According to the person on EARTH - Takes a longer time for the light to reach the spaceship back due to a V shape

    According to the person on the SPACESHIP - Takes a shorter while for the light to return back to the ship

    The person on EARTH sees the time passed as MORE

    The person on the SPACESHIP sees the time passed as LESS

    The person on EARTH sees the time passed as MORE (1 hour),

    The person on the SPACESHIP sees the time passed as LESS (1 minute)


    FOR the person ON EARTH, the clock on the spaceship is SLOWER (BECAUSE for her, the time passed on HER clock is 1 HOUR, but the time passed on HIS clock is JUST 1 MINUTE, hence his is SLOWER compared to hers, hence time is moving SLOWER on HIS clock and FASTER on HER clock)



    FOR the person ON THE SPACESHIP, the clock on the EARTH is FASTER - Right??

    How does Adam view the clock on Earth as SLOW?

    Can someone please explain this..Thanks!
     
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  3. Dec 3, 2014 #2

    Matterwave

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    1) This is the quantum mechanics forum. The special relativity and general relativity forum is ----> thataway

    2) It is unclear which effects the video is discussing. There is a gravitational time dilation happening in this scenario because one person is on Earth, in a gravitational well, and one person is in space. In addition, there is a special relativistic time-dilation happening because the person in the spaceship is presumably moving. The general relativistic effect, gravitational time dilation, will have the person in the spaceship observing the person on Earth's time to be slow compared to his own. The special relativistic effect will also have the person in the spaceship observing the person on Earth's time to slow. So both effects will add, and he will see the person on Earth to have a time which is slower than his own.
     
  4. Dec 3, 2014 #3

    DREAMWALKER

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    This is exactly what I don't understand. How can it be slow for the person in the spacecraft? Shouldn't it be FAST because

    less time passed = slower clock

    more time = faster clock

    Or am I missing something?
     
  5. Dec 4, 2014 #4

    Matterwave

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    Less time will have ticked off on the Earth's clock compared to the ship's clock, in the frame of the guy on the ship. So the guy on the ship might see his own clock tick 10 seconds, but the clock on Earth only tick 9 seconds.
     
  6. Dec 4, 2014 #5

    DREAMWALKER

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    That's opposite of what the video says:



    The video says, the person on earth will see that the beam takes longer to bounce back to the ship. The Person the spacecraft sees it come quicker. Hence, MORE time has ticked off on the clock on EARTH (takes longer for the beam?) and LESS time on the clock of this ship.
     
  7. Dec 4, 2014 #6

    Matterwave

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    Where in the video does it say this? I don't really have time right now to watch a 15 minute youtube video front to end.
     
  8. Dec 4, 2014 #7

    DREAMWALKER

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    From 2:58 to 5:55.
     
  9. Dec 4, 2014 #8

    A.T.

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    That part of the video deals only with kinetic time dilation from Special Relativity, which is symmetrical. Gravitational time dilation from General Relativity is ignored here. They could just as well show this in space. far from any big masses.

    Just apply the same logic in reverse: How will a light beam going vertically up-down from Sarah's perspective look from Adam's perspective?
     
    Last edited: Dec 4, 2014
  10. Dec 4, 2014 #9

    DREAMWALKER

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    Nope. Didn't get it. For me, it's pretty simple.
    This is how I break it down:

    less time passed = slower clock

    more time = faster clock

    So, according to Adam, the EARTH clock SHOULD be fast and not slow.
     
  11. Dec 4, 2014 #10

    A.T.

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    “Everything should be made as simple as possible, but no simpler.” A. Einstein (paraphrased)

    It depends on the reference frame which clock is the faster clock, so you must identify them by name, not their rate.
     
  12. Dec 4, 2014 #11

    DREAMWALKER

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    Ok, but can we talk only in terms of this example..this video particularly? I want to understand this, but this is the only point I am not able to understand.
     
  13. Dec 4, 2014 #12

    A.T.

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    You are asking about something, that the video doesn't show: Sarah's light-clock from Adam’s perspective. You have to draw you some pictures yourself or find a better video that shows the symmetry.
     
  14. Dec 4, 2014 #13

    DREAMWALKER

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    I just shared the screenshot above...That's the crux of the problem...I can understand everything, except that bit. It doesn't show the clock, but it DOES show that the time on Sarah's clock is SLOWER as compared to Adam. Which I don't think is right. Adam should see EARTH clock FASTER. Right? More time passed remember relative to Adam's clock?
     
  15. Dec 4, 2014 #14

    A.T.

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    That "bit" is the whole point of relativity. It's relative whose clock is slower.

    It is right in Adam's frame of reference.
     
  16. Dec 4, 2014 #15

    DREAMWALKER

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    I dont agree then. Because my calculation tells me Adam should see Sarah's clock going FASTER and not SLOWER. Because again, Sarah's clock RELATIVE TO Adam's clock, took longer. Which means her clock is faster compared to Adam's clock which took a shorter time.
     
  17. Dec 4, 2014 #16

    A.T.

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    Then you have done something wrong.
     
  18. Dec 4, 2014 #17

    DREAMWALKER

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    Ok then tell me what's wrong in this:

    Adam- Space

    Sarah- Ground

    Beam of light shone from the spaceship

    according to the person on EARTH - Takes a longer time for the light to reach the spaceship back due to a V shape

    according to the person on the SPACESHIP - Takes a shorter while for the light to return back to the ship

    the person on EARTH sees the time passed as MORE

    the person on the SPACESHIP sees the time passed as LESS

    the person on EARTH sees the time passed as MORE (1 hour),

    the person on the SPACESHIP sees the time passed as LESS (1 minute)

    FOR the person ON EARTH, the clock on the spaceship is SLOWER (BECAUSE for her, the time passed on HER clock is 1 HOUR, but the time passed on HIS clock is JUST 1 MINUTE, hence his is SLOWER compared to hers, hence time is moving SLOWER on HIS clock and FASTER on HER clock)

    FOR the person ON THE SPACESHIP, the clock on the EARTH is FASTER ...
     
  19. Dec 4, 2014 #18

    A.T.

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    But what measures the time according to the person on EARTH? You need a second light-clock that bounces light vertically in the EARTH frame, and therefore V-shaped in the SPACESHIP frame. That clock represents the EARTH time, and in the SPACESHIP-frame this EARTH-clock is slower than the SPACESHIP-clock because of the V-shape.
     
  20. Dec 4, 2014 #19

    DREAMWALKER

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    Man..sorry, but you are just repeating that. I need to know how? and why? I think we have already agreed to disagree on the EARTH clock being slower. But I need an explanation. I am sorry, but I am not quite convinced by your explanation.
     
  21. Dec 4, 2014 #20

    A.T.

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    I just told you how. You need to consider a second light clock, which is not shown in the video. And you need to distinguish the two clocks clearly. So far you just keep talking about "time" without specifying according to which clock.
     
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