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Light is slower if object moves fast

  1. Sep 17, 2013 #1
    My book says this:-
    So objects move slower actually means that we perceive them as slower. It is not "actually" moving slower.
    I interpret this is this way : Objects move at the same speed but since we depend on light to see objects, it seems as if they are moving slower as light takes longer. Am I right?

    Now this creates lot of confusion. In the famous twin paradox it is said that the twin ages more that the other.
    According to me, the aging for both is same but the still guy only perceives the other one to be slower i.e. light is taking more time to reach him. In this case both should be same age.

    I'm kind of lost here.
  2. jcsd
  3. Sep 17, 2013 #2


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    No, if you synchronize a very accurate clock with your own, accelerate it away from you, then bring it back, LESS time will have passed according to that clock when it returns. What rate the clock appears to be ticking at while it is moving away from and then towards you is a separate effect.

    Edit: To be clear, you don't need a clock for this to happen. The aging of one twin slower than the other would still occur as per the twin paradox.
  4. Sep 17, 2013 #3


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    Both guys will perceive the other ones clock to be running (ticking) slower than their own by the exact same amount during the departure. When the traveling guy turns around, he immediately perceives the other guy's clock to be running (ticking) faster than his own but the guy that remained at home does not perceive this same change in the traveling guy's clock until a long time later. That's why, when they finally get back together, their clocks have accumulated different times on them.

    Do you see how their two perceptions are different?
  5. Sep 17, 2013 #4
    My doubt is different. The problem is with the word "slower".

    When we say the clock moves "slower", what do you mean by that? What I understand is that the clock moves at its normal rate but since light takes longer to reach it is perceived as "slower".
    So considering this, there shouldn't be any affect because clock moved at its normal rate all the time (it just seemed slower because it was moving).
  6. Sep 17, 2013 #5


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    Your quote and my response did not say 'the clock moves "slower"'. Please use the term "moves" to apply to the speed of an object. Your quote used the term "running slower" and I clarified that with "ticking" slower to emphasize that we are talking about the passage of time on the clock.

    With that in mind, when two clocks are moving away from each other, do you agree that both observers will each see the other clock as ticking slower than their own? And do you agree that if two clocks have been moving toward each other for a long time, do you agree that each observer will see the other clock ticking faster than their own? And do you agree that when the traveler turns around, he immediately sees the other clock ticking faster but the other guy doesn't see that happen until more than half the time the traveler is gone?

    There is actually more to this than what I just described but I want to make use you at least agree with these points.
  7. Sep 17, 2013 #6
    Actually, the clock does indeed tick slower but on top of that the perception of how fast it is ticking is affected by the Doppler effect which is what the book is talking about. The slowing down of the moving clock is as real as real gets.
  8. Sep 17, 2013 #7
    Ok, so how do we know it ticks slower? The explanation I have seems flawed.
    "Part" of what it says is: -
    In a moving train let a light source be placed in the center. Then in the frame of reference of train, light reaches both the ends at the same time.
    But in the frame of reference of track (i.e. outside the train), the one end is going away from light and other end is moving towards the light. So light reaches one end faster than other.
    Doesn't this imply that light is actually moving faster (c + u where u is the velocity of train)?
  9. Sep 17, 2013 #8
    No, it implies that the distance traveled is smaller, which will take less time since the light speed doesn't change
  10. Sep 17, 2013 #9
    Right, thanks. I think I have cleared my doubt now.

    Relativity is messing up my concepts! I'm currently reading "It's about time" by David Mermin. Is it good enough?
  11. Sep 17, 2013 #10
    No idea
  12. Sep 17, 2013 #11


    Staff: Mentor

    No, this is incorrect. Your book is describing the Doppler effect. This is true in classical mechanics as well and has nothing to do with relativity (although the relativistic expression is different from the pre-relativistic one).

    The relativistic effects of time dilation and length contraction are what remain AFTER taking account of the optical effects from the finite speed of light.
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