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Verify special relativity by concept light clock

  1. Jun 17, 2009 #1
    Verify special relativity by concept "light clock"

    Hello.

    For verify the special theory of relativity most books use a "light-clock":
    Light-clock.png

    Obviously, the way the light covers, when the clock is moved is bigger, than the way the light covers when the clock is not move.
    Because speed of light is constant, time must decrease (when the velocity in not 0).


    But what happens with clock which are not based on the principle of constant speed of light?
    I mean it would be much easier to say "Light-clocks are not real clocks, because they do not give the right time when they are moved. Despite a pendulum clock seems to be a real clock."


    You will say that the theory of special relativity was proofed by atomic clocks which were moved around the world in planes.
    Nevertheless, an atomic clocks is based on the principle of constant speed of electrodynamic waves, too: E.g. caesium emits measureable microwaves, lasers are used, etc..


    Where is my mistake?

    Mark
     
  2. jcsd
  3. Jun 17, 2009 #2
    Re: Verify special relativity by concept "light clock"

    No that is incorrect.

    Unaccelerated movement is relative in SR.
     
  4. Jun 17, 2009 #3

    Cyosis

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    Re: Verify special relativity by concept "light clock"

    What Jennifer is saying is that you need to specify two frames. One observer in frame 1 sees "something" the other observer in frame 2 sees "something else".
     
  5. Jun 17, 2009 #4
    Re: Verify special relativity by concept "light clock"

    I meant that the ways seems to be bigger for an external observer. When the clock is moved, the ways seems to become bigger, while speed of light remain constant. Hence an external observer follows that time goes faster.

    Mark
     
  6. Jun 17, 2009 #5
    Re: Verify special relativity by concept "light clock"

    Yes, I know.
    But my problem is to find the reason why special relativity is always true and not only when light-clocks are used.
     
  7. Jun 17, 2009 #6

    Cyosis

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    Re: Verify special relativity by concept "light clock"

    Perhaps the light clock is a deceptive name. Imagine person A and person B sync there watches, after which person A boards a train. On that train there are two mirrors and a light beam bouncing in between just as in your picture. When the train starts moving person A sees the mirror/light beam assembly like in picture 1. He measures the time it takes for a beam to bounce up and down. Person B sees the clock the way your second picture depicts it. He too measures the time it takes for a light beam to bounce back and forth. They will not agree on the times they measured.
     
    Last edited: Jun 17, 2009
  8. Jun 17, 2009 #7
    Re: Verify special relativity by concept "light clock"

    Thank you for your replies.

    Why they do not agree? Because special relativity tells us? No, we are not allowed to use the predictions of special relativity as long as we are not sure that the theory is true. (And we want to proof time-dillation with this experiment).
    ___________________________________________
     
  9. Jun 17, 2009 #8

    Cyosis

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    Re: Verify special relativity by concept "light clock"

    I didn't use the theory of special relativity as you may think I did. All I used was that the speed of light is constant. From this time dilation will follow for exactly the reason I said.

    You said it yourself, although worded poorly, that person B sees the light travel a long distance. Since the speed of light is constant person B will measure the time interval between emitted light and returning light to be longer than person A. After all from person B's view light had to travel less distance.
     
  10. Jun 17, 2009 #9
    Re: Verify special relativity by concept "light clock"

    Thank you, Cyosis.
    Nevertheless I think my problem did not become clear.
    I will try it again with another experiment:


    Assume that there are two clocks on the train.
    A real clock and a fake-clock.
    The fake-clock ist not a real clock, because it can find out, whether the train is moved (relative to the earth) or not (by "looking" out of the window or s.th like that).
    If the train is not moved, the fake-clock will show normal time.
    If the train is moved, the fake-clock will show the time t'=t*k (0<k<1).

    Assume that the fake-clock is identical with the light-clock!
    -> If the light-clock is moved, it gives the wrong time, although no other "real" clock is influenced.

    It is not possible to distinguish whether this assumption or special relativity is true.


    Again, where is my mistake? :-)
     
  11. Jun 17, 2009 #10

    Cyosis

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    Re: Verify special relativity by concept "light clock"

    No, in my example person A and B both used a normal clock (no light clock), synced their watches,stepped on the train and did their measurements. The word light clock as you know it was not used at all.
     
  12. Jun 17, 2009 #11
    Re: Verify special relativity by concept "light clock"

    Mh, ok.

    Thank you very much.
     
  13. Jun 17, 2009 #12

    Cyosis

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    Re: Verify special relativity by concept "light clock"

    Keep asking until all doubt vanishes! Special relativity at first is hard, conceptually (not only at first).
     
    Last edited: Jun 17, 2009
  14. Jun 17, 2009 #13
    Re: Verify special relativity by concept "light clock"

    Okay, maybe another question:

    All opportunities to derive time-dillation has to do something with light.
    Even in your derivation you use a light beam, bouncing between two mirrors.
    Assume you take a ball which bounce between two walls with the velocity 0,5 c. The distance between the walls is only half of the distance of the mirror, so that the event of reflecting the light beam and the reflecting of the ball are always at the same time.

    The speed of light is constant for every observer, the speed of the ball obviously not.

    An outstanding observer sees that the travelled distance of the light rises, because speed is constant -> time-dillation

    The travvelled distance of the ball rises, too, but the speed of the ball is not constant.
    -> Changing of the ball's velocity (rises).

    Where do we find the time-dillation in the "ball-clock"?


    Why are you such an expert in special relativity? Do you/ Did you study physics?

    Mark
     
    Last edited: Jun 17, 2009
  15. Jun 17, 2009 #14

    JesseM

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    Re: Verify special relativity by concept "light clock"

    Mark1991, are you aware that SR is based on two fundamental postulates, the first of which says all laws of physics should work the same in every inertial frame (meaning if two observers are in windowless ships moving inertially relative to one another, and they both perform the same experiment on board their own ship, they should always see the same result), and the second of which says the speed of light is the same in every frame? You have only been talking about the second postulate, but it's the first postulate that makes clear why non-light-clocks should be expected to agree with light clocks (assuming the postulates actually hold true of course). If you place a non-light-clock at rest next to a light-clock in one inertial frame and they keep the same time, then this still must hold true if you do the same experiment in any other inertial frame.
     
  16. Jun 17, 2009 #15
    Re: Verify special relativity by concept "light clock"

    Yes, I know thar the theory of special relativity is based on this postulates.

    By the way: Why we kann be really, really sure that laws of relativity must work in every inertial frame? Only the fact nobody ever has observed an inertial frame, in which laws of physics does not work, does not proof that the postulate is true, does it?
     
  17. Jun 17, 2009 #16

    JesseM

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    Re: Verify special relativity by concept "light clock"

    OK, so do you see why a non-light-clock should experience the same time dilation as a light clock if the two postulates are both true?
    We can't be totally sure, but we can check whether the equations of the fundamental laws of physics as expressed in one inertial frame have the mathematical property of "Lorentz symmetry", if they do that means they'll obey the same equations in all the other inertial frames related to the first one by the Lorentz transformation. All the fundamental laws of physics we know of today have this property.
     
  18. Jun 17, 2009 #17

    A.T.

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    Re: Verify special relativity by concept "light clock"

    Well, yes. The constancy of light speed for every observer is the only reason why we need this annoying counterintuitive stuff.
    If the co-moving light- and ball-clock are in certain sync for one observer, they obviously are so for every other observer. Observers cannot disagree on their relative tick rate, as it would create contradictions. So it follows that every clock is dilated like the light clock.
    This is true for every law in physics. They all cannot be proven.
     
  19. Jun 17, 2009 #18
    Re: Verify special relativity by concept "light clock"

    Why is it counterintuitive?


    You're inside of a perfectly insulated room, you have no way to observe the outside of the room, and you have a tennis ball.

    Can you perform any experiment to show you're moving?

    Toss the ball up in the air, if it falls straight back down to your hand, you can not tell if you are at rest, or moving at a uniform velocity.

    If you tossed it again and it fell away from you, you could determine you had been in motion and were decelerating.

    If you were observing a light clock, while tossing the ball up and down, you could time your tosses by the clock.

    If you were in motion, and the light was taking a longer path, but the ball still went straight up and fell straight back down, you could determine you were in motion if the light clock were observed ticking slower in your reference frame.

    If you add in the postulate that light is always measured to travel at a constant velocity, then the only solution is that your motion through time is a variable.

    Special Relativity, simple as that.


    My motion through space is variable, why would motion through time be any different? Moving through one impairs motion through the other, moving to the left impairs my motion through any other direction, why is that counterintuitive?

    Time is just a direction, not a process. Time is not something that happens to you at a certain rate.

    If it was, I could ask you "What is the speed of time?"

    Unfortunately, that would make just as much sense as asking you "What is the speed of left?"
     
  20. Jun 17, 2009 #19

    A.T.

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    Re: Verify special relativity by concept "light clock"

    What is intuitive, is a subjective perception. But most people find the constant speed of light for every observer counterintuitive, because it contradicts expectations based on everyday experience.

    Edit: deleted stuff that belong in the other thread.
     
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