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One way speed of light and simultaneity

  1. Feb 23, 2012 #1
    Hello,

    There are two observers. A is stationary and B is on train. Train is moving.
    B is at middle of train. If B flashes a beam to front and back, and at the both end there are detectors, then what does B measure one way speed of light in both direction?

    Thanks.
     
  2. jcsd
  3. Feb 23, 2012 #2

    Dale

    Staff: Mentor

    Assuming that the detectors are synchronized using Einstein's convention then they measure c, by definition.
     
  4. Feb 24, 2012 #3
    Ok, We assume left end of train as A, right end is B and middle is M. Observer is at M.
    When train is at rest. O flashes two pulse in direction of A and B to synchronize both clocks with O's clock. clock of A and B has some little difference with respect to O's clock because of time taken by light to reach at ends.

    Suppose, that O flashes two pulse to both clocks at [itex]t_{o}=0[/itex].
    pulse reaches to both clock and set [itex]t_{a}=0[/itex] and [itex]t_{b}=0[/itex], but now O's clock displays [itex]t_{o}=1[/itex]. (We can say that the values of t is in time-unit, and train's length is 2 light-time-unit).

    Now, train is moving to the right with the speed of 0.5 light-time-unit/time-unit and suppose O flashes two pulse in both direction at [itex]t_{o}=5[/itex]. We know that reading of both end clock should be [itex]t_{a}=4[/itex] and [itex]t_{b}=4[/itex] now.

    When pulse reach to the clocks, is both clocks reading different or same?
    If different that what would be values of [itex]t_{a}[/itex] and [itex]t_{b}[/itex]?
     
    Last edited: Feb 24, 2012
  5. Feb 24, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    O is at M?

    It would have made more sense to set the clocks equal to 1 when the pulses arrived, that way the three clocks would be synchronized in the train frame.

    "Now" in the train frame.
    I'd say they'd read 5.

    The same. Why would you think otherwise?
     
  6. Feb 24, 2012 #5
    yes.
    yes, you are right. we know the length of train, so can actually.
    Yes, we are talking about train frame.
    My problem is different, I am going to elaborate it.

    Two event occurs simultaneously, but train observer have no way to prove it. So he has to conclude that the event was not simultaneous because speed of light is always c.

    But, if in moving train both light pulse take same time to reach clocks, then why simultaneous events occurred in rest seems one by one in moving frame?
     
  7. Feb 24, 2012 #6

    Doc Al

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    Staff: Mentor

    What two events? Simultaneous according to whom? On what basis does the train observer draw his conclusion?
    Simultaneity is frame dependent. In the train frame, the pulses (in your original example) reach the ends of the train at the same time. But to an observer watching the moving train go by, those pulses reach the ends of the train at different times. He also sees the train clocks as being out of synch.
     
  8. Feb 24, 2012 #7
    Suppose, train is moving, if lightning happens at the two ends simultaneously with respect to rest frame what is the clocks' reading in train frame? same of different?
     
  9. Feb 24, 2012 #8

    Doc Al

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    Staff: Mentor

    Assuming the train clocks are synchronized, the two end clocks will read different times when the lightning strikes them.
     
  10. Feb 24, 2012 #9
    Sorry, but I cannot understand this. Please, can you give me some more detail?
     
  11. Feb 24, 2012 #10

    Doc Al

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    Staff: Mentor

    What don't you understand? Are you familiar with the relativity of simultaneity?

    According to the ground frame, the train clocks are not synchronized. (The clock in the rear of the train is ahead of the clock in the front of the train.) So if the lightning strikes the ends of the train at the same time according to the ground frame, the train clocks will show different times.
     
  12. Feb 24, 2012 #11
    I have read it, but I cannot understand it.
    Is this situation changes when train coming to rest observer, going far from rest observer and a moment when it is on the middle of rest observer?

    I understand that clocks on the train is not synchronized with rest clock. But the two clocks have same dilation effect with respect to rest clock.
    Two train's clock is not synchronized with each other with respect to rest observer is strange to me.
    Can you please explain me why does it happen?
     
    Last edited: Feb 24, 2012
  13. Feb 24, 2012 #12

    Dale

    Staff: Mentor

    This is not the Einstein synchronization convention. Using this convention the speed of light outwards would be infinite and the speed of light inwards would be c/2.

    Einstein synchronization is described in section 1 of this paper:
    http://www.fourmilab.ch/etexts/einstein/specrel/www/
     
  14. Feb 24, 2012 #13

    Doc Al

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    Staff: Mentor

    No, the time offset of the two train clocks according to the ground observer does not depend on where the train is in its motion.
    That's true: All train clocks display the same dilation according to ground observers.
    Try reading this explanation: Special Relativity: Synchronizing Clocks
     
  15. Feb 24, 2012 #14
    @Doc Al

    If train observer synchronize the clocks when train at rest, both observer agree that clocks are synchronized.
    If we accelerate the train to some speed and then make speed constant. Now, both frame are inertial.
    Train observer doesn't synchronize the clock in the moving frame.
    Is both clock still not synchronized with each other with respect to rest observer?
     
    Last edited: Feb 24, 2012
  16. Feb 24, 2012 #15
    Yes, you are right, that is why I followed Doc Al and correct myself by making all three clocks value 0.
     
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