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Direct observation of lorentz length contraction

  1. Feb 2, 2012 #1
    There is a train(think of having only two boggie for a claer visualization ) in which two rod which are attached to the front and rear end of the train(say, the length between these rod is the length of the train). These are lowered by a single lever to touch the ground. These rods are at different length from the body of the train, so that when lowered it will touched the ground at different distance from the track of the train, drawing two lines along the track.

    The distance between these two starting lines will be the same as that of the length between the rod or to say the length of the train at normal speed.

    As the train approaches say .8c speed, the lever is lowered to touch the ground.

    Would the length between the two starting lines drawn along the track, would actually contract, which would be a direct observation of the lorentz length contraction?

  2. jcsd
  3. Feb 2, 2012 #2
    I'm sorry, but I can't decipher this. Could you maybe post a diagram, or draw some sort of picture?
  4. Feb 2, 2012 #3


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    They would make marks that are farther apart on the ground. Remember, as far as the train is concerned, the ground is what is contracted so when it lowers the rods at the same time, the rods are farther apart as far as the ground is concerned.
  5. Feb 3, 2012 #4
    You also have to remember this: if the two marks are made at the same time relative to the observer in the train, the ground observer will not see the marks being made at the same time.

    What the ground observer will measure is that:
    - The train is shorter than it was when it was at rest relative to the ground.
    - The rear mark will be made before the front mark.
    - The train moves forwards between making the two marks, enough that the distance between the marks is more than it was when the train was at rest relative to the ground.
  6. Feb 3, 2012 #5
    the diagram

    Attached Files:

  7. Feb 3, 2012 #6
    As the train moves fast the ground observer see the train length contracts.

    So isn't the ground observer would have found the train length contract, from the marker on the ground.
  8. Feb 3, 2012 #7
    I agree.
    But here the statement seems contradicts with the former statement.
  9. Feb 3, 2012 #8


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    Michael C explained why your scheme won't work, because the timing is based on the train.

    In order for the ground observer to measure the contraction of the train, he has to use his own timing. Remember, they each measure the other one as contracted.
  10. Feb 3, 2012 #9
    There's no contradiction. The point is that the two events are simultaneous for the train observer, but not for the ground observer. The ground observer sees the rear mark being made before he sees the front mark being made.
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