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Am i moving

  1. Jun 5, 2009 #1
    In this example I make 2 assumptions

    1. if an object is moving through space at time T0 it will be at point x,y,z,T0 and at a different time T1 it will be at a different position x,y,z,T1.

    The other assumption I make is that light is non ballistic.

    see the associated pics

    Referring to fig 1

    There is a box that may be moving through space. In the box is a strobe light at the red square, a mirror at the blue square and an observer with a clock at the green square. All objects are at rest wrt the box.

    The strobe flashes briefly once. Light will travel from the strobe to the observer.

    Light will also travel to the mirror reflect off the mirror to the observer. So the observer will see two flashes of light separated by a time period.

    Using the clock the observer measures the period between the two flashes of light.

    In fig 1 the light source is to the left of the direction of travel, the mirror on the right.

    With this configuration the observer fires the strobe and measures the time period he sees between the two flashes of light. Lets say he gets a reading of X

    The observer now changes the position of the mirror and the light. He places the mirror where the light source was and the lgiht source where the mirror was. The observer then places himslf at the psoition he was in in fig 1. The only thing that changes is the position of the mirror and the light all distances between object are the same as in fig 1.
    With this configuration the observer fires the strobe and measures the time period he sees between the two flashes of light. I calculate that the observer will not get a reading of X.

    Why is this or am I wrong?
     

    Attached Files:

  2. jcsd
  3. Jun 5, 2009 #2

    JesseM

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    Science Advisor

    This has nothing to do with relativity, the time interval would be different even if we look at things in the rest frame of the boxcar. For example, suppose the distance from the source to the observer is 13 light-seconds, the distance from the source to the mirror is 14 light-seconds, and the distance from the mirror to the observer is 15 light-seconds. In this case it will take 13 seconds for the light to travel from the source to the observer and 14+15=29 seconds for the light to travel from the source to the mirror to the observer, a difference of 16 seconds. If you now switch the positions of the source and the mirror, that means it takes 15 seconds for the light to travel from the source to the observer, and 14+13=27 seconds for the light to travel from the source to the mirror to the observer, a difference of only 12 seconds. It's just a consequence of basic geometry, if you have a triangle with three unequal sides A, B, and C, there's no reason to expect that (B + C) - A should equal (B + A) - C.

    Or did you want to specify that the distance from the mirror to the observer is the same as the distance from the source to the observer?
     
  4. Jun 5, 2009 #3
    ...and if you do assume that the mirror and source are the same distance from the observer, then since the speed of light is constant, the time delay will be the same either way. There is no experiment that you can perform in a box that will be affected in any way by its state of uniform motion. Uniform motion and rest are the same thing.
     
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