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Homework Help: Invariance of the y coordinate for a boost along the x axis

  1. Jan 26, 2013 #1
    1. The problem statement, all variables and given/known data
    I've been reading through Spacetime Physics by Taylor & Wheeler, but this argument about the invariance of the [itex]y[/itex] coordinate for inertial frames, one moving relative to the other on the [itex]x[/itex] axis, is tripping me up. I'll just write the text word for word:

    I'm just not understanding this argument at all. There is a way to distinguish between whether the lab clocks were painted by the rocket or not if the lines painted at [itex]y=1[/itex] (rocket frame) weren't at [itex]y=1[/itex] (lab frame)??? Is that the experiment to distinguish the two frames?

    2. Relevant equations
    The laws of physics are the same in any two inertial frames

    3. The attempt at a solution
    Just lost trying to understand how non-invariance of the [itex]y[/itex] coordinate makes the laws of physics different in between the inertial lab and rocket frames.
    Last edited: Jan 26, 2013
  2. jcsd
  3. Jan 27, 2013 #2


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    welcome to pf!

    hi homerT! welcome to pf! :smile:

    it's a little confusing, but i think what they're saying is this …

    all we've stipulated is that the y = 0 line (the x-axis) is the same in both systems

    in other words, if one clock on y = 0 was using paint, then we've stipulated that the paint must fall on the clocks on y = 0 in the other frame

    we haven't stipulated anything about clocks on y = 1

    but if the paint from one clock on y = 1 fell inside the clocks on the other frame on y = 1, then that would be a way to distinguish between the frames, which can't happen, so the paint must fall exactly on the other frame's y = 1 clocks :wink:
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