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Thought Experiment

  1. Jan 28, 2009 #1
    There is a hollow cylinder out in space (Cylinder A), shaped like a giant soda straw. Inside the cylinder are mirror-laser clocks keeping time by bouncing light back and forth across the Cylinder A's diameter.

    A similar cylinder of slightly smaller diameter (Cylinder B) is traveling at .9999 of the speed of light relative to Cylinder A. In addition to the mirror clocks, it has several evenly spaced holes along its sides, like the windows of an airliner. From a relatively stationary point outside of Cylinder A, Cylinder B appears to be traveling from right to left. Cylinder B also has a series of laser clocks, some placed opposite windows and some placed opposite mirrors within the cylinder, so its lasers can point at a mirror inside itself or shoot a laser out of a window.

    Cylinder C is similar to Cylinder B, except it is slightly smaller in diameter. Cylinder C is traveling at .9999 of the speed of light from left to right relative to Cylinder A.

    Cylinder B passes through Cylinder A and Cylinder C passes through Cylinder B, which is also inside of Cylinder A, so that all three cylinders appear to line up perfectly for an instant for an observer standing relatively stationary outside of Cylinder A.

    If the light from a laser clock in Cylinder A travels through the windows of the other nested cylinders' walls, ABCCBABCCBA, will the time recorded be different than when light just bounces back and forth through the diameter of an empty Cylinder A?

    If "No", because only the space actually occupied by the material moving with B and C (the cylinder walls) is in a different frame of reference than A, then what if Cylinder C held a plasma in a magnetic field that A's light passed through?

    I'm sure you can think of many more variations of this scenario, like half-silvered mirrors bouncing light off different combinations of mirrors in different cylinders and making the light zig zag left and right down the length of the B and C cylinders before returning to a clock in A.

    Thanks for your input!
    -Paul Murphy
  2. jcsd
  3. Jan 29, 2009 #2


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    Welcome to PF!

    Hi Paul! Welcome to PF! :smile:
    "the space actually occupied … is in a different frame of reference than A" …

    nooo … space is in all reference frames …

    a reference frame isn't a physical object, it's just a way of measuring … it measures anything, at any speed, and in any combinations.

    light passing through space in cylinder B (or C) isn't like light passing through air … the air will be moving at the same speed as B, and the speed of light through that air will depend on that speed … but the speed of light through (empty) space is c anyway. :wink:
    plasma slows light, just like air (not sure about the magnetic field, though :redface:)
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