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Question about approaching light speed of a rotating object

  1. Sep 4, 2011 #1
    this is my first post so please be gentile with me

    what would a wheel or a disc turning at a speed approaching that of light look like

    my knowledge of the subject it quite small

    but say the disc was rotating clock wise then looking at it from its right side would the top half look like it was blue shifted and would the bottom half look like it was red shifted
    also would that mean that the blue shifted area of the disc be ageing more quickly and the red shifted area be ageing more slowly.

    and looking at it straight on would the centre of the disc look different to the edges of it because it the centre is rotating slower than the edge.

    any thought's would be appreciated

    sorry about the lack of punctuation if this is in the wrong part of the forum i Apologize if some one could direct me to a more appropriate section it would be appreciated
  2. jcsd
  3. Sep 4, 2011 #2


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  4. Sep 5, 2011 #3
    thanks for the link ghwell jr its sort of helps i understand that spinning a disc up to that sort of speed is near impossible but if it was. what would the observable effects be. but thx all the same :)
  5. Sep 5, 2011 #4


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    I think the point is that you cannot use a theory that predicts one thing to ask what that theory would predict if it predicted something different. There are lots more threads that deal with this subject if you search on "disc" in this forum.
  6. Sep 6, 2011 #5


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    I wonder whether any galaxies (some of which are disc shaped) spin fast enough that stars on the rim are moving at relativistic speeds relative to the centre, or to stars antipodally opposite them.

    Regarding your specific questions, I would expect the answers to be as follows:

    If you are looking along the axis you will see no red or blue shift, because the direction of motion of the disc's rim is perpendicular to the line from you to it.

    If you are looking at a nonzero angle to the axis, there will be some part of the rim that has a maximal component of velocity towards you and the antipodal part will have a maximal component of velocity away from you. You will see light travelling from those parts to you as blue and red shifted respectively, but the effect will probably be too small to measure for an artificial disc. For a galaxy, it may be measurable, depending on the answer to the first question above.

    The parts moving towards you would be aging (in your observer's reference frame) fast and those moving away would be aging slowly. So in a complete circuit I would expect the amount of aging you perceive to be the same as in the comoving reference frame (maybe not, though, as it's an accelerated frame). Here I'm thinking of a pulsar on the rim of said galaxy. It will appear to be pulsing faster (than in its own co-moving reference frame) when it's moving towards you and more slowly when moving away.
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