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Homework Help: Relativity physics questions

  1. Jan 19, 2006 #1
    I've been researching relativity out of interest for a while now. I am quite fammiliar with most of the basic principals. But I have a few questions,

    1)If there was a large wheel with about the same circumference of the earth, and you managed to spin in it so that the edge of the wheel was traveling at near the speed of light relative to the center (so it was spining 7rps), what effects would we see happening? And if its impossible to reach the speed of light because you'd need an infinate ammount of energy, would it therefore be impossible to make is so the inside of the wheel was turning at a speed that would insure the edge is traveling at the speed of light?

    2)With the twin paradox, the twin in space ages less because of the relativistic doppler shift which relies on traveling in the right direction to work. What if you were to make that twin blast off and just travel in circles at great speeds, would we see the same effects?

    3)If photons travel at the speed of light relative to everything, then shouldnt all time stop therefore making light from one source to an object instantaneus?
    Last edited by a moderator: Jan 7, 2014
  2. jcsd
  3. Jan 20, 2006 #2

    Tom Mattson

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    You certainly couldn't make any point on the wheel move at the speed of light, but there's something subtle that you're missing. In relativity the rigidity of an object is relative. Objects that are perfectly rigid in one frame are not rigid in any other frame. If a wheel that is rigid in its own rest frame starts rotating in some other frame, then in that latter frame the spokes aren't straight radial lines anymore.

    The relativistic doppler shift is not used to explain the twin paradox. But yes, you would see the same type of effect if one twin traveled in a circle and then returned.

    Relativity does not answer this question. In relativity the rest frame of the photon is not defined. So if you ask what happens in the rest frame of the photon, you are outside of the scope of relativity.
  4. Jan 20, 2006 #3
    Thanks. What does explain the twin paradox then? I heard it was the realtivistic dopple shift effect.
  5. Jan 20, 2006 #4


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    to explain the twin paradox so that it's not a paradox anymore,
    you have to include the acceleration of the traveling twin.
    So the twin paradox is not generally explained in special relativity.

    I recommend the "Special and General Relativity Forum" for this topic
    (which they put under the Astronomy and Astrophysics category).
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