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Speed of light question

  1. Dec 14, 2014 #1
    Sorry if this was asked but I couldn't find a answer!
    I heard that when you travel at near the speed of light (0.999...c, wrt the earth or Sun maybe?) the universe would be seen blueshifted (even the CMB would be visible) and length contracted in front of you.
    However I also heard that we are traveling right now to 0.9999...c wrt a reference frame. Why don't I see stars or CMB blueshifted or length contracted in front of me?
    Many thanks!
  2. jcsd
  3. Dec 14, 2014 #2


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  4. Dec 14, 2014 #3
    I may be wrong but this all appears paradoxical in the same manner as the twin paradox.
    In the time dilation problem of twin paradox, the solution is that it is only the inertial observer on earth who is qualified to apply time dilation. The moving twin aboard a rocket close to speed of light does not remain inertial at all times, for doing so would imply constant velocity and thus it would be impossible to return back to earth - there must be some kind of acceleration involved.

    See the section here http://www.einstein-online.info/spotlights/Twins titled 'turning the tables' and the subsequent section to see whats going on.
  5. Dec 14, 2014 #4
    Thanks for your reply
    I don't understand what is "relative to the CMB", doesn't it travel at c?
    Also do all galaxies share the same reference frame? (even when receding due to expansion?)
  6. Dec 14, 2014 #5


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    We are traveling at .9999c wrt "a" reference frame, but we are not traveling that fast wrt the CMB's reference frame. It is how fast we travel wrt the CMB that matters for looking at blueshift/redshift of the CMB.
  7. Dec 14, 2014 #6
    Thanks Salford

    So the key is the acceleration, right? As it is absolute and the galaxies and such do not, without accounting expansion as I accelerate to near c wrt the earth so I would be like the twin who leaves it?
  8. Dec 14, 2014 #7


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    The phrase "relative to the CMB" is just shorthand for "the local reference frame where the CMB radiation has no dipole anisotropy".
  9. Dec 14, 2014 #8


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    The key to the whole issue is "wrt a reference frame." Which reference frame? The cmb's reference frame. You don't see the CMB blueshifted because you aren't traveling near c wrt its reference frame. Broader: all speed measurements are between two - any two - reference frames.
  10. Dec 15, 2014 #9
    thanks for your replies I think I am understanding now, so the CMB FoR is that which doesn't have any redshift or blueshift in any direction am I facing, right?

    So can the CMB FoP be considered an "special" frame or is it just a regular one for this case?
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