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Absolute Velocity of the Earth?

  1. Jul 5, 2005 #1
    A few questions from a non-scientist:

    From what I've read there is a dipole in the microwave background radiation. Does anyone consider this to be very significant? Were any scientists very surprised that there was such a dipole? Would it have been considered weird if the background radiation had no detectable polarity?

    Isn't the main point of Special Relativity that you can't determine an absolute velocity of something? But if there's radiation coming out of everywhere that is is only uniform relative to some speed, then what is that if not a privileged reference frame? Could there be anything else significant about that frame?

    Is there a polarity detected in redshift (or function of redshift and distance) when looking at distant objects, like galaxies, in different directions? (Presumably with the same pole directions as in the background radiation?)
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  3. Jul 6, 2005 #2


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    Hi WilliamP, welcom to PF! A very good question! The CMB dipole shows earth has a slight relative motion with respect to CMB photons. That makes the CMB a convenient, but not absolute reference frame. We could just as easily substitute our motion relative to the sun, or the galactic core, to the same effect. It does not impact the validity of SR.
  4. Jul 6, 2005 #3


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    yes, in fact Hubble himself discovered it

    it is a slight imbalance in the recessionspeeds of distant galaxies
    (in the direction of the const. Leo they are not receding as fast as in the other direction)

    the expansion of the U, called the "hubble flow", determines a preferred frame, even before the CMB dipole was measured astronomers would talk about being "at rest with respect to the Hubble flow"

    the solar system is traveling about 360 km per second in the direction of Leo wrt. hubble flow

    and also wrt. CMB

    Special Rel was replaced by General Rel circa 1915.
    In Gen Rel, when you get a solution to the Einst. eqn that defines a spacetime, then typically that spacetime HAS A PREFERRED FRAME and you can determine an absolute velocity. The metric which normal working cosmologists use, the FRW-metric for Friedman Robertson Walker, has a built in absolute rest, absolute velocity notion

    Typically in popularized science books they say "there is no absolute velocity" because they never get beyond special rel, and that is a dogma of special rel. but it simply is not true. In an expanding universe there is a preferred frame, or class of preferred frames (there is no preferred origin).

    So it's a major pedagogical screwup. In highschool, and in pop sci books, and in Freshman Physics, and all that they tell you there is no absolute velocity. And then in junior or senior year they turn around and say "Oh, but the universe is expanding, so there is an absolute velocity after all! "
    Last edited: Jul 6, 2005
  5. Jul 6, 2005 #4


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    A clarification, if I may. Semantical issues sometimes cloud matters. The crucial assumption of relativity is there are no reference frames where the laws of physics are different. There clearly is a reference frame where the CMB is at rest. You could call this the rest frame of the universe, but that sounds a lot more important than it really is. Observers in that reference frame have no 'priveleged' view of the universe and there is nothing any more 'absolute' about the velocity of an object with respect to that frame than with respect to any other reference frame. No experiment done in the CMB rest frame would yield a different result than the same experiment done in any other reference frame. All the laws of physics operate exactly the same as they do in the earth rest frame, or any other reference frame. You will still have all the usual arguments about simultaneity, etc., with observers in other reference frames, and vice versa.
  6. Jul 7, 2005 #5


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    Welcome to Physics Forums, WilliamLP!

    moving this thread to the Relativity forum...
  7. Jul 28, 2011 #6
    Could an accurate absolute velocity of the earth be worked out by the amount of force required to accelerate the ions in the LHC? As the ions fly round the ring they will be moving with or against the motion and rotation of the earth. This will either make them move faster or slower. As they approach the speed of light the fluctuations in mass should increase so at some points, depending on the time of day and year they will have more mass and at other times they'll have less mass. Am i wrong here?

    Many thanks

    Tom Vines
  8. Jul 28, 2011 #7
    I think it has a frame preferred by cosmologists.

    A frame is nothing more than a point of view, a chart, a map. It is not a primary constituent of nature.
  9. Jul 28, 2011 #8


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    When we talk about hypothetical absolute reference frames, we are thinking in terms of inertial reference frames, which move at constant velocity (straight line at constant speed) with respect to each other. The earth's surface is not an inertial reference frame, because of the earth's rotation. We can detect this motion in many ways, without having to go so far as to use the LHC.
  10. Jul 28, 2011 #9
    Marcus makes an important point, and this is something too many people seems to have muddled.
    Special relativity as its own name indicates is constrained to a special case, that of interactions that don't involve gravity. So it is perfectly valid when applied in the right context. Now for the general case (with gravity since we know for certain that Minkoski spacetime is not the model for our universe) we have the general theory of relativity (again as it is implicit in its name).
    But it is important to stress that what is generalized in the general theory is not the special principle of relativity, but the field of application of the the theory. The special theory is valid only for a uniform velocity, no-gravity(flat) universe, and the general theory generalizes to the actual case of a universe with gravity (curved).

    Of pots and holes: Einstein’s bumpy road to general relativity
    by Michel Janssen
    https://netfiles.umn.edu/users/janss011/home%20page/potsandholes.pdf?ticket=t_ycAqaW0A [Broken]
    Last edited by a moderator: May 5, 2017
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