Doesn't cosmic microwave background radiation provide an absolute reference point for motion?

Physics 101 teaches that, if Bob is moving at speed X relative to Mary, both Bob and Mary are perfectly entitled to claim that they are standing still and the other is moving. Neither view point is right or wrong because there is no absolute space to measure motion against. However, doesn't cosmic microwave background radiation provide this absolute space? If Bob wanted to confirm whether or not he was really at rest or not, couldn't he just check for dopplar effect of this radiation? If there wasn't any, then he could claim he was at rest and Mary was moving.

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  • 1 decade ago
    Favourite answer

    Yes and no. Yes, you can in fact do measurements to determine whether you're moving with respect to the frame in which the cosmic microwave background exhibits no Doppler shift. But while finding this frame of reference might be interesting, it is still not an absolute frame of reference. That is, there's no difference in the laws of motion whether you're in a CMB-Doppler-shifted frame or a CMB-Doppler-unshifted frame. That is the more fundamental point. So we might decide to call the CMB-Doppler-unshifted frame a "standard" frame of reference, but it is still not an "absolute" frame of reference.

    Incidentally, my understanding is that we measure very little Doppler shift in the CMB in our galaxy. However, even though we can measure very significant Doppler shifts in the light from distant galaxies, observers in these galaxies would also measure very little Doppler shift in the CMB. This is because the universe is expanding more or less symmetrically, so the "horizon" from which the CMB comes is essentially the same distance away no matter where you are. So an observer on Earth and an observer in a distant galaxy could both claim to be at rest with respect to the CMB standard frame, yet still be moving at relativistic speeds with respect to each other!

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  • 4 years ago

    Absolute Frame Of Reference

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  • Anonymous
    1 decade ago

    In a sense you are correct. If Bob could determine that there was more red shift in one direction than in the opposite direction, he would know that he is moving. However, the observations show that there is no such variation with respect to distant galaxies and the CMB. However, knowing that one is moving with respect to the CMB would not provide a coordinate system.

    For example consider a simple three dimensional cartesian coordinate system. Knowing that you are moving with respect to the three axes does not provide sufficient information to know where the origin of the three axes is located.

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  • 1 decade ago

    Yes. By measuring CBR's red or blue shift we can measure Earth's motion relative to it. That does NOT eliminate the relativity of motion vs. light speed and inertial experiments.

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