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A Question Considering Motion

  1. Apr 29, 2007 #1
    I understand that motion is relative and that everything is in motion.

    My Questions are :

    Is there such a thing as universal motion? In other words, is the space-time continuum rotating or moving in any way as a whole entity?

    Can this be determined using the cosmic background radiation?

    Would this motion have an effect on space-time?

    If so, what would it be?
  2. jcsd
  3. Apr 30, 2007 #2


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    Since motion is, as you say, all relative, this question can't really be answered since for everything to be moving it would have to be moving with respect to something that was stationary, but the stationary thing would be part of the everything and hence would have to be moving as well. I hope that made sense! The short answer is no :wink:

    The CMB does define a rest frame, but there is nothing special about this rest frame. It's not a rest frame in the way you might think though. Consider this; you can be at rest with respect to the CMB by ensuring that the wavelength of the CMB photons you see is the same in all directions (this isn't exactly true for us, there is a 'hot' and 'cold' side to the CMB due to local motions such as our orbit of the Sun and our galaxies motion towards nearby large clusters). Now, consider a distant galaxy. By Hubbles law the galaxy is moving away from us however if you went to that galaxy you would find that it was also at rest with respect to the CMB (except for small local motions that it experiences).

    In this way the CMB doesn't define a global rest frame in the way you might normally think of a rest frame. To be at rest with respect to it, things distant from each other must be in motion with respect to each other.
  4. May 1, 2007 #3

    Thanks for the response. You first say the question is unanswerable but then give the answer, No. This is confusing.

    For the sake of discussion, lets say in the same way we determined the rotation of the Milky Way, We were able to observe a rotation of the continuum. Would this affect the computer models of the universe. Maybe help resolve a part of the pioneer anomaly or some other problems with the modeling.

    I know this is highly speculative and complicated but it is mild compared to some of the thoughts I have concerning the unknowns.
  5. May 1, 2007 #4


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    Apologies for the confusion, what I was trying to point out is that if everything was moving uniformly there would be not reference point to check against that this motion was happening. Hence the answer 'No' referred to the fact that you cannot sensibly define a uniform motion of the entirety of space-time.

    I think you are mixing concepts here. We can observe the rotation of the Milky Way by seeing the gas in the spiral arms moving (through the doppler shift in the Hydrogen 21cm line). We do not observe any motion of some underlying space-time continuum, we observe motion of gas particle through space-time, not the rotation of space-time.

    We don't observe any such large scale rotation in the redshift data of distant galaxies.
  6. May 17, 2007 #5


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    I am not a physicist and I don't know how the rotation of the MW is determined; so I will rephrase your statement as: "For the sake of discussion, lets say we were able to determine a rotation of the continuum. Would this affect the computer models of the universe. Maybe help resolve a part of the pioneer anomaly or some other problems with the modeling." ("assume" may be a better word here than "determine" -- highlighting the point that this is a hypothetical thought experiment).

    Perhaps a model that assumed rotational acceleration could explain gravity as acceleration -- rather than (merely) "indistinguishable from acceleration" (as per Einstein's space elevator), which quoted expression is an epistemological statement. A rotating universe model could (perhaps) make the ontological statement that gravity is acceleration.

    (Or it could prove that gravity is not rotational acceleration.)

    As a second-best, it might determine that the question "is gravity really acceleration?" cannot be answered by a rotating universe model.
    Last edited: May 17, 2007
  7. May 17, 2007 #6

    Chris Hillman

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    Attempt to answer some questions...

    That is a caricature of what relativity theory says, one which unfortunately can be particularly misleading in the context of cosmology.

    It would be better to say that relativistic kinematics (str) and Galilean kinematics agree that inertial motion is relative.

    Well, there are a couple of phenomena you might be asking about.

    First, roughly speaking, our solar system is in a sense moving linearly with respect to the CMB, and this can be detected and has been. For those who are familiar with the FRW models, which are some of the simplest models employed in relativistic cosmology, as an exercise you can write down a frame field which is moving linearly wrt a frame comoving with the dust particles; the latter in the FRW idealization in some sense provides a kind of notion of absolute rest wrt the CMB.

    Second, there are many different things one might mean by "rotating matter" in relativistic cosmology, and unfortunately it is not easy to briefly explain the distinctions. One possibility: the FRW models are homogeneous and isotropic (that's why they are so simple), but one can consider more elaborate models which are neither homogeneous nor isotropic. In particular, one can consider models in which the matter in the universe is a sense rotating about a prefered axis (which you can think of as a definite place in the universe).

    Such models can in fact be obtained as perturbations of the FRW models, which are good approximations to the universe we observe, so these models can be plausible candidates to try to match to observations. When that is done, one can come up with limits to the rate at which the universe could be "rotating" in the sense intended. See Wheeler and Cuifolini, Gravitation and Inertia.

    According to gtr (and many closely related theories), rotating matter (in most of the various senses I mentioned) produces characteristic gravitational effects.

    Well, frame-dragging, for example. One can write down models in which observers would in principle find that their local inertial gyrostabilized frame is slowly spinning with respect to the distant stars. The buzzword you are probably looking for is "Mach principles" (note that many Mach principles have been suggested). See http://www.arxiv.org/abs/gr-qc/9607009
    Last edited: May 17, 2007
  8. May 19, 2007 #7
    In regards to original question, one might be reminded of Hudson M. et al 2004 arXiv.org/astro-ph work with SMAC (streaming motion of Abell clusters) studies and the idea of any large scale peculiar velocity of such clusters, other than just infall into respective superclusters. This year more input from ACT (Atacama Cosmology Telescope), South Polar Telescope, and perhaps Planck Surveyor's better resolution of CMB, suitable for total Suneyev-Zeldovich effect, a redshift independent distance indicator, might enlighten matters. The latter effect is another standard candle, wherein one has CMB scattering off intra-cluster hot gas. So large scale peculiar velocity studies might address any speculation as to any sort of larger scale coherent component of motion of such clusters. Cheers.
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