Measuring speed..

Discussion in 'General Science & Technology' started by ZMacZ, Mar 27, 2018.

  1. Q-reeus Valued Senior Member

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    3,052
    Well a fairly brief search turns up these first few:

    http://www.sciforums.com/threads/no...g-at-an-accelerated-rate.158122/#post-3412310

    http://www.sciforums.com/threads/is-there-anything-faster-than-light.155933/page-4#post-3374691 (yes even danshawen got it right here and there)

    http://www.sciforums.com/threads/cosmological-red-shift.155893/page-9#post-3373463 (ditto)

    http://www.sciforums.com/threads/gravitational-time-dilation.145889/page-13#post-3298347 (the best of this bunch)

    http://www.sciforums.com/threads/wh...-doesnt-make-sense.144835/page-9#post-3284862

    I'm not picking on you exchemist re that particular matter - it's no sin to forget when the last 'refresh' was some time ago. What really irritates is to have put in a lot of effort correcting what will remain unnamed folks who then quickly return to repeating the original error again and again - sometimes in the same thread let alone subsequent ones. All too prevalent at SF. A not-so-merry-go-round from my pov. But those that treat the place as a carnival would likely disagree.

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  3. exchemist Valued Senior Member

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    Thanks for these. I've now read them. (To be fair to myself, the first two involved danshawen, paddo and Trivedi so that may excuse my not having read them at the time!)

    Most seem to address only the issue of the dipole anisotropy in the CMBR as observed from Earth, which I now understand, I think. So clearly one can speak of motion relative to a "universal" CMBR background.

    The last one is more interesting as it deals with Fednis's question (I have learnt that any question Fednis asks is worth paying close attention to!), about what can be meant by the idea of a CMBR "rest frame" . This is very much the same thing NotEinstein was talking to me about. In other words, how can it be a "rest frame" in the relativity sense if, due to the expansion of the metric, objects "at rest" in this "frame" at different locations are retreating from one another!

    I'm not sure I understand your response exactly. Would I be right to interpret what you say about this to be that the term "rest frame" is actually used loosely (=wrongly?) in this context and that strictly speaking it isn't one?
     
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  5. sweetpea Valued Senior Member

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    I see what you mean there, and using different words to say the same thing I would say... The CMBR photon density is decreasing due to spacetime expansion. Or, every CMBR photon is the centre of its own observable universe. Where does that leave the idea that the CMBR is an absolute frame if every CMBR photon is receding from every other CMBR photon? Just like galactic clusters?
     
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  7. Q-reeus Valued Senior Member

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    Well in the GR sense, anyone or anything experiencing zero g-forces is in a rest frame i.e in 'free-fall'. We standing on Earth's surface are not, but move up a scale and Earth's center of mass is in free fall wrt the rest of the Solar system, and in turn our Solar system is in free fall wrt the rest of the galaxy, and in turn our galaxy is in free fall wrt our local galactic cluster, in turn in free fall wrt our super cluster. There are a few disconcerting exceptions but once you get to a few hundred million light-years in scale, things become essentially homogeneous and then out it's believed the (observable) universe can be treated as a uniformly dense continuum.

    The cosmological principle then says an observer anywhere in the universe considers themselves to be at a center where all other large distance entities, as part of that uniformly dense continuum, are moving away from that observer in a spherically symmetric manner. And as you know, the further out from that center, the faster the apparent recession speed of distant objects becomes.Cosmological redshift.
    What spoils it is the inhomogeneity at those smaller scales - being attracted to some higher density galaxy cluster/super-cluster leading to locally observed 'peculiar velocities' wrt the ultimate reference we have defining almost perfect homogeneity - the CMBR. It's because the fossil radiation of that CMBR has experienced negligible gravitation 'clumping' on it's journey over billions of years, and was initially so uniform at ~ 300,000 years post BB, it acts as that reference backdrop from which we can gauge our local deviation from perfectly symmetric Hubble flow centered about us.

    Drop down to the expanding balloon analogy. Almost uniformly spaced dots on that balloon will see every other dot moving away with an apparent speed proportional to distance from it. But now allow that dot to move across the balloon surface, and moreover feel 2D gravity tugs from neighboring dots. If the distribution were entirely uniform, the reference dot will have no tendency to drift across the surface owing to symmetry of the collective tugs. Hence each is 'at rest' wrt the balloon surface. But with significant inhomogeneity of dot spacing, chances are our reference dot will feel a net tug in some direction and will thus start moving toward there. If we now throw in a diffuse, surface confined radiation, the dot motion is detectable via Doppler shift anisotropy - a so-called dipole anisotropy in particular.
    Note the dot is still in 'free fall' thus 'at rest' in the sense of being 'weightless', but nevertheless has a motion wrt the balloon average dot distribution. In that sense it is 'not at rest'.
    Hope I haven't confused you further!
     
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  8. DaveC426913 Valued Senior Member

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    This is not true. This is not the definition of rest frame. My rest frame is sitting here on this couch, at full Earth g.

    I wonder if what you mean is inertial frame.
     
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  9. Q-reeus Valued Senior Member

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  10. DaveC426913 Valued Senior Member

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    (whew! I wasn't nearly as sure as I sounded.)
     
  11. exchemist Valued Senior Member

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    Yes, so objects at rest with respect to the CMBR background will be retreating from each other at the Hubble flow rate.

    I presume from your answer that while one can define a rest frame for a single object that is aligned with the locally prevailing CMBR, the CMBR-aligned rest frame of an object at a different location is in a different rest frame, moving with the respect to the first at the Hubble flow rate.

    So the CMBR does not define a rest frame that can be applied across multiple locations in space.

    Is that a fair statement?
     
  12. Q-reeus Valued Senior Member

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    All good except for the last bit. Hubble flow rate is a conversion factor given by the Hubble 'constant' H such that v = Hd. Where v is the recession speed and d is the distance to some distant object moving at v away from us but assumed itself moving with it's own local Hubble flow - that is it has no locally detectable CMBR anisotropy.
    Deviation from that simple equation occurs for two basic reasons - accelerated expansion, and 'peculiar velocities' owing to gravitational clumpiness as described earlier. The former dominates at very large distances, the latter relatively closer in.
    Yes. There can be no absolute center of the universe - not of one conforming to modern cosmological modelling.
     
  13. exchemist Valued Senior Member

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    OK I think I have it straight now.

    But regarding your first comment, what you seem to be talking about is why there are deviations from the rate of separation predicted by Hubble flow (dark energy etc). All I was trying to say is that if you had, for whatever reason, 2 locations that did follow that rate of separation, they would both be in the local rest frame of the CMBR.
     
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  14. Q-reeus Valued Senior Member

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    Yes indeed. Easiest to visualize that via the 2D expanding balloon analogy - with all dots uniformly distributed. Let's hope there's no Capricious Divinity with a Cosmic Pin ready to prick our universe.

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