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CMBr and absolute motion query

  1. Nov 7, 2014 #1
    Why can't the CMBr 'wall' that is as if we were in the middle of a sphere surrounded by it, be used as a reference point for absolute motion in space?
  2. jcsd
  3. Nov 7, 2014 #2


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    You can use the CMBR as a reference if you want. But there are lots of other references you can use instead, and they are all equally as good as each other as far as the laws of physics are concerned.
  4. Nov 7, 2014 #3


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    Why can't mount Everest be used as a reference point for absolute motion in space?

    EDIT: scooped by DrGreg!
  5. Nov 7, 2014 #4
    I really want to understand this so I suppose I would answer that Mt Everest, or any other point in the Universe may or may not be in motion but the CMBr 'wall' seems to me that it is not in motion. Or is it? And if it is, is it in motion the same way that an object in the Universe could be said to be in motion.

    I mean, if I was floating freely in intergalactic space and I wanted to call myself at rest then I can't see how I would be able to perceive the CMBr 'wall' which surrounds me like a sphere, as being in motion. Or should I perceive it as such?
  6. Nov 7, 2014 #5
    Because that would imply or assume you were in "the middle" of something, If you can't prove it's that way in any consistent fashion it is pointless as a "star map" if that is what you were asking...
  7. Nov 7, 2014 #6
  8. Nov 7, 2014 #7
    But that's just it, aren't I in the middle of the Universe, as is every other point? Isn't the CMBr equidistant from me in all directions as it is for every other place in the Universe? Which is why I thought that it would have a unique property that would enable it to be used as an absolute reference point.

    Is the problem something to do with me trying to use 3 dimensional analogies in 4 D space/time?
  9. Nov 7, 2014 #8
    Nope the CMBr might be of use in the t=? area of interest but x,y,z are always relative and indifferent as far as we can tell. Who knows for sure we could imagine being in another galaxy and it looks very different, perhaps that "cold spot" is alot bigger...
  10. Nov 7, 2014 #9
    Who knows we could go the opposite direction and see another "cold spot" start to form in front of us and the only logical conclusion I could draw from that would be that we are very close to the center of a finite universe.
  11. Nov 7, 2014 #10


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    In addition to points already made, I add the following:

    1) You can't talk about the the CMBR as a 'thing' you are moving relative to. What you can say is that if you have a body the measures the CMBR as isotropic, then a another body moving rapidly by the first body, will see the CMBR as non-isotropic. However there is nothing in the laws of physics that favors the first body as being at rest, any more than you would claim that a body that feels no average wind in the earth's atmosphere is universally at rest.

    2) Are you aware that two bodies at great distance from each other, that are locally at rest per the CMBR as described in (1) have relative motion? In this sense it does not make any sense to talk about the CMBR as defining a global rest - bodies that locally see isotropic CMBR are in relative motion to each other (as defined by either red shift or growth of distance between them, or via comparing vectors by parallel transport).
  12. Nov 7, 2014 #11


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    But "seems to me" is not a reason, it is just restating your assumptions. The CMBR is just part of the "terrain" of the universe. You certainly can measure your velocity relative to it (or rather the local frame where it is isotropic), but that is no more "absolute" than measuring your velocity relative to any other piece of "terrain".
  13. Nov 7, 2014 #12
    @https://www.physicsforums.com/threads/cmbr-and-absolute-motion-query.780670/members/pallen.275028/ [Broken], OK, no I don't fully see it yet, but I will give what you have said some more research and consideration until I get it.

    @https://www.physicsforums.com/threads/cmbr-and-absolute-motion-query.780670/members/dalespam.43978/ [Broken], yes I see, 'seems to me' is not a good thing for me to say.
    Last edited by a moderator: May 7, 2017
  14. Feb 21, 2015 #13
    Sorry to bring this thread back but I did not want to start another thread similar to this. I have just come up with another query that I'm not sure about...

    I was thinking about the quantum vacuum and virtual particles which come into existence for a very short time. It seems to me that this would be an all pervading static field because the virtual particles exist for such a short time that they can't go anywhere. So...... can the quantum field of virtual particle be used as a reference for absolute rest. And if not is it because there's nothing special about the virtual particle field, or is it because there's no way to get a fix on it because of the brevity of it's existence. In other words is there a reference for absolute rest there but it remains forever out of our ability to use because of being hidden behind a quantum wall of brevity.
  15. Feb 21, 2015 #14

    Vanadium 50

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    "It seems to me" is still not a reason. And the vacuum is invariant; it's properties do not change with velocity.
  16. Feb 21, 2015 #15


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    With that, thread closed.
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