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Absolute velocity, CMB, and doppler shift

  1. Aug 30, 2009 #1
    I've heard it said that it is impossible to determine an objects absolute velocity, all one can do is find it relative to some other object.....

    but, if the CMB is everywhere, why could you not just measure the doppler shift in all directions, and adjust your velocity so the CMB has no doppler shift in any direction? Wouldn't this tell you that your absolute velocity is 0?
     
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  3. Aug 30, 2009 #2
    You kinda answered ypour own question there.
    "The CMB is everywhere"
    "measure the doppler shift in all directions"
    "adjust your velocity so the CMB has no doppler shift in any direction"

    Moving towards one point in the universe would lower the doppler shift magnitude in that direction, but at the same time, increase it in the opposite direction and various trigonometrical ratios of all angles in between, whilst 90 degree doppler shifts would remain unchanged.

    You would need to move in a velocity that moves in all directions at the same time (Impossible, as velocity is a measure of the magnitude of motion in a definite direction). This would give a relative velocity to the rest of the universe as 0, which is perhaps as close to an absolute velocity as one could obtain, but is impossible all the same.
     
  4. Aug 30, 2009 #3

    marcus

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    Sure. There is no problem with what you say. In effect, astronomers do this. They deduct the solar system's motion from the data, when making measurements that our motion relative to CMB would mess up.

    How it works is the CMB has an average temperature over the whole sky. Around 2.73 kelvin (exact figure doesn't matter.) But in a certain direction (marked by the constellation Leo in the sky) the temperature is a fraction of a percent hotter----the wavelengths are a fraction of a percent shorter. Roughly a tenth of a percent.

    What this means is the solar system is moving in the direction of Leo, relative to CMB, at that same fraction of speed of light (roughly a tenth of a percent). So in cases where our individual motion would distort things, they deduct that motion, or correct for it, so that it gets taken out of the data. And then the data is just as if we were observing from AT REST relative CMB.

    That is just what you were talking about, you imagined an observer at "absolute" rest, where absolute means relative to the CMB.
    It is a good perspective to imagine taking.
    Actually the solar system is moving about 370 kilometers per second relative to CMB, and the earth is orbiting at a speed of about 30 km/s.
    But with the help of a computer we can correct for all that giddy motion and effectively take a look at the universe as though we were standing still.

    Nothing is "absolute" in some extreme philosophical sense, but the CMB is a pretty good substitute, as your post suggests. It is the light from the matter of the early universe which was nearly uniformly spread out, in a hot cloud, and all at nearly the same temperature----at the time it was about 3000 kelvin (before that the cloud was too hot and too dense so the light was kind of trapped in the fog, we only see light from when it cooled to 3000 and became transparent). So that uniform cloud of ancient matter, roughly the same everywhere in all directions, is what we are taking as our reference when we measure our motion relative CMB.
     
    Last edited: Aug 31, 2009
  5. Aug 31, 2009 #4

    Chronos

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    The CMB reference frame is based on observation of a "hotspot" in one direction, and a "coldspot" in the opposite direction from earth. This is what makes it attractive as a sort of universal coordinate system.
     
  6. Aug 31, 2009 #5

    Chalnoth

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    Right, the CMB has a particular reference frame. But that frame is just as arbitrary as any other. As near as we can tell, the laws of physics all work out in exactly the same way no matter what speed you're moving at.
     
  7. Aug 31, 2009 #6
    Please explain. Your statement doesn't make sense to me, but I am a little dense.


    PS - My very uncertain opinions on expansion of the universe are posted here:

    https://www.physicsforums.com/showthread.php?t=328143
     
    Last edited by a moderator: Feb 14, 2014
  8. Aug 31, 2009 #7

    Chalnoth

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    Well, like the earth. We can measure our velocity with respect to the earth just fine. This doesn't mean that we have an "absolute" velocity. It means we have a velocity with respect to the earth.

    The CMB is no different. We have a velocity with respect to the CMB. But that doesn't make such a velocity "absolute": it's just a velocity with respect to the CMB.
     
  9. Aug 31, 2009 #8
    That makes no sense to me.

    If CMB is everywhere and an object moves relative to CMB, than that seems to me like motion relative to the old concept of ether.
     
    Last edited by a moderator: Feb 14, 2014
  10. Aug 31, 2009 #9

    Chalnoth

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    Just because it's everywhere that doesn't make it anything remotely like the concept of the ether. The CMB is just a gas of photons. Surely you can see the possibility of moving with respect to a gas?
     
  11. Aug 31, 2009 #10
    Yes, I can "see the possibility of moving with respect to a gas". Historically that gas was called "luminiferous aether" or "ether."
     
    Last edited by a moderator: Feb 14, 2014
  12. Aug 31, 2009 #11

    Chalnoth

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    You seem to be misunderstanding the concept of the ether. The ether was not a gas of photons, or anything of the sort. The ether was a hypothetical material that was believed to be the medium through which electromagnetic waves propagate, in the same way that air is the medium through which sound waves propagate. If this were the case, then we could have detected our motion through the ether by measuring differences in the speed of light in different directions.

    The basic idea is this: from E&M we can compute the speed of light. It was believed at the time that this was due to the properties of this "ether", and that this speed of light was with respect to the ether. Therefore, most any observer measuring the speed of light should be able to measure their speed with respect to the ether.

    But that's not what happens. When we do measure the speed of light in different directions, we always end up with the exact same result. Light doesn't move with respect to any background, but is a constant speed to all observers.

    The fact that there's a gas of photons out there doesn't change this result one iota.
     
  13. Aug 31, 2009 #12
    You may be right.

    Or you may be grasping at straws.

    As Marcus noted above, Nothing is "absolute" in some extreme philosophical sense, but the CMB is a pretty good substitute, as your post suggests.
     
    Last edited by a moderator: Feb 14, 2014
  14. Aug 31, 2009 #13

    Chalnoth

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    Oh, I'm quite confident that what I've written above is accurate. The CMB is a reference frame. And it's a rather convenient one to use in cosmology (though not so much for most other purposes). But there is no possibility that it is in any way, shape, or form related to the falsified concept of the luminiferous aether. The two are entirely different concepts.
     
  15. Aug 31, 2009 #14

    sylas

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    He certainly is right.

    The background radiation is nothing like the hypothetical and now disproved notion of the aether, which was postulated as the medium within which waves could exist. The fact that we are moving with respect to the background and that we continue to measure the speed of light as being the same in every direction shows immediately that they are not the same.

    He is not grasping at straws; but you are. To mix up the notions of aether with background simply because it can stand as a kind of substitute for an absolute is an obvious case of grasping at straws.

    Cheers -- sylas
     
    Last edited: Aug 31, 2009
  16. Sep 1, 2009 #15
    I realize that you are spouting "standard" dogma, but I personally am not impressed.

    Nonsense is nonsense.

    That is my opinion,
     
    Last edited by a moderator: Feb 14, 2014
  17. Sep 1, 2009 #16

    russ_watters

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    Indeed, nonsense is nonsense and yours is not welcome here. Thread locked.
     
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