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Natural frames of reference

  1. Jul 2, 2010 #1
    I'm looking for a list of "natural" frames of reference on the cosmic scale. Something that can be used as "the" frame of reference for everyone in the universe that isn't arbitrary.

    So let's brainstorm about some potential candidates. The more the better. If need be, start with an idea and people can try to disambiguate it and/or turn it into an objective property. Once we have an objective fully clarified idea, let's try to come up with a suitable name and simple snappy definition. After all that, we can start arguing about which one is "best".

    As far as I know there are three parameters that determine our frame of reference (at least that are relevant to relativity) Position, Orientation, and Velocity. (or is it Momentum?)

    We can choose each independently, though it might be more "natural" to choose them all in some related manner.

    Some food for thought:
    Average over some relevant or "natural" domain.
    Landmarks (think center of biggest galaxy or void)

    Here's an idea that a few of you are probably already thinking:

    1) Universal Centroid

    A: Average position and momentum of every thing that has a position and momentum. OR
    B: Average position of every thing that has a position and average momentum of every thing that has momentum. (Some things don't have momentum right?)

    Some other domain? Only background radiation particles? Only neutrinos (for some reason)? Only hadrons?

    A potential problem I see with these is quantum mechanical funkiness may make that frame indeterminate or something. I'm not sure. Can we get around that? Also, this doesn't say anything about orientation/direction. Maybe an arrow from the center point to the furthest known/knowable thing?

    Something to do with the beginning or end of the universe? Or maybe the shape? Something to do with background radiation? Something to do with a mathematically special frame of reference? Something to do with the biggest or best civilization?
  2. jcsd
  3. Jul 3, 2010 #2


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    Cosmologists use a natural reference, the CMB.
    the ancient light emitted by the most ancient matter we can see which was a hot gas at about 3000 kelvin roughly uniformly distributed.

    You know that you are at rest relative to the CMB light, and therefore at rest relative to the ancient matter (the glowing hot gas) if you look around and see the CMB is all approx. the same temperature. No doppler hotspot.

    It turns out the solar system is moving in a certain known direction at the known speed of about 370 km/s. (because we see a hotspot in the microwave sky ahead of us and a cold spot behind us). So the solar system's motion is FACTORED OUT. Astronomers convert the data, by compensating for the solar system motion, so it is as if seen by an observer who is at rest relative to the redshifted glow from the ancient matter.

    Also cosmology uses a model which has a preferred reference. It is called LCDM and that is a version of the Friedmann model. The Friedmann model has an idea of rest, and an idea of universe-wide simultaneity (time measured by all the observers who are at rest.)
    The Friedmann model dates from around 1925.

    So you are right on target to be looking for a natural or absolute time, and an absolute rest criterion, for cosmology. Only that the cosmologists have had this and been using it for over 50 years. Hubble himself had something like an absolute rest criterion even before the CMB was observed in around 1970. It just wasn't so accurate. One could not say exactly 370 km/s.

    So welcome to the club, and congratulations for suspecting this on your own. :biggrin:
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