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Frame of reference and speed

  1. Jan 8, 2007 #1
    Our planet is moving through our solar system and our solar system is moving through our galaxy and our galaxy is moving within the local group and are local group is moving through our universe, so how fast are we moving right now, and if i then jump in to my space ship and accelerate am i accelerating or slowing down relative to the motion i already had gained through everything else's motion within the universe.

    we can travel at what we believe to be a certain speed but that speed is only relative to the one point in space which is not fixed which we have decided to use as our frame of reference but that one frame of reference cant be absolute as every point of space has a different frame of reference. so who has the correct frame of reference, no one .

    So is speed real ,is it possible to say that something anythings travelling within the universe at 1 10th the speed of light for instance.
    Last edited: Jan 8, 2007
  2. jcsd
  3. Jan 9, 2007 #2
    Speed, or velocity, is only measurable relative to some reference object. There is no preferred background for measuring velocities, or in other words, every /inertial/ (non-accelerating) frame of reference is the "correct" frame of reference.

    So in answer to your last question something can only be travelling through the univers at 0.1c relative to some reference frame which is not accelerating.

    Hope this helped.
  4. Jan 9, 2007 #3


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    Staff: Mentor

    You can, of course, calculate all those velocities if you want - the information is available via some googling. But as Jheriko says, it though it is real, it isn't real meaningful.
  5. Jan 9, 2007 #4

    Chris Hillman

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    Science Advisor

    No preferred frame?

    Actually, in a sense, in the context of cosmological models like the FRW dust solutions of the EFE, there is a kind of "preferred motion": motion comoving with the dust particles can be distinguished from motion which is not.

    Moving from these highly idealized models to the real universe, this means that in a sense motion with respect to the "spatially averaged" cosmic background radiation can in principle be detected. Astronomers have in fact done this; this "dipole anisotropy" must be allowed for in analyzing the COBE data. In case anyone is curious: our solar system is moving toward the constellation Virgo, wrt the background radiation, at a fairly impressive clip.

    Moving back to the FRW models, it is instructive to take a frame field http://en.wikipedia.org/w/index.php?title=Frame_fields_in_general_relativity&oldid=42117350 which is moving with comoving constant velocity wrt the dust particles. The energy-momentum tensor then acquires some off-diagonal components (momentum flux of the dust wrt our new observers) and pressure terms, but the eigenvalues still reveal that in a comoving frame, the pressure vanishes, so that the source of the gravitational field is a pressure perfect fluid (dust), and this is indeed a physical feature which is independent of the observer.
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