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It seems like everything in the universe is in motion, at least relative to some other object. That leads me to believe that all objects are in motion. But how do we measure motion when we are talking about galaxies, is there any reference point that's not moving?

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4 Answers 4

up vote 5 down vote accepted

You are correct: everything is in motion (or not) based on the reference frame. Motion is a relative concept, so you are never "moving" but only "moving with respect to something".

Find a good basic primer here: http://en.wikipedia.org/wiki/Principle_of_relativity

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Why the down vote? You don't need to invoke SR or GR to answer the question. –  Sklivvz Dec 9 '10 at 22:12

Well, all objects are in motion with respect to some other object and none of them are superior frames of reference due to Special Relativity.

Also, you may think of the expansion of the universe. Everything in the universe that is far enough is moving away from every observer in the universe in a macroscopic scale. To clarify what I mean, if an object is far enough from you, you can easily detect that it is moving away from you using redshifting and you would see that everything distant enough is moving away from you without any exceptions, although "distant enough" is dependent on some other factors.

Edit: Motion of galaxies are measured with respect to another galaxy, or if it's the rotation you're talking about it's measured with respect to the rotation axis.

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Actually, even Galilean Relativity states that all inertial frames are equal! –  Noldorin Dec 9 '10 at 18:58
Indeed. Filler letters. –  Cem Dec 9 '10 at 19:01

The CMB definitely gives a preferred inertial frame at any point in the universe. However, this gets a little trickier when you include GR, and then if you are sitting in/near a deep potential well (e.g., a cluster of galaxies), the CMB may never appear isotropic, regardless of your velocity

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The inertial frame defined by isotropy of the CMB isn't a "preferred" frame in the relativistic sense, though. Physical laws still work the same whether the CMB is isotropic or not. (Depending on the problems you need to solve, I could certainly agree that you might prefer to use that frame, I'm just saying that you can't really argue that nature itself prefers that frame for some fundamental physical reason.) –  David Z Dec 10 '10 at 21:56

But, are all inertial frames really equal? Place two objects (or reference frames if you prefer) close together, but with a high relative velocity. Do they see the same cosmic background radiation? Does the one moving relative to the other see a doplar effect in their measurements of the CMB, and in redshifts of distant objects? If the answer isn't no (i.e. we can detect velocity wrt the CMB), then there is some prefered velocity for each point in spacetime.

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