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why cosmic background radiation is not ether? I mean it's everywhere and it' a radiation then we can measure Doppler effect by moving with a velocity.

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up vote 13 down vote accepted

The luminiferous aether was, by definition, a hypothesized medium that was needed for electromagnetic waves to propagate through space. The cosmic microwave background isn't needed for photons to move; indeed, they move through space even if one removes (shields) the cosmic microwave radiation. So that's why CMS isn't luminiferous aether.

On the other hand, the aether also made a particular prediction, the existence of a preferred reference frame. In this sense, the CMB plays the same role as the aether. Cosmologists use the reference frame associated with the CMB as the preferred coordinate system. However, this ability to pick a "preferred" coordinate system depends on the environment – the CMB is just some property of the environment that could possibly be different as well (e.g. if you shield it). In this respect, it still differs from the luminiferous aether that couldn't have been shielded and that guaranteed the existence of a preferred coordinate system in any situation, regardless of details of the environment.

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It may be worth noting that CMB's "rest frame" is still a non-intertial frame. This is trivial to show in GR, as no frame can be globally inertial. Equivlently, one can reason that two ships at rest with respect to the CMD "rest frame", but seperated by supercluster-scale distances will still not appear at rest with respect to each other (due to hubble expansion) which conflicts with the fact that being at rest is a transitive property in an inertial frame. – Kevin Cathcart Apr 12 '13 at 16:58
Agreed. The relative motion of the CMB frames defined at various points - the Hubble expansion - is the very same obstruction as the spacetime curvature, after all. ;-) That's what prevents us from using the flat spacetime of special relativity to describe a generic cosmology, i.e. a curved background of general relativity, using any coordinates. It's just physically not flat i.e. not special relativistic. – Luboš Motl Apr 13 '13 at 7:17

There are a couple of misconceptions here. The CMB radiation is thought to be the (historical) light left over after the big bang. Due to space expansion, it has been red-shifted towards the microwave region of spectrum. While on the other hand, ether is the medium required for light to travel when it was thought as a longitudinal pressure wave (like sound)...

In light of your question, you're saying that the red-shifted EM radiation acts as a medium for light to travel... - which is certainly untrue..!

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Why cosmic background is no ether is not a question it is a statement.

A question would be is the cosmic background an ether?

For one thing it is an inertial system. This inertial system can be found by anyone in the universe. One can find(calculate) his or her velocity to it.

Since a inertial system is sufficiant to describe physical phenomena in it has all the properties to state that Lightvelocity is constant c in it.

If Michelson knew what we now know about Cosmic Background Radiation he would not have build his large interferrometers to measure the velocity of the solar system to the universe and special relativity would not have emerged.

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This is total nonsense. How in the world can you claim that the existence of the CMB is inconsistent with relativity? – Ben Crowell Sep 24 '13 at 23:33

protected by Qmechanic Sep 24 '13 at 22:59

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