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In order to determine the relative motion between the Earth and the ether (the medium through which light supposedly propagated. It has zero density and complete transparency), scientists used the concept of the aberration of starlight. If the Earth frame of reference was not dragging the ether frame of reference, then the direction of starlight as seen from both frames of reference separately would be different. Whereas if the Earth frame of reference was indeed dragging the ether frame of reference, then the starlight observed from both frames of reference separately would appear to come from the same direction. This is all intuitive. But how do we prove whether either is the case, when we are unable to identify the ether frame of reference itself, and conduct experiments within this frame of reference to determine the observed direction of starlight from within this frame of reference?

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A comment by @Nathaniel:"Ether drag" was an alternative explanation for the Michelson-Morley experiment. It said that the experiment couldn't detect any motion of the Earth through the ether because a friction-like effect caused the local region of ether surrounding the Earth to move with it. (Actual drag would cause dissipation of the Earth's kinetic energy, but I'm sure the theory had a way around that. – anna v Mar 17 '13 at 7:33
continued: Luminiferous ether theory was a lot more coherent and mathematically developed than we tend to give it credit for today.) The OP is asking how to distinguish between this possibility and special relativity./end Nathaniel's comment. – anna v Mar 17 '13 at 7:34
This historical framework should be referred to by any answer, as current experimental data have completely falsified the luminiferous aether theory: a permeating inertial framework against which everything moved. – anna v Mar 17 '13 at 7:36

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