Aether drag hypothesis
![Lodge's ether machine. Light from a sensitive common path interferometer was guided between the rapidly rotating disks.](/Images/godic/202501/14/Sir_Oliver_Lodge's_ether_machine3353.png")
![Complete aether dragging is inconsistent with the phenomenon of stellar aberration. In this illustration, imagine the stars to be infinitely distant. Aberration occurs when the observer's velocity has a component that is perpendicular to the line traveled by the light incoming from the star. As seen in the animation on the left, the telescope must be tilted before the star will appear in the center of the eyepiece. As seen in the animation of the right, if the aether is dragged in the vicinity of the earth, then the telescope must be pointed directly at the star for the star to appear in the center of the eyepiece.](/Images/godic/202501/14/Stellar_aberration_versus_the_dragged_aether3353.gif")
In the 19th century, the theory of the luminiferous aether as the hypothetical medium for the propagation of light was widely discussed. An important part of this discussion was the question concerning the state of motion of Earth with respect to this medium. The aether drag hypothesis dealt with the question whether the luminiferous aether is dragged by or entrained within moving matter. According to the first variant no relative motion exists between Earth and aether; according to the second one, relative motion exists and thus the [speed of light] should depend on the speed of this motion ("aether wind"), which should be measurable by instruments at rest on Earth's surface. Specific aether models were invented by Augustin-Jean Fresnel who in 1818 proposed that the aether is partially entrained by matter. The other one was proposed by George Stokes in 1845, in which the aether is completely entrained within or in the vicinity of matter.