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Michelson–Morley experiment
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== Detecting the aether == Physics theories of the 19th century assumed that just as surface water waves must have a supporting substance, i.e., a "medium", to move across (in this case water), and audible [[sound]] requires a medium to transmit its wave motions (such as air or water), so light must also require a medium, the "[[luminiferous aether]]", to transmit its wave motions. Because light can travel through a vacuum<!-- the article "a" is optional, and grammatically ok, so please do not remove it as a grammar correction. See [[wp:RETAIN]]-->, it was assumed that even a vacuum must be filled with aether. Because the [[speed of light]] is so great, and because material bodies pass through the ''aether'' without obvious friction or drag, it was assumed to have a highly unusual combination of properties. Designing experiments to investigate these properties was a high priority of 19th-century physics.<ref group=A name=Whittaker />{{rp|411ff}} [[Earth]] orbits around the [[Sun]] at a speed of around {{Convert|30|km/s|abbr = on}}, or {{Convert| 107000|km/h|abbr = on}}. The Earth is in motion, so two main possibilities were considered: (1) The aether is stationary and only partially [[drag (physics)|dragged]] by Earth (proposed by [[Augustin-Jean Fresnel]] in 1818), or (2) the aether is completely dragged by Earth and thus shares its motion at Earth's surface (proposed by [[Sir George Stokes, 1st Baronet]] in 1844).<ref group=A name=Jan /> In addition, [[James Clerk Maxwell]] (1865) recognized the [[Electrodynamics|electromagnetic]] nature of light and developed what are now called [[Maxwell's equations]], but these equations were still interpreted as describing the motion of waves through an aether, whose state of motion was unknown. Eventually, Fresnel's idea of an (almost) stationary aether was preferred because it appeared to be confirmed by the [[Fizeau experiment]] (1851) and the [[aberration of light|aberration of star light]].<ref group=A name=Jan /> [[Image:aetherWind.svg|thumb|right|300px|A depiction of the concept of the "[[aether wind]]"]] According to the stationary and the partially dragged aether hypotheses, Earth and the aether are in relative motion, implying that a so-called "aether wind" (Fig. 2) should exist. Although it would be theoretically possible for the Earth's motion to match that of the aether at one moment in time, it was not possible for the Earth to remain at rest with respect to the aether at all times, because of the variation in both the direction and the speed of the motion. At any given point on the Earth's surface, the magnitude and direction of the wind would vary with time of day and season. By analyzing the return speed of light in different directions at various different times, it was thought to be possible to measure the motion of the Earth relative to the aether. The expected relative difference in the measured speed of light was quite small, given that the velocity of the Earth in its orbit around the Sun has a magnitude of about one hundredth of one percent of the speed of light.<ref group=A name=Whittaker />{{rp|417ff}} During the mid-19th century, measurements of aether wind effects of first order, i.e., effects proportional to ''v''/''c'' (''v'' being Earth's velocity, ''c'' the speed of light) were thought to be possible, but no direct measurement of the speed of light was possible with the accuracy required. For instance, the [[Fizeau wheel]] could measure the speed of light to perhaps 5% accuracy, which was quite inadequate for measuring directly a first-order 0.01% change in the speed of light. A number of physicists therefore attempted to make measurements of indirect first-order effects not of the speed of light itself, but of variations in the speed of light (see [[Luminiferous aether#First order experiments|First order aether-drift experiments]]). The [[Hoek experiment]], for example, was intended to detect [[interferometry|interferometric]] [[fringe shift]]s due to speed differences of oppositely propagating light waves through water at rest. The results of such experiments were all negative.<ref group="A" name="laub" /> This could be explained by using [[Aether drag hypothesis#Partial aether dragging|Fresnel's dragging coefficient]], according to which the aether and thus light are partially dragged by moving matter. Partial aether-dragging would thwart attempts to measure any first order change in the speed of light. As pointed out by Maxwell (1878), only experimental arrangements capable of measuring second order effects would have any hope of detecting aether drift, i.e., effects proportional to ''v''<sup>2</sup>/''c''<sup>2</sup>.<ref group=A name=maxa /><ref group=A name=maxb /> Existing experimental setups, however, were not sensitive enough to measure effects of that size.{{citation needed|date=May 2025}}
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