Editing Luminiferous aether (section)

====Second-order experiments====
[[File:Michelson-Morley experiment conducted with white light.png|right|thumb|The Michelson–Morley experiment compared the time for light to reflect from mirrors in two orthogonal directions.]]
While the ''first''-order experiments could be explained by a modified stationary aether, more precise ''second''-order experiments were expected to give positive results. However, no such results could be found.

The famous [[Michelson–Morley experiment]] compared the source light with itself after being sent in different directions and looked for changes in phase in a manner that could be measured with extremely high accuracy. In this experiment, their goal was to determine the velocity of the Earth through the aether.<ref group=E name=michel1 /><ref group=E name=michel3 /> The publication of their result in 1887, the [[null result]], was the first clear demonstration that something was seriously wrong with the aether hypothesis (Michelson's first experiment in 1881 was not entirely conclusive). In this case the MM experiment yielded a shift of the fringing pattern of about 0.01 of a [[Fringe shift|fringe]], corresponding to a small velocity. However, it was incompatible with the expected aether wind effect due to the Earth's (seasonally varying) velocity which would have required a shift of 0.4 of a fringe, and the error was small enough that the value may have indeed been zero.  Therefore, the [[null hypothesis]], the hypothesis that there was no aether wind, could not be rejected.  More modern experiments have since reduced the possible value to a number very close to zero, about 10<sup>−17</sup>.

{{Blockquote|It is obvious from what has gone before that it would be hopeless to attempt to solve the question of the motion of the solar system by observations of optical phenomena at the surface of the earth.|A. Michelson and E. Morley. "On the Relative Motion of the Earth and the Luminiferous Æther". ''[[Philosophical Magazine]]'' S. 5. Vol. 24. No. 151. December 1887.<ref>{{cite web|url=http://www.aip.org/history/gap/PDF/michelson.pdf|title=Selected Papers of Great American Physicists|website=www.aip.org|access-date=30 April 2018|url-status=live|archive-url=https://web.archive.org/web/20150715063415/https://www.aip.org/history/gap/PDF/michelson.pdf|archive-date=15 July 2015}}</ref>}}

A series of experiments using similar but increasingly sophisticated apparatuses all returned the null result as well. Conceptually different experiments that also attempted to detect the motion of the aether were the [[Trouton–Noble experiment]]&nbsp;(1903),<ref group=E name=Trouton2 /> whose objective was to detect [[Torsion (mechanics)|torsion]] effects caused by electrostatic fields, and [[Experiments of Rayleigh and Brace|the experiments of Rayleigh and Brace]] (1902, 1904),<ref group=E name=Rayleigh2 /><ref group=E name=Brace1 /> to detect [[double refraction]] in various media. However, all of them obtained a null result, like Michelson–Morley (MM) previously did.

These "aether-wind" experiments led to a flurry of efforts to "save" aether by assigning to it ever more complex properties, and only a few scientists, like [[Emil Cohn]] or [[Alfred Bucherer]], considered the possibility of the abandonment of the aether hypothesis. Of particular interest was the possibility of "aether entrainment" or "aether drag", which would lower the magnitude of the measurement, perhaps enough to explain the results of the Michelson–Morley experiment. However, as noted earlier, aether dragging already had problems of its own, notably aberration. In addition, the interference experiments of [[Oliver Lodge|Lodge]] (1893, 1897) and [[Ludwig Zehnder]] (1895), aimed to show whether the aether is dragged by various, rotating masses, showed no aether drag.<ref group=E name=Lodge /><ref group=E name=Lodge2 /><ref group=E name=Zehnder /> A more precise measurement was made in the [[Hammar experiment]] (1935), which ran a complete MM experiment with one of the "legs" placed between two massive lead blocks.<ref group=E name=Hammar /> If the aether was dragged by mass then this experiment would have been able to detect the drag caused by the lead, but again the null result was achieved. The theory was again modified, this time to suggest that the entrainment only worked for very large masses or those masses with large magnetic fields. This too was shown to be incorrect by the [[Michelson–Gale–Pearson experiment]], which detected the Sagnac effect due to Earth's rotation (see [[Aether drag hypothesis]]).

Another completely different attempt to save "absolute" aether was made in the [[Lorentz–FitzGerald contraction hypothesis]], which posited that ''everything'' was affected by travel through the aether. In this theory, the reason that the Michelson–Morley experiment "failed" was that the apparatus contracted in length in the direction of travel. That is, the light was being affected in the "natural" manner by its travel through the aether as predicted, but so was the apparatus itself, cancelling out any difference when measured. FitzGerald had inferred this hypothesis from a paper by [[Oliver Heaviside]]. Without referral to an aether, this physical interpretation of relativistic effects was [[Kennedy–Thorndike experiment|shared by Kennedy and Thorndike]] in 1932 as they concluded that the interferometer's arm contracts and also the frequency of its light source "very nearly" varies in the way required by relativity.<ref group=E name=kenn /><ref>They commented in a footnote: "From [the Michelson–Morley] experiment it is not inferred that the velocity of the earth is but a few kilometers per second, but rather that the dimensions of the apparatus vary very nearly as required by relativity. From the present experiment we similarly infer that the frequency of light varies conformably to the theory."</ref>

Similarly, the [[Sagnac effect]], observed by G. Sagnac in 1913, was immediately seen to be fully consistent with special relativity.<ref group=E name=Sagnac1 /><ref group=E name=Sagnac2 /> In fact, the [[Michelson–Gale–Pearson experiment]] in 1925 was proposed specifically as a test to confirm the relativity theory, although it was also recognized that such tests, which merely measure absolute rotation, are also consistent with non-relativistic theories.<ref>The confusion over this point can be seen in Sagnac's conclusion that "in the ambient space, light is propagated with a velocity V0, independent of the movement as a whole of the luminous source O and the optical system. That is a property of space which experimentally characterizes the luminiferous aether." The invariance of light speed, independent of the movement of the source, is also one of the two fundamental principles of special relativity.</ref>

During the 1920s, the experiments pioneered by Michelson were repeated by [[Dayton Miller]], who publicly proclaimed positive results on several occasions, although they were not large enough to be consistent with any known aether theory. However, other researchers were unable to duplicate Miller's claimed results. Over the years the experimental accuracy of such measurements has been raised by many orders of magnitude, and no trace of any violations of Lorentz invariance has been seen. (A later re-analysis of Miller's results concluded that he had underestimated the variations due to temperature.)

Since the Miller experiment and its unclear results there have been many more experimental attempts to detect the aether. Many experimenters have claimed positive results. These results have not gained much attention from mainstream science, since they contradict a large quantity of high-precision measurements, all the results of which were consistent with special relativity.<ref>Roberts, Schleif (2006); Physics FAQ: [http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Experiments_not_consistent_with_SS Experiments that Apparently are NOT Consistent with SR/GR] {{webarchive|url=https://web.archive.org/web/20091015153529/http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html |date=2009-10-15 }}</ref>