Editing Luminiferous aether (section)

===Einstein's views on the aether===
When Einstein was still a student in the Zurich Polytechnic in 1900, he was very interested in the idea of aether. His initial proposal of research thesis was to do an experiment to measure how fast the Earth was moving through the aether.<ref>{{Cite book|title=Einstein: His life and Universe|url=https://archive.org/details/einsteinhislifeu0000isaa|url-access=registration|last=Isaacson|first=Walter|publisher=Simon & Schuster|year=2007|location=New York|pages=[https://archive.org/details/einsteinhislifeu0000isaa/page/47 47]–48}}</ref>  "The velocity of a wave is proportional to the square root of the elastic forces which cause [its] propagation, and inversely proportional to the mass of the aether moved by these forces."<ref>Albert Einstein's 'First' Paper (1894 or 1895), http://www.straco.ch/papers/Einstein%20First%20Paper.pdf {{Webarchive|url=https://web.archive.org/web/20200727021612/http://www.straco.ch/papers/Einstein |date=2020-07-27 }}</ref>

In 1916, after Einstein completed his foundational work on [[general relativity]], Lorentz wrote a letter to him in which he speculated that within general relativity the aether was re-introduced. In his response Einstein wrote that one can actually speak about a "new aether", but one may not speak of motion in relation to that aether. This was further elaborated by Einstein in some semi-popular articles (1918, 1920, 1924, 1930).<ref group="A" name="einsta" /><ref group="A" name="einstb" /><ref group="A" name="einstc" /><ref group="A" name="einstd" /><ref group="B" name="kosta" /><ref group="B" name="stach" /><ref group="B" name="kostb" />

In 1918, Einstein publicly alluded to that new definition for the first time.<ref group=A name=einsta /> Then, in the early 1920s, in a lecture which he was invited to give at Lorentz's university in Leiden, Einstein sought to reconcile the theory of relativity with [[Lorentz ether theory|Lorentzian aether]]. In this lecture Einstein stressed that special relativity took away the last mechanical property of the aether: immobility. However, he continued that special relativity does not necessarily rule out the aether, because the latter can be used to give physical reality to acceleration and rotation. This concept was fully elaborated within [[general relativity]], in which physical properties (which are partially determined by matter) are attributed to space, but no substance or state of motion can be attributed to that "aether" (by which he meant curved space-time).<ref group=B name=kostb /><ref group=A name=einstb /><ref>Einstein 1920: ''We may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an aether. According to the general theory of relativity space without aether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this aether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.''</ref>

In another paper of 1924, named "Concerning the Aether", Einstein argued that Newton's absolute space, in which acceleration is absolute, is the "Aether of Mechanics". And within the electromagnetic theory of Maxwell and Lorentz one can speak of the "Aether of Electrodynamics", in which the aether possesses an absolute state of motion. As regards special relativity, also in this theory acceleration is absolute as in Newton's mechanics. However, the difference from the electromagnetic aether of Maxwell and Lorentz lies in the fact that "because it was no longer possible to speak, in any absolute sense, of simultaneous states at different locations in the aether, the aether became, as it were, four-dimensional since there was no objective way of ordering its states by time alone". Now the "aether of special relativity" is still "absolute", because matter is affected by the properties of the aether, but the aether is not affected by the presence of matter. This asymmetry was solved within general relativity. Einstein explained that the "aether of general relativity" is not absolute, because matter is influenced by the aether, just as matter influences the structure of the aether.<ref group=A name=einstc />

The only similarity of this relativistic aether concept with the [[Aether (classical element)|classical aether]] models lies in the presence of physical properties in space, which can be identified through [[Geodesics in general relativity|geodesics]]. As historians such as [[John Stachel]] argue, Einstein's views on the "new aether" are not in conflict with his abandonment of the aether in 1905. As Einstein himself pointed out, no "substance" and no state of motion can be attributed to that new aether. Einstein's use of the word "aether" found little support in the scientific community, and played no role in the continuing development of modern physics.<ref group=B name=kosta /><ref group=B name=stach /><ref group=B name=kostb />