Editing Luminiferous aether (section)

==End of aether==

===Special relativity===
Aether theory was dealt another blow when the Galilean transformation and Newtonian dynamics were both modified by [[Albert Einstein]]'s [[Special relativity|special theory of relativity]], giving the mathematics of [[Lorentz ether theory|Lorentzian electrodynamics]] a new, "non-aether" context.<ref group=A name=einselektro /> Unlike most major shifts in scientific thought, special relativity was adopted by the scientific community remarkably quickly, consistent with Einstein's later comment that the laws of physics described by the Special Theory were "ripe for discovery" in 1905.<ref group=B name=born /> Max Planck's early advocacy of the special theory, along with the elegant formulation given to it by [[Hermann Minkowski]], contributed much to the rapid acceptance of special relativity among working scientists.

Einstein based his theory on Lorentz's earlier work. Instead of suggesting that the mechanical properties of objects changed with their constant-velocity motion through an undetectable aether, Einstein proposed to deduce the characteristics that any successful theory must possess in order to be consistent with the most basic and firmly established principles, independent of the existence of a hypothetical aether. He found that the Lorentz transformation must transcend its connection with Maxwell's equations, and must represent the fundamental relations between the space and time coordinates of [[inertial frame of reference|inertial frames of reference]]. In this way he demonstrated that the laws of physics remained invariant as they had with the Galilean transformation, but that light was now invariant as well.

With the development of the special theory of relativity, the need to account for a single universal [[frame of reference]] had disappeared – and acceptance of the 19th-century theory of a luminiferous aether disappeared with it. For Einstein, the Lorentz transformation implied a conceptual change: that the concept of position in space or time was not absolute, but could differ depending on the observer's location and velocity.

Moreover, in another paper published the same month in 1905, Einstein made several observations on a then-thorny problem, the [[photoelectric effect]]. In this work he demonstrated that light can be considered as particles that have a "wave-like nature". Particles obviously do not need a medium to travel, and thus, neither did light. This was the first step that would lead to the full development of [[quantum mechanics]], in which the wave-like nature ''and'' the particle-like nature of light are both considered as valid descriptions of light. A summary of Einstein's thinking about the aether hypothesis, relativity and light quanta may be found in his 1909 (originally German) lecture "The Development of Our Views on the Composition and Essence of Radiation".<ref group=A name=einsdev />

Lorentz on his side continued to use the aether hypothesis. In his lectures of around 1911, he pointed out that what "the theory of relativity has to say ... can be carried out independently of what one thinks of the aether and the time". He commented that "whether there is an aether or not, electromagnetic fields certainly exist, and so also does the energy of the electrical oscillations" so that, "if we do not like the name of 'aether', we must use another word as a peg to hang all these things upon". He concluded that "one cannot deny the bearer of these concepts a certain substantiality".<ref>Lorentz wrote: "One cannot deny to the bearer of these properties a certain substantiality, and if so, then one may, in all modesty, call true time the time measured by clocks which are fixed in this medium, and consider simultaneity as a primary concept." However, he went on to say that this was based on his conception of "infinite velocity", which according to his own theory is not physically realizable. Lorentz also admitted that the postulate of an absolute but undetectable rest frame was purely metaphysical, and had no empirical consequences.</ref><ref group=B name=miller />

Nevertheless, in 1920, Einstein gave an address at [[Leiden University]] in which he commented "More careful reflection teaches us however, that the special theory of relativity does not compel us to deny ether. We may assume the existence of an ether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. We shall see later that this point of view, the conceivability of which I shall at once endeavour to make more intelligible by a somewhat halting comparison, is justified by the results of the general theory of relativity". He concluded his address by saying that "according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable."<ref>{{Cite web|url=https://mathshistory.st-andrews.ac.uk/Extras/Einstein_ether/|title=Einstein: Ether and Relativity|website=Maths History|accessdate=7 August 2023}}</ref>

=== Other models ===
{{Main|Aether theories}}
In later years there have been a few individuals who advocated a neo-Lorentzian approach to physics, which is Lorentzian in the sense of positing an absolute true state of rest that is undetectable and which plays no role in the predictions of the theory. (No violations of [[Lorentz covariance]] have ever been detected, despite strenuous efforts.) Hence these theories resemble the 19th century aether theories in name only. For example, the founder of quantum field theory, [[Paul Dirac]], stated in 1951 in an article in Nature, titled "Is there an Aether?" that "we are rather forced to have an aether".<ref>Dirac wrote about his theory: "We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an aether."</ref><ref group=A name=dirac /> However, Dirac never formulated a complete theory, and so his speculations found no acceptance by the scientific community.

===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 />