Historical models
 Luminiferous aetherIn the 19th century, luminiferous aether (or ether), meaning light-bearing aether, was the term used to describe a medium for the propagation of light (electromagnetic radiation). However, a series of increasingly complex experiments had been carried out in the late 1800s like the Michelson-Morley experiment in an attempt to detect the motion of earth through the aether, and had failed to do so. A range of proposed aether-dragging theories could explain the null result but these were more complex, and tended to use arbitrary-looking coefficients and physical assumptions. Hendrik Lorentz and George Francis FitzGerald offered within the framework of Lorentz ether theory a more elegant solution to how the motion of an absolute aether could be undetectable (length contraction), but if their equations were correct, Albert Einstein's 1905 special theory of relativity could generate the same mathematics without referring to an aether at all. This led most physicists to conclude that the classical notion of aether was not a useful concept.
 Mechanical gravitational aetherFrom the 16th until the late 19th century, gravitational phenomena had also been modeled utilizing an aether. The most well-known formulation is Le Sage's theory of gravitation, although other models were proposed by Isaac Newton, Bernhard Riemann, and Lord Kelvin. None of those concepts is considered to be viable by the scientific community today.
 Non-standard interpretations in modern physics
 General relativityEinstein sometimes used the word aether for the gravitational field within general relativity, but this terminology never gained widespread support.
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.
 Quantum vacuumQuantum mechanics can be used to describe spacetime as being non-empty at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. It has been suggested by some such as Paul Dirac that this quantum vacuum may be the equivalent in modern physics of a particulate aether. However, Dirac's aether hypothesis was motivated by his dissatisfaction with quantum electrodynamics, and it never gained support by the mainstream scientific community.
Robert B. Laughlin, Nobel Laureate in Physics, endowed chair in physics, Stanford University, had this to say about ether in contemporary theoretical physics:
It is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed [..] The word 'ether' has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum. . . . Relativity actually says nothing about the existence or nonexistence of matter pervading the universe, only that any such matter must have relativistic symmetry. [..] It turns out that such matter exists. About the time relativity was becoming accepted, studies of radioactivity began showing that the empty vacuum of space had spectroscopic structure similar to that of ordinary quantum solids and fluids. Subsequent studies with large particle accelerators have now led us to understand that space is more like a piece of window glass than ideal Newtonian emptiness. It is filled with 'stuff' that is normally transparent but can be made visible by hitting it sufficiently hard to knock out a part. The modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo.”
 Pilot wavesLouis de Broglie stated:
 Dark Energy as AetherThere has been a lot of discussion about dark energy and dark matter as concepts to explain certain anomalies in physics such as between mass and gravity. Some scientists are starting to see dark energy as a new reference to the concept of the aether.
One such article was published in Physics Letters B in 2012. As the abstract states, "In the generalized Einstein-aether theories by taking a special form of the Lagrangian density of aether field, the possibility of Einstein-aether theory as an alternative to dark energy model is discussed in detail, that is, taking a special aether field as a dark energy candidate."
Earlier, New Scientist reported on research by a team at the University of Oxford seeking to link dark energy and the aether to resolve a problem with gravity and mass. Keep in mind though that New Scientist is a non-peer reviewed weekly publication which focuses on sensationalism to sell ad space.
- Starkman and colleagues Tom Zlosnik and Pedro Ferreira of the University of Oxford are now reincarnating the ether in a new form to solve the puzzle of dark matter, the mysterious substance that was proposed to explain why galaxies seem to contain much more mass than can be accounted for by visible matter. They posit an ether that is a field, rather than a substance, and which pervades space-time.
- This is not the first time that physicists have suggested modifying gravity to do away with this unseen dark matter. The idea was originally proposed by Mordehai Milgrom while at Princeton University in the 1980s. He suggested that the inverse-square law of gravity only applies where the acceleration caused by the field is above a certain threshold, say a0. Below that value, the field dissipates more slowly, explaining the observed extra gravity. "It wasn't really a theory, it was a guess," says cosmologist Sean Carroll at the University of Chicago in Illinois.
- Now Starkman's team has reproduced Bekenstein's results using just one field - the new ether (www.arxiv.org/astro-ph/ 0607411). Even more tantalisingly, the calculations reveal a close relationship between the threshold acceleration a0 - which depends on the ether - and the rate at which the universe's expansion is accelerating. Astronomers have attributed this acceleration to something called dark energy, so in a sense the ether is related to this entity. That they have found this connection is a truly profound thing, says Bekenstein. The team is now investigating how the ether might cause the universe's expansion to speed up.
- Andreas Albrecht, a cosmologist at the University of California, Davis, believes that this ether model is worth investigating further. "We've hit some really profound problems with cosmology Ð with dark matter and dark energy," he says. "That tells us we have to rethink fundamental physics and try something new."
 See also
- "Aether", American Heritage Dictionary of the English Language.
- Born, Max (1964), Einstein's Theory of Relativity, Dover Publications, ISBN 0-486-60769-0
- Kostro, L. (1992), "An outline of the history of Einstein's relativistic ether concept", in Jean Eisenstaedt & Anne J. Kox, Studies in the history of general relativity, 3, Boston-Basel-Berlin: Birkäuser, pp. 260–280, ISBN 0-8176-3479-7
- Einstein, Albert: "Ether and the Theory of Relativity" (1920), republished in Sidelights on Relativity (Methuen, London, 1922)
- Dirac, Paul: "Is there an Aether?", Nature 168 (1951), p. 906.
- Kragh, Helge (2005). Dirac. A Scientific Biography. Cambridge: Cambridge University Press. pp. 200-203. ISBN 0-521-01756-4.
- Laughlin, Robert B. (2005). A Different Universe: Reinventing Physics from the Bottom Down. NY, NY: Basic Books. pp. 120–121. ISBN 978-0-465-03828-2.
- Annales de la Fondation Louis de Broglie, Volume 12, no.4, 1987
- Foundations of Physics, Volume 13, Issue 2. Springer. 1983. pp. 253-286. doi:10.1007/BF01889484. "It is shown that one can deduce the de Broglie waves as real collective Markov processes on the top of Dirac's aether"
- Meng, Xinhe; Du, Xiaolong (April). "Einstein-aether theory as an alternative to dark energy model?". Physics Letters B 710 (4-5): 493-499. doi:10.1016/j.physletb.2012.03.024. http://adsabs.harvard.edu/abs/2012PhLB..710..493M. Retrieved 27 August 2012.
- Zeeya, Morali (26). "Ether returns to oust dark matter". New Scientist. http://www.eurekalert.org/pub_releases/2006-08/ns-ert082306.php. Retrieved 27 August 2012.
 Further reading
- Whittaker, Edmund Taylor (1910), A History of the theories of aether and electricity (1 ed.), Dublin: Longman, Green and Co., http://www.archive.org/details/historyoftheorie00whitrich
- Schaffner, Kenneth F. (1972), Nineteenth-century aether theories, Oxford: Pergamon Press, ISBN 0-08-015674-6
- Darrigol, Olivier (2000), Electrodynamics from Ampére to Einstein, Oxford: Clarendon Press, ISBN 0-19-850594-9
- Maxwell, James Clerk (1878), "Ether", Encyclopædia Britannica Ninth Edition 8: 568–572, http://en.wikisource.org/wiki/Encyclop%C3%A6dia_Britannica_Ninth_Edition/Ether
- Harman, P.H. (1982), Energy, Force and Matter: The Conceptual Development of Nineteenth Century Physics, Cambridge: Cambridge University Press, ISBN 0-521-28812-6
- Decaen, Christopher A. (2004), "Aristotle's Aether and Contemporary Science", The Thomist 68: 375–429, http://www.thomist.org/jour/2004/July/2004%20July%20A%20Dec.htm, retrieved 2011-03-05.
- Joseph Larmor, "Ether", Encyclopædia Britannica, Eleventh Edition (1911).
- Oliver Lodge, "Ether", Encyclopædia Britannica, Thirteenth Edition (1926).
- "A Ridiculously Brief History of Electricity and Magnetism; Mostly from E. T. Whittaker’s A History of the Theories of Aether and Electricity". (PDF format)
- Epple, M. Topology, Matter, and Space, I: Topological Notions in 19th-Century Natural Philosophy. Arch. Hist. Exact Sci. 52 (1998) 297–392.