## Monday, 15 October 2012

### Aether Theory

Aether theories in physics propose the existence of a medium, the aether (also spelled ether, from the Greek word (αἰθήρ), meaning "upper air" or "pure, fresh air"[1]), a space-filling substance or field, thought to be necessary as a transmission medium for the propagation of electromagnetic or gravitational forces. The assorted aether theories embody the various conceptions of this "medium" and "substance". This early modern aether has little in common with the aether of classical elements from which the name was borrowed. Since the development of special relativity, theories using a substantial aether are not used any more in modern physics, and are replaced by more abstract models.[2]

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## Historical models

### Luminiferous aether

In 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 aether

From 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 relativity

Einstein sometimes used the word aether for the gravitational field within general relativity, but this terminology never gained widespread support.[3]
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.[4]

### Quantum vacuum

Quantum 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[5] 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.[6]
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.”[7]

### Pilot waves

Louis de Broglie stated:
any particle, even isolated, has to be imagined as in continuous “energetic contact” with a hidden medium[8][9]

### Dark Energy as Aether

There 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[citation needed]. Some scientists are starting to see dark energy as a new reference to the concept of the aether[citation needed].
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."[10]
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.
Then in 2004 this idea of modified Newtonian dynamics (MOND) was reconciled with general relativity by Jacob Bekenstein at the Hebrew University in Jerusalem, Israel (New Scientist, 22 January 2005, p 10), making MOND a genuine contender in the eyes of some physicists...
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."[11]

## References

1. ^ "Aether", American Heritage Dictionary of the English Language.
2. ^ Born, Max (1964), Einstein's Theory of Relativity, Dover Publications, ISBN 0-486-60769-0
3. ^ 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
4. ^ Einstein, Albert: "Ether and the Theory of Relativity" (1920), republished in Sidelights on Relativity (Methuen, London, 1922)
5. ^ Dirac, Paul: "Is there an Aether?", Nature 168 (1951), p. 906.
6. ^ Kragh, Helge (2005). Dirac. A Scientific Biography. Cambridge: Cambridge University Press. pp. 200-203. ISBN 0-521-01756-4.
7. ^ 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.
8. ^ Annales de la Fondation Louis de Broglie, Volume 12, no.4, 1987
9. ^ 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"
10. ^ 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. Retrieved 27 August 2012.
11. ^ Zeeya, Morali (26). "Ether returns to oust dark matter". New Scientist. Retrieved 27 August 2012.