Aether theories

Aether theories in early modern physics proposed 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 waves. 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.

Theories

Although hypotheses of the aether vary somewhat in detail they all have certain characteristics in common. In 19th century theories, aether is considered to be a physical medium occupying every point in space, including within material bodies. A second essential feature is that aether's properties give rise to the electric and magnetic phenomena and determines the propagation velocity of their effects. Therefore the speed of light and all other propagating effects are determined by the physical properties of the aether at the relevant location, analogous to the way that gaseous, liquid and solid media affect the propagation of sound waves. The aether is considered the overall reference frame for the universe and thus velocities are all absolute relative to its rest frame. Therefore, in this view, any physical consequences of those velocities are considered as having an absolute, i. e. real effects.

John Bell, interviewed by Paul Davies in "The Ghost in the Atom" has suggested that an aether theory might help resolve the EPR paradox by allowing a reference frame in which signals go faster than light.^[2] He suggests Lorentz contraction is perfectly coherent, not inconsistent with relativity, and could produce an aether theory perfectly consistent with the Michelson-Morley experiment. Bell suggests the aether was wrongly rejected on purely philosophical grounds: "what is unobservable does not exist" [p.49]. Einstein found the non-aether theory simpler and more elegant, but Bell suggests that doesn't rule it out. Besides the arguments based on his interpretation of quantum mechanics, Bell also suggests resurrecting the aether because it is a useful pedagogical device. That is, many problems are solved more easily by imagining the existence of an aether.

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.

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

Contemporary Ideas

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.”^[3]

Modern analogies

In physics there is no concept considered exactly analogous to the aether. However, dark energy is sometimes called quintessence due to its similarity to the classical aether.

Quantum mechanics can be used to describe spacetime as being "bitty" at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. Instead of being "smooth", the vacuum is described as looking like "quantum foam". It has been suggested that this seething mass of virtual particles may be the equivalent in modern physics of a particulate aether.

References

^ "Aether", American Heritage Dictionary of the English Language.
^ As suggested by Bell, Ribaric and Sustersic used a sort of a Feynman medium to put forward a finite, relativistic theory of quantum scattering of fundamental particles which also resolves the FRD paradox.
^ Laughlin, Robert B. (2005). A Different Universe: Reinventing Physics from the Bottom Down. NY, NY: Basic Books. pp. 120–121. ISBN 978-0465038282.

Simpson, Thomas K. Maxwell on the Electomagnetic Field, Rutgers University Press (2001)
"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)
Maxwell, James Clerk, "On Physical Lines of Force". 1861.
Epple, M. Topology, Matter, and Space, I: Topological Notions in 19th-Century Natural Philosophy. Arch. Hist. Exact Sci. 52 (1998) 297–392.